Polymeric Materials for Corrosion Control - American Chemical Society

15 Cathodic Delamination of Protective Coatings: Cause and Control J. S. Thornton, J. F. Cartier, and R. W. Thomas

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch015

Texas Research Institute, Inc., 9063 Bee Caves Road, Austin, TX 78733

The results of some recent tests, directed at understanding the role of the hydroxide ion in the cathodic delamination of thick rubber adherends, are discussed. In addition to contributing to the breaking of adhesive bonds, hydroxide ions appear to cause some components of commercially available adhesive systems to become swollen. Thus, at the debond interface, where the cathodic reaction is producing a strongly basic solution, the adhesive bond may be additionally strained by volume changes. Evidence is presented which suggests that the selection of an adhesive for marine applications which includes exposure of the metal substrate to a cathodic potential, should be preceded by an examination of the predisposition of the adhesive system to volume changes in the presence of high concentrations of hydroxide ion.

C a t h o d i c p r o t e c t i o n i s a common approach t o r e d u c i n g c o r r o s i o n o f m e t a l s i n marine s e r v i c e . V i r t u a l l y e v e r y s t e e l s h i p i n the U.S. F l e e t i s p r o t e c t e d by the placement o f z i n c anodes w h i c h c o r r o d e s a c r i f i c i a l l y , t h e r e b y p r o t e c t i n g the s t e e l . The b e n e f i t s o f c a t h o d i c p r o t e c t i o n are enormous - s t e e l h u l l s would be q u i t e s h o r t l i v e d without i t . There are c o m p l i c a t i o n s however. The c a t h o d i c p o t e n t i a l e f f e c t i v e l y s t o p s c o r r o s i o n on the m e t a l s u b s t r a t e b u t i t a l s o c o n t r i b u t e s t o e a r l y debonding o f adherends, the development o f l e a k p a t h s under s e a l s , and the b l i s t e r i n g and p e e l i n g o f c o a t i n g s . D e g r a d a t i o n o f p r o t e c t i v e c o a t i n g s i s a b a s i c l i f e - l i m i t i n g problem f o r underwater equipment exposed t o a c a t h o d i c p o t e n t i a l . I n t h i s paper we are concerned w i t h the a d h e s i v e systems b e n e a t h t h i c k r u b b e r adherends. U n l i k e the t e c h n o l o g y f o r the p a i n t i n d u s t r y , where the s c i e n c e o f q u a l i f i c a t i o n t e s t i n g f o r d u r a b l e t h i n c o a t i n g s has produced p a i n t s w i t h 10, p o s s i b l y 20 y e a r s l i f e 0097-6156/86/0322-0169506.00/ 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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POLYMERIC MATERIALS FOR CORROSION CONTROL

e x p e c t a n c y , the a d h e s i v e s f o r underwater use are n o t so w e l l q u a l i f i e d . The r e l a t i o n s h i p between f o r m u l a t i o n and performance r e l i a b i l i t y c e r t a i n l y i s n o t obvious f o r t h i s p a r t i c u l a r h a r s h environment. There i s a need f o r an a c c u r a t e assessment o f the mechanism o f c a t h o d i c debonding o f t h i c k adherends. T h i s would c o n t r i b u t e t o b e t t e r c o n t r o l over the s e l e c t i o n o f f o r m u l a t i o n s r e s i s t a n t t o a t t a c k . One o f the o b j e c t i v e s o f t h i s work was t o d e v e l o p an a c c e l e r a t e d s c r e e n i n g t e s t . The s c r e e n i n g t e s t was used t o e v a l u a t e the r e l a t i v e performance p r o f i l e s o f a number o f commercial a d h e s i v e s w h i c h were recommended f o r marine a p p l i c a t i o n s . It is hoped t h a t a s c r e e n i n g t e s t such as t h i s one c o u l d be used t o i s o l a t e measurable p r i m e r p r o p e r t i e s w h i c h can be r e l a t e d t o the l o n g term p r o s p e c t u s o f the p r i m e r , and the v a r i a b i l i t y o f t h i s p r o s p e c t u s under p e r m u t a t i o n o f s u b s t r a t e type o r a d d i t i v e s . The R o l e o f the H y d r o x i d e I o n i n C a t h o d i c D e l a m i n a t i o n The r e l a t i o n s h i p between performance r e l i a b i l i t y and a d h e s i v e f o r m u l a t i o n i s n o t s i m p l e . The key s t e p i n i m p r o v i n g the r e l i a b i l i t y o f a d h e s i v e s on c a t h o d i c a l l y p r o t e c t e d s u b s t r a t e s i s f u l l y u n d e r s t a n d i n g the 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 . V a r i o u s mechanisms have been proposed i n the l i t e r a t u r e . A l a r g e number o f i n v e s t i g a t o r s have f o c u s e d a t t e n t i o n on the damage h y d r o x i d e i o n does t o c o a t i n g a d h e s i o n . D u r i n g the 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 t h e r e a r e two i m p o r t a n t r e a c t i o n s w h i c h can o c c u r a t the cathode and w h i c h a r e c a t a l y z e d on the t h i n l a y e r o f m e t a l o x i d e w h i c h c o v e r s the cathode s u r f a c e . These r e a c t i o n s a r e ; 1/2 0

2H 0 3

2

+

+ H 0 + 2e" - 2 OH"

(1)

+2e"

(2)

2

- H

2

+ 2H 0 2

E i t h e r r e a c t i o n w i l l r e s u l t i n an i n c r e a s e o f the pH n e a r the r e a c t i o n s i t e . The hydrogen r e a c t i o n w i l l p r o c e e d even i n s t r o n g l y b a s i c s o l u t i o n s i f the a p p l i e d p o t e n t i a l i s i n c r e a s e d s u f f i c i e n t l y . Which r e a c t i o n predominates depends upon the c i r c u m s t a n c e s . The e q u i l i b r i u m p o t e n t i a l f o r the oxygen r e d u c t i o n r e a c t i o n i s 1.24V more p o s i t i v e t h a n the e q u i l i b r i u m p o t e n t i a l f o r hydrogen r e d u c t i o n . On the o t h e r hand, the exchange c u r r e n t d e n s i t i e s f o r hydrogen e v o l u t i o n on c o r r o d i b l e m e t a l s u r f a c e s a r e f a r g r e a t e r t h a n the c o r r e s p o n d i n g v a l u e s f o r oxygen r e d u c t i o n . From cathode p o l a r i z a t i o n c u r v e s f o r s t e e l i n 0.6M s a l t water (1) i t can be seen t h a t the oxygen r e d u c t i o n r e a c t i o n i s f a v o r e d a t p o t e n t i a l s l e s s t h a n -0.8V ( v e r s u s a s t a n d a r d c a l o m e l e l e c t r o d e ) and the hydrogen r e d u c t i o n r e a c t i o n i s f a v o r e d a t p o t e n t i a l s more n e g a t i v e t h a n -1.0V.

Thus, i n n e u t r a l o r b a s i c s o l u t i o n s , where the H^0

+

c o n c e n t r a t i o n i s low, where d i s s o l v e d oxygen i s p r e s e n t and where an a p p l i e d v o l t a g e l e s s t h a n -0.8V i s p r e s e n t , we e x p e c t t o f i n d the oxygen r e d u c t i o n r e a c t i o n d o m i n a t i n g .



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I t i s g e n e r a l l y (2-4) agreed t h a t t h e f o r m a t i o n o f t h e OH- i s a c r u c i a l step i n p r o g r e s s i o n o f circumstances which l e a d t o d e l a m i n a t i o n . As l o n g as t h e r e i s a growing p o c k e t o f c a u s t i c s o l u t i o n s e q u e s t e r e d between t h e c o a t i n g and s u b s t r a t e l a y e r s , f u r t h e r d e l a m i n a t i o n i s o c c u r i n g . However, t h i s i s where t h e agreement ends. The a c t u a l mechanism by w h i c h OH- i n i t i a t e s t h e debond i s n o t c l e a r . V a r i o u s t h e o r i e s e x i s t . These t h e o r i e s g e n e r a l l y r e v o l v e around two c e n t r a l t e n e t s : e i t h e r t h e OH- i s a t t a c k i n g t h e polymer s u r f a c e and d i s r u p t i n g polymer t o m e t a l bonds, o r i t i s a t t a c k i n g t h e m e t a l oxide l a y e r that covers the metal surface. I n support o f the f i r s t mechanism, u s i n g s u r f a c e a n a l y s i s t e c h n i q u e s , D i c k i e , Hammond, and Holubka (5) have r e p o r t e d t h a t c a r b o x y l a t e d s p e c i e s p r e s e n t a t t h e i n t e r f a c e c a n be seen as a r e s u l t o f OH- a t t a c k o f t h e polymer. On the o t h e r hand, L e i d h e i s e r (4) r e p o r t s t h a t R i t t e r has o b s e r v e d t h e a t t a c k o f the metal oxide u s i n g e l l i p s o m e t r i c techniques t o study a p o l y b u t a d i e n e c o a t i n g on s t e e l . I n s u p p o r t o f t h i s , R i t t e r and K r u g e r (1) have measured pH v a l u e s as h i g h as 14 a t t h e d e l a m i n a t i o n s i t e under n a t u r a l c o r r o s i o n c o n d i t i o n s - t h i s i s c e r t a i n l y h i g h enough t o cause t h e d i s s o l u t i o n o f some m e t a l o x i d e s . K o e h l e r (6) p r e s e n t e d arguments f o r t h e case t h a t t h e r o o t cause o f c a t h o d i c d e l a m i n a t i o n i s t h e d i s p l a c e m e n t o f t h e c o a t i n g by a h i g h pH aqueous f i l m t h a t grows i n t h e i n t e r f a c i a l r e g i o n . I n t h a t d e s c r i p t i o n t h e i n t e r f a c i a l w a t e r d r a s t i c a l l y reduces t h e d i s p e r s i o n f o r c e s between polymer and m e t a l . I t i s a l s o t r u e t h a t i f t h e m e t a l t o polymer bonds were p r i n c i p a l l y o f i o n i c c h a r a c t e r , t h e n t h e water w h i c h forms a t t h e i n t e r f a c e would s e r i o u s l y degrade these bonds due t o t h e h i g h s o l v a t i o n energy r e l e a s e d d u r i n g t h e d i s s o l u t i o n o f i o n i c bonds. I t has been o b s e r v e d (7) t h a t t h e commercial a d h e s i v e system Chemlok 205/220 used f o r b o n d i n g r u b b e r t o m e t a l w i l l f a i l a a d h e s i v e tape p e e l t e s t a f t e r submersion i n seawater and exposure t o c a t h o d i c p o t e n t i a l . However, i t may r e c o v e r up t o 80% o f t h e o r i g i n a l bond i f i t i s d r y e d f o r s e v e r a l days b e f o r e s u b j e c t i n g i t t o t h e p e e l t e s t . T h i s r e v e r s i b l i t y s t r o n g l y s u g g e s t s t h a t i o n i c bonds o r d i s p e r s i o n f o r c e s a r e a more i m p o r t a n t s o u r c e o f b o n d i n g s t r e n g t h ( i n a t l e a s t the case o f t h i s Chemlok s y s t e m ) . Other e x p l a n a t i o n s o f t h e n a t u r e o f t h e polymer t o m e t a l bond i n c l u d e ; m e c h a n i c a l a d h e s i o n due t o m i c r o s c o p i c p h y s i c a l i n t e r l o c k i n g o f t h e two f a c e s , c h e m i c a l b o n d i n g due t o a c i d / b a s e r e a c t i o n s o c c u r i n g a t t h e i n t e r f a c e , hydrogen b o n d i n g a t t h e i n t e r f a c e , and e l e c t r o s t a t i c f o r c e s b u i l t up between t h e m e t a l f a c e and t h e d i e l e c t r i c polymer. I t i s r e a s o n a b l e t o assume t h a t a l l o f t h e s e k i n d s o f i n t e r a c t i o n s , t o one degree o r a n o t h e r , a r e needed t o e x p l a i n the f a i l u r e o f adhesion i n the cathodic delamination process. I n a d d i t i o n , we have o b s e r v e d t h a t t h e OH- appears t o be r e s p o n s i b l e f o r a s u r p r i s i n g degree o f i n c r e a s e d water a b s o r p t i o n . S t u d i e s c o n d u c t e d on t h e Chemlok 205/220 b o n d i n g system showed s u b s t a n t i a l l y i n c r e a s e d w e i g h t g a i n s when conducted i n 0.1 Ν NaOH o v e r t h o s e o b s e r v e d when conducted i n water o r seawater. The importance o f t h e OH- i o n i s n o t r e a l l y d i s p u t e d . I t i s t h e r o l e o f t h e OH- w h i c h i s i n q u e s t i o n . To f u r t h e r u n d e r s t a n d t h e e f f e c t o f t h e OH- on t h e a d h e s i o n o f m e t a l s t o p o l y m e r s , some e x p l o r a t o r y t e s t s were conducted. D e l a m i n a t i o n r a t e s were compared



172

t o s t u d y t h e e f f e c t o f c a t h o d i c a c t i o n and t h e e f f e c t o f h y d r o x i d e ion concentration.


D e l a m i n a t i o n Rate

Studies

Much o f t h e f o c u s o f t h e f o l l o w i n g work was aimed a t t h e p r e v i o u s l y mentioned Chemlok a d h e s i v e system. T h i s was because i t h a d been u s e d e x t e n s i v e l y i n a f a m i l i a r marine a p p l i c a t i o n , we were a c q u a i n t e d w i t h i t s l o n g term performance so i t c o u l d be u s e d as a b a s e l i n e f o r c o m p a r i s o n w i t h o t h e r commercial a d h e s i v e f o r m u l a t i o n s . A l l o f t h e commercial a d h e s i v e s used, i n c l u d i n g Chemlok 205/220, a r e p r o p r i e t a r y f o r m u l a t i o n s , thus t h e e x a c t f u n c t i o n a l n a t u r e o f each adhesive i s not a v a i l a b l e . A s e r i e s o f d e l a m i n a t i o n r a t e t e s t s were c o n d u c t e d on t h e Chemlok 205/220 a d h e s i v e system. The t e s t specimen was d e v e l o p e d from a m o d i f i c a t i o n o f t h e s t a n d a r d (ASTM D-429) p e e l t e s t . Each specimen h a d two one i n c h square bond a r e a s on a monel s u b s t r a t e . The t e s t specimens were suspended i n f i v e d i f f e r e n t t e s t t a n k s . One t a n k h a d 1.0 Ν NaOH, two h a d 0.25N NaOH and two h a d 3.5% (by wt.) N a C l s o l u t i o n . One 0.25N NaOH and one 3.5% NaCl s o l u t i o n tank were p u r g e d o f oxygen w i t h a n i t r o g e n atmosphere (oxygen c o n c e n t r a t i o n o f l e s s t h a n 3%), t h e complimentary p a i r o f tanks h a d n a t u r a l a i r e x p o s u r e (oxygen c o n c e n t r a t i o n 2 0 % ) . A summary o f t h e i n i t i a l s t a t u s o f each tank i s g i v e n i n T a b l e I . Table I. Tank

Initial

C o n d i t i o n s i n t h e D e l a m i n a t i o n Rate Tanks

Solution

Oxygen*

Conductivity

Current

pH

1

IN NaOH

22%

1..8x10 mohm

1., 0 amp

13..1

2

0.25N NaOH

23%

0..5

1..95

12..95

3

3.5%

23%

0..6

3.,6

8..8

4

0.25N NaOH

2%

0..6

1..5

12..92

5

3.5%

2%

0..6

3..5

8,.75

NaCl

NaCl

* Temperature o f t h e a l l f o u r t a n k s was m a i n t a i n e d a t 35 C. The oxygen c o n t e n t r e f e r s t o t h e degree o f s a t u r a t i o n o f t h e s o l u t i o n f o r that temperation o f s o l u t i o n . Each tank was c o n t i n u o u s l y s c r u b b e d o f CO^ and p o t e n t i o s t a t s were employed t o m a i n t a i n an a p p l i e d p o t e n t i a l o f -1.2 v o l t s v s SCE ( a t y p i c a l v a l u e f o r t h e p o t e n t i a l o f monel o r s t e e l on a s h i p ' s h u l l i n c l o s e p r o x i m i t y t o a z i n c anode). H a v i n g e l i m i n a t e d t h e p o s s i b i l i t y o f b u i l d u p o f c a r b o n a t e i o n s and z i n c i o n s , t h e t e s t c o u l d c o n t i n u e f o r extended p e r i o d s without changing the b a l a n c e o f i o n s i n t h e e l e c t r o l y t e . T h i s approximates a c t u a l a p p l i c a t i o n c o n d i t i o n s where t h e i n s t r u m e n t a t i o n i s immersed i n t h e ocean which p r o v i d e s a r e l a t i v e l y c o n s t a n t e l e c t r o l y t e environment. Each tank



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c o n t a i n e d a s e t o f f o u r d e l a m i n a t i o n t e s t specimens, each sample p a i n t e d everywhere e x c e p t under t h e r u b b e r adherend. D e l a m i n a t i o n was measured on a l l b o n d l i n e s . T h i s y i e l d e d t h e n 16 measurements p e r tank. The c o n c e n t r a t i o n o f t h e 0.25N NaOH was s e l e c t e d because i t has e q u i v a l e n t c o n d u c t i v i t y t o t h e 3.5% NaCl s o l u t i o n . The i n i t i a l c u r r e n t l e v e l s i n t h e NaCl v e r s u s NaOH tanks were d i f f e r e n t d e s p i t e the e q u i v a l e n t c o n d u c t i v i t i e s and t h e e q u a l a p p l i e d p o t e n t i a l . The d i f f e r e n c e was due t o t h e d i f f e r e n c e i n pH. The more b a s i c s o l u t i o n r e d u c e d t h e exchange c u r r e n t d e n s i t y f o r t h e hydrogen r e a c t i o n . A t t h i s p o t e n t i a l , t h e p r i n c i p a l r e a c t i o n i s t h e hydrogen r e a c t i o n . When t h e pH i s i n c r e a s e d , d e c r e a s i n g t h e c o n c e n t r a t i o n o f hydronium i o n , thereby decreasing the c o n c e n t r a t i o n o f the r e a c t a n t s , the r e a c t i o n i s slowed down. F i g u r e 1 i l l u s t r a t e s t h e c o m p a r i t i v e d e l a m i n a t i n g r a t e i n each o f t h e f i v e t a n k s . The d e l a m i n a t i o n r a t e s i n t h e NaCl s o l u t i o n s were s l i g h t l y g r e a t e r t h a n t h e NaOH s o l u t i o n s o f e q u i v a l e n t c o n d u c t i v i t y . The b a s i c e l e c t r o c h e m i c a l d i f f e r e n c e between t h e s o l u t i o n s was t h e c u r r e n t . F i g u r e 2 i l l u s t r a t e s t h e c u r r e n t due t o t h e c a t h o d i c r e a c t i o n i n each o f t h e f i v e t a n k s . The c u r r e n t i n t h e NaCl s o l u t i o n s o f e q u i v a l e n t c o n d u c t i v i t y was much h i g h e r because o f t h e lower pH, c o n s e q u e n t l y h i g h e r hydronium i o n c o n c e n t r a t i o n , f a v o r i n g the hydrogen r e a c t i o n . Thus, t h e c a t h o d i c r e a c t i o n was p r o c e e d i n g more r a p i d l y i n t h e NaCl s o l u t i o n and d e l a m i n a t i o n i s p r o c e e d i n g c o r r e s p o n d i n g l y , more r a p i d l y . The r a t e o f t h e c a t h o d i c r e a c t i o n i s n o t t h e o n l y f a c t o r i n f l u e n c i n g t h e debonding however. C o n s i d e r t h e IN NaOH s o l u t i o n . The 1 Ν NaOH s o l u t i o n i s c l e a r l y d e l a m i n a t i n g t h e f a s t e s t , d e s p i t e i t s lower c u r r e n t . The lower c u r r e n t i s due t o t h e f a c t t h a t t h e hydrogen r e a c t i o n i s hampered by t h e h i g h e r c o n c e n t r a t i o n o f h y d r o x i d e i o n s . The o t h e r tanks have h i g h e r average c u r r e n t d e n s i t i e s b u t s l o w e r debond r a t e s . As a r e s u l t we see t h a t t h e h y d r o x i d e i o n c o n c e n t r a t i o n i s a s e p a r a t e and d i s t i n c t a c c e l e r a t i n g factor. D e c r e a s i n g t h e a v a i l a b i l i t y o f d i s s o l v e d oxygen i n t h e NaCl and e q u i v a l e n t NaOH tanks h a d t h e e x p e c t e d e f f e c t . I n each c a s e , l o w e r i n g t h e oxygen c o n c e n t r a t i o n reduced t h e c a t h o d i c c u r r e n t . However, a t t h i s p o t e n t i a l t h e hydrogen r e a c t i o n i s c l e a r l y d o m i n a t i n g and t h e r e d u c t i o n i n oxygen i s n o t o f g r e a t m a g n i t u t e . These r e s u l t s i n d i c a t e t h a t t h e h y d r o x i d e i o n i s an a c c e l e r a t i n g f a c t o r o f i t s own r i g h t . I n t h e n e x t s e c t i o n we p r e s e n t some e v i d e n c e t h a t s u g g e s t s t h a t one o f t h e reasons h y d r o x i d e i o n i s a d e l a m i n a t i n g agent i s t h a t i t causes u n u s u a l s w e l l i n g o f some adhesives. S w e l l i n g o f A d h e s i v e s i n t h e Presence o f H y d r o x i d e To f u r t h e r u n d e r s t a n d t h e a c t i o n o f t h e h y d r o x i d e i o n s on t h e p r i m e r p r o p e r t i e s , w e i g h t g a i n t e s t s were u n d e r t a k e n on v a r i o u s , c o m m e r c i a l l y a v a i l a b l e a d h e s i v e s . We attempted t o p r e p a r e n e a t samples o f t e n types o f a d h e s i v e agents ( i e t o p c o a t s and p r i m e r s ) w h i c h h a d been recommended by t h e m a n u f a c t u r e r s as s u i t a b l e f o r marine a p p l i c a t i o n s . The neat samples were p r e p a r e d w i t h t h e use o f a commercial a d j u s t a b l e wet f i l m a p p l i c a t o r s u p p l i e d by t h e P a u l Gardner Company. T h i s a p p l i c a t o r w i l l l a y f i l m s up t o 0.25inches


174

POLYMERIC MATERIALS FOR CORROSION CONTROL .250 η


.225-

0

900

1800

2700

3600

4500

5400

6300

7200

8100

9000

TIME (MINUTES)

Figure 1

D e l a m i n a t i o n o f p e e l t e s t specimens i n f i v e tanks versus time.

Figure 2

C u r r e n t v e r s u s time i n the f i v e t a n k s .



15.

THORNTON ET AL.


175

t h i c k and s i x i n c h e s wide. The f i l m s were c a s t on a g l a s s s u b s t r a t e . The f i l m s were composed o f two l a y e r s o f 0.035 i n c h e s wet t h i c k n e s s . M u l t i p l e l a y e r s were l a i d down. A f t e r d r y i n g f o r about f o u r h o u r s , the s l i g h t l y wet f i l m was c u t i n t o one i n c h s t r i p s and removed from the g l a s s s u b s t r a t e . I t was i m p o r t a n t t o remove t h e s t r i p s b e f o r e they h a d c o m p l e t e l y d r i e d o r they would become b r i t t l e o r s t i c k t o the g l a s s s u r f a c e . The f r e e f i l m s were p l a c e d between t e f l o n s h e e t s and a l l o w e d t o d r y f o r s e v e r a l days. T h i s method gave r e p r o d u c i b l e d r y f i l m t h i c k n e s s e s o f 0.020 i n c h e s f o r t h e p r i m e r and 0.015 i n c h e s f o r t h e t o p c o a t . Cured n e a t samples were o b t a i n e d by p r e s s u r e c u r i n g t h e specimens a t 315 F f o r 50 minutes under 25000 pounds o f p l a t e n p r e s s u r e . The average sample s i z e was .8" X 1.00" χ 0.07", and t h e w e i g h t s ranged from 1.3 t o 1.8g. Three o f t h e a d h e s i v e systems were n o t p r e p a r e d because they were so v i s c o u s i t was n o t p o s s i b l e t o draw down a t h i n f i l m . Another system was d e l e t e d because i t s n e a t sample was t o o porous a f t e r b e i n g c u r e d . The r e m a i n i n g s i x a d h e s i v e agents were t e s t e d f o r t h e i r w e i g h t g a i n and volume change p r o p e r t i e s i n 3.5% N a C l , IN NaOH and d e i o n i z e d w a t e r . Two specimens o f each type were p l a c e d i n each o f the s o l u t i o n s . The s o l u t i o n s were m a i n t a i n e d a t 35°C and c o n d i t i o n s m o n i t o r e d i n c l u d e d : pH, temperature, s p e c i f i c g r a v i t y and e l e c t r i c a l c o n d u c t i v i t y . D e n s i t y changes and volume changes were measured by f i r s t w e i g h i n g each sample i n a i r , then i n water. A l l samples were r i n s e d t h o r o u g h l y t o p r e v e n t c o n t a m i n a t i o n o f t h e water used f o r w e i g h i n g . B e f o r e w e i g h i n g i n a i r they were d r i e d w i t h a t h i n stream o f n i t r o g e n u n t i l t h e y were v i s i b l y d r y . F i g u r e s 3-4 a r e r e p r e s e n t a t i v e o f t h e range o f r e s u l t s on t h e w e i g h t g a i n t e s t s . The range o f r e s u l t s was d r a m a t i c . T a b l e I I i n c l u d e s a t a b u l a t i o n o f t h e volume changes o b s e r v e d i n t h e s i x a d h e s i v e s . Some o f t h e n e a t samples showed extreme i n c r e a s e s i n w e i g h t and volume i n t h e OH

s o l u t i o n over t h e NaCl s o l u t i o n . I f

you r e g a r d t h e 0H~ s o l u t i o n t o be t y p i c a l o f t h e s o l u t i o n c o n d i t i o n s a t t h e s u b s t r a t e s u r f a c e where t h e c a t h o d i c r e a c t i o n s have made t h e l o c a l environment v e r y b a s i c , then t h e OH curve i n F i g u r e 3-4 p o r t r a y what i s g o i n g on a t t h e bond l i n e . Thus, p r i m e r s , l i k e 205, w i l l become s w o l l e n and s t r e t c h e d w i t h i n c r e a s e d water a b s o r p t i o n a t t h e bond l i n e . T h i s s w e l l i n g c a n c o n t r i b u t e t o t h e s t r e s s e s t h a t l e a d t o debond. On t h e o t h e r hand a few o f t h e n e a t samples showed a l m o s t no change i n volume, i n n e i t h e r t h e NaCl n o r the NaOH s o l u t i o n s . I t might be e x p e c t e d t h a t rubber t o m e t a l bonds employing t h e s e agents would have l e s s s t r a i n and b e t t e r bond preformance t h a n t h e s w o l l e n ones.


176


20.0-

15.0-

β 10.0—1

o NaCl


3 ζ

° NaOH

u 5.0-

M

0.0-

-5.0-

I 200 TIME (HOURS)

Figure 3

300

Change i n volume o f Chemlok 205 p r i m e r i n IN NaOH s o l u t i o n and i n 3.5%(Wt.) s o l u t i o n o f N a C l . Thus, t h e upper c u r v e , showing t h e volume change i n t h e presence o f h y d r o x i d e i o n , w i l l be t y p i c a l o f the behavior o f the primer a t the debond d u r i n g c a t h o d i c d e l a m i n a t i o n .

ο NaCl ° NaOH

g

>

5.0-

-5.0-

I 100

200

TIME (HOURS)

F i g u r e 4 Change i n volume o f Chemlok 220 t o p c o a t i n I N NaOH s o l u t i o n and i n 3.5%(Wt.) s o l u t i o n o f N a C l . Volume changes were m i n i m a l , i l l u s t r a t i n g t h e p o t e n t i a l range o f responses o f a d h e s i v e agents t o exposure t o h y d r o x i d e i o n .


15.

THORNTON HT AL.

Table I I . SUPPLIER Lord

Corp.


Dayton C h e m i c a l s


Commercial V u l c a n i z i n g PRODUCT

177

Adhesives VOLUME CHANGE

Chemlok

205

15%

Chemlok

220

3%

Chemlok

252

16%

Thixon

Ρ-10

6%

Thixon

511T

10%

Thixon

GPO

2%

A l l t h i s e v i d e n c e p o i n t s t o the c o n c l u s i o n i o n may be r e s p o n s i b l e f o r b r e a k i n g m e t a l o x i d e a l s o a c t s t o i n c r e a s e the degreee o f s a t u r a t i o n water. I t may, i n f a c t , be s w e l l i n g the p r i m e r , p o o l i n g water a t the i n t e r f a c e . Screening Test f o r Adhesives

and

that while hydroxide o r polymer bonds, i t o f the polymer w i t h weakening bonds and

Additives

I n i t i a l l y , we t r i e d t o d e v i s e a s c r e e n i n g t e s t f o r a d h e s i v e s and a d d i t i v e s w h i c h i m i t a t e d n a t u r a l l y o c c u r i n g c o n d i t i o n s . The environment we were i m i t a t i n g was c h a r a c t e r i z e d by s a l t water immersion, t e m p e r a t u r e s r a n g i n g from j u s t above f r e e z i n g t o 30°C, and a c a t h o d i c p o t e n t i a l r a n g i n g anywhere from -0.8 t o -1.2 volts. In l i g h t o f the e v i d e n c e about the importance o f the h y d r o x i d e i o n i n s w e l l i n g the a d h e s i v e , a r t i f i c i a l enhancement o f 0Hc o n c e n t r a t i o n was t a k e n as an a c c e l e r a t i n g f a c t o r . Thus, i n s t e a d o f the a r t i f i c i a l seawater s o l u t i o n , a IN s o l u t i o n o f NaOH was used. The a d h e s i v e s were a p p l i e d a c c o r d i n g t o m a n u f a c t u r e r s d i r e c t i o n s t o s c a l e d down models o f an a p p l i c a t i o n . F a i l u r e o f the v u l c a n i z e d , r u b b e r t o m e t a l bond was d e t e c t e d by a l o s s o f r e s i s t a n c e r e s u l t i n g from the e s t a b l i s h m e n t o f a l e a k p a t h under the r u b b e r . The samples were c o n n e c t e d e l e c t r i c a l l y t o the z i n c anode. The temperature, s p e c i f i c g r a v i t y , e l e c t r i c a l c o n d u c t i v i t y and pH o f the s o l u t i o n s were m o n i t o r e d d u r i n g the t e s t . A f t e r a s e v e n t y seven day exposure, f a i l u r e s were o b s e r v e d i n the q u a n t i t i e s l i s t e d i n T a b l e I I I . We see from t h i s t a b l e t h a t T h i x o n P-10/GPO i s c l e a r l y the b e s t p e r f o r m i n g system. It is i n s t r u c t i v e t o compare the f a i l u r e r a t e s from the s c r e e n i n g t e s t w i t h the volume changes o b s e r v e d i n the p r i m e r s i n the water a b s o r p t i o n t e s t . Where the p r i m e r i s r e s i s t a n t t o volume changes i n the p r e s e n c e o f h y d r o x i d e i o n , the bond g e n e r a l l y has a low i n c i d e n c e o f f a i l u r e . There i s one n o t a b l e e x c e p t i o n t o t h i s g e n e r a l i z a t i o n . The Chemlok 205/252 system has a p r i m e r d i s t i n g u i s h e d by l a r g e volume changes, y e t when combined w i t h the t o p c o a t 252, i t p e r f o r m s r e a s o n a b l y w e l l . T h i s i s p u z z l i n g and needs to be i n v e s t i g a t e d . One p o s s i b l e e x p l a n a t i o n i s t h a t the volume


178


changes i n the t o p c o a t a r e a l s o i m p o r t a n t . I f the t o p c o a t i s v e r y much s t i f f e r t h a n the p r i m e r , s t r e s s w i l l develop and c o n t r i b u t e t o debonding when the p r i m e r s t a r t s t o expand w i t h m o i s t u r e . Comparing the behavor o f t h e t o p c o a t and p r i m e r i n the 205/252 system we see t h e y share s i m i l a r e x p a n s i o n t e n d e n c i e s i n the h i g h pH environment. Thus t h e t o p c o a t may be r e d u c i n g s t r e s s i n the p r i m e r by accommodating the volume changes more e a s i l y . A f u r t h e r i n v e s t i g a t i o n o f t h i s phenomenon would c o n t r i b u t e t o o u r u n d e r s t a n d i n g o f t h e c o r r e c t procedure f o r s e l e c t i n g t h e t o p c o a t t o go a l o n g w i t h a chosen p r i m e r .


Table I I I .

C o r r e l a t i o n Between the I n c i d e n c e o f F a i l u r e i n t h e S c r e e n i n g T e s t and the A b s o r p t i o n o f Water i n t h e Presence o f OH- Ions WATER ABSORPTION TESTS SCREENING TEST

Adhesive

System #failed

Chemlok 205/220 Chemlok 205/220/A1100 Chemlok 252 T h i x o n P-10/511T T h i x o n P-10/GPO Chemlok 205/252

83% 100% 66% 17% 0% 33%

%volume change s i n Primer Topcoat 15% 15% 6% 6% 15%

3% 3% 16% 10% 2% 16%

These r e s u l t s i n d i c a t e t h a t t h e degree o f volume change due t o the uptake o f water i s r e l a t e d t o t h e r e s i s t a n c e o f the bond t o c a t h o d i c a c t i o n . T e n t a t i v e l y we c a n c o n c l u d e from t h i s t e s t , p e n d i n g f u r t h e r information, that a primer w i t h high r e s i s t a n c e t o s w e l l i n g i n the p r e s e n c e o f OH- and a t o p c o a t w i t h s i m i l a r volume change b e h a v i o r i n the p r e s e n c e o f OH-, w i l l p e r f o r m b e s t as an a d h e s i v e system. Conclusions The p r e c e d i n g h a s been a d e s c r i p t i o n o f o u r e f f o r t s t o d e v e l o p a s c r e e n i n g t e s t f o r a d h e s i v e s w h i c h w i l l be used on s t e e l o r monel s u b s t r a t e s w h i c h a r e m a i n t a i n e d a t a c a t h o d i c p o t e n t i a l , immersed i n seawater a t temperatures r a n g i n g from j u s t above f r e e z i n g t o 30° C. Because o f the q u a n t i t y o f e v i d e n c e t h a t p o i n t s t o the h y d r o x i d e i o n , n o t o n l y as a p o t e n t i a l bond b r e a k i n g agent, b u t a l s o as t h e agent w h i c h i n d u c e s s w e l l i n g and c o n c e n t r a t e s s t r e s s a t t h e b o n d l i n e , the h y d r o x i d e i o n was t a k e n t o be a u s e f u l a c c e l e r a t i n g factor f o r the screening test. A t l e a s t one c o n c l u s i o n can be drawn from a l l o f t h i s . P r i m e r s t h a t e x p e r i e n c e l a r g e i n c r e a s e s i n s w e l l i n g i n the presence o f h y d r o x i d e i o n , d e s p i t e s t a b i l i t y and c o n s i d e r a b l e bond s t r e n g t h i n s a l t water, would probably perform p o o r l y i f a c a t h o d i c p o t e n t i a l was a p p l i e d . Thus, t h e q u a l i f i c a t i o n o f an a d h e s i v e system f o r a marine a p p l i c a t i o n t h a t i n c l u d e s exposure o f the m e t a l s u b s t r a t e t o a cathodic p o t e n t i a l , should include examination o f the


15.

THORNTON ET AL.


179

p r e d i s p o s i t i o n o f the a d h e s i v e t o volume changes i n t h e p r e s e n c e o f hydroxide i o n .

Literature Cited 1.


2. 3. 4. 5.

6. 7.

J . S. Thornton, and R. W Thomas, "Adhesive Systems - Durability Testing", Sonar Transducer Reliability Improvement Program STRIP 4'th Qtr. Rpt., NRL Memo. Rpt., R. W. Timme, Ed., Oct., 1985. E. L. Koehler, Corrosion, 40, 5 (1984). E. L. Koehler, Corrosion, 33, 209 (1977). H. Leidheiser, J r . , Corrosion, 38,374 (1980). J . S. Hammond, J . W. Holubka, and R. A. Dickie, Org. Coatings Plastics Chem. 39,506 (1978); J . Coatings Tech., 51,655 (1979); Org. Coatings Plastics Chem. 41,499 (1979); J . S. Hammond, J. W. Holubka, J . E. DeVries, and R. A. Dickie, Corrosion S c i ., 21, 239 (1981). E. L. Koehler, Corrosion 40, 984, TRI 128 D. A. Dillard, and H. F. Brinson "Adhesive Systems - Mechanical and Thermal Properties", Sonar Transducer Reliability Improvement Program STRIP 4'th Qtr. Rpt., NRL Memo. Rpt., R. W. Timme, Ed., Oct., 1985.

RECEIVED January 22, 1986


Polymeric Materials for Corrosion Control - American Chemical Society

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