The Technology of Anodizing Aluminum [2 ed.] 0905228081, 9780905228082

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THE TECHNOLOGY OF ANODIZING ALUMINIUM A. W. Brace B.Sc. (Econ.)., C.G.I.A., C. Eng.,

F.I.M.F..F.1. Cofr. T.

P. G. Sheasby B.Sc.. F.I.M.F.

lj|echnicopy

Limited

1 ■ .. c - . IV J '

of tanks used for anodizing operations.

Fig. 30. Aluminium bus bars and anode rails with bolted connections on plant for producing bright anodized bumpers for the Volvo 144. [Courtesy: Svenska Metallverken, Vdsteras

Cathode and bus-bar protection

Experience has been that aluminium bus-bars give very satisfactory service in anodizing plants but where strong electrolytes are being used, e.g. 250 - 350 g/1 acid, there are dangers of attack on the aluminium by the spray from the anodizing solution. However, it should also be borne in mind that it is in any case quite good practice to coat with acid-resistant paint all those surfaces that are exposed to the acid atmosphere of the plant. It is obviously necessary to remember in any case that dissolution of copper 93

in the electrolyte appears to be harmful, according to work that has been carried out by several investigators and which is referred to in the next chapter, and therefore there is a strong argument in favour of using aluminium cathode rails and cathode strips exclusively. If there are reasons why copper cathode rails are preferred, including their use in conjunction with lead cathodes, then every precaution must be taken to protect the copper from attack by the acid so that there is no build-up of copper salt in the anodizing bath. The design and installation of bus-bars is considered on p. 105.

to the shape and size of the tank, and also to the degree of agitation produced. It will be indicated in the next section that one of the methods of controlling the temperature of the anodizing electrolyte is to pump out electrolyte and pass it through an external heat exchanger and then return it to the main tank. If this method is used it can again provide agitation so long as the rate is high enough. Generally speaking, the rate of pumping will depend upon the total heat input, due to the current used by the rectifier and the operating temperature, as well as the ratio of bath volume to kVA input. In general, however, air agitation is used as well as the movement produced by the acid circulation.

Agitation Refrigeration and temperature control It is also necessary to provide for means of agitation to each anodizing tank so that a constant temperature is maintained within the bath. The most commonly employed method is use of a perforated PVC or polypropylene tube held at the bottom of the anodizing tank and with holes of about 3 mm ( 1/8") diameter at approximately 100 - 150 mm (4" - 6") intervals. The diameter of the tube is usually in the order of 18 - 32 mm (% " - 1 % "). The tube is fed by compressed air through a pressure reducing valve, but oil traps must be fitted in the air line as great care must be taken to avoid oil being carried into the anodizing electrolyte. In larger plants it is now much more usual to use blowers of the positive displacement or centrifugal type. However care must be taken in choosing a blower as some are extremely noisy, and the latter type are preferred for this reason. Blowers are favoured because they produce a large volume of low pressure air and this is what is required for agitation purposes. The high pressure produced by a compressor can easily lead to uneven agitation in a long tank, for example in an architectural plant. In addition, if a push-pull fume extraction system is used, the blower can be used as the source of the ‘push’ air. One factor that is frequently overlooked in respect of agitation is that this is a much more important variable than is generally realised, and that even a rating of the air pressure entering the tank may by no means be adequate for really satisfactory control. There are available flow meters which will measure the rate of flow of air into the anodizing tank and it is better to control the agitation by controlling the flow rate rather than by the air pressure. It is difficult to know the agitation required in order to ensure consistent properties of the anodic oxide coating, but Turner 1 suggests a level of 0.09 - 0.22 m 3 of air per m 2 of tank surface in rinse tanks, and 0.22 - 0.45 m 3 per m 2 of tank surface in anodizing tanks. Other means of agitation have been employed and there are circumstances in which these may well merit careful consideration. It is quite possible to provide agitation by means of a mechanical double paddle. This arrangement can work quite satisfactorily on small tanks, but its use on larger tanks requires quite a degree of attention to the actual design and positioning of the paddle in relation 94

As has been indicated earlier there is an inherent tendency for the temperature of the anodizing electrolyte to rise progressively during use since not only does the current required to produce the anodic oxide coating become converted to heat in the anodizing bath, but it is further supplemented by approximately 400 K.cals/gm due to the heat of formation of the oxide coating. The temperature of the electrolyte affects the porosity of the anodic coating and can also affect the hardness and the thickness of the film. Under limiting conditions the coating will become softer and more powdery as the temperature rises since the effective temperature at the surface between the anodic coating and the electrolyte will be correspondingly higher, and this will also limit the maximum thickness attainable. It is, therefore, necessary to provide some means of keeping the temperature of the anodizing bath constant by suitable cooling. For a number of years anodizers used mains water to provide this necessary cooling. This water was fed through suitable cooling coils placed along the sides of the tank, usually lead coils. The flow of water was adjusted to keep the temperature reasonably constant as indicated by a thermometer. This means of control has been found to be somewhat inadequate against the current needs of high output per unit of floor space, with the consequent high loading and current employed. In any case the temperature of mains water can vary widely and in particular it can rise in the summer to the point at which it is quite incapable of providing sufficient cooling, although obviously there are many local geographical factors that can modify these remarks. A further problem that has arisen is the increasing shortage of water in the main industrial areas and consequently higher cost. Some firms have been in the fortunate position of being able to drill a bore-hole and tap spring water at a depth where its temperature remains surprisingly constant throughout the year. If this temperature is in the order of 8 - 10°C and the supply adequate, this source may prove suitable. On the other hand, the trend has become increasingly towards providing large-scale refrigeration plants to ensure the necessary degree of cooling and, with it, the means of controlling the temperature to within very precise limits. 95

in the electrolyte appears to be harmful, according to work that has been carried out by several investigators and which is referred to in the next chapter, and therefore there is a strong argument in favour of using aluminium cathode rails and cathode strips exclusively. If there are reasons why copper cathode rails are preferred, including their use in conjunction with lead cathodes, then every precaution must be taken to protect the copper from attack by the acid so that there is no build-up of copper salt in the anodizing bath. The design and installation of bus-bars is considered on p. 105.

to the shape and size of the tank, and also to the degree of agitation produced. It will be indicated in the next section that one of the methods of controlling the temperature of the anodizing electrolyte is to pump out electrolyte and pass it through an external heat exchanger and then return it to the main tank. If this method is used it can again provide agitation so long as the rate is high enough. Generally speaking, the rate of pumping will depend upon the total heat input, due to the current used by the rectifier and the operating temperature, as well as the ratio of bath volume to kVA input. In general, however, air agitation is used as well as the movement produced by the acid circulation.

Agitation Refrigeration and temperature control It is also necessary to provide for means of agitation to each anodizing tank so that a constant temperature is maintained within the bath. The most commonly employed method is use of a perforated PVC or polypropylene tube held at the bottom of the anodizing tank and with holes of about 3 mm ( 1/8") diameter at approximately 100 - 150 mm (4" - 6") intervals. The diameter of the tube is usually in the order of 18 - 32 mm (% " - 1 % "). The tube is fed by compressed air through a pressure reducing valve, but oil traps must be fitted in the air line as great care must be taken to avoid oil being carried into the anodizing electrolyte. In larger plants it is now much more usual to use blowers of the positive displacement or centrifugal type. However care must be taken in choosing a blower as some are extremely noisy, and the latter type are preferred for this reason. Blowers are favoured because they produce a large volume of low pressure air and this is what is required for agitation purposes. The high pressure produced by a compressor can easily lead to uneven agitation in a long tank, for example in an architectural plant. In addition, if a push-pull fume extraction system is used, the blower can be used as the source of the ‘push’ air. One factor that is frequently overlooked in respect of agitation is that this is a much more important variable than is generally realised, and that even a rating of the air pressure entering the tank may by no means be adequate for really satisfactory control. There are available flow meters which will measure the rate of flow of air into the anodizing tank and it is better to control the agitation by controlling the flow rate rather than by the air pressure. It is difficult to know the agitation required in order to ensure consistent properties of the anodic oxide coating, but Turner 1 suggests a level of 0.09 - 0.22 m 3 of air per m 2 of tank surface in rinse tanks, and 0.22 - 0.45 m 3 per m 2 of tank surface in anodizing tanks. Other means of agitation have been employed and there are circumstances in which these may well merit careful consideration. It is quite possible to provide agitation by means of a mechanical double paddle. This arrangement can work quite satisfactorily on small tanks, but its use on larger tanks requires quite a degree of attention to the actual design and positioning of the paddle in relation 94

As has been indicated earlier there is an inherent tendency for the temperature of the anodizing electrolyte to rise progressively during use since not only does the current required to produce the anodic oxide coating become converted to heat in the anodizing bath, but it is further supplemented by approximately 400 K.cals/gm due to the heat of formation of the oxide coating. The temperature of the electrolyte affects the porosity of the anodic coating and can also affect the hardness and the thickness of the film. Under limiting conditions the coating will become softer and more powdery as the temperature rises since the effective temperature at the surface between the anodic coating and the electrolyte will be correspondingly higher, and this will also limit the maximum thickness attainable. It is, therefore, necessary to provide some means of keeping the temperature of the anodizing bath constant by suitable cooling. For a number of years anodizers used mains water to provide this necessary cooling. This water was fed through suitable cooling coils placed along the sides of the tank, usually lead coils. The flow of water was adjusted to keep the temperature reasonably constant as indicated by a thermometer. This means of control has been found to be somewhat inadequate against the current needs of high output per unit of floor space, with the consequent high loading and current employed. In any case the temperature of mains water can vary widely and in particular it can rise in the summer to the point at which it is quite incapable of providing sufficient cooling, although obviously there are many local geographical factors that can modify these remarks. A further problem that has arisen is the increasing shortage of water in the main industrial areas and consequently higher cost. Some firms have been in the fortunate position of being able to drill a bore-hole and tap spring water at a depth where its temperature remains surprisingly constant throughout the year. If this temperature is in the order of 8 - 10°C and the supply adequate, this source may prove suitable. On the other hand, the trend has become increasingly towards providing large-scale refrigeration plants to ensure the necessary degree of cooling and, with it, the means of controlling the temperature to within very precise limits. 95

There are two primary means of cooling employed. For small anodizing tanks one of the most common is that of placing lead covered coils carrying coolant along the sides of the anodizing tank (fig. 31). Some idea of the area of cooling coil required has been given in the C.I.D.A./E.W.A.A. specifications 2. These suggest that the area of cooling surface in m 2 12 x applied current in amps

ation system then consists of a primary direct-expansion system which cools a reservoir of suitable liquid by means of cooling coils. The cooled liquid is then pumped through the cooling coils in the anodizing tank, the circulating pump being actuated by a thermostat in the tank according to the anodizing bath temperature. Cooling coils

/ 18 + water temperature in °C 250 | 21 --------------------------------------\ 2

Anodizing tank

Compressor

Direct system with cooling coils in tank Cooling coils

Anodizing tank

Pump --------- ---------------Chilled water Compressor reservoir

Indirect system with cooling coils in tank

Fig. 32. Refrigeration systems using internal cooling coils in the anodizing tank

Fig. 31. Refrigeration system on small return type automatic with refrigerant pumped through coils immersed in the anodizing tank. [Courtesy: W. Canning & Co. Ltd. In other words, a 1 000 amp. anodizing bath using water at 10 °C for cooling requires a surface area of coil of 6.9 m 2. With cooling coils in the tank there are still two possibilities, using either a direct or indirect system. These are illustrated diagrammatically in fig. 32. With a direct system the refrigerant is circulating directly in the coils in the anodizing tank, and, if a coil becomes accidently damaged and the refrigerant leaks into the electrolyte, some refrigerants can have quite harmful effects. For this reason many companies employ an indirect system employing a coolant system, using chilled water, a water-glycerine mixture or a water-glycol mixture. The refriger96

However with larger plants, which often have a high amperage load per unit volume of anodizing electrolyte, it has become more common to pump the electrolyte out of the tank and cool it externally by means of a heat exchanger before returning it to the tank. This method involves additional engineering problems but can give very effective control, particularly for large architectural anodizing plants and hard anodizing installations. Again there are two main systems which are shown diagrammatically in fig. 33. The first is a direct system involving a single heat exchanger with refrigerant circulating through one side of the exchanger and the sulphuric acid electrolyte through the other. This requires a specialised heat exchanger material and lead coated copper tubes are often used, but great care is necessary in the construction of the exchanger, as, if the tubes fail, acid can be carried directly to the compressor. The acid is pumped continuously through the exchanger, and the compressor is switched on and off as necessary to control the electrolyte temperature. 97

There are two primary means of cooling employed. For small anodizing tanks one of the most common is that of placing lead covered coils carrying coolant along the sides of the anodizing tank (fig. 31). Some idea of the area of cooling coil required has been given in the C.I.D.A./E.W.A.A. specifications 2. These suggest that the area of cooling surface in m 2 12 x applied current in amps

ation system then consists of a primary direct-expansion system which cools a reservoir of suitable liquid by means of cooling coils. The cooled liquid is then pumped through the cooling coils in the anodizing tank, the circulating pump being actuated by a thermostat in the tank according to the anodizing bath temperature. Cooling coils

/ 18 + water temperature in °C 250 | 21 --------------------------------------\ 2

Anodizing tank

Compressor

Direct system with cooling coils in tank Cooling coils

Anodizing tank

Pump --------- ---------------Chilled water Compressor reservoir

Indirect system with cooling coils in tank

Fig. 32. Refrigeration systems using internal cooling coils in the anodizing tank

Fig. 31. Refrigeration system on small return type automatic with refrigerant pumped through coils immersed in the anodizing tank. [Courtesy: W. Canning & Co. Ltd. In other words, a 1 000 amp. anodizing bath using water at 10 °C for cooling requires a surface area of coil of 6.9 m 2. With cooling coils in the tank there are still two possibilities, using either a direct or indirect system. These are illustrated diagrammatically in fig. 32. With a direct system the refrigerant is circulating directly in the coils in the anodizing tank, and, if a coil becomes accidently damaged and the refrigerant leaks into the electrolyte, some refrigerants can have quite harmful effects. For this reason many companies employ an indirect system employing a coolant system, using chilled water, a water-glycerine mixture or a water-glycol mixture. The refriger96

However with larger plants, which often have a high amperage load per unit volume of anodizing electrolyte, it has become more common to pump the electrolyte out of the tank and cool it externally by means of a heat exchanger before returning it to the tank. This method involves additional engineering problems but can give very effective control, particularly for large architectural anodizing plants and hard anodizing installations. Again there are two main systems which are shown diagrammatically in fig. 33. The first is a direct system involving a single heat exchanger with refrigerant circulating through one side of the exchanger and the sulphuric acid electrolyte through the other. This requires a specialised heat exchanger material and lead coated copper tubes are often used, but great care is necessary in the construction of the exchanger, as, if the tubes fail, acid can be carried directly to the compressor. The acid is pumped continuously through the exchanger, and the compressor is switched on and off as necessary to control the electrolyte temperature. 97

------------------------ Pump Anodizing tank

Heat Exchanger

Fig.34. Refrigeration plant serving an architectural anodizing installation capable of handling extrusions up to 24 feet in length. In the foreground can be seen the condenser with pressure valve controlling the flow of cooling water and the compressor with its three stages of unloading. At the bottom right is a centrifugal pump for circulating chilled water while in the middle foreground is the chilled water by-pass with flow meter safety device'.

Compressor

Direct system with acid circulation

3 - Way valve

Anodizing tank

Pum

P Heat Pump Chilled Compressor Exchanger water reservoir

Indirect system with acid circlation

Hi?.

33. Refrigeration systems using acid circulation

The second is an indirect method involving two heat exchange stages. It is inevitably the more elaborate and often the more expensive system but is capable of very good control and a great deal of flexibility. With this system the electrolyte is pumped continuously through a stainless steel or carbon block heat exchanger. The other side of the heat exchanger is fed with cooled water from a reservoir which in turn is cooled by cooling coils through which the refrigerant is pumped. Temperature control is achieved by controlling the flow of chilled water through the heat exchanger by means of a 3-way valve, which can allow partial or complete by-passing of the heat exchanger. The system is flexible because more than one compressor can be used to maintain the temperature of the chilled water tank, and the chilled water can be pumped, via separate circuits, through more than one heat exchanger. Thus, if, for example, one compressor breaks down, the plant can usually continue working if only at reduced capacity (fig. 34).

The refrigeration system to be adopted in a particular plant needs very serious consideration and requires the help of specialists in the field. However a general idea of the quantity of refrigeration required can be obtained using the information provided in Survila’s useful article on this subject. 4 This suggests that: — Refrigeration capacity required

electrical load in watts x 3.42 12 000

tons

The motive power is then approximately 1.2 h.p. per ton of refrigeration. For example an anodizing bath with a capacity of 5 000 amp. at 20 volts requires: 5 000 x 20 x 3.42 , £ £ . ----------------- tons of refrigeration or approximately 30 tons or 36 h.p. of refrigeration. Before leaving this subject it is important to remember that the compressor itself is water cooled and a large volume of water is required for this purpose. It is typically about 12 litres /min. per ton of refrigeration?

98 99

------------------------ Pump Anodizing tank

Heat Exchanger

Fig.34. Refrigeration plant serving an architectural anodizing installation capable of handling extrusions up to 24 feet in length. In the foreground can be seen the condenser with pressure valve controlling the flow of cooling water and the compressor with its three stages of unloading. At the bottom right is a centrifugal pump for circulating chilled water while in the middle foreground is the chilled water by-pass with flow meter safety device'.

Compressor

Direct system with acid circulation

3 - Way valve

Anodizing tank

Pum

P Heat Pump Chilled Compressor Exchanger water reservoir

Indirect system with acid circlation

Hi?.

33. Refrigeration systems using acid circulation

The second is an indirect method involving two heat exchange stages. It is inevitably the more elaborate and often the more expensive system but is capable of very good control and a great deal of flexibility. With this system the electrolyte is pumped continuously through a stainless steel or carbon block heat exchanger. The other side of the heat exchanger is fed with cooled water from a reservoir which in turn is cooled by cooling coils through which the refrigerant is pumped. Temperature control is achieved by controlling the flow of chilled water through the heat exchanger by means of a 3-way valve, which can allow partial or complete by-passing of the heat exchanger. The system is flexible because more than one compressor can be used to maintain the temperature of the chilled water tank, and the chilled water can be pumped, via separate circuits, through more than one heat exchanger. Thus, if, for example, one compressor breaks down, the plant can usually continue working if only at reduced capacity (fig. 34).

The refrigeration system to be adopted in a particular plant needs very serious consideration and requires the help of specialists in the field. However a general idea of the quantity of refrigeration required can be obtained using the information provided in Survila’s useful article on this subject. 4 This suggests that: — Refrigeration capacity required

electrical load in watts x 3.42 12 000

tons

The motive power is then approximately 1.2 h.p. per ton of refrigeration. For example an anodizing bath with a capacity of 5 000 amp. at 20 volts requires: 5 000 x 20 x 3.42 , £ £ . ----------------- tons of refrigeration or approximately 30 tons or 36 h.p. of refrigeration. Before leaving this subject it is important to remember that the compressor itself is water cooled and a large volume of water is required for this purpose. It is typically about 12 litres /min. per ton of refrigeration?

98 99

Good thermostatic control equipment is an essential part of any installation. The control requirements are exacting since control is required to within ± 1 ° C of the nominal temperature, but this can be achieved using properly designed equipment. The most common problems are either undersized refrigeration units, insufficient flow rates, particularly of the electrolyte, with external cooling systems, or insufficient cooling coil area with internal systems. Another point that is frequently overlooked is the fact that over a week-end or holiday shut-down, the temperature of the electrolyte may fall considerably, particularly during the winter. It cannot be over-emphasised that it is not advisable to produce work outside the temperature range laid down by specification, whether this specification is one imposed by an outside inspecting authority or one arrived at by the company for purposes of control. Means should therefore be provided on any anodizing tank for raising the temperature of the tank up to the required operating level. The method of doing this will depend upon exact local conditions, but one simple means is to provide the anodizing tank with a small steam-heated coil placed near the bottom of the tank. If the anodizing tank is small it may be equally practical to use a lead-covered or silica-sheathed immersion heater to bring the bath up to temperature fairly rapidly. Any such heating arrangement should incorporate a thermostatic control device which will cut off heat when operating temperature is reached. Current supply and control

Sulphuric acid anodizing needs a D.C. source capable of giving a controlled output within the range 12 - 24 volts. In the early stages of anodizing, field excited and shunt controlled motor generators were used, but rectifiers have almost completely replaced motor generators and are invariably specified for new plants. For many years a 16V supply was considered adequate, but with more exacting demands being made in terms of alloys to be processed and film thicknesses required, 20V and 24V are becoming increasingly common-place as sources for decorative and protective anodizing, and are to be preferred. Special processes, such as hard anodizing and self-colour anodizing, require outputs of up to 75V (or even 120 volts for difficult alloys). Selenium rectifiers have given many years of satisfactory service but with the trend for higher voltages, silicon rectifiers have become increasingly attractive in terms of cost and reliability. As the operating voltage required increases and also the ampere output, silicon rectifiers come into their own in terms of installed cost, operating efficiency and reliability. They are very compact and are usually air-cooled, but some water cooled versions are also available with large capacity units. Some are now thyristor controlled, but care must be taken that adequate smoothing circuits are used or a very distorted wave-form may be produced, particularly with small loads. 100

The control of rectifier output is the most important factor in determining the effective current density on the work and hence the rate of growth of the anodic film. The rule-of-thumb method of controlling by voltage can give problems, but for many anodizers it is still the most practical form of anodizing control. Provided that it is realised that the relationship between voltage and current density is controlled by the resistivity of the anodizing electrolyte, and in some circumstances the load size also, good control can be obtained. The main factors affecting solution resistivity are the sulphuric acid concentration and the electrolyte temperature, and these must be controlled within close limits if good reproducibility is to be achieved. However, if these are controlled, the relationship between the voltage applied and the current density produced is essentially constant, and, as the current density controls film growth, the rate of build-up of oxide can be predicted under any particular condition. The great advantage of this system is that it takes into account the ‘effective’ current density on the load, rather than the current density which would be applied on the basis of the apparent load area. In addition, if work loses contact during the anodizing process, the rest of the work will continue to receive the correct current density for the rate of anodizing predicted. The alternative to voltage control is some form of current or current density control, and, whilst this is nearly always used in a laboratory, there are practical problems when it comes to a production plant. The simplest method is to calculate the surface area of the load concerned and, using the current density desired, work out the current to be applied to the load. This current is then applied and maintained during the anodizing period either manually or automatically. The snag is that, apart from the difficulty of calculating area on complex loads, it is also very difficult to assess how the current will be distributed over the load, i.e. to assess the ‘effective’ current. Hollow sections provide a particular problem in this regard and anodizers have to make a rough estimate of how much current will flow to the inside of the hollow. Generally figures of 1/3 to 1 / 4 of the outside area are used, but this can be a long way out on some loads. Nevertheless many anodizers use this method of control. The other main current control system is to use an aluminium control electrode of accurately known area in parallel with the load. This has to be of similar alloy to that being anodized and of the same general surface texture, so a new electrode is usually used with each load. A known current density is applied to the electrode and it is assumed that the current density on the rest of the load is similar. However care and experience is necessary in the positioning of the electrode relative to the load, as misleading results can be obtained. All these factors indicate how important it is to use suitable control devices. The obvious method is that of manual control by an operator, but it is rarely practical nor economic to make this the sole concern of one man. In any case it still suffers from the fact that all operators have human failings and their reliability 101

Good thermostatic control equipment is an essential part of any installation. The control requirements are exacting since control is required to within ± 1 ° C of the nominal temperature, but this can be achieved using properly designed equipment. The most common problems are either undersized refrigeration units, insufficient flow rates, particularly of the electrolyte, with external cooling systems, or insufficient cooling coil area with internal systems. Another point that is frequently overlooked is the fact that over a week-end or holiday shut-down, the temperature of the electrolyte may fall considerably, particularly during the winter. It cannot be over-emphasised that it is not advisable to produce work outside the temperature range laid down by specification, whether this specification is one imposed by an outside inspecting authority or one arrived at by the company for purposes of control. Means should therefore be provided on any anodizing tank for raising the temperature of the tank up to the required operating level. The method of doing this will depend upon exact local conditions, but one simple means is to provide the anodizing tank with a small steam-heated coil placed near the bottom of the tank. If the anodizing tank is small it may be equally practical to use a lead-covered or silica-sheathed immersion heater to bring the bath up to temperature fairly rapidly. Any such heating arrangement should incorporate a thermostatic control device which will cut off heat when operating temperature is reached. Current supply and control

Sulphuric acid anodizing needs a D.C. source capable of giving a controlled output within the range 12 - 24 volts. In the early stages of anodizing, field excited and shunt controlled motor generators were used, but rectifiers have almost completely replaced motor generators and are invariably specified for new plants. For many years a 16V supply was considered adequate, but with more exacting demands being made in terms of alloys to be processed and film thicknesses required, 20V and 24V are becoming increasingly common-place as sources for decorative and protective anodizing, and are to be preferred. Special processes, such as hard anodizing and self-colour anodizing, require outputs of up to 75V (or even 120 volts for difficult alloys). Selenium rectifiers have given many years of satisfactory service but with the trend for higher voltages, silicon rectifiers have become increasingly attractive in terms of cost and reliability. As the operating voltage required increases and also the ampere output, silicon rectifiers come into their own in terms of installed cost, operating efficiency and reliability. They are very compact and are usually air-cooled, but some water cooled versions are also available with large capacity units. Some are now thyristor controlled, but care must be taken that adequate smoothing circuits are used or a very distorted wave-form may be produced, particularly with small loads. 100

The control of rectifier output is the most important factor in determining the effective current density on the work and hence the rate of growth of the anodic film. The rule-of-thumb method of controlling by voltage can give problems, but for many anodizers it is still the most practical form of anodizing control. Provided that it is realised that the relationship between voltage and current density is controlled by the resistivity of the anodizing electrolyte, and in some circumstances the load size also, good control can be obtained. The main factors affecting solution resistivity are the sulphuric acid concentration and the electrolyte temperature, and these must be controlled within close limits if good reproducibility is to be achieved. However, if these are controlled, the relationship between the voltage applied and the current density produced is essentially constant, and, as the current density controls film growth, the rate of build-up of oxide can be predicted under any particular condition. The great advantage of this system is that it takes into account the ‘effective’ current density on the load, rather than the current density which would be applied on the basis of the apparent load area. In addition, if work loses contact during the anodizing process, the rest of the work will continue to receive the correct current density for the rate of anodizing predicted. The alternative to voltage control is some form of current or current density control, and, whilst this is nearly always used in a laboratory, there are practical problems when it comes to a production plant. The simplest method is to calculate the surface area of the load concerned and, using the current density desired, work out the current to be applied to the load. This current is then applied and maintained during the anodizing period either manually or automatically. The snag is that, apart from the difficulty of calculating area on complex loads, it is also very difficult to assess how the current will be distributed over the load, i.e. to assess the ‘effective’ current. Hollow sections provide a particular problem in this regard and anodizers have to make a rough estimate of how much current will flow to the inside of the hollow. Generally figures of 1/3 to 1 / 4 of the outside area are used, but this can be a long way out on some loads. Nevertheless many anodizers use this method of control. The other main current control system is to use an aluminium control electrode of accurately known area in parallel with the load. This has to be of similar alloy to that being anodized and of the same general surface texture, so a new electrode is usually used with each load. A known current density is applied to the electrode and it is assumed that the current density on the rest of the load is similar. However care and experience is necessary in the positioning of the electrode relative to the load, as misleading results can be obtained. All these factors indicate how important it is to use suitable control devices. The obvious method is that of manual control by an operator, but it is rarely practical nor economic to make this the sole concern of one man. In any case it still suffers from the fact that all operators have human failings and their reliability 101

is usually less than that of an instrument, so automation is preferred. Whichever control system is used, the rectifier is usually remotely controlled from a cubicle close to the anodizing tank. The output can be raised manually or automatically to the desired voltage or current level, but it is useful to have a timer controlling the anodizing time and for the rectifier to be automatically wound back to zero at the end of the cycle. The main control systems available are discussed overleaf. Controlling ammeters These are the simplest control instruments available. They consist of an ammeter fitted with additional upper and lower limit contact points which can be set to required values within the range ± 1 per cent to ± 5 per cent of the required current depending upon the current to be carried and the ammeter used. By means of a shunt across the ammeter a small current energises a relay on the control motor when the current falls to the preset minimum and cuts off the motor when the maximum is reached. Given suitable design of equipment this system is simple, cheap and effective. Its limitation is that the load required on the rectifier must be known previously with accuracy. With work of complex shape, or small and varied articles, it may be impracticable to calculate each load. With large loads of standard items, such as architectural sections, this system can be used but will generally be found to work satisfactorily only on loads up to 1 000 amps. Constant voltage control Some Government specifications, normally for chromic acid anodizing, call for a constant voltage over a major part of the anodizing cycle. In conjunction with timing devices the voltage can be changed at any pre-determined point in the anodizing cycle, and is stepped up to definite preset levels. In this case the actual anodic film thickness achieved is not usually critical, provided that it is sufficient to give the protection required. On the other hand, with sulphuric acid anodizing the anodic film thickness requirement is nearly always a major part of the specification and adequate thickness is required on all significant surfaces. The rate of anodizing must therefore be known and voltages used must be related to the current density which they produce. The simplest way to do this is to have a test load of accurately known area (often a sheet load) and this is placed in the anodizing bath and the calculated current applied. The voltage on this load is read after about 5 minutes anodizing, when a more or less steady state is reached, and this voltage is then used for the subsequent loads processed on that shift or on that day. Provided that the anodizing temperature is controlled within ± 1 ° C and the acid concentration within ± 5 g/litre, consistent anodic film thicknesses will be obtained. The alloy used for the 102

test load must preferably be the same as that used in the subsequent loads, but similar alloys can be used without too much error. For example commercial purity aluminium sheet (SIC) can be used to calibrate the bath for HE9 extrusions. Many anodizers believe that the aluminium content of the anodizing electrolyte also effects the required voltage, but in the authors’ experience it has very little effect on the voltage-current density relationship for normal sulphuric acid anodizing. With this type of system it is preferable to measure the voltage at the anodizing tank, in order to avoid errors due to variable voltage drops in the bus-bar system. Methods of securing constant voltage vary between equipment manufacturers. In recent years solid state devices have become a favoured feature on control units. In one such transductor controlled supply, the control instrument consists of two basically identical units. One unit energises the regulator motor so as to increase the voltage when it is below the set value and the other is arranged to achieve the opposite effect. With simple on-off controllers close control can be difficult to achieve because of ‘hunting’. This can be overcome by the use of proportional control of the commutator motor on the control device. As the voltage approaches the set value the speed of the control motor is proportionally decreased." In a direct thyristor controlled supply proportional control is much easier as the entire operation is carried out using solid state devices. Another system of control utilises the Wheatstone Bridge principle in that the voltage between a secondary electrode and the work is used to control the voltage across the bath. Using suitable relays the controlling galvanometer will raise or lower the voltage supplied from the rectifier. A similar system can be used for current control. Constant current control One method of constant current control is that employing a Contacting Ammeter (see previous section, p. 102). Another method is to use a suitable core reactor. In this device the current output is proportional to the degree of magnetisation of the saturable core. By using a control device which controls at a preset level of magnetisation and operates controlling relays, the current output of the unit can be kept constant (fig. 35). The inclusion of an ampere-minute device which can be preset to give an alarm signal on completion of anodizing is useful as an additional guarantee of the correct thickness, if the area of work being anodized is known. Constant current density Constant current density controllers operate on the basis of an assumed slope to the voltage-current density curve. The reference voltage can be adjusted to a value needed to provide the initial required current density and a suitable slope 103

is usually less than that of an instrument, so automation is preferred. Whichever control system is used, the rectifier is usually remotely controlled from a cubicle close to the anodizing tank. The output can be raised manually or automatically to the desired voltage or current level, but it is useful to have a timer controlling the anodizing time and for the rectifier to be automatically wound back to zero at the end of the cycle. The main control systems available are discussed overleaf. Controlling ammeters These are the simplest control instruments available. They consist of an ammeter fitted with additional upper and lower limit contact points which can be set to required values within the range ± 1 per cent to ± 5 per cent of the required current depending upon the current to be carried and the ammeter used. By means of a shunt across the ammeter a small current energises a relay on the control motor when the current falls to the preset minimum and cuts off the motor when the maximum is reached. Given suitable design of equipment this system is simple, cheap and effective. Its limitation is that the load required on the rectifier must be known previously with accuracy. With work of complex shape, or small and varied articles, it may be impracticable to calculate each load. With large loads of standard items, such as architectural sections, this system can be used but will generally be found to work satisfactorily only on loads up to 1 000 amps. Constant voltage control Some Government specifications, normally for chromic acid anodizing, call for a constant voltage over a major part of the anodizing cycle. In conjunction with timing devices the voltage can be changed at any pre-determined point in the anodizing cycle, and is stepped up to definite preset levels. In this case the actual anodic film thickness achieved is not usually critical, provided that it is sufficient to give the protection required. On the other hand, with sulphuric acid anodizing the anodic film thickness requirement is nearly always a major part of the specification and adequate thickness is required on all significant surfaces. The rate of anodizing must therefore be known and voltages used must be related to the current density which they produce. The simplest way to do this is to have a test load of accurately known area (often a sheet load) and this is placed in the anodizing bath and the calculated current applied. The voltage on this load is read after about 5 minutes anodizing, when a more or less steady state is reached, and this voltage is then used for the subsequent loads processed on that shift or on that day. Provided that the anodizing temperature is controlled within ± 1 ° C and the acid concentration within ± 5 g/litre, consistent anodic film thicknesses will be obtained. The alloy used for the 102

test load must preferably be the same as that used in the subsequent loads, but similar alloys can be used without too much error. For example commercial purity aluminium sheet (SIC) can be used to calibrate the bath for HE9 extrusions. Many anodizers believe that the aluminium content of the anodizing electrolyte also effects the required voltage, but in the authors’ experience it has very little effect on the voltage-current density relationship for normal sulphuric acid anodizing. With this type of system it is preferable to measure the voltage at the anodizing tank, in order to avoid errors due to variable voltage drops in the bus-bar system. Methods of securing constant voltage vary between equipment manufacturers. In recent years solid state devices have become a favoured feature on control units. In one such transductor controlled supply, the control instrument consists of two basically identical units. One unit energises the regulator motor so as to increase the voltage when it is below the set value and the other is arranged to achieve the opposite effect. With simple on-off controllers close control can be difficult to achieve because of ‘hunting’. This can be overcome by the use of proportional control of the commutator motor on the control device. As the voltage approaches the set value the speed of the control motor is proportionally decreased." In a direct thyristor controlled supply proportional control is much easier as the entire operation is carried out using solid state devices. Another system of control utilises the Wheatstone Bridge principle in that the voltage between a secondary electrode and the work is used to control the voltage across the bath. Using suitable relays the controlling galvanometer will raise or lower the voltage supplied from the rectifier. A similar system can be used for current control. Constant current control One method of constant current control is that employing a Contacting Ammeter (see previous section, p. 102). Another method is to use a suitable core reactor. In this device the current output is proportional to the degree of magnetisation of the saturable core. By using a control device which controls at a preset level of magnetisation and operates controlling relays, the current output of the unit can be kept constant (fig. 35). The inclusion of an ampere-minute device which can be preset to give an alarm signal on completion of anodizing is useful as an additional guarantee of the correct thickness, if the area of work being anodized is known. Constant current density Constant current density controllers operate on the basis of an assumed slope to the voltage-current density curve. The reference voltage can be adjusted to a value needed to provide the initial required current density and a suitable slope 103

control chosen to suit the alloy and process conditions. Most devices incorporate a process timer and this can be arranged to run down the regulator at the end of the anodizing cycle to a low voltage ready for removal of work and commencement of the next cycle (fig. 36). An alternative method is employ a reference electrode of known area from which the current density is computed. Systems employing an inert reference electrode are to be viewed with some suspicion since the resistance of the anodic coating is a significant factor in controlling the effective current density. The use as the basis of the control system of a standard test area of material similar to that being anodized is much to be recommended provided an adequate area is employed.

Bus-bars

Fig. 35. Automatic current/ voltage controller.

\Courtesy: Howard Wall Ltd Where very heavy currents are to be carried, as in large automatic plants for bright anodizing or in large architectural anodizing plants, a channel-type twin bus-bar may prove advantageous in avoiding the necessity for very deep cathode rails which can be difficult to mount on the anodizing tank. For example it required two flat bars 8" x i/2 " Or four bars 4" x i/2 " to carry 5 450 amps and 5 100 amps respectively whereas two channel bus-bars 5" deep x 0.317" channel spaced 5" apart can carry 5 540 amps. The ratings for a range of double-channel bus-bar installations are set out in Table 10. Apart from their use supplying current from the rectifier to the anodizing tanks such bus-bars can also be use for substation work and for carrying current between transformers and rectifiers. Supply of current from rectifier to anodizing or electropolishing baths is achieved by means of bus-bars. Copper has been a traditional material for such purposes, but aluminium has become competitive on economic grounds. Reference has already been made to the potentially adverse effects of dissolved copper in the electrolyte, and taking all factors into account the balance of advantage lies with aluminium. The main data showing the relation between aluminium bus-bars and their current-carrying capacity are summarised in Tables 5 & 6. Further detail on bus-bar design and installation is contained in a reference work on the subject 2 while their protection has already been considered (p. 93).

Fig. 36. Standard constant current density controller.

[Courtesy: Volfield Ltd.

Aluminium bus-bars are jointed by bolting using normal techniques. To secure uniform pressures without overtightening use of a pre-set torque spanner is recommended. Some typical recommended bolting schedules for various bus-bar widths are given in Table 11.

104 105

control chosen to suit the alloy and process conditions. Most devices incorporate a process timer and this can be arranged to run down the regulator at the end of the anodizing cycle to a low voltage ready for removal of work and commencement of the next cycle (fig. 36). An alternative method is employ a reference electrode of known area from which the current density is computed. Systems employing an inert reference electrode are to be viewed with some suspicion since the resistance of the anodic coating is a significant factor in controlling the effective current density. The use as the basis of the control system of a standard test area of material similar to that being anodized is much to be recommended provided an adequate area is employed.

Bus-bars

Fig. 35. Automatic current/ voltage controller.

\Courtesy: Howard Wall Ltd Where very heavy currents are to be carried, as in large automatic plants for bright anodizing or in large architectural anodizing plants, a channel-type twin bus-bar may prove advantageous in avoiding the necessity for very deep cathode rails which can be difficult to mount on the anodizing tank. For example it required two flat bars 8" x i/2 " Or four bars 4" x i/2 " to carry 5 450 amps and 5 100 amps respectively whereas two channel bus-bars 5" deep x 0.317" channel spaced 5" apart can carry 5 540 amps. The ratings for a range of double-channel bus-bar installations are set out in Table 10. Apart from their use supplying current from the rectifier to the anodizing tanks such bus-bars can also be use for substation work and for carrying current between transformers and rectifiers. Supply of current from rectifier to anodizing or electropolishing baths is achieved by means of bus-bars. Copper has been a traditional material for such purposes, but aluminium has become competitive on economic grounds. Reference has already been made to the potentially adverse effects of dissolved copper in the electrolyte, and taking all factors into account the balance of advantage lies with aluminium. The main data showing the relation between aluminium bus-bars and their current-carrying capacity are summarised in Tables 5 & 6. Further detail on bus-bar design and installation is contained in a reference work on the subject 2 while their protection has already been considered (p. 93).

Fig. 36. Standard constant current density controller.

[Courtesy: Volfield Ltd.

Aluminium bus-bars are jointed by bolting using normal techniques. To secure uniform pressures without overtightening use of a pre-set torque spanner is recommended. Some typical recommended bolting schedules for various bus-bar widths are given in Table 11.

104 105

cm

mm

cm m m

m m M" la M> M>

1------------1 g

QOOQOQ M" CM CM O LA O CM rx. LA -M- o CM CM m m ■M’ LA

COOlAO t- m ST- LA O CO LA v- v- V-

M2 O O CM LACMCotm LA M2 CO

LAOOO O M> O m CO CM w h* ■ 1 ;

mmm

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R5=«

mmm

f

CM v- O CO M" CO m T LA la F-. CO

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rtferMenfo

C

2"

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m sss

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4

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FILLET RADIUS

>------------1 g | _______| r< “

SS3§ OmSrT- v CM

01 Hi

INNER FLAT SURFACE

4

HI

FLANGE WIDTH

1_______1

y-r-'cMCM

106

m o la m2

REACTANCE

S38888 co m O' * cm m m

Two Channels

8383SS O\ V- 'O Q O' i-rMcimTM"

|

88?S mcbrNsO cm cm m m

WEB AND FLANGE THICKNESS t

888S 'r- t- cm

'ONC'O

Dimensions e t c . o f a Single Channel

▼

Table 6. Current ratings for aluminium channel bus-bar in 6101 A alloy 5

cm m ▼ la M) co

Ratings are for 50°C rise over 35 °C ambient temperature in still but unconfined air. For multiple-bar arrangements, the space between bars is equal to the bar thickness. A.C. ratings are based on spacings at which proximity-effect is negligible. Ratings may be increased by approximately 20% if the bus-bar is painted with a non-metallic finish paint. To obtain the current rating forCIE bus-bar, multiply the appropriate figure above by 1.03. These are English units but in the United Kingdom these sizes are now supplied in the nearest metric equivalent.

cMm + m

Notes: 1 . 2. 3. 4. 5. 6.

£

i- v- cm cm

1_______1 |______ | O m CO CM w h* ■ 1 ;

mmm

s- V- CM

R5=«

mmm

f

CM v- O CO M" CO m T LA la F-. CO

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2"

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m sss

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FILLET RADIUS

>------------1 g | _______| r< “

SS3§ OmSrT- v CM

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4

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FLANGE WIDTH

1_______1

y-r-'cMCM

106

m o la m2

REACTANCE

S38888 co m O' * cm m m

Two Channels

8383SS O\ V- 'O Q O' i-rMcimTM"

|

88?S mcbrNsO cm cm m m

WEB AND FLANGE THICKNESS t

888S 'r- t- cm

'ONC'O

Dimensions e t c . o f a Single Channel

▼

Table 6. Current ratings for aluminium channel bus-bar in 6101 A alloy 5

cm m ▼ la M) co

Ratings are for 50°C rise over 35 °C ambient temperature in still but unconfined air. For multiple-bar arrangements, the space between bars is equal to the bar thickness. A.C. ratings are based on spacings at which proximity-effect is negligible. Ratings may be increased by approximately 20% if the bus-bar is painted with a non-metallic finish paint. To obtain the current rating forCIE bus-bar, multiply the appropriate figure above by 1.03. These are English units but in the United Kingdom these sizes are now supplied in the nearest metric equivalent.

cMm + m

Notes: 1 . 2. 3. 4. 5. 6.

£

i- v- cm cm

1_______1 |______ | 1 I

w

4

Fig. 39. Further example of jig for small and medium sized components. [Courtesy: Acorn Anodizing Co. Ltd.

■ ’’'M

110 Ill

order to ensure adequate film quality and minimum operating costs. Aluminium is the material most commonly used for jigging purposes, usually in the form of wire or extruded sections. Small components present particular problems and are sometimes wired and bunched together on hooks (fig 37)

for lighter sections. The clamps used are often made of aluminium, but the threads are rapidly eaten away during processing and they have to be replaced frequently. Titanium clamps provide an expensive but longer lasting answer, but the threads can easily seize up if they are not machined correctly. Increasingly, therefore, plastic clamps are being used; these make use of special plastics which will stand up to all the process solutions involved and the heat of the sealing bath, and they are long lasting and not too expensive.

Fig. 38. Typical aluminium jigs for small aluminium articles for decorative anodizing. [Courtesy: Alumilite and Alzak Ltd.

■ ‘

K

Fig. 37. Typical examples of small articles being wired for anodizing. [Courtesy: Acorn Anodizing Co. Ltd. However the point at which the wire is in contact with the component remains unanodized and one of the skills of jigging is to leave as small a contact area as possible. More complicated pin jigs or spring jigs are therefore often preferred (figs. 3S and 39). This type of jig also provides a very positive contact, as the slightest movement of the article during anodizing can mean loss of electrical contact and hence insufficient anodic film thickness or even complete lack of anodizing. Large quantities of small articles may be anodized in perforated barrel containers which have a central hollow tube carrying compressed air for agitation ( ig- 40). A detailed description of this anodizing technique has been published by Flusm 6. Hollow articles are usually jigged on the inside to avoid contact marks on the readily-visible outer surface, and are arranged so as to provide easy drainage. The danger of trapping air in one part of the component must also be considered with this sort of article. Large sections intended for architectural use are usually clamped or bolted onto heavy vertical splines or frames, which in turn are attached to the flight bar which carries work through the plant (fig. 41). Pin jigs are also used, particularly

I

> 1 I

w

4

Fig. 39. Further example of jig for small and medium sized components. [Courtesy: Acorn Anodizing Co. Ltd.

■ ’’'M

110 Ill

Fig. 40. Barrel' or basket anodizing. This photograph shows the perforated barrel container being lowered into the electrolyte with tube attached for circulating cooled electrolyte through the work 6.

Fig. 41. Jigging area of large architectural anodizing plant. [Courtesy: Alcan Booth Extrusions Ltd., Banbury

112

The main problem with aluminium jigs is that the jigs themselves are anodized during the process and this anodic film has to be stripped, at least in the contact areas, after each anodizing operation. The usual way to do this is to place the jigs in the caustic etching solution until the oxide has been removed, but inevitably this means significant loss of aluminium from the jigs with use and consequent eventual replacement. It must also be remembered that the larger the jig area the more of the applied current is being used to anodize the jigs and not the work, so jigs with an unnecessarily large area are wasteful of current. On the other hand if the jigs are too small in area, the splines will overheat and may even melt and the pin contacts may burn away. An alternative to aluminium as the main jig material is titanium (fig. 42). This has the advantage that the oxide film produced during anodizing is sufficiently thin not to give contact problems, so the jigs do not have to be stripped after use. They therefore have a long life, but are of course expensive. They have good resistance to the aggressive action of the phosphoric-nitric brightening solutions and are extensively used in bright anodizing plants for trim components. They can be made adjustable, thus reducing the total number of jigs required to handle a variety of articles (fig. 43). However the current carrying capacity of titanium is not as good as that of aluminium and all-titanium jigs are not normally suitable when producing thick anodic films (more than 15 microns). They are also not suitable with high voltage anodizing processes as the titanium oxide film breaks down under these conditions. In general, in the architectural anodizing field, titanium is best used to locate or hold the work against an aluminium spline which is carrying the current and providing electrical contact.

Fig. 42. Titanium jigs for anodizing lengths of light extruded section. [Courtesy: Norman Radcliffe Ltd.

113

Fig. 40. Barrel' or basket anodizing. This photograph shows the perforated barrel container being lowered into the electrolyte with tube attached for circulating cooled electrolyte through the work 6.

Fig. 41. Jigging area of large architectural anodizing plant. [Courtesy: Alcan Booth Extrusions Ltd., Banbury

112

The main problem with aluminium jigs is that the jigs themselves are anodized during the process and this anodic film has to be stripped, at least in the contact areas, after each anodizing operation. The usual way to do this is to place the jigs in the caustic etching solution until the oxide has been removed, but inevitably this means significant loss of aluminium from the jigs with use and consequent eventual replacement. It must also be remembered that the larger the jig area the more of the applied current is being used to anodize the jigs and not the work, so jigs with an unnecessarily large area are wasteful of current. On the other hand if the jigs are too small in area, the splines will overheat and may even melt and the pin contacts may burn away. An alternative to aluminium as the main jig material is titanium (fig. 42). This has the advantage that the oxide film produced during anodizing is sufficiently thin not to give contact problems, so the jigs do not have to be stripped after use. They therefore have a long life, but are of course expensive. They have good resistance to the aggressive action of the phosphoric-nitric brightening solutions and are extensively used in bright anodizing plants for trim components. They can be made adjustable, thus reducing the total number of jigs required to handle a variety of articles (fig. 43). However the current carrying capacity of titanium is not as good as that of aluminium and all-titanium jigs are not normally suitable when producing thick anodic films (more than 15 microns). They are also not suitable with high voltage anodizing processes as the titanium oxide film breaks down under these conditions. In general, in the architectural anodizing field, titanium is best used to locate or hold the work against an aluminium spline which is carrying the current and providing electrical contact.

Fig. 42. Titanium jigs for anodizing lengths of light extruded section. [Courtesy: Norman Radcliffe Ltd.

113

along the top of the tank and actuated when a load is lifted out of the rinse, can also reduce overall water comsumption. Air agitation in most rinses is desirable and helps to prevent stagnant areas and concentration or pH gradients in the rinses. Examples of the sort of problem which can occur as a result of faulty rinsing are shown in figs. 44 and 45. In one case insufficient or uneven rinsing between anodizing and colouring has led to uneven dye uptake, and in the other case contamination of the rinses in the pretreatment stages, particularly by chlorides, has led to rapid but shallow pitting attack 10. Fig. 43. Adjustable titanium jig designed for various sizes of holloware lids. [Courtesy: Norman Radcliffe Ltd.

Fig. 44. Patchy appearance caused by inadequate rinsing before colouring

Rinsing Again this is not strictly a part of the sulphuric acid anodizing plant itself, but rinsing operations throughout the line are very important in ensuring satisfactory appearance and quality in the final product. In general rinsing is carried out with water at ambient temperature but in special cases, such as after chemical brightening or after some cleaning processes, warm water (50° - 60°C) rinsing is desirable. In most cases it is preferable to run fresh water through the rinsing tanks, continuously introducing it at the bottom of the tank at one end and overflowing it over a weir at the top at the other end. The amount of water used will depend on the process concerned and the amount of solution carried over from the previous process, but with the increasing cost of water many plants are looking closely at the amount of water which they use. Counterflow and double rinsing systems can reduce water consumption considerably as can automatic conductivity controls 7 8 , and plants for recycling some or all of the rinse water are availble from specialist suppliers 9. Spray rinses, placed

Fig. 45. Rinse water corrosion on etched and anodized HE9 extrusion

114 115

along the top of the tank and actuated when a load is lifted out of the rinse, can also reduce overall water comsumption. Air agitation in most rinses is desirable and helps to prevent stagnant areas and concentration or pH gradients in the rinses. Examples of the sort of problem which can occur as a result of faulty rinsing are shown in figs. 44 and 45. In one case insufficient or uneven rinsing between anodizing and colouring has led to uneven dye uptake, and in the other case contamination of the rinses in the pretreatment stages, particularly by chlorides, has led to rapid but shallow pitting attack 10. Fig. 43. Adjustable titanium jig designed for various sizes of holloware lids. [Courtesy: Norman Radcliffe Ltd.

Fig. 44. Patchy appearance caused by inadequate rinsing before colouring

Rinsing Again this is not strictly a part of the sulphuric acid anodizing plant itself, but rinsing operations throughout the line are very important in ensuring satisfactory appearance and quality in the final product. In general rinsing is carried out with water at ambient temperature but in special cases, such as after chemical brightening or after some cleaning processes, warm water (50° - 60°C) rinsing is desirable. In most cases it is preferable to run fresh water through the rinsing tanks, continuously introducing it at the bottom of the tank at one end and overflowing it over a weir at the top at the other end. The amount of water used will depend on the process concerned and the amount of solution carried over from the previous process, but with the increasing cost of water many plants are looking closely at the amount of water which they use. Counterflow and double rinsing systems can reduce water consumption considerably as can automatic conductivity controls 7 8 , and plants for recycling some or all of the rinse water are availble from specialist suppliers 9. Spray rinses, placed

Fig. 45. Rinse water corrosion on etched and anodized HE9 extrusion

114 115

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10

T. Turner, Aluminium Association Finishing Seminar, Detroit, February 1968 Paper AN-2. European Wrought Aluminium Association, “Recommendations on Anodized Aluminium for Architectural Purposes” 2nd Edition (1969). Electroplating and Metal Finishing, 1965, 18, 390. E. Survila, Aluminium Federation Symposium on Anodizing Aluminium, April 1967, Paper 14. A. G. Thomas and P. J . H. Rata, ‘Aluminium bus-bar’, Hutchinson Scientific and Technical for Alcan Industries Ltd., (1967). F. Flusin, Metal Ind., 1956, 89, 413-416. D. A. Swalheim, Plating and Surface Finishing, 1976, 63(12), 24-30. A. W. Brace, Electroplating and Met. Finishing, 1966, 19, 319-321. R. Pinner, Electroplating and Met. Finishing, 1967, 20, 208-222. 247-253, 280-285. E. P. Short and A. J . Bryant, Trans. Inst. Met. Finishing, 1975, 53, 169-177.

Readers will also find of interest the section on Plant in the book “The Practical Anodizing of Aluminium” by W. Huber and A. Schiltknecht, Macdonald and Evans, London (1960).

Chapter 9 . Sulphuric acid anodizing

technology

By far the greatest area of anodizing is accounted for by work which is anodized in a sulphuric acid electrolyte. If judged by installed amperage it is unlikely that, excluding the special electrolytes used for anodizing condenser foil, all other processes account for more than 5 per cent of anodizing capacity. The low cost and general ease of operation of this process are the reasons for its widespread use. Literature concerning the process is abundant, but it is only relatively recently that adequate research has been undertaken to offer the practical man sufficient data, systematically collected, to enable him to have a reasonably reliable guide as to the variables that have to be controlled and to their effect on subsequent operations and on the final product. The main ones are acid concentration, impurities in the anodizing bath, electrolyte temperature, anodizing voltage and current density, agitation of the electrolyte, and the composition and condition of the alloy being anodized. These factors will each be considered in terms of the relevance of research findings to practical requirements. Influence of acid concentration Researches have been carried out and experience obtained with a wide range of acid concentrations ranging from as low as 10 g/1 (1 per cent weight) to 700 g/1 (70 per cent weight), but most anodizing is carried out using electrolytes ranging from 150 g/1 to 250 g/1 H 2SO4. One of the factors affecting the choice of electrolyte can be deduced from fig. 46 showing the conductivity of sulphuric acid electrolytes. 0.7

Specific conductivity

0.4

Fig. 46. Specific conductivity of H 2 S(f solutions.

0.5 0.4

0.3 0.2

10

20

30

40

50

60

70

00

90

100 %

Ha S0 4 concentration

116

117

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10

T. Turner, Aluminium Association Finishing Seminar, Detroit, February 1968 Paper AN-2. European Wrought Aluminium Association, “Recommendations on Anodized Aluminium for Architectural Purposes” 2nd Edition (1969). Electroplating and Metal Finishing, 1965, 18, 390. E. Survila, Aluminium Federation Symposium on Anodizing Aluminium, April 1967, Paper 14. A. G. Thomas and P. J . H. Rata, ‘Aluminium bus-bar’, Hutchinson Scientific and Technical for Alcan Industries Ltd., (1967). F. Flusin, Metal Ind., 1956, 89, 413-416. D. A. Swalheim, Plating and Surface Finishing, 1976, 63(12), 24-30. A. W. Brace, Electroplating and Met. Finishing, 1966, 19, 319-321. R. Pinner, Electroplating and Met. Finishing, 1967, 20, 208-222. 247-253, 280-285. E. P. Short and A. J . Bryant, Trans. Inst. Met. Finishing, 1975, 53, 169-177.

Readers will also find of interest the section on Plant in the book “The Practical Anodizing of Aluminium” by W. Huber and A. Schiltknecht, Macdonald and Evans, London (1960).

Chapter 9 . Sulphuric acid anodizing

technology

By far the greatest area of anodizing is accounted for by work which is anodized in a sulphuric acid electrolyte. If judged by installed amperage it is unlikely that, excluding the special electrolytes used for anodizing condenser foil, all other processes account for more than 5 per cent of anodizing capacity. The low cost and general ease of operation of this process are the reasons for its widespread use. Literature concerning the process is abundant, but it is only relatively recently that adequate research has been undertaken to offer the practical man sufficient data, systematically collected, to enable him to have a reasonably reliable guide as to the variables that have to be controlled and to their effect on subsequent operations and on the final product. The main ones are acid concentration, impurities in the anodizing bath, electrolyte temperature, anodizing voltage and current density, agitation of the electrolyte, and the composition and condition of the alloy being anodized. These factors will each be considered in terms of the relevance of research findings to practical requirements. Influence of acid concentration Researches have been carried out and experience obtained with a wide range of acid concentrations ranging from as low as 10 g/1 (1 per cent weight) to 700 g/1 (70 per cent weight), but most anodizing is carried out using electrolytes ranging from 150 g/1 to 250 g/1 H 2SO4. One of the factors affecting the choice of electrolyte can be deduced from fig. 46 showing the conductivity of sulphuric acid electrolytes. 0.7

Specific conductivity

0.4

Fig. 46. Specific conductivity of H 2 S(f solutions.

0.5 0.4

0.3 0.2

10

20

30

40

50

60

70

00

90

100 %

Ha S0 4 concentration

116

117

§ § §

100

s

Coating weight (mg/dm 2 )

Maximum conductivity is obtained at approximately 350 g/1. Using this electrolyte the voltage used to produce a given current density will be a minimum and total electrical power used in anodizing will therefore be at its lowest. With the cost of all forms of energy steadily increasing this might be considered the most desirable acid concentration to use, but power costs still represent a small proportion of the total anodizing cost and many other factors, not least anodic film quality, affect the choice of acid concentration.

Temperature (° F)

80

Fig. 47. Relations between temperature, concentration and voltage for a current density of 1.3 A/ dm 2 (12 A / ft 2 ) in a sulphuric acid type electrolyte on 99.99% aluminium sheet 1.

Volts

100 Metal l o s s (mg/dm 2 ) 1 0 ------- Anodizing time (min.) -----------«-60

10

60

Fig. 48. Effect of acid concentration on anodic oxide coating weight after anodizing at 1.6 A/ dm 2 (15 A/ ft 2 ) and 21 QC. 2

40 20

20

100 33 % 10

20

30

40

16 % Hg SO4

h 2 s o4

50

5557 -H25

Sulphuric acid (weight %)

A further effect of increasing the electrolyte concentration over the range 8.5 per cent to 33 per cent H3SO4 has been found’ to decrease the abrasion resistance by over 30 per cent (and, incidentally, to improve image clarity: fig 49). This film softening is a reflection of the attack and re-dissolution of the coating which occurs during anodizing — a tendency which increases sharply with rising acid concentration and temperature. If electrolyte concentration is held constant, lowering the electrolyte temperature increases abrasion resistance and hardness, as shown by Speiser 5, and also reduces the tendency for softening of the outer layers of the anodic oxide coating (fig. 50). 118

5557-0

Abrasion resistance, Grams

Within normal limits the voltage required to produce a given current density generally decreases with increase in electrolyte concentration and also with increase in electrolyte temperature1 (fig. 47). However, the degree of attack of the outer part of the anodic film increases at the same time. For example it has been shown that the weight (and therefore the thickness) of coating formed decreases with increase in electrolyte concentration as anodizing time is increased 2 so that, at the end of 60 minutes anodizing, a 30 per cent weight (330 g/1) electrolyte produces only about half the film thickness obtained from a 15 per cent weight (165 g/1) electrolyte at 30°C due to the re-dissolution of the coating in the acid. The temperature used in these experiments was chosen to enhance this effect, but even at 21 °C the differences are still quite marked (e.g. fig. 48).

h 2 s o4

60

40 10

12

14

16

18

20

Anodizing voltage

Fig. 49. Effect of electrolyte concentration on image clarity and abrasion resistance. Alloy 5557-H25 and 5557-0 with 7p,[0.3 mil] thick anodic coatings. 1 Dori % is the reading on the Alcoa Dori-meter. 119

§ § §

100

s

Coating weight (mg/dm 2 )

Maximum conductivity is obtained at approximately 350 g/1. Using this electrolyte the voltage used to produce a given current density will be a minimum and total electrical power used in anodizing will therefore be at its lowest. With the cost of all forms of energy steadily increasing this might be considered the most desirable acid concentration to use, but power costs still represent a small proportion of the total anodizing cost and many other factors, not least anodic film quality, affect the choice of acid concentration.

Temperature (° F)

80

Fig. 47. Relations between temperature, concentration and voltage for a current density of 1.3 A/ dm 2 (12 A / ft 2 ) in a sulphuric acid type electrolyte on 99.99% aluminium sheet 1.

Volts

100 Metal l o s s (mg/dm 2 ) 1 0 ------- Anodizing time (min.) -----------«-60

10

60

Fig. 48. Effect of acid concentration on anodic oxide coating weight after anodizing at 1.6 A/ dm 2 (15 A/ ft 2 ) and 21 QC. 2

40 20

20

100 33 % 10

20

30

40

16 % Hg SO4

h 2 s o4

50

5557 -H25

Sulphuric acid (weight %)

A further effect of increasing the electrolyte concentration over the range 8.5 per cent to 33 per cent H3SO4 has been found’ to decrease the abrasion resistance by over 30 per cent (and, incidentally, to improve image clarity: fig 49). This film softening is a reflection of the attack and re-dissolution of the coating which occurs during anodizing — a tendency which increases sharply with rising acid concentration and temperature. If electrolyte concentration is held constant, lowering the electrolyte temperature increases abrasion resistance and hardness, as shown by Speiser 5, and also reduces the tendency for softening of the outer layers of the anodic oxide coating (fig. 50). 118

5557-0

Abrasion resistance, Grams

Within normal limits the voltage required to produce a given current density generally decreases with increase in electrolyte concentration and also with increase in electrolyte temperature1 (fig. 47). However, the degree of attack of the outer part of the anodic film increases at the same time. For example it has been shown that the weight (and therefore the thickness) of coating formed decreases with increase in electrolyte concentration as anodizing time is increased 2 so that, at the end of 60 minutes anodizing, a 30 per cent weight (330 g/1) electrolyte produces only about half the film thickness obtained from a 15 per cent weight (165 g/1) electrolyte at 30°C due to the re-dissolution of the coating in the acid. The temperature used in these experiments was chosen to enhance this effect, but even at 21 °C the differences are still quite marked (e.g. fig. 48).

h 2 s o4

60

40 10

12

14

16

18

20

Anodizing voltage

Fig. 49. Effect of electrolyte concentration on image clarity and abrasion resistance. Alloy 5557-H25 and 5557-0 with 7p,[0.3 mil] thick anodic coatings. 1 Dori % is the reading on the Alcoa Dori-meter. 119

500

Other impurities arise during the operation of the anodizing bath. Aluminium itself, which builds up in proportion to the ampere-hours of anodizing carried out, must be regarded as an impurity. Practical experience has shown that the presence of a small amount of aluminium in the electrolyte is advantageous. Tests carried out by one of the Authors have shown a higher brightness to be obtained after anodizing brightened work in an electrolyte containing 5 g/1 Al compared with anodizing in an electrolyte containing no aluminium. Dissolved aluminium is generally thought to decrease the conductivity of the electrolyte, but in the Authors experience the effect is very small within the normal limits of aluminium content. It has also been stated that dissolved aluminium tends to decrease the corrosion resistance. It is good practice to arrange to keep the aluminium content constant by daily draw-off of old electrolyte and replacement with new. A nominal 10 g/1 is a level at which it is convenient to control since the presence of a minimum of 5 g/1 Al is advisable and it is preferable not to exceed 15 g/1 Al. Some British process specifications (e.g. DEF 151) require that a maximum aluminium content dependent upon H 2SO4 content, must not be exceeded (fig. 51).

8

Micro hardness (kp/mm 2 )

in the acid supplied. The acid may also contain traces of nitrous acid and chloride, and these should not exceed 30 ppm. The grade of acid supplied in Great Britain is R.O.V. (Raw Oil of Vitriol) and usually has a relatively high heavy metal content, particularly Fe, and is not recommended. A High Purity Grade is available from several sources which meets the necessary specification and at only a small price premium over R.O.V.

0

-» --------------------L—

20

30

|

40

Thickness (g)

Fig. 50. In fluence of electrolyte temperature on the micro-hardness of sulphuric acid (GS) films. AlMgSi 0.5 anodized at 1.5 A/ dm 2* . 40

Effect of impurities in solution

4

Commercial sulphuric acid is not free from impurities although much depends upon the grade of acid supplied. The impurities present depend upon the age of the plant in which the acid was made and the process used, and also upon the lining used in the tanks for storing and transporting the acid. The literature on the effects of impurities is very limited'”8 but practical experience of one of the Authors indicates that more importance should be attached to this matter. Iron is one of the most commonly-encountered impurities and it has been found to impair the finish obtained after anodizing a previously-brightened surface when present in amounts of around 200 ppm. For best results, the iron content should not exceed 50 ppm and, allowing for some build-up in the bath, the acid supplied should preferably not exceed 25 ppm Fe. Other heavy metals should also be kept low such that the total of Pb, As, Cu, etc., does not exceed 10 ppm

Fig. 51. Maximum aluminium content of sulphuric acid electrolytes [at 15°C} as specified in DEF 151. 20

10 100

200

300

400

Ha so 4 (g/1)

Copper, zinc, and other metallic impurities may become dissolved in the electrolyte during operation. Copper may be present either from copper-containing

120 121

500

Other impurities arise during the operation of the anodizing bath. Aluminium itself, which builds up in proportion to the ampere-hours of anodizing carried out, must be regarded as an impurity. Practical experience has shown that the presence of a small amount of aluminium in the electrolyte is advantageous. Tests carried out by one of the Authors have shown a higher brightness to be obtained after anodizing brightened work in an electrolyte containing 5 g/1 Al compared with anodizing in an electrolyte containing no aluminium. Dissolved aluminium is generally thought to decrease the conductivity of the electrolyte, but in the Authors experience the effect is very small within the normal limits of aluminium content. It has also been stated that dissolved aluminium tends to decrease the corrosion resistance. It is good practice to arrange to keep the aluminium content constant by daily draw-off of old electrolyte and replacement with new. A nominal 10 g/1 is a level at which it is convenient to control since the presence of a minimum of 5 g/1 Al is advisable and it is preferable not to exceed 15 g/1 Al. Some British process specifications (e.g. DEF 151) require that a maximum aluminium content dependent upon H 2SO4 content, must not be exceeded (fig. 51).

8

Micro hardness (kp/mm 2 )

in the acid supplied. The acid may also contain traces of nitrous acid and chloride, and these should not exceed 30 ppm. The grade of acid supplied in Great Britain is R.O.V. (Raw Oil of Vitriol) and usually has a relatively high heavy metal content, particularly Fe, and is not recommended. A High Purity Grade is available from several sources which meets the necessary specification and at only a small price premium over R.O.V.

0

-» --------------------L—

20

30

|

40

Thickness (g)

Fig. 50. In fluence of electrolyte temperature on the micro-hardness of sulphuric acid (GS) films. AlMgSi 0.5 anodized at 1.5 A/ dm 2* . 40

Effect of impurities in solution

4

Commercial sulphuric acid is not free from impurities although much depends upon the grade of acid supplied. The impurities present depend upon the age of the plant in which the acid was made and the process used, and also upon the lining used in the tanks for storing and transporting the acid. The literature on the effects of impurities is very limited'”8 but practical experience of one of the Authors indicates that more importance should be attached to this matter. Iron is one of the most commonly-encountered impurities and it has been found to impair the finish obtained after anodizing a previously-brightened surface when present in amounts of around 200 ppm. For best results, the iron content should not exceed 50 ppm and, allowing for some build-up in the bath, the acid supplied should preferably not exceed 25 ppm Fe. Other heavy metals should also be kept low such that the total of Pb, As, Cu, etc., does not exceed 10 ppm

Fig. 51. Maximum aluminium content of sulphuric acid electrolytes [at 15°C} as specified in DEF 151. 20

10 100

200

300

400

Ha so 4 (g/1)

Copper, zinc, and other metallic impurities may become dissolved in the electrolyte during operation. Copper may be present either from copper-containing

120 121

Various foreign anions can also be present in the electrolyte. One of the commonest is chloride which can arise when the bath is made up from chlorinated supply waters or from brackish water. Under some circumstances loss by spray and evaporation can result in a build-up of chloride ions in the electrolyte. A current British specification (DEF 151) limits chloride to the equivalent of 0.2 g/1 NaCl (approximately 0.08 g/1 Cl) in the sulphuric acid electrolyte. Depending upon precise conditions chloride contents only slightly above this limit can produce pitting of the anodic oxide coating (fig. 52). With steps being taken to fluoridise water supplies, information is needed on the effects of fluorine in the electrolyte since they could be similar to those of chlorine.

.

** * * * >

* * * ♦

*

70

50

30

80

*

Fzg. 52. Pitting on HE9 extrusion, occurring as a result of chloride present in anodizing electrolyte. Other anions may enter the electrolyte accidentally. Incomplete rinsing can result in traces of nitric acid being present, but these do not seem to give rise 122

It can be appreciated that there is a high degree of interdependence between the various factors. Reference to fig. 47 (p. 118) shows that increasing the electrolyte temperature reduces the voltage required to produce a given current density at any given electrolyte concentration. Keeping electrolyte concentration and density constant at 165 g/1 and 1.3 A/dm z (12 A/ft 2 ) respectively. Alcoa workers have shown that high electrolyte temperatures decrease abrasion resistance but increase the brightness of bright anodized work (fig. 53).

IO

*' *

Electrolyte temperature

Abrasion resistance Grams

**

to any practical problems. Chromic acid, either as such or from chromates used in a desmut bath, can be an occasional impurity. Here again indications are that their practical importance is small and that it would require upwards of 0.1 g/1 (100 ppm) before any effect would be observed. This is likely to take the form of some loss of brightness and the need for a higher operating voltage.

Dori, per cent

alloys or from the use of copper bus-bars and work rails. Both Russian and German workers have found high copper concentrations harmful to the corrosion resistance of the anodic oxide coating produced when the Cu content of the electrolyte reaches 0.125 g/1 (125 ppm). It is likely that the brightness of bright anodized work would show some reduction below this level. Zinc can also be dissolved from certain alloys and this is considered harmful at 500 ppm. No data have been published on manganese but there are indications that it can have similar, but less pronounced effects than iron. There are no indications that magnesium is harmful, but silicon is a rather suspect impurity at 100 ppm upwards although more data on this element would be helpful in establishing limits.

70

60

10

12

M

f6

ia

20

Anodizing voltage

Fig. 53. Effect of electrolyte temperature on image clarity and abrasion resistance. Alloy 5357-H25 with 7p [0.3 mil] thick anodic coatings* Dori % is the reading on the Alcoa Dori-meter and measures the distinctness of the reflected image. They have also demonstrated that, at a constant current density (1.3 A/dm 2), increase of temperature reduces the amount of anodic coating produced in a given 123

Various foreign anions can also be present in the electrolyte. One of the commonest is chloride which can arise when the bath is made up from chlorinated supply waters or from brackish water. Under some circumstances loss by spray and evaporation can result in a build-up of chloride ions in the electrolyte. A current British specification (DEF 151) limits chloride to the equivalent of 0.2 g/1 NaCl (approximately 0.08 g/1 Cl) in the sulphuric acid electrolyte. Depending upon precise conditions chloride contents only slightly above this limit can produce pitting of the anodic oxide coating (fig. 52). With steps being taken to fluoridise water supplies, information is needed on the effects of fluorine in the electrolyte since they could be similar to those of chlorine.

.

** * * * >

* * * ♦

*

70

50

30

80

*

Fzg. 52. Pitting on HE9 extrusion, occurring as a result of chloride present in anodizing electrolyte. Other anions may enter the electrolyte accidentally. Incomplete rinsing can result in traces of nitric acid being present, but these do not seem to give rise 122

It can be appreciated that there is a high degree of interdependence between the various factors. Reference to fig. 47 (p. 118) shows that increasing the electrolyte temperature reduces the voltage required to produce a given current density at any given electrolyte concentration. Keeping electrolyte concentration and density constant at 165 g/1 and 1.3 A/dm z (12 A/ft 2 ) respectively. Alcoa workers have shown that high electrolyte temperatures decrease abrasion resistance but increase the brightness of bright anodized work (fig. 53).

IO

*' *

Electrolyte temperature

Abrasion resistance Grams

**

to any practical problems. Chromic acid, either as such or from chromates used in a desmut bath, can be an occasional impurity. Here again indications are that their practical importance is small and that it would require upwards of 0.1 g/1 (100 ppm) before any effect would be observed. This is likely to take the form of some loss of brightness and the need for a higher operating voltage.

Dori, per cent

alloys or from the use of copper bus-bars and work rails. Both Russian and German workers have found high copper concentrations harmful to the corrosion resistance of the anodic oxide coating produced when the Cu content of the electrolyte reaches 0.125 g/1 (125 ppm). It is likely that the brightness of bright anodized work would show some reduction below this level. Zinc can also be dissolved from certain alloys and this is considered harmful at 500 ppm. No data have been published on manganese but there are indications that it can have similar, but less pronounced effects than iron. There are no indications that magnesium is harmful, but silicon is a rather suspect impurity at 100 ppm upwards although more data on this element would be helpful in establishing limits.

70

60

10

12

M

f6

ia

20

Anodizing voltage

Fig. 53. Effect of electrolyte temperature on image clarity and abrasion resistance. Alloy 5357-H25 with 7p [0.3 mil] thick anodic coatings* Dori % is the reading on the Alcoa Dori-meter and measures the distinctness of the reflected image. They have also demonstrated that, at a constant current density (1.3 A/dm 2), increase of temperature reduces the amount of anodic coating produced in a given 123

time (30 minutes). This is expressed as the coating ratio, which is the ratio of oxide formed to metal dissolved. British Defence specifications limit the maximum operating temperature of sulphuric acid electrolytes as in fig. 54 where the upper limit depends upon the H2SO4 concentration.

K» 00 NJ Ch NJ -N. NJ N)

Fig. 54. Maximum permissible temperature of operation of sulphuric acid electrolytes as specified in DEF 151.

Maximum temperature (with agitation) ° C

30

20 0

100

200

300

165 g/1 electrolyte at 20 °C, and specular reflectivity was considered to be equally high. The effect on voltage of electrolyte temperature and concentration is fairly well known but the way in which they influence the appearance of anodic coatings is less obvious. An important factor here is the presence in all commercial aluminium alloys of second phase constituents, such as Mg2Si, ALFe-Si compounds, FeAl 3 , MnAl 6 , CuA12, Si and others. The way in which these react during anodizing has been studied by several workers 10 11 12 but the aspect of their behaviour which should be emphasized is their effect on the metal/oxide interface. This effect was first discussed in detail by Cooke 13 who suggested that inert constituents particularly common ones such as Al-Fe-Si, Si itself and MnAl b, have a higher conductivity than the matrix around them, and therefore cause a concentration of current, which results in the formation of what he called a ‘conical asperity’ beneath them. The growth of such an asperity is illustrated in fig. 55. This can grow to many times the size of the original particle and the extent to which it produces roughening at the metal/oxide interface is strongly dependent on the anodizing voltage used, roughening increasing considerably as voltage rises. Thus the lower anodizing voltages, used as a result of higher acid concentrations and temperatures and lower current densities, are in large part responsible for the improved brightening obtained.

400

h s S04 (g/1) //•/////

Anodizing voltage and current density

Increase in the anodizing voltage decreases the porosity of the anodic oxide coating produced and increases coating hardness. At the same time it also increases the total heat evolved and with it the refrigeration capacity required. The latter affects both capital and running costs. Increased voltages arise from lowering the anodizing temperature or lowering the acid concentration but keeping the current density constant, or from increasing the current density under any particular electrolyte condition. Since fig. 53 shows a fall in brightness with increase in voltage and reduction in anodizing temperature, means have been sought to combine adequate abrasion resistance with high brightness and production at low voltage. Alcoa workers1 have demonstrated that lower anodizing voltages increase brightness but have noted that the abrasion resistance falls by 12 - 20 per cent when the temperature of a 165 g/1 electrolyte is increased from 21°C to 30°C and the anodizing voltage thereby decreased from 15 volts to 11 volts with a constant current density of 1.3 A/dm 2 (12 A/ft 2). Further work at British Aluminium Co. Ltd.’s Research Laboratories 9 has shown that using a 130 g/1 (7 per cent volume) electrolyte at 25°C and 1.3 A/dm 2 (13 V) gave as good abrasion resistance as the

/

(a) Particle on surface (oxide not shown in sketches a-d)

(c) Condition where cone strongly affects reflectance and image clarity see note: for (b)

(b) Initiation of cone formation NOTE; length of arrows indicates magnitude of current at each location

(d) Isolation of particle in film

(e) Anodic film with dimple due to unoxidized particle and aluminium

Fig. 55. Various stages of isolation of non- oxidizing, semiconducting particle in an anodic film.

124 125

time (30 minutes). This is expressed as the coating ratio, which is the ratio of oxide formed to metal dissolved. British Defence specifications limit the maximum operating temperature of sulphuric acid electrolytes as in fig. 54 where the upper limit depends upon the H2SO4 concentration.

K» 00 NJ Ch NJ -N. NJ N)

Fig. 54. Maximum permissible temperature of operation of sulphuric acid electrolytes as specified in DEF 151.

Maximum temperature (with agitation) ° C

30

20 0

100

200

300

165 g/1 electrolyte at 20 °C, and specular reflectivity was considered to be equally high. The effect on voltage of electrolyte temperature and concentration is fairly well known but the way in which they influence the appearance of anodic coatings is less obvious. An important factor here is the presence in all commercial aluminium alloys of second phase constituents, such as Mg2Si, ALFe-Si compounds, FeAl 3 , MnAl 6 , CuA12, Si and others. The way in which these react during anodizing has been studied by several workers 10 11 12 but the aspect of their behaviour which should be emphasized is their effect on the metal/oxide interface. This effect was first discussed in detail by Cooke 13 who suggested that inert constituents particularly common ones such as Al-Fe-Si, Si itself and MnAl b, have a higher conductivity than the matrix around them, and therefore cause a concentration of current, which results in the formation of what he called a ‘conical asperity’ beneath them. The growth of such an asperity is illustrated in fig. 55. This can grow to many times the size of the original particle and the extent to which it produces roughening at the metal/oxide interface is strongly dependent on the anodizing voltage used, roughening increasing considerably as voltage rises. Thus the lower anodizing voltages, used as a result of higher acid concentrations and temperatures and lower current densities, are in large part responsible for the improved brightening obtained.

400

h s S04 (g/1) //•/////

Anodizing voltage and current density

Increase in the anodizing voltage decreases the porosity of the anodic oxide coating produced and increases coating hardness. At the same time it also increases the total heat evolved and with it the refrigeration capacity required. The latter affects both capital and running costs. Increased voltages arise from lowering the anodizing temperature or lowering the acid concentration but keeping the current density constant, or from increasing the current density under any particular electrolyte condition. Since fig. 53 shows a fall in brightness with increase in voltage and reduction in anodizing temperature, means have been sought to combine adequate abrasion resistance with high brightness and production at low voltage. Alcoa workers1 have demonstrated that lower anodizing voltages increase brightness but have noted that the abrasion resistance falls by 12 - 20 per cent when the temperature of a 165 g/1 electrolyte is increased from 21°C to 30°C and the anodizing voltage thereby decreased from 15 volts to 11 volts with a constant current density of 1.3 A/dm 2 (12 A/ft 2). Further work at British Aluminium Co. Ltd.’s Research Laboratories 9 has shown that using a 130 g/1 (7 per cent volume) electrolyte at 25°C and 1.3 A/dm 2 (13 V) gave as good abrasion resistance as the

/

(a) Particle on surface (oxide not shown in sketches a-d)

(c) Condition where cone strongly affects reflectance and image clarity see note: for (b)

(b) Initiation of cone formation NOTE; length of arrows indicates magnitude of current at each location

(d) Isolation of particle in film

(e) Anodic film with dimple due to unoxidized particle and aluminium

Fig. 55. Various stages of isolation of non- oxidizing, semiconducting particle in an anodic film.

124 125

As might be expected from the above discussion, increasing the anodizing current density without changing electrolyte strength or temperature involves increasing the anodizing voltage, which in turn increases abrasion resistance but decreases the brightness of brightened work (fig. 53). Anodizing at higher current densities under ideal conditions improves the efficiency of the process as expressed by the coating ratio and correspondingly the weight of coating produced in a given time. In fact, hard anodizing depends upon combining all the above factors to give the hardest possible coating in the minimum time consistent with operational requirements. Hard anodizing electrolytes are usually relatively dilute, and are operated at low temperatures (0 - 10°C) and high current densities (2.6 A/dm 2 upwards)? One factor that has also to be taken into account is that, due to the resistivity of the anodic oxide coating, the voltage required to maintain a given current density must be increased slowly over the period of anodizing. The adjustment required and the initial and final voltages are affected by the alloy being anodized, as can be seen from fig. 56.

load becomes an important operational art. Agitation of the electrolyte has also been found to be an important, and sometimes critical operational factor which can be decisive in determining the consistency of the coating and the maximum current density which can be used with safety. Summarising the effect of all these variables, there is always a balance between oxide film formation and its re-dissolution in the sulphuric acid electrolyte and this controls the upper limit of thickness which can be produced under any particular anodizing conditions. High electrolyte temperatures, high electrolyte concentrations and low anodizing current densities all encourage film re-dissolution, thus leading to softer films, lower anodizing efficiency and lower anodizing voltages, but a brighter appearance. Low electrolyte temperatures, low acid concentrations and higher current densities favour film growth, leading to harder films, higher anodizing efficiency, higher anodizing voltages and a rougher metal/oxide interface. The way in which these parameters are altered in practice is shown for the typical cases of bright, architectural and hard anodizing in Table 8. Table 8. Variation in sulphuric acid anodizing conditions according to product application

20

Electrolyte Current Electrolyte concentration temperature density (A/ft 2 ) (°C) (%by wt. Product ofH 2 S0 4 ) 10-11 22-24 Bright trim 18-20 14-18 Architectural 18-22 15-18 20-30 0-5 15-16 Hard anodizing

19

Fig. 56. Voltages required to maintain a current density of 1.3 A/ dm 2 (12 A/ ft 2 ) for various aluminium alloys anodized in 15% H, SO, at 25° C."

Typical voltage (volts) 14-15 17-19 25-50

Approximate limiting film thickness (microns) 20-25 35-40 50 +

Influence of agitation It is surprising that, despite the everyday use of agitation, technological studies of its quantitative effects are very few indeed. One of the earliest studies was made by Spooner 4 who, with a 275 gallon bath containing an electrolyte at 21°C and using 18 volts potential, noted the current densities obtained with various degrees of agitation (Table 9). Table 9. Effect of air agitation on anodizing current density Agitation 0

10

20

30

40

50

60

Anodizing time (minutes)

Under practical conditions other factors have to be taken into account. Jigging to secure consistent contact pressures and uniform current density over the work126

None Light Moderate Vigorous

Air flow (cu.ft/ min. /gal. ) 0.0 0.07 0.14 0.19

Current density (amp/ ft 2 .) 23.2 19.6 14.0 14.0 127

As might be expected from the above discussion, increasing the anodizing current density without changing electrolyte strength or temperature involves increasing the anodizing voltage, which in turn increases abrasion resistance but decreases the brightness of brightened work (fig. 53). Anodizing at higher current densities under ideal conditions improves the efficiency of the process as expressed by the coating ratio and correspondingly the weight of coating produced in a given time. In fact, hard anodizing depends upon combining all the above factors to give the hardest possible coating in the minimum time consistent with operational requirements. Hard anodizing electrolytes are usually relatively dilute, and are operated at low temperatures (0 - 10°C) and high current densities (2.6 A/dm 2 upwards)? One factor that has also to be taken into account is that, due to the resistivity of the anodic oxide coating, the voltage required to maintain a given current density must be increased slowly over the period of anodizing. The adjustment required and the initial and final voltages are affected by the alloy being anodized, as can be seen from fig. 56.

load becomes an important operational art. Agitation of the electrolyte has also been found to be an important, and sometimes critical operational factor which can be decisive in determining the consistency of the coating and the maximum current density which can be used with safety. Summarising the effect of all these variables, there is always a balance between oxide film formation and its re-dissolution in the sulphuric acid electrolyte and this controls the upper limit of thickness which can be produced under any particular anodizing conditions. High electrolyte temperatures, high electrolyte concentrations and low anodizing current densities all encourage film re-dissolution, thus leading to softer films, lower anodizing efficiency and lower anodizing voltages, but a brighter appearance. Low electrolyte temperatures, low acid concentrations and higher current densities favour film growth, leading to harder films, higher anodizing efficiency, higher anodizing voltages and a rougher metal/oxide interface. The way in which these parameters are altered in practice is shown for the typical cases of bright, architectural and hard anodizing in Table 8. Table 8. Variation in sulphuric acid anodizing conditions according to product application

20

Electrolyte Current Electrolyte concentration temperature density (A/ft 2 ) (°C) (%by wt. Product ofH 2 S0 4 ) 10-11 22-24 Bright trim 18-20 14-18 Architectural 18-22 15-18 20-30 0-5 15-16 Hard anodizing

19

Fig. 56. Voltages required to maintain a current density of 1.3 A/ dm 2 (12 A/ ft 2 ) for various aluminium alloys anodized in 15% H, SO, at 25° C."

Typical voltage (volts) 14-15 17-19 25-50

Approximate limiting film thickness (microns) 20-25 35-40 50 +

Influence of agitation It is surprising that, despite the everyday use of agitation, technological studies of its quantitative effects are very few indeed. One of the earliest studies was made by Spooner 4 who, with a 275 gallon bath containing an electrolyte at 21°C and using 18 volts potential, noted the current densities obtained with various degrees of agitation (Table 9). Table 9. Effect of air agitation on anodizing current density Agitation 0

10

20

30

40

50

60

Anodizing time (minutes)

Under practical conditions other factors have to be taken into account. Jigging to secure consistent contact pressures and uniform current density over the work126

None Light Moderate Vigorous

Air flow (cu.ft/ min. /gal. ) 0.0 0.07 0.14 0.19

Current density (amp/ ft 2 .) 23.2 19.6 14.0 14.0 127

It becomes evident that agitation is important under practical conditions and that it seems necessary to provide sufficient agitation to ensure that slight variation in air flow will not change the current density. In other words, the degree of agitation must correspond at least to that rated as ‘moderate’ in Table 10. Agitation is also important in controlling the film thickness obtained in that it affects the degree of re-dissolution of the anodic oxide coating during anodizing. This is demonstrated by figures given by Spooner, employing a constant current density of 1.6 A/dm 2 (15 A/ft 2 ), reproduced in Table 10. Table 10. Effect of air agitation on bath voltage and anodic oxide film thickness

Agitation None Light Moderate

4

Bath voltage Film thickness required (v) (M) 16.6 16.2 16.9 17.5 18.3 18.7

Later studies by Prati, Sacchi and Paolini 15 with electrolytes of 165 - 275 g/1 H SO show substantially similar effects. Generally, for decorative anodizing, a perforated pipe with holes of approximately 3 mm (1/8") diameter at 7.5 - 15 cm (3 - 6") intervals is used to supply air to the electrolyte. Kape 1(1 has used porous ceramic tube for this purpose which gives very uniform agitation. Practical experience confirms the advantage of these porous ceramic diffusers, but they are rather expensive. Throwing power

This subject is of considerable practical importance but little work on it has been published. This is probably due to the fact that the throwing power of an anodizing solution is high compared with that of a bright chromium plating bath or even a nickel bath. Nevertheless, when anodizing long hollow sections throwing power has considerable importance in determining the distribution of the current between the inside and outside surfaces. Sacchi and Oliva 1 anodized square tubes 25 X 25 mm (approximately 1" square), 45 X 45 mm (approximately 1 3A" square) and 80 x 80 mm (approximately 3 'A" square) by 600 mm (approximately 2 ft), 1 200 mm (approximately 4 ft), 2 400 mm (approximately 8 ft) and 4 800 mm (approximately 16 ft) long. Anodizing at 1.3 A/dm 2 (approximately 12 A/ft 2) they found that some 20 - 30 per cent of the current was taken by the inside of the shorter or larger crosssectional area tubes. This could diminish to 2 - 8 per cent on the 4 800 mm (16 ft) tubes depending upon their diameter. Generally speaking anodizers are advised to allow for at least 25 per cent of the current required for the outside being taken 128

by the inside surface on larger hollow sections of medium length, and 10 per cent for long narrow sections or tubes. For short lengths, i.e. up to 4 ft, 50 per cent should be allowed. With more general items such as milk churns, containers, tubs, etc., many favour the use of an internal cathode to ensure adequate thickness of anodizing on the internal surface, and this is essential if the inside only is to be anodized. With external cathodes only, the film on the outside will always tend to be thicker than on the inside and, particularly with long narrow articles, it will be substantially so. Throwing power is also important in determining the spread of film thickness occurring over a large or complex load. In most cases, for example in architectural anodizing, a survey of anodic film thickness over the full load surface will show the highest film thickness around the perimeter of the load and the lowest film thicknesses in the centre. If more than one line of work is processed the inner faces will usually have a lower film thickness than the outer faces. In order to meet current specifications, which usually specify a minimum local thickness on any significant surface, it is important to know the film thickness variation which is likely to occur. Under normal architectural anodizing conditions the variation can easily be as much as 10 microns on a 25 micron load, and even larger variations occur with the high current densities used for hard anodizing. In effect this means that the load must be over-anodized in order to achieve the desired minimum film thickness on the less accessible parts of the load. Sulphuric-oxalic acid anodizing

Over many years attempts have been made to combine the advantages of sulphuric and oxalic acid anodizing. 30‘3’ The higher hardness of oxalic acid films, or at least the fact that films comparable in hardness with sulphuric acid can be produced at higher electrolyte temperatures, has always attracted attention. By adding sulphuric acid to an oxalic acid electrolyte the voltage required to produce a normal anodizing current density is lowered, or if only small amounts of oxalic acid are added to a sulphuric acid electrolyte the voltage required is only slightly affected. One of the electrolytes favoured has been the 12 per cent sulphuric - 2 per cent oxalic acid electrolyte which forms the basis of the process for Alcoa 225 and 226 hard anodizing coatings. Voltage-current density curves for this electrolyte at various temperatures have been determined using pure aluminium, as recorded in fig. 57. Additions of 10 - 40 g/1 oxalic acid to electrolytes of 200 - 250 g/1 H7SO4 have been used in Continental countries 32 33. Unpublished work by Brace has shown that a 60 g/1 H 2SO4 - 50 g/1 (COOH) 2 electrolyte operated at 30° 35 °C gives a coating of comparable hardness to that of a 165 g/1 (8.5 per cent volume) H 2SO 4 electrolyte at 21° - 25°C. Alternatively, operated at room temperature it produces harder coatings. Voltage-current density curves for this electrolyte at various temperatures are shown in fig. 58. These findings appear in accordance with Japanese work along similar lines.34 129

It becomes evident that agitation is important under practical conditions and that it seems necessary to provide sufficient agitation to ensure that slight variation in air flow will not change the current density. In other words, the degree of agitation must correspond at least to that rated as ‘moderate’ in Table 10. Agitation is also important in controlling the film thickness obtained in that it affects the degree of re-dissolution of the anodic oxide coating during anodizing. This is demonstrated by figures given by Spooner, employing a constant current density of 1.6 A/dm 2 (15 A/ft 2 ), reproduced in Table 10. Table 10. Effect of air agitation on bath voltage and anodic oxide film thickness

Agitation None Light Moderate

4

Bath voltage Film thickness required (v) (M) 16.6 16.2 16.9 17.5 18.3 18.7

Later studies by Prati, Sacchi and Paolini 15 with electrolytes of 165 - 275 g/1 H SO show substantially similar effects. Generally, for decorative anodizing, a perforated pipe with holes of approximately 3 mm (1/8") diameter at 7.5 - 15 cm (3 - 6") intervals is used to supply air to the electrolyte. Kape 1(1 has used porous ceramic tube for this purpose which gives very uniform agitation. Practical experience confirms the advantage of these porous ceramic diffusers, but they are rather expensive. Throwing power

This subject is of considerable practical importance but little work on it has been published. This is probably due to the fact that the throwing power of an anodizing solution is high compared with that of a bright chromium plating bath or even a nickel bath. Nevertheless, when anodizing long hollow sections throwing power has considerable importance in determining the distribution of the current between the inside and outside surfaces. Sacchi and Oliva 1 anodized square tubes 25 X 25 mm (approximately 1" square), 45 X 45 mm (approximately 1 3A" square) and 80 x 80 mm (approximately 3 'A" square) by 600 mm (approximately 2 ft), 1 200 mm (approximately 4 ft), 2 400 mm (approximately 8 ft) and 4 800 mm (approximately 16 ft) long. Anodizing at 1.3 A/dm 2 (approximately 12 A/ft 2) they found that some 20 - 30 per cent of the current was taken by the inside of the shorter or larger crosssectional area tubes. This could diminish to 2 - 8 per cent on the 4 800 mm (16 ft) tubes depending upon their diameter. Generally speaking anodizers are advised to allow for at least 25 per cent of the current required for the outside being taken 128

by the inside surface on larger hollow sections of medium length, and 10 per cent for long narrow sections or tubes. For short lengths, i.e. up to 4 ft, 50 per cent should be allowed. With more general items such as milk churns, containers, tubs, etc., many favour the use of an internal cathode to ensure adequate thickness of anodizing on the internal surface, and this is essential if the inside only is to be anodized. With external cathodes only, the film on the outside will always tend to be thicker than on the inside and, particularly with long narrow articles, it will be substantially so. Throwing power is also important in determining the spread of film thickness occurring over a large or complex load. In most cases, for example in architectural anodizing, a survey of anodic film thickness over the full load surface will show the highest film thickness around the perimeter of the load and the lowest film thicknesses in the centre. If more than one line of work is processed the inner faces will usually have a lower film thickness than the outer faces. In order to meet current specifications, which usually specify a minimum local thickness on any significant surface, it is important to know the film thickness variation which is likely to occur. Under normal architectural anodizing conditions the variation can easily be as much as 10 microns on a 25 micron load, and even larger variations occur with the high current densities used for hard anodizing. In effect this means that the load must be over-anodized in order to achieve the desired minimum film thickness on the less accessible parts of the load. Sulphuric-oxalic acid anodizing

Over many years attempts have been made to combine the advantages of sulphuric and oxalic acid anodizing. 30‘3’ The higher hardness of oxalic acid films, or at least the fact that films comparable in hardness with sulphuric acid can be produced at higher electrolyte temperatures, has always attracted attention. By adding sulphuric acid to an oxalic acid electrolyte the voltage required to produce a normal anodizing current density is lowered, or if only small amounts of oxalic acid are added to a sulphuric acid electrolyte the voltage required is only slightly affected. One of the electrolytes favoured has been the 12 per cent sulphuric - 2 per cent oxalic acid electrolyte which forms the basis of the process for Alcoa 225 and 226 hard anodizing coatings. Voltage-current density curves for this electrolyte at various temperatures have been determined using pure aluminium, as recorded in fig. 57. Additions of 10 - 40 g/1 oxalic acid to electrolytes of 200 - 250 g/1 H7SO4 have been used in Continental countries 32 33. Unpublished work by Brace has shown that a 60 g/1 H 2SO4 - 50 g/1 (COOH) 2 electrolyte operated at 30° 35 °C gives a coating of comparable hardness to that of a 165 g/1 (8.5 per cent volume) H 2SO 4 electrolyte at 21° - 25°C. Alternatively, operated at room temperature it produces harder coatings. Voltage-current density curves for this electrolyte at various temperatures are shown in fig. 58. These findings appear in accordance with Japanese work along similar lines.34 129

Current density (amp / f t 2 )

Fig. 57. Voltage-current density relations for commercial aluminium anodized in 12% sulphuric acid - 2%o oxalic acid elect rolvte,

The main disadvantages of these mixed electrolytes are three-fold. Oxalic acid is more expensive than sulphuric acid. It is also decomposed by the passage of current and so there is need to control carefully both the oxalic acid and sulphuric acid contents. Further, the. presence of oxalic acid in the electrolyte may reduce the colour fastness or change the shade of some dyestuffs and these effects can only be checked by careful testing. However there is no doubt that use of the mixed electrolyte makes film quality much less dependent on anodizing temperature and these processes are now widely used. In most cases relatively small amounts of oxalic acid are being added to what is essentially a sulphuric acid electrolyte, so clear coatings are usually obtained, but Japanese workers have suggested low concentration sulphuric-oxalic acid mixtures which can be used for either clear or integral colour anodizing according to the conditions. 35 The electrolyte contains 25 - 27 g/l oxalic acid, 1.5 - 1.7 g/l sulphuric acid and 1.8 - 2.0 g/l aluminium ions and is used at 20 -55 volts and 25° - 40°C. The same workers recommend a similar electrolyte composition used at a low anodizing temperature (0 - 12 °C) for integral colour anodizing. 36 1

IO

12

14

Alternating current anodizing

Potential (volts)

Current density (amp /ft 2 )

For many years there has been recurrent interest in the possibility of using alternating current for anodizing, but it has only found occasional specialised application. In recent years there has been useful systematic work carried out evaluating the properties of films formed other than by straight D.C. Tajima and colleagues reported 18 the results shown in Table 11 overleaf from tests carried out on a variety of current forms. Other studies by Japanese workers 14 have shown that the factors affecting the properties of D.C. films are equally, if not more, significant under A.C. anodizing conditions. Properties such as corrosion and abrasion resistance decreased with higher acid concentrations and temperatures. The A.C. films showed abrasion resistances about one-half of those produced by the D.C. process. Sacchi and Paolini 20 reported the comparative voltages to produce equivalent current densities at various temperatures using a 20 per cent (w/v) electrolyte to be as shown in fig. 59 . Using electron micrographs they established the pore diameters of coatings produced by D.C. and A.C. in the above electrolyte to be those recorded in Table 12. Potential (volts)

Fig. 58. Voltage-current density' relations for commercial aluminium anodized in a 60 g/l sulphuric acid50 g/l oxalic acid electrolyte at various temperatures. 130

Measurements made on the surface of films of varying thickness showed the initial pore size of the A.C. films to be greater, but the difference diminished with increase in thickness. Films produced by A.C. dye readily but tend to have a slight yellowish colour. They are less abrasion resistant than D.C. films and it was not possible to produce compact films thicker than 12p. Corrosion resistance 131

Current density (amp / f t 2 )

Fig. 57. Voltage-current density relations for commercial aluminium anodized in 12% sulphuric acid - 2%o oxalic acid elect rolvte,

The main disadvantages of these mixed electrolytes are three-fold. Oxalic acid is more expensive than sulphuric acid. It is also decomposed by the passage of current and so there is need to control carefully both the oxalic acid and sulphuric acid contents. Further, the. presence of oxalic acid in the electrolyte may reduce the colour fastness or change the shade of some dyestuffs and these effects can only be checked by careful testing. However there is no doubt that use of the mixed electrolyte makes film quality much less dependent on anodizing temperature and these processes are now widely used. In most cases relatively small amounts of oxalic acid are being added to what is essentially a sulphuric acid electrolyte, so clear coatings are usually obtained, but Japanese workers have suggested low concentration sulphuric-oxalic acid mixtures which can be used for either clear or integral colour anodizing according to the conditions. 35 The electrolyte contains 25 - 27 g/l oxalic acid, 1.5 - 1.7 g/l sulphuric acid and 1.8 - 2.0 g/l aluminium ions and is used at 20 -55 volts and 25° - 40°C. The same workers recommend a similar electrolyte composition used at a low anodizing temperature (0 - 12 °C) for integral colour anodizing. 36 1

IO

12

14

Alternating current anodizing

Potential (volts)

Current density (amp /ft 2 )

For many years there has been recurrent interest in the possibility of using alternating current for anodizing, but it has only found occasional specialised application. In recent years there has been useful systematic work carried out evaluating the properties of films formed other than by straight D.C. Tajima and colleagues reported 18 the results shown in Table 11 overleaf from tests carried out on a variety of current forms. Other studies by Japanese workers 14 have shown that the factors affecting the properties of D.C. films are equally, if not more, significant under A.C. anodizing conditions. Properties such as corrosion and abrasion resistance decreased with higher acid concentrations and temperatures. The A.C. films showed abrasion resistances about one-half of those produced by the D.C. process. Sacchi and Paolini 20 reported the comparative voltages to produce equivalent current densities at various temperatures using a 20 per cent (w/v) electrolyte to be as shown in fig. 59 . Using electron micrographs they established the pore diameters of coatings produced by D.C. and A.C. in the above electrolyte to be those recorded in Table 12. Potential (volts)

Fig. 58. Voltage-current density' relations for commercial aluminium anodized in a 60 g/l sulphuric acid50 g/l oxalic acid electrolyte at various temperatures. 130

Measurements made on the surface of films of varying thickness showed the initial pore size of the A.C. films to be greater, but the difference diminished with increase in thickness. Films produced by A.C. dye readily but tend to have a slight yellowish colour. They are less abrasion resistant than D.C. films and it was not possible to produce compact films thicker than 12p. Corrosion resistance 131

132 Direct current 1-P half wave 3-P non-reversedinterruptcd 1-P imperfectly rectified (2: 1 ) (3:1) (4:1) 3-P imperfectly rectified (2:1) (3:1) (4:1) Superimposed A.C. + D.C. Normal A.C.

Current form

Voltage D.C. A.C.

1.22

1.16 1.25

Coating ratio

2.76

7.5 CN n CM CM n QO s0 q Tt

ri ri CM CM CM

ri

Lt. yellow-gold Lt. yellow-opaque Lt. yellow-opaque Lt. yellow-opaque Lt. yellow-opaque Lt. yellow-opaque Clear transparent Light yellowish

54 46 64 48 47 47 78 43

lg lg lg

LG

LG CM

LG

TT oc cro q q

370 155

r- c x

2.50 2.70

Clear transparent

80

640

O QO

7 5.5

Clear transparent Clear transparent

Appearance

Hardness (Vickers micro) 78 84

Abrasion resistance (seconds) 445 435

LG (N

1.13 1.04

2.46 2.66

Density

8 8

Thickness (p)

using various proportions of A.C. and various waveforms "

Table 11. Properties of films obtained by anodizing 99.99% aluminium in 165 g/1 H 2SO4 at 1 A/ dm 2 and 30° for 30 minutes

LG O

of A.C. films was compared to that of D.C. films but A.C. films had a greater tendency to bloom on weathering. 26

24

tO 1.0

Type of current

D.C.

A.C.

16°

2.0 3.0

7S°-

20

16° 16° 21°

ddc A / d m 2

Fig. 59. Comparative voltages to produce equivalent current densities at various temperatures using a 20% w/v electrolyte 20.

Current density 1 A/dm 3] 1 1.5 2.5 Voltage (V) Pore number (X 10™ /cm 2 )

11.9 14.1 16.1 12.24 9.88 8.12

1.54* 2.3* 3.8* 18.1 20.0 23.0 13.36 10.64 7.58

4.0

Table 12. Results of pore diameter measurements on A.C. and D.C. films on 99.99% Al anodized in 220 g/1 H 2SO4 at 21°C 20

lg

133

132 Direct current 1-P half wave 3-P non-reversedinterruptcd 1-P imperfectly rectified (2: 1 ) (3:1) (4:1) 3-P imperfectly rectified (2:1) (3:1) (4:1) Superimposed A.C. + D.C. Normal A.C.

Current form

Voltage D.C. A.C.

1.22

1.16 1.25

Coating ratio

2.76

7.5 CN n CM CM n QO s0 q Tt

ri ri CM CM CM

ri

Lt. yellow-gold Lt. yellow-opaque Lt. yellow-opaque Lt. yellow-opaque Lt. yellow-opaque Lt. yellow-opaque Clear transparent Light yellowish

54 46 64 48 47 47 78 43

lg lg lg

LG

LG CM

LG

TT oc cro q q

370 155

r- c x

2.50 2.70

Clear transparent

80

640

O QO

7 5.5

Clear transparent Clear transparent

Appearance

Hardness (Vickers micro) 78 84

Abrasion resistance (seconds) 445 435

LG (N

1.13 1.04

2.46 2.66

Density

8 8

Thickness (p)

using various proportions of A.C. and various waveforms "

Table 11. Properties of films obtained by anodizing 99.99% aluminium in 165 g/1 H 2SO4 at 1 A/ dm 2 and 30° for 30 minutes

LG O

of A.C. films was compared to that of D.C. films but A.C. films had a greater tendency to bloom on weathering. 26

24

tO 1.0

Type of current

D.C.

A.C.

16°

2.0 3.0

7S°-

20

16° 16° 21°

ddc A / d m 2

Fig. 59. Comparative voltages to produce equivalent current densities at various temperatures using a 20% w/v electrolyte 20.

Current density 1 A/dm 3] 1 1.5 2.5 Voltage (V) Pore number (X 10™ /cm 2 )

11.9 14.1 16.1 12.24 9.88 8.12

1.54* 2.3* 3.8* 18.1 20.0 23.0 13.36 10.64 7.58

4.0

Table 12. Results of pore diameter measurements on A.C. and D.C. films on 99.99% Al anodized in 220 g/1 H 2SO4 at 21°C 20

lg

133

The yellowish colour of the film may be due to the presence of sulphur or sulphides in the film. There is no doubt that some of the sulphate is reduced to sulphide and Budiloff 2’ patented the colouring of films produced by A.C. by dipping the coating into a solution of metallic salt which deposits an insoluble sulphide in the pores of the coating. Kape 22 has explored this technique in the laboratory and reports good colours when the films are dipped in, for example, 2.5 per cent lead acetate (mahogany), 1 per cent cobalt acetate (black), 2 per cent ferric ammonium oxalate (black), 2.5 per cent copper sulphate (green) or 1 per cent cadmium acetate (yellow). Black finishes produced from ferric ammonium oxalate and ammonium sulphide 23 have been used in production and a variety of sulphide colours have been suggested in combination with lacquer coatings in Japan. 24 Although some occasional use has been made of A.C. anodizing for decorative work, the low process efficiency and the softer and more coloured film usually produced reduces its economic appeal. It has been used on commercial wire anodizing plants in Great Britain, 25 26 and also on a development unit built in Canada 27. However Kape has recently revived interest in the process with conditions which are said to give improved efficiency and eliminate sulphide in the film 28 29. He recommends an electrolyte containing about 120 g/1 sulphuric acid and about 30 - 50 g/1 of a ferric salt or an oxidising agent such as a chromate or permanganate, which will react with sulphur compounds formed during anodizing. Good quality, clear coatings can then be obtained at a wide range of current densities on a variety of alloys. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

134

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

A. Prati, F. Sacchi and G. Paolini, Electroplating and Metal Finishing, 1963, 16 (2) 44. J . M. Kape, Metal Ind., 1957,26(10) 198. F. Sacchi and N. Oliva, Alluminio, 1962, 31 (12) 573. S. Tajima, F. Satch, N. Baba and T. Fukushima, J . Electrochem. Soc, (Japan), 1959, 27, E.262. T. Kunimato, E. Ideda and H. Nishimura, Light Metals(Japan), 1953 (17) 66; 1954 (10) 46. F. Sacchi and G. Paolini, Trans. Inst. Met. Finishing, 1963, 40 (5) 229. D . B . P . 967 919(1953). J . M. Kape, Electroplating and Metal Finishing, 1961, 14 (11) 410. Brit. Pat. 884 447. U.S. Pat. 3 930 966. N. D. Pullen, Electroplating and Metal Finishing, 1948, 3(1)3. H. Vevers, Proc. Conference on Anodizing, Aluminium Development Association, 1961, p. 153. A. A. Defoe, Wire & Wire Products, 1957, 31 (11) 1341 . Brit. Pat. 1 439 933. J . M. Kape, Trans. Inst. Met. Finishing, 1977, 55, 25-30. U.S. Pat. 1,965,683. R. B. Mason andC. J . Slunder, Ind. and Eng. Chem., 1947, 39 (12), 1602. F. Sacchi, Alluminio. 1960, 29(10), 447. L. Bosdorf, Metall, 1964, 18(10), 1087. I. Takashi and O. Ichiro, Light Metals (Japan), 1960, 10 (5), 42. Brit. Pat. 1,489,482. Brit. Pat. 1,391,808.

W.C.Cochran and F. Keller, Proc. A.E.S., 1961, 48, 82. R. C. Spooner, J . Electrochem. Soc., 1955, 102 (4) 156. N. D. Tomashov and A. V. Byalobzheskii, Investigations into the Corrosion of Metals, Vol. 1, Akad. Nauk. S.S.S.R., Moscow (1951). R. C. Spooner, Metal Ind., 1952, 81 (13) 248-250. C. Th. Speiser, Aluminium, 1966, 42 (7) 422. M. Schenk, ‘Werkstoff Aluminium und seine Anodische Oxidation’, A. Franke A.G., Berne (1948). F. M. Felmonovich, Ukrain. Khim. Shur. (USSR), 1957, 23, 97. J . Elze, Galvanotechnik, 1962, 53 (8) 374. B. A. Scott, Trans. Inst. Met. Finishing, 1962, 39 (3) 109. J . C6fe, E. E. Howlett, M. J . Wheeler andH. J . Lamb, Plating, 1969, 56, 386-394. J . Cote, E. E. Howlett and H . J . Lamb, Plating, 1970, 57, 484-496. R. D. Guminski, P. G. Sheasby and H. J . Lamb, Trans. Inst. Met. Finishing, 1968, 46, 44-48. W. E. Cooke, Plating, 1962, 49, 1157-64. G . H . Kissin, B. E. Deal and R. V. Paulson in ‘Finishing of Aluminum’ G. H . Kissin (ed), Reinhold Publishing Corp., New York, Chapter 2, 13-31.

135

The yellowish colour of the film may be due to the presence of sulphur or sulphides in the film. There is no doubt that some of the sulphate is reduced to sulphide and Budiloff 2’ patented the colouring of films produced by A.C. by dipping the coating into a solution of metallic salt which deposits an insoluble sulphide in the pores of the coating. Kape 22 has explored this technique in the laboratory and reports good colours when the films are dipped in, for example, 2.5 per cent lead acetate (mahogany), 1 per cent cobalt acetate (black), 2 per cent ferric ammonium oxalate (black), 2.5 per cent copper sulphate (green) or 1 per cent cadmium acetate (yellow). Black finishes produced from ferric ammonium oxalate and ammonium sulphide 23 have been used in production and a variety of sulphide colours have been suggested in combination with lacquer coatings in Japan. 24 Although some occasional use has been made of A.C. anodizing for decorative work, the low process efficiency and the softer and more coloured film usually produced reduces its economic appeal. It has been used on commercial wire anodizing plants in Great Britain, 25 26 and also on a development unit built in Canada 27. However Kape has recently revived interest in the process with conditions which are said to give improved efficiency and eliminate sulphide in the film 28 29. He recommends an electrolyte containing about 120 g/1 sulphuric acid and about 30 - 50 g/1 of a ferric salt or an oxidising agent such as a chromate or permanganate, which will react with sulphur compounds formed during anodizing. Good quality, clear coatings can then be obtained at a wide range of current densities on a variety of alloys. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

134

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

A. Prati, F. Sacchi and G. Paolini, Electroplating and Metal Finishing, 1963, 16 (2) 44. J . M. Kape, Metal Ind., 1957,26(10) 198. F. Sacchi and N. Oliva, Alluminio, 1962, 31 (12) 573. S. Tajima, F. Satch, N. Baba and T. Fukushima, J . Electrochem. Soc, (Japan), 1959, 27, E.262. T. Kunimato, E. Ideda and H. Nishimura, Light Metals(Japan), 1953 (17) 66; 1954 (10) 46. F. Sacchi and G. Paolini, Trans. Inst. Met. Finishing, 1963, 40 (5) 229. D . B . P . 967 919(1953). J . M. Kape, Electroplating and Metal Finishing, 1961, 14 (11) 410. Brit. Pat. 884 447. U.S. Pat. 3 930 966. N. D. Pullen, Electroplating and Metal Finishing, 1948, 3(1)3. H. Vevers, Proc. Conference on Anodizing, Aluminium Development Association, 1961, p. 153. A. A. Defoe, Wire & Wire Products, 1957, 31 (11) 1341 . Brit. Pat. 1 439 933. J . M. Kape, Trans. Inst. Met. Finishing, 1977, 55, 25-30. U.S. Pat. 1,965,683. R. B. Mason andC. J . Slunder, Ind. and Eng. Chem., 1947, 39 (12), 1602. F. Sacchi, Alluminio. 1960, 29(10), 447. L. Bosdorf, Metall, 1964, 18(10), 1087. I. Takashi and O. Ichiro, Light Metals (Japan), 1960, 10 (5), 42. Brit. Pat. 1,489,482. Brit. Pat. 1,391,808.

W.C.Cochran and F. Keller, Proc. A.E.S., 1961, 48, 82. R. C. Spooner, J . Electrochem. Soc., 1955, 102 (4) 156. N. D. Tomashov and A. V. Byalobzheskii, Investigations into the Corrosion of Metals, Vol. 1, Akad. Nauk. S.S.S.R., Moscow (1951). R. C. Spooner, Metal Ind., 1952, 81 (13) 248-250. C. Th. Speiser, Aluminium, 1966, 42 (7) 422. M. Schenk, ‘Werkstoff Aluminium und seine Anodische Oxidation’, A. Franke A.G., Berne (1948). F. M. Felmonovich, Ukrain. Khim. Shur. (USSR), 1957, 23, 97. J . Elze, Galvanotechnik, 1962, 53 (8) 374. B. A. Scott, Trans. Inst. Met. Finishing, 1962, 39 (3) 109. J . C6fe, E. E. Howlett, M. J . Wheeler andH. J . Lamb, Plating, 1969, 56, 386-394. J . Cote, E. E. Howlett and H . J . Lamb, Plating, 1970, 57, 484-496. R. D. Guminski, P. G. Sheasby and H. J . Lamb, Trans. Inst. Met. Finishing, 1968, 46, 44-48. W. E. Cooke, Plating, 1962, 49, 1157-64. G . H . Kissin, B. E. Deal and R. V. Paulson in ‘Finishing of Aluminum’ G. H . Kissin (ed), Reinhold Publishing Corp., New York, Chapter 2, 13-31.

135

Chapter 10: Anodizing in other electrolytes Many electrolytes have been mentioned in the literature other than sulphuric acid or mixtures based primarily on sulphuric acid. Some of these can only be regarded as of academic interest, others as being developed for very specialised applications, and only a few have been relatively widely used. Of these chromic acid merits further consideration because of its continued use for military equipment where its value as an inspection tool is probably at least as important as its protection value. Oxalic acid was widely used for some years in countries where chromic acid anodizing was not available because of patent restrictions, and it is still used industrially. Chromic acid anodizing The process of chromic acid anodizing has remained very largely unchanged since it was discovered in England by Bengough and Stuart.1 It has been used principally for the treatment of aircraft parts in the United States, Russia and Great Britain but it has been less widely employed for this purpose in other European countries. Its main attractions are that it does not leave a corrosive residue in riveted joints, and that the distinctive brown colour of the electrolyte makes it useful as an inspection tool since it is readily seen when it seeps out of pores, cracks, etc. Because of its attractive opaque appearance, it is also the process mainly used for anodizing knitting needles, and a Bengough-Stuart type of process is still employed.

Bengough-Stuart process In England the process has been only slightly modified from the original and is based on the use of a 3 - 5 per cent CrO3,electrolyte maintained at 40° ± 2°C. The voltage is raised gradually to 40 volts over 10 minutes, held constant at 40 volts for the next 20 minutes, raised to 50 volts during the following five minutes, and then maintained at 50 volts for five minutes. When treating casting alloys, particularly those high in copper, a lower electrolyte temperature (25 - 30°C) and slightly modified voltage cycle are recommended. As a general rule the voltage on casting alloys should be such that where the area of the work can be accurately measured, the current density obtained is only 0.5 - 0.7 A/dm 2. This can also be important on AlZnMgCu wrought alloys where only about 15 volts are required. Many casting alloys require only 15 - 20 volts. 136

137

Chapter 10: Anodizing in other electrolytes Many electrolytes have been mentioned in the literature other than sulphuric acid or mixtures based primarily on sulphuric acid. Some of these can only be regarded as of academic interest, others as being developed for very specialised applications, and only a few have been relatively widely used. Of these chromic acid merits further consideration because of its continued use for military equipment where its value as an inspection tool is probably at least as important as its protection value. Oxalic acid was widely used for some years in countries where chromic acid anodizing was not available because of patent restrictions, and it is still used industrially. Chromic acid anodizing The process of chromic acid anodizing has remained very largely unchanged since it was discovered in England by Bengough and Stuart.1 It has been used principally for the treatment of aircraft parts in the United States, Russia and Great Britain but it has been less widely employed for this purpose in other European countries. Its main attractions are that it does not leave a corrosive residue in riveted joints, and that the distinctive brown colour of the electrolyte makes it useful as an inspection tool since it is readily seen when it seeps out of pores, cracks, etc. Because of its attractive opaque appearance, it is also the process mainly used for anodizing knitting needles, and a Bengough-Stuart type of process is still employed.

Bengough-Stuart process In England the process has been only slightly modified from the original and is based on the use of a 3 - 5 per cent CrO3,electrolyte maintained at 40° ± 2°C. The voltage is raised gradually to 40 volts over 10 minutes, held constant at 40 volts for the next 20 minutes, raised to 50 volts during the following five minutes, and then maintained at 50 volts for five minutes. When treating casting alloys, particularly those high in copper, a lower electrolyte temperature (25 - 30°C) and slightly modified voltage cycle are recommended. As a general rule the voltage on casting alloys should be such that where the area of the work can be accurately measured, the current density obtained is only 0.5 - 0.7 A/dm 2. This can also be important on AlZnMgCu wrought alloys where only about 15 volts are required. Many casting alloys require only 15 - 20 volts. 136

137

Constant voltage processes In the United States work was carried out by Buzzard to devise a constant voltage process. 2 It was found that if the chromic acid content was kept between 5 and 10 per cent a constant potential of 40 volts could be used with the treatment time being reduced to 30 minutes. Similar conclusions were reached by a Russian worker 3 who also found that if the pH of the solution was 0.15 - 0.60 then 30 volts were adequate, but at pH 0.6 - 0.8 an increase to 40 volts was necessary. Investigations by American workers 4 did not completely concur with the use of 30 volts and subsequent American practice was based mainly on a 9.5 per cent CrO3 electrolyte with pH control.

2

2

3

80

120

160

0

More recently Lewsey 7 reported that useful decorative coatings could be obtained from 10 per cent CrO3 electrolytes if the temperature were increased to 54 °C. Constant voltage processing conditions using potentials of between 20 and 30 volts were recommended. This modified process was investigated in more detail by Peck and Brace 8 who found that films of up to 10 thickness could be obtained with 30 volts at 54°C, compared with a maximum of 50 volts for the BengoughStuart and Buzzard processes. The films from the former process are an attractive opaque grey-white and have good dyeing characteristics. The dyed films have an enamel-like appearance. Aluminium Francais 9 have described a quite different process in which even higher chromic acid concentrations are used (15 - 19% by weight), with low bath temperatures (below 30°C) and low anodizing voltages (preferably 21 - 23 V). The cathodic surface again has to be small and is related to the electrolyte volume. With this process the consumption of chromic acid is said to be very much reduced as a result of peroxidic reactions occurring at the anode and addition of hydrogen peroxide to the bath is apparently beneficial. The rate of anodizing is slow but films up to 6 microns in thickness can be produced after 120 minutes anodizing.

2

40

the cathode area was kept small relative to the anode area, the reduction of chromic acid to trivalent chromium was minimised (Fig. 50). The British Specification DEF 151 advises cathode : anode ratios from 5 : 1 to 10 : 1.

40

More recently still National Research Development Corporation 10 have described a chromic acid process which is capable of producing anodic films more than 15 microns thick, and even up to 25 microns. Very high anodizing voltages (100 - 150 V) are used with low chromic acid concentrations (0.2 - 0.3 M) and high anodizing temperatures (58° - 62 °C). The process can be operated on either a constant voltage or constant current density basis and good film appearance and properties are claimed.

A T / V (dm 2 x h r / I )

A B C D

Fig. 60. Change in the concentration of principal constituents with continued use of chromic acid anodizing baths 6. 1 Hexavalent chromium 5% CrO3 , large cathode 2 Free chromic acid 5% CrO3 , small cathode 3 Aluminium 10% CrO 3, large cathode 4 Trivalent chromium 10% CrO3, small cathode

Further Russian work' found that up to 200 g/1 CrO 3 (free and combined) was without harmful effect and a method was devised of regenerating the bath using lead anodes and steel cathodes, a current density of 0.25 A/dm 2 , a cathode : anode ratio of 1 : 40 and a regeneration time of 24 hours. To reduce the pH sufficiently the cathodes may have to be stripped of their lead chromate film and the process continued for a further 24 hours. American investigations 0 also showed that if 138

Control of impurities The purity of the chromic acid used is of importance and in the electrolyte a limit of 0.2 g/1 chloride, estimated as sodium chloride, is normally specified. Sulphite, a common impurity, increases the consumption of chromic acid by promoting reduction of hexavalent chromium at the cathode. To minimise this it is preferable to line the anodizing tank with a glass or plastics shield and to use steel cathodes of an area that will result in a high cathodic current density. Anode : cathode ratios of at least 20 : 1 are preferred. Sulphate also has a significant effect on the appearance of the coating. Jackson" has reported that Bengough-Stuart films are slightly opaque up to 0.1 g/1 SO4 and opaque at 0.1 - 0.3 g/1; beyond this the films become transparent. The sulphate range which gives opaque films is slightly wider (0.1 - 0.4 g/1) for 10 per cent CrO3 electrolytes. The opacity is due to etching at the metal-oxide interface but the mechanism 139

Constant voltage processes In the United States work was carried out by Buzzard to devise a constant voltage process. 2 It was found that if the chromic acid content was kept between 5 and 10 per cent a constant potential of 40 volts could be used with the treatment time being reduced to 30 minutes. Similar conclusions were reached by a Russian worker 3 who also found that if the pH of the solution was 0.15 - 0.60 then 30 volts were adequate, but at pH 0.6 - 0.8 an increase to 40 volts was necessary. Investigations by American workers 4 did not completely concur with the use of 30 volts and subsequent American practice was based mainly on a 9.5 per cent CrO3 electrolyte with pH control.

2

2

3

80

120

160

0

More recently Lewsey 7 reported that useful decorative coatings could be obtained from 10 per cent CrO3 electrolytes if the temperature were increased to 54 °C. Constant voltage processing conditions using potentials of between 20 and 30 volts were recommended. This modified process was investigated in more detail by Peck and Brace 8 who found that films of up to 10 thickness could be obtained with 30 volts at 54°C, compared with a maximum of 50 volts for the BengoughStuart and Buzzard processes. The films from the former process are an attractive opaque grey-white and have good dyeing characteristics. The dyed films have an enamel-like appearance. Aluminium Francais 9 have described a quite different process in which even higher chromic acid concentrations are used (15 - 19% by weight), with low bath temperatures (below 30°C) and low anodizing voltages (preferably 21 - 23 V). The cathodic surface again has to be small and is related to the electrolyte volume. With this process the consumption of chromic acid is said to be very much reduced as a result of peroxidic reactions occurring at the anode and addition of hydrogen peroxide to the bath is apparently beneficial. The rate of anodizing is slow but films up to 6 microns in thickness can be produced after 120 minutes anodizing.

2

40

the cathode area was kept small relative to the anode area, the reduction of chromic acid to trivalent chromium was minimised (Fig. 50). The British Specification DEF 151 advises cathode : anode ratios from 5 : 1 to 10 : 1.

40

More recently still National Research Development Corporation 10 have described a chromic acid process which is capable of producing anodic films more than 15 microns thick, and even up to 25 microns. Very high anodizing voltages (100 - 150 V) are used with low chromic acid concentrations (0.2 - 0.3 M) and high anodizing temperatures (58° - 62 °C). The process can be operated on either a constant voltage or constant current density basis and good film appearance and properties are claimed.

A T / V (dm 2 x h r / I )

A B C D

Fig. 60. Change in the concentration of principal constituents with continued use of chromic acid anodizing baths 6. 1 Hexavalent chromium 5% CrO3 , large cathode 2 Free chromic acid 5% CrO3 , small cathode 3 Aluminium 10% CrO 3, large cathode 4 Trivalent chromium 10% CrO3, small cathode

Further Russian work' found that up to 200 g/1 CrO 3 (free and combined) was without harmful effect and a method was devised of regenerating the bath using lead anodes and steel cathodes, a current density of 0.25 A/dm 2 , a cathode : anode ratio of 1 : 40 and a regeneration time of 24 hours. To reduce the pH sufficiently the cathodes may have to be stripped of their lead chromate film and the process continued for a further 24 hours. American investigations 0 also showed that if 138

Control of impurities The purity of the chromic acid used is of importance and in the electrolyte a limit of 0.2 g/1 chloride, estimated as sodium chloride, is normally specified. Sulphite, a common impurity, increases the consumption of chromic acid by promoting reduction of hexavalent chromium at the cathode. To minimise this it is preferable to line the anodizing tank with a glass or plastics shield and to use steel cathodes of an area that will result in a high cathodic current density. Anode : cathode ratios of at least 20 : 1 are preferred. Sulphate also has a significant effect on the appearance of the coating. Jackson" has reported that Bengough-Stuart films are slightly opaque up to 0.1 g/1 SO4 and opaque at 0.1 - 0.3 g/1; beyond this the films become transparent. The sulphate range which gives opaque films is slightly wider (0.1 - 0.4 g/1) for 10 per cent CrO3 electrolytes. The opacity is due to etching at the metal-oxide interface but the mechanism 139

of this etching has never been explained. With use the composition of the electrolyte changes in two respects; there is a steady increase in aluminium content and also an increase in the total chromate and trivalent chromium contents. A simple pH - titrimetric method has been described 12 in which the total and free CrO3 concentrations and Al content can be determined by one titration. There is no general agreement as to the maximum total chromate content which should be permitted. In England a total of 10 per cent free and combined chromic acid is permitted with the Bengough-Stuart process but in the United States a limit of 20 per cent is frequently observed for the 10 per cent CrO3 (nominal) electrolyte. Peek and Brace 8 have stated that with the modified 10 per cent CrO3 process, even this limit can be exceeded provided the free acid content is maintained at 10 per cent. To maintain more constant electrolyte conditions appropriate formulae have been devised 13 which enable the bath analysis to be used to decide the additions of fresh acid, or the amount of used electrolyte which should be discarded to maintain the bath at optimum efficiency. More recently ion exchange has been applied to the control and regeneration of chromic acid anodizing solutions 14 using cation exchange resins. Proprietary cation exchange resins such as Permutit Q, Zeo-Karb 225, Amberlite IR-20 and Rowex 50 are suitable. The pumps, tanks and piping used for operating the regeneration system are normally PVC lined. In addition anion exchange resins can be used to remove chromic acid from the rinse waters. With larger plants especially, use of such exchange systems can result in very significant cost savings as well as reducing effluent problems.

produced are rather thin. The structure of phosphoric acid coatings has been studied by O’Sullivan and Wood 19. Oxalic acid anodizing This electrolyte 20 was widely used in Germany and Japan prior to 1939 but it is now used only to a limited extent, although oxalic acid is frequently added to sulphuric acid anodizing electrolytes (see next section). It does not usually give the clear, transparent coating obtained from sulphuric acid anodizing. The colour of the coating is dependent upon the electrolyte temperature, current density, film thickness and alloy being anodized, varying from little or no colour at 5 p t o a deep colour with films of 25 - 50/z, increasing in depth of colour with increase in current density, but decreasing as the anodizing temperature is raised. Oxalic acid anodizing is influenced by similar factors to those discussed under sulphuric acid anodizing, so the same principles, slightly modified, apply to control of anodic film properties. The effects of oxalic acid concentration on film thickness obtained after 60 minutes anodizing at constant current density are shown in Fig. 61, similar information for constant voltage conditions being summarised in Fig. 62. Anodizing with alternating current has been used with this electrolyte, primarily to obtain a more strongly coloured film. Films produced in this electrolyte are softer and more porous than those produced with direct current. 40

30

Phosphoric acid anodizing was first used during attempts made to electroplate aluminium. It was used as a pretreatment, with 10 minutes anodizing in a 354 g/1 phosphoric acid electrolyte at 32°C and 1.3 A/dm 2 being recommended 15. Industrial experience reported by Bunce 16 led to some modification of the process, depending on the alloy to be plated and the metal to be deposited. Some alloys gave better results if the temperature was lowered to 27°C or even 24°C. Spooner and Seraphim 17 studied the process in detail and recommended anodizing in a 250 g/1 phosphoric acid electrolyte at 1.1 - 1.6 A/dm 2 and 25°C, also for a time of 10 minutes. A film thickness of about 3 microns was produced but, as might be expected, the solubility of the films in phosphoric acid was much higher than in sulphuric acid and thinner and more porous films resulted. These processes are rarely used now as types of zincate or stannate pretreatment are more often used, but phosphoric acid anodizing is used in the preparation of printing plates for the lithographic printing industry 18. Similar anodizing conditions to those used for plating pretreatment are employed but the coatings 140

Thickness (p)

Phosphoric acid anodizing

20 mA/cm‘

1 5 mA/cm

20

10 mA/ cm' 10 5 mA/ cm

0.71 1-43 2.14 2.86 3.57 4.29 5.00 5.71

Concentration (%)

Fig. 61. Film thickness resulting after 60 mins, anodizing at constant current density in various concentrations of oxalic acid at 20° C 21. 141

of this etching has never been explained. With use the composition of the electrolyte changes in two respects; there is a steady increase in aluminium content and also an increase in the total chromate and trivalent chromium contents. A simple pH - titrimetric method has been described 12 in which the total and free CrO3 concentrations and Al content can be determined by one titration. There is no general agreement as to the maximum total chromate content which should be permitted. In England a total of 10 per cent free and combined chromic acid is permitted with the Bengough-Stuart process but in the United States a limit of 20 per cent is frequently observed for the 10 per cent CrO3 (nominal) electrolyte. Peek and Brace 8 have stated that with the modified 10 per cent CrO3 process, even this limit can be exceeded provided the free acid content is maintained at 10 per cent. To maintain more constant electrolyte conditions appropriate formulae have been devised 13 which enable the bath analysis to be used to decide the additions of fresh acid, or the amount of used electrolyte which should be discarded to maintain the bath at optimum efficiency. More recently ion exchange has been applied to the control and regeneration of chromic acid anodizing solutions 14 using cation exchange resins. Proprietary cation exchange resins such as Permutit Q, Zeo-Karb 225, Amberlite IR-20 and Rowex 50 are suitable. The pumps, tanks and piping used for operating the regeneration system are normally PVC lined. In addition anion exchange resins can be used to remove chromic acid from the rinse waters. With larger plants especially, use of such exchange systems can result in very significant cost savings as well as reducing effluent problems.

produced are rather thin. The structure of phosphoric acid coatings has been studied by O’Sullivan and Wood 19. Oxalic acid anodizing This electrolyte 20 was widely used in Germany and Japan prior to 1939 but it is now used only to a limited extent, although oxalic acid is frequently added to sulphuric acid anodizing electrolytes (see next section). It does not usually give the clear, transparent coating obtained from sulphuric acid anodizing. The colour of the coating is dependent upon the electrolyte temperature, current density, film thickness and alloy being anodized, varying from little or no colour at 5 p t o a deep colour with films of 25 - 50/z, increasing in depth of colour with increase in current density, but decreasing as the anodizing temperature is raised. Oxalic acid anodizing is influenced by similar factors to those discussed under sulphuric acid anodizing, so the same principles, slightly modified, apply to control of anodic film properties. The effects of oxalic acid concentration on film thickness obtained after 60 minutes anodizing at constant current density are shown in Fig. 61, similar information for constant voltage conditions being summarised in Fig. 62. Anodizing with alternating current has been used with this electrolyte, primarily to obtain a more strongly coloured film. Films produced in this electrolyte are softer and more porous than those produced with direct current. 40

30

Phosphoric acid anodizing was first used during attempts made to electroplate aluminium. It was used as a pretreatment, with 10 minutes anodizing in a 354 g/1 phosphoric acid electrolyte at 32°C and 1.3 A/dm 2 being recommended 15. Industrial experience reported by Bunce 16 led to some modification of the process, depending on the alloy to be plated and the metal to be deposited. Some alloys gave better results if the temperature was lowered to 27°C or even 24°C. Spooner and Seraphim 17 studied the process in detail and recommended anodizing in a 250 g/1 phosphoric acid electrolyte at 1.1 - 1.6 A/dm 2 and 25°C, also for a time of 10 minutes. A film thickness of about 3 microns was produced but, as might be expected, the solubility of the films in phosphoric acid was much higher than in sulphuric acid and thinner and more porous films resulted. These processes are rarely used now as types of zincate or stannate pretreatment are more often used, but phosphoric acid anodizing is used in the preparation of printing plates for the lithographic printing industry 18. Similar anodizing conditions to those used for plating pretreatment are employed but the coatings 140

Thickness (p)

Phosphoric acid anodizing

20 mA/cm‘

1 5 mA/cm

20

10 mA/ cm' 10 5 mA/ cm

0.71 1-43 2.14 2.86 3.57 4.29 5.00 5.71

Concentration (%)

Fig. 61. Film thickness resulting after 60 mins, anodizing at constant current density in various concentrations of oxalic acid at 20° C 21. 141

lOOV

90V

60V

Fig. 62. Film thickness resulting after 60 mins, anodizing at constant voltage in various concentrations of oxalic acid at 20° 2\

Fig. 63. Effect of anodizing bath temperature on limiting thickness of film obtained in 5% oxalic acid at 1.5 A/ dm 2 2 2. I = D.C. anodizing II— A.C. anodizing

20

30

40

50

60

Bath temperature (° C )

50_v

40V 10

1.4 3 2.14

2.86

3.57 4.29

5.71

Concentration (%)

As with sulphuric acid, there is a decided re-dissolution of the oxide film in oxalic acid, although it is less than with the former electrolyte. This effectively sets a working limit to the electrolyte temperature (see Fig. 63). This can be seen more clearly in Fig. 64 which is due to Hubner, 22 who points out that the pH and aluminium content also have an effect. The increased thickness for the higher temperatures at the higher pH values is due to the lower dissolution rate.

Several standard processes were developed using oxalic acid electrolytes and these are still practiced. The electrolyte used normally contains 3 - 5 per cent of crystalline acid of technical grade but some anodizers use 1 per cent concentration. These processes are summarised in Table 16 which includes a typical sulphuric acid process for comparison. 142

Thickness (g )

Dissolved aluminium lowers the conductivity of the solution linearly with concentration but has little effect on corrosion resistance, showing a slight fall up to 2 g/1 according to Japanese workers 23 and then a slight increase with further build-up. They also report that abrasion resistance falls steadily with aluminium content. It is recommended by one authority 24 that the aluminium content should not exceed 2.5 g/1 and that chloride should also be not more than 0.02 g/1, and experience has shown that chloride is critical in causing pitting above this level.

Fig. 64. Effect of pH and temperature on thickness obtained on 99.5% aluminium anodized in 5% oxalic acid electrolyte 22

pH

143

lOOV

90V

60V

Fig. 62. Film thickness resulting after 60 mins, anodizing at constant voltage in various concentrations of oxalic acid at 20° 2\

Fig. 63. Effect of anodizing bath temperature on limiting thickness of film obtained in 5% oxalic acid at 1.5 A/ dm 2 2 2. I = D.C. anodizing II— A.C. anodizing

20

30

40

50

60

Bath temperature (° C )

50_v

40V 10

1.4 3 2.14

2.86

3.57 4.29

5.71

Concentration (%)

As with sulphuric acid, there is a decided re-dissolution of the oxide film in oxalic acid, although it is less than with the former electrolyte. This effectively sets a working limit to the electrolyte temperature (see Fig. 63). This can be seen more clearly in Fig. 64 which is due to Hubner, 22 who points out that the pH and aluminium content also have an effect. The increased thickness for the higher temperatures at the higher pH values is due to the lower dissolution rate.

Several standard processes were developed using oxalic acid electrolytes and these are still practiced. The electrolyte used normally contains 3 - 5 per cent of crystalline acid of technical grade but some anodizers use 1 per cent concentration. These processes are summarised in Table 16 which includes a typical sulphuric acid process for comparison. 142

Thickness (g )

Dissolved aluminium lowers the conductivity of the solution linearly with concentration but has little effect on corrosion resistance, showing a slight fall up to 2 g/1 according to Japanese workers 23 and then a slight increase with further build-up. They also report that abrasion resistance falls steadily with aluminium content. It is recommended by one authority 24 that the aluminium content should not exceed 2.5 g/1 and that chloride should also be not more than 0.02 g/1, and experience has shown that chloride is critical in causing pitting above this level.

Fig. 64. Effect of pH and temperature on thickness obtained on 99.5% aluminium anodized in 5% oxalic acid electrolyte 22

pH

143

Table 13. Typical oxalic acid anodizing processes with a sulphuric acid process for comparison

Electrolyte

Process

Potential (V)

Temp.

(°O GS GX GXh WX WGX D.C. + A.C.

H 2SO 4 (COOH) 2 (COOH) 2 (COOH) 2

21 20 35 35

(COOH) 2

25

17D.C. 40-60D.C. 30-35D.C. 40 - 6 0 A. C. 30-60D.C. 40 - 6 0 A. C.

Current density (A/dm 2 )

Time (min. )

1 . 0 - 1.5 1.0-2.0 1.5-2.0 2.0 - 3 . 0 2.0 -3.0 1 . 0 - 2 .0

30-40 40-60 20-30 40-60 15-30 15-30

Kwh consumed 0.5 - 2 . 0 3-12 1-3.5 5-9

Table 15. Influence of anodizing conditions on the colour obtained from anodizing in a 7% (COOH)2.2H 2O electrolyte

Direct

2 - 10

The hardness and abrasion resistance of the coatings produced by these processes 25 is seen in Table 14.

Alternating

Table 14. Scratch hardness and [rubbing] abrasion resistance of oxalic and sulphuric acid coatings compared- s Process

Thickness

W

Scratch hardness [kg]

GS

14.7

38

GX GXh WX WGX

35.3 39.0 5.9 14.7

105 41 5.2 14.5

Abrasion resistance [double movements] 85 000 444 40 4 57

000 000 000 000

Specific Specific scratch abrasion hardness resistance [kg/ m m 2 ] [strokes/ mm] 2.6 2.9 1.1 0.9 1.0

5 800 12 500 10 000 680 3 900

Coloured coatings The tendency of oxalic acid to produce coloured coatings has been exploited to achieve specific decorative effects. One of the earliest investigations showed the relevant factors affecting depth of colour (Table 15). A process formerly used to develop self-coloured coatings for architectural anodizing (the ‘Alcanodox’ process) utilises a saturated oxalic acid electrolyte at a current density of 1.6 - 2.7 A/dm 2 and room temperature 26 . To obtain a reasonable depth of colour the film thickness must b e at least 15 and the depth of colour increases with film thickness. This process is still used for architectural purposes in Sweden. 144

Colour Film Current Potential Electrolyte Time temp [mins. ] thickness density [V] [A/dm 2 ] [°C] almost 1 60 45/47 19/20 20.5 colourless light gold 15 20/20.5 21.5 58/63 4 gold .7.5 22.5 66/72 20/22.5 8 yellow-gold 70/80 3 21 20 15/23 deep gold 77/92 1.5 20 40 19.5/29

Current

1 4 8 20

13/15 67/72 73/77 89/102

28/30 30/32 30/32 20/34

60 15 7.5 3

6.3 6.3 6.6 6.2

40

107/120

23/35

1.5

6.0

colourless brassy-gold darker brass dark brass/ gold golden brown

Table 16. Effect of oxalic acid concentration on colour obtained and voltage required to maintain a current density of 15 A/ft 3 at 20°C for various aluminium alloys 27 (25// film)

Alloy

0.5% acid*

1.0% acid

No. 1 sheetl 15 - 265V opaque 9 0 - 155V light (99.52 Al) grey grey-brown No. 2 sheet 30 -225V blue 3 0 - 160V slate (5% Si) grey grey No. 3 sheet 96 - 188V dark 30 - 225V dark grey (5% Si) grey 100 -120V silver 110 -240V dark NS 3 sheet grey (1%% Mn) grey 70 -210V pale HE 9 8 0 - 165V light (AlMgSi) grey grey-brown B33SWP 35 - 252V . blue 30- 142V greygrey (5%Si + Mg) brown

1.5% acid 72- 105V greenish yellow

6.6% acid 50 - 56V light gold

33 - 108V greenish 28 - 77V greyish yellow yellow 36 - 72V dark 30- 122V dark grey grey 60 - 97V silver grey

36 - 72V dark grey

70- 115V greenish yellow

50 - 52V light gold

28- 122V medium 45 - 66V medium grey grey

* Figures are for anhydrous (COOH) 2 1 % anhydrous acid = approximately 1 .4% hydrated crystals

145

Table 13. Typical oxalic acid anodizing processes with a sulphuric acid process for comparison

Electrolyte

Process

Potential (V)

Temp.

(°O GS GX GXh WX WGX D.C. + A.C.

H 2SO 4 (COOH) 2 (COOH) 2 (COOH) 2

21 20 35 35

(COOH) 2

25

17D.C. 40-60D.C. 30-35D.C. 40 - 6 0 A. C. 30-60D.C. 40 - 6 0 A. C.

Current density (A/dm 2 )

Time (min. )

1 . 0 - 1.5 1.0-2.0 1.5-2.0 2.0 - 3 . 0 2.0 -3.0 1 . 0 - 2 .0

30-40 40-60 20-30 40-60 15-30 15-30

Kwh consumed 0.5 - 2 . 0 3-12 1-3.5 5-9

Table 15. Influence of anodizing conditions on the colour obtained from anodizing in a 7% (COOH)2.2H 2O electrolyte

Direct

2 - 10

The hardness and abrasion resistance of the coatings produced by these processes 25 is seen in Table 14.

Alternating

Table 14. Scratch hardness and [rubbing] abrasion resistance of oxalic and sulphuric acid coatings compared- s Process

Thickness

W

Scratch hardness [kg]

GS

14.7

38

GX GXh WX WGX

35.3 39.0 5.9 14.7

105 41 5.2 14.5

Abrasion resistance [double movements] 85 000 444 40 4 57

000 000 000 000

Specific Specific scratch abrasion hardness resistance [kg/ m m 2 ] [strokes/ mm] 2.6 2.9 1.1 0.9 1.0

5 800 12 500 10 000 680 3 900

Coloured coatings The tendency of oxalic acid to produce coloured coatings has been exploited to achieve specific decorative effects. One of the earliest investigations showed the relevant factors affecting depth of colour (Table 15). A process formerly used to develop self-coloured coatings for architectural anodizing (the ‘Alcanodox’ process) utilises a saturated oxalic acid electrolyte at a current density of 1.6 - 2.7 A/dm 2 and room temperature 26 . To obtain a reasonable depth of colour the film thickness must b e at least 15 and the depth of colour increases with film thickness. This process is still used for architectural purposes in Sweden. 144

Colour Film Current Potential Electrolyte Time temp [mins. ] thickness density [V] [A/dm 2 ] [°C] almost 1 60 45/47 19/20 20.5 colourless light gold 15 20/20.5 21.5 58/63 4 gold .7.5 22.5 66/72 20/22.5 8 yellow-gold 70/80 3 21 20 15/23 deep gold 77/92 1.5 20 40 19.5/29

Current

1 4 8 20

13/15 67/72 73/77 89/102

28/30 30/32 30/32 20/34

60 15 7.5 3

6.3 6.3 6.6 6.2

40

107/120

23/35

1.5

6.0

colourless brassy-gold darker brass dark brass/ gold golden brown

Table 16. Effect of oxalic acid concentration on colour obtained and voltage required to maintain a current density of 15 A/ft 3 at 20°C for various aluminium alloys 27 (25// film)

Alloy

0.5% acid*

1.0% acid

No. 1 sheetl 15 - 265V opaque 9 0 - 155V light (99.52 Al) grey grey-brown No. 2 sheet 30 -225V blue 3 0 - 160V slate (5% Si) grey grey No. 3 sheet 96 - 188V dark 30 - 225V dark grey (5% Si) grey 100 -120V silver 110 -240V dark NS 3 sheet grey (1%% Mn) grey 70 -210V pale HE 9 8 0 - 165V light (AlMgSi) grey grey-brown B33SWP 35 - 252V . blue 30- 142V greygrey (5%Si + Mg) brown

1.5% acid 72- 105V greenish yellow

6.6% acid 50 - 56V light gold

33 - 108V greenish 28 - 77V greyish yellow yellow 36 - 72V dark 30- 122V dark grey grey 60 - 97V silver grey

36 - 72V dark grey

70- 115V greenish yellow

50 - 52V light gold

28- 122V medium 45 - 66V medium grey grey

* Figures are for anhydrous (COOH) 2 1 % anhydrous acid = approximately 1 .4% hydrated crystals

145

A later paper by Kape 27 has summarised the effect of the oxalic acid content of the electrolyte on the colour produced and the voltage required to maintain 1.6 A/dm 2 at 20°C (Table 16). He also notes the fact that with the weaker electrolytes especially, the colour obtained is to a large extent dependent upon the final voltage reached in anodizing. The structure of oxalic acid films has been studied by Bailey and Wood 28, who concluded that, as with all high voltage processes, the pore size, barrier layer thickness and cell size of oxalic acid films were all larger than those of sulphuric acid coatings. Further development of oxalic acid anodizing was carried out by Alcan29 , who showed that attractive deep gold, bronze, dark bronze and black finishes could be produced on HE9 sections by using pulsed currents for anodizing. Ratios of peak current to mean current used were as high as 15 : 1 (i.e. peak currents of nearly 25 A/dm 2) and no detrimental effect on coating quality was observed. The power supply for this process was thyristor controlled but its cost was inevitably very high and the procedure was never used industrially. Pulsed current anodizing in sulphuric acid has, however, apparently been used for producing coloured coatings in Japan 30 31 . Although it is not widely used oxalic acid anodizing is an interesting process, the potential of which has probably never been fully utilised. It is surprising, for example, that despite the high anodizing voltages used, a polished finish remains smooth and polished after oxalic acid anodizing. This is in marked contrast to most high voltage processes which produce considerable roughening of the aluminium/aluminium oxide interface. The corrosion resistance of the films formed in oxalic acid is excellent and possibly superior to that of similar sulphuric acid coatings. Ematal process A well known modification of the oxalic acid process is the Ematal process 32 in which an electrolyte containing 40 g/1 potassium titanium oxalate, 1.2 g/1 oxalic acid, 8 g/1 boric acid and 1 g/1 citric acid is used at 55° - 60°C and pH 1.6 - 3.0. To provide a current density of lA/dm 2 potentials up to 120V may be required. The process gives an attractive enamel-like appearance. A later process 33 of a similar type uses an electrolyte containing 30 - 60 g/1 potassium titanium oxalate, 4 - 5 g/1 cobalt acetate, 3 - 4 g/1 chromic acid and 1 - 1.8 g/1 nickel acetate or nickel sulphate. Again high voltages (130 - 140V) are used and the metal deposits in the film are said to make it electrically conductive.

Integral colour anodizing processes Integral colour anodizing processes are those in which colour is produced during the anodizing step itself by use of special organic-acid based electrolytes. They 146

are also sometimes referred to as ‘self-colour anodizing’ or ‘hard colour anodizing’ processes. They are all used at high voltages (up to 100V or more) and, as a result, the hardness and abrasion resistance are higher than that of films produced in a sulphuric acid electrolyte, hence the name ‘hard colour’. They were developed industrially in North America and were a response to the demand for highly durable colour anodized finishes for monumental buildings. They are used in the same sort of applications as the later developed electrolytically coloured finishes, and, like them, most processes are covered by patent applications and operated under licence. Most processes produce ranges of bronze, gold, grey and black finishes, according to the alloys and anodizing conditions used. In many cases special alloys have been developed for use with these processes. The earliest processes used were the oxalic acid anodizing processes developed in Japan and mentioned earlier in this Chapter, but only gold to bronze finishes were produced at that stage. The colours were considered difficult to control and were, in fact, thought to be a disadvantage, so conditions producing the minimum film colour were usually used. The same was true of German investigations in the 1930’s by Schering 3 4 35 and, although many of the mixtures examined are close to the sort of processes used now, no industrial use was made of them. 'Kalcolor 1 process The first industrial integral colour anodizing process was the ‘Kalcolor’ process of Kaiser Aluminum, which was introduced in the late 1950’s 36 37. This process, which has probably been more widely used than any other, was initially based on a mixture of 70 - 150 g/1 sulphosalicylic acid and 3 - 40 g/1 sulphuric acid. As with many of the integral processes, sulphuric acid, or certain metal sulphates, is added to increase the conductivity of the electrolyte and hence reduce the operating voltage. The electrolyte was used at a temperature of about 25°C with current densities of 1 - 10 A/dm 2 and voltages of 20 - 120 volts. Under these conditions attractive bronze finishes are obtained on HE9 (6063) extrusions, greyish finishes on NS3 (3003) sheet and black or grey-black finishes on NS5 (5086) sheet or HE20 (6061) extrusions. These and other details are shown in Table 17. As with all integral anodizing processes, the actual shade produced depends on the anodic film thickness and the anodizing conditions used as well as on the alloy concerned, darker shades generally being produced with high anodizing voltages, lower anodizing temperatures and high film thicknesses, and lighter colours with lower voltages, higher temperatures and lower thicknesses. On the HE9 extrusions normally used for anodizing, it was quickly clear that the darker bronze shades were very popular, and in the early stages of the process this meant producing films as much as 40 microns in thickness. Much effort has therefore been centred on producing dark colours with more normal thicknesses of say 20 - 25 microns. In practice this has meant use of electrolytes containing lower sulphosalicylic acid (60 - 70 g/1) and sulphuric acid (5 - 6 g/1) content at voltages up to 147

A later paper by Kape 27 has summarised the effect of the oxalic acid content of the electrolyte on the colour produced and the voltage required to maintain 1.6 A/dm 2 at 20°C (Table 16). He also notes the fact that with the weaker electrolytes especially, the colour obtained is to a large extent dependent upon the final voltage reached in anodizing. The structure of oxalic acid films has been studied by Bailey and Wood 28, who concluded that, as with all high voltage processes, the pore size, barrier layer thickness and cell size of oxalic acid films were all larger than those of sulphuric acid coatings. Further development of oxalic acid anodizing was carried out by Alcan29 , who showed that attractive deep gold, bronze, dark bronze and black finishes could be produced on HE9 sections by using pulsed currents for anodizing. Ratios of peak current to mean current used were as high as 15 : 1 (i.e. peak currents of nearly 25 A/dm 2) and no detrimental effect on coating quality was observed. The power supply for this process was thyristor controlled but its cost was inevitably very high and the procedure was never used industrially. Pulsed current anodizing in sulphuric acid has, however, apparently been used for producing coloured coatings in Japan 30 31 . Although it is not widely used oxalic acid anodizing is an interesting process, the potential of which has probably never been fully utilised. It is surprising, for example, that despite the high anodizing voltages used, a polished finish remains smooth and polished after oxalic acid anodizing. This is in marked contrast to most high voltage processes which produce considerable roughening of the aluminium/aluminium oxide interface. The corrosion resistance of the films formed in oxalic acid is excellent and possibly superior to that of similar sulphuric acid coatings. Ematal process A well known modification of the oxalic acid process is the Ematal process 32 in which an electrolyte containing 40 g/1 potassium titanium oxalate, 1.2 g/1 oxalic acid, 8 g/1 boric acid and 1 g/1 citric acid is used at 55° - 60°C and pH 1.6 - 3.0. To provide a current density of lA/dm 2 potentials up to 120V may be required. The process gives an attractive enamel-like appearance. A later process 33 of a similar type uses an electrolyte containing 30 - 60 g/1 potassium titanium oxalate, 4 - 5 g/1 cobalt acetate, 3 - 4 g/1 chromic acid and 1 - 1.8 g/1 nickel acetate or nickel sulphate. Again high voltages (130 - 140V) are used and the metal deposits in the film are said to make it electrically conductive.

Integral colour anodizing processes Integral colour anodizing processes are those in which colour is produced during the anodizing step itself by use of special organic-acid based electrolytes. They 146

are also sometimes referred to as ‘self-colour anodizing’ or ‘hard colour anodizing’ processes. They are all used at high voltages (up to 100V or more) and, as a result, the hardness and abrasion resistance are higher than that of films produced in a sulphuric acid electrolyte, hence the name ‘hard colour’. They were developed industrially in North America and were a response to the demand for highly durable colour anodized finishes for monumental buildings. They are used in the same sort of applications as the later developed electrolytically coloured finishes, and, like them, most processes are covered by patent applications and operated under licence. Most processes produce ranges of bronze, gold, grey and black finishes, according to the alloys and anodizing conditions used. In many cases special alloys have been developed for use with these processes. The earliest processes used were the oxalic acid anodizing processes developed in Japan and mentioned earlier in this Chapter, but only gold to bronze finishes were produced at that stage. The colours were considered difficult to control and were, in fact, thought to be a disadvantage, so conditions producing the minimum film colour were usually used. The same was true of German investigations in the 1930’s by Schering 3 4 35 and, although many of the mixtures examined are close to the sort of processes used now, no industrial use was made of them. 'Kalcolor 1 process The first industrial integral colour anodizing process was the ‘Kalcolor’ process of Kaiser Aluminum, which was introduced in the late 1950’s 36 37. This process, which has probably been more widely used than any other, was initially based on a mixture of 70 - 150 g/1 sulphosalicylic acid and 3 - 40 g/1 sulphuric acid. As with many of the integral processes, sulphuric acid, or certain metal sulphates, is added to increase the conductivity of the electrolyte and hence reduce the operating voltage. The electrolyte was used at a temperature of about 25°C with current densities of 1 - 10 A/dm 2 and voltages of 20 - 120 volts. Under these conditions attractive bronze finishes are obtained on HE9 (6063) extrusions, greyish finishes on NS3 (3003) sheet and black or grey-black finishes on NS5 (5086) sheet or HE20 (6061) extrusions. These and other details are shown in Table 17. As with all integral anodizing processes, the actual shade produced depends on the anodic film thickness and the anodizing conditions used as well as on the alloy concerned, darker shades generally being produced with high anodizing voltages, lower anodizing temperatures and high film thicknesses, and lighter colours with lower voltages, higher temperatures and lower thicknesses. On the HE9 extrusions normally used for anodizing, it was quickly clear that the darker bronze shades were very popular, and in the early stages of the process this meant producing films as much as 40 microns in thickness. Much effort has therefore been centred on producing dark colours with more normal thicknesses of say 20 - 25 microns. In practice this has meant use of electrolytes containing lower sulphosalicylic acid (60 - 70 g/1) and sulphuric acid (5 - 6 g/1) content at voltages up to 147

about 70V and current densities of 2 - 3 A/dm 2 38. In conjunction with these conditions special alloys have been developed to give faster colouring rates. The main extrusion alloy used is still of the HE9 type, but it contains deliberate additions of about 0.25% of manganese and copper 39. These additions have the effect of producing more small second-phase particles in the metal, which are then built into the oxide structure during anodizing. Special sheet alloys have also been developed 40 41 and even special alloys for casting purposes 42. More recently a special alloy of the HE9 type with the addition of about 0.4% manganese has been developed for producing grey finishes 43. All these alloys must obviously be carefully controlled during fabrication in order to ensure consistent anodizing response, and heat treatment practices are particularly important 44.

Table 17. Anodizing conditions and colours produced with a range of alloys in the ‘Kalcolor’ process Alloy

Colour

5005 clad with 5005 (sheet)

37

Initial current current density (A/dm 2 )

Time to maximum volts (minutes)

Maximum volts

Total anodizing time (minutes)

Amber grey

2.6

20

50

30

5005 clad with 5005 (sheet)

Charcoal brown

2.6

35

60

45

3003 clad with 3003 (sheet)

Dove grey

2.6

10

50

20

3003 clad with 3003 (sheet)

Charcoal grey

2.6

20

65

40

5086 (sheet)

Black

2.6

25

65

40

5086 (sheet)

Grey

2.6

15

50

20

5357 (sheet)

Light brown

2.6

20

50

30

5357 (sheet)

Brown

2.6

30

60

45

6063-T5 (extrusion)

Amber

2.6

20

50

30

6063-T5 (extrusion)

Light brown

2.6

35

60

45

until a preset maximum value is reached, at which point the voltage is kept constant for the remainder of the anodizing period 37 45. This has become the standard control method for many integral anodizing processes, but automatic voltage programming devices are also used 46. Another factor which must be closely controlled with most integral colour anodizing processes is the build-up of aluminium in the electrolyte, as this significantly reduces its conductivity and raises the operating voltage required. The aluminium content has to be maintained at a much lower level than with sulphuric acid anodizing, and has to be kept within close limits. In practice it is held within ±0.25 g/1 of a chosen value in the range 1 . 5 - 3 g/1 of aluminium. This control is achieved by passing at least a portion of the electrolyte at regular intervals through a suitable cation exchange column 4'. Duranodic ' process Soon after the introduction of ‘Kalcolor’, Alcoa developed their ‘Duranodic’ 300 process. This used a very similar electrolyte, which contained sulphophthallic acid instead of sulphosalicylic acid, but the former acid, which is mentioned in both Alcoa48 and Kaiser 49 patents, seemed to give the desired increase in rate of colouring and therefore easier production of dark colours with standard alloys. Typically the electrolyte contains 80 - 100 g/1 sulphophthallic acid and 5 - 8 g/1 sulphuric acid and it is generally used under similar conditions to those required for ‘Kalcolor’. Again the electrolyte is sensitive to aluminium build-up and this is removed by ion exchange. Despite the high colouring power of the electrolyte, special alloys are often used particularly for sheet products, and alloys to produce grey finishes50 and black finishes 51 have been developed. ‘Permanodic ' process This process was developed by Kawneer and is thought to be very similar to the ‘Kalcolor’ and ‘Duranodic’ processes, but no details have been published. The successful introduction of integral colour anodizing processes in North America generated considerable research effort in Europe also, and by the mid1960’s several processes were in production use. These included Alcan’s ‘Alcanodox’, VAW’s ‘Veroxal’, Alusuisse’s ‘Permalux’ and Pechiney’s ‘Eurocolor 100’. 'Alcanodox ' process

In addition, close control of anodizing conditions is very important and various control methods have been suggested. Unlike sulphuric acid anodizing, with integral colour anodizing the voltage required to produce a particular current density increases considerably during the anodizing period, and therefore some method of automatically adjusting the voltage is desirable. Constant current density anodizing is usually used in the initial stages, the voltages being raised 148

This process has already been referred to in broad principle in the section on oxalic acid anodizing. It uses a saturated, or near saturated, oxalic acid electrolyte 26 (7 - 9% by wt), at about 20°C and a current density of 1.3 - 2.7 A/dm 2. Voltages up to 60 - 70V are again required. The main difference from the other processes is that it produces much lighter colours, gold and light bronze being typical, and because the architectural trend was for darker colours it was not widely 149

about 70V and current densities of 2 - 3 A/dm 2 38. In conjunction with these conditions special alloys have been developed to give faster colouring rates. The main extrusion alloy used is still of the HE9 type, but it contains deliberate additions of about 0.25% of manganese and copper 39. These additions have the effect of producing more small second-phase particles in the metal, which are then built into the oxide structure during anodizing. Special sheet alloys have also been developed 40 41 and even special alloys for casting purposes 42. More recently a special alloy of the HE9 type with the addition of about 0.4% manganese has been developed for producing grey finishes 43. All these alloys must obviously be carefully controlled during fabrication in order to ensure consistent anodizing response, and heat treatment practices are particularly important 44.

Table 17. Anodizing conditions and colours produced with a range of alloys in the ‘Kalcolor’ process Alloy

Colour

5005 clad with 5005 (sheet)

37

Initial current current density (A/dm 2 )

Time to maximum volts (minutes)

Maximum volts

Total anodizing time (minutes)

Amber grey

2.6

20

50

30

5005 clad with 5005 (sheet)

Charcoal brown

2.6

35

60

45

3003 clad with 3003 (sheet)

Dove grey

2.6

10

50

20

3003 clad with 3003 (sheet)

Charcoal grey

2.6

20

65

40

5086 (sheet)

Black

2.6

25

65

40

5086 (sheet)

Grey

2.6

15

50

20

5357 (sheet)

Light brown

2.6

20

50

30

5357 (sheet)

Brown

2.6

30

60

45

6063-T5 (extrusion)

Amber

2.6

20

50

30

6063-T5 (extrusion)

Light brown

2.6

35

60

45

until a preset maximum value is reached, at which point the voltage is kept constant for the remainder of the anodizing period 37 45. This has become the standard control method for many integral anodizing processes, but automatic voltage programming devices are also used 46. Another factor which must be closely controlled with most integral colour anodizing processes is the build-up of aluminium in the electrolyte, as this significantly reduces its conductivity and raises the operating voltage required. The aluminium content has to be maintained at a much lower level than with sulphuric acid anodizing, and has to be kept within close limits. In practice it is held within ±0.25 g/1 of a chosen value in the range 1 . 5 - 3 g/1 of aluminium. This control is achieved by passing at least a portion of the electrolyte at regular intervals through a suitable cation exchange column 4'. Duranodic ' process Soon after the introduction of ‘Kalcolor’, Alcoa developed their ‘Duranodic’ 300 process. This used a very similar electrolyte, which contained sulphophthallic acid instead of sulphosalicylic acid, but the former acid, which is mentioned in both Alcoa48 and Kaiser 49 patents, seemed to give the desired increase in rate of colouring and therefore easier production of dark colours with standard alloys. Typically the electrolyte contains 80 - 100 g/1 sulphophthallic acid and 5 - 8 g/1 sulphuric acid and it is generally used under similar conditions to those required for ‘Kalcolor’. Again the electrolyte is sensitive to aluminium build-up and this is removed by ion exchange. Despite the high colouring power of the electrolyte, special alloys are often used particularly for sheet products, and alloys to produce grey finishes50 and black finishes 51 have been developed. ‘Permanodic ' process This process was developed by Kawneer and is thought to be very similar to the ‘Kalcolor’ and ‘Duranodic’ processes, but no details have been published. The successful introduction of integral colour anodizing processes in North America generated considerable research effort in Europe also, and by the mid1960’s several processes were in production use. These included Alcan’s ‘Alcanodox’, VAW’s ‘Veroxal’, Alusuisse’s ‘Permalux’ and Pechiney’s ‘Eurocolor 100’. 'Alcanodox ' process

In addition, close control of anodizing conditions is very important and various control methods have been suggested. Unlike sulphuric acid anodizing, with integral colour anodizing the voltage required to produce a particular current density increases considerably during the anodizing period, and therefore some method of automatically adjusting the voltage is desirable. Constant current density anodizing is usually used in the initial stages, the voltages being raised 148

This process has already been referred to in broad principle in the section on oxalic acid anodizing. It uses a saturated, or near saturated, oxalic acid electrolyte 26 (7 - 9% by wt), at about 20°C and a current density of 1.3 - 2.7 A/dm 2. Voltages up to 60 - 70V are again required. The main difference from the other processes is that it produces much lighter colours, gold and light bronze being typical, and because the architectural trend was for darker colours it was not widely 149

used. However it was used in Britain and in Sweden, and in the latter country some major buildings were clad with oxalic acid anodized material. Even today it is still being used occasionally for major architectural projects. 'Veroxal 'process Of the Continental processes the most widely used are the VAW ‘Veroxal’ process and the Alusuisse ‘Permalux’ process, both of which are based on maleic acid containing electrolytes. The original ‘Veroxal’ process was based on a sulphosalicylic acid, sulphuric acid, maleic acid mixture 52 , but this was quickly abandoned in favour of a mixture containing 100 - 300 g/1 maleic acid, 10 - 30 g/1 oxalic acid and 3 g/1 sulphuric acid 53 used at temperatures of 15° - 25°C, current densities of 1.6 - 3.2 A/dm 2 and voltages of 30 - 80V. A full range of bronze and black finishes could again be produced and the general operation of the process was similar to that of other integral colour processes. Aluminium content was again critical and it was suggested that the electrolyte could be regenerated by precipitating the aluminium with potassium fluoride 54. With maleic acid based electrolytes there is, however, an additional problem, in that the maleic acid is reduced by the hydrogen evolved at the cathode to succinic acid, and this rapidly precipitates out on the cathodes and agitation pipes, giving rise to increased colouring voltages and poor colour uniformity. The maleic acid can be rapidly consumed in this way and the initial solution was to reduce the cathode area 55. Later, more sophisticated systems were developed, in which the cathode was isolated from the bulk electrolyte by means of a porous diaphragm 56 or an ion exchange membrane . With all integral anodizing processes the lower throwing power of the electrolyte can cause colour uniformity problems and VAW have suggested the use of a short sulphuric acid anodizing stage (producing a thickness of 2 - 10 microns) before integral colour anodizing, in order to improve colour uniformity"8. The same technique has been suggested by Kaiser to give more lustrous integral colour finishes 59. It is also believed that such double anodizing treatments are sometimes used when light coloured finishes are required with thick anodic coatings (20 25 microns). 'Permalux’ process As already indicated the ‘Permalux’ process is also maleic acid based and an electrolyte containing 100 - 400 g/1 maleic acid and 1 - 10 g/1 sulphuric acid is suggested 60. The high maleic acid content is apparently necessary in order to get uniform coating formation and the balance between maleic acid and sulphuric acid content is also important. Addition of calcium or barium hydroxide is suggested to control the sulphate content 61. Normal integral anodizing conditions are used and a normal range of coloured finishes obtained. No indications of the methods used to overcome succinic acid production have been indicated. 150

'Acadai 'process This process, developed in Italy by ISML, is apparently very similar to ‘Permalux’, and patents from ISML on integral colour anodizing talk of electrolytes containing 300 g/1 maleic acid and 3 g/1 sulphuric acid. As usual the problem of succinic acid production is discussed, and a two compartment cell with a porous baffle between the two halves 62 and use of hollow cathodes which are cooled to below the electrolyte temperature65 are claimed to give benefits. 'Eurocolor 100’ process Coming close to the other European processes, Pechiney’s ‘Eurocolor 100’ process utilises an electrolyte containing 50 - 200 g/1 sulphomaleic acid and 0.5 - 8 g/1 sulphuric acid 64. Anodizing conditions are normal for this type of process and the shade desired can be controlled by suitable adjustment of the anodizing current density and electrolyte temperature. No indication is given as to whether sulphomaleic acid gives rise to the same precipitation problems as maleic acid. 'Sumitone ’ process Sumitomo in Japan claim a cheaper integral colour anodizing process using a mixture of 120 g/1 phenol sulphonic acid and 1 g/1 sulphuric acid 65. A.c. superimposed on d.c. is suggested and maximum anodizing voltages are said to be 45 60V. Removal of aluminium from the solution is by means of cation exchange resins. Other integral colour anodizing electrolytes Many other integral colour anodizing processes exist and many other suitable electrolytes have been suggested both in patents and in published literature, but most have either not been used industrially or only in particular plants and areas. They have been reviewed extensively by Wernick and Pinner 66. Most of these electrolytes contain mixtures of acids, but Kape has studied the anodizing characteristics of many single organic acid electrolytes 67" 69. He concluded that the acid strength is of prime importance in deciding if an organic acid will act as an anodizing electrolyte and the electrolyte’s conductivity will give an indication of its general anodic behaviour. Few organic acids have practical significance when used by themselves, although apart from oxalic acid, malonic acid has been extensively studied 6 . However a range of acids, such as malonic acid, glyoxylic acid, tartaric acid, citric acid, glycollic acid and mellitic acid, when used with small additions of oxalic acid, show promising results 69. A patent on the same subject 0 indicates how the colour and abrasion resistance of the coating are affected by the anodizing electrolyte used (Table 18). It can be seen from this Table that all the organic acid electrolytes give coatings of significantly higher abrasion resistance than that of coatings produced in a sulphuric acid electrolyte. 151

used. However it was used in Britain and in Sweden, and in the latter country some major buildings were clad with oxalic acid anodized material. Even today it is still being used occasionally for major architectural projects. 'Veroxal 'process Of the Continental processes the most widely used are the VAW ‘Veroxal’ process and the Alusuisse ‘Permalux’ process, both of which are based on maleic acid containing electrolytes. The original ‘Veroxal’ process was based on a sulphosalicylic acid, sulphuric acid, maleic acid mixture 52 , but this was quickly abandoned in favour of a mixture containing 100 - 300 g/1 maleic acid, 10 - 30 g/1 oxalic acid and 3 g/1 sulphuric acid 53 used at temperatures of 15° - 25°C, current densities of 1.6 - 3.2 A/dm 2 and voltages of 30 - 80V. A full range of bronze and black finishes could again be produced and the general operation of the process was similar to that of other integral colour processes. Aluminium content was again critical and it was suggested that the electrolyte could be regenerated by precipitating the aluminium with potassium fluoride 54. With maleic acid based electrolytes there is, however, an additional problem, in that the maleic acid is reduced by the hydrogen evolved at the cathode to succinic acid, and this rapidly precipitates out on the cathodes and agitation pipes, giving rise to increased colouring voltages and poor colour uniformity. The maleic acid can be rapidly consumed in this way and the initial solution was to reduce the cathode area 55. Later, more sophisticated systems were developed, in which the cathode was isolated from the bulk electrolyte by means of a porous diaphragm 56 or an ion exchange membrane . With all integral anodizing processes the lower throwing power of the electrolyte can cause colour uniformity problems and VAW have suggested the use of a short sulphuric acid anodizing stage (producing a thickness of 2 - 10 microns) before integral colour anodizing, in order to improve colour uniformity"8. The same technique has been suggested by Kaiser to give more lustrous integral colour finishes 59. It is also believed that such double anodizing treatments are sometimes used when light coloured finishes are required with thick anodic coatings (20 25 microns). 'Permalux’ process As already indicated the ‘Permalux’ process is also maleic acid based and an electrolyte containing 100 - 400 g/1 maleic acid and 1 - 10 g/1 sulphuric acid is suggested 60. The high maleic acid content is apparently necessary in order to get uniform coating formation and the balance between maleic acid and sulphuric acid content is also important. Addition of calcium or barium hydroxide is suggested to control the sulphate content 61. Normal integral anodizing conditions are used and a normal range of coloured finishes obtained. No indications of the methods used to overcome succinic acid production have been indicated. 150

'Acadai 'process This process, developed in Italy by ISML, is apparently very similar to ‘Permalux’, and patents from ISML on integral colour anodizing talk of electrolytes containing 300 g/1 maleic acid and 3 g/1 sulphuric acid. As usual the problem of succinic acid production is discussed, and a two compartment cell with a porous baffle between the two halves 62 and use of hollow cathodes which are cooled to below the electrolyte temperature65 are claimed to give benefits. 'Eurocolor 100’ process Coming close to the other European processes, Pechiney’s ‘Eurocolor 100’ process utilises an electrolyte containing 50 - 200 g/1 sulphomaleic acid and 0.5 - 8 g/1 sulphuric acid 64. Anodizing conditions are normal for this type of process and the shade desired can be controlled by suitable adjustment of the anodizing current density and electrolyte temperature. No indication is given as to whether sulphomaleic acid gives rise to the same precipitation problems as maleic acid. 'Sumitone ’ process Sumitomo in Japan claim a cheaper integral colour anodizing process using a mixture of 120 g/1 phenol sulphonic acid and 1 g/1 sulphuric acid 65. A.c. superimposed on d.c. is suggested and maximum anodizing voltages are said to be 45 60V. Removal of aluminium from the solution is by means of cation exchange resins. Other integral colour anodizing electrolytes Many other integral colour anodizing processes exist and many other suitable electrolytes have been suggested both in patents and in published literature, but most have either not been used industrially or only in particular plants and areas. They have been reviewed extensively by Wernick and Pinner 66. Most of these electrolytes contain mixtures of acids, but Kape has studied the anodizing characteristics of many single organic acid electrolytes 67" 69. He concluded that the acid strength is of prime importance in deciding if an organic acid will act as an anodizing electrolyte and the electrolyte’s conductivity will give an indication of its general anodic behaviour. Few organic acids have practical significance when used by themselves, although apart from oxalic acid, malonic acid has been extensively studied 6 . However a range of acids, such as malonic acid, glyoxylic acid, tartaric acid, citric acid, glycollic acid and mellitic acid, when used with small additions of oxalic acid, show promising results 69. A patent on the same subject 0 indicates how the colour and abrasion resistance of the coating are affected by the anodizing electrolyte used (Table 18). It can be seen from this Table that all the organic acid electrolytes give coatings of significantly higher abrasion resistance than that of coatings produced in a sulphuric acid electrolyte. 151

This is a feature of all integral colour coatings and is due to the high anodizing voltages used and the low rate of attack on the oxide by the electrolyte. It was mentioned in the original ‘Kalcolor’ patent 36 (Table 19) and has been widely publicized as an advantageous feature of such finishes in architectural applications. Table 18. Anodizing characteristics of a range of organic acid electrolytes

(

The presence of formic acid in an anodizing electrolyte often causes pitting rather than oxide formation, but Fujisash Industries in Japan have developed formic acid - sulphosalicylic acid - sulphuric acid mixtures for integral colour anodizing 71. Electrolytes typically contain 60 - 70 g/1 sulphosalicylic acid, 1 5 - 1 8 g/1 formic acid and about 1 g/1 sulphuric acid and rather yellowish bronzes are produced. Integral colour anodizing plant

Electrolyte

8 % sulphuric acid

Voltage range 19-20

Colour of coating

Thickness of coating (microns)

Silver

Specific abrasion resistance* (Schuh and Kern)

25-35

2.0

6 % tartaric 1 .5% oxalic

100-130

Brown-grey

20

7.5

5 % tartaric + 0.3% oxalic

110-155

Medium-grey

20

6.75

5 % citric + 1 .% oxalic

85-104

Greenish-yellovv

25

6.9

5 % citric + 0.3% oxalic

110-180

Grey-brown

15

8.3

5 % itaconic 41 .5% oxalic

120-150

Grey

25

6.9

5 % malic + 1.5% oxalic

180-200

Dark-grey

25

7.45

5 % diglycollic + 1.5% oxalic

100-130

Yellow-grey

25

7.25

5 % diglycollic + 1 .2% oxalic

100-140

Yellow-brown

30

10.5

5 % thiomalic + 1 .% oxalic

88-162

Grey-bronze

25

7.6

The equipment required for integral colour anodizing is generally similar to that required for sulphuric acid anodizing, but many aspects of equipment design are rather more critical. Tanks can be constructed of suitably lined concrete or steel but stainless steel tanks, appropriately shielded at the ends and bottom, have also been used. Acid resistant rubber linings are often used but plastic and Fibreglass lined tanks can also be suitable. The cathodes used are usually stainless steel but, as already indicated, special cathode designs may be necessary, for example with maleic acid based electrolytes. Aluminium cathodes are not used because of the sensitivity of the electrolytes to aluminium build-up. High voltage rectifiers, are always required and a 70V supply is a minimum requirement for many processes. The desirability of automatic control systems has already been mentioned. With this increased wattage input to the bath, the capacity of the refrigeration equipment has to be considerably increased, usually by a factor of at least 3. It also means that circulation of the electrolyte through an external heat exchanger, rather than the use of cooling coils, is essential. Jigging systems also have to be carefully designed, and clamped or bolted contacts are usually necessary. Bus-bar to flight bar contacts have to be well designed and maintained, and agitation of the anodizing electrolyte must be good. In addition, regular or continuous treatment of the electrolyte by ion-exchange or other means is nearly always necessary to keep the aluminium content within close limits.

♦After sealing for 5 minutes in 8 g/1 nickel acetate at 80°C plus 60 minutes in hot water at 95 °C

Table 19. Abrasion resistance* of samples anodized in sulphosalicylic acid based electrolytes Electrolyte 10% 15% sulphosalicylic sulphosalicylic acid acid 2% sulphuric 5% ferric acid sulphate

Alloy

10% sulphosalicylic acid 1% sulphuric acid

1100

33.0

19.0

27.9

11.5

30.3

12.0

31.6 —

11.5

5052

22.5

6061-T6

25.0

22.5 19.5

7075-T6

31.0

21.5

15% sulphuric acid

Inevitably these factors mean that the equipment for an integral colour anodizing bath is considerably more expensive than that for a conventional sulphuric acid anodizing process, and it is this high cost (and the high energy consumption during operation) that is making anodizers look carefully at alternative systems such as electrolytic colouring. The mechanism of integral colour anodizing processes

7.0

♦Abrasive jet method, ASTM Bulletin No. 208 (1955) figures quoted are time in seconds to penetrate coating).

152

36

In the past little information has been available on the way in which colours were produced during integral anodizing processes, but it was clear that a common mechanism was operating in most processes, as essentially similar bronze, gold grey or black finishes were obtained. The alloying elements present in the aluminium were important, but not essential, as coloured finishes could still be obtained on super purity aluminium. As long ago as 1944, Fischer and Budiloff 72 in153

This is a feature of all integral colour coatings and is due to the high anodizing voltages used and the low rate of attack on the oxide by the electrolyte. It was mentioned in the original ‘Kalcolor’ patent 36 (Table 19) and has been widely publicized as an advantageous feature of such finishes in architectural applications. Table 18. Anodizing characteristics of a range of organic acid electrolytes

(

The presence of formic acid in an anodizing electrolyte often causes pitting rather than oxide formation, but Fujisash Industries in Japan have developed formic acid - sulphosalicylic acid - sulphuric acid mixtures for integral colour anodizing 71. Electrolytes typically contain 60 - 70 g/1 sulphosalicylic acid, 1 5 - 1 8 g/1 formic acid and about 1 g/1 sulphuric acid and rather yellowish bronzes are produced. Integral colour anodizing plant

Electrolyte

8 % sulphuric acid

Voltage range 19-20

Colour of coating

Thickness of coating (microns)

Silver

Specific abrasion resistance* (Schuh and Kern)

25-35

2.0

6 % tartaric 1 .5% oxalic

100-130

Brown-grey

20

7.5

5 % tartaric + 0.3% oxalic

110-155

Medium-grey

20

6.75

5 % citric + 1 .% oxalic

85-104

Greenish-yellovv

25

6.9

5 % citric + 0.3% oxalic

110-180

Grey-brown

15

8.3

5 % itaconic 41 .5% oxalic

120-150

Grey

25

6.9

5 % malic + 1.5% oxalic

180-200

Dark-grey

25

7.45

5 % diglycollic + 1.5% oxalic

100-130

Yellow-grey

25

7.25

5 % diglycollic + 1 .2% oxalic

100-140

Yellow-brown

30

10.5

5 % thiomalic + 1 .% oxalic

88-162

Grey-bronze

25

7.6

The equipment required for integral colour anodizing is generally similar to that required for sulphuric acid anodizing, but many aspects of equipment design are rather more critical. Tanks can be constructed of suitably lined concrete or steel but stainless steel tanks, appropriately shielded at the ends and bottom, have also been used. Acid resistant rubber linings are often used but plastic and Fibreglass lined tanks can also be suitable. The cathodes used are usually stainless steel but, as already indicated, special cathode designs may be necessary, for example with maleic acid based electrolytes. Aluminium cathodes are not used because of the sensitivity of the electrolytes to aluminium build-up. High voltage rectifiers, are always required and a 70V supply is a minimum requirement for many processes. The desirability of automatic control systems has already been mentioned. With this increased wattage input to the bath, the capacity of the refrigeration equipment has to be considerably increased, usually by a factor of at least 3. It also means that circulation of the electrolyte through an external heat exchanger, rather than the use of cooling coils, is essential. Jigging systems also have to be carefully designed, and clamped or bolted contacts are usually necessary. Bus-bar to flight bar contacts have to be well designed and maintained, and agitation of the anodizing electrolyte must be good. In addition, regular or continuous treatment of the electrolyte by ion-exchange or other means is nearly always necessary to keep the aluminium content within close limits.

♦After sealing for 5 minutes in 8 g/1 nickel acetate at 80°C plus 60 minutes in hot water at 95 °C

Table 19. Abrasion resistance* of samples anodized in sulphosalicylic acid based electrolytes Electrolyte 10% 15% sulphosalicylic sulphosalicylic acid acid 2% sulphuric 5% ferric acid sulphate

Alloy

10% sulphosalicylic acid 1% sulphuric acid

1100

33.0

19.0

27.9

11.5

30.3

12.0

31.6 —

11.5

5052

22.5

6061-T6

25.0

22.5 19.5

7075-T6

31.0

21.5

15% sulphuric acid

Inevitably these factors mean that the equipment for an integral colour anodizing bath is considerably more expensive than that for a conventional sulphuric acid anodizing process, and it is this high cost (and the high energy consumption during operation) that is making anodizers look carefully at alternative systems such as electrolytic colouring. The mechanism of integral colour anodizing processes

7.0

♦Abrasive jet method, ASTM Bulletin No. 208 (1955) figures quoted are time in seconds to penetrate coating).

152

36

In the past little information has been available on the way in which colours were produced during integral anodizing processes, but it was clear that a common mechanism was operating in most processes, as essentially similar bronze, gold grey or black finishes were obtained. The alloying elements present in the aluminium were important, but not essential, as coloured finishes could still be obtained on super purity aluminium. As long ago as 1944, Fischer and Budiloff 72 in153

vestigated the colouring of oxalic acid anodized samples, and concluded that the gold colour produced was due to colloidal carbon in the film arising from breakdown of the organic electrolyte. Alcoa workers 73 74 have shown, however, that three factors contribute to the colour of integral coatings. The first is the presence of intermetallic particles, or their reaction products, dispersed in the oxide. These particles, of course, come from the aluminium alloy itself and this aspect of colouring has been used to increase the rate of colour production as described earlier. In general the more fine particles that remain trapped in the anodic coating the darker the colour produced for any particular anodizing condition. Secondly colouring of the oxide matrix by ions such as chromium, copper or manganese, which are present in solid solution in the alloy, also occurs. Finally the oxide matrix can be coloured by the electrochemical process and it is this reaction that is often dominant with integral colour anodizing processes. The first two reactions both occur to some extent with sulphuric acid anodizing, grey finishes on aluminium-silicon alloys being produced by particle dispersion in the oxide and pale gold finishes being produced from chromium-containing alloys. However it has now been shown that the third reaction involves the presence of metallic aluminium in the oxide. The quantity of aluminium present controls the depth of bronze colour achieved, the free aluminium originating in the barrier layer of the coating. Conditions which favour the formation of metallic aluminium in the oxide are a high rate of oxidation and a low rate of field assisted dissolution of the oxide. This explains the need for relatively high anodic current densities and low sulphuric acid contents if rapid colouring is to be achieved with these electrolytes.

15 - 25% sulphuric acid, 1 - 2% glycollic acid and 1 - 2% glycerol can be used for everything from bright trim anodizing to architectural and hard anodizing. It is claimed that all these finishes can be obtained merely by appropriately modifying current density and anodizing time. Films produced at normal temperatures can be dyed to produce decorative finishes, and hard coatings up to 125 microns in thickness can be produced at 21°C and 40V maximum using high anodizing current densities (3.9 A/ dm 2). 78 79 Acorn Anodizing Co. have used sulphuric acid - nitric acid electrolytes A typical electrolyte contains 140 g/1 sulphuric acid and 14 g/1 nitric acid which is said to allow the use of lower operating voltages for a particular current density and to make the anodizing of difficult alloys, such as those containing large amounts of copper or silicon, easier. Again the electrolyte has applications in the hard anodizing field as well as being used for general anodizing purposes. References 1. 2. 3. 4. 5. 6. 7.

An interesting suggestion by Reynolds Metals77 is that one electrolyte should be used for a variety of purposes, and it is recommended that a bath containing 154

M Gj W

Many of the electrolytes used for integral colour anodizing purposes are almost as interesting for the film properties that they produce as for the appearance of the coating. The hardness and abrasion resistance of the coatings, in particular, is not dissimilar to that produced under true hard anodizing conditions. Many hard anodizing electrolytes use sulphuric acid and oxalic acid, and mellitic acid is also added. In France there has been some use of mixed electrolytes for producing films of high wear resistance and with good electrical insulation properties They consist of approximately 240 g/1 sodium bisulphate and 100 g/1 citric acid or 80 g/1 oxalic acid and 55 g/1 formic acid. They are unconventional in several respects including the fact that they can be operated at 15° and above, and are claimed to be capable of producing films up to 200 microns thick at a current density of 6 A/dm 2.

M W

Miscellaneous anodizing processes

25. 26.

Brit. Pat. 223,994/5 (1923) R. W. Buzzard, J . Res. Nat. Bureau, 1937, 18, 251 A. 1. Utyanskaya, Novaya Teknol. v. Aviostroenii, Pervoe Glavnoe, Upravlenie N.K.A.P., 1939(4)41 O. F. Tarr, M. Darrin and L. G. Tubbs, Ind. and Eng. Chem. (Indust. Edn.), 1941, 33(12) 1957 A. I. Utyanskaya and Z. I. Shivea, Aviaprom. 1940 (10) 50 R. W. Buzzard and J . H. Wilson, J . Res. Nat. Bureau Standards, 1937, 18, 53 B. C. Lewsey, Electroplating and Metal Finishing, 1952, 5 (8) 250 R. Peek and A. W. Brace, Trans. Inst. Met. Finishing, 1957, 34, 232 Brit. Pat. 1,067,373 Brit. Pat. 1,464,857 D. Jackson, J . Electrodepos. Tech. Soc., 1945, 20, 177 M. Sang, Monthly Rev., American Electroplaters’ Soc., 1941, 28, (9), 709 W. M. Hartford, Ibid., 1942, 29, (10), 831 Electroplating and Met. Fin., 1953, 6, 121-130 Aluminum Co. of America, ‘Electroplating of Aluminium and its Alloys . 1946 B. E. Bunce, Electroplating and Met. Fin., 1953, 6, 317 R. C. Spooner and D. P. Seraphim, Trans. Inst. Met Finishing, 1954, 31, 29-45 Brit. Pat. 1,244,723 J . P. O’Sullivan andG. C. Wood, Proc. Roy. Soc. Lond., 1970, 317, 511-543 Brit. Pat. 226,536 A. Jenny, ‘Die Electrolytische Oxydation des Aluminiums und seiner Legierungen , Dresden, Theodor Steinkopff Verfag, 1938 W. Hubner and A. von Zeerleder, Chimia, 1949, 3, (4), 77 E. Ikeda, N. Hirosha and H. Ono, Light Metals (Japan), 1955, (17), 79 W. Hubner and A. Schiltkneckt, ‘Die Praxis der Anodischen Oxydation des Aluminiums’. Aluminium Verlag GmbH, Dusseldorf, 1956 H. Fischer, Aluminium, 1937, 19, 358 Brit. Pat. 970,500

155

vestigated the colouring of oxalic acid anodized samples, and concluded that the gold colour produced was due to colloidal carbon in the film arising from breakdown of the organic electrolyte. Alcoa workers 73 74 have shown, however, that three factors contribute to the colour of integral coatings. The first is the presence of intermetallic particles, or their reaction products, dispersed in the oxide. These particles, of course, come from the aluminium alloy itself and this aspect of colouring has been used to increase the rate of colour production as described earlier. In general the more fine particles that remain trapped in the anodic coating the darker the colour produced for any particular anodizing condition. Secondly colouring of the oxide matrix by ions such as chromium, copper or manganese, which are present in solid solution in the alloy, also occurs. Finally the oxide matrix can be coloured by the electrochemical process and it is this reaction that is often dominant with integral colour anodizing processes. The first two reactions both occur to some extent with sulphuric acid anodizing, grey finishes on aluminium-silicon alloys being produced by particle dispersion in the oxide and pale gold finishes being produced from chromium-containing alloys. However it has now been shown that the third reaction involves the presence of metallic aluminium in the oxide. The quantity of aluminium present controls the depth of bronze colour achieved, the free aluminium originating in the barrier layer of the coating. Conditions which favour the formation of metallic aluminium in the oxide are a high rate of oxidation and a low rate of field assisted dissolution of the oxide. This explains the need for relatively high anodic current densities and low sulphuric acid contents if rapid colouring is to be achieved with these electrolytes.

15 - 25% sulphuric acid, 1 - 2% glycollic acid and 1 - 2% glycerol can be used for everything from bright trim anodizing to architectural and hard anodizing. It is claimed that all these finishes can be obtained merely by appropriately modifying current density and anodizing time. Films produced at normal temperatures can be dyed to produce decorative finishes, and hard coatings up to 125 microns in thickness can be produced at 21°C and 40V maximum using high anodizing current densities (3.9 A/ dm 2). 78 79 Acorn Anodizing Co. have used sulphuric acid - nitric acid electrolytes A typical electrolyte contains 140 g/1 sulphuric acid and 14 g/1 nitric acid which is said to allow the use of lower operating voltages for a particular current density and to make the anodizing of difficult alloys, such as those containing large amounts of copper or silicon, easier. Again the electrolyte has applications in the hard anodizing field as well as being used for general anodizing purposes. References 1. 2. 3. 4. 5. 6. 7.

An interesting suggestion by Reynolds Metals77 is that one electrolyte should be used for a variety of purposes, and it is recommended that a bath containing 154

M Gj W

Many of the electrolytes used for integral colour anodizing purposes are almost as interesting for the film properties that they produce as for the appearance of the coating. The hardness and abrasion resistance of the coatings, in particular, is not dissimilar to that produced under true hard anodizing conditions. Many hard anodizing electrolytes use sulphuric acid and oxalic acid, and mellitic acid is also added. In France there has been some use of mixed electrolytes for producing films of high wear resistance and with good electrical insulation properties They consist of approximately 240 g/1 sodium bisulphate and 100 g/1 citric acid or 80 g/1 oxalic acid and 55 g/1 formic acid. They are unconventional in several respects including the fact that they can be operated at 15° and above, and are claimed to be capable of producing films up to 200 microns thick at a current density of 6 A/dm 2.

M W

Miscellaneous anodizing processes

25. 26.

Brit. Pat. 223,994/5 (1923) R. W. Buzzard, J . Res. Nat. Bureau, 1937, 18, 251 A. 1. Utyanskaya, Novaya Teknol. v. Aviostroenii, Pervoe Glavnoe, Upravlenie N.K.A.P., 1939(4)41 O. F. Tarr, M. Darrin and L. G. Tubbs, Ind. and Eng. Chem. (Indust. Edn.), 1941, 33(12) 1957 A. I. Utyanskaya and Z. I. Shivea, Aviaprom. 1940 (10) 50 R. W. Buzzard and J . H. Wilson, J . Res. Nat. Bureau Standards, 1937, 18, 53 B. C. Lewsey, Electroplating and Metal Finishing, 1952, 5 (8) 250 R. Peek and A. W. Brace, Trans. Inst. Met. Finishing, 1957, 34, 232 Brit. Pat. 1,067,373 Brit. Pat. 1,464,857 D. Jackson, J . Electrodepos. Tech. Soc., 1945, 20, 177 M. Sang, Monthly Rev., American Electroplaters’ Soc., 1941, 28, (9), 709 W. M. Hartford, Ibid., 1942, 29, (10), 831 Electroplating and Met. Fin., 1953, 6, 121-130 Aluminum Co. of America, ‘Electroplating of Aluminium and its Alloys . 1946 B. E. Bunce, Electroplating and Met. Fin., 1953, 6, 317 R. C. Spooner and D. P. Seraphim, Trans. Inst. Met Finishing, 1954, 31, 29-45 Brit. Pat. 1,244,723 J . P. O’Sullivan andG. C. Wood, Proc. Roy. Soc. Lond., 1970, 317, 511-543 Brit. Pat. 226,536 A. Jenny, ‘Die Electrolytische Oxydation des Aluminiums und seiner Legierungen , Dresden, Theodor Steinkopff Verfag, 1938 W. Hubner and A. von Zeerleder, Chimia, 1949, 3, (4), 77 E. Ikeda, N. Hirosha and H. Ono, Light Metals (Japan), 1955, (17), 79 W. Hubner and A. Schiltkneckt, ‘Die Praxis der Anodischen Oxydation des Aluminiums’. Aluminium Verlag GmbH, Dusseldorf, 1956 H. Fischer, Aluminium, 1937, 19, 358 Brit. Pat. 970,500

155

27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

156

J . M. Kape, The use of integral colour processes for architectural colour anodizing, Proc. Aluminium Federation Conference on Anodizing, 1967 G. Bailey and G. C. Wood, Trans. Inst. Met. Fin., 1974, 52, 187-199 Brit. Pat. 1,150,882 Brit. Pat. 1,469,294 T. Takahashi and J . Saitoh, Plating and Surface Fin., 1977, 64, 36-41 Brit. Pat. 455,412 and 492,618 Brit. Pat. 1,180,299 German Pat. 657,902 German Pat. 664,240 Brit. Pat. 850,576 Brit. Pat. 957,865 Brit. Pat. 1,344,192 Brit. Pat. 1,129,676 Brit. Pat. 1,084,988 Brit. Pat. 1,436,437 Brit. Pat. 1,239,788 Brit. Pat. 1,267,235 R. C. Dorward, Light Metal Age, 1972, 30, (9,10), 7-10 Brit. Pat. 1,023,535 Brit. Pat. 1,344,192 Brit. Pat. 971,275 Brit. Pat. 962,048 Brit. Pat. 1,023,535 Brit. Pat. 1,328,368 Brit. Pat. 1,448,146 Brit. Pat. 973,391 Brit. Pat. 1,026,609 Brit. Pat. 1,170,914 Brit. Pat. 1,191,090 Brit. Pat. 1,319,658 and 1,319,659 Brit. Pat. 1,356,544 Brit. Pat. 1,097,356 Brit. Pat. 1,147,907 Swiss Pat. 462,584 Brit. Pat. 1,099,769 Brit. Pat. 1,274,104 Brit. Pat. 1,339,878 Brit. Pat. 1,213,114 S. Terai, T. Suzuki and Y. Hayashi, Sumitomo Light Metal Technical Reports, 1974, 15(3), 60-66 S. Wernick and R. Pinner, ‘Surface Treatment of Aluminium’, 4th Edition, 1972 Robert Draper Ltd. J . M. Kape, Metallurgia, 1959, 60, 181-191 J . M. Kape, Electroplating and Met. Finishing, 1961, 14, 407-415 J . M. Kape, Trans. Inst. Met. Finishing, 1967, 45, 34-42 Brit. Pat. 1,173,597 Brit. Pat. 1,426,423

72. 73. 74. 75. 76. 77. 78. 79.

H. Fischer and N. Budiloff, Korrosion und Metallschutz, 1944, 20, (3), 115-119 K. Wefers and W. T. Evans, Plating and Surf. Fin., 1975, 62, 951-957 K. Wefers and P. F. Wallace, Aluminium, 1976, 52, 485-489 French Pat. 1,005,592 P. Lelong, R. Segond and J . Herenguel, Proc. A.E.S., 1959, 46, 226 K. H. Dale, Plating, 1972, 59, 843-844 Brit. Pat. 1,114,463 Brit. Pat. 1,215,314

157

27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

156

J . M. Kape, The use of integral colour processes for architectural colour anodizing, Proc. Aluminium Federation Conference on Anodizing, 1967 G. Bailey and G. C. Wood, Trans. Inst. Met. Fin., 1974, 52, 187-199 Brit. Pat. 1,150,882 Brit. Pat. 1,469,294 T. Takahashi and J . Saitoh, Plating and Surface Fin., 1977, 64, 36-41 Brit. Pat. 455,412 and 492,618 Brit. Pat. 1,180,299 German Pat. 657,902 German Pat. 664,240 Brit. Pat. 850,576 Brit. Pat. 957,865 Brit. Pat. 1,344,192 Brit. Pat. 1,129,676 Brit. Pat. 1,084,988 Brit. Pat. 1,436,437 Brit. Pat. 1,239,788 Brit. Pat. 1,267,235 R. C. Dorward, Light Metal Age, 1972, 30, (9,10), 7-10 Brit. Pat. 1,023,535 Brit. Pat. 1,344,192 Brit. Pat. 971,275 Brit. Pat. 962,048 Brit. Pat. 1,023,535 Brit. Pat. 1,328,368 Brit. Pat. 1,448,146 Brit. Pat. 973,391 Brit. Pat. 1,026,609 Brit. Pat. 1,170,914 Brit. Pat. 1,191,090 Brit. Pat. 1,319,658 and 1,319,659 Brit. Pat. 1,356,544 Brit. Pat. 1,097,356 Brit. Pat. 1,147,907 Swiss Pat. 462,584 Brit. Pat. 1,099,769 Brit. Pat. 1,274,104 Brit. Pat. 1,339,878 Brit. Pat. 1,213,114 S. Terai, T. Suzuki and Y. Hayashi, Sumitomo Light Metal Technical Reports, 1974, 15(3), 60-66 S. Wernick and R. Pinner, ‘Surface Treatment of Aluminium’, 4th Edition, 1972 Robert Draper Ltd. J . M. Kape, Metallurgia, 1959, 60, 181-191 J . M. Kape, Electroplating and Met. Finishing, 1961, 14, 407-415 J . M. Kape, Trans. Inst. Met. Finishing, 1967, 45, 34-42 Brit. Pat. 1,173,597 Brit. Pat. 1,426,423

72. 73. 74. 75. 76. 77. 78. 79.

H. Fischer and N. Budiloff, Korrosion und Metallschutz, 1944, 20, (3), 115-119 K. Wefers and W. T. Evans, Plating and Surf. Fin., 1975, 62, 951-957 K. Wefers and P. F. Wallace, Aluminium, 1976, 52, 485-489 French Pat. 1,005,592 P. Lelong, R. Segond and J . Herenguel, Proc. A.E.S., 1959, 46, 226 K. H. Dale, Plating, 1972, 59, 843-844 Brit. Pat. 1,114,463 Brit. Pat. 1,215,314

157

Chapter 11: Hard anodizing Reference has been made previously to hard anodizing which basically bears a similar relation to decorative anodizing to that which hard chromium plating bears to decorative chromium plating. Essentially the process employs a refrigerated electrolyte maintained below 10°C, vigorous agitation and close control of processing conditions. Within the scope of broad principles already discussed, considerable variation in the detail of operating practice has been recorded in the literature. Final choice of process will be dictated by the material being processed, the geometry of the surfaces to be hard anodized, dimensional tolerances specified and equipment available. Anodizing electrolytes A number of electrolytes have been used industrially for hard anodizing. Much of the early American work was carried out using a 165 g/1 H,SO 4 electrolyte. This electrolyte gives good results on the more readily anodizable alloys but gives rise to difficulties on high copper alloys of the Duralumin type. Russian work' 2 has tended to favour the use of electrolytes of 200 - 250 g/1 H SO4 , which is of the same order as the 230 g/1 nominal concentration of the German ‘GS’ process. Current densities of 2.0 to 5.0 A/dm 2 (approximately 18 - 45 A/ft 2 ) have been employed in conjunction with electrolyte temperatures ranging from - 1 0 ° to +10°C. The limiting film thickness obtained before burning occurred, progressively decreased as electrolyte strength was increased over the range 2N - 6N H SO, (96 -294 g/1) (Fig. 65). An interesting piece of work has been reported from Hungary 3 in which a 10 g/1 H SO, electrolyte maintained at 20°C has been used for hard anodizing. Although such an electrolyte would be expected to give hard coatings, throwing power difficulties in recesses and blind holes would be expected under practical conditions. The difficulty of compromise between the various factors involved leads some companies to prefer to keep one high concentration bath for difficult alloys and another lower concentration for general work. Alternatively a compromise is reached with a concentration of approximately 250 g/1. One factor that is not fully appreciated in hard anodizing is the reduced solubility of aluminium salts, such as complex acid sulphates produced as a residue in the anodizing tank. In general terms the aluminium content of the bath should not be allowed to rise above 5 - 8 g/1 depending upon the exact conditions chosen. The higher the H 2SO 4 content and the lower the temperature the lower the level at which the aluminium content 158

159

Chapter 11: Hard anodizing Reference has been made previously to hard anodizing which basically bears a similar relation to decorative anodizing to that which hard chromium plating bears to decorative chromium plating. Essentially the process employs a refrigerated electrolyte maintained below 10°C, vigorous agitation and close control of processing conditions. Within the scope of broad principles already discussed, considerable variation in the detail of operating practice has been recorded in the literature. Final choice of process will be dictated by the material being processed, the geometry of the surfaces to be hard anodized, dimensional tolerances specified and equipment available. Anodizing electrolytes A number of electrolytes have been used industrially for hard anodizing. Much of the early American work was carried out using a 165 g/1 H,SO 4 electrolyte. This electrolyte gives good results on the more readily anodizable alloys but gives rise to difficulties on high copper alloys of the Duralumin type. Russian work' 2 has tended to favour the use of electrolytes of 200 - 250 g/1 H SO4 , which is of the same order as the 230 g/1 nominal concentration of the German ‘GS’ process. Current densities of 2.0 to 5.0 A/dm 2 (approximately 18 - 45 A/ft 2 ) have been employed in conjunction with electrolyte temperatures ranging from - 1 0 ° to +10°C. The limiting film thickness obtained before burning occurred, progressively decreased as electrolyte strength was increased over the range 2N - 6N H SO, (96 -294 g/1) (Fig. 65). An interesting piece of work has been reported from Hungary 3 in which a 10 g/1 H SO, electrolyte maintained at 20°C has been used for hard anodizing. Although such an electrolyte would be expected to give hard coatings, throwing power difficulties in recesses and blind holes would be expected under practical conditions. The difficulty of compromise between the various factors involved leads some companies to prefer to keep one high concentration bath for difficult alloys and another lower concentration for general work. Alternatively a compromise is reached with a concentration of approximately 250 g/1. One factor that is not fully appreciated in hard anodizing is the reduced solubility of aluminium salts, such as complex acid sulphates produced as a residue in the anodizing tank. In general terms the aluminium content of the bath should not be allowed to rise above 5 - 8 g/1 depending upon the exact conditions chosen. The higher the H 2SO 4 content and the lower the temperature the lower the level at which the aluminium content 158

159

should be controlled. A further factor is that in addition to aluminium, other metallic salts will be present from the alloys processed, such as those of iron, manganese, copper and silicon. These are unwelcome impurities and practical experience has shown that, even if there are no other effects, they reduce the throwing power of the electrolyte.

Having arrived at a suitable anodizing electrolyte the next essential feature of the process is the choice and control of the current supplied to the work. The general aspects of such control have been discussed earlier, but special problems arise in hard anodizing. One of the simplest operating techniques is that of constant current anodizing. With this method the area of the work to be anodized is accurately measured, using a profilometer if required. The total area of the load is then calculated and used for control purposes, either by manual control or by use of a contacting ammeter. Alternatively there are control devices for current density control using either an inert sensing element of standard area which provides a controlling element in the system once the desired current density has been set or alternatively a test piece of standard area of the same material may be used in the control system. With either system thickness obtained varies approximately linearly with time for films of up to 35 pt but will usually be non-linear for greater thicknesses. George and Powers 6 have pointed out that even differences in temper of an alloy can affect the current density achieved at any given voltage (see Fig. 66) and that therefore the current density control specimen must be identical in every way with the material being processed.

8

£ r

Current control

\ \ \

100

80

Fig. 65. Effect of acid concentration on limiting film thickness at 0 ° C \

5

60 3

20

Current 1 2 3

40

60

80

2

density (amp / ft ) 2N H 2 SO4 4N Hs SO4 6N H2 SO4

$ § w 1 5

40

u

20

Current density 6061- T6 control panel

75

50

bC O

Current density 6061 - T651 exp. panel

25

Voltage

Two of the difficulties associated with hard anodizing are failure to produce a hard coating and, more calamitously, actual burning (i.e. rapid electrochemical dissolution) of part or the whole component, especially with high copper alloys. To reduce these difficulties various mixed electrolytes have been recommended. Wiesner and Mears4 found additions of 11 - 15 g/1 oxalic acid to a 385 g/1 H2SO4 electrolyte to give the best results when hard anodizing high copper alloys such as 2017, 2024 and X2219. Another mixture used commercially, forming the basis of the Sandford process 5, is sulphuric-mellitic acid, with about 30 g/1 mellitic acid added to sulphuric acid of 150 - 250 g/1 concentration. Some companies prefer to use a sulphuric acid concentration of 320 - 350 g/1 (i.e. around maximum conductivity) without additions for high copper alloys (i.e. 2024 alloy). 160

10

20

30

40

50

Time, min

Fig. 66. Current density voltage-time relationships developed with “parallel circuit ' ' anodizing control system? One of the practical drawbacks to constant current (or constant current density) hard anodizing is that over the anodizing time, the total wattage input continues to rise This means that the rectifier required must be able to cope with a peak load rather than a much lower average demand load and that the heating of the film 161

should be controlled. A further factor is that in addition to aluminium, other metallic salts will be present from the alloys processed, such as those of iron, manganese, copper and silicon. These are unwelcome impurities and practical experience has shown that, even if there are no other effects, they reduce the throwing power of the electrolyte.

Having arrived at a suitable anodizing electrolyte the next essential feature of the process is the choice and control of the current supplied to the work. The general aspects of such control have been discussed earlier, but special problems arise in hard anodizing. One of the simplest operating techniques is that of constant current anodizing. With this method the area of the work to be anodized is accurately measured, using a profilometer if required. The total area of the load is then calculated and used for control purposes, either by manual control or by use of a contacting ammeter. Alternatively there are control devices for current density control using either an inert sensing element of standard area which provides a controlling element in the system once the desired current density has been set or alternatively a test piece of standard area of the same material may be used in the control system. With either system thickness obtained varies approximately linearly with time for films of up to 35 pt but will usually be non-linear for greater thicknesses. George and Powers 6 have pointed out that even differences in temper of an alloy can affect the current density achieved at any given voltage (see Fig. 66) and that therefore the current density control specimen must be identical in every way with the material being processed.

8

£ r

Current control

\ \ \

100

80

Fig. 65. Effect of acid concentration on limiting film thickness at 0 ° C \

5

60 3

20

Current 1 2 3

40

60

80

2

density (amp / ft ) 2N H 2 SO4 4N Hs SO4 6N H2 SO4

$ § w 1 5

40

u

20

Current density 6061- T6 control panel

75

50

bC O

Current density 6061 - T651 exp. panel

25

Voltage

Two of the difficulties associated with hard anodizing are failure to produce a hard coating and, more calamitously, actual burning (i.e. rapid electrochemical dissolution) of part or the whole component, especially with high copper alloys. To reduce these difficulties various mixed electrolytes have been recommended. Wiesner and Mears4 found additions of 11 - 15 g/1 oxalic acid to a 385 g/1 H2SO4 electrolyte to give the best results when hard anodizing high copper alloys such as 2017, 2024 and X2219. Another mixture used commercially, forming the basis of the Sandford process 5, is sulphuric-mellitic acid, with about 30 g/1 mellitic acid added to sulphuric acid of 150 - 250 g/1 concentration. Some companies prefer to use a sulphuric acid concentration of 320 - 350 g/1 (i.e. around maximum conductivity) without additions for high copper alloys (i.e. 2024 alloy). 160

10

20

30

40

50

Time, min

Fig. 66. Current density voltage-time relationships developed with “parallel circuit ' ' anodizing control system? One of the practical drawbacks to constant current (or constant current density) hard anodizing is that over the anodizing time, the total wattage input continues to rise This means that the rectifier required must be able to cope with a peak load rather than a much lower average demand load and that the heating of the film 161

increases significantly with time. A further problem is that if burning occurs on one component, this may be undetected with automatic control. Certainly with the higher wattage input towards the end of the process it is difficult on some alloys to avoid softness in the outer layers of the coating. Instead of using continuous control an alternative suggested in a Sandford Processes Inc. patent is to allow the current to fall by up to a fixed percentage of the original before raising the controlling voltage. One approach advocated to minimise this problem has been that of constant wattage control 8. With this method the electrical supply is controlled by an integrator which measures voltage and amperage and controls the supply to keep them constant. In this way the current density may be high initially (3 - 5 A/dm 2 ) and fall by the end of the process to less than one-half of these values. Whilst the method avoids the worst of the problems it can give rise to burning in the critical initial first 10 minutes of anodizing before anodizing conditions have become stabilised. A somewhat simplified version of this approach is contained in a patent 4 covering a constant voltage process in which the voltage is steadily increased in the first 3 - 4 minutes until a current density of approximately 30 A/dm 2 is obtained. The voltage (about 60V) is then held constant for about 15 minutes during which the current density falls progressively until about 1.5 A/dm 2 is reached. The high current density is obtained by using a reciprocating anode rail operating at 50 - 60 strokes/minute in conjunction with a 185 g/1 H,SO 4 electrolyte maintained at -5 to-10°C. A further elaboration in the electrical conditions forms the basis of the Hardas process 10. In this a.c. is superimposed on d.c., the proportions of each being varied to suit the alloy. In general terms the more highly alloyed the material (or the more difficult to anodize), the higher the proportion of a.c. used. Normal 50 or 60 cycle current is used and no particular advantage appears to accrue from the use of interrupted or other sophisticated systems. The exact role of the a.c. component still remains a subject of speculation. A possible clue is contained in a paper describing Russian work" which measured the gas evolved at the anode and cathode under hard anodizing conditions. This showed that up to the practical limit of film growth, the gas evolved for a given set of anodizing conditions is almost exclusively hydrogen. Then, as the voltage required to maintain the current rises sharply, there is a marked evolution of oxygen at the anode. The oxygen polarisation is believed to be the factor limiting film growth. If this is so it may well be that the function of the a.c. is primarily one of suppressing, partially or completely, this polarisation. A more recent description of the process has been given by Thompson14 who also deals with pretreatment and the effect of alloy composition on component dimensions. 162

Refrigeration requirements

The general requirements of refrigeration plant have been discussed previously. It is generally agreed that successful hard anodizing depends upon the ability to maintain uniform conditions throughout the anodizing cycle and rapidly to remove the heat produced. This means that the capacity of the refrigeration system must always exceed the maximum heat evolution of the anodizing bath plus heat gain from the atmosphere. It therefore becomes important that hard anodizing tanks should be lagged to insulate them from heat transfer from the atmosphere. Traditionally slabs of cork have been used for this purpose, but foamed plastics, such as polystyrene foam or foamed phenolics, have been used more recently with success and usually at lower cost.

The system of refrigeration used depends upon the size of installation and partly on the anodizing conditions selected. For small to medium sized baths cooling is achieved by cooling coils immersed directly in the anodizing bath. These cooling coils are used in conjunction with the secondary circuit of a direct expansion refrigeration system. Choice of refrigerant in the cooling coils is important since there is always the possibility of damage to the coil resulting in leakage of the refrigerant into the electrolyte. A water-glycol mixture (about 20 per cent glycol) is often employed since small quantities of glycol in the electrolyte are without effect. If there is a leak the characteristic colour, due to an indicator normally added to commercial glycol, makes its presence obvious. Water-glyccrine mixtures have also been used successfully for this purpose and offer no real hazard, although addition of colouring matter is useful for detecting leaks. Brine solutions are not advised because of the pitting of the anodic coating caused by even small amounts of sodium chloride in the electrolyte. Most ‘Freon’ refrigerants are chlorinated or fluorinated organic compounds which hydrolyse to the corresponding acid, and such materials should also be avoided if they are likely to be able to leak and come into contact with the anodizing electrolyte. Their use is normally confined to the primary circuit on an indirect system.

With medium- to large-sized hard anodizing tanks there are drawbacks to the direct cooling method just discussed. The area of cooling coil becomes considerable and it can be difficult to avoid some temperature gradient across the bath. Some installations have employed cooling by external heat exchanger (Fig. 67). The external heat exchanger provides for a flow of electrolyte over heat exchange surfaces cooled internally by a suitable refrigerant. Heat exchanger elements are normally of the tubular or plate type and materials such as graphite, titanium and stainless steel have been successfuly employed. 163

increases significantly with time. A further problem is that if burning occurs on one component, this may be undetected with automatic control. Certainly with the higher wattage input towards the end of the process it is difficult on some alloys to avoid softness in the outer layers of the coating. Instead of using continuous control an alternative suggested in a Sandford Processes Inc. patent is to allow the current to fall by up to a fixed percentage of the original before raising the controlling voltage. One approach advocated to minimise this problem has been that of constant wattage control 8. With this method the electrical supply is controlled by an integrator which measures voltage and amperage and controls the supply to keep them constant. In this way the current density may be high initially (3 - 5 A/dm 2 ) and fall by the end of the process to less than one-half of these values. Whilst the method avoids the worst of the problems it can give rise to burning in the critical initial first 10 minutes of anodizing before anodizing conditions have become stabilised. A somewhat simplified version of this approach is contained in a patent 4 covering a constant voltage process in which the voltage is steadily increased in the first 3 - 4 minutes until a current density of approximately 30 A/dm 2 is obtained. The voltage (about 60V) is then held constant for about 15 minutes during which the current density falls progressively until about 1.5 A/dm 2 is reached. The high current density is obtained by using a reciprocating anode rail operating at 50 - 60 strokes/minute in conjunction with a 185 g/1 H,SO 4 electrolyte maintained at -5 to-10°C. A further elaboration in the electrical conditions forms the basis of the Hardas process 10. In this a.c. is superimposed on d.c., the proportions of each being varied to suit the alloy. In general terms the more highly alloyed the material (or the more difficult to anodize), the higher the proportion of a.c. used. Normal 50 or 60 cycle current is used and no particular advantage appears to accrue from the use of interrupted or other sophisticated systems. The exact role of the a.c. component still remains a subject of speculation. A possible clue is contained in a paper describing Russian work" which measured the gas evolved at the anode and cathode under hard anodizing conditions. This showed that up to the practical limit of film growth, the gas evolved for a given set of anodizing conditions is almost exclusively hydrogen. Then, as the voltage required to maintain the current rises sharply, there is a marked evolution of oxygen at the anode. The oxygen polarisation is believed to be the factor limiting film growth. If this is so it may well be that the function of the a.c. is primarily one of suppressing, partially or completely, this polarisation. A more recent description of the process has been given by Thompson14 who also deals with pretreatment and the effect of alloy composition on component dimensions. 162

Refrigeration requirements

The general requirements of refrigeration plant have been discussed previously. It is generally agreed that successful hard anodizing depends upon the ability to maintain uniform conditions throughout the anodizing cycle and rapidly to remove the heat produced. This means that the capacity of the refrigeration system must always exceed the maximum heat evolution of the anodizing bath plus heat gain from the atmosphere. It therefore becomes important that hard anodizing tanks should be lagged to insulate them from heat transfer from the atmosphere. Traditionally slabs of cork have been used for this purpose, but foamed plastics, such as polystyrene foam or foamed phenolics, have been used more recently with success and usually at lower cost.

The system of refrigeration used depends upon the size of installation and partly on the anodizing conditions selected. For small to medium sized baths cooling is achieved by cooling coils immersed directly in the anodizing bath. These cooling coils are used in conjunction with the secondary circuit of a direct expansion refrigeration system. Choice of refrigerant in the cooling coils is important since there is always the possibility of damage to the coil resulting in leakage of the refrigerant into the electrolyte. A water-glycol mixture (about 20 per cent glycol) is often employed since small quantities of glycol in the electrolyte are without effect. If there is a leak the characteristic colour, due to an indicator normally added to commercial glycol, makes its presence obvious. Water-glyccrine mixtures have also been used successfully for this purpose and offer no real hazard, although addition of colouring matter is useful for detecting leaks. Brine solutions are not advised because of the pitting of the anodic coating caused by even small amounts of sodium chloride in the electrolyte. Most ‘Freon’ refrigerants are chlorinated or fluorinated organic compounds which hydrolyse to the corresponding acid, and such materials should also be avoided if they are likely to be able to leak and come into contact with the anodizing electrolyte. Their use is normally confined to the primary circuit on an indirect system.

With medium- to large-sized hard anodizing tanks there are drawbacks to the direct cooling method just discussed. The area of cooling coil becomes considerable and it can be difficult to avoid some temperature gradient across the bath. Some installations have employed cooling by external heat exchanger (Fig. 67). The external heat exchanger provides for a flow of electrolyte over heat exchange surfaces cooled internally by a suitable refrigerant. Heat exchanger elements are normally of the tubular or plate type and materials such as graphite, titanium and stainless steel have been successfuly employed. 163

is fitted in the air supply and the air flow controlled at a definite value established by previous tests. Low agitation rates can give soft patches above the airholes due to the air being warmer than the electrolyte. Another method of increasing the agitation provided is by the use of a reciprocating anode bar, similar to that used in electroplating or electropolishing. Suitable equipment should provide for 40 - 60 strokes/minute with a stroke of about 100 mm (4"). This has previously been referred to as a feature in a hard anodizing patent 9. Campbell 13 studied the importance of agitation in the development of his hard anodizing process which employs d.c. with a.c. superimposed in order to be able to use higher current densities. With this anodizing system the electrolyte is pumped from the anodizing tank to an external heat exchanger and then returned. Using suitable controls the rate of electrolyte flow past the anode surface can be varied. An important finding which emerged from his investigations was the dependence of flow rate on thickness of material being anodized, thin sections requiring very significantly higher flow rates than thick sections to avoid burning

i r l

* ■nd Fig. 67. Installation for operating the 'Hardas' hard anodizing process showing the electrical controls, process tank and cooled electrolyte being returned from an external heat exchanger. This plant is producing a high precision uniform coating simultaneously on eight bores in a guided missile component. Each bore has a central cathode with individual trimmer resistance and ammeter to ensure identical current conditions in each. [Courtesy Hawker Siddeley Dynamics Ltd.. Lostock and Hard Aluminium Surfaces Ltd. ]

2 P a r t s A.C.

Plus

Agitation Agitation performs an essential role in the removal of heat from the surface being anodized. On many installations it is provided by compressed air fed into a pipe of 25 - 35 mm (1" - P/2") diameter with holes about 0.3 mm (1/8") diameter at 100 - 150 mm (4" - 6") intervals. Kape 13 has reported that porous ceramic diffusers having pores ~ 25 p diameter provided a more uniform agitation due to the more widely dispersed fine bubbles produced. The degree of agitation is normally determined by experience but better control is obtained if a flowmeter 164

Fig. 68. Diagram showing the relation between sheet thickness, proportion of A. C. component and speed of agitation in the Hardas process. The solid and dotted curves show the relation between speed of agitation (in terms of rate of flow in feet per minute per foot of anode surface). When agitation is increased the A.C. can be reduced. The increase of the shaded area to the right represents the tendency for thinner sheet to burn J:. 165

is fitted in the air supply and the air flow controlled at a definite value established by previous tests. Low agitation rates can give soft patches above the airholes due to the air being warmer than the electrolyte. Another method of increasing the agitation provided is by the use of a reciprocating anode bar, similar to that used in electroplating or electropolishing. Suitable equipment should provide for 40 - 60 strokes/minute with a stroke of about 100 mm (4"). This has previously been referred to as a feature in a hard anodizing patent 9. Campbell 13 studied the importance of agitation in the development of his hard anodizing process which employs d.c. with a.c. superimposed in order to be able to use higher current densities. With this anodizing system the electrolyte is pumped from the anodizing tank to an external heat exchanger and then returned. Using suitable controls the rate of electrolyte flow past the anode surface can be varied. An important finding which emerged from his investigations was the dependence of flow rate on thickness of material being anodized, thin sections requiring very significantly higher flow rates than thick sections to avoid burning

i r l

* ■nd Fig. 67. Installation for operating the 'Hardas' hard anodizing process showing the electrical controls, process tank and cooled electrolyte being returned from an external heat exchanger. This plant is producing a high precision uniform coating simultaneously on eight bores in a guided missile component. Each bore has a central cathode with individual trimmer resistance and ammeter to ensure identical current conditions in each. [Courtesy Hawker Siddeley Dynamics Ltd.. Lostock and Hard Aluminium Surfaces Ltd. ]

2 P a r t s A.C.

Plus

Agitation Agitation performs an essential role in the removal of heat from the surface being anodized. On many installations it is provided by compressed air fed into a pipe of 25 - 35 mm (1" - P/2") diameter with holes about 0.3 mm (1/8") diameter at 100 - 150 mm (4" - 6") intervals. Kape 13 has reported that porous ceramic diffusers having pores ~ 25 p diameter provided a more uniform agitation due to the more widely dispersed fine bubbles produced. The degree of agitation is normally determined by experience but better control is obtained if a flowmeter 164

Fig. 68. Diagram showing the relation between sheet thickness, proportion of A. C. component and speed of agitation in the Hardas process. The solid and dotted curves show the relation between speed of agitation (in terms of rate of flow in feet per minute per foot of anode surface). When agitation is increased the A.C. can be reduced. The increase of the shaded area to the right represents the tendency for thinner sheet to burn J:. 165

MJ

when hard anodized at 250 - 300 A/ft 2. This is shown in Fig. 68 where the minimum rate of flow necessary to avoid burning is plotted against section thickness and also the proportion of a.c. required to maintain a current density of 280 A/ft 2 using the ‘Hardas’ process. Hard anodizing conditions demand more rigorous control of agitation. At lower current densities than used by Campbell, means of agitation other than pumping have been employed. Wiesner and Mears 16 employed anode rod oscillation in the range of 35 - 50 rpm with a 3" stroke when hard anodizing at 36 A/ft 2 in a 12 per cent (130 g/1) H2SO 4 + 1 per cent (10 g/1) oxalic acid electrolyte. Similar recommendations have been made in another hard anodizing patent 1 . Where external cooling by heat exchanger is employed the agitation may be conveniently combined with the refrigeration system provided the flow rate is sufficiently high. Campbell considers this an essential feature of the ‘Hardas’ process and his patent specifies the use of a minimum of 50 ft/minute relative movement between the article being anodized and the electrolyte. This movement must be adjusted to suit the thickness of the material being anodized but it also affects the amount of a.c. required in this process, reducing it with increase in agitation rate as shown in Fig. 68. Jigging and stopping-off

Fig. 69. Example of hard anodizing jig employing stop-off .

&

These techniques have parallels with those used in hard chromium plating. Jigs for hard anodizing should be robustly constructed and of adequate area to carry the current required by the component. It is desirable to make the jig of the same alloy as the component so as to get good current distribution. If the jig is not stopped-off this requirement is essential since a pure aluminium jig will anodize in preference to a less readily anodizable component held on it. Titanium jigs have to be used with considerable care and unless they provide a generous contact area and firmly clamp the component, there is a danger of local burning. In general, the use of titanium is confined to components not carrying heavy currents. In many cases a simple stopping-off technique using waxes as in hard chromium plating, can be employed. Areas not to be stopped-off are covered with a French chalk-vaseline mixture and the unit pre-heated in a warm oven, and then dipped in wax. With a sharp knife or scalpel, wax can be removed from the areas to be anodized. Some skill is required to produce a clean edge to the anodized area. Various types of proprietary PVC coating compositions have also been used successfully. On simple components application of PVC adhesive tape has proved satisfactory. An example of a hard anodizing jig employing wax stopping-off is illustrated in Figs. 69 and 70. 166

Fig. 70. Preparing Jaguar(Dunlop) disc brakes for hard anodizing. {Courtesy Hard Aluminium Surfaces Ltd. ) 167

MJ

when hard anodized at 250 - 300 A/ft 2. This is shown in Fig. 68 where the minimum rate of flow necessary to avoid burning is plotted against section thickness and also the proportion of a.c. required to maintain a current density of 280 A/ft 2 using the ‘Hardas’ process. Hard anodizing conditions demand more rigorous control of agitation. At lower current densities than used by Campbell, means of agitation other than pumping have been employed. Wiesner and Mears 16 employed anode rod oscillation in the range of 35 - 50 rpm with a 3" stroke when hard anodizing at 36 A/ft 2 in a 12 per cent (130 g/1) H2SO 4 + 1 per cent (10 g/1) oxalic acid electrolyte. Similar recommendations have been made in another hard anodizing patent 1 . Where external cooling by heat exchanger is employed the agitation may be conveniently combined with the refrigeration system provided the flow rate is sufficiently high. Campbell considers this an essential feature of the ‘Hardas’ process and his patent specifies the use of a minimum of 50 ft/minute relative movement between the article being anodized and the electrolyte. This movement must be adjusted to suit the thickness of the material being anodized but it also affects the amount of a.c. required in this process, reducing it with increase in agitation rate as shown in Fig. 68. Jigging and stopping-off

Fig. 69. Example of hard anodizing jig employing stop-off .

&

These techniques have parallels with those used in hard chromium plating. Jigs for hard anodizing should be robustly constructed and of adequate area to carry the current required by the component. It is desirable to make the jig of the same alloy as the component so as to get good current distribution. If the jig is not stopped-off this requirement is essential since a pure aluminium jig will anodize in preference to a less readily anodizable component held on it. Titanium jigs have to be used with considerable care and unless they provide a generous contact area and firmly clamp the component, there is a danger of local burning. In general, the use of titanium is confined to components not carrying heavy currents. In many cases a simple stopping-off technique using waxes as in hard chromium plating, can be employed. Areas not to be stopped-off are covered with a French chalk-vaseline mixture and the unit pre-heated in a warm oven, and then dipped in wax. With a sharp knife or scalpel, wax can be removed from the areas to be anodized. Some skill is required to produce a clean edge to the anodized area. Various types of proprietary PVC coating compositions have also been used successfully. On simple components application of PVC adhesive tape has proved satisfactory. An example of a hard anodizing jig employing wax stopping-off is illustrated in Figs. 69 and 70. 166

Fig. 70. Preparing Jaguar(Dunlop) disc brakes for hard anodizing. {Courtesy Hard Aluminium Surfaces Ltd. ) 167

Another technique which has been used successfully employs a chromic acid anodizing treatment before hard anodizing. The work is anodized by the original process involving raising the voltage to 40V over the first 10 minutes, holding at 40 volts for 40 minutes, raising to 50 volts over the next 5 minutes and a final 5 minutes at 50 volts. The work is then rinsed only in cold water and sealed for 30 minutes in boiling deionised water at pH 5.5 - 6.5. By light honing or grinding or by some similar finishing operation the coating is removed from the areas to be hard anodized. Provided the remaining coating is not scratched or damaged the chromic anodized film will provide adequate stopping-off for normal hard anodizing at 1.6- 4.2 A/dm 2 (15-40 A/ft 2 ) . Hard anodizing procedure

The following represents an outline of the practice that might be followed for the production of hard anodized components. As will be appreciated from the foregoing discussion there are a number of variants and refinements that can be introduced. Consequently it is proposed only to outline a simple basic technique which companies with limited experience in this field can use and develop further in the light of experience. On receipt of the work the components should be carefully inspected to ensure that they are undamaged and correspond to the drawings of the work which should be supplied with the components. The specification of the material should be clearly stated on the drawing and should also be given on internal paperwork and on instructions to the hard anodizing department. The area of the surfaces to be hard anodized should be calculated and noted for internal costing and processing instructions. The initial processing follows conventional lines in that, after jigging, a suitable cleaning procedure should be followed. This will usually consist of vapour degrease, alternatively a non-etching cleaner of the emulsion or inhibited-sulphuric type, followed by approximately 10 minutes immersion in a chromic-sulphuric acid bath (e.g. to DEF. 151 ) maintained at 50°C. After rinsing, the following alternative procedures can be applied.

75 - 90 minutes on some alloys of high copper content. At the conclusion of anodizing the current is reduced to zero, the agitation cut off and the work transferred to a running water rinse tank. After thorough rinsing, a hot water dip may be given and the work dried off or, if specified, sealed in a dichromate or lubricant solution. (ii ) Work sulphuric acid anodized with local stopping-off The service conditions of some components may call for areas not hard anodized to be sulphuric acid anodized and dichromate sealed. The prodedure employed is the following: — Degrease and clean Sulphuric acid anodize Dichromate seal Grind areas to be hard anodized to 0.001 in below required size Apply stopping-off to all other areas Degrease and clean Hard anodize to 50p ( 0.002" ) coating thickness Remove stop-off Seal if specified Final hone on hard anodized surfaces if micro-inch finish required. Instead of grinding-off in some cases, such as cylinder bores, the anodic coating can be stripped chemically in a chrome-phosphoric acid mixture after stoppingoff is complete.

(iii) Work using chromic acid anodizing as stop-off For many aerospace applications chromic acid anodizing is frequently specified as a basic protective treatment. It can be used additionally, under the modified conditions previously given, as a stop-off coating prior to hard anodizing. The following is the recommended procedure: —

(i) Work hard anodized completely The work is firmly attached to the anode rail of the anodizing bath, preferably by bolting or clamping, and agitation commenced. The current is then turned on and so controlled that current density is initially not greater than 1.5 A/dm 2 (14 A/ft 2 ); as conditions become stabilised the current density is increased to 2.5 A/dm 2 (24 A/ft 2 ). With high copper alloys it may be found advantageous to maintain a current density of 1.5 A/dm 2 for 10-15 minutes before increasing the current density. Using these conditions it requires approximately 60 minutes to produce a 50p (0.002") thickness coating on the more readily anodized alloys and 168

Extended cycle chromic acid anodize Rinse Seal 30 minutes in boiling deionised water Dry Hone off anodic coating where hard anodizing required (dimensions 0.001 in less than required if 0.002 in thickness specified). Hard anodize Seal if specified Final honing (of other super-finishing technique) of hard anodized surface. 169

Another technique which has been used successfully employs a chromic acid anodizing treatment before hard anodizing. The work is anodized by the original process involving raising the voltage to 40V over the first 10 minutes, holding at 40 volts for 40 minutes, raising to 50 volts over the next 5 minutes and a final 5 minutes at 50 volts. The work is then rinsed only in cold water and sealed for 30 minutes in boiling deionised water at pH 5.5 - 6.5. By light honing or grinding or by some similar finishing operation the coating is removed from the areas to be hard anodized. Provided the remaining coating is not scratched or damaged the chromic anodized film will provide adequate stopping-off for normal hard anodizing at 1.6- 4.2 A/dm 2 (15-40 A/ft 2 ) . Hard anodizing procedure

The following represents an outline of the practice that might be followed for the production of hard anodized components. As will be appreciated from the foregoing discussion there are a number of variants and refinements that can be introduced. Consequently it is proposed only to outline a simple basic technique which companies with limited experience in this field can use and develop further in the light of experience. On receipt of the work the components should be carefully inspected to ensure that they are undamaged and correspond to the drawings of the work which should be supplied with the components. The specification of the material should be clearly stated on the drawing and should also be given on internal paperwork and on instructions to the hard anodizing department. The area of the surfaces to be hard anodized should be calculated and noted for internal costing and processing instructions. The initial processing follows conventional lines in that, after jigging, a suitable cleaning procedure should be followed. This will usually consist of vapour degrease, alternatively a non-etching cleaner of the emulsion or inhibited-sulphuric type, followed by approximately 10 minutes immersion in a chromic-sulphuric acid bath (e.g. to DEF. 151 ) maintained at 50°C. After rinsing, the following alternative procedures can be applied.

75 - 90 minutes on some alloys of high copper content. At the conclusion of anodizing the current is reduced to zero, the agitation cut off and the work transferred to a running water rinse tank. After thorough rinsing, a hot water dip may be given and the work dried off or, if specified, sealed in a dichromate or lubricant solution. (ii ) Work sulphuric acid anodized with local stopping-off The service conditions of some components may call for areas not hard anodized to be sulphuric acid anodized and dichromate sealed. The prodedure employed is the following: — Degrease and clean Sulphuric acid anodize Dichromate seal Grind areas to be hard anodized to 0.001 in below required size Apply stopping-off to all other areas Degrease and clean Hard anodize to 50p ( 0.002" ) coating thickness Remove stop-off Seal if specified Final hone on hard anodized surfaces if micro-inch finish required. Instead of grinding-off in some cases, such as cylinder bores, the anodic coating can be stripped chemically in a chrome-phosphoric acid mixture after stoppingoff is complete.

(iii) Work using chromic acid anodizing as stop-off For many aerospace applications chromic acid anodizing is frequently specified as a basic protective treatment. It can be used additionally, under the modified conditions previously given, as a stop-off coating prior to hard anodizing. The following is the recommended procedure: —

(i) Work hard anodized completely The work is firmly attached to the anode rail of the anodizing bath, preferably by bolting or clamping, and agitation commenced. The current is then turned on and so controlled that current density is initially not greater than 1.5 A/dm 2 (14 A/ft 2 ); as conditions become stabilised the current density is increased to 2.5 A/dm 2 (24 A/ft 2 ). With high copper alloys it may be found advantageous to maintain a current density of 1.5 A/dm 2 for 10-15 minutes before increasing the current density. Using these conditions it requires approximately 60 minutes to produce a 50p (0.002") thickness coating on the more readily anodized alloys and 168

Extended cycle chromic acid anodize Rinse Seal 30 minutes in boiling deionised water Dry Hone off anodic coating where hard anodizing required (dimensions 0.001 in less than required if 0.002 in thickness specified). Hard anodize Seal if specified Final honing (of other super-finishing technique) of hard anodized surface. 169

Sealing hard anodic oxide coatings

There is some loss of hardness when hard anodized work is sealed in water or an aqueous sealant but fatigue properties are improved. Sealing in a dichromate solution is preferred for aerospace applications except where contact with H.T.P (concentrated hydrogen peroxide) is involved. The following sealing treatments are required by a British specification (Ministry of Defence DEF. 151):— A Sodium/potassium dichromate 70 - 100 g/1 1 Sodium carbonate 8 g/1 (of sodium hydroxide 13 g/1 pH (glass electrode) 6.3 - 7.4 alternatively: B

Sodium/potassium dichromate pH (glass electrode)

40 - 60 g/1 5.6 - 6.0

Immerse parts for 5 - 1 0 minutes in solution A or for a time equal to the anodizing time in solution B, both solutions to be at 96° - 100°C. Sealing in silicone oil of low to medium viscosity is also used, especially when the component operates at slightly elevated temperatures. Other organic sealants such as lubricating oils containing fatty acid soaps or molybdenum disulphide, have been used as well as colloidal graphite. References

6. 8.

J

10. 11. 12. 13. 14. 15. 16. 18.

170

G. Kurganov and N. 1. Varzhemskaya, Metalloved, i. Obrabotka Mctallov., 1956 (1) 45 I. I. Moroz, I. P. Kharlamov and L. S. Sogolava, Stanki i Instrument. 1961. 32. (11)32 25 P. Csokan, Trans. Inst. Met. Finishing. 1964. 41, (1)51 H. J . Wiesner and H. A. Mears, Proc. A.E.S.. 1958, 45. 105 U.S. Pat. 2,918,416 (1959) D. J . George and J . H. Powers, Plating. Nov. 1969. p. 1240 U.S. Pat. 2,897,125(1954) Brit. Pat. 727,749 (1953) Brit. Pat. 892,768(1959) Brit. Pat. 716,759 (1952) N. D. Tomashov and A. V. Byalobzheskii, Trudy Inst. Fiz. Khim.. Akad, Nauk. S.S.S.R. No. 5, Issledovan Korrozi, 1955, (4) 114 J . M. Kape, Metal Ind., 1957, 26, (10) 198 W. J . Campbell, Proc. A.D.A. Conference on Anodizing (1961) p. 137 D. A. Thompson, Trans. Inst. Met. Finishing 1976, 54, 94 W. J . Campbell, Proc. Electrodepos. Tech. Soc., 1952, 28, 273 H. J . Wiesner and H. A. Mears, Proc. A.E.S., 1958, 45, 105 Brit. Pat. 716.554(1953) Electroplating and Metal Finishing, 1961, 14, (2) 93

Chapter 12: Dyeing anodic oxide coatings Almost simultaneously with disclosure in 1923 of the suitability of chromic acid electrolytes for producing porous anodic oxide coatings came a patent for dyeing these coatings in dyestuffs such as anthracene blue or Turkey red 1. Since then many organic dyestuffs have been investigated and a number of them, ranging from the older alizarin-based dyestuffs to the newer metal-complex dyes, are in regular use and give a wide colour range. Some years later it was discovered that colours could be produced by precipitating an inorganic pigment in the pores of the anodic coating by a double-dipping technique 2 ’ The most popular process of this kind consists of first dipping the anodized article in a warm solution of cobalt acetate and then, after a short rinsing, into a potassium permanganate solution whereby a bronze colour is produced. In 1936 Budiloff discovered that a gold tone could be obtained by a single immersion in a warm solution of a hydrolysable iron salt, an observation which forms the basis for current techniques for producing inorganic ‘gold’ shades? It has already been mentioned that oxalic acid electrolytes can produce brassbronze coloured anodic coatings particularly when alternating currents are used"' and that other electrolytes can also provide coloured coatings; these colours modify the effect of any subsequent dyeing. Colours due to constituent colouring (aluminium - silicon and aluminium - chromium alloys) have a similar but not identical effect. It has also been observed" that a.c. anodizing results in the production of hydrogen sulphide and that sufficient sulphide is retained in the film for a black colour to be developed when it is dipped in cobalt acetate solution. A number of special effects are also possible by making use of well-established techniques employed for colouring other materials and using them in a unique way. For example, it is possible to use silk screen printing techniques to provide multi-colour designs, whilst offset lithographic techniques have been employed for coloured work, mainly as a means of applying resists to stopped-off portions of the coating which are to be left uncoloured or subsequently dyed in a second colour. By precipitating silver salts in the anodic oxide coating the anodized surface can be used as a photographic plate. Theory of inorganic pigment colouring

It is convenient to consider colouring techniques under two headings - those in which inorganic pigments produce the colour and those where organic dyestuffs are used. Despite the widespread use of these techniques, little has been published on the theory or mechanism of dyeing anodic oxide coatings. 171

Sealing hard anodic oxide coatings

There is some loss of hardness when hard anodized work is sealed in water or an aqueous sealant but fatigue properties are improved. Sealing in a dichromate solution is preferred for aerospace applications except where contact with H.T.P (concentrated hydrogen peroxide) is involved. The following sealing treatments are required by a British specification (Ministry of Defence DEF. 151):— A Sodium/potassium dichromate 70 - 100 g/1 1 Sodium carbonate 8 g/1 (of sodium hydroxide 13 g/1 pH (glass electrode) 6.3 - 7.4 alternatively: B

Sodium/potassium dichromate pH (glass electrode)

40 - 60 g/1 5.6 - 6.0

Immerse parts for 5 - 1 0 minutes in solution A or for a time equal to the anodizing time in solution B, both solutions to be at 96° - 100°C. Sealing in silicone oil of low to medium viscosity is also used, especially when the component operates at slightly elevated temperatures. Other organic sealants such as lubricating oils containing fatty acid soaps or molybdenum disulphide, have been used as well as colloidal graphite. References

6. 8.

J

10. 11. 12. 13. 14. 15. 16. 18.

170

G. Kurganov and N. 1. Varzhemskaya, Metalloved, i. Obrabotka Mctallov., 1956 (1) 45 I. I. Moroz, I. P. Kharlamov and L. S. Sogolava, Stanki i Instrument. 1961. 32. (11)32 25 P. Csokan, Trans. Inst. Met. Finishing. 1964. 41, (1)51 H. J . Wiesner and H. A. Mears, Proc. A.E.S.. 1958, 45. 105 U.S. Pat. 2,918,416 (1959) D. J . George and J . H. Powers, Plating. Nov. 1969. p. 1240 U.S. Pat. 2,897,125(1954) Brit. Pat. 727,749 (1953) Brit. Pat. 892,768(1959) Brit. Pat. 716,759 (1952) N. D. Tomashov and A. V. Byalobzheskii, Trudy Inst. Fiz. Khim.. Akad, Nauk. S.S.S.R. No. 5, Issledovan Korrozi, 1955, (4) 114 J . M. Kape, Metal Ind., 1957, 26, (10) 198 W. J . Campbell, Proc. A.D.A. Conference on Anodizing (1961) p. 137 D. A. Thompson, Trans. Inst. Met. Finishing 1976, 54, 94 W. J . Campbell, Proc. Electrodepos. Tech. Soc., 1952, 28, 273 H. J . Wiesner and H. A. Mears, Proc. A.E.S., 1958, 45, 105 Brit. Pat. 716.554(1953) Electroplating and Metal Finishing, 1961, 14, (2) 93

Chapter 12: Dyeing anodic oxide coatings Almost simultaneously with disclosure in 1923 of the suitability of chromic acid electrolytes for producing porous anodic oxide coatings came a patent for dyeing these coatings in dyestuffs such as anthracene blue or Turkey red 1. Since then many organic dyestuffs have been investigated and a number of them, ranging from the older alizarin-based dyestuffs to the newer metal-complex dyes, are in regular use and give a wide colour range. Some years later it was discovered that colours could be produced by precipitating an inorganic pigment in the pores of the anodic coating by a double-dipping technique 2 ’ The most popular process of this kind consists of first dipping the anodized article in a warm solution of cobalt acetate and then, after a short rinsing, into a potassium permanganate solution whereby a bronze colour is produced. In 1936 Budiloff discovered that a gold tone could be obtained by a single immersion in a warm solution of a hydrolysable iron salt, an observation which forms the basis for current techniques for producing inorganic ‘gold’ shades? It has already been mentioned that oxalic acid electrolytes can produce brassbronze coloured anodic coatings particularly when alternating currents are used"' and that other electrolytes can also provide coloured coatings; these colours modify the effect of any subsequent dyeing. Colours due to constituent colouring (aluminium - silicon and aluminium - chromium alloys) have a similar but not identical effect. It has also been observed" that a.c. anodizing results in the production of hydrogen sulphide and that sufficient sulphide is retained in the film for a black colour to be developed when it is dipped in cobalt acetate solution. A number of special effects are also possible by making use of well-established techniques employed for colouring other materials and using them in a unique way. For example, it is possible to use silk screen printing techniques to provide multi-colour designs, whilst offset lithographic techniques have been employed for coloured work, mainly as a means of applying resists to stopped-off portions of the coating which are to be left uncoloured or subsequently dyed in a second colour. By precipitating silver salts in the anodic oxide coating the anodized surface can be used as a photographic plate. Theory of inorganic pigment colouring

It is convenient to consider colouring techniques under two headings - those in which inorganic pigments produce the colour and those where organic dyestuffs are used. Despite the widespread use of these techniques, little has been published on the theory or mechanism of dyeing anodic oxide coatings. 171

The production of a ‘gold’ colour by a single immersion in a solution of a hydrolysable iron salt is an exception to the normal way of producing an inorganic pigment colour in that only a single immersion is required. When the anodic oxide coating is immersed in the iron solution, some of the aluminium oxide is dissolved by the anions present and either this salt itself is more alkaline than pH 5.5 or it hydrolyses to produce a sufficiently high pH for the iron to be precipitated as ferric hydroxide. Solutions of all iron salts hydrolyse and are slowly affected by light so that in due course there is some precipitation of ferric hydroxide in the bath. The attraction of a currently used process 7, which employs ferric ammonium oxalate, is that additions of oxalic acid enable any precipitated ferric hydroxide to be redissolved, whilst ammonia additions enable the bath to be restored to its optimum pH range. A detailed investigation into inorganic pigment colouring has been reported by Kape and Mills 8 who offer some suggestions on its mechanism. They consider that the amount of pigment precipitated in the anodic oxide coating will largely depend on the absorption and diffusion of anions or cations into its pores. This diffusion should be governed by the surface charge (zeta potential) of the anodic coating with respect to a given solution. The zeta potential of the anodic coating in water is positive but in electrolytes its charge may be changed. Thus, in the double dipping cobalt acetate - potassium permanganate technique, it was found that it was not possible to obtain colouring if the work was first dipped into the permanganate solution. This is assumed to be due to the potassium cation being absorbed preferentially so that manganate ions were not available to react with the cobalt acetate solution. With all reagents except potassium ferrocyanide, potassium ferricyanide, potassium chromate, tannic acid and gallic acid it was necessary to immerse the anodic coating first into the solution contributing the positively charged part of the pigment, i.e. the metallic cation, if a deep colour was to be obtained. From this it was concluded that in such solutions the zeta potential of the anodic coating is negative. There is little doubt that the zeta potential has an important influence on precipitation of inorganic pigments. It is also necessary to remember that some colouring reactions which may be theoretically possible will not be produced from solutions with either a low or a high pH value. If these are used there will be dissolution and hydrolysis of the surface of the anodic coating producing a soft chalky layer on which pigment may precipitate. The colour thus produced may either disappear on sealing or rub off easily in use. Mechanism of organic dyestuff dyeing There are a number of different types of dyestuffs. Speiser 9 has pointed out that anodized aluminium can be coloured with both acid and substantive dyestuffs but that these are only absorbed and their fastness to light and weather is not good. Dyestuffs such as Alizarin Bordeaux R form a lake with the aluminium oxide thus: 172

Al OH O O

OH 0 OH

OH OH O

OH O OH

and such lakes have quite good fastness. Wherever chemical combinations with the aluminium occurs the colour is usually of good fastness. Dyestuffs of the metalcomplexing acid-colour type come in this category such as Chrome Fast OrangeR: COONa N = N-

■N = N-

OH

SO 3 Na In a further category are those known as metal-complexing colours, of which Palatine Bordeaux RN is an example: O

SO HO■N = NCl

NH It is uncertain whether they combine with aluminium to form a new complex or whether aluminium combines at hydroxyl or sulphonate groupings. Giles and his colleagues 10 11 using mainly sulphonated dyestuffs, have arrived at some general conclusions as to the mechanism of dyeing of anodic coatings. They conclude that dyestuffs can be broadly divided into chelating and nonchelating dyestuffs. A typical aluminium chelate complex is that formed with alizarin. The absorption mechanism of the non-chelating dyes is probably covalent bond formation, each sulphonate group replacing an hydroxyl group or an SO4 or CrO4 group in the oxide, together with some ion-exchange adsorption of anionic micelles. The sequence of events in adsorption from water appears to be: (i) film etching by rapid initial dissolution of the surface layer of the anodic coating, (ii) rapid adsorption of the external layer of solute, followed by (iii) slow inward diffusion of solute from the external layer to form a complete monolayer, (iv) with sulphonated dyestuffs, penetration and ultimate breakdown of the oxide crystal 173

The production of a ‘gold’ colour by a single immersion in a solution of a hydrolysable iron salt is an exception to the normal way of producing an inorganic pigment colour in that only a single immersion is required. When the anodic oxide coating is immersed in the iron solution, some of the aluminium oxide is dissolved by the anions present and either this salt itself is more alkaline than pH 5.5 or it hydrolyses to produce a sufficiently high pH for the iron to be precipitated as ferric hydroxide. Solutions of all iron salts hydrolyse and are slowly affected by light so that in due course there is some precipitation of ferric hydroxide in the bath. The attraction of a currently used process 7, which employs ferric ammonium oxalate, is that additions of oxalic acid enable any precipitated ferric hydroxide to be redissolved, whilst ammonia additions enable the bath to be restored to its optimum pH range. A detailed investigation into inorganic pigment colouring has been reported by Kape and Mills 8 who offer some suggestions on its mechanism. They consider that the amount of pigment precipitated in the anodic oxide coating will largely depend on the absorption and diffusion of anions or cations into its pores. This diffusion should be governed by the surface charge (zeta potential) of the anodic coating with respect to a given solution. The zeta potential of the anodic coating in water is positive but in electrolytes its charge may be changed. Thus, in the double dipping cobalt acetate - potassium permanganate technique, it was found that it was not possible to obtain colouring if the work was first dipped into the permanganate solution. This is assumed to be due to the potassium cation being absorbed preferentially so that manganate ions were not available to react with the cobalt acetate solution. With all reagents except potassium ferrocyanide, potassium ferricyanide, potassium chromate, tannic acid and gallic acid it was necessary to immerse the anodic coating first into the solution contributing the positively charged part of the pigment, i.e. the metallic cation, if a deep colour was to be obtained. From this it was concluded that in such solutions the zeta potential of the anodic coating is negative. There is little doubt that the zeta potential has an important influence on precipitation of inorganic pigments. It is also necessary to remember that some colouring reactions which may be theoretically possible will not be produced from solutions with either a low or a high pH value. If these are used there will be dissolution and hydrolysis of the surface of the anodic coating producing a soft chalky layer on which pigment may precipitate. The colour thus produced may either disappear on sealing or rub off easily in use. Mechanism of organic dyestuff dyeing There are a number of different types of dyestuffs. Speiser 9 has pointed out that anodized aluminium can be coloured with both acid and substantive dyestuffs but that these are only absorbed and their fastness to light and weather is not good. Dyestuffs such as Alizarin Bordeaux R form a lake with the aluminium oxide thus: 172

Al OH O O

OH 0 OH

OH OH O

OH O OH

and such lakes have quite good fastness. Wherever chemical combinations with the aluminium occurs the colour is usually of good fastness. Dyestuffs of the metalcomplexing acid-colour type come in this category such as Chrome Fast OrangeR: COONa N = N-

■N = N-

OH

SO 3 Na In a further category are those known as metal-complexing colours, of which Palatine Bordeaux RN is an example: O

SO HO■N = NCl

NH It is uncertain whether they combine with aluminium to form a new complex or whether aluminium combines at hydroxyl or sulphonate groupings. Giles and his colleagues 10 11 using mainly sulphonated dyestuffs, have arrived at some general conclusions as to the mechanism of dyeing of anodic coatings. They conclude that dyestuffs can be broadly divided into chelating and nonchelating dyestuffs. A typical aluminium chelate complex is that formed with alizarin. The absorption mechanism of the non-chelating dyes is probably covalent bond formation, each sulphonate group replacing an hydroxyl group or an SO4 or CrO4 group in the oxide, together with some ion-exchange adsorption of anionic micelles. The sequence of events in adsorption from water appears to be: (i) film etching by rapid initial dissolution of the surface layer of the anodic coating, (ii) rapid adsorption of the external layer of solute, followed by (iii) slow inward diffusion of solute from the external layer to form a complete monolayer, (iv) with sulphonated dyestuffs, penetration and ultimate breakdown of the oxide crystal 173

structure, and finally (v) slow subsequent ‘sealing’ of the film pores by growth of boehmite crystals. The apparent activation energy of diffusion in the film falls with increase in size of the unsulphonated residue. The rate-determining part of the process is diffusion of the dyestuff inside the pores of the coatings.

Most dyestuffs used are ones which have unsaturated groups capable of forming metal complexes thus: R N=N I

Fig. 71. Effect of dyeing time and pH on iron absorbed by a 25panodic oxide coating dyed in a 12.5 g/ 1 solution of ferric ammonium oxalate.

Fe Absorbed mg/drn 2

The conclusion reached by Skulikidis and his colieages 12 13 as to the mechanism of dyeing with a mordant dye is that the exchange of places between the dye and the aluminium oxide is the governing reaction.

▲ pH varied • Time varied pH = 5

+ 2H

N=N' i R Whilst these combinations may be largely with aluminium there will always be groupings available after dyeing capable of forming covalent bonds with a metallic ion. It has therefore become established practice to ‘fix’ most films dyed with organic dyestuffs in a nickel acetate solution 14 although more recently it has been claimed that certain dyestuffs of the metallic-organic complex type give higher light fastness if sealed in solutions of lead, copper or chromium salts". Inorganic pigment colouring techniques The most widely used inorganic pigment colouring technique is that of producing a gold colour by a single immersion of the anodic oxide coating in a ferric ammonium oxalate solution. It has also been found that either by additions to this solution or by a further ‘toning’ treatment other colours are obtained. Colouring techniques based on ferric ammonium oxalate treatment will therefore be described separately from other double-dipping processes. Colours based on ferric ammonium oxalate The gold colour produced by immersion in a solution of this compound can vary from a pale brass to a red-orange colour according to the conditions of dyeing. The solution is normally used at concentrations of 5 - 50 g/1 and at 30° - 60°C. The lower the concentration and temperature the paler the colour obtained. The depth of colour also increases with increasing anodic film thickness. It has been shown by estimating the iron content of anodic coatings treated in such a solution, that the depth of colour does not increase linearly with time (Fig. 71) but rises rapidly in the first minute after which uptake proceeds more slowly. 174

8 Mins/i)H dyeing

Dyeing times of less than a minute make control of colour difficult. It has further been established that the pH of the bath influences the depth of colour obtained for a given time of immersion (see also Fig. 71). The sharp fall above pH 5.5 is due to the precipitation of ferric hydroxide. For practical purposes a pH value of 4.5 ± 0.5 is the best one at which the bath should be controlled. When ferric hydroxide is precipitated from the bath it can be redissolved by the addition of 5 g/1 oxalic acid and a little hydrogen peroxide to the warm solution. It can be adjusted to the correct pH value by additions of ammonium hydroxide. Other colours can be obtained from treatments based on ferric ammonium oxalate, either by additions to this solution or by a further dip in another bath (see Table 20). It should be remembered that the organic acids listed in this table, as well as pyrogallol, are not very stable and tend to decompose. The ferric ammonium oxalate - pyrogallol mixture is particularly unstable and will not remain usable for more than a few hours, or at best a working day. The processes using two-stage dipping are more practicable but even so the organic bath, even at room temperature, will require frequent renewal. Colours produced by precipitation of inorganic pigments [double-dipping} These have been mentioned in the literature for many years and in particular the production of bronze shades by a double-dipping process involving dipping first 175

structure, and finally (v) slow subsequent ‘sealing’ of the film pores by growth of boehmite crystals. The apparent activation energy of diffusion in the film falls with increase in size of the unsulphonated residue. The rate-determining part of the process is diffusion of the dyestuff inside the pores of the coatings.

Most dyestuffs used are ones which have unsaturated groups capable of forming metal complexes thus: R N=N I

Fig. 71. Effect of dyeing time and pH on iron absorbed by a 25panodic oxide coating dyed in a 12.5 g/ 1 solution of ferric ammonium oxalate.

Fe Absorbed mg/drn 2

The conclusion reached by Skulikidis and his colieages 12 13 as to the mechanism of dyeing with a mordant dye is that the exchange of places between the dye and the aluminium oxide is the governing reaction.

▲ pH varied • Time varied pH = 5

+ 2H

N=N' i R Whilst these combinations may be largely with aluminium there will always be groupings available after dyeing capable of forming covalent bonds with a metallic ion. It has therefore become established practice to ‘fix’ most films dyed with organic dyestuffs in a nickel acetate solution 14 although more recently it has been claimed that certain dyestuffs of the metallic-organic complex type give higher light fastness if sealed in solutions of lead, copper or chromium salts". Inorganic pigment colouring techniques The most widely used inorganic pigment colouring technique is that of producing a gold colour by a single immersion of the anodic oxide coating in a ferric ammonium oxalate solution. It has also been found that either by additions to this solution or by a further ‘toning’ treatment other colours are obtained. Colouring techniques based on ferric ammonium oxalate treatment will therefore be described separately from other double-dipping processes. Colours based on ferric ammonium oxalate The gold colour produced by immersion in a solution of this compound can vary from a pale brass to a red-orange colour according to the conditions of dyeing. The solution is normally used at concentrations of 5 - 50 g/1 and at 30° - 60°C. The lower the concentration and temperature the paler the colour obtained. The depth of colour also increases with increasing anodic film thickness. It has been shown by estimating the iron content of anodic coatings treated in such a solution, that the depth of colour does not increase linearly with time (Fig. 71) but rises rapidly in the first minute after which uptake proceeds more slowly. 174

8 Mins/i)H dyeing

Dyeing times of less than a minute make control of colour difficult. It has further been established that the pH of the bath influences the depth of colour obtained for a given time of immersion (see also Fig. 71). The sharp fall above pH 5.5 is due to the precipitation of ferric hydroxide. For practical purposes a pH value of 4.5 ± 0.5 is the best one at which the bath should be controlled. When ferric hydroxide is precipitated from the bath it can be redissolved by the addition of 5 g/1 oxalic acid and a little hydrogen peroxide to the warm solution. It can be adjusted to the correct pH value by additions of ammonium hydroxide. Other colours can be obtained from treatments based on ferric ammonium oxalate, either by additions to this solution or by a further dip in another bath (see Table 20). It should be remembered that the organic acids listed in this table, as well as pyrogallol, are not very stable and tend to decompose. The ferric ammonium oxalate - pyrogallol mixture is particularly unstable and will not remain usable for more than a few hours, or at best a working day. The processes using two-stage dipping are more practicable but even so the organic bath, even at room temperature, will require frequent renewal. Colours produced by precipitation of inorganic pigments [double-dipping} These have been mentioned in the literature for many years and in particular the production of bronze shades by a double-dipping process involving dipping first 175

in a warm cobalt acetate solution followed by a short rinse and immersion in warm potassium permanganate solution. To obtain the desired shade this process may have to be repeated several times. Table 20. Colours obtained from treatments based on ferric ammonium oxalate solutions Colour

Gold

Bronze

Bronze

Blue

Black

Treatment in Bath 1 P/2-5 min. in 10-50 g/1 ferric ammonium oxalate at 50°C, pH 4.5 As for gold

1 min. in 20 g/1 ferric ammonium oxalate + 1 g/1 pyrogallol at 60-70°C, pH 5.0

Treatment in Bath 2

Remarks

Ref.

—

—

(7)

1-3 min. in 0.2% gallic acid at 60°C

—

ature those listed in Table 21 are those which are both practicable and also produce pigments of high light fastness. It should be remembered that nearly all the above compounds are to some degree toxic and operators should be warned accordingly and protected with rubber gloves and protective clothing. The ammonium sulphide solution should be used at room temperature and the tank provided with exhaust; the other tanks do not require this. Potassium dichromate sealing of the Prussian blue provides a significant improvement in light fastness but the dichromate concentration in the sealing tank should not exceed 0.5 per cent or it will noticeably change the final colour. Table 22. Inorganic pigment colours produced by double-dipping techniques 4 Bath 1

Colour

Gallic acid slowly decomposes

(16)

Bath decomposes

(17)

2 min. in 50 g/1 ferric ammonium oxalate at 35°C

1-2 min. in 3% potas-Post dip in 25% (volsium ferrocyanide ume)HNOa . Sealing (19) solution at 35 °C in 0.5% K,Cr2 O7 preferred

2 min. in 50 g/1 ferric ammonium oxalate at 50°C

3 min. in 10% tannic Tannic acid solution acid at room temper- decomposes (18) ature

Bath 2

Temp. °C

Bronze

20-50 g/1 cobalt acetate 20-50 g/1 potassium permanganate

30-50

Yellow

50-100 g/1 lead nitrate

50-100 g/1 potassium chromate

40-50

Yellow-brown

50-100 g/1 lead nitrate

50-100 g/1 potassium permanganate

50

Green

25-50 g/1 copper sulphate

1-10 g/1 ammonium sulphide*

35-50

Prussian blue

50 g/1 ferric sulphate

25-50 g/1 potassium ferrocyanide

40-50t

* Second bath should be kept at room temperature.

As a general principle the higher the concentration of cobalt acetate and the temperature of the solution the deeper the colour. Although the same principles apply to the permanganate solution, conditions are less critical in this bath. The time of rinsing between immersion in the two baths is critical since prolonged rinsing will result in appreciable desorption of the cobalt ions whilst too short rinsing will result in uneven surface precipitation rather than precipitation in the pores. The pH of the rinse is also often critical. These comments apply to all doubledipping techniques other than those based on ferric ammonium oxalate. A number of colours can be produced by double-dipping methods. These have been extensively investigated by Kape and Mills8 whilst a useful general summary of these techniques is also contained in an account by Hermann and Hiibner 21 and a standard work on anodizing 20. Of the numerous colours mentioned in the liter176

t Sealing in 0.5% potassium dichromate recommended.

Dyeing with organic dyestuffs Since the number of dyestuffs available which are suitable for colouring anodic oxide coatings is steadily increasing, no useful purpose is served by cataloguing those currently available. It should also be noted that dyestuffs of virtually the same composition are available from more than one manufacturer under different trade names. Reference to the Colour Index 22 will provide all the known information about the composition of most proprietary dyestuffs. The dyeing of anodized aluminium to a consistent shade of colour and to meet performance standards requires careful preliminary work in arriving at a technique for using a particular dyestuff. The first significant factor is the customer’s 177

in a warm cobalt acetate solution followed by a short rinse and immersion in warm potassium permanganate solution. To obtain the desired shade this process may have to be repeated several times. Table 20. Colours obtained from treatments based on ferric ammonium oxalate solutions Colour

Gold

Bronze

Bronze

Blue

Black

Treatment in Bath 1 P/2-5 min. in 10-50 g/1 ferric ammonium oxalate at 50°C, pH 4.5 As for gold

1 min. in 20 g/1 ferric ammonium oxalate + 1 g/1 pyrogallol at 60-70°C, pH 5.0

Treatment in Bath 2

Remarks

Ref.

—

—

(7)

1-3 min. in 0.2% gallic acid at 60°C

—

ature those listed in Table 21 are those which are both practicable and also produce pigments of high light fastness. It should be remembered that nearly all the above compounds are to some degree toxic and operators should be warned accordingly and protected with rubber gloves and protective clothing. The ammonium sulphide solution should be used at room temperature and the tank provided with exhaust; the other tanks do not require this. Potassium dichromate sealing of the Prussian blue provides a significant improvement in light fastness but the dichromate concentration in the sealing tank should not exceed 0.5 per cent or it will noticeably change the final colour. Table 22. Inorganic pigment colours produced by double-dipping techniques 4 Bath 1

Colour

Gallic acid slowly decomposes

(16)

Bath decomposes

(17)

2 min. in 50 g/1 ferric ammonium oxalate at 35°C

1-2 min. in 3% potas-Post dip in 25% (volsium ferrocyanide ume)HNOa . Sealing (19) solution at 35 °C in 0.5% K,Cr2 O7 preferred

2 min. in 50 g/1 ferric ammonium oxalate at 50°C

3 min. in 10% tannic Tannic acid solution acid at room temper- decomposes (18) ature

Bath 2

Temp. °C

Bronze

20-50 g/1 cobalt acetate 20-50 g/1 potassium permanganate

30-50

Yellow

50-100 g/1 lead nitrate

50-100 g/1 potassium chromate

40-50

Yellow-brown

50-100 g/1 lead nitrate

50-100 g/1 potassium permanganate

50

Green

25-50 g/1 copper sulphate

1-10 g/1 ammonium sulphide*

35-50

Prussian blue

50 g/1 ferric sulphate

25-50 g/1 potassium ferrocyanide

40-50t

* Second bath should be kept at room temperature.

As a general principle the higher the concentration of cobalt acetate and the temperature of the solution the deeper the colour. Although the same principles apply to the permanganate solution, conditions are less critical in this bath. The time of rinsing between immersion in the two baths is critical since prolonged rinsing will result in appreciable desorption of the cobalt ions whilst too short rinsing will result in uneven surface precipitation rather than precipitation in the pores. The pH of the rinse is also often critical. These comments apply to all doubledipping techniques other than those based on ferric ammonium oxalate. A number of colours can be produced by double-dipping methods. These have been extensively investigated by Kape and Mills8 whilst a useful general summary of these techniques is also contained in an account by Hermann and Hiibner 21 and a standard work on anodizing 20. Of the numerous colours mentioned in the liter176

t Sealing in 0.5% potassium dichromate recommended.

Dyeing with organic dyestuffs Since the number of dyestuffs available which are suitable for colouring anodic oxide coatings is steadily increasing, no useful purpose is served by cataloguing those currently available. It should also be noted that dyestuffs of virtually the same composition are available from more than one manufacturer under different trade names. Reference to the Colour Index 22 will provide all the known information about the composition of most proprietary dyestuffs. The dyeing of anodized aluminium to a consistent shade of colour and to meet performance standards requires careful preliminary work in arriving at a technique for using a particular dyestuff. The first significant factor is the customer’s 177

requirement; it may be essential for the anodizer to be able to exactly match a sample provided by the customer. This necessitates that the anodizer must establish a technique to meet this requirement and there may be only one dyestuff which can be used. In other instances it may be that some other property, such as fastness to light or to heat or ability to be used in a printing process or to dye at room temperature, will be the overwhelming factor provided the shade is within broad limits similar to that which will be suitable for the end use. Given that the colour and performance specification are known the anodizer will generally have one or more dyestuffs which appear suitable. Manufacturers generally give helpful advice as to appropriate conditions under which a dyestuff may be used and often provide colour samples showing the depth of colour obtained at two or three dyestuff concentrations together with the recommended pH value for the dyebath. This will enable initial tests to be made. Experience suggests that initially the dyebath temperature and pH should be fixed and tests made with two or three levels of dye concentration and dyeing time. These initial tests will usually indicate a dye concentration and dyeing time which is likely to be suitable. Schenkel and Speiser 26 have contributed a useful study of the practical effects on dyeing of varying conditions, as well as of contaminants in the dyebath. pH value The next step is to check the effects on colour of departure from the pH value used for the initial tests, since some change in pH can be expected under production conditions. If a variation of ± 0.5 in the pH value produces no visible difference in the hue obtained, and if a range of ± 1.0 shows only a just noticeable difference, then the dye will give few problems from this cause. Other factors have also to be considered, such as the method used to control dyebath pH value. The best practice for each individual dyestuff needs to be established to suit the dye and the shop operating conditions. For some dyestuffs manufacturers recommend buffering with sodium acetate in the presence of some acetic acid. Practical experience indicates that there are dyes where this is not always the best practice. Some dyes show some change of hue or colour in the presence of sodium acetate and may be sensitive to the amount of sodium acetate present. A further factor is that even in combined form, there is volatility of acetic acid to be taken into account as the salt hydrolyses. Unless the dyestuff is one which definitely responds favourably to buffering with acetate the Authors prefer to adjust the pH by additions of a dilute solution of an alkali metal hydroxide (e.g. caustic soda) or sulphuric acid. Dyebath contaminants It should be remembered that sodium sulphate is a contaminant normally present in dyestuffs, so that correction using the above reagents avoids introducing a fresh contaminant. However, there are dyes which can be somewhat sensitive to the 178

presence of sulphate. One of these is Acid Yellow 3, which shows a progressive diminution in depth of shade as the sulphate content increases. Some dyes can be sensitive to sulphuric acid; for example Acid Green 25 dye has a decided tendency to precipitate, depending upon dye and sulphate concentrations and pH of operation. In this case buffering with acetate is beneficial. On the other hand the phthalocyanine blue dyes such as Direct Blue 86 and Acid Blue 197 seem to require the presence of sulphate to secure full depth of shade. Giles and Datye 2 have shown that: (a) Sodium chloride and potassium iodide do not retard dyeing. (b) All sulphates are effective in retarding dyeing. (c) Trisodium phosphate has a greater retarding effect than sulphates. (d) The effect of sodium sulphate increases with the.number of sulphonate groups in the dye ion. A further factor of significance can be the effect of dissolved substances in the supply water used for making up the dyebath if deionised water is not available. One of the most commonly encountered ions is calcium which can form complexes with some organic dyes. An example of this behaviour is that of Mordant Red 3, an alizarin-based dyestuff. Fig. 72 shows the transmission curves obtained with an optical wedge spectrometer of solutions of the dyestuff made up with deionised water and the corresponding curves of the same dyestuff with the addition of a calcium salt, at two pH values. The presence of the calcium complex affects the final colour of the dyed work, and of course calcium can be concentrated as fresh water is added to make good evaporation losses. Another important source of contamination can be the anodic coating itself. The dyeing reaction introduces some aluminium salts into the dyestuffs and occasionally this can change the shade produced. It occurs but rarely and mainly with a few metal complex dyes where the aluminium can exchange with the metalion complex in the dye. Other metals such as iron and copper (including brass) can also interfere, but they should not normally be encountered unless they have been used for suspending the work in the dyebath. Deliberate addition of from 0.1 to 1.0 g/1 Al (or other metal salts) to the dye and comparing work dyed in it with the original can give an indication of potential difficulties from this source since some slight dissolution of the coating occurs in the dyebath. Dyebath operation An important pre-requisite to good dyeing is that the anodizing should be carried out under closely controlled conditions and the rinsing of the work be adequate. Prolonged rinsing is not necessary since any free acid diffuses out of the coating and of crevices in a few minutes with a well-agitated rinse. Although neutralising rinses using ammonium or sodium bicarbonate have been advocated, these can also give changes of shade on dyeing. 179

requirement; it may be essential for the anodizer to be able to exactly match a sample provided by the customer. This necessitates that the anodizer must establish a technique to meet this requirement and there may be only one dyestuff which can be used. In other instances it may be that some other property, such as fastness to light or to heat or ability to be used in a printing process or to dye at room temperature, will be the overwhelming factor provided the shade is within broad limits similar to that which will be suitable for the end use. Given that the colour and performance specification are known the anodizer will generally have one or more dyestuffs which appear suitable. Manufacturers generally give helpful advice as to appropriate conditions under which a dyestuff may be used and often provide colour samples showing the depth of colour obtained at two or three dyestuff concentrations together with the recommended pH value for the dyebath. This will enable initial tests to be made. Experience suggests that initially the dyebath temperature and pH should be fixed and tests made with two or three levels of dye concentration and dyeing time. These initial tests will usually indicate a dye concentration and dyeing time which is likely to be suitable. Schenkel and Speiser 26 have contributed a useful study of the practical effects on dyeing of varying conditions, as well as of contaminants in the dyebath. pH value The next step is to check the effects on colour of departure from the pH value used for the initial tests, since some change in pH can be expected under production conditions. If a variation of ± 0.5 in the pH value produces no visible difference in the hue obtained, and if a range of ± 1.0 shows only a just noticeable difference, then the dye will give few problems from this cause. Other factors have also to be considered, such as the method used to control dyebath pH value. The best practice for each individual dyestuff needs to be established to suit the dye and the shop operating conditions. For some dyestuffs manufacturers recommend buffering with sodium acetate in the presence of some acetic acid. Practical experience indicates that there are dyes where this is not always the best practice. Some dyes show some change of hue or colour in the presence of sodium acetate and may be sensitive to the amount of sodium acetate present. A further factor is that even in combined form, there is volatility of acetic acid to be taken into account as the salt hydrolyses. Unless the dyestuff is one which definitely responds favourably to buffering with acetate the Authors prefer to adjust the pH by additions of a dilute solution of an alkali metal hydroxide (e.g. caustic soda) or sulphuric acid. Dyebath contaminants It should be remembered that sodium sulphate is a contaminant normally present in dyestuffs, so that correction using the above reagents avoids introducing a fresh contaminant. However, there are dyes which can be somewhat sensitive to the 178

presence of sulphate. One of these is Acid Yellow 3, which shows a progressive diminution in depth of shade as the sulphate content increases. Some dyes can be sensitive to sulphuric acid; for example Acid Green 25 dye has a decided tendency to precipitate, depending upon dye and sulphate concentrations and pH of operation. In this case buffering with acetate is beneficial. On the other hand the phthalocyanine blue dyes such as Direct Blue 86 and Acid Blue 197 seem to require the presence of sulphate to secure full depth of shade. Giles and Datye 2 have shown that: (a) Sodium chloride and potassium iodide do not retard dyeing. (b) All sulphates are effective in retarding dyeing. (c) Trisodium phosphate has a greater retarding effect than sulphates. (d) The effect of sodium sulphate increases with the.number of sulphonate groups in the dye ion. A further factor of significance can be the effect of dissolved substances in the supply water used for making up the dyebath if deionised water is not available. One of the most commonly encountered ions is calcium which can form complexes with some organic dyes. An example of this behaviour is that of Mordant Red 3, an alizarin-based dyestuff. Fig. 72 shows the transmission curves obtained with an optical wedge spectrometer of solutions of the dyestuff made up with deionised water and the corresponding curves of the same dyestuff with the addition of a calcium salt, at two pH values. The presence of the calcium complex affects the final colour of the dyed work, and of course calcium can be concentrated as fresh water is added to make good evaporation losses. Another important source of contamination can be the anodic coating itself. The dyeing reaction introduces some aluminium salts into the dyestuffs and occasionally this can change the shade produced. It occurs but rarely and mainly with a few metal complex dyes where the aluminium can exchange with the metalion complex in the dye. Other metals such as iron and copper (including brass) can also interfere, but they should not normally be encountered unless they have been used for suspending the work in the dyebath. Deliberate addition of from 0.1 to 1.0 g/1 Al (or other metal salts) to the dye and comparing work dyed in it with the original can give an indication of potential difficulties from this source since some slight dissolution of the coating occurs in the dyebath. Dyebath operation An important pre-requisite to good dyeing is that the anodizing should be carried out under closely controlled conditions and the rinsing of the work be adequate. Prolonged rinsing is not necessary since any free acid diffuses out of the coating and of crevices in a few minutes with a well-agitated rinse. Although neutralising rinses using ammonium or sodium bicarbonate have been advocated, these can also give changes of shade on dyeing. 179

O

C>

The initial make-up of the dyebath is important in that care should be taken to ensure that the dyestuff is first made into a smooth thick paste by the addition of a small amount of hot water and careful stirring to ensure complete mixing. This should then be added a little at a time to water in the dyebath until completely dissolved. The final strength of the dye should be checked and adjusted, if necessary, by the photo-optical method described later (p. 182).

O

—

O

O

O

O

Optical density % NJ GJ -KLn

phosphoric, oxalic, citric or tartaric acid for this purpose, especially in connection with nameplate manufacture.

0.03% Alizarin red S

0.03% Alizarin red S pH 3.6

500

600

500

700

600

700

Wavelength ( n m )

Wavelength ( n m )

0.03% Alizarin red S + Ca pH 5.4

(d)

NJ o

Optical density %

Optical density % GJ -f*Lr o o c

0.03% Alizarin red S + C a pH 3.6

500 600 Wavelength ( n m )

700

500 600 Wavelength ( n m )

Fig. 72. [

The initial make-up of the dyebath is important in that care should be taken to ensure that the dyestuff is first made into a smooth thick paste by the addition of a small amount of hot water and careful stirring to ensure complete mixing. This should then be added a little at a time to water in the dyebath until completely dissolved. The final strength of the dye should be checked and adjusted, if necessary, by the photo-optical method described later (p. 182).

O

—

O

O

O

O

Optical density % NJ GJ -KLn

phosphoric, oxalic, citric or tartaric acid for this purpose, especially in connection with nameplate manufacture.

0.03% Alizarin red S

0.03% Alizarin red S pH 3.6

500

600

500

700

600

700

Wavelength ( n m )

Wavelength ( n m )

0.03% Alizarin red S + Ca pH 5.4

(d)

NJ o

Optical density %

Optical density % GJ -f*Lr o o c

0.03% Alizarin red S + C a pH 3.6

500 600 Wavelength ( n m )

700

500 600 Wavelength ( n m )

Fig. 72. [ 10 ppm SiO? were reported by Richaud 7 to adversely affect sealing while Scott 19 reported that > 20 ppm of silica (added as sodium metasilicate) was definitely deleterious. Sheasby 20 has subsequently demonstrated that the effect of silica (added as sodium metasilicate) is significantly influenced by pH. The effect of silica content and pH on seal quality, as indicated by measurement of weight loss in the acidified sulphite solution test (B.S. 1615: 1972, Appendix E), are shown in Table 25. Table 25. Effect of pH and silica content of seal water on weight loss (mg/ cm2 ) in Kape test on Al-Mg-Si alloy anodized to 25 pi 20 SiO2 ppm 0 2.5 5 10 20

pH 5.0

pH 6.0

pH 7.0

0.22 0.80 1.09 1.22 1.22

0.07 0.13 0.19

0.05 0.02 0.04 0.04 0.09

A weight loss of 0.20mg/cm 2or higher indicates unsatisfactory sealing. It seems necessary, therefore, to control the pH of the seal at around 6.0 - 6.5 to minimize the possible effect of silica contamination. It is important to realise that the effect of silica on sealing quality will not be detected by all sealing tests. In particular measurement of admittance or impedance is not sensitive to silica level, and very misleading results can be obtained if this is the only production control test which is being used (see Table 26). The same is true of the presence of phosphate in the sealing water, but fortunately dye spot and acid dissolution tests will indicate unacceptable quality. It is possible, however, to arrange for automatic control devices which will result in automatic regeneration of the colomn before silica breakthrough is likely to occur. Such devices depend upon the ability of a special electrode to detect sodium at a very low level (< 0.01 ppm) which appears when the exchange columns are in the first stage of exhaustion. By such means the anion and cation columns are regenerated about 30 per cent more frequently than would be the case using 223

W

”

"W

Hf nr

fl*

The reason for this can be seen from Table 24 which lists the effect of various ions on water sealing.

Table 24. Effect of various ions on water sealing Ion

Mg + +

F' Cl' Si0

3' PO/' SO4 "

1

*

Effect No reported effect on seal quality but aluminium hydroxide precipitates at relatively low levels and this can affect appearance of work. None Variable effect reported. Some workers claim improvement in seal quality above 100 p.p.m. . but others say it is deleterious at above 5 p.p.m. Tends to produce chalkiness but otherwise no reported effect. Harmful at above 10 p.p.m. Harmful at above 10 p.p.m. Harmful at above 20 - 30 p.p.m. Decreases corrosion resistance. Inhibits sealing at above 10 p.p.m. Inhibits scaling at above 5 p.p.m. Some doubt but said to be harmful above 50 p.p.m. . Present after short time in most scaling water and build-up could be cause of water rejection.

It seems probable that potassium ions exhibit a similar behaviour to sodium ions. Ammonia and acetic acid are commonly used for adjusting the pH of the seal bath so these ions are not deleterious, and Sheasby and Bancroft have shown that addition of 1 g/1 ammonium acetate to the sealing water can give improved seal quality by buffering the pH. Phosphate ions, in addition to inhibiting sealing, can also strip anodic oxide coatings if their concentration exceeds 100 p.p.m.24 Effect of silica in the water The significance of the problem of silica in sealing water has only recently been adequately appreciated. Silica is invariably present in supply waters collected from areas where granitic rocks predominate. Thus in the United Kingdom, the water used in areas such as Glasgow, Manchester and Birmingham comes largely from such sources. From most practical considerations it is a good quality water for metal finishing operations, being low in dissolved solids and of a pH around 6.0. However, it is quite firmly established that it is possible to commence the week with work passing acceptance test and by the middle or end of the week to find work being rejected. This is due to silica concentrating in the seal water as a result of adding further water to make good evaporation losses. 222

Even if a deionised water supply is provided this does not necessarily ensure freedom from silica in the final outgoing water. Silica is usually present in water in the hydrated form, silicic acid, which is a weak acid, and a strong base anion exchange resin is needed to remove it at all. Even then it is the first substance not to be removed by the anion column when the latter is approaching the point at which regeneration is needed. This can happen even before the conductivity value indicates the necessity for regeneration. Under certain operating conditions it is possible to obtain the phenomenon of silica breakthrough whereby silica adsorbed on the anion column may be suddenly released into the outgoing water. Values of > 10 ppm SiO? were reported by Richaud 7 to adversely affect sealing while Scott 19 reported that > 20 ppm of silica (added as sodium metasilicate) was definitely deleterious. Sheasby 20 has subsequently demonstrated that the effect of silica (added as sodium metasilicate) is significantly influenced by pH. The effect of silica content and pH on seal quality, as indicated by measurement of weight loss in the acidified sulphite solution test (B.S. 1615: 1972, Appendix E), are shown in Table 25. Table 25. Effect of pH and silica content of seal water on weight loss (mg/ cm2 ) in Kape test on Al-Mg-Si alloy anodized to 25 pi 20 SiO2 ppm 0 2.5 5 10 20

pH 5.0

pH 6.0

pH 7.0

0.22 0.80 1.09 1.22 1.22

0.07 0.13 0.19

0.05 0.02 0.04 0.04 0.09

A weight loss of 0.20mg/cm 2or higher indicates unsatisfactory sealing. It seems necessary, therefore, to control the pH of the seal at around 6.0 - 6.5 to minimize the possible effect of silica contamination. It is important to realise that the effect of silica on sealing quality will not be detected by all sealing tests. In particular measurement of admittance or impedance is not sensitive to silica level, and very misleading results can be obtained if this is the only production control test which is being used (see Table 26). The same is true of the presence of phosphate in the sealing water, but fortunately dye spot and acid dissolution tests will indicate unacceptable quality. It is possible, however, to arrange for automatic control devices which will result in automatic regeneration of the colomn before silica breakthrough is likely to occur. Such devices depend upon the ability of a special electrode to detect sodium at a very low level (< 0.01 ppm) which appears when the exchange columns are in the first stage of exhaustion. By such means the anion and cation columns are regenerated about 30 per cent more frequently than would be the case using 223

conductivity control. A simpler and less sophisticated method is to determine the capacity between regenerations and fix an alarm and cut-off which operates at 80 per cent of this level. This is normally an adequate control.

formation is strongly pH dependent. Above pH 6.5 the level of sealing smut tends to increase considerably, although seal test results are generally good, and acid dissolution tests may even give a small weight gain rather than a weight loss.

Table 26. Effect of silicate in seal water on seal test results

However, with use the pH of the sealing water tends to drop, as a result of carry-over of small amounts of sulphuric acid in the anodic film. Frequent adjustment of pH is therefore required to keep it within working limits and this presents certain practical problems. Firstly the chemicals used to adjust pH are important. Normally either ammonia or sodium hydroxide solutions are used to adjust the pH upwards. The former has the disadvantage that it is relatively volatile so that it may be partially lost over a period of a few hours. Use of the latter raises the sodium content of the water and significantly increases its conductivity. Whilst this has no effect at low levels, the effect on film properties at high levels is less clear. Equally, if the pH is over-adjusted, either acetic acid or dilute sulphuric acid can be used to adjust it downwards.

Sealing water

Anodic film Silicate content Thickness of water in microns mg /litre

Admittance value in micro-siemens

Admittance value x thickness

Dye stain result Acidified (Aluminium Sulphite green GLW) Wt. loss in mg/cm 2

Town water

14

19.5

24

336

Heavy stain

0.31

Plant deionised water

13

19.5

20

261

Heavy stain

0.30

14

none

19

266

No stain

0.03

Laboratory deionised water

Continuous deionisation Because of the importance of consistently high water quality and taking into consideration the fact that deionised water in the United Kingdom costs around 6p per gallon if running costs and capital charges are taken into account, it can be advantageous on large plants to arrange for continuous deionisation. Such a system involves pumping the seal water out into a cooling tank and then passing it through the deionising unit, and maintaining a storage tank reservoir of deionised water from which the sealing tank is supplied. A level controller in the seal tank is linked to an outlet valve in the storage tank so as to maintain a constant level. It is not, of course, possible to use continuous deionisation if sealing additives are used. Because of this, and the problems involved in cooling and reheating water for continuous circulation, some plants continuously deionise the last rinse before sealing. This has the advantage that the water can be circulated through the ion exchange columns directly and any water then carried into the seal tank is relatively pure. However, as explained earlier, the aluminium and sulphate content of the seal bath will still build up as a result of the sealing operation itself, and the water will eventually have to be changed. Control of pH will, however, be simplified. pH control Control of the pH of the seal water presents probably the greatest practical problem in operating a production sealing bath. It is generally agreed that the most satisfactory operating range is between 5.5 and 6.5 Many workers have shown that below 5.5 the sealing quality falls off rapidly, with very poor quality sealing being produced at pH 4, and this is in agreement with the fact that bohmite 224

Acetic acid and ammonia provide a greater buffering action than sodium hydroxide and sulphuric acid, and are preferred pH adjustors, and this fact has been utilised by Sheasby and Bancroft 21, who investigated an ammonium acetate addition to the seal bath, in order to buffer it against changes in pH. They showed that a 1 g/1 addition was sufficient to significantly reduce the number of pH adjustments required with use, and unexpectedly they showed an increase in the useful life of the seal bath, possibly because of the fewer adjustments made. This practice is now quite widely used in production plants. Secondly it is important to ensure that the pH is consistent throughout the seal bath and that it is maintained within the desired limits. With large seal tanks, particularly, consistency throughout the bath can be difficult to achieve, especially since often only the ends of the bath are accessible for pH checking. The chemicals used for adjustment are usually added at the same points, so care must be taken to mix the bath thoroughly before carrying out further checks. Air or other forms of agitation are essential for this purpose, though it is usually undesirable to agitate the bath continuously because of the increased heat losses produced. If the chemicals can be added at a number of points along the length of the bath, then it is desirable to do so. Consistency of pH over the operating period can only be achieved by frequent checking and this means several times per shift in many plants. However, automatic pH control systems are available, though the location of the pH measuring unit and its construction can present problems. Direct immersion of the pH probe in the seal tank at a suitable position near the mid-point of the long side is desirable, but this presents difficulties in finding suitable probes to withstand continuous exposure to high temperatures. Some installations have therefore used a by-pass system in which the water is cooled before the pH is measured. As with the current density probes, however, it can only operate satisfactorily if the pH of the water at the measuring point is typical of the seal bath pH as a whole. 225

conductivity control. A simpler and less sophisticated method is to determine the capacity between regenerations and fix an alarm and cut-off which operates at 80 per cent of this level. This is normally an adequate control.

formation is strongly pH dependent. Above pH 6.5 the level of sealing smut tends to increase considerably, although seal test results are generally good, and acid dissolution tests may even give a small weight gain rather than a weight loss.

Table 26. Effect of silicate in seal water on seal test results

However, with use the pH of the sealing water tends to drop, as a result of carry-over of small amounts of sulphuric acid in the anodic film. Frequent adjustment of pH is therefore required to keep it within working limits and this presents certain practical problems. Firstly the chemicals used to adjust pH are important. Normally either ammonia or sodium hydroxide solutions are used to adjust the pH upwards. The former has the disadvantage that it is relatively volatile so that it may be partially lost over a period of a few hours. Use of the latter raises the sodium content of the water and significantly increases its conductivity. Whilst this has no effect at low levels, the effect on film properties at high levels is less clear. Equally, if the pH is over-adjusted, either acetic acid or dilute sulphuric acid can be used to adjust it downwards.

Sealing water

Anodic film Silicate content Thickness of water in microns mg /litre

Admittance value in micro-siemens

Admittance value x thickness

Dye stain result Acidified (Aluminium Sulphite green GLW) Wt. loss in mg/cm 2

Town water

14

19.5

24

336

Heavy stain

0.31

Plant deionised water

13

19.5

20

261

Heavy stain

0.30

14

none

19

266

No stain

0.03

Laboratory deionised water

Continuous deionisation Because of the importance of consistently high water quality and taking into consideration the fact that deionised water in the United Kingdom costs around 6p per gallon if running costs and capital charges are taken into account, it can be advantageous on large plants to arrange for continuous deionisation. Such a system involves pumping the seal water out into a cooling tank and then passing it through the deionising unit, and maintaining a storage tank reservoir of deionised water from which the sealing tank is supplied. A level controller in the seal tank is linked to an outlet valve in the storage tank so as to maintain a constant level. It is not, of course, possible to use continuous deionisation if sealing additives are used. Because of this, and the problems involved in cooling and reheating water for continuous circulation, some plants continuously deionise the last rinse before sealing. This has the advantage that the water can be circulated through the ion exchange columns directly and any water then carried into the seal tank is relatively pure. However, as explained earlier, the aluminium and sulphate content of the seal bath will still build up as a result of the sealing operation itself, and the water will eventually have to be changed. Control of pH will, however, be simplified. pH control Control of the pH of the seal water presents probably the greatest practical problem in operating a production sealing bath. It is generally agreed that the most satisfactory operating range is between 5.5 and 6.5 Many workers have shown that below 5.5 the sealing quality falls off rapidly, with very poor quality sealing being produced at pH 4, and this is in agreement with the fact that bohmite 224

Acetic acid and ammonia provide a greater buffering action than sodium hydroxide and sulphuric acid, and are preferred pH adjustors, and this fact has been utilised by Sheasby and Bancroft 21, who investigated an ammonium acetate addition to the seal bath, in order to buffer it against changes in pH. They showed that a 1 g/1 addition was sufficient to significantly reduce the number of pH adjustments required with use, and unexpectedly they showed an increase in the useful life of the seal bath, possibly because of the fewer adjustments made. This practice is now quite widely used in production plants. Secondly it is important to ensure that the pH is consistent throughout the seal bath and that it is maintained within the desired limits. With large seal tanks, particularly, consistency throughout the bath can be difficult to achieve, especially since often only the ends of the bath are accessible for pH checking. The chemicals used for adjustment are usually added at the same points, so care must be taken to mix the bath thoroughly before carrying out further checks. Air or other forms of agitation are essential for this purpose, though it is usually undesirable to agitate the bath continuously because of the increased heat losses produced. If the chemicals can be added at a number of points along the length of the bath, then it is desirable to do so. Consistency of pH over the operating period can only be achieved by frequent checking and this means several times per shift in many plants. However, automatic pH control systems are available, though the location of the pH measuring unit and its construction can present problems. Direct immersion of the pH probe in the seal tank at a suitable position near the mid-point of the long side is desirable, but this presents difficulties in finding suitable probes to withstand continuous exposure to high temperatures. Some installations have therefore used a by-pass system in which the water is cooled before the pH is measured. As with the current density probes, however, it can only operate satisfactorily if the pH of the water at the measuring point is typical of the seal bath pH as a whole. 225

Time and temperature of sealing Having established factors controlling the quality of water used in sealing and the significance of pH and impurity control it is necessary to consider further the influence of sealing time and temperature on the overall quality of sealing.

i

E z?

20

100° I E 10

1.0

15

2.0

showed that sealing times of more than 20 minutes and seal bath temperatures of over 94 °C were necessary (fig. 88) in order to obtain adequate seal quality with 25 micron films in the laboratory. Longer times and/or higher temperatures would of course be necessary under production conditions. Birtel and Leute 23 24 , Bradshaw and co-workers 25 and many others have examined the effects of sealing parameters on admittance and dissipation factor measurements and have again shown the same general dependence on sealing time and temperature. In the latter work it was shown that the acceptance level suggested for admittance values over a range of 10 - 40 microns lay between the values achieved with 2 minutes/micron sealing time and 1 minute/micron. Unpublished work carried out by Brace 26 used weight gain measurements as a means of following the progress of sealing using commercial purity aluminium panels anodized to 25p thickness in a 165 g/l H2SO 4 electrolyte at 21°C and 1.6 A/dm 2 (15 A/ft 2 ). From Fig. 89 it can be seen that the weight gain tends to flatten out after 30 minutes sealing, and also that the weight gain is temperature dependent, being further increased if sealing is continued in water under pressure at 115°C. The significance of the steam sealing results will be discussed later.

Weight loss (mg/cm 2 ) •Or

— Water at l l ! r C . __ Steam at 115° C

\

\

Fig. 88 a. Effect of temperature of sealing on weight loss of anodic films in sodium sulphite solution sealing test 22

Water at 100°C Water at “ 80° C

20 J E 20

i 1

io

30

5 mins sealing

I 0

03

Fig. 89. Effect of time and temperature on weight gain in sealing 25 p coatings produced in 165 g/l sulphuric acid electrolyte at 21°C and 1.7 A/ dm 2 2b.

Specific sealing gain-mg/gm

30

Steam at 100°C

10

Weight loss (mg/cm 2 )

f z

Fig. 88b. Effect of time of sealing on weight loss of anodic films m sodium sulphite solution sealing test 22 Again, in this field, many workers have examined the effects of time and temperature with the general conclusion that higher temperatures and longer times are beneficial. Using the sulphite acid dissolution test, Sheasby and co-workers 22 226

o

* ................................................................. 20 30 40 50 *O

IO

Sealing time - mins

Such results tend to give rise to empirical rules for finding the duration of sealing required. In this instance one might conclude that 1 minute per micron was adequate. Further experiments involving anodizing under similar conditions but in a 360 g/l H2SO, electrolyte produced a very different set of curves and levelling 227

Time and temperature of sealing Having established factors controlling the quality of water used in sealing and the significance of pH and impurity control it is necessary to consider further the influence of sealing time and temperature on the overall quality of sealing.

i

E z?

20

100° I E 10

1.0

15

2.0

showed that sealing times of more than 20 minutes and seal bath temperatures of over 94 °C were necessary (fig. 88) in order to obtain adequate seal quality with 25 micron films in the laboratory. Longer times and/or higher temperatures would of course be necessary under production conditions. Birtel and Leute 23 24 , Bradshaw and co-workers 25 and many others have examined the effects of sealing parameters on admittance and dissipation factor measurements and have again shown the same general dependence on sealing time and temperature. In the latter work it was shown that the acceptance level suggested for admittance values over a range of 10 - 40 microns lay between the values achieved with 2 minutes/micron sealing time and 1 minute/micron. Unpublished work carried out by Brace 26 used weight gain measurements as a means of following the progress of sealing using commercial purity aluminium panels anodized to 25p thickness in a 165 g/l H2SO 4 electrolyte at 21°C and 1.6 A/dm 2 (15 A/ft 2 ). From Fig. 89 it can be seen that the weight gain tends to flatten out after 30 minutes sealing, and also that the weight gain is temperature dependent, being further increased if sealing is continued in water under pressure at 115°C. The significance of the steam sealing results will be discussed later.

Weight loss (mg/cm 2 ) •Or

— Water at l l ! r C . __ Steam at 115° C

\

\

Fig. 88 a. Effect of temperature of sealing on weight loss of anodic films in sodium sulphite solution sealing test 22

Water at 100°C Water at “ 80° C

20 J E 20

i 1

io

30

5 mins sealing

I 0

03

Fig. 89. Effect of time and temperature on weight gain in sealing 25 p coatings produced in 165 g/l sulphuric acid electrolyte at 21°C and 1.7 A/ dm 2 2b.

Specific sealing gain-mg/gm

30

Steam at 100°C

10

Weight loss (mg/cm 2 )

f z

Fig. 88b. Effect of time of sealing on weight loss of anodic films m sodium sulphite solution sealing test 22 Again, in this field, many workers have examined the effects of time and temperature with the general conclusion that higher temperatures and longer times are beneficial. Using the sulphite acid dissolution test, Sheasby and co-workers 22 226

o

* ................................................................. 20 30 40 50 *O

IO

Sealing time - mins

Such results tend to give rise to empirical rules for finding the duration of sealing required. In this instance one might conclude that 1 minute per micron was adequate. Further experiments involving anodizing under similar conditions but in a 360 g/l H2SO, electrolyte produced a very different set of curves and levelling 227

off did not occur until 50 - 60 minutes sealing had taken place. Other tests showed results to be further affected by anodizing temperature and film thickness.

scratch resistance than chemical resistance and there is no indication that a film ‘sealed’ in such a way would be suitable for exposure or decorative use.

In trying to summarise this and other work it must be remembered that most of the results are laboratory results obtained under conditions in which only one variable at a time is being studied, and all the other conditions are close to the optimum. In production more than one aspect of sealing practice is likely to be varying and in order to be safe many anodizers seal longer than is theoretically necessary. In practice it is useful to relate sealing time to the anodic film thickness, on the reasonable assumption that it takes longer to seal a thick film than a thin one. On this basis, sealing times of 2 - 2Vi minutes per micron are suggested for general sealing practice with a minimum time of 10 minutes for very thin films. However, particular customer or specification requirements may vary and sealing conditions will have to be modified to meet these. On the continent, for example, some countries set architectural sealing specifications at levels which can require 3 - 4 minutes per micron for sealing times. The same is generally true of sealing water temperature. A minimum temperature of 95° - 96°C is essential for good quality sealing, and the best advice is to use as high a temperature as possible, without the water actually boiling. It must be remembered, however, that a large load of cold anodized components placed in the seal baths can drop the temperature sharply, and the heating available should be such that the required temperature is reached again as quickly as possible.

The evidence for any superiority between steam and water sealing is inconclusive, and there appear to have been few controlled tests in which steam and water have been carefully compared under optimum conditions for each, particularly at temperatures over 100°C. The judgement on steam versus water sealing must therefore be made on practical grounds which include consideration of investment economics and production costs. An efficient steam sealing unit is more expensive to build than a water sealing tank. It is essential to ensure that the tank is properly sealed when the load is being treated so that the steam pressure is maintained at the required level, usually 0.7 - 1 atmosphere (10 - 15 lb/in 2), during the operation. The supply of heat must be such that there is a very rapid build-up of temperature and pressure. Normally this should not exceed 5 minutes even on a large plant and should preferably be 2 - 3 minutes on typical batch anodizing plants. Steam sealing presents considerable engineering difficulties on a normal return-type automatic plant and does not appear to have been used on such plants. With programmed automatic in-line plants operating on a batch principle it appears technically feasible to use steam sealing, although it could prove costly in terms of plant.

Steam sealing

Work in Japan' on oxalic acid coatings resulted in the use of sealing in steam under pressure. A later patent included reference to subjecting the sealing chamber to the action of a vacuum before allowing the steam to enter. Subsequent experience has shown that this is not necessary, but the temperature (and corresponding pressure) of the steam are important. The data given in Fig. 89 show a low weight gain for sealing in steam at 100°C. This was due to the construction of the sealing chamber which was not fully closed and resulted in wet steam condensing on the surface. Spooner 29 obtained better results with steam at 1 atmosphere (15 lb/in 2), assessed by measurement of cathodic current passing when the sealed specimen was made cathodic in a 2 per cent (volume) nitric acid solution for 3 minutes. Brace 20 found increased weight gain on panels steam sealed at 115 °C compared with 100°C, but found that identical panels placed in water at the same temperature gave higher values and were more difficult to strip. Lenz 30 has reported a continuing improvement in corrosion resistance with increase in temperature of steam sealing up to 150°C and stressed the importance of avoiding condensation of water on the work if optimum results were to be obtained. Schmecken 31 reports impedance values which show that, in general, impedance values for water-sealed films are higher than for those that are steam-sealed. General Electric 32 suggest even higher temperatures (300 - 400 °C) for only 1 minute, but in the absence of steam. However, they seemed more concerned with 228

The main claim for steam sealing has been freedom from contamination and a greater degree of reliability. In practice, where searching test methods such as SO 2 - humidity tests, acid dissolution tests or impedance testing are used, the advantages of steam sealing are by no means clear-cut. Design and correct operation of the plant are very important and can give rise to operational problems. Although the use of steam avoids problems such as effects of silica in dissolved water it can bring its own problems in that, if the water remaining on the work after rinsing contains some sulphuric acid, this can have significant vapour pressure at sealing temperatures and can interfere with sealing. Another problem is that steam condenses on the flight bar, splines and upper sections on the load, and the water dripping down onto lower sections can cause serious staining. To avoid this, in some architectural plants in the Far East, work is removed from the jigs before sealing and placed on suitable stillages which are then placed in the empty steam sealing chamber. When the chamber is full it is closed and steam under pressure is fed into it. In this way the work of a whole shift can be sealed in one operation without the necessity of opening and refilling a vessel with steam each time a load is processed. It is said to be economical in operation but the plant must obviously be designed to operate in this way, and great care must be taken in handling and stacking the work before sealing in order to avoid staining. Sealing smut formation and anti-smutting additives

Early in this chapter mention was made of the formation of sealing smut on the surface of the anodized aluminium. This smut formation is a normal part of the sealing process and it has been shown to be largely bohmite. It is visible as an 229

off did not occur until 50 - 60 minutes sealing had taken place. Other tests showed results to be further affected by anodizing temperature and film thickness.

scratch resistance than chemical resistance and there is no indication that a film ‘sealed’ in such a way would be suitable for exposure or decorative use.

In trying to summarise this and other work it must be remembered that most of the results are laboratory results obtained under conditions in which only one variable at a time is being studied, and all the other conditions are close to the optimum. In production more than one aspect of sealing practice is likely to be varying and in order to be safe many anodizers seal longer than is theoretically necessary. In practice it is useful to relate sealing time to the anodic film thickness, on the reasonable assumption that it takes longer to seal a thick film than a thin one. On this basis, sealing times of 2 - 2Vi minutes per micron are suggested for general sealing practice with a minimum time of 10 minutes for very thin films. However, particular customer or specification requirements may vary and sealing conditions will have to be modified to meet these. On the continent, for example, some countries set architectural sealing specifications at levels which can require 3 - 4 minutes per micron for sealing times. The same is generally true of sealing water temperature. A minimum temperature of 95° - 96°C is essential for good quality sealing, and the best advice is to use as high a temperature as possible, without the water actually boiling. It must be remembered, however, that a large load of cold anodized components placed in the seal baths can drop the temperature sharply, and the heating available should be such that the required temperature is reached again as quickly as possible.

The evidence for any superiority between steam and water sealing is inconclusive, and there appear to have been few controlled tests in which steam and water have been carefully compared under optimum conditions for each, particularly at temperatures over 100°C. The judgement on steam versus water sealing must therefore be made on practical grounds which include consideration of investment economics and production costs. An efficient steam sealing unit is more expensive to build than a water sealing tank. It is essential to ensure that the tank is properly sealed when the load is being treated so that the steam pressure is maintained at the required level, usually 0.7 - 1 atmosphere (10 - 15 lb/in 2), during the operation. The supply of heat must be such that there is a very rapid build-up of temperature and pressure. Normally this should not exceed 5 minutes even on a large plant and should preferably be 2 - 3 minutes on typical batch anodizing plants. Steam sealing presents considerable engineering difficulties on a normal return-type automatic plant and does not appear to have been used on such plants. With programmed automatic in-line plants operating on a batch principle it appears technically feasible to use steam sealing, although it could prove costly in terms of plant.

Steam sealing

Work in Japan' on oxalic acid coatings resulted in the use of sealing in steam under pressure. A later patent included reference to subjecting the sealing chamber to the action of a vacuum before allowing the steam to enter. Subsequent experience has shown that this is not necessary, but the temperature (and corresponding pressure) of the steam are important. The data given in Fig. 89 show a low weight gain for sealing in steam at 100°C. This was due to the construction of the sealing chamber which was not fully closed and resulted in wet steam condensing on the surface. Spooner 29 obtained better results with steam at 1 atmosphere (15 lb/in 2), assessed by measurement of cathodic current passing when the sealed specimen was made cathodic in a 2 per cent (volume) nitric acid solution for 3 minutes. Brace 20 found increased weight gain on panels steam sealed at 115 °C compared with 100°C, but found that identical panels placed in water at the same temperature gave higher values and were more difficult to strip. Lenz 30 has reported a continuing improvement in corrosion resistance with increase in temperature of steam sealing up to 150°C and stressed the importance of avoiding condensation of water on the work if optimum results were to be obtained. Schmecken 31 reports impedance values which show that, in general, impedance values for water-sealed films are higher than for those that are steam-sealed. General Electric 32 suggest even higher temperatures (300 - 400 °C) for only 1 minute, but in the absence of steam. However, they seemed more concerned with 228

The main claim for steam sealing has been freedom from contamination and a greater degree of reliability. In practice, where searching test methods such as SO 2 - humidity tests, acid dissolution tests or impedance testing are used, the advantages of steam sealing are by no means clear-cut. Design and correct operation of the plant are very important and can give rise to operational problems. Although the use of steam avoids problems such as effects of silica in dissolved water it can bring its own problems in that, if the water remaining on the work after rinsing contains some sulphuric acid, this can have significant vapour pressure at sealing temperatures and can interfere with sealing. Another problem is that steam condenses on the flight bar, splines and upper sections on the load, and the water dripping down onto lower sections can cause serious staining. To avoid this, in some architectural plants in the Far East, work is removed from the jigs before sealing and placed on suitable stillages which are then placed in the empty steam sealing chamber. When the chamber is full it is closed and steam under pressure is fed into it. In this way the work of a whole shift can be sealed in one operation without the necessity of opening and refilling a vessel with steam each time a load is processed. It is said to be economical in operation but the plant must obviously be designed to operate in this way, and great care must be taken in handling and stacking the work before sealing in order to avoid staining. Sealing smut formation and anti-smutting additives

Early in this chapter mention was made of the formation of sealing smut on the surface of the anodized aluminium. This smut formation is a normal part of the sealing process and it has been shown to be largely bohmite. It is visible as an 229

uneven chalky or powdery deposit which is most easily seen on colour anodized work or bright anodized finishes. It occurs to varying extents with all normal sealing practices and its absence is often an indication of a fault in the sealing process. However, it adversely affects the appearance of the anodized work and must usually be removed before sending the work to a customer. In the past this has always been done by hand wiping the work with such things as a 5% lanoline solution in white spirit or fine pumice powder and water. However, with the increasing cost of labour, anodizers have been looking for chemical ways of eliminating smut formation.

that much stricter control of sealing parameters must be exercised. The additives are usually difficult to determine analytically and, as more has to be added during use, it is easy to build up an undesirable level in the seal bath. Added to this is the fact that certain seal tests, notably admittance and impedance tests, are not sensitive to excessive amounts of most anti-smutting additives, so that they give no indication of seal quality problems. It is therefore important to use dye spot or acid dissolution tests for checking seal quality whenever additives of this type are being used, and in general it is advisable to use the minimum possible addition to obtain the appearance desired.

There are two broad approaches to this problem; either smut formation of a particular type is created during sealing and is then dissolved in an acid solution afterwards, or additives which inhibit the formation of smut are added to the sealing solution itself. Both these approaches have their problems, and it is fair to say that sealing practices become generally more critical with either method, if good seal quality is to be achieved.

The simplest example of the effect of additives is that of phosphate. Very small additions (less than 3 ppm) will effectively prevent smut formation, but it has already been shown that amounts above this seriously inhibit sealing. As this balance changes with age of the sealing bath it is almost impossible to maintain, and some plants have produced very clean surfaces at the cost of almost total loss of sealing! Many proprietary sealing additives are, however, available and these are of many chemical types, including acrylic acid 3'1, dextrin 37, sulphonic acids 38 39, hydroxy-carboxylic acids 40 and phosphonic acids 41. Some of these additives are used in conjunction with hydrolysable metal salts such as nickel acetate or nickel sulphate. It is difficult to know which of these are most effective or even to know which are present in most proprietary formulations, but Friedemann and colleagues have published useful information on some additives 42 43 44 .

The basic difficulty is that smut is bohmite, and it has already been indicated that this represents the completion of the sealing reaction at least at the surface of the film. This ‘surface sealing’ is very important, as it represents the main chemical resistance of the film. If bohmite formation is seriously prevented within the pores, the possibility of chemical attack will increase, acid dissolution and dye drop test results will be unsatisfactory and weathering performance of the material may be adversely affected. Mineral acid dissolution of smut has been mainly examined by Alcoa workers and patents suggest nitric acid as the most suitable medium. Baker 33 has shown in an extensive investigation that the smut formed in a normal boiling water seal does not always dissolve very readily in nitric acid, but that produced by nickel acetate sealing does, so a system of sealing first in 5 g/1 nickel acetate solution and then dipping for 3 minutes in 40% by volume nitric acid is suggested 34. An alternative is provided by first sealing in a 1 - 10 ml/1 triethanolamine solution and then nitric acid dipping 35. Both these methods give good appearance and good quality if the initial film and sealing quality are good, but significant loss of film can occur in the nitric acid if sealing quality is poor, or if the anodic film is at all soft. The practice is therefore more readily applicable to integral colour coatings than to normal sulphuric acid films. It cannot, of course, be used with dyed work, where significant loss of colour can take place in the nitric acid dip. The alternative approach, of adding chemicals to the sealing solution to prevent smut formation, has been widely investigated, but it must again be realised that most additives are inhibiting bohmite formation as well as providing a smut-free appearance, so some degree of seal quality is usually sacrificed. There is therefore a delicate balance between adequate smut elimination and achievement of adequate seal quality, and too much additive will, in most cases, impair seal quality and too little will allow some smut to form. In practice this usually means 230

Generally it is not, of course, possible to use anti-smutting additives with steam sealing operations, but the acrylic acid type of additive can be used in a hot rinse prior to steam sealing. Whilst this produces some reduction in smut level, it is usually less effective than additions in the hot water seal itself. Sealing in metal salt solutions

A wide variety of substances has been proposed for sealing anodic oxide coatings, such as solutions of various metal salts which, when absorbed into the anodic oxide coating, are hydrolysed and precipitated into the pores as hydroxides 4? 46 47 . Patent literature refers to the use of acetates of nickel, cobalt, cadmium, chromium, zinc, copper, aluminium and lead as well as to sulphates, fluorides, chlorides, nitrates, oxalates, citrates, tartrates and sulphonates of these elements, and these and other additions have been extensively reviewed byKape 4*. Probably the most widely used addition has been nickel acetate, sometimes also in conjunction with cobalt acetate, but nickel sulphate is apparently widely used in Italy. Concentrations ranging from 0.01 per cent to 2.0 per cent have been mentioned in the literature, and there have also been some comparisons made betwen nickel acetate and nickel sulphate 49 50 . The pH range recommended is 5.0 - 6.5 with the range pH 5.5 - 5.8 being preferred. Additions of boric acid up to 1 per cent to buffer the bath around these values have also been recommended. Considerable confusion has existed as to the role of these substances in sealing. 231

uneven chalky or powdery deposit which is most easily seen on colour anodized work or bright anodized finishes. It occurs to varying extents with all normal sealing practices and its absence is often an indication of a fault in the sealing process. However, it adversely affects the appearance of the anodized work and must usually be removed before sending the work to a customer. In the past this has always been done by hand wiping the work with such things as a 5% lanoline solution in white spirit or fine pumice powder and water. However, with the increasing cost of labour, anodizers have been looking for chemical ways of eliminating smut formation.

that much stricter control of sealing parameters must be exercised. The additives are usually difficult to determine analytically and, as more has to be added during use, it is easy to build up an undesirable level in the seal bath. Added to this is the fact that certain seal tests, notably admittance and impedance tests, are not sensitive to excessive amounts of most anti-smutting additives, so that they give no indication of seal quality problems. It is therefore important to use dye spot or acid dissolution tests for checking seal quality whenever additives of this type are being used, and in general it is advisable to use the minimum possible addition to obtain the appearance desired.

There are two broad approaches to this problem; either smut formation of a particular type is created during sealing and is then dissolved in an acid solution afterwards, or additives which inhibit the formation of smut are added to the sealing solution itself. Both these approaches have their problems, and it is fair to say that sealing practices become generally more critical with either method, if good seal quality is to be achieved.

The simplest example of the effect of additives is that of phosphate. Very small additions (less than 3 ppm) will effectively prevent smut formation, but it has already been shown that amounts above this seriously inhibit sealing. As this balance changes with age of the sealing bath it is almost impossible to maintain, and some plants have produced very clean surfaces at the cost of almost total loss of sealing! Many proprietary sealing additives are, however, available and these are of many chemical types, including acrylic acid 3'1, dextrin 37, sulphonic acids 38 39, hydroxy-carboxylic acids 40 and phosphonic acids 41. Some of these additives are used in conjunction with hydrolysable metal salts such as nickel acetate or nickel sulphate. It is difficult to know which of these are most effective or even to know which are present in most proprietary formulations, but Friedemann and colleagues have published useful information on some additives 42 43 44 .

The basic difficulty is that smut is bohmite, and it has already been indicated that this represents the completion of the sealing reaction at least at the surface of the film. This ‘surface sealing’ is very important, as it represents the main chemical resistance of the film. If bohmite formation is seriously prevented within the pores, the possibility of chemical attack will increase, acid dissolution and dye drop test results will be unsatisfactory and weathering performance of the material may be adversely affected. Mineral acid dissolution of smut has been mainly examined by Alcoa workers and patents suggest nitric acid as the most suitable medium. Baker 33 has shown in an extensive investigation that the smut formed in a normal boiling water seal does not always dissolve very readily in nitric acid, but that produced by nickel acetate sealing does, so a system of sealing first in 5 g/1 nickel acetate solution and then dipping for 3 minutes in 40% by volume nitric acid is suggested 34. An alternative is provided by first sealing in a 1 - 10 ml/1 triethanolamine solution and then nitric acid dipping 35. Both these methods give good appearance and good quality if the initial film and sealing quality are good, but significant loss of film can occur in the nitric acid if sealing quality is poor, or if the anodic film is at all soft. The practice is therefore more readily applicable to integral colour coatings than to normal sulphuric acid films. It cannot, of course, be used with dyed work, where significant loss of colour can take place in the nitric acid dip. The alternative approach, of adding chemicals to the sealing solution to prevent smut formation, has been widely investigated, but it must again be realised that most additives are inhibiting bohmite formation as well as providing a smut-free appearance, so some degree of seal quality is usually sacrificed. There is therefore a delicate balance between adequate smut elimination and achievement of adequate seal quality, and too much additive will, in most cases, impair seal quality and too little will allow some smut to form. In practice this usually means 230

Generally it is not, of course, possible to use anti-smutting additives with steam sealing operations, but the acrylic acid type of additive can be used in a hot rinse prior to steam sealing. Whilst this produces some reduction in smut level, it is usually less effective than additions in the hot water seal itself. Sealing in metal salt solutions

A wide variety of substances has been proposed for sealing anodic oxide coatings, such as solutions of various metal salts which, when absorbed into the anodic oxide coating, are hydrolysed and precipitated into the pores as hydroxides 4? 46 47 . Patent literature refers to the use of acetates of nickel, cobalt, cadmium, chromium, zinc, copper, aluminium and lead as well as to sulphates, fluorides, chlorides, nitrates, oxalates, citrates, tartrates and sulphonates of these elements, and these and other additions have been extensively reviewed byKape 4*. Probably the most widely used addition has been nickel acetate, sometimes also in conjunction with cobalt acetate, but nickel sulphate is apparently widely used in Italy. Concentrations ranging from 0.01 per cent to 2.0 per cent have been mentioned in the literature, and there have also been some comparisons made betwen nickel acetate and nickel sulphate 49 50 . The pH range recommended is 5.0 - 6.5 with the range pH 5.5 - 5.8 being preferred. Additions of boric acid up to 1 per cent to buffer the bath around these values have also been recommended. Considerable confusion has existed as to the role of these substances in sealing. 231

It is well known that nickel, cobalt, etc., can be detected in the pores of anodic oxide coatings sealed in solutions of their acetates or sulphates. In such solutions therefore, two processes are proceeding simultaneously, sealing by the formation of bohmite and plugging of the pores by precipitation of a metallic hydroxide. Nickel acetate The main advantage of nickel acetate is found in the sealing of dyed anodic coatings. It is well known that some dyestuffs tend to leach-out badly in the seal, and the addition of nickel acetate is often a satisfactory means of avoiding this. It is also considered by Spieser*1 that sealing in nickel acetate improves light fastness by forming co-valent bonds with the dyestuff where suitable groups exist. The precipitation of the nickel hydroxide can result in a heavy whitish smut on the surface of the dyed work, and this can be removed by lightly mopping with a clean mop, dressed with a little Tripoli compound, in order to obtain a good appearance. It is preferable for the dyed work to be first ‘fixed’ by immersion for 1 - 5 minutes in a 0.5% (weight) nickel acetate, 0.5% (weight) boric acid solution at 75° 80°C, and then sealed in hot water in the usual way. Smut formation is then apparently less severe than with nickel salt sealing. As in the case of water sealing the use of organic additives, such as lignosulphonates or sulphonic acids, has been suggested for eliminating smut52 5354 but the same problems of seal quality and control occur. Probably the greatest benefit of nickel salt sealing is that it can make the process less sensitive to the common sealing variables, and, in some cases, as Spooner has shown 55, it can make use of tap water, rather than deionised water, possible for sealing. On the other hand, experiences of exposure testing of films sealed in nickel acetate or sulphate indicate that their primary drawback is their effect on surface appearance during weathering. Compared with water- or steam-sealed specimens, those sealed in nickel salt solutions develop a chalky surface deposit after a short period of weathering, and although it can be largely removed by cleaning with a suitable lightly abrasive cleaner, the ‘chalk’ soon reappears. Nickel salt-sealed films tend eventually to break down and show numerous fine pin-points of corrosion whereas water- or steam-sealed films show few pits which, however, tend to be larger. Chromate sealing Sealing in chromate or dichromate solutions was the subject of early patents 56 57 and this treatment was claimed to provide good corrosion inhibition, particularly when used on anodized aluminium-copper alloys, but it imparts a characteristic yellow colouration to the coating. The pricipal contribution to knowledge of the mechanism of chromate sealing is that of Tomashov and Tyukina 58. They concluded that an essential feature was the formation of either aluminium oxydichromate: OA1.OH + KHCrO, = OAl.HCrO4 + KOH 232

or aluminium oxychromate: OA1.OH OAI.OH + K H C r 0
CrO

4

+ KOH + H 2O

The oxydichromate is presumably formed at lower pH values and the oxychromate in the more alkaline solutions. A decrease in the pH causes the equilibrium to be shifted and favours formation of oxydichromate whilst leaching occurs when the oxydichromate or oxychromate is hydrolysed. It appears, however, that simultaneously with oxychromate formation normal hydration to bohmite also proceeds. These investigations showed that optimum sealing, expressed in terms of weight gain, is produced in a solution whose pH lies between 6.32 and 6.64, with complete sealing being achieved in 10 minutes. These conditions also give maximum corrosion resistance. At higher pH values sealing proceeds more rapidly but at a pH value of 8.5 film dissolution occurs. From the results of this work two solutions were recommended: (1 ) For highest corrosion resistance K2 Cr2 O 7 NaOH pH Temperature

100 g/1 13 g/1 6.0 -7.0 94°-98°C

(2) For reduced chromate consumption K 2Cr 2O 7 NaOH pH Temperature

15g/l 3 g/1 6.5 -7.5 94° - 98°C

The corrosion inhibiting properties of chromate are well known, but chromate sealing apparently also has less effect on the mechanical properties of anodized aluminium than water sealing. Chromate sealing is therefore generally recommended for aluminium in military or aeronautical use and is the main sealing method called for in DEF 151 59. Other aqueous sealants A number of other aqueous sealants have been mentioned in the literature and been reviewed by Wood 60 and Kape 48. Sodium silicate solutions were the subject of early patents 56 57 and were investigated in detail by Whitby He found that the optimum ratio was Na 2O : SiO2 = 1 : 3.3, and that silicates having a ratio less than 1 : 2.9 were unsuitable because of their high alkalinity. He concluded that aluminium silicate was formed by the action of precipitated silicic acid on the anodic coating or by double decomposition of sodium silicate with aluminium sulphate in a sulphuric acid anodic coating. This latter mechanism, in the light of current knowledge, seems unlikely since there is little evidence for free aluminium sulphate 233

It is well known that nickel, cobalt, etc., can be detected in the pores of anodic oxide coatings sealed in solutions of their acetates or sulphates. In such solutions therefore, two processes are proceeding simultaneously, sealing by the formation of bohmite and plugging of the pores by precipitation of a metallic hydroxide. Nickel acetate The main advantage of nickel acetate is found in the sealing of dyed anodic coatings. It is well known that some dyestuffs tend to leach-out badly in the seal, and the addition of nickel acetate is often a satisfactory means of avoiding this. It is also considered by Spieser*1 that sealing in nickel acetate improves light fastness by forming co-valent bonds with the dyestuff where suitable groups exist. The precipitation of the nickel hydroxide can result in a heavy whitish smut on the surface of the dyed work, and this can be removed by lightly mopping with a clean mop, dressed with a little Tripoli compound, in order to obtain a good appearance. It is preferable for the dyed work to be first ‘fixed’ by immersion for 1 - 5 minutes in a 0.5% (weight) nickel acetate, 0.5% (weight) boric acid solution at 75° 80°C, and then sealed in hot water in the usual way. Smut formation is then apparently less severe than with nickel salt sealing. As in the case of water sealing the use of organic additives, such as lignosulphonates or sulphonic acids, has been suggested for eliminating smut52 5354 but the same problems of seal quality and control occur. Probably the greatest benefit of nickel salt sealing is that it can make the process less sensitive to the common sealing variables, and, in some cases, as Spooner has shown 55, it can make use of tap water, rather than deionised water, possible for sealing. On the other hand, experiences of exposure testing of films sealed in nickel acetate or sulphate indicate that their primary drawback is their effect on surface appearance during weathering. Compared with water- or steam-sealed specimens, those sealed in nickel salt solutions develop a chalky surface deposit after a short period of weathering, and although it can be largely removed by cleaning with a suitable lightly abrasive cleaner, the ‘chalk’ soon reappears. Nickel salt-sealed films tend eventually to break down and show numerous fine pin-points of corrosion whereas water- or steam-sealed films show few pits which, however, tend to be larger. Chromate sealing Sealing in chromate or dichromate solutions was the subject of early patents 56 57 and this treatment was claimed to provide good corrosion inhibition, particularly when used on anodized aluminium-copper alloys, but it imparts a characteristic yellow colouration to the coating. The pricipal contribution to knowledge of the mechanism of chromate sealing is that of Tomashov and Tyukina 58. They concluded that an essential feature was the formation of either aluminium oxydichromate: OA1.OH + KHCrO, = OAl.HCrO4 + KOH 232

or aluminium oxychromate: OA1.OH OAI.OH + K H C r 0
CrO

4

+ KOH + H 2O

The oxydichromate is presumably formed at lower pH values and the oxychromate in the more alkaline solutions. A decrease in the pH causes the equilibrium to be shifted and favours formation of oxydichromate whilst leaching occurs when the oxydichromate or oxychromate is hydrolysed. It appears, however, that simultaneously with oxychromate formation normal hydration to bohmite also proceeds. These investigations showed that optimum sealing, expressed in terms of weight gain, is produced in a solution whose pH lies between 6.32 and 6.64, with complete sealing being achieved in 10 minutes. These conditions also give maximum corrosion resistance. At higher pH values sealing proceeds more rapidly but at a pH value of 8.5 film dissolution occurs. From the results of this work two solutions were recommended: (1 ) For highest corrosion resistance K2 Cr2 O 7 NaOH pH Temperature

100 g/1 13 g/1 6.0 -7.0 94°-98°C

(2) For reduced chromate consumption K 2Cr 2O 7 NaOH pH Temperature

15g/l 3 g/1 6.5 -7.5 94° - 98°C

The corrosion inhibiting properties of chromate are well known, but chromate sealing apparently also has less effect on the mechanical properties of anodized aluminium than water sealing. Chromate sealing is therefore generally recommended for aluminium in military or aeronautical use and is the main sealing method called for in DEF 151 59. Other aqueous sealants A number of other aqueous sealants have been mentioned in the literature and been reviewed by Wood 60 and Kape 48. Sodium silicate solutions were the subject of early patents 56 57 and were investigated in detail by Whitby He found that the optimum ratio was Na 2O : SiO2 = 1 : 3.3, and that silicates having a ratio less than 1 : 2.9 were unsuitable because of their high alkalinity. He concluded that aluminium silicate was formed by the action of precipitated silicic acid on the anodic coating or by double decomposition of sodium silicate with aluminium sulphate in a sulphuric acid anodic coating. This latter mechanism, in the light of current knowledge, seems unlikely since there is little evidence for free aluminium sulphate 233

existing in any quantity in the anodic coating. Schenk has given a figure of 1 : 3.86 as the optimum Na,0 : SiO2 ratio. Whilst coatings sealed in this way have a higher alkali resistance than normal, Sacchi b2 has reported that they give poor weathering performance. A later British patent 63 has sought to combine the advantages of nickel salt sealing with the high alkali resistance of silicate-sealed films in that the work is first partially sealed in a nickel salt solution and sealing is completed in an alkali metal silicate solution. Practical experience shows that the high alkalinity of the silicate seal gives problems with staining and partial attack of the film when articles are being removed from the seal prior to a final hot rinse. Although limited tests indicate some value for such treatment where contact with alkali washing powder solutions is involved, exposure to the atmosphere, particularly urban atmospheres, produces an unsightly white film on the surface. At best silicate sealing might be useful on applications such as washing machine lids, but otherwise its value is doubtful. Spooner has shown that molybdate solutions confer a measure of improved corrosion resistance as judged by acetic acid-salt spray resistance, although corrosion resistance in industrial atmospheres was not enhanced as greatly as was indicated by the accelerated test. He has also subsequently shown 1,4 that sealing in water for 3 - 15 minutes followed by 5 minutes in a boiling 5 g/1 disodium hydrogen phosphate (Na ? HPO4. 7H2O) solution gave enhanced acetic acid-salt spray resistance, but that phosphate sealing alone severely lowered the degree of sealing and the corrosion resistance of the anodic film. The detrimental effect of phosphate is confirmed by Hunter and many other workers?5 though Amore and Murphy 66 surprisingly reported no adverse effects. Other double sealing treatments have been suggested but few have been used industrially, though more recently Lockheed 6" have developed a system based on an initial short treatment in a hot sodium tungstate/chromic acid solution followed by nickel acetate sealing. This sealing method is claimed to give improved corrosion resistance of the coating whilst still maintaining an essentially clear finish. With the current emphasis on low energy consumption, patents are now appearing on low temperature sealing systems, something that has long been a desirable goal. Alcoa suggest use of ammonia vapour under pressure for this purpose 68. This is claimed not only to give a smut-free surface, but also to be capable of room temperature sealing. Sealing times of several hours under these conditions are necessary but at a temperature of about 50°C only 30 minutes is required. The equipment for such a process is obviously closer to steam sealing equipment than water sealing, as a closed, pressurised vessel is needed and the ammonia then has to be extracted before the vessel is opened. More easily utilised is Alcoa’s later development 69 using a solution containing 3 - 6 g/1 of a hydrolysable metal salt (e.g. nickel acetate), 8 - 12 ml/1 of an ethanolamine (e.g. triethanol234

amine) and 1000 - 1500 mg/1 of soluble sulphate (e.g. sulphuric acid). This bath is used at a pH of 6.8 - 7.2 for 30 minutes at a temperature of about 70°C. Sealing smut is formed in this treatment and this is removed by a dip in a mineral acid such as nitric acid. Running costs of about 50% of those for normal hot water sealing are claimed. Non-aqueous sealants

Where resistance to outdoor weathering is involved, no sealing treatment has been found which is as good as that obtained from aqueous sealants, but the pores of the film can also be partially or completely filled with organic materials and this is sometimes required for special applications. It has been established 70 that there is some loss of infra-red reflectivity of bright anodized aluminium when sealed in water, and sealing in wax has been found to overcome this. Ensor 71 recommends heating the anodized aluminium to 160°C and then dipping the hot article into a mixture consisting of: stearic acid oleic acid turpentine butanol petrol

5 parts 3 parts 5 parts 1 part 1 part

The stearic acid should first be mixed into the oleic acid and then the solvents added. Another technique reported 72 is to neutralise the anodized work in 2 per cent sodium carbonate solution, rinse, dry and then immerse the work for 2 minutes in petroleum jelly (vaseline) heated to 160°C. Any surplus grease is subsequently wiped off. Hot lanolin or a lanolin-white spirit mixture has also been used. An American research worker has reported 73 superior corrosion resistance (judged by resistance to corrosion in salt spray) of anodized specimens, coupled to copper discs, which had been sealed for 5 minutes in boiling water containing 5 ml/1 alkylaryl polyethylene glycol followed by sealing in 5 per cent sodium dichromate solution, as compared with simple dichromate sealing. He also reported that sealing in suspensions of tetrafuorethylene, wax or diglycol stearate significantly reduced the coefficient of friction of the anodized surface. Silicone-containing waxes have a similar effect and are used for treatment of refrigerator trays for ice cubes. Another possibility which has been explored is to form an organic polymer in the pores of the anodic coating by exposing it to suitable mixtures of vapours above 95°C. Mixtures of phenol and urea acetaldehyde, phthalic anhydride and glycerol, styrene and furfuryl alcohol have been quoted as combinations suitable for this technique 4. Another means proposed of producing an organic polymer coating 75 is to immerse the anodized work in an aqueous emulsion of acrylic and methacrylic 235

existing in any quantity in the anodic coating. Schenk has given a figure of 1 : 3.86 as the optimum Na,0 : SiO2 ratio. Whilst coatings sealed in this way have a higher alkali resistance than normal, Sacchi b2 has reported that they give poor weathering performance. A later British patent 63 has sought to combine the advantages of nickel salt sealing with the high alkali resistance of silicate-sealed films in that the work is first partially sealed in a nickel salt solution and sealing is completed in an alkali metal silicate solution. Practical experience shows that the high alkalinity of the silicate seal gives problems with staining and partial attack of the film when articles are being removed from the seal prior to a final hot rinse. Although limited tests indicate some value for such treatment where contact with alkali washing powder solutions is involved, exposure to the atmosphere, particularly urban atmospheres, produces an unsightly white film on the surface. At best silicate sealing might be useful on applications such as washing machine lids, but otherwise its value is doubtful. Spooner has shown that molybdate solutions confer a measure of improved corrosion resistance as judged by acetic acid-salt spray resistance, although corrosion resistance in industrial atmospheres was not enhanced as greatly as was indicated by the accelerated test. He has also subsequently shown 1,4 that sealing in water for 3 - 15 minutes followed by 5 minutes in a boiling 5 g/1 disodium hydrogen phosphate (Na ? HPO4. 7H2O) solution gave enhanced acetic acid-salt spray resistance, but that phosphate sealing alone severely lowered the degree of sealing and the corrosion resistance of the anodic film. The detrimental effect of phosphate is confirmed by Hunter and many other workers?5 though Amore and Murphy 66 surprisingly reported no adverse effects. Other double sealing treatments have been suggested but few have been used industrially, though more recently Lockheed 6" have developed a system based on an initial short treatment in a hot sodium tungstate/chromic acid solution followed by nickel acetate sealing. This sealing method is claimed to give improved corrosion resistance of the coating whilst still maintaining an essentially clear finish. With the current emphasis on low energy consumption, patents are now appearing on low temperature sealing systems, something that has long been a desirable goal. Alcoa suggest use of ammonia vapour under pressure for this purpose 68. This is claimed not only to give a smut-free surface, but also to be capable of room temperature sealing. Sealing times of several hours under these conditions are necessary but at a temperature of about 50°C only 30 minutes is required. The equipment for such a process is obviously closer to steam sealing equipment than water sealing, as a closed, pressurised vessel is needed and the ammonia then has to be extracted before the vessel is opened. More easily utilised is Alcoa’s later development 69 using a solution containing 3 - 6 g/1 of a hydrolysable metal salt (e.g. nickel acetate), 8 - 12 ml/1 of an ethanolamine (e.g. triethanol234

amine) and 1000 - 1500 mg/1 of soluble sulphate (e.g. sulphuric acid). This bath is used at a pH of 6.8 - 7.2 for 30 minutes at a temperature of about 70°C. Sealing smut is formed in this treatment and this is removed by a dip in a mineral acid such as nitric acid. Running costs of about 50% of those for normal hot water sealing are claimed. Non-aqueous sealants

Where resistance to outdoor weathering is involved, no sealing treatment has been found which is as good as that obtained from aqueous sealants, but the pores of the film can also be partially or completely filled with organic materials and this is sometimes required for special applications. It has been established 70 that there is some loss of infra-red reflectivity of bright anodized aluminium when sealed in water, and sealing in wax has been found to overcome this. Ensor 71 recommends heating the anodized aluminium to 160°C and then dipping the hot article into a mixture consisting of: stearic acid oleic acid turpentine butanol petrol

5 parts 3 parts 5 parts 1 part 1 part

The stearic acid should first be mixed into the oleic acid and then the solvents added. Another technique reported 72 is to neutralise the anodized work in 2 per cent sodium carbonate solution, rinse, dry and then immerse the work for 2 minutes in petroleum jelly (vaseline) heated to 160°C. Any surplus grease is subsequently wiped off. Hot lanolin or a lanolin-white spirit mixture has also been used. An American research worker has reported 73 superior corrosion resistance (judged by resistance to corrosion in salt spray) of anodized specimens, coupled to copper discs, which had been sealed for 5 minutes in boiling water containing 5 ml/1 alkylaryl polyethylene glycol followed by sealing in 5 per cent sodium dichromate solution, as compared with simple dichromate sealing. He also reported that sealing in suspensions of tetrafuorethylene, wax or diglycol stearate significantly reduced the coefficient of friction of the anodized surface. Silicone-containing waxes have a similar effect and are used for treatment of refrigerator trays for ice cubes. Another possibility which has been explored is to form an organic polymer in the pores of the anodic coating by exposing it to suitable mixtures of vapours above 95°C. Mixtures of phenol and urea acetaldehyde, phthalic anhydride and glycerol, styrene and furfuryl alcohol have been quoted as combinations suitable for this technique 4. Another means proposed of producing an organic polymer coating 75 is to immerse the anodized work in an aqueous emulsion of acrylic and methacrylic 235

esters in which the emulsified phase has a particle size of 0.1 - 4 microns. Coatings anodized but unsealed and impregnated with lacquers to improve resistance to specific chemicals or foodstuffs, find limited specialised application where the corrosion resistance of normal anodized and sealed work is inadequate. In the last 5 or 6 years certain unsealed but lacquered coatings are also finding architectural use especially in Japan. Waxing or lacquering of conventionally sealed work with clear polythene waxes or lacquers of the methacrylic or cellulose aceto-butyrate type is, of course, well known, and is advantageous where protection against handling damage and staining from alkaline building materials is required. The lacquer is usually sprayed onto the sections to give a coating thickness of about 10 - 15 microns and is air dried. It is not normally removed on site but left to weather away naturally, and it can have a protective life of 5 - 6 years even in an industrial atmosphere. Lacquers must, however, be chosen with care so that they do not yellow or peel during exposure. However, in Japan, lacquering systems in which the anodized but unsealed work (clear or colour) is thoroughly rinsed and then placed in a water-based resin which is applied electrophoretically to the anodic film surface, are now very widespread 76. The clear lacquer is finally stoved in order to give it optimum weather resistance. About 10 microns of anodic film and 10 microns of lacquer are usually used and good weathering properties, and, of course, alkali resistance are claimed. Patent literature in this field is becoming very extensive and covers many combinations of anodic film and resin type, so only a few relevant specifications are quoted 77 ' 80. Probably the most widely known of these processes is Honny Chemicals ‘Honnylite’ process. With this, work is pretreated and anodized normally to a thickness of about 10 microns. It is then very thoroughly rinsed and placed in a water-based acrylic resin bath. The work is made anodic in this bath and a d.c. voltage of about 200V is applied for a period of 2 - 3 mins. A clear lacquer thickness of about 10 microns is built up and the work is finally stoved for 20 - 30 minutes at a temperature of about 180°C. Rinsing before coating is very critical as ions dragged into the resin bath can seriously affect its conductivity and it is continuously filtered, and purified when necessary by ion exchange. At present such processes are mainly used in Japan and even there are mostly used for domestic architectural applications, but this market is so large in Japan that enormous tonnages of aluminium extrusions are processed in this way. In the future, however, no doubt it will see more widespread use.

References 1. Brit. Pat. 406,988(1932) 2. Brit. Pat. 407,457(1932) 3. J . P. O’Sullivan, J . A. Hockey and G. C. Wood, Trans. Faraday Soc., 1969, 65, 535 4. E. Detombe and M. Pourbaix, Corrosion, 1958, 14, 498

236

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

D. P. Dobychin, Dopov, Akad, Nauk. Ukrain, 1955(12), 744 D. Altenpohl, Aluminium, 1955, 31, 10-14 and 62-69 H. Richaud, Proc. A.D.A. Conference on Anodizing, 1961, 181-185 U.S. Pat. 1,946,149(1931) R. C. Spooner, Nature, 1956, 178, 1113 R. C. Spooner and W. J . Forsyth, Plating, 1968, 55, 336-340, 341-344 and 463-471 T. P. Hoar and G. C. Wood, Proc. A.D.A. Conference on Anodizing, 1961, 186-200 G. C. Wood and V. J . J . Marron, Trans. Inst. Met. Finishing, 1967, 45, 17-20, 107-114 G. C. Wood and J . P. O’Sullivan, J.Electrochem. Soc., 1969, 116, 1351-1357 J . P. O’Sullivan and G. C. Wood, Trans. Inst. Met. Finishing, 1969, 47, 142-144 K. Wefers, Aluminium, 1973, 49, 553-561 and 622-624 G. E. Thompson, R. C. Furneaux and G. C. Wood, Trans. Inst. Met. Finishing, 1975, 53, 97-102 J . M. Kape, Met. Ind., 1959, 95, 115-118, 122 L. W. Owen and H. G. Cole, Trans. Inst. Met. Finishing, 1974, 52, 177-183 B. A. Scott, Electroplating and Met. Finishing, 1965, 18, 47-50 P. G. Sheasby, Electroplating and Met. Finishing, 1966, 19, 104 P. G. Sheasby and G. Bancroft, Trans. Inst. Met. Finishing, 1970, 48, 140-144 P. G. Sheasby, R. D. Guminski and T. K. Castle, Trans. Inst. Met. Finishing, 1966, 44, 50-56 H. Birtel and W. Leute, Aluminium, 1967, 43, 93-98 H. Birtel and W. Leute, Aluminium, 1969, 45, 413-418 D. H. Bradshaw, P. G. Sheasby, G. Bancroft and D. F. Hack, Trans. Inst. Met. Finishing, 1972,50, 87-94 A. W. Brace, unpublished work S. Setoh and A. Miyata, Sci. Papers Inst. Phys. Chcm. Res. (Tokyo), 1932, 17, 189 Brit. Pat. 496,436 (1938) R. C. Spooner, Proc. A.E.S., 1957, 44, 132 D. Lenz, Aluminium, 1956, 32, 126-135, 190-201 H. Schmecken, Aluminium, 1966, 42, 436-437 Brit. Pat. 1,053,216 B. R. Baker, Plating and Surface Fin., 1977, 64, 36-42 Brit. Pat. 1,350,292 U.S. Pat. 3,822,156 Brit. Pat. 1,265,424 Brit. Pat. 1,302,288 Brit. Pat. 1,368,336 Brit. Pat. 1,310,644 Brit. Pat. 1,398,589 Brit. Pat. 1,419,597 W. Friedemann, H. G. Germscheid and R. Geisler, Aluminium, 1971, 47, 245-253 W. Friedemann and H. G. Germscheid, Aluminium, 1973, 49, 300-301 W. Friedemann. Aluminium, 1974, 50, 150-153 U.S. Pat. 1,943,153(1934) U.S. Pat. 2,008,733(1933) Brit. Pat. 388,787(1931) J . M. Kape, Finishing Industries, 1977, 1, 13-20, 38-43, 49 F. Sacchi, G. Paolini and A. Prati, Electroplating and Met. Finishing, 1963, 16, 108-114,

esters in which the emulsified phase has a particle size of 0.1 - 4 microns. Coatings anodized but unsealed and impregnated with lacquers to improve resistance to specific chemicals or foodstuffs, find limited specialised application where the corrosion resistance of normal anodized and sealed work is inadequate. In the last 5 or 6 years certain unsealed but lacquered coatings are also finding architectural use especially in Japan. Waxing or lacquering of conventionally sealed work with clear polythene waxes or lacquers of the methacrylic or cellulose aceto-butyrate type is, of course, well known, and is advantageous where protection against handling damage and staining from alkaline building materials is required. The lacquer is usually sprayed onto the sections to give a coating thickness of about 10 - 15 microns and is air dried. It is not normally removed on site but left to weather away naturally, and it can have a protective life of 5 - 6 years even in an industrial atmosphere. Lacquers must, however, be chosen with care so that they do not yellow or peel during exposure. However, in Japan, lacquering systems in which the anodized but unsealed work (clear or colour) is thoroughly rinsed and then placed in a water-based resin which is applied electrophoretically to the anodic film surface, are now very widespread 76. The clear lacquer is finally stoved in order to give it optimum weather resistance. About 10 microns of anodic film and 10 microns of lacquer are usually used and good weathering properties, and, of course, alkali resistance are claimed. Patent literature in this field is becoming very extensive and covers many combinations of anodic film and resin type, so only a few relevant specifications are quoted 77 ' 80. Probably the most widely known of these processes is Honny Chemicals ‘Honnylite’ process. With this, work is pretreated and anodized normally to a thickness of about 10 microns. It is then very thoroughly rinsed and placed in a water-based acrylic resin bath. The work is made anodic in this bath and a d.c. voltage of about 200V is applied for a period of 2 - 3 mins. A clear lacquer thickness of about 10 microns is built up and the work is finally stoved for 20 - 30 minutes at a temperature of about 180°C. Rinsing before coating is very critical as ions dragged into the resin bath can seriously affect its conductivity and it is continuously filtered, and purified when necessary by ion exchange. At present such processes are mainly used in Japan and even there are mostly used for domestic architectural applications, but this market is so large in Japan that enormous tonnages of aluminium extrusions are processed in this way. In the future, however, no doubt it will see more widespread use.

References 1. Brit. Pat. 406,988(1932) 2. Brit. Pat. 407,457(1932) 3. J . P. O’Sullivan, J . A. Hockey and G. C. Wood, Trans. Faraday Soc., 1969, 65, 535 4. E. Detombe and M. Pourbaix, Corrosion, 1958, 14, 498

236

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

D. P. Dobychin, Dopov, Akad, Nauk. Ukrain, 1955(12), 744 D. Altenpohl, Aluminium, 1955, 31, 10-14 and 62-69 H. Richaud, Proc. A.D.A. Conference on Anodizing, 1961, 181-185 U.S. Pat. 1,946,149(1931) R. C. Spooner, Nature, 1956, 178, 1113 R. C. Spooner and W. J . Forsyth, Plating, 1968, 55, 336-340, 341-344 and 463-471 T. P. Hoar and G. C. Wood, Proc. A.D.A. Conference on Anodizing, 1961, 186-200 G. C. Wood and V. J . J . Marron, Trans. Inst. Met. Finishing, 1967, 45, 17-20, 107-114 G. C. Wood and J . P. O’Sullivan, J.Electrochem. Soc., 1969, 116, 1351-1357 J . P. O’Sullivan and G. C. Wood, Trans. Inst. Met. Finishing, 1969, 47, 142-144 K. Wefers, Aluminium, 1973, 49, 553-561 and 622-624 G. E. Thompson, R. C. Furneaux and G. C. Wood, Trans. Inst. Met. Finishing, 1975, 53, 97-102 J . M. Kape, Met. Ind., 1959, 95, 115-118, 122 L. W. Owen and H. G. Cole, Trans. Inst. Met. Finishing, 1974, 52, 177-183 B. A. Scott, Electroplating and Met. Finishing, 1965, 18, 47-50 P. G. Sheasby, Electroplating and Met. Finishing, 1966, 19, 104 P. G. Sheasby and G. Bancroft, Trans. Inst. Met. Finishing, 1970, 48, 140-144 P. G. Sheasby, R. D. Guminski and T. K. Castle, Trans. Inst. Met. Finishing, 1966, 44, 50-56 H. Birtel and W. Leute, Aluminium, 1967, 43, 93-98 H. Birtel and W. Leute, Aluminium, 1969, 45, 413-418 D. H. Bradshaw, P. G. Sheasby, G. Bancroft and D. F. Hack, Trans. Inst. Met. Finishing, 1972,50, 87-94 A. W. Brace, unpublished work S. Setoh and A. Miyata, Sci. Papers Inst. Phys. Chcm. Res. (Tokyo), 1932, 17, 189 Brit. Pat. 496,436 (1938) R. C. Spooner, Proc. A.E.S., 1957, 44, 132 D. Lenz, Aluminium, 1956, 32, 126-135, 190-201 H. Schmecken, Aluminium, 1966, 42, 436-437 Brit. Pat. 1,053,216 B. R. Baker, Plating and Surface Fin., 1977, 64, 36-42 Brit. Pat. 1,350,292 U.S. Pat. 3,822,156 Brit. Pat. 1,265,424 Brit. Pat. 1,302,288 Brit. Pat. 1,368,336 Brit. Pat. 1,310,644 Brit. Pat. 1,398,589 Brit. Pat. 1,419,597 W. Friedemann, H. G. Germscheid and R. Geisler, Aluminium, 1971, 47, 245-253 W. Friedemann and H. G. Germscheid, Aluminium, 1973, 49, 300-301 W. Friedemann. Aluminium, 1974, 50, 150-153 U.S. Pat. 1,943,153(1934) U.S. Pat. 2,008,733(1933) Brit. Pat. 388,787(1931) J . M. Kape, Finishing Industries, 1977, 1, 13-20, 38-43, 49 F. Sacchi, G. Paolini and A. Prati, Electroplating and Met. Finishing, 1963, 16, 108-114,

50. 51. 52. 53. 54 . 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80.

146-149, 187-189 L. Campanella, Trattamenti e Finitura, 1964, 4, 65 C. Th. Speiser, Aluminium, 1955, 31, 8-9 U.S. Pat. 2,755,239(1959) U.S. Pat. 2,888,388 (1952) French Pat . 898, 158(1 943 ) R. C. Spooner, Metal Finishing, 66, (12), 44-49, 53 and 67, (1 ), 80-83, 91 U.S. Pat. 1,946,152(1934) Brit. Pat. 393,996 (1931) N. D. Tomashov and A. Tyukina, Light Metals, 1946, 9, 22-35 Defence Specification DEF- 151, August 1972 G. C. Wood, Trans. Inst. Met. Finishing, 1959, 36, 220-229 L. Whitby, Metal Ind., 1948, 72, 400-403 F. Sacchi, Trans. Inst. Met. Finishing, 1964, 41, 182-189 Brit. Pat. 965,837(1964) R. C. Spooner, Chemistry in Canada, April 1960 M. S. Hunter, P. F. Towner and D. L. Robinson, Proc. A.E.S., 1959, 46, 220-225 C. J . Amore and J . F. Murphy, Metal Finishing, 1965, 63, (11 ), 50-55, 60 Brit. Pat. 1,428,048 U.S. Pat. 4,031,275 U.S. Pat. 4,045,599 A. G. C. Gwyer and N. D. Pullen, Metal Industry, 1940, 56, 7-10, 33-35 A. H. Ensor, Production Engineering, 1943(12), 36 W. Briese, Metallwaren Industrie und Galvanotechnik, 1954, 45, (8), 373 F. Pearlstein, Metal Finishing, 1960, 58, (8), 40-43 U.S. Pat. 2,662,034(1950) French Pat. 1,191,839(1958) J . Patrie, Revue de L’Aluminium, 1976, 448, 77-88 Brit. Pat. 1,126,855 U.S. Pat. 3,622,473 Brit. Pat. 1,134,000 Brit. Pat. 1,379,798

Chapter 17: Tests and properties of anodic oxide coatings Part 1: Thickness, sealing and accelerated corrosion resistance

The range of tests and the properties evaluated tend to widen continuously as new applications for anodized finishes are found and the conditions of service become more arduous. Testing methods have some underlying philosophy behind them; thus they can only be considered in relation to the property being tested and to the production techniques applicable to a given product so as to enable suitable control to be exercised over that property. It is widely acknowledged that the thickness of the anodic oxide coating and the quality of sealing are the two most important properties from performance considerations. These are also fairly closely linked with corrosion resistance. This Chapter is concerned exclusively with these properties; other characteristics such as density, hardness, mechanical and electrical properties are considered in the next chapter which also includes a review of the weather resistance of anodized aluminium. It may be felt by some that this last should be discussed as part of corrosion resistance, but the Authors consider that this is a property which can only be evaluated by weathering. Accelerated tests of corrosion are merely a convenient way of checking that the sum of properties of a coating produced under conditions previously established as providing the required weather resistance, is in fact being reproduced in production. Thickness testing Thickness is the most frequent specified property and its importance lies not only in its effect on service life, but also that it is a major factor in determining the price at which anodized work is sold to a customer. Failure to produce the minimum required thickness or recording of values significantly in excess of the required level provide a warning that the anodizing process is not under control. Too thin coatings despatched to a customer are a form of unfulfilled contract, whilst too thick coatings represent lost profit to the anodizer. Early test methods were essentially destructive and depended upon either the use of separate test-pieces, the removal of coating from the sample being tested, or sectioning through the article. This restricted both the ability to guarantee uniformity within a batch and knowledge of factors requiring control to ensure such uniformity. In recent years non-destructive techniques have been developed which have made possible the routine checking of film thicknesses on individual

238

239

50. 51. 52. 53. 54 . 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80.

146-149, 187-189 L. Campanella, Trattamenti e Finitura, 1964, 4, 65 C. Th. Speiser, Aluminium, 1955, 31, 8-9 U.S. Pat. 2,755,239(1959) U.S. Pat. 2,888,388 (1952) French Pat . 898, 158(1 943 ) R. C. Spooner, Metal Finishing, 66, (12), 44-49, 53 and 67, (1 ), 80-83, 91 U.S. Pat. 1,946,152(1934) Brit. Pat. 393,996 (1931) N. D. Tomashov and A. Tyukina, Light Metals, 1946, 9, 22-35 Defence Specification DEF- 151, August 1972 G. C. Wood, Trans. Inst. Met. Finishing, 1959, 36, 220-229 L. Whitby, Metal Ind., 1948, 72, 400-403 F. Sacchi, Trans. Inst. Met. Finishing, 1964, 41, 182-189 Brit. Pat. 965,837(1964) R. C. Spooner, Chemistry in Canada, April 1960 M. S. Hunter, P. F. Towner and D. L. Robinson, Proc. A.E.S., 1959, 46, 220-225 C. J . Amore and J . F. Murphy, Metal Finishing, 1965, 63, (11 ), 50-55, 60 Brit. Pat. 1,428,048 U.S. Pat. 4,031,275 U.S. Pat. 4,045,599 A. G. C. Gwyer and N. D. Pullen, Metal Industry, 1940, 56, 7-10, 33-35 A. H. Ensor, Production Engineering, 1943(12), 36 W. Briese, Metallwaren Industrie und Galvanotechnik, 1954, 45, (8), 373 F. Pearlstein, Metal Finishing, 1960, 58, (8), 40-43 U.S. Pat. 2,662,034(1950) French Pat. 1,191,839(1958) J . Patrie, Revue de L’Aluminium, 1976, 448, 77-88 Brit. Pat. 1,126,855 U.S. Pat. 3,622,473 Brit. Pat. 1,134,000 Brit. Pat. 1,379,798

Chapter 17: Tests and properties of anodic oxide coatings Part 1: Thickness, sealing and accelerated corrosion resistance

The range of tests and the properties evaluated tend to widen continuously as new applications for anodized finishes are found and the conditions of service become more arduous. Testing methods have some underlying philosophy behind them; thus they can only be considered in relation to the property being tested and to the production techniques applicable to a given product so as to enable suitable control to be exercised over that property. It is widely acknowledged that the thickness of the anodic oxide coating and the quality of sealing are the two most important properties from performance considerations. These are also fairly closely linked with corrosion resistance. This Chapter is concerned exclusively with these properties; other characteristics such as density, hardness, mechanical and electrical properties are considered in the next chapter which also includes a review of the weather resistance of anodized aluminium. It may be felt by some that this last should be discussed as part of corrosion resistance, but the Authors consider that this is a property which can only be evaluated by weathering. Accelerated tests of corrosion are merely a convenient way of checking that the sum of properties of a coating produced under conditions previously established as providing the required weather resistance, is in fact being reproduced in production. Thickness testing Thickness is the most frequent specified property and its importance lies not only in its effect on service life, but also that it is a major factor in determining the price at which anodized work is sold to a customer. Failure to produce the minimum required thickness or recording of values significantly in excess of the required level provide a warning that the anodizing process is not under control. Too thin coatings despatched to a customer are a form of unfulfilled contract, whilst too thick coatings represent lost profit to the anodizer. Early test methods were essentially destructive and depended upon either the use of separate test-pieces, the removal of coating from the sample being tested, or sectioning through the article. This restricted both the ability to guarantee uniformity within a batch and knowledge of factors requiring control to ensure such uniformity. In recent years non-destructive techniques have been developed which have made possible the routine checking of film thicknesses on individual

238

239

pieces at a low cost. This has represented a significant advance for the industry.

Thickness measurement with an optical split-beam microscope

Strip and weigh method

A microscope has been developed by Zeiss1 which depends upon measuring the separation produced when two parallel monochromatic beams of polarised light strike an anodized surface. One of these is reflected from the air-oxide face and the other from the metal-oxide interface. The thickness is calculated from the formula: T = S7(2n 2 - 1) where S = distance separating the two fringes and n = the refractive index of the coating (1.60)

This was one of the first methods used and is based on the fact that a solution consisting of: phosphoric acid (S.G. 1.75) 3.5% (w/v) chromic anhydride (CrO, ) A.R. 2.0% (w/v) used at 85° - 100°C will strip an anodic coating from the metal without attacking the base. With this method the anodized sample is first weighed and then stripped in the above solution, washed, dried and preferably re-immersed in the above solution until constant weight is recorded. The thickness is calculated from the expression: 1

_ 100 x W “ Ax d

when t = anodic coating thickness in microns W = weight of anodic coating removed in grams A = surface area of anodic coating in cm 2 d = density of coating in g / cm 3 (This is taken as 2.6 for normally-sealed coatings) This method is specified in B.S. 1615:1972, Appendix B and in I.S.O. 2106. It gives an average coating thickness over the test area and is the referee test for coatings of 5 microns and lower, which cannot readily be measured in other ways. Thickness by micro- sectioning

The Zeiss instrument has a calibrated eyepiece which enables a direct thickness reading to be obtained. It is a useful non-destructive test but can only be used where the surface is flat or almost so, and it is difficult to use on matt surfaces of low reflectivity. It is a useful non-destructive test which can best be used where the surfaces are relatively smooth and the coating reasonably clear. It is difficult to use on surfaces of low' reflectivity. The method is specified in B.S. 1615:1972, Appendix D and in I.S.O. 2128. Another allied method had been described 2 using a special microscope in which the thickness is obtained by counting the number of interference bands (Newton’s rings) produced when a suitably-designed optical system is built into the equipment. Eddy current thickness testing This has become the most widely used non-destructive testing technique in production use, and has the advantage that it can be used on a wide range of components and is not affected by the brightness or mattness of the surface, the presence of a dye, etc. It is based on the fact that at a suitably chosen high frequency the film thickness is directly proportional to the capacitance:

c = This is the classical metallurgical laboratory technique in which a piece is removed from the anodized article by careful cutting and is mounted in one of the usual mounting media so that when polished it provides a cross-section through the anodic coating at right angles to the surface being checked for thickness. Care and some experience are needed in preparing and mounting the specimen. Sometimes a thin piece of unanodized foil or sheet is placed against the edge being measured to protect the anodic coating from crumbling during mounting and preparation, and to provide a readily identifiable edge to the coating. After careful polishing and light etching the thickness of the coating can be read directly through a suitably calibrated eyepiece, such as one giving a direct reading at x 100. This method is specified in B.S. 1615;1972, Appendix A and in I.S.O. 1463. It is a measure only of local thickness but is the referee method for most anodic coating thicknesses. 240

_F£ 4 zzd

where

C — capacitance £ = dielectric constant of the coating F = the area under test d = the thickness of the insulated film. The instruments are direct-reading and give the anodic film thickness when the test probe is placed on the surface of the anodic film. The reading on the instrument is relative to a nominal zero obtained on unanodized aluminium of similar surface texture and alloy to the w ork being tested, and of the same contour. All instruments are provided with means of adjusting the zero point to suit the material being tested. Once the zero has been set for a given piece of work it should not require further adjustment. After setting the zero the calibration is checked, preferably on anodized standards, the thickness of which has been determined by micro241

pieces at a low cost. This has represented a significant advance for the industry.

Thickness measurement with an optical split-beam microscope

Strip and weigh method

A microscope has been developed by Zeiss1 which depends upon measuring the separation produced when two parallel monochromatic beams of polarised light strike an anodized surface. One of these is reflected from the air-oxide face and the other from the metal-oxide interface. The thickness is calculated from the formula: T = S7(2n 2 - 1) where S = distance separating the two fringes and n = the refractive index of the coating (1.60)

This was one of the first methods used and is based on the fact that a solution consisting of: phosphoric acid (S.G. 1.75) 3.5% (w/v) chromic anhydride (CrO, ) A.R. 2.0% (w/v) used at 85° - 100°C will strip an anodic coating from the metal without attacking the base. With this method the anodized sample is first weighed and then stripped in the above solution, washed, dried and preferably re-immersed in the above solution until constant weight is recorded. The thickness is calculated from the expression: 1

_ 100 x W “ Ax d

when t = anodic coating thickness in microns W = weight of anodic coating removed in grams A = surface area of anodic coating in cm 2 d = density of coating in g / cm 3 (This is taken as 2.6 for normally-sealed coatings) This method is specified in B.S. 1615:1972, Appendix B and in I.S.O. 2106. It gives an average coating thickness over the test area and is the referee test for coatings of 5 microns and lower, which cannot readily be measured in other ways. Thickness by micro- sectioning

The Zeiss instrument has a calibrated eyepiece which enables a direct thickness reading to be obtained. It is a useful non-destructive test but can only be used where the surface is flat or almost so, and it is difficult to use on matt surfaces of low reflectivity. It is a useful non-destructive test which can best be used where the surfaces are relatively smooth and the coating reasonably clear. It is difficult to use on surfaces of low' reflectivity. The method is specified in B.S. 1615:1972, Appendix D and in I.S.O. 2128. Another allied method had been described 2 using a special microscope in which the thickness is obtained by counting the number of interference bands (Newton’s rings) produced when a suitably-designed optical system is built into the equipment. Eddy current thickness testing This has become the most widely used non-destructive testing technique in production use, and has the advantage that it can be used on a wide range of components and is not affected by the brightness or mattness of the surface, the presence of a dye, etc. It is based on the fact that at a suitably chosen high frequency the film thickness is directly proportional to the capacitance:

c = This is the classical metallurgical laboratory technique in which a piece is removed from the anodized article by careful cutting and is mounted in one of the usual mounting media so that when polished it provides a cross-section through the anodic coating at right angles to the surface being checked for thickness. Care and some experience are needed in preparing and mounting the specimen. Sometimes a thin piece of unanodized foil or sheet is placed against the edge being measured to protect the anodic coating from crumbling during mounting and preparation, and to provide a readily identifiable edge to the coating. After careful polishing and light etching the thickness of the coating can be read directly through a suitably calibrated eyepiece, such as one giving a direct reading at x 100. This method is specified in B.S. 1615;1972, Appendix A and in I.S.O. 1463. It is a measure only of local thickness but is the referee method for most anodic coating thicknesses. 240

_F£ 4 zzd

where

C — capacitance £ = dielectric constant of the coating F = the area under test d = the thickness of the insulated film. The instruments are direct-reading and give the anodic film thickness when the test probe is placed on the surface of the anodic film. The reading on the instrument is relative to a nominal zero obtained on unanodized aluminium of similar surface texture and alloy to the w ork being tested, and of the same contour. All instruments are provided with means of adjusting the zero point to suit the material being tested. Once the zero has been set for a given piece of work it should not require further adjustment. After setting the zero the calibration is checked, preferably on anodized standards, the thickness of which has been determined by micro241

sectioning, and, apart from an hourly check on the zero, no further adjustment should be necessary. However, the zero can drift on some instruments and it is always advisable to check this characteristic.

than none at all, they must be regarded as a rough-and-ready workshop guide. In some anodizing plants where the materials being anodized are always closely similar, such as automobile bright trim materials, they have served as helpful workshop tools, but the superiority of the eddy-current methods can hardly be disputed for acceptance testing purposes. Sealing tests In most applications the quality of sealing is the other critical property of anodized coatings. To a large extent sealing tests have also tended to be indirect tests of the serviceability of the product. Thus, the original dye-stain test was essentially stimulated by service experience in which it was observed that improperly sealed anodic oxide coatings stained if colouring matter was spilt on them. In addition, tests have tried to simulate external environments, particularly the effect on the surface of condensation of moisture and the action of sulphur dioxide. More work has been carried out on sealing tests in the last few years than on any other aspect of anodic film quality, and this has largely arisen from the need to include suitable referee and control tests for sealing quality in anodizing specifications. At the present time the widely used sealing tests can be divided into three broad categories:

Fzg. 90. A portable anodic film thickness tester operating on the eddy current principle. (Courtesy: Wyndham Instruments Most instruments are available in a mains or battery-operated form so they can be readily used on the production line, or even on site in the case of architectural anodizing. Choice of a suitable instrument can be difficult but important aspects are probe design, instrument stability and scale suitability. Good guidance on these and other aspects of eddy-current instruments is provided by Latter ’. This method is specified in B.S. 1615:1972, Appendix C and in I.S.O. 2360. Thickness by electrical breakdow n voltage or resistance It had been observed by early investigators that there was an approximately linear relationship between film thickness and electrical breakdown voltage of anodic oxide coatings 4 . Beyond 15 - 25p however, the relationship becomes increasingly non-linear. Later work has shown that this relationship is at best only approximate and that it is influenced by the alloy, degree of sealing and ageing of the film. Low-cost thickness testers have been developed which either employ the breakdown voltage principle or else utilize the fact that the electrical resistance is also roughly proportional to the film thickness. Even so, whilst such tests arc better 242

(a) acid dissolution tests (b) dye spot tests (c) electrical tests Many authorities feel that the acid dissolution tests give the most accurate guidance on sealing quality and they are invariably used as the referee tests in anodizing specifications. They are based on measurement of loss in weight from the anodized specimen when immersed for a suitable time in a suitable acid solution. However, they are destructive and relatively complicated to carry out in comparison with other sealing tests, so they are not usually used for routine production control. Dye spot tests provide a rapid, non-destructive test for sealing assessment, but they involve comparison of the colour produced in the test area with suitable colour spot charts, and so are subjective to some extent. They are also unsuitable for use with deeply coloured coatings. The electrical tests involve measurement of the admittance or the impedance of anodic coatings and give a rapid, quantitative result. At first sight they provide the ideal method of routine production control of sealing quality but, unfortunately, the electrical changes which they measure do not always correspond to changes in sealing quality and vice-versa. However, they are widely used and with care can give good guidance on sealing quality. 243

sectioning, and, apart from an hourly check on the zero, no further adjustment should be necessary. However, the zero can drift on some instruments and it is always advisable to check this characteristic.

than none at all, they must be regarded as a rough-and-ready workshop guide. In some anodizing plants where the materials being anodized are always closely similar, such as automobile bright trim materials, they have served as helpful workshop tools, but the superiority of the eddy-current methods can hardly be disputed for acceptance testing purposes. Sealing tests In most applications the quality of sealing is the other critical property of anodized coatings. To a large extent sealing tests have also tended to be indirect tests of the serviceability of the product. Thus, the original dye-stain test was essentially stimulated by service experience in which it was observed that improperly sealed anodic oxide coatings stained if colouring matter was spilt on them. In addition, tests have tried to simulate external environments, particularly the effect on the surface of condensation of moisture and the action of sulphur dioxide. More work has been carried out on sealing tests in the last few years than on any other aspect of anodic film quality, and this has largely arisen from the need to include suitable referee and control tests for sealing quality in anodizing specifications. At the present time the widely used sealing tests can be divided into three broad categories:

Fzg. 90. A portable anodic film thickness tester operating on the eddy current principle. (Courtesy: Wyndham Instruments Most instruments are available in a mains or battery-operated form so they can be readily used on the production line, or even on site in the case of architectural anodizing. Choice of a suitable instrument can be difficult but important aspects are probe design, instrument stability and scale suitability. Good guidance on these and other aspects of eddy-current instruments is provided by Latter ’. This method is specified in B.S. 1615:1972, Appendix C and in I.S.O. 2360. Thickness by electrical breakdow n voltage or resistance It had been observed by early investigators that there was an approximately linear relationship between film thickness and electrical breakdown voltage of anodic oxide coatings 4 . Beyond 15 - 25p however, the relationship becomes increasingly non-linear. Later work has shown that this relationship is at best only approximate and that it is influenced by the alloy, degree of sealing and ageing of the film. Low-cost thickness testers have been developed which either employ the breakdown voltage principle or else utilize the fact that the electrical resistance is also roughly proportional to the film thickness. Even so, whilst such tests arc better 242

(a) acid dissolution tests (b) dye spot tests (c) electrical tests Many authorities feel that the acid dissolution tests give the most accurate guidance on sealing quality and they are invariably used as the referee tests in anodizing specifications. They are based on measurement of loss in weight from the anodized specimen when immersed for a suitable time in a suitable acid solution. However, they are destructive and relatively complicated to carry out in comparison with other sealing tests, so they are not usually used for routine production control. Dye spot tests provide a rapid, non-destructive test for sealing assessment, but they involve comparison of the colour produced in the test area with suitable colour spot charts, and so are subjective to some extent. They are also unsuitable for use with deeply coloured coatings. The electrical tests involve measurement of the admittance or the impedance of anodic coatings and give a rapid, quantitative result. At first sight they provide the ideal method of routine production control of sealing quality but, unfortunately, the electrical changes which they measure do not always correspond to changes in sealing quality and vice-versa. However, they are widely used and with care can give good guidance on sealing quality. 243

Acid dissolution tests

Dye- spot tests

These tests have developed from the sodium sulphite beaker test suggested by Kape6 . This involves immersion of the work being tested in a 10 g/1 sodium sulphite solution suitably acidified with acetic acid and sulphuric acid to pH 2.5. The solution is used at 90° - 92°C and the normal immersion time is 20 minutes. Initally assessment of sealing quality was carried out qualitatively by noting the degree of bloom formation that occurred, but Sheasby et al 7 developed a quantitative version of the test which is now the form used. (B.S. 1615:1972, Appendix E and I.S.O. 2932). In addition to making the test quantitative, they suggested that a 10-minute predip in 50% (v/v) nitric acid at room temperature should be used in order to increase the sensitivity of the test. This nitric acid predip had little effect on the weight loss in the sulphite solution with well-sealed work but it increased the weight loss produced with borderline or poorly sealed work. The maximum weight loss allowed in the test for satisfactory sealing quality is generally set at 20 mg/dm 2 of anodized surface.

The earliest tests for controlling the quality of sealing were dye-spot tests in which, initially, the dye used was Anthraquinone Violet or Methyl Violet. In the early tests two drops of Anthraquinone Violet R (Acid Violet 34) of 20 g/1 concentration were placed on the cleaned anodized surface and left there for 5 minutes. After thorough rinsing and cleaning with detergent the specimen was dried and examined. Any signs of colouring of the anodic coating were grounds for rejection.

At about the same time a quantitative test was developed in France, which involved 15 minutes immersion in a boiling acetic acid/sodium acetate solution adjusted to pH 2.3 - 2.5 (I.S.O. 2932). This gave similar results to the acidified sodium sulphite test and again a maximum acceptable weight loss of 20 mg/dm 2 has been generally used. This test did not originally include a nitric acid predip, but the I.S.O. specification (2932) is now being revised and is expected to require that both tests will then include a 10-minute predip in nitric acid. More recently Manhart and Cochran 8 have developed an acid dissolution test based on the chromic acid/phosphoric acid solution used for stripping anodic coatings. They recommended measurement of loss of weight after 15 minutes immersion in a 35 ml/1 phosphoric acid, 25 g/1 chromic acid solution used at 38°C. Because of its simplicity and reproducibility this test is rapidly becoming the most widely used acid dissolution test and it is likely to be the referee test of future specifications. It is already the referee sealing test in B.S. 3987:1974, Appendix E and is covered by I.S.O. 3210. It is slightly more aggressive than the other acid dissolution tests and a maximum acceptable weight loss of 30 mg/dm 2 is normally allowed. A nitric acid predip was not used originally with, this test, but it is likely that this will be introduced in future to bring conformity with the other tests. It is often useful to measure the weight loss in the nitric acid predip also, as it can give indications of anodic films which are softer than usual. Normally weight losses in the nitric acid predip are small (well below 10 mg/dm 2 ), but losses significantly higher than this can indicate a poor quality anodic coating (often produced at too high an anodizing temperature). However, very long sealing times and high pH sealing can also produce weight losses in the nitric acid which are higher than normal. 244

This test was found to have many limitations as it was not really sufficiently discriminating. The results were shown to depend on the time which had elapsed between sealing and testing 9, whilst the use of nickel salts for sealing gave good test results which were not necessarily reflected in service behaviour. Even so, it can be a useful test for work not subjected to severe service conditions, for example many domestic articles. Later, a 2% alcoholic solution of Methyl Violet was used. In this case, a spot of 50% (v/v) nitric acid is applied to the part to be tested first and, after thorough rinsing, the dye solution is applied to the same spot. Although this method has been widely included in specifications (B.S. 1615:1972 Appendix F, and I.S.O. 2143), it is still not always sufficiently discriminating for testing architectural work. Another modification which has been widely used is the so-called ‘Green Dye Test’ which was developed in Germany 10. In its present form, a spot of 50% (v/v) nitric acid is placed on the degreased anodized surface and left in contact for 10 minutes at room temperature. After thorough washing, a few drops of 10 g/1 Aluminium Green GLW dyestuff are placed in the same spot and left for 2 minutes, and then the surface is thoroughly washed and dried. Again seal quality is assessed by comparison of the stain (if any) with suitable control charts. In its original form this test was an immersion test in the green dye bath but Schmecken 11 has suggested that the results could be assessed by measuring the changed reflectivity of the surface. Whilst this form of the test has been used in some German standards, it has not generally been found to give reliable results. The green dye test has been used in both the automotive and architectural industries and is specified in B.S. 1615:1972 Appendix F and I.S.O. 2143, but it has been found to be very dependent on the surface roughness of the anodic coating and generally, in contrast to the violet tests, will give some staining on almost all anodized surfaces. Because of this, and because the particular dyestuff is no longer commercially available, the test is now less and less used. A later variant of this type of test was described by Scott 12. This used a pretreatment spot of 10 g/1 potassium fluoride and 25 ml/1 sulphuric acid solution applied to the surface for exactly 1 minute at room temperature. After rinsing, a few drops of 10 g/1 Aluminium Red B3LW dyestuff are applied to the same spot and again left in contact for 1 minute. As usual, the surface is then thoroughly washed and dried and the degree of staining assessed. Here again well-sealed 245

Acid dissolution tests

Dye- spot tests

These tests have developed from the sodium sulphite beaker test suggested by Kape6 . This involves immersion of the work being tested in a 10 g/1 sodium sulphite solution suitably acidified with acetic acid and sulphuric acid to pH 2.5. The solution is used at 90° - 92°C and the normal immersion time is 20 minutes. Initally assessment of sealing quality was carried out qualitatively by noting the degree of bloom formation that occurred, but Sheasby et al 7 developed a quantitative version of the test which is now the form used. (B.S. 1615:1972, Appendix E and I.S.O. 2932). In addition to making the test quantitative, they suggested that a 10-minute predip in 50% (v/v) nitric acid at room temperature should be used in order to increase the sensitivity of the test. This nitric acid predip had little effect on the weight loss in the sulphite solution with well-sealed work but it increased the weight loss produced with borderline or poorly sealed work. The maximum weight loss allowed in the test for satisfactory sealing quality is generally set at 20 mg/dm 2 of anodized surface.

The earliest tests for controlling the quality of sealing were dye-spot tests in which, initially, the dye used was Anthraquinone Violet or Methyl Violet. In the early tests two drops of Anthraquinone Violet R (Acid Violet 34) of 20 g/1 concentration were placed on the cleaned anodized surface and left there for 5 minutes. After thorough rinsing and cleaning with detergent the specimen was dried and examined. Any signs of colouring of the anodic coating were grounds for rejection.

At about the same time a quantitative test was developed in France, which involved 15 minutes immersion in a boiling acetic acid/sodium acetate solution adjusted to pH 2.3 - 2.5 (I.S.O. 2932). This gave similar results to the acidified sodium sulphite test and again a maximum acceptable weight loss of 20 mg/dm 2 has been generally used. This test did not originally include a nitric acid predip, but the I.S.O. specification (2932) is now being revised and is expected to require that both tests will then include a 10-minute predip in nitric acid. More recently Manhart and Cochran 8 have developed an acid dissolution test based on the chromic acid/phosphoric acid solution used for stripping anodic coatings. They recommended measurement of loss of weight after 15 minutes immersion in a 35 ml/1 phosphoric acid, 25 g/1 chromic acid solution used at 38°C. Because of its simplicity and reproducibility this test is rapidly becoming the most widely used acid dissolution test and it is likely to be the referee test of future specifications. It is already the referee sealing test in B.S. 3987:1974, Appendix E and is covered by I.S.O. 3210. It is slightly more aggressive than the other acid dissolution tests and a maximum acceptable weight loss of 30 mg/dm 2 is normally allowed. A nitric acid predip was not used originally with, this test, but it is likely that this will be introduced in future to bring conformity with the other tests. It is often useful to measure the weight loss in the nitric acid predip also, as it can give indications of anodic films which are softer than usual. Normally weight losses in the nitric acid predip are small (well below 10 mg/dm 2 ), but losses significantly higher than this can indicate a poor quality anodic coating (often produced at too high an anodizing temperature). However, very long sealing times and high pH sealing can also produce weight losses in the nitric acid which are higher than normal. 244

This test was found to have many limitations as it was not really sufficiently discriminating. The results were shown to depend on the time which had elapsed between sealing and testing 9, whilst the use of nickel salts for sealing gave good test results which were not necessarily reflected in service behaviour. Even so, it can be a useful test for work not subjected to severe service conditions, for example many domestic articles. Later, a 2% alcoholic solution of Methyl Violet was used. In this case, a spot of 50% (v/v) nitric acid is applied to the part to be tested first and, after thorough rinsing, the dye solution is applied to the same spot. Although this method has been widely included in specifications (B.S. 1615:1972 Appendix F, and I.S.O. 2143), it is still not always sufficiently discriminating for testing architectural work. Another modification which has been widely used is the so-called ‘Green Dye Test’ which was developed in Germany 10. In its present form, a spot of 50% (v/v) nitric acid is placed on the degreased anodized surface and left in contact for 10 minutes at room temperature. After thorough washing, a few drops of 10 g/1 Aluminium Green GLW dyestuff are placed in the same spot and left for 2 minutes, and then the surface is thoroughly washed and dried. Again seal quality is assessed by comparison of the stain (if any) with suitable control charts. In its original form this test was an immersion test in the green dye bath but Schmecken 11 has suggested that the results could be assessed by measuring the changed reflectivity of the surface. Whilst this form of the test has been used in some German standards, it has not generally been found to give reliable results. The green dye test has been used in both the automotive and architectural industries and is specified in B.S. 1615:1972 Appendix F and I.S.O. 2143, but it has been found to be very dependent on the surface roughness of the anodic coating and generally, in contrast to the violet tests, will give some staining on almost all anodized surfaces. Because of this, and because the particular dyestuff is no longer commercially available, the test is now less and less used. A later variant of this type of test was described by Scott 12. This used a pretreatment spot of 10 g/1 potassium fluoride and 25 ml/1 sulphuric acid solution applied to the surface for exactly 1 minute at room temperature. After rinsing, a few drops of 10 g/1 Aluminium Red B3LW dyestuff are applied to the same spot and again left in contact for 1 minute. As usual, the surface is then thoroughly washed and dried and the degree of staining assessed. Here again well-sealed 245

work should result in little or no staining of the film. In his work, Scott evaluated a number of dyestuffs but found that the red dye was superior to Aluminium Green GLW, in that results were more reproducible and there was better discrimination. These conclusions have been borne out in practice and this test has set the pattern for most current dye-spot testing. The fluoridecontaining predip seems to give the discrimination necessary for architectural anodizing and the red dye gives the reproducibility required. This test is specified in B.S. 1615:1972, Appendix F.

urements to verify their electrical analogue of an anodic oxide coating and to follow the progress of sealing (fig. 91). This led to further work on impedance by Engelhart and Sowinski , 7 in America and on admittance by Birtel and Leute 18 in Germany, which resulted in the development of commercial instruments for testing sealing quality see (fig. 92.)

A similar test, developed in Switzerland 13 uses a 2.5% (v/v) fluoro-silicic acid (H2SiFb ) pretreatment in combination with an Aluminium Blue 2LW dyestuff. Again the benefits of the fluoride pretreatment are obtained and the blue dyestuff gives similar reproducibility to the red. This, and the red-dye test, form the basis of the revision of I.S.O. 2143 which is currently in progress. Actual colour spot charts to indicate different levels of sealing quality have been prepared and will be issued in conjunction with the I.S.O. standard, so hopefully assessment will be much less subjective. Electrical tests The use of electrical measurements for assessing the quality of sealing of anodized aluminium has become widespread in the last few years. The speed with which results can be obtained and the quantitative nature of the results have made it an ideal method of production control, but, unfortunately, it is not always in agreement with other sealing tests. Fig. 92. The Anospec 'S' Admittance meter for testing the quality of sealing of anodic coatings. \Courtesy: Metal Information Services Ltd., Stonehouse

Fig. 91. Modified electrical analogue for sealed porous fibnslb .

Sealed

The property usually measured is the impedance of the anodic coating or its reciprocal value, admittance. This development is based on the early work of Jason and J . L. Wood’5 and Hoar and G. C. Wood’6 who used a.c. impedance meas246

Data recorded by the above authors showed linear relationships between impedance values and important sealing parameters such as time and temperature of sealing and the pH of the sealing bath. Anodic film thickness also had a linear effect on the electrical values obtained so this had to be taken into account in evaluating the results. However, Birtel and Leute felt that measurement of admittance more closely represented the changes taking place in the anodic film during sealing than did impedance changes. For example, impedance values reported by these authors18 show' a linear increase with sealing time, so therefore, the period of 20 to 40 minutes sealing produces the same numerical increase as does the period of 0“20 minutes. In practice, sealing is known to proceed relatively rapidly at first and then to proceed more slowly. In their later work' 9 they showed that admittance values reflect more accurately the process of sealing, in that the values reported fall non-linearly with sealing time and with changes in other parameters such as temperature and pH. It is for these reasons that admittance measurements are used most commonly for sealing assessment in Europe. 247

work should result in little or no staining of the film. In his work, Scott evaluated a number of dyestuffs but found that the red dye was superior to Aluminium Green GLW, in that results were more reproducible and there was better discrimination. These conclusions have been borne out in practice and this test has set the pattern for most current dye-spot testing. The fluoridecontaining predip seems to give the discrimination necessary for architectural anodizing and the red dye gives the reproducibility required. This test is specified in B.S. 1615:1972, Appendix F.

urements to verify their electrical analogue of an anodic oxide coating and to follow the progress of sealing (fig. 91). This led to further work on impedance by Engelhart and Sowinski , 7 in America and on admittance by Birtel and Leute 18 in Germany, which resulted in the development of commercial instruments for testing sealing quality see (fig. 92.)

A similar test, developed in Switzerland 13 uses a 2.5% (v/v) fluoro-silicic acid (H2SiFb ) pretreatment in combination with an Aluminium Blue 2LW dyestuff. Again the benefits of the fluoride pretreatment are obtained and the blue dyestuff gives similar reproducibility to the red. This, and the red-dye test, form the basis of the revision of I.S.O. 2143 which is currently in progress. Actual colour spot charts to indicate different levels of sealing quality have been prepared and will be issued in conjunction with the I.S.O. standard, so hopefully assessment will be much less subjective. Electrical tests The use of electrical measurements for assessing the quality of sealing of anodized aluminium has become widespread in the last few years. The speed with which results can be obtained and the quantitative nature of the results have made it an ideal method of production control, but, unfortunately, it is not always in agreement with other sealing tests. Fig. 92. The Anospec 'S' Admittance meter for testing the quality of sealing of anodic coatings. \Courtesy: Metal Information Services Ltd., Stonehouse

Fig. 91. Modified electrical analogue for sealed porous fibnslb .

Sealed

The property usually measured is the impedance of the anodic coating or its reciprocal value, admittance. This development is based on the early work of Jason and J . L. Wood’5 and Hoar and G. C. Wood’6 who used a.c. impedance meas246

Data recorded by the above authors showed linear relationships between impedance values and important sealing parameters such as time and temperature of sealing and the pH of the sealing bath. Anodic film thickness also had a linear effect on the electrical values obtained so this had to be taken into account in evaluating the results. However, Birtel and Leute felt that measurement of admittance more closely represented the changes taking place in the anodic film during sealing than did impedance changes. For example, impedance values reported by these authors18 show' a linear increase with sealing time, so therefore, the period of 20 to 40 minutes sealing produces the same numerical increase as does the period of 0“20 minutes. In practice, sealing is known to proceed relatively rapidly at first and then to proceed more slowly. In their later work' 9 they showed that admittance values reflect more accurately the process of sealing, in that the values reported fall non-linearly with sealing time and with changes in other parameters such as temperature and pH. It is for these reasons that admittance measurements are used most commonly for sealing assessment in Europe. 247

In practice admittance measurements are made using a suitable bridge circuit operating at a frequency of 1 kHz. The instruments are equipped with two electrodes, one of which is a contact screw by which a contact can be made to the basis metal of the sample, and the other is a pencil-like probe. This probe is placed in an electrolytic cell on the sample to be tested. The cell is usually made up of an adhesive rubber ring attached to the sample and fitted with a suitable electrolyte (usually a 35 g/1 potassium sulphate solution). When the second probe is dipped into this solution a direct reading of the admittance value is obtained. This reading is affected by anodic film thickness so a measurement of this will need to be made close to the site of the admittance test. It is also affected by the measuring area used and by the electrolyte temperature, and corrections need to be made for these factors if necessary. The method is specified in B.S. 1615:1972, Appendix G and in I.S.O. 2931 (admittance or impedance). The maximum acceptable value of admittance has aroused considerable debate and acceptance values still vary from one country to another. In general, the levels set seem to represent the sealing practices used in a particular country, rather than the real level that is necessary. Thus, in Germany, where very long sealing times are common, the maximum value is microsiemens, where t is the anodic film thickness in microns. Bradshaw et al 20 investigated the correlation of admittance values with acidified sulphite tests and dye-spot tests in the U.K. and concluded that a value of gave a good correlation. They showed that this relationship represented a sealing time of between 1 and 2 minutes per micron (fig. 93) which was representative of general sealing practice, and this is the value adopted in B.S. 1615. An international compromise may well be found in the ',(’0 figure specified in the Qualanod regulations.

Admittance E - m i c r o m h o s

\

O & A

T h e proposed a c c e p t a n c e limit c u r v e E x p e r i m e n t a l readings obtained with anodic f i l m s s e a l e d for 1 m i n / p E x p e r i m e n t a l readings obtained with anodic film s e a l e d for 2 m i n / p

Film thickness - microns

Fig. 93. The acceptance limit curve based on 500/ t proposed by Bradshaw et al. 248

The simplicity of admittance measurement has meant that it has been widely applied to many different types of anodized finish, but this has produced many problems in correlating the results with those of other sealing tests. Engelhart and Sowinski 17, Birtel and Lcute 19 and many others have shown that electrical measurements are sensitive to ageing of anodic coatings. This results in an increase of impedance values and decrease in admittance values with time. Even poorly sealed coatings age to the extent that the difference between a well-sealed and an inadequately-sealed coating narrows considerably with long exposure to the atmosphere. This has led to a restriction in some specifications that measurements are to be made within 48 hours of sealing. The work of Bradshaw et al 20 showed that admittance measurements were less sensitive to the effect of silicate and phosphate contamination of the sealing bath than were acid dissolution tests, and the effect of pH variation was also less. On the other hand, admittance is very much more sensitive to the presence of electrolytically deposited colouring metals, such as tin and copper 21, owing to their high conductivity, and special allowance has to be made when testing such work. The presence of anti-smutting additives in the sealing bath can also present problems, since admittance measurements are generally not sensitive to excessive amounts of these additives, whereas acid dissolution and dye-spot tests will indicate a poor sealing result. In order to try to overcome some of the difficulties with admittance values, work in Germany 22 23 was extended to cover also the influence of dissipation factor (the ratio of resistance to capacitance, measured from the phase shift between current and voltage). This property was said to be mainly dependent on sealing time and temperature and not on factors such as sealing bath contamination and pH, so it was felt that using both admittance and dissipation factor measurements gave additional information on anodic film quality. However, Bradshaw et al 20 in the U.K., found it difficult to interpret the dissipation factor values obtained in their work, and it may be that the long sealing times used commonly in Germany are necessary in order to obtain reproducible values. A number of authors have carried out comparative tests using many of the sealing tests described, notably Survila 14 24 and Gohausen 25. In general, reasonable agreement has been obtained between acid dissolution and dye spot tests but more anomalies are shown up by admittance measurements. It seems that acid dissolution and dye-spot tests are a measure of the chemical resistance of the outer part of the anodic coating and therefore very dependent on the degree of bohmite formation at the surface. The electrical tests, on the other hand, are dependent on the overall resistance and capacitance of the film and are probably more affected by the pore-filling mechanism than by surface changes. Thus, there are many occasions where they are not measuring precisely the same property. However, whatever method is used for checking sealing quality, it is advisable not to rely on one method alone. All methods have their drawbacks and limitations 249

In practice admittance measurements are made using a suitable bridge circuit operating at a frequency of 1 kHz. The instruments are equipped with two electrodes, one of which is a contact screw by which a contact can be made to the basis metal of the sample, and the other is a pencil-like probe. This probe is placed in an electrolytic cell on the sample to be tested. The cell is usually made up of an adhesive rubber ring attached to the sample and fitted with a suitable electrolyte (usually a 35 g/1 potassium sulphate solution). When the second probe is dipped into this solution a direct reading of the admittance value is obtained. This reading is affected by anodic film thickness so a measurement of this will need to be made close to the site of the admittance test. It is also affected by the measuring area used and by the electrolyte temperature, and corrections need to be made for these factors if necessary. The method is specified in B.S. 1615:1972, Appendix G and in I.S.O. 2931 (admittance or impedance). The maximum acceptable value of admittance has aroused considerable debate and acceptance values still vary from one country to another. In general, the levels set seem to represent the sealing practices used in a particular country, rather than the real level that is necessary. Thus, in Germany, where very long sealing times are common, the maximum value is microsiemens, where t is the anodic film thickness in microns. Bradshaw et al 20 investigated the correlation of admittance values with acidified sulphite tests and dye-spot tests in the U.K. and concluded that a value of gave a good correlation. They showed that this relationship represented a sealing time of between 1 and 2 minutes per micron (fig. 93) which was representative of general sealing practice, and this is the value adopted in B.S. 1615. An international compromise may well be found in the ',(’0 figure specified in the Qualanod regulations.

Admittance E - m i c r o m h o s

\

O & A

T h e proposed a c c e p t a n c e limit c u r v e E x p e r i m e n t a l readings obtained with anodic f i l m s s e a l e d for 1 m i n / p E x p e r i m e n t a l readings obtained with anodic film s e a l e d for 2 m i n / p

Film thickness - microns

Fig. 93. The acceptance limit curve based on 500/ t proposed by Bradshaw et al. 248

The simplicity of admittance measurement has meant that it has been widely applied to many different types of anodized finish, but this has produced many problems in correlating the results with those of other sealing tests. Engelhart and Sowinski 17, Birtel and Lcute 19 and many others have shown that electrical measurements are sensitive to ageing of anodic coatings. This results in an increase of impedance values and decrease in admittance values with time. Even poorly sealed coatings age to the extent that the difference between a well-sealed and an inadequately-sealed coating narrows considerably with long exposure to the atmosphere. This has led to a restriction in some specifications that measurements are to be made within 48 hours of sealing. The work of Bradshaw et al 20 showed that admittance measurements were less sensitive to the effect of silicate and phosphate contamination of the sealing bath than were acid dissolution tests, and the effect of pH variation was also less. On the other hand, admittance is very much more sensitive to the presence of electrolytically deposited colouring metals, such as tin and copper 21, owing to their high conductivity, and special allowance has to be made when testing such work. The presence of anti-smutting additives in the sealing bath can also present problems, since admittance measurements are generally not sensitive to excessive amounts of these additives, whereas acid dissolution and dye-spot tests will indicate a poor sealing result. In order to try to overcome some of the difficulties with admittance values, work in Germany 22 23 was extended to cover also the influence of dissipation factor (the ratio of resistance to capacitance, measured from the phase shift between current and voltage). This property was said to be mainly dependent on sealing time and temperature and not on factors such as sealing bath contamination and pH, so it was felt that using both admittance and dissipation factor measurements gave additional information on anodic film quality. However, Bradshaw et al 20 in the U.K., found it difficult to interpret the dissipation factor values obtained in their work, and it may be that the long sealing times used commonly in Germany are necessary in order to obtain reproducible values. A number of authors have carried out comparative tests using many of the sealing tests described, notably Survila 14 24 and Gohausen 25. In general, reasonable agreement has been obtained between acid dissolution and dye spot tests but more anomalies are shown up by admittance measurements. It seems that acid dissolution and dye-spot tests are a measure of the chemical resistance of the outer part of the anodic coating and therefore very dependent on the degree of bohmite formation at the surface. The electrical tests, on the other hand, are dependent on the overall resistance and capacitance of the film and are probably more affected by the pore-filling mechanism than by surface changes. Thus, there are many occasions where they are not measuring precisely the same property. However, whatever method is used for checking sealing quality, it is advisable not to rely on one method alone. All methods have their drawbacks and limitations 249

and none can be used in all circumstances. Ultimate reliance is presently on the referee acid dissolution tests which are certainly a measure of chemical resistance and therefore should be related to weathering performance. Dye-spot tests measure a similar property and generally give good correlation, but admittance tests should always be used in conjunction with one or other of these tests. Other tests The other main method of assessing sealing quality has been by using various forms of SO2- humidity test. Condensation of moisture and atmospheric pollution with SOx are known to be major factors controlling attack of anodic coatings in the weather, so it is not surprising that attempts have been made to accelerate this effect. The Kesternich test, in which a cabinet containing a humid atmosphere of CO2 and SOz is used, has been used to some extent in Germany 26 , but a problem is that the SO2 level is not constant throughout the test period. Edwards 27, working on corrosion tests for electroplated coatings, devised a test cabinet in which the concentration of SO2 and the humidity could be closely controlled. Using this apparatus (fig. 94) Brace and Pocock 28 found that it provided a good means of assessing sealing quality, the criterion being the virtual absence of bloom after testing (see fig. 95). This method is described in B.S. 1615:1972, Appendix H, but it is now little used outside the automotive industry. The main reason for this is the long testing time required (usually 24 hours) in comparison with the other sealing tests.

Fig. 94. A sulphur dioxide- humidity cabinet used for testing the sealing of anodic oxide coatings (B.S. 1615)

' ’

1

Fig. 95. Panels after SO2 - humidity testing. All panels sealed for 30 minutes. Left to right (i) water sealed, IOO°C, pH 3.9; (ii) water sealed, 80°C, pH 5.5; (Hi) water sealed, 100QC, pH 5.5; (iv) steam sealed, 100°C . Top row: SIB, (99,5% Al), Bottom row: D57S, (99. 99% Al + 1.2% Mg). More recently, Britnell and Ward 29 have suggested a spot dissolution test for sealing quality. This works on the principle that the time required to penetrate an anodic coating by an acid solution is dependent on the sealing quality. It employs a hydrofluoric acid-ammonium fluoride-potassium permanganate mixture which becomes colourless when the film is penetrated. Its main drawback, apart from handling difficulties, is that the anodic film thickness also has a strong effect on the dissolution time, and, to overcome this, test solutions with different levels of hydrofluoric acid have to be used for films of different thickness. Accelerated corrosion tests

(Courtesy: W. Canning & Co. Ltd.)

250

It should be appreciated that there are limitations to the conclusions which can be drawn from any accelerated corrosion test. Any attempt to correlate the results of accelerated tests with service behaviour is certain to run into difficulties and may well result in misleading conclusions being drawn. Accelerated tests should be chosen primarily as a means of ensuring that the overall quality of the product, i.e. the sum of the properties of the material and of the anodizing and sealing conditions, is consistent. A prime requirement of any such test is, therefore, a high degree of reproducibility, which, unfortunately, is not easily attained. 251

and none can be used in all circumstances. Ultimate reliance is presently on the referee acid dissolution tests which are certainly a measure of chemical resistance and therefore should be related to weathering performance. Dye-spot tests measure a similar property and generally give good correlation, but admittance tests should always be used in conjunction with one or other of these tests. Other tests The other main method of assessing sealing quality has been by using various forms of SO2- humidity test. Condensation of moisture and atmospheric pollution with SOx are known to be major factors controlling attack of anodic coatings in the weather, so it is not surprising that attempts have been made to accelerate this effect. The Kesternich test, in which a cabinet containing a humid atmosphere of CO2 and SOz is used, has been used to some extent in Germany 26 , but a problem is that the SO2 level is not constant throughout the test period. Edwards 27, working on corrosion tests for electroplated coatings, devised a test cabinet in which the concentration of SO2 and the humidity could be closely controlled. Using this apparatus (fig. 94) Brace and Pocock 28 found that it provided a good means of assessing sealing quality, the criterion being the virtual absence of bloom after testing (see fig. 95). This method is described in B.S. 1615:1972, Appendix H, but it is now little used outside the automotive industry. The main reason for this is the long testing time required (usually 24 hours) in comparison with the other sealing tests.

Fig. 94. A sulphur dioxide- humidity cabinet used for testing the sealing of anodic oxide coatings (B.S. 1615)

' ’

1

Fig. 95. Panels after SO2 - humidity testing. All panels sealed for 30 minutes. Left to right (i) water sealed, IOO°C, pH 3.9; (ii) water sealed, 80°C, pH 5.5; (Hi) water sealed, 100QC, pH 5.5; (iv) steam sealed, 100°C . Top row: SIB, (99,5% Al), Bottom row: D57S, (99. 99% Al + 1.2% Mg). More recently, Britnell and Ward 29 have suggested a spot dissolution test for sealing quality. This works on the principle that the time required to penetrate an anodic coating by an acid solution is dependent on the sealing quality. It employs a hydrofluoric acid-ammonium fluoride-potassium permanganate mixture which becomes colourless when the film is penetrated. Its main drawback, apart from handling difficulties, is that the anodic film thickness also has a strong effect on the dissolution time, and, to overcome this, test solutions with different levels of hydrofluoric acid have to be used for films of different thickness. Accelerated corrosion tests

(Courtesy: W. Canning & Co. Ltd.)

250

It should be appreciated that there are limitations to the conclusions which can be drawn from any accelerated corrosion test. Any attempt to correlate the results of accelerated tests with service behaviour is certain to run into difficulties and may well result in misleading conclusions being drawn. Accelerated tests should be chosen primarily as a means of ensuring that the overall quality of the product, i.e. the sum of the properties of the material and of the anodizing and sealing conditions, is consistent. A prime requirement of any such test is, therefore, a high degree of reproducibility, which, unfortunately, is not easily attained. 251

Salt spray tests These represent one of the traditional methods of corrosion testing and were used in early researches on anodic oxide coatings, particularly those produced on alloys of poor corrosion resistance, such as the Duralumin -type alloys. Work reported by J . D. Edwards 10 showed that on a Duralumin alloy (14S-T) 200 hours testing was required before a 5 film pitted, but on an A1-2 per cent Mg alloy (57S) for a 7 coating or thicker, upwards of 700 hours of exposure were required. Breakdown of the anodic oxide coating by pitting can be accelerated by the addition of acetic acid to a 3 per cent NaCl solution to give a pH of 3.2 - 3.5. Brace and Pocock*8 reported the time required for pitting on films of 5 - 26/7 thickness on 99.5 per cent Al and of 5 - 10/ithickness on 99.99 per cent Al - 1 per cent Mg. For the thinner films these ranged up to 100 hours (when tested at room temperature), depending upon the material anodized. Cooke and Spooner ”, using the ASTM Acidified Salt Spray Test (Method B286-61) which employs 5 per cent NaCl + acetic acid at 35 °C. reported that a 7p bright trim alloy (AA5457) withstood a 24-hour test without deterioration and also showed up poor sealing as bloom. More recently, Engelhart and Sowinski 52 have expressed the view that the discrimination of this test is not good, but it forms the basis of I.S.O. 3769 and is specified in B.S. 1615:1972, Appendix K. In Germany immersion in a solution of 25 g/1 NaCl. 15 ml/I glacial acetic acid and 3.3 ml/1 H2Oi (30% volume) for six days has been the accepted method of evaluating corrosion resistance. It appears to be much more effective as a means of discriminating between materials or of showing up important differences in film thickness than as providing a general quality control. Such a test can sometimes present difficulties in reproducibility since it can be influenced by the metallurgical characteristics of the material being anodized as well as by the anodizing and sealing conditions. The need to develop improved corrosion tests for electroplated automobile parts has led to the use of the CASS test which has also been applied to anodized aluminium. This test is essentially the ASTM acidified salt spray test to which a small amount of copper salt has been added, and it is operated at 50 c C (120'F) instead of 35°C (95°F). Carter 33 has reported good correlation between pitting resistance in industrial atmospheres in England and the results of the CASS test. He also concurs with Cooke and Spooner 31 that the test does not offer any useful evaluation of sealing quality, a view also expressed by Cochran 3* and by Bolmer and Graue 3*. Later work by Nakayama et al u has suggested that the CASS test would be more suitable for use with anodized aluminium if a lower solution pH of about 2 was used. This gives a faster rate of attack and greater reproducibilitv than the normal pH of 3.2. They also stress the importance of using a constant angle of exposure in order to obtain good reproducibility. The CASS test is now specified in B.S. 1615:1972. Appendix J and is the preferred method of corrosion testing, and is also specified in I.S.O. 3770. Both these specifications, however. 252

still recommend the generally used pH of 3.2. The Corrodkote test has proved rather unsatisfactory in all respects, giving poor discrimination in respect of the effects of both thickness and sealing. Electrolytic cathodic corrosion tests A cathodic dissolution test has been developed by the Ford Motor Co. of Detroit and used by them as an acceptance specification test 37. A drop of CASS solution is placed over a small area of anodized surface which is made the cathode, and a high d.c. potential is applied between a platinum anode and the test panel. An integrator, measuring the area under the current/time curve, provides an E.Q.T. number (Electrolytic Quality Test) for the 3 minute test period. J . D. Edwards has noted 3 that this test tends to exaggerate the influence of base metal purity, and Cooke and Spooner 31 found that the values obtained were not much influenced by variations in sealing time. On the other hand, Carter concludes 33 38 that the test does provide a rapid and simple means of differentiating between coating thicknesses on well-sealed articles and of distinguishing variations in the quality of sealing. Correlation with industrial exposure results are also reported as good, but the test was found to exaggerate the effects of basis metal purity. In practice, however, perhaps because of this latter characteristic, the test is no longer used even for automotive components. Kape 39 reported a few years ago a test in which a standard area of anodic oxide coating is made anodic, with the application of 40V potential, in a 5 per cent solution of formic acid. In this case the total number of coulombs passing during a 5 minute test period is taken as a measure of corrosion resistance. The higher the corrosion resistance, the lower the number of coulombs passing. This test appears more soundly based than the Ford test in terms of its basic principles, but again it has been little used in practice. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

W. Illig, Metalloberflache, 1959, 13, (2), 33 G. Elssner, Aluminium, 1959, 35 (4), 202-204 T. D. T. Latter, Finishing Industries, 1977, (2), 13-18, 22 N. D. Pullen, J . Electrodepos, Tech. Soc., 1939, 15, 69 W. L. Mauscher, Machinist (European edit.) 1945, 89 (25), 881 J . M. Kape, Metal Industry, 1959, 95(6), 115-118, 122 P. G. Sheasby, R. D. Guminski and T. K. Castle, Trans. Inst. Met. Finishing, 1966, 44(2), 50-56 J . H. Manhart and W. C. Cochran, Plating, 1971, 58(3), 219-224 F. Sacchi, G. Paolini and A. Prati, Alluminio, 1961, 30 (12), 597-611 H. Neunzig and V. Rohrig, Aluminium, 1962, 38 (3), 150-154 H. Schmecken, Aluminium, 1965, 41 (12), 760-764 B. A. Scott, Electroplating and Met. Finishing, 1965, 18(2), 47-50 C. Th. Speiser, Aluminium, 1973, 49 (10), 671-673

253

Salt spray tests These represent one of the traditional methods of corrosion testing and were used in early researches on anodic oxide coatings, particularly those produced on alloys of poor corrosion resistance, such as the Duralumin -type alloys. Work reported by J . D. Edwards 10 showed that on a Duralumin alloy (14S-T) 200 hours testing was required before a 5 film pitted, but on an A1-2 per cent Mg alloy (57S) for a 7 coating or thicker, upwards of 700 hours of exposure were required. Breakdown of the anodic oxide coating by pitting can be accelerated by the addition of acetic acid to a 3 per cent NaCl solution to give a pH of 3.2 - 3.5. Brace and Pocock*8 reported the time required for pitting on films of 5 - 26/7 thickness on 99.5 per cent Al and of 5 - 10/ithickness on 99.99 per cent Al - 1 per cent Mg. For the thinner films these ranged up to 100 hours (when tested at room temperature), depending upon the material anodized. Cooke and Spooner ”, using the ASTM Acidified Salt Spray Test (Method B286-61) which employs 5 per cent NaCl + acetic acid at 35 °C. reported that a 7p bright trim alloy (AA5457) withstood a 24-hour test without deterioration and also showed up poor sealing as bloom. More recently, Engelhart and Sowinski 52 have expressed the view that the discrimination of this test is not good, but it forms the basis of I.S.O. 3769 and is specified in B.S. 1615:1972, Appendix K. In Germany immersion in a solution of 25 g/1 NaCl. 15 ml/I glacial acetic acid and 3.3 ml/1 H2Oi (30% volume) for six days has been the accepted method of evaluating corrosion resistance. It appears to be much more effective as a means of discriminating between materials or of showing up important differences in film thickness than as providing a general quality control. Such a test can sometimes present difficulties in reproducibility since it can be influenced by the metallurgical characteristics of the material being anodized as well as by the anodizing and sealing conditions. The need to develop improved corrosion tests for electroplated automobile parts has led to the use of the CASS test which has also been applied to anodized aluminium. This test is essentially the ASTM acidified salt spray test to which a small amount of copper salt has been added, and it is operated at 50 c C (120'F) instead of 35°C (95°F). Carter 33 has reported good correlation between pitting resistance in industrial atmospheres in England and the results of the CASS test. He also concurs with Cooke and Spooner 31 that the test does not offer any useful evaluation of sealing quality, a view also expressed by Cochran 3* and by Bolmer and Graue 3*. Later work by Nakayama et al u has suggested that the CASS test would be more suitable for use with anodized aluminium if a lower solution pH of about 2 was used. This gives a faster rate of attack and greater reproducibilitv than the normal pH of 3.2. They also stress the importance of using a constant angle of exposure in order to obtain good reproducibility. The CASS test is now specified in B.S. 1615:1972. Appendix J and is the preferred method of corrosion testing, and is also specified in I.S.O. 3770. Both these specifications, however. 252

still recommend the generally used pH of 3.2. The Corrodkote test has proved rather unsatisfactory in all respects, giving poor discrimination in respect of the effects of both thickness and sealing. Electrolytic cathodic corrosion tests A cathodic dissolution test has been developed by the Ford Motor Co. of Detroit and used by them as an acceptance specification test 37. A drop of CASS solution is placed over a small area of anodized surface which is made the cathode, and a high d.c. potential is applied between a platinum anode and the test panel. An integrator, measuring the area under the current/time curve, provides an E.Q.T. number (Electrolytic Quality Test) for the 3 minute test period. J . D. Edwards has noted 3 that this test tends to exaggerate the influence of base metal purity, and Cooke and Spooner 31 found that the values obtained were not much influenced by variations in sealing time. On the other hand, Carter concludes 33 38 that the test does provide a rapid and simple means of differentiating between coating thicknesses on well-sealed articles and of distinguishing variations in the quality of sealing. Correlation with industrial exposure results are also reported as good, but the test was found to exaggerate the effects of basis metal purity. In practice, however, perhaps because of this latter characteristic, the test is no longer used even for automotive components. Kape 39 reported a few years ago a test in which a standard area of anodic oxide coating is made anodic, with the application of 40V potential, in a 5 per cent solution of formic acid. In this case the total number of coulombs passing during a 5 minute test period is taken as a measure of corrosion resistance. The higher the corrosion resistance, the lower the number of coulombs passing. This test appears more soundly based than the Ford test in terms of its basic principles, but again it has been little used in practice. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

W. Illig, Metalloberflache, 1959, 13, (2), 33 G. Elssner, Aluminium, 1959, 35 (4), 202-204 T. D. T. Latter, Finishing Industries, 1977, (2), 13-18, 22 N. D. Pullen, J . Electrodepos, Tech. Soc., 1939, 15, 69 W. L. Mauscher, Machinist (European edit.) 1945, 89 (25), 881 J . M. Kape, Metal Industry, 1959, 95(6), 115-118, 122 P. G. Sheasby, R. D. Guminski and T. K. Castle, Trans. Inst. Met. Finishing, 1966, 44(2), 50-56 J . H. Manhart and W. C. Cochran, Plating, 1971, 58(3), 219-224 F. Sacchi, G. Paolini and A. Prati, Alluminio, 1961, 30 (12), 597-611 H. Neunzig and V. Rohrig, Aluminium, 1962, 38 (3), 150-154 H. Schmecken, Aluminium, 1965, 41 (12), 760-764 B. A. Scott, Electroplating and Met. Finishing, 1965, 18(2), 47-50 C. Th. Speiser, Aluminium, 1973, 49 (10), 671-673

253

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31 . 32. 33. 34. 35. 36. 37. 38. 39.

E. Survila and D. P. Andrews, Trans. Inst. Met. Finishing, 1976, 54 (4), 163-173 A. C. Jason and J . L. Wood, Proc. Phys. Soc., 1955(B), 68, 1105 T. P. Hoar and G. C. Wood. Proc. A.D.A. Conference on Anodizing, 1961, 186-200 E. T. Engelhart and G. Sowinski, Modern Metals, 1964, 20 (7), 56-60 H. Birtel and W. Leute, Aluminium, 1965, 41 (1), 52-53 H. Birtel and W. Leute, Aluminium, 1967, 43 (2), 93-98 D. H. Bradshaw, P. G. Sheasby, G. Bancroft and D. F. Hack, Trans. Inst. Met. Finishing, 1972,50, 87-94 P. G. Sheasby and W. E. Cooke, Trans. Inst. Met. Finishing, 1974, 52, 103-106 H. Birtel and W. Bunk, Aluminium, 1968, 44 (12), 736-737 H. Birtel and W. Leute, Aluminium, 1969, 45 (7), 413-418 E. Survila, Trans. Inst. Met. Finishing, 1972, 50, 215-222 H. J . Gohausen, Aluminium, 1977, 53 (5), 317-321 F. Baumann and R. Lattey, Aluminium, 1955, 31 (5), 199-204 J. D. Edwards, Trans. Inst. Met. Finishing, 1958, 35, 55-78 A. W. Brace and K. Pocock. Trans. Inst. Met. Finishing, 1958, 35, 277-297 D. M. Britnell and J . J . B. Ward, Paper presented to Institute of Metal Finishing Conference, 1975 J . D. Edwards, Proc. A.S.T.M., 1945, 45, 146 W. E. Cooke and R. C. Spooner, Plating, 1961, 48 (1), 42 E. T. Engelhart and G. Sowinski, Paper to Automotive Engineering Congress, Detroit, S.A.E.J., January 1964 V. E. Carter, Trans. Inst. Met. Finishing, 1967, 45 (2), 64-74 W. C. Cochran, Test Methods for Anodized Aluminium and their significance. Paper to Aluminium Federation Anodizing Symposium, Aston, 1967 P. W. Bolmer and J . W. Graue, Plating, 1970, 57 (3), 241-244 T. Nakayama, M. Mushiro and K. Suzuki, Aluminium, 1972, 48 (12), 790-796 and 1974, 50 (8), 524-525 Ford Motor Company, Quality Laboratory and Chem. Eng. Phys. Test Method. MA-P, BQ7 V. E. Carter and J . Edwards, Trans. Inst. Met. Finishing, 1965, 43, 97-105 J . M. Kape, ‘Testing of Anodic Coatings’, Paper to S.T.S. International Symposium for the Standardisation of Surface Treatments of Metallic Materials, Turin, October 1961

Chapter 18: Tests and properties of anodic oxide coatings Part 2. Other tests and properties For routine control and acceptance testing checks on thickness of coating and quality of sealing are sufficient to ensure quality, although reference to some overall quality test such as the CASS test may be desirable as well. Some specific applications may call for additional end-use performance tests which additionally evaluate some further property such as reflectivity, resistance to staining, abrasion resistance or electrical insulation value. Most of these properties can be evaluated using relatively rapid and simple tests. Resistance of an anodized article to weathering whilst in service can ultimately only be evaluated by actual long-term experience and testing. It may then be possible to relate this behaviour to some accelerated test, but to extrapolate accelerated test results to predict a hypothetical service life is a step not to be advised unless considerable background experience is available. Weathering resistance Anodic oxide coatings are frequently required to provide protection of a component or assembly against deterioration under the influence of the weather and other environmental factors. One of the early uses of aluminium was for lighting reflectors and the British Aluminium Co. published data showing that, even with only a 3 - 5 t coating, an aluminium reflector showed negligible loss of reflectivity after 45 weeks outdoor service and was superior to alternative materials such as silver, rhodium or chromium plating after the same exposure period (see fig. 96). 100

Reflectivity - p e r cent ? 9 ? 5

&RYTAL ON SUPER PURITY ALUMINIUM

LACQUERf D SILVER PUTt

STAINLESS STEEL

CHROMIUM PLATE

PLAIN POLISHED ALUMINIUM,

20

30

40

Time- weeks

Fig. 96. Effect of atmospheric exposure on the reflectivity values for various reflector materials (relative to silvered glass as 100). 254

255

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31 . 32. 33. 34. 35. 36. 37. 38. 39.

E. Survila and D. P. Andrews, Trans. Inst. Met. Finishing, 1976, 54 (4), 163-173 A. C. Jason and J . L. Wood, Proc. Phys. Soc., 1955(B), 68, 1105 T. P. Hoar and G. C. Wood. Proc. A.D.A. Conference on Anodizing, 1961, 186-200 E. T. Engelhart and G. Sowinski, Modern Metals, 1964, 20 (7), 56-60 H. Birtel and W. Leute, Aluminium, 1965, 41 (1), 52-53 H. Birtel and W. Leute, Aluminium, 1967, 43 (2), 93-98 D. H. Bradshaw, P. G. Sheasby, G. Bancroft and D. F. Hack, Trans. Inst. Met. Finishing, 1972,50, 87-94 P. G. Sheasby and W. E. Cooke, Trans. Inst. Met. Finishing, 1974, 52, 103-106 H. Birtel and W. Bunk, Aluminium, 1968, 44 (12), 736-737 H. Birtel and W. Leute, Aluminium, 1969, 45 (7), 413-418 E. Survila, Trans. Inst. Met. Finishing, 1972, 50, 215-222 H. J . Gohausen, Aluminium, 1977, 53 (5), 317-321 F. Baumann and R. Lattey, Aluminium, 1955, 31 (5), 199-204 J. D. Edwards, Trans. Inst. Met. Finishing, 1958, 35, 55-78 A. W. Brace and K. Pocock. Trans. Inst. Met. Finishing, 1958, 35, 277-297 D. M. Britnell and J . J . B. Ward, Paper presented to Institute of Metal Finishing Conference, 1975 J . D. Edwards, Proc. A.S.T.M., 1945, 45, 146 W. E. Cooke and R. C. Spooner, Plating, 1961, 48 (1), 42 E. T. Engelhart and G. Sowinski, Paper to Automotive Engineering Congress, Detroit, S.A.E.J., January 1964 V. E. Carter, Trans. Inst. Met. Finishing, 1967, 45 (2), 64-74 W. C. Cochran, Test Methods for Anodized Aluminium and their significance. Paper to Aluminium Federation Anodizing Symposium, Aston, 1967 P. W. Bolmer and J . W. Graue, Plating, 1970, 57 (3), 241-244 T. Nakayama, M. Mushiro and K. Suzuki, Aluminium, 1972, 48 (12), 790-796 and 1974, 50 (8), 524-525 Ford Motor Company, Quality Laboratory and Chem. Eng. Phys. Test Method. MA-P, BQ7 V. E. Carter and J . Edwards, Trans. Inst. Met. Finishing, 1965, 43, 97-105 J . M. Kape, ‘Testing of Anodic Coatings’, Paper to S.T.S. International Symposium for the Standardisation of Surface Treatments of Metallic Materials, Turin, October 1961

Chapter 18: Tests and properties of anodic oxide coatings Part 2. Other tests and properties For routine control and acceptance testing checks on thickness of coating and quality of sealing are sufficient to ensure quality, although reference to some overall quality test such as the CASS test may be desirable as well. Some specific applications may call for additional end-use performance tests which additionally evaluate some further property such as reflectivity, resistance to staining, abrasion resistance or electrical insulation value. Most of these properties can be evaluated using relatively rapid and simple tests. Resistance of an anodized article to weathering whilst in service can ultimately only be evaluated by actual long-term experience and testing. It may then be possible to relate this behaviour to some accelerated test, but to extrapolate accelerated test results to predict a hypothetical service life is a step not to be advised unless considerable background experience is available. Weathering resistance Anodic oxide coatings are frequently required to provide protection of a component or assembly against deterioration under the influence of the weather and other environmental factors. One of the early uses of aluminium was for lighting reflectors and the British Aluminium Co. published data showing that, even with only a 3 - 5 t coating, an aluminium reflector showed negligible loss of reflectivity after 45 weeks outdoor service and was superior to alternative materials such as silver, rhodium or chromium plating after the same exposure period (see fig. 96). 100

Reflectivity - p e r cent ? 9 ? 5

&RYTAL ON SUPER PURITY ALUMINIUM

LACQUERf D SILVER PUTt

STAINLESS STEEL

CHROMIUM PLATE

PLAIN POLISHED ALUMINIUM,

20

30

40

Time- weeks

Fig. 96. Effect of atmospheric exposure on the reflectivity values for various reflector materials (relative to silvered glass as 100). 254

255

256 8 9

7

K 2Cr 2 O K 2Cr 2 O 7 none Water

H , S O4 H 2S O4 CrO, H 2S O 4

Anodic film thickness (p) 5

Anodizing Thickness treatment (p) [mins. ] F.

B.

F.

B** B.

F.

Condition Ct

o tn 1 o m o — — CM (N co m o rn tJ-

on pitting produced

272-3 yrs F 5-6 yrs D - E P NP NP NP NP NP

2 l/ 2 - 3 y r s F 5-6 yrs E NP NP NP NP NP NP moinoinoino - m n in

£

m
co on CS (N fN

52S 14S-T 14S-T 14S-T

Sealant

Electrolyte

SSSg

X EES

6

No.

Anodizing treatment

Table 27. Corrosion resistance of anodic oxide coatings on 52S ( AlMg 2) and 14S-T (Dural) exposed in various North American Locations 1

o co on CS (N fN

52S 14S-T 14S-T 14S-T

Sealant

Electrolyte

SSSg

X EES

6

No.

Anodizing treatment

Table 27. Corrosion resistance of anodic oxide coatings on 52S ( AlMg 2) and 14S-T (Dural) exposed in various North American Locations 1

o 20%20°C

90

0.16

80

25%25°C\

0.101. 0.12

I Ofi

i 0.16 x

018

0-9

8

T-

&

012

S

70 Reflectivity (%)

01lf

t/2

30 20 ' 10 0

0.4

0.6 0.8 1.0

2

4

6

8

10

Wavelength (p)

Fig. 100. Comparative reflectivities oj anodized aluminium, vacuum evaporated aluminium, silver and platinum over the range 0.4 - 10p r'. A ~ Vacuum evaporated aluminium B — Anodized aluminium

% White / on On 3335 35

on 2 5

on 335 Fast

ye

Red

B3LW dye

on 25

Fig. 99. Colour triangle showing position of colours of aluminium (2S) and aluminium —5% silicon (33S) anodized and dyed with turquoise PLWS of fast red B3LW dye. 266

More usually the reflectivity of anodized aluminium is measured by an optical technique which consists of directing a controlled beam of light on to the metal surface, measuring first the total amount of lijght reflected and then introduces an aperture which measures only the specularly reflected light, or a blackened cap which is intended to trap the light specularly reflected and to enable the diffusely reflected light to be measured. Scott 3'- has investigated the factors influencing the measurement of specular reflectivity and has demonstrated the importance of the diameter of the aperture orifice in determining the value obtained. Using a reflectivity head with an aperture of 1/16" to measure specular reflectivity and with total reflectivity measured with a separate head consisting of a circular photocell of relatively large area closely adjacent and parallel to the test surface, which is illuminated by diffuse light through a relatively large central 267

hole, he reports 37 values summarised in Table 33. The apparatus used is described in British Standard 1615 (1972), Methods P & Q and is illustrated in fig. 101 Table 33. Typical reflectivities of bright anodized aluminium and other reflector materials37 7? ef I e c t i v i t y

Material

Treatment

99.99% Al

Polished and electrobrightened *’

99.7% Al + 1% Mg

Polished and Phosbrite 159 treated

Silver-plated brass Chromium-plated brass Polished stainless steel

Film thicknes w

Total % incident light

Specular % of incident light

Specular % of total reflectivity

4 7-8 10-12 20

90 90 90 90

77 76 75 74

85.6 84.4 83.3 83.3

Data for total and specular reflectivity of aluminium of varying purities anodized to 5 z and for the effect of film thickness are given in an account by Lattey 54 which also describes the measuring instrument normally used in Germany. Additional data are given in a further account 38 giving reflectivity values for Raffmal (99.99 per cent Al), Reflectal 0.5 (99.99 per cenj Al + 0.5 per cent Mg), and Reflectal 2 (99.99 per cent Al + 2 per cent Mg) given various brightening treatments. In the United States other approaches to the measurement of brightness have been made. One of the most comprehensive of these is that reported by Barkman 39 using the Gardner Modified Pivotable Sphere Hazemeter, which consists of a tungsten lamp source in conjunction with a double filter (blue and amber), a pivotable magnesium oxide coated sphere and photocells. The sphere is first held on the 0° position for diffuse reflection measurement (D.R.) and is then moved through 8° to obtain a total reflectivity reading (T.R.). From these measurements a Specular Reflection Factor can be computed thus: T R

5 7-8 10-12 20

88 87 87 84

65 62 60 52

73.9 71.3 69.0 61.9

-

98 65 60

86 62 53

87.8 95.4 88.3

s

( T R 3 - D R s) -- D R s + ------------------------D R Ag (T R Ag - D R Ag)

S R F s — ioo

TR

S

However, if an instrument correction factor C has been determined by use of a known specular standard (e.g. a silvered mirror): D R Ag

C = -------------------T R Ag - D R Ag (T R

s

- D R s ) ( i + C)

Then S R F % = ioo

where SRFs

I 1 I - ,1

=

TR S

=

DR S

=

TR g

= =

Fig.101. Apparatus for measurement of total and specular reflectivity of anodized aluminium to the method of B. S. 1615. (Courtesy: Sheen Instruments Ltd. 268

specular reflectance factor of the sample surface expressed as % of the silver mirror standard. total reflectance of sample surface expressed as % of the magnesium oxide standard diffuse reflectance of the sample surface expressed as % of the magnesium oxide standard. total reflectance of the silver mirror standard expressed as % of the magnesium oxide standard. diffuse reflectance of the silver mirror standard expressed as % of the magnesium oxide standard.

With the same apparatus the specular brightness factor can be calculated from the equation: 269

hole, he reports 37 values summarised in Table 33. The apparatus used is described in British Standard 1615 (1972), Methods P & Q and is illustrated in fig. 101 Table 33. Typical reflectivities of bright anodized aluminium and other reflector materials37 7? ef I e c t i v i t y

Material

Treatment

99.99% Al

Polished and electrobrightened *’

99.7% Al + 1% Mg

Polished and Phosbrite 159 treated

Silver-plated brass Chromium-plated brass Polished stainless steel

Film thicknes w

Total % incident light

Specular % of incident light

Specular % of total reflectivity

4 7-8 10-12 20

90 90 90 90

77 76 75 74

85.6 84.4 83.3 83.3

Data for total and specular reflectivity of aluminium of varying purities anodized to 5 z and for the effect of film thickness are given in an account by Lattey 54 which also describes the measuring instrument normally used in Germany. Additional data are given in a further account 38 giving reflectivity values for Raffmal (99.99 per cent Al), Reflectal 0.5 (99.99 per cenj Al + 0.5 per cent Mg), and Reflectal 2 (99.99 per cent Al + 2 per cent Mg) given various brightening treatments. In the United States other approaches to the measurement of brightness have been made. One of the most comprehensive of these is that reported by Barkman 39 using the Gardner Modified Pivotable Sphere Hazemeter, which consists of a tungsten lamp source in conjunction with a double filter (blue and amber), a pivotable magnesium oxide coated sphere and photocells. The sphere is first held on the 0° position for diffuse reflection measurement (D.R.) and is then moved through 8° to obtain a total reflectivity reading (T.R.). From these measurements a Specular Reflection Factor can be computed thus: T R

5 7-8 10-12 20

88 87 87 84

65 62 60 52

73.9 71.3 69.0 61.9

-

98 65 60

86 62 53

87.8 95.4 88.3

s

( T R 3 - D R s) -- D R s + ------------------------D R Ag (T R Ag - D R Ag)

S R F s — ioo

TR

S

However, if an instrument correction factor C has been determined by use of a known specular standard (e.g. a silvered mirror): D R Ag

C = -------------------T R Ag - D R Ag (T R

s

- D R s ) ( i + C)

Then S R F % = ioo

where SRFs

I 1 I - ,1

=

TR S

=

DR S

=

TR g

= =

Fig.101. Apparatus for measurement of total and specular reflectivity of anodized aluminium to the method of B. S. 1615. (Courtesy: Sheen Instruments Ltd. 268

specular reflectance factor of the sample surface expressed as % of the silver mirror standard. total reflectance of sample surface expressed as % of the magnesium oxide standard diffuse reflectance of the sample surface expressed as % of the magnesium oxide standard. total reflectance of the silver mirror standard expressed as % of the magnesium oxide standard. diffuse reflectance of the silver mirror standard expressed as % of the magnesium oxide standard.

With the same apparatus the specular brightness factor can be calculated from the equation: 269

100

•S B F s %

----------------- | . (T R | T R a. - D R A g[

s

- D R s)

where SBF = specular brightness factor (image brightness of sample surface expressed as % of silver mirror standard.

measurements on surfaces ranging from low to high gloss, as well as on coloured surfaces. Methods for measuring light reflectivity are standardized in BS 1615 (1972), Appendices P, Q and R and in I.S.O. 2767 (with further methods in preparation). MgO 100

The range of values obtained for various bright trim materials, including both anodized aluminium and chromium plating, is shown in figs. 102 and 103. 80

80 MgO 0

/

+ SPECULAR ALUMINIUM

Diffuse reflectance %

80 O CHROME PLATE

40

60

OH80

Diffuse reflectance %

100

60

ALLOY O

O 4043 A L L O Y

60

40

40

20 20 60

40

/

Ag

MIRROR 80

40

20

0

Ag

’mirror

0 0

100

20

Total reflectance %

Fig.102. Results obtained by Barkman on Gardner Modified Pivotable Hazemeter for SRF and SBF factors o f anodized bright trim and other materials** . The graph shows the relationship of total and diffuse reflectance values at 0 - 8° as related to the adopted (MgO and black) primary standards and the silver mirror. S R F . designates the diffuse reflectance factor, and S B . F . % the diffuse brightness factor as percentages of the silver standard. Another apparatus developed by the German firm Dr Lange, which appears to be a development of the Scott instrument, has been described by Drapier and Lelong 82 who have used it to assess uniformity of appearance of anodized extrusions. Moller and Briicker 8' have adapted the head to give readings at a 20°, 60° and 85° reflection angle which it is claimed overcomes the problem of 270

60

80

100

Total reflectance %

Fig.103. Shows the reflectance triangle of Fig. 84 with diffuse terms in solid lines constituting diffuse reflectance factor values (D.R.F. = 100 — S.R.F.) and dashes constituting diffuse brightness factors. (The value for a matt silver surface is shown ( + ) in comparison with the silver standard. The 1180 surface designates commercially pure aluminium as etched and anodized; 4043 designates a grey architectural type aluminium alloy.)

Heat reflectivity Whilst brightened and anodized aluminium is a good reflector of ultra-violet and visible light the reflectivity to heat is mainly dependent upon the thickness of the anodic oxide coating (but not the purity of the metal). This effect is well demonstrated by fig. 104. The thickness of anodic film on heating reflectors is therefore kept to the absolute minimum consistent with retaining the reflective surface. 271

100

•S B F s %

----------------- | . (T R | T R a. - D R A g[

s

- D R s)

where SBF = specular brightness factor (image brightness of sample surface expressed as % of silver mirror standard.

measurements on surfaces ranging from low to high gloss, as well as on coloured surfaces. Methods for measuring light reflectivity are standardized in BS 1615 (1972), Appendices P, Q and R and in I.S.O. 2767 (with further methods in preparation). MgO 100

The range of values obtained for various bright trim materials, including both anodized aluminium and chromium plating, is shown in figs. 102 and 103. 80

80 MgO 0

/

+ SPECULAR ALUMINIUM

Diffuse reflectance %

80 O CHROME PLATE

40

60

OH80

Diffuse reflectance %

100

60

ALLOY O

O 4043 A L L O Y

60

40

40

20 20 60

40

/

Ag

MIRROR 80

40

20

0

Ag

’mirror

0 0

100

20

Total reflectance %

Fig.102. Results obtained by Barkman on Gardner Modified Pivotable Hazemeter for SRF and SBF factors o f anodized bright trim and other materials** . The graph shows the relationship of total and diffuse reflectance values at 0 - 8° as related to the adopted (MgO and black) primary standards and the silver mirror. S R F . designates the diffuse reflectance factor, and S B . F . % the diffuse brightness factor as percentages of the silver standard. Another apparatus developed by the German firm Dr Lange, which appears to be a development of the Scott instrument, has been described by Drapier and Lelong 82 who have used it to assess uniformity of appearance of anodized extrusions. Moller and Briicker 8' have adapted the head to give readings at a 20°, 60° and 85° reflection angle which it is claimed overcomes the problem of 270

60

80

100

Total reflectance %

Fig.103. Shows the reflectance triangle of Fig. 84 with diffuse terms in solid lines constituting diffuse reflectance factor values (D.R.F. = 100 — S.R.F.) and dashes constituting diffuse brightness factors. (The value for a matt silver surface is shown ( + ) in comparison with the silver standard. The 1180 surface designates commercially pure aluminium as etched and anodized; 4043 designates a grey architectural type aluminium alloy.)

Heat reflectivity Whilst brightened and anodized aluminium is a good reflector of ultra-violet and visible light the reflectivity to heat is mainly dependent upon the thickness of the anodic oxide coating (but not the purity of the metal). This effect is well demonstrated by fig. 104. The thickness of anodic film on heating reflectors is therefore kept to the absolute minimum consistent with retaining the reflective surface. 271

! ,----,

Reflectance

----r— -I

I----1----!----,

• Super purity aluminium % 99.5% aluminium

O | ----1----r

% Reflectivity for infra-red radiation (1000°C)

io

4

6

8

10

12

14

16

18

Thickness of anodic film in microns

Fig.104. Effect of anodic film thickness on reflectivity of aluminium for infra-red radiation from a source at I 000QC b\

69 59 A - 55 • - 15

-

Aluminum - 1075 Sulphuric acid, sealed Boric acid Chromic acid Hard coat, sealed

Wavelength - microns

Fig 106- Effect of anodizing process on infra-red reflectance of aluminium*. 09

o

Reflectivity (%)

B

Rhodium plate

20 Copper

0.2

0

100

200

300

400

500

600

Temperature (°C)

Fig. 105. Comparison of heat reflectivity of electrobrightened Brytal with other reflector materials over the range 20 - 600° C. 272

Aluminum - 7075 alloy - H a SO 4 • - 63 - Room temp. A - 63 - 300° F V ■ 63 - 600° F + - 63 - 825° F ■ - 63 - Room temp. - Final

0J

0

.t

»

Abb

.7 -•

9 I

1

3

«

ft

S 7 B • K)

Wavelength - microns

20

SO

Fig. 107. Effect of heating on reflectance of sulphuric acid anodized aluminium 4". 273

! ,----,

Reflectance

----r— -I

I----1----!----,

• Super purity aluminium % 99.5% aluminium

O | ----1----r

% Reflectivity for infra-red radiation (1000°C)

io

4

6

8

10

12

14

16

18

Thickness of anodic film in microns

Fig.104. Effect of anodic film thickness on reflectivity of aluminium for infra-red radiation from a source at I 000QC b\

69 59 A - 55 • - 15

-

Aluminum - 1075 Sulphuric acid, sealed Boric acid Chromic acid Hard coat, sealed

Wavelength - microns

Fig 106- Effect of anodizing process on infra-red reflectance of aluminium*. 09

o

Reflectivity (%)

B

Rhodium plate

20 Copper

0.2

0

100

200

300

400

500

600

Temperature (°C)

Fig. 105. Comparison of heat reflectivity of electrobrightened Brytal with other reflector materials over the range 20 - 600° C. 272

Aluminum - 7075 alloy - H a SO 4 • - 63 - Room temp. A - 63 - 300° F V ■ 63 - 600° F + - 63 - 825° F ■ - 63 - Room temp. - Final

0J

0

.t

»

Abb

.7 -•

9 I

1

3

«

ft

S 7 B • K)

Wavelength - microns

20

SO

Fig. 107. Effect of heating on reflectance of sulphuric acid anodized aluminium 4". 273

Comparison of anodized aluminium with other reflector materials is provided infig. 105. The use of aluminium in spacecraft has led to more extensive investigation of the properties of anodized aluminium in the infra-red range . The effect of the anodizing process is shown in fig. 106, whilst fig. 107 shows the effect of heating on reflectivity, particularly the fact that at 825°F (441°C) an apparently irreversible change takes place which improves reflectance in the 1 0 - 2 0 micron range. A method for measuring infra-red reflectivity is given in B.S. 1615 (1972), Appendix S.

Temperature stability The anodic coating is virtually unaffected chemically by heating up to the melting point of the aluminium (except for some loss of water), but owing to its low co-efficient of expansion crazing of the coating occurs. The temperature at which this becomes visible has been shown by Lelong and Herenguel 42 to be influenced by the thickness of the anodic film and the anodizing conditions which, if suitably chosen, could produce a 5/i film which would withstand 329°C without crazing (figs. 109 and 110).

Heat emissivity

Anodized

DEFORMS

Z

AND

CRAZNG

§

SURFACE

g

s

s

Energy curve Emission value

300

§

------- Sandblasted—♦—Matt —o— Polished—

350

Temperature °C

The emissivity of anodized aluminium is significantly higher than for either polished or etched metal and increases with wavelength. This means that it is particularly suitable for use in hot water radiators and heat exchangers. The emissivity of anodized aluminium over an energy range of 2 - 10 microns is shown in fig. 108 4!

CRAZING VIRTUALLY

§

Emissivity

§

§

ABSENT

»

15

20

25

30

Film thickness - microns

S

S

Fig.109.Effect of temperature and film thickness on the susceptibility to crazing of an Al-l% Mg alloy anodized in 180 g/l H 2 S(f at 15°C and 0. 75 A/ dm 1 and sealed in a fatty acid ester. Assessment after a 1 000 hour test.42

23456789

Wavelength (u) at 400° C

Fig. 108. Heat emissivity of anodized aluminium with various surface conditions compared with that of black copper 4'. 274

5

Michelson 83 84 investigated the effect of anodizing conditions, film thickness and heating conditions (time and temperature) on crazing, particularly under stove enamelling conditions. He found that lower electrolyte temperature (21°C as against 27°C) reduced crazing and gives recommendations on means of avoiding crazing on trim to which a paint finish has to be applied locally. 275

Comparison of anodized aluminium with other reflector materials is provided infig. 105. The use of aluminium in spacecraft has led to more extensive investigation of the properties of anodized aluminium in the infra-red range . The effect of the anodizing process is shown in fig. 106, whilst fig. 107 shows the effect of heating on reflectivity, particularly the fact that at 825°F (441°C) an apparently irreversible change takes place which improves reflectance in the 1 0 - 2 0 micron range. A method for measuring infra-red reflectivity is given in B.S. 1615 (1972), Appendix S.

Temperature stability The anodic coating is virtually unaffected chemically by heating up to the melting point of the aluminium (except for some loss of water), but owing to its low co-efficient of expansion crazing of the coating occurs. The temperature at which this becomes visible has been shown by Lelong and Herenguel 42 to be influenced by the thickness of the anodic film and the anodizing conditions which, if suitably chosen, could produce a 5/i film which would withstand 329°C without crazing (figs. 109 and 110).

Heat emissivity

Anodized

DEFORMS

Z

AND

CRAZNG

§

SURFACE

g

s

s

Energy curve Emission value

300

§

------- Sandblasted—♦—Matt —o— Polished—

350

Temperature °C

The emissivity of anodized aluminium is significantly higher than for either polished or etched metal and increases with wavelength. This means that it is particularly suitable for use in hot water radiators and heat exchangers. The emissivity of anodized aluminium over an energy range of 2 - 10 microns is shown in fig. 108 4!

CRAZING VIRTUALLY

§

Emissivity

§

§

ABSENT

»

15

20

25

30

Film thickness - microns

S

S

Fig.109.Effect of temperature and film thickness on the susceptibility to crazing of an Al-l% Mg alloy anodized in 180 g/l H 2 S(f at 15°C and 0. 75 A/ dm 1 and sealed in a fatty acid ester. Assessment after a 1 000 hour test.42

23456789

Wavelength (u) at 400° C

Fig. 108. Heat emissivity of anodized aluminium with various surface conditions compared with that of black copper 4'. 274

5

Michelson 83 84 investigated the effect of anodizing conditions, film thickness and heating conditions (time and temperature) on crazing, particularly under stove enamelling conditions. He found that lower electrolyte temperature (21°C as against 27°C) reduced crazing and gives recommendations on means of avoiding crazing on trim to which a paint finish has to be applied locally. 275

Hs S0 4 concentration g / 1

% V \

CRAZtMG VIRTUALLY ABSENT

5

»

15

20

25

Campbell 85 has shown that it is essential to make measurements across the film, especially since the greatest hardness is found in the layers near the surface. The relevant British Standard B.S. 5599 (1978) gives detailed instructions on the preparation and carrying out of the Vickers micro-hardness test and requires that the readings so obtained shall not be less than 350. SEVERE GENERAL

Wear resistance

CRA3NG

The wear resistance of the anodic coating is dependent upon anodizing conditions. Using a sulphuric acid electrolyte under hard anodizing conditions a wear resistance comparable with that ot cyanide hardened steel can be obtained, judged by performance figures by Gillig46 using the Taber Abrader(see fig. 111J.Tomashov and Tyukina 43 report the results shown in Table 34 from a five-hour test with a hard anodized high copper alloy (AK-4) in contact with steel using oil lubrication. 30

35

/oo.ooo

Temperature ° C

Mechanical properties Hardness Measurement of the hardness of a thin film is difficult and there are few reliable figures available. Micro-hardness measurements made on anodic coatings reported by three investigators are given below:

Coating of Raffinal (88.89% Al) Coating on aluminium (99.5%) Coating on high strength alloy AK4 Measured on Vickers machine Measured on Knoop machine Imputed from Taber Abrader Hardness on Moh’s scale

micro-hardness (kg/ mm) 1 500 43 600 41 350 43 400 -500 44 70044 1 200 44 7 - 9 45

60,000 50, OOO

Number of wear c y c l e s Taber Abraser CS - 17 Wheels, 1000 G load )

Fig. 110. Effect of anodizing temperature and H2SO4 concentration on the craze resistance of an 8p anodic oxide film produced under otherwise similar conditions to those in Fig 87 and tested for I 000 hours at 250° C 42.

90,000 60,000 70.000

40,000 30,000

20,000

t o , OOO 9,000 6,000 7, OOO 6,000 5,000 4,000 3. OOO

O.OOI

0.002

Amount of w e a r , in.

Fig. 111.Comparative wear resistance of hard anodized (Hard Coat) aluminium and other materials using the Taber Abrader lest 277

Hs S0 4 concentration g / 1

% V \

CRAZtMG VIRTUALLY ABSENT

5

»

15

20

25

Campbell 85 has shown that it is essential to make measurements across the film, especially since the greatest hardness is found in the layers near the surface. The relevant British Standard B.S. 5599 (1978) gives detailed instructions on the preparation and carrying out of the Vickers micro-hardness test and requires that the readings so obtained shall not be less than 350. SEVERE GENERAL

Wear resistance

CRA3NG

The wear resistance of the anodic coating is dependent upon anodizing conditions. Using a sulphuric acid electrolyte under hard anodizing conditions a wear resistance comparable with that ot cyanide hardened steel can be obtained, judged by performance figures by Gillig46 using the Taber Abrader(see fig. 111J.Tomashov and Tyukina 43 report the results shown in Table 34 from a five-hour test with a hard anodized high copper alloy (AK-4) in contact with steel using oil lubrication. 30

35

/oo.ooo

Temperature ° C

Mechanical properties Hardness Measurement of the hardness of a thin film is difficult and there are few reliable figures available. Micro-hardness measurements made on anodic coatings reported by three investigators are given below:

Coating of Raffinal (88.89% Al) Coating on aluminium (99.5%) Coating on high strength alloy AK4 Measured on Vickers machine Measured on Knoop machine Imputed from Taber Abrader Hardness on Moh’s scale

micro-hardness (kg/ mm) 1 500 43 600 41 350 43 400 -500 44 70044 1 200 44 7 - 9 45

60,000 50, OOO

Number of wear c y c l e s Taber Abraser CS - 17 Wheels, 1000 G load )

Fig. 110. Effect of anodizing temperature and H2SO4 concentration on the craze resistance of an 8p anodic oxide film produced under otherwise similar conditions to those in Fig 87 and tested for I 000 hours at 250° C 42.

90,000 60,000 70.000

40,000 30,000

20,000

t o , OOO 9,000 6,000 7, OOO 6,000 5,000 4,000 3. OOO

O.OOI

0.002

Amount of w e a r , in.

Fig. 111.Comparative wear resistance of hard anodized (Hard Coat) aluminium and other materials using the Taber Abrader lest 277

Table 34. Wear of un-anodized and hard anodized aluminium in contact with steel for 5-hour test period 43 Wear (mg/ cm 2 ) AK4 Unanodized Anodized

4.0, 7.0, 4.0 0

Coeff of friction

Steel 1.0*, 3.0*, 3.5* 0

0.12 0.13

microhardness and the erosion resistance. Films on an aluminium-magnesium alloy had higher erosion resistance than those produced on a high strength Al-Cu alloy. Polishing the anodized surface with chromic oxide polishing material further improved the erosion resistance. Table 35. Comparative Schuh and Kern abrasion resistance of decorative and hard anodic coatings on various alloys 46

* weight gain. However, Wright 47 has found that unless the heat produced from high speed movement is quickly dissipated by contact with a steel or other mating surface, the coating quickly cracks and seizure follows. Presence of a soap-forming substance, e.g. oleic acid or aluminium oleate itself, reduced the coefficient of friction. It was concluded that hard anodized surfaces are useful in applications which avoid high sliding temperatures and impact loads. Hard coatings have been used successfully in a number of items of hydraulic and fuel pumping systems. The reproducibility of the Taber Abrader test has been challenged by investigators and currently the Erichsen apparatus — model 317 Al — is now the preferred test method. The first results with this apparatus were reported by Witt and Gerken 86 , and further tests by Witt and Sautter87 have been recently reported. Gohausen 88 89 has investigated in detail the effects of anodizing and sealing conditions on wear resistance as determined by this apparatus. The method is now under consideration for standardisation by the LS.O. It is also preferred to the Schuh and Kern and similar tests. Abrasion resistance, measured by the Schuh and Kern method (in which 180 mesh silicon carbide is directed as a jet under 5 cm air pressure on to the anodized surface and the weight of abrasive required to penetrate the coating is measured), has also been used as a tool to test for wear resistance but it appears to be essentially a measure of erosion resistance. Comparative tests on various alloys having a normal decorative coating and a hard coating are listed in Table 35. This test is covered by B.S. 1615 (1972) Appendix L. 48

Spooner has shown that Schuh and Kern abrasion resistance is increased by 40 per cent by raising the current density from 12 to 24 A/ft 2 (1.3 - 2.6 A/dm 2 ) and by 15 per cent by lowering the electrolyte temperature from 30°C to 21°C, whilst Brace and Pocock 49 have obtained a nearly 40 per cent increase by lowering the temperature from 21 °C to 15°C. Both workers used a 3.3N H ?SO4 electrolyte. Tomashov, Schreider and Byalobzheskii 50 have used a similar technique but employing a higher particle speed (approximately 275 m/sec) to measure erosion resistance. They found that films prepared under hard anodizing conditions had a considerably higher resistance but there was no relation between the measured 278

Alloy 2S-H18 3S-H18 61S-T6 75S-T6 26S-T3

Abrasion resistance (g/mil) Alcoa 226 (hard) Alcoa 204 decorative process process 75 62 89 102 54

173 158 169 168 58

All the above authors are in agreement that homogeneous Al-Mg-Si and Al-Mg alloys give resistance superior to heterogeneous complex Al-Cu alloys, and from Herenguel’s work 51 it also appears that even homogeneous binary Al-Cu alloys do not give good compact films. Ductility Anodic coatings are essentially brittle and although much work has been undertaken in an effort to produce ‘ductile’ coatings, it has not been successful. Hill and Mason 52 found that when tested in tension, cracks appeared in the sealed anodic oxide coatings produced in sulphuric and oxalic electrolytes at a strain of 0.003 - 0.0045 in/in but coatings from a chromic acid bath showed no cracks until strained 0.0155 in/in. Kohler 53 studied the deformation of coatings by tensile testing and by the Erichsen cupping test using both the microscope and aural detection supplemented by the application of a 24V potential to the specimen immersed in sodium alizarin sulphonate, which produced a red colouration in the cracks. He concluded that it was practically impossible to deform anodic oxide coatings without producing cracking irrespective of anodizing conditions, and that only small deformations were required to produce cracks. Pullen 55 has shown that the apparent flexibility decreases with thickness and gives a value of 0.3 - 0.4 per cent for the actual elongation of the oxide film before cracking. 279

Table 34. Wear of un-anodized and hard anodized aluminium in contact with steel for 5-hour test period 43 Wear (mg/ cm 2 ) AK4 Unanodized Anodized

4.0, 7.0, 4.0 0

Coeff of friction

Steel 1.0*, 3.0*, 3.5* 0

0.12 0.13

microhardness and the erosion resistance. Films on an aluminium-magnesium alloy had higher erosion resistance than those produced on a high strength Al-Cu alloy. Polishing the anodized surface with chromic oxide polishing material further improved the erosion resistance. Table 35. Comparative Schuh and Kern abrasion resistance of decorative and hard anodic coatings on various alloys 46

* weight gain. However, Wright 47 has found that unless the heat produced from high speed movement is quickly dissipated by contact with a steel or other mating surface, the coating quickly cracks and seizure follows. Presence of a soap-forming substance, e.g. oleic acid or aluminium oleate itself, reduced the coefficient of friction. It was concluded that hard anodized surfaces are useful in applications which avoid high sliding temperatures and impact loads. Hard coatings have been used successfully in a number of items of hydraulic and fuel pumping systems. The reproducibility of the Taber Abrader test has been challenged by investigators and currently the Erichsen apparatus — model 317 Al — is now the preferred test method. The first results with this apparatus were reported by Witt and Gerken 86 , and further tests by Witt and Sautter87 have been recently reported. Gohausen 88 89 has investigated in detail the effects of anodizing and sealing conditions on wear resistance as determined by this apparatus. The method is now under consideration for standardisation by the LS.O. It is also preferred to the Schuh and Kern and similar tests. Abrasion resistance, measured by the Schuh and Kern method (in which 180 mesh silicon carbide is directed as a jet under 5 cm air pressure on to the anodized surface and the weight of abrasive required to penetrate the coating is measured), has also been used as a tool to test for wear resistance but it appears to be essentially a measure of erosion resistance. Comparative tests on various alloys having a normal decorative coating and a hard coating are listed in Table 35. This test is covered by B.S. 1615 (1972) Appendix L. 48

Spooner has shown that Schuh and Kern abrasion resistance is increased by 40 per cent by raising the current density from 12 to 24 A/ft 2 (1.3 - 2.6 A/dm 2 ) and by 15 per cent by lowering the electrolyte temperature from 30°C to 21°C, whilst Brace and Pocock 49 have obtained a nearly 40 per cent increase by lowering the temperature from 21 °C to 15°C. Both workers used a 3.3N H ?SO4 electrolyte. Tomashov, Schreider and Byalobzheskii 50 have used a similar technique but employing a higher particle speed (approximately 275 m/sec) to measure erosion resistance. They found that films prepared under hard anodizing conditions had a considerably higher resistance but there was no relation between the measured 278

Alloy 2S-H18 3S-H18 61S-T6 75S-T6 26S-T3

Abrasion resistance (g/mil) Alcoa 226 (hard) Alcoa 204 decorative process process 75 62 89 102 54

173 158 169 168 58

All the above authors are in agreement that homogeneous Al-Mg-Si and Al-Mg alloys give resistance superior to heterogeneous complex Al-Cu alloys, and from Herenguel’s work 51 it also appears that even homogeneous binary Al-Cu alloys do not give good compact films. Ductility Anodic coatings are essentially brittle and although much work has been undertaken in an effort to produce ‘ductile’ coatings, it has not been successful. Hill and Mason 52 found that when tested in tension, cracks appeared in the sealed anodic oxide coatings produced in sulphuric and oxalic electrolytes at a strain of 0.003 - 0.0045 in/in but coatings from a chromic acid bath showed no cracks until strained 0.0155 in/in. Kohler 53 studied the deformation of coatings by tensile testing and by the Erichsen cupping test using both the microscope and aural detection supplemented by the application of a 24V potential to the specimen immersed in sodium alizarin sulphonate, which produced a red colouration in the cracks. He concluded that it was practically impossible to deform anodic oxide coatings without producing cracking irrespective of anodizing conditions, and that only small deformations were required to produce cracks. Pullen 55 has shown that the apparent flexibility decreases with thickness and gives a value of 0.3 - 0.4 per cent for the actual elongation of the oxide film before cracking. 279

Russian workers have also investigated this phenomenon and report, in Table 36, the effect of anodizing temperature on the ability of an anodized specimen to withstand bending without cracking. Table 36. Bend angle to produce cracking on 4N H2SO4 coatings produced by anodizing at 1 A/dm 2 for 20 minutes 50 Temp. °C To first cracks 2.5 10 20 30 40

0.75 0.60 1.00 1.75 2.25

Bend angle to continuous cracks 1.62 1.50 2.0 3.0 3.6

Patrie1*0 has developed an apparatus for determining the flexibility of anodic coatings and obtained values of 0.32 - 0.35% elongation to fracture on an Al-2% Cu-Mg alloy and of 0.23 - 0.26% on an Al-Zn-Mg alloy. Fatigue resistance Data on the effect of anodic oxide coatings on fatigue properties are conflicting in detail and the figures given below should be treated with caution. In addition to the anodizing technique used and the thickness of coating, factors such as alloy, surface treatment, shape of specimen and method of test all affect the values obtained. Some general trends appear to be discernible from the results of various investigations. Effect of anodizing technique and alloy It would seem that anodizing in chromic acid electrolytes produces some improvement in fatigue properties. Gerard and Sutton* reported a 20 per cent increase in endurance strength for Duralumin specimens so anodized and similar improvements for the same material and anodizing conditions were also noted by Krenig and Ambartsumyan 57. Schreider et al 52 reported an 11 per cent increase in fatigue resistance at 150 x 106 cycles in pure bending for AK-6 alloy (1.8 2.6 per cent Cu. 0.4 - 0.8 per cent Mg. 0.4 - 0.8 per cent Mn. 0. - 1.2 per cent Si) anodized in 3 per cent chromic acid. On the other hand anodic oxide films produced in sulphuric acid and oxalic acid electrolyes appear generally to lower the fatigue resistance. Mauksch 59 reported no change in fatigue properties with Al-Cu and Al -Cu-Mg alloys anodized in sulphuric or oxalic acid, but it appears that the films were rather thin. Muller reported reductions of 4 - 20 per cent in properties of Al-Mg 280

and Al-Cu-Mg alloy samples anodized in sulphuric acid, and similar reductions were reported by Igarashi and Fukai 61 for Duralumin anodized in oxalic acid. Stickley and Howell 62 , in a more recent investigation using Al-Cu-Mg-Mn and Al-Zn-Mg-Cu alloys, found that thin films, approximately 3g thick, gave a slight improvement in fatigue properties but with thicker films the properties fell depending on the load. The fall was found to be small with light loads but large at high stresses. Effect of suface preparation Abadie and Vidal 63 have carried out exhaustive tests with Schenk ‘Webi’-type fatigue specimens on the effects of surface condition on the fatigue resistance of two high strength aluminium alloys, AU4SG (Duralumin-type) and AZ5GU (Al-5 per cent Zn-2 per cent Mg-Cu-Mn). The basic treatment used was to prepare the specimens by normal machine milling to size and shape and then to degrease and process. The results are summarised in Table 37. The acid etching process used comprised a 5 minute immersion in 30 per cent H 3PO4 -30 per cent HF at room temperature followed by a nitric acid dip. The anodizing treatment was for 60 minutes in a chromic acid electrolyte with 50V constant voltage. Electropolishing was carried out for 12 minutes in a phosphoricchromic acid proprietary electrolyte. Hockenhull, Gupta and Hurst 91 have reported fatigue test results on forged specimens of alloy RR58, sulphuric acid anodized and dichromate sealed, in which the effects of film thickness and relative humidity on fatigue strength and crack initiation are detailed. Table 37. Fatigue strengths of two high-strength alloys prepared in various ways 63 Preparation

Alternating stress (Kg/ mm 2 ) to produce fracture in cycles: AU4SG

Machined Machined + anodized Machined 4- acid etched Machined + acid etched + anodized Machined + electropolished Machined + electro- * polished + anodized Machined + Alodine 1200

4

10 s 22 22 20

10 6 14 15 12

107 11 14 10

17.5 17

35

22

16

13

25

15

14

35

22

14

11

22 24

16 16

15 15

37 33

24 19

17 11

15 9

10 5 23 26 24

10 13 17.5 14

38

23

38 40 40

10 39 43 41

AZ5GU 10 4 38 33 36

6

107 12 17 13

281

Russian workers have also investigated this phenomenon and report, in Table 36, the effect of anodizing temperature on the ability of an anodized specimen to withstand bending without cracking. Table 36. Bend angle to produce cracking on 4N H2SO4 coatings produced by anodizing at 1 A/dm 2 for 20 minutes 50 Temp. °C To first cracks 2.5 10 20 30 40

0.75 0.60 1.00 1.75 2.25

Bend angle to continuous cracks 1.62 1.50 2.0 3.0 3.6

Patrie1*0 has developed an apparatus for determining the flexibility of anodic coatings and obtained values of 0.32 - 0.35% elongation to fracture on an Al-2% Cu-Mg alloy and of 0.23 - 0.26% on an Al-Zn-Mg alloy. Fatigue resistance Data on the effect of anodic oxide coatings on fatigue properties are conflicting in detail and the figures given below should be treated with caution. In addition to the anodizing technique used and the thickness of coating, factors such as alloy, surface treatment, shape of specimen and method of test all affect the values obtained. Some general trends appear to be discernible from the results of various investigations. Effect of anodizing technique and alloy It would seem that anodizing in chromic acid electrolytes produces some improvement in fatigue properties. Gerard and Sutton* reported a 20 per cent increase in endurance strength for Duralumin specimens so anodized and similar improvements for the same material and anodizing conditions were also noted by Krenig and Ambartsumyan 57. Schreider et al 52 reported an 11 per cent increase in fatigue resistance at 150 x 106 cycles in pure bending for AK-6 alloy (1.8 2.6 per cent Cu. 0.4 - 0.8 per cent Mg. 0.4 - 0.8 per cent Mn. 0. - 1.2 per cent Si) anodized in 3 per cent chromic acid. On the other hand anodic oxide films produced in sulphuric acid and oxalic acid electrolyes appear generally to lower the fatigue resistance. Mauksch 59 reported no change in fatigue properties with Al-Cu and Al -Cu-Mg alloys anodized in sulphuric or oxalic acid, but it appears that the films were rather thin. Muller reported reductions of 4 - 20 per cent in properties of Al-Mg 280

and Al-Cu-Mg alloy samples anodized in sulphuric acid, and similar reductions were reported by Igarashi and Fukai 61 for Duralumin anodized in oxalic acid. Stickley and Howell 62 , in a more recent investigation using Al-Cu-Mg-Mn and Al-Zn-Mg-Cu alloys, found that thin films, approximately 3g thick, gave a slight improvement in fatigue properties but with thicker films the properties fell depending on the load. The fall was found to be small with light loads but large at high stresses. Effect of suface preparation Abadie and Vidal 63 have carried out exhaustive tests with Schenk ‘Webi’-type fatigue specimens on the effects of surface condition on the fatigue resistance of two high strength aluminium alloys, AU4SG (Duralumin-type) and AZ5GU (Al-5 per cent Zn-2 per cent Mg-Cu-Mn). The basic treatment used was to prepare the specimens by normal machine milling to size and shape and then to degrease and process. The results are summarised in Table 37. The acid etching process used comprised a 5 minute immersion in 30 per cent H 3PO4 -30 per cent HF at room temperature followed by a nitric acid dip. The anodizing treatment was for 60 minutes in a chromic acid electrolyte with 50V constant voltage. Electropolishing was carried out for 12 minutes in a phosphoricchromic acid proprietary electrolyte. Hockenhull, Gupta and Hurst 91 have reported fatigue test results on forged specimens of alloy RR58, sulphuric acid anodized and dichromate sealed, in which the effects of film thickness and relative humidity on fatigue strength and crack initiation are detailed. Table 37. Fatigue strengths of two high-strength alloys prepared in various ways 63 Preparation

Alternating stress (Kg/ mm 2 ) to produce fracture in cycles: AU4SG

Machined Machined + anodized Machined 4- acid etched Machined + acid etched + anodized Machined + electropolished Machined + electro- * polished + anodized Machined + Alodine 1200

4

10 s 22 22 20

10 6 14 15 12

107 11 14 10

17.5 17

35

22

16

13

25

15

14

35

22

14

11

22 24

16 16

15 15

37 33

24 19

17 11

15 9

10 5 23 26 24

10 13 17.5 14

38

23

38 40 40

10 39 43 41

AZ5GU 10 4 38 33 36

6

107 12 17 13

281

Effect of hard anodizing The use of thick hard anodic coatings in aircraft hydraulic and fuel systems has caused some further investigations to be made. Gillig46 has confirmed Stickley and Howell’s findings regarding the effect of stress on endurance limit using unsealed hard anodized Al-Zn-Mg-Cu (76S) material, but found that sealing for 15 minutes in 5 per cent K2 Cr2 O7 at 95° - 100°C alleviated the loss of fatigue properties due to anodizing, although the lowering (20 - 30 per cent) was still significant and there was some loss of hardness. Schreider et al 58 have also published some detailed test results for hard coatings summarised in Table 38. Table 38. Effect of hard anodizing on fatigue resistance of various Russian alloys tested to 150 x 10 cycles after hard anodizing in 20% w/v H SO 58 Film thickness

Treatment after anodizing

Change in fatigue strength (%)

170 100 80 10

None None None K 2Cr 2Ch sealed

-45.5 -33.3 -12.5 -18

Alloy AMg AK4-1 AL 4 AMg

under conditions of high humidity. Sealing of the anodic oxide coating in boiling water or in one of the many aqueous sealants reduces this effect considerably but does not appear to eliminate it. There are, however, no published data that provide adequate information on the stability of sealed anodic oxide coatings. Sealing in wax, epoxy resin or other organic material has been mentioned frequently in the literature, but this treatment brings with it the disadvantage of uncertain, or loss of high temperature insulation properties, one of the major advantages of anodic oxide coatings. Dielectric constant Values of 7 - 8 are generally quoted for this property, but there are indications that the value is affected by the anodizing process and possibly also by sealing. Using oxalic acid anodic coatings one worker has reported a value of 8.0 64, and another, values of 7.4 - 7.6 65. Lower values of 5 - 6 have been quoted 66 for sulphuric acid anodic oxide coatings. Dielectric strength One paper 67 has quoted a value of 47.5V//Z for the dielectric strength of 5 - 10/z coatings produced by continuous anodizing in sulphuric acid electrolytes. This is broadly in agreement with values of 30 - 45V//z quoted by Mauscher 72 for 12/z sulphuric acid anodic oxide coatings produced conventionally, but values 16 - 25V/g were reported for high Si alloys. Another source 66 quotes 30 - 45V//z for pure Al and alloys in solid solution but only 21 - 30V /jz for alloys containing Si or Cu in amounts such that these elements are not taken into solid solution.

It was observed that crumbling of the oxide film took place in the region of the fracture on the test pieces and that there was a roughening of the metal surface. It was considered by these workers that cracks develop in the film and act as foci for cracking. This appears a highly probable explanation of the lowering of the endurance limit, but there is probably also a contributory effect associated with the roughening of the metal at the metal-oxide interface and there is need for more careful investigation as to the extent to which this can be influenced by anodizing conditions.

Using wound coils of wire continuously anodized either in sulphuric acid alone (Type 100) or with an addition of MgCl (Type 200), Smits 68 has investigated the effects of heating on dielectric strength, and his results are summarised in Table 39.

Electrical properties

Table 39. Effect of heating on the dielectric strength of continuously anodized wire

Although strictly physical properties the various electrical characteristics of anodic oxide coatings are sufficiently important and unrelated to other properties to merit separate treatment. Despite continuing interest in electrical applications over 30 years there is still a paucity of data, reflecting in part the limited application of anodizing for electrical insulation purposes but also some of the problems inherent in making measurements with anodic oxide coatings. One of the basic difficulties is that much work has been carried out on unsealed anodic oxide coatings, but very little with sealed coatings. Unsealed anodic oxide coatings are very hygroscopic and their electrical properties are influenced considerably by atmospheric humidity, having decidedly inferior insulation properties 282

Temperature

Room 1 hr. at 100°C 1 hr. at 200°C 1 hr. at 300°C 1 hr. at 400°C 1 hr. at 500°C 1 hr. at 600°C

Breakdown voltage Type 100 Type 200 coatings coatings 310 315 300 305 320 285 345

330 320 300 290 315 315 325 283

Effect of hard anodizing The use of thick hard anodic coatings in aircraft hydraulic and fuel systems has caused some further investigations to be made. Gillig46 has confirmed Stickley and Howell’s findings regarding the effect of stress on endurance limit using unsealed hard anodized Al-Zn-Mg-Cu (76S) material, but found that sealing for 15 minutes in 5 per cent K2 Cr2 O7 at 95° - 100°C alleviated the loss of fatigue properties due to anodizing, although the lowering (20 - 30 per cent) was still significant and there was some loss of hardness. Schreider et al 58 have also published some detailed test results for hard coatings summarised in Table 38. Table 38. Effect of hard anodizing on fatigue resistance of various Russian alloys tested to 150 x 10 cycles after hard anodizing in 20% w/v H SO 58 Film thickness

Treatment after anodizing

Change in fatigue strength (%)

170 100 80 10

None None None K 2Cr 2Ch sealed

-45.5 -33.3 -12.5 -18

Alloy AMg AK4-1 AL 4 AMg

under conditions of high humidity. Sealing of the anodic oxide coating in boiling water or in one of the many aqueous sealants reduces this effect considerably but does not appear to eliminate it. There are, however, no published data that provide adequate information on the stability of sealed anodic oxide coatings. Sealing in wax, epoxy resin or other organic material has been mentioned frequently in the literature, but this treatment brings with it the disadvantage of uncertain, or loss of high temperature insulation properties, one of the major advantages of anodic oxide coatings. Dielectric constant Values of 7 - 8 are generally quoted for this property, but there are indications that the value is affected by the anodizing process and possibly also by sealing. Using oxalic acid anodic coatings one worker has reported a value of 8.0 64, and another, values of 7.4 - 7.6 65. Lower values of 5 - 6 have been quoted 66 for sulphuric acid anodic oxide coatings. Dielectric strength One paper 67 has quoted a value of 47.5V//Z for the dielectric strength of 5 - 10/z coatings produced by continuous anodizing in sulphuric acid electrolytes. This is broadly in agreement with values of 30 - 45V//z quoted by Mauscher 72 for 12/z sulphuric acid anodic oxide coatings produced conventionally, but values 16 - 25V/g were reported for high Si alloys. Another source 66 quotes 30 - 45V//z for pure Al and alloys in solid solution but only 21 - 30V /jz for alloys containing Si or Cu in amounts such that these elements are not taken into solid solution.

It was observed that crumbling of the oxide film took place in the region of the fracture on the test pieces and that there was a roughening of the metal surface. It was considered by these workers that cracks develop in the film and act as foci for cracking. This appears a highly probable explanation of the lowering of the endurance limit, but there is probably also a contributory effect associated with the roughening of the metal at the metal-oxide interface and there is need for more careful investigation as to the extent to which this can be influenced by anodizing conditions.

Using wound coils of wire continuously anodized either in sulphuric acid alone (Type 100) or with an addition of MgCl (Type 200), Smits 68 has investigated the effects of heating on dielectric strength, and his results are summarised in Table 39.

Electrical properties

Table 39. Effect of heating on the dielectric strength of continuously anodized wire

Although strictly physical properties the various electrical characteristics of anodic oxide coatings are sufficiently important and unrelated to other properties to merit separate treatment. Despite continuing interest in electrical applications over 30 years there is still a paucity of data, reflecting in part the limited application of anodizing for electrical insulation purposes but also some of the problems inherent in making measurements with anodic oxide coatings. One of the basic difficulties is that much work has been carried out on unsealed anodic oxide coatings, but very little with sealed coatings. Unsealed anodic oxide coatings are very hygroscopic and their electrical properties are influenced considerably by atmospheric humidity, having decidedly inferior insulation properties 282

Temperature

Room 1 hr. at 100°C 1 hr. at 200°C 1 hr. at 300°C 1 hr. at 400°C 1 hr. at 500°C 1 hr. at 600°C

Breakdown voltage Type 100 Type 200 coatings coatings 310 315 300 305 320 285 345

330 320 300 290 315 315 325 283

A limitation to anodic oxide coatings is that existing continuous anodizing processes do not produce films greater than 10/z in thickness having a breakdown voltage of 300 - 350V. Where higher values are required components can be individually thickly anodized. Whilst conventional anodizing gives a breakdown voltage of around 1 000V for a 25 p film, hard anodizing gives lower values for similar thickness but higher overall values can be achieved because of the greater thicknesses that can be obtained. Campbell 44 has reported values of 950V for an unsealed 50 film, increased to 1 200V by water sealing and to 1 500V if sealing is followed by a dip in molten paraffin wax, with values up to 2 000V for thicker coatings. Values as high as 3 750V have been claimed for the MHC hard anodized coating. Leakage current

Breakdown voltage measurements are a somewhat crude method of assessing the insulation resistance of anodic oxide coatings. For many applications the more important criterion is the voltage a coating will withstand before there is a significant current flow between adjacent turns in a coil. Smits 68 has measured the voltage required to produce various leakage currents through two layers of a 0.13 mm (0.0508") diameter Type 100 and Type 200 wire wound on a 50.8 (2") x 25.4 mm(l") diameter bobbin at 3 per cent,52per cent and 100 per cent relative humidity. The results are reported in Table 40. It should be remembered that most coils operate at temperatures of 50°C upwards, and with many of them the operating conditions will not involve exposure to high humidity. The poorer performance of the Type 200 coating may be due either to entrapment of small particles of unoxidised metal in the coating or to its higher porosity providing a greater surface area for moisture absorption. Table 40. D.C. voltage required at various humidities to cause stated leakage currents to flow in a two-layer anodized wire coil

Wire Type

284

Specific resista:nee

Information on this property is very limited. The values shown in Table 41 were obtained 1by Franckenstein 69 on a 50p oxalic acid coating measured at various temperatures. Table 41. S]pecific resistance of 50/z oxalic coating at various temperatures

Temp. (°C)

60

Specific resistance (ohm/ cm /cm 2 )

20 100 200 300

4 8 1.1 9

x x x x

IO15 10 14 IO'4 IO'2

Some measu rements were also made on sulphuric a7oS h ° V ' A ' V ' S c h r e i d e r a n d A - V - Byalobzheskii, Zhur, Priklad. Khim,, 1953, 51. J . Herenguel and P. Lelong, Rev. Aluminium, 1957, 34 (249) 1197 52. H. N. Hill and R. B. Mason, Metals and Alloys, 1942, 15 (6) 972 53. W. Kohler, Werkstoffe u. Korrosion. 1955, 6(4) 169 54. R. Lattey, Proc, A.E.S., 1959, 46, 233 55. N. D. Pullen, J. Electrodepos. Tech. Soc., 1939, 15, 69 56. I. J . Gerard and H. Sutton, J . Inst. Metals, 1935, 56, 35 57 ' msSRwiOdn a n d R ' S ' A r n b a r t s u m >'a n ' 'Corrosion of Metals in Aircraft’, Oborgoniz 58. A. V Schreider, A. V. Byalobzheskii, Z. T. Zagritsensko and B. V. Serebrennikov, Metallov. i. Obrabotka Metallov. (UUSR), 1956, (4) 14 59. W. Mauksch, Aluminium, 1941, 23, 285 60. W. Muller, Schweiz. Archiv., 1939, 5, 294 61 . I. Igarashi and S. Fukai, Trans. Soc. Meeh. Eng., Japan, 1940, 6, 1 62. G. W. Stickley and F. M. Howell, Proc. A.S.T.M., 1950, 50, 735 ’ 63. G. Abadie and G. Vidal, Proc. A.E.S., 1959, 46, 277 64. W. Hermann, Wiss, Veroff, Siemens-Werken, 1940, p. 188 65 (194o" n y ’ A n O d ' C O , l i d a t i O n ° f A l u m i n i u m a n d >ts Alloys’, Griffin & Co. Ltd., London 66. Aluminium Development Assocn., ‘Anodic Oxidation of Aluminium and its Alloys’,

287

Londong (1961) p. 139 A. A. Defoe, Wire and Wire Products, 1957, 31 (11) 134; (12) 1401 P. Smits, Modern Metals, 1958, 14 (7) 30 G. Franckenstein, Ann. Physik, 1936, 26 (5) 17 S . S . Gutin, Zhur.Teckn.Fiz., 1933,3(8) J . J . Lee, Proc. Conference on Anodizing, A.D.A. (1961) p. 130 W. Mauscher, Machinist (European Ed.), 1945, 89 (25) 881 E. A. G. Liddiard, G. Sanderson and J . E. Penn, Trans. Inst. Met. Finishing 1974, 49 (5) 200 74. V. E. Carter, J . Inst, of Metals, 1972, (7) 75 75. V. E. Carter, Br. Corrosion J . 1974, (1) 10 76. J . Patrie, Trans. Inst. Met. Finishing, 1975, 53 (1) 28 77. L. Knutsson and K. Dahlberg, Trans. Inst. Met. Finishing, 1976, 54 (1) 28 78. C. Th. Speiser and H. Schenkel, Metals and Materials, 1976, 165 79. C. Th. Speiser, Trans. Inst. Met. Finishing, 1977 55, (4), 165 80. G. E. Gardam, Trans. Inst. Met. Finishing, 1964, 41 (5) 190 81. K. H. Moller and F. Briicker, Aluminium 1977,53(5)314 82. C. Drapier and P. Lelong, Metal Finishing J . 1969, (4) 134 83. C. E. Michelson. Thermal Crazing of Anodic Coatings on Aluminium, Paper to S.A.E. Mid-Year Meeting, Chicago, 17-21 May, 1965 84. C. E. Michelson, S.A.E. Journal, 1965, (10) 60 85. W. J . Campbell, Proc. Conference on Anodizing Aluminium, Aluminium Development Assocn., 1961, p. 137 86. C. A. Witt and G. Gerken, Aluminium 1971, 47, 748 87. C. A. Witt and W. Sautter, Aluminium, 1978, 54 (8) 510 88. H. J . Gohausen, Trans. Inst. Met. Finishing, 1978, 56, 57 89. H. J . Gohausen, Aluminium, 1978, 54(5)337 90. J . Patrie, Aluminium, 1976, 52(8)500 91. B. S. Hockenhull, S. S. Gupta and R. C. Hurst, Trans. Inst. Met. Finishing, 1976, 54 (3) 123 92. B. G. Sheasby and P. G. Carter, Trans. Inst. Met. Finishing 1978, 56 (1) 18

67. 68. 69. 70. 71 . 72. 73.

Appendix: 1 Methods of analysis used in the production of anodized finishes 1.

Caustic soda etching bath

This method is intended for the analysis of etching baths which either contain only caustic soda or are proprietary solutions based on caustic soda plus a complexing agent such as gluconate or other non-interfering additions such as dextrin 01 mannitol. Substances such as carbonates, phosphates or silicates need to be estimated separately. This method provides a figure for total alkalinity from which the amount of other additions separately determined has to be deducted to determine the NaOH content. The main factor that has to be taken into account is the aluminium content of the bath. Apparatus pH meter, 200 ml beaker, 50 ml burette, 5 ml pipette. N sulphuric acid or hydrochloric acid.

Procedure Pipette 5 ml of the etchant into a 200 ml beaker and dilute to approximately 100 ml of solution with distilled water. Titrate with 1.0N sulphuric acid to a pH of 11.0 and read the titre (A ml); this corresponds to the titration of free sodium hydroxide. Continue the titration with 1.0 N sulphuric acid to a pH of 8.0 and read the second titre (B ml) (total volume of H 2SO 4 used); this represents completion of the precipitation of aluminium hydroxide. The results are calculated as follows:— Free NaOH =

Titre

A

x

40

x

40

e/3 g/i

oor r

Titre A x 4 % „ w /v / ---------------5

o/l 1

or

-------------

5 Total alkalinity =

Titre

B 5

Al content of bath 288

=

Titre B x 4

%m

w

/y

5

(Titre B - Titre A) x 5.4 g /i 289

Londong (1961) p. 139 A. A. Defoe, Wire and Wire Products, 1957, 31 (11) 134; (12) 1401 P. Smits, Modern Metals, 1958, 14 (7) 30 G. Franckenstein, Ann. Physik, 1936, 26 (5) 17 S . S . Gutin, Zhur.Teckn.Fiz., 1933,3(8) J . J . Lee, Proc. Conference on Anodizing, A.D.A. (1961) p. 130 W. Mauscher, Machinist (European Ed.), 1945, 89 (25) 881 E. A. G. Liddiard, G. Sanderson and J . E. Penn, Trans. Inst. Met. Finishing 1974, 49 (5) 200 74. V. E. Carter, J . Inst, of Metals, 1972, (7) 75 75. V. E. Carter, Br. Corrosion J . 1974, (1) 10 76. J . Patrie, Trans. Inst. Met. Finishing, 1975, 53 (1) 28 77. L. Knutsson and K. Dahlberg, Trans. Inst. Met. Finishing, 1976, 54 (1) 28 78. C. Th. Speiser and H. Schenkel, Metals and Materials, 1976, 165 79. C. Th. Speiser, Trans. Inst. Met. Finishing, 1977 55, (4), 165 80. G. E. Gardam, Trans. Inst. Met. Finishing, 1964, 41 (5) 190 81. K. H. Moller and F. Briicker, Aluminium 1977,53(5)314 82. C. Drapier and P. Lelong, Metal Finishing J . 1969, (4) 134 83. C. E. Michelson. Thermal Crazing of Anodic Coatings on Aluminium, Paper to S.A.E. Mid-Year Meeting, Chicago, 17-21 May, 1965 84. C. E. Michelson, S.A.E. Journal, 1965, (10) 60 85. W. J . Campbell, Proc. Conference on Anodizing Aluminium, Aluminium Development Assocn., 1961, p. 137 86. C. A. Witt and G. Gerken, Aluminium 1971, 47, 748 87. C. A. Witt and W. Sautter, Aluminium, 1978, 54 (8) 510 88. H. J . Gohausen, Trans. Inst. Met. Finishing, 1978, 56, 57 89. H. J . Gohausen, Aluminium, 1978, 54(5)337 90. J . Patrie, Aluminium, 1976, 52(8)500 91. B. S. Hockenhull, S. S. Gupta and R. C. Hurst, Trans. Inst. Met. Finishing, 1976, 54 (3) 123 92. B. G. Sheasby and P. G. Carter, Trans. Inst. Met. Finishing 1978, 56 (1) 18

67. 68. 69. 70. 71 . 72. 73.

Appendix: 1 Methods of analysis used in the production of anodized finishes 1.

Caustic soda etching bath

This method is intended for the analysis of etching baths which either contain only caustic soda or are proprietary solutions based on caustic soda plus a complexing agent such as gluconate or other non-interfering additions such as dextrin 01 mannitol. Substances such as carbonates, phosphates or silicates need to be estimated separately. This method provides a figure for total alkalinity from which the amount of other additions separately determined has to be deducted to determine the NaOH content. The main factor that has to be taken into account is the aluminium content of the bath. Apparatus pH meter, 200 ml beaker, 50 ml burette, 5 ml pipette. N sulphuric acid or hydrochloric acid.

Procedure Pipette 5 ml of the etchant into a 200 ml beaker and dilute to approximately 100 ml of solution with distilled water. Titrate with 1.0N sulphuric acid to a pH of 11.0 and read the titre (A ml); this corresponds to the titration of free sodium hydroxide. Continue the titration with 1.0 N sulphuric acid to a pH of 8.0 and read the second titre (B ml) (total volume of H 2SO 4 used); this represents completion of the precipitation of aluminium hydroxide. The results are calculated as follows:— Free NaOH =

Titre

A

x

40

x

40

e/3 g/i

oor r

Titre A x 4 % „ w /v / ---------------5

o/l 1

or

-------------

5 Total alkalinity =

Titre

B 5

Al content of bath 288

=

Titre B x 4

%m

w

/y

5

(Titre B - Titre A) x 5.4 g /i 289

la.

Caustic soda-sodium nitrate etching bath

The free sodium hydroxide content of the bath and its aluminium content are determined in exactly the same way as for the simple caustic soda bath above. The sodium nitrate level, however, must be determined separately and is measured by titration of excess ferrous sulphate solution with potassium permanganate.

2.

Caustic soda-trisodium phosphate etch cleaner

Take 10 ml of solution, dilute to 150 ml and titrate with N / l sulphuric acid to the red end point with methyl orange indicator. Let this be A ml. (This accounts for all the caustic soda plus two-thirds of the trisodium phosphate). Any carbon dioxide is then boiled off, the solution cooled and then back titrated with N'NaOH using phenolphthalein as indicator. Let this be B ml. Then: (A — 2B) x 0.40 = g/1 NaOH

Apparatus

B x 1 . 6 4 = g/1 Na 2 PO4.

pH meter, chemical balance, 50 ml burette, 1 litre volumetric flask, 50 ml volumetric flask, 600 ml conical flask, 25 ml pipette, 10 ml pipette, 50 ml measuring cylinder, glass beads. Ferrous sulphate (FeSO 4.7 H 2O), sodium chloride (NaCl), 0.1 N potassium permanganate (KMnO 4 ), concentrated sulphuric acid.

3.

Nitric acid in nitric acid desmut baths

First make a standard ferrous sulphate solution by dissolving 55.6g of FeSO4. 7H2O and 40g NaCl in water, adding 250 ml of concentrated sulphuric acid and making up to 1 litre with distilled water.

This method is intended for use as a control method. The nitric acid bath will normally not require frequent checking, and sometimes a check on its density is adequate. For example a 250 g/1 HNO, solution has a specific gravity of 1.13 compared with 1.103 for 200 g/L However tnere is some dissolution of aluminium in the bath which increases S.G. as does phosphoric acid from chemical brightening baths. The method below allows for dissolved aluminium but not for phosphoric acid. If there is reason to suspect significant contamination with this acid this should be confirmed with a molybdate test.

Pipette 10 ml of the filtered etch solution into a 50 ml volumetric flask and make up to the mark with distilled water.

Apparatus

Procedure

Carry out a blank test on the reagents as follows before testing the etch solution: Pipette 25 ml of ferrous sulphate solution into a 600 ml conical flask and add 25 ml of distilled water and 20 ml of concentrated sulphuric acid, together with 2 or 3 glass beads. Bring to the boil and allow to boil for exactly 3 minutes. Cool the solution, dilute with water to about 250 ml and titrate with 0.1 N potassium permanganate to a faint pink end point (Titre A). Test the etch bath sample in a similar manner by pipetting 25 ml of ferrous sulphate solution and 10 ml of the diluted etch solution into a 600 ml conical flask. Add 15 ml of water and 20 ml of concentrated sulphuric acid, together with 2 or 3 glass beads, and boil for exactly 3 minutes. Cool and titrate in the same manner as the blank determination with 0.1 N potassium permanganate (Titre B). Calculate the result as follows:Sodium nitrate =

290

0.142 x (A — B) % w/v or 1.42 x (A — B)g/1

250 ml Volumetric flask, 250 ml conical beaker, 25 ml pipette, 50 ml burette. N caustic soda, potassium fluoride, thymol blue indicator. Procedure Pipette 25 ml of solution from the bath into a 250 ml volumetric flask, dilute up to the mark with water and mix. From this diluted solution pipette 25 ml into a 250 ml conical beaker and dilute to about 50 ml, then add approximately 1 g potassium fluoride and dissolve. Add a few drops of thymol blue indicator and titrate with IN caustic soda to the blue end point of the indicator. If the amount used = B ml: the free HNO3 content % vol = B ml N-NaOH x 2.52 or, as g/1 = B ml N’NaOH X 25.2 Using a further 25 ml of the diluted solution, dilute to approximately 100 ml and titrate with A ml of IN caustic soda: the Al content in g/1 = 0.36 (A — B). 291

la.

Caustic soda-sodium nitrate etching bath

The free sodium hydroxide content of the bath and its aluminium content are determined in exactly the same way as for the simple caustic soda bath above. The sodium nitrate level, however, must be determined separately and is measured by titration of excess ferrous sulphate solution with potassium permanganate.

2.

Caustic soda-trisodium phosphate etch cleaner

Take 10 ml of solution, dilute to 150 ml and titrate with N / l sulphuric acid to the red end point with methyl orange indicator. Let this be A ml. (This accounts for all the caustic soda plus two-thirds of the trisodium phosphate). Any carbon dioxide is then boiled off, the solution cooled and then back titrated with N'NaOH using phenolphthalein as indicator. Let this be B ml. Then: (A — 2B) x 0.40 = g/1 NaOH

Apparatus

B x 1 . 6 4 = g/1 Na 2 PO4.

pH meter, chemical balance, 50 ml burette, 1 litre volumetric flask, 50 ml volumetric flask, 600 ml conical flask, 25 ml pipette, 10 ml pipette, 50 ml measuring cylinder, glass beads. Ferrous sulphate (FeSO 4.7 H 2O), sodium chloride (NaCl), 0.1 N potassium permanganate (KMnO 4 ), concentrated sulphuric acid.

3.

Nitric acid in nitric acid desmut baths

First make a standard ferrous sulphate solution by dissolving 55.6g of FeSO4. 7H2O and 40g NaCl in water, adding 250 ml of concentrated sulphuric acid and making up to 1 litre with distilled water.

This method is intended for use as a control method. The nitric acid bath will normally not require frequent checking, and sometimes a check on its density is adequate. For example a 250 g/1 HNO, solution has a specific gravity of 1.13 compared with 1.103 for 200 g/L However tnere is some dissolution of aluminium in the bath which increases S.G. as does phosphoric acid from chemical brightening baths. The method below allows for dissolved aluminium but not for phosphoric acid. If there is reason to suspect significant contamination with this acid this should be confirmed with a molybdate test.

Pipette 10 ml of the filtered etch solution into a 50 ml volumetric flask and make up to the mark with distilled water.

Apparatus

Procedure

Carry out a blank test on the reagents as follows before testing the etch solution: Pipette 25 ml of ferrous sulphate solution into a 600 ml conical flask and add 25 ml of distilled water and 20 ml of concentrated sulphuric acid, together with 2 or 3 glass beads. Bring to the boil and allow to boil for exactly 3 minutes. Cool the solution, dilute with water to about 250 ml and titrate with 0.1 N potassium permanganate to a faint pink end point (Titre A). Test the etch bath sample in a similar manner by pipetting 25 ml of ferrous sulphate solution and 10 ml of the diluted etch solution into a 600 ml conical flask. Add 15 ml of water and 20 ml of concentrated sulphuric acid, together with 2 or 3 glass beads, and boil for exactly 3 minutes. Cool and titrate in the same manner as the blank determination with 0.1 N potassium permanganate (Titre B). Calculate the result as follows:Sodium nitrate =

290

0.142 x (A — B) % w/v or 1.42 x (A — B)g/1

250 ml Volumetric flask, 250 ml conical beaker, 25 ml pipette, 50 ml burette. N caustic soda, potassium fluoride, thymol blue indicator. Procedure Pipette 25 ml of solution from the bath into a 250 ml volumetric flask, dilute up to the mark with water and mix. From this diluted solution pipette 25 ml into a 250 ml conical beaker and dilute to about 50 ml, then add approximately 1 g potassium fluoride and dissolve. Add a few drops of thymol blue indicator and titrate with IN caustic soda to the blue end point of the indicator. If the amount used = B ml: the free HNO3 content % vol = B ml N-NaOH x 2.52 or, as g/1 = B ml N’NaOH X 25.2 Using a further 25 ml of the diluted solution, dilute to approximately 100 ml and titrate with A ml of IN caustic soda: the Al content in g/1 = 0.36 (A — B). 291

4. Control of nitric-hydrofluoric etching/ desmut solutions

5.

These solutions are used for smut removal on alloys high in silicon or copper and also for certain types of matt etching. This method (detailed by K. E. Langford, ‘Analysis of Electroplating and Related Solutions’, 3rd Ed. (1962) p. 285. Robert Draper Ltd., Teddington) depends upon the formation of fluosilicic acid when silica gel is added to the HF-HNOi mixture:

The method is based on the well-known reaction of nitric acid with ferrous sulphate in the ‘brown ring’ test. Apparatus 5 ml Pipette, 25 ml pipette, 100 ml measuring flask, 1 litre volumetric flask. Ferrous sulphate solution, phosphoric acid S.G. 1.75, N/2 potassium dichromate, 1 per cent potassium ferrocyanide (as indicator).

6HF + SiO2 = H 2SiF 6 + 2H 2O Fluosilicic acid can be titrated in two stages: (1) H 2SiF6 + 2K0H = K2SiF 6 + 2H 2O (2) K2 SiF(, + 4K0H = 6KF + SiO2 + 2H 2O (3) H SiF6 + 6K0H = 6KF + SiO2 + 4H 2O

Nitric acid in phosphoric-base chemical brightening baths

(in the cold) (boiling) (boiling)

The titration method given below is in two stages, the first in the cold giving the HNOj content + 16 H F and the second the remaining % HF.

Apparatus 10 ml Polythene measure, 450 ml Phillips beaker, 50 ml burette. Phenophthalein indicator, ethyl alcohol (industrial spirit), 1.0 N KOH.

Procedure Take a 5 ml sample from a 10 ml polythene measure and pour into the Phillips beaker; add to it a slight excess of fresh silica gel prepared by diluting some commercial 40 per cent fluosilicic acid with water, raising to the boil and adding NKOHsolution until just alkaline to phenolphthalein, The gel is filtered through a No. 41 Whatman paper, washed well with boiling water and stored in a stoppered bottle. To the cooled sample of gel and mixed acids are added a few spots of phenolphthalein and a volume of ethyl alcohol equal to the contents of the beaker. These are then titrated with N*KOH solution (not caustic soda) until a pink colouration is produced. The latter is rather transient, fading quickly but the first flush of colour permeating the whole liquid should be taken as the end point. Let the volume of alkali added be A ml.

Procedure The ferrous sulphate solution is made up by adding 265 g FeSO 4.7H 2O (A.R.) to a solution of 50 ml of concentrated sulphuric acid dissolved in 700 ml of distilled water, and then the whole made up to 1 litre. This solution should be standardised weekly (or otherwise be freshly made up). Standardising is carried out by titrating 25 ml of N/2 potassium dichromate with the ferrous sulphate solution. Spots of potassium ferrocyanide indicator are placed on a white tile and at intervals a drop of the titrated solution is removed and mixed with it. The end point is reached when a bluish-green colour is produced. To estimate the nitric acid content 5 ml of the brightener are transferred to a dry 250 ml beaker, preferably using a burette. Add 100 ml phosphoric acid, stir and heat to 40° - 50°C and titrate with ferrous sulphate until the first permanent golden-brown colour is formed. Towards the end of the titration the acid boils locally with each addition of ferrous sulphate and reddish-yellow fumes are evolved; at the end point no fumes are evolved. Let A = ml of ferrous sulphate used to titrate against potassium dichromate B = ml of ferrous sulphate required to neutralise the sample of brightener p then % HNCh (vol) = ( ) x 5.3 B A A (This method is that advised by Albright & Wilson Ltd., Oldbury, Warley, West Midlands.)

Add 20 ml of water to the liquid in the flask, raise to the boil (bumping is prevalent) and again titrate with N KOH to the pink end point, which this time is more persistent. Let the new volume of alkali used be B ml. Then (A— y ) x 12.6 = g/1 HNO and (B X =) X 4.0 = g/1 HF 292

293

4. Control of nitric-hydrofluoric etching/ desmut solutions

5.

These solutions are used for smut removal on alloys high in silicon or copper and also for certain types of matt etching. This method (detailed by K. E. Langford, ‘Analysis of Electroplating and Related Solutions’, 3rd Ed. (1962) p. 285. Robert Draper Ltd., Teddington) depends upon the formation of fluosilicic acid when silica gel is added to the HF-HNOi mixture:

The method is based on the well-known reaction of nitric acid with ferrous sulphate in the ‘brown ring’ test. Apparatus 5 ml Pipette, 25 ml pipette, 100 ml measuring flask, 1 litre volumetric flask. Ferrous sulphate solution, phosphoric acid S.G. 1.75, N/2 potassium dichromate, 1 per cent potassium ferrocyanide (as indicator).

6HF + SiO2 = H 2SiF 6 + 2H 2O Fluosilicic acid can be titrated in two stages: (1) H 2SiF6 + 2K0H = K2SiF 6 + 2H 2O (2) K2 SiF(, + 4K0H = 6KF + SiO2 + 2H 2O (3) H SiF6 + 6K0H = 6KF + SiO2 + 4H 2O

Nitric acid in phosphoric-base chemical brightening baths

(in the cold) (boiling) (boiling)

The titration method given below is in two stages, the first in the cold giving the HNOj content + 16 H F and the second the remaining % HF.

Apparatus 10 ml Polythene measure, 450 ml Phillips beaker, 50 ml burette. Phenophthalein indicator, ethyl alcohol (industrial spirit), 1.0 N KOH.

Procedure Take a 5 ml sample from a 10 ml polythene measure and pour into the Phillips beaker; add to it a slight excess of fresh silica gel prepared by diluting some commercial 40 per cent fluosilicic acid with water, raising to the boil and adding NKOHsolution until just alkaline to phenolphthalein, The gel is filtered through a No. 41 Whatman paper, washed well with boiling water and stored in a stoppered bottle. To the cooled sample of gel and mixed acids are added a few spots of phenolphthalein and a volume of ethyl alcohol equal to the contents of the beaker. These are then titrated with N*KOH solution (not caustic soda) until a pink colouration is produced. The latter is rather transient, fading quickly but the first flush of colour permeating the whole liquid should be taken as the end point. Let the volume of alkali added be A ml.

Procedure The ferrous sulphate solution is made up by adding 265 g FeSO 4.7H 2O (A.R.) to a solution of 50 ml of concentrated sulphuric acid dissolved in 700 ml of distilled water, and then the whole made up to 1 litre. This solution should be standardised weekly (or otherwise be freshly made up). Standardising is carried out by titrating 25 ml of N/2 potassium dichromate with the ferrous sulphate solution. Spots of potassium ferrocyanide indicator are placed on a white tile and at intervals a drop of the titrated solution is removed and mixed with it. The end point is reached when a bluish-green colour is produced. To estimate the nitric acid content 5 ml of the brightener are transferred to a dry 250 ml beaker, preferably using a burette. Add 100 ml phosphoric acid, stir and heat to 40° - 50°C and titrate with ferrous sulphate until the first permanent golden-brown colour is formed. Towards the end of the titration the acid boils locally with each addition of ferrous sulphate and reddish-yellow fumes are evolved; at the end point no fumes are evolved. Let A = ml of ferrous sulphate used to titrate against potassium dichromate B = ml of ferrous sulphate required to neutralise the sample of brightener p then % HNCh (vol) = ( ) x 5.3 B A A (This method is that advised by Albright & Wilson Ltd., Oldbury, Warley, West Midlands.)

Add 20 ml of water to the liquid in the flask, raise to the boil (bumping is prevalent) and again titrate with N KOH to the pink end point, which this time is more persistent. Let the new volume of alkali used be B ml. Then (A— y ) x 12.6 = g/1 HNO and (B X =) X 4.0 = g/1 HF 292

293

6. Constituents of phosphoric-acid based polishing baths

These methods are used where it is necessary to undertake a more detailed analysis of the bath than control of nitric acid in chemical polishing baths or viscosity and density of Battelle or similar electropolishing baths.

Phosphate: Transfer a 2 ml aliquot to a 500 ml Erlenmeyer flask. Add 5 ml HN0 3 (sp. g. 1.42), 50 ml of H2 O, and 5 g. of NH 4 NO3 and heat to boiling. Add 125 ml of molybdate reagent, shake several minutes and let stand for two hours at room temperature (or until the supernatant liquid is clear). Filter through a plug of paper pulp, using suction. Wash the precipitate three times with 2 per cent HNO 3 and then with 2 per cent KNO3 solution until the washings are neutral. Dissolve the precipitate in an excess of N/10 NaOH and titrate the excess with N/10 HNO3.

Apparatus 1 mlofN/lONaOH = 0.0004131 g PO 4. 500 ml Volumetric flask, 600 ml beakers, 500 ml Erlenmeyer flask, various pipettes, 50 ml burette, analytical balance. Reagents are listed at the end of this section.

Transfer approximately 15 g of sample (weighed to the nearest milligram) to a 500 ml volumetric flask. Dilute to the mark and mix well.

Total chromium: Transfer an approximately 0.5 gram sample (taken as an aliquot from the dilution of a larger sample) to a 600 ml beaker. Add 60 ml chromium acid mixture, 10 ml 0.25 per cent AgNO3 solution and one drop 5 per cent MnSO4 solution. Dilute to 300 ml with hot water, heat to boiling and add 20 ml (NH 4)2S2O8 solution. Boil 10 minutes after the red MnO4 colour develops. Add 5 ml HC1 (1 : 3) and boil 5 minutes after red colour has disappeared. Cool, add 25 ml titrating mix, and run in standard N/10 ferrous ammonium sulphate solution until the liquid takes on a greenish coloration.

Total acid: Transfer with a pipette an aliquot of the above solution (or a 1 g sample of bath dissolved in small amount of water) to a beaker or flask and titrate to phenolphthalein end point with standard sodium hydroxide solution.

Add three drops orthophenanthroline indicator and continue the addition of ferrous ammonium sulphate until the indicator assumes a red colour, adding several ml in excess. It is convenient to add an amount of ferrous ammonium sulphate that is evenly divisible by 10. Titrate the excess with standard KMnO 4 solution to a clear green end point. Record as titre A.

Procedure

% acid in bath

(normality of NaOH ) x ( ml of NaOH) x 4.9 (g bath used)

Add to the same solution the same amount of ferrous ammonium sulphate that was initially added and again titrate to the clear green end point with KMnO 4. Record the KMnO 4 required as titre B. (titre B - titre A) x Cr value of 1 ml KMnO 4 x 100 -------------------------- --------------wt of sample in grams

or. for N/l NaOH and a 1 g sample:

_ — % of Cr in the sample

% acid in bath = ml NaOH used x 4.9 Phosphoric acid is dibasic toward phenolphthalein. If PO/' is to be calculated from total acid, SO/' and metallic figures, this fact must be taken into consideration. Other indicators of the same pH may be used. Sulphate: Transfer a 25 ml aliquot to a 600 ml beaker. Add 400 ml of hot water and 3 ml of HC1 (sp. g 1.19). Heat to boiling and add 25 ml of 10 per cent BaCl. solution. Digest and filter through a tight paper washing with hot water until free from Cl. Ignite and weight as BaSO4. Ignition should be started at a low temperature and finished at about 850~C. (wt of BaSO 4 )x 41.15 (g of bath used) 294

°o SO4. in sample.

1 mlN/10KMnO 4 = 0.001734 g Cr. Hexavalent chromium: Transfer an approximately 0.5 g sample (preferably an aliquot from the dilution of a large sample) to a 600 ml beaker. Add 600 ml chromium acid mixture, 300 ml cold water, and 5 ml HC1 (1 : 3). Titrate as described in the determination of total chromium. Tnvalent chromium: % Total chromium - % hexavalent chromium = %trivalent chromium. Aluminium. Mercury cathode method: Transfer an approximately 0.5 g sample (preferably an aliquot) to a 600 ml beaker. Add 2 ml H 2SO4 ( 1 : 1 ) and 300 ml cold water. Electrolyse on a mercury cathode until the solution is free from Cr (1 to 2 295

6. Constituents of phosphoric-acid based polishing baths

These methods are used where it is necessary to undertake a more detailed analysis of the bath than control of nitric acid in chemical polishing baths or viscosity and density of Battelle or similar electropolishing baths.

Phosphate: Transfer a 2 ml aliquot to a 500 ml Erlenmeyer flask. Add 5 ml HN0 3 (sp. g. 1.42), 50 ml of H2 O, and 5 g. of NH 4 NO3 and heat to boiling. Add 125 ml of molybdate reagent, shake several minutes and let stand for two hours at room temperature (or until the supernatant liquid is clear). Filter through a plug of paper pulp, using suction. Wash the precipitate three times with 2 per cent HNO 3 and then with 2 per cent KNO3 solution until the washings are neutral. Dissolve the precipitate in an excess of N/10 NaOH and titrate the excess with N/10 HNO3.

Apparatus 1 mlofN/lONaOH = 0.0004131 g PO 4. 500 ml Volumetric flask, 600 ml beakers, 500 ml Erlenmeyer flask, various pipettes, 50 ml burette, analytical balance. Reagents are listed at the end of this section.

Transfer approximately 15 g of sample (weighed to the nearest milligram) to a 500 ml volumetric flask. Dilute to the mark and mix well.

Total chromium: Transfer an approximately 0.5 gram sample (taken as an aliquot from the dilution of a larger sample) to a 600 ml beaker. Add 60 ml chromium acid mixture, 10 ml 0.25 per cent AgNO3 solution and one drop 5 per cent MnSO4 solution. Dilute to 300 ml with hot water, heat to boiling and add 20 ml (NH 4)2S2O8 solution. Boil 10 minutes after the red MnO4 colour develops. Add 5 ml HC1 (1 : 3) and boil 5 minutes after red colour has disappeared. Cool, add 25 ml titrating mix, and run in standard N/10 ferrous ammonium sulphate solution until the liquid takes on a greenish coloration.

Total acid: Transfer with a pipette an aliquot of the above solution (or a 1 g sample of bath dissolved in small amount of water) to a beaker or flask and titrate to phenolphthalein end point with standard sodium hydroxide solution.

Add three drops orthophenanthroline indicator and continue the addition of ferrous ammonium sulphate until the indicator assumes a red colour, adding several ml in excess. It is convenient to add an amount of ferrous ammonium sulphate that is evenly divisible by 10. Titrate the excess with standard KMnO 4 solution to a clear green end point. Record as titre A.

Procedure

% acid in bath

(normality of NaOH ) x ( ml of NaOH) x 4.9 (g bath used)

Add to the same solution the same amount of ferrous ammonium sulphate that was initially added and again titrate to the clear green end point with KMnO 4. Record the KMnO 4 required as titre B. (titre B - titre A) x Cr value of 1 ml KMnO 4 x 100 -------------------------- --------------wt of sample in grams

or. for N/l NaOH and a 1 g sample:

_ — % of Cr in the sample

% acid in bath = ml NaOH used x 4.9 Phosphoric acid is dibasic toward phenolphthalein. If PO/' is to be calculated from total acid, SO/' and metallic figures, this fact must be taken into consideration. Other indicators of the same pH may be used. Sulphate: Transfer a 25 ml aliquot to a 600 ml beaker. Add 400 ml of hot water and 3 ml of HC1 (sp. g 1.19). Heat to boiling and add 25 ml of 10 per cent BaCl. solution. Digest and filter through a tight paper washing with hot water until free from Cl. Ignite and weight as BaSO4. Ignition should be started at a low temperature and finished at about 850~C. (wt of BaSO 4 )x 41.15 (g of bath used) 294

°o SO4. in sample.

1 mlN/10KMnO 4 = 0.001734 g Cr. Hexavalent chromium: Transfer an approximately 0.5 g sample (preferably an aliquot from the dilution of a large sample) to a 600 ml beaker. Add 600 ml chromium acid mixture, 300 ml cold water, and 5 ml HC1 (1 : 3). Titrate as described in the determination of total chromium. Tnvalent chromium: % Total chromium - % hexavalent chromium = %trivalent chromium. Aluminium. Mercury cathode method: Transfer an approximately 0.5 g sample (preferably an aliquot) to a 600 ml beaker. Add 2 ml H 2SO4 ( 1 : 1 ) and 300 ml cold water. Electrolyse on a mercury cathode until the solution is free from Cr (1 to 2 295

Solution B Dilute 1 040 ml HNO3 (sp. g 1.42) to 2 700 ml. Add solution A to solution B slowly through a tube that dips below the surface of B, agitating with a vigorous stream of air. Continue passage of air one to two hours. Filter before using.

hours). Remove the electrolyte and precipitate Al as phosphate as directed under the direct-precipitation method. Aluminium. Direct precipitation method: This method assumes the absence of interference from Fe, Mn, Ca, etc. Transfer an approximately 0.5 g sample (aliquot) to a 600 ml beaker. Add 10 ml H 2SO4 (1 : 1), 300 ml hot water, and 10 ml 0.25 per cent AgNO3 solution. Heat to boiling, add 4 g (NH4 )2S 2O8 and boil for 10 minutes. Add 5 ml HC1 (1 : 3), boil for 2 minutes and cool. Filter through a Whatman No. 42 filter paper, washing the paper and residue five times with cold water. Add 5 ml HC1 to the filtrate and two grams (NH4 )2HPO 4. Make barely alkaline with NH 4OH (1 : 1) and then add 4 ml HC1 (1 : 4). Dilute to 500 ml with hot water and heat to boiling. Add 30 ml 25 per cent ammonium acetate solution and boil for five minutes. Filter hot through a Whatman No. 41 filter paper, using paper pulp. Wash the precipitate with hot 5 per cent NH 4 NO3 solution until the washings are free from chloride. Dry and ignite, initially at 100°C raising gradually to 850°C and obtain constant weight, cool in a desiccator, and weigh as A1PO4. A1PO4 x 0.22116 = Al

Nitric acid (2%)

20 ml HNO3 (sp. g 1.42) + 980 ml o f H 2O.

Orthophenanthroline indicator solution (ferrous complex 0.025 molar)

Sold by G. Frederick Smith Chemical Company, Columbus, Ohio, U.S.A.

Potassium nitrate solution (1%)

10 g KNO 3 dissolved in 1 000 ml H 2O

Silver nitrate solution (0.25%)

2.5g AgNO 3 dissolved in 1 000 ml H 2O.

Silver nitrate solution (approximately N/20)

8.5 g AgNO 3 dissolved in H 2O and diluted to 1 000 ml.

Sodium hydroxide solution (N/ 10)

4.0 g NaOH in 100 ml of H2 O (CO2 -free). Standardise against potassium acid phthalate.

Standard ferrous ammonium sulphate (approximately N/10)

40 g of FeSO4 (NH4 )2SO4.6H 2O and 100 ml H,SO< (1 : 1) diluted to 1 000 ml. 3.3 g KMnO, in 1 000 ml H2O. Let stand several days and siphon into a clean container. Standardise against Bureau of Standards sodium oxalate.

Reagents and solution required Ammonium acetate solution (25%)

25 g CH3COONH 4 dissolved in 100 ml H 2O.

Ammonium hydroxide (1 : 1)

1 volume NH4OH (sp. gr. 0.90) + 1 volume H2O.

Ammonium nitrate solution (5%)

50 g NH4 NO3 dissolved in 1 litre H 2O and the solution made alkaline to methyl red with NH4OH.

Ammonium persulphate solution

15 g (NH 4)2S2O8 dissolved in 85 ml H 2O.

Barium chloride solution (10%)

100 g BaCl 2 .2H 2 O dissolved in 1 000 ml of H 2O. Filter, if necessary, before using.

Chromium acid mixture

320mlH 2SO4 (l : 1) + 80 ml H,PO 4 + 600 ml H2O.

Hydrochloric acid (1 : 3)

1 volume HC1 (sp. g. 1.19) + 3 volumes H 2O.

Manganous sulphate solution (5%)

5 g MnSO 4 dissolved in 100 ml H2 O.

Molybdate reagent

Solution A Dissolve 183 g 85 per cent MoO3 in 776 ml NH4OH ( 1 : 1 ) and dilute to 1 150 ml with H 2 O.

Standard potassium permanganate

Sulphuric acid ( 1 : 1 )

1 volume of H 2SO 4 (sp. gr. 1.84) added to 1 volume H 2O.

297 296

Solution B Dilute 1 040 ml HNO3 (sp. g 1.42) to 2 700 ml. Add solution A to solution B slowly through a tube that dips below the surface of B, agitating with a vigorous stream of air. Continue passage of air one to two hours. Filter before using.

hours). Remove the electrolyte and precipitate Al as phosphate as directed under the direct-precipitation method. Aluminium. Direct precipitation method: This method assumes the absence of interference from Fe, Mn, Ca, etc. Transfer an approximately 0.5 g sample (aliquot) to a 600 ml beaker. Add 10 ml H 2SO4 (1 : 1), 300 ml hot water, and 10 ml 0.25 per cent AgNO3 solution. Heat to boiling, add 4 g (NH4 )2S 2O8 and boil for 10 minutes. Add 5 ml HC1 (1 : 3), boil for 2 minutes and cool. Filter through a Whatman No. 42 filter paper, washing the paper and residue five times with cold water. Add 5 ml HC1 to the filtrate and two grams (NH4 )2HPO 4. Make barely alkaline with NH 4OH (1 : 1) and then add 4 ml HC1 (1 : 4). Dilute to 500 ml with hot water and heat to boiling. Add 30 ml 25 per cent ammonium acetate solution and boil for five minutes. Filter hot through a Whatman No. 41 filter paper, using paper pulp. Wash the precipitate with hot 5 per cent NH 4 NO3 solution until the washings are free from chloride. Dry and ignite, initially at 100°C raising gradually to 850°C and obtain constant weight, cool in a desiccator, and weigh as A1PO4. A1PO4 x 0.22116 = Al

Nitric acid (2%)

20 ml HNO3 (sp. g 1.42) + 980 ml o f H 2O.

Orthophenanthroline indicator solution (ferrous complex 0.025 molar)

Sold by G. Frederick Smith Chemical Company, Columbus, Ohio, U.S.A.

Potassium nitrate solution (1%)

10 g KNO 3 dissolved in 1 000 ml H 2O

Silver nitrate solution (0.25%)

2.5g AgNO 3 dissolved in 1 000 ml H 2O.

Silver nitrate solution (approximately N/20)

8.5 g AgNO 3 dissolved in H 2O and diluted to 1 000 ml.

Sodium hydroxide solution (N/ 10)

4.0 g NaOH in 100 ml of H2 O (CO2 -free). Standardise against potassium acid phthalate.

Standard ferrous ammonium sulphate (approximately N/10)

40 g of FeSO4 (NH4 )2SO4.6H 2O and 100 ml H,SO< (1 : 1) diluted to 1 000 ml. 3.3 g KMnO, in 1 000 ml H2O. Let stand several days and siphon into a clean container. Standardise against Bureau of Standards sodium oxalate.

Reagents and solution required Ammonium acetate solution (25%)

25 g CH3COONH 4 dissolved in 100 ml H 2O.

Ammonium hydroxide (1 : 1)

1 volume NH4OH (sp. gr. 0.90) + 1 volume H2O.

Ammonium nitrate solution (5%)

50 g NH4 NO3 dissolved in 1 litre H 2O and the solution made alkaline to methyl red with NH4OH.

Ammonium persulphate solution

15 g (NH 4)2S2O8 dissolved in 85 ml H 2O.

Barium chloride solution (10%)

100 g BaCl 2 .2H 2 O dissolved in 1 000 ml of H 2O. Filter, if necessary, before using.

Chromium acid mixture

320mlH 2SO4 (l : 1) + 80 ml H,PO 4 + 600 ml H2O.

Hydrochloric acid (1 : 3)

1 volume HC1 (sp. g. 1.19) + 3 volumes H 2O.

Manganous sulphate solution (5%)

5 g MnSO 4 dissolved in 100 ml H2 O.

Molybdate reagent

Solution A Dissolve 183 g 85 per cent MoO3 in 776 ml NH4OH ( 1 : 1 ) and dilute to 1 150 ml with H 2 O.

Standard potassium permanganate

Sulphuric acid ( 1 : 1 )

1 volume of H 2SO 4 (sp. gr. 1.84) added to 1 volume H 2O.

297 296

7. ‘Brytal’* electrobrightening bath

Procedure A 5 ml sample of the electrolyte is diluted with 40 ml water and titrated in a 250 ml conical flask against N/10 NaOH in the presence of a few drops of p-nitrophenol until a pale yellow coloration is obtained. Let the NaOH required by A ml.

The method depends upon estimation of total alkalinity by titrating against sulphuric acid, then heating to remove carbon dioxide, thus freeing NaOH previously combined as Na2 C O , and back titrating with alkali to estimate the alkali from this source (Na 2CO? * 2NaOH + CO2).

Phenolphthalein is then added and the titration continued until a red colour is produced. Let the further NaOH now used be B ml.

Apparatus

3 ml of glycerine (or 1 gm mannitol) are added and the solution re-titrated until the colour changes to orange. Let this additional amount of NaOH be C ml. Then:

500 ml Conical flask, 20 ml pipette, 50 ml burette. 2N Sulphuric acid, 2N caustic soda, methyl orange, thymolphthalein indicator.

The HBF4 content (g/1) — 1.76 (A — B) The A12 O3 content (%) = 0.034 (C — B) — 0.05 A silver nitrate test should also show the electrolyte to be free from chloride.

Procedure Dilute 20 ml of the electrobrightening bath to about 200 ml, add four drops methyl orange and titrate with 2N H3SO4 until the solution turns permanently red. Let A ml of 2N H3SO4 be required. Heat the solution for 5 minutes to near boiling to remove the carbon dioxide and cool to room temperature. Add 10 drops of 0.1 per cent thymolphthalein solution in alcohol and titrate against 2N NaOH until the colour changes to blue. Let B ml of 2N NaOH be required. Then: Na3 CO3 g/l —5.3 (A — 2B) Na3 PO4 g/1 = 16.4 B

9.

Sulphuric acid electrolytes

A 5 ml sample of the electrolyte is pipetted into a 400 ml beaker and diluted to approximately 200 ml. With the pH electrode inserted the diluted sample is titrated against N / l sodium hydroxide solution until a pH of 3.6 is reached, the solution being kept continuously stirred. Record the amount of NaOH used as A ml. Continue titrating until a pH of 10.0 is reached and record the new titration as B ml of NaOH. A x N x 9.80 = g/!H,SO 4 (B — A) x N x 1.35 = g/1 Al Where N is the normality of the sodium hydroxide solution (if this is 1.0 or very close to that value the factor may be ignored)

8. ‘Alzak’t electrobrightening bath The method depends upon determining the total and free acidity by titration against N/10 NaOH using p- nitrophenol as indicator for total acidity.

Apparatus 5 ml Pipette, 250 ml conical flask, 50 ml burette. N/10 Caustic soda, p-nitrophenol, glycerine, phenolphthalein indicator.

* Registered trade mark of British Aluminium Co. Ltd. t Registered trade mark of The Aluminum Company of America. 298

299

7. ‘Brytal’* electrobrightening bath

Procedure A 5 ml sample of the electrolyte is diluted with 40 ml water and titrated in a 250 ml conical flask against N/10 NaOH in the presence of a few drops of p-nitrophenol until a pale yellow coloration is obtained. Let the NaOH required by A ml.

The method depends upon estimation of total alkalinity by titrating against sulphuric acid, then heating to remove carbon dioxide, thus freeing NaOH previously combined as Na2 C O , and back titrating with alkali to estimate the alkali from this source (Na 2CO? * 2NaOH + CO2).

Phenolphthalein is then added and the titration continued until a red colour is produced. Let the further NaOH now used be B ml.

Apparatus

3 ml of glycerine (or 1 gm mannitol) are added and the solution re-titrated until the colour changes to orange. Let this additional amount of NaOH be C ml. Then:

500 ml Conical flask, 20 ml pipette, 50 ml burette. 2N Sulphuric acid, 2N caustic soda, methyl orange, thymolphthalein indicator.

The HBF4 content (g/1) — 1.76 (A — B) The A12 O3 content (%) = 0.034 (C — B) — 0.05 A silver nitrate test should also show the electrolyte to be free from chloride.

Procedure Dilute 20 ml of the electrobrightening bath to about 200 ml, add four drops methyl orange and titrate with 2N H3SO4 until the solution turns permanently red. Let A ml of 2N H3SO4 be required. Heat the solution for 5 minutes to near boiling to remove the carbon dioxide and cool to room temperature. Add 10 drops of 0.1 per cent thymolphthalein solution in alcohol and titrate against 2N NaOH until the colour changes to blue. Let B ml of 2N NaOH be required. Then: Na3 CO3 g/l —5.3 (A — 2B) Na3 PO4 g/1 = 16.4 B

9.

Sulphuric acid electrolytes

A 5 ml sample of the electrolyte is pipetted into a 400 ml beaker and diluted to approximately 200 ml. With the pH electrode inserted the diluted sample is titrated against N / l sodium hydroxide solution until a pH of 3.6 is reached, the solution being kept continuously stirred. Record the amount of NaOH used as A ml. Continue titrating until a pH of 10.0 is reached and record the new titration as B ml of NaOH. A x N x 9.80 = g/!H,SO 4 (B — A) x N x 1.35 = g/1 Al Where N is the normality of the sodium hydroxide solution (if this is 1.0 or very close to that value the factor may be ignored)

8. ‘Alzak’t electrobrightening bath The method depends upon determining the total and free acidity by titration against N/10 NaOH using p- nitrophenol as indicator for total acidity.

Apparatus 5 ml Pipette, 250 ml conical flask, 50 ml burette. N/10 Caustic soda, p-nitrophenol, glycerine, phenolphthalein indicator.

* Registered trade mark of British Aluminium Co. Ltd. t Registered trade mark of The Aluminum Company of America. 298

299

10. Sulphuric-oxalic acid electrolytes

11. Oxalic acid electrolytes

The method depends upon the separate estimation of free sulphuric acid content by the method previously detailed and a permanganate titration for estimation of the oxalic acid content.

The method depends upon the reduction of potassium permanganate for the estimation of free oxalic acid and the freeing of the combined aluminium by warming to 60° - 70°C with an excess of sulphuric acid to obtain the total oxalate and aluminium contents.

Apparatus

Apparatus

250 ml Conical flask, 10 ml pipette, 100 ml measure, 50 per cent sulphuric acid solution, N/10 potassium permanganate, N/5 caustic soda, 50 per cent potassium fluoride solution. Phenolphthalein indicator, bromothymol blue indicator.

Procedure A 10 ml sample of the electrolyte is diluted to 100 ml. The total oxalic acid content is determined by taking 10 ml of the diluted electrolyte sample, adding 70 ml of water and 50 ml of 50 per centH 2SO4 solution, heating to 70°C and titrating against N/10 permanganate until a pink coloration is obtained. Let the volume of N/10 KMnO.i solution used be A ml. Then:

Oxalic acid content (g/1) = 4.5 A.

The total acidity is obtained by titrating a further 10 ml of diluted electrolyte with N/5 NaOH using phenolphthalein indicator. Let the caustic soda solution required be B ml. Then:

250 ml Conical beaker, 2.5 ml pipette, 25 ml pipette, 50 ml burette. N Caustic soda, 2N sulphuric acid, phenolphthalein indicator. Procedure The free oxalic acid content is obtained from a 25 ml sample of the electrolyte cooled to 15 °C, transferred to a 250 ml conical beaker and diluted to approximately 100 ml. This solution is titrated against N caustic soda using phenolphthalein as indicator until a permanent pink coloration is obtained. Let the volume of N / l NaOH used be A ml. Then: Free (COOH), (g/1) — 2.52 A. The total oxalic acid content is obtained by diluting 2.5 ml of the electrolyte to 25 ml and adding 25 ml of 2N H2SO4 solution; after warming to 60° -70°C it is titrated against N/10 KMnO 4 until a permanent violet coloration is obtained. Let the amount of N/10 KMnO 4 used be B ml. Then: Total (COOH)2 (g/1) = 2 . 5 2 B. Aluminium content (g/1) = 0.35 (B — A)

Total acidity (as H 2SO 4 g/1) — 9.8 B.

The free sulphuric acid is obtained by adding 20 ml of the 50 per cent KF solution to 10 ml of the diluted electrolyte and titrating with N/5 NaOH using bromothymol blue as indicator. Let the caustic soda solution required be C ml. Then:

12. Chromic acid electrolytes Free H,SO4 (g/1) = 9.8 (C — y )

The aluminium content is then calculated as follows: Aluminium (g/1) = 1.8 (B — C)

pH titration method: This method enables free and total CrO3 contents as well as the aluminium contents to be obtained in one titration. The first titre gives the free CrO3 while the second titre gives the CrOj combined with aluminium. Apparatus pH Meter, 25 ml pipette, 50 ml burette, 250 ml conical beaker. N / l Caustic soda solution.

300

301

10. Sulphuric-oxalic acid electrolytes

11. Oxalic acid electrolytes

The method depends upon the separate estimation of free sulphuric acid content by the method previously detailed and a permanganate titration for estimation of the oxalic acid content.

The method depends upon the reduction of potassium permanganate for the estimation of free oxalic acid and the freeing of the combined aluminium by warming to 60° - 70°C with an excess of sulphuric acid to obtain the total oxalate and aluminium contents.

Apparatus

Apparatus

250 ml Conical flask, 10 ml pipette, 100 ml measure, 50 per cent sulphuric acid solution, N/10 potassium permanganate, N/5 caustic soda, 50 per cent potassium fluoride solution. Phenolphthalein indicator, bromothymol blue indicator.

Procedure A 10 ml sample of the electrolyte is diluted to 100 ml. The total oxalic acid content is determined by taking 10 ml of the diluted electrolyte sample, adding 70 ml of water and 50 ml of 50 per centH 2SO4 solution, heating to 70°C and titrating against N/10 permanganate until a pink coloration is obtained. Let the volume of N/10 KMnO.i solution used be A ml. Then:

Oxalic acid content (g/1) = 4.5 A.

The total acidity is obtained by titrating a further 10 ml of diluted electrolyte with N/5 NaOH using phenolphthalein indicator. Let the caustic soda solution required be B ml. Then:

250 ml Conical beaker, 2.5 ml pipette, 25 ml pipette, 50 ml burette. N Caustic soda, 2N sulphuric acid, phenolphthalein indicator. Procedure The free oxalic acid content is obtained from a 25 ml sample of the electrolyte cooled to 15 °C, transferred to a 250 ml conical beaker and diluted to approximately 100 ml. This solution is titrated against N caustic soda using phenolphthalein as indicator until a permanent pink coloration is obtained. Let the volume of N / l NaOH used be A ml. Then: Free (COOH), (g/1) — 2.52 A. The total oxalic acid content is obtained by diluting 2.5 ml of the electrolyte to 25 ml and adding 25 ml of 2N H2SO4 solution; after warming to 60° -70°C it is titrated against N/10 KMnO 4 until a permanent violet coloration is obtained. Let the amount of N/10 KMnO 4 used be B ml. Then: Total (COOH)2 (g/1) = 2 . 5 2 B. Aluminium content (g/1) = 0.35 (B — A)

Total acidity (as H 2SO 4 g/1) — 9.8 B.

The free sulphuric acid is obtained by adding 20 ml of the 50 per cent KF solution to 10 ml of the diluted electrolyte and titrating with N/5 NaOH using bromothymol blue as indicator. Let the caustic soda solution required be C ml. Then:

12. Chromic acid electrolytes Free H,SO4 (g/1) = 9.8 (C — y )

The aluminium content is then calculated as follows: Aluminium (g/1) = 1.8 (B — C)

pH titration method: This method enables free and total CrO3 contents as well as the aluminium contents to be obtained in one titration. The first titre gives the free CrO3 while the second titre gives the CrOj combined with aluminium. Apparatus pH Meter, 25 ml pipette, 50 ml burette, 250 ml conical beaker. N / l Caustic soda solution.

300

301

Method From a sample of the electrolyte take 25 ml and dilute with distilled water to approximately 100 ml. Titrate with constant stirring using N caustic soda; the amount of alkali needed to reach pH 3.2 is titre A and that required to produce pH 4.8 is titre B. Then: Free CrO3 g/1 = 0.4 X A per cent w/v Total CrO3 g/1 = 0.4 X B per cent w/v Al content = 0.68B — 0.694A per cent w/v

Chromic acid. Method specified in Ministry of Defence Specification DEF151 This method depends upon the estimation of free CrO by titration against a sodium carbonate solution to the point at which sodium dichromate has been formed by neutralisation of the CrO3 and insoluble ‘chromates’ start to be thrown out of solution. The total chromic acid content is estimated in an acidified solution oxidised with persulphate and titrated against ferrous ammonium sulphate. This method is included in Ministry of Defence Specification DEF 151 . Apparatus 250 and 500 m Conical flasks, 10 ml and 25 ml pipettes, 50 ml burette. N/10 Potassium dichromate, N/10 ferrous ammonium sulphate, 3 per cent silver nitrate, 25 per cent sulphuric acid and N/2 sodium carbonate solutions, ammonium persulphate, N -phenylanthranilic acid indicator. Procedure The ferrous ammonium sulphate is first standardised by adding 25 ml of N/10 K2Cr2O to a solution of 45 ml of 25 per cent H2SO4 and 200 ml water in a 500 ml conical flask. To this five drops of N-phenylanthranilic acid indicator are added and the mixture is titrated with the approximately N/10 FeSO 4.(NH 4 )2SO 4 until the colour changes from reddish-purple to green. Towards the end point care should be taken to add the ferrous ammonium sulphate drop-wise and to allow a few seconds to elapse between each addition. Let the amount of FeSO 4.(NH 4 ) 2SO 4 used be A ml. Used electrolytes The total chromic acid content of a used electrolyte is obtained by diluting 20 ml of the electrolyte to 250 ml in a volumetric flask. After mixing, 25 ml of the diluted electrolyte are placed in a 500 ml conical flask containing 150 ml of water

and 45 ml of 25 per cent H 2SO 4. To this are added 10 ml of 3 per cent AgNO solution and 2 g of (NH4 )2S 2O 8; the whole is then boiled for 20 minutes to decompose the ammonium persulphate. After cooling to room temperature add five drops of N-phenylanthranilic acid indicator and titrate with the ferrous ammonium sulphate as detailed above. If B ml are required: •25 B xv Total CrO, (g/1) = — — y Free Chromic acid is obtained by adding 10 ml of electrolyte to 100 ml of water contained in a 250 ml conical flask and titrating with N/2 Na2CO., until the first signs of permanent turbidity are obtained. If C ml of N/2 Na2CO, are required: Free CrCE (g/1) = SC. Unused electrolytes For an unused electrolyte, 25 ml are diluted to 250 ml in a volumetric flask. To 45 ml of 25 per cent H2SO4 and 200 ml of water in a 500 ml conical flask add 50 ml of diluted electrolyte and five drops of N-phenylanthranilic acid indicator and titrate with N/10 FeSO, .(NH ) 2SO, as described above. Let D ml be the amount of ferrous ammonium sulphate required: Free CrO, (g/1) = y

x

y

Chloride is determined as for a sulphuric acid electrolyte (see DEF 151-AppendixA). Sulphate is determined by precipitation as barium sulphate. Apparatus 400 ml (squat) beaker, filter, 10 ml and 25 ml pipettes, 50 ml burette. Hydrochloric acid of S.G. 1.16, glacial acetic acid, 10 per cent barium chloride solution, pure ethoanl or sulphur-free methylated spirits. Procedure To 100 ml of filtered electrolyte in a 400 ml (squat) beaker add 10 ml of HC1, 25 ml of glacial acetic acid and 20 ml of ethanol and boil gently for 15 minutes to expel aldehydes and excess ethanol. Dilute the solution to about 200 ml and raise slowly to boiling point. Add slowly 10 ml of BaCl 2 solution (10 per cent w/v), boil gently for 15 minutes and allow the precipitate to settle overnight. Filter the precipitate on a small ashless paper pulp, wash well with warm water and transfer the pulp and precipitate to a weighed crucible; ignite and weigh as BaSO4. Let E be the weight in grams of the BaSO4. Then: Sulphate (as Na 2SO 4, g/1) = 6.1 E. 303

302

Method From a sample of the electrolyte take 25 ml and dilute with distilled water to approximately 100 ml. Titrate with constant stirring using N caustic soda; the amount of alkali needed to reach pH 3.2 is titre A and that required to produce pH 4.8 is titre B. Then: Free CrO3 g/1 = 0.4 X A per cent w/v Total CrO3 g/1 = 0.4 X B per cent w/v Al content = 0.68B — 0.694A per cent w/v

Chromic acid. Method specified in Ministry of Defence Specification DEF151 This method depends upon the estimation of free CrO by titration against a sodium carbonate solution to the point at which sodium dichromate has been formed by neutralisation of the CrO3 and insoluble ‘chromates’ start to be thrown out of solution. The total chromic acid content is estimated in an acidified solution oxidised with persulphate and titrated against ferrous ammonium sulphate. This method is included in Ministry of Defence Specification DEF 151 . Apparatus 250 and 500 m Conical flasks, 10 ml and 25 ml pipettes, 50 ml burette. N/10 Potassium dichromate, N/10 ferrous ammonium sulphate, 3 per cent silver nitrate, 25 per cent sulphuric acid and N/2 sodium carbonate solutions, ammonium persulphate, N -phenylanthranilic acid indicator. Procedure The ferrous ammonium sulphate is first standardised by adding 25 ml of N/10 K2Cr2O to a solution of 45 ml of 25 per cent H2SO4 and 200 ml water in a 500 ml conical flask. To this five drops of N-phenylanthranilic acid indicator are added and the mixture is titrated with the approximately N/10 FeSO 4.(NH 4 )2SO 4 until the colour changes from reddish-purple to green. Towards the end point care should be taken to add the ferrous ammonium sulphate drop-wise and to allow a few seconds to elapse between each addition. Let the amount of FeSO 4.(NH 4 ) 2SO 4 used be A ml. Used electrolytes The total chromic acid content of a used electrolyte is obtained by diluting 20 ml of the electrolyte to 250 ml in a volumetric flask. After mixing, 25 ml of the diluted electrolyte are placed in a 500 ml conical flask containing 150 ml of water

and 45 ml of 25 per cent H 2SO 4. To this are added 10 ml of 3 per cent AgNO solution and 2 g of (NH4 )2S 2O 8; the whole is then boiled for 20 minutes to decompose the ammonium persulphate. After cooling to room temperature add five drops of N-phenylanthranilic acid indicator and titrate with the ferrous ammonium sulphate as detailed above. If B ml are required: •25 B xv Total CrO, (g/1) = — — y Free Chromic acid is obtained by adding 10 ml of electrolyte to 100 ml of water contained in a 250 ml conical flask and titrating with N/2 Na2CO., until the first signs of permanent turbidity are obtained. If C ml of N/2 Na2CO, are required: Free CrCE (g/1) = SC. Unused electrolytes For an unused electrolyte, 25 ml are diluted to 250 ml in a volumetric flask. To 45 ml of 25 per cent H2SO4 and 200 ml of water in a 500 ml conical flask add 50 ml of diluted electrolyte and five drops of N-phenylanthranilic acid indicator and titrate with N/10 FeSO, .(NH ) 2SO, as described above. Let D ml be the amount of ferrous ammonium sulphate required: Free CrO, (g/1) = y

x

y

Chloride is determined as for a sulphuric acid electrolyte (see DEF 151-AppendixA). Sulphate is determined by precipitation as barium sulphate. Apparatus 400 ml (squat) beaker, filter, 10 ml and 25 ml pipettes, 50 ml burette. Hydrochloric acid of S.G. 1.16, glacial acetic acid, 10 per cent barium chloride solution, pure ethoanl or sulphur-free methylated spirits. Procedure To 100 ml of filtered electrolyte in a 400 ml (squat) beaker add 10 ml of HC1, 25 ml of glacial acetic acid and 20 ml of ethanol and boil gently for 15 minutes to expel aldehydes and excess ethanol. Dilute the solution to about 200 ml and raise slowly to boiling point. Add slowly 10 ml of BaCl 2 solution (10 per cent w/v), boil gently for 15 minutes and allow the precipitate to settle overnight. Filter the precipitate on a small ashless paper pulp, wash well with warm water and transfer the pulp and precipitate to a weighed crucible; ignite and weigh as BaSO4. Let E be the weight in grams of the BaSO4. Then: Sulphate (as Na 2SO 4, g/1) = 6.1 E. 303

302

13. Nickel acetate sealing baths This is the classical method of determination of nickel gravimetrically by precipitation with dimethylglyoxime.

Appendix: 2 Important Anodizing Specifications

Apparatus 10 ml Pipette, 25 ml pipette, 250 ml volumetric flask. Nitric acid, ammonia, 25 per cent tartaric acid, 1 per cent dimethylglyoxime (ammoniacal solution) Gooch crucible and filter.

B.S. 1615:1972

Anodic oxidation coatings on Aluminium.

B.S. 3987:1974

Anodic oxide coatings on wrought Aluminium for external architectural applications.

B.S. 5599:1978

Hard anodic oxide coatings on aluminium for engineering purposes

EURAS/EWAA

Specifications for a quality sign. for anodic oxidation coatings on aluminium for architectural purposes (for use with the Qualanod scheme).

DEF-151

Anodizing of aluminium and aluminium alloys.

Procedure Dilute 10 ml of the sealing solution to 250 ml in the volumetric flask, add 5 ml of nitric acid and bring to the boil, cool and filter if necessary. Add 10 ml of tartaric acid, neutralise with ammonium hydroxide, add 30 ml of dimethylglyoxime solution and allow to stand for 30 minutes. Filter the precipitate through a weighed Gooch crucible. Wash with hot water, dry at 125°C for IV2 hours, cool and reweigh. Nickel acetate (g/1) = weight of precipitate x 61.

Aluminum Association (U.S.A.) Standards for Anodized Architectural Aluminum Architectural Aluminum Manufacturers Association (U.S.A.) Voluntary Guide Specification and Inspection Methods for Integral anodic finishes for architectural aluminum.

14. Sodium dichromate sealing baths The method used is the normal one of titration against a standardised ferrous ammonium sulphate solution.

Voluntary Guide Specification and Inspection Methods for electrolytically deposited colour anodic finishes for architectural aluminum.

Apparatus 10 ml Pipette, 25 ml pipette, 50 ml burette, 250 ml volumetric flask, 500 ml conical flask. N/10 Ferrous ammonium sulphate, N/10 potassium permanganate, 50 per cent sulphuric acid solution. Procedure Take a 25 ml sample of the electrolyte and dilute to 250 ml in the volumetric flask. Transfer 10 ml of the diluted sample to the conical flask, dilute to 200 ml and add 10 ml 50 per cent H2SO4. Titrate against N/10 ferrous ammonium sulphate to excess. This can be determined by a spot test against a 1 per cent potassium ferricyanide solution which results in blue-green colour when neutralised. A further 1 - 2 ml ferrous ammonium sulphate should then be added. Let the total used be A ml. Back titrate the contents of the flask with N/10 KMnO 4 solution until a faint pink colour appears. Let the volume required be B ml. Chromate (as CrOj) g/1 = (A — B) x 1.32 Chromate (as KTCr2 O7) g/1 = (A — B) X 4.9 304

Methods of testing properties given in the above specifications:

Abrasion Resistance: Schuh and Kern - B.S. 1615:1972, Appendix L. Erichsen - I.S.O. draft in preparation. Anodic film thickness: Eddy current method - BS. 1615: 1972 Appendix C, BS. 5411 Part 3: 1976, and ISO. 2360. Gravimetric method - BS. 1615: 1972, Appendix B, and ISO. 2106. Microsection method - BS. 1615: 1972 Appendix A, BS. 5411 Part 5: 1976 and ISO. 1463. Split-beam microscope method - BS. 1615: 1972 Appendix D, and ISO. 2128. 305

13. Nickel acetate sealing baths This is the classical method of determination of nickel gravimetrically by precipitation with dimethylglyoxime.

Appendix: 2 Important Anodizing Specifications

Apparatus 10 ml Pipette, 25 ml pipette, 250 ml volumetric flask. Nitric acid, ammonia, 25 per cent tartaric acid, 1 per cent dimethylglyoxime (ammoniacal solution) Gooch crucible and filter.

B.S. 1615:1972

Anodic oxidation coatings on Aluminium.

B.S. 3987:1974

Anodic oxide coatings on wrought Aluminium for external architectural applications.

B.S. 5599:1978

Hard anodic oxide coatings on aluminium for engineering purposes

EURAS/EWAA

Specifications for a quality sign. for anodic oxidation coatings on aluminium for architectural purposes (for use with the Qualanod scheme).

DEF-151

Anodizing of aluminium and aluminium alloys.

Procedure Dilute 10 ml of the sealing solution to 250 ml in the volumetric flask, add 5 ml of nitric acid and bring to the boil, cool and filter if necessary. Add 10 ml of tartaric acid, neutralise with ammonium hydroxide, add 30 ml of dimethylglyoxime solution and allow to stand for 30 minutes. Filter the precipitate through a weighed Gooch crucible. Wash with hot water, dry at 125°C for IV2 hours, cool and reweigh. Nickel acetate (g/1) = weight of precipitate x 61.

Aluminum Association (U.S.A.) Standards for Anodized Architectural Aluminum Architectural Aluminum Manufacturers Association (U.S.A.) Voluntary Guide Specification and Inspection Methods for Integral anodic finishes for architectural aluminum.

14. Sodium dichromate sealing baths The method used is the normal one of titration against a standardised ferrous ammonium sulphate solution.

Voluntary Guide Specification and Inspection Methods for electrolytically deposited colour anodic finishes for architectural aluminum.

Apparatus 10 ml Pipette, 25 ml pipette, 50 ml burette, 250 ml volumetric flask, 500 ml conical flask. N/10 Ferrous ammonium sulphate, N/10 potassium permanganate, 50 per cent sulphuric acid solution. Procedure Take a 25 ml sample of the electrolyte and dilute to 250 ml in the volumetric flask. Transfer 10 ml of the diluted sample to the conical flask, dilute to 200 ml and add 10 ml 50 per cent H2SO4. Titrate against N/10 ferrous ammonium sulphate to excess. This can be determined by a spot test against a 1 per cent potassium ferricyanide solution which results in blue-green colour when neutralised. A further 1 - 2 ml ferrous ammonium sulphate should then be added. Let the total used be A ml. Back titrate the contents of the flask with N/10 KMnO 4 solution until a faint pink colour appears. Let the volume required be B ml. Chromate (as CrOj) g/1 = (A — B) x 1.32 Chromate (as KTCr2 O7) g/1 = (A — B) X 4.9 304

Methods of testing properties given in the above specifications:

Abrasion Resistance: Schuh and Kern - B.S. 1615:1972, Appendix L. Erichsen - I.S.O. draft in preparation. Anodic film thickness: Eddy current method - BS. 1615: 1972 Appendix C, BS. 5411 Part 3: 1976, and ISO. 2360. Gravimetric method - BS. 1615: 1972, Appendix B, and ISO. 2106. Microsection method - BS. 1615: 1972 Appendix A, BS. 5411 Part 5: 1976 and ISO. 1463. Split-beam microscope method - BS. 1615: 1972 Appendix D, and ISO. 2128. 305

Breakdown Voltage: B.S. 1615:1972, Appendix T and I.S.O. 2376.

SUBJECT INDEX

Continuity of thin coatings [Copper sulphate test] I.S. 0. 2085 Corrosion re si stance : CASS method - BS. 1615: 1972 Appendix J , BS. 5466: Part 3: 1977 and ISO. 3770. Acetic acid salt spray method - BS. 1615: 1972 Appendix K, BS. 5466: Part 2: 1977 and ISO. 3769. Ductility of anodic coatings:

I.S.O. 3211 .

Fastness to Light: B.S. 1615:1972, Appendix M and I.S.O. 2135. Infra-red Reflectivity : B.S. 1615:1972, Appendix S. Light Reflectivity: B.S. 1615:1972, Appendices P, Q and R, also I.S.O. 2726 (further I.S.O. drafts in preparation). Micro-hardness: B.S. 5599:1978, Appendix D and I.S.O. 4516. Sealing quality Weight loss methods: Acidified sodium sulphite - BS. 1615 Appendix E and ISO. 2932. Sodium acetate/ acetic acid - ISO. 2932. Chromic acid/phosphoric acid - BS. 3987: 1974 Appendix E and ISO. 3210. Dye spot methods - BS. 1615: 1972 Appendix F and ISO. 2143 (under revision). Admittance methods - BS. 1615: 1972 Appendix G and ISO. 2931 (also impedance). SO 2 humidity method - BS. 1615: Appendix H. U. V. Light Fastness: B.S. 1615:1972, Appendix N, I.S.O. draft in preparation.

Except where otherwise stated, references refer to sulphuric acid anodizing or to coatings produced by sulphuric acid anodizing Abrasion resistance (see also Erosion resistance; Hardness) 119, 123, 124, 131, 142, 152, 154,277-9 Absorption curves for dyebaths 183 A.C. anodizing 131, 141, 211, 213 Accelerated corrosion tests 251 electrolytic cathodic 253 salt spray 252 Acetic acid and acetates buffering dyestuff solutions 178 in chemical polishing solutions 59 in sealing solutions 225, 231. 232 reviving aged anodic oxide films before dyeing 180 Acid Blue 197 164 Acid cleaners and pickles 42, 46 Acid Green 25 179 Acid Yellow 3 179 Acid Yellow 167 184 Admittance test 206, 246ff. Agitation in anodizing 4, 94, 127 in chemical polishing 61 in cleaning 41, 47 in dyeing 181 in electropolishing 79, 80 in etch make-up 47 in hard anodizing 159, 164, 168 porous ceramic tubes for 164 in sealing 221 Air (see also Agitation) rate of flow into anodizing tank 94 Alcanodox Self-Colour Anodizing Process 144, 149 Alcoa 225 and 226 Hard Anodizing Process 129 Alizarin Red-S 180 Alkaline electropolishing see Brytal Alkylaryl polyethylene glycol sealing with 235 Alloys see Aluminium Alpol chemical polishing process 60 Aluflex electrolpolishing process 84

Aluminium (and alloys)

References to specific alloys are given at the end of this entry bus bars 93, 105, 109 cast 8 cast, chromic acid anodizing 137 cathodes 76,92 in Alzak electrobrightening bath determining in chemical polishing solutions, determining 295 in chromic acid electrolytes, determining 301 in electrobrightening baths, determining 298 in etchants, determining 290, 291 foil, printing before anodizing 188 in oxalic and sulphuric-oxalic acid electrolytes 300, 301 jigs see Racks permissible concentration in: chromic acid anodizing solutions 140 oxalic acid anodizing solutions 142 sulphuric acid anodizing solutions 121, 159 racks see Racks removing oxide produced by heat treatment 40 removing temporary protectives 41 residues from semi-manufacturing operations, removing 40 response of cast alloys to anodizing 8 response of wrought alloys to anodizing 7 salts affecting dyebath 179 Al-Cr alloys colour of anodic coating on 177 Al-Cu alloys chromic acid anodizing 137 desmutting 53 electropolishing 81 fatigue properties 280 hard anodizing 160, 168 307

306

Breakdown Voltage: B.S. 1615:1972, Appendix T and I.S.O. 2376.

SUBJECT INDEX

Continuity of thin coatings [Copper sulphate test] I.S. 0. 2085 Corrosion re si stance : CASS method - BS. 1615: 1972 Appendix J , BS. 5466: Part 3: 1977 and ISO. 3770. Acetic acid salt spray method - BS. 1615: 1972 Appendix K, BS. 5466: Part 2: 1977 and ISO. 3769. Ductility of anodic coatings:

I.S.O. 3211 .

Fastness to Light: B.S. 1615:1972, Appendix M and I.S.O. 2135. Infra-red Reflectivity : B.S. 1615:1972, Appendix S. Light Reflectivity: B.S. 1615:1972, Appendices P, Q and R, also I.S.O. 2726 (further I.S.O. drafts in preparation). Micro-hardness: B.S. 5599:1978, Appendix D and I.S.O. 4516. Sealing quality Weight loss methods: Acidified sodium sulphite - BS. 1615 Appendix E and ISO. 2932. Sodium acetate/ acetic acid - ISO. 2932. Chromic acid/phosphoric acid - BS. 3987: 1974 Appendix E and ISO. 3210. Dye spot methods - BS. 1615: 1972 Appendix F and ISO. 2143 (under revision). Admittance methods - BS. 1615: 1972 Appendix G and ISO. 2931 (also impedance). SO 2 humidity method - BS. 1615: Appendix H. U. V. Light Fastness: B.S. 1615:1972, Appendix N, I.S.O. draft in preparation.

Except where otherwise stated, references refer to sulphuric acid anodizing or to coatings produced by sulphuric acid anodizing Abrasion resistance (see also Erosion resistance; Hardness) 119, 123, 124, 131, 142, 152, 154,277-9 Absorption curves for dyebaths 183 A.C. anodizing 131, 141, 211, 213 Accelerated corrosion tests 251 electrolytic cathodic 253 salt spray 252 Acetic acid and acetates buffering dyestuff solutions 178 in chemical polishing solutions 59 in sealing solutions 225, 231. 232 reviving aged anodic oxide films before dyeing 180 Acid Blue 197 164 Acid cleaners and pickles 42, 46 Acid Green 25 179 Acid Yellow 3 179 Acid Yellow 167 184 Admittance test 206, 246ff. Agitation in anodizing 4, 94, 127 in chemical polishing 61 in cleaning 41, 47 in dyeing 181 in electropolishing 79, 80 in etch make-up 47 in hard anodizing 159, 164, 168 porous ceramic tubes for 164 in sealing 221 Air (see also Agitation) rate of flow into anodizing tank 94 Alcanodox Self-Colour Anodizing Process 144, 149 Alcoa 225 and 226 Hard Anodizing Process 129 Alizarin Red-S 180 Alkaline electropolishing see Brytal Alkylaryl polyethylene glycol sealing with 235 Alloys see Aluminium Alpol chemical polishing process 60 Aluflex electrolpolishing process 84

Aluminium (and alloys)

References to specific alloys are given at the end of this entry bus bars 93, 105, 109 cast 8 cast, chromic acid anodizing 137 cathodes 76,92 in Alzak electrobrightening bath determining in chemical polishing solutions, determining 295 in chromic acid electrolytes, determining 301 in electrobrightening baths, determining 298 in etchants, determining 290, 291 foil, printing before anodizing 188 in oxalic and sulphuric-oxalic acid electrolytes 300, 301 jigs see Racks permissible concentration in: chromic acid anodizing solutions 140 oxalic acid anodizing solutions 142 sulphuric acid anodizing solutions 121, 159 racks see Racks removing oxide produced by heat treatment 40 removing temporary protectives 41 residues from semi-manufacturing operations, removing 40 response of cast alloys to anodizing 8 response of wrought alloys to anodizing 7 salts affecting dyebath 179 Al-Cr alloys colour of anodic coating on 177 Al-Cu alloys chromic acid anodizing 137 desmutting 53 electropolishing 81 fatigue properties 280 hard anodizing 160, 168 307

306

reduced corrosion resistance of anodized 258 wear resistance of anodized 277 weather resistance of anodized 255 Al-Mg alloys chemical brightening 57, 70 electrobrightening 76 electropolishing 81 , 85 fatigue properties 280 reflectivity 268 salt spray testing 252 temperature stability 275 weather resistance 258 Al-Si alloys colour of anodic oxide coating on 145, 171 desmutting 46, 53 electropolishing 81 weather resistance 258 Al (superpure) and alloys chemical brightening 57, 70, 73 electropolishing 76, 81 reflectivity 268 Aluminium Black MLW 183 Aluminium Fast Yellow G3LW 183, 184 Aluminium Green GLW sealing test using 246 Aluminium Red B3LW sealing test using 245 Aluminium Turquoise PLW 184 Alupol II chemical polishing process 59 Alupol IV chemical polishing process 57 Alupol V chemical polishing process 57 Alzak electropolishing process 78 solution analysis 298 Ammeters, contacting 98 Ammonium borate, phosphate or tartrate electrolytes 1 , 273 Ammonium sulphide-copper sulphate colouring process 177 Ammonium salt etches 46 Ammonium sulphide-lead acetate colouring process 177 Analysis Brytal electrobrightening bath 298 caustic soda etching bath 289 caustic soda-sodium nitrate etching bath 290 caustic soda-trisodium phosphate etch cleaner 291 chromic acid electrolytes 301 nickel acetate sealing baths 304 nitric acid desmut baths 291 nitric-hydrofluoric etching/ desmut solutions 292 oxalic acid electrolytes 301 phosphoric-base chemical brightening baths 294 sodium dichromate sealing baths 304 sulphuric acid electrolytes 299 308

sulphuric-oxalic acid electrolytes 300 Aniline in chemical polishing solutions 72 Annual budget 32 Anode to cathode distance in electropolishing 80 Anode to cathode ratio in chromic acid anodizing 138 Anode movement (see also Agitation) 80, 84, 166 Anodic oxide coatings blooming, chalking, etc. 217, 225, 229, 230 colour (see also Colouring) 131, 139, 143, 144, 146, 147, 148, 150, 264 colouring and dyeing see Colouring; Dyeing composition 262 conversion to bohmite 246 exposure behaviour 255-260 impregnating with lacquers, etc, see Lacquers; Waxes; etc. impregnating with light-sensitive materials 193 opaque 137, 146 properties and tests (indexed under Properties; Tests; and detailed under the individual properties and tests) 239 sealing see Sealing silk screen printing 189 thickness see Thickness weight see Thickness Anodizing (Sulphuric acid except where otherwise stated; detailed under the individual headings) Alcanodox 144, 149 alternating current 131, 143, 144, 145, 211,213 architectural see Architectural anodizing Bengough-Stuart 137 Buzzard 138 chemical polishing before 57 chromic acid 137 cleaning before 39 combined d.c. + a.c. 144, 146, 162, 212 conditions affecting chemical composition of coating 262 conditions affecting density of coating 264 constant current 99, 128 constant current density 103, 148, 161 constant voltage 102 constant wattage 162 Duranodic 149 economics see Business aspects electropolishing before 74 Ematal 146 fatigue affected by 280 hard 159

holding work awaiting 70 integral colour 146 miscellaneous electrolytes 154 operating conditions see Technology oxalic acid 141 plant for 89 strip 209 racking for 109 rinsing after 114 self-colour 146 solution analysis 289 solution composition see Technology sulphuric-oxalic acid 129 technology 117 Veroxal 150 wire 209 Anodizing industry barriers to entry 15 economic characteristics 12 outworking anodizers 15 Anthraquinone Violet dye sealing test 245 Apparent density anodic oxide coatings 264 Architectural anodizing automation in 25 capital costs 24 lacquering after 236 plant economics 21 plant size 24 plant utilization/profitability 25 process 144, 147 protecting against alkaline building materials 236 room temperature dyeing not recommended 182 service life 255 types of plant for 22, 99, 113 Arsenic affecting anodizing 120 Asphalt resist in multi-colouring process 188 Automation economics of 26 Barrel (basket) anodizing 110 Barrier-layer films 1 Battelle electropolishing process 79 Al build-up 82 analysis 294 control 81 current loading 80 defects and remedies 83 plant 79 practice 81 solution composition and operating condition 79 solution viscosity 81, 83 Bend tests 280

Bichromates see Chromates; Dicromate sealing Bisulphite in bleaching dyed anodic oxide coatings 184 Black colours 147, 176, 183, 200 affected by Fe contamination 184 Bleaching dyed anodic oxide coatings 184 Blooming 217, 225, 229, 230 a.c. films worse than d.c. 133 produced by SO2 - humidity tests 250 Blue colours 176, 179, 183, 184 Bohmite 6, 218 Borate electrolytes 1 , 273 Bores hard anodizing 164 Boric acid in chemical polishing 58 in electrolytic colouring 201 reduced burning in sulphuric acid anodizing 194 Break-down voltage see Electrical break-down voltage Brightening see Chemical polishing; Electropolishing Bright etched finishes chemical brightening solutions producing 60, 68 Brightness sec Reflectivity Brillotalu chemical polishing process 57 Bronze colours 146, 147, 150, 153, 176, 177, 200, 202 Brown colours 148, 177, 201 Brytal electropolishing process 75 bath control 78 gassing affecting brightening 77 irridescent film removal 77 plant 76 process conditions 76 sludge formation and filtration 78 solution analysis 298 Building materials protecting against alkali in 236 Burning 160, 166, 194 Bus bars 93, 105 Business aspects anodizing as an industry 10 architectural anodizing plants 22 automation 25, 26 budgeting 32 capital costs 15 captive v outworking shops 21 cost control 31, 36 customer credit 31 economic characteristics 15 objectives 9 plant investment 21 profitability 29, 37 uncertainties 16 309

reduced corrosion resistance of anodized 258 wear resistance of anodized 277 weather resistance of anodized 255 Al-Mg alloys chemical brightening 57, 70 electrobrightening 76 electropolishing 81 , 85 fatigue properties 280 reflectivity 268 salt spray testing 252 temperature stability 275 weather resistance 258 Al-Si alloys colour of anodic oxide coating on 145, 171 desmutting 46, 53 electropolishing 81 weather resistance 258 Al (superpure) and alloys chemical brightening 57, 70, 73 electropolishing 76, 81 reflectivity 268 Aluminium Black MLW 183 Aluminium Fast Yellow G3LW 183, 184 Aluminium Green GLW sealing test using 246 Aluminium Red B3LW sealing test using 245 Aluminium Turquoise PLW 184 Alupol II chemical polishing process 59 Alupol IV chemical polishing process 57 Alupol V chemical polishing process 57 Alzak electropolishing process 78 solution analysis 298 Ammeters, contacting 98 Ammonium borate, phosphate or tartrate electrolytes 1 , 273 Ammonium sulphide-copper sulphate colouring process 177 Ammonium salt etches 46 Ammonium sulphide-lead acetate colouring process 177 Analysis Brytal electrobrightening bath 298 caustic soda etching bath 289 caustic soda-sodium nitrate etching bath 290 caustic soda-trisodium phosphate etch cleaner 291 chromic acid electrolytes 301 nickel acetate sealing baths 304 nitric acid desmut baths 291 nitric-hydrofluoric etching/ desmut solutions 292 oxalic acid electrolytes 301 phosphoric-base chemical brightening baths 294 sodium dichromate sealing baths 304 sulphuric acid electrolytes 299 308

sulphuric-oxalic acid electrolytes 300 Aniline in chemical polishing solutions 72 Annual budget 32 Anode to cathode distance in electropolishing 80 Anode to cathode ratio in chromic acid anodizing 138 Anode movement (see also Agitation) 80, 84, 166 Anodic oxide coatings blooming, chalking, etc. 217, 225, 229, 230 colour (see also Colouring) 131, 139, 143, 144, 146, 147, 148, 150, 264 colouring and dyeing see Colouring; Dyeing composition 262 conversion to bohmite 246 exposure behaviour 255-260 impregnating with lacquers, etc, see Lacquers; Waxes; etc. impregnating with light-sensitive materials 193 opaque 137, 146 properties and tests (indexed under Properties; Tests; and detailed under the individual properties and tests) 239 sealing see Sealing silk screen printing 189 thickness see Thickness weight see Thickness Anodizing (Sulphuric acid except where otherwise stated; detailed under the individual headings) Alcanodox 144, 149 alternating current 131, 143, 144, 145, 211,213 architectural see Architectural anodizing Bengough-Stuart 137 Buzzard 138 chemical polishing before 57 chromic acid 137 cleaning before 39 combined d.c. + a.c. 144, 146, 162, 212 conditions affecting chemical composition of coating 262 conditions affecting density of coating 264 constant current 99, 128 constant current density 103, 148, 161 constant voltage 102 constant wattage 162 Duranodic 149 economics see Business aspects electropolishing before 74 Ematal 146 fatigue affected by 280 hard 159

holding work awaiting 70 integral colour 146 miscellaneous electrolytes 154 operating conditions see Technology oxalic acid 141 plant for 89 strip 209 racking for 109 rinsing after 114 self-colour 146 solution analysis 289 solution composition see Technology sulphuric-oxalic acid 129 technology 117 Veroxal 150 wire 209 Anodizing industry barriers to entry 15 economic characteristics 12 outworking anodizers 15 Anthraquinone Violet dye sealing test 245 Apparent density anodic oxide coatings 264 Architectural anodizing automation in 25 capital costs 24 lacquering after 236 plant economics 21 plant size 24 plant utilization/profitability 25 process 144, 147 protecting against alkaline building materials 236 room temperature dyeing not recommended 182 service life 255 types of plant for 22, 99, 113 Arsenic affecting anodizing 120 Asphalt resist in multi-colouring process 188 Automation economics of 26 Barrel (basket) anodizing 110 Barrier-layer films 1 Battelle electropolishing process 79 Al build-up 82 analysis 294 control 81 current loading 80 defects and remedies 83 plant 79 practice 81 solution composition and operating condition 79 solution viscosity 81, 83 Bend tests 280

Bichromates see Chromates; Dicromate sealing Bisulphite in bleaching dyed anodic oxide coatings 184 Black colours 147, 176, 183, 200 affected by Fe contamination 184 Bleaching dyed anodic oxide coatings 184 Blooming 217, 225, 229, 230 a.c. films worse than d.c. 133 produced by SO2 - humidity tests 250 Blue colours 176, 179, 183, 184 Bohmite 6, 218 Borate electrolytes 1 , 273 Bores hard anodizing 164 Boric acid in chemical polishing 58 in electrolytic colouring 201 reduced burning in sulphuric acid anodizing 194 Break-down voltage see Electrical break-down voltage Brightening see Chemical polishing; Electropolishing Bright etched finishes chemical brightening solutions producing 60, 68 Brightness sec Reflectivity Brillotalu chemical polishing process 57 Bronze colours 146, 147, 150, 153, 176, 177, 200, 202 Brown colours 148, 177, 201 Brytal electropolishing process 75 bath control 78 gassing affecting brightening 77 irridescent film removal 77 plant 76 process conditions 76 sludge formation and filtration 78 solution analysis 298 Building materials protecting against alkali in 236 Burning 160, 166, 194 Bus bars 93, 105 Business aspects anodizing as an industry 10 architectural anodizing plants 22 automation 25, 26 budgeting 32 capital costs 15 captive v outworking shops 21 cost control 31, 36 customer credit 31 economic characteristics 15 objectives 9 plant investment 21 profitability 29, 37 uncertainties 16 309

Business management applied to anodizing 9 Butyl rubber tank linings 71, 73 Calcium see Water Capacity see Size Capital investment 18, 25 Captive v out-working shops 17 Carbon (see also Graphite) heat exchangers 78, 98 origin of colour in oxalic anodic coatings 150 resistance heaters 71 Carbonaceous deposits removing 42 Carbonate in Brytal bath, determining 298 CASS test 252, 255 Castings cleaning 40 Cathode to anode distance in electropolishing 80 Cathode to anode ratio in chromic acid anodizing 138 Cathodes anodizing 92, 105 electropolishing 79, 84 for integral colour anodizing 153 internal 129, 164 Cathodic dissolution tests 253 Caustic soda see Sodium hydroxide Cells anodic oxide 1 , 2 Ceramic tubes porous, for air agitation 164 Chalk cleaning off 40 Chalking (in sealing) 217, 222, 230 Chemical composition anodic oxide coating 262 Chemical polishing 57 Alpol process 60 Alupol II process 59 Alupol IV process 57 Alupol V process 57 Brillotalu process 57 cleaning before 61, 62, 72 copper salts in 58, 59, 69 critical dimensions limitations 60 Erftwerk process 70 fume extraction 61, 69, 71 fumeless 59 giving bright etched appearance 60. 68 grease increasing gassing 68 holding work between brightening and anodizing 72 milky appearance due to water carryover 66 nitric -hydrofluoric acid processes (see

310

below) Phosbrite 150 process (see also Phosphoric acid-base processes) 59, 62, 63 Phosbrite 155 process 60 Phosbrite 156 process 60 Phosbrite 159 process 57,61-66 Phosbrite 161 process 64 Phosbrite 171 process 58 practice 61, 72 safety precautions 68 solution analysis 294 nitric-hydrofluoric acid processes 70 control 73 defects and remedies 73 Erftwerk process 70 lead essential 71 nitric acid concentration /weight loss 70 plant 71, 73 practice 72, 73 ‘S’ process 72 safety precautions 72 phosphoric acid-base processes 57 ageing effects 63 aluminium content 63, 64, 66 control 64, 69 exhaust requirements 61, 69 nitric acid/weight los 65 nomogram for control 67 operating problems 64, 66, 68 phosphoric-acetic-nitric mixtures 59 phosphoric-sulphuric-nitric mixtures 57 phosphoric-water-nitric mixtures 58 plant 60 practice 61 safety precautions 68 solution analysis 68, 293, 294 specific gravity 66. 67 temperature 58, 63 temperature/weight loss 63 time 62 time/weight loss 63 water affecting 66 Chemical resistance 264 in electropolishing solutions 79, 84 preventing cathode reduction 84 Chromic acid anodizing 137 Al and Cr permissible concentrations 140 applications 137 Bengough-Stuart process 137 Buzzard process 138 constant voltage process 138 impurities affecting 139 solution analysis 301 solution control 140

solution regeneration 138, 140 stop-off treatment before hard anodizing 168 sulphuric acid increasing transparency 139 Citric acid in anodizing electrolyte 154 in chemical polishing solutions 58 reviving aged anodic oxide films before dyeing 180 Cleaners analysis 291 applications 42 detergents in 41 formulations 48 make-up 52 need to add water softener 47 organic solvent vapours 41, 42, 61 phosphates in 41, 43 silicate causing difficulties 40 types 42 wetting agents in 41 Cleaning 39 agitation in 41, 47 before brightening 61 before chemical polishing 61, 72 before etching 47 desmutting after 52 drag-out 48 before electropolishing 81 operating conditions 47 optimum temperature 40 plant for 46 practice 47 principles 39 solution replenishment 48 scum produced in 48 vapour degreasing 41, 42, 61 Coatings see Anodic oxide coatings Cobalt acetate colouring solution 134, 171, 172, 177 in electrolytic colouring 200, 205, 206 sealing 231 Coils see Heating Colour of anodic oxide films as anodized 134, 137, 139, 144, 146, 148-153, 264 alternating current affecting 134, 144 origin oxalic acid films 144 integral colour 153 Colouring anodic oxide films dyeing see Dyeing with inorganic salts 134, 171, 174 Colour measurement 264 Composition of anodic oxide coatings 262 Conductivity sulphuric acid solutions 117 Conductivity test see Admittance test Contact marks 110, 166

Cooling anodizing solutions 95, 118, 120, 126, 153, 163 electropolishing solutions 78, 80 graphite coils for 78, 98 lead coils for 96 Copper (see also Al-Cu alloys) affecting anodizing 120 cathode rails 92 bus bars 92, 105 in chemical polishing solutions 58, 59 in electrolytic colouring 197, 200, 201 etch accelerator 45, 46 joining to aluminium 108 salts affecting dyes 178 Copper sulphate colouring agent for anodic oxide coating 177, 201 Corrodkote fest 253 Corrosion (see also Accelerated corrosion tests; Pitting; Weather resistance) spots produced after nitric acid desmutting 53 Costing 36 full v marginal 37 internal 36 invoice price 37 Crack detection chromic acid anodizing used for 137 fluids causing cleaning problems 40 Crazing due to temperature rise 275 Credit cost of customer 31 Current density chromic acid anodizing 137 control 100, 161 electropolishing 76, 78, 79, 81, 85 hard anodizing 159, 160, 162, 166 initial surge 3 in integral colour anodizing 146-151 oxalic acid anodizing 141, 144, 146 self-colour processes 147 sulphuric acid anodizing 4, 124 factors affecting 118, 124, 126, 133 factors influenced by 6, 118, 123,262, 264 factors influencing voltage required to produce 6, 118, 124, 126, 128 Current distribution in anodizing tube interiors 128 in electropolishing 84 Current ratings for bus bars 105 Current supply and control 100 alternating current 131, 145, 162 bus bars for 105, 153 for hard anodizing 161 rectifiers, etc. 100, 153

311

Business management applied to anodizing 9 Butyl rubber tank linings 71, 73 Calcium see Water Capacity see Size Capital investment 18, 25 Captive v out-working shops 17 Carbon (see also Graphite) heat exchangers 78, 98 origin of colour in oxalic anodic coatings 150 resistance heaters 71 Carbonaceous deposits removing 42 Carbonate in Brytal bath, determining 298 CASS test 252, 255 Castings cleaning 40 Cathode to anode distance in electropolishing 80 Cathode to anode ratio in chromic acid anodizing 138 Cathodes anodizing 92, 105 electropolishing 79, 84 for integral colour anodizing 153 internal 129, 164 Cathodic dissolution tests 253 Caustic soda see Sodium hydroxide Cells anodic oxide 1 , 2 Ceramic tubes porous, for air agitation 164 Chalk cleaning off 40 Chalking (in sealing) 217, 222, 230 Chemical composition anodic oxide coating 262 Chemical polishing 57 Alpol process 60 Alupol II process 59 Alupol IV process 57 Alupol V process 57 Brillotalu process 57 cleaning before 61, 62, 72 copper salts in 58, 59, 69 critical dimensions limitations 60 Erftwerk process 70 fume extraction 61, 69, 71 fumeless 59 giving bright etched appearance 60. 68 grease increasing gassing 68 holding work between brightening and anodizing 72 milky appearance due to water carryover 66 nitric -hydrofluoric acid processes (see

310

below) Phosbrite 150 process (see also Phosphoric acid-base processes) 59, 62, 63 Phosbrite 155 process 60 Phosbrite 156 process 60 Phosbrite 159 process 57,61-66 Phosbrite 161 process 64 Phosbrite 171 process 58 practice 61, 72 safety precautions 68 solution analysis 294 nitric-hydrofluoric acid processes 70 control 73 defects and remedies 73 Erftwerk process 70 lead essential 71 nitric acid concentration /weight loss 70 plant 71, 73 practice 72, 73 ‘S’ process 72 safety precautions 72 phosphoric acid-base processes 57 ageing effects 63 aluminium content 63, 64, 66 control 64, 69 exhaust requirements 61, 69 nitric acid/weight los 65 nomogram for control 67 operating problems 64, 66, 68 phosphoric-acetic-nitric mixtures 59 phosphoric-sulphuric-nitric mixtures 57 phosphoric-water-nitric mixtures 58 plant 60 practice 61 safety precautions 68 solution analysis 68, 293, 294 specific gravity 66. 67 temperature 58, 63 temperature/weight loss 63 time 62 time/weight loss 63 water affecting 66 Chemical resistance 264 in electropolishing solutions 79, 84 preventing cathode reduction 84 Chromic acid anodizing 137 Al and Cr permissible concentrations 140 applications 137 Bengough-Stuart process 137 Buzzard process 138 constant voltage process 138 impurities affecting 139 solution analysis 301 solution control 140

solution regeneration 138, 140 stop-off treatment before hard anodizing 168 sulphuric acid increasing transparency 139 Citric acid in anodizing electrolyte 154 in chemical polishing solutions 58 reviving aged anodic oxide films before dyeing 180 Cleaners analysis 291 applications 42 detergents in 41 formulations 48 make-up 52 need to add water softener 47 organic solvent vapours 41, 42, 61 phosphates in 41, 43 silicate causing difficulties 40 types 42 wetting agents in 41 Cleaning 39 agitation in 41, 47 before brightening 61 before chemical polishing 61, 72 before etching 47 desmutting after 52 drag-out 48 before electropolishing 81 operating conditions 47 optimum temperature 40 plant for 46 practice 47 principles 39 solution replenishment 48 scum produced in 48 vapour degreasing 41, 42, 61 Coatings see Anodic oxide coatings Cobalt acetate colouring solution 134, 171, 172, 177 in electrolytic colouring 200, 205, 206 sealing 231 Coils see Heating Colour of anodic oxide films as anodized 134, 137, 139, 144, 146, 148-153, 264 alternating current affecting 134, 144 origin oxalic acid films 144 integral colour 153 Colouring anodic oxide films dyeing see Dyeing with inorganic salts 134, 171, 174 Colour measurement 264 Composition of anodic oxide coatings 262 Conductivity sulphuric acid solutions 117 Conductivity test see Admittance test Contact marks 110, 166

Cooling anodizing solutions 95, 118, 120, 126, 153, 163 electropolishing solutions 78, 80 graphite coils for 78, 98 lead coils for 96 Copper (see also Al-Cu alloys) affecting anodizing 120 cathode rails 92 bus bars 92, 105 in chemical polishing solutions 58, 59 in electrolytic colouring 197, 200, 201 etch accelerator 45, 46 joining to aluminium 108 salts affecting dyes 178 Copper sulphate colouring agent for anodic oxide coating 177, 201 Corrodkote fest 253 Corrosion (see also Accelerated corrosion tests; Pitting; Weather resistance) spots produced after nitric acid desmutting 53 Costing 36 full v marginal 37 internal 36 invoice price 37 Crack detection chromic acid anodizing used for 137 fluids causing cleaning problems 40 Crazing due to temperature rise 275 Credit cost of customer 31 Current density chromic acid anodizing 137 control 100, 161 electropolishing 76, 78, 79, 81, 85 hard anodizing 159, 160, 162, 166 initial surge 3 in integral colour anodizing 146-151 oxalic acid anodizing 141, 144, 146 self-colour processes 147 sulphuric acid anodizing 4, 124 factors affecting 118, 124, 126, 133 factors influenced by 6, 118, 123,262, 264 factors influencing voltage required to produce 6, 118, 124, 126, 128 Current distribution in anodizing tube interiors 128 in electropolishing 84 Current ratings for bus bars 105 Current supply and control 100 alternating current 131, 145, 162 bus bars for 105, 153 for hard anodizing 161 rectifiers, etc. 100, 153

311

Dangers see Hazards Degreasing see Cleaning Deionised water see Water Density anodic coatings 264 Designs — multi-colour 187 Desmutting and desmudging treatments after cleaning or etching 52 after electropolishing 78, 81 solution analysis 291, 292 Detergents in etches 44, 45 Dextrin in Erftwerk solutions 70 etch inhibitor 45 Dichromates see Chromates Dichromate sealing baths analysis 304 Diecastings cleaning 40 Dielectric constant 283 Dielectric strength 283 Direct Blue 86 184 Discounted cash flow 22, 30 Double dipping colouring techniques 175 Double sealing techniques 234 Drawing lubricants - removing 41 Ductility tests 279 Dullness defect in electropolishing 84 Duranodic integral anodizing process 149 Dyed anodic oxide films bleaching 184 light- and heat-fastness 178, 182, 250 Dyeing anodic oxide films (organic and inorganic colours) 171 acetic acid buffering 178 ageing affecting 180 agitation 181 aluminium salts affecting 179 calcium affecting 179 contamination affecting 178 Cu and Fe affecting 179 double-dipping techniques 175 dyebath control 182 dyebath make-up 181 dyebath operation 179 ferric ammonium oxalate colours 176 film thickness affecting 174, 181 historical and general 171 inorganic pigment colouring techniques

room temperature 181 sulphate affecting 179 temperature of 181 times 175, 181 Dyes Acid Blue 197 179 Acid Green 25 179 Acid Yellow 3 179 Acid Yellow 167 183 Alizarin Red S 180 Aluminium Black MLW 183 Aluminium Fast Yellow G3LW 183 Aluminium Green GLW 245, 248 Aluminium Turquoise PLWS 183 black colours 147, 176, 183, 200 blue colours 176, 179, 183, 184 bronze colours 146, 147, 150, 153, 176, 200, 202 brown colours 148, 177, 201 contamination affecting 178, 184 Direct Blue 86 179,184 green colours 177, 245 Mordant Black 85 183 Mordant Red 3 179 Palatine Bordeau RN 173 phthalocyanine blue dyes 179, 184 Dye-spot tests for sealing of anodic oxide coatings 224, 245

Economic characteristics of the anodizing industry 12 Eddy current thickness test 241 Electrical break-down voltage 242, 283 sealing effect 284 as thickness test 242 Electrical impedance tests 246 Electrical properties 282 Electrical resistance 285 Electrolytic cathodic corrosion tests 253 Electrolytic colouring a.c. process 197, 199 buffering agents 201 continuous processes 207 d.c. process 200 counter electrodes 197, 200 electrolytes 200-3 bismuth -cobalt 200 cobalt 200 copper 200 indium 201 174 iron 197 inorganic pigment colouring theory 171 manganese 197 jigging for 181 molybdate 200 multi-colouring techniques (detailed under nickel 197, 200,201,203 Multi-colouring techniques) 187 nickel-tin 200, 202 organic dyestuff dyeing mechanism 172 selenium 200 patchy 39 silver 200 pH affecting 175, 178 tin 197, 200,201 with photosensitive materials 193 tungstate 200 rinsing before 1 79

312

historical development 197 mechanism 203 operations 200 plant 199 power supply 199 processes (list of proprietary below) 198 Anolok Carmiol Colinol 200 Colorox Electrocolor Endacolor Eurocolor 800, 1000 lonkol Korundalor Metacolor Metoxal Oxicolor Sailox Variocolor properties 205 spalling 202 Electrolytic condensers 1 Electroplating anodizing as a basis for 1. 140 Electropolishing 75 affecting fatigue 281 with alkaline solutions 75 Aluflex process 84 Alzak bath, analysis 298 Alzak process 78 Battellc process 79 Brytal bath, analysis 298 Brytal process 75 with chromic-sulphuric-phosphoric acid solution 84 cleaning solution 81 etching before 76 with fluoboric acid solution 78 fume extraction in 76, 80 G IV process 84 history 75 metal removal in 81 plant and practice see the individual electropolishing processes polishing before 76 sludge produced in 78, 82, 87 smoothing in 86 with sulphuric-chromic acid 84 used as electromachining process 75 V.A.W. alkaline process 75 Ematal process 146 Emulsification a s cleaning action 40, 41 Emulsion cleaners 41, 42 Erichsen abrasion tests 278 Erosion resistance see Abrasion resistance Esters in electropolishing solutions 84 Etchants 44

acid 46 alkaline 45 analysis 289, 290 detergents in 45 formulation 44 increasing etch rate 45 long-life 51 make-up 47 modifiers in 45 producing relief patterns 178 scale inhibitors 45 sequestering agents in 45 when to renew 51 Etching 44 affecting fatigue 281 between mechanical buffering and colouring 76 bloom 53, 54 desmutting after 52 drag-over 52 operating conditions 47 patchy 51 , 54 plant for 46 practice 47 scum produced in 48 spangle effect 51 solution replenishment 53 uneven 39 white etch bloom 54 Extruded sections anodizing 22, 144, 147-152 FACT test see Ford corrosion test Fatigue properties anodizing affecting 280 hard anodizing affecting 281 sealing improving 281 surface preparation affecting 281 Ferric ammonium oxalate for gold colouring anodic oxide coatings 172, 174, 176 Ferric chloride in relief etchant 192 Ferric sulphate desmut 53 Ferrocyanide colouring process 176 photographic process 195 Film continuity test 305 Film dissolution (see also Burning) 3, 118 Film growth 3, 119, 142, 148 Film thickness see Thickness Finger marking 212 Fluoboric acid electropolishing process see Alzak electropolishing process Fluorides in alkaline etches 45 in chemical polishing solutions 70 in desmutting solutions 53 in sealing in 222 tanks for 53, 71, 73, 78

313

Dangers see Hazards Degreasing see Cleaning Deionised water see Water Density anodic coatings 264 Designs — multi-colour 187 Desmutting and desmudging treatments after cleaning or etching 52 after electropolishing 78, 81 solution analysis 291, 292 Detergents in etches 44, 45 Dextrin in Erftwerk solutions 70 etch inhibitor 45 Dichromates see Chromates Dichromate sealing baths analysis 304 Diecastings cleaning 40 Dielectric constant 283 Dielectric strength 283 Direct Blue 86 184 Discounted cash flow 22, 30 Double dipping colouring techniques 175 Double sealing techniques 234 Drawing lubricants - removing 41 Ductility tests 279 Dullness defect in electropolishing 84 Duranodic integral anodizing process 149 Dyed anodic oxide films bleaching 184 light- and heat-fastness 178, 182, 250 Dyeing anodic oxide films (organic and inorganic colours) 171 acetic acid buffering 178 ageing affecting 180 agitation 181 aluminium salts affecting 179 calcium affecting 179 contamination affecting 178 Cu and Fe affecting 179 double-dipping techniques 175 dyebath control 182 dyebath make-up 181 dyebath operation 179 ferric ammonium oxalate colours 176 film thickness affecting 174, 181 historical and general 171 inorganic pigment colouring techniques

room temperature 181 sulphate affecting 179 temperature of 181 times 175, 181 Dyes Acid Blue 197 179 Acid Green 25 179 Acid Yellow 3 179 Acid Yellow 167 183 Alizarin Red S 180 Aluminium Black MLW 183 Aluminium Fast Yellow G3LW 183 Aluminium Green GLW 245, 248 Aluminium Turquoise PLWS 183 black colours 147, 176, 183, 200 blue colours 176, 179, 183, 184 bronze colours 146, 147, 150, 153, 176, 200, 202 brown colours 148, 177, 201 contamination affecting 178, 184 Direct Blue 86 179,184 green colours 177, 245 Mordant Black 85 183 Mordant Red 3 179 Palatine Bordeau RN 173 phthalocyanine blue dyes 179, 184 Dye-spot tests for sealing of anodic oxide coatings 224, 245

Economic characteristics of the anodizing industry 12 Eddy current thickness test 241 Electrical break-down voltage 242, 283 sealing effect 284 as thickness test 242 Electrical impedance tests 246 Electrical properties 282 Electrical resistance 285 Electrolytic cathodic corrosion tests 253 Electrolytic colouring a.c. process 197, 199 buffering agents 201 continuous processes 207 d.c. process 200 counter electrodes 197, 200 electrolytes 200-3 bismuth -cobalt 200 cobalt 200 copper 200 indium 201 174 iron 197 inorganic pigment colouring theory 171 manganese 197 jigging for 181 molybdate 200 multi-colouring techniques (detailed under nickel 197, 200,201,203 Multi-colouring techniques) 187 nickel-tin 200, 202 organic dyestuff dyeing mechanism 172 selenium 200 patchy 39 silver 200 pH affecting 175, 178 tin 197, 200,201 with photosensitive materials 193 tungstate 200 rinsing before 1 79

312

historical development 197 mechanism 203 operations 200 plant 199 power supply 199 processes (list of proprietary below) 198 Anolok Carmiol Colinol 200 Colorox Electrocolor Endacolor Eurocolor 800, 1000 lonkol Korundalor Metacolor Metoxal Oxicolor Sailox Variocolor properties 205 spalling 202 Electrolytic condensers 1 Electroplating anodizing as a basis for 1. 140 Electropolishing 75 affecting fatigue 281 with alkaline solutions 75 Aluflex process 84 Alzak bath, analysis 298 Alzak process 78 Battellc process 79 Brytal bath, analysis 298 Brytal process 75 with chromic-sulphuric-phosphoric acid solution 84 cleaning solution 81 etching before 76 with fluoboric acid solution 78 fume extraction in 76, 80 G IV process 84 history 75 metal removal in 81 plant and practice see the individual electropolishing processes polishing before 76 sludge produced in 78, 82, 87 smoothing in 86 with sulphuric-chromic acid 84 used as electromachining process 75 V.A.W. alkaline process 75 Ematal process 146 Emulsification a s cleaning action 40, 41 Emulsion cleaners 41, 42 Erichsen abrasion tests 278 Erosion resistance see Abrasion resistance Esters in electropolishing solutions 84 Etchants 44

acid 46 alkaline 45 analysis 289, 290 detergents in 45 formulation 44 increasing etch rate 45 long-life 51 make-up 47 modifiers in 45 producing relief patterns 178 scale inhibitors 45 sequestering agents in 45 when to renew 51 Etching 44 affecting fatigue 281 between mechanical buffering and colouring 76 bloom 53, 54 desmutting after 52 drag-over 52 operating conditions 47 patchy 51 , 54 plant for 46 practice 47 scum produced in 48 spangle effect 51 solution replenishment 53 uneven 39 white etch bloom 54 Extruded sections anodizing 22, 144, 147-152 FACT test see Ford corrosion test Fatigue properties anodizing affecting 280 hard anodizing affecting 281 sealing improving 281 surface preparation affecting 281 Ferric ammonium oxalate for gold colouring anodic oxide coatings 172, 174, 176 Ferric chloride in relief etchant 192 Ferric sulphate desmut 53 Ferrocyanide colouring process 176 photographic process 195 Film continuity test 305 Film dissolution (see also Burning) 3, 118 Film growth 3, 119, 142, 148 Film thickness see Thickness Finger marking 212 Fluoboric acid electropolishing process see Alzak electropolishing process Fluorides in alkaline etches 45 in chemical polishing solutions 70 in desmutting solutions 53 in sealing in 222 tanks for 53, 71, 73, 78

313

in water, affecting anodizing 122 Foil (aluminium) anodizing continuous 209 printing before anodizing 188 Ford corrosion test 253 Forgings - cleaning 40 Formic acid in anodizing electrolyte 153, 154 French chalk stopping-off material 166 Fume extraction chemical polishing tanks 61, 69, 71 electropolishing tanks 75, 80 etching tanks 47 Fume suppressants in chemical polishing solutions 58 in etchants 45 G IV elcctropolishing process 84 Gallic acid colouring process 176 reducing burning in sulphuric acid anodizing 194 Gardner Modified Pivotable Sphere Hazemeter 269 Gassing in anodizing, polarisation affect 162 in chemical polishing 62, 69, 71 in electropolishing 77, 80, 83, 85 in etching 47 Gelatin in photographic process 189 Gluconates - scale inhibitor 45 Glycerine refrigerant 96, 163 thickening agent for dyestuff solutions 171 Glycol refrigerant 96, 130, 163 Glycollic acid anodizing 151 Glyoxylic acid anodizing 152 Goggles sec Hazards Gold colours by inorganic pigment colouring 171, ' 174, 176 by integral colour anodizing by oxalic acid anodizing 145, 146 Graphite (see also Carbon) anodizing cathodes 92 cooling coils 78, 98 Grease cleaning-off 40 in chemical polishing solutions Green colours 177, 179 Green dye test 224, 245, 246 Gum Arabic in Erftwerk solutions 70 scale inhibitor 45 Hard anodizing 159 affecting fatigue 280

314

agitation 164 Al content 159 bores 164 cooling and refrigeration 163 electrolytes for 159 Hardas process 162, 165 high and low concentration baths 159 high copper alloys 159, 168 impurities affecting 160 mixed electrolytes 160 practice 135 Sandford process 160 sealing 170 stopped-off work 166 wear resistance 277 Hardas hard anodizing process 162, 165 Hardness (see also Abrasion resistance) 120,276 oxalic acid coatings 144 sulphuric acid coatings 120 Hazards in chemical polishing 68, 72, 73 in cleaner or etchant make-up 48 in elcctropolishing 79 in inorganic colouring processes 177 Hazemeter 269 Heat generated in anodizing 4, 95 insulation, hard anodizing tanks 163 Heat emissivity 274 Heating anodizing solutions 100 carbon heat exchangers for 78, 98 chemical brightening plant 60, 69, 71 cleaning and etching tanks 46 coils 47, 60, 69, 80, 84, 220 with high pressure hot water 47 scale and sludge produced by 47 scaling tanks 220 with steam 46 Heat reflectivity (see also Reflectivity) 235,271 Heat treatment removing oxide films produced by 40 Heptonates - scale inhibitor 45 Hollow articles anodizing 110 long, anodizing 128 Hydrochloric acid in relief etchant 192 Hydrofluoric acid in acid etchants 46 in chemical polishing solutions 70, 72 68 in desmutting solutions 53 in desmutting solutions, determining 292 in etches 45, 46 tanks for 53 Hydrogen peroxide sealing for contact with 169 Hypochlorite

in bleaching dyed anodic oxide coatings 184 Image clarity see Reflectivity Impedance see Electrical impedance Impregnation of anodic oxide coatings see Dyeing; Lacquers; Photographic processes; Waxes; etc. Impurities in chromic acid anodizing solutions 139 in oxalic acid anodizing 142 in sulphuric acid anodizing solutions 120, 125, 160 Infrared reflectivity (see also Reflectivity) 235, 271 Inhibitors in etches 45 Inks for multi-colouring process 188, 190 Insulation see Electrical insulation Interferometric thickness test 241 Integral colour anodizing 146 Acadai process 151 Alcanodox process 149 Duranodic process 149 Eurocolor JOO process 151 history 197 Kalcolor process 147 mechanism 153 miscellaneous processes 154 Permalux process 150 Permanodic process 149 plant 153 Sumitome process 151 Veroxal process 150 Investment see Plant investment Invoice price factors affecting 37 Ion exchange (see also Water) regenerating chromic acid anodizing solutions 140 regenerating ‘Kalcolor’ electrolyte 149 regenerating ‘Veroxal’ electrolyte 150 treatment of water for sealing 223, 224 Iron (see also Ferric ammonium oxalate; Ferric chloride) affecting anodizing 120, 126, 160 affecting sealing 222 etch accelerator 45 salts affecting dyes 179, 184 salts in colouring solutions 171 Japan anodizing in 1, 141, 151 Jigs see Racks Kalcolor integral colour anodizing process 147 Kape test 244

Kesternich test 250 Lacquers cleaning off 41 impregnating anodic oxide coatings with 236 improving corrosion resistance 260 Lanolin cleaning-off 41 sealing with 235 Lead affecting anodizing 120 anodizing cathodes 92 antimonial, affecting chemical polishing 69 counter-electrodes in electrolytic colouring 200 electropolishing cathodes 80, 84 essential in Erftwerk polishing 71 tank linings 69, 73, 80, 84 Lead acetate colouring processes for anodic oxide coatings 177, 187 Leakage current Lettering reproducing 187, 193 Lids (holloware) anodizing 114 Light reflectivity (see also Reflectivity) 255, 267 Lubricants cleaning-off 41 sealing with 169, 235 Leakage current 281 Machining lubricants removing 41 Magnesia film effect in etching 54 Magnesium salts in water, affecting sealing 222 Maleic acid in anodizing electrolytes 150, 151 Malic acid anodizing 152 Malonic acid anodising 151 Manganese affecting hard anodizing 160 Marginal costing 37 Masking see Stopping-off Material to be anodized see Aluminium Matt finishes etchants producing 46 Mechanical polishing before electropolishing 75, 76 Mechanism of anodizing 1-4 Microhardness see Hardness Micro-sectioning thickness test 240 Military equipment chromic acid anodizing as inspection

315

in water, affecting anodizing 122 Foil (aluminium) anodizing continuous 209 printing before anodizing 188 Ford corrosion test 253 Forgings - cleaning 40 Formic acid in anodizing electrolyte 153, 154 French chalk stopping-off material 166 Fume extraction chemical polishing tanks 61, 69, 71 electropolishing tanks 75, 80 etching tanks 47 Fume suppressants in chemical polishing solutions 58 in etchants 45 G IV elcctropolishing process 84 Gallic acid colouring process 176 reducing burning in sulphuric acid anodizing 194 Gardner Modified Pivotable Sphere Hazemeter 269 Gassing in anodizing, polarisation affect 162 in chemical polishing 62, 69, 71 in electropolishing 77, 80, 83, 85 in etching 47 Gelatin in photographic process 189 Gluconates - scale inhibitor 45 Glycerine refrigerant 96, 163 thickening agent for dyestuff solutions 171 Glycol refrigerant 96, 130, 163 Glycollic acid anodizing 151 Glyoxylic acid anodizing 152 Goggles sec Hazards Gold colours by inorganic pigment colouring 171, ' 174, 176 by integral colour anodizing by oxalic acid anodizing 145, 146 Graphite (see also Carbon) anodizing cathodes 92 cooling coils 78, 98 Grease cleaning-off 40 in chemical polishing solutions Green colours 177, 179 Green dye test 224, 245, 246 Gum Arabic in Erftwerk solutions 70 scale inhibitor 45 Hard anodizing 159 affecting fatigue 280

314

agitation 164 Al content 159 bores 164 cooling and refrigeration 163 electrolytes for 159 Hardas process 162, 165 high and low concentration baths 159 high copper alloys 159, 168 impurities affecting 160 mixed electrolytes 160 practice 135 Sandford process 160 sealing 170 stopped-off work 166 wear resistance 277 Hardas hard anodizing process 162, 165 Hardness (see also Abrasion resistance) 120,276 oxalic acid coatings 144 sulphuric acid coatings 120 Hazards in chemical polishing 68, 72, 73 in cleaner or etchant make-up 48 in elcctropolishing 79 in inorganic colouring processes 177 Hazemeter 269 Heat generated in anodizing 4, 95 insulation, hard anodizing tanks 163 Heat emissivity 274 Heating anodizing solutions 100 carbon heat exchangers for 78, 98 chemical brightening plant 60, 69, 71 cleaning and etching tanks 46 coils 47, 60, 69, 80, 84, 220 with high pressure hot water 47 scale and sludge produced by 47 scaling tanks 220 with steam 46 Heat reflectivity (see also Reflectivity) 235,271 Heat treatment removing oxide films produced by 40 Heptonates - scale inhibitor 45 Hollow articles anodizing 110 long, anodizing 128 Hydrochloric acid in relief etchant 192 Hydrofluoric acid in acid etchants 46 in chemical polishing solutions 70, 72 68 in desmutting solutions 53 in desmutting solutions, determining 292 in etches 45, 46 tanks for 53 Hydrogen peroxide sealing for contact with 169 Hypochlorite

in bleaching dyed anodic oxide coatings 184 Image clarity see Reflectivity Impedance see Electrical impedance Impregnation of anodic oxide coatings see Dyeing; Lacquers; Photographic processes; Waxes; etc. Impurities in chromic acid anodizing solutions 139 in oxalic acid anodizing 142 in sulphuric acid anodizing solutions 120, 125, 160 Infrared reflectivity (see also Reflectivity) 235, 271 Inhibitors in etches 45 Inks for multi-colouring process 188, 190 Insulation see Electrical insulation Interferometric thickness test 241 Integral colour anodizing 146 Acadai process 151 Alcanodox process 149 Duranodic process 149 Eurocolor JOO process 151 history 197 Kalcolor process 147 mechanism 153 miscellaneous processes 154 Permalux process 150 Permanodic process 149 plant 153 Sumitome process 151 Veroxal process 150 Investment see Plant investment Invoice price factors affecting 37 Ion exchange (see also Water) regenerating chromic acid anodizing solutions 140 regenerating ‘Kalcolor’ electrolyte 149 regenerating ‘Veroxal’ electrolyte 150 treatment of water for sealing 223, 224 Iron (see also Ferric ammonium oxalate; Ferric chloride) affecting anodizing 120, 126, 160 affecting sealing 222 etch accelerator 45 salts affecting dyes 179, 184 salts in colouring solutions 171 Japan anodizing in 1, 141, 151 Jigs see Racks Kalcolor integral colour anodizing process 147 Kape test 244

Kesternich test 250 Lacquers cleaning off 41 impregnating anodic oxide coatings with 236 improving corrosion resistance 260 Lanolin cleaning-off 41 sealing with 235 Lead affecting anodizing 120 anodizing cathodes 92 antimonial, affecting chemical polishing 69 counter-electrodes in electrolytic colouring 200 electropolishing cathodes 80, 84 essential in Erftwerk polishing 71 tank linings 69, 73, 80, 84 Lead acetate colouring processes for anodic oxide coatings 177, 187 Leakage current Lettering reproducing 187, 193 Lids (holloware) anodizing 114 Light reflectivity (see also Reflectivity) 255, 267 Lubricants cleaning-off 41 sealing with 169, 235 Leakage current 281 Machining lubricants removing 41 Magnesia film effect in etching 54 Magnesium salts in water, affecting sealing 222 Maleic acid in anodizing electrolytes 150, 151 Malic acid anodizing 152 Malonic acid anodising 151 Manganese affecting hard anodizing 160 Marginal costing 37 Masking see Stopping-off Material to be anodized see Aluminium Matt finishes etchants producing 46 Mechanical polishing before electropolishing 75, 76 Mechanism of anodizing 1-4 Microhardness see Hardness Micro-sectioning thickness test 240 Military equipment chromic acid anodizing as inspection

315

tool 137 Modifiers used in etches 45 Molybdate sealing solutions 234 Mordant Black 85 183 Mordant Green 50 246 Mordant Red 3 179 Multi-colouring techniques 187 on aluminium foil 188 mottled effects 187 on name plates 189 photographic processes (detailed under Photographic processes) 193 random patterns 187 with relief 191 without relief 191 Seo-Druck process 188 using off-set litho printed-on resists 188 using silk screen printing 189, 193

alternating current 141, 143 colour 145 combined d.c. + a.c. 144 Ematal process 146 grade of acid to use 142 impurity contents 142 oxalic-sulphuric acid anodizing 129 solution analysis 301 thickness /concentration relations 141, 142 thickness/pH and temperature relations 143 Oxide films removing before anodizing 42 stripping 240 Oxygen polarisation in anodizing 162

Passivity before dyeing 180 Patterns producing, on anodic oxide coatings 187 Nameplates 184, 189 Palatine Bordeau RN 173 Neoprene see Rubber Pay-back period for plant 29 Nickel chloride Perchloroethylene degreasing 42pH in relief etchant 192 colouring solutions 175, 178 Nickel salts in electrolytic colouring 197, 200 sealing solutions 217, 218, 223, 224, 233 Nickel salt sealing 231, 232 Phosbrite 150 process 59, 62, 63 non-correlation between sealing test Phosbrite 155 process 60 and service behaviour with 245 Phosbrite 156 process 16 solution analysis 304 Phosbrite 159 process 57, 61-67 Nitrates Phosbrite 171 process 58 etch modifiers 45 Phosphate electrolytes 1 Nitric acid Phosphates in acid pickles 46 in Brytal bath, determining 298 for bleaching dyed anodic oxide in chemical polishing solutions, coatings 184 determining 294 in chemical brighteners, determining 293 in cleaners 40, 43, 291 in chemical polishing solutions 57, 58, 60, in electropolishing solutions 74, 76 62, 65, 67, 70, 72 in etches 46 in desmutting solutions 54 inhibiting sealing 222, 234 in desmutting solutions, determining in sealing solutions 222 52,53 Phosporic acid in etches 46 in acid pickles 43, 44 treating aged anodic films before anodizing solutions 140 dyeing 180 in chemical polishing solutions 57 Nitrous acid in chemical polishing solutions, in sulphuric acid for anodizing 121 determining 294 in electropolishing solutions 79 Off-set litho printing in etches 44 of resists in multi-colouring techniques in relief etchant 192 188 reviving aged anodic oxide films Opaque coatings 139, 146 before dyeing 180 Organic acid electrolytes 141, 147-155 Photographic processes 193 Organic sealants 204 dye-line process 195 Outworking v captive shops 17 Seo-Foto process 193 Oxalic acid using bichromate-gelatine or -albumen 195 anodizing see Oxalic acid anodizing using light-sensitive ferric salts 195 reviving aged anodic oxide films before using silver salts 193 dyeing 181 Phthalocyanine blue dyes 179, 184 Oxalic acid anodizing 141 Pickling (see also Desmutting; Etching) Alcanodox process 144, 146, 149

316

chrome-sulphuric pickle 53 Pitting (see also Corrosion) by outdoor exposure 257-260 in chemical polishing 63, 65, 71 induced by salt spray testing 252 in oxalic acid anodizing 142 in Battelle electropolishing 82 in sulphuric acid anodizing 122 Plant (for sulphuric acid anodizing) 89 agitation (see also Agitation) 94 automatic, economics and justification 26 bus bars 93, 105 cathodes (see also Cathodes) 92 cooling 95, 100, 153, 163 current supply and control 100-105, 153, 1 heating (see also Heating) 47, 60, 71, 100, 220 programmed, economics 28 racks (see also Racks) 109 refrigeration (see also Cooling) 95 tanks and linings (see also Tanks and the individual lining materials) 89 temperature control 95 Plant (for other purposes; see also Tanks) agitation see Agitation chemical polishing 60, 71, 73 electrolytic colouring 199 electropolishing 76, 78, 79, 84 fume extraction see Fume extraction integral colour anodizing 153 sealing 220 Plant investment and profitability (see also Profit and profitability) architectural plants 24 economics of automation 26 principles 21 profitability criteria 29 Plant utilization affecting profitability 25 Polishing see Chemical polishing; Electropolishing; Mechanical polishing Polishing compound cleaning-off 40 reducing brightness in chemical polishing 68 Polarisation effect in anodizing 5, 162 Polyphosphates see Phosphates Polypropylene tank linings 91, 199 Polythene tank linings 71 Pores and porosity alternating current affecting 131 anodizing voltage affecting 6/124, 146 in cell structure 3 chromic acid more than sulphuric acid coatings 194 dimensions 4

inducing, in coatings for photographic purposes 194 mechanism of formation 2 volume 6 Potassium chromate and dichromate see Chromates; Dichromate sealing Potassium ferrocyanide see Ferrocyanide Potassium permanganate in bleaching dyed anodic oxide coatings 184 colouring solution 176, 177 Power factor 286 Precautions see Hazards Precipitation of inorganic pigments 175 Pricing anodized work 41 Printed designs 187 Printing inks for 188 in multi-colouring process 189 Process cycle significance 88 Profit and profitability architectural plants 25 automation affecting 26 criteria 29 linked to plant size and utilization 24, 28 normal level of profit 9 plant investment 21 principles 21 profitability concept 9 Programmed plant economics 28 Properties of anodic oxide coatings (see also Tests) chemical resistance 264 colour 264 composition 262 density 264 dielectric properties 283 ductility 279 electrical resistance 285 fatigue resistance 280 hardness 276 heat emissivity 274 heat reflectivity 271 leakage current 284 light reflectivity 269 power factor 286 refrective index 264 temperature stability 275 P.V.C. binding bus bars with 108 stopping-off material 166 tank linings 71, 91 , 153 Quenching oil causing cleaning problems 40 R5 chemical process 59

317

tool 137 Modifiers used in etches 45 Molybdate sealing solutions 234 Mordant Black 85 183 Mordant Green 50 246 Mordant Red 3 179 Multi-colouring techniques 187 on aluminium foil 188 mottled effects 187 on name plates 189 photographic processes (detailed under Photographic processes) 193 random patterns 187 with relief 191 without relief 191 Seo-Druck process 188 using off-set litho printed-on resists 188 using silk screen printing 189, 193

alternating current 141, 143 colour 145 combined d.c. + a.c. 144 Ematal process 146 grade of acid to use 142 impurity contents 142 oxalic-sulphuric acid anodizing 129 solution analysis 301 thickness /concentration relations 141, 142 thickness/pH and temperature relations 143 Oxide films removing before anodizing 42 stripping 240 Oxygen polarisation in anodizing 162

Passivity before dyeing 180 Patterns producing, on anodic oxide coatings 187 Nameplates 184, 189 Palatine Bordeau RN 173 Neoprene see Rubber Pay-back period for plant 29 Nickel chloride Perchloroethylene degreasing 42pH in relief etchant 192 colouring solutions 175, 178 Nickel salts in electrolytic colouring 197, 200 sealing solutions 217, 218, 223, 224, 233 Nickel salt sealing 231, 232 Phosbrite 150 process 59, 62, 63 non-correlation between sealing test Phosbrite 155 process 60 and service behaviour with 245 Phosbrite 156 process 16 solution analysis 304 Phosbrite 159 process 57, 61-67 Nitrates Phosbrite 171 process 58 etch modifiers 45 Phosphate electrolytes 1 Nitric acid Phosphates in acid pickles 46 in Brytal bath, determining 298 for bleaching dyed anodic oxide in chemical polishing solutions, coatings 184 determining 294 in chemical brighteners, determining 293 in cleaners 40, 43, 291 in chemical polishing solutions 57, 58, 60, in electropolishing solutions 74, 76 62, 65, 67, 70, 72 in etches 46 in desmutting solutions 54 inhibiting sealing 222, 234 in desmutting solutions, determining in sealing solutions 222 52,53 Phosporic acid in etches 46 in acid pickles 43, 44 treating aged anodic films before anodizing solutions 140 dyeing 180 in chemical polishing solutions 57 Nitrous acid in chemical polishing solutions, in sulphuric acid for anodizing 121 determining 294 in electropolishing solutions 79 Off-set litho printing in etches 44 of resists in multi-colouring techniques in relief etchant 192 188 reviving aged anodic oxide films Opaque coatings 139, 146 before dyeing 180 Organic acid electrolytes 141, 147-155 Photographic processes 193 Organic sealants 204 dye-line process 195 Outworking v captive shops 17 Seo-Foto process 193 Oxalic acid using bichromate-gelatine or -albumen 195 anodizing see Oxalic acid anodizing using light-sensitive ferric salts 195 reviving aged anodic oxide films before using silver salts 193 dyeing 181 Phthalocyanine blue dyes 179, 184 Oxalic acid anodizing 141 Pickling (see also Desmutting; Etching) Alcanodox process 144, 146, 149

316

chrome-sulphuric pickle 53 Pitting (see also Corrosion) by outdoor exposure 257-260 in chemical polishing 63, 65, 71 induced by salt spray testing 252 in oxalic acid anodizing 142 in Battelle electropolishing 82 in sulphuric acid anodizing 122 Plant (for sulphuric acid anodizing) 89 agitation (see also Agitation) 94 automatic, economics and justification 26 bus bars 93, 105 cathodes (see also Cathodes) 92 cooling 95, 100, 153, 163 current supply and control 100-105, 153, 1 heating (see also Heating) 47, 60, 71, 100, 220 programmed, economics 28 racks (see also Racks) 109 refrigeration (see also Cooling) 95 tanks and linings (see also Tanks and the individual lining materials) 89 temperature control 95 Plant (for other purposes; see also Tanks) agitation see Agitation chemical polishing 60, 71, 73 electrolytic colouring 199 electropolishing 76, 78, 79, 84 fume extraction see Fume extraction integral colour anodizing 153 sealing 220 Plant investment and profitability (see also Profit and profitability) architectural plants 24 economics of automation 26 principles 21 profitability criteria 29 Plant utilization affecting profitability 25 Polishing see Chemical polishing; Electropolishing; Mechanical polishing Polishing compound cleaning-off 40 reducing brightness in chemical polishing 68 Polarisation effect in anodizing 5, 162 Polyphosphates see Phosphates Polypropylene tank linings 91, 199 Polythene tank linings 71 Pores and porosity alternating current affecting 131 anodizing voltage affecting 6/124, 146 in cell structure 3 chromic acid more than sulphuric acid coatings 194 dimensions 4

inducing, in coatings for photographic purposes 194 mechanism of formation 2 volume 6 Potassium chromate and dichromate see Chromates; Dichromate sealing Potassium ferrocyanide see Ferrocyanide Potassium permanganate in bleaching dyed anodic oxide coatings 184 colouring solution 176, 177 Power factor 286 Precautions see Hazards Precipitation of inorganic pigments 175 Pricing anodized work 41 Printed designs 187 Printing inks for 188 in multi-colouring process 189 Process cycle significance 88 Profit and profitability architectural plants 25 automation affecting 26 criteria 29 linked to plant size and utilization 24, 28 normal level of profit 9 plant investment 21 principles 21 profitability concept 9 Programmed plant economics 28 Properties of anodic oxide coatings (see also Tests) chemical resistance 264 colour 264 composition 262 density 264 dielectric properties 283 ductility 279 electrical resistance 285 fatigue resistance 280 hardness 276 heat emissivity 274 heat reflectivity 271 leakage current 284 light reflectivity 269 power factor 286 refrective index 264 temperature stability 275 P.V.C. binding bus bars with 108 stopping-off material 166 tank linings 71, 91 , 153 Quenching oil causing cleaning problems 40 R5 chemical process 59

317

Racks and racking for anodizing 109, 126 for dyeing 181 for electropolishing 77, 85 for hard anodizing 166 oscillation see Agitation for integral colour anodizing 153 Rectifiers size/capital costs 25 size/plant costs and profitability 24 for anodizing 100, 153 for electrolytic colouring 199 Reflectivity anodizing affecting 119, 123, 268 Cr in sulphuric acid reducing 123 infra-red (heat) 235, 271 light 119, 123,267 sulphuric acid concentration affecting 119, 123 temperature affecting 123 voltage affecting 119, 123 Refrective index 264 Refrigeration (see also Cooling) anodizing solutions 95, 153 Relief patterns 191 Resistance (electrical) 285 Resists in multi-colouring techniques 187 Return-type plant economics 28 Rinsing after chromic acid anodizing 217 after chemical brightening 63, 72 after electropolishing 81 after etching 49 after sulphuric acid anodizing 114 before dyeing 179 Rolling lubricants removing 40 Rubber aprons and boots see Hazards tank linings 53,71,73,90, 153, 199 Rubbing resistance see Abrasion resistance Safety precautions see Hazards Sales budget 33 Salt spray tests 252 Saponification as cleaning action 40, 41 Scale on heating coils 47 ‘S’ chemical polishing process 72 Schuh and Kern abrasion test 278 Scott’s sealing test 245 Scratch hardness see Hardness Sealing anodic oxide coatings 217 acetate buffering 225 acetate leading to stripping 217 after inorganic pigment colouring 177 anti-smut additives 229 avoiding leaching of dyestuff 232

318

Ca and Mg salts affecting 222 causing chalking 217, 222 in chromate solutions 232 chromate concentration affecting colour 177 dichromate improving fatigue properties 170 dichromate, not beneficial in industrial environments 258 deionised water for 221, 224 electrical analogue for 246 exposure behaviour 232, 234, 258 hard coatings 1 70 history 217 improving fatigue properties 170, 282 in metal salt solutions 231 in multi-colouring processes 191, 192 in nickel acetate solution 231, 232, 234 in non-aqueous solutions 235 pH affecting 223 pH control 224 phosphate deleterious 222 silica affecting 222 smut formation 229 in sodium silicate solutions 233 solution analysis 304 stickiness after 217 theory 218 time and temperature affecting 226 water quality affecting 221, 222 water sealing (see also Water sealing) 220 with alkylaryl polyethylene glycol/ dichromate 235 with lacquers 236 with organic polymers 235 with silicone or polythene waxes 228 with steam 235 with vaseline or lanolin 205 Sealing tests 243 acid dissolution 244 dye spot 243, 245 electrical analogue for 246 impedance 246 Kape 244 Kesternich 250 referee test 244 sodium sulphite 244 sulphur dioxide-humidity 250 time between sealing and testing affecting 249 Self-coloured anodizing see Integral colour anodizing Semi-manufacturing operations removing residues from 40 Seo-Druck multi-colouring process 188 Seo-Druck photographic process 193 Sequestering agents in etches 45

Silica affecting sealing 222 Silicate causing difficulties 40 in cleaners 43 sodium, sealing in 233 Silicofluorides in etches 46 Silicon see Al-Si alloys Silicones sealing with 170, 235 Silk screen printing on anodized aluminium 189 Silver salts in photographic processes 193 Size of anodizing concern affecting economics 15 of architectural anodizing plant 22 Sludge in chemical polishing 71 in electropolishing 78, 82, 85 in etching 47, 51 Smudge films see Desmutting treatments Soak cleaners 42, 43 Sodium acetate see Acetate acid and Acetates Sodium chromate see Chromates; Chromic acid Sodium hydroxide in cleaners, determining 291 in etchants 44, 45 in etchants, determining 289, 290 Sodium silicate sealing 233 Sodium sulphite test 244 Soils types of 39 Solution analysis 289 Specific resistance 285 Spectrophotmetric control of dyebaths 182 Speed of anodizing (see individual anodizing processes) Spinning lubricants removing 40 Spray suppressants in etchants 51 Staining in etching 49 nitric acid desmutting causing 53 Stainless steel cathodes in electropolishing 80 counter electrodes 200 dye tanks, galvanic affect 181 heat exchangers 98, 163 heating coils 60 tanks 60, 153 types suitable for chemical polishing tanks 60 Steam sealing 228 Stopping-off

for hard anodizing 166 in multi-colouring techniques 187 Strip-and-weigh thickness test 240 Strip, continuous, anodizing 209 Aluminum Co. of Canada plant 213 British Aluminium process 211 International Anodizing process 213 Nordisk Aluminium Industri plant 209 Pechiney process 211 United Anodising process 210 Stripping anodic oxide coatings due to acetate in sealing solution 217 solution for 240 Sulphates (see also Sulphuric acid) affecting dyes 179 in chemical polishing solutions, determining 294 in chromic acid electrolytes, determining 301 sealing in 333 Sulphide colouring process 177 Sulphides etch modifiers 45 Sulphonates sealing in 231 Sulpho-phthalic acid anodizing 149 Sulpho-salicylic acid anodizing 147 Sulphur dioxide-humidity test 250 Sulphuric acid (see also Sulphates) in acid pickles 41, 44 affecting dyes 179 in chemical polishing solutions 57, 68 concentration in anodizing solutions 1, 119,123 in desmut solutions 53 in electropolishing solutions 79, 84 impurities in, affecting anodizing 120 impurity in chromic acid anodizing solutions 139 Sulphuric acid anodizing (detailed under the individual headings) alternating current 131 chemical polishing before 57 cleaning before 39 combined d.c. + a.c. anodizing 162, 165 constant current 103, 161 constant current density 103, 161 constant voltage 102, 162 constant wattage 162 electropolishing before 75 etching before 44 hard 159 holding work awaiting 72 plant for 89 racking for 109 solution analysis 299 technology 117 wire and strip 209 Sulphuric-chromic acid electropolishing

319

Racks and racking for anodizing 109, 126 for dyeing 181 for electropolishing 77, 85 for hard anodizing 166 oscillation see Agitation for integral colour anodizing 153 Rectifiers size/capital costs 25 size/plant costs and profitability 24 for anodizing 100, 153 for electrolytic colouring 199 Reflectivity anodizing affecting 119, 123, 268 Cr in sulphuric acid reducing 123 infra-red (heat) 235, 271 light 119, 123,267 sulphuric acid concentration affecting 119, 123 temperature affecting 123 voltage affecting 119, 123 Refrective index 264 Refrigeration (see also Cooling) anodizing solutions 95, 153 Relief patterns 191 Resistance (electrical) 285 Resists in multi-colouring techniques 187 Return-type plant economics 28 Rinsing after chromic acid anodizing 217 after chemical brightening 63, 72 after electropolishing 81 after etching 49 after sulphuric acid anodizing 114 before dyeing 179 Rolling lubricants removing 40 Rubber aprons and boots see Hazards tank linings 53,71,73,90, 153, 199 Rubbing resistance see Abrasion resistance Safety precautions see Hazards Sales budget 33 Salt spray tests 252 Saponification as cleaning action 40, 41 Scale on heating coils 47 ‘S’ chemical polishing process 72 Schuh and Kern abrasion test 278 Scott’s sealing test 245 Scratch hardness see Hardness Sealing anodic oxide coatings 217 acetate buffering 225 acetate leading to stripping 217 after inorganic pigment colouring 177 anti-smut additives 229 avoiding leaching of dyestuff 232

318

Ca and Mg salts affecting 222 causing chalking 217, 222 in chromate solutions 232 chromate concentration affecting colour 177 dichromate improving fatigue properties 170 dichromate, not beneficial in industrial environments 258 deionised water for 221, 224 electrical analogue for 246 exposure behaviour 232, 234, 258 hard coatings 1 70 history 217 improving fatigue properties 170, 282 in metal salt solutions 231 in multi-colouring processes 191, 192 in nickel acetate solution 231, 232, 234 in non-aqueous solutions 235 pH affecting 223 pH control 224 phosphate deleterious 222 silica affecting 222 smut formation 229 in sodium silicate solutions 233 solution analysis 304 stickiness after 217 theory 218 time and temperature affecting 226 water quality affecting 221, 222 water sealing (see also Water sealing) 220 with alkylaryl polyethylene glycol/ dichromate 235 with lacquers 236 with organic polymers 235 with silicone or polythene waxes 228 with steam 235 with vaseline or lanolin 205 Sealing tests 243 acid dissolution 244 dye spot 243, 245 electrical analogue for 246 impedance 246 Kape 244 Kesternich 250 referee test 244 sodium sulphite 244 sulphur dioxide-humidity 250 time between sealing and testing affecting 249 Self-coloured anodizing see Integral colour anodizing Semi-manufacturing operations removing residues from 40 Seo-Druck multi-colouring process 188 Seo-Druck photographic process 193 Sequestering agents in etches 45

Silica affecting sealing 222 Silicate causing difficulties 40 in cleaners 43 sodium, sealing in 233 Silicofluorides in etches 46 Silicon see Al-Si alloys Silicones sealing with 170, 235 Silk screen printing on anodized aluminium 189 Silver salts in photographic processes 193 Size of anodizing concern affecting economics 15 of architectural anodizing plant 22 Sludge in chemical polishing 71 in electropolishing 78, 82, 85 in etching 47, 51 Smudge films see Desmutting treatments Soak cleaners 42, 43 Sodium acetate see Acetate acid and Acetates Sodium chromate see Chromates; Chromic acid Sodium hydroxide in cleaners, determining 291 in etchants 44, 45 in etchants, determining 289, 290 Sodium silicate sealing 233 Sodium sulphite test 244 Soils types of 39 Solution analysis 289 Specific resistance 285 Spectrophotmetric control of dyebaths 182 Speed of anodizing (see individual anodizing processes) Spinning lubricants removing 40 Spray suppressants in etchants 51 Staining in etching 49 nitric acid desmutting causing 53 Stainless steel cathodes in electropolishing 80 counter electrodes 200 dye tanks, galvanic affect 181 heat exchangers 98, 163 heating coils 60 tanks 60, 153 types suitable for chemical polishing tanks 60 Steam sealing 228 Stopping-off

for hard anodizing 166 in multi-colouring techniques 187 Strip-and-weigh thickness test 240 Strip, continuous, anodizing 209 Aluminum Co. of Canada plant 213 British Aluminium process 211 International Anodizing process 213 Nordisk Aluminium Industri plant 209 Pechiney process 211 United Anodising process 210 Stripping anodic oxide coatings due to acetate in sealing solution 217 solution for 240 Sulphates (see also Sulphuric acid) affecting dyes 179 in chemical polishing solutions, determining 294 in chromic acid electrolytes, determining 301 sealing in 333 Sulphide colouring process 177 Sulphides etch modifiers 45 Sulphonates sealing in 231 Sulpho-phthalic acid anodizing 149 Sulpho-salicylic acid anodizing 147 Sulphur dioxide-humidity test 250 Sulphuric acid (see also Sulphates) in acid pickles 41, 44 affecting dyes 179 in chemical polishing solutions 57, 68 concentration in anodizing solutions 1, 119,123 in desmut solutions 53 in electropolishing solutions 79, 84 impurities in, affecting anodizing 120 impurity in chromic acid anodizing solutions 139 Sulphuric acid anodizing (detailed under the individual headings) alternating current 131 chemical polishing before 57 cleaning before 39 combined d.c. + a.c. anodizing 162, 165 constant current 103, 161 constant current density 103, 161 constant voltage 102, 162 constant wattage 162 electropolishing before 75 etching before 44 hard 159 holding work awaiting 72 plant for 89 racking for 109 solution analysis 299 technology 117 wire and strip 209 Sulphuric-chromic acid electropolishing

319

advantages 87 practice 84 process control 85 smoothing properties 86 Sulphuric-oxalic acid anodizing 129 Surge in initial anodizing current 3 Tanks (see also Plant) for acid electropolishing 78, 79, 84 for alkaline electropolishing 76 for Alzak electropolishing 78 for anodizing 89 for chemical polishing solutions 60, 69, 71,73 for electrolytic colouring 199 for solutions containing fluorides 53 71,73 for integral colour anodizing 153 for water sealing 220 brick and concrete 90, 91 lead linings 69, 73, 79, 84, 89 polythene linings for 71 P.V.C. linings for 53, 71,91, 153, 199 rubber linings for 53, 71, 73, 90, 153, 199 Tannic acid in colouring process 176 Tartaric acid in chemical polishing solutions 58 reviving aged anodic oxide films before dyeing 181 Tartrates - scale inhibitor 45 Technology of sulphuric acid anodizing acid concentration 117 burning 159, 160, 162, 166 current density 124 impurities 120, 160 influence of agitation 127, 165 influence of alternating current 131 permissible Al content 121, 159 temperature 123 throwing power 128 voltage 118, 124 Temperature stability 275 Temporary protectives cleaning-off 40 Temperature (see also Cooling; Refrigeration) anodizing 118, 123, 124, 126, 129. 133, 137, 140, 141, 143, 145, 147, 150, 151, 159 chemical polishing 63, 69, 71, 72, control in anodizing 95 cleaning 43 desmutting 53 double dipping colouring 177 electropolishing 75, 78, 79, 84 etching 45 ferric ammonium oxalate dyeing 174 organic dyeing 181 rise during anodizing 4

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sealing 226 Test methods Tests for (see also Properties) accelerated corrosion 252, 253 hardness 277 heat reflectivity 271, 274 light reflectivity 267 sealing 243 thickness 239 wear resistance 277 weather resistance 255 Thermal stability 275 Thickness acid concentration affecting 117, 119, 123 133, 143, 159 agitation affecting 127, 164 colour relations 147, 174 current density relations 5, 130, 133, 143, 160 general notes 1 growth mechanism 2 growth rate 3 hardness relations 120 limitations 4, 118 temperature relations 123 tests for 239 solutions for producing thick films 154, 160 Thickness tests 239 break-down voltage 242 eddy current 241 interferometric 241 micro-sectioning 240 strip-and-weigh 240 Thiomalic acid anodizing 152 Throwing power 128, 159 Tin etch accelerator 45 Titanium in Ematal electrolyte 146 heat exchangers for hard anodizing 163 racks 113 o-Toluidine in chemical polishing solutions 72 Trichloroethylene degreasing 41. 61 Tricresyl phosphate in cleaners 40 Trim anodized, reflectivity 270 Tripoli polishing compound reducing brightness in chemical polishing 68 Tubes anodizing 128 porous ceramic, for air agitation 128, 164 Utilization see Plant utilization

sealing with 235 V. A. W. electropolishing process 75 Veroxal self-colour anodizing process 150 Voltage control 102, 162 factors affected by 124 factors affecting 6, 118, 123, 124, 126, 130, 133, 137, 138, 142, 145, 147, 161 Water affecting chemical polishing solution 66 calcium in, affecting deystuffs 179 Cl and F in, affecting anodizing 122, 139, 142 Cl affecting electrobrightening 76 cooling anodizing solutions 95 for electropolishing solutions 80 hard, depositing calcium soaps 40 sealing in see Water sealing silicate in, causing difficulties 40, 222 Water sealing 220 agitation in 221 acetates in 225 ammonia in 225 deionization in 224 ethanolamine in 234 ions affecting 222 pH affecting 223, 224 pH control 224 quality tests for 243 silica affecting 222 tank heating 220 temperature 226

time 226 water quality affecting 220, 221 weight gain in 226 Wax sealing with 235 stopping-off material 166 Wear resistance see Abrasion resistance Weather resistance 225 Wetting agents in chemical polishing solutions 68 in cleaners 43 in electropolishing solutions 84 in etches 45 Wire anodizing 183 Aluminum Co. of Canada plant 213 British Aluminium process 211 International Anodizing process 213 Nordisk Aluminium Industri plant 209 Pechiney process 211 United Anodising process 210 with alternating current 211 Wiring-up 110 Working budget 33 Work movement (see also Agitation) 62, 76, 80, 85, 165 Yellow colours 177, 179, 183, 184, 191 Zinc affect in anodizing 122 (-tin) electrolytic colouring 200

Addenda: Chloride in alkaline electropolishing solutions 76 in chromic acid anodizing 139 in dyebaths 179, 181 in oxalic acid electrolytes 142 in sulphuric acid for anodizing 122 Chromates and dichromates in electropolishing solutions determining 295 in desmut solutions 53 in photographic processes 195

in sealing solutions 169, 176, 232, 233, 235 sealing not beneficial in industrial environments 258 Chrome-sulphuric pickle 53, 76, 78, 85 Chromic acid in acid pickles 41 affect on sulphuric acid anodizing 123 anodizing (detailed under Chromic acid anodizing 137 in electropolishing solutions determining 295 in desmut solutions 53 determining 301

Vapour degreasing 41, 61 Vaseline

321

advantages 87 practice 84 process control 85 smoothing properties 86 Sulphuric-oxalic acid anodizing 129 Surge in initial anodizing current 3 Tanks (see also Plant) for acid electropolishing 78, 79, 84 for alkaline electropolishing 76 for Alzak electropolishing 78 for anodizing 89 for chemical polishing solutions 60, 69, 71,73 for electrolytic colouring 199 for solutions containing fluorides 53 71,73 for integral colour anodizing 153 for water sealing 220 brick and concrete 90, 91 lead linings 69, 73, 79, 84, 89 polythene linings for 71 P.V.C. linings for 53, 71,91, 153, 199 rubber linings for 53, 71, 73, 90, 153, 199 Tannic acid in colouring process 176 Tartaric acid in chemical polishing solutions 58 reviving aged anodic oxide films before dyeing 181 Tartrates - scale inhibitor 45 Technology of sulphuric acid anodizing acid concentration 117 burning 159, 160, 162, 166 current density 124 impurities 120, 160 influence of agitation 127, 165 influence of alternating current 131 permissible Al content 121, 159 temperature 123 throwing power 128 voltage 118, 124 Temperature stability 275 Temporary protectives cleaning-off 40 Temperature (see also Cooling; Refrigeration) anodizing 118, 123, 124, 126, 129. 133, 137, 140, 141, 143, 145, 147, 150, 151, 159 chemical polishing 63, 69, 71, 72, control in anodizing 95 cleaning 43 desmutting 53 double dipping colouring 177 electropolishing 75, 78, 79, 84 etching 45 ferric ammonium oxalate dyeing 174 organic dyeing 181 rise during anodizing 4

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sealing 226 Test methods Tests for (see also Properties) accelerated corrosion 252, 253 hardness 277 heat reflectivity 271, 274 light reflectivity 267 sealing 243 thickness 239 wear resistance 277 weather resistance 255 Thermal stability 275 Thickness acid concentration affecting 117, 119, 123 133, 143, 159 agitation affecting 127, 164 colour relations 147, 174 current density relations 5, 130, 133, 143, 160 general notes 1 growth mechanism 2 growth rate 3 hardness relations 120 limitations 4, 118 temperature relations 123 tests for 239 solutions for producing thick films 154, 160 Thickness tests 239 break-down voltage 242 eddy current 241 interferometric 241 micro-sectioning 240 strip-and-weigh 240 Thiomalic acid anodizing 152 Throwing power 128, 159 Tin etch accelerator 45 Titanium in Ematal electrolyte 146 heat exchangers for hard anodizing 163 racks 113 o-Toluidine in chemical polishing solutions 72 Trichloroethylene degreasing 41. 61 Tricresyl phosphate in cleaners 40 Trim anodized, reflectivity 270 Tripoli polishing compound reducing brightness in chemical polishing 68 Tubes anodizing 128 porous ceramic, for air agitation 128, 164 Utilization see Plant utilization

sealing with 235 V. A. W. electropolishing process 75 Veroxal self-colour anodizing process 150 Voltage control 102, 162 factors affected by 124 factors affecting 6, 118, 123, 124, 126, 130, 133, 137, 138, 142, 145, 147, 161 Water affecting chemical polishing solution 66 calcium in, affecting deystuffs 179 Cl and F in, affecting anodizing 122, 139, 142 Cl affecting electrobrightening 76 cooling anodizing solutions 95 for electropolishing solutions 80 hard, depositing calcium soaps 40 sealing in see Water sealing silicate in, causing difficulties 40, 222 Water sealing 220 agitation in 221 acetates in 225 ammonia in 225 deionization in 224 ethanolamine in 234 ions affecting 222 pH affecting 223, 224 pH control 224 quality tests for 243 silica affecting 222 tank heating 220 temperature 226

time 226 water quality affecting 220, 221 weight gain in 226 Wax sealing with 235 stopping-off material 166 Wear resistance see Abrasion resistance Weather resistance 225 Wetting agents in chemical polishing solutions 68 in cleaners 43 in electropolishing solutions 84 in etches 45 Wire anodizing 183 Aluminum Co. of Canada plant 213 British Aluminium process 211 International Anodizing process 213 Nordisk Aluminium Industri plant 209 Pechiney process 211 United Anodising process 210 with alternating current 211 Wiring-up 110 Working budget 33 Work movement (see also Agitation) 62, 76, 80, 85, 165 Yellow colours 177, 179, 183, 184, 191 Zinc affect in anodizing 122 (-tin) electrolytic colouring 200

Addenda: Chloride in alkaline electropolishing solutions 76 in chromic acid anodizing 139 in dyebaths 179, 181 in oxalic acid electrolytes 142 in sulphuric acid for anodizing 122 Chromates and dichromates in electropolishing solutions determining 295 in desmut solutions 53 in photographic processes 195

in sealing solutions 169, 176, 232, 233, 235 sealing not beneficial in industrial environments 258 Chrome-sulphuric pickle 53, 76, 78, 85 Chromic acid in acid pickles 41 affect on sulphuric acid anodizing 123 anodizing (detailed under Chromic acid anodizing 137 in electropolishing solutions determining 295 in desmut solutions 53 determining 301

Vapour degreasing 41, 61 Vaseline

321