[acs symposium series] polymeric materials for corrosion control volume 322 || evaluation of coating...

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

Malcolm L. White1 and Henry Leidheiser, Jr. 2

1Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA 18015 2Department of Chemistry and Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA 18015

Three types of coatings--a vinyl ester, a polyester and four epoxies--were coated on steel and exposed to 0.1M H 2 S O 4 at 60°C. Measurements of corrosion potential , AC conductance, tensile adhesion and weight gain were made on the coated substrates after 1000 hours of exposure, and the values were compared with the observed corrosion of the steel substrate. The best correlation of parameter values with corrosion was found with conductance. Corrosion potential did not show a consistent relationship, and weight gain and tensile adhesion showed no correlation with corrosion. It was concluded that the most important properties for coatings to be used in acid media are low permeability and resistance to degradation by acid. The vinyl ester, a bisphenol A epoxy cured with an aliphatic amine, and a novolac epoxy cured with a mixed aromatic/cycloaliphatic amine provided the best corrosion protection. The saturated polyester and a bisphenol A epoxy cured with a polyamide amine showed significant deterioration in acid and corrosion of the underlying steel. Two novolac epoxies cured with aromatic amines showed intermediate performance.

T h e m e c h a n i s m f o r t h e i n i t i a l c o r r o s i o n o f s t e e l i n n e u t r a l o r a l k a l i n e s o l u t i o n s i s g e n e r a l l y a c c e p t e d as t h e o x i d a t i o n o f i r o n f r o m t h e m e t a l l i c s t a t e t o t h e f e r r o u s i o n :

Fe - F e " " + 2 e " ( 1 )

w i t h t h e a t t e n d a n t r e d u c t i o n r e a c t i o n b e i n g t h e f o r m a t i o n o f h y d r o x i d e i o n f r o m o x y g e n and w a t e r [ 1 J :

1 / 2 0 2 + H 2 0 + 2e~ - 2 0 H " ( 2 )

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

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

78 POLYMERIC MATERIALS FOR CORROSION CONTROL

A v a r i e t y o f t e c h n i q u e s has b e e n d e v e l o p e d t o m e a s u r e t h e c o n d i t i o n o f a c o a t i n g s o t h a t some e v a l u a t i o n o f i t s p r o t e c t i v e a b i l i t y c a n be made. Many o f t h e s e a r e b a s e d on e l e c t r o c h e m i c a l m e a s u r e m e n t s [ 2 ] . T h e f o u r t e c h n i q u e s u s e d i n t h i s s t u d y a r e ( 1 ) c o r r o s i o n p o t e n t i a l , ( 2 ) AC c o n d u c t a n c e , ( 3 ) t e n s i l e a d h e s i o n , and ( 4 ) w e i g h t g a i n .

The c o r r o s i o n p o t e n t i a l i s d e t e r m i n e d by t h e p o t e n t i a l a t w h i c h t h e a n o d i c a n d c a t h o d i c r e a c t i o n s o c c u r a t t h e s a m e r a t e [ 3 ] . T h e AC c o n d u c t a n c e i s a m e a s u r e o f t h e e a s e w i t h w h i c h c h a r g e i s t r a n s m i t t e d t h r o u g h t h e c o a t i n g [ 4 ] . The a d h e s i v e s t r e n g t h o f t h e c o a t i n g t o t h e s t e e l s u r f a c e i s a f f e c t e d b y r e a c t i o n s o c c u r r i n g a t t h e i n t e r f a c e . The w e i g h t g a i n o f c o a t i n g s has b e e n s t u d i e d by F u n k e and Haagen [ 5 ] who h a v e s h o w n t h a t a w e i g h t g a i n e x c e e d i n g t h a t o f a f r e e f i l m i n d i c a t e s a n a c c u m u l a t i o n o f w a t e r a t t h e i n t e r f a c e [5].

T h e s e t e c h n i q u e s a r e f r e q u e n t l y u s e d t o s t u d y c o r r o s i o n u n d e r c o a t i n g s i n n e u t r a l s o l u t i o n s and e n j o y s p o r a d i c s u c c e s s , d e p e n d i n g p r i m a r i l y on t h e t y p e and t h i c k n e s s o f c o a t i n g b e i n g s t u d i e d .

T h e m e c h a n i s m o f s t e e l c o r r o s i o n i n a c i d s o l u t i o n s , h o w e v e r , i s d i f f e r e n t f r o m t h a t i n n e u t r a l s o l u t i o n s i n t h a t t h e r e d u c t i o n r e a c t i o n i s t h e f o r m a t i o n o f h y d r o g e n f r o m h y d r o g e n i o n :

2 H + + 2e~ + H 2 ( 3 )

P r e v i o u s w o r k i n t h i s l a b o r a t o r y h a s e s t a b l i s h e d t h a t f o r e p o x y and f 1 u o r o p o l y m e r c o a t i n g s e x p o s e d t o d i l u t e s u l f u r i c a c i d , t h e r e i s m o v e m e n t o f a c i d t h r o u g h t h e c o a t i n g t o t h e s t e e l s u r f a c e s o t h a t E q u a t i o n 3 i s t h e p r e d o m i n a n t r e d u c t i o n r e a c t i o n [ 6 ] .

B e c a u s e o f t h i s d i f f e r e n c e i n c o r r o s i o n m e c h a n i s m i n a c i d s o l u t i o n , t h e u s e f u l n e s s o f t h e f o u r e v a l u a t i o n t e c h n i q u e s d i s c u s s e d a b o v e may b e d i f f e r e n t t h a n i n n e u t r a l s o l u t i o n s . T h e p u r p o s e o f t h i s w o r k w a s t o e v a l u a t e t h e s e f o u r t e c h n i q u e s f o r p r e d i c t i n g t h e b e h a v i o r o f c o a t i n g r e s i n s i n a c i d s o l u t i o n s . I n a d d i t i o n , t h e a b i l i t y o f s e v e r a l d i f f e r e n t t y p e s o f c o a t i n g r e s i n s t o p r o t e c t s t e e l a g a i n s t c o r r o s i o n i n a c i d s o l u t i o n was e v a l u a t e d .

E x p e r i m e n t a l

The f o l l o w i n g c o a t i n g r e s i n s w e r e u s e d : ( 1 ) a v i n y l e s t e r ( D e r a -k a n e 4 7 0 f r o m Dow C h e m i c a l ) ; (2) a p o l y e s t e r ( A t l a c 3 8 2 - 0 5 AC f r o m I C I ) ; and ( 3 ) f o u r e p o x y r e s i n / h a r d e n e r c o m b i n a t i o n s . The d e t a i l s o f t h e r e s i n s and h a r d e n e r s u s e d a r e s h o w n i n T a b l e I. One o f t h e e p o x y / h a r d e n e r c o m b i n a t i o n s was r e p r e s e n t e d by m a t e r i a l s f r o m t w o s o u r c e s .

The c o a t i n g s w e r e a p p l i e d t o o n e s i d e o f a s t e e l s u b s t r a t e by means o f a s p r a y g u n f o r t h e l o w e r v i s c o s i t y c o a t i n g s , o r b y d o c t o r b l a d i n g w i t h an a d j u s t a b l e G a r d n e r k n i f e f o r t h e h i g h e r v i s c o s i t y m a t e r i a l s . A c a s t i n g t e c h n i q u e w a s a l s o u s e d i n w h i c h a k n o w n v o l u m e o f t h e c o a t i n g m a t e r i a l was p o u r e d i n t o a known a r e a d e f i n e d by h e a v y t a p e and was a l l o w e d t o s p r e a d w h i l e on a l e v e l s u r f a c e .

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Tabl

e I.

Coat

ing

Mate

rial

s

Su

pp

lie

r D

esi

gn

ati

on

Re

sin

Ha

rde

ne

r

Re

sin

: H

ard

en

er

Ra

tio

Cu

red

Th

ick

ne

ss

(mil

s)

(my)

V

inyl

E

ste

r Do

w C

he

mic

al

(De

rak

an

e 4

70

)

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ste

r IC

I A

me

rica

s (A

tla

c 3

82

-05

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)

Ep

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es

CON

/CH

EM

Cib

a/

Ge

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Ma

rte

k

VE

PE

BP

A/A

L

NO

V/AR

1

NO

V/A

R

2

BP

A/P

A

NO

V/A

R/

AL

Bis

ph

en

ol

A V

iny

l E

ste

r +

.15

% C

oNap

;

36%

Sty

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ph

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um

ara

te

Po

lye

ste

r +

1%

CoN

ap*

+ D

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5

0%

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vola

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tha

na

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(6%

).

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10

0:2

10

0:2

10

0:2

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0

10

0:4

6

2.7

7:1

1.8

2:1

7-

9

6-1

2

9-1

0

10

-12

10

-11

8-

12

8-9

17

8-2

29

15

2-3

05

22

9-2

54

25

4-3

05

25

4-2

79

20

3-3

05

20

3-2

29

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T h e c o a t i n g s w e r e c u r e d i n t w o s t e p s : f i r s t , a r o o m t e m p e r a t u r e e x p o s u r e f o r a t l e a s t f i v e h o u r s t o a l l o w a n y s o l v e n t p r e s e n t t o e v a p o r a t e a n d / o r t h e r e s i n t o g e l a n d , s e c o n d , an e l e v a t e d t e m p e r a t u r e c u r e . T h e v i n y l e s t e r , p o l y e s t e r a n d e p o x i e s w e r e b a k e d a t 6 0 ° C f o r 4 - 1 6 h o u r s . The c o a t i n g t h i c k n e s s a f t e r c u r i n g was m e a s u r e d w i t h a m i c r o m e t e r , s u b t r a c t i n g t h e s u b s t r a t e t h i c k n e s s . The t h i c k n e s s o f t h e c o a t i n g s r a n g e d f r o m 6 t o 12 m i l s ( 1 5 0 - 3 0 0 urn) and i s s h o w n f o r e a c h t y p e i n T a b l e I.

The s u b s t r a t e s w e r e c o l d r o l l e d , l o w - c a r b o n SAE 1010 s t e e l , 32 m i l s ( 0 . 8 mm) t h i c k (Q P a n e l s ) . T h e y w e r e s a n d b l a s t e d on b o t h s i d e s t o a 6 pm p r o f i l e w i t h s i l i c a s a n d . No f u r t h e r c l e a n i n g was d o n e . A c i r c u l a r d i s k 3 . 3 3 cm i n d i a m e t e r was p r e p a r e d f r o m a l a r g e r c o a t e d p a n e l w i t h a p u n c h a n d d i e s e t . T h e d i s k w a s p l a c e d o n a 1 2 5 m l w i d e m o u t h s c r e w - c a p p o l y p r o p y l e n e b o t t l e , u s i n g a r u b b e r g a s k e t t o m a k e a t i g h t s e a l , w i t h t h e c o a t e d s i d e f a c i n g t h e i n s i d e o f t h e b o t t l e . The d i s k and g a s k e t w e r e h e l d on t h e b o t t l e w i t h t h e c a p , f r o m w h i c h t h e c e n t r a l p o r t i o n had b e e n r e m o v e d , s o t h e b a c k ( u n -c o a t e d ) s i d e o f t h e s t e e l s u b s t r a t e w a s e x p o s e d . T h e b o t t l e w a s i n v e r t e d and h a l f f i l l e d w i t h 0.1 M H 0 S O 4 t h r o u g h a h o l e d r i l l e d i n t h e b o t t o m o f t h e b o t t l e i n o r d e r t o c o n t a c t t h e a c i d w i t h t h e c o a t i n g . The i n v e r t e d b o t t l e was p l a c e d i n an o v e n a t 6 0 ° C .

The c o r r o s i o n p o t e n t i a l was m e a s u r e d by p u t t i n g a s a t u r a t e d c a l o m e l e l e c t r o d e / s a l t b r i d g e i n t o t h e s o l u t i o n t h r o u g h t h e h o l e i n t h e b o t t o m o f t h e p l a s t i c b o t t l e a n d c o n t a c t i n g t h e b a c k s i d e o f t h e s u b s t r a t e t o c o m p l e t e t h e c i r c u i t as s h o w n i n F i g u r e 1. A K e i t h l e y 600A e l e c t r o m e t e r was u s e d f o r t h e m e a s u r e m e n t . The AC c o n d u c t a n c e w a s d e t e r m i n e d b y i n s e r t i n g a c a r b o n r o d i n t o t h e s o l u t i o n a n d m e a s u r i n g t h e c o n d u c t a n c e a t 2 kHz f r e q u e n c y and 2 0 0 mv p o t e n t i a l w i t h an E x t e c h M o d e l 4 4 0 D i g i t a l C o n d u c t i v i t y M e t e r . The c o n d u c t a n c e v a l u e s w e r e c o n v e r t e d t o s p e c i f i c c o n d u c t i v i t y by m u l t i p l y i n g by t h e t h i c k n e s s and d i v i d i n g by t h e a r e a ( 5 c m 2 ) .

The a d h e s i o n was m e a s u r e d by f a s t e n i n g a l e a d a n c h o r o f known a r e a ( 2 . 8 4 c m 2 ) t o t h e c o a t i n g w i t h a c y a n o a c r y l a t e a d h e s i v e ( L o c t i t e 4 1 4 ) and a f t e r c u r i n g , p u l l i n g i t o f f n o r m a l t o t h e s u r f a c e w i t h a D i l l o n t e n s i l e t e s t e r . The f o r c e t o r e m o v e t h e c o a t i n g was d i v i d e d b y t h e a r e a o f a t t a c h m e n t t o c o n v e r t i t t o a n o r m a l i z e d t e n s i l e a d h e s i o n v a l u e .

The w e i g h t g a i n was m e a s u r e d by w e i g h i n g t h e c o a t e d d i s k a f t e r i t was r e m o v e d f r o m t h e p l a s t i c b o t t l e , f o l l o w i n g a w a t e r r i n s e and r e m o v a l o f s u r f a c e w a t e r .

T h e c o a t i n g a n d s u b s t r a t e w e r e o b s e r v e d t h r o u g h t h e h o l e i n t h e b o t t l e d u r i n g t h e e x p o s u r e t o t h e a c i d . S i n c e t h e c o a t i n g s w e r e t r a n s p a r e n t , i t w a s p o s s i b l e t o o b s e r v e a n y v i s i b l e c o r r o s i o n o c c u r r i n g o n t h e s t e e l s u b s t r a t e . T h e c o r r o s i o n p r o d u c t s o n t h e s t e e l w e r e g r a y o r b l a c k , e x c e p t w h e n t h e c o a t i n g b l i s t e r e d a n d s o m e r u s t i n g w a s s e e n . A s c o r r o s i o n p r o g r e s s e d d u r i n g t h e a c i d e x p o s u r e , t h e s t e e l s u r f a c e g r a d u a l l y d a r k e n e d f r o m t h e i n i t i a l l i g h t g r a y o f t h e o r i g i n a l s a n d b l a s t e d s u r f a c e t o an a l m o s t b l a c k s u r f a c e . T h e e x t e n t o f c o r r o s i o n w a s e s t i m a t e d f r o m t h e a m o u n t o f d a r k e n i n g o b s e r v e d on t h e s t e e l s u r f a c e .

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WHITE A N D LEIDHEISER Evaluation of Coating Resins

F i g u r e 1. T e c h n i q u e f o r e l e c t r o c h e m i c a l m e a s u r e m e n t s . R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f e r e n c e 1 2 . C o p y r i g h t 1 9 8 5 , N a t i o n a l A s s o c i a t i o n o f C o r r o s i o n E n g i n e e r s .

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R e s u l t s

F i g u r e 2 s u m m a r i z e s t h e v a l u e s o b t a i n e d f o r t h e f o u r m e a s u r e m e n t s on t h e s e v e n c o a t i n g s a f t e r e x p o s u r e t o 0.1 M H 0 S O 4 a t 6 0 ° C f o r 1 0 0 0 h o u r s . The o r d i n a t e s h o w s t h e v a l u e s m e a s u r e c f f o r e a c h o f t h e f o u r t e c h n i q u e s , a n d t h e a b c i s s a r e p r e s e n t s t h e a m o u n t o f c o r r o s i o n o b s e r v e d o n t h e s t e e l u n d e r e a c h o f t h e c o a t i n g s a f t e r t h e a c i d e x p o s u r e , w i t h t h e a m o u n t o f o b s e r v e d c o r r o s i o n d e c r e a s i n g f r o m l e f t t o r i g h t .

The c o r r o s i o n p o t e n t i a l s show a g e n e r a l t r e n d o f i n c r e a s i n g v a l u e s w i t h d e c r e a s i n g s u b s t r a t e c o r r o s i o n , w i t h t h e e x c e p t i o n o f t h e p o l y e s t e r a n d t h e n o v o l a c e p o x y c u r e d w i t h an a r o m a t i c / c y c l o a l i -p h a t i c a m i n e .

T h e s p e c i f i c AC c o n d u c t i v i t y v a l u e s s h o w a g e n e r a l l y d e c r e a s i n g v a l u e w i t h d e c r e a s i n g s u b s t r a t e c o r r o s i o n , w i t h o n e e x c e p t i o n : t h e n o v o l a c e p o x y c u r e d w i t h an a r o m a t i c / c y c l o a l i p h a t i c a m i n e . T h i s i s o n e o f t h e c o a t i n g s t h a t a l s o d i d n o t f i t i n t o t h e t r e n d f o r t h e c o r r o s i o n p o t e n t i a l v a l u e s .

The t e n s i l e a d h e s i o n v a l u e s show no c o r r e l a t i o n w i t h t h e e x t e n t o f c o r r o s i o n ; t h e b i s p h e n o l A e p o x y c u r e d w i t h a p o l y a m i d e a m i n e s h o w e d b l i s t e r i n g , w h i c h r e p r e s e n t s a c o m p l e t e l o s s o f a d h e s i o n . T h e p o l y e s t e r s h o w e d c o h e s i v e f a i l u r e a t l e s s t h a n 1 0 0 0 h o u r s o f e x p o s u r e , s o a t r u e a d h e s i o n v a l u e c o u l d n o t b e d e t e r m i n e d . T h e o t h e r e p o x i e s a n d t h e v i n y l e s t e r a l l had v a l u e s i n t h e 1 5 0 - 2 0 0 p s i r a n g e , w i t h no a p p a r e n t r e l a t i o n s h i p t o t h e a m o u n t o f c o r r o s i o n .

W e i g h t c h a n g e d a t a w e r e o b t a i n e d f o r o n l y f o u r o f t h e s e v e n c o a t i n g s , and t h o s e d a t a s h o w e d no c o r r e l a t i o n w i t h t h e e x t e n t o f s t e e l s u b s t r a t e c o r r o s i o n . T h e p o l y e s t e r s h o w e d a w e i g h t l o s s , r a t h e r t h a n a w e i g h t g a i n , p r o b a b l y d u e t o a n a t t a c k a n d d i s s o l u t i o n o f t h e e p o x y by t h e a c i d .

I t s h o u l d be n o t e d t h a t t h e e l e c t r o c h e m i c a l m e a s u r e m e n t s ( c o r r o s i o n p o t e n t i a l and c o n d u c t i v i t y ) f o r t h e t w o n o v o l a c e p o x i e s c u r e d w i t h a n a r o m a t i c a m i n e f r o m d i f f e r e n t s o u r c e s s h o w e d g o o d a g r e e m e n t , a l t h o u g h t h e t e n s i l e a d h e s i o n a n d w e i g h t g a i n v a l u e s w e r e n o t a s r e p r o d u c i b l e .

D i s c u s s i o n

The b e s t p e r f o r m i n g c o a t i n g s w e r e t h e v i n y l e s t e r , t h e b i s p h e n o l A e p o x y c u r e d w i t h an a l i p h a t i c a m i n e , and a n o v o l a c e p o x y c u r e d w i t h a m i x e d a r o m a t i c / c y c l o a l i p h a t i c a m i n e . The s a t u r a t e d p o l y e s t e r , and a b i s p h e n o l A e p o x y c u r e d w i t h a p o l y a m i d e a m i n e s h o w e d s i g n i f i c a n t d e t e r i o r a t i o n o f t h e c o a t i n g m a t e r i a l i n t h e a c i d , and c o r r o s i o n o f t h e u n d e r l y i n g s t e e l . T w o t y p e s o f n o v o l a c e p o x i e s c u r e d w i t h a r o m a t i c a m i n e s s h o w e d i n t e r m e d i a t e p e r f o r m a n c e .

O n l y o n e o f t h e f o u r t e c h n i q u e s — t h e c o n d u c t i v i t y — s h o w e d any c o r r e l a t i o n w i t h t h e o b s e r v e d e x t e n t o f c o r r o s i o n . The l a c k o f c o r r e l a t i o n o f t h e t e n s i l e a d h e s i o n v a l u e s w i t h c o r r o s i o n i s a r e s u l t o f t h e f a c t t h a t t h e m e t h o d i n t e g r a t e s a d h e s i o n l o s s a t t h e s u b s t r a t e

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i n t e r f a c e and t h e c o h e s i o n l o s s d u e t o d e t e r i o r a t i o n o f t h e p o l y m e r b y a c i d . A l s o , t h e c o r r o s i o n o f t e n i s l o c a l i z e d a n d t h e a r e a a t t a c k e d i s a s m a l l f r a c t i o n o f t h e t o t a l i n t e r f a c i a l a r e a . T h e w e i g h t g a i n i s n o t a r e l i a b l e m e a s u r e o f c o r r o s i o n p r o t e c t i o n b e c a u s e t h e r e may b e an a t t a c k a n d s o l u b i l i z a t i o n o f t h e c o a t i n g m a t e r i a l i t s e l f ( a s o c c u r r e d w i t h t h e p o l y e s t e r ) , s o t h a t t h e o b s e r v e d c h a n g e i n w e i g h t w i l l be t h e n e t r e s u l t o f a w e i g h t l o s s f r o m s o l u b i l i z a t i o n a n d a w e i g h t g a i n f r o m w a t e r a n d a c i d e n t r y i n t o t h e c o a t i n g . T h e o n l y w a y t o s e p a r a t e t h e t w o e f f e c t s i s t o m e a s u r e t h e w e i g h t c h a n g e o c c u r r i n g w i t h a f r e e f i l m o f t h e c o a t i n g a n d a s s u m e i t w i l l b e t h e s a m e a s a c o a t i n g o n a s u b s t r a t e ; t h i s was t h e t e c h n i q u e u s e d by F u n k e [ 5 ] . T h e r e i s much more l i k e l i h o o d o f a t t a c k o f c o a t i n g m a t e r i a l s i n a c i d s o l u t i o n s b e c a u s e o f i n c r e a s e d r a t e s o f h y d r o l y s i s r e a c t i o n s a t l o w pH*s.

T h e i n t e r p r e t a t i o n o f c o r r o s i o n p o t e n t i a l has a l w a y s been d i f f i c u l t . W o l s t e n h o l m e [ 3 ] c o n c l u d e d t h a t t h e c o r r o s i o n p o t e n t i a l was n o t a n u n a m b i g u o u s i n d i c a t o r o f t h e a m o u u n t o f c o r r o s i o n , a n d C e r i s o l a a n d B o n o r a [ 7 ] d e s c r i b e d t h e m e a s u r e m e n t a s o n e w i t h no q u a n t i t a t i v e r e l a t i o n s h i p t o a m o u n t o f c o r r o s i o n . T h e r e s u l t s s h o w n i n F i g u r e 2 c o n f i r m t h e q u e s t i o n a b l e v a l u e o f p o t e n t i a l m e a s u r e m e n t s i n c o r r e l a t i o n s w i t h t h e c o r r o s i o n o f an u n d e r l y i n g s u b s t r a t e .

The c o a t i n g c o n d u c t a n c e , on t h e o t h e r h a n d , has b e e n r e p o r t e d by a n u m b e r o f p e o p l e t o b e r e l a t e d t o t h e e x t e n t o f c o r r o s i o n u n d e r a c o a t i n g [ 8 - 1 1 ] . The c o n d u c t a n c e , e i t h e r AC o r DC, i s a f u n c t i o n o f t h e a m o u n t o f c h a r g e t h a t c a n p a s s t h r o u g h t h e c o a t i n g a n d t h i s a m o u n t o f c h a r g e i s a f u n c t i o n o f t h e a m o u n t o f a q u e o u s p h a s e i n t h e c o a t i n g t h a t p e r m i t s c h a r g e m o t i o n .

T h u s , i n a c i d s o l u t i o n i t a p p e a r s t h a t a n i m p o r t a n t p r o p e r t y o f a c o a t i n g f o r c o r r o s i o n p r o t e c t i o n i s i t s p e r m e a b i l i t y t o a c i d . T h i s v a r i a t i o n i n p e r m e a b i l i t y i s t h o u g h t t o b e t h e r e a s o n f o r t h e d i f f e r e n c e i n b e h a v i o r o f c o a t i n g s o b s e r v e d d u r i n g e x p o s u r e t o a c i d e n v i r o n m e n t s [ 1 2 J . The p e r m e a b i l i t y i s a l s o a f f e c t e d by t h e d e g r a d a t i o n o f t h e c o a t i n g as c a u s e d by r e a c t i o n w i t h t h e a c i d .

C o n c l u s i o n s

O f t h e f o u r t e c h n i q u e s s t u d i e d f o r e v a l u a t i n g c o a t i n g s on s t e e l f o r c o r r o s i o n c o n t r o l ( c o r r o s i o n p o t e n t i a l , c o n d u c t a n c e , a d h e s i o n a n d w e i g h t g a i n ) , t h e m o s t u s e f u l was c o n d u c t a n c e . C o r r o s i o n p o t e n t i a l d i d n o t show a c o n s i s t e n t r e l a t i o n s h i p , and w e i g h t g a i n and t e n s i l e a d h e s i o n s h o w e d no c o r r e l a t i o n w i t h c o r r o s i o n .

The b e s t p e r f o r m i n g c o a t i n g s s t u d i e d w e r e a v i n y l e s t e r , a b i s p h e n o l A e p o x y c u r e d w i t h an a l i p h a t i c a m i n e , a n d a n o v o l a c e p o x y c u r e d w i t h a m i x e d a r o m a t i c / c y c l o a l i p h a t i c a m i n e . A s a t u r a t e d p o l y e s t e r , and a b i s p h e n o l A e p o x y c u r e d w i t h a p o l y a m i d e a m i n e s n o w e d s i g n i f i c a n t d e t e r i o r a t i o n i n t h e a c i d and c o r r o s i o n o f t h e u n d e r l y i n g s t e e l . T w o t y p e s o f n o v o l a c e p o x i e s c u r e d w i t h a r o m a t i c a m i n e s s h o w e d i n t e r m e d i a t e p e r f o r m a n c e .

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7. WHITE A N D LEIDHEISER Evaluation of Coating Resins 85

A c k n o w l e d g m e n t s

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

Literature Cited

1. Fontana, M. G.; Greene, N. D. "Corrosion Engineering"; McGraw-Hill: New York, 1978; 2nd ed.

2. Leidheiser, H. Jr. Prog. Org. Coatings 1979, 7, 79-104. 3. Wolstenholme, J . Corr. Sci. 1973, 13, 521-30. 4. Mansfeld, F.; Kendig, M. W.; Tsai, S. Corrosion 1982, 38,

478-85. 5. Funke, W.; Haagen, H. Ind. Eng. Chem. Prod. Res. Dev. 1978,

17, 50-53. 6. White, M. L.; Paper given at ACS Meeting, Philadelphia, PA,

August 1984. Submitted to Ind. Eng. Prod. Res. Dev. for publication in 1986.

7. Cerisola, G.; Bonora, P. L. Mater. Chem. 1982, 7, 241-48. 8. D. J. Mills. In Coatings and Surface Treatment for Corro

sion and Wear Resistance; Strafford, K. N., Ed.; Horwood: Chichester, England, 1984; pp. 315-30.

9. Rajagopalan, K. S.; Guruviah, S.; Rajagopalan, C. S. J. Oil Col. Chem. Assoc. 1980, 63, 144-48.

10. Touhsaent, R. E.; Leidheiser, H. Jr. Corrosion 1972, 28, 435-40.

11. Vertere, V.; Rozados, E.; Carbonari, R. J. Oil Col. Chem. Assoc. 1978, 61, 419-26.

12. White, M. L.; Leidheiser, H. Jr., Materials Performance 1985, 24, 9-16.

RECEIVED January 27, 1986

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[acs symposium series] polymeric materials for corrosion control volume 322 || evaluation of coating...

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