Chemical Studies of the Organic Coating-Steel Interface After

13 Chemical Studies of the Organic Coating-Steel Interface After Exposure to Aggressive Environments Ray A . Dickie

Downloaded by UNIV OF ROCHESTER on June 23, 2017 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch013

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.


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


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

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 ) .


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 .


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

Chemical Studies of the Organic Coating-Steel Interface After

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