13 Chemical Studies of the Organic Coating-Steel Interface After Exposure to Aggressive Environments Ray A . Dickie
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Ford Motor Company, Dearborn, MI 48121
The chemical composition and morphology of organic coating/steel interfacial surfaces have been examined 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
Dickie and Floyd; Polymeric Materials for Corrosion Control 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.
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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 s s u r f a c e a n a l y t i c a l methods t o a d h e s i o n and c o r r o s i o n problems have been e x t e n s i v e l y r e v i e w e d ( e . g . , 2, 7-12). 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
Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
POLYMERIC MATERIALS FOR CORROSION CONTROL
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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
A d h e s i o n Loss
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 ) .
Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
13.
DICKIE
The Organic Coating- Steel Interface After Exposure
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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 .
Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Dickie and Floyd; Polymeric Materials for Corrosion Control 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
