A study of undercoatings for nickel in the plating of zinc base die castings

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A STUDY OF UNDERCOATIMGS FOR NICKEL IN THE ✓ PLATING OF ZINC BASE DIE CASTINGS

Adolph Fischbach

A Thesis submitted to the Faculty of the Graduate School of Indiana University in partial fulfillment of the requirements for the degree Doctor of Philosophy January 1942

ProQ uest Num ber: 10295099

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uest. ProQ uest 10295099 Published by ProQ uest LLC (2016). C opyright of th e Dissertation is held by th e Author. All rights reserved. This work is p ro te c te d a g a in s t unauthorized copying u n d e r Title 17, United S tates C o d e Microform Edition © ProQ uest LLC. ProQ uest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346

The writer wishes to express his sincere gratitude and appreciation for the advice and invaluable assistance rendered by Dr. Frank C. Mathers in the study of this problem.

TABLE OF CONTENT S

Introduction ........

. . . . . . . .

1

Review of the Literature.................. Apparatus

5

........................

14

P r o c e d u r e ...........................

16

E x p e r i m e n t s ............................... Nickel Undercoating . . * • • •

...........

18 19

Copper Undercoating ........................

21

Copper-Lead Alloy Undercoating

25

...........

Zinc-Cadmium Alloy Undercoating • • • • • • Lead Undercoating

......................... 30

Tin Undercoat i n g ..................

• . .

Silver Undercoating ........................ Cadmium Undercoating

27

34 39

................... 41

Conclusion to E x p e r i m e n t s ....................... 43

INTRODUCTION

-

2-

With the wide use of zinc base die castings in the auto­ mobile industry in recent years, nickel plating of these alloys had become general.

Here again the chromium finish was used

for appearance purposes** Door handles, radiator grills, car­ buretor bodies, fuel pumps and filters, accelerator parts, and a host of others were conspicuous examples of the use of zinc base die castings.

Besides the automotive industry, the

zinc base die castings had become important in the production of domestic oil burners and radios, in scientific instruments, and in toy manufacture. The zinc base die castings possess exceptional utility for a wide range of uses, some of the chief reasons being: (1) Adaptability to rapid production at moderate cost and within closer dimensional limits as compared to stampings and forgings, (2) Adaptability to a wide range of shapes and sizes, including complex parts with difficult core work.

Irregular contours and structural

elements, which though not so strong as wrought parts, have unusual toughness as compared to most low cost castings, (3) The die castings in some instances may be pro­ duced with surfaces so smooth as to require little or no polishing or even buffing prior to plating or other finishings.

(4) The die castings may be produced in remark­ ably thin sections, yet with a stiffness not attained with stamped parts. (5) The relative ease of machining the die castings, when machining is required, and the minimum waste in metal removed are important from an expense outlook. (6) Comparative resistance to corrosion under most conditions of service.

Any corrosion that does

occur is confined to the surface and rarely af­ fects the strength or serviceability of the part. There are, however, limitations to the use of zinc alloys, which involve temperature changes during service. Prolonged use at temperatures above 300°F has not been recom­ mended.

Although the impact strength is high at normal tem­

peratures, it decreases rapidly at very low temperatures} but with a return to normal temperatures, the impact strength is restored.

Despite this fact, however, breakage attributed

to temperature in service has rarely been reported. Most finishings on automotive parts are of the plated type.

In recent years practically the entire automotive

industry has standardized on certain definite specifications for plated coatings on zinc base die castings.

In general

they all require heavy coatings, the minimum varying from

-

•0008” to .001".

4-

This may be composed of a series of copper,

nickel, and chromium, or of nickel and chromium, or the option being allowed in most cases*

“Where the sequence is copper,

nickel, and chromium, the minimum copper thickness is specified as .0002" or .0003".

The nickel deposit is specified as a

minimum of .0003" to .0005", and the chromium in most cases is required to be a minimum of .00001".

It cannot be too

strongly emphasized that the satisfaction being obtained with plated die castings in the automotive industry depends upon the conscientious fulfillment of these specifications. The big difficulty with the plating of zinc base die castings is the subsequent blistering of the electrodeposit, when the plated object is coated with organic finishes and baked at 150°C.

Microscopic and X-ray examinations have shown

that the zinc and copper form a diffusion layer, which con­ tains a brittle film of the alloy.

The brittleness of this

film is responsible for the blistering of the deposit.

The

diffusion of copper and zinc takes place slowly at ordinary temperatures, but the effect is greatly increased at the ele­ vated temperatures.

This blistering is a tremendous expense

to the industry. In the litere.ture, copper and, only recently, brass were found to be used as an undercoating for nickel.

Thus, this

work is concerned mainly with undercoatings other than copper and brass in the nickel plating of zinc base die castings.

REVIEW OF THE LITERATURE

-

6-

1 R. J. Wirshing

from a study of the corrosion of electro-

^Monthly Rev. Am, Electroplaters' Soc. 19, No. 8, (1932) Chemical Abstracts 26, 5503 (1932) plated steel drew the following conclusions!

(l) The total

deposits should be at least .001" thick; (2) no thin or "flash" deposits should be used with the exception of the final chromium coating; and (3) at least two dissimilar me­ tals should be plated exclusive of the chromium "flash." Mien these conclusions were applied to the plating of zinc base die castings, using a thickness of .0005" copper and .0005" nickel, followed by the usual chromium flash, extreme­ ly good corrosion-resisting deposits were obtained.

The use

of the thick copper deposit enabled the low pH solution to be used for the nickel plating in place of the usual high pH nickel plating solution required when only a thin copper de­ posit was used, or when the nickel was plated directly on the zinc die castings. Carl Hussner

2

reviewed the standards which should be

^ Monthly R q v . Am. Electroplaters1 Soc. Chemical Abstracts 2-8, 1930 (1934) maintained to obtain satisfactory products.

20, No. 4,(1933)

He discussed the

preliminary finishing and machining as well as the cleaning and plating of the zinc die castings.

Results were given to

-

7-

show that the life of a plated casting was in proportion to the thickness of the nickel plate, while other results showed that the thickness of the nickel plate was not the only factor to be considered.

The discrepancies observable between

the life of a plated part under the salt-spray test and in actual service were noted. •Z

William F. Castell0 stated that the zinc base die castings

^ Trans. Electrochem. Soc.

66, 11

(1934)

were generally copper plated, then nickel and chromium plated. Ee described a diffusion action that took place between the electroplated copper and the zinc in the base metal.

The rate

of this diffusion depended largely upon the temperature to which the plated parts were exposed.

At elevated temperatures

between 105°F and 232°F the diffusion was rapid, while under normal temperatures only a very small amount had been found alloyed with the zinc.

Failures of the plated deposit on zinc

base die castings on outdoor exposure were found to be due to surface defects both in the casting and in the electroplates. Herbert Chase^ discussed the relative merits of different

4 Machinery 42, 120-2, 181-4 (1935) Chemical Abstracts 30, 62 (1936)

methods designed to finish zinc and aluminum die castings for purposes of appearance and resistance to corrosion.

He des­

cribed low cost procedures for cleaning, buffing, plating, and

-

polishing.

8-

Nickel baths for plating the zinc and aluminum

die casting? were given, together with a recommended proce­ dure for etching prior to plating.

Methods of applying

enamel, lacquer, and varnish coatings were also discussed. After buffing and cleaning, Charles H. Costello

5 Monthly Rev. Am. Electroplaters1 Soc. 22, No. 10 (1935) Chemical Abstracts 30, 8043 *("1936)

etched the zinc base die castings in acid. 5-10$ HC1, or 5$ HgSO^ by volume were used.

From 1-5$ HF, The die castings

were then plated with copper in a cyanide bath and finally plated with nickel in a solution containing Na 2 S 0 ^.. £ Karl B. Thews

stated that an undercoat of copper gave

® Metallwaren - Ind. Galvano-Tech. 37, No. 16, (1939) Chemical Abstracts 33, 9149 (19397

less desirable results than plating nickel directly on zinc die castings.

He discussed the compositions of an electro­

lytic degreasing solution, three pickling solutions, four nickel baths, and one copper bath. 7

W. B. Stoddard

stripped copper from zinc base die cast­

ings in a chromium plating solution with a 50-60 cycle alter-

^ Trans. Electrochem. Soc*

nating current. ratio CrOgjSO^

77, 3

(1940)

The bath consisted of 200-500 g./L. CrO^, 100:1.

The current density was 7-14 amp./sq.dm.

-

and the temperature 20-25° C.

9-

After stripping, the zinc sur­

face was activated by dipping the articles into cold, dilute HC1 (10 cc./liter of HC1, sp. gr* 1*19). According to A. C* West®, the alkaline cathodic clean-

^ Monthly Rev* Am. Electroplaters * Soc* Chemical Abstracts 34, 4672 (1940)

_27, 262-6 (1940)

ing commonly given to metals preparatory to plating was some­ times the cause of poor adherence of the coating.

The ab­

sence of a water break on the surface merely showed that greasy material was absent.

There may be a film on the sur­

face which cannot be removed by ordinary cyanide or acid dips. These films could be removed by making the metal anodic for several seconds in another alkaline bath.

This treatment im­

proved the adhesion of subsequent metal coatings.

Zinc base

die castings, which had been degreased, could be cleaned with an anodic treatment of 5-10 second^, cathodic cleaning being omitted.

Care had to be taken to avoid etching of the sur­

face of the casting. The plated coatings on die castings were subject to

Q

Proc. Am. Electroplaters * Soc.

7 (preprint)

(1940)

blistering, especially when heated to about 150-180°C for 9 baking lacquer coatings.

F. F. Oplinger

described the fol­

lowing procedures and precautions which should be observed to

-

prevent blistering.

10-

The casting should be plated very soon

after polishing to prevent formation of tarnish on the sur­ face.

Solvent degreasing was used to remove buffing com­

pounds, and this followed by a mild alkaline cleaning.

The

solvent should not attack or discolor the surface of the casting.

The cathodic cleaning was limited from 30 seconds to two

minutes in a cleaner containing 2.0 oz./gal. of Na^PQ^ and ^a 2 ^

3

* or 10-20 seconds in a cleaner containing 6.0 oz./gal.

Anodic cleaning or simple immersion could also be used. acid dip consisted of 0.5 - 1*0/ HCl. tions had to be avoided.

The

Stronger acid solu­

A strike in a copper cyanide bath

was recommended before plating in either the rochelle or high­ speed copper bath.

When a casting was properly cleaned and

plated, there was no diffusion of copper and zinc at 120° in 60 hours.

At 170°C. slight diffusion occurred in two hours

and complete diffusion of a coating .0005n thick in 100-150 hours.

Diffusion took place more readily when the casting

had been subjected to alkaline cleaning and to pickling for too long a time. Edmund T* Richards^ gave a summary of recommended prac-

10 Werksleiter 34,117-18 (1940) Chemical Abstracts 34, 7754 (1940)

tices in the plating of zinc die castings.

This included di­

rections regarding cleaning, degreasing, pickling, composition

-

11-

of pickling and plating baths, current density, pH of baths, buffers, and thickness of coatings.

He also discussed the

causes for brittleness and methods of avoiding streakiness. 11

E. A. Anderson and C. E. Reinhard

^

described a procedure

Monthly Rev. Am. Electroplaters1 Soc. 27, 175-33 (1940)

for plating zinc base die castings.

The die castings were

usually given a preliminary coating of copper, then nickel, and finally a chromium coating.

When the plated objects were

coated with organic finishes and baked at 150°C., blistering of the electrodeposit frequently occurred.

Microscopic and

X-ray examinations showed that the zinc and copper formed a diffusion layer, which contained a brittle zinc-rich alloy. The brittleness of this alloy was responsible for the blister­ ing.

The amount of alloying between the zinc and copper was

reduced by buffing the die casting before plating and by de­ creasing the length of time in the alkaline cleaner.

As an

alternative the strength of the cleaner should be reduced. Zinc base die castings required a preliminary preparation prior to plating.""

The articles were usually buffed and then

12

Burns and Schuh, Protective Coatings for Metals, Rein­ hold Publishing Corp., New York (1939)

generally degreased with trichlorethylene in a three-stage system involving hot dipping, cold rinsing, and a final vapor

-

rinse#

12-

All platers used an alkaline cleaner, commonly at

180°F., and in the majority of cases, the castings were cathodically treated in such solutions.

An acid dip of 5-10$

IiCl was used to remove alkaline salts or superficial oxides, the work being left in solution until there was evidence of gas evolution. The solution for plating nickel directly was a low nickel, high sodium sulfate solution containing ammonium chloride and buffered in the pH range 4.9-5,7 by boric acid.

The copper

solution used was the rochelle salt alkaline cyanide bath. It was common practice to dip all copper-plated work in di­ lute sulfuric acid in order to neutralise traces of remaining alkali. According to Burns and Schuh the nickel coatings have shown a tendency to peel off or blister under severe atmospheric cor­ rosion, if the coating were porous or abraded. C. S. Wernlundx

13 u.

s.

described a method comprising the use of

Patent 2,221,502

November 12, 1940.

a v/hite brass plate as an undercoat for nickel deposits, es­ pecially bright nickel on sine base die castings.

The deposit

should be at least .0001" thick, and should contain 19-31$ hy weight of copper.

It was claimed that the deposit would not

diffuse into the base metal. M. M. Thompson^ began an extensive research program to

14 Thompson, M. M . , Private Communication, National Cush Register Company (1941)

determine the best method of cleaning zinc base die castings prior to copper, nickel, and chromium plating. The best cleaning procedure is that in which both the electrolytic and the washing machine cleaner have a compara­ tively low (13$ - 25$) free NagO content— .the electrolytic cleaner to be used anodically at high voltage (11-12) and to be of such a concentration and temperature that spotting will not occur.

This will ordinarily be between five and six ounces

per gallon of cleaner and a temperature of 195° to 20G°F.

The

concentration of the washing machine cleaner is best carried out at two ounces per gallon vtith a temperature of 160° to 170°F. preceded by a two minute soak in a soap solution, which serves to soften any hard lumps of dirt.

APPARATUS

Experiments were carried out in 250 c.c* beakers, each containing 200 c.c. of solution.

Electric hot plates were

used to control temperatures of the plating baths. Each cell contained one anode and one cathode.

Only

one side of the cathode was calculated as exposed area.

The

average anode area was 4 square inches, the cathode 1 3/4 square inches.

The anode in each bath was the same as the

metal or metals being deposited from solution. The current densities were controlled by means of lamp banks and carbon pile rheostats. The standard nickel bath consisted of NiSO^.TE^Q 105 g./L. 1MH4C1

15 g./L., NiClg.SHgO

15 g./L., H3B03

thiourea as a brightening agent.

15 g./L. and 0.1

The nickel was plated to a

thickness of *0007" using a current density of 30-32 amp./sq.ft The zinc base die castings used in the experiments were of the following composition: Copper

0. 1$ max.

Aluminum

3.5 - 4.3$

Magnesium

0.03- 0.08$

Iron

0.1$ max.

Lead

0.007/i max.

Cadmium

0.005$ max.

Zinc

remainder

PROCEDURE

17-

Before plating, it was necessary for the zinc die cast­ ing cathode to undergo a cleaning procedure to insure the op­ timum conditions for the subsequent electrodeposition of metals. In the procedure the zinc die castings were first treated cathodically for one minute in a mild alkaline solution heated to 180°F.

They were then given a warm rinse, a cold rinse,

and an acid dip consisting of b% hydrochloric acid.

The pur­

pose of the acid dip was to remove alkaline salts or superficial oxides.

The article was left in the acid dip until there m s

evidence of a uniform evolution of gas bubbles from its surface, usually 5 seconds.

A final warm rinse was given before the die

casting was ready for electroplating. The die casting was then plated directly to the desired thickness with the metal or alloy to be used as an undercoat­ ing for the nickel, and then rinsed and plated with nickel to a thickness of .0007".

After the plating operations, the ca­

thode was put into an oven at 190°C. for one hour.

The article

was then inspected for blistering, cracking, or peeling.

EXPERIMENTS

19-

Nickel Undercoating

In the literature it was seen that zinc base die castings have been plated either directly with nickel or with a com­ posite copper-nickel coating.

This experiment of plating

nickel directly to zinc base die castings was a repetition of previous work, but it was done in order to grasp a better un­ derstanding of the problem. The article to be plated was cleaned in the manner des­ cribed in the procedure, plated in the standard nickel bath, and baked for one hour at 190°C. in the oven.

In the several

experiments tried, the nickel deposit was controlled to a thickness of from .0003" to .0008".

Conclusion. Although the appearance and adherence of the deposit on removal from the nickel bath seemed good, there was a tendency for the deposit to crack and peel when subjected to a tempera­ ture of 190°C. for any length of time.

This peeling or crack­

ing seemed to have a direct relationship to the time of pickling in the acid solution.

At first the time of pickling ranged from

thirty seconds to two minutes and in every case the deposit peeled.

When the article was pickled longer than twenty se­

conds, the die casting turned dark due to the presence on its surface of a residual, non-adherent film, "soot", which was probably the cause for the subsequent peeling of the deposit.

-

20-

By cutting the pickling time to five seconds, long enough for a uniform evolution of gas bubbles from the surface of the die casting, there was no evidence of cracking or peeling.

-

21-

Copper Undercoating

A bath consisting of CuCN and NagCOg

22.5 g./L., NaCN

34 g./L.,

15 g./L. was prepared to deposit copper on the

zinc base die casting cathode.

The temperature of the bath

during electrolysis was 30-40°C.

Table 1 The copper was deposited to a thickness of .000311 at a current density of 22-25 amp./sq. ft. The nickel was de­ posited to a thickness of .0007** at a current density of 30-32 amp./sq.ft. The article was then tested in the oven for one hour at a temperature of 190°C, Time of acid pickle 5/ HC1 room temp.

Experiment

Result

1

5 minutes

2

4

"

ti

3

3

"

ii

4

2

"

ti

5

1 1/2 "

ti

6

1

ti

»

20 seconds

7

5

8

"

blistered

good H

The copper baths were stirred at 300 r.p.m., the nickel bath at 100 r.p.m.

The copper deposit was bright in appearance.

Table 1 shows the effects of pickling on the results of the experiments.

As in the previous experiments with nickel,

-

22-

it was found that just so long as the article to be plated was darkened by the loose, non-adherent "soot" caused by excessive acid pickling, the deposit would blister or peel when sub­ jected to high temperatures in the oven*

Five seconds in the

acid pickle was sufficient time to remove any superficial ox­ ides that might have resulted from the alkaline cleaner* On examining the deposit closely, it was found that the zinc had diffused into the copper.

This process took place

slowly at ordinary temperatures, but was accelerated at the higher temperatures.

Where the article had been pickled too

long, so as to cause the appearance of the loose "soot” on its surface, the copper would diffuse with this non-adherent film forming a very thin and brittle layer*

It was this layer that

would crack and cause the subsequent nickel deposit to blister when subjected to the heat treatment. In an experiment where the article had been over-pickled, the copper was plated from the cyanide solution to a thickness of .0003” , and then put into the oven for one hour.

On remo­

val the article was dark gray in appearance, the bright copper having diffused with the zinc.

Nickel was plated on the die

casting, and it was again put into the oven for one hour.

The

deposit cracked and peeled, the result being the same as the previous experiments where there was evidence of over-pickling. In other experiments the die castings were subjected to the high temperatures immediately after over-pickling.

The

-23-

copper and nickel were then plated to the desired thickness. However, in all cases the effect of over-pickling caused the deposits to blister. A copper alkaline tartrate bath consisting of Cu (N03 )2 25 g./L., Tartaric Acid

20 g./L., and NaOH

15 g./L. was pre­

pared to deposit copper on the zinc base die castings. bath was run at 40°C.

This

Each cathode was cleaned in the standard

manner and pickled only five seconds in the acid solution. In the first experiments with this bath the article was plated to a thickness of .0005” copper, nickel plated, and then tested in the oven.

The result was good.

The copper deposit was

bright; however, a fluffy precipitate was present in the bath and the solution was turbid.

This was probably due to a de­

ficiency of caustic soda in the bath.

Below 22 and above

30 amp./sq.ft., the copper deposit was black.

Table 2 After plating with copper, the article was plated with nickel to a thickness of .0007” at a current density of 30-32 amp./sq. ft. and then tested in the oven for one hour at a temperature of 190°C.

Exp.

Time of Acid Pickle 5$ HC1 loom Temp.

C.D. (Cu) Thickness of amp./sq.ft. Cu deposit

Result

1

5 seconds

28-30

.0002"

peeled

2

5

10-11

.0004”

peeled

3

5

"

22-25

.0005”

good

4

5

"

22-25

.0006”

good

5

5

»

22-25

.0008”

good

-24-

Another alkaline tartrate bath was prepared with a caus­ tic soda concentration of 50 g./L. of these experiments.

Table 2 gives the results

Pickling only 5 seconds and with a suf­

ficiently thick copper undercoating, the final result on re­ moval from the oven was good. Conclusion. 1)

The copper cyanide bath was the best solution from

which to plate bright copper as an undercoating for nickel. It was plated to a thickness of .0003” . 2)

Five seconds in the acid pickle was sufficient time

to remove any superficial oxides from the surface of the ca­ thode.

Longer pickling caused the formation of a loose non­

adherent "soot” , which resulted in the final blistering of the deposit. 3)

When using the alkaline tartrate bath, the copper

must be plated to a thickness of at least .0005” and the cur­ rent density must not fall below 22 nor above 30 amp./sq.ft.

-25-

Copper-Lead Alloy Undercoating

Lead is a metal -which does not diffuse with zinc, either at ordinary or at elevated temperatures*

It was thought that

if an alloy of lead and copper could be deposited on the zinc base die castings, the formation of the brittle diffusion layer between the copper and the zinc, which resulted in the subsequent blistering of the nickel deposit, could be pre­ vented* A bath to deposit such an alloy was prepared consisting of CuCNOr^g

25 g*/L., PbCNOgJg

100 g*/L., Tartaric Acid

20 g*/L., and NaOH pellets

150 g*/L.

a temperature of 30-40°C.

Copper anodes were used in the ex­

periments*

The bath was kept at

The cathodes were cleaned in the standard manner

and pickled only 5 seconds in the acid solution*

Table 3

Experiment

Time of Acid C.D. (Cu-Pbj Pickle amp./sq. ft* 5% HC1 room temp.

Condition of alloy deposit

1

5 seconds

15

black and spongy

2

5

"

40

black and spongy

3

5

"

60

black and spongy

The deposit from this bath was in the form of a loose, thick, spongy mass.

On simple immersion lead tended to de­

posit on the zinc die casting by primary action.

This resulted

-26-

in the formation of the spongy deposit.

Closing the circuit

and dipping the cathode into the bath at higher current den­ sities failed to remedy or prevent primary action.

These ex­

periments were discontinued, as there seemed no chance of finding a lead bath that would not deposit by primary action.

Conclusion. Lead tended to deposit on the sine die castings by pri­ mary action.

Because of this an alloy of lead and copper could

not be adequately plated, and therefore, could not serve suc­ cessfully as an undercoating for nickel.

-27-

Zinc-Cadmium Alloy Undercoating

A bath to deposit an alloy of zinc and cadmium was pre­ pared consisting of Z11SO4 H^SO^

15 g./L.

70 g./L., CdSO^

3 g./L., and

The bath was kept at a temperature of 40-50°C.

Cadmium anodes were used.

The cathodes were cleaned in the

standard manner*

Table 4

Experiment

Time of Acid Pickle 5c/o HG1 room temp.

C.D. (Zn-Cd) amp./sq.ft*

Condition of Alloy Deposit

1

5 seconds

10-12

powdery, non-adherent

2

5

"

22-25

powdery, non-adherent

3

5

"

5- 6

spongy, non-adherent

The deposits of the zinc-cadmium alloy in each of the ex­ periments were poor* obtained in each case.

A loose, powdery, or spongy deposit was Such a condition was not adequate for

plating nickel successfully.

Therefore, these experiments were

discontinued. Another bath was prepared in a similar manner, but run at room temperature*

-

28-

Table 5

®XP*

Time of Acid C.D. (Zn-Cd) Pickle amp./sq. ft* 5/ HC1 room temp.

Oven 190°C.

Result

1

5 seconds

10-12

1 hr. cracked, peeled

2

5

"

10-12

1 hr. cracked, peeled

3

5

”

10-12

1 hr. cracked, peeled

4

5

B

3-4

1 hr. cracked, peeled

The zinc-cadmium deposits were somewhat better when the bath was run at room temperature.

Qualitative tests showed

the presence of both zinc and cadmium in the deposits.

The

alloy was plated to various thicknesses on the 2inc base die castings, nickel plated, and then tested in the oven.

The

deposit cracked and peeled in each test. In other experiments the zinc-cadmium deposits were coated with a layer of copper from the cyanide bath (page 21), then nickel plated, and tested in the oven.

The final re­

sults showed tendencies to crack and peel along the edges. A cadmium zinc cyanide bath was prepared consisting of GdO

25 g./L., Zn(CN)^

Oil

15 drops/L.

150 g./L., NaCN

75 g./L., Turkey Red

The bath was operated at a temperature of

35-40°C. at a current density of 32-33 amp./sq. ft. In all experiments the alloy deposits from the cyanide bath were good; however, after nickel plating and heating m the oven, the nickel deposit either cracked or blistered.

-29-

Conclusion. 1)

A suitable deposit of a zinc-cadmium alloy could not

be obtained on the zinc base die castings. 2)

An intermediate layer of copper between the alloy and

the nickel deposit didn't seem to better the result to any ap­ preciable extent* 3)

Good deposits were obtained from the cyanide bath;

however, in each experiment the nickel deposit either cracked or blistered after having been subjected to high temperatures in the oven.

-30-

Lead Undercoating

In a previous experiment, an attempt was made to deposit an alloy of lead and copper on the zinc base die castings* Because of primary action it was impossible to use the bath. It appeared that if a suitable bath could be found, the lead deposit would serve as a good basis for the final nickel de­ posit.

Since lead does not diffuse with zinc, the cracking

or blistering occurring after the heat treatment could be el­ iminated. A sodium plumbite bath was used, consisting of Pb(N0„)C) O u

25 g./L., NaOH

25 g./L-, and Rosin

10 g./L.

The temperature

during electrolysis was kept at 80°C. In the preparation of this bath the amount of caustic soda was varied until the minimum quantity which would de­ crease primary action during simple immersion was found. Twenty-five grams lead nitrate were dissolved in 500 c.c. distilled water and 25 grams sodium hydroxide in another 500 c.c. distilled water kept in a separate vessel.

The lead ni­

trate solution was slowly added to the caustic solution, and the bath was left to stand for one hour.

Any crystalline lead

hydroxide which had precipitated during the standing period was removed by filtration. The cathode was cleaned as described and incerted in the lead bath at a current density of 22-25 amp./ sq. ft*

After

one minute it was noticed that the deposit on the lower extre­

-31-

mities of the cathode was in the form of minute globules of sponge lead.

The cathode was wiped clean of the sponge me­

tal with a soft rag, rinsed, and inserted again into the bath at a current density of 4-5 amp./sq. ft.

A grey deposit was

obtained, which appeared to adhere rather well to the cathode. It was nickel plated and tested in the oven.

The result was

a blistering and cracking deposit. In attempting to duplicate the experiment, it was noted that even at 4-5 amp./sq* ft. the lead deposit was in the form of sponge metal.

After wiping the cathode of sponge metal,

electrolysis proceeded in a satisfactory manner. The appearance of the sponge lead indicated that primary action was evidently still occurring.

UVhen a soft cloth was

used to wipe the sponge metal from the cathode, a thin film of lead was probably spread over the surface,which eliminated primary action on further electrolysis.

This film, which be­

cause of primary action was not adherent to the cathode, was probably responsible for the blistering effect during the heating period in the oven.

The current density was varied

from very high currents, where the cathode was given a momen­ tary flash, to very low currents; but in all cases the effects of primary action was evident.

Heating the cathode in the

oven before electrolysis failed to prevent the deposition of the metal in a sponge condition. Another lead bath was prepared consisting of PbCNOgJg 20 g./L., NaCN

35 g./L; and Na9C0 M

O

15 g./L.

The sodium car-

-32-

bonate was added in an attempt to keep the lead in the form of sodium plumbite and to make the bath alkaline.

The heavy

precipitate resulting in the bath was removed before electro­ lysis. On the first run electrolysis continued for twenty minutes at a current density of 4-5 amp./sq. ft* before a deposit of lead could be seen on the cathode.

The length of time of elec­

trolysis indicated that practically all the lead had been pre­ cipitated from solution during the preparation of the bath. The deposit was dark in color and appeared to have been deposit­ ed without the interference of primary action. nickel plated and heated in the oven.

The cathode was

The result was good.

The

lead deposit must have been an adherent one, therefore making it a good basis for the nickel deposition.

Table 6

Experiment

C. D. (Pb) amp./sq.ft.

1

4-5

2 3

Time of electrolysis for Pb deposition

Result

20 minutes

good

10

18

”

good

15

18

n

good

The experiment was repeated with the lead bath heated to 80°C.

The lead deposition was better from the hot solution.

It was difficult to calculate an accurate thickness of the lead deposit because of the length of time of electrolysis required

-33-

before an indication of lead could be observed on the cathode. The lead content of the bath was very low. A bath was prepared consisting of HaCU ^a2 ^ 3

£*/L.

35 g./L., and

An attempt was made to see if lead could be

deposited from such a solution using a lead anode at a current density of 4-5 amp./sq. ft. coated black.

After an hour the cathode was

After being nickel plated and tested in the

oven, the result proved satisfactory. ever, was very slight.

The lead deposit, how­

A qualitative test of the

black depo­

sit proved the presence of lead.

Conclusion. 1)

Due to primary action lead was deposited in a sponge

condition when plated from a sodium plumbite solution. 2)

Lead deposited from a cyanide solution was adherent

and served as a good basis for nickel plating zinc base die castings.

The lead was deposited in a very thin film, and

only after lengthy electrolysis. 3)

Using lead as an undercoating, there was no danger of

over-pickling, in the cleaning process.

Although it was diffi­

cult to deposit lead from a cyanide solution, it served very well as an undercoating for nickel.

If a suitable solution

could be found to deposit lead without interference of primary action, the plating of zinc base die castings would be greatly simplified.

-34-

Tin Undercoating

Prom preliminary experiments it appeared as though tin might serve as a good intermediate deposit for the nickel plating of zinc base die castings.

There

seemed to be little

or no diffusion of the tin with the zinc. A tin bath was prepared consisting of N&gSnOg 90 g./L., NaAc

15 g./L., NaOH

15 g./L., and 3/ hgOg

25 c.c./L.

The

temperature of the bath was kept at 70-80°C. to facilitate a better deposition of the metal. The first experiment consisted in depositing both tin and copper as an undercoating for the nickel.

The cathode was

purposely over-pickled in the acid solution and then plated in the tin hath to

a thickness of .0003” .It was then

with .0003” copper

(copper cyanide bath), nickel plated, and

tested in the oven for one hour at 190°C. good.

coated

The result was very

The tin deposit evidently prevented any diffusion of the

zinc and copper to form the brittle film responsible for the ultimate blistering of the nickel coating. In running the bath at room temperature, a dark gray tin deposit was obtained.

However, this dark deposit didn't seem

to affect the final results of the experiment.

When run at

7Q-80°C., a very light gray, almost white deposit was obtained. Table 7 gives the minimum thickness to which the tin must be deposited in order that the final result be good.

By com­

paring experiments 1 and 2, and also 3 and 4, it was found

-35-

Table 7 After plating with tin, the article was nickel plated to a thickness of .0007" at a current density of 30-32 amp./sq.ft, and then tested in the oven for one hour at a temperature of 190°C.

Exp.

Time of Acid C.D. (Sn) Thickness Pickle amp./sq.ft. of Sn 5/ HC1 room temp. deposit

Result

1

5 seconds

32-33

.0004"

2

1 minute

32-33

.0004”

it

it

3

5 seconds

32-33

.0003"

tt

ii

4

1 minute

32-33

.0003"

n

»i

5

1 minute

32-33

.0002"

n

ii

6

5 seconds

12-13

.0001"

ti

it

7

5 seconds

32-33

.00004”

ii

it

8

5 seconds

12-13

.00002"

n

n

9

5 seconds

4- 5

.000015"

ii

it

10

5 seconds

4- 5

.00001"

11

5 seconds

4- 5

very good

tendency to crack and peel .000005" blistered

that over-pickling had no ill effects on the outcome of the experiment.

Using tin as an undercoating for the nickel el­

iminated the danger of over-pickling in the acid solution. By employing 0.1 gram thiourea in the nickel bath, the nickel was found to be deposited as a bright plate. used in the above experiments.

This was

-

36-

Another tin bath was prepared consisting of SnCLCh w

50 g./L., (NH^JgCgO^ Peptone 2*5 grams.

S*/L», Oxalic Acid

fzr

15 g./L., and

The SnC^O^ was prepared by dissolving

SnClg in distilled water acidified with HC1, and precipitat­ ing the tin oxalate by adding the required weight of oxalic acid.

The precipitate was filtered, washed, and dried.

Table 8 After depositing tin from the oxalate bath, the article was nickel plated to a thickness of .0007'* at a current den­ sity of 30-32 amp./sq.ft., and then tested in the oven for one hour at a temperature of 190°C. Time of Acid Pickle 5/ HC1 rm.temp.

Exp •

G.D. (Sn) amp./sq.ft.

Thickness of Sn deposit

Result

1

1 minute

12-13

.0003"

2

5 seconds

4-5

.0001"

3

5 seconds

8-9

.0001"

4

5 seconds

12-13

.0015"

it

5

5 seconds

12-13

.002"

ti

6

5 seconds

12-13

.005"

7

5 seconds

22-23

sponge deposit

—

8

5 seconds

15-16

sponge deposit

—

blistered tt ii

11

The bath was prepared by dissolving the ammonium oxalate and oxalic acid in distilled water and then dissolving the tin oxalate in the solution.

The addition agent was added to the

-37-

bath in solid form.

The bath was kept at room temperature.

Current densities greater than 12-13 amp./sq.ft. caused the tin to deposit in a sponge condition from the oxalate bath.

The tin deposit was white in appearance and appeared

to have good adherence, but the final nickel deposit blistered after the heating period in the oven. It was thought that the acidity of the oxalate solution affected the blistering of the deposit.

The solution was

neutralized with ammonium hydroxide, but even this failed to prevent the ultimate result. In other experiments with the oxalate bath, a copper coating of .0003" was deposited on the tin, but again the nickel blistered under the heat treatment.

The temperature

of the bath was raised to 50°C. and then 75°C., but the re­ sult was the same.

It was probable that the adherence of the

tin from the oxalate bath was not as firm as that from the stannate bath. Attempts were made to deposit the tin in a bright condi­ tion.

Addition agents such as selenium fused in NaOH, NagC^O^,

Ua S 0 , la S, and Propionic Acid were added to the alkaline 2 2 3

2

stannate bath.

Two to three grams of NagCgO^ or NagSgOg gave

a very white deposit.

Five to ten c.c. Propionic Acid tended

to brighten the deposit to a small degree.

38-

Conclusion^ 1)

Tin when deposited from a hot stannate hath served

as an excellent basis for the deposition of nickel*

Using

tin as an undercoating eliminated the danger of over-pickling in the cleaning process*

The minimum thickness to which tin

from the stannate bath could be deposited without any ill ef­ fects on the result was *0002". 2)

Tin when deposited from the oxalate bath was not

found to be a successful undercoating for nickel plating on zinc base die castings.

In every experiment where the tin

undercoating was deposited from the oxalate bath, the nickel deposit blistered.

-39-

Silver Undercoating

As in the case of lead, silver tended to be precipitated from its common salts in the form of coarse or sponge metal due to primary action.

It was thought that the deposition

of silver on zinc base die castings could best be done from a cyanide solution. 23 g./L., Na2C03

A bath was prepared consisting of NaCN

19 g./L., and AgCN

13 g./L.

The tempera­

ture of the bath was kept at 40°C.

Table 9 The cathode was pickled for five seconds in 5% HC1 at room temperature and then silver plated at a current density of 1-2 amp./sq.ft. It was then plated with nickel to a thick­ ness of .0007" at a current density of 30-32 amp./sq.ft., and tested in the oven for one hour at a temperature of 190°C.

Experiment

Thickness of Ag deposit

Result

1

.001"

peeled

2

.0006"

peeled

3

.0003"

tendency to crack

4

.0001"

tendency to crack

Above 1-2 amp./sq.ft. the silver deposit turned black. Over-pickling caused the deposit to crack and peel. had poor throwing power.

The bath

This was easily seen by the nature

of the deposit when allowed to remain in the bath for periods exceeding two minutes.

The deposit was thickest on the ele­

vated areas of the cathode.

-40-

Because there was a tendency for silver to deposit by primary action, the cathode was lowered into a closed circuit system*

Where only a thin coating of metal was deposited, the

final result showed neither peeling nor blistering.

There

were, however, faint indications of cracking along the outer edges of the cathode,

Where the silver deposit exceeded

,0003u thickness, the nickel deposit cracked or peeled.

The

adherence of the silver to the zinc base die castings from such a bath was poor.

Conclusion, Silver could not be used successfully as an undercoating in the nickel plating of zinc base die castings.

Because of

primary action, the poor throwing power of the bath, and the danger of over-pickling, there was no certainty of obtaining a suitable result.

41-

Cadmium. Under coating

Cadmium, plating has found a widespread use in the last decade*

It was possible to deposit cadmium from solutions

of the chlorides and sulphate, but the deposits were in a coarsely crystalline form and definitely unsuited as a basis for the nickel plating of zinc base die castings.

The cyanide

solution is the solution usually employed commercially. A bath was prepared consisting of CdO

25 g./L., NaCN

85 g./L., and Turkey Red Oil as an addition agent 3 drops. The temperature of the bath was kept at 35-40°C.

Table 10 The cathode was pickled for five seconds in 5% EC1 at room temperature and then cadmium plated at a current density of 32-33 amp.^q.ft. It was then plated with nickel to a thickness of .0007'1 at a current density of 30-32 amp./sq.ft., ant tested in the oven for one hour at a temperature of 190°C.

Experiment

Thickness of Cd deposit

Result

faint cracks

1

.0006"

2

.0005"

M

tt

3

.00025"

tt

11

4

.0001"

It

tt

5

.00006"

tt

tt

-42-

The cadmium deposit out of the cyanide bath was a very bright deposit*

However, after nickel plating and testing

in the oven, faint cracks were visible at the outer edges of the cathode.

Conclusion. There was a tendency for the nickel deposit to crack when b&dmium was used as the undercoating.

Various thick­

nesses of the cadmium were applied to the zinc base die cast­ ings, but the result was the same in every case.

CONCLUSION TO EXPERIMENTS

-44-

It was found that in the nickel plating of zinc base die castings, certain factors had to be controlled in order to obtain a deposit free from blistering, cracking, or peeling* It was necessary to control the time of pickling in the cleaning process.

Over-pickling resulted in the formation of

a loose non-adherent film, ’’soot11, on the surface of the ca­ thode, which was responsible for the subsequent blistering of the nickel deposit. When using as undercoating a metal such as copper, which diffused easily with the zinc, it was especially imperative to remove the danger of over-pickling.

The copper metal was

found to diffuse with the loose non-adherent film to form a brittle film, which when subjected to the heat treatment caused the nickel deposit to blister. Nickel, deposited directly on the zinc die casting, tended to crack or peel when subjected to a temperature of 190°C. for any length of time.

An alloy of lead and copper

could not be deposited successfully because of the tendency of lead to deposit by primary action. non-adherent, spongy mass.

The result was a loose,

Although an alloy of zinc and cad­

mium could be deposited easily from a cyanide solution of both metals, it was found that in each case, the nickel deposit blistered at the elevated temperatures.

Silver was deposited,

but the adherence and throwing power of the bath was poor.

-45-

Cadmium could be deposited in a bright condition, but again the nickel deposit would crack or peel when heated in the oven* Lead when deposited from a cyanide solution served as a good basis for the deposition of nickel.

With lead as an

undercoating the danger of over-pickling was eliminated.

How­

ever, the lead content of the bath was very low, and the time of electrolysis to deposit only a thin film was great.

The ef­

ficiency of the bath was poor. In later experiments tin was found to serve best as an undercoating for the nickel plating of zinc base die castings, but only when plated from a hot alkaline stannate bath.

Using

tin as an undercoating removed entirely the danger of over­ pickling in the cleaning process.

Furthermore, it was found

that tin could be deposited to a minimum thickness of .00002” without danger of the nickel deposit blistering during the heat treatment. The best tin plate was deposited from a solution of the following composition;

Na^SnO^

90 g./L.

NaC2H302

15 g./L.

NaQH

15 g./L.

25 c.c./L. Two to three grams ^a2^2^4 0l< ^a2^2^3 anc^ 5-10 c.c# Propionic Acid could be employed as addition agents to im­ prove the appearance of the deposit.

The bath was maintained

at a temperature of 70-80°C., and electrolysis proceeded at 32-33 amp./sq.ft.