Polymeric Materials for Corrosion Control 9780841209985, 9780841211599, 0-8412-0998-7

Table of contents :
Title Page ......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Polymeric Materials for Corrosion Control: An Overview......Page 8
Metallic Corrosion......Page 9
Mechanisms of Corrosion Control......Page 10
Methods of Evaluating Polymer Performance......Page 11
Barrier Aspects of Corrosion Control......Page 14
Interfacial and Adhesion Aspects of Corrosion Control......Page 15
Effects of Polymer Composition on Corrosion Control......Page 18
Critical Issues......Page 19
Literature Cited......Page 20
2 Alternating Current Impedance and Underfilm Darkening Studies on Acidic Water-Based Anticorrosive Paints......Page 24
Experimental......Page 26
Results and Discussion......Page 27
Underfilm Darkening......Page 29
Conclusions......Page 35
Appendix......Page 36
3 New Methods in Electrochemical Assessment of Polymer Coatings on Steel......Page 37
Results and Discussion......Page 38
Literature Cited......Page 41
4 Application of Electrochemical Noise Measurements to Coated Systems......Page 42
Instrumentation......Page 43
Experimental......Page 45
Discussion......Page 48
Literature Cited......Page 53
5 Electrochemical Characterization of Photocured Coatings......Page 54
ExperimentaI......Page 55
Corrosion Resistance of the Photocured Coatings......Page 56
Salt Fog Corrosion Resistance......Page 58
Nobility and the Degree of Corrosion Resistance......Page 60
Rust Rating - Temperature Correlation......Page 61
Literature Cited......Page 62
6 Alternating Current Impedance: Utility in Evaluating Phosphate Coating, Phosphorus-Chromium Rinse, and Paint Performance......Page 64
THEORETICAL......Page 65
EXPERIMENTAL......Page 68
RESULTS AND DISCUSSION......Page 69
LITERATURE CITED......Page 81
7 Evaluation of Coating Resins for Corrosion Protection of Steel Exposed to Dilute Sulfuric Acid......Page 83
Experimental......Page 84
Discussion......Page 88
Conclusions......Page 90
Literature Cited......Page 91
8 Comparison of Laboratory Tests and Outdoor Tests of Paint Coatings for Atmospheric Exposure......Page 92
Experimental......Page 94
Results and Discussion......Page 96
Literature Cited......Page 106
9 Degradation of Organic Protective Coatings on Steel......Page 107
Degradation of Amine-Cured Epoxy Coating on Cold-Rolled Steel......Page 108
Degradation of Polybutadiene Coating on Cold-Rolled Steel......Page 112
LITERATURE CITED......Page 115
10 Permeabilities of Model Coatings: Effect of Cross-link Density and Polarity......Page 116
Experimental......Page 117
Results and Discussion......Page 118
Literature Cited......Page 120
Experimental......Page 121
Results and Discussions......Page 122
Literature Cited......Page 128
12 Mechanisms of De-adhesion of Organic Coatings from Metal Surfaces......Page 129
De-adhesion Processes......Page 130
Migration of Species Through Coatings......Page 136
Acknowledgment......Page 139
Literature Cited......Page 140
13 Chemical Studies of the Organic Coating-Steel Interface After Exposure to Aggressive Environments......Page 141
Surface Studies of Interfacial Composition......Page 142
Humidity-induced Adhesion Loss......Page 143
Corrosion-induced Adhesion Loss......Page 147
Literature Cited......Page 158
14 Adhesion Loss of Ultraviolet-Cured Lacquer on Nickel-Plated Steel Sheets......Page 160
Experimental Method......Page 161
Results and Discussion......Page 162
Conclusions......Page 171
Literature Cited......Page 173
15 Cathodic Delamination of Protective Coatings: Cause and Control......Page 174
The Role of the Hydroxide Ion in Cathodic Delamination......Page 175
Delamination Rate Studies......Page 177
Swelling of Adhesives in the Presence of Hydroxide......Page 178
Screening Test for Adhesives and Additives......Page 182
Conclusions......Page 183
Literature Cited......Page 184
16 Effect of Surface Preparation on the Durability of Structural Adhesive Bonds......Page 185
Experimental Details Substrates......Page 186
Aging Environments......Page 187
Durability......Page 188
Literature Cited......Page 198
17 Effects of Corrosive Environments on the Locus and Mechanism of Failure of Adhesive Joints......Page 199
Results and Discussion......Page 200
Literature Cited......Page 207
18 New Polymeric Materials for Metal Conversion Coating Applications......Page 208
Background - Present Industrial Practice......Page 209
Chrome-Free Post-Treatments......Page 210
Polyvinylphenol Post-Treatments......Page 212
Acknowledgment......Page 214
Literature Cited......Page 215
19 Enhancement of Acid-Chloride Resistance in a Chromate Conversion Coating......Page 216
Experimental......Page 217
Results......Page 218
Discussion......Page 222
Conclusions......Page 223
Literature Cited......Page 224
The Electrochemical Mechanism......Page 225
The Physicochemical mechanism......Page 226
The Adhesional Mechanism......Page 227
The Combination Of Different Protective Mechanisms......Page 228
Literature Cited......Page 231
21 Improving the Performance of Zinc-Pigmented Coatings......Page 232
Results and Discussion......Page 233
Literature Cited......Page 236
22 Organic Corrosion Inhibitors to Improve the Durability of Adhesion Between Aluminum and Polymeric Coatings......Page 237
Epoxy Systems......Page 239
Results......Page 240
Adhesive Bond Durability......Page 244
Discussion......Page 248
Literature Cited......Page 251
23 Inhibition of Copper Corrosion by Azole Compounds in Acidic Aqueous Solutions......Page 253
Experimental......Page 254
Results and Discussion......Page 256
Literature Cited......Page 268
24 Corrosion Protection on Copper by Polyvinylimidazole......Page 271
Experimental......Page 272
Molecular Structure of PVI/Copper(II) Complexes......Page 273
Comparison of Benzotriazole, Undecylimidazole and PVIs (150 to 300°C)......Page 275
High Temperature Study of PVIs (330 to 450°C)......Page 278
Literature Cited......Page 283
25 N-(Hydroxyalkyl)acrylamide Copolymers for Corrosion Control......Page 286
Experimental Section......Page 287
Results and Discussion......Page 289
Conclusion......Page 292
Literature Cited......Page 293
26 Performance Aspects of Plasma-Deposited Films......Page 294
Experimental......Page 295
a) Moisture barrier properties......Page 296
b) Corrosion protection......Page 298
Acknowledgment......Page 300
Literature cited......Page 301
27 Humidity Testing of Silicone Polymers for Corrosion Control of Implanted Medical Electronic Prostheses......Page 302
Mechanisms of Implant Failure......Page 304
Interface Properties of Silicone Polymers......Page 306
Silicones for Electronic Encapsulation......Page 307
Methods......Page 308
Results......Page 309
Discussion......Page 310
Literature cited......Page 315
28 Corrosion Behavior of Epoxy and Unsaturated Polyester Resins in Alkaline Solution......Page 317
CORROSION BEHAVIOR OF EPOXY RESINS......Page 318
CORROSION BEHAVIOR OF UNSATURATED POLYESTER RESIN......Page 323
PREDICTION OF STRENGTH OF RESINS IN CORROSIVE ENVIRONMENT......Page 326
CONCLUSIONS......Page 327
Literature Cited......Page 329
29 Structure-Property Relationships in Tin-Based Antifouling Paints......Page 330
EXPERIMENTAL MODELLING AND SIMULATION......Page 332
POLYMER SYNTHESIS......Page 333
POLYMER TESTING......Page 334
RESULTS AND DISCUSSION......Page 335
Literature Cited......Page 343
30 Polyurethane Foam Component Lifetimes......Page 344
Background - Corrosion Study of Formulation 1......Page 345
Discussion - Accelerated Aging Study of Formulation 2......Page 346
Summary And Conclusions......Page 350
Literature Cited......Page 351
31 Rubber Coatings for Fiberglass Protection in an Alkaline Environment......Page 352
Experimental......Page 353
Results and Discussion......Page 357
Acknowledgments......Page 361
Literature Cited......Page 362
Author Index......Page 363
A......Page 364
C......Page 366
Ε......Page 368
G......Page 369
L......Page 370
O......Page 371
Ρ......Page 372
R......Page 373
S......Page 374
X......Page 375
Ζ......Page 376

Citation preview

Polymeric Materials for Corrosion Control

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ACS

SYMPOSIUM

SERIES

Polymeric Materials for Corrosion Control Ray A. Dickie, EDITOR F. Louis Floyd, EDITOR Glidden Coatings and Resins

Developed from a symposium sponsored by the Division of Polymeric Materials Science and Engineering at the 190th Meeting of the American Chemical Society, Chicago, Illinois, September 8-13, 1985

American Chemical Society, Washington, DC 1986 In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

322

Library of Congress Cataloging-in-Publication Data Polymeric materials for corrosion control. (ACS symposium series, ISSN 0097-6156; 322) "Developed from a symposium sponsored by the Division of Polymeric Materials, Science and Engineering at the 190th Meeting of the American Chemical Society, Chicago, Illinois, September 8-13, 1985." Includes bibliographies and index. 1. Corrosion and anti-corrosives—Congresses 2. Polymers and polymerization—Congresses 3. Protective coatings—Congresses. I. Dickie, R. Α., 1940. II. Floyd, F. Louis, 1945. III. American Chemical Society. Division of Polymeric Materials: Science and Engineering. IV. American Chemical Society. Meeting (190th: 1985: Chicago, Ill.) V. Series. TA462.P57 1985 ISBN 0-8412-0998-7

620.1'9204223

86-20646

Copyright © 1986 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN T H E UNITED STATES O F A M E R I C A

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ACS Symposium Series M . Joan Comstock, Series Editor Advisory Board Harvey W. Blanch

Donald E Moreland

University of California—Berkele

Alan Elzerman

W. H. Norton

Clemson University

J. T. Baker Chemical Company

John W. Finley

James C. Randall

Nabisco Brands, Inc.

Exxon Chemical Company

Marye Anne Fox

W. D. Shults

The University of Texas—Austin

Oak Ridge National Laboratory

Martin L. Gorbaty

Geoffrey K. Smith

Exxon Research and Engineering Co.

Rohm & Haas Co.

Roland F. Hirsch

Charles S.Tuesday

U.S. Department of Energy

General Motors Research Laboratory

Rudolph J. Marcus

Douglas B. Walters

Consultant, Computers & Chemistry Research

National Institute of Environmental Health

Vincent D. McGinniss

C. Grant Willson

Battelle Columbus Laboratories

IBM Research Department

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FOREWORD The ACS S Y M P O S I U M S E R I E S was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing A D V A N C E S IN C H E M I S T R Y S E R I E S except that, in order to save time, the papers are not typeset but reproduced the submitted by the authors in camera-read the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

PREFACE THERE A R E T H R E E C R I T I C A L S T E P S in obtaining satisfactory performance from corrosion protective materials: design of the device, component, or structure; selection of materials for the intended application; and control of the manufacturing process or application conditions. The principal concern of the present volume is evaluating and selecting materials for use in corrosive environments. Intelligent materials selection requires a thorough understanding of protection and failure mechanisms and relies on the availability of appropriate test methodology Papers concerning these topics make up the bulk of this volume materials or unexpected applications of polymeric corrosion-protective materials. The symposium on which this book is based was organized to provide a forum for discussion of recent advances in the use of polymeric materials in corrosion control. Most of the papers presented in the symposium are included in this volume. Several chapters have been added. These include an introductory overview as well as separate review chapters on how organic coating systems protect against corrosion, on mechanisms of adhesion loss of organic coatings, and on the interfacial chemistry of adhesion loss in aggressive environments. This volume provides a slice-in-time view of the progress in the science and technology of polymeric materials for corrosion control. The editors hope it will prove thought provoking and will contribute to a continuing discussion within the polymeric materials community on improved methods for achieving corrosion control. The editors wish to express their appreciation to Ford Motor Company and to Glidden Coatings and Resins Division, S C M Corporation, for support during organization of the symposium and preparation of this volume. Special thanks are due Cathy Ciarrocchi and Diane DeSimone for their secretarial assistance. R A Y A. D I C K I E

Ford Motor Company Dearborn, MI 48121 F. Louis FLOYD Glidden Coatings and Resins Division SCM Corporation Strongsville, OH 44136 ix

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1 Polymeric Materials for Corrosion Control: An Overview 1

Ray A. Dickie and F. Louis Floyd 1

2

2

Ford Motor Company, Dearborn, MI 48121 Glidden Coatings and Resins Division, SCM Corporation, 16651 Sprague Road, Strongsville, OH 44136

Polymeric materials are widely used to control the corrosion of metals, both to maintain appearance and to prevent loss of structural integrity. In this chapter, the fundamentals of metallic corrosion are briefly reviewed. Methods of studying corrosion, and of evaluating the performance of polymeric materials used in corrosion protection, are outlined. Factors that influence the corrosion protective performance of polymeric materials are discussed, and some of the research needs and important unsolved problems are highlighted. The economic c o s t s and e n v i r o n m e n t a l impact o f m e t a l l i c c o r r o s i o n are w e l l known, and need not be d i s c u s s e d i n depth h e r e . I t has been e s t i m a t e d (1) t h a t the t o t a l c o s t o f c o r r o s i o n i n the U n i t e d S t a t e s may be as much as 4% o f the g r o s s n a t i o n a l p r o d u c t , and t h a t about 15% o f t h e t o t a l c o s t might be a v o i d a b l e t h r o u g h the economic use o f a v a i l a b l e technology. Most s t u d i e s o f c o r r o s i o n and i t s e f f e c t s understandably concentrate on the c o s m e t i c and s t r u c t u r a l e f f e c t s o f m e t a l l i c c o r r o s i o n ; most o f the papers i n the p r e s e n t volume f a l l i n t o t h i s category. I t s h o u l d be n o t e d , however, t h a t m e t a l l i c c o r r o s i o n and the p r o d u c t s o f m e t a l l i c c o r r o s i o n can d e l e t e r i o u s l y a f f e c t the p r o p e r t i e s o f n o n - m e t a l l i c m a t e r i a l s , p a r t i c u l a r l y a t j o i n t s between m e t a l s and non-metals. There a r e a l s o e n v i r o n m e n t a l d e g r a d a t i o n phenomena t h a t c a n a f f e c t n o n - m e t a l l i c m a t e r i a l s such as p l a s t i c s , c o m p o s i t e s and g l a s s d i r e c t l y ; some o f t h e s e phenomena resemble m e t a l l i c c o r r o s i o n p r o c e s s e s i n the e f f e c t s o b s e r v e d on appearance and structural integrity. S e v e r a l papers i n t h i s volume d e a l w i t h c o r r o s i o n e f f e c t s on a d h e s i v e j o i n t s and n o n - m e t a l l i c m a t e r i a l s . The present chapter begins w i t h a b r i e f overview o f m e t a l l i c c o r r o s i o n and mechanisms o f c o r r o s i o n c o n t r o l . Methods o f e v a l u a t i n g polymer performance and e l e c t r o c h e m i c a l c h a r a c t e r i z a t i o n t e c h n i q u e s are d i s c u s s e d . B a r r i e r and a d h e s i o n a s p e c t s o f c o r r o s i o n c o n t r o l are r e v i e w e d , and some c r i t i c a l i s s u e s n e e d i n g f u r t h e r s t u d y are o u t l i n e d . 0097-6156/ 86/ 0322-0001 $06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC MATERIALS FOR CORROSION C O N T R O L

2 Metallic

Corrosion

M e t a l l i c c o r r o s i o n has been t h e s u b j e c t o f many t e x t b o o k s and s c h o l a r l y compendia (e.g., 2-4), and a number o f i n t r o d u c t o r y t r e a t m e n t s d e a l i n g w i t h c o r r o s i o n and c o r r o s i o n p r o t e c t i o n a r e a l s o a v a i l a b l e (e.g., 5-7). I n t h i s c o n t e x t , t h e term " c o r r o s i o n " r e f e r s t o t h e c h e m i c a l d e g r a d a t i o n o f a m e t a l by i t s environment. The r e a c t i o n s a r e most o f t e n heterogeneous redox r e a c t i o n s and o c c u r a t t h e m e t a l - e n v i r onment i n t e r f a c e . The a n o d i c r e a c t i o n i s t y p i c a l l y t h e o x i d a t i o n o f the m e t a l ; t h e c a t h o d i c r e a c t i o n i s r e d u c t i o n o f a non-metal, t y p i c a l l y oxygen. I f t h e p r o d u c t o f t h e m e t a l o x i d a t i o n forms a t i g h t and adherent f i l m , t h e c o r r o s i o n p r o c e s s may be s e l f - l i m i t i n g . I f the products o f the c o r r o s i o n r e a c t i o n are s o l u b l e i n the c o r r o s i v e medium, o r a r e permeable t o i t , t h e n m e t a l l i c c o r r o s i o n c a n proceed. Corrosion i s often represented i n terms o f a s i m p l e e l e c t r o c h e m i c a l model. The a n o d i c and c a t h o d i c h a l f r e a c t i o n s o f t h e c o r r o s i o n c e l l may o c c u r a t a d j a c e n t o the e l e c t r i c a l c i r c u i t i the c o r r o d i n g m e t a l and i o n i c c o n d u c t i o n w i t h i n t h e aqueous e l e c t r o lyte. I n n a t u r a l c o r r o s i o n , i t i s common f o r t h e s i t e s o f t h e a n o d i c and cathodic c o r r o s i o n r e a c t i o n s t o become more o r l e s s w i d e l y separated. I n such c a s e s , t h e a n o d i c s i t e s t e n d t o become a c i d i c and the c a t h o d i c s i t e s t e n d t o become b a s i c . These changes i n pH can be l a r g e , and c a n have s e r i o u s i m p l i c a t i o n s f o r t h e performance o f polymeric materials. The c o r r o s i o n o f i r o n i s one o f t h e most w i d e s p r e a d and technol o g i c a l l y i m p o r t a n t examples o f m e t a l l i c c o r r o s i o n . I n t h e presence o f w a t e r and oxygen, t h e c o r r o s i o n o f i r o n proceeds t o form a c o m p l i c a t e d m i x t u r e o f h y d r a t e d i r o n o x i d e s and r e l a t e d s p e c i e s ; a complete d e s c r i p t i o n i s beyond t h e scope o f t h e p r e s e n t d i s c u s s i o n , and the i n t e r e s t e d reader i s r e f e r r e d t o the p r e v i o u s l y c i t e d general r e f e r ences on c o r r o s i o n as w e l l as t o t h e w e l l known d e s c r i p t i o n s o f e l e c t r o c h e m i c a l e q u i l i b r i a i n aqueous s o l u t i o n g i v e n by P o u r b a i x ( 8 , 9). I r o n i s a base m e t a l , s u b j e c t t o c o r r o s i o n i n aqueous s o l u t i o n s . In the presence o f o x i d i z i n g s p e c i e s , i r o n surfaces c a n be p a s s i v a t e d by the formation o f an o x i d e l a y e r ; i f t h e o x i d e l a y e r formed i s i m p e r f e c t , r a p i d c o r r o s i o n may o c c u r . In simplest form, t h e r e a c t i o n o f i r o n t o form i r o n o x i d e c a n be w r i t t e n a s : 4 Fe + 2 H 0 +3 0 2

-> 2 F e 0 . H 0

2

2

3

2

The f i r s t step i n the c o r r o s i o n process i s the d i s s o l u t i o n o f i r o n to form f e r r o u s i o n : Fe

-> F e

+ +

+ 2 e"

I n g e n e r a l , t h e pH d e c r e a s e s a t h y d r o l y s i s r e a c t i o n s such a s : Fe

+ +

+ H 0 -> FeOH 2

+

+ H

sites of

anodic d i s s o l u t i o n

due t o

+

The c a t h o d i c r e a c t i o n s commonly o b s e r v e d a r e t h e e v o l u t i o n o f hydrogen and t h e r e d u c t i o n o f oxygen; hydrogen e v o l u t i o n i s k i n e t i c a l l y f a v o r e d under a c i d i c c o n d i t i o n s , w h i l e oxygen r e d u c t i o n i s k i n e t i c a l l y f a v o r e d under n e u t r a l and b a s i c c o n d i t i o n s .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1.

DICKIE A N D

3

Overview

FLOYD

H 0+ + e"

-> 1/2

H

2

+

H0

(acid

H0

+ e"

-> 1/2

H

2

+

OH"

(neutral/basic

1/2

0

2

+ H30

1/2

0

2

+ H0

3

2

2

+

+ 2 e" + 2 e"

2

solutions) solutions)

-> 3 H 0

(acid solutions)

-> 2 OH'

(neutral/basic

2

solutions)

The pH a t c a t h o d i c s i t e s tends to i n c r e a s e due t o the p r o d u c t i o n of h y d r o x i d e i o n and/or consumption o f hydrogen i o n . I t i s i n t e r e s t i n g t o c o n t r a s t the b e h a v i o r o f i r o n w i t h t h a t of aluminum. Aluminum i s a v e r y base m e t a l ; y e t aqueous s o l u t i o n s i n the n e u t r a l pH range t h a t are q u i t e a g g r e s s i v e toward i r o n o f t e n have l i t t l e e f f e c t on aluminum. I n the p r e s e n c e o f a c i d s o l u t i o n s , aluminum d i s s o l v e s w i t h the formation o f A l ^ i o n s , w h i l e under a l k a l i n e c o n d i t i o n s , i t d i s s o l v e s as a l u m i n a t e i o n s A 1 0 " Over the mid-range o f pH from abou o x i d e tends t o form. Th depend on the c o n d i t i o n s under w h i c h i t i s formed, and the c o r r o s i o n performance o f aluminum tends t o be dominated by the performance o f the oxide l a y e r . C e r t a i n s o l u t i o n s p e c i e s , n o t a b l y c h l o r i d e , can d i s r u p t the o x i d e l a y e r and cause l o c a l i z e d p i t t i n g . C o n t r o l and m o d i f i c a t i o n o f the aluminum s u r f a c e o x i d e l a y e r has been e x t e n s i v e l y s t u d i e d , and i s o f p a r t i c u l a r importance i n the p r o t e c t i o n o f aluminum substrates. +

Mechanisms o f C o r r o s i o n

Control

Corrosion can be c o n t r o l l e d by i s o l a t i o n o f the m e t a l from the c o r r o s i v e environment; by s u p p r e s s i o n o f the anodic d i s s o l u t i o n of m e t a l ; and by s u p p r e s s i o n o f the c o r r e s p o n d i n g c a t h o d i c r e a c t i o n . I s o l a t i o n o f c o r r o s i o n prone m e t a l s from c o r r o s i v e environments i s p r o b a b l y the most g e n e r a l mechanism o f the corrosion protection a f f o r d e d by p a i n t f i l m s , s e a l e r s , and s i m i l a r polymer-based m a t e r i a l s . Effective i s o l a t i o n requires that polymeric materials have good b a r r i e r p r o p e r t i e s and remain adherent i n the p r e s e n c e o f w a t e r and the p r o d u c t s o f m e t a l l i c c o r r o s i o n . B a r r i e r p r o p e r t i e s and a d h e s i o n a s p e c t s o f c o r r o s i o n c o n t r o l are d i s c u s s e d in detail i n subsequent sections. The a n o d i c d i s s o l u t i o n o f m e t a l can be s u p p r e s s e d by l o w e r i n g the p o t e n t i a l so t h a t o x i d a t i o n o f the m e t a l i s t h e r m o d y n a m i c a l l y impossible; this i s the p r i n c i p l e o f c a t h o d i c p r o t e c t i o n of steel. C a t h o d i c p r o t e c t i o n r e l i e s on e i t h e r an e x t e r n a l s o u r c e o f e l e c t r i c c u r r e n t or c o u p l i n g o f the m e t a l t o be p r o t e c t e d w i t h a more a c t i v e m e t a l (e.g., s t e e l i s p r o t e c t e d by coupling to z i n c ) . W i t h the e x c e p t i o n o f some z i n c - c o n t a i n i n g organic coatings a p p l i e d to s t e e l , c a t h o d i c p r o t e c t i o n i s not a major mechanism o f p r o t e c t i o n by p o l y meric m a t e r i a l s . The mechanism o f a c t i o n o f z i n c c o n t a i n i n g c o a t i n g s has been the s u b j e c t o f some disagreement. P a r t o f the e f f e c t i v e n e s s o f z i n c pigmented c o a t i n g s may be due t o the f o r m a t i o n o f z i n c c o r r o s i o n p r o d u c t s a f t e r an i n i t i a l p e r i o d of true cathodic protection. The a c t i o n o f the z i n c compounds formed has been v a r i o u s l y a s c r i b e d to a b l o c k i n g o f the pores o f the f i l m and t o p a s s i v a t i o n of the s u r f a c e . The e v a l u a t i o n ( u s i n g impedance methods, see a l s o Ref.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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POLYMERIC MATERIALS FOR CORROSION C O N T R O L

10) and m o d i f i c a t i o n o f z i n c pigmented c o a t i n g s a r e d i s c u s s e d i n t h i s volume by Szauer and M i s z c z y k . The chemistry o f z i n c - r i c h and m o d i f i e d z i n c - r i c h c o a t i n g s were a l s o d i s c u s s e d by Fawcett i n the symposium on w h i c h t h i s book i s based. The paper was not made a v a i l a b l e f o r i n c l u s i o n i n t h i s volume, b u t the p r e p r i n t v e r s i o n i s a v a i l a b l e (11). S u p p r e s s i o n o f the a n o d i c r e a c t i o n can be a c h i e v e d by the use o f o x i d i z i n g i n h i b i t o r s ; e s s e n t i a l l y , the i n h i b i t o r i s c a l l e d upon t o form (and m a i n t a i n ) an i m p e r v i o u s and p a s s i v a t i n g o x i d e f i l m on the s u r f a c e o f the m e t a l . I f the o x i d e f i l m formed i s i m p e r f e c t , however, i t i s p o s s i b l e f o r r a p i d l o c a l i z e d c o r r o s i o n t o t a k e p l a c e . I n the case o f aluminum, i n h i b i t o r s can be used t o s t a b i l i z e the o x i d e f i l m a g a i n s t h y d r a t i o n , as d i s c u s s e d i n t h i s volume by M a t i e n z o e t a l . Organic coatings designed f o r c o r r o s i o n p r o t e c t i o n of f e r r o u s metals o f t e n i n c o r p o r a t e m e t a l chromâtes as o x i d i z i n g i n h i b i t o r s . The use o f i n h i b i t o r s i n c o a t i n g s , and the r e q u i r e m e n t s f o r an i d e a l i n h i b i t o r , have been d i s c u s s e d by L e i d h e i s e t h i s volume and elsewher i n h i b i t i n g pigments i s open t o q u e s t i o n : the b i n d e r s used f o r p a i n t s c o n t a i n i n g c o r r o s i o n i n h i b i t i n g pigments must be somewhat w a t e r permeable f o r the pigments t o work, a t l e a s t p a r t i a l l y v i t i a t i n g the b a r r i e r e f f e c t o f the c o a t i n g . A d s o r p t i o n i n h i b i t o r s a c t by f o r m i n g a f i l m on the m e t a l s u r f a c e . The a c t i o n of t r a d i t i o n a l o i l - b a s e d red lead p a i n t formulations presumably i n v o l v e s the f o r m a t i o n o f soaps and the p r e c i p i t a t i o n o f complex f e r r i c s a l t s t h a t r e i n f o r c e the o x i d e f i l m . There has been s u b s t a n t i a l i n t e r e s t i n r e c e n t y e a r s i n development o f replacements f o r lead-based and chromate-based i n h i b i t o r systems. Adsorption i n h i b i t o r s based on polymers have been o f p a r t i c u l a r i n t e r e s t . In t h i s volume, Johnson e t a l . and Eng and I s h i d a d i s c u s s i n h i b i t o r s f o r copper; 2 - u n d e c y l i m i d a z o l e i s shown t o be e f f e c t i v e i n a c i d media, where i t s u p p r e s s e s the oxygen r e d u c t i o n r e a c t i o n a l m o s t c o m p l e t e l y . P o l y v i n y l i m i d a z o l e s are shown t o be e f f e c t i v e o x i d a t i o n i n h i b i t o r s f o r copper a t e l e v a t e d t e m p e r a t u r e s . A l s o i n t h i s volume, Chen d i s c u s s e s the use o f N - ( h y d r o x y a l k y l ) a c r y l a m i d e copolymers i n c o n j u n c t i o n w i t h phosphate-orthophosphate i n h i b i t o r systems f o r c o o l i n g systems. I n many i n d u s t r i a l c o a t i n g a p p l i c a t i o n s , i n o r g a n i c c o n v e r s i o n c o a t i n g s a r e used as s u r f a c e p r e t r e a t m e n t s f o r metals. Such t r e a t ments t y p i c a l l y r e s u l t i n the f o r m a t i o n o f an i n s o l u b l e m e t a l chromate or phosphate on the m e t a l s u r f a c e . The e f f e c t i v e n e s s o f z i n c phosphate c o n v e r s i o n c o a t i n g s has been r e l a t e d t o t h e i r r o l e i n supp r e s s i n g the c a t h o d i c r e d u c t i o n o f oxygen ( 1 4 ) . Bender e t a l . (15) have r e v i e w e d the l i t e r a t u r e e x t e n s i v e l y . The performance o f i n o r g a n i c c o n v e r s i o n c o a t i n g systems i s dependent on b a t h c o m p o s i t i o n and d e p o s i t i o n c o n d i t i o n s , on the i n i t i a l c o n d i t i o n o f the s u b s t r a t e , and on the f i n a l r i n s e o r p o s t - t r e a t m e n t used. I n t h i s volume, L i n d e r t and Maurer d i s c u s s a n o v e l f i l m - f o r m i n g o r g a n i c p o s t - t r e a t m e n t f o r i n o r g a n i c phosphate c o n v e r s i o n coatings. Agarwala discusses a m o d i f i e d chromate c o n v e r s i o n c o a t i n g f o r aluminum. Methods o f E v a l u a t i n g Polymer Performance Performance T e s t s . The underlying goal of c o r r o s i o n t e s t i n g i s g e n e r a l l y the p r e d i c t i o n o f s e r v i c e performance, whether d i r e c t l y f o r a d e v i c e o r system, o r i n d i r e c t l y i n the d e s i g n o r f o r m u l a t i o n o f a

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1.

DICKIE A N D

FLOYD

Overview

5

new m a t e r i a l o r p r o c e s s . The u l t i m a t e t e s t i s performance i n the i n t e n d e d a p p l i c a t i o n ; a c l o s e second i s exposure o f t e s t p a n e l s t o the normal s e r v i c e environment. T e s t i n g based on n a t u r a l exposure i s , o f c o u r s e , time consuming, and a l a r g e number o f l a b o r a t o r y t e s t methods have been d e v e l o p e d t o a s s e s s a s p e c t s o f polymer p r o p e r t i e s and c o r r o s i o n p r o t e c t i o n system performance. Performance t e s t s g e n e r a l l y i n v o l v e exposure o f a system, component, o r t e s t p i e c e t o a s i m u l a t e d or a c c e l e r a t e d c o r r o s i o n environment; e v a l u a t i o n o f r e s u l t s i s t y p i c a l l y based on an assessment o f the type and e x t e n t of corrosion failure. Property t e s t s g e n e r a l l y i n v o l v e the measurement o f a s i n g l e , i s o l a b l e , material property, o r o f a change i n m a t e r i a l p r o p e r t y w i t h exposure t o an a g g r e s s i v e environment. E v a l u a t i o n o f r e s u l t s i s t y p i c a l l y i n terms o f a c o r r e l a t i o n w i t h performance t e s t s o r f i e l d performance d a t a . The fundamental problems o f a c c e l e r a t e d performance t e s t i n g are the s e l e c t i o n o f a p p r o p r i a t e t e s t c o n d i t i o n s , and the d e t e r m i n a t i o n and v a l i d a t i o n o f a c c e l e r a t i o s h o u l d be s e l e c t e d t o a c c e l e r a t and p h y s i c a l p r o c e s s e s e q u a l l y . The d e t e r m i n a t i o n o f a c c e l e r a t i o n f a c t o r s t y p i c a l l y r e q u i r e s , and hence poses the same problems a s , performance t e s t s under n a t u r a l exposure c o n d i t i o n s . Organic coatings a r e commonly e v a l u a t e d u s i n g s a l t w a t e r immers i o n , s a l t f o g o r s p r a y , m o d i f i e d s a l t exposure t e s t s ( e . g . , s a l t fog w i t h added SO2), and v a r i o u s c y c l i c exposure t e s t s . H u m i d i t y exposure and w a t e r immersion, and, f o r many a p p l i c a t i o n s , p h y s i c a l r e s i s t a n c e t e s t s (adhesion, impact r e s i s t a n c e , e t c . ) a r e w i d e l y u s e d p r e l i m i n a r y tests. S t a n d a r d methods f o r most o f t h e s e t e s t s a r e g i v e n i n c o m p i l a t i o n s of standard t e s t s such as the A n n u a l Book o f ASTM Standards ( 1 6 ) . T e s t methods have been e x t e n s i v e l y r e v i e w e d (e.g., 17-23). D e s p i t e t h e i r l o n g and common use, none o f the p o p u l a r l a b o r a t o r y c o r r o s i o n t e s t s are e n t i r e l y s a t i s f a c t o r y . The use o f s t a n d a r d l a b o r a t o r y t e s t s to e s t a b l i s h comparative rankings o f the c o r r o s i o n performance o f d i f f e r e n t m a t e r i a l s i s e s p e c i a l l y s u s c e p t i b l e t o e r r o r . I n one r e c e n t s t u d y , f o r example, the r e s u l t s o f e x t e r i o r exposure and s t a n d a r d l a b o r a t o r y t e s t s were used t o compare c o r r o s i o n i n h i b i t i v e p r i m e r s ( 2 4 ) ; i t was found t h a t s t a n d a r d s a l t f o g t e s t s showed s u b s t a n t i a l d i f f e r e n c e s i n p a i n t performance t h a t were n o t observed under f i e l d exposure c o n d i t i o n s . C y c l i c exposure t e s t s have been p r o p o s e d t h a t i n c o r p o r a t e p e r i o d s o f exposure t o h u m i d i t y , s a l t water immersion, temperature c y c l i n g , and d i r t (25, 2 6 ) ; such t e s t s a r e s u b s t a n t i a l l y more c o m p l i c a t e d t h a n c o n v e n t i o n a l l a b o r a t o r y t e s t s , b u t i n some c a s e s g i v e b e t t e r agreement w i t h c o r r o s i o n performance i n service. C y c l i c exposure t e s t s have a l s o been a p p l i e d t o p r e c o a t e d s t e e l s , b o t h w i t h and w i t h o u t p a i n t c o a t i n g s (27.). The mechanism o f failure i n c y c l i c exposure t e s t i n g has been the s u b j e c t o f some discussion. S t a n d i s h (2£) argues t h a t the c y c l i c t e s t allows corrosion products l i k e those o b s e r v e d i n s e r v i c e t o form under the c o a t i n g ; such c o r r o s i o n p r o d u c t s a r e t y p i c a l l y n o t o b s e r v e d i n s a l t spray. Jones (29) has a l s o d i s c u s s e d the f o r m a t i o n o f a b u l k y o x i d e l a y e r . Elsewhere i n t h i s volume, D i c k i e d i s c u s s e s surface a n a l y t i c a l r e s u l t s on the c y c l i c immersion f a i l u r e o f o r g a n i c c o a t i n g s on phosphated s t e e l s u b s t r a t e s ; d e l a m i n a t i o n o f the c o a t i n g i s found t o be a s s o c i a t e d w i t h d i s s o l u t i o n o f the c o n v e r s i o n c o a t i n g . T h i s r e s u l t i s c o n s i s t e n t w i t h o b s e r v a t i o n s o f v a n O o i j (30) on l o c u s and mechanism o f c o a t i n g d e l a m i n a t i o n on phosphated s t e e l .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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CORROSION C O N T R O L

Electrochemical C h a r a c t e r i z a t i o n Techniques. S i n c e c o r r o s i o n i s an e l e c t r o c h e m i c a l p r o c e s s , i t i s not s u r p r i s i n g that a considerable amount o f work has been r e p o r t e d o v e r the y e a r s on e l e c t r i c a l and e l e c t r o c h e m i c a l t e c h n i q u e s f o r the s t u d y o f the c o r r o s i o n p r o c e s s . L e i d h e i s e r (31) and Szauer (32, 33) have p r o v i d e d good r e v i e w s o f the p r i n c i p a l techniques. W a l t e r has r e c e n t l y p r o v i d e d a r e v i e w o f DC e l e c t r o c h e m i c a l t e s t s f o r p a i n t e d m e t a l s (34)· B o t h AC and DC methods have been employed t o s t u d y a v a r i e t y o f i s s u e s r e l a t e d t o c o r r o s i o n and corrosion protection. DC t e c h n i q u e s are e s p e c i a l l y u s e f u l f o r s t u d y i n g s u b s t r a t e p r o c e s s e s , w h i l e AC impedance t e c h n i q u e s are most u s e f u l f o r s t u d y i n g p r o c e s s e s r e l a t i n g t o c o a t e d s u b s t r a t e s and the performance o f c o a t i n g s . DC t e c h n i q u e s i n c l u d e measurement o f DC r e s i s t a n c e , d e t e r m i n a t i o n o f p o l a r i z a t i o n b e h a v i o r , and measurement o f p o l a r i z a t i o n r e s i s t a n c e . C o a t i n g r e s i s t a n c e has been c o r r e l a t e d w i t h c o r r o s i o n performance by a number o f w o r k e r s . As summarized by L e i d h e i s e r ( 3 1 ) the r e s u l t s o f s e v e r a l independent i n v e s t i g a t i o n below about 10^ ohm/cm^ u n d e r - f i l m c o r r o s i o n . P a r a l l e l DC r e s i s t a n c e measurements on t h i n f i l m m e t a l s u b s t r a t e s have been u s e d t o s t u d y the d e t e r i o r a t i o n o f c o a t e d m e t a l s ; the technique s u c c e s s f u l l y detected the e f f e c t s o f w a t e r a f t e r m i g r a t i o n t o the c o a t i n g / m e t a l i n t e r f a c e (3j>) . P o l a r i z a t i o n methods i n v o l v e c h a n g i n g the p o t e n t i a l ( o r c u r r e n t ) o f a c o r r o d i n g system i n b o t h the a n o d i c and c a t h o d i c d i r e c t i o n s w h i l e monitoring current (or p o t e n t i a l ) . By m a n i p u l a t i n g the r e s u l t i n g i n f o r m a t i o n , an u n d e r s t a n d i n g o f the c o r r o s i o n p r o c e s s can be o b t a i n ed. For example, Beese (36) has used the l i n e a r p o l a r i z a t i o n t e c h n i q u e t o d e v e l o p i n f o r m a t i o n r e l a t e d t o c o r r o s i o n i n b e e r and beverage cans t h a t are c o a t e d w i t h an o r g a n i c enamel. Such i n f o r m a t i o n was u l t i m a t e l y employed t o develop improved c o a t i n g s f o r the c o n t a i n e r . G r o s e c l o s e e t a l . (37) employed an a n o d i c p o l a r i z a t i o n t e c h n i q u e t o q u a n t i f y the q u a l i t y and v a r i a b i l i t y o f b o t h c o l d r o l l e d and hot rolled steels. This i n f o r m a t i o n was used t o a c c u r a t e l y p r e d i c t the r e l a t i v e s a l t s p r a y performance o f the s u b s e q u e n t l y c o a t e d s t e e l s , and evaluate the e f f e c t o f a b r a s i v e p o l i s h i n g and s a n d b l a s t i n g o f the substrate. The p o l a r i z a t i o n r e s i s t a n c e method w i d e l y used f o r s t u d y i n g m e t a l c o r r o s i o n has a l s o been a p p l i e d t o p a i n t e d m e t a l s . In p r i n c i p l e , the p o l a r i z a t i o n r e s i s t a n c e i s i n v e r s e l y p r o p o r t i o n a l to the c o r r o s i o n r a t e o f the m e t a l . For c o a t e d m e t a l s , the method i s c o m p l i c a t e d by the c o r r e c t i o n f o r ohmic p o t e n t i a l drop, d i f f u s i o n l i m i t a t i o n s , and changes i n f i l m p r o p e r t i e s under the a p p l i e d potent i a l (32). AC t e c h n i q u e s are h i g h l y v a r i e d , b u t t e n d t o converge upon the use o f impedance s p e c t r o s c o p y . I n r e c e n t p u b l i c a t i o n s , Hubrecht e t a l . ( 3 8 ) , M a n s f i e l d and K e n d i g ( 3 9 ) , and K e n d i g e t a l . (40) have r e v i e w e d the a p p l i c a t i o n o f impedance s p e c t r o s c o p y t o c o a t i n g systems. By e x a m i n i n g the AC impedance o f the c o a t e d system as a f u n c t i o n o f f r e q u e n c y , u s e f u l i n f o r m a t i o n i s o b t a i n e d r e g a r d i n g b o t h the b a r r i e r p r o p e r t i e s o f the c o a t i n g and the c o r r o s i o n s u s c e p t i b i l i t y o f the substrate. Under p r o p e r c o n d i t i o n s , i n f o r m a t i o n can be e x t r a c t e d r e l a t i n g t o the i n t e r f a c i a l l a y e r as w e l l . I n f o r m a t i o n can a l s o be e x t r a c t e d r e l a t i v e t o the p r e s e n c e o f w a t e r and i o n s i n p a i n t f i l m s as shown by L i n d q v i s t ( 4 1 ) . A good example o f the l a t t e r has been g i v e n by Padget and M o r e l a n d ( 4 2 ) . I n most c a s e s , b a r r i e r p r o p e r t i e s o f c o a t i n g s are u l t i m a t e l y found t o be h i g h l y i m p o r t a n t t o the p r e v e n t i o n

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1.

DICKIE A N D

FLOYD

Overview

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o f c o r r o s i o n o f the s u b s t r a t e . I n t h i s p a r t i c u l a r c a s e , i t was a l s o shown t h a t the p r e s e n c e o f a b a r r i e r f i l m seems t o augment the f o r m a t i o n o f a s t r o n g l y p a s s i v e l a y e r a t the i n t e r f a c e between the c o a t i n g and the s u b s t r a t e . An i n t e r e s t i n g a p p l i c a t i o n o f e l e c t r o c h e m ical techniques i n v o l v e s the c h a r a c t e r i z a t i o n o f z i n c r i c h p a i n t s . F e r n a n d e z - P r i n i and K a p i s t a (43) and L i n d q v i s t e t a l . (44) d e s c r i b e b o t h DC and AC techniques f o r c h a r a c t e r i z i n g z i n c r i c h coatings i n such a way t h a t subsequent s a l t spray t e s t i n g i s r a t i o n a l i z e d . The p r e s e n t volume c o n t a i n s a number o f papers r e l a t i n g t o the i s s u e o f e l e c t r o c h e m i c a l t e s t i n g . M o r c i l l o e t a l . compare the r e s u l t s o f AC impedance measurements w i t h a c c e l e r a t e d and o u t d o o r exposure test results. V i j a y a n r e p o r t s the use o f AC impedance t e s t i n g to s t u d y the e f f e c t s o f v a r i o u s components o f the p h o s p h a t i n g p r e t r e a t ment p r o c e s s , p a i n t t h i c k n e s s , and t e s t v a r i a b l e s on subsequent s a l t s p r a y r e s u l t s . M o r e l a n d and Padget update t h e i r work on AC impedance as i t p e r t a i n s t o the study o f the p a s s i v e l a y e r w h i c h forms between a b a r r i e r c o a t i n g and a s t e e d e s c r i b e t h e i r use o f D s a l t s p r a y d a t a as a f u n c t i o n o f p o s t - b a k e temperature o f UV c u r e d coatings. Eden and co-workers d e s c r i b e their studies involving e l e c t r o c h e m i c a l n o i s e measurements t o s t u d y c o r r o s i o n as i t progresses. The a u t h o r s ' p o s i t i o n i s t h a t the c o a t i n g b r e a k d o w n / f a i l u r e on a s t e e l s u b s t r a t e i s accompanied by a change i n the e l e c t r o c h e m i c a l noise s i g n a l , w h i c h g i v e s a r a p i d i n d i c a t i o n o f the s t a t e o f the coating. Lomas e t a l . d e s c r i b e t h e i r n o v e l work w i t h harmonic a n a l y s i s , combined w i t h AC impedance t e s t i n g i n an attempt t o d e t e c t c o r r o s i o n of t h i c k l y coated substrates. B a r r i e r Aspects of Corrosion

Control

The r e l a t i v e importance o f the b a r r i e r f u n c t i o n o f o r g a n i c c o a t i n g s i n c o r r o s i o n p r o t e c t i o n has been debated f o r y e a r s . I t i s clear that, i f a m e t a l l i c s u b s t r a t e c o u l d be c o m p l e t e l y i s o l a t e d from i t s e n v i r o n ment, no c o r r o s i o n would o c c u r . The degree t o w h i c h a p r o t e c t i o n system b a r s oxygen, w a t e r , and i o n s from the s u b s t r a t e would seem l i k e l y t o be a measure o f the e f f e c t i v e n e s s o f the system i n p r e v e n t ing c o r r o s i o n . H i s t o r i c a l l y , a number o f d i f f e r e n t t h e o r i e s r e g a r d i n g the r o l e o f the b a r r i e r f u n c t i o n i n c o r r o s i o n p r o t e c t i o n have emerged. Studies by Mayne and h i s co-workers (45-49), Bacon e t a l . ( 5 0 ) , and Cherry (51) i n d i c a t e d t h a t n e i t h e r the p e r m e a b i l i t y o f w a t e r nor the permeab i l i t y o f oxygen c o u l d be the r a t e d e t e r m i n i n g factor i n corrosion c o n t r o l by organic c o a t i n g s , s i n c e n e i t h e r was s u f f i c i e n t l y low to p r o v i d e e f f e c t i v e i s o l a t i o n o f the m e t a l l i c s u b s t r a t e . P r o t e c t i o n was a t t r i b u t e d t o the h i g h e l e c t r i c a l r e s i s t a n c e and low i o n i c p e r m e a b i l i t y o f c o a t i n g s t h a t a f f o r d e d good p r o t e c t i o n . To v a r y i n g e x t e n t s , G u r u v i a h ( 5 2 ) , Bauman ( 5 3 ) , and K r e s s e (54) d i s a g r e e d w i t h the e a r l i e r w o r k e r s r e g a r d i n g the l i m i t s o f oxygen and w a t e r p e r m e a b i l i t y i n f i l m s . Haagen.and Funke (55, 56) agreed w i t h G u r u v i a h and Bauman t h a t oxygen p e r m e a b i l i t y was the c o n t r o l l i n g f a c t o r ; t h e y o b s e r v e d t h a t w a t e r p e r m e a b i l i t y was the d e t e r m i n i n g f a c t o r f o r the l o s s o f adhes i o n , but not f o r c o r r o s i o n . More r e c e n t l y , workers i n the f i e l d have r e c o g n i z e d the p r o b a b l e need f o r a m u l t i p l e parameter model t o u n d e r s t a n d the c o r r o s i o n p r o t e c t i o n process. Funke (57) p r o p o s e d a model b a s e d on water

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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p e r m e a b i l i t y , oxygen p e r m e a b i l i t y , and a d h e s i o n under h i g h h u m i d i t y c o n d i t i o n s . The model was used t o r a t i o n a l i z e the r a n k o r d e r o f s a l t spray r e s u l t s o f seven d i f f e r e n t e l e c t r o c o a t i n g systems. No mathemati c a l t r e a t m e n t o f the d a t a was offered. F l o y d e t a l . (58, 59) i n t r o d u c e d a m a t h e m a t i c a l a n a l y s i s o f a wide range o f p r o p e r t i e s i n c o m p a r i s o n w i t h s a l t s p r a y r e s u l t s . A b a r r i e r mechanism f o r c o r r o s i o n c o n t r o l was postulated. This technique was a l s o a p p l i e d t o F u n k e s e a r l i e r d a t a , w i t h a s i m i l a r r e s u l t . The model was f u r t h e r e l a b o r a t e d t o take i n t o account the e x i s t e n c e o f an e l e c t r o c h e m i c a l component i n the model as a back-up t o the p r i m a r y b a r r i e r component. F l o y d e t a l . f u r t h e r o b s e r v e d t h a t no adequate c h a r a c t e r i z a t i o n o f t h i s e l e c t r o c h e m i c a l i n t e r a c t i o n between p a i n t and s u b s t r a t e e x i s t e d . The p e r m e a b i l i t y o f polymer systems i s i n f l u e n c e d by the propert i e s o f the polymer, by the p r e s e n c e o f pigments o r f i l l e r s , and by the i n t e r a c t i o n between polymer and f i l l e r s . Hulden and Hansen (60) have r e c e n t l y r e v i e w e d w a t e r p e r m e a t i o n i n c o a t i n g s R e g u l a r i t y of structure, c r y s t a l l i n i t y g i v e low p e r m e a b i l i t y . r e s u l t i n g i n reduced p e r m e a b i l i t y , b u t r e s u l t s presented i n this volume by M u i z e b e l t and H e u v e l s l a n d suggest t h a t c r o s s - l i n k d e n s i t y may be i r r e l e v a n t i n t h i s r e s p e c t . As Funke n o t e s e l s e w h e r e i n t h i s volume, some o f the f a c t o r s t h a t c o n t r i b u t e t o low p e r m e a b i l i t y may i n t e r f e r e w i t h a d h e s i o n ; i n p a r t i c u l a r , p o l a r f u n c t i o n a l groups appear t o be e s s e n t i a l t o a c h i e v i n g good a d h e s i o n , b u t are l i k e l y t o i n c r e a s e p e r m e a b i l i t y and c o n t r i b u t e t o w a t e r s e n s i t i v i t y . P i g m e n t a t i o n can have a p r o f o u n d e f f e c t on p e r m e a b i l i t y . The use o f b a r r i e r pigments has been s u g g e s t e d as an a l t e r n a t i v e t o the use o f a c t i v e i n h i b i t i v e pigments, many o f w h i c h are o b j e c t i o n a b l e on e n v i r o n m e n t a l grounds ( 1 3 ) . F l a k e shaped pigments a r e p a r t i c u l a r l y e f f e c t i v e , b u t pigment geometry i s not the o n l y i m p o r t a n t f a c t o r . If w a t e r can accumulate a t the p i g m e n t - b i n d e r i n t e r f a c e , as e v i d e n t l y happens i n the case o f m i c a , w a t e r a b s o r p t i o n tends t o i n c r e a s e w i t h pigment volume c o n c e n t r a t i o n and, although permeability i s s t i l l r e d u c e d by i n c o r p o r a t i o n o f the pigment, the e f f e c t i s much s m a l l e r t h a n w i t h , f o r example, comparable l o a d i n g s o f aluminum f l a k e ( 1 3 ) . The i n f l u e n c e o f i n e r t pigments on p e r m e a b i l i t y and corrosion protect i v e p r o p e r t i e s has been r e v i e w e d b r i e f l y by Hulden and Hansen ( 6 0 ) , and has been d i s c u s s e d i n a number o f papers by K r e s s e ( e . g . , 61, 62). The mechanism o f a c t i o n o f i n e r t b a r r i e r pigments i s commonly s t a t e d t o be t o i n c r e a s e the d i f f u s i o n pathway t o the s u b s t r a t e ; i t i s a l s o p o s s i b l e t h a t pigments may t e n d t o b l o c k o r p r e v e n t the f o r m a t i o n o f pathways f o r d i r e c t i o n i c c o n d u c t i o n t o the s u b s t r a t e . 1

I n t e r f a c i a l and A d h e s i o n A s p e c t s o f C o r r o s i o n

Control

B a s i c Mechanisms o f A d h e s i o n : A c i d - B a s e I n t e r a c t i o n s . The understandi n g o f polymer a d h e s i o n has been g r e a t l y advanced i n r e c e n t y e a r s by the r e c o g n i t i o n o f the c e n t r a l r o l e o f a c i d - b a s e i n t e r a c t i o n s . The c o n c e p t o f an a c i d was broadened by G. N. Lewis t o i n c l u d e those atoms, m o l e c u l e s , o r i o n s i n w h i c h a t l e a s t one atom has a v a c a n t o r b i t a l i n t o w h i c h a p a i r o f e l e c t r o n s can be a c c e p t e d . S i m i l a r l y , a base i s r e g a r d e d as an e n t i t y w h i c h p o s s e s s e s a p a i r o f e l e c t r o n s which are not already i n v o l v e d i n a c o v a l e n t bond. The p r o d u c t s o f acid-base i n t e r a c t i o n s have been c a l l e d c o o r d i n a t i o n compounds, adducts, acid-base complexes, and o t h e r such names. The concept t h a t

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1.

DICKIE A N D

FLOYD

Overview

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a c i d s and bases v a r y i n t h e i r a b i l i t y t o i n t e r a c t w i t h one a n o t h e r was i n t r o d u c e d by P e a r s o n i n 1968 (63, 64). He i n t r o d u c e d the concept o f p o l a r i z a b i l i t y o f the a c i d o r base u n i t , d e s c r i b i n g i t on a h a r d - s o f t scale. Hard a c i d s a r e t h o s e o f h i g h e l e c t r o n e g a t i v i t y and low polarizability. S o f t a c i d s , i n c o n t r a s t , a r e l a r g e i n s i z e , have h i g h p o l a r i z a b i l i t y , and low e l e c t r o n e g a t i v i t y . For the purposes o f t h i s book, i t i s i m p o r t a n t t o remember t h a t h a r d a c i d s r e a c t most r e a d i l y and form the s t r o n g e s t complexes w i t h h a r d b a s e s , w h i l e s o f t a c i d s r e a c t most r e a d i l y and form the s t r o n g e s t complexes w i t h s o f t bases. Drago and co-workers i n t r o d u c e d an e m p i r i c a l c o r r e l a t i o n t o c a l c u l a t e the e n t h a l p y o f adduct f o r m a t i o n o f Lewis a c i d s and bases ( 6 5 ) . I n 1971, he and h i s co-workers expanded the concept t o a c o m p u t e r - f i t t e d s e t o f parameters t h a t a c c u r a t e l y c o r r e l a t e d over 200 e n t h a l p i e s o f adduct f o r m a t i o n ( 6 6 ) . These parameters were t h e n used t o p r e d i c t o v e r 1200 e n t h a l p i e s o f i n t e r a c t i o n . The parameters Ε and C a r e l o o s e l y i n t e r p r e t e d t o r e l a t e t o the degree o f e l e c t r o s t a t i c and c o v a l e n t n a t u r e o f the i n t e r a c t i o model was used t o g e n e r a l i z h a r d - s o f t a c i d - b a s e model and r e n d e r i t more q u a n t i t a t i v e l y a c c u r a t e . I n 1975, Sorensen (67) used the a c i d - b a s e i n t e r a c t i o n concept t o r a t i o n a l i z e c o l o r s t r e n g t h , g l o s s , and f l o c c u l a t i o n p r o p e r t i e s o f c o a t i n g systems h a v i n g b i n d e r s o f d i f f e r i n g a c i d - b a s e c h a r a c t e r i s t i c s . Anomalies t h a t appear when u s i n g s o l u b i l i t y parameter c o n c e p t s were s u c c e s s f u l l y e x p l a i n e d by the a c i d - b a s e c o n c e p t . Drago e t a l . (68) were s t a r t i n g t o address the i s s u e o f c o r r e c t i o n s t o the s o l u b i l i t y parameter concept u s i n g t h i s t e c h n i q u e a t about the same t i m e . A good r e v i e w o f the s u b j e c t was w r i t t e n i n 1978 by J e n s e n (69). The a p p l i c a t i o n o f a c i d - b a s e i n t e r a c t i o n s t o the phenomenon o f a d h e s i o n was d i s c u s s e d by J e n s e n a t an ACS meeting i n 1981 (70). Fowkes and co-workers had a l r e a d y been d i s c u s s i n g the c o m p e t i t i v e a b s o r p t i o n o f polymers onto pigment s u r f a c e s i n the c o n t e x t o f a c i d - b a s e i n t e r a c ­ t i o n s by t h i s time (e. g., 7 1 ) . Manson (72) expanded the concept t o the s o l i d s t a t e by o b s e r v i n g t h a t the s t r e n g t h o f composite m a t e r i a l s a l s o depended upon the a c i d - b a s e i n t e r a c t i o n between c o n t i n u o u s and d i s p e r s e d phases. More d i r e c t l y , Vanderhoff et a l . (73) a d d r e s s e d the i s s u e o f a d h e s i o n o f polymeric materials to corroded s t e e l . They s y n t h e s i z e d e i g h t c o r r o s i o n p r o d u c t s o f i r o n , and used the i n t e r a c t i o n scheme d e v e l o p e d by Fowkes and Manson f i r s t t o c h a r a c t e r i z e t h e i r o n c o r r o s i o n p r o d u c t s as Lewis a c i d s o r bases and t h e n t o s e l e c t polymer v e h i c l e s f o r p r a c t i c a l c o a t i n g systems. Such r e s u l t s were employed t o enhance the a d h e s i o n o f epoxy systems t o s u b s t r a t e s w h i c h were p r e d o m i n a n t l y i r o n oxide i n nature. A good o v e r v i e w o f t h e s e i s s u e s was p r e s e n t e d by Fowkes i n 1983 ( 7 4 ) . Fowkes and co-workers a l s o c l e a r l y demonstrated t h a t the p h y s i c a l i n t e r a c t i o n o f polymers w i t h n e i g h b o r i n g m o l e c u l e s was d e t e r m i n e d by o n l y two k i n d s o f i n t e r a c t i o n s : London d i s p e r s i o n f o r c e s and Lewis acid-base i n t e r a c t i o n s ( 7 5 ) . C a l c u l a t i o n s b a s e d on t h i s concept were shown t o c o r r e c t many o f the problems i n h e r e n t i n the s o l u b i l i t y approach. They were a l s o a b l e t o use the c o n c e p t t o s t u d y the d i s t r i b u t i o n o f molar h e a t s o f a b s o r p t i o n o f v a r i o u s polymers onto f e r r i c o x i d e s , and t h e r e b y more a c c u r a t e l y d e s c r i b e d the r e q u i r e m e n t s f o r adequate a d h e s i o n t o s t e e l s u b s t r a t e s (7j>) . I n the symposium on w h i c h t h i s book i s based, Fowkes summarized work showing t h a t the p o l a r i n t e r a c t i o n s between polymers and m e t a l s u r f a c e s t h a t are

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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important t o a d h e s i o n are e n t i r e l y o f Lewis a c i d - b a s e c h a r a c t e r . C a l o r i m e t r i c and i n f r a - r e d s p e c t r o s c o p i c methods f o r d e t e r m i n i n g the Ε and C c o n s t a n t s f o r polymers and m e t a l o x i d e s were p r e s e n t e d . The f u l l m a n u s c r i p t was not made a v a i l a b l e f o r p u b l i c a t i o n i n t h i s volume, and the r e a d e r i s t h e r e f o r i n v i t e d t o c o n s u l t the p r e p r i n t m a n u s c r i p t (77). Role of Adhesion i n Corrosion P r o t e c t i o n . Many o f the t h e o r i e s r e g a r d i n g the mechanism o f c o r r o s i o n f a i l u r e suggest t h a t the l o s s o f a d h e s i o n p r e c e d e s the o n s e t o f c o r r o s i o n , and i s t h e r e f o r e o f c r i t i c a l importance i n u n d e r s t a n d i n g the p r o c e s s ; Parker and G e r h a r t (78) c o n s i d e r e d a d h e s i o n t o be c r u c i a l t o c o r r o s i o n performance. For o r g a n i c c o a t i n g s , the s t r e n g t h o f the a d h e s i v e bond between c o a t i n g and s u b s t r a t e does not appear t o be the c r i t i c a l i s s u e ; what does appear t o be i m p o r t a n t i s t h a t d u r i n g and a f t e r e n v i r o n m e n t a l exposure the c o a t i n g s h o u l d be a b l e t o w i t h s t a n d the forces a p p l i e d to i t i n i t s intended application a d v e r s e l y a f f e c t e d by exposur (79) f o u n d t h a t the a d h e s i v e s t r e n g t h o f a wide v a r i e t y o f c o a t i n g s dropped from 20 t o 40 MPa t o 5 t o 15 MPa i n a d i r e c t p u l l - o f f t e s t a f t e r exposure t o humid environments. The i n i t i a l ( d r y ) bond s t r e n g t h was n o t a good p r e d i c t o r o f performance. Haagen and Funke (56) o b s e r v e d t h a t good p r o t e c t i o n was o b t a i n e d i f wet a d h e s i o n was good, even i f the p a i n t was h i g h l y w a t e r permeable. The importance o f a d h e s i o n t o c o r r o s i o n p r o t e c t i o n i s f u r t h e r d i s c u s s e d i n t h i s volume and elsewhere (80) by Funke. Mechanisms o f a d h e s i o n l o s s under v a r i o u s exposure c o n d i t i o n s have been e x t e n s i v e l y s t u d i e d . A survey i s given elsewhere i n t h i s volume by L e i d h e i s e r , and s p e c i f i c examples o f a d h e s i o n l o s s are d i s c u s s e d i n d e t a i l by T h o r n t o n e t a l . , Maeda e t a l . , and Troyk e t a l . , among o t h e r s . A c o u s t i c e m i s s i o n has been u s e d t o s t u d y c o a t i n g a d h e s i o n and the e f f e c t s o f w a t e r immersion on c o a t i n g s on w a t e r (see, - g-» 81-83)· I n t h i s volume, C a l l o w and S c a n t l e b u r y d i s c u s s the p o s s i b i l i t y of using acoustic emission as a m o n i t o r i n g t o o l to i n v e s t i g a t e c o r r o s i o n - i n d u c e d debonding. Modern s u r f a c e a n a l y t i c a l methods have l e d t o much more d e t a i l e d u n d e r s t a n d i n g o f the i n t e r f a c i a l c h e m i s t r y o f a d h e s i o n l o s s p r o c e s s e s . Surface a n a l y t i c a l studies of i n t e r f a c i a l chemistry are reviewed i n t h i s volume by D i c k i e ; i n t h i s paper, as i n a r e c e n t paper by C a s t l e and Watts ( 8 4 ) , i t i s c o n c l u d e d t h a t no s i n g l e c h e m i c a l mechanism a d e q u a t e l y a c c o u n t s f o r a l l o f the o b s e r v e d b e h a v i o r . I n the s i m p l e s t c a s e s , l o s s o f a d h e s i o n appears t o i n v o l v e d i s p l a c e m e n t by water. D i s p l a c e m e n t o f c o a t i n g s by c o r r o s i o n g e n e r a t e d h y d r o x i d e , chemical degradation o f the o r g a n i c c o a t i n g , and chemical a t t a c k on the underlying substrate surface or conversion c o a t i n g have a l s o been o b s e r v e d . F u r t h e r examples o f i n t e r f a c i a l s t u d i e s are g i v e n i n t h i s volume by Maeda e t a l . , and a d i s c u s s i o n o f the r e a c t i o n s i n conver­ s i o n c o a t i n g s d u r i n g c o r r o s i o n has been g i v e n by van O o i j ( 3 0 ) . A r e l a t e d but l i t t l e s t u d i e d a r e a o f a d h e s i o n and c o r r o s i o n p r o t e c t i o n i n v o l v e s the c h e m i c a l e f f e c t s o f m e t a l s u b s t r a t e s on c o a t i n g s and o t h e r p o l y m e r i c m a t e r i a l s and conversely of polymeric m a t e r i a l s on m e t a l s . I n the c u r i n g o f c e r t a i n a i r - o x i d i z i n g c o a t i n g s on s t e e l , f o r example, r e d u c t i o n o f f e r r i c t o f e r r o u s s p e c i e s i n the s u r f a c e m e t a l o x i d e , s u b s t a n t i a l t h i n n i n g o f the o x i d e , and o x i d a t i o n o f the c o a t i n g m a t e r i a l have been r e p o r t e d t o o c c u r i n the i n t e r f a c i a l e

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region. These phenomena have been s t u d i e d by i n f r a - r e d and X-ray photoelectron spectroscopic techniques, and are d i s c u s s e d i n t h i s volume by Nguyen and B y r d and by D i c k i e . The s t o r e d components used to make p o l y u r e t h a n e foams are s u b j e c t t o l o n g term c h e m i c a l degradat i o n ; the d e g r a d a t i o n p r o d u c t s have been a s s o c i a t e d w i t h the c o r r o s i o n o f s t o r a g e c o n t a i n e r s . Wischmann d i s c u s s e s the problem, and suggests f o r m u l a t i o n changes f o r improved performance. Adhesives and s e a l e r s can be an i m p o r t a n t part of a t o t a l c o r r o s i o n p r o t e c t i o n system. S t r u c t u r a l bonding procedures and adhesives f o r aluminum, polymer composites, and t i t a n i u m are w e l l e s t a b l i s h e d i n the aerospace i n d u s t r y . S t r u c t u r a l b o n d i n g o f s t e e l i s g a i n i n g i n c r e a s i n g prominence i n the a p p l i a n c e and a u t o m o t i v e i n d u s tries. The d u r a b i l i t y o f a d h e s i v e bonds has been d i s c u s s e d by a number o f a u t h o r s ( s e e , e.g., 85)· The e f f e c t s o f a g g r e s s i v e e n v i r o n ments on a d h e s i v e bonds are o f p a r t i c u l a r concern. Minford (86) has p r e s e n t e d a comparative e v a l u a t i o n o f aluminum j o i n t s i n s a l t water exposure; Smith (87) ha hydrothermal s t r e s s ; Drai p r e s e n t e d r e s u l t s on the e f f e c t s o f w a t e r on performance o f v a r i o u s adhesive/substrate combinations. I n t h i s volume, the d u r a b i l i t y o f a d h e s i v e bonds i n the p r e s e n c e o f water and i n c o r r o s i v e environments i s d i s c u s s e d by Matienzo et a l . , G o s s e l i n , and Holubka e t a l . The e f f e c t s o f a g g r e s s i v e environments on a d h e s i v e l y bonded s t e e l s t r u c t u r e s have a number o f f e a t u r e s i n common w i t h t h e i r e f f e c t s on c o a t e d s t e e l , b u t the m e c h a n i c a l r e q u i r e m e n t s p l a c e d on a d h e s i v e bonds add an a d d i t i o n a l l e v e l of complication. E f f e c t s o f Polymer C o m p o s i t i o n

on C o r r o s i o n C o n t r o l

Polymer c o m p o s i t i o n poses not one b u t many c r i t i c a l i s s u e s f o r the development o f m a t e r i a l s f o r c o r r o s i o n c o n t r o l . As outlined in p r e v i o u s s e c t i o n s o f t h i s c h a p t e r , the elements o f m o l e c u l a r d e s i g n f o r good a d h e s i o n , good b a r r i e r p r o p e r t i e s , and e f f e c t i v e use o f pigments i n o r g a n i c c o a t i n g s are o f t e n i n c o n f l i c t . There does not appear t o be a u n i f y i n g t h e o r e t i c a l b a s i s on w h i c h these c o n f l i c t i n g f a c t o r s can be r e s o l v e d , and an e m p i r i c a l b a l a n c i n g o f p r o p e r t i e s remains an e s s e n t i a l p a r t o f new product development f o r c o r r o s i o n control. I t i s not s u r p r i s i n g t h a t the d e t a i l s o f c o m p o s i t i o n c r i t i c a l t o performance o f t e n remain p r o p r i e t a r y o r appear o n l y i n the patent l i t e r a t u r e . In a d d i t i o n t o the customary d e s i r e f o r improved m a t e r i a l p e r f o r m a n c e , the development o f new m a t e r i a l s has, i n r e c e n t y e a r s , been shaped by the demand f o r n o n - p o l l u t i n g o r e c o l o g i c a l l y n e u t r a l m a t e r i a l s . R e s t r i c t i o n s on s o l v e n t e m i s s i o n s from i n d u s t r i a l and maintenance p a i n t s , and l i m i t a t i o n s on l e a d - b a s e d and chromate c o r r o s i o n - i n h i b i t i v e pigments has had a major impact on c o r r o s i o n p r o t e c t i v e m a t e r i a l technology. I n the f i e l d o f o r g a n i c c o a t i n g s , t h e r e has been major emphasis on water-borne and s o - c a l l e d h i g h s o l i d s c o a t i n g s . H i l l and Wicks (90) have d i s c u s s e d d e s i g n c r i t e r i a f o r h i g h s o l i d s c o a t i n g s ; a r e c e n t book on r e a c t i v e o l i g o m e r s discusses a number o f polymer systems o f i n t e r e s t i n h i g h s o l i d s c o a t i n g s ( 9 1 ) . A v a r i e t y o f w a t e r - s o l u b l e and w a t e r - d i s p e r s i b l e r e s i n s have been d e s c r i b e d i n the l i t e r a t u r e ( e . g . , 90-96); the T e c h n i c a l Committee o f the New England S o c i e t y f o r C o a t i n g s Technology has p u b l i s h e d a s e r i e s o f a r t i c l e s on the d e s i g n o f waterborne c o a t i n g s f o r the c o r r o s i o n

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p r o t e c t i o n o f s t e e l (97-99). The development and commercial implem e n t a t i o n o f a n o d i c and, subsequently, of cathodic e l e c t r o d e p o s i t i o n coatings binders f o r e l e c t r o d e p o s i t i o n have made p o s s i b l e major improvements i n the c o r r o s i o n p r o t e c t i o n o f a p p l i a n c e s and motor v e h i c l e s . The c h e m i s t r y o f b i n d e r s f o r e l e c t r o d e p o s i t i o n has been r e v i e w e d by Schenk e t a l . (100) and by Kordomenos and Nordstrom (101): the c a t h o d i c e l e c t r o d e p o s i t i o n p r o c e s s has been d i s c u s s e d by Wismer e t a l . (102). I n the p r e s e n t volume, s e v e r a l papers d e a l w i t h unique m a t e r i a l s or a p p l i c a t i o n s : S c h r e i b e r d e s c r i b e s work on p l a s m a - d e p o s i t e d f i l m s from o r g a n o - s i l i c o n e and i n o r g a n i c (SiN) s t a r t i n g m a t e r i a l s . M o r e l a n d and Padget d i s c u s s studies of a c h l o r i n e - c o n t a i n i n g v i n y l a c r y l i c copolymer t h a t i s a p p l i e d as an a c i d i c aqueous f o r m u l a t i o n and t h a t promotes i n s i t u f o r m a t i o n o f a p r o t e c t i v e f i l m . White and L e i d h e i s e r d i s c u s s c o a t i n g r e s i n s f o r the p r o t e c t i o n o f s t e e l exposed t o s u l f u r i c a c i d ; Hojo e t a l . , the b e h a v i o r o f epoxy and polyester resins i n alkaline solution. Dreyfu designed to p r o t e c t glas discuss structure-property relationships i n tin-based a n t i - f o u l i n g paints. The p r o t e c t i o n o f m i c r o e l e c t r o n i c s from the e f f e c t s o f h u m i d i t y and c o r r o s i v e environments p r e s e n t s e s p e c i a l l y demanding r e q u i r e m e n t s on p r o t e c t i v e c o a t i n g s and e n c a p s u l a n t s . S i l i c o n e polymers, epoxies, and i m i d e r e s i n s are among the m a t e r i a l s t h a t have been used f o r the encapsulation of microelectronics. The p h y s i o l o g i c a l environment t o which implanted medical e l e c t r o n i c devices are exposed poses an e s p e c i a l l y c h a l l e n g i n g p r o t e c t i o n problem. I n t h i s volume, Troyk e t a l . o u t l i n e the demands p l a c e d on such systems i n m e d i c a l a p p l i c a t i o n s , and d i s c u s s the p r o p e r t i e s o f a v a r i e t y o f s i l i c o n e - b a s e d encapsulants. Critical

Issues

A t o p i c a l symposium p r o v i d e s a forum f o r the r e v i e w and u p d a t i n g o f work i n a g i v e n field, and provides an o p p o r t u n i t y to i d e n t i f y c r i t i c a l issues. The e d i t o r s o f t h i s volume w o u l d l i k e t o suggest t h a t the f o l l o w i n g i s s u e s are among those n e e d i n g a d d i t i o n a l s t u d y : Systems. C o r r o s i o n i s u s u a l l y s t u d i e d i n an i s o l a t e d f a s h i o n i n the l a b o r a t o r y , but i n p r a c t i c e i s c l e a r l y the r e s u l t o f i n t e r a c t i n g systems i n the environment. S t u d i e s need t o be c o n d u c t e d on the way i n w h i c h the component p a r t s o f c o r r o d i n g systems i n t e r a c t under a c t u a l e n v i r o n m e n t a l c o n d i t i o n s , and on the way i n w h i c h the compone n t s o f the environment i n t e r a c t w i t h the t o t a l c o r r o d i n g system. T h i s would suggest not o n l y d e s i g n work f o r c o r r o s i o n p r o t e c t i o n systems, b u t a l s o a d d i t i o n a l work on the s e n s i n g and m o n i t o r i n g o f c o r r o s i o n i n r e a l h o s t i l e environments. Methods. To observe t h a t c o r r o s i o n t e s t i n g i n the l a b o r a t o r y f r e q u e n t l y f a i l s t o p r e d i c t what happens i n r e a l - w o r l d environments i s to admit t h a t the mechanisms c o n t r o l l i n g c o r r o s i o n i n such environments are not u n d e r s t o o d , even a t t h i s l a t e date o f s t u d y . Mechanism-based t e s t methods f o r m o n i t o r i n g c o r r o s i o n are needed t h a t w i l l p r o v i d e r e l i a b l e and r a p i d p r e d i c t i o n of s e r v i c e l i f e f o r corrosion-suscept i b l e systems. I t i s e x p e c t e d t h a t s t a t i s t i c a l a n a l y s i s w i l l p l a y a

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l a r g e r o l e i n any such e f f o r t , s i n c e the f a i l u r e modes o b s e r v e d o f t e n vary widely w i t h i n a s t a t i s t i c a l d i s t r i b u t i o n . Such a r e c o g n i t i o n has l e d t o the development o f s t a t i s t i c a l methods f o r the d e s c r i p t i o n o f m e c h a n i c a l f a i l u r e o f m a t e r i a l s , and i t i s s u g g e s t e d t h a t a s i m i l a r e f f o r t would b e a r f r u i t i n the a n a l y s i s o f c o r r o s i o n phenomena. P a i n t s . E p o x i e s and c a t h o d i c e l e c t r o c o a t s r e p r e s e n t major advances i n the f i e l d o f c o r r o s i o n c o n t r o l by o r g a n i c c o a t i n g s . The performance of these coatings represents a p l a t e a u w h i c h has not been d e p a r t e d from i n over a decade. A g r e a t d e a l o f a t t e n t i o n has been devoted t o i n t e r f a c i a l p r o c e s s e s i n c o r r o s i o n over the l a s t several years, y e t c o n t r o l o f t h e s e p r o c e s s e s remains an e l u s i v e g o a l . I t i s c o n c e i v a b l e t h a t , i f means can be found t o c o n t r o l i n t e r f a c i a l f a i l u r e p r o c e s s e s , a new g e n e r a t i o n o f c o a t i n g s c a n be d e v e l o p e d t h a t w i l l establish a considerably higher p l a t e a u o f performance. C o n v e r s i o n c o a t i n g s and s u r f a c e t r e a t m e n t s may p l a y a v i t a l r o l e i n t h i s development Plastics. Part o f the substrates f o r m e t a l s c a n be a t t r i b u t e d t o a d e s i r e t o a v o i d the p r o c e s s o f m e t a l l i c c o r r o s i o n and subsequent f a i l u r e . Relatively little a t t e n t i o n has been c a l l e d t o the p o s s i b l e f a i l u r e modes o f p l a s t i c s under environments c o n s i d e r e d corrosive t o metals. More e x t e n s i v e work s h o u l d be conducted on the d u r a b i l i t y and l i f e expect a n c y o f p l a s t i c and composite m a t e r i a l s under end-use environments. A f u r t h e r c o n s i d e r a t i o n i s the p o t e n t i a l f o r polymer d e g r a d a t i o n by the products o f metal c o r r o s i o n i n h y b r i d s t r u c t u r e s comprising m e t a l and polymer components. Since i t i s expected that coatings w i l l continue t o be used t o p r o t e c t p l a s t i c and composite s u b s t r a t e s , a n c i l l a r y programs need t o be conducted on the mechanisms b y w h i c h c o a t i n g s c a n p r o t e c t such s u b s t r a t e s . A d h e s i v e s . I n many a p p l i c a t i o n s , t h e r e are s u b s t a n t i a l f u n c t i o n a l and economic r e a s o n s t o p r e f e r a d h e s i v e b o n d i n g o v e r m e c h a n i c a l f a s t e n i n g o f m e t a l s , o f p l a s t i c s , and o f m i x e d - s u b s t r a t e j o i n t s . The r o l e o f c o r r o s i o n i n the f a i l u r e o f a d h e s i v e bonds i s t h e r e f o r e becoming an i n c r e a s i n g l y c r u c i a l one. The performance demands p l a c e d on a d h e s i v e bonds b y the c o m b i n a t i o n o f m e c h a n i c a l l o a d i n g and a g g r e s s i v e e n v i r o n ments are p a r t i c u l a r l y s e v e r e , and i t i s c l e a r t h a t s t u d i e s i n v o l v i n g combined mode t e s t i n g need t o be g r e a t l y expanded. I t i s a n t i c i p a t e d t h a t i n f o r m a t i o n d e v e l o p e d i n the s t u d y o f a d h e s i v e s and the study o f coatings should inter-relate i n a s u f f i c i e n t l y strong fashion that b o t h f i e l d s w i l l b e n e f i t from such s t u d i e s .

Literature Cited 1. NBS Special Publication 511-1. Economic Effects of Metallic Corrosion in the United States. A Report to Congress by the National Bureau of Standards. SD Stock No. SN-003-01926-7, 1978; NBS Special Publication 511-2. Economic Effect of Metallic Corrosion in the United States. Appendix B. A report to NBS by Battelle Columbus Laboratories. SD Stock No. SN-00301927-5, 1978. 2. Evans, U. R. "The Corrosion and Oxidation of Metals", St. Martins Press: New York, 1960; ibid.. 1st Supplementary Volume,

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3. 4. 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.

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St. Martins Press: New York, 1968; ibid., 2nd Supplementary Volume, Edward Arnold: London, 1976. Uhlig, H. H. "Corrosion and Corrosion Control", 2nd ed., Wiley: New York, 1971. Shreir, L. L. "Corrosion", 2nd ed., Newnes-Butterworths: London, 1976. Evans, U. R. "An Introduction to Metallic Corrosion", 2nd ed., Edward Arnold: London, 1972. Scully, J. C. "The Fundamentals of Corrosion", 2nd ed., Perg­ amon: Oxford, 1975. Van Delindes, L. S., Ed. "Corrosion Basics: An Introduction", National Association of Corrosion Engineers: Houston, 1984. Pourbaix, M. "Lectures on Electrochemical Corrosion", Plenum: New York, 1973. Pourbaix, M. "Atlas of Electrochemical Equilibria in Aqueous Solutions", 2nd English ed. National Association of Corrosion Engineers: Houston Szauer, T.; Brandt Fawcett, N. C. Polym. Mat. Sci. Eng. 1985, 53, 855. Leidheiser, H., Jr. J. Coat. Technol. 1981, 53(678), 29. Funke, W. J. Coat. Technol. 1983, 55(705), 31. Zurilla, R. W.; Hospadaruk, V. Trans. SAE 1978, 87, 762. Bender, H. S.; Cheever, G. D; Wojtkowiak, J. J. Prog. Org. Coat. 1980, 8, 241. "Annual Book of ASTM Standards", Part 27, "Paint - Tests for Formulated Products and Applied Coatings", American Society for Testing and Materials: Philadelphia, issued annually. Von Fraunhofer, J. Α.; Boxall, J. "Protective Paint Coatings for Metals", Portcullis Press: Redhill, 1976. Funke, W. J. Oil Col. Chem. Assoc. 1979, 62, 63. Funke, W. Farbe Lack 1978, 84, 380. Funke, W.; Machunsky, E.; Handloser, G. Ibid., 1979, 84, 498. Funke, W.; Zatloukal, H. Ibid., 1979, 84, 584. Funke, W. in "Corrosion Control by Coatings", Leidheiser, Η., Jr., Ed.; Science Press: Princeton, 1979, p. 35. Rowe, L. C.; Chance, R. L. in "Automotive Corrosion by De-icing Salts", Baboian, R., Ed.; National Association of Corrosion Engineers: Houston, 1981, p. 133. Athey, R; Duncan, R.; Harmon, E.; Hartmann, M.; Iszak, D.; Nakabe, H.; Ochoa, J . ; Shaw, P.; Specht, T.; Tostenson, P.; Warness, R. J. Coat. Technol. 1985, 57(726), 71. Opinsky, A. J . ; Thompson, R. F.; Boegehold, A. L. ASTM Bull. 1953 (Jan), 47. Hospadaruk, V.; Huff, J . ; Zurilla, R. W.; Greenwood, H. T. Trans. SAE. 1978, 87, 755. Lambert, M. R.; Townsend, H. E.; Hart, R. G.; Frydrych, D. J. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 378. Standish, J. V. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 1985. Jones, D. A. Polym. Mat. Sci. Eng. 1985, 53, 470. van Ooij, W. Polym. Mat. Sci. Eng. 1985, 53, 698. Leidheiser, Η., Jr. Prog. Org. Coat. 1979, 7, 79. Szauer, T. Prog. Org. Coat. 1982, 10, 157. Szauer, T. Prog. Org. Coat. 1982, 10, 171. Walter, G. W. Corr. Sci. 1986, 26, 39.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Overview

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35. McIntyre, J. F.; Leidheiser, Η., Jr. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 348. 36. Beese, R. E.; Allman, J. C. in "Modern Container Coatings", Strand, R. C., Ed.; ACS Symposium Series No. 78, American Chemical Society: Washington, DC, 1978. 37. Groseclose, R. G.; Frey, C. M; Floyd, F. L. J. Coatings Tech­ nol. 1984, 56(714), 31. 38. Hubrecht, J . ; Vereecken, J . ; Piens, M. J. Electrochem. Soc. 1984, 131, 2010; see also Piens, M.; Verbist, R.; Vereecken, J . ; in "Organic Coatings: Science and Technology"; Parfitt, G. D.; Patsis, Α. V., Eds.; Dekker: New York, 1984; Vol. 7, p. 249. 39. Mansfield, F.; Kendig, M. W. Werkst. Korros. 1985, 36, 473. 40. Kendig, M. W.; Allen, A. T.; Jeanjaquet, S. L.; Mansfield, F. NACE Paper No. 74, presented at National Association of Cor­ rosion Engineers National Meeting, Boston, 1985. 41. Lindqvist, S. A. Corrosion. 1985, 41(2), 69. 42. Padget, J. C.; Moreland 55(698), 39. 43. Fernandez-Prini, R.; Kapista, S. J. Oil Col. Chem. Assoc. 1979, 62, 93. 44. Lindqvist, S. Α.; Meszaros, L.; Svenson, L. J. Oil Col. Chem. Assoc. 1985, 68, 10. 45. Mayne, J. E. O. J. Oil Col. Chem. Assoc. 1957, 40, 183. 46. Mayne, J. Ε. O.; Cherry, Β. W. Intern. Congr. Metallic Cor­ rosion. 1st. London. Engl., Butterworths: London, 1961. p. 539. 47. Mayne, J. E. O.; Maitland, C. C. Off. Dig.. Fed. Socs. Paint Technol. 1962, 34, 972. 48. Mayne, J. E. O. Trans Inst. Metal Finishing 1964, 41(4), 121. 49. Mayne, J. E. O.; Cherry, B. W. Off. Dig. Fed. Soc. Paint Technol. 1965, 37, 13. 50. Bacon, R. C.; Smith, J. J . ; Rugg, F. M. Ind. Eng. Chem. 1948, 40, 161. 51. Cherry, B. W. Australas. Corr. Eng. 1974, 18(10), 23. 52. Guruviah, S. J. Oil Col. Chem. Assoc. 1970, 53, 660. 53. Bauman, K. Plast. Kautsch., 1972, 19, 455. 54. Kresse, P. Pigm. Resin Technol. 1973, 2(11), 21. 55. Haagen, H.; Funke, W. J. Oil Col. Chem. Assoc. 1975, 58, 359. 56. Funke, W.; Haagen, H. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 50. 57. Funke, W. J. Oil Col. Chem. Assoc. 1979, 62, 63. 58. Floyd, F. L.; Frey, C. M. Org. Coatings Plastics Chem. 1980, 43, 580. 59. Floyd, F. L.; Groseclose, R. G.; Frey, C. M. J. Oil Col. Chem. Assoc. 1983, 66, 329. 60. Hulden, M.; Hansen, C. M. Prog. Org. Coat. 1985, 13, 171. 61. Kresse, P. Farbe Lack 1974, 80, 817. 62. Kresse, P. Ibid. 1977, 83, 85. 63. Pearson, R. G. J. Chem. Ed. 1968, 45, 581. 64. Pearson, R. G. Ibid. 1969, 45, 643. 65. Drago, R. S.; Wayland, Β. B. J. Am. Chem. Soc. 1965, 87, 3571. 66. Drago, R. S.; Vogel, G. C.; Needham, T. E. J. Am. Chem. Soc. 1971, 93, 6014. 67. Sorensen, P. J. Coat. Technol. 1975, 47(602), 31. 68. Drago, R. S.; Parr, L. B.; Chamberlain, C. S. J. Am. Chem. Soc. 1977, 99, 3203.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC MATERIALS FOR CORROSION C O N T R O L

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69. Jensen, W. B. Chem. Rev. 1978, 78, 1. 70. Jensen, W. B. Rubber Chem. Technol. 1981, 55, 881. 71. Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3. 72. Manson, J. Α.; Lin, J. S.; Tibureio, A. Org. Coat. Appl. Polym. Sci. Proc. 1982, 46, 121. 73. Vanderhoff, J. W.; Bennetch, L. M.; Cantow, M. J.; Earhart, K. Α.; El-Aasser, M. S.; Huang, T. C.; Kang, M. H.; Micale, F. J.; Shaffer, O. L.; Timmons, D. W. Org. Coat. Appl. Polym. Sci. Proc. 1982, 46, 12. 74. Fowkes, F. M. in "Physicochemical Aspects of Polymer Surfaces", Mittal, K. L., Ed.; Plenum: New York, 1983. p. 583. 75. Fowkes, F. M. J. Polym. Sci. Polym. Chem. Ed. 1984, 22, 547. 76. Joslin, S. T.; Fowkes, F. M. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 369. 77. Fowkes, F. M. Polym. Mat. Sci. Eng. 1986, 53, 560. 78. Parker, E.; Gerhart 79. Walker, P. Off. Dig 80. Funke, W. J. Oil Col. Chem. Assoc. 1985, 68, 229. 81. Rawlings, R. D.; Strivens, T. A. J. Oil Col. Chem. Assoc. 1980, 63, 412. 82. Rooum, J. Α.; Rawlings, R. D. J. Mater. Sci. 1982, 17, 1745. 83. Rooum, J. Α.; Rawlings, R. D. J. Coat. Technol. 1982, 54(695), 43. 84. Castle, J. E.; Watts, J. F. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 361. 85. Kinloch, A. J., Ed. "Durability of Adhesive Bonds", Applied Science Publishers: London, 1983. 86. Minford, J. D. J. Adhesion 1985, 18, 19. 87. Smith, T. J. Adhesion 1984, 17, 1. 88. Drain, K. F.; Guthrie, J.; Leung, C. L.; Martin, F. R.; Otterbrun, M. S. J. Adhesion 1984, 17, 71. 89. Dodiuk, H; Drori, L.; Miller, J. J. Adhesion 1985, 18, 1. 90. Hill, L. W.; Wicks, Z. W. Prog. Org. Coat. 1982, 10, 55. 91. Harris, F. W.; Spinelli, H. J., Eds. "Reactive Oligomers", American Chemical Society: Washington, 1985. 92. Krishnamurti, K. Prog. Org. Coat. 1983, 11, 167. 93. Hopwood, J.J. J. Oil Col. Chem. Assoc. 1965, 48, 157. 94. Lerman, M. A. J. Coat. Technol. 1976, 48(623), 35. 95. Qaderi, S. Β. Α.; Bauer, D. R.; Holubka, J. W.; Dickie, R. A. J. Coat. Technol. 1984, 56(719), 71. 96. Woo, J. T. K.; Ting, V.; Evans, J.; Marcinko, R.; Carlson, G.; Ortiz, C. J. Coat. Technol. 1982, 54(689), 41. 97. New England Society for Coatings Technology Technical Committee J. Coat. Technol. 1981, 53(683), 27. 98. Ibid. 1982, 54(684), 63. 99. Lein, M. M.; Brakke, B.; Keltz, G.; Kiezulas, M. P.; Leavy, C. M.; Marderosian, R.; Withington, D. Ibid. 1983, 55(703), 81. 100. Schenk, H. U; Spoor, H.; Marx, M. Prog. Org. Coat. 1979, 7, 1. 101. Kordomenos, P. I.; Nordstrom, J. D. J. Coat. Technol. 1982, 54(686), 33. 102. Wismer, M.; Pierce, P. C.; Bosso, J. F.; Christenson, R. M.; Jerabek, R. D.; Zwack, R. R. J. Coat. Technol. 1982, 54(688), 35. RECEIVED

June 16, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2 Alternating Current Impedance and Underfilm Darkening Studies on Acidic Water-Based Anticorrosive Paints P. J. Moreland and J. C. Padget Imperial Chemical Industries PLC, Mond Division, Technical Department, P.O. Box 8, The Heath, Runcorn, Cheshire, England

The protective properties and interface reactions on mild steel substrates of an acidically formulated (pH5) water-borne paint based upon a chlorine containing vinyl acrylic copolymer have been examined using a variety of techniques. Traditional electrochemical polarisation curves as well as ac impedance studies were used to investigate the corrosion process in "wet" formulations associated with the occurrence or absence of "flash rusting". Investigation of an underfilm darkening phenomenon observed upon exposure testing of some similarly formulated coatings and associated with excellent long term protective performance are also presented. An arrest in a corrosion process, after some period involving insignificant metal loss was observed, evidenced the formation of a protective interface film. The characterisation of the film showed that its properties were in accord with the recognised protective performance of the coating system. An a c r y l a t e m o d i f i e d v i n y l c h l o r i d e - v i n y l i d e n e c h l o r i d e l a t e x copolymer ( H a l o f l e x 202) has been developed i n our l a b o r a t o r y (1,2) s p e c i f i c a l l y f o r t h e p r e p a r a t i o n o f water-borne a n t i - c o r r o s i v e primer paints. This c a r e f u l l y designed copolymer, hereafter r e f e r r e d t o as a c h l o r i n e - c o n t a i n i n g v i n y l a c r y l i c copolymer, e x h i b i t s a low water vapour p e r m e a b i l i t y (detached f i l m ) o f a p p r o x i m a t e l y 100 f o l d l e s s than t h a t o f t y p i c a l a c r y l i c l a t e x polymers intended f o r t h e p r e p a r a t i o n o f a n t i - c o r r o s i v e p r i m e r s , and when f o r m u l a t e d i n t o paint i s capable o f g i v i n g e x c e l l e n t a n t i - c o r r o s i v e performance on smooth and b l a s t e d s t e e l . C h l o r o p o l y m e r l a t i c e s d i f f e r from o t h e r l a t i c e s i n t h a t they undergo a d e h y d r o c h l o r i n a t i o n r e a c t i o n a t a l k a l i n e pH; t h e h i g h e r the pH t h e h i g h e r t h e r a t e o f d e h y d r o c h l o r i n a t i o n . Thus when such a l a t e x i s formulated i n t o a p a i n t a t t h e t y p i c a l p a i n t pH (7-9) 0097-6156/86/0322-0018S06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Acidic Water-Based Anticorrosive Paints

19

t h e r e i s a downward d r i f t i n pH and an i n c r e a s e i n c h l o r i d e i o n concentration i n the aqueous phase. A l t h o u g h the r a t e of d e h y d r o c h l o r i n a t i o n can be reduced by r e d u c i n g the c h l o r i n e c o n t e n t o f the polymer, i n our e x p e r i e n c e t h i s r e d u c t i o n i s a t the expense o f both the b a r r i e r p r o p e r t i e s and the a n t i - c o r r o s i v e performance. We have shown t h a t the r a t e o f d e h y d r o c h l o r i n a t i o n o f the h i g h c h l o r i n e content copolymer i s n e g l i g i b l y s m a l l at pH ^ 4.5. A c c o r d i n g l y a c i d i c p a i n t f o r m u l a t i o n s were developed (1,3) (see A p p e n d i x ) , which e x h i b i t e d v e r y l i t t l e change i n pH or c h l o r i d e i o n concentrations during storage. Such a c i d i c paint fomulations require the dispersed components (ie polymer and pigment p a r t i c l e s ) t o be s t r o n g l y s t e r i c a l l y s t a b i l i s e d i f they are t o remain c o l l o i d s t a b l e . Ethylene oxide-propylene-ethylene oxide b l o c k copolymers were found t o be p a r t i c u l a r l y s u i t a b l e as they i n c r e a s e d the r a t e and e x t e n t o f l a t e x p a r t i c l e c o a l e s c e n c e due t o a s u r f a c e p l a s t i c i z a t i o n effect(£) w i t h o u t downgrading the b a r r i e r p r o p e r t i e s or a n t i - c o r r o s i v ac impedance study on t h i A l t h o u g h a c i d i c p a i n t f o r m u l a t i o n s based on the c h l o r i n e - c o n t a i n i n g vinyl acrylic l a t e x copolymer give excellent anti-corrosive performance, they do e x h i b i t two u n u s u a l f e a t u r e s not p r e s e n t i n the c o r r e s p o n d i n g a l k a l i n e f o r m u l a t i o n s : a)

The e x t e n t o f f l a s h r u s t i n g on g r i t b l a s t e d m i l d s t e e l d u r i n g t h e "wet" f i l m c o n d i t i o n p r o g r e s s i v e l y decreases with d e c r e a s i n g pH, b e i n g e x t e n s i v e a t pH ^7 and b e i n g very s l i g h t a t pH ' 10 ohm on 10 cm , but the apparent f i l m r e s i s t a n c e decreased upon exposure and e x h i b i t e d a minimum between 9 and 23 days exposure. The f i l m ' s b e h a v i o u r can be m o d e l l e d as a Randies e q u i v a l e n t RC c i r c u i t i n which i o n i c f i l m r e s i s t a n c e R d e c r e a s e d and f i l m c a p a c i t a n c e C i n c r e a s e d w i t h exposure time up t o 9 days. The depressed semi c i r c u l a r b e h a v i o u r at 4 and 9 days i n d i c a t e d a d i s p e r s i o n i n the time c o n s t a n t RC f o r the f i l m . Between 9 and 23 days i t i s e v i d e n t t h a t the f i l m r e s i s t a n c e i n c r e a s e d and c o n t i n u e d t o do so up t o the 51 day measurement. S i m i l a r b e h a v i o u r was o b t a i n e d f o r t h i s t y p e o f c o a t i n g under c o n s t a n t immersion c o n d i t i o n s ( F i g u r e 8) though the r e c o v e r y t o h i g h e r impedance at 51 days was not as marked. In a l l c a s e s , even at minimum impedance v a l u e s the f i l m r e s i s t a n c e was h i g h at > 5 χ 1 0 ohms on u n i t a r e a and compared f a v o u r a b l y w i t h the r e s i s t a n c e c r i t e r i o n f o r a p r o t e c t i v e c o a t i n g ( 9 ) . 8

2

7

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

M O R E L A N D A N D PADGET

F i g u r e 4.

Acidic Water-Based Anticorrosive Paints

F o i l r e s i s t a n c e changes-constant immersion 3% N a C l .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

26

P O L Y M E R I C M A T E R I A L S FOR CORROSION

CONTROL

16Γ4

10

0

20

30

40

5

0

Time, days

F i g u r e 5.

F o i l r e s i s t a n c e changes-alternate

immersion i n 3% NaCl.

0 13

5.2

"» • 0

10

F i g u r e 6.

20

Time, days

cT

30

Mean c o r r o s i o n r a t e - t i m e

40

curves.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

J 50

2.

M O R E L A N D A N D PADGET

9 Days

Acidic Water-Based Anticorrosive Paints

23 Days

27

51 Days

10 cm

F i g u r e 8.

Nyquist p l o t s - c h l o r i n e containing v i n y l c o a t i n g c o n s t a n t l y immersed i n 3% NaCl.

acrylic

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Area

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

28

The i n i t i a l decrease i n i o n i c f i l m r e s i s t a n c e and i n c r e a s e i n c a p a c i t a n c e can be a s s o c i a t e d w i t h e i t h e r NaCl e l e c t r o l y t e o r water e n t r y i n t o t h e f i l m . From ER measurements t h i s p e r i o d i s a s s o c i a t e d w i t h a m e t a l l o s s p r o c e s s a t t h e s u b s t r a t e s u r f a c e . However, between 9 t o 23 days t h e i o n i c f i l m r e s i s t a n c e i n c r e a s e s , which i s a s s o c i a t e d w i t h an a r r e s t i n m e t a l l o s s a t t h e s u b s t r a t e s u r f a c e i n ER measurements. I t a p p e a r s , t h e r e f o r e , t h a t w i t h t h e knowledge o f an underfilm darkening phenomenon occurring at the substrate/coating i n t e r f a c e , a f i l m of a protective ( i e passive or h i g h i o n i c r e s i s t a n c e ) n a t u r e i s produced d u r i n g exposure. As shown i n F i g u r e 9 t h e impedance o f t h e a c r y l i c coating i m m e d i a t e l y showed low v a l u e s which d i d n o t i n c r e a s e . The c o a t i n g showed marked r u s t i n g and e x f o l i a t i o n . C h l o r i n a t e d rubber c o a t i n g s maintained a high impedance similar to that of the c h l o r i n e - c o n t a i n i n g v i n y l - a c r y l i c c o a t i n g s though t h e development of a pinhole a f t e r lon i n F i g u r e 9. The t h r e e c o a t i n g systems were a l s o exposed t o h o t s a l t s p r a y . I n t h i s c a s e , i t appeared t h a t t h e minimum impedance o f t h e c h l o r i n e - c o n t a i n i n g v i n y l - a c r y l i c coating occurred w i t h i n the f i r s t 5 hours exposure and t h e r e a f t e r t h e impedance remained h i g h (>10 ohms). T h i s b e h a v i o u r i s p r o b a b l y due t o f a s t e n t r y o f e l e c t r o l y t e and/or water i n t o t h e f i l m under t h e more a g g r e s s i v e c o n d i t i o n s t o form an i n t e r f a c e f i l m . As i n p r e v i o u s experiments t h e a c r y l i c c o a t i n g had low impedance (10 ohm) unless a pinhole developed. 7

8

Acrylic

2E9

2E8

2E7 10 cnr Area

F i g u r e 9.

N y q u i s t - a c r y l i c and c h l o r i n a t e d rubber a l t e r n a t i v e l y immersed i n 3% NaCl.

coatings

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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29

Acidic Water-Based Anticorrosive Paints

Chemical Characterisation. Chemical characterisation of the u n d e r f i l m d a r k e n i n g beneath c h l o r i n e - c o n t a i n i n g v i n y l - a c r y l i c f i l m s was sought u s i n g a v a r i e t y o f t e c h n i q u e s on the s u b s t r a t e s u r f a c e as w e l l as the b a c k s i d e o f s t r i p p e d f i l m s . The s u b s t r a t e s were g r i t b l a s t e d and p l a i n m i l d s t e e l Q p a n e l s exposed up t o 98 days i n hot s a l t spray and examined w i t h i n hours o f r e m o v a l . XRD i d e n t i f i e d Fe 0 p l u s p a i n t c o n s t i t u e n t s o f BaSO , T i 0 and Zn (?0 ) t o g e t h e r w i t h a l e s s i d e n t i f i a b l e major phase (7.7A, 2.68A, 2.36A). T h i s phase i s now b e l i e v e d t o be a member o f a c l a s s o f compounds r e f e r r e d t o as the p y r o a u r i t e group. These compounds have the g e n e r a l f o r m u l a : 3

3

li

h

u

2

2

Μ

χ

R

y

(0H)

2 x + 3 y

_

2 z

(A*-) .2H 0 Z

2

where M i s a d i v a l e n t c a t i o n , R i s a t r i v a l e n t c a t i o n and A i s an a n i o n , commonly C 0 ^ ~ , The members of t h i s grou w h i c h p y r o a u r i t e has t h e f o r m u l a Mg F e (0H) (CO)^ 3 H 0. The l a y e r s t r u c t u r e of such compounds a l l o w s the accommodation o f a v a r i e t y of a n i o n s and cations (1_0) and the l a r g e number of h y d r o x y l groups may p r o v i d e a b u f f e r i n g c a p a c i t y . T h i s b u f f e r i n g c a p a c i t y has been r e c o g n i s e d as a p r o b a b l e i n f l u e n t i a l f a c t o r i n the p r o t e c t i v e p r o p e r t y afforded by s i m i l a r h y d r o t a l c i t e type f i l m s on aluminium i n sea water ( JJ_). The b u f f e r i n g p r o p e r t i e s c o u l d a l s o c o n s t r a i n l o c a l i s e d a t t a c k and promote l a t e r a l movement o f the c o r r o s i o n p r o c e s s and f i l m f o r m a t i o n . 2

6

2

1 6

2

SEM s t u d i e s s u p p o r t e d the above i n o b s e r v a t i o n o f p l a t e l e t type c r y s t a l s c o n t a i n i n g Fe and CI (by EDAX). ESCA r e v e a l e d little d e t a i l but SIMS i d e n t i f i e d a number of hydroxy and_oxychloride s p e c i e s i n c l u d i n g Fe(0H)+, Fe0+, FeO-, FeOCl- and F e C l ^ t o s u p p o r t the presence of c h l o r i d e i n the p y r o a u r i t e type f i l m . LIMA i n d i c a t e d a number Fe 0 + peaks w i t h χ as y as h i g h as 2 or 3 χ y whereas χ and y are g e n e r a l l y 2 f o r FeOOH. Conclusions F l a s h r u s t i n g e x h i b i t e d i n n e u t r a l t o a l k a l i n e water borne formulations appears t o occur t h r o u g h a l o c a l i s e d corrosion p r o c e s s p r o b a b l y i n v o l v i n g g r i t " a c t i v i t y " p r e s e n t from b l a s t i n g , e i t h e r d i r e c t l y or i n d i r e c t l y , i n an e l e c t r o c h e m i c a l p r o c e s s . At such pH the r a p i d o x i d a t i o n o f f e r r o u s t o f e r r i c i o n produces intense l o c a l p r e c i p i t a t i o n of f e r r i c hydroxide evidenced as f l a s h - r u s t s p o t s . The p r o c e s s can be e l i m i n a t e d by f o r m u l a t i n g at a lower pH, eg pH 4.5 which g i v e s r i s e t o a u n i f o r m c o r r o s i o n p r o c e s s at the s u b s t r a t e s u r f a c e . I t has been shown t h a t a c h l o r i n e c o n t a i n i n g v i n y l a c r y l i c c o a t i n g can be s a t i s f a c t o r i l y f o r m u l a t e d at t h i s pH. Under t h i s c o n d i t i o n , o x i d a t i o n t o f e r r i c i o n w i t h subsequent p r e c i p i t a t i o n does not o c c u r and hence f l a s h r u s t i n g i s not o b s e r v e d . I t appears t h a t under a g g r e s s i v e c o r r o s i v e c o n d i t i o n s e l e c t r o l y t e may enter the f i l m and s t i m u l a t e c o r r o s i o n but the c h l o r i n e

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

30

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

c o n t a i n i n g v i n y l a c r y l i c c o a t i n g q u i c k l y promotes f o r m a t i o n o f a p r o t e c t i v e f i l m , as e v i d e n t by ER, ac impedance and i r o n p i c k u p measurments, w i t h i n s i g n i f i c a n t o v e r a l l m e t a l l o s s . Indeed, ac impedance measurements on t y p i c a l s u b s t r a t e s u r f a c e s i n d i c a t e d f i l m r e s i s t a n c e s t o remain v e r y h i g h even d u r i n g f o r m a t i o n o f t h e interface film. The l o n g term protective f i e l d performance behaviour o f such c o a t i n g r e f l e c t s the p r o t e c t i v e c h a r a c t e r o f t h i s system. However, water borne systems f o r m u l a t e d a t pH 7-9 eg c o n v e n t i o n a l a c r y l i c s a r e both capable o f p r o d u c i n g f l a s h r u s t i n g d u r i n g c o a t i n g and i n c a p a b l e o f p r o d u c i n g i n - s i t u p r o t e c t i v e f i l m s i n the presence o f c o r r o s i v e e n v i r o n m e n t s .

Literature Cited 1 2 3 4 5 6 7 8 9 10 11

Burgess, A J; Caldwell, D; Padget, J C.; JOCCA, 1981, 64, 175. US Patent 4,341,679 European Patent 0,035,316 Padget, J C.; Moreland, Ρ J; J Coating Technol, 1983, 55, 39. Piens, M; Verbist, R; In "Corrosion Control by Organic Coatings", NACE, Houston, Texas, 1980; p 163. Dravnieks, A; Cataldi, H A; Corrosion, 1954, 10, 224. Sykes, J; Lewis, G; unpublished work at Oxford University. Minegishi, T; Asaki, Z; Higuchi, B; Konds, Y; Met Trans B, 1983, 14B, 17. Bacon, R C; Smith J J; Rugg, F M; Ind Eng Chem, (1948), 40, 161. Taylor, H F W; Mineralogical Mag, 1973, 39, 377. Austing, C E; Pritchard, A M; Wilkins, Ν J M; Desalination, 1973, 12, 251.

Appendix Primer Latex

Formulation

Based on C h l o r i n e C o n t a i n i n g V i n y l Acrylic

Ingredient V i n y l A c r y l i c Copolymer ( H a l o f l e x 202)

% w/w

59.7

N o n - i o n i c b l o c k copolymer 3.1 surfactant

Ingredient

% w/w

M i c r o n i s e d Z i n c Phosphate

5.8

Micronised barytes Titanium dioxide

H y d r o x y - p r o p y l methyl cellulose

0.2

Water

De-foamer

0.2

pH

Butyl glycol

2.0

Pigment volume concentration

15.6 2.6 10.8

5

20

The name Haloflex is a trademark, the p r o p e r t y o f I m p e r i a l Chemical I n d u s t r i e s PLC. RECEIVED March 5, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3 New Methods in Electrochemical Assessment of Polymer Coatings on Steel J. P. Lomas, L. M . Callow, and J. D. Scantlebury Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Sackville Street, Manchester M60 1QD England

Harmonic Analysi as a corrosion monitoring test following the original theoretical work by Meszaros. This study looks at the possibility of using this technique to enhance the accuracy of Impedance based corrosion rate measurements on painted metal electrodes. A series of 12 chlorinated rubber coated specimens (of 2 different formulations) have been allowed to degrade under immersion for over 3 years and exhibit a variety of defects. This paper aims to show whether a combination of Harmonic Analysis and Impedance tests, can give a realistic ranking of the degree of degradation. Some specimens have remained intact and the Impedance measurements are purely capacitive, such specimens do not exhibit a stable rest potential and no additional information can be obtained from Harmonic Analysis. Those specimens showing certain forms of corrosion attack, give corrosion rates that are determinable using both techniques. In other instances, the results obtained are widely divergant. The use o f t h e impedance t e c h n i q u e i n the s t u d y o f polymer c o a t e d s t e e l , has been t h o r o u g h l y d e s c r i b e d elsewhere(1). The p r e s e n t paper compares t h i s t e c h n i q u e w i t h t h a t o f harmonic a n a l y s i s , o r i g i n a l l y proposed by M e s z a r o s T h e a u t h o r s have p r e s e n t e d p r e l i m i n a r y d a t a u s i n g t h e l a t t e r technique(3) w h e r e i n t h e e a r l y s t a g e s o f polymer breakdown have been s t u d i e d . The c u r r e n t paper extends t h i s work t o polymers w h i c h have been immersed f o r a c o n s i d e r a b l e p e r i o d o f t i m e . The harmonic method g i v e s i n f o r m a t i o n not a v a i l a b l e from the impedance t e c h n i q u e i n the T a f e l s l o p e s and the c o r r o s i o n c u r r e n t a r e d i r e c t l y measurable. A b r i e f summary of the harmonic method and t h e e q u a t i o n s used a r e g i v e n below. A small sinusoidal perturbation i s applied p o t e n t i o s t a t i c a l l y t o the system under i n v e s t i g a t i o n and t h e r e s u l t i n g c u r r e n t s i n e wave i s a n a l y s e d i n terms o f i t s second and t h i r d harmonics (±2 and 13), i i b e i n g the fundamental. The c o r r o s i o n c u r r e n t i s c a l c u l a t e d 0097-6156/ 86/ 0322-0031 $06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

32 from the f o l l o w i n g

equation: _JL

corr

S and

48

>Λΐ.

i

-i

2 2

the T a f e l s l o p e s a and c from e q u a t i o n s o f the type shown below. 1

-

3a

1 2U

1 ο

i

χ

corr

+

4i

2

Ι­ 1

where U

= ο magnitude of the perturbing sine wave.

Experimental 12 c h l o r i n a t e d r u b b e r c o a t e aluminium f l a k e d pigmen o r i g i n a l l y 6 p a n e l s , cut i n h a l f a f t e r p a i n t i n g . D e t a i l s of the p r e p a r a t i o n and masking l e a v i n g an exposed p a i n t a r e a of 34 cm^, are g i v e n elsewhere W. The p a n e l s were immersed i n a r t i f i c i a l sea water f o r 1042 days as p a r t of a s e p a r a t e s e r i e s of e x p e r i m e n t s . Harmonic a n a l y s i s was c a r r i e d out on the specimens 7 days a f t e r the impedance measurements i n o r d e r to a l l o w the specimens to s e t t l e down a g a i n . An Ono S o k k i CF 910 d u a l c h a n n e l FFT a n a l y s e r was used i n c o n j u n c t i o n w i t h a p o t e n t i o s t a t (Thompson M i n i s t a t 251) to h o l d the specimen a t i t s r e s t p o t e n t i a l and to p r o v i d e the low frequency s i n e wave p e r t u r b a t i o n . The second c h a n n e l was used to measure the harmonic c o n t e n t of the r e s u l t i n g c u r r e n t . The Ono S o k k i produces a d i g i t i a l l y generated h i g h p u r i t y s i n e wave a t a chosen f r e q u e n c y , i n t h i s i n s t a n c e , 0.5 Hz. The t o t a l harmonic c o n t e n t of the i n p u t s i n e wave was l e s s than 0.45% measured over 10 harmonics. Only the f i r s t 3 harmonics are used to c a l c u l a t e the corrosion current. For each specimen an average over 122 c y c l e s were t a k e n , but because of the h i g h impedances p r e s e n t , a p e r t u r b i n g v o l t a g e of 115 mV was used. T h i s i s somewhat h i g h e r than the t h e o r e t i c a l v a l u e a r r i v e d a t by Meszaros(2) but n e c e s s a r y because of the low l e v e l of harmonic s i g n a l from the specimens. The v i s u a l appearance of each e l e c t r o d e was n o t e d f o r comparison w i t h the e l e c t r o c h e m i c a l d a t a . R e s u l t s and

Discussion

For c e r t a i n specimens, no s t a b l e r e s t p o t e n t i a l c o u l d be measured and f o r these e l e c t r o d e s , the p o t e n t i a l was h e l d a t -610 mV (SCE) d e s i g n a t e d by @. I t can be seen from T a b l e 1 t h a t a l l those specimens w h i c h c o u l d not s u p p o r t a s t a b l e r e s t p o t e n t i a l , a l s o showed impedance p l o t s t h a t were of a p u r e l y c a p a c i t i v e t y p e . T h i s impedance was a l s o o b t a i n e d from o t h e r specimens of b o t h p i g m e n t a t i o n , w h i c h e x h i b i t e d s t a b l e r e s t p o t e n t i a l s (A12 & G6). The d i a m e t e r s of the s e m i c i r c l e s ( g i v e n as R ) are a l l l a r g e , c l a s s i c a l l y i n d i c a t i v e of h i g h l y r e s i s t i v e f i l m s and no c o r r o s i o n r e a c t i o n . R e g a r d l e s s of whether the p l o t i s p u r e l y c a p a c i t i v e or shows s e m i c i r c u l a r b e h a v i o u r , the c a p a c i t a n c e v a l u e s may be a t t r i b u t e d t o t h a t r e s u l t i n g from the sc

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3.

L O M A S ET A L .

Electrochemical Assessment of Polymer Coatings on Steel 33

polymer f i l m i t s e l f . With the e x c e p t i o n of G4, w h i c h gave an anomalous v a l u e of 320 pF/cm . 2

Table 1.

Potential (SCE)

Specimen

Al

Impedance and Rest P o t e n t i a l Data

1

Impedance plot

5.6E7

-390 ^

2 3

-505

1E8

L

5

-631

6

(3-610

1

(§-610

2

@-610

3

(a-610

4

L L

E

6

-680

35.5 — .

5

—

36.6

—

35.8

—

31.7

—

44.6

1E7 1E7

L

1E7 1.6E5

-550 @-610

-

3E7

^ 5

18

2.2E6

4

Grey

2

1.8E7

l ^ -640

Capacitance pF/cm

1 E 7

-590

4

(Ohms cm2) Rsc

L L

2E5

1E7

—

320 53.3 —

1E7

I t can be seen t h a t i t was a g a i n d i f f i c u l t t o o b t a i n r e s u l t s from specimens where no s t a b l e r e s t p o t e n t i a l c o u l d be measured. The harmonic c u r r e n t s i n a l l cases were low and f o r c e r t a i n specimens were of the same o r d e r as the d i s t o r t i o n r e s u l t i n g from the i n p u t s i n e wave. The T a f e l s l o p e s o b t a i n e d were i n g e n e r a l anomalously h i g h and the c o r r o s i o n r a t e s v a r i e d over s e v e r a l o r d e r s of magnitude. As the impedance and harmonic a n a l y s i s t e c h n i q u e s gave d i f f e r e n t types of d a t a from each o t h e r , a d i r e c t comparison between T a b l e s 1 and 2 i s d i f f i c u l t . However, i t s h o u l d be anoted t h a t G4 gave b o t h s e m i c i r c u l a r b e h a v i o u r and a r e a s o n a b l y h i g h c o r r o s i o n r a t e . This s i m i l a r i t y i s a l s o t r u e f o r A14 and A15. A l l and A13 showed s m a l l e r but s t i l l measurable c o r r o s i o n r a t e s , t o g e t h e r w i t h s e m i c i r c u l a r impedance b e h a v i o u r and the a b s o l u t e v a l u e s measured v a r i e d s i m i l a r l y as b e f o r e . Any e l e c t r o c h e m i c a l t e c h n i q u e t h a t c o u l d be employed as a means of r a p i d c o a t i n g d e g r a d a t i o n assessment s h o u l d i d e a l l y be a b l e t o c o r r e l a t e w i t h the a c t u a l o b s e r v a b l e d e g r a d a t i o n . The v i s u a l appearance of the specimens i s o u t l i n e d i n Table 3. This appearance has been r e l a t i v e l y s t a b l e f o r the p a s t 600 days. I t may be seen from t h i s t a b l e t h a t of the A l f l a k e pigmented

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

34

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

T a b l e 2.

Harmonic A n a l y s i s Data

i Specimen P o t e n t i a l Fundamental Second Third (SCE) ( F i r s t h a r ­ Harmonic Harmonic c o r r (mV) monic (mV) (mV) (uA) Input s i n e wave Al 1 2 3 4 5 6 Grey 1 2 3 4 5 6

-

-390 -505 -590 -640 -630 @ 610 @ 610 @ 610 @ 610 -550 @ 610 -680

117 15.9 6.57 87.15 5.58E3 366

--

.01 .58 .2 .24 181 .4

.01 .17 .15 .12 15 .2

-1.8

-

b

b a

-

c

-

.18 .005 4.69 135 17.6

275 171 724 706 1025

204 145 666 393 986

-

-

--

-

-

-

1.4

30.9

742

569

-

984

907

2 1.003E3

161.4

-

.42

-

.11

7.3

-

Table 3. V i s u a l Appearance o f Specimens Specimen Al

1 2 3 4 5 6 Grey 1 2 3 6 5 4

Description l x l m m + l x 3 m m d i a m e t e r orange spots 2 χ 1 mm d i a m e t e r b l a c k s p o t s 2 χ 2 mm d i a m e t e r orange s p o t s Orange " r o s e t t e " 1.5 cm t o t a l d i a m e t e r no c o r r o s i o n

Paint b l i s t e r e d , very rusty

specimens, A14 e x h i b i t e d extreme breakdown i n the form o f a c o r r o s i o n r o s e t t e , w h i l s t G4 had b l i s t e r e d and s u f f e r e d c o n s i d e r a b l e c o r r o s i o n a t t a c k . The r e m a i n i n g specimens e i t h e r showed no o b s e r v a b l e d e g r a d a t i o n o r e x h i b i t e d s m a l l , s t a b l e and t h e r e f o r e r e l a t i v e l y i n s i g n i f i c a n t point s i t e attack. Comparison between the impedance and harmonic d a t a shows t h a t the r o s e t t e was d e t e c t a b l e i n the impedance d a t a by the emergence of Warburg type b e h a v i o u r a t low f r e q u e n c i e s a l t h o u g h no q u a n t i f i a b l e d a t a as t o the r a t e o f t h i s r e a c t i o n c o u l d be o b t a i n e d . I n the harmonic d a t a , the c o r r o s i o n r a t e was the h i g h e s t o f those measured, at 135 uA.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3.

LOMAS ET AL.

Electrochemical Assessment of Polymer Coatings on Steel

Conclusions I t can be seen t h a t f o r s e v e r e l y degraded specimens, b o t h t h e harmonic a n a l y s i s and impedance t e c h n i q u e s a r e c a p a b l e o f d e t e c t i n g the presence o f g r o s s c o r r o s i o n . The harmonics method p r o v i d e s a r e a s o n a b l e e s t i m a t i o n o f t h e c o r r o s i o n r a t e when t h e impedance d a t a e x h i b i t s Warburg type b e h a v i o u r . F o r l e s s s e v e r e l y degraded specimens, e s p e c i a l l y those e x h i b i t i n g b l i s t e r a t t a c k , t h e impedance method i s n o t as s u c c e s s f u l as t h e harmonic a n a l y s i s t e c h n i q u e . Where v e r y l i t t l e c o r r o s i o n a t t a c k has o c c u r r e d , n e i t h e r method i s c a p a b l e o f p r o v i d i n g r e l i a b l e q u a n t i t a t i v e d a t a . The n o n - a p p l i c a b i l i t y i n c e r t a i n i n s t a n c e s o f t h e impedance t e c h n i q u e as a m o n i t o r i n g t o o l has been r e p o r t e d p r e v i o u s l y . Further experience w i t h the harmonic a n a l y s i s t e c h n i q u e may be c a p a b l e o f r e f i n i n g t h e r e s u l t s obtained. Acknowledgments Drs. C a l l o w and Lomas would l i k e t o thank I n t e r n a t i o n a l P a i n t p i c f o r f i n a n c i a l s u p p o r t . A l l a u t h o r s would l i k e t o thank Dr. R.P.M. Procter f o r provision of laboratory f a c i l i t i e s .

Literature Cited 1. Callow, L.M. and Scantlebury, J.D., J.O.C.C.A., 64, 83 (1981). 2. Gill, J.S., Callow, L.M. and Scantlebury, J.D. Corrosion, 39, 61 (1983). 3. Devay, J. and Meszaros, L. Acta Chim. Acad. Sci., Hungary, 104, No. 13. 311 (1980). 4. Lomas, J.P., Callow, L.M., Scantlebury, J.D. and G.A.M. Sussex. Presented at 160th Meeting of Electrochem. Soc., Denver, Col. (Oct 1981). RECEIVED January

22, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

35

4 Application of Electrochemical Noise Measurements to Coated Systems 1

1

2

D. A. Eden , M . Hoffman , and B. S. Skerry 1

Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Sackville Street, Manchester, M60 1QD, England Sherwin Williams Company Research Center, 10909 South Cottage Grove Avenue, Chicago, IL 60628

2

This pape applicatio electrochemical techniques to studies of paint films on steel substrates exposed to aqueous environments. Simultaneous monitoring of the self-generated electrochemical potential and current noise using analogue and digital techniques has been evaluated as a tool for monitoring coating performance. These data obtained have been compared with those from a.c. impedance techniques. Laboratory measurement procedures used for electrochemical data acquisition and analysis during the monitoring exercise are outlined, and particular emphasis is placed on the electrochemical noise techniques. Electrochemical current noise has been monitored between two identical electrodes and the potential noise between the 'working' electrodes and a reference electrode. Digital noise measurements have been obtained by use of a microcomputer controlling the sampling rate of a sensitive digital voltmeter employed to measure the potential or current fluctuations. The subsequent analysis of the derived time records is described. Analogue noise measurements have been made using high gain amplifier/ filter circuits which permit examination of low frequency fluctuations on a 'real-time' basis. Electrochemical noise monitoring techniques have been used p r e v i o u s l y i n studies of c o r r o s i o n processes o c c u r r i n g on metals i n a v a r i e t y o f environments. I n i t i a l l y , work was d i r e c t e d towards t h e m o n i t o r i n g of p o t e n t i a l noise f l u c t u a -

0097-6156/ 86/ 0322-0036S06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EDEN ET AL.

Application of Electrochemical Noise Measurements

t i o n s , and was u s e d p a r t i c u l a r l y i n t h e i d e n t i f i c a t i o n o f t h e o n s e t o f l o c a l i s e d a t t a c k ( i . e . p i t t i n g o r c r e v i c e type a t t a c k ) Q-.3] . C u r r e n t n o i s e measurements have been u s e d i n t h e s t u d i e s o f e l e c t r o c r y s t a l l i s a t i o n [4] and p i t t i n g [5] w i t h t h e specimens b e i n g h e l d under p o t e n t i o s t a t i c c o n t r o l . Recent work [ 6 ,_7 ] has been d i r e c t e d towards t h e s i m u l t a neous m o n i t o r i n g o f p o t e n t i a l and c u r r e n t n o i s e , where t h e c u r r e n t n o i s e s i g n a l i s g e n e r a t e d by c o u p l i n g two n o m i n a l l y i d e n t i c a l e l e c t r o d e s w i t h a z e r o r e s i s t a n c e ammeter (ZRA), and the p o t e n t i a l n o i s e of the couple i s monitored with r e s p e c t t o a reference electrode. In t h i s manner no e x t e r n a l l y a p p l i e d signal i s required. The potential n o i s e s i g n a l provides i n f o r m a t i o n pert a i n i n g t o t h e t y p e o f a t t a c k , whereas t h e c u r r e n t n o i s e p r o v i d e s d a t a which i n d i c a t e t h e r a t e o f c o r r o s i o n and t h e t y p e of attack. When used i n p a r a l l e l t h e two n o i s e measurements may be used t o e s t i m a t i n t e r f a c e b e i n g examined When a p p l i e d t o c o a t e d m e t a l s , t h e f l u c t u a t i o n s o b s e r v e d i n the c u r r e n t n o i s e s i g n a l are g e n e r a l l y low i n magnitude w i t h t h e b a s e l i n e o f d e t e c t i o n e s s e n t i a l l y b e i n g l i m i t e d by the s e n s i t i v i t y o f t h e e l e c t r o n i c i n t e r f a c e . For t h e s t u d i e s c i t e d , t h e lower l i m i t o f t h e c u r r e n t n o i s e s i g n a l i s some 10 pico-amps. F o r t h e p u r p o s e s o f t h i s study t h e r e s p o n s e s o f a v a r i e t y o f i n t a c t and d e f e c t i v e c o a t i n g s were m o n i t o r e d and the r e s u l t s a r e compared w i t h a.c. impedance d a t a . The a.c. impedance t e c h n i q u e i s u s e f u l f o r m o n i t o r i n g changes o c c u r r i n g i n c o a t e d systems, and t h e v a r i o u s t y p e s o f response may be summarised b r i e f l y as f o l l o w s : a)

b) c) d)

I n t a c t c o a t i n g s (no pores) v e r y h i g h impedance p r o d u c e s almost p u r e l y c a p a c i t i v e r e s p o n s e , d i f f i c u l t y i n e s t i m a t i n g d.c. component o f r e s i s t a n c e . I n t a c t c o a t i n g s (as ( a ) ) , w i t h water uptake c a p a c i t a n c e i n c r e a s e s due t o d i e l e c t r i c c o n s t a n t changes. C o a t i n g s w i t h minor d e f e c t s u s u a l l y produce w e l l d e f i n e d r e s p o n s e w i t h r e s i s t i v e as w e l l as c a p a c i t i v e components. C o a t i n g s w i t h major d e f e c t s show response i n which comp l e x b e h a v i o u r i s observed, t h e c o a t i n g r e s p o n s e moving t o h i g h e r f r e q u e n c i e s due t o s m a l l e r v a l u e s o f r e s i s t a n c e , and i n a d d i t i o n , charge t r a n s f e r and diffusion e f f e c t s may become e v i d e n t .

Instrumentation D i g i t a l electrochemical noise. The d i g i t a l i n s t r u m e n t a t i o n used f o r t h e n o i s e s t u d i e s comprised t h e f o l l o w i n g : A H e w l e t t P a c k a r d HP85 Microcomputer A Hewlett Packard 3478A D i g i t a l V o l t m e t e r A "custom b u i l t " m u l t i p l e x e r A schematic diagram f o r t h e e x p e r i m e n t a l i n F i g u r e 1.

s e t up

is illustrated

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

38

POLYMERIC M A T E R I A L S FOR CORROSION

Multiplexer and Zero Resistance Ammeter

CONTROL

Microprocessor

8 sets of inputs

To working electrodes

*To reference

electrode

F i g u r e 1. Schemati using multiplexed electrodes

The i n p u t m u l t i p l e x e r was d e s i g n e d t o a l l o w m u l t i - c h a n n e l c a p a b i l i t y and was c o n f i g u r e d t o m o n i t o r b o t h p o t e n t i a l and c u r r e n t n o i s e f l u c t u a t i o n s s e q u e n t i a l l y on a maximum o f e i g h t p a i r s o f samples. The s a m p l i n g r a t e o f t h e d i g i t a l v o l t m e t e r (DVM) was cont r o l l e d by t h e m i c r o p r o c e s s o r and c h a n n e l s e l e c t i o n f o r monit o r i n g was o b t a i n e d by u t i l i s i n g a p u l s e o u t p u t from t h e DVM. Time r e c o r d s o f t h e c o u p l i n g c u r r e n t and p o t e n t i a l f o r t h e r e s p e c t i v e samples were o b t a i n e d and s t o r e d f o r f u r t h e r analysis. Analogue electrochemical noise. Analogue instrumentation m o n i t o r i n g low f r e q u e n c y n o i s e i n a s p e c i f i e d bandwidth was u s e d f o r t h e a n a l o g u e measurements. The s c h e m a t i c diagram ( F i g u r e 2) i l l u s t r a t e s t h e b a s i c c o n f i g u r a t i o n o f t h e i n s t r u mentation. The rms v a l u e s o f t h e n o i s e s i g n a l s were l o g g e d and s e n t as a 0 - 10V s i g n a l t o a c o n v e n t i o n a l c h a r t r e c o r der. The s i g n a l s e n s i t i v i t y c o r r e s p o n d i n g t o t h e f u l l s c a l e

To workina electrodes

Zero Resistance Ammeter

High Impedance Buffer

To reference

F i g u r e 2. monitoring.

Recorder

Filter

electrode

System

f o r analogue

current

and

potential

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

noise

EDEN ET AL.

Application of Electrochemical Noise Measurements

response o f the c h a r t r e c o r d e r was 1uV minimum (OV o u t p u t ) t o 10mV (10V output) c o v e r i n g f o u r decades a t 2.5V p e r decade. The c u r r e n t n o i s e s i g n a l was m o n i t o r e d by u s i n g a s e n s i ­ t i v e , low n o i s e z e r o r e s i s t a n c e ammeter (ZRA) t o c o u p l e p a i r s o f i d e n t i c a l e l e c t r o d e s ; the ZRA a c t i n g as a c u r r e n t t o v o l ­ tage converter. T h i s d e r i v e d p o t e n t i a l s i g n a l was t h e n f e d i n t o a p o t e n t i a l noise monitor. A.c. impedance. Impedance measurements were made u s i n g a S o l a r t r o n 1250 f r e q u e n c y r e s p o n s e a n a l y s e r under computer con­ t r o l u s i n g a Hewlett Packard HP85 microcomputer and commer­ c i a l l y a v a i l a b l e software. The c o a t i n g s were s t u d i e d i n t h e t h r e e e l e c t r o d e mode u s i n g a Thompson M i n i s t a t . Figure 3 i l ­ l u s t r a t e s s c h e m a t i c a l l y t h e e x p e r i m e n t a l arrangement.

F i g u r e 3. Working arrangement f o r 3 e l e c t r o d e e l e c t r o c h e m i c a l impedance s t u d i e s .

Ssmple p r e p a r a t i o n F o r t h e p u r p o s e s o f t h i s study a v a r i e t y o f c o a t i n g s a p p l i e d t o m i l d s t e e l s u b s t r a t e s were u s e d . The c o a t i n g s were chosen t o p r o v i d e a range o f p r o t e c t i o n from p o o r t o e x c e l l e n t . The c o a t i n g s s t u d i e d were: 1.

Polyurethane

(unpigmented)

2.

Polyurethane

(pigmented)

3. 3.

Bitumen Bitumen o v e r z i n c r i c h

paint

1 2 1 2 1 1

c o a t ~ 40μπι c o a t s ~ 80urn c o a t ~ 45um c o a t s ~ 90μπι c o a t ~ 20um c o a t ~ 20μπι

Expérimental P l a s t i c c e l l s o f d i m e n s i o n s 5 χ 5 χ 7.5cms were f i x e d t o t h e c o a t e d specimens u s i n g s i l i c o n e r u b b e r s e a l a n t . The s i l i c o n e

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

40

r u b b e r was a l l o w e d t o c u r e f o r a t l e a s t two days p r i o r t o f i l ­ l i n g with e l e c t r o l y t e . Three p e r c e n t sodium c h l o r i d e s o l u ­ t i o n i n d e m i n e r a l i s e d water was added t o t h e c e l l s which were p r e p a r e d as i d e n t i c a l p a i r s . C o u p l i n g between t h e p a i r s o f e l e c t r o d e s was a c h i e v e d u s i n g a sodium c h l o r i d e / a g a r salt bridge. P o t e n t i a l s o f t h e specimens were m o n i t o r e d u s i n g silver/silver chloride reference electrodes. A platinum c o u n t e r e l e c t r o d e was i n t r o d u c e d i n t o i n d i v i d u a l c e l l s when m o n i t o r i n g t h e a.c. impedance r e s p o n s e . A typical c e l l ar­ rangement i s shown i n F i g u r e 4. Platinum Reference

electrode

electrode NaCl/Agar s a l t

bridge

Cell

— Coated specimen

F i g u r e 4.

C e l l arrangement f o r e l e c t r o c h e m i c a l s t u d i e s .

D u r i n g t h e p e r i o d o f immersion o f t h e samples i n sodium c h l o r i d e e l e c t r o l y t e , e l e c t r o c h e m i c a l n o i s e measurements were made u s i n g t h e e l e c t r o n i c a p p a r a t u s p r e v i o u s l y d e s c r i b e d . The time r e c o r d s o b t a i n e d were a n a l y s e d u s i n g s t a t i s t i c a l t e c h ­ n i q u e s t o d e r i v e mean, s t a n d a r d d e v i a t i o n and c o e f f i c i e n t o f variance. The d e r i v e d v a l u e o f p o l a r i s a t i o n r e s i s t a n c e was e v a l u ­ a t e d from t h e r a t i o o f t h e s t a n d a r d d e v i a t i o n o f t h e p o t e n t i a l noise s i g n a l t o the standard d e v i a t i o n of the current noise signal, i.e. :

OV oi Data i s presented g r a p h i c a l l y t o i l l u s t r a t e the v a r i a t i o n i n d.c. potential ^Figure 5) mean d.c. c o u p l i n g c u r r e n t , i ( F i g u r e 6) and — τ « Rp ( F i g u r e 7) w i t h t i m e . T y p i c a l anaîogue n o i s e t r a c e s a r e i l l u s t r a t e d i n F i g u r e s

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Application of Electrochemical Noise Measurements

EDEN ET AL.

-0.5

15 Time (days) Figure 5. Potential vs time for coated specimens in 37 NaCl. Key: · , bitumen;A> Zn r i c h + bitumen;Q , polyurethane, one coat (unpigmented); ψ , polyurethane, two coats (unpigmented); 0, polyurethane, one coat (pigmented); and X, polyurethane, two coats (pigmented). c

-5

Time (days) Figure 6. Log i vs time for coated specimens in 3% Key: same as for Figure 5.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

42

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

8 and 9. Impedance d a t a F i g u r e s 10, 11 and 12.

for typical

cells

are

presented

in

Discussion From t h e d a t a o b t a i n e d f o r t h e d i f f e r e n t specimens i t can be seen t h a t t h e r e i s s i g n i f i c a n t l y d i f f e r e n t b e h a v i o u r between t h e p o o r , p o r o u s c o a t i n g s (bitumen) and t h e p o l y u r e t h a n e p a i n t samples. Of t h e p o l y u r e t h a n e samples o n l y one showed any e v i ­ dence o f c o r r o s i o n b e n e a t h t h e c o a t i n g d u r i n g t h e d u r a t i o n o f the t e s t and t h i s was an unpigmented s i n g l e c o a t specimen.

Potential (rms)

L

1mV

Current (rms)

1uA

.

L 100nA

I IOOuV J"

10nA

10pV

ο Time

F i g u r e 8. Analogue p o t e n t i a l bitumen on m i l d s t e e l . Day 1.

and

current

(hours)

noise

traces

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

for

.

4.

EDEN ET AL.

Application of Electrochemical Noise Measurements

potential (rms)

Current (rms)

-

1mV

-

-

1nA

-

-

100μν

-

- 100pA

-

.

10μν

.

.

Figure 9. Analogue p o t e n t i a l and c u r r e n t p o l y u r e t h a n e 2 c o a t s unpigmented. Day 2.

noise

10pA .

traces f o r

Z Imaginary (ohms)

15000 Ζ Real (ohms)

Ζ Imagi nary (ohms) 500 r -

1000 Ζ Real (ohms)

Figure 10. Nyquist Day 0 and Day 50.

plots

f o r bitumen

coated

mild

steel.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

44

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

Z Imoglnory (ohms)

150000 Ζ Real (ohme) Ζ Imaginary (ohms)

300

Figure steel.

11. Day

Nyquist p l o t s 0 and Day 50.

for zinc

rich

400 Ζ Real (ohme)

and

bitumen

on

mild

Even so, t h e low f r e q u e n c y impedance as d e r i v e d from t h e n o i s e measurements was s t i l l two t o t h r e e o r d e r s o f magnitude h i g h e r t h a n t h e bitumen c o a t e d system. The b e t t e r c o a t i n g s e x h i b i t e d low f r e q u e n c y impedances some f o u r t o f i v e o r d e r s o f magnitude h i g h e r t h a n t h e bitumen. The d.c. p o t e n t i a l s , however, o n l y i n d i c a t e d whether t h e m a t e r i a l b e i n g s t u d i e d was i n a c o r r o s i o n regime, both t h e b i t u m e n and unpigmented s i n g l e c o a t p o l y u r e t h a n e assumed v e r y s i m i l a r p o t e n t i a l s o v e r the p e r i o d o f the t e s t , even though t h e c o r r o s i o n r a t e s were g r o s s l y d i f f e r e n t . The impedance d a t a i l l u s t r a t e d i n F i g u r e s 10, 11 and 12 have been chosen t o i l l u s t r a t e t h e w i d e l y d i f f e r i n g b e h a v i o u r o f the d i f f e r e n t c o a t i n g systems. In F i g u r e 10, which shows t h e impedance b e h a v i o u r o f t h e bitumen c o a t i n g on m i l d s t e e l , i t i s a p p a r e n t t h a t a t day 0, t h e c o a t i n g i s immediately showing s i g n s o f major d e f e c t i v e a r e a s , w i t h the impedance response b e i n g governed by what ap­ p e a r t o be d i f f u s i v e e f f e c t s . The response a t high fre­ q u e n c i e s p r o b a b l y b e i n g due t o t h e c o a t i n g i t s e l f . The r e s i s ­ tance o f t h e system a t t h i s s t a g e i s g r e a t e r t h a n 15000 ohms. A f t e r 50 days' exposure, the impedance response has changed t o one i n d i c a t i n g charge t r a n s f e r and d i f f u s i o n e f ­ f e c t s w i t h a r e s i s t a n c e g r e a t e r than 1000 ohms. In c o m p a r i ­ son, t h e b e h a v i o u r o f t h e bitumen c o a t i n g on z i n c r i c h p a i n t ( F i g u r e 11) i n d i c a t e s t h a t a t day 0, the c o a t i n g i s showing

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EDEN ET AL.

Application of Electrochemical Noise Measurements

2 Imaginary (ohms)

100000 r

50000

50000

200000 Ζ Real (ohme)

2 Imaginary (ohme) 50000

50000

100000 Ζ Real (ohme)

F i g u r e 12. N y q u i s t p l o t s f o r p o l y u r e t h a n e (1 coat) u n p i g ­ mented system a f t e r 50 d a y s , i l l u s t r a t i n g c o a t i n g breakdown on P a n e l B. m i n o r d e f e c t s w i t h an impedance o f some 150,000 ohms t y p i c a l of a c o a t i n g response, although e x h i b i t i n g a f r e e c o r r o s i o n p o t e n t i a l (-806mV), which i s i n d i c a t i v e o f a porous c o a t i n g . A f t e r exposure f o r 50 days t h i s system i s showing t o t a l l y d i f ­ f e r e n t b e h a v i o u r w i t h charge t r a n s f e r and d i f f u s i o n e f f e c t s becoming much more e v i d e n t . Figure 12 i l l u s t r a t e s t h e d i f f e r e n c e i n impedance be­ h a v i o u r between t h e two samples o f unpigmented p o l y u r e t h a n e ( a p p l i e d a t ~ 40um) a f t e r 50 days exposure. P a n e l A gave a r e s p o n s e i n d i c a t i v e o f a good, i n t a c t c o a t i n g (almost p u r e l y

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

46

P O L Y M E R I C M A T E R I A L S FOR CORROSION

CONTROL

c a p a c i t i v e ) , whereas p a n e l Β was showing s i g n s o f breakdown, w i t h an e s t i m a t e d r e s i s t a n c e o f some 75,000 ohms. W i t h t h e n o i s e t e c h n i q u e s , b o t h a n a l o g u e and d i g i t a l , no e x t e r n a l l y a p p l i e d s i g n a l i s r e q u i r e d , and measurement o f t h e f l u c t u a t i o n s around t h e f r e e c o r r o s i o n p o t e n t i a l p r o v i d e s a l l the i n f o r m a t i o n . Hie n o i s e t e c h n i q u e i s u s e f u l i n t h a t i t a l l o w s a f a i r l y r a p i d e s t i m a t i o n o f t h e e l e c t r o c h e m i c a l im­ pedance o f t h e system b e i n g s t u d i e d , whereas, w i t h f o r i n ­ s t a n c e , a.c. impedance t e c h n i q u e s , v e r y o f t e n t h e minimum f r e ­ quency s t u d i e d i s s t i l l n o t low enough t o p r o v i d e s u f f i c i e n t i n f o r m a t i o n t o a l l o w an a c c u r a t e e s t i m a t i o n o f t h e impedance. With e l e c t r o c h e m i c a l n o i s e measurements t h e d.c. p o t e n ­ t i a l o f two c o u p l e d i d e n t i c a l e l e c t r o d e s i s governed by t h e sample w i t h t h e l o w e s t impedance. I t i s t h i s lower v a l u e o f impedance w h i c h i s m o n i t o r e d by t h e n o i s e t e c h n i q u e , i . e . t h a t o f t h e worst c o a t i n g o F o r t h e systems s t u d i e d the mean l e v e l o f c o u p l i n g appear y u s e f u l a s a means o f s t u d y i n g h i g h impedance systems, b u t t h i s can cause problems i f t h e c u r r e n t f l u c t u a t e s around z e r o and changes p o l a r i t y . G e n e r a l l y , i t would appear t o be a b e t t e r approach t o u t i l i s e t h e v a l u e o f s t a n d a r d d e v i a t i o n o f t h e c u r r e n t s i g n a l as a measure o f c o r r o s i o n r a t e . The c o e f f i ­ c i e n t o f v a r i a n c e f o r t h e c u r r e n t s i g n a l g i v e s some i n d i c a t i o n o f t h e s t a b i l i t y o f t h e d.c. c o u p l i n g c u r r e n t . I f we c o n s i d e r t h e a n a l a g o u s n o i s e e q u a t i o n s d e r i v e d f o r e l e c t r o n i c components a t t h e low f r e q u e n c y end o f t h e spec­ trum, one o f t h e e q u a t i o n s u s e d t o d e s c r i b e t h e n o i s e i s :

1 V where : l"d.c. Rs Kl f

η

= Κχ/— . I . Rs r d.c.

= d.c. c u r r e n t f l o w i n g t h r o u g h = source r e s i s t a n c e = constant = frequency

Correspondingly I

n

(1)

device

the equation f o r the current noise i s : = Ki/J . I. „ f d.c. A

(2)

I f we u t i l i s e t h e above e q u a t i o n s t o d e s c r i b e t h e low f r e ­ quency n o i s e s i g n a l s o b s e r v e d w i t h e l e c t r o c h e m i c a l systems, i t i s apparent t h a t t h e p o t e n t i a l n o i s e s i g n a l w i l l p r o v i d e i n ­ f o r m a t i o n p e r t a i n i n g t o t h e v a l u e o f t h e S t e r n Geary c o n s t a n t since: =r

corr where: i corr Β

—

R Ρ

corrosion current S t e r n Geary c o n s t a n t polarisation resistance

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

4.

EDEN ET AL.

and hence:

Application of Electrochemical Noise Measurements

Vn = K^/*

. Β

whereas t h e c u r r e n t n o i s e s i g n a l w i l l p r o v i d e i n f o r m a t i o n r e ­ l a t i n g to the c o r r o s i o n r a t e . I t i s t h e r e f o r e , not s u r p r i s i n g t h a t t h e low f r e q u e n c y p o t e n t i a l n o i s e s i g n a l s o n l y t e n d t o v a r y o v e r a few d e c a d e s , whereas t h e c u r r e n t n o i s e s i g n a l s may v a r y o v e r many o r d e r s o f magnitude. Conclusions 1.

2.

3.

E l e c t r o c h e m i c a l n o i s e measurements have shown g r e a t p r o ­ mise as a m o n i t o r i n g t o o l i n s t u d i e s o f c o r r o d i n g m e t a l s i n a v a r i e t y of environments. The a p p l i c a t i o n o f t h e s e s e n s i t i v e t e c h n i q u e s t o e v a l u a t e t h e p e r f o r m a n c e o f c o a t e d specimens would appear t o be a p p r o p r i a t e f o r th also f o r the monitorin tion. S i n c e the n o i s e s i g n a l s a r e g e n e r a t e d by the specimens t h e m s e l v e s c o a t i n g f a i l u r e i s accompanied by a change i n t h e e l e c t r o c h e m i c a l n o i s e s i g n a l which g i v e s a r a p i d i n d i c a t i o n o f the s t a t e of the c o a t i n g . Statis­ t i c a l a n a l y s i s o f t h e d a t a p r o v i d e s a r a p i d method o f a s s e s s i n g the noise levels without the n e c e s s i t y f o r t r a n s p o s i t i o n o f t h e d a t a i n t o t h e f r e q u e n c y domain by, f o r i n s t a n c e , FFT t e c h n i q u e s . Simultaneous m o n i t o r i n g o f c u r r e n t and p o t e n t i a l n o i s e and d e r i v a t i o n o f low f r e q u e n c y v a l u e s o f impedance a l ­ lows, i n some i n s t a n c e s , d i r e c t comparison w i t h p o l a r i s a ­ t i o n r e s i s t a n c e v a l u e s d e r i v e d from, f o r example, a.c. impedance t e c h n i q u e s .

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Hladky, Κ., and Dawson, J.L., Corr. Sci 22, p317 (1981). Hladky, Κ., and Dawson, J.L., ibid, 23, p231 (1982). Dawson, J.L, Hladky, Κ., and Eden, D.A., Paper presented at "On line Monitoring of Continuous Process Plant", London, June 1983. Bindra, P., Fleischmann, Μ., Oldfield, J.W. and Singleton, D., Discussion of Faraday Soc. 56 (1974). Williams, D.E., Westcott, C., Fleischmann, Μ., Passivity of Metals and Semi Conductors, p217-228, Elsevier Science publishers, Ed. M. Froment. Farrell, D.M., Cox, W.M., Stott, F.H., Eden, D.A., Dawson, J.L., and Wood, G.C., High Temperature Technology Vol. 3, No. 1, February 1985. John, D.G., Hladky, Κ., Eden, D.A., and Dawson, J.L., Paper presented at Research Sciences Symposium NACE/Corrosion 84, New Orleans, April 1984.

RECEIVED March 6,

1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5 Electrochemical Characterization of Photocured Coatings R. Bartoszek-Loza and R. J. Butler Research and Development Laboratory, The Standard Oil Company, 4440 Warrensville Center Road, Cleveland, OH 44128

Correlations were observe solvent-borne, high n i t r i l e coatings are well known for their barrier properties. The compositions were applied to Bonderite 40 coated steel panels and photocured at room temperature. Dramatic increases in rust rating were observed upon post-thermal treatment of N-vinyl pyrrolidone-based photocured coatings. Furthermore, the open circuit potentials of these coatings correlate with salt spray data over a broad postbake temperature range (25°C-220°C). The same trends were observed for gamma-butyrolactone-based coatings although the extent was not as dramatic. This indicates that electrochemical measurements can be utilized to systematically characterize photocured high nitrile polymer-based coatings. Metallic corrosion is an electrochemical process associated with the flow of current between surface sites having a difference in electrochemical potential. The assessment and evaluation of organic coatings to prevent metal corrosion has traditionally been accomplished through salt fog testing (ASTM B-117) and long term exposure tests in particular service environments. Electrochemical techniques have often been considered (1), but are not routinely employed in practice. A b s o l u t e c o r r e l a t i o n s between s e r v i c e performance and e l e c t r o c h e m i c a l measurements do n o t appear f r e q u e n t l y i n t h e l i t e r a t u r e . Based on 300 t e s t systems, Bacon and coworkers ( 2 ) , c o r r e l a t e d e l e c t r o c h e m i c a l r e s i s t a n c e w i t h exposure t i m e . Recently, M i l l s (3) a l s o observed a c o r r e l a t i o n between s a l t f o g c o r r o s i o n and e l e c t r o c h e m i c a l r e s i s t a n c e . We have found open c i r c u i t p o t e n t i a l measurements t o be e x t r e m e l y u s e f u l f o r t h e r o u t i n e e v a l u a t i o n o f h i g h - n i t r i l e polymer-based photocured c o a t i n g s . Organic coatings f u n c t i o n as e i t h e r i n h i b i t o r s , s a c r i f i c i a l c o a t i n g s o r b a r r i e r s ( 4 ) . W h i l e i n h i b i t o r and s a c r i f i c i a l c o a t i n g s p r o t e c t t h e s u b s t r a t e f r o m d e t e r i o r a t i o n by p r e f e r e n t i a l c o r r o s i o n of t h e c o a t i n g system, b a r r i e r c o a t i n g s f u n c t i o n by i s o l a t i n g t h e s u b s t r a t e from t h e c o r r o s i v e environment. High n i t r i l e polymers a r e known t o p o s s e s s h i g h r e s i s t a n c e t o water and oxygen permeation (5). 0097-6156/ 86/ 0322-0048$06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5.

B A R T O S Z E K - L O Z A A N D BUTLER

Characterization of Photocured Coatings 49

These b a r r i e r p r o p e r t i e s a r e b e l i e v e d t o r e s u l t from s t r o n g , compact -CN d i p o l e s . C l o s e p a c k i n g o f t h e polymer c h a i n produces powerful intermolecular forces (6). M e t a l l i c c o r r o s i o n can be c h a r a c t e r i z e d by two e l e c t r o c h e m i c a l q u a n t i t i e s , c u r r e n t and p o t e n t i a l . The c u r r e n t a s s o c i a t e d w i t h a s i n g l e e l e c t r o d e r e a c t i o n on a metal s u r f a c e i s r e l a t e d t o t h e p o t e n t i a l o f t h e metal by: Ε = a

+ b (log I)

where Ε i s t h e p o t e n t i a l ( v o l t s ) , I i s t h e c u r r e n t (amps) and a and b a r e c o n s t a n t s ( 7 ) . A t e q u i l i b r i u m , a l l a n o d i c and c a t h o d i c r e a c t i o n s proceed a t an e q u a l , f i n i t e r a t e . The n e t c u r r e n t f l o w i s z e r o . The v o l t a g e c o r r e s p o n d i n g t o t h i s z e r o n e t c u r r e n t i s t h e open c i r c u i t o r c o r r o s i o n p o t e n t i a l . As a p p l i e d v o l t a g e i s changed, t h e r e s u l t i n g c u r r e n t can be recorded t o produce a p o l a r i z a t i o n c u r v e (8) . Electrochemical Coatιngs

E v a l u a t i o n o f C o r r o s i o n P r o t e c t i o n by O r g a n i c

I n 1979, L e i d h e i s e r (9) reviewed t h e use o f c o r r o s i o n p o t e n t i a l measurements w i t h r e g a r d s t o t h e p r e d i c t i o n o f c o r r o s i o n a t meta I o r g a n i c c o a t i n g i n t e r f a c e s . Wolstenholme had l a s t reviewed t h i s l i t e r a t u r e i n 1970 (10). Work i n t h e 1930-1940's f o c u s e d on t h e magnitude o f t h e c o r r o s i o n p o t e n t i a l and how i t changed w i t h t i m e (11-14). N e g a t i v e p o t e n t i a l s w i t h r e s p e c t t o uncoated s u b s t r a t e s were i n d i c a t i v e o f c o r r o s i o n beneath t h e c o a t i n g . P o s i t i v e p o t e n t i a l s w i t h r e s p e c t t o uncoated s u b s t r a t e s were i n d i c a t i v e o f t h e absence o f c o r r o s i o n . Anomalous c a s e s were noted i n which t h i s g e n e r a l i z a t i o n d i d n o t h o l d . These very e m p i r i c a l measurements were f o l l o w e d by more thorough s t u d i e s ( 1 5 ) . T h i n p a i n t f i l m s w i t h very low e l e c t r i c a l r e s i s t a n c e show a c t i v e c o r r o s i o n p o t e n t i a l s which become more p o s i t i v e a s t h e p a i n t f i l m was i n c r e a s e d i n t h i c k n e s s . Shapes o f t h e p o t e n t i a I/time c u r v e s were m i s l e a d i n g as a g u i d e t o u l t i m a t e coating protective properties. Kendig and L e i d h e i s e r (16) e l e c t r o c h e m i c a I l y e v a l u a t e d t h i n (9 micron) p o l y b u t a d i e n e c o a t i n g s on s t e e l . They c o n c l u d e d t h a t movement o f t h e c o r r o s i o n p o t e n t i a l i n t h e n o b l e d i r e c t i o n was i n d i c a t i v e o f an i n c r e a s i n g c a t h o d i c / a n o d i c s u r f a c e area r a t i o . Oxygen and water p e n e t r a t e t h e c o a t i n g t o produce t h e c a t h o d i c r e a c t i o n a t t h e meta I / c o a t i n g i n t e r f a c e . Expérimenta I The e l e c t r o n i c components f o r t h e measurements c o n s i s t e d o f EG&G Model 173 P o t e n t i o s t a t equipped w i t h s l o w sweep o p t i o n (0.1 mv/sec) and EG&G Model 376 L o g a r i t h m i c C u r r e n t C o n v e r t e r . An EG&G Model 175 U n i v e r s a l Programmer s u p p l i e d t h e waveform f o r r u n n i n g t h e p o l a r i z a t i o n e x p e r i m e n t . The o u t p u t from t h e e l e c t r o m e t e r o f t h e 173 and t h e log o u t p u t o f t h e 376 were connected t o a H e w l e t t P a c k a r d Model 7035B X-Y Recorder and t h e p o t e n t i a l p l o t t e d v e r s u s log c u r r e n t .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

50

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

Open c i r c u i t p o t e n t i a l s ( p o t e n t i a l a t z e r o c u r r e n t ) were r e c o r d e d a f t e r a p e r i o d of 10 minutes a t which p o i n t t h e r e a d i n g s were c o n s t a n t f o r c a . 5 seconds. S i g n c o n v e n t i o n s d e s c r i b e d i n ASTM G5-72 (17) a r e used t o r e p o r t t h e d a t a . P o l a r i z a t i o n c u r v e s were obtainecT~250 mv a n o d i c and c a t h o d i c t o t h e open c i r c u i t p o t e n t i a l . The e l e c t r o l y t e s o l u t i o n was 0.5M NaCI ( d i s t i l l e d water) as d e s c r i b e d i n ASTM G5-72. A l l p o t e n t i a l s a r e r e p o r t e d r e l a t i v e t o a s a t u r a t e d calomel e l e c t r o d e (SCE). A c o n v e n t i o n a l t h r e e compartment c o r r o s i o n c e l l was used. A p o l y t e t r a f I u o r o e t h y l e n e s l e e v e i n s e r t e d through t h e 24/40 j o i n t h o l d s t h e t e s t specimen w i t h a known g e o m e t r i c s u r f a c e a r e a o f t h e specimen exposed t o s a l t s o l u t i o n . An 0 - r i n g i n t h e j o i n t e l i m i n a t e s t h e i n f l u e n c e of edge e f f e c t s . E l e c t r i c a l c o n n e c t i o n i s made t o t h e t e s t sample w i t h a s p r i n g loaded w i r e i n c o n t a c t w i t h t h e r e a r (unexposed) s i d e of t h e specimen. A g r a p h i t e rod i n t r o d u c e d through t h e t o p of t h e c e l l s e r v e d as t h e c o u n t e r e l e c t r o d e A s a t u r a t e d calomel e l e c t r o d e w i t h c i r c u i t completed throug a p p r o x i m a t e l y 1 mm fro A l l ni t r i l e - b a s e d c o a t i n g s r e p o r t e d i n t h i s s t u d y weçe a p p l i e d t o B o n d e r i t e 40 c o a t e d s t e e l (B40) p a n e l s (150-300 mg/ft z i n c phosphate p r e t r e a t m e n t ; The P a r k e r Company). A commercial h i g h n i t r i l e polymer (Barex 210) was employed as t h e base r e s i n . Nv i n y I p y r r o l i d o n e ( A l d r i c h ) and gamma-butyrolactone ( A l d r i c h ) were employed as r e a c t i v e d i l u e n t s . A l l o r g a n i c c o a t e d B40 p a n e l s were photocured f o r t h e same l e n g t h o f t i m e e n s u r i n g t h e same amount of c u r e . P a n e l s were then baked a t t e m p e r a t u r e s r a n g i n g from 40°C t o 220 C ( i n 10 C increments) f o r 5, 10 or 15 minutes. Two 5/16" d i s c s were punched from each panel f o r e l e c t r o c h e m i c a l a n a l y s i s . They were p l a c e d i n t h e T e f l o n c e l l h o l d e r d e s c r i b e d above. The o r g a n i c c o a t e d B40 panel edges were then masked and p l a c e d under t h e s a l t f o g environment f o r 24 hours. C o r r o s i o n performance was e v a l u a t e d u s i n g ASTM D 610-68 ( 1 8 ) . A r a t i n g o f 10 was g i v e n f o r no a p p r e c i a b l e r u s t . A r a t i n g o f 0 was g i v e n f o r 100% r u s t i n g . The s c a l e i s l o g a r i t h m i c between t h e two extreme e n d p o i n t s . Resυ I t s B o n d e r i t e 40 - Open C i r c u i t P o t e n t i a l

Measurements

Twelve d i f f e r e n t B o n d e r i t e 40 c o a t e d s t e e l (B40) p a n e l s were examined t o p r o v i d e a s t a t i s t i c a l l y v a l i d v a l u e f o r t h e open c i r c u i t p o t e n t i a l . T h e i r average r e s t p o t e n t i a l was -0.578 V ( v s . SCE) w i t h an average d e v i a t i o n of 20 mv. A f t e r r e c o r d i n g t h e open c i r c u i t p o t e n t i a l , p o l a r i z a t i o n c u r v e s were o b t a i n e d ( F i g u r e 1 ) . C o r r o s i o n R e s i s t a n c e o f t h e Photocured

Coatings

The c o a t i n g c o m p o s i t i o n s c o n t a i n i n g 25 weight p e r c e n t Barex 210 r e s i n (B210) i n e i t h e r N - v i n y l p y r r o l i d o n e (NVP) o r gammab u t y r o l a c t o n e (GBL) were a p p l i e d t o t h e B40 p a n e l s and photocured a t room t e m p e r a t u r e . The c o r r o s i o n r e s i s t a n c e (18) f o r t h e s e two systems was v a s t l y d i f f e r e n t . The B40 p a n e l s showed 100% r u s t i n g (an ASTM r u s t r a t i n g o f 0) a f t e r 24 hours s a l t f o g exposure. The

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

BARTOSZEK-LOZA AND BUTLER

Characterization of Photocured Coatings

l o g CURRENT F i g u r e 1. panels.

Polarization

curve f o r Bonderite

40 c o a t e d

steel

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

52

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

B210/GBL system showed no r u s t (an ASTM r u s t r a t i n g o f 10) a f t e r 24 hours exposure. However, a p p r o x i m a t e l y 50% s u r f a c e r u s t (an ASTM r u s t r a t i n g of 1) was observed f o r t h e B210/NVP system. Post-Thermal Treatment of t h e Photocured C o a t i n g s A f t e r t h e B210 systems were p o s t - t h e r m a l l y t r e a t e d , they were a s s e s s e d e l e c t r o c h e m i c a I l y and by 24 hour exposure t o t h e s a l t f o g environment. P o s t t h e r m a l l y t r e a t e d B40 p a n e l s show no dependence of p o s t thermal t r e a t m e n t t e m p e r a t u r e on e i t h e r c o r r o s i o n performance ( a l l p a n e l s showed 100% r u s t i n g ) o r open c i r c u i t potential (Figure 2). For t h e B210/NVP system, l i t t l e change i n r u s t r a t i n g o c c u r r e d as t h e p o s t - t h e r m a l t r e a t m e n t temperature i n c r e a s e d from 40 C t o 9 0 C (F i g u r e 2 ) . A t t e m p e r a t u r e s g r e a t e r than 90°C, t h e r u s t r a t i n g i n c r e a s e d , r e a c h i n g a maximum a t 140°C, d e c r e a s e d t o a minimum a t 170 C, t h e n i n c r e a s e d a g a i n t h e t h e r m a l l y t r e a t e d sample same b e h a v i o r was o b s e r v e d . That i s , t h e r e s t p o t e n t i a l t r a c k e d t h e r u s t r a t i n g w i t h t h e same minimum and maximum. T h i s b e h a v i o r was observed independent of t h e t i m e (5, 10 o r 15 min) t h a t t h e sample was heated. Exposure of t h e B210/CBL p o s t - t h e r m a l l y t r e a t e d system t o t h e s a l t f o g environment f o r 24 hours gave no r u s t i n g o f any of t h e p a n e l s . A l l p a n e l s had a r u s t r a t i n g of 10. When t h e open c i r c u i t p o t e n t i a l s were p l o t t e d ( F i g u r e 3 ) , minor changes i n p o t e n t i a l were observed which p a r a l l e l e d t h e B210/NVP system. V

P i s c u s s i on I n a s e a r c h f o r r e l i a b l e a c c e l e r a t e d t e s t methods f o r d e t e r m i n i n g c o a t i n g performance, e l e c t r o c h e m i c a l t e c h n i q u e s have o f t e n been e x p l o r e d . The c o r r o s i o n r e s i s t a n c e of a c o a t e d s t e e l panel i s a c o m p o s i t e of t h e s t e e l q u a l i t y , i t s s u r f a c e f i n i s h and t h e q u a l i t y of t h e c o a t i n g . For t h i s r e a s o n , B o n d e r i t e 40 c o a t e d s t e e l p a n e l s were i n c l u d e d i n our work. They were employed p r i m a r i l y t o a i d i n t h e i n t e r p r e t a t i o n of t h e e l e c t r i c a l measurements f o r t h e n i t r i l e based photocured samples. S a l t Fog C o r r o s i o n R e s i s t a n c e The 24 hour s a l t f o g c o r r o s i o n r e s i s t a n c e f o r t h e photocured B210/NVP and B210/GBL systems were v a s t l y d i f f e r e n t . U s i n g NVP as a d i l u e n t , 50% r u s t i n g of t h e sample was seen, w h i l e t h e GBL d i l u e n t showed no r u s t i n g . T h i s d i f f e r e n c e i s a t t r i b u t e d t o t h e i n h e r e n t water s o l u b i l i t y of p o l y v i n y l p y r r o l i d o n e ( 1 9 ) . P r e v i o u s work has shown t h a t t h e r m a l l y c u r e d h i g h ni t r i l e polymer c o a t i n g s have good thermal r e s i s t a n c e ( 2 0 ) . Thus, t h e B210/NVP photocured p a n e l s were then heated i n an a t t e m p t t o improve t h e i r performance. The 24 hour r u s t r a t i n g of t h e s e t h e r m a l l y t r e a t e d t e s t p a n e l s v a r i e d depending on t h e t e m p e r a t u r e of t h e treatment (Figure 2 ) .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5. BARTOSZEK-LOZA A N D BUTLER

Characterization of Photocured Coatings 53

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200

175

F i g u r e 2. Changes i n open c i r c u i t p o t e n t i a l (OPC) and r u s t r a t i n g ( R R ) w i t h temperature f o r p o s t - t h e r m a l l y t r e a t e d B o n d e r i t e 40 coated s t e e l (B40) p a n e l s and Barex 210/Nv i n y l p y r r o l i d o n e (B210/NVP) photocured c o a t i n g s on B40 p a n e l s . -0.41

ο—ο- -

B210/NVP on Β40

LU

ο

(Λ

•

—

—

π

B210/GBL on Β40

-0.47

Δ

Β40

Α

Δ

r \

< Ζ

-0.53

Ο D Ο

ÇC ο ζ

-0.59

LU

α Ο

-0.65 25

50

75

100

125

150

175

200

225

TEMPERATURE (C)

F i g u r e 3. Changes i n open c i r c u i t p o t e n t i a l (OPC) w i t h tempera­ t u r e f o r p o s t - t h e r m a l l y t r e a t e d B40 p a n e l s , Barex 210/Nv i n y l p y r r o l i d o n e (B210/NVP) and Barex 210/gamma-butyrolactone (B210/GBL) photocured c o a t i n g s on B40 p a n e l s .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

54

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

Open C i r c u i t P o t e n t i a l Measurements The open c i r c u i t p o t e n t i a l r e p r e s e n t s t h e p o t e n t i a l o f t h e system t a k e n a f t e r a p p r o x i m a t e l y a 10 minute e q u i l i b r a t i o n p e r i o d . I n a d d i t i o n t o open c i r c u i t p o t e n t i a l measurements, p o l a r i z a t i o n c u r v e s were r u n f o r each sample. I t was hoped a c o r r e l a t i o n c o u l d be seen between t h e v i s u a l assessment o f c o r r o s i o n and p o l a r i z a t i o n . U n f o r t u n a t e l y , o n l y open c i r c u i t p o t e n t i a l s c o u l d be c o r r e l a t e d w i t h performance. T h e r e f o r e , no a t t e m p t s were made t o c o r r e l a t e t h e c o r r o s i o n c u r r e n t d e n s i t y o r t h e shapes o f t h e p o l a r i z a t i o n c u r v e s w i t h performance. The open c i r c u i t p o t e n t i a l i s t h e sum o f a l l p o s s i b l e a n o d i c and c a t h o d i c r e a c t i o n s o f t h e system. I n t h e most i d e a l c a s e , t h e r e a r e a t l e a s t f o u r pathways t o complete t h e e l e c t r i c a l c i r c u i t between t h e s o l u t i o n and t h e metal s u b s t r a t e t o b r i n g about c o r r o s i o n : A. t h r o u g h t h e o r g a n i c c o a t i n g and phosphate p r e t r e a t m e n t B. t h r o u g h t h e phosphat in t h e organic coating C. t h r o u g h t h e o r g a n i c c o a t i n g where t h e r e i s a p i n h o l e i n t h e phosphate p r e t r e a t m e n t . D. t h r o u g h a p i n h o l e i n both t h e o r g a n i c c o a t i n g and phosphate pretreatment. W h i l e d e f i n i t i v e w e i g h t s cannot be p l a c e d on each f a c t o r , a b e t t e r u n d e r s t a n d i n g o f t h e i r i n f l u e n c e can be made by c o n s i d e r i n g t h e t h i c k n e s s o f t h e system components. U s i n g an average d e n s i t y v a l u e o f 3.335 g/cm f o r z i n c phosphate ( h o p e i t e ) and assuming a B40 panel coverage o f 150-300 mg/ft , t h e z i n c phosphate c o n v e r s i o n c o a t i n g t h i c k n e s s can range from 0.53-1.07 m i c r o n s . The t h i c k n e s s of t h e B210/NVP and B210/GBL systems ranged from 2.5-5.0 m i c r o n s . O v e r a l l , t h i s i s a very t h i n system and r a p i d c o m p l e t i o n o f t h e e l e c t r o c h e m i c a l c i r c u i t i n a very s h o r t t i m e p e r i o d i s v i a b l e . Based on t h e l i t e r a t u r e , t h e c o r r e l a t i o n between open c i r c u i t p o t e n t i a l r u s t r a t i n g and temperature was unexpected. To r u l e o u t t h e f a c t t h a t t h e open c i r c u i t p o t e n t i a l measurements d i d n o t s i m p l y r e p r e s e n t changes i n t h e phosphated s u b s t r a t e due t o t e m p e r a t u r e , B40 p a n e l s were a l s o s u b j e c t e d t o heat t r e a t m e n t . As shown i n F i g u r e 2, no dependence o f t h e open c i r c u i t p o t e n t i a l on p o s t thermal t r e a t m e n t t e m p e r a t u r e i s o b s e r v e d . T h i s suggested t h a t t h e open c i r c u i t p o t e n t i a l measurements r e f l e c t e d changes a t t h e c o a t i n g / B 4 0 panel i n t e r f a c e and n o t i n t h e B40 panel a l o n e . N o b i l i t y and t h e Degree o f C o r r o s i o n R e s i s t a n c e High n i t r i l e photocured c o a t i n g s on B40 c o a t e d s t e e l p a n e l s e x h i b i t c o r r o s i o n r e s i s t a n c e when t h e i r open c i r c u i t p o t e n t i a l s a r e e i t h e r more n o b l e o r l e s s n o b l e t h a n t h e B40 c o a t e d s t e e l panel ( F i g u r e 3 ) . A t f i r s t , we assumed t h a t enhanced n o b i l i t y s h o u l d g i v e r i s e t o enhanced c o r r o s i o n r e s i s t a n c e , i . e . t h e g a l v a n i c s e r i e s . G r o s e c l o s e (21) used a n o d i c p o l a r i z a t i o n t o e l e c t r o c h e m i c a l l y r a t e uncoated s t e e l . I t was n e c e s s a r y t o employ a s p e c i f i c e l e c t r o l y t e (lOmM NaCI/25 mM sodium t e t r a f I u o r o b o r a t e ) and e l e c t r o c h e m i c a I l y c l e a n e d c o l d r o l l e d s t e e l . Two d i f f e r e n t l o t s o f s t e e l w i t h i d e n t i c a l primer e x h i b i t e d g r o s s l y d i f f e r e n t s a l t f o g performance. The bottom h a l v e s (uncoated) o f t h e same panel t e s t e d

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5.

Characterization of Photocured Coatings

BARTOSZEK-LOZA AND BUTLER

i n t h e s a l t s p r a y was e l e c t r o c h e m i c a 1 1 y t e s t e d . I n g e n e r a l , t h e "good" l o t o f s t e e l p a n e l s e x h i b i t e d p a s s i v a t i o n ( i . e . n e a r l y c o n s t a n t c u r r e n t ) over most o f t h e 250 mv a n o d i c s c a n . The "poor" l o t e x h i b i t e d l o s s of p a s s i v a t i o n . E x a m i n a t i o n of t h e r e s p e c t i v e rest potentials i n d i c a t e d t h e "poor" s t e e l l o t had more n o b l e r e s t p o t e n t i a l s t h a n t h e "good" s t e e l l o t . Our e l e c t r o c h e m i c a l work d i f f e r e d d r a s t i c a l l y from t h e G r o s e c l o s e work i n t h a t polymer c o a t e d metal samples were employed. F u r t h e r m o r e , we found t h a t c o a t i n g s can have c o r r o s i o n r e s i s t a n c e when t h e i r r e s t p o t e n t i a l s a r e e i t h e r more n o b l e (B210/NVP) o r l e s s n o b l e (B210/GBL) t h a n t h e uncoated s u b s t r a t e . L e i d h e i s e r (22,23) examined z i n c phosphate p r e t r e a t e d p a n e l s w i t h a u t o m o t i v e p r i m e r a f t e r 10 days exposure t o t h e s a l t s p r a y . The s t a r t and f i n i s h r e s t p o t e n t i a l s of t h e samples w i t h good p a i n t performance were c o n s i s t e n t l y more n e g a t i v e t h a n t h o s e samples w i t h poor p a i n t performance:

E

Poor samples Good samples

start(

v o l t s

-0.566 -0.629

>

E

finish(™

l t s

>

-0.757 -0.770

The e f f e c t was more pronounced a t t h e s t a r t i n g p o t e n t i a l t h a n a t t h e f i n i s h p o t e n t i a l . L e i d h e i s e r suggested t h a t t h e b e s t performance i s o b t a i n e d when t h e cathode/anode s u r f a c e a r e a r a t i o i s t h e same as t h e uncoated m e t a l . Inadequate performance i s o b t a i n e d when t h e cathode/anode a r e a r a t i o becomes l a r g e r . Our work a g r e e s w i t h L e i d h e i s e r ' s h y p o t h e s i s . The B210/GBL c o a t i n g s have r e s t p o t e n t i a l s l e s s n o b l e t h a n t h e B40 c o a t e d s t e e l p a n e l s and p e r f o r m b e s t i n t h e s a l t f o g environment. R u s t R a t i n g - Temperature

Correlation

F i g u r e 2 was s e p a r a t e d i n t o f o u r r e g i o n s : Region I (room t e m p e r a t u r e t o 90°C), Region I I (100°C-120°C), Region I I I (130°C-170°C) and Region IV (170 C and a b o v e ) . I n Region I , heat has no e f f e c t . I n Region I I , t h e enhancement i n performance can be e x p l a i n e d by changes i n polymer m a t r i x c r y s t a l I i n i t y . G l a s s t r a n s i t i o n t e m p e r a t u r e s f o r t h e homopolymer c o n s t i t u e n t s of t h e B210/NVP m a t r i x range from 86°C t o 105 C. T h i s i n c r e a s e i n amorphous n a t u r e of t h e m a t r i x s h o u l d r e s u l t i n a more t o r t u o u s path f o r water and i o n permeation and i n c r e a s e t h e c o r r o s i o n r e s i s t a n c e of t h e c o a t i n g . I n Region I I I , t h e maximum i n t h e r u s t r a t i n g i s o b s e r v e d . The maximum i s p o s s i b l e due t o t h e combined changes i n t h e o r g a n i c c o a t i n g m a t r i x and t h e B40 p a n e l . P o l y v i n y l p y r r o l i d o n e becomes water i n s o l u b l e when heated t o 150°C due t o c r o s s I i n k i n g ( 1 9 ) . Recent e v i d e n c e a l s o s u g g e s t s t h a t p o l y v i n y l p y r r o l i d o n e i t s e l f may a c t as a c o r r o s i o n i n h i b i t o r ( 2 4 ) . As t h e t e m p e r a t u r e i s i n c r e a s e d , f u r t h e r c r o s s I i n k i n g may r e s u l t i n t h e b u i l d u p o f s t r e s s i n t h e c o a t i n g . I f c o h e s i v e breakdown were t o o c c u r , a pathway f o r water and i o n permeation becomes a v a i l a b l e .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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56

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

With r e g a r d s t o t h e c o n v e r s i o n c o a t e d s u b s t r a t e , W i t t e l (25) o b s e r v e d t h a t a t t e m p e r a t u r e s g r e a t e r t h a n 140°C, t e t r a h y d r a t e z i n c phosphates l o s e p a r t o f t h e i r water o f h y d r a t i o n . I t i s likely t h a t t h e water o f h y d r a t i o n l i b e r a t e d i n t h e phosphate r e c r y s t a 1 1 i z a t i o n p r o c e s s has a n e g a t i v e e f f e c t on t h e a d h e s i o n o f the polymer m a t r i x t o t h e B40 p a n e l . In Region I V , performance a g a i n i n c r e a s e s . CrystaIlographic t r a n s f o r m a t i o n s i n phosphate c o n v e r s i o n c o a t i n g s a t 180 C a r e known t o a d v e r s e l y a f f e c t phosphate However, n i t r i l e c y c l i z a t i o n may be i m p o r t a n t a t t h e s e h i g n e r t e m p e r a t u r e s . C y c l i z a t i o n i s b e l i e v e d t o enhance b a r r i e r p r o p e r t i e s o f h i g h n i t r i l e polymers (27,28,29). R u s t R a t i n g - Open C i r c u i t P o t e n t i a l - Temperature

Correlation

The open c i r c u i t p o t e n t i a l d a t a f o r t h e B210/NVP system m i r r o r s t h e behavior of the r u s t r a t i n g plausible explanation o as f o l l o w s . As t e m p e r a t u r e i s i n c r e a s e d , t h e c o m p o s i t i o n o f t h e v a r i o u s o x i d e s and h y d r o x i d e s which make up t h e z i n c phosphate c o n v e r s i o n l a y e r and t h e base i r o n o x i d e l a y e r undergo changes. These c o m p o s i t i o n a l changes a r e r e f l e c t e d i n t h e c h a n g i n g open c i r c u i t potentials. I n terms o f n o b i l i t y , t h e open c i r c u i t p o t e n t i a l s f o r t h e B210/NVP system a t a l l t e m p e r a t u r e s a r e more p o s i t i v e t h a n t h e B40 p a n e l s . The open c i r c u i t p o t e n t i a l s f o r t h e B210/GBL system a t a l l t e m p e r a t u r e s a r e g e n e r a l l y more n e g a t i v e t h a n t h e B40 p a n e l s . The open c i r c u i t p o t e n t i a l t r e n d s f o r t h e B210/GBL system mimic t h e B210/NVP system. However, t h e e f f e c t s a r e n o t a s pronounced. We propose t h a t i n t h e GBL system t h e r e i s a r e d u c t i o n i n t h e cathode/anode a r e a r a t i o a s suggested by L e i d h e i s e r ( 1 6 ) . In c o n c l u s i o n , we have shown t h a t a s i m p l e , f a s t , e l e c t r o c h e m i c a l measurement, t h e open c i r c u i t p o t e n t i a l , can be extremely useful i n assessing t h e e f f e c t of d i l u e n t f o r high n i t r i l e p h o t o c u r e d c o a t i n g s . F u r t h e r work i s underway t o e l u c i d a t e t h e u n d e r l y i n g reasons behind t h e t e m p e r a t u r e induced changes observed i n c o r r o s i o n performance. Literature Cited 1. "Corrosion Control by Organic Coatings"; Leidheiser, H. Jr. Ed.; National Association of Corrosion Engineers: Houston, Texas, 1981. 2. Bacon, R. C . ; Smith, J . J.; Rugg, F. M. Ind. Eng. Chem. 1948, 40, 161. 3. Mills, D. J . Pitture E Vernici 1984, 7, 102. 4. Hare, C. FEDERATION OF SOCIETIES FOR COATINGS TECHNOLOGY Units 26 and 27, 1978. 5. Nemphos, S. P.; Salame, M.; Steingeiser In "Encyclopedia of Polymer Technology"; Bikales, N. M. Ed.; Wiley-Interscience: New York, 1976; p. 65. 6. American Cyanamid Co. "The Chemistry of Acrylonitrile"; 1959. 7. Tafel, J . Z. Physik. Chem. 1905, 50, 641

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5. BARTOSZEK-LOZA AND BUTLER

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

Characterization of Photocured Coatings

"Electrochemical Techniques for Corrosion"; Baboian, J . Ed.; National Association for Corrosion Engineers: Houston, Texas, 1978. Leidheiser, H. Jr. In "Corrosion Control by Coatings"; Leidheiser, H. Jr. Ed.; Science Press: Princeton, New Jersey, 1979; p. 143-170. Wolstenholme, J . Corrosion Science 1973, 13, 521. Burns, R. M.; Haring, Η. Ε. Trans. Electrochem. Soc. 1936, 69, 169. Haring, Η. E . ; Gibney, R. B. Trans. Electrochem. Soc. 1939, 76, 287. Whitby, L. Paint Research Asscn. 1939, Tech. Paper No. 125. Zahn, H. Corrosion 1947, 3, 233. Wormwell, F . ; Brasher, D. M. J . Iron Steel Inst. 1950, 164, 141. Kendig, M. W.; Leidheiser H Jr J Electrochem Soc 1976 123, 982. "Standard Referenc Potentiodynamic Anodic Polarization Measurements", ASTM G5-72. "Evaluating Degree of Rusting on Painted Steel Surfaces", ASTM D610-68. Lorenz, D. H. Encyclopedia of Polymer Technology; Βikales, N. M. Ed.; Wiley-Interscience: New York, 1976; p. 239. Talsma, H; Giffen, M. W. U. S. Patent 4 329 401, 1980. Groseclose, R. G.; Frey, C. M.; Floyd, F. L. J . Ctg. Tech. 1984, 56, 31. Iezzi, R. Α.; Leidheiser, H. Jr. Corrosion 1981, 37, 28. Leidheiser, H. Jr. Corrosion 1983, 39, 189. Mostafa, A. El-Khair B. Corr. Prev. and Control 1983, 30, 14. Wittel, K. Ind. Lackier Betrieb 1983, 51, 169. van Ooij, W. J . Proc. 10th Conf. in Organic Coatings Science and Technology, 1984, p. 381. Grassie, N.; Hay, J . N. J . Polym. Sci. 1962, 56, 189. Coleman, M. M.; Sivy, G. T. Carbon 1981, 19, 123. Fochler, H. S.; Mooney, J . R.; Ball, L. E.; Boyer, R. D.; Grasselli, J . G. Spectrochemica Acta, 1985, 41A, 1/2, 271.

RECEIVED January 21, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

57

6 Alternating Current Impedance: Utility in Evaluating Phosphate Coating, Phosphorus-Chromium Rinse, and Paint Performance C. P. Vijayan, D. Noël, and J.-J. Hechler Industrial Materials Research Institute, National Research Council of Canada, 75, Boulevard de Mortagne, Boucherville Quebec Canada J4B 6Y4

Surface preparation methods are related to impedance behavior of painted aluminum-killed 1006 steel in a 3 weight percent sodium chloride solution. Variation of phosphating time, anodic phosphating, phosphochromic rinse, paint thickness, temperature of test solution and immersion time are studied using two different types of paints. For porous coatings, the beneficial effects of phospho-chromic rinse is confirmed. Decrease in coating resistance and increase in capacitance are observed with increasing time of immersion as well as with increasing test temperature. Progress in deterioration is indicated by the appearance of Warburg-type behavior. Good protection is obtained by phosphating for 5 minutes.

S i m p l i c i t y and r e l i a b i l i t y o f o p e r a t i o n make AC impedance measurements a t t r a c t i v e as a t e c h n i q u e i n the evaluation of coating i n t e g r i t y . As opposed t o c l a s s i c a l s a l t s p r a y t e s t , a n a l y s i s t i m e s are s h o r t e r w i t h t h e AC impedance t e c h n i q u e and q u a n t i t a t i v e d a t a are o b t a i n e d permitting relevant mechanistic i n f o r m a t i o n t o be derived. Impedance t e s t methods a r e l i k e l y t o f i n d many a p p l i c a t i o n s i n the r e s o l u t i o n o f unsolved p r a c t i c a l problems U_). V a r i o u s p u b l i c a t i o n s have appeared i n r e c e n t years r e g a r d i n g the a p p l i c a t i o n o f AC impedance and p o l a r i z a t i o n r e s i s t a n c e t e s t methods i n t h e s t u d y o f s u r f a c e m o d i f i c a t i o n s such as o x i d a t i o n , p a s s i v a t i o n , c a t h o d i c d e p o s i t i o n , c o a t i n g s and t h e s t u d y o f c o r r o sion reactions (2-8). A n a l y s i s o f impedance d a t a p r o v i d e s c l u e s r e g a r d i n g t h e mechanism o f r e a c t i o n s l i k e l y t o be t a k i n g p l a c e a t d i f f e r e n t i n t e r f a c e s i n t h e system ( 9 , 1 0 ) . E l e c t r i c a l equivalent c i r c u i t s proposed, i n c o r p o r a t i n g impedance c o r r e s p o n d i n g t o e l e c t r o l y t e / s u r f a c e c o a t i n g as w e l l as s u r f a c e c o a t i n g / m e t a l s u r f a c e , are o f g r e a t h e l p i n e v a l u a t i n g and i m p r o v i n g t h e nature o f c o a t i n g s (11-17). They a r e a l s o u s e f u l i n d e t e r m i n i n g compatible c o a t i n g / e n v i r o n m e n t combinations· This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6.

VIJAYAN ET AL.

59

Evaluation by AC Impedance

In t h e present work, s t e e l s u r f a c e s p o l i s h e d , phosphated and p a i n t e d a r e s t u d i e d u s i n g AC impedance t e c h n i q u e i n order to e v a l u a t e t h e p r o t e c t i o n e f f i c i e n c y o f a commercial phosphating solution. The AC impedance b e h a v i o r o f p a i n t e d metal has been c o r r e l a t e d w i t h t h e immersion time i n t h e phosphating s o l u t i o n and w i t h t h e d e s i r a b i l i t y o f a phospho-chromic r i n s e (18-23). A comparison o f t h e impedance b e h a v i o r o f two d i f f e r e n t types o f commercial p a i n t s i s made f o r v a r i o u s d u r a t i o n s o f immersion i n sodium c h l o r i d e s o l u t i o n a t room t e m p e r a t u r e , and a l s o f o r v a r i o u s temperatures a t a g i v e n d u r a t i o n o f immersion. M e c h a n i s t i c a n a l y s e s proposed by G a b r i e l l i e t a l (25) and S l u y t e r s (26) a r e made use o f i n u n d e r s t a n d i n g the r e s u l t s o b t a i n e d i n t h i s work. THEORETICAL An examination of th d i s s o l u t i o n and f o r t h enables t h e r a t e - d e t e r m i n i n g s t e p o f t h e c o r r o s i o n r e a c t i o n t o be identified. I t i s then p o s s i b l e t o s e p a r a t e l y s t u d y t h e r a t e determining step i n order to f i n d a s u i t a b l e i n h i b i t o r or a suitable surface coating. B o c k r i s e t a l (24) proposed a two s t e p mechanism f o r i r o n d i s s o l u t i o n i n v o l v i n g an adsorbed i n t e r m e d i a t e s p e c i e s ( F e 0 H ) . a d s

ki = ^

Fe + OH-

(Fe0H)

(Fe0H)

—+

a d s

k

+ e

a d s

(Fe0H)

+

+ e

2

In t h e b u l k o f t h e s o l u t i o n , t h e r e a c t i o n k

3 — ^

(FeOH)+

F e * * + OH"

takes p l a c e but i t i s n o t c o n s i d e r e d t o i n t e r v e n e i n t h e e l e c t r o d e kinetics. T h i s mechanism can be r e l a t e d t o t h e f o r m a l d i s s o l u t i o n model proposed by G a b r i e l l i e t a l (25) i n terms o f f a r a d a i c impedance. I f χ r e p r e s e n t s t h e s u r f a c e c o n c e n t r a t i o n of - ( F e 0 H ) i t can be shown t h a t a d s

- F I kk x

1

f

(1-*) - ( k _ ' - k ' ) l T - ( k ^ f k . k 1

2

r

2

)

Ζ F χ = k /(k +k_ +k ) 1

1

1

2

and dx _ k i ' d - x ) - (k-χ· - k dE

f 2

)x

k +k- +k +j 3 1

1

2

w

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

60

POLYMERIC MATERIALS FOR CORROSION CONTROL

where Z - f a r a d a i c impedance; F = Faraday c o n s t a n t ; k - r a t e constants; k d e r i v a t i v e s of rate constants with respect to the p o t e n t i a l , Ε; ω * 2wf where f i s t h e f r e q u e n c y i n Hz; 8 - c o n s t a n t c o u p l i n g t h e s u r f a c e c o n c e n t r a t i o n o f ( F e 0 H ) j and t h e f r a c t i o n ­ a l s u r f a c e coverage. The appearance o f c a p a c i t i v e o r i n d u c t i v e impedance depends e s s e n t i a l l y on t h e v a l u e o f t h e r a t e c o n s t a n t s . Low f r e q u e n c y l o o p s , i n a g e n e r a l c a s e , a r e a l l v e r y s e n s i t i v e t o t h e pH o f t h e electrolyte. The d i f f e r e n t time c o n s t a n t s a r e a t t r i b u t e d t o t h e r e l a x a t i o n o f s u r f a c e coverage by a c o r r e s p o n d i n g number o f r e a c ­ tion intermediates. Rehbach and S l u y t e r s ( 2 6 ) suggest the following general e x p r e s s i o n f o r impedance Ζ (-Ζ* + j Z ) F

f s s

a (

s

l f

R

+aa>~i - 1 fwC (σω ic

(R +σω~*)

2

2

+a C

+σω"^Ί

+1) dl

d l ct

with σ - σ + σ ; - ohmic c e l l r e s i s t a n c e ; R t - charge transfer resistance; σ Warburg c o e f f i c i e n t ; σ , σ - s e p a r a t e Warburg c o e f f i c i e n t s f o r o x i d i s e d and reduced species; C ^ i double l a y e r c a p a c i t a n c e . F o r g i v e n v a l u e s o f double l a y e r c a p a c i t a n c e C ^ i , s o l u t i o n r e s i s t a n c e R^ and Warburg c o e f f i c i e n t σ, p l o t s o f - Z versus Z have been made f o r s e l e c t e d v a l u e s o f charge t r a n s f e r r e s i s t a n c e , Rct ( 2 6 ) . I t i s observed t h a t a t s m a l l e r v a l u e s o f R ^ (~10 Ω cm ) r e l a x a t i o n due t o R c t ~ d l and Warburg d i f f u s i o n b e h a v i o r a r e both c l e a r l y seen. The a n a l y s i s proposed by G a b r i e l l i (25) does n o t t a k e d i f ­ f u s i o n e f f e c t s i n t o c o n s i d e r a t i o n . However, t h i s model t o g e t h e r w i t h t h e d i s s o l u t i o n mechanism proposed by B o c k r i s show how r e l a ­ t i v e v a r i a t i o n s i n t h e v a l u e s o f r a t e c o n s t a n t s can g i v e r i s e t o d i f f e r e n t types o f N y q u i s t d i a g r a m s . I n o t h e r words, i t i s p o s s i ­ b l e t o e v a l u a t e r a t e c o n s t a n t s f o r a p a r t i c u l a r system by l o o k i n g at t h e N y q u i s t diagram i f t h e experiment has p r o p e r l y been designed. In systems where d i f f u s i o n phenomena a r e o f s i g n i f i c a n c e , t h e m e c h a n i s t i c s t u d y i s f a c i l i t a t e d by u s i n g t h e g e n e r a l e x p r e s s i o n f o r impedance Ζ ( 2 6 ) . T h i s e q u a t i o n shows f o r i n s t a n c e how t h e Warburg c o e f f i c i e n t can be e v a l u a t e d by c o n d u c t i n g impedance s t u d i e s a t v e r y low f r e q u e n c i e s . These c o e f f i c i e n t s i n t u r n enable the e v a l u a t i o n o f d i f f u s i o n c o e f f i c i e n t s f o r the d i f f u s i n g s p e c i e s . Thus i t appears t h a t by i n c o r p o r a t i n g parameters such as pore r e s i s t a n c e and c o a t i n g c a p a c i t a n c e t o the e x i s t i n g t h e o r e t i c a l i m ­ pedance model d e a l i n g w i t h m e t a l d i s s o l u t i o n one would o b t a i n v a l u ­ a b l e o v e r a l l i n f o r m a t i o n (14,27). Complemented by r e s u l t s from r e ­ g u l a r immersion and s a l t s p r a y t e s t s i t s h o u l d be p o s s i b l e t o f i n d s a t i s f a c t o r y s o l u t i o n s t o c o r r o s i o n problems o f coated metals ( 9 ) . A g e n e r a l i s e d model o f e l e c t r i c a l e q u i v a l e n t c i r c u i t f o r p a i n t e d s u r f a c e s has been c o n s i d e r e d i n many o f the r e c e n t p u b l i c a ­ t i o n s . Googan (2) used i t t o s t u d y v i n y l c o a t i n g s f r e e o f d e f e c t s and coatings containing defects. E l e c t r o c o a t i n g s were a l s o e v a l u a t e d . M u s i a n i e t a l (27) i n t h e i r i n v e s t i g a t i o n o f m i l d s t e e l 0

Γ

c

β

0

Γ

, f

c

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

?

6.

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Evaluation by AC Impedance

c o a t e d w i t h e l e c t r o c h e m i c a l l y s y n t h e s i s e d polyoxyphenylenes used an e l e c t r i c a l equivalent c i r c u i t i n c o r p o r a t i n g elements such as coating capacitance C and pore r e s i s t a n c e R r e Certain samples showed f a s t e r c o r r o s i o n r a t e s p r o b a b l y due t o poor a d h e s i o n to the metal substrate. Conversion or pseudo-conversion phosphating w h i c h forms a p r o t e c t i v e l a y e r on the base m e t a l u s u a l l y improves t h e f i n a l performance of p a i n t s by i n c r e a s i n g t h e contact area and thus adhesion and r e d u c i n g b l i s t e r i n g and underskin c o r r o s i o n . P i e n s ( 9 ) suggested a similar equivalent c i r c u i t slightly modifying the i n c o r p o r a t i o n of e l e c t r o l y t e r e s i s t a n c e . Chlorinated rubber pigmented w i t h i r o n o x i d e was a p p l i e d t o s a n d - b l a s t e d steel. Impedance measurements were taken a f t e r 24 hours o f immers i o n i n 0.5M N a C l , a t the c o r r o s i o n p o t e n t i a l . Two c a p a c i t i v e s e m i - c i r c l e s c o v e r i n g t h e frequency range 1 0 t o 280 Hz and 280 Hz t o 0.28 Hz were o b t a i n e d The l i n e a r s e c t i o n l y i n g between 0.28 Hz and 1 0 " Hz i s c h a r a c t e r i s t i The v a l u e s of C o f s e n s i b i l i t y o f a c o a t i n g t o water. T h e i r v a r i a t i o n e n a b l e s one t o compare v a r i o u s c o a t i n g s as water b a r r i e r s . The r e s i s t a n c e Rpore c o a t i n g i s o f t e n so h i g h t h a t the impedance diagram i s no l o n g e r a complete s e m i - c i r c l e , but r a t h e r an a r c i n t e r s e c t i n g t h e o r i g i n o f t h e axes a t h i g h f r e q u e n c i e s . I n such c a s e , t h e v a l u e s f o r Rpore c measured a t a g i v e n frequency a r e used t o e v a l u a t e c o a t i n g s . These v a l u e s a r e i n f l u e n c e d by water a b s o r p t i o n , t h e i n f l u e n c e b e i n g pronounced a t lower f r e q u e n c i e s . Parameters such as a g e i n g , i n f l u e n c e o f pigment c o n c e n t r a t i o n and i n f l u e n c e o f c o a l e s c e n c e o f e m u l s i o n p a i n t s can a l s o be s t u d i e d u s i n g AC impedance t e s t methods. Mansfeld and Kendig 05) evaluated different surface p r e t r e a t m e n t s o f s t e e l and aluminum a l l o y s . They suggest t h e usage o f curve f i t t i n g methods such as CIRFIT program i n o r d e r t o overcome t h e d i f f i c u l t i e s posed by e x p e r i m e n t a l d a t a scatter. T h e i r s t u d y o f phosphated and coated s t e e l i n d i c a t e s the appearance of a Warburg-type impedance a t l o n g e r exposure t i m e s . A slope of -1/2 o r -1/4 i s observed i n the Bode p l o t w i t h a c o r r e s p o n d i n g phase angle maximum of 45° o r 22.5°. These b e h a v i o r s perhaps represent respectively semi-infinite linear diffusion and s e m i - i n f i n i t e d i f f u s i o n i n pores. Padget and Moreland (11) showed that films cast from c h l o r i d e - c o n t a i n i n g v i n y l a c r y l i c l a t e x copolymers e x h i b i t e d low uptake of l i q u i d water and i o n s when the degree o f p a r t i c l e c o a l e s c e n c e was h i g h . AC impedance measurements supplemented by s a l t s p r a y and outdoor exposure r e s u l t s showed a r e l a t i o n s h i p between the permeation characteristics and t h e a n t i c o r r o s i v e performance o f l a t e x f i l m s and l a t e x p a i n t s , and showed the advantages t o be gained by u s i n g low p e r m e a b i l i t y c h l o r i n e c o n t a i n i n g polymer. D e t e r i o r a t i o n o f t h e c o a t i n g s was found t o pass t h r o u g h a s e r i e s of s t a g e s each c h a r a c t e r i z e d by i t s own d i s t i n c t i v e N y q u i s t plot. I n t h e f i r s t s t a g e , t h e impedance diagram had l i t t l e c u r v a t u r e and corresponded t o a h i g h impedance v a l u e . The p l o t then became curved due t o the i n g r e s s o f i o n s i n t o t h e c o a t i n g , f i l m e

c

p 0

5

2

o f

a

a n d

c

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

62

resistance f e l l progressively u n t i l rate controlling diffusion p r o c e s s e s were apparent and the f i l m had a v e r y low impedance. C o a l t a r epoxy and p l a s t i c i z e d c h l o r i n a t e d rubber l a q u e r c o a t e d on m i l d s t e e l were s t u d i e d by S c a n t l e b u r y e t a l ( 2 8 ) . Impedance p l o t s show a g r a d u a l decrease i n the v a l u e of R t and the onset of Warburg-type b e h a v i o r w i t h i n c r e a s i n g immersion time i n 3 weight percent sodium c h l o r i d e s o l u t i o n . Appearance of an i n d u c t i v e l o o p when the c o a l - t a r epoxy had a p i n - h o l e was c l e a r l y demonstrated. The present s t u d y d e a l s w i t h two types of p a i n t s a p p l i e d on phosphated s t e e l , the i n f l u e n c e of temperatures upto 90 °C and immersion times upto 10 days. The r e s u l t s o b t a i n e d are a n a l y z e d i n the l i g h t of p u b l i s h e d i n f o r m a t i o n b r i e f l y d e s c r i b e d i n t h i s s e c t i o n . An e l e c t r i c a l e q u i v a l e n t c i r c u i t s i m i l a r t o the one used by M u s i a n i e t a l (27) i s c o n s i d e r e d s u i t a b l e f o r the a n a l y s i s . c

EXPERIMENTAL AC impedance system 368 (EG&G PARC, P r i n c e t o n , NJ) w i t h F a s t F o u r i e r Transform a n a l y s i s was used a l o n g w i t h a p o t e n t i o s t a t EG&G model 273. The p o t e n t i o s t a t was coupled to an Apple 11+ computer t h r o u g h an IEEE-488 i n t e r f a c e . A f r e q u e n c y range of 0.01 Hz t o 1 0 Hz was used f o r many of the e x p e r i m e n t s . The e l e c t r o l y t e used was a 3 weight percent sodium c h l o r i d e s o l u t i o n and was prepared u s i n g ACS c e r t i f i e d c h e m i c a l s and d e i o n i z e d w a t e r . A l u m i n u m - k i l l e d 1006 s t e e l sheet (Sidbec-Dosco, C o n t r e c o e u r , Québec, Canada) was cut t o 1 cm X 1 cm s i z e , s o l d e r e d t o a copper w i r e , embedded i n epoxy, p o l i s h e d t o 600 g r i t , washed r e s p e c t i v e l y i n t a p - w a t e r , methanol and d e i o n i z e d w a t e r , d r i e d i n a stream of a i r and was then p r e s e r v e d i n a d e s i c c a t o r . D e t a i l s of p h o s p h a t i n g (Oxy-Plus 84 DRS, L a b o r a t o i r e Brabant Inc., V i l l e St-Pierre, Québec, Canada), r i n s i n g and p a i n t i n g (Tremclad Rust P a i n t , Tremco Ltd., T o r o n t o , O n t a r i o , Canada) used i n the f i r s t s e t of specimens are shown i n Table I . P a i n t e d specimens were d r i e d f o r 2 days a t room t e m p e r a t u r e . The average t h i c k n e s s o f s i n g l e l a y e r p a i n t was 20 ]im f o r t h e s e specimens as measured u s i n g an i n s t r u m e n t NEO-DERM, Model 179-711 ( M i t u t o y o Mfg. Co. L t d . , Tokyo, J a p a n ) . Impedances of c o a t e d specimens were measured a f t e r 4 hours of immersion i n 3% NaCl at the r e q u i r e d t e m p e r a t u r e . Coated specimens were p l a c e d i n an open t h r e e - e l e c t r o d e electrochemical c e l l . A f t e r 4 hours of immersion a t ambient temperature, o p e n - c i r c u i t p o t e n t i a l s were noted and impedance measurements were made on d u p l i c a t e samples. Specimens were t e s t e d i n a s a l t s p r a y t e s t c a b i n e t (ASTM Bl17-73) f o r 1, 17 and 96 hours r e s p e c t i v e l y and t h e i r s u r f a c e s photographed i n o r d e r t o c a l c u l a t e the percentage of s u r f a c e covered by c o r r o d e d s p o t s and b l i s t e r s (ASTM D610-68). Table I I shows the d e t a i l s of p r e p a r a t i o n f o r the second s e t of 10 p a i n t e d s t e e l specimens. A p o l y u r e t h a n e p a i n t (Marinox SR-2, Mabraco I n t e r n a t i o n a l , 20 des N a v i g a t e u r s , Québec, Canada) a l o n g with an initial coating of an aluminum containing paint (prépolymère d'aluminium, Mabraco I n t e r n a t i o n a l ) was the second type of p a i n t system used i n t h i s s t u d y . 5

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Evaluation by AC Impedance

The method of p r e p a r a t i o n f o r the second type of p a i n t con­ s i s t e d of m e c h a n i c a l p o l i s h i n g , c l e a n i n g , p h o s p h a t i n g f o r 5 minutes w i t h Oxy-Plus 84 DRS s o l u t i o n , d r y i n g f o r 30 minutes w i t h o u t r i n s ­ i n g , a p p l i c a t i o n of one l a y e r of p r e c o a t f o l l o w e d by a i r d r y i n g f o r 4 hours a t room temperature and a p p l i c a t i o n of one l a y e r of Marinox SR-2 p a i n t f o l l o w e d by c u r i n g f o r 8 days a t room t e m p e r a t u r e . P o l a r i z a t i o n r e s i s t a n c e , R , of specimens #25 and #27 was measured as a f u n c t i o n of immersion time u s i n g C o r r o s i o n Console 350A (EG&G PARC, P r i n c e t o n , N J ) . A l l specimens a n o d i c a l l y phosphated were i n i t i a l l y p o l i s h e d t o 600 g r i t , c l e a n e d and immersed i n the p h o s p h a t i n g s o l u t i o n Oxy-Plus 84 DRS, b e f o r e a p p l y i n g a p o t e n t i a l of +0.8V/SCE. T h i s p o t e n t i a l c o r r e s p o n d s to the p a s s i v e zone i n the cyclovoltammogram o b t a i n e d f o r the t e s t s t e e l i n the p h o s p h a t i n g s o l u t i o n . * p

RESULTS AND DISCUSSION T a b l e I shows the d e t a i l w i t h t h e c a l c u l a t e d v a l u e s of t o t a l r e s i s t a n c e R and e f f e c t i v e c a p a c i t a n c e C. For specimens w i t h i n i t i a l m e c h a n i c a l s u r f a c e p r e p a r a t i o n , the N y q u i s t impedance p l o t shows t h e c h a r a c t e r i s t i c s e m i c i r c u l a r b e h a v i o r w i t h a r e s i s t a n c e of the o r d e r of 1800 Ω cm and a c a p a c i t a n c e of about 40 yF cm . As d i f f e r e n t s u r f a c e t r e a t ­ ments are i n c o r p o r a t e d on a s e q u e n t i a l b a s i s , the complex plane diagram shows a g r a d u a l e v o l u t i o n . Comparison of the c a p a c i t a n c e v a l u e s of specimens phosphated f o r 5 minutes and 30 minutes r e s p e c t i v e l y and r i n s e d i n phosphochromic s o l u t i o n (specimens #6 and #8) w i t h a non-phosphated s p e c i ­ men (#1) shows v a l u e s of the same o r d e r of magnitude i . e . 20 and 32 yF cm" as compared to 40 yF cm"" . On the o t h e r hand, the s p e c i ­ mens #7 and #9 w h i c h are phosphated f o r 5 and 30 minutes r e s p e c t i ­ v e l y but not r i n s e d , show h i g h e r v a l u e s of c a p a c i t a n c e per u n i t s u r f a c e i n the m i l l i f a r a d range. S i n c e these specimens are not r i n s e d a f t e r p h o s p h a t i n g and s i n c e the p h o s p h a t i n g s o l u t i o n con­ t a i n s a p r o p r i e t a r y i n h i b i t o r , perhaps a c e r t a i n amount of the i n h i b i t o r i s r e t a i n e d on the s u r f a c e c a u s i n g t h i s change i n c a p a c i ­ tance. S i m i l a r t r e n d i s observed i n specimens c o a t e d w i t h one l a y e r of p a i n t (specimens #10, #12, #14, #16) but prepared w i t h and without r i n s i n g . The c o n c l u s i o n h o l d s a l s o f o r specimens c o a t e d w i t h two l a y e r s of p a i n t . R e s i s t a n c e v a l u e s are seen t o be h i g h e r f o r specimens t h a t have been washed w i t h phospho-chromic s o l u t i o n a f t e r p h o s p h a t i n g . S i n c e the r i n s i n g s o l u t i o n i s supposed t o s e a l the openings e x i s t i n g between phosphate c r y s t a l s ( 1 9 ) , i t i s l o g i ­ cal t h a t specimens s u b j e c t e d to r i n s i n g show h i g h e r r e s i s t a n c e values ( i . e . less tendency for effecting charge transfer reactions)· On comparing the r e s u l t s of specimens s u b j e c t e d to r i n s i n g w i t h phospho-chromic s o l u t i o n but h a v i n g same p a i n t t h i c k n e s s (#10 and #14) i t i s seen t h a t p h o s p h a t i n g f o r 30 minutes does not pro­ v i d e any a p p r e c i a b l e b e n e f i t . The same i s t r u e on comparing s p e c i ­ mens #11 and #15. I f one l o o k s at specimens #12 and #16 the c o n c l u s i o n s made above r e g a r d i n g n o n - r i n s e d specimens h o l d . The same i s t r u e f o r specimens #13 and #17 w h i c h a r e a l s o n o n - r i n s e d but have two l a y e r s 2

2

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

64

POLYMERIC MATERIALS FOR CORROSION CONTROL

of p a i n t . These r e s u l t s thus appear t o i n d i c a t e t h a t prolonged phosphating does n o t p r o v i d e a p p r e c i a b l e a d d i t i o n a l b e n e f i t from the p o i n t o f v i e w o f t h e c o r r o s i o n r e a c t i o n l i k e l y t o take p l a c e a t the m e t a l s u r f a c e . S i n c e p a i n t a d h e s i o n on t h i c k e r phosphate f i l m s i s o f poor q u a l i t y and s i n c e t h e r e i s no s i g n i f i c a n t improvement i n impedance b e h a v i o r due t o t h i c k e r phosphate l a y e r s , i t appears t h a t o n l y a t h i n phosphate c o a t i n g s h o u l d be used whenever p o s s i b l e . Double l a y e r p a i n t p r o v i d e s a d d i t i o n a l p r o t e c t i o n s i n c e such c o a t i n g s would be l e s s porous than s i n g l e l a y e r p a i n t . I t i s a l s o noted t h a t i n a l l specimens t h a t a r e not r i n s e d t h e r e i s a tendency to show i n d u c t i v e l o o p s i n t h e impedance p l o t . I t i s not c l e a r i f t h i s i s due t o the a d s o r p t i o n o f i n h i b i t o r on s t e e l s u r f a c e o r due to t h e f o r m a t i o n o f o x i d e s o r due t o i n c r e a s e d p o r o s i t y ( 2 8 ) . Many specimens (#1, #6, #8, #10 and #12) a l s o show t h e i n i t i a t i o n o f Warburg impedance b e h a v i o r a t t h e lower end o f t h e f r e q u e n c y range covered i n t h i s s t u d y Specimens t h a t wer withdrawn a f t e r 1, 17 these specimens was r a t e d as per ASTM procedure D610-68 which i s a measure o f t h e s u r f a c e c l e a r l y a t t a c k e d o r showing f o r m a t i o n o f b l i s t e r s (Table I ) . Here a g a i n i t i s noted t h a t t h e c o n c l u s i o n s made e a r l i e r h o l d good: phosphated n o n - r i n s e d specimens behave p o o r l y as compared t o r i n s e d specimens, thus e s t a b l i s h i n g t h e n e c e s s i t y o f phos pho-chromic s o l u t i o n r i n s e . S a l t spray t e s t r e s u l t s do a l s o i n d i c a t e t h a t phosphating f o r 30 minutes does n o t p r o v i d e any a p p r e c i a b l e improvement as compared t o 5 minutes phosphating. Impedance measurements t a k e n on specimens a f t e r 96 hours exposure t o s a l t s p r a y show a c o m b i n a t i o n o f R-C and Warburg d i f f u s i o n behavior. T h i s i s i n agreement w i t h t h e o b s e r v a t i o n elsewhere ( 9 , 1 1 ) . T e s t s conducted on specimens phosphated f o r 5 minutes b u t p r o v i d e d w i t h a d d i t i o n a l p a i n t t h i c k n e s s u s i n g Tremclad o r M a r i n o x SR-2 a r e summarized i n Table I I . Specimens (#18) r i n s e d w i t h phospho-chromic s o l u t i o n show h i g h e r v a l u e s o f r e s i s t a n c e and lower v a l u e s o f c a p a c i t a n c e w i t h r e s p e c t t o n o n - r i n s e d specimens (#20) both h a v i n g 80 urn o f p a i n t t h i c k n e s s . Specimen #19 t e s t e d f o r impedance a f t e r exposure t o s a l t s p r a y t e s t s shows a l o w charge t r a n s f e r r e s i s t a n c e o f 4300 Ω c m and a f a i r l y h i g h double l a y e r c a p a c i t a n c e o f 316 yF cm"" . T h i s shows the i n c r e a s e d tendency o f NaCl s o l u t i o n t o p e n e t r a t e t h e p a i n t f i l m exposed t o s a l t s p r a y t e s t . The c a p a c i t a n c e i s even h i g h e r f o r specimen #21 prepared w i t h no phospho-chromic r i n s e . I t i s c l e a r on a n a l y z i n g t h e r e s u l t s o b t a i n e d from specimens #18 and #19 as w e l l as from #20 and #21 t h a t f i l m d e g r a d a t i o n and c o a t i n g i n t e g r i t y can be f o l l o w e d more e f f i c i e n t l y by impedance measurements than by s a l t s p r a y t e s t i n g (Table I I and F i g u r e s 1 and 2). R e s u l t s o f impedance t e s t s conducted on specimens w i t h t h e p o l y u r e t h a n e p a i n t always showed c a p a c i t a n c e i n t h e pF cm"* range, in s p i t e o f no r i n s i n g o p e r a t i o n being carried o u t . The r e s i s t a n c e s a r e i n t h e 10 -10 Ω cm range. AC impedance measured u s i n g a few specimens a n o d i c a l l y phos­ phated and p a i n t e d w i t h Tremclad o r Mar i n o x p a i n t a r e shown Table I I and F i g u r e s 3 and 4. C y c l o v o l t a m m e t r y i n d i c a t e d a n o t a b l e f a l l 2

2

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6.

VIJAYAN ET AL.

TABLE I .

Spec. #

65

Evaluation by AC Impedance

RESULTS OF AC IMPEDANCE AND SALT SPRAT TESTS FOR DIFFERENT SURFACE TREATMENTS

R Details of surface treatment (Gem ) and coating (a) 2

(F

C rust cm" ) grade (d) 2

6

40x10"

M e c h a n i c a l p o l i s h i n g + no a d d i t i o n a l treatment

1800

6

5min. phosphating ( b ) + r i n s e (c)

8000

20xl0-

7

5min. phosphating + no r i n s e

460

lxlO-

8

30min. phosphatin

9

30min. phosphating + no r i n s e

280

0.32xl0-

1

-

10

5min. phosphating + r i n s e + 20 ym p a i n t

4.5X10 *

11

5min. p h o s p h a t i n g + r i n s e + 40 ym p a i n t

4.5xl0

12

5min. p h o s p h a t i n g + no r i n s e + 20 ym p a i n t

3.0x10**

13

5min. phosphating + no r i n s e + 40 ym p a i n t

2.2xl0

14

30min. p h o s p h a t i n g + r i n s e + 20 ym p a i n t

5.0X10 *

15

30min. phosphating + r i n s e + 40 ym p a i n t

2.5xl0

16

30min. phosphating + no r i n s e + 20 ym

17

30min. phosphating + no r i n s e + 4.0X10 * 40 ym

1

5

5

1

5

8000

1

-

3

-

6

9

6

9

10x10"

lxlO"

20x10"

6

1

4x10"

6

4

20x10"

6

9

6

9

50x10"

6

1

3x10"

6

3

lxlO"

a)

A l l specimens

b)

Specimens a r e phosphated u s i n g a commercial s o l u t i o n , Oxy-Plus 84 DRS, r e c e i v e d from Laboratoire Brabant Inc., V i l l e S t - P i e r r e , Québec, Canada.

c)

D i l u t e s o l u t i o n o f phospho-chromic r i n s i n g operation.

(d) A h i g h e r r u s t corrosion.

are mechanically polished

6

grade

to 600 g r i t .

a c i d m i x t u r e i s used f o r the

number i n d i c a t e s

better

protection

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

against

66

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

TABLE I I . RESULTS OF AC IMPEDANCE MEASUREMENTS MADE ON SAMPLES WITH THICKER COATINGS

Spec. #

*

Details of surface treatment and coating (a)

R corr (V/SCE) ( Ω α · )

C

E

2

2

am- )

3.2x10"

9

14x10"

6

3

724x10"

6

8.0xl0

7

50x10"

l.OxlO

9

32x10-

9

5min. a n o d i c p h o s p h a t i n g * + r i n s e -0.601 + 32 um p a i n t (Tremclad)

l.lxlO *

40x10-

6

-0.599

2.3x10**

32x10"

6

18

5min.phosphating + r i n s e + 80 ym p a i n t (Tremclad)

19

5min. phosphatin p a i n t (Tremclad) spray t e s t

20

7

(F

-0.114

1.5xl0

5min. phosphating + no r i n s e + 80 um p a i n t (Tremclad)

-0.509

7.0X10 *

21

5min. phosphating + no r i n s e + 80 um p a i n t (Tremclad) + lOOh. s a l t spray t e s t

-0.502

3.5xl0

22

5min. phosphating + no r i n s e + 1 p r e c o a t + 1 Marinox SR-2 coat (100 um c o a t i n g )

-0.126

23

5min. phosphating + no r i n s e + 1 p r e c o a t + 1 Marinox SR-2 coat + lOOh s a l t s p r a y t e s t

-0.173

24

25

30min. a n o d i c p h o s p h a t i n g * + r i n s e + 24 ym p a i n t (Tremclad)

26

5min. a n o d i c p h o s p h a t i n g * + r i n s e -0.170 + 1 precoat + 1 Marinox SR-2 coat (170 ym c o a t i n g )

8.0xl0

7

160x10-

12

27

30min. a n o d i c p h o s p h a t i n g * + r i n s e + 1 precoat + 1 Marinox SR-2 coat (160 ym c o a t i n g )

-0.285

2.0xl0

8

316xl0"

1 2

1

1

See t e x t f o r d e t a i l s .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1 2

Evaluation by AC Impedance

VIJAYAN ET AL.

Figure

1.

Bode p l o t s f o r specimens #18 ( · See T a b l e I I f o r d e t a i l s .

) and #20 (

2000

1600|-

«

1200 Ι­

Ε

Ν 400 h I

0

400

Τ

I

800

ι

I

1200

ι

I

1600

ι

I

2000

ι

L

2400

2800

2

Z' (ohm cm ) Figure

2.

N y q u i s t p l o t s f o r specimens See T a b l e I I f o r d e t a i l s .

#19 ( # ) and #21 ( • ) .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

68

POLYMERIC MATERIALS FOR CORROSION CONTROL

12000

°

8000

ε ο ^

4000

NI ι 0 0

4000

8000

12000 16000 20000 24000 28000

Ζ ' (oh

2

)

F i g u r e 3. N y q u i s t p l o t See Table I

•

\

s

-

O

7.0

•

-

1 • ï

-

Ε Ο,

f •

Ν _ Ο) 60

•

•

•

•

5.0

•

•

•t -

-2.0

.

t

l

-1.0

i

l

.

0.0

I

1.0

ι

I

2.0

ι

1

3.0

,

I

4.0

•1 ι

·

5.0

log f (Hz)

F i g u r e 4. Bode p l o t s f o r specimens #26 ( φ ) and #27 ( • *). See Table I I f o r d e t a i l s .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6.

VUAYAN ET AL.

69

Evaluation by AC Impedance

i n the a n o d i c c u r r e n t beyond +0.5 V/SCE i n t h e phosphating s o l u t i o n Oxy-plus 84 DRS. However, a n o d i c p h o s p h a t i n g a t +0.8 V/SCE f o r 5 and 30 minutes r e s p e c t i v e l y d i d n o t produce any improvement i n t h e ultimate performance of painted specimens i n spite of the b e n e f i c i a l a s p e c t s o f t h i s process c i t e d i n l i t e r a t u r e ( 2 9 , 3 0 ) . AC impedance measurements t a k e n on t h e same specimen a t d i f ­ f e r e n t temperatures i n t h e range 25-90 °C a r e shown i n T a b l e I I I . A specimen w i t h no s u r f a c e t r e a t m e n t o t h e r t h a n m e c h a n i c a l p o l i s h i n g shows Cdi*40yF cm"" a t 25°C but t h e v a l u e i n c r e a s e s a p p r e c i a b l y w i t h i n c r e a s i n g temperature. The v a l u e s o f R t f o r d i f f e r e n t specimens (#28,#29) show a s y s t e m a t i c d e c r e a s e w i t h i n c r e a s i n g temperature whereas t h e v a l u e s o f C ^ i show a system­ a t i c increase. F i g u r e s 5 and 6 show t h e e v o l u t i o n o f impedance p l o t s as a f u n c t i o n o f t e m p e r a t u r e . In addition to the v a r i a t i o n i n t h e v a l u e s o f R ^ and C , i t i s n o t i c e d t h a t t h e Warburg2

c

d l

TABLE I I I . AC IMPEDANCE MEASUREMENTS AT DIFFERENT TEMPERATURES

Spec. #

Details of surface Treatment and coating

Temp. c o r r (°C) V/SCE E

C

R 2

(Gem )

2

(F cm" )

63xio-«; 166x10251xl0" 550x10-

5min. p h o s p h a t i n g + r i n s e + 40 ym p a i n t (Tremclad) 4h. immersion i n 3% NaCl a t room temperature

25 45 65 90

-0.588 -0.587 -0.580 -0.545

2.1X10

5min. phosphating + no r i n s e + 1 precoat + 1 M a r i n o x SR-2 coat 4h. immersion i n 3% NaCl a t room temperature (100 ym c o a t i n g )

25 45 65 90

-0.126 -0.534 -0.612 -0.708

8.0xl0 6.5xl0 β.ΟχΙΟ * 5.0x1ο *

30

5min.phosphating + r i n s e + 40 ym p a i n t (Tremclad) 4h immersion i n 3% NaCl a t 45°C.

45

-0.586

1.2x1ο *

251xl0-

31

5min. p h o s p h a t i n g + r i n s e + 40 ym p a i n t (Tremclad) 4h. immersion i n 3% NaCl a t 65°C.

65

-0.438

3500

398x10"

32

5min. p h o s p h a t i n g + r i n s e + 40 ym p a i n t (Tremclad) 4h immersion i n 3% NaCl a t 90°C.

90

-0.450

2060

832xl0"

28

29

11

7000 5000 3000

6

6

6

7

5

1

1

1

0.50x102.5x1016xl0400x10""

9

9

9

9

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6

6

6

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

2000

3000

Z' (ohm

6000

4000

cm ) 2

F i g u r e 5a. N y q u i s t plots f o r specimens #28 a t d i f f e r e n t temperatures. See Table I I f o r d e t a i l s . Legend: 25°C ( · ); 45°C ( • ); 65°C ( ο ) and 90°C ( • ) . 1000

ε ο

800

-

600

I-·

ε

-· · ·

Ι-'

• •

Ο, 400 Ν

200 0

• Λ ο " ο " ο · • ι 200 I ι 400 I 9600 I ι

I

ι

I

ι

I

.

I

,

800 I ι1000 I ι 1200 1400 1600 1800 2000

Z' (ohm cm ) 2

F i g u r e 5b. Enlargement o f t h e i n i t i a l

p o r t i o n o f F i g u r e 5a.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

VIJAYAN ET AL.

Evaluation by AC Impedance

6000

0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Z' (ohm cm ) 2

F i g u r e 6a. N y q u i s t p l o t s f o r specimens i d e n t i c a l l y p r e p a r e d but immersed i n 3% NaCl f o r 4 hours a t t h e temperature o f impedance measurement. See T a b l e I I I f o r d e t a i l s . Legend: specimen #28 ( · ), # 30 ( • ); #31 ( ο ) and #32 ( • ).

800

1200

1600

Τ (ohm cm ) 2

F i g u r e 6b. Enlargement

of the i n i t i a l

portion of Figure 6a.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

72

POLYMERIC MATERIALS FOR CORROSION CONTROL

type b e h a v i o r a t l o w f r e q u e n c i e s g r a d u a l l y become more prounonced at h i g h e r t e m p e r a t u r e s . F i g u r e 5 has been o b t a i n e d from a specimen (#28) m a i n t a i n e d a t 25 °C f o r 4 hours i n 3% NaCl s o l u t i o n b e f o r e t a k i n g impedance measurements. The temperature was then r a i s e d t o the d e s i r e d v a l u e f o r each s t e p . In t h e case o f F i g u r e 6 s e p a r a t e specimens identically prepared were i n d i v i d u a l l y m a i n t a i n e d a t 25,45,65 and 90°C f o r 4 hours b e f o r e impedances were measured a t t h e p a r t i c u l a r tempera­ t u r e . The v a r i a t i o n i n impedance as shown i n F i g u r e s 5 and 6 w i t h temperature appears t o be l o g i c a l . An i n c r e a s e i n temperature decreases the r e s i s t a n c e but i n c r e a s e s the c a p a c i t a n c e . In Figure 6, specimen #30 i s an e x c e p t i o n i n t h a t t h e impedance a t 45°C i s h i g h e r than t h a t a t 25°C. T h i s i s perhaps caused by unknown e r r o r s i n manipulation. I t i s remarked t h a t specimens m a i n t a i n e d a t 65°C and 90°C f o r 4 hours show lower r e s i s t a n c e the specimen was m a i n t a i n e temperature r a i s e d t o 65°C and then t o 90°C. T h i s i s a l s o l o g i c a l because maintenance a t a h i g h e r temperature f o r a l o n g e r time causes a c c e l e r a t e d d e g r a d a t i o n . F i g u r e 7 shows t h e impedance p l o t s as a f u n c t i o n o f tempera­ t u r e o b t a i n e d u s i n g a specimen h a v i n g t h e p o l y u r e t h a n e paint coating. The r e s i s t a n c e i s o f t h e o r d e r o f 1 0 Ω c m and t h e c a p a c i t a n c e i s i n the nanofarad range. Higher values of r e s i s t a n c e and v e r y l o w v a l u e s o f c a p a c i t a n c e show t h e b e t t e r p r o t e c t i o n o f f e r e d by t h i s type o f p a i n t a t a l l o f the temperatures s t u d i e d . Apparent a c t i v a t i o n e n e r g i e s f o r the d e g r a d a t i o n r e a c t i o n was c a l c u l a t e d u s i n g specimens #28 and specimens #30, #31 and #32. A v a l u e o f ~7 k c a l / m o l was o b t a i n e d . T h i s e v a l u a t i o n was based on the v a r i a t i o n o f r e s i s t a n c e as a f u n c t i o n o f temperature. A s i m i l a r v a l u e was o b t a i n e d a l s o w i t h specimen #11 (Table I ) a f t e r i t was exposed t o s a l t s p r a y t e s t (100 h o u r s ) . When a t h i c k c o a t i n g (-100-160 um) o f p o l y u r e t h a n e p a i n t i s a p p l i e d ( e . g . specimen #29), the a c t i v a t i o n energy i s o f the o r d e r of 26 k c a l / m o l . T h i s i n d i c a t e s the h i g h e r energy b a r r i e r presented by a non-porous c o a t i n g . F i g u r e 8 p r e s e n t s t h e impedance b e h a v i o r o f p o l y u r e t h a n e p a i n t e d specimen a f t e r 4 hours o f immersion and a f t e r 10 days o f immersion. The impedance spectrum does n o t show any a p p r e c i a b l e v a r i a t i o n . F u r t h e r i n f o r m a t i o n on the b e h a v i o r o f the Tremclad and the M a r i n o x systems (specimens #25 and #27) f o r immersion times upto 10 days i s g i v e n i n Table I V . The v a r i a t i o n i n c o r r o s i o n p o t e n t i a l i s n o t v e r y s y s t e m a t i c but t h e changes i n C^]^ R t and R appear t o f o l l o w a l o g i c a l t r e n d . F o r t h e Tremclad c o a t i n g , t h e r e s i s t a n c e decreases and c a p a c i t a n c e i n c r e a s e s w i t h immersion t i m e . This i n d i c a t e s i n c o r p o r a t i o n of e l e c t r o l y t e i n t o the p a i n t f i l m . For the p o l y u r e t h a n e c o a t i n g , the r e s i s t a n c e s t a y s i n the 10 Ω cm range and the c a p a c i t a n c e i n the 400 pF cm" range i n d i c a t i n g r e s i s t a n c e t o e l e c t r o l y t e i n c o r p o r a t i o n i n t o the f i l m . The r e s u l t s a r e i n concordance w i t h p u b l i s h e d i n f o r m a t i o n a v a i l a b l e r e g a r d i n g c o a t i n g d e t e r i o r a t i o n . I t was c o n f i r m e d t h a t a f a l l i nR as w e l l as t h e appearance o f Warburg-type o f b e h a v i o r at low f r e q u e n c i e s , e s p e c i a l l y a t higher temperatures, i s c l e a r l y an i n d i c a t i o n o f l a c k o f p r o t e c t i o n . A p r o p e r l y prepared s u r f a c e 5

2

c

p

c t

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6.

73

Evaluation by AC Impedance

VIJAYAN ET A L . 8.0

Ε υ Ε Ο,

M

6.0 ••••

ο

5.0

Ό θ θ

J

-2.0

ι

-1.0

L

0.0

J

,

1.0

0

Ο Ο Ο Ο Ο θ ο

I

ι

2.0

0

I

ο

ι

3.0

"

L

4.0

5.0

log f (Hz) F i g u r e 7.

Bode plots f o r specimen #29: influence of temperature.See T a b l e I I I f o r d e t a i l s . Legend: 25°C ( · ); 45°C ( • ); 65°C ( Q ) and 90°C ( • ).

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

log f (Hz) F i g u r e 8.

Bode p l o t s f o r specimen #27: i n f l u e n c e of immersion t i m e . See T a b l e IV f o r d e t a i l s . Legend: 4 h ( t ); 10 days ( • ).

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

74

POLYMERIC MATERIALS FOR CORROSION CONTROL

TABLE IV· AC IMPEDANCE AND DC POLARIZATION RESISTANCE MEASUREMENTS (25°C)

Spec. Immersion time (h.) #

25

27

R

E

corr V/SCE

C (F cm" )

4

-0.599

24

-0.708

48

-0.70

72

-0.732

96

-0.734

168

-0.585

192

-0.589

216

-0.574

240

-0.578

4

-0.122

24

-0.234

48

-0.410

-

72

-0.442

3.4 χ 1 0

7

320 χ 1 0 -

120

-0.404

4.0 χ 1 0

7

794 χ 1 0 -

144

-0.382

5.0 χ 1 0

7

200 χ 10"

168

-0.412

4.0 χ 1 0

7

631 χ 1 0 "

192

-0.310

1.6 χ 1 0

7

320 χ 1 0 -

216

-0.290

5.0 χ 1 0

7

320 χ ΙΟ"

240

-0.278

5.0 χ 1 0

7

251 χ Ι Ο

2.3 χ 10*

2

2

-

-

-

5

7.4 χ 10*

5.4 χ 10* 4.4 χ 10* 3.4 χ 10* 4.5 χ ΙΟ* 4.0 χ 10*

398 χ 1 0 "

8

> 10

32 χ 10"6

-

χ 10

Rp ( Ω cm )

2

2

(flcm )

6

3.0 χ 10*

>10

5

>10

5

>10

5

1 2

>10

5

1 2

>10

5

>10

5

1 2

>10

5

1 2

>10

5

1 2

>10

5

- 1 2

>10

5

316 χ 1 0 -

1 2

-

1 2

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6. VIJAYAN ET A L .

Evaluation by AC Impedance

75

p r o v i d e d w i t h a t h i c k p o l y u r e t h a n e c o a t i n g shows good performance. The economic a s p e c t s o f t h e problem and t h e investment i n v o l v e d i n such p a i n t systems need o f course t o be taken i n t o c o n s i d e r a t i o n . CONCLUSION 1)

2)

3)

4) 5) 6)

AC impedance measurements enable the d e t e r m i n a t i o n o f charge t r a n s f e r r e s i s t a n c e and double l a y e r c a p a c i t a n c e and o t h e r parameters r e l a t e d t o coated systems. Decrease i n charge t r a n s f e r r e s i s t a n c e and i n c r e a s e i n double l a y e r c a p a c i t a n c e i s observed w i t h i n c r e a s i n g time o f immers i o n o r w i t h i n c r e a s i n g t e s t temperature and g i v e s i n f o r m a t i o n on t h e degree o f p r o t e c t i o n e f f i c i e n c y o f a c o a t i n g . Appearance o f Warburg-type b e h a v i o r shows t h a t d i f f u s i o n phenomena become predominant i n some c o a t i n g s as t h e i r d e t e r i o r a t i o n progresses Longer phosphatin v i d e any a p p r e c i a b l Phospho-chromic r i n s e has a b e n e f i c i a l e f f e c t on t h e l i f e o f a coated s u r f a c e . S t e e l s u r f a c e phosphated f o r 5 m i n u t e s , r i n s e d and p r o v i d e d w i t h a p o l y u r e t h a n e c o a t i n g shows good r e s i s t a n c e t o 3% NaCl solution.

ACKNOWLEDGMENTS The a u t h o r s thanks B. Harvey f o r a s s i s t a n c e i n c o n d u c t i n g some o f the t e s t s and i n t a k i n g p i c t u r e s o f t h e c o r r o d e d specimens. S p e c i a l thanks a r e expressed t o D. La j e u n e s s e f o r h e r h e l p i n c o r r e c t i n g a few problems r e l a t e d t o t h e e l e c t r i c a l system, t o D. Simard f o r h i s a s s i s t a n c e w i t h p h o t o g r a p h i c work. Samples o f phosphating s o l u t i o n r e c e i v e d from L a b o r a t o i r e Brabant Canada I n c . ( V i l l e S t - P i e r r e , Québec, Canada) and samples o f a l u m i n u m - k i l l e d 1006 s t e e l sheet r e c e i v e d from Sidbec-Dosco ( C o n t r e c o e u r , Québec, Canada) a r e g r a t e f u l l y acknowledged. LITERATURE CITED 1. 2. 3. 4. 5. 6.

7. 8.

Leidheiser J r . , H. Corrosion (NACE) 1982, 38, 374-383. Googan, C.G. Proc. UK National Corrosion Conference, London, England, 1982, pp. 13-18. Groseclose, R.G.; Frey, C.M.; Floyd, F . L . J. Coat. Technol. 1984, 56, 31-43. Bombara, G.; Lunazzi, G.C.; Martini, B. Werkst. Korros. 1982, 33, 610-617. Mansfeld, F . ; Kendig, M. Proc. 9th Int. Congr. Met. Corros., Toronto, 1984, Vol. 3, 74-84. Macdonald, D.D.; McKubre, M.C.H. In "Electrochemical Corrosion Testing"; Mansfeld, F . , Bertocci, U., Eds., ASTM STP 727, American Society for Testing and Materials: Philadelphia, 1981; pp. 110-149. Chao, C . Y . ; Lin, L . F . ; Macdonald, D.D. J. Electrochem Soc. 1982, 129, 1874-1879. Mansfeld, F. Corrosion (NACE) 1981, 36, 301-307.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

76 9. 10. 11. 12.

13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

27. 28.

29. 30.

POLYMERIC MATERIALS FOR CORROSION CONTROL

Piens, M.; Verbist, R.; Vereecken, J . Proc. 9th Int. Conf. Org. Coat. Sci. Technol., Athens, Greece, 1983, p. 137. Hubrecht, J.; Vereecken, J.; Piens, M. J . Electrochem Soc. 1984, 131, 2010-2015. Padget, J . C . ; Moreland, P.J. J . Coat. Technol. 1983, 55, 39-51. Epelboin, I.; Gabrielli, C.; Keddam, M.; Takenouti, H. In "Electrochemical Corrosion Testing"; Mansfeld, F . ; Bertocci, U., Eds.; ASTM STP 727 American Society for Testing and Materials: Philadelphia, 1981; pp. 150-166. Haruyama, S.; Tsuru, T. In "Electrochemical Corrosion Testing"; Mansfeld, F . ; Bertocci, U., Eds.; ASTM STP 727, American Society for Testing and Materials: Philadelphia, 1981; pp.167-186. Rothstein, Μ., personal communication and Application Note AC-1, EG&G PARC, Princeton N.J Kendig, M.W.; Meyer Corros.Sci. 1983, 23 Mansfeld, F . ; Kendig, M.W. Werkst. Korros. 1983, 34, 397-401. Kendig, M., Mansfeld, F . ; Tsai, S. Corros. Sci. 1983, 23, 317-329. Gordon, D.C. Corros. Prev. Control Oct. 1984, 7-10. Lorin, G. "La phosphatation des métaux"; Eyrolles: Paris, 1979; p.168. Ottaviani, R.A. Proc. Org. Coat. Appl. Polym. S c i . , 1981, 46, 50-55. Dickie, R.A. In "Adhesion Aspects of Polymeric Coatings"; Mittal, K . L . , Ed.; Plenum: New York, 1983, pp. 319-327. de Vries, J . E . ; Riley, T . L . ; Holubka, J.W.; Dickie, R.A. Surf. Interface Anal. 1985, 7, 111-116. Kuehner, M.A. Met. Finish. 1985, 83, 15-18. Bockris, J.O'M.; Kita, H. J. Electrochem. Soc. 1961, 108, 676. Gabrielli, C.; Keddam, M.; Takenouti, H. In "Treatise on Materials Science and Technology"; Scully, J . C . , Ed.; Academic: New York, 1983, Vol. 23, pp. 395-451. Sluyters-Rehbach, M.; Sluyters, J.H. In "Comprehensive Treatise of Electrochemistry"; Yeager, E . ; Bockris, J.O'M.; Conway, B . E . ; Sarangapani, S., Eds.; Plenum: New York, 1984, Vol. 9, pp. 183-191. Musiani, M.M.; Pagura, C.; Mengoli, G. Electrochim. Acta 1985, 30, 501-509. Scantlebury, J.D.; Ho, K.N.; Eden, D.A. In "Electrochemical Corrosion Testing"; Mansfeld, F . ; Bertocci, U., Eds.; ASTM STP 727, American Society for Testing and Materials: Philadelphia, 1981; pp.187-197. Rajagopalan, Κ.S.; Krithivasan, Ν.; Rajagopal, C.; Janaki, M.E.K. Corros. Maint. 1983, 6, 259-266. Gabe,D.R.; Johal, C.P.S.; Akanni, K.A. Met. Finish. 1985, 83, 41-44.

RECEIVED January 22, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

7 Evaluation of Coating Resins for Corrosion Protection of Steel Exposed to Dilute Sulfuric Acid 1

Malcolm L. White and Henry Leidheiser, Jr.

2

1

Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA 18015 Department of Chemistry and Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA 18015

2

Three types of coatings--a vinyl ester, a polyester and four epoxies--were coated on steel and exposed to 0.1M H S O at 60°C. Measurements of corrosion potent i a l , AC conductance, tensile adhesion and weight gain were made on the coated substrates after 1000 hours of exposure, and the values were compared with the observed corrosion of the steel substrate. The best correlation of parameter values with corrosion was found with conductance. Corrosion potential did not show a consistent relationship, and weight gain and tensile adhesion showed no correlation with corrosion. It was concluded that the most important properties for coatings to be used in acid media are low permeability and resistance to degradation by acid. The vinyl ester, a bisphenol A epoxy cured with an aliphatic amine, and a novolac epoxy cured with a mixed aromatic/cycloaliphatic amine provided the best corrosion protection. The saturated polyester and a bisphenol A epoxy cured with a polyamide amine showed significant deterioration in acid and corrosion of the underlying steel. Two novolac epoxies cured with aromatic amines showed i n termediate performance. 2

4

The m e c h a n i s m f o r t h e i n i t i a l c o r r o s i o n o f s t e e l i n n e u t r a l o r a l k a l i n e s o l u t i o n s i s g e n e r a l l y a c c e p t e d as t h e o x i d a t i o n o f iron from the m e t a l l i c s t a t e to the f e r r o u s i o n : Fe -

Fe""

+

w i t h the attendant reduction r e a c t i o n d r o x i d e i o n f r o m o x y g e n and w a t e r [ 1 J : 1/20

2

+ H 0 2

+

2e"

(1)

being

2e~ -

the

formation

20H"

0097-6156/86/0322-0077S06.00/0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

of

hy-

(2)

POLYMERIC MATERIALS FOR CORROSION CONTROL

78

A v a r i e t y o f t e c h n i q u e s has been d e v e l o p e d t o m e a s u r e t h e c o n d i t i o n o f a c o a t i n g s o t h a t some e v a l u a t i o n o f i t s p r o t e c t i v e a b i l i t y c a n be made. M a n y o f t h e s e a r e b a s e d on e l e c t r o c h e m i c a l m e a s u r e m e n t s [2]. The f o u r t e c h n i q u e s u s e d i n t h i s s t u d y a r e (1) corrosion p o t e n t i a l , ( 2 ) AC c o n d u c t a n c e , ( 3 ) t e n s i l e a d h e s i o n , and ( 4 ) w e i g h t gain. The c o r r o s i o n p o t e n t i a l i s d e t e r m i n e d by t h e p o t e n t i a l a t w h i c h t h e a n o d i c and c a t h o d i c r e a c t i o n s o c c u r a t t h e same r a t e [ 3 ] . T h e AC conductance i s a measure of the ease w i t h which charge i s t r a n s m i t ted through the coating [4]. The a d h e s i v e s t r e n g t h o f t h e c o a t i n g t o t h e s t e e l s u r f a c e i s a f f e c t e d by r e a c t i o n s o c c u r r i n g a t t h e interface. The w e i g h t g a i n o f c o a t i n g s h a s b e e n s t u d i e d by F u n k e and H a a g e n [ 5 ] who h a v e s h o w n t h a t a w e i g h t g a i n e x c e e d i n g t h a t o f a f r e e f i l m i n d i c a t e s an a c c u m u l a t i o n o f w a t e r a t t h e i n t e r f a c e

[5]. These t e c h n i q u e s are f r e q u e n t l c o a t i n g s i n n e u t r a l s o l u t i o n s and e n j o y s p o r a d i c s u c c e s s , d e p e n d i n g p r i m a r i l y on t h e t y p e and t h i c k n e s s o f c o a t i n g b e i n g s t u d i e d . The m e c h a n i s m o f s t e e l c o r r o s i o n i n a c i d s o l u t i o n s , h o w e v e r , is d i f f e r e n t from that in neutral s o l u t i o n s in that the reduction r e a c t i o n i s the f o r m a t i o n of hydrogen from hydrogen i o n : 2H

+

+

2e~

+

H

2

(3)

P r e v i o u s w o r k i n t h i s l a b o r a t o r y has e s t a b l i s h e d t h a t f o r e p o x y and f 1 uoropolymer c o a t i n g s exposed to d i l u t e s u l f u r i c acid, there i s movement o f a c i d t h r o u g h t h e c o a t i n g t o t h e s t e e l s u r f a c e so t h a t Equation 3 i s the predominant reduction r e a c t i o n [6]. Because of t h i s d i f f e r e n c e i n c o r r o s i o n mechanism i n a c i d s o l u t i o n , t h e u s e f u l n e s s o f t h e f o u r e v a l u a t i o n t e c h n i q u e s d i s c u s s e d above may be d i f f e r e n t t h a n i n n e u t r a l s o l u t i o n s . The p u r p o s e o f t h i s w o r k was t o e v a l u a t e t h e s e f o u r t e c h n i q u e s f o r p r e d i c t i n g t h e behavior of c o a t i n g r e s i n s in a c i d s o l u t i o n s . In a d d i t i o n , t h e a b i l i t y of several d i f f e r e n t types of c o a t i n g r e s i n s to p r o t e c t s t e e l a g a i n s t c o r r o s i o n i n a c i d s o l u t i o n was e v a l u a t e d . Experimental The f o l l o w i n g c o a t i n g r e s i n s w e r e u s e d : (1) a v i n y l e s t e r ( D e r a k a n e 4 7 0 f r o m Dow C h e m i c a l ) ; ( 2 ) a p o l y e s t e r ( A t l a c 3 8 2 - 0 5 AC f r o m ICI); and (3) f o u r e p o x y r e s i n / h a r d e n e r c o m b i n a t i o n s . The d e t a i l s o f t h e r e s i n s and h a r d e n e r s u s e d a r e s h o w n i n T a b l e I. One o f t h e e p o x y / h a r d e n e r c o m b i n a t i o n s was r e p r e s e n t e d by m a t e r i a l s f r o m t w o sources. The c o a t i n g s w e r e a p p l i e d t o o n e s i d e o f a s t e e l s u b s t r a t e by means o f a s p r a y gun f o r t h e l o w e r v i s c o s i t y c o a t i n g s , o r by d o c t o r b l a d i n g w i t h an a d j u s t a b l e G a r d n e r k n i f e f o r t h e h i g h e r viscosity materials. A c a s t i n g t e c h n i q u e was a l s o used i n w h i c h a known v o l u m e o f t h e c o a t i n g m a t e r i a l was p o u r e d i n t o a known a r e a d e f i n e d b y h e a v y t a p e a n d w a s a l l o w e d t o s p r e a d w h i l e on a l e v e l s u r f a c e .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Cobalt

Martek

Naphthanate

Ciba/Geigy

Epoxies CON/CHEM

(6%).

NOV/AR/AL

BPA/PA

NOV/AR 2

NOV/AR 1

BPA/AL

PE

Polyester ICI Americas (Atlac 382-05

AC)

VE

Designation

Vinyl Ester Dow C h e m i c a l (Derakane 470)

Supplier Resin

I.

A

Novolac

Bisphenol

A

Multifunctional Novolac

Multifunctional

Bisphenol

Bisphenol A-Fumarate P o l y e s t e r + 1% C o N a p * + DMA; 50% S t y r e n e

Cycloaliphatic/ A r o m a t i c Amine

Polyamideamine

Aromatic Amine

Aromatic Amine

Aliphatic Amine

MEKP

Cumene Hydroperoxide

Hardener

Coating Materials

Bisphenol A Vinyl E s t e r + . 1 5 % CoNap ; 36% S t y r e n e

Table

1.82:1

2.77:1

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8-9

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10-11

10-12

9-10

6-12

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203-229

203-305

254-279

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80

POLYMERIC MATERIALS FOR CORROSION CONTROL

The c o a t i n g s w e r e c u r e d i n t w o s t e p s : f i r s t , a room t e m p e r a t u r e e x p o s u r e f o r a t l e a s t f i v e h o u r s t o a l l o w any s o l v e n t p r e s e n t t o e v a p o r a t e a n d / o r t h e r e s i n t o g e l a n d , s e c o n d , an e l e v a t e d t e m p e r a t u r e c u r e . The v i n y l e s t e r , p o l y e s t e r and e p o x i e s w e r e b a k e d a t 60°C f o r 4-16 hours. The c o a t i n g t h i c k n e s s a f t e r c u r i n g was m e a sured w i t h a micrometer, s u b t r a c t i n g the substrate thickness. The t h i c k n e s s o f t h e c o a t i n g s r a n g e d f r o m 6 t o 12 m i l s ( 1 5 0 - 3 0 0 urn) and i s shown f o r each t y p e i n T a b l e I. The s u b s t r a t e s w e r e c o l d r o l l e d , l o w - c a r b o n SAE 1 0 1 0 s t e e l , 32 m i l s ( 0 . 8 mm) t h i c k (Q P a n e l s ) . T h e y w e r e s a n d b l a s t e d on b o t h s i d e s t o a 6 pm p r o f i l e w i t h s i l i c a s a n d . No f u r t h e r c l e a n i n g was d o n e . A c i r c u l a r d i s k 3 . 3 3 cm i n d i a m e t e r was p r e p a r e d f r o m a l a r g e r c o a t e d p a n e l w i t h a punch and d i e s e t . The d i s k was p l a c e d on a 1 2 5 ml widemouth screw-cap polypropylene b o t t l e , using a rubber gasket to make a t i g h t s e a l , w i t h t h e c o a t e d s i d e f a c i n g t h e i n s i d e o f t h e bottle. The d i s k and g a s k e from which the c e n t r a l c o a t e d ) s i d e o f t h e s t e e l s u b s t r a t e was e x p o s e d . The b o t t l e was i n v e r t e d and h a l f f i l l e d w i t h 0.1 M H 0 S O 4 t h r o u g h a h o l e d r i l l e d i n the bottom of the b o t t l e in order to contact the a c i d w i t h the coating. The i n v e r t e d b o t t l e was p l a c e d i n an o v e n a t 6 0 ° C . The c o r r o s i o n p o t e n t i a l was m e a s u r e d b y p u t t i n g a s a t u r a t e d c a l o m e l e l e c t r o d e / s a l t bridge i n t o the s o l u t i o n through the hole in the b o t t o m o f t h e p l a s t i c b o t t l e and c o n t a c t i n g t h e b a c k s i d e o f t h e s u b s t r a t e t o c o m p l e t e t h e c i r c u i t as s h o w n i n F i g u r e 1. A K e i t h l e y 600A e l e c t r o m e t e r w a s u s e d f o r t h e m e a s u r e m e n t . T h e AC c o n d u c t a n c e was d e t e r m i n e d by i n s e r t i n g a c a r b o n r o d i n t o t h e s o l u t i o n and m e a s u r i n g t h e c o n d u c t a n c e a t 2 kHz f r e q u e n c y and 2 0 0 mv p o t e n t i a l w i t h an E x t e c h M o d e l 4 4 0 D i g i t a l C o n d u c t i v i t y M e t e r . The c o n d u c t a n c e v a l u e s w e r e c o n v e r t e d t o s p e c i f i c c o n d u c t i v i t y by m u l t i p l y i n g by t h e t h i c k n e s s a n d d i v i d i n g by t h e a r e a ( 5 c m ) . 2

The a d h e s i o n w a s m e a s u r e d b y f a s t e n i n g a l e a d a n c h o r o f known a r e a (2.84 cm ) t o the c o a t i n g w i t h a c y a n o a c r y l a t e adhesive (Loctite 4 1 4 ) and a f t e r c u r i n g , p u l l i n g i t o f f n o r m a l t o t h e s u r f a c e w i t h a Dillon tensile tester. The f o r c e t o r e m o v e t h e c o a t i n g was d i v i d e d by t h e a r e a o f a t t a c h m e n t t o c o n v e r t i t t o a n o r m a l i z e d t e n s i l e adhesion value. 2

The w e i g h t g a i n was m e a s u r e d by w e i g h i n g t h e c o a t e d d i s k a f t e r it was removed f r o m t h e p l a s t i c b o t t l e , f o l l o w i n g a w a t e r r i n s e and removal of surface water. The c o a t i n g and s u b s t r a t e w e r e o b s e r v e d t h r o u g h t h e h o l e i n t h e b o t t l e during the exposure to the acid. Since the coatings were transparent, i t was p o s s i b l e t o o b s e r v e any v i s i b l e corrosion o c c u r r i n g on t h e s t e e l s u b s t r a t e . The c o r r o s i o n p r o d u c t s on t h e s t e e l w e r e g r a y o r b l a c k , e x c e p t when t h e c o a t i n g b l i s t e r e d and some r u s t i n g was s e e n . As c o r r o s i o n p r o g r e s s e d d u r i n g t h e a c i d exposure, the steel surface gradually darkened from the i n i t i a l l i g h t g r a y o f t h e o r i g i n a l s a n d b l a s t e d s u r f a c e t o an a l m o s t black surface. The e x t e n t o f c o r r o s i o n was e s t i m a t e d f r o m t h e a m o u n t o f d a r k e n i n g o b s e r v e d on t h e s t e e l s u r f a c e .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

WHITE A N D LEIDHEISER

Evaluation of Coating Resins

F i g u r e 1. Technique f o r e l e c t r o c h e m i c a l measurements. Reproduced w i t h p e r m i s s i o n from R e f e r e n c e 12. Copyright 1985, National A s s o c i a t i o n of Corrosion Engineers.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

82

POLYMERIC MATERIALS FOR CORROSION CONTROL

Results F i g u r e 2 summarizes the values o b t a i n e d f o r the f o u r measurements on t h e s e v e n c o a t i n g s a f t e r e x p o s u r e t o 0.1 M H 0 S O 4 a t 6 0 ° C f o r 1 0 0 0 hours. The o r d i n a t e s h o w s t h e v a l u e s m e a s u r e c f f o r e a c h o f t h e f o u r t e c h n i q u e s , and t h e a b c i s s a r e p r e s e n t s t h e amount o f c o r r o s i o n o b s e r v e d on t h e s t e e l u n d e r e a c h o f t h e c o a t i n g s a f t e r t h e a c i d exposure, w i t h the amount of observed c o r r o s i o n d e c r e a s i n g from l e f t to right. The c o r r o s i o n p o t e n t i a l s s h o w a g e n e r a l t r e n d o f i n c r e a s i n g v a l u e s with d e c r e a s i n g s u b s t r a t e c o r r o s i o n , with the exception of the p o l y e s t e r a n d t h e n o v o l a c e p o x y c u r e d w i t h an a r o m a t i c / c y c l o a l i p h a t i c amine. T h e s p e c i f i c AC c o n d u c t i v i t y v a l u e s s h o w value with decreasing substrat n o v o l a c epoxy c u r e d w i t one o f t h e c o a t i n g s t h a t a l s o d i d not f i t corrosion potential values.

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The t e n s i l e a d h e s i o n v a l u e s show no c o r r e l a t i o n w i t h t h e e x t e n t o f c o r r o s i o n ; the b i s p h e n o l A epoxy cured w i t h a p o l y a m i d e amine showed b l i s t e r i n g , w h i c h r e p r e s e n t s a complete l o s s of adhesion. The p o l y e s t e r s h o w e d c o h e s i v e f a i l u r e a t l e s s t h a n 1000 h o u r s o f e x p o s u r e , so a t r u e a d h e s i o n v a l u e c o u l d n o t be d e t e r m i n e d . The o t h e r e p o x i e s a n d t h e v i n y l e s t e r a l l had v a l u e s i n t h e 1 5 0 - 2 0 0 p s i r a n g e , w i t h no a p p a r e n t r e l a t i o n s h i p t o t h e a m o u n t o f c o r r o s i o n . Weight change d a t a were o b t a i n e d f o r o n l y f o u r o f t h e seven c o a t i n g s , a n d t h o s e d a t a s h o w e d no c o r r e l a t i o n w i t h t h e e x t e n t o f s t e e l substrate corrosion. The p o l y e s t e r s h o w e d a w e i g h t l o s s , rather t h a n a w e i g h t g a i n , p r o b a b l y d u e t o an a t t a c k and d i s s o l u t i o n o f t h e e p o x y by t h e a c i d . I t s h o u l d be n o t e d t h a t t h e e l e c t r o c h e m i c a l m e a s u r e m e n t s ( c o r r o s i o n p o t e n t i a l and c o n d u c t i v i t y ) f o r t h e t w o n o v o l a c e p o x i e s c u r e d w i t h an a r o m a t i c a m i n e f r o m d i f f e r e n t s o u r c e s s h o w e d g o o d a g r e e m e n t , a l t h o u g h t h e t e n s i l e a d h e s i o n and w e i g h t g a i n v a l u e s w e r e n o t as reproducible. Discussion The b e s t p e r f o r m i n g c o a t i n g s w e r e t h e v i n y l e s t e r , t h e b i s p h e n o l A e p o x y c u r e d w i t h an a l i p h a t i c a m i n e , and a n o v o l a c e p o x y c u r e d w i t h a mixed a r o m a t i c / c y c l o a l i p h a t i c amine. The s a t u r a t e d p o l y e s t e r , and a b i s p h e n o l A e p o x y c u r e d w i t h a p o l y a m i d e a m i n e s h o w e d s i g n i f i c a n t d e t e r i o r a t i o n o f t h e c o a t i n g m a t e r i a l i n t h e a c i d , and c o r r o s i o n o f t h e u n d e r l y i n g s t e e l . Two t y p e s o f n o v o l a c e p o x i e s c u r e d w i t h a r o m a t i c a m i n e s showed i n t e r m e d i a t e p e r f o r m a n c e . O n l y one relation l a t i o n of the f a c t

o f t h e f o u r t e c h n i q u e s — t h e c o n d u c t i v i t y — s h o w e d any c o r w i t h t h e o b s e r v e d e x t e n t o f c o r r o s i o n . The l a c k o f c o r r e the t e n s i l e adhesion values w i t h c o r r o s i o n i s a r e s u l t of t h a t t h e method i n t e g r a t e s a d h e s i o n l o s s a t t h e s u b s t r a t e

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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i n t e r f a c e and t h e c o h e s i o n l o s s d u e t o d e t e r i o r a t i o n o f t h e p o l y m e r by a c i d . A l s o , t h e c o r r o s i o n o f t e n i s l o c a l i z e d and t h e a r e a a t t a c k e d i s a s m a l l f r a c t i o n of the t o t a l i n t e r f a c i a l area. The weight gain i s not a r e l i a b l e measure of c o r r o s i o n protection b e c a u s e t h e r e may be an a t t a c k a n d s o l u b i l i z a t i o n o f t h e c o a t i n g m a t e r i a l i t s e l f (as o c c u r r e d w i t h the p o l y e s t e r ) , so t h a t the o b s e r v e d c h a n g e i n w e i g h t w i l l be t h e n e t r e s u l t o f a w e i g h t l o s s f r o m s o l u b i l i z a t i o n and a w e i g h t g a i n f r o m w a t e r and a c i d e n t r y into the c o a t i n g . The o n l y way t o s e p a r a t e t h e t w o e f f e c t s i s t o measure the w e i g h t change o c c u r r i n g w i t h a f r e e f i l m o f t h e c o a t i n g and a s s u m e i t w i l l be t h e s a m e as a c o a t i n g on a s u b s t r a t e ; this was t h e t e c h n i q u e u s e d by F u n k e [ 5 ] . T h e r e i s much m o r e l i k e l i h o o d of attack of c o a t i n g m a t e r i a l s in a c i d s o l u t i o n s because of i n c r e a s e d r a t e s o f h y d r o l y s i s r e a c t i o n s a t l o w pH*s. T h e i n t e r p r e t a t i o n o f c o r r o s i o n p o t e n t i a l has a l w a y s b e e n d i f f i c u l t . Wolstenholme [3] n o t an u n a m b i g u o u s i n d i c a t o C e r i s o l a a n d B o n o r a [ 7 ] d e s c r i b e d t h e m e a s u r e m e n t a s o n e w i t h no q u a n t i t a t i v e r e l a t i o n s h i p to amount of c o r r o s i o n . The r e s u l t s shown i n F i g u r e 2 c o n f i r m the q u e s t i o n a b l e v a l u e of p o t e n t i a l m e a s u r e m e n t s i n c o r r e l a t i o n s w i t h t h e c o r r o s i o n o f an u n d e r l y i n g substrate. The c o a t i n g c o n d u c t a n c e , on t h e o t h e r h a n d , h a s b e e n r e p o r t e d by n u m b e r o f p e o p l e t o be r e l a t e d t o t h e e x t e n t o f c o r r o s i o n u n d e r coating [8-11]. The c o n d u c t a n c e , e i t h e r AC o r DC, i s a f u n c t i o n t h e amount o f c h a r g e t h a t can p a s s t h r o u g h t h e c o a t i n g and t h amount of charge i s a f u n c t i o n of the amount of aqueous phase the c o a t i n g t h a t p e r m i t s charge motion.

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T h u s , i n a c i d s o l u t i o n i t a p p e a r s t h a t an i m p o r t a n t p r o p e r t y o f a coating for corrosion protection i s i t s permeability to acid. This v a r i a t i o n i n p e r m e a b i l i t y i s t h o u g h t t o be t h e r e a s o n f o r the d i f f e r e n c e in behavior of coatings observed during exposure to a c i d environments [12J. The p e r m e a b i l i t y i s a l s o a f f e c t e d by t h e d e g r a d a t i o n o f t h e c o a t i n g as c a u s e d b y r e a c t i o n w i t h t h e a c i d . Conclusions O f t h e f o u r t e c h n i q u e s s t u d i e d f o r e v a l u a t i n g c o a t i n g s on s t e e l f o r c o r r o s i o n c o n t r o l ( c o r r o s i o n p o t e n t i a l , c o n d u c t a n c e , a d h e s i o n and w e i g h t g a i n ) , t h e m o s t u s e f u l was c o n d u c t a n c e . Corrosion potential d i d n o t show a c o n s i s t e n t r e l a t i o n s h i p , and w e i g h t g a i n a n d t e n s i l e a d h e s i o n s h o w e d no c o r r e l a t i o n w i t h c o r r o s i o n . The b e s t p e r f o r m i n g c o a t i n g s s t u d i e d w e r e a v i n y l e s t e r , a b i s p h e n o l A e p o x y c u r e d w i t h an a l i p h a t i c a m i n e , a n d a n o v o l a c e p o x y cured w i t h a mixed a r o m a t i c / c y c l o a l i p h a t i c amine. A s a t u r a t e d p o l y e s t e r , and a b i s p h e n o l A e p o x y c u r e d w i t h a p o l y a m i d e a m i n e s n o w e d s i g n i f i c a n t d e t e r i o r a t i o n i n t h e a c i d and c o r r o s i o n o f t h e underlying steel. Two t y p e s o f n o v o l a c e p o x i e s c u r e d w i t h a r o m a t i c amines showed i n t e r m e d i a t e p e r f o r m a n c e .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

7.

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Evaluation of Coating Resins

85

Acknowledgments The a u t h o r s a r e i n d e b t e d t o t h e E l e c t r i c P o w e r R e s e a r c h I n s t i t u t e , P a l o A l t o , C a l i f o r n i a , f o r s u p p o r t i n g t h i s w o r k and t o B. C. S y r e t t of t h a t o r g a n i z a t i o n f o r h e l p f u l d i s c u s s i o n s during the study.

Literature Cited 1. Fontana, M. G.; Greene, N. D. "Corrosion Engineering"; McGraw-Hill: New York, 1978; 2nd ed. 2. Leidheiser, H. Jr. Prog. Org. Coatings 1979, 7, 79-104. 3. Wolstenholme, J . Corr. Sci. 1973, 13, 521-30. 4. Mansfeld, F.; Kendig, M. W.; Tsai, S. Corrosion 1982, 38, 478-85. 5. Funke, W.; Haagen, H. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 50-53. 6. White, M. L.; Pape August 1984. Submitte publication in 1986. 7. Cerisola, G.; Bonora, P. L. Mater. Chem. 1982, 7, 241-48. 8. D. J. Mills. In Coatings and Surface Treatment for Corrosion and Wear Resistance; Strafford, K. N., Ed.; Horwood: Chichester, England, 1984; pp. 315-30. 9. Rajagopalan, K. S.; Guruviah, S.; Rajagopalan, C. S. J. Oil Col. Chem. Assoc. 1980, 63, 144-48. 10. Touhsaent, R. E.; Leidheiser, H. Jr. Corrosion 1972, 28, 43540. 11. Vertere, V.; Rozados, E.; Carbonari, R. J. Oil Col. Chem. Assoc. 1978, 61, 419-26. 12. White, M. L.; Leidheiser, H. Jr., Materials Performance 1985, 24, 9-16. RECEIVED January 27, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8 Comparison of Laboratory Tests and Outdoor Tests of Paint Coatings for Atmospheric Exposure 1

1

1

1

2

M . Morcillo , J. Simancas , J. M . Bastidas , S. Feliu , C. Blanco , and F. Camón

2

1

Centro Nacional de Investigaciones Metalúrgicas, Ciudad Universitaria, 28040-Madrid, Spain Instituto Nacional de Técnica Aeroespacial "Esteban Terradas," Torrejon de Ardoz, Madrid, Spain

2

Many a c c e l e r a t e devised to determine the s u s c e p t i b i l i t y of paint f i l m s to breakdown by atmospheric weathering, however, the demand for a g e n e r a l l y a p p l i c a b l e t e s t e x i s t s . In t h i s study d i f f e r e n t t y p i c a l paint systems have been subjected to various n a t u r a l environments and l a b o r a t o r y t e s t s (DEF-1053 Method No. 26, ASTM G53-77, S a l t Spray and E l e c t r o c h e m i c a l Impedance Measurements). The r e s u l t s i n d i cate that e l e c t r o c h e m i c a l impedance measurements provide a s a t i s f a c t o r y c o r r e l a t i o n with the behaviour of paint coatings as evaluated by v i s u a l examination. In a d d i t i o n , i t appears t h a t , i n c e r t a i n cases, data obtained by t h i s technique w i l l allow p r e d i c t i o n of the m e t a l l i c c o r r o s i o n underneath the paint c o a t i n g when no changes i n the appearance of the c o a t i n g can be e x t e r n a l l y observed. In p r a c t i c e , most p a i n t c o a t i n g s e x h i b i t some r e d u c t i o n i n protective properties during their service l i f e . Paint c o a t i n g s exposed t o t h e atmosphere undergo a p r o g r e s s i v e d e g r a d a t i o n which u l t i m a t e l y r e s u l t s i n complete l o s s o f protective action. This degradation i s attributable t oa number o f e n v i r o n m e n t a l f a c t o r s , t h e most i m p o r t a n t b e i n g atmospheric contamination, u l t r a v i o l e t l i g h t , moisture, and t e m p e r a t u r e f l u c t u a t i o n s . It i s extremely important t o p r e d i c t c o a t i n g service l i f e w i t h some d e g r e e o f c e r t a i n t y . C l e a r l y , the best 0097-6156/ 86/ 0322-0086506.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8.

MORCILLO ET AL.

Tests of Paint Coatings for Atmospheric Exposure

87

assesment o f a c o a t i n g ' s p r o t e c t i v e e f f i c i e n c y i s p r o longed exposure t o t h e proposed s e r v i c e c o n d i t i o n s combined w i t h r e g u l a r i n s p e c t i o n s a t p r e d e t e r m i n e d time i n t e r v a l s . However, as t h e u s e f u l l i f e o f t h e o r g a n i c c o a t i n g s under atmospheric exposure i s o f t e n i n excess o f t e n y e a r s , i t i s n o t a l w a y s r e a l i s t i c t o t e s t new d e v e l o p m e n t s u n t i l t h e y have f a i l e d i n s e r v i c e . Consequently accelerated t e s t are necessary t o determine t h e s u s c e p t i b i l i t y o f p a i n t f i l m s t o breakdown by weathering o r o t h e r c l i m a t i c f a c t o r s . Many a c c e l e r a t e d l a b o r a t o r y t e s t s have been d e v i s e d f o r t h i s p u r p o s e . A r t i f i c i a l Weathering Tests. P r o g r e s s i n t h i s a r e a has been a c h i e v e d as c h e m i s t o f t h e d e g r a d a t i o n mechanism S i n c e a d i r e c t r e l a t i o n s h i p between t h e i n t e n s i t y o f s u n l i n g h t and t h e r a t e o f d e t e r i o r a t i o n o f p a i n t seemed t o e x i s t , an i n t e n s e l i g h t s o u r c e was c o n s i d e r e d t o be the p r i m a r y p r e r e q u i s i t e t o s i m u l a t e o r a c c e l e r a t e t h e degradation of polymers i n the l a b o r a t o r y . P r e s e n t l y , t h e r e i s a g r e a t number o f a p p a r a t u s and methods f o r a c c e l e r a t i n g w e a t h e r i n g i n t h e l a b o r a t o r y (DEF - 1053 No 26, ASTM G23 C a r b o n - a r c t y p e , ASTM G26 x e n o n - a r c t y p e , ASTM G53 f l u o r e s c e n t U V - C o n d e n s a t i o n type, etc.). However, even t h o u g h t h e s e d e v i c e s e x i s t , i t i s d i f f i c u l t t o o b t a i n r e p r o d u c i b l e r e s u l t s . Hence, t h e r a t h e r p e s s i m i s t i c s t a t e m e n t o f P a p e n r o t h (1_) : "A general a c c e l e r a t e d w e a t h e r i n g method w h i c h i s v a l i d i n a l l c a s e s does n o t e x i s t t o d a y and t h e r e w i l l n e v e r be one". A c c e l e r a t e d w e a t h e r i n g t e s t s a r e performed i n special t e s t chambers a l l o f w h i c h a r e s i m i l a r i n t h a t a c o a t e d s p e c i m e n i s e x p o s e d i n a c o n t r o l l e d e n v i r o n m e n t containing a h i g h i n t e n s i t y u l t r a v i o l e t l i g h t s o u r c e and a f a c i l i t y for s p r a y i n g water onto the t e s t s u r f a c e s . Accelerated Corrosion Tests. T h e r e a r e as many as a d o z e n methods ( s a l t f o g , K e s t e r n i c h , e t c . ) t h a t a r e c u r r e n t l y b e i n g used t o i n v e s t i g a t e c o r r o s i o n r e s i s t a n c e o f c o a t i n g s y s t e m s and a need t o d e v e l o p a b e t t e r and more d e p e n d a b l e method t o p r e d i c t i n - u s e s e r v i c e . A severe drawback o f a l l these t e s t s i s t h a t t h e i r r e s u l t s o f t e n compare u n s a t i s f a c t o r i l y w i t h p r a c t i c a l experience. One r e a s o n f o r t h e d i s c r e p a n c i e s i s assumed t o be t h e v a r i a b i l i t y of n a t u r a l exposure c o n d i t i o n s . Accordingly, c y c l i c t e s t i n g p r o c e d u r e s have been d e v e l o p e d w i t h w h i c h e x p o s u r e c o n d i t i o n s , e s p e c i a l l y t e m p e r a t u r e and h u m i d i t y ,

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

88

POLYMERIC MATERIALS FOR CORROSION CONTROL

are s i s t e m a t i c a l l y v a r i e d . R e s u l t s o b t a i n e d w i t h such c y c l i c t e s t s a g r e e much b e t t e r w i t h p r a c t i c a l performance t h a n t h o s e f r o m c o n s t a n t c o n d i t i o n t e s t . However a t present t h e r e i s no s i n g l e a c c e l e r a t e d t e s t w h i c h c o r r e l a t e s s a ­ t i s f a c t o r i l y w i t h a c t u a l l o n g term performance i n any e n v i r o n m e n t ; t h e demand f o r a g e n e r a l l y a p p l i c a b l e c o r r o ­ s i o n t e s t e x i s t s (2) . Electrochemical Corrosion Tests. Electrochemical proce­ dures are very w e l l e s t a b l i s h e d f o r i n v e s t i g a t i n g the c o r r o s i o n b e h a v i o u r o f p u r e m e t a l s , a l l o y s and m e t a l l i c c o a t i n g s . The a p p l i c a t i o n o f t h e s e t e c h n i q u e s t o p a i n t c o a t i n g s i s , however, f a r l e s s common. I t was d i s a p p o i n t ­ i n g t o f i n d t h a t u t 1 973 (3) e l e c t r o c h e m i c a l t e s t d i d not p r o v i d e much u s e f u was f u l l y j u s t i f i a b l e a decade ago when d . c . e l e c t r o c h e m i c a l methods were a l m o s t e x c l u s i v e l y u s e d , t h e p i c t u r e seems t o have changed r a d i c a l l y when r e c e n t r e s u l t s o b ­ t a i n e d by a . c . impedance measurements a r e e x a m i n e d . The a d v a n t a g e o f t h e s e impedance measurements o v e r t h e c l a ­ s s i c a l t e s t s m e n t i o n e d i s i t s non d e s t r u c t i v e n a t u r e ; i n a d d i t i o n , c o r r o s i o n and c o a t i n g damage may be d e t e r m i n e d prior to i t s visual manifestation. This coincides with t h e p r e s e n t t r e n d t o w a r d s t h e d e v e l o p m e n t o f methods which enable e a r l y p r e d i c t i o n o f the paint c o a t i n g per­ formance, even b e f o r e t h e o c c u r r e n c e o f any s u b s t a n t i a l changes i n i t s appearance. Experimental T e s t s a m p l e s have been p r e p a r e d f r o m a 3 mm h o t - r o l l e d s t e e l s h e e t w h i c h showed an i n t a c t m i l l s c a l e (Degree A of Swedish Standards, SIS-055900). T h e r e a f t e r t h e y were s h o t b l a s t e d w i t h S-280 t o r e a c h t h e ASa3 s t a n d a r d , p r i o r to the a p p l i c a t i o n of the paint coating. Surface prepa­ r a t i o n Β S t 2 was o b t a i n e d by w i r e b r u s h i n g a s t e e l s h e e t o f Grade Β o b t a i n e d i n t u r n by o x i d a t i o n o f Grade A s h e e t i n a contaminant f r e e atmosphere. I n "Table I . " , t h e c h a r a c t e r i s t i c s o f t h e p a i n t s used i n t h i s s t u d y a r e shown. To s t u d y t h e e f f e c t o f c o n t a m i n a n t s ( c h l o r i d e s and s u l p h a t e s ) a t t h e i n t e r f a c e m e t a l / c o a t i n g , a s e t ofpanels ( s u r f a c e A Sa 3) was p r e p a r e d and d o s e d w i t h s o l u t i o n s o f N a C l and F e S 0 i n d i s t i l l e d w a t e r and m e t h a n o l . Subse­ q u e n t l y , two p a i n t s y s t e m s ( c h l o r i n a t e d r u b b e r and p o l y u ­ r e t h a n e ) were a p p l i e d on t h e s e c o n t a m i n a t e d s u r f a c e s . 4

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8.

MORCILLO ET AL.

Table I .

Tests of Paint Coatings for Atmospheric Exposure

C h a r a c t e r i s t i c s of paint

Paint System

System Designation

Primer

systems.

Undercoat

Topcoat

Total Thickness (μη)

0/A

INTA 164103

—

INTA 16421 8

90-120

Alkyd

A

INTA 164201

—

INTA 16421 8

80-110

Chlorinated Rubber

CR

INTA 164705

INTA 164701A

INTA 164704A

80-100

Vinyl

V

INTA

INTA

INTA

100-120

Polyurethane

Ρ

Epoxy/Polyur,

E/P

Oil/Alkyd

RENFE 03.323 .125 MIL C-82407

RENFE 03.323.125 «

120-1 50 90-120

The f o l l o w i n g t e s t s were u s e d : 1 .

2. 2.1.

N a t u r a l w e a t h e r i n g t e s t s i n v o l v i n g d i f f e r e n t environ­ ments: r u r a l , u r b a n , i n d u s t r i a l and m a r i n e . At p r e s e n t o n l y r e s u l t s f o r two y e a r s o f f i e l d exposure are a v a i l a b l e . A r t i f i c i a l w e a t h e r i n g t e s t s i n c l i m a t i c chambers i n accordance w i t h the f o l l o w i n g s p e c i f i c a t i o n s : DEF-1053 Method No. 26. A l t e r n a t i v e r u n n i n g o f car­ bon a r c s i n 6-hour p e r i o d s , w i t h a chamber t e m p e r a ­ t u r e o f about 402C. In*between carbon a r c o p e r a ­ t i o n a l p e r i o d s , d e i o n i z e d water s p r a y i n g w i t h a r e ­ s i s t i v i t y over 300.000JX.cm . Testing period: 3500 h o u r s . ASTM G53. C o n d e n s a t i o n (44^C) and c o o l i n g ( 6 5 C ) c y c l e s were u s e d o f 4 - h o u r d u r a t i o n e a c h , w i t h a 0.5 h o u r i n b e t w e e n . An u l t r a v i o l e t r a d i a t i o n was c o n t i n u o u s l y produced. Testing period: 2300 h o u r s . S a l t S p r a y ( f o g ) t e s t (ASTM Β 117) w i t h a t e s t i n g p e r i o d o f 3200 h o u r s . Impedance d i a g r a m t e c h n i q u e . The p o l a r i z a t i o n c e l l c o n s i s t e d i n a t r a n s p a r e n t p l a s t i c t u b e t h a t was a d h e r e d t o t h e p a i n t s u r f a c e by means o f a s i l i c o n e sealer. The t u b e c o n t a i n e d d i s t i l l e d w a t e r and a 25 c m p l a t i n i z e d t i t a n i u m s h e e t , w h i c h was u s e d as auxiliary electrode. M e a s u r e m e n t s were made w i t h 2

2.2.

3. 4.

Q

2

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

89

90

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

t h e d o u b l e e l e c t r o d e t e c h n i q u e ( 4 ) and t h e h e l p o f a H.P. 4800-A V e c t o r Impedance M e t e r , o v e r a f r e q u e n c y range o f 50 K H t o 5 H . Z

z

R e s u l t s and D i s c u s s i o n R e g u l a r i n s p e c t i o n s o f r u s t i n g and b l i s t e r i n g g r a d e s shown on p a i n t e d s u r f a c e s were c a r r i e d o u t . F a i l u r e was e v a l u a ­ t e d a c c o r d i n g t o ASTM D610 and ASTM D 714 s t a n d a r d s . The r e s u l t s o b t a i n e d a r e g i v e n i n " T a b l e s I I and I I I . " As i s a p p a r e n t , no v i s i b l e damage was o b s e r v e d on ASa 3 s t e e l p a n e l s ("Table I I I " . ) a f t e r two y e a r s ' t e s t e x p o s u r e t o the v a r i o u s atmospheres. The s a l t f o g t e s t , e n a b l e d us to arrange t h e d i f f e r e n t p a i n t system accordin t th exposure time e l a p s e cular rating level. We have c h o s e n r u s t Grade 8 ( 0 . 1 % r u s t ) since i ti s a reasonably accepted i n d i c a t o r i n t h i s type of s t u d i e s . I t was seen t h a t s y s t e m s Ρ and E/P r e m a i n u n a l t e r e d a f t e r 3200 h o u r s t e s t i n g . The r e m a i n i n g systems f a i l a t s h o r t e r t e s t t i m e s . W i t h r e g a r d t o t h e ASTM G 53 and DEF-1052-26 a c c e l e ­ r a t e d w e a t h e r i n g t e s t s , no c l o s e agreement was o b s e r v e d between them. Whereas i n t h e ASTM G53 t e s t t h e CR and V s y s t e m s show r u s t p o i n t s a t 800-1000 h o u r s , i n t h e DEF-1053-26 t e s t b o t h s y s t e m s keep i n a l m o s t p e r f e c t condition. However, i n t h i s t e s t t h e 0/A and A s y s t e m s a r e t h e o n l y ones w h i c h show a s l i g h t c o r r o s i o n a t 1900 hours. I n e i t h e r o f t h e two c a s e s o n l y a s l i g h t c o r r o s i o n i s i n v o l v e d w h i c h does n o t r e a c h Grade 8 i n t h e ASTM range. Of t h e BSt2 s t e e l p a n e l s under a t m o s p h e r i c e x p o s u r e ("Table I I " . ) o n l y t h e CR s y s t e m shows e a r l y r u s t s p o t s (6-12 m o n t h s ) . A f t e r t h e f i r s t 3-6 months, r u s t b l i s t e r s a p p e a r on t h e p a i n t s u r f a c e . L a t e r these b l i s t e r breaks due t o a c c u m u l a t i o n o f c o r r o s i o n p r o d u c t s i n s i d e them. The r u s t Grade i s h i g h e r as i s t h e a t m o s p h e r i c c o r r o s i v i t y W i t h t h i s p a i n t s y s t e m (CR) l a b o r a t o r y t e s t s c o r r e ­ l a t e q u i t e w e l l w i t h t h e r e s u l t s i n t h e atmosphere. So, i n t h e s a l t f o g chamber t h e w o r s t p a i n t p e r f o r m a n c e i s shown by t h i s s y s t e m , w h i c h i s a l s o t h e o n l y one t h a t shows s l i g h t r u s t i n g i n t h e ASTM G 53 t e s t . The DEF-1053 t e s t p r o m o t e s b l i s t e r i n g i n t h i s s y s t e m , as w e l l as i n the 0/A,A and V s y s t e m s . When m a k i n g an o v e r a l l e x a m i n a t i o n o f " T a b l e s I I . and I I I . " t o c o r r e l a t e t h e r e s u l t s o b t a i n e d i n t h e l a b o ­ r a t o r y w i t h t h o s e i n t h e n a t u r a l e n v i r o n m e n t s , we a r e faced w i t h the problem o f t h e s h o r t time e l a p s e d i n the

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

NF

NF NF

>3200 ^3200

NF

NF

Ρ

E/P

NF = No f a i l u r e s i n t i m e

specified

NF

1000 h r s . : pinholes i n f i l m and rusting

2400

NF

160 h r s . : slight blistering

NF

800 h r s . : light rusting

CR

2100

1900 h r s . slight rusting

NF

1900 h r s . slight rusting

NF

NF

DEF-1053(26) (3500 h r s . )

ASTM G53 (2300 h r s . )

1 800

NF

Salt Spray (hrs. t o rust Grade 8) 800

NF

0/A

Outdoor (2 years)

Outdoor and laboratory t e s t s of paint coatings applied over ASa 3 s t e e l panel

System Designation

Table 11(A).

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

specified

NF

>3200

NF

E/P

NF = No f a i l u r e s i n t i m e

NF

>3200

NF

>3200

NF

NF

1000 h r s , slight rusting

NF

800

500

NF

6-12 months : rust coming through b l i s t e r s . Rust Grade increases with atmospheric corrosivity

NF

NF

ASTM G53 (2300 h r s . )

Salt Spray (hrs. t o rust Grade 8) 1 400

Ρ

CR

0/A

Outdoor (2 y e a r s )

NF

NF

160 h r s . : slight blistering

1900 h r s . : blistering

1900 h r s . : slight blis­ tering

1900 h r s . : slight blis­ t e r i n g and rusting

DEF-1053(26) (3500 h r s . )

Outdoor and laboratory t e s t s of paint coatings applied over BSt 2 s t e e l panels

System Designation

Table 11(B).

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

NF NF

1500 1500

700 500

NF NF

ASa3, 500 mg/m FeSO^

ASa3, 1000 mg/m FeSO^

NF = No f a i l u r e s i n t i m e s p e c i f i e d

2

2

NF

1500

700

2

NF

NF

1100 (heavy rusting)

500

ASa3, 250 mg/m FeSO^

ASa3, 500 mg/m NaCl

12-18 months : blistering and r u s t i n g

NF

1200

700

NF

ASa3, 100 mg/m NaCl

2

NF

800

700

NF

ASa3, 20 mg/m NaCl

2

NF

1000

500

6-12 months : rusting

BSt 2

NF

NF

ASa 3

Spray ASTM G53 (hrs. t o rust (hrs. t o s l i Grade 8) ght rusting) 800 2100

Surface Condition

S a l t

O u t d o o r and l a b o r a t o r y t e s t s o f p a i n t c o a t i n g s a p p l i e d o v e r and B S t 2 s t e e l p a n e l s . P a i n t system: C h l o r i n a t e d Rubber.

Outdoor (2 years)

Table I I I ( A ) .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2400 2400

NF NF

ASa3, 250 mg/m FeSO^

NF

NF

NF

NF

NF

NF

NF

NF

NF

NF

NF

NF = No f a i l u r e s i n t i m e specified. χ = B l i s t e r Grade a c c o r d i n g c o n v e r s i o n T a b l e o f ASTM D714 t o a numerical scale (5).

ASa3, 1000 mg/m FeSO

2

ASa3, 500 mg/m FeSO^

2

2

2400

1 50

12-18 months : slight blistering

ASa3, 500 mg/m NaCl

NF

NF

800

NF

2

ASa3, 100 mg/m NaCl

2

ASa3, 20 mg/m NaCl

NF

NF

2

1 600

NF

BSt 2

NF

DEF-1053(26) A S ™ G53 (hrs. t o b l i s t e r ) (hrs. t o b l i s t e r )

NF

NF

ASa 3

Salt Spray (hrs. t o b l i s t e r Grade 6*) 1800 400

Outdoor (2 years)

O u t d o o r and l a b o r a t o r y t e s t s o f p a i n t c o a t i n g s a p p l i e d o v e r A S a and BSt 2 s t e e l p a n e l s . P a i n t system: Polyurethane.

Surface Condition

Table I I I ( B ) .

8.

M O R C I L L O ET AL.

Tests of Paint Coatings for Atmospheric Exposure

95

l a t t e r i n o r d e r t o r e a c h s i g n i f i c a n t d i f f e r e n c e s among the d i f f e r e n t p a i n t s y s t e m s . Among t h e v a r i o u s t e s t s c a r r i e d o u t , t h e s a l t s p r a y t e s t i s t h e one t h a t b e s t c o r r e l a t e s with atmospheric exposure. A p p a r e n t l y i t i s the most a g g r e s i v e o f a l l t e s t s , and t h e o n l y a c c e l e r a t e d c o r r o s i o n t e s t i n c o m p a r i s o n w i t h t h e r e m a i n i n g ones c o n s i d e r e d s p e c i f i c a l l y as a c c e l e r a t e d w e a t h e r i n g t e s t s . I n t h e a c c e l e r a t e d w e a t h e r i n g t e s t s some o f t h e s y s t e m s showed a c e r t a i n d e g r a d a t i o n ( c h a n g e s i n c o l o u r a n d / o r b r i g h t n e s s , c h a l k i n g , e t c . ) w h i c h have n o t been i n c l u d e d i n t a b l e s s i n c e t h e y were n o t c o n s i d e r e d of i n t e r e s t f o r our purpose. However, s u c h t e s t s p r o v i d e p e r haps t h e most u s e f u l i n f o r m a t i o n f o r e v a l u a t i n g t h e s e type of degradation. p e r h a p s t h e most s u i t a b l t h e c o a t i n g s y s t e m i t s e l f , but s u c h t e s t s do not a l w a y s o f f e r an a c c u r a t e i n d i c a t i o n o f t h e c o r r o s i o n p r o t e c t i o n p r o v i d e d by t h e c o a t i n g . These methods a r e not by thems e l v e s c o r r o s i o n t e s t s and a r e o n l y l i k e l y t o c a u s e s i g n i f i c a n t s u r f a c e d e t e r i o r a t i o n o f t h e p a i n t c o a t i n g (6,7). E l e c t r o c h e m i c a l impedance measurements were u s e d t o determine the e f f e c t of a c c e l e r a t e d w e a t h e r i n g of the p a i n t c o a t i n g on t h e c o r r o s i o n o f t h e s t e e l s u b s t r a t e . F i g u r e s 1 and 2 g i v e t h e impedance d i a g r a m s a t d i f f e r e n t t i m e s f o r t h e v a r i o u s p a i n t s y s t e m s c a r r i e d out on non w e a t h e r e d c o a t i n g s (N.W.) and on c o a t i n g s a f t e r weathering by means o f t h e ASTM G 53 and DEF-1053- 26 t e s t s f o r 2300 and 3500 h r s , r e s p e c t i v e l y . I n a c c o r d w i t h the v i s u a l o b s e r v a t i o n s , t h e CR and V s y s t e m s i n t h e ASTM G 53 t e s t and 0/A and A s y s t e m s i n t h e DEF-1053 -26 t e s t , show a diagram i n a s e m i c i r c l e form, which i n d i c a t e s the e x i s tance of paths or channels of e l e c t r o l y t e p e n e t r a t i o n a c r o s s t h e f i l m p e r m i t i n g t h e movement o f t h e c h e m i c a l s p e c i e s t o and f r o m t h e m e t a l s u b s t r a t e ( c o r r o s i o n , d i f f u s i o n , e t c . ) ( 8 ) . The o t h e r s y s t e m s i n d i c a t e a p a i n t f i l m i n p e r f e c t c o n d i t i o n and t h e impedance d i a g r a m t a k e s t h e shape o f a s t r a i g h t l i n e f o r m i n g a c e r t a i n a n g l e w i t h t h e imaginary a x i s . This response i s s i m i l a r to t h a t of a c a p a c i t o r , w i t h phase a n g l e s n e a r t o 90^ and v e r y h i g h v a l u e s of t h e impedance m o d u l u s . Thus, t h i s m e a s u r i n g technique r e v e a l s q u i t e s a t i s f a c t o r i l y the c o r r o s i o n damage o b s e r v e d by t h e naked eye on p a i n t e d s u r f a c e s . E f f e c t o f P o l l u t a n t s on t h e M e t a l / P a i n t I n t e r f a c e . "Table I I I . " gives the r e s u l t s o f d i f f e r e n t t e s t s c a r r i e d o u t on CR f i l m s a p p l i e d t o r u s t e d s t e e l s u r f a c e s (BSt 2) o r surfaces c o n t a m i n a t e d w i t h N a C l and FeSO^. I n t h e c a s e o f r u s t

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC MATERIALS FOR CORROSION CONTROL

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coating

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cc(STP)cm g_çm g cm (cm Hg)sec cm (cm Hg)sec cm^(cm Hg) 2

ΒIB/HMMM (BI) B/HMMM BTB/HMMM BTB + (BT) B/HMMM 3

2

(BT) B/HMMM GTG/HMMM GCG/HMMM NIN/HMMM BIB/isocyanate GTG/isocyanate NIN/isocyanate B/isocyanate N/isocyanate 2

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R e s u l t s and D i s c u s s i o n Water vapour p e r m e a b i l i t y . The most n o t a b l e phenomenon o v e r ­ l o o k i n g t h e d a t a p r e s e n t e d i n T a b l e I i s t h a t t h e w a t e r vapour p e r m e a b i l i t i e s o f t h e HMMM-based c o a t i n g s a r e n o t w i d e l y d i f f e r ­ e n t . The i s o c y a n a t e c o a t i n g s show somewhat l a r g e r d i f f e r e n c e s . GTG/isocyanate and t h e c o a t i n g s made from b u t a n e d i o l and n e o p e n t y l g l y c o l a r e more permeable. The e x p e r i m e n t a l p e r m e a b i l i t y i s t h e p r o d u c t o f t h e c o e f f i ­ c i e n t o f d i f f u s i o n and s o l u b i l i t y (P - D χ S ) . When t h e measured s o l u b i l i t i e s a r e t a k e n i n t o c o n s i d e r a t i o n i t appears t h a t t h e d i f ­ f e r e n c e s i n p e r m e a b i l i t y o b s e r v e d c a n m a i n l y be a t t r i b u t e d t o t h i s f a c t o r . The c a l c u l a t e d d i f f u s i o n c o e f f i c i e n t s d i f f e r a t most a f a c t o r o f t h r e e . However, i f i t i s r e a l i z e d t h a t t h i s c o e f f i c i e n t i s d e r i v e d from two e x p e r i m e n t a l l y observed v a r i a b l e s and t h a t t h e

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

10.

MUIZEBELT AND HEUVELSLAND

Permeabilities of Model Coatings

113

s o l u b i l i t y measurements were somewhat l e s s r e p r o d u c i b l e t h a n t h e p e r m e a b i l i t i e s , i t i s q u e s t i o n a b l e whether t h e d i f f e r e n c e s i n d i f fusion coefficients are significant. We t h e r e f o r e tend t o c o n c l u d e t h a t t h e d i f f e r e n c e s i n permeab i l i t y observed a r e due t o d i f f e r e n c e s i n s o l u b i l i t y r a t h e r t h a n v a r i a t i o n s i n the d i f f u s i o n c o e f f i c i e n t . Differences i n s o l u b i l i t y o f w a t e r may be a t t r i b u t e d t o d i f f e r e n c e s i n p o l a r i t y o f t h e medium. I t w i l l be c l e a r t h a t t h e i s o c y a n a t e c o a t i n g s made from but a n e d i o l o r n e o p e n t y l g l y c o l c o n t a i n a h i g h e r c o n c e n t r a t i o n o f pol a r urethane l i n k a g e s t h a n those made from t h e o l i g o m e r s (although these c o n t a i n p o l a r e s t e r groups). A l s o isocyanate coatings a r e more p o l a r t h a n those w i t h HMMM, w h i c h c o n t a i n l e s s p o l a r e t h e r groups. The main c o n c l u s i o n we would l i k e t o advance i s t h a t t h e d i f f u s i o n o f w a t e r i n t h e c o a t i n g i s n o t n o t i c e a b l y a f f e c t e d by t h e c r o s s l i n k density of the f i l m s This conclusion i s i n contrast t o t h o s e o f Gordon and Ravv l i n k d e n s i t y on oxygen b i l i t y o f n a t u r a l v u l c a n i z a t e s f o r v a r i o u s gases was found t o be s t r o n g l y dependent on t h e amount o f s u l f u r used (18,19). I t must be c o n c l u d e d t h a t a l t h o u g h t h e c r o s s l i n k d e n s i t i e s o f o u r m a t e r i a l s a r e i n t h e range o f t h e a c r y l a t e s s t u d i e d by Gordon and Ravve, the d i f f e r e n c e s i n c r o s s l i n k d e n s i t i e s do n o t l e a d t o s i m i l a r e f f e c t s on space f i l l i n g c h a r a c t e r o r network t i g h t n e s s . The d i f f e r e n c e w i t h t h e v u l c a n i z a t e s (18,19) c o u l d c o n c e i v a b l y be a matt e r o f g l a s s t r a n s i t i o n t e m p e r a t u r e . Our measurements were c a r r i e d out below Tg whereas t h e o b s e r v a t i o n s on t h e v u l c a n i z a t e s were c a r r i e d o u t above Tg. Oxygen p e r m e a b i l i t y . Oxygen p e r m e a b i l i t y measurement r e q u i r e d a l a r g e r p i e c e o f c o a t i n g w i t h a g r e a t e r chance o f l e a k s . T h e r e f o r e i t was o f t e n n o t p o s s i b l e t o p e r f o r m t h e s e measurements. The fewer d a t a f o r oxygen p e r m e a b i l i t y i n Table I i n d i c a t e s m a l l e r v a l u e s f o r t h e i s o c y a n a t e c o a t i n g s than f o r t h o s e based on HMMM. T h i s w i l l be due t o t h e d i f f e r e n c e i n p o l a r i t y , w h i c h i n f l u e n c e s t h e s o l u b i l i t y t h e o p p o s i t e way as i n t h e case o f w a t e r . Oxygen, as a n o n - p o l a r m o l e c u l e , d i s s o l v e s b e t t e r i n media w i t h l o w e r p o l a r i t y i n c o n t r a s t t o w a t e r . T h e r e f o r e t h e p e r m e a b i l i t y o f oxygen i s a l s o l a r g e r i n media o f lower p o l a r i t y . S a l t spray t e s t . The model c o a t i n g s o f T a b l e I a r e o f t h e h i g h s o l i d type used i n a u t o m o t i v e t o p c o a t s . T h e i r p r i m a r y f u n c t i o n i s not c o r r o s i o n p r o t e c t i o n s i n c e t h i s i s f i r s t o f a l l a m a t t e r o f phosphate l a y e r , e l e c t r o c o a t and/or p r i m e r . However, t h e t o p c o a t s may c o n t r i b u t e t o c o r r o s i o n p r o t e c t i o n by t h e i r b a r r i e r f u n c t i o n f o r w a t e r , oxygen and s a l t s . T h e r e f o r e t h e i r p e r m e a b i l i t y i s i m p o r t a n t as one o f t h e f a c t o r s i n t h e c o r r o s i o n p r o t e c t i o n by t h e t o t a l c o a t i n g system. We f e e l t h a t a s a l t s p r a y t e s t o f t h e model coatings d i r e c t l y a p p l i e d t o a s t e e l surface i s of l i t t l e r e l e vance f o r t h e i r c o r r o s i o n p r o t e c t i o n performance i n a r e a l system. N e v e r t h e l e s s we d i d a number o f t e s t s o f o u r model c o a t i n g s d i r e c t l y a p p l i e d t o Bonder 101 p a n e l s . The p a n e l s were g i v e n a s t a n d a r d s c r a t c h j u s t below t h e m e t a l s u r f a c e a f t e r w h i c h they

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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POLYMERIC MATERIALS FOR CORROSION CONTROL

were exposed i n the s a l t spray t e s t . The corrosion protection performance was at best moderate and no s i g n i f i c a n t differences between the various coatings could be seen. This i s i n accord with the small differences i n permeability observed. On t h i s basis we do not expect s i g n i f i c a n t differences when the coatings are tested on panels provided with a proper electrocoat primer, although the corrosion protection by the complete system may be expected to be on a much higher l e v e l . Acknowledgment Experimental assistance was given by Rianne Willems and Mark Buurman· Solid state

NMR spectra were taken by I r . H. Angad Gaur.

Literature Cited 1. W. Funke, J.O.C.C.A. 62, 63 (1979). 2. W. Funke and H. Haagen, Ind. Eng. Chem. Prod. Res. Dev. 17, 50 (1978). 3. H. Haagen and W. Funke, J.O.C.C.A. 58, 359 (1975). 4. F . L . Floyd, R.G. Groseclose and C.M. Frey, J.O.C.C.A. 66, 329 (1983). 5. M. Yaseen and K.V.S.N. Raju, J.O.C.C.A. 67, 185 (1984). 6. S. Guruviah, J.O.C.C.A. 63, 669 (1970). 7. D.Y. Perera and S. Pelier, Progr. Org. Coat. 1, 57 (1973). 8. P.W. Morgan, Ind. Eng. Chem. 2296 (1953). 9. W.L.H. Moll, Kolloid Zeitschr. 195, 43 (1964). 10. W. Funke and C. Carfagna, J.O.C.C.A. 67, 102 (1984). 11. K.A. v. Oeteren, Fette, Seife, Anstrichmittel 84, 242 (1982). 12. J . E . Fitzwater, J . Coat. Techn. 53 (683) 27 (1981). 13. G.A. Gordon and A. Ravve, Polymer Eng. and Sci. 20, 70 (1980). 14. H.W. Hässlin, M. Dröscher and G. Wegner, Makromol. Chem. 181, 301 (1980). 15. M. Yaseen and W. Funke, J.O.C.C.A. 61, 284 (1978). 16. M. Lomax, Polymer Testing 1, 105 (1980). 17. P.E. Cassidy, T.M. Aminabhari and C.M. Thompson, Rub. Chem. Techn. 56, 594 (1983). 18. R.M. Barrer and G. Skirrow, J. Pol. Sci. 3, 549 (1948). 19. A. Aitken and R.M. Barrer, Trans. Farad. Soc. 51. 116 (1955).

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

11 Using Acoustic Emission to Investigate Disbonding at the Polymer-Metal Interface L. M . Callow and J. D. Scantlebury Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Sackville Street, Manchester, M6O 1QD, England

There has been som signals result from the fracture that occurs when adhesion is lost at the paint-metal interface during blistering and cathodic disbonding. In the present study, very sensitive Acoustic Emission measurements have been made in systems exhibiting these types of breakdown. A lacquer was applied to a panel, which subsequently suffered gross blistering, electrochemical voltage noise and acoustic emission were monitored simultaneously. The former showed large peaks as the blisters grew, whilst the latter produced no signals that were directly attributable to blistering. In the second series of experiments, a r t i f i c i a l holidays were introduced into a commercially available epoxy lacquer, which was potentiostatically polarised to -1000 mV (SCE). Large disbonded areas grew rapidly around each holiday, but once again, no acoustic signals were detectable. P r e v i o u s work on the a p p l i c a t i o n of the a c o u s t i c e m i s s i o n t e c h n i q u e t o c o r r o s i o n p r o c e s s e s has been c a r r i e d out by M a n s f e l d and S t o c k e r ( D who were a b l e t o show t h a t the detachment o f hydrogen b u b b l e s from a p o l a r i s e d m e t a l s u r f a c e gave measurable s i g n a l s . The a p p l i c a t i o n o f the t e c h n i q u e t o the detachment o f a p a i n t from a s t r e s s e d s u b s t r a t e has been shown t o be v i a b l e by S t r i v e n s ^ ) and o t h e r s . The work p r e s e n t e d h e r e i n i s an attempt t o extend the above work i n t o a r e a s where the s u b s t r a t e i s u n s t r e s s e d and the p a i n t becomes detached a s a r e s u l t o f s t r e s s e s i n the f i l m o r s t r e s s e s h a v i n g an e l e c t r o c h e m i c a l o r i g i n . Experimental The a c o u s t i c e m i s s i o n equipment used i n t h i s s e r i e s o f experiments i s d e s c r i b e d below. Two types o f A.E. t r a n s d u c e r were employed, the f i r s t , B r u e l and K j a e r type 8313 covered the f r e q u e n c y range 50 t o 600 kHz, w h i l s t the second, B r u e l and K j a e r type 8314 covered 0097-6156/ 86/ 0322-0115S06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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the h i g h e r frequency range 0.5 t o 1.1 MHz. The frequency response curve c h a r a c t e r i s t i c s a r e shown i n B r u e l and K j a e r l i t e r a t u r e ^ . ) . The output from these t r a n s d u c e r s were f e d i n t o a B r u e l and K j a e r pre-amp (model 2637) which had a g a i n o f 40 dB. The s i g n a l was f u r t h e r a m p l i f i e d by a B r u e l and K j a e r c o n d i t i o n i n g a m p l i f i e r (model 2638) capable o f g i v i n g an e x t r a 60 dB o f g a i n and g i v i n g a f l a t frequency response between 0.1 Hz and 2MHz. The t o t a l a m p l i f i c a t i o n i n the system was l i m i t e d by s t r a y n o i s e and susceptability t o Radio Frequency I n t e r f a c e . (R.F.I.),so s u p p r e s s i o n was i n t r o d u c e d i n t h e system t o l i m i t t h e problem. Any A.E. t r a n s i e n t s o c c u r r i n g were c a p t u r e d by a D a t a l a b (DL 910) t r a n s i e n t r e c o r d e r ; i t s A/D C o n v e r t e r was capable o f o p e r a t i n g t o 20 MHz and 8k byte l o n g t r a n s i e n t s c o u l d be s t o r e d . F u r t h e r a n a l y s i s was s u b s e q u e n t l y c a r r i e d out on t h e s t o r e d d a t a u s i n g a BBC model Β microcomputer, programmed t o p e r f o r m F a s t F o u r i e r Transforms ( F . F . T . ) P r e l i m i n a r y experiments were c a r r i e d out i n which m i l d s t e e (Thompson m i n i s t a t 251 e v o l u t i o n on t h e specimen. F o l l o w i n g t h i s s u c c e s s f u l t e s t , i t was d e c i d e d t o extend t h e use o f t h e a p p a r a t u s i n t o d i s b o n d i n g . A d e l i b e r a t e l y underbound z i n c - r i c h p a i n t was a p p l i e d t o a s t a n d a r d t e s t p a n e l g i v i n g a wet f i l m t h i c k n e s s g r e a t e r than 3 mm. As t h e p a i n t d r i e d , extreme "mud c r a c k i n g " was e x h i b i t e d . The p r o g r e s s o f the c r a c k i n g and subsequent detachment o f t h e f i l m from t h e s t e e l s u r f a c e was observed u s i n g a v i d e o camera i n c o n j u n c t i o n w i t h t h e A.E. equipment. F u r t h e r experiments were c a r r i e d out i n which s m a l l f l a k e s o f a 40 um t h i c k grey mica pigmented l a c q u e r were m e c h a n i c a l l y detached from m i l d s t e e l , t h e r e s u l t i n g t r a n s i e n t s were c a p t u r e d and a n a l y s e d . As A.E. s i g n a l s had been observed i n t h e above e x p e r i m e n t s , i n v e s t i g a t i o n proceeded w i t h t h e grey mica pigmented l a c q u e r w h i c h was prone t o b l i s t e r i n g under immersion. Specimens w i t h f i l m t h i c k n e s s e s o f 50 - 110 um were semi-immersed i n 3% NaCl s o l u t i o n , by suspending t h e e l e c t r o d e from e l a s t i c bands, I n o r d e r t o conserve any s i g n a l s produced. S e v e r a l attachment s i t e s f o r t h e t r a n s d u c e r were t r i e d on t h e specimen, i n c l u d i n g b o t h the p a i n t e d and n o n - p a i n t e d s i d e s ; no d e t e c t a b l e d i f f e r e n c e s due t o t r a n s d u c e r l o c a t i o n were observed d u r i n g t h e s e v e r a l weeks immersion time o f the specimen. I n a second s e r i e s o f e x p e r i m e n t s , an epoxy c o a t e d specimen was employed, w h i c h had 5 s m a l l (1 mm d i a m e t e r ) h o l e s d r i l l e d down to the s t e e l . E l e c t r i c a l c o n n e c t i o n s were made t o t h e upper edge and t h e specimen was p o t e n t i o s t a t i c a l l y p o l a r i s e d t o -1050 mV (SCE) and a l l o w e d t o d i s b o n d . A g a i n , t h i s p r o c e s s was m o n i t o r e d u s i n g t h e a c o u s t i c e m i s s i o n t r a n s d u c e r . A l l the immersed specimens were masked u s i n g a m i x t u r e o f beeswax and colophony r e s i n , as d e s c r i b e d elsewhere (4). R e s u l t s and D i s c u s s i o n s Many o f t h e problems encountered w i t h t h e a p p l i c a t i o n o f t h e A c o u s t i c E m i s s i o n (A.E) t e c h n i q u e t o c o r r o s i o n problems a r e due t o t h e f a c t t h a t t h e s i g n a l l e v e l was o f a s i m i l a r o r d e r o f magnitude t o t h e background n o i s e l e v e l o f the i n s t r u m e n t a t i o n .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

11. CALLOW AND SCANTLEBURY

Acoustic Emission to Investigate Disbonding

F i g u r e 1 shows t h e n o i s e l e v e l o b t a i n e d w i t h the maximum u s a b l e g a i n o f 70 dB. F i g u r e 2 i s a F.F.T. o f t h e time domain d a t a o f the p r e v i o u s f i g u r e . T h i s shows the f r e q u e n c y d i s t r i b u t i o n o f the background n o i s e . When t h e t r a n s d u c e r was connected t o a p i e c e o f m i l d s t e e l t h a t was c a t h o d i c a l l y p o l a r i s e d t o e v o l v e hydrogen, t h e time domain t r a c e o f f i g u r e 3 was o b t a i n e d . I t can be seen t h a t e x t r a peaks o c c u r r e d on a r e g u l a r b a s i s t h a t c o u l d be a t t r i b u t e d t o t h e f o r m a t i o n o f hydrogen b u b b l e s , as shown arrowed i n f i g u r e 3. When the grey p a i n t f i l m was l e v e r e d from t h e s u b s t r a t e m e c h a n i c a l l y , t h e time domain d a t a o f f i g u r e 4 was o b t a i n e d . The c o r r e s p o n d i n g t r a n s f o r m i s shown i n f i g u r e 5. I t can be seen t h a t w i t h a d r y p a i n t f i l m t h e major A.E. events took p l a c e i n t h e f r e q u e n c y band between 10 KHz and 30 KHz. The magnitude o f these events was 10 times g r e a t e r than those seen i n t h e hydrogen e v o l u t i o n e x p e r i m e n t s . T a k i n g t h e hydrogen e v o l u t i o n experiments and t h e m e c h a n i c a l detachment experiments a u s i n g these on a p a i n t know f r e q u e n c y b a n d s ) . When t h e p o o l o f z i n c - r i c h p a i n t f i r s t began t o d r y , c r a c k s formed i n t h e s u r f a c e l e a v i n g t h e p a i n t below f l u i d . These c r a c k s d i d n o t g i v e r i s e t o any o b s e r v a b l e a c o u s t i c e m i s s i o n . With time t h e c r a c k s e v e n t u a l l y p e n e t r a t e d t o t h e s u r f a c e o f t h e s u b s t r a t e and t h e e n t i r e p o o l became t o u c h d r y . I t was o n l y when the i n t e r n a l s t r e s s e s i n t h e c r a c k e d polymer caused i t t o c u r l up and d e t a c h i t s e l f from t h e s u b s t r a t e t h a t A.E. s i g n a l s were observed. A t y p i c a l t r a n s i e n t i s shown i n f i g u r e 6, and t h e c o r r e s p o n d i n g t r a n s f o r m i n f i g u r e 7. Comparison o f t h i s d a t a w i t h the r e s u l t s o b t a i n e d from m e c h a n i c a l detachment shows t h a t t h e amount o f energy i n t h e two s p e c t r a i s s i m i l a r b u t mud c r a c k i n g o c c u r s a t lower f r e q u e n c i e s and has a more u n i f o r m d i s t r i b u t i o n through t h e spectrum. The immersed grey p a i n t b l i s t e r e d r e a d i l y . W i t h i n a p e r i o d of 24 hours s m a l l b l i s t e r s were observed and a f t e r a p e r i o d o f a week most o f t h e s u r f a c e o f t h e specimen was b l i s t e r e d . No A.E. t r a n s i e n t s were observed a t any time d u r i n g the course o f t h i s blistering. Sharp, s h o r t t r a n s i e n t s o f t h e type shown i n f i g u r e 8 were observed from time t o time throughout the course o f t h i s experiment. C o n s i d e r a b l e e f f o r t s were made t o e l i m i n a t e R F I , t h e s e i n c l u d e d t h e use o f an e l e c t r i c a l l y i s o l a t e d room, t r a n s i e n t suppressed mains s u p p l i e s , screened l e a d s and p a s s i v e s u p p r e s s i o n d e v i c e s . N e v e r t h e l e s s , these s i g n a l s may be a t t r i b u t e d t o r a d i o f r e q u e n c y i n t e r f e r e n c e , as a l l these t r a n s i e n t s o c c u r r e d throughout the w o r k i n g day and none were r e c o r d e d o v e r n i g h t . S e c o n d l y , t h e length of the t r a n s i e n t s d i d not i n d i c a t e that " r i n g i n g " of the s u b s t r a t e took p l a c e . When t h e epoxy c o a t e d p a n e l c o n t a i n i n g a r t i f i c i a l h o l i d a y s was a l l o w e d t o c a t h o d i c a l l y d i s b o n d , a r e a s o f g r e a t e r than one square c e n t i m e t r e became detached i n l e s s than 7 days. Once a g a i n a t no time were r e a l i s t i c A.E. t r a n s i e n t s o b s e r v e d . I n i t i a l l y , w i t h t h e grey p a i n t , i t was thought t h a t t h e t h i n n e s s o f t h e c o a t i n g combined w i t h i t s p l a s t i c i t y was c a u s i n g the p a i n t t o p e e l from t h e s u r f a c e i n a d u c t i l e manner, i t b e i n g f u r t h e r p l a s t i c i s e d by t h e uptake o f w a t e r . These o b s e r v a t i o n s were n o t t r u e f o r t h e epoxy c o a t e d p a n e l s as t h e f i l m t h i c k n e s s

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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POLYMERIC MATERIALS FOR CORROSION CONTROL

2E-5mV,

-2E-5mV

J

20mS

F i g u r e 1.

Background n o i s e

l e v e l from equipment.

3E-5mV

01 Figure

2.

100 Fourier transform

o f the background n o i s e .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

CALLOW AND SCANTLEBURY

2E-5mV

Acoustic Emission to Investigate Disbonding 1

r

-2E-5mV

20mS

F i g u r e 3. Time domain t r a c e r e s u l t i n g from hydrogen e v o l u t i o n a t -1700 mV SCE.

AE-AmVi-

0

iMM^^i

4E-4mV'

40mS

F i g u r e 4. Time domain t r a c e r e s u l t i n g from the m e c h a n i c a l d i s b o n d i n g o f a c h l o r i n a t e d rubber s u b s t i t u t e .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC MATERIALS FOR CORROSION CONTROL

120

|4E-4mV

F i g u r e 5.

F.F.T. o f F i g u r e 4.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

11.

CALLOW AND SCANTLEBURY

Acoustic Emission to Investigate Disbonding

2E-4mV

F i g u r e 7.

F i g u r e 8.

F.F.T. o f F i g u r e 6.

S p u r i o u s t r a n s i e n t a t t r i b u t e d t o R.F.I.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

121

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POLYMERIC MATERIALS FOR CORROSION CONTROL

and i t s i m p e r v i o u s n a t u r e , t o g e t h e r w i t h t h e i n h e r e n t b r i t t l e n e s s of t h e f i l m , made p l a s t i c i s a t i o n u n l i k e l y . I f the disbonding o f the epoxy under these c i r c u m s t a n c e s was g i v i n g r i s e t o A.E. t r a n s i e n t s then t h e s e must e i t h e r be below t h e n o i s e l e v e l o f t h e equipment o r they were b e i n g absorbed by t h e water o r t h e p a n e l itself. However, t h e l i k e l y e x p l a n a t i o n f o r the l a c k o f A.E. i n f o r m a t i o n from specimens u n d e r g o i n g c a t h o d i c d i s b o n d i n g o r b l i s t e r i n g i s t h a t t h e d i s b o n d i n g p r o c e s s i s a c h e m i c a l r a t h e r than a m e c h a n i c a l phenomenon. Conclusions The A.E. t e c h n i q u e i s c a p a b l e o f m o n i t o r i n g t h e p r o g r e s s o f hydrogen e v o l u t i o n and mud c r a c k i n g t o g e t h e r w i t h t h e m e c h a n i c a l d i s b o n d i n g of p a i n t from t h e m e t a l s u b s t r a t e . When t h i s s u b s t r a t e was immersed and t h e d i s b o n d i n g was c a r r i e d o u t by e l e c t r o c h e m i c a l means then no A.E. t r a n s i e n t s were chemical disbonding i s Acknowledgments L.M. C a l l o w would l i k e t o thank I n t e r n a t i o n a l P a i n t p i c f o r f i n a n c i a l support. Both a u t h o r s would l i k e t o thank Dr. R.P.M. P r o c t e r f o r the p r o v i s i o n of l a b o r a t o r y f a c i l i t i e s .

Literature Cited 1. 2. 3.

Mansfeld, F. and Stocker, P . J . , J. Electrochem. Soc., 77, p. 1301-2, Vol. 24. No. 8. Strivens, T. and Rawlings, S. J.O.C.C.A., 63, 412-418 (1980). Bruel & Kjaer, Instrumentation Handbook.

4.

Callow, L.M. and Scantlebury, J . D . , J.O.C.C.C.A,

64 83 (1981).

RECEIVED January 22, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

12 Mechanisms of De-adhesion of Organic Coatings from Metal Surfaces Henry Leidheiser, Jr. Department of Chemistry and Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA 18015

Organic coatings many processes. consequenc de-adhesion is corrosion of the metal beneath the coating. Among the important de-adhesion processes are: loss of adhesion when wet, cathodic delamination, cathodic blistering, swelling of the polymer, gas b l i s tering by corrosion, osmotic blistering, thermal cycling and anodic undermining. Real-life and laboratory examples of these phenomena are given and the principles which govern the behavior are discussed. The de-adhesion processes require, with the possible exception of thermal cycling, that reactive species such as water, oxygen and ions penetrate through the coating. New studies on the migration of species through organic coatings are discussed.

S t e e l o b j e c t s , when exposed t o humid atmospheres o r when immersed i n e l e c t r o l y t e s , c o r r o d e a t a r a p i d r a t e . F o r example, a b r a s i v e l y p o l i s h e d , c o l d - r o l l e d s t e e l p a n e l s w i l l show s i g n s o f r u s t w i t h i n 15 minutes when immersed i n d i l u t e c h l o r i d e s o l u t i o n s w i t h pH i n t h e range o f 7-10. One o f t h e methods used t o c o n t r o l t h i s r a p i d corros i o n i s t o coat t h e m e t a l w i t h a p o l y m e r i c f o r m u l a t i o n such as a p a i n t . The r o l e o f t h e p a i n t i s t o serve p r i m a r i l y as a b a r r i e r t o e n v i r o n m e n t a l c o n s t i t u e n t s such as w a t e r , oxygen, s u l f u r d i o x i d e , and i o n s and s e c o n d a r i l y as a r e s e r v o i r f o r c o r r o s i o n i n h i b i t o r s . Some formulations contain very high concentrations of m e t a l l i c zinc o r m e t a l l i c aluminum such t h a t t h e c o a t i n g p r o v i d e s g a l v a n i c protection as w e l l as b a r r i e r p r o t e c t i o n , b u t such f o r m u l a t i o n s a r e not d i s cussed i n t h i s paper. The c o r r o s i o n p r o c e s s t h a t o c c u r s i n de-adhered r e g i o n s under paint i s d r i v e n by an e l e c t r o c h e m i c a l p r o c e s s i n w h i c h a p o r t i o n o f the a r e a i s a n o d i c i n n a t u r e and another p o r t i o n i s c a t h o d i c i n nature. The r e a l i t y o f t h i s e l e c t r o - c h e m i c a l p r o c e s s can be conf i r m e d when pH i n d i c a t o r s o r substances s e n s i t i v e t o i r o n i o n s a r e placed beneath t h e c o a t i n g such t h a t t h e sharp d i s t i n c t i o n between 0097-6156/ 86/ 0322-0124S06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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LEIDHEISER

Mechanisms of De-adhesion of Organic Coatings

125

the anodic and cathodic regions i s v i v i d l y i l l u s t r a t e d , A good example i s shown i n the color photograph on the cover of the A p r i l 1983 issue of Materials Performance. The fact that the corrosion process is electrochemical in nature i s s i g n i f i c a n t i n two respects. F i r s t , i t i s necessary that aqueous phase water be present at the interface between the coating and the paint. Second, water must migrate through the coating or through a defect i n the coating and there must be a mechanism for condensation, or aqueous phase development, at the interface. The purpose of this paper i s to describe i n a very general way the p r i n c i p l e s underlying the de-adhesion, when such p r i n c i p l e s are known, and to emphasize the lack of understanding when the p r i n c i p l e s are not yet recognized. Eight different types of de-adhesion processes w i l l be discussed: loss of adhesion when wet, cathodic delamination, cathodic b l i s t e r i n g , swelling of the polymer, gas b l i s tering by corrosion, osmotic b l i s t e r i n g thermal c y c l i n g and anodic undermining. De-adhesion

Processes

Water Aggregation. An interesting question arises at the outset as to what constitutes an aqueous phase. How many water molecules are required before an electrochemical process can be activated? Conversations with many well-known electrochemists have led us to use a 1M solution as a reference. Another basis for using 1M i s the observation that the pH at the active front under a cathodically delaminating coating approaches a value of s l i g h t l y under 14, i . e . , approximately 1M i n hydroxy1 ions. A 1M solution i s 55M with respect to water so that i n a 1M solution of NaCl^ the r a t i o of water to ions is 55 molecules of water for each Na and CI p a i r . We are thus using as our working guide that an aggregate of the order of 50 molecules of water represents the minimum number of water molecules that can be considered to have the properties of an aqueous phase. This small number of water molecules thus requires a very small void at the interface between the coating and the metal i n which to form a condensate. Voids may be formed where the wetting behavior of the coating i s i n s u f f i c i e n t to penetrate into notches at grain boundaries, into fine scratches, into voids at boundaries between inclusions and the metal matrix, or into small recesses following abrasive b l a s t i n g . Voids s u f f i c i e n t to contain 50 or more molecules of water are certainly present at the coating/metal interface and the important question i s what are the mechanisms by which water w i l l condense within the existing voids. Now l e t us consider some of the processes which promote the development of an aqueous phase at the interface, assuming that there are voids at the interface s u f f i c i e n t l y large to accommodate the nucleus of an aqueous phase. Loss of Adhesion When Wet. Many coatings, p a r t i c u l a r l y those applied to a roughened surface, show excellent tensile adhesion to steel but lose this adhesion after exposure to pure water at room or elevated temperatures. A thin f i l m of water at the interface i s apparently responsible for the loss of adhesion. If the coating i s allowed to dry without destructively testing the adhesion, the dried coating often exhibits the o r i g i n a l t e n s i l e adhesion. The phenomenon i s

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reversible: the a d h e s i o n i s poor when the c o a t i n g i s wet and i s s a t i s f a c t o r y when i t i s d r y . Some p r e l i m i n a r y experiments by a s t u d e n t , George Rommal, i n d i ­ c a t e t h a t a s i m i l a r phenomenon can be observed a t a g l a s s / p o l y m e r i n t e r f a c e . He s t u d i e d a t h i n , s p i n - a p p l i e d a c r y l i c c o a t i n g on glass and found t h a t the water c o l l e c t e d under the c o a t i n g i n d i s c r e t e , a p p r o x i m a t e l y c i r c u l a r r e g i o n s . The water c o l l e c t e d under the coat­ i n g i n a m a t t e r of minutes and the c i r c u l a r r e g i o n s s l o w l y grew i n s i z e . The adherence, as measured by a p p l y i n g a d h e s i v e t a p e , was good when the c o a t i n g was dry and was poor when the c o a t i n g was wet. When the experiment was done r e p e a t e d l y on the same sample a f t e r c y c l i c wetting and d r y i n g , i t was observed t h a t the water c o l l e c t e d i n the same r e g i o n s . I t appeared t h a t water p e n e t r a t e d t h r o u g h channels i n the coating and i t was i n t e r p r e t e d t h a t the development of glass/water/polymer i n t e r f a c e represented a negative f r e e energy change r e l a t i v e t o the g l a s s / p o l y m e r i n t e r f a c e Wet a d h e s i o n phenomen research s i n c e so l i t t l are: (1) How does one measure q u a n t i t a t i v e l y the magnitude of the a d h e s i o n when the c o a t i n g i s wet? (2) What i s the g o v e r n i n g p r i n c i ­ p l e t h a t determines whether or not water c o l l e c t s a t an organic coating/metal interface? (3) What i s the t h i c k n e s s of the water l a y e r at the i n t e r f a c e and what determines the t h i c k n e s s ? A recent paper (JL) c o r r e l a t e s the wet a d h e s i o n p r o p e r t i e s of a phosphated s u r ­ f a c e w i t h the c r y s t a l l i n e n a t u r e of the z i n c phosphate a t the m e t a l surface. Cathodic Delamination. Most o r g a n i c c o a t i n g s on most m e t a l surfaces l o s e t h e i r adherence when a l k a l i i s g e n e r a t e d a t a d e f e c t i n the c o a t i n g or a t weak s p o t s i n the c o a t i n g . A l k a l i can be generated by the c a t h o d i c h a l f o f the c o r r o s i o n r e a c t i o n o r by d r i v i n g the c a t h o d ­ i c r e a c t i o n by means of an a p p l i e d p o t e n t i a l . P r e v i o u s publications (2-5) have r e p o r t e d e x t e n s i v e l y on the c a t h o d i c d e l a m i n a t i o n phe­ nomenon and o n l y a b r i e f summary w i l l be g i v e n h e r e . The a l k a l i i s g e n e r a t e d by the c a t h o d i c r e a c t i o n , H0 2

+ 1/2

0

2

+ 2 e"

=

2

OH"

w h i c h o c c u r s a t a d e f e c t i n the c o a t i n g o r through an electrolytic pathway a t weak spots i n the c o a t i n g . I t occurs a t c a t h o d i c p o t e n ­ t i a l s of -0.7 t o -1.5 ν ( v s . SCE) on a l l c o a t i n g s , except one, that we have i n v e s t i g a t e d . I t has been observed on a l k y d , a c r y l i c , epoxy, epoxy powder, bitumen, v i n y l e s t e r , f l u o r o c a r b o n , p o l y e s t e r , polybu­ t a d i e n e , and polyethylene coatings. The o n l y c o a t i n g i n w h i c h no d e l a m i n a t i o n o c c u r r e d i n 0.5M NaCl w i t h an a p p l i e d p o t e n t i a l of -1.5 ν vs. SCE f o r 60 days a t room temperature i s an e l e c t r o s t a t i c a l l y a p p l i e d epoxy c o a t i n g , 50 um t h i c k , on a p r o p r i e t a r y copper sub­ strate. The reason f o r t h i s l a c k of s e n s i t i v i t y t o cathodic d e l a m i n a t i o n i s unknown, a l t h o u g h i t i s suspected t h a t the coating has a low degree of p e r m e a b i l i t y t o water and i o n s . Some of the important f a c t s about c a t h o d i c d e l a m i n a t i o n are sum­ m a r i z e d i n the f o l l o w i n g i t e m i z e d s t a t e m e n t s : (1) When the c a t h o d i c r e a c t i o n occurs under the c o a t i n g , the pH of the s o l u t i o n under the c o a t i n g may approximate 14. (2) The important c a t h o d i c r e a c t i o n under the c o a t i n g i n most c i r -

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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cumstances i s t h e oxygen r e d u c t i o n r e a c t i o n and no s i g n i f i c a n t d e l a m i n a t i o n occurs i n t h e absence o f oxygen when t h e p o l a r i z a tion potential i s i n s u f f i c i e n t t o d r i v e t h e hydrogen e v o l u t i o n reaction at a s i g n i f i c a n t rate. (3) No s i g n i f i c a n t d e l a m i n a t i o n i s observed i n t h e absence o f m e t a l c a t i o n . No c a t h o d i c d e l a m i n a t i o n occurs i n pure a c i d s o l u t i o n s . (4) The r a t e o f d e l a m i n a t i o n i s s t r o n g l y a f u n c t i o n o f t h e c a t a l y t i c a c t i v i t y o f t h e s u r f a c e f o r t h e oxygen r e d u c t i o n r e a c t i o n . The a c t i v i t y can be decreased by s u r f a c e treatment of the metal p r i o r to the a p p l i c a t i o n of the coating. (5) R e a c t i v e s p e c i e s r e a c h t h e d e l a m i n a t i o n f r o n t by m i g r a t i o n through t h e c o a t i n g . (6) The a r e a delaminated i s g e n e r a l l y l i n e a r l y r e l a t e d t o t h e time a t c o n s t a n t temperature and c o n s t a n t p o t e n t i a l . (7) The r a t e o f d e l a m i n a t i o n i n c r e a s e s w i t h i n c r e a s e i n t h e a p p l i e d potential. (8) The r a t e o f d e l a m i n a t i o The a c t i v a t i o n energ s t e e l i s a p p r o x i m a t e l y 12 k c a l / m o l e . (9) F o r c o a t i n g s t h i c k e r than a p p r o x i m a t e l y 30 um, t h e r e i s an i n c u b a t i o n p e r i o d , or delay time, before the delaminated area i n c r e a s e s l i n e a r l y w i t h time. T h i s d e l a y time decreases with i n c r e a s e i n temperature o r i n c r e a s e i n a p p l i e d p o t e n t i a l . (10) The o r g a n i c c o a t i n g a t t h e m e t a l i n t e r f a c e i s m o d i f i e d c h e m i c a l l y by t h e s t r o n g a l k a l i n e medium t h a t i s generated under t h e coating. (11) The r a t e o f d e l a m i n a t i o n i s a f u n c t i o n o f t h e s u b s t r a t e metal and i s v e r y low i n t h e case o f aluminum s u b s t r a t e s . (12) The r a t e o f d e l a m i n a t i o n i s a f u n c t i o n o f t h e type o f c o a t i n g and i t s t h i c k n e s s . The major unknown i n t h e c a t h o d i c d e l a m i n a t i o n p r o c e s s i s the mechanism by which t h e i n t e r f a c i a l bond i s b r o k e n . A l k a l i n e a t t a c k of t h e polymer, s u r f a c e energy c o n s i d e r a t i o n s , and a t t a c k o f t h e o x i d e a t t h e i n t e r f a c e have a l l been proposed, but none o f t h e a v a i l a b l e evidence a l l o w s an u n e q u i v o c a l answer. Cathodic B l i s t e r i n g . I n t h e absence o f a purposely-imposed d e f e c t i n the c o a t i n g , t h e c a t h o d i c d e l a m i n a t i o n phenomenon i s known as c a t h o d i c b l i s t e r i n g . An example o f c a t h o d i c b l i s t e r i n g as a f u n c t i o n o f time i s shown i n F i g u r e 1. S w e l l i n g o f t h e Polymer. Some polymer f o r m u l a t i o n s have t h e p r o p e r t y of s w e l l i n g , i . e . , i n c r e a s i n g i n dimension, when exposed t o c e r t a i n environments. An example o f t h i s e f f e c t i s t h e s w e l l i n g o f some epoxy c o a t i n g s when exposed t o s t r o n g s u l f u r i c a c i d s o l u t i o n s a t e l e v a t e d temperatures. Exposure o f such a c o a t i n g on s t e e l results i n t h e f o r m a t i o n o f m u l t i p l e b l i s t e r s when t h e s u b s t r a t e i s sand b l a s t e d b e f o r e t h e a p p l i c a t i o n o f t h e c o a t i n g and i n a s i n g l e large b l i s t e r when t h e s u b s t r a t e i s s i m p l y abraded. The p r o c e s s i s f a c i l i t a t e d i f t h e c o a t i n g i s permeable t o gaseous atmospheric c o n s t i t u e n t s t h a t may f i l l t h e v o i d . An i n t e r e s t i n g way t o d i s t i n g u i s h b l i s t e r i n g by s w e l l i n g o f t h e polymer from c o r r o s i o n - i n d u c e d b l i s t e r i n g i s t o a p p l y t h e c o a t i n g t o a t h i n l e a d s u b s t r a t e and c o n f i n e t h e a r e a o f exposure t o a c i r c u l a r r e g i o n o f t h e o r d e r o f 2-3 cm i n diameter. The a r e a may be c o n f i n e d

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F i g u r e 1. An example o f c a t h o d i c b l i s t e r i n g . The c o a t i n g was a z i n c chromate a l k y d p r i m e r m a t e r i a l . The e l e c t r o l y t e was 0.5M KC1 and t h e p o t e n t i a l o f t h e m e t a l was m a i n t a i n e d a t - 1.0 ν v s . SCE.

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by cementing a c y l i n d e r t o t h e c o a t i n g and f i l l i n g t h e c y l i n d e r w i t h the a g g r e s s i v e l i q u i d . I f t h e adherence t o t h e l e a d i s s u f f i c i e n t l y good, t h e s w e l l i n g o f t h e polymer w i l l cause t h e l e a d t o deform i n the same shape as t h e b l i s t e r . An example o f t h e d e f o r m a t i o n o f a l e a d p a n e l by t h i s p r o c e s s i s shown i n F i g u r e 2, Gas B l i s t e r i n g by C o r r o s i o n . T h i s phenomenon has been observed i n a very few c a s e s . An example i s shown i n F i g u r e 3 f o r a c o a t i n g exposed t o s t r o n g s u l f u r i c a c i d a t 60 C. The e f f e c t was a t t r i b u t e d t o gas b l i s t e r i n g r a t h e r than s w e l l i n g o f t h e polymer because t h e b l i s t e r c o n t a i n e d a l a r g e q u a n t i t y o f hydrogen as judged by e x t r a c ­ t i o n o f t h e gas i n t h e b l i s t e r w i t h a hypodermic n e e d l e f o l l o w e d by gas chromatographic a n a l y s i s . The b l i s t e r i n g must o c c u r as a conse­ quence o f r a p i d p e n e t r a t i o n o f t h e c o a t i n g by hydrogen i o n s and slow d i f f u s i o n o f t h e hydrogen gas out through t h e c o a t i n g . The b l i s t e r ­ i n g r e q u i r e s t h a t t h e c o a t i n g possess a degree o f d u c t i l i t y s i n c e a b r i t t l e c o a t i n g would b Osmotic B l i s t e r i n g . Osmotic p r e s s u r e s a r e v e r y p o w e r f u l and a r e a driving f o r c e f o r b l i s t e r i n g . They a r e e s p e c i a l l y d e s t r u c t i v e under c o n d i t i o n s where a s o l u b l e s a l t i m p u r i t y i s p r e s e n t beneath t h e c o a t ­ i n g and t h e coated m e t a l i s exposed t o water w i t h a low i o n i c content. The d r i v i n g f o r c e i s t h e attempt by t h e system t o e s t a b l i s h two l i q u i d s , one under t h e c o a t i n g and t h e o t h e r e x t e r n a l t o t h e c o a t i n g , w i t h t h e same thermodynamic a c t i v i t y . The d i r e c t i o n o f water f l o w through t h e c o a t i n g i s inwards s i n c e d i l u t i o n o f t h e con­ c e n t r a t e d s o l u t i o n a t t h e i n t e r f a c e i s t h e mechanism by which t h e two l i q u i d s s t r i v e f o r equal thermodynamic a c t i v i t y . A q u o t a t i o n from a p r e v i o u s a r t i c l e ( 6 ) i s w o r t h r e p e a t i n g h e r e . A d i s c u s s i o n p a r t i c i p a n t a t t h e C o r r o s i o n 81 meeting i n Toronto p r o ­ v i d e d t h e f o l l o w i n g example o f osmotic b l i s t e r i n g . ~k s h i p was painted i n Denmark and made a voyage i m m e d i a t e l y t h e r e a f t e r a c r o s s the A t l a n t i c and i n t o t h e Great Lakes. When i t reached p o r t , a b l i s ­ t e r p a t t e r n i n t h e form o f a h a n d p r i n t was observed above t h e water l i n e . A p p a r e n t l y , t h e p a i n t was a p p l i e d over a h a n d p r i n t . No b l i s ­ t e r i n g o c c u r r e d d u r i n g exposure t o s e a water because o f t h e h i g h s a l t content o f t h e w a t e r , b u t when t h e s h i p was exposed t o f r e s h w a t e r , the o s m o t i c f o r c e s became s i g n i f i c a n t and t h e b l i s t e r i n g occurred."' Another good example o f osmotic e f f e c t s i s shown i n F i g u r e 4. Cathodic delamination s t u d i e s were c a r r i e d out on a pigmented epoxy c o a t i n g a t an a p p l i e d p o t e n t i a l o f -0.8 ν v s . SCE. Coatings of e q u a l t h i c k n e s s were s t u d i e d i n 0.001, 0.01, 0.1, and 0.5M NaCl s o l u ­ t i o n . I t w i l l be noted t h a t t h e r a t e s o f d e l a m i n a t i o n ( s l o p e o f t h e c u r v e ) i n c r e a s e d i n t h e o r d e r 0.001, 0.01, 0.1 = 0.5M. However, t h e i n t e r s e c t i o n p o i n t o f t h e curves w i t h t h e time a x i s ( t h e s o - c a l l e d delay time) increased i n t h e order 0.001 = 0.01, 0.1, 0.5M, T h i s l a t t e r e f f e c t i s a t t r i b u t e d t o t h e f a c t t h a t t h e d e l a y time i s a s s o ­ c i a t e d w i t h t h e time r e q u i r e d t o form a s t e a d y - s t a t e d i f f u s i o n g r a d i e n t a c r o s s t h e c o a t i n g . The most important component i n a c h i e v ­ i n g t h i s steady s t a t e i s water s i n c e i o n m i g r a t i o n and oxygen m i g r a t i o n p r o b a b l y f o l l o w aqueous pathways i n t h e c o a t i n g . The o s m o t i c f o r c e s dominate i n e s t a b l i s h i n g t h i s d i f f u s i o n g r a d i e n t and thus t h e more c o n c e n t r a t e d s o l u t i o n s r e q u i r e a l o n g e r time t o e s t a b ­ lish this gradient. Once t h e g r a d i e n t i s e s t a b l i s h e d , t h e r a t e o f d e l a m i n a t i o n i s determined by t h e r a t e a t w h i c h c a t i o n s can d i f f u s e

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F i g u r e 2. Epoxy c o a t i n g on a l e a d s u b s t r a t e . Coated m e t a l was exposed t o 1M I^SO^ a t 60 C f o r 3 days. View i s from the l e a d substrate s i d e Note t h a t s w e l l i n g o f the c o a t i n g caused a d e f o r ­ m a t i o n o f the l e a d . 0

F i g u r e 4. C a t h o d i c d e l a m i n a t i o n o f pigmented epoxy c o a t i n g s on steel. A d e f e c t was p l a c e d i n the c o a t i n g and the coated m e t a l was m a i n t a i n e d a t a p o t e n t i a l o f - 0.8 ν v s . SCE w h i l e immersed i n NaCl s o l u t i o n s o f d i f f e r e n t c o n c e n t r a t i o n s .

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through t h e c o a t i n g and a c t as charge c a r r i e r s and c o u n t e r i o n s t o a l l o w t h e c a t h o d i c r e a c t i o n t o o c c u r beneath t h e c o a t i n g . The f l u x of c a t i o n s a c r o s s t h e c o a t i n g i n c r e a s e s w i t h i n c r e a s e i n c o n c e n t r a t i o n of the d i f f u s i n g c a t i o n . Thermal C y c l i n g . Coatings t h a t a r e b r i t t l e and have d i f f e r e n t c o e f f i c i e n t s of expansion than the substrate metal are very s u s c e p t i b l e t o d i s b o n d i n g upon t h e r m a l c y c l i n g . T h i s d i s b o n d i n g may o c c u r l o c a l l y i n s m a l l areas o r i t may o c c u r i n t h e most d r a s t i c cases over v e r y l a r g e a r e a s . A v i n y l e s t e r c o a t i n g t h a t has r e c e n t l y been s t u d i e d i n our l a b o r a t o r y e x h i b i t e d v e r y low r a t e s o f water t r a n s m i s s i o n but t h e bonding had a t e n s i l e s t r e n g t h o f t h e o r d e r o f 70 kg/m , a low v a l u e compared t o t e n s i l e s t r e n g t h s observed w i t h many o t h e r c o a t i n g s . As might be e x p e c t e d , t h i s c o a t i n g tends t o l o s e adherence upon thermal c y c l i n g as shown i n a r e c e n t paper by T a t e r ( 7 ) . There i s good r e a s o n t o b e l i e v e t h a t one o f t h e f u n c t i o n s o f t h e rough s u r f a c e g e n e r a t e d by a b r a s i v t h a t reduce t h e l i k e l i h o o cycling. Stresses leading t o disbonding of a b r i t t l e c o a t i n g may a l s o o r i g i n a t e a t welded j o i n t s o r i n c o a t i n g s on t h i n s u b s t r a t e s t h a t s u f f e r f l e x i n g during s e r v i c e . A n o d i c Undermining. A n o d i c undermining r e r e s e n t s t h a t c l a s s o f c o r r o s i o n r e a c t i o n s underneath an o r g a n i c c o a t i n g i n which t h e major s e p a r a t i o n p r o c e s s i s t h e a n o d i c c o r r o s i o n r e a c t i o n under t h e c o a t ing. An o u t s t a n d i n g example i s t h e d i s s o l u t i o n o f t h e t h i n t i n c o a t i n g between t h e o r g a n i c l a c q u e r and t h e s t e e l s u b s t r a t e i n a food container. I n such c i r c u m s t a n c e s , t h e c a t h o d i c r e a c t i o n may i n v o l v e a component i n t h e f o o d s t u f f o r a d e f e c t i n t h e t i n c o a t i n g may expose i r o n w h i c h then s e r v e s as t h e cathode. The t i n i s s e l e c t i v e l y d i s s o l v e d and t h e c o a t i n g s e p a r a t e s from t h e m e t a l and l o s e s i t s p r o tective character. Another example i s t h e v e r y s l i g h t d e l a m i n a t i o n t h a t occurs when a t h i n copper l a y e r i s o v e r c o a t e d w i t h an o r g a n i c c o a t i n g such as a p h o t o r e s i s t and t h e system i s made a n o d i c . The r a t e o f d i s b o n d i n g i s a f u n c t i o n o f t h e a p p l i e d p o t e n t i a l and hence t h e r a t e of d i s s o l u t i o n of t h e copper beneath t h e c o a t i n g . A n o d i c d e l a m i n a t i o n occurs very s l o w l y r e l a t i v e t o cathodic delamination a t equal p o t e n t i a l d i f f e r ences from t h e c o r r o s i o n p o t e n t i a l . A n o d i c undermining has not been s t u d i e d as e x t e n s i v e l y as cathodic delamination because t h e r e do n o t appear t o be any m y s t e r ies. Galvanic e f f e c t s and p r i n c i p l e s w h i c h a p p l y to crevice c o r r o s i o n p r o v i d e a s u i t a b l e e x p l a n a t i o n f o r observed cases o f a n o d i c undermining. M i g r a t i o n o f Species Through

Coatings

C o r r o s i o n beneath an o r g a n i c c o a t i n g r e q u i r e s t h a t t h e r e be an aqueous phase, t h a t t h e r e be a n i o n s and c a t i o n s t o p r o v i d e c o n d u c t i v i t y i n t h e aqueous phase and t h a t t h e r e be oxygen f o r t h e c a t h o d i c reaction. These s p e c i e s must a l l f i n d m i g r a t i o n pathways t h r o u g h t h e c o a t i n g . Some r e c e n t e x p e r i m e n t s t h a t p r o v i d e some i n t e r e s t i n g f a c t s about t h e m i g r a t i o n o f s p e c i e s through o r g a n i c c o a t i n g s w i l l be described.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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M i g r a t i o n o f Water. Water uptake by a c o a t i n g may be f o l l o w e d bv impedance measurements (8) and by d i e l e c t r i c s p e c t r o s c o p y i n t h e 10 Hz r e g i o n (9.). An important concern i s what types o f pathways do t h e water m o l e c u l e s f o l l o w i n t h e m i g r a t i o n through t h e c o a t i n g . A r e these p r e - e x i s t i n g pathways t h a t remain as t h e s o l v e n t i s removed from t h e c o a t i n g ? Or does t h e water f o l l o w a random walk through t h e organic matrix? Immersion o f some c o a t i n g s i n r a d i o a c t i v e s o l u t i o n s f o l l o w e d by exposure o f t h e c o a t i n g t o h i g h r e s o l u t i o n p h o t o g r a p h i c f i l m suggests t h a t t h e r e a r e p r e f e r r e d pathways i n t h e c o a t i n g through w h i c h water may move r e l a t i v e l y r a p i d l y . I t i s t h e w o r k i n g h y p o t h e s i s i n our l a b o r a t o r y t h a t t h e major means by which water may move t h r o u g h an o r g a n i c c o a t i n g i s by p r e - e x i s t i n g pathways where on a s u b m i c r o s c o p i c s c a l e t h e d e n s i t y o f t h e c o a t i n g i s low. A g r a d u a t e s t u d e n t , H y a c i n t h Vedage, i s c u r r e n t l y s t u d y i n g t h e pH o f t h e l i q u i d beneath an o r g a n i c c o a t i n g u s i n g an o x i d i z e d i r i d i u m w i r e , implanted t h r o u g h t h e s t e e l s u b s t r a t e so as t o be f l u s h with the p l a n e o f t h e s u b s t r a t e / c o a t i n i n s u l a t e d from t h e s t e e of t h e l i q u i d was determined from a c a l i b r a t i o n curve by measuring the p o t e n t i a l o f t h e w i r e r e l a t i v e t o a r e f e r e n c e electrode i n the solution. I n t h e case o f a v i n y l e s t e r c o a t i n g on s t e e l immersed i n 0.1M s u l f u r i c a c i d a t 60 C, i t r e q u i r e d a p p r o x i m a t e l y 60 days before the e l e c t r o d e y i e l d e d a s t a b l e p o t e n t i a l . The p o t e n t i a l i n d i c a t e d t h a t t h e pH was a p p r o x i m a t e l y 6. I t r e q u i r e d a day o r two b e f o r e t h e p o t e n t i a l achieved a v a l u e c o r r e s p o n d i n g t o a pH o f 2. These meas­ urements, which were r e p r o d u c i b l e , suggested t h a t i n t h i s c a s e , t h e water m i g r a t e d t h r o u g h t h e c o a t i n g f i r s t and t h e i o n i c components d i f f u s e d t o t h e i n t e r f a c e a f t e r t h e water pathway was e s t a b l i s h e d . T h i s work i s c o n t i n u i n g w i t h o t h e r c o a t i n g systems. I t s h o u l d be p o i n t e d out t h a t t h i s t e c h n i q u e i s u s e f u l n o t o n l y f o r d e t e r m i n i n g the pH under t h e c o a t i n g but t h e time t o o b t a i n a s t e a d y - s t a t e c o r r o ­ s i o n p o t e n t i a l i n d i c a t e s t h e l e n g t h o f time b e f o r e an aqueous phase develops a t t h e i n t e r f a c e i n t h e v i c i n i t y o f t h e s e n s i n g e l e c t r o d e . A n o t h e r i n t e r e s t i n g f e a t u r e about water m i g r a t i o n i s t h a t an a p p l i e d c a t h o d i c p o t e n t i a l i n c r e a s e s t h e r a t e o f uptake o f water by the c o a t i n g ( 1 0 ) . Data l e a d i n g t o t h i s c o n c l u s i o n a r e summarized i n T a b l e I f o r t h r e e d i f f e r e n t c o a t i n g systems. I n a l l cases t h e water uptake as e s t i m a t e d from impedance measurements was more than one order o f magnitude g r e a t e r a t an a p p l i e d p o t e n t i a l o f -0.8 ν v s . Ag/AgCl compared t o open c i r c u i t c o n d i t i o n s where t h e c o r r o s i o n p o t e n t i a l was -0.62 v . No e x p l a n a t i o n f o r t h e i n c r e a s e d r a t e o f water p e n e t r a t i o n w i t h t h e a p p l i c a t i o n o f a m i l d a p p l i e d p o t e n t i a l i s apparent a t t h e p r e s e n t t i m e . Companion measurements u s i n g r a d i o a c ­ tive Na i n d i c a t e d t h a t t h e a p p l i e d p o t e n t i a l i n c r e a s e d t h e r a t e o f m i g r a t i o n o f sodium t h e same o r d e r o f magnitude as t h e i n c r e a s e i n the r a t e o f m i g r a t i o n o f w a t e r . The e f f e c t o f t h e a p p l i e d p o t e n t i a l on water uptake may be a d i r e c t consequence o f t h e development o f more e f f e c t i v e d i f f u s i o n pathways through t h e c o a t i n g . No d i s c r i m i ­ n a t i o n among t h e s e p o s s i b i l i t i e s o r o t h e r s can be made a t p r e s e n t . M i g r a t i o n o f C a t i o n s . Data a r e g i v e n i n T a b l e I I f o r t h e r a t e o f uptake o f Na and C s w i t h and w i t h o u t an a p p l i e d c a t h o d i c p o t e n t i a l of -0.8 ν v s . Ag/AgCl. I n a l l cases i t w i l l be noted t h a t t h e uptake was i n c r e a s e d approximately one o r d e r o f magnitude w i t h an a p p l i e d p o t e n t i a l . T h i s r e s u l t i s j u s t what one might expect because +

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

12.

LEIDHEISER

133

Mechanisms of De-adhesion of Organic Coatings

Table I. The Effect of an Applied Cathodic Potential on the Rate of Uptake of Water by Organic Coatings

Type of Coating

Thickness, um

Without Applied Potential 7

6.7

χ

10"

8

1.3 χ

10"

5

7

3.8 χ

10"

6

Alkyd Topcoat

37 - 40

5.9 χ

10"

Two layers of primer plus alkyd topcoat

62-67

3.9 χ

10"

Polybutadiene

10-12

1.6 χ

10"

Conditions: 0.5M -0.8

With Applied Potential b

NaCl, room temperature, cathode potential = ν vs. Ag/AgCl, several days exposure.

Table I I . The Effect of an Applied Cathodic Potential on the Rate of Uptake of Cations by Organic Coatings (10)

Cation Uptake, mol/h Type of Coating

Alkyd Topcoat

Thickness, um

37

- 40

Cation

+

Na

Cs Two layers of primer plus alkyd topcoat

62 - 67

Polybutadiene

10 - 12

+

Na

Cs Na

+

+

+

Cs

+

Without Applied Potential

With Applied Potential

9.1

X

10" •9

6.9

X

ίο"

8

1.8

X

10" •8

1.1

X

ίο"

7

6.3

X

10" •10

9.3

X

ίο"

9

3.9

X

10" •10

6.3

X

10"

3.5

X

10" •10

7.3

X

ΙΟ"

9

3.3

X

10" 9

3.9

X

ΙΟ"

9

9

Conditions: 0.5M a l k a l i metal chloride; room temperature; applied potential = - 0.8 ν vs. Ag/AgCl, 1 0 - 2 5 day exposure.

the potential of the metal substrate i s such as to attract p o s i t i v e l y charged ions. It i s s t r i k i n g that such an increase occurs when the magnitude of the applied potential i s so small, i . e . , 180 mv d i f f e r ­ ence between the steady state potential and the applied p o t e n t i a l . In a l l cases studied to date, the rate of cathodic delamination i s greater in CsCl solutions than in NaCl solutions of the same molarity. The increased rate has been attributed to the greater rate

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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POLYMERIC MATERIALS FOR CORROSION CONTROL

of d i f f u s i o n o f t h e hydrated cesium i o n t h r o u g h t h e c o a t i n g than t h a t of t h e h y d r a t e d sodium i o n . U n f o r t u n a t e l y , t h e data i n T a b l e I I a r e not p r e c i s e enough t o make a comparison between t h e r e s u l t s f o r N a and Cs , The u n c e r t a i n t y i s l a r g e because t h e t o t a l uptake o f t h e c a t i o n i s so s m a l l . The data a l s o do n o t d i s c r i m i n a t e between t h e r a d i o t r a c e r i o n s t h a t a r e p r e s e n t i n t h e c o a t i n g o r i n t h e aqueous phase a t t h e i n t e r f a c e between t h e c o a t i n g and t h e s u b s t r a t e . These data show t h a t i n c r e a s e d r a t e s o f m i g r a t i o n o f c a t i o n s o c c u r w i t h s m a l l a p p l i e d p o t e n t i a l s . One may a l s o e x t r a p o l a t e t h e s e data and i n f e r t h a t c a t i o n m i g r a t i o n , and hence charge îlow, i s i n c r e a s e d by d i f f e r e n c e s i n p o t e n t i a l a t l o c a l anodes and cathodes e x i s t i n g a t t h e m e t a l s u r f a c e i n t h e absence o f an a p p l i e d p o t e n t i a l .

+

M i g r a t i o n o f Oxygen. Our r e s e a r c h on t h e m i g r a t i o n o f oxygen t h r o u g h o r g a n i c c o a t i n g s has had a v e r y l i m i t e d o b j e c t i v e and some background i s i n o r d e r . The r a t e o f c a t h o d i delaminatio f different types o f c o a t i n g s i f u n c t i o n of the a l k a l i metal delamination under e q u i v a l e n t experimental conditions increases i n the o r d e r : L i < N a < K < C s . The most l i k e l y explanation f o r t h i s c a t i o n e f f e c t i s the r e l a t i v e r a t e s of d i f f u s i o n of the hydrated c a t i o n through t h e c o a t i n g o r w i t h i n t h e t h i n l i q u i d i n t h e d e l a m i nated r e g i o n between t h e c o a t i n g and t h e m e t a l . Other e x p l a n a t i o n s f o r t h i s e f f e c t have a l s o been c o n s i d e r e d and one t h a t was amenable t o t e s t was an e x p l a n a t i o n based on t h e r a t e s o f d i f f u s i o n o f oxygen t h r o u g h t h e c o a t i n g as a f u n c t i o n o f t h e type o f c a t i o n under c o n d i t i o n s where t h e r e were a s i m u l t a n e o u s c o n c e n t r a t i o n g r a d i e n t and p o t e n t i a l g r a d i e n t , b o t h o f w h i c h would be i n t h e same d i r e c t i o n through t h e c o a t i n g as t h e oxygen c o n c e n t r a t i o n g r a d i e n t . The i d e a has been t e s t e d w i t h f r e e f i l m s o f p o l y e t h y l e n e and an a c r y l i c spray c o a t i n g . These f i l m s were mounted between two chambers i n w h i c h t h e l e f t hand chamber c o n t a i n e d an oxygen probe, an e l e c t r o d e , and a 0.005M s o l u t i o n o f t h e a l k a l i m e t a l c h l o r i d e . The r i g h t hand chamber c o n t a i n e d an e l e c t r o d e , an a i r b u b b l e r and a 0.5M s o l u t i o n o f t h e a l k a l i m e t a l c h l o r i d e . The e l e c t r o d e i n t h e l e f t hand chamber was m a i n t a i n e d a t a p o t e n t i a l o f -1.2 ν v s . a Ag/AgCl e l e c ­ t r o d e so t h a t t h e g r a d i e n t between the two chambers was a p p r o x i m a t e l y 600 mv. The l e f t hand chamber was d e a e r a t e d b e f o r e t h e experiment began and t h e oxygen c o n c e n t r a t i o n i n t h e chamber was t h e n m o n i t o r e d c o n t i n u o u s l y w i t h t h e probe as a f u n c t i o n o f time. The r a t e o f oxygen d i f f u s i o n through t h e c o a t i n g s i n b o t h cases was i n t h e order Κ > Na > L i i n t h e presence o f t h e p o t e n t i a l and c o n c e n t r a t i o n g r a d i e n t but t h e d i f f e r e n c e s between t h e lowest and h i g h e s t r a t e s were o f t h e o r d e r o f 35%. I n t h e absence o f an a p p l i e d p o t e n t i a l , t h e r a t e s were a p p r o x i m a t e l y t h e same w i t h a maximum spread o f 15%. These r e s u l t s a r e s u g g e s t i v e t h a t t h e a l k a l i m e t a l c a t i o n s do a f f e c t t h e m i g r a t i o n o f oxygen through a c o a t i n g when t h e r e e x i s t s b o t h a c o n c e n t r a t i o n g r a d i e n t and a p o t e n t i a l g r a d i e n t . However, many more experiments must be performed b e f o r e a c o n c l u s i v e statement can be made. +

+

+

+

Acknowledgment Much o f t h e work r e p o r t e d h e r e i n was o b t a i n e d i n a r e s e a r c h program supported by t h e O f f i c e o f N a v a l Research. We a r e indeed g r a t e f u l

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Mechanisms of De-adhesion of Organic Coatings

135

f o r t h i s support. Colleagues and s t u d e n t s who have contributed importantly t o t h e work d e s c r i b e d h e r e i n i n c l u d e Dr. R i c h a r d Granat a , Dr. M a l c o l m W h i t e , D r . Douglas E a d l i n e , Dr. Jeffrey Parks, Wayne B i l d e r , Hyacinth Vedage, Mark A t k i n s o n , P h i l i p Deck, George Rommal, and Valmore R o d r i g u e z .

Literature Cited 1. Miyoshi, Y . ; Kitayama, M.; Nishimura, K.; Naito, S. "Cosmetic Corrosion Mechanism of Zinc and Zinc Alloy Coated Steel Sheet for Automobiles"; paper presented at Society of Automotive Engineers, March, 1985. 2. Leidheiser, Η., J r . ; Wang, W. J. Coatings Technol. 1981, 53 (672), 77. 3. Leidheiser, Η., J r . ; Wang, W.; Igetoft, L. Prog. Org. Coatings 1983, 11, 19 4. Leidheiser, Η., J r . ganic Coatings Scienc Α. V . , Eds.; Dekker: New York, 1984; Vol. 7, p. 327. 5. Wang, W.; Leidheiser, Η., Jr. In "Equilibrium Diagrams and Localized Corrosion"; Frankenthal, R. P.; Kruger, J., Eds.; Electrochemical Society: Pennington, N. J., 1984; p. 255. 6. Leidheiser, Η., Jr. Corrosion 1982, 38, 374. 7. Tater, Κ. B. Am. Painting Contractor 1982, 11. 8. Leidheiser, Η., J r . ; Kendig, M. W. Corrosion 1976, 32, 69. 9. Eadline, D. J.; Leidheiser, Η., Jr. Rev. Sci. Instrum. 1985, 56, 1432. 10. Parks, J.; Leidheiser, Η., Jr. Ind. Eng. Chem. Prod. Res. Dev. in press. RECEIVED March 7, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13 Chemical Studies of the Organic Coating-Steel Interface After Exposure to Aggressive Environments Ray A . Dickie Ford Motor Company, Dearborn, MI 48121

The chemical compositio coating/steel interfacia following adhesion failure in various aggressive environments. The analytical techniques employed have included X-ray photoelectron spectroscopy, dynamic secondary ion mass spectrometry, and scanning electron microscopy. Examples of cohesive and adhesive coating failure have been observed in each of several test modes in studies of model thermoset coatings. Typically, analyses of interfacial surfaces generated by simple mechanical removal of coatings from their substrates, and of those formed as a result of humidity-induced adhesion failure, indicate that there is l i t t l e or no chemical change associated with the loss of adhesion. Exposure to corrosive environments can result in substantial changes in interfacial surface composition and morphology. In some instances, chemical degradation of organic coatings has been observed in the interfacial region. Chemical degradation of inorganic conversion coatings has also been observed, and appears to dominate the corrosion-induced paint adhesion loss process in some cases. The c o r r o s i o n p r o t e c t i o n a f f o r d e d t o s t e e l b y o r g a n i c c o a t i n g s i s w e l l known t o be dependent on s u b s t r a t e c o m p o s i t i o n and s u r f a c e p r e p a r a t i o n , organic coating composition, and t e s t o r exposure c o n d i t i o n s , among o t h e r v a r i a b l e s . Organic coatings provide p r o t e c t i o n t o metal s u b s t r a t e s b o t h b y a c t i n g as b a r r i e r s between t h e s u b s t r a t e and t h e environment and b y p r e v e n t i n g t h e spread o f c o r r o s i o n from an i n i t i a l or i n c i p i e n t c o r r o s i o n s i t e . I n g e n e r a l , good c o r r o s i o n p r o t e c t i o n r e q u i r e s t h e e s t a b l i s h m e n t o f good c o a t i n g a d h e s i o n . For continued p r o t e c t i o n , a d h e s i o n must be m a i n t a i n e d i n t h e presence o f water, e l e c t r o l y t e , and t h e v a r i o u s p r o d u c t s o f the c o r r o s i o n reactions. Once c o r r o s i o n s t a r t s , t h e r e i s o f t e n a p r o g r e s s i v e d i s r u p t i o n o f c o a t i n g adhesion; t h e mechanism and r a t e o f t h e c o r r o s i o n induced a d h e s i o n l o s s p r o c e s s has l o n g been t h e s u b j e c t o f r e s e a r c h (see R e f . 1 f o r a r e c e n t r e v i e w ) . The n a t u r e o f t h e c h e m i c a l p r o c e s s e s responsThis chapter not subject to U.S. copyright. Published 1986, American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13.

DICKIE

The Organic Coating-Steel Interface After Exposure

137

i b l e f o r the a p p a r e n t l o s s o f a d h e s i o n has been the s u b j e c t o f a number o f i n v e s t i g a t i o n s u s i n g modern s u r f a c e a n a l y t i c a l t e c h n i q u e s (see, e.g., 2-6). T h i s paper d i s c u s s e s r e c e n t c h e m i c a l s t u d i e s o f the o r g a n i c c o a t i n g / s t e e l i n t e r f a c e , w i t h p a r t i c u l a r r e f e r e n c e t o the e f f e c t o f changes i n the m o l e c u l a r s t r u c t u r e o f the o r g a n i c c o a t i n g on the r a t e and mechanism o f h u m i d i t y - and c o r r o s i o n - i n d u c e d a d h e s i o n loss. Surface Studies of I n t e r f a c i a l

Composition

The l o c u s and c h e m i s t r y of adhesion l o s s have been s t u d i e d u s i n g a wide range o f a n a l y t i c a l t e c h n i q u e s . Among the most u s e f u l have been s u r f a c e s e n s i t i v e s p e c t r o s c o p i c methods, i n c l u d i n g X - r a y p h o t o e l e c t r o n spectroscopy (XPS o r ESCA), Auger e l e c t r o n s p e c t r o s c o p y (AES), and secondary i o n mass s p e c t r o m e t r y (SIMS). Conventional microscopic t o o l s ( e s p e c i a l l y s c a n n i n g e l e c t r o n m i c r o s c o p y ) have a l s o been w i d e l y used. A p p l i c a t i o n s o f v a r i o u and c o r r o s i o n problems hav XPS a l l o w s a q u a n t i t a t i v e e l e m e n t a l a n a l y s i s o f the topmost m o l e c u l a r l a y e r s and can a l s o g i v e u s e f u l , i f somewhat l i m i t e d , m o l e c u l a r i n f o r m a t i o n . A f u r t h e r advantage o f XPS i s t h a t beam damage and charging e f f e c t s are r e l a t i v e l y minor, a l l o w i n g s t r a i g h t f o r w a r d a n a l y s i s o f o r g a n i c m a t e r i a l s . For polymers t y p i c a l l y u s e d i n o r g a n i c c o a t i n g s , f o r example, h i g h r e s o l u t i o n c a r b o n s p e c t r a can y i e l d i n f o r m a t i o n on the p r e s e n c e and r e l a t i v e abundance o f a number o f common f u n c t i o n a l groups, i n c l u d i n g e t h e r , e s t e r , c a r b o x y l a t e , and c a r b o n a t e m o i e t i e s ( c f . F i g u r e 1 ) . The major d i s a d v a n t a g e o f XPS as a p p l i e d i n most p u b l i s h e d s t u d i e s i s i t s poor l a t e r a l r e s o l u t i o n ( c a . 5 mm), a l t h o u g h r e c e n t advances i n equipment have r e s u l t e d i n a s u b s t a n t i a l reduction i n a n a l y s i s area. AES can a l s o p r o v i d e an e l e m e n t a l a n a l y s i s o f the topmost l a y e r s o f a sample, and i n addition can p r o v i d e images w i t h a l a t e r a l r e s o l u t i o n on the o r d e r o f 0.1 μια. A p p l i c a t i o n o f AES t o o r g a n i c m a t e r i a l s has been l i m i t e d i n p a r t due

BE , eU

F i g u r e 1. species.

Experimental

CARBON

SPECIES

C Is b i n d i n g energies f o r s e l e c t e d carbon

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC MATERIALS FOR CORROSION CONTROL

138

to the p a u c i t y o f m o l e c u l a r i n f o r m a t i o n t h a t can be o b t a i n e d , and i n p a r t due t o beam damage e f f e c t s d u r i n g measurement. SIMS, and more r e c e n t l y , dynamic imaging SIMS, w h i c h p r o v i d e g r e a t e r s u r f a c e s e n s i t i v i t y and s u b s t a n t i a l l y b e t t e r l a t e r a l r e s o l u t i o n t h a n XPS have been a p p l i e d t o c o r r o s i o n and a d h e s i o n problems (10, 11, 13, 1 4 ) . Although o n l y a q u a l i t a t i v e a n a l y s i s can be o b t a i n e d u s i n g SIMS t e c h n i q u e s , t h e i o n images o b t a i n e d can p r o v i d e new i n f o r m a t i o n on the d i s t r i b u t i o n and r e l a t i o n s h i p between s u r f a c e s p e c i e s . A s e r i o u s problem w i t h the a p p l i c a t i o n o f most s u r f a c e s e n s i t i v e s p e c t r o s c o p i c t e c h n i q u e s t o the s t u d y o f a d h e s i o n and c o r r o s i o n phenomena i s t h a t i n s i t u measurements are n o t p o s s i b l e : t y p i c a l l y , the c o a t i n g f i l m must be removed from the substrate p r i o r t o a n a l y s i s . This r e s t r i c t i o n i s avoided by o p t i c a l methods, such as o p t i c a l m i c r o s c o p y and e l l i p s o m e t r y ( e . g . , 15, 16), b u t these methods l a c k c h e m i c a l a n a l y s i s c a p a b i l i t y and, for i n situ measurements, are l i m i t e d t o t r a n s p a r e n t c o a t i n g s . Humidity-induced

Adhesio

Good i n i t i a l o r d r y a d h e s i o n o f a c o a t i n g t o a s u b s t r a t e does not ensure good performance upon exposure t o humid o r c o r r o s i v e environments. Exposure t o h i g h h u m i d i t y i s w e l l known t o reduce the adhesion o f organic coatings t o s t e e l (17). Studies o f moisture a b s o r p t i o n k i n e t i c s suggest s p e c i f i c i n v o l v e m e n t o f the i n t e r f a c i a l r e g i o n i n humidity induced adhesion l o s s (18). I n s t u d i e s o f humidity induced adhesion f a i l u r e o f o r g a n i c c o a t i n g s on c l e a n , b a r e s t e e l , examples o f e s s e n t i a l l y a d h e s i v e f a i l u r e , w i t h l i t t l e o r no c o a t i n g residue remaining on the s u b s t r a t e , and o f c o h e s i v e f a i l u r e o f the c o a t i n g f i l m have been found ( 1 9 ) . T y p i c a l l y , l i t t l e o r no c h e m i c a l change a s s o c i a t e d w i t h h u m i d i t y i n d u c e d a d h e s i o n l o s s i s d e t e c t a b l e i n the o r g a n i c c o a t i n g . Of c o u r s e , the presence o f water-soluble i n o r g a n i c s a l t s as s u r f a c e contaminants p r o f o u n d l y a l t e r s the i n t e r f a c i a l chemistry and c a n l e a d t o osmotic b l i s t e r i n g and v a r i o u s c o r r o s i o n - r e l a t e d b l i s t e r i n g and a d h e s i o n - l o s s phenomena ( 2 0 ) . F i g u r e 2 i s r e p r e s e n t a t i v e o f the h i g h r e s o l u t i o n C I s s p e c t r a o b t a i n e d i n XPS a n a l y s e s o f t h e i n t e r f a c i a l s u r f a c e s generated by humidity-induced adhesion failure; a l s o i n c l u d e d i n the f i g u r e are s p e c t r a o b t a i n e d from a r e f e r e n c e ( u n t e s t e d ) c o a t i n g s u r f a c e and from i n t e r f a c i a l surfaces generated b y m e c h a n i c a l - and c o r r o s i o n - i n d u c e d adhesion f a i l u r e . These s p e c t r a were o b t a i n e d i n a study o f a thermoset c o a t i n g b a s e d on a m e l a m i n e - r e s i n - c r o s s l i n k e d o l i g o u r e t h a n e r e s i n ( d e t a i l s o f the r e s i n s t r u c t u r e and c o a t i n g f o r m u l a t i o n are g i v e n i n Ref. 19). E s s e n t i a l l y i d e n t i c a l s p e c t r a were o b t a i n e d from the r e f e r e n c e c o a t i n g s u r f a c e and from the i n t e r f a c i a l s u r f a c e s a f t e r adhesion f a i l u r e . C o r r e s p o n d i n g s p e c t r a o f the s u b s t r a t e i n t e r f a c i a l s u r f a c e s suggest t h a t m e c h a n i c a l removal and h u m i d i t y - i n d u c e d f a i l u r e leave l i t t l e o r no c o a t i n g r e s i d u e . There i s l i t t l e evidence f o r chemical change d u r i n g h u m i d i t y induced adhesion loss. Similar r e s u l t s , w h i c h a r e summarized i n T a b l e I , have been p r e s e n t e d f o r c o a t i n g s b a s e d on v a r i o u s o t h e r r e s i n systems (19) . F o r the c o a t i n g s s t u d i e d , t h e r e was a s t r i k i n g dependence o f a d h e s i o n , o r more c o r r e c t l y , o f r e s i s t a n c e to humidity-induced adhesion l o s s , on the r a t i o o f r e s i n h y d r o x y l t o c r o s s l i n k e r a l k o x y groups; o n l y c o a t i n g s f o r w h i c h t h i s r a t i o was g r e a t e r than about one were a b l e t o w i t h s t a n d condensi n g h u m i d i t y exposure ( 1 9 ) .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13.

DICKIE

The Organic Coating- Steel Interface After Exposure

A

ι ι 281

B C

139

D

• I LlJ d l_ 283 285 287 289 291 BINDING ENERGY, eV

F i g u r e 2. XPS C I s s p e c t r a f o r o l i g o u r e t h a n e b a s e d c o a t i n g s : (a) untested oligomer c o a t i n g s u r f a c e ; (b) i n t e r f a c i a l c o a t i n g s u r f a c e a f t e r m e c h a n i c a l l y induced a d h e s i o n l o s s ; (c) i n t e r f a c i a l c o a t i n g s u r f a c e a f t e r h u m i d i t y induced a d h e s i o n l o s s . S p e c t r a l components A, B, C, and D a t t r i b u t e d t o methyl/methylene, e t h e r , melamine, and u r e t h a n e c a r b o n y l c a r b o n s , r e s p e c t i v e l y . Reproduced from Ref. 19, c o p y r i g h t 1984, American Chemical S o c i e t y .

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 24h

Epoxy-diol adduct

>240h (NF)

>240h (NF)

>48h (NF)

Epoxy-fatty a c i d adduct

Epoxy-alkanol amine adduct

Epoxy e s t e r amine adduct

(P)

l-2h (G)

Oligoester

50 % of the test surface) and the reproducibility of the test results was very good. C r i t i c a l Surface Tension of Wetting. A c c o r d i n g to Zisman s method (7), the c r i t i c a l surface tension ( Y ) on oiled C D C - N i plated sheets was measured using a homologous series of liquid (alcohol/water). T h e advancing contact angle of a drop of the liquid was determined on the test surfaces of the N i - p l a t e d sheets with E r m a C o n t a c t A n g l e m e t e r , Goniometer T y p e , Model G - l . The c r i t i c a l surface tension ( Y ) was determined by the value of Ί\ (surface tension o f the liquid) at the intercept o f the plot o f cos θ vs. Ύ\ with the horizontal line, cos θ= 1. T

c

c

Results and Discussion X P S Spectrum of Surface. Figure 2 shows X P S spectra of C D C n i c k e l - p l a t e d sheets with various p r e t r e a t m e n t observed in the surface o ATBC. T h e carbon spectra show ester c a r b o n y l carbon and hydrocarbon species on both samples. T h e ester carbonyl carbon reflects the ester bond o f D O S ( C g H n C O O C g H i e O C O C g H n ) and A T B C ( ( C 3 H C O O ) 3 C ( C H ) O C O C H 3 ) . 7

2

T h e c h r o m i u m and n i c k e l are identified as chromium hydroxide and n i c k e l (III) oxide, respectively. Prebaking usually decreases the hydrocarbon and increases the ester c a r b o n y l carbon (polar group) for D O S - o i l e d sheets, indicating the oxidation and evaporation of surface o i l . Prebaking also caused the oxidation of surface n i c k e l . Although not shown, the samples that were first tight prebaked and then flame heated underwent the same changes as open-prebaked samples. U V preradiation and hexane washing after tight prebaking hardly changed the surface carbon concentration and the polar group c o n c e n t r a t i o n . T h e samples oiled with A T B C which is originally higher in polar group r a t i o than D O S showed a higher polar group concentration on the surfaces, and both surface carbon and polar group concentration changed l i t t l e under either open or tight prebaking conditions as shown in F i g . 2. T h e high resolution C l s s p e c t r a for t y p i c a l D O S and A T B C - o i l e d sheets are shown in F i g . 3 and the surface composition (atomic %) of the oiled C D C N i - p l a t e d sheets with various pre treatments is summarized in " T a b l e 1". According to the high resolution spectra ( F i g . 3), the apparent percentage o f the ester species (286. 5eV) appears smaller than that of the carbonyl species (288.8eV) on both D O S - a n d A T B C - o i l e d sheets, indicating the contribution of the contamination by adventitious hydrocarbon species. The measured percentage of the carbonyl carbon of a s - r e c e i v e d sheets is 10.9 % for D O S and 18.0 % for A T B C , those of which roughly correspond to the calculated ratios of D O S (9.2 %) and A T B C (21.0 %), respectively. But the e f f e c t of contamination makes it d i f f i c u l t to have further insight into the deviation from the calculated values. Identification of L o c u s of Adhesion F a i l u r e . T o c l a r i f y whether the disruption is cohesive failure o f the lacquer or i n t e r f a c i a l failure between the substrate and lacquer, the lacquer and metal sides of the fracture surface were both measured by X P S . The results are shown in F i g . 4. F o r the purpose of comparison, U V C lacquer coated and D O S oiled n i c k e l - p l a t e d sheets are shown in the top and bottom of the d i a g r a m , respectively. T h e contribution at c a . 286.5 eV of the lacquer surface is attributed to carbon singly bonded to oxygen

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

158

POLYMERIC MATERIALS FOR CORROSION CONTROL

90° take off angle

d = Xsin9

Figure

1. R e l a t i v e sampling depth for angular dependent X P S .

N12O3

DOS C1s — —

01s

As received After baking (Open)

C1s

(Tight)

-CH;

01s

As received After baking (Open) (Tight)

-CH2-

Cr(OH) Ni 03 Ni(OH)2 C*O.J

3

2

280

285

290

530

280

535

285

290

Cr2p3/2 Cr 0 2

Cr2p3/2

3

NJ2P3/2

Cr(OH)

3

NÎ2P3/2

W N 1 2 O 3

Cr20

3

Cr(OH)

Nj0

u

Ni 0 NiO I Ni(OH) 2

N i

575

j,Ni(OH>2

3

580 Binding

3

2

l/'*i

850 855 energy,eV

860

575

580 850 855 Binding energy, eV

Figure 2. X P S spectra of D O S (left) and A T B C plated sheets before and after baking.

860

(right) oiled N i -

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

C1s

V

81.4

C1s

° 0 S As received

—ΓΗ I

f

159

Adhesion Loss of Ultraviolet-Cured Lacquer

MAEDA ET AL.

c

" ° "

c

_

\\

ATBC As received

\v

^

L69.2

%

!

°P

_

2

V i

\

Λ \

-CHj

C H

ι

* 1

V\

:

P baking

e n

re

-

\V -c-o-?-

/ 71.0

ϊ\ Λχ >95%ΛΧ

-

-

119.5%V^

-

Tight prebaking

-Cn2~

\\ 9 \ \ -c-o-c-

W \ \

\

\V /

-y

280

285

Binding

Energy (eV)

290

280

'

I

64.9

χ

285

290

Binding Energy (eV)

Figure 3. High resolution spectra of CIS for DOS (left) and ATBC (right) oiled surfaces.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

POLYMERIC MATERIALS FOR CORROSION CONTROL Table I. Oil

Surface composition of the oiled C D C - N i plated sheets Pretreatments

Surface composition (at 96) COC=0)

DOS

Cr

Ni

0

As-received

51.8(5.6)

7.5

1.8

38.9

Open prebaking

43.7(7.9)

9.4

3.5

43.5

Tight prebaking

56.3(6.8)

7.1

1.8

34.7

Tight prebaking t F l a m e heating

39.3(7.9)

9.9

3.1

47.6

Tight prebaking r U V preradiation

51.8(6.8)

7.8

2.2

37.3

Hexane washing

49.4(5.9)

7.9

2.8

39.9

As-received

37.9(6.8)

9.0

1.7

51.3

Tight prebaking Hexane washin

ATBC

Open prebaking

38.5(7.8)

9.0

3.9

48.6

Tight prebaking

38.7(7.2)

9.2

2.4

49.6

Figure 4 . X P S spectra showing the i n t e r f a c i a l coating surface and the i n t e r f a c i a l m e t a l surface (Top and bottom are U V C lacquer and D O S oiled surface)

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

MAEDA ET AL.

161

Adhesion Loss of Ultraviolet-Cured Lacquer

(characteristics o f ether and ester species), but the observed greater intensity of this species relative to ester c a r b o n y l carbon (at 288.8eV) c l e a r l y shows the contribution of ether carbon o f lacquer component. D e t e c t i o n o f no chromium and n i c k e l in the peeled lacquer side indicates that the fracture o c c u r r e d at and near the substrate/coating i n t e r f a c e . Identification of the ether bond ( - C 0 - C - ) , a component o f the lacquer, on the substrate, however, shows that the lacquer component remained on the substrate to some degree. Figure 5 shows the X P S angular dependence o f the fracture surface o f a sample. T h e ratios of the ester carbonyl carbon and ether carbon to the t o t a l amount o f carbon are plotted on the ordinate. T h e 3 0 ° angle means that the thickness measured is assumed to be about a half o f that with an angle of 6 0 ° . T h e results show that the concentration o f the ester bond observed on the i n t e r f a c i a l substrate surface decreases toward the fracture interface, suggesting that the hydrocarbon groups are oriented toward the outside (lacquer side). T h e concentration o f the ether group (a component o f the lacquer) on the i n t e r f a c i a l substrate surface is about one-third that on the i n t e r f a c i a l lacquer surface islands on the substrate surfac On the i n t e r f a c i a l lacquer surface, the concentration o f the ester group decreased toward the outside (the fracture interface), suggesting the diffusion o f D O S into the lacquer f i l m . S E M observation o f the fracture surface of m e t a l side is shown in F i g . 6. It c a n be seen that some lacquer remains as an island state. T h e apparent disagreement with X P S d a t a seems to be due to the presence of invisible lacquer by this magnification (x 550). A c c o r d i n g to X P S results and S E M observation the locus o f failure may be s c h e m a t i c a l l y represented as shown in F i g . 7. Change in Composition o f Surface F i l m with T r e a t m e n t . Since trivalent chromium hydroxide c a n be generally expressed as C r 2 0 3 n H 2 0 , the composition o f the outermost surface layer is assumed to be expressed by " E q u a t i o n 2" and the hydration degree η and covering rate α o f chromium hydroxide are calculated by "Equations 3 and 4" from the a t o m i c percent o f each o f the elements measured by X P S . (The escape depth o f n i c k e l is smaller than that o f chromium and oxygen. Although i t may pose a problem, s t r i c t l y speaking, the e f f e c t o f this condition is neglected because the a t o m i c concentration o f n i c k e l is low.) e

a C r 2 0 3 - n H 0 + (1-α)Νΐ2θ3 2

( 2 )

When to!/ for ] = X and [ N i V t C r 1 = Y , α = 1/(Y + 1)

( 3 )

η = 2X - 3Y - 3

(4 )

where [ C r i , [ N i l a n d [Olare the a t o m i c concentration o f each o f the element, and [Ol= [O] t o t a l " ^ coo* T h e oxygen bonded with carbon is assumed to be approximately twice as much as the a t o m i c percent o f c a r b o n y l carbon (288.8 eV) which seems to be more reliable than that of ester carbon (286.5 eV). A c c o r d i n g to the binding energy o f N i 2P (8), n i c k e l oxide was identified as N12O3 but the possibility o f the state as Ni(OH)2 is not always denied. If Ni(OH)2 is adopted for analysis, "Equations 3 and 4" take a different formula (9), leading to slightly smaller value for both α and η , but the relative values among the individual sheets are invaliable. 2

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

162

POLYMERIC MATERIALS FOR CORROSION CONTROL

*

30

Metal side

Coating side

A Ether (+ Ester ) • Ester carbonyl

UVC lacquer

/

/ ο DOS

•^-•C:

Take off angle ( θ ) Figure 5. V a r i a t i o n o f the polar groups (ester c a r b o n y l and ether) with photoelectron take o f f angle.

Figure 550 )

6.

S E M observation o f the fracture surface of m e t a l side ( χ

Figure

7.

S c h e m a t i c model showing a locus of failure

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

MAEDA ET AL.

Adhesion Loss of Ultraviolet-Cured Lacquer

163

T h e covering rate α and hydration degree η of chromium hydroxide and the polar group r a t i o [C ] / [ C 1 total wn in F i g . 8. It is found that η = 3.7 and ROH + "O-C-R'

0 II R-O-C-NH-R' + 20H"

-

ROH + CO3- + NH

2

- R'

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

17.

Locus and Mechanism of Failure of Adhesive Joints

HOLUBKA ET AL.

Table I.

Adhesive

Adhesive

Chemistry

Structure/Property Studies S t r e n g t h (MPa) Initial S a l t Spray (14 Days)

Locus o f Failure

A

Epoxy/Dicy

11.,96

5.,21

Cohesive

Β

Epoxy/Dicy

16..24

8.,23

Cohesive

C

Urethane

8..04

3.,69

Adhesive

D

Epoxy/Polyamide

9,.62

7..55

90% C o h e s i v e 10% A d h e s i v e

Ε

Epoxy/Amidoamin

F

Vinyl

6,.27

0,.69

Adhesive

G

Acrylic

8 .96

1 .03

Adhesive

Plastisol

The p r e s e n c e o f t h e s e i o n i c m a t e r i a l s a t the i n t e r f a c e , as w e l l as the c o r r e s p o n d i n g r e d u c t i o n o f polymer c r o s s l i n k d e n s i t y a s s o c i a t e d w i t h t h i s d e g r a d a t i v e p r o c e s s , l i k e l y c o n t r i b u t e t o the o b s e r v e d r e d u c t i o n i n bond d u r a b i l i t y . In a s p e c i f i c example o f a d h e s i v e bonds between c o l d r o l l e d s t e e l and SMC adherends ( T a b l e I I ) an a d h e s i v e b a s e d on h y d r o l y s i s r e s i s t a n t epoxy c h e m i s t r y ( i . e . , a d h e s i v e E) was compared w i t h an a d h e s i v e b a s e d on h y d r o l y s i s prone u r e t h a n e c h e m i s t r y ( i . e . , a d h e s i v e C) i n composite t o c o l d r o l l e d s t e e l bonds. A f t e r corro­ s i o n t e s t i n g , a s i g n i f i c a n t d i f f e r e n c e i n both r e t e n t i o n of i n i t i a l bond s t r e n g t h and l o c u s o f f a i l u r e was o b s e r v e d . F o r bonds p r e p a r e d w i t h a d h e s i v e E, l i t t l e i f any r e d u c t i o n o f the i n i t i a l bond s t r e n g t h was o b s e r v e d a f t e r c o r r o s i o n t e s t i n g . The l o c u s o f f a i l u r e f o r b o t h the t e s t e d and u n t e s t e d bonds was l a r g e l y i n the T a b l e I I . Composite t o M e t a l Bonding: Urethane v s . Epoxy A d h e s i v e s

Test

Urethane Failure Strength (MPa)

Urethane Strength Failure (MPa) 4.97

Fiber

Tear

Adhesion Loss to S t e e l

4.48

Fiber

Tear

Fiber

4.87

Fiber

Tear

Unexposed SMC/CRS

5.23

Fiber

Corrosion Exposed SMC/CRS

3.21

Untested SMC/SMC

5.36

Tear

Tear

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

197

198

POLYMERIC MATERIALS FOR CORROSION CONTROL

SMC adherend ( f i b e r t e a r out o b s e r v e d ) ; o n l y about 10% o f the f a i l u r e was a t t h e a d h e s i v e - m e t a l i n t e r f a c e . The l o c u s f a i l u r e was t y p i c a l o f bonds i n v o l v i n g composite m a t e r i a l s . Bond s t r e n g t h v a l u e s f o r these c o m p o s i t e / m e t a l bonds compared f a v o r a b l y w i t h s i m i l a r bonds i n v o l v i n g c o m p o s i t e / c o m p o s i t e bonds where t h e e f f e c t s of c o r r o s i o n r e a c t i o n s are not present. F o r bonds p r e p a r e d w i t h a d h e s i v e C ( h y d r o l y s i s prone m a t e r i a l ) , n e a r l y a 40% r e d u c t i o n i n bond s t r e n g t h was o b s e r v e d and t h e f a i l u r e was e n t i r e l y a t the m e t a l - a d h e s i v e i n t e r f a c e . The o b s e r v e d r e s u l t s w i t h composite m e t a l bonds i n c o r r o s i o n a r e c o n s i s t e n t w i t h a c o r r o s i o n i n d u c e d degradat i o n o f t h e a d h e s i v e a t the m e t a l - a d h e s i v e i n t e r f a c e t h a t reduces the o v e r a l l s t r e n g t h o f t h e a d h e s i v e below t h e c o h e s i v e s t r e n g t h o f the composite (hence, the change i n l o c u s o f bond f a i l u r e ) . The i n t e r f a c i a l a d h e s i v e bond s u r f a c e s g e n e r a t e d as a r e s u l t o f c o r r o s i o n i n d u c e d f a i l u r e ( f o r a d h e s i v e s C and E) have been examined using x-ray photoelectron spectroscopy. The r e s u l t s o f these s t u d i e s a r e shown i n T a b l T a b l e I I I . X-Ray P h o t o e l e c t r o n S p e c t r o s c o p y o f Composite t o M e t a l Bond F a i l u r e i n C o r r o s i o n Sample Ç

P e r c e n t Atomic C o m p o s i t i o n 0 Ν Fe

N/0

Adhesive C Unexposed

80.2

17.3

2.5

--

.145

Adhesive C After Corrosion Polymer I n t e r f a c e

73.0

24.9

2.1

--

.084

Adhesive C After Corrosion Metal Interface

51.7

42.5

1.7

4.1

.040

Adhesive Ε Unexposed

78.5

15.4

6.1

--

.396

Adhesive Ε After Corrosion Polymer I n t e r f a c e

82.0

12.6

5.4

--

.428

Adhesive Ε After Corrosion Metal Interface

49.9

45.4

--

4.7

0

e l e m e n t a l c o m p o s i t i o n were o b s e r v e d w i t h t h e u r e t h a n e b a s e d adhesive a f t e r c o r r o s i o n induced adhesion f a i l u r e . The most n o t a b l e change o b s e r v e d i s i n the N/0 r a t i o . There i s a marked r e d u c t i o n o f n i t r o g e n c o n c e n t r a t i o n ( i . e . , N/0 r a t i o f o r u n t e s t e d a d h e s i v e C s u r f a c e o f 0.145 reduced t o N/0 r a t i o o f 0.084 f o r i n t e r f a c i a l a d h e s i v e s u r f a c e a f t e r c o r r o s i o n ) . The lower n i t r o g e n c o n c e n t r a t i o n on t h e i n t e r f a c i a l a d h e s i v e C s u r f a c e a f t e r c o r r o s i o n i s c o n s i s t e n t w i t h a d e g r a d a t i o n o f polymer w i t h l o s s o f n i t r o g e n - c o n t a i n i n g

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

17.

Locus and Mechanism of Failure of Adhesive Joints

HOLUBKA ET AL.

291.0

2890

287.0

2850

BINDING ENERGY, eV

F i g u r e 1. C I s X-Ray P h o t o e l e c ­ t r o n S p e c t r a (XPS) o f i n t e r f a c i a l s u r f a c e s o f a d h e s i v e C showing (a) C I s XPS spectrum o f u n t e s t e d A d h e s i v e C s u r f a c e h a v i n g peaks at 285.0 eV, 285.8 eV, 286.8 eV, and 289.3 eV; (b) C I s XPS s p e c ­ trum o f i n t e r f a c i a l A d h e s i v e C polymer s u r f a c e a f t e r c o r r o s i o n showing peaks i d e n t i c a l t o ( a ) ; (c) C I s XPS spectrum o f i n t e r f a c i a l A d h e s i v e C metal s u r f a c e a f t e r c o r r o s i o n showing com­ ponents a t 285.0 eV, 286.1 eV, 287.3 eV, and 288.9 eV; (d) C I s XPS spectrum o f c o l d r o l l e d s t e e l s t a n d a r d showing peaks a t 285.0 eV, 286.5 eV, and 288.7 eV.

287 0

285.0

BINDING ENERGY, eV

F i g u r e 2. C I s XPS o f i n t e r f a c i a l s u r f a c e s o f A d h e s i v e Ε showing ( a ) C I s XPS spectrum o f u n t e s t e d A d h e s i v e Ε s u r f a c e h a v i n g peaks at 285.0 eV, 285.7 eV, 286.7 eV, and 287.9 eV; (b) C I s XPS s p e c ­ trum o f i n t e r f a c i a l A d h e s i v e Ε polymer s u r f a c e a f t e r c o r r o s i o n showing peaks i d e n t i c a l t o ( a ) ; ( c ) C I s XPS spectrum o f i n t e r f a c i a l Adhesive Ε metal surface a f t e r c o r r o s i o n showing com­ ponents a t 285.0 eV, 286.1 eV, 287.3 eV, and 288.9 eV; (d) C I s XPS spectrum o f c o l d r o l l e d s t e e l s t a n d a r d showing peaks a t 285.0 eV, 286.5 eV, and 288.7 eV.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

199

200

POLYMERIC MATERIALS FOR CORROSION CONTROL

s p e c i e s d u r i n g c o r r o s i o n . The presence o f n i t r o g e n on the i n t e r f a c i a l m e t a l s u r f a c e i s c o n s i s t e n t w i t h the presence o f polymer r e s i d u e on t h a t s i d e o f the i n t e r f a c e . The h i g h r e s o l u t i o n C I s XPS s p e c t r a f o r a d h e s i v e C ( F i g u r e 1) show t h a t the i n t e r f a c i a l s u r f a c e s g e n e r a t e d as a r e s u l t o f bond f a i l u r e ( F i g u r e s IB and 1C) d i f f e r from the u n t e s t e d a d h e s i v e . The o b s e r v e d changes i n the s p e c t r a are c o n s i s t e n t w i t h a h y d r o l y s i s p r o c e s s t h a t c l e a v e s a w a t e r s o l u b l e a m i n e - c o n t a i n i n g component from the c r o s s l i n k e d network o f a d h e s i v e C. I n c o n t r a s t , the d e l a m i n a t e d p o r t i o n o f the a d h e s i v e Ε bond t h a t f a i l e d a d h e s i v e l y shows o n l y minor changes i n c o m p o s i t i o n (Table I I I ) . N/0 r a t i o s remain e s s e n t i a l l y unchanged f o r b o t h u n t e s t e d a d h e s i v e Ε s u r f a c e and the i n t e r f a c i a l a d h e s i v e s u r f a c e g e n e r a t e d as a r e s u l t o f bond f a i l u r e . Bond f a i l u r e i s l i k e l y t o r e s u l t from a s i m p l e d i s p l a c e m e n t o f the a d h e s i v e from the s t e e l adherend by water o r by a s i m p l e p e e l p r o c e s s d u r i n g the bond f a i l u r e t h a t o c c u r s w i t h o u t polymer d e g r a d a t i o n . H i g h r e s o l u t i o n C I s s p e c t r a f o r adhesiv f o r b o t h u n t e s t e d and t e s t e s i g n i f i c a n t polymer d e g r a d a t i o n i n t h i s c o r r o s i o n t e s t . The absence o f n i t r o g e n on the i n t e r f a c i a l m e t a l s u r f a c e as w e l l as a C I s n e a r l y i d e n t i c a l t o s t e e l s t a n d a r d f u r t h e r i n d i c a t e s t h a t bond f a i l u r e o c c u r r e d w i t h o u t polymer d e g r a d a t i o n . The E f f e c t o f A d h e s i v e P r i m e r s . I n p r a c t i c e , a d h e s i v e bonds i n v o l v i n g m e t a l adherends o f t e n use p r i m e r s as p r e t r e a t m e n t s o f the m e t a l s u r f a c e p r i o r t o bonding. Table IV shows the d u r a b i l i t y o f c o m p o s i t e - m e t a l bonds p r e p a r e d w i t h a d h e s i v e C o v e r a s e r i e s o f p r i m e r s ( o f v a r y i n g c o r r o s i o n r e s i s t a n c e ) i n 240 hour s a l t s p r a y t e s t . The r e s u l t s i n d i c a t e t h a t the performance o f bonds i s d i r e c t l y r e l a t e d t o the c o r r o s i o n r e s i s t a n c e o f the p r i m e r used t o p r e p a r e the adherend s u r f a c e . I n g e n e r a l , the a d h e s i o n o f the p r i m e r t o the s t e e l adherend, r a t h e r t h a n the a d h e s i v e c h e m i s t r y , T a b l e IV. E f f e c t o f P r i m e r C h e m i s t r y on Bond S t r e n g t h o f SMC/Primed S t e e l Epoxy A d h e s i v e Bonds Primer Chemistry

Primer Adhesion Loss i n SS*(mnO

Epoxy Ecoat

0

Epoxy E s t e r

Bond S t r e n g t h (MPa) Initial A f t e r SS

Failure

6.27

5.86

Fiber Tear

2-3

6.59

5.29

Fiber Tear + Primer

Urethane

4-5

6.36

2.73

Primer

Epoxy

Urethane

2 fog chamber tests showed excessive corrosion and p i t t i n g within one week on chromate conversion coated (CGC) 7075-T6 A l a l l o y panels. The CMT coated panels were almost uncorroded and without any p i t s . The plates i n Figure 1 show the conditions of the panels a f t e r 7 and 14 days' exposure i n t h i s environment. Even a f t e r 14 days' exposure the CMT panels were s t i l l f a r better than CCC panels. Weight Loss : A quantitative evaluation of the coatings, both CGC and CMT, was also made. The panels exposed i n salt/SC>2 fog chamber were measured and weighed before and a f t e r the test to calculate corrosion losses. A l l exposed panels were cleaned i n 1:1 HNO3 f o r after-the-test weight according to ASTM Standard Practice f o r Preparation, Cleaning, and Evaluating Corrosion Test Specimens (Gl-81, para 7.2.2). This i s to remove the corrosion products f o r c a l c u l a t i n g weight losses. The r e s u l t s of these measurements are given i n Table 1. Each panel was also examined v i s u a l l y and the

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

214

POLYMERIC MATERIALS FOR CORROSION CONTROL

2

number of p i t s were counted i n a s p e c i f i e d area, i . e . , 50cm surface area; an average of 4 specimens f o r each coating was determined and reported i n Table 1. Both the weight loss and p i t t i n g data f o r CMT showed s i g n i f i c a n t inprovement over the OCC. In fact, the p i t t i n g resistance for the CMT coating was excellent. In a 14-day period, i t showed only 10 p i t s / c m compared to 75 pits/cm for the OCC. 2

2

Table 1 - Corrosion test results of coated 7075-T6 A l a l l o y exposed to salt/S02 fog environment.

Coating

2

*Weight Loss, mg/cm /48 hrs.

# Pits/cm (in 14 days)

2

Bare

1.1

CCC

0.65 ± 0.10

75 ± 20

CMT

0.25 + 0.05

10+3

* Based on specimen s i z e of 8 χ 6 χ 0.3cm

Stress Corrosion Cracking : Results of the proving ring tests of the coated specimens i n both the neutral and a c i d i f i e d s a l t solutions are given i n Table 2. Although there i s no s i g n i f i c a n t difference i n the f a i l u r e times f o r both the coated specimens i n neutral 1% NaCl, the differences were remarkable at low pH. The CMT coating was able to extend the stress corrosion cracking resistance of the chromate coated material from 60 to 90 hours at pH 2.

Table 2 - Stress corrosion cracking properties of coated 7075-T6 A l a l l o y from proving ring tests (stress = 40 k s i or 275 MPa).

Coating

*Time to F a i l u r e , Hours 1% NaCl 1% NaCl(pH 2)

CCC

300

60 ± 15

CMT

300

90 ± 15

*

Mean average of 10 specimens.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

AGARWALA

Enhancement of Acid-Chloride Resistance

Electrochemical Behavior : The corrosion (open c i r c u i t ) p o t e n t i a l values for the coatings were as given i n Table 3· For CMT coating, the corrosion p o t e n t i a l was l e a s t a c t i v e . Generally a less active (less negative) p o t e n t i a l indicates more corrosion r e s i s t a n t properties of the surface. The s t a b i l i t y of the CMT coating was excellent since i t showed no change in i t s corrosion p o t e n t i a l (-0.700V) as the environment was changed from neutral to a c i d i c or high to low chloride concentrations (cf. Table 3). The differences i n the corrosion resistant properties of the CQC and CMT coatings were better characterized from the p o t e n t i o s t a t i c anodic and cathodic p o l a r i z a t i o n behaviors i n a c i d i c s a l t solutions. The t y p i c a l p l o t s of the p o t e n t i a l vs current density are shown i n Figure 2. The curves i n Figure 2 also include a p l o t for untreated (bare) 7075-T6 aluminum a l l o y for comparison Although both the coatings CCC and CMT, showed s i g n i f i c a n p o l a r i z a t i o n curves whe they were almost the same among themselves. The s h i f t s i n the cathodic p o l a r i z a t i o n curves (cf. Figure 2) between the CMT and CCC were highly s i g n i f i c a n t . The cathodic current densities for the CMT coated material were almost an order of magnitude lower than for CCC coated material. In other words, the corrosion rate (determined as the point of intersection of the anodic and cathodic T a f e l slopes) for the CMT w i l l be lower by an order of magnitude than f o r CCC i n 1% NaCl solution of pH 2. In 3.5% NaCl solution (pH 2), the p o l a r i z a t i o n p l o t s were almost the same but indicated an even greater s h i f t of the cathodic p o l a r i z a t i o n curve toward lower current densities for the CMT coating.

Table 3 - E f f e c t of chemical conversion coatings on corrosion potentials of 7075-T6 A l a l l o y .

Ecorr. vs SCE, Bare

CCC

volt

Environment

pH

CMT

3.5% NaCl

6

-0.725

-0.700

-0.700

3.5%

NaCl

2

-0.750

-0.720

-0.700

1.0%

NaCl

2

-0.760

-0.720

-0.700

Coating S t a b i l i t y : The s t a b i l i t y of a chemical conversion coating i s best described by the non-leaching character of i t s corrosion i n h i b i t i n g constituents and the insoluble nature of the oxides of the substrate metal. Chromate conversion coatings suffer from the lack of these properties. Thus, a study was conducted i n which a l l the coated panels

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

216

-I Z

'

Ι Ο "

6

'

ι

» I I I 1 11 Ι Ο " *

5

CURRENT

I

1 1 I I I I I Ι Ο * "

DENSITY,

I

1

I

I I I I I 1 I O "

4

Amfr/cm

I

I

I

I I I I I I

3

2

Figure 2. Electrochemical p o l a r i z a t i o n behavior of chemical conversion coatings on 7075-T6 aluminum a l l o y i n 1% NaCl solution at pH 2.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

10'

AGARWALA

Enhancement of Acid-Chloride Resistance

were immersed i n 3.5% NaCl solutions of d i f f e r e n t pH for various exposure periods. A chemical analysis of the solutions by atomic absorption spectroscopy (AAS) showed that the amount of Cr leached out from the CMT coatings was approximately 1/3 of that from CCC. In a c i d i f i e d s a l t solutions (pH 2), leaching of Cr was much greater for CGC. Based on 14 days exposure i n 3.5% NaCl (pH 2), the OCC coating l o s t approximately 2.0 pg /cm of Cr compared to 0.4 pg/cm for CMT. The i n i t i a l chromium concentration i n both the CCC and CMT coatings was approximately 7-8 pg/cm . This was determined by stripping the coatings with 1:1 HN0 and analyzing the solution by AAS. 1

2

2

2

3

Surface Characterization : The morphology of the coating surface was examined by scanning electron microscopy and the composition of the films were determined form X-ray photoelectron microscopi coatings were a l i k e chemical contents. Well pronounced peaks for Mo and Cr were found i n the CMT coating during energy dispersive X-ray analysis. Of course, there was no Mo detected i n the CCC coating. Elemental d i s t r i b u t i o n of Mo and Cr i n the CMT was very uniform. The surface composition of coatings, determined by XPS, was as follows: Coating

Cr

Mo

CCC

Cr203

—

CMT

CrOOH

M0O4"

Others M 0 , M (OH) X

"

Y

ΜχΟ

γ

These results indicate the presence of Cr(III) i n both the coatings but a less soluble hydrated oxide (CrOOH) i n CMT. Presence of Cr(VI) was not detected by the XPS technique. This was in agreement with the findings of Glass { §_ ), and Matienzo and Holub ( 1_ ). In the CMT coating, the presence of Mo (VI) as molybdate was s i g n i f i c a n t . I t was also noted that the OCC had at least two d i f f e r e n t forms of other metal (M) oxides ,probably aluminum, while the CMT had only one. Although no quantitative estimation of the individual species present was made, the approximation was that Cr and Mo concentrations were not s i g n i f i c a n t l y d i f f e r e n t i n the CMT coating. Discussion The mechanism of developing corrosion protective properties in an inorganic coating p r i n c i p a l l y consists of forming insoluble oxides on the metal surface. Additionally, oxides must have c e r t a i n corrosion i n h i b i t i o n (redox) properties which can protect the metal substrate from corrosive species l i k e CI" and SO4"." In the case of chromate conversion coating, CCC, the oxides of aluminum and chromium have been responsible for their corrosion i n h i b i t i v e properties which were derived from t h e i r soluble and insoluble portions of the

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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chromate f i l m . In acid-chloride environments, the protective properties of chromate become limited because the oxides of aluminum which hold the chromium oxides i n place are no longer insoluble. In t h i s study, these considerations were taken into account to a l t e r the composition of the surface f i l m s . In the CMT process, the chromate conversion coating was re-inforced with other substances which contained molybdates. This surface f i l m was f i v e times more stable than the conventional chromate coating i n both the neutral and acid-chloride environments. XPS studies and those by other investigators ( T_ ) suggest that chromium in chromate coatings i s mostly present as (^203. In the CMT coating i t was found to be i n CrOOH form. The hydrated oxides of chromium are known to be more stable than 0^03 ; they tend to form polymer type l i n k s ( 8_ ) and most probably protect the substrate metal by forming a b a r r i e r A d d i t i o n a l l y Mo i n the CMT coating, whic of forming polymeric presence of Mo(VI) i n the f i l m enhances the protection by counteracting the aggresive nature of C I " ions. Most probably Mo(VI) t i e s up CI" ions during the process. XPS analysis showed a very even d i s t r i b u t i o n of Cr and Mo oxides with very l i t t l e aluminum oxide on the top layers of the f i l m . In contrast, the CCC coating contained a s i g n i f i c a n t amount of both aluminum and chromium oxides. The surfaces which contain more stable and insoluble oxides can o f f e r better resistance to acid-chloride environments. The high corrosion r e s i s t a n t behavior of the CMT coating supports t h i s conclusion. Electrochemical studies showed a marked reduction i n the cathodic p o l a r i z a t i o n behavior which means the CMT coating o f f e r s a higher resistance b a r r i e r (potential drop) than the OCC to achieve the same cathodic reaction rates (current d e n s i t i e s ) . Superiority of the new coating was exhibited i n i t s a b i l i t y to reduce stress corrosion cracking s u s c e p t i b i l i t y of 7075-T6 A l a l l o y i n acid-chloride environments. Conclusions The conventional chromate conversion coating i s non-protective i n acid-chloride environments. A new coating c a l l e d CMT which contains oxides of Mo and Cr, was developed for aluminum A l l o y s . An evaluation of t h i s coating was made with the following conclusions: (1) In 5% NaCl + SO2 spray there was no corrosion of the substrate metal for up to two weeks; (2) CMT enhanced the stress corrosion cracking resistance of 7075-T6 A l a l l o y s i g n i f i c a n t l y ; (3) the CMT coating was more stable than CCC and showed almost no leaching of Cr or Mo i n acid-chloride environments; and (4) The composition of the CMT f i l m was mostly Mo as MoOj" and Cr as CrOOH, and had almost no oxides of aluminum on the surface.

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

AGARWALA

Enhancement of Acid-Chloride Resistance

Acknowledgments The author thanks the assistance of the co-op students and P. J . Sabatini of Naval A i r Development Center (NADC); L. J . Matienzo of Martin-Marietta Corporation i n performing XPS analysis; and support of the NADC Independent Research program for f i n a n c i a l support.

Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9.

Wernick, S. and Pinner, R. In "Surface Treatment of Aluminum"; Draper, Robert, Ed.; Teddington, U. Κ., 1972; Vol. I, pp. 233-290. . Katzman, Η. Α . , Malouf, G. M. and Stupian, G. W., Applications Surf. Sci. , 1979, 2, 416-432. Agarwala, V. S. In "Atmospheric Corrosion" Ailor, W. H . , Ed. N.Y., 1982; pp. 183-192 Montle, J. F. and Hasser, M.D., Materials Performance , 1976, 50, 15-18. Ketcham, S. J . and Jankowsky, E. J., "ASTM Symposium on Laboratory Corrosion Tests and Standards", Bal Harbor, FL, 14-17 Nov. 1983. Glass, A. L., " A Radiochemical Investigation of the Leaching of Cr from Chemically Chromated A1 Alloy Surfaces, Part III, The "Uptake" of the Chloride Ion Into Such Surfaces," Naval Air Development Center, Warminster, PA; Report No. NADC-MA-6702, August 1967. Matienzo, L. J. and Holub, K. J., Applications Surf. Sci. , 1981, 9, 47-73. Lollar, R. M. In "Chromium-Chemistry of Chromium and Its Compounds"; Udy, M. J., Ed.; Reinhold Publishing, New York, N.Y., 1956; Vol. I, p. 306. Cotton, F. A. and Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley Interscience, New York, N.Y., 1972; pp. 965-972.

RECEIVED February 24, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

20 How Organic Coating Systems Protect Against Corrosion Werner Funke Forschungsinstitut für Pigment Republic of Germany

The electrochemical, physicochemical and adhesional aspects of corrosion protection by organic coatings are shortly discussed. Attention i s drawn to some inconsistancies in the interpretation of protective mechanisms and suggestions are given how protective principles may be optimally realized in practical systems. There are e s s e n t i a l l y three important mechanisms by which organic coating systems protect against metal corrosion: The electrochemical, the phy s iœchemical and the adhesional mechanism. In order t o obtain optimum protection, i t i s commonly proposed to incorporate as many as possible of these mechanisms i n a coating system. I t w i l l be discussed how f a r t h i s strategy i s tenable i n p r a c t i c a l paint formulation and whether i t i s reasonable i n the l i g h t of a c r i t i c a l judgement. For t h i s purpose i t i s h e l p f u l to r e c a l l how these mechanisms work and what the requirements are f o r t h e i r operat i o n . Correlations of permeability with anticorrosive action i n corrosion protection by organic coatings have been recently d i s cussed (J_). The Electrochemical Mechanism The elec±rochemical mechanism i s generally connected with the presence of a c t i v e anticorrosive pigments, l i k e red lead o r zinc chromate, and occasionally also t o corrosion i n h i b i t o r s , which are added to the base coat of the system. I t i s a wide-spread opinion that such anticorrosive pigments are almost indispensable f o r a s a t i s f a c t o r y corrosion protection because "no organic coating i s impermeable to water" (2) . Therefore anticorrosive pigments are considered t o be an ultimate l i n e of defense f o r corrosion protection. Active a n t i c o r rosive agents act only i n presence of water, which dissolves a small f r a c t i o n of them and makes them a v a i l a b l e at the coating/metal i n t e r This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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face ( 3 , 4 ) . Passivation o r corrosion i n h i b i t i o n of metal surfaces i s mostly achieved by supporting the i n s i t u formation of t h i n l a y ers of insoluble corrosion products ( 4 , 5 ) , which cover corroding areas and s t i f l e the action of the corrosion elements. For a continued corrosion i n h i b i t i o n the anticorrosive solution must keep steady contact with the metal surface. Substitution by normal water usually i n i t i a t e s corrosion again. To allow d i f f u s i o n o f the dissolved anticorrosive agent to the coating/metal i n t e r f a c e , the binder should be permeable t o water. This requirement c l e a r l y contradicts the other requirement of corrosion protective coatings, namely to prevent the access of water as a corrosive agent to the metal surface. Frequently binders used i n p r a c t i c a l corrosion protective coating systems scarcely swell by wat e r and are only s l i g h t l y permeable to i t . Accordingly the protectiv i s not a v a i l a b l e i n s u f f i c i e n Good protective properties claimed i n these cases are rather due to other protective mechanisms than to the electrochemical one. -

2-

Unfortunately some corrosion stimulants, l i k e C l , S0^ or NO.. , strongly oppose i n h i b i t i o n by anticorrosive pigments and i n h i b i t o r s (6). Steel corrodes i n saturated aqueous solutions o f an anticorrosive pigment i n presence of small amounts of these stimul a n t s , e.g. 1% w/w NaCl i s s u f f i c i e n t to make a saturated aqueous extract o f zinc chromate corrosive (1). Therefore, i r r e s p e c t i v e of environmental requirements, the usefulness of a c t i v e anticorrosive pigments and i n h i b i t o r s as well has become questionable. The Physicochemical

mechanism

The physicochemical mechanism consists i n blocking up d i f f u s i o n of corrosive agents, l i k e water and oxygen, and of corrosion stimulants. This b a r r i e r a c t i o n of organic coatings may be enhanced s i g n i f i c a n t l y by pigments, f i l l e r s or extenders which, due to a f l a k y o r p l a t e l i k e geometrical shape, greatly increase the length of d i f f u s i o n a l pathways through the cross section of the coating f i l m . In order to avoid d i f f u s i o n i n the pigment/binder i n t e r f a c e , i n t e r f a c i a l bonds between both phases should be as water-resistant as possible. I f permeability i s taken as a measure, properly formul a t e d b a r r i e r coatings may compare i n corrosion protective e f f i c i e n c y with normal coatings, the thickness of which i s two o r three times as high. The binder contributes a l s o to the b a r r i e r e f f e c t of a coating system. Permeability of the binder depends on the r i g i d i t y and pol a r i t y of i t s macromolecular structure and also on the density of the molecular packing. Accordingly permeability decreases by decreasing the chain mobility, e.g. by c r o s s l i n k i n g , by decreasing the hydrophilic character of the macromolecules and by increasing the density of molecular packing up to c r y s t a l l i n e o r c r y s t a l l i n e - l i k e structures.

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Considering the measures to be taken i n binders f o r ensuring an optimum b a r r i e r e f f e c t , they obviously oppose the requirements f o r the a n t i c o r r o s i v e function o f pigments, which need water-permeable and swellable binders. One may argue, however, that concurrently with improving the b a r r i e r properties of a coating system the a n t i corrosive function of pigments o r i n h i b i t o r s i s l e s s challenged. Furthermore the r i g i d i t y of the macromolecular structure of the binder may oppose the demand f o r mechanical strength and shock r e sistance o f the coating f i l m . Increasing the r i g i d i t y by c r o s s l i n k ing leads to i n t e r n a l stresses, which accumulate with increasing f i l m thickness. A way out of t h i s dilemma may be the use of very t h i n but highly cros s i inked base coats (8). The Adhesional Mechanism The adhesional mechanis attention i n corrosion protectio adhesion o f the base coat t o the metal surface i s unchanged no corrosion can take place below a coating. Too much emphasis has been given t o adhesion under dry cond i t i o n s . However, corrosion i s only possible i f enough water i s present i n the coating/metal i n t e r f a c e to provide the e l e c t r o l y t e f o r the corrosion elements to operate. This condition i s hardly imaginable without a previous s i g n i f i c a n t reduction o r even the l o s s of adhesion. Therefore "wet adhesion" i s considered to be o f c r u c i a l importance to corrosion protection by organic coatings (9). I t i s generally agreed that due t o the polar nature of o x i d i c metal surfaces good dry adhesion i s only possible by incorporating polar groups i n the binder molecules. However, these polar groups may e f f e c t water s e n s i v i t y of the coating/metal i n t e r f a c e thus causing poor wet adhesion. That water accumulates at the i n t e r f a c e coating/metal s u b s t a n t i a l l y , has been shown by comparing water absorption o f f r e e and supported f i l m s (10). One way to make waters e n s i t i v e interfaces r e s i s t a n t against water i s to adsorb polar groups which are attached to r i g i d polymer backbone chains. I t i s s t i l l not known f o r c e r t a i n , whether on exposure to water adhesion i s uniformly reduced over the exposed area o r only l o c a l l y l o s t at channels providing the e l e c t r o l y t i c pathways between anodic and cathodic areas of the metal surface. In choosing binders with good adhesion, again protective prop e r t i e s are encountered, which exclude each other, e.g. non-polar macromolecules with low permeability would be benif i c i a l t o the b a r r i e r e f f e c t but objective to good dry as well as wet adhesion i . e . t o the adhesional mechanism. On the other hand p o l a r groups supporting dry adhesion are required despite o f t h e i r weakness i n presence of water. The question remains how the adhesional i n t e r action may be s t a b i l i z e d t o r e s i s t the attack of water. For the sake of good adhesion a metal surface should be clean and f r e e of water-soluble substances. On the other hand, f o r the

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protective action of anticorrosive pigments a soluble f r a c t i o n , i . e . an "impurity", must be present at the i n t e r f a c e . I t i s hard to r e concile t h i s requirement with good wet adhesion of the base coat. Corrosion protective base coats with hydrophilic binders, such as water-borne coatings drying a t ambient temperatures, usually exhibit, high water permeability and poor wet adhesion. In these cases a c t i v e anticorrosive pigments are needed. Protective properties can be improved by delaying the access of water to the coating /metal interface, e.g. by increasing f i l m thickness, incorporating b a r r i e r pigments o r applying b a r r i e r top coats (Figure 1). However, even then these systems are l a t e n t l y weak and may f a i l on prolonged exposure to water o r high humidity, e s p e c i a l l y i f mechanical stressess simultaneously act on the coatings and place excessive demands on t h e i r adhesion. The Combination Of D i f f e r e n The combination o f d i f f e r e n t protective mechanisms i n one coat o r one coating system i s frequently recommended f o r optimal r e s u l t s i n c o r rosion protection. However, ways and measures t o optimize protective mechanisms may be d i f f e r e n t and sometimes even exclude each other. For example t r y i n g to combine good wet adhesion and corrosion i n h i b i t i o n by an a c t i v e anticorrosive pigment i n the same base coat does not make much sense, despite being frequently postulated t o explain protective properties of commercial paint systems (Figure 2). Binders with good wet adhesion lock i n the anticorrosive pigment and therefore dindnish o r even prevent i t s corrosion i n h i b i t i n g e f f e c t . Sometimes i t i s claimed that p r a c t i c a l experience disproves t h i s statement, but i t cannot be excluded i n these cases that protection i s mostly due to good b a r r i e r properties and/or good wet adhesion. On the other hand i t i s advantageous to choose a primer e x h i b i t i n g optimal wet adhesion and simultaneously optimal b a r r i e r properties. The b a r r i e r mechanism not only decreases water permeating to the coating/metal i n t e r f a c e but likewise retards the release o f solvents from a coating. In order t o avoid delayed f i l m formation f o r t h i s combination solventless paint systems are most s u i t a b l e . In two-layer coating systems the best choice i s to endow both layers with the b a r r i e r e f f e c t and choose a binder having good wet adhesion. Other combinations are l e s s e f f e c t i v e or even not reasonable (Figure 3 ) . The use o f a c t i v e anticorrosive pigments i s only j u s t i f i e d t o prevent corrosion at scratches, pinholes o r s i m i l a r coating defects and even then only i n absence o f v i r t u a l amounts of corrosion stimulants. I t i s commonly assumed by paint technologists that prot e c t i v e e f f e c t s incorporated i n each l a y e r of a coating system add together i n preventing corrosion at the coated metal surface. However, i n coating systems, which base coats protect by an e l e c t r o chemical mechanism (Figure 4 ) , the successive layers including the top layer rather should prevent any rhysicochemical or e l e c t r o chemical reaction a t the base coat/metal i n t e r f a c e . Considering the

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In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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high demands on the metal surface pretreatment to achieve good adhesion, i t i s not conceivable to allow this interface being kind of a reaction vessel. Layers succeeding to electrochemical protecting base coats should prevent any reaction in the base-coat and especially at i t s interface to the metal surface. The base coat should only come into action at coating defects extending down to the metal surface. Likewise coating systems suitable for cathodic protection should prevent any reaction at the metal surface below the intact coating system. Otherwise cathodic delamination i s unavoidable. In a l l these cases we actually have not a "twofold-protection" but interdependent as well as œmplementary mechanisms. The intact coating system protect by i t s barrier action and, possible, by good wet adhesion, whereas the electrochemical mechanism must be restricted to coating defects. Literature Cited 1.

F . L . Floyd, R.G. Groseclose, C.M. Frey, J. O i l Col. Chem. Assoz., 1983, 329 2. J.E.O. Mayne, Pigment Handbook Vol. III, Edited by T.C Patton, Wiley Interscience Publ. 1973, p. 459 3. J.E.O. Mayne, E.H. Ramshaw, J. Appl. Chem. 13, 1969, 553 4. H. Leidheiser, J. Coatings Technol., 53 No. 678, 1981, 29 5. J.E.O. Mayne, Pigment Handbook Vol. III, Edited by T.C. Patton, Wiley Interscience Publ. 1973, p. 457-464 6. L.A. Buckowiecke, Schweizer Archiv f. Wissenschaft u. Technik (6), 1954, 1 7. W. Funke, unpublished results 8. W. Funke, J. O i l Col. Chem. Assoz., 1985, 229 9. W. Funke, J. Coatings Technol., 55 No. 705, 1983, 31 10. W. Funke, Fette, Seifen, Anstrichmittel, 64, 1962, 714 RECEIVED March 5, 1986

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

21 Improving the Performance of Zinc-Pigmented Coatings T. Szauer and A. Miszczyk Institute of Inorganic Chemistry and Technology, Technical University of Gdánsk, 80-952 Gdánsk, Majakowskiego 11, Poland

The i n v e s t i g a t i o n coatings with of t h e i r a c t i o n and l o o k i n g f o r ways o f impro v i n g p r o t e c t i v e p r o p e r t i e s . With the use o f i m pedance technique the mechanism f o r two p e r i o d s of a c t i v i t y has been proven w i t h these c o a t i n g s . Zn pigmented c o a t i n g s impose the full c a t h o d i c p r o t e c t i o n d u r i n g the first p e r i o d o f a c t i o n , while i n the second p e r i o d the s e a l i n g and inhibiting p r o p e r t i e s of Zn pigment c o r r o s i o n products are the main f a c t o r s i n f l u e n c i n g the p r o tection. The work has been focused on l o o k i n g f o r ways o f improving the c o a t i n g s by i n f l u e n c i n g both p e r i o d s . P o s i t i v e r e s u l t s have been obtained w i t h z i n c phosphate used as an c o a t i n g a d d i t i v e to modify the second p e r i o d o f the p r o t e c t i o n .

Organic and inorganic coatings containing metallic zinc dust are extensively and successfully used for antloorrosion s t e e l protection i n various agressive media such as sea water, i n d u s t r i a l and sea atmosphere (J[). They comprise frequently a prime layer i n multilayer coatings or a paint for temporary protection. Their unique feature i s s t e e l protection even i n the course of minor damages. Standard layers contain from 85 to 95 % of zinc with respect to dry mass of the coating. Such a large content of zinc i n the form of duet makes the coating e l e c t r i c a l l y conducting and porous ( 2.3 )• According to generally accepted concepts, the protective action of a highly zinc pigmented coating has a two step mechanism which can be distinguished (1-6). The f i r s t one, which i s r e l a t i v e l y short, i s a period of cathodic protection. In z i n c - s t e e l microoells, d i s solution of zinc at the anode i s accompanied by protect i o n of s t e e l at the cathode. For i t s existence several faotore are necessary: the contact between zinc p a r t i c les as well as between zinc p a r t i c l e s and s t e e l substra0097-6156/86/0322-0229$06.00/0 © 1986 American Chemical Society

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turn and wetting of zlno and s t e e l surface* These condi­ tions are f u l f i l l e d due to the large zinc content and porosity of the coating. The decrease of effectiveness of cathodic protection i s associated with the loss of contact between s t e e l and zinc dust ( X ) or between par­ t i c l e s of zinc due to formation of the corrosion pro­ ducts of low conductivity. The second stage, several times longer than the f i r s t one (6), i s associated with the blocking action of zinc corrosion products i n pores existing i n the coating (4 8.