Mechanisms of De-adhesion of Organic Coatings ... - ACS Publications

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 31, 2015 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch012

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 lose adherence to a metal substrate by many processes. The eventual consequence of this 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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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 phenomena r e p r e s e n t a p o t e n t i a l l y f r u i t f u l a r e a of research s i n c e so l i t t l e i s known. Some of the important q u e s t i o n s 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

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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 -



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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 n i n c r e a s e s w i t h i n c r e a s e i n temperature. The a c t i v a t i o n energy i n t h e case o f 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 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 be expected t o f r a c t u r e r a t h e r than t o deform. 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 .


12.

LEIDHEISER


131


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 e b l a s t i n g i s t o p r o v i d e many anchor p o i n t s t h a t reduce t h e l i k e l i h o o d o f l a r g e - a r e a d i s b o n d i n g upon thermal 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.



132


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 g i n t e r f a c e . The i r i d i u m w i r e i s i n s u l a t e d from t h e s t e e l by an o r g a n i c c o a t i n g on t h e w i r e . The pH 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 +


12.

LEIDHEISER

133


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



134


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 c delamination o f many d i f f e r e n t types o f c o a t i n g s i n a l k a l i metal h a l i d e s o l u t i o n s i s s t r o n g l y a f u n c t i o n of the a l k a l i metal. I n a l l cases s t u d i e d , t h e r a t e o f 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


12.

LEIDHEISER


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 . ; Igetoft, L . ; Wang, W.; Weber, K. In "Or ganic Coatings Science and Technology"; Parfitt, G. D.; Patsis, 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


Mechanisms of De-adhesion of Organic Coatings ... - ACS Publications

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