Applied Polymer Science - ACS Publications - American Chemical

32 An Introduction to Corrosion Control by Organic Coatings R. A. DICKIE

Downloaded by EMORY UNIV on August 12, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch032

Ford Motor Company, Dearborn, MI 48121

Fundamentals of Corrosion Chemistry P r i n c i p a l Corrosion Reactions Theoretical Bases of Corrosion Control Types of Corrosion Phenomena Methods of Studying Corrosion Control Goals of Testing and C l a s s i f i c a t i o n of Test Methods Nonelectrochemical Methods Electrochemical and Electrical Methods B a r r i e r C h a r a c t e r i s t i c s of Coatings Adhesion of Organic Coatings Transport Properties of Coatings Other Film Properties Corrosion Resistance of Painted Metals Metal Surface Preparation and Conversion Coatings Corrosion Control by Pigments Effects of Resin Structure on Corrosion Control

The term "corrosion" has been applied to a wide variety of processes that have in common a degradation of the chemical or physical state of a material upon exposure to an aggressive environment. More commonly, the term i s restricted to chemical degradation of a metal by its environment. The technological and economic importance of corrosion, and hence of corrosion protection, are enormous. A National Bureau of Standards study (1) estimates that the total cost of corrosion i n the United States i n 1975 was $70 billion, about 4.2% of the gross national product; of t h i s amount, the study concludes that about 15% might be avoidable through the economic use of currently available technology. Organic coatings have long been used to provide corrosion protection. Smith (2) mentions the use of melted pitch by the Romans and goes on to describe reports from the early nineteenth century of the a p p l i c a t i o n of varnish to smoked s t e e l and of a composition of r e s i n , olive oil, and ground bricks used to protect a water carrier from rust. This chapter presents an introduction to the fundamentals of corrosion chemistry and t h e i r 0097-6156/85/0285-0773$07.75/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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APPLIED POLYMER SCIENCE

a p p l i c a t i o n to the understanding of c o r r o s i o n c o n t r o l by o r g a n i c coatings. The scope of the s u b j e c t p r e c l u d e s a comprehensive r e v i e w ; a d d i t i o n a l sources of i n f o r m a t i o n i n c l u d e w e l l - k n o w n compendia on c o r r o s i o n by Evans (3), U h l i g (4), and S h r e i r (J5), the p r e p r i n t and proceedings volumes of r e c e n t symposia (j>), and, of course, the p e r i o d i c a l l i t e r a t u r e (7). Many introductory treatments of corrosion and corrosion protection are a l s o a v a i l a b l e (e.g., 813). Leidheiser (14) has r e c e n t l y surveyed current research on the corrosion of painted metals.


Fundamentals of Corrosion P r i n c i p a l Corrosion Reactions. Organic coatings play many r o l e s i n c o r r o s i o n p r o t e c t i o n . A l t h o u g h the primary f u n c t i o n may be to prevent the onset of corrosion by i s o l a t i n g the substrate metal from the environment, c o n t r o l of the spread of corrosion from an i n i t i a l corrosion s i t e can a l s o be of c r i t i c a l importance. An understanding of the nature of c o r r o s i o n r e a c t i o n s i s a necessary f o u n d a t i o n to understanding t h e i r c o n t r o l . Metal/environment corrosion reactions are most often redox r e a c t i o n s o c c u r r i n g at a m e t a l / n o n m e t a l interface. The metal i s oxidized, and the nonmetal, u s u a l l y oxygen, i s reduced. I f the p r o d u c t O f the metal oxidation forms an adherent and i n e r t f i l m , the corrosion process may be s e l f - l i m i t i n g , and the metal may appear to be r e l a t i v e l y unreactive. I f the oxide does not form an adherent f i l m , or i f the m e t a l c o r r o s i o n products are s o l u b l e i n the c o r r o s i v e medium, m e t a l l i c corrosion proceeds. In general terras, the oxidation of a metal M can be written as M —> M

z +

+ ζ e"

and the reduction of a nonmetal Q can be written as z

Q + ζ e" —> Q ~ for the o v e r a l l corrosion process M + Q —> MQ The oxidation and reduction reactions occur at the same rate; there i s no net accumulation of charge. C o r r o s i o n can be represented i n terms of a s i m p l e e l e c t r o chemical c e l l as i l l u s t r a t e d i n Figure 1 for aqueous corrosion. In the electrochemical c e l l , the e l e c t r i c a l c i r c u i t i s completed by the transport of charge through the aqueous e l e c t r o l y t e . The source of p o t e n t i a l may be, f o r e x a m p l e , a d i f f e r e n c e i n e l e c t r o d e c o m p o s i t i o n . For m e t a l s exposed to a c o r r o s i v e environment, the anodic and c a t h o d i c r e a c t i o n s can occur at adjacent or w i d e l y separated s i t e s on a s i n g l e metal, or on different metals that are e l e c t r i c a l l y connected. As i n the e l e c t r o c h e m i c a l c e l l , the e l e c t r i c a l c i r c u i t i s t y p i c a l l y completed by an aqueous e l e c t r o l y t e ; sources of p o t e n t i a l i n c l u d e d i f f e r e n c e s i n e l e c t r o l y t e concentration and i n electrode composition.



32. DICKIE

775 An Introduction to Corrosion Control by Organic Coatings

CATIONS

®

I2É ANODE

ANIONS θ aqueous electrolyte

CATHODE

Figure 1,

aqueous electrolyte

ANODE

CATHODE

Schematic comparison of a c o r r o s i o n s i t e ( r i g h t ) and simple electrochemical c e l l ( l e f t ) .


776


T h e o r e t i c a l Bases of C o r r o s i o n C o n t r o l . This section provides a b r i e f r e v i e w of thermodynamic and k i n e t i c c o n s i d e r a t i o n s i n the c o n t r o l of c o r r o s i o n . S h r e i r (5) or one of the other g e n e r a l references (3-12) can be consulted for d e t a i l e d discussion. In thermodynamic terms, f o r the metal d i s s o l u t i o n r e a c t i o n to proceed, the f r e e energy change a s s o c i a t e d w i t h the o x i d a t i o n r e a c t i o n must be n e g a t i v e . In terms of the u s u a l c o n v e n t i o n , the free energy change associated with reduction of the metal ion formed by metal d i s s o l u t i o n must be p o s i t i v e . The free energy change AG i s given by


AG = -zeF where ζ i s the number of e l e c t r o n s i n v o l v e d i n the r e a c t i o n , F i s the Faraday constant, and E, the p o t e n t i a l , i s given by Ε « 0 - F e

+ 2

+ 2e~

F e r r i c s p e c i e s are formed i n subsequent h y d r o l y s i s and o x i d a t i o n reactions i n c l u d i n g , for example, the f o l l o w i n g : 2

Fe+ + H 0 —> FeOH* + H

+

2


Fe

+ 2

—> F e

+ 3

+ e"

In g e n e r a l , the pH at s i t e s of anodic d i s s o l u t i o n decreases. The process of i r o n corrosion i s i n fact extremely complicated; Pourbaix (15) g i v e s some 29 e q u i l i b r i a for the corrosion of i r o n i n aqueous media. The c a t h o d i c r e d u c t i o n r e a c t i o n s commonly observed are the hydrogen e v o l u t i o n reactions: H 0

+

+ e~ —> 1/2 H + H 0 (acid solutions)

3

2

2

H 0 + e~ —> 1/2 H + 0H~ (neutral/basic solutions) 2

2

and the oxygen reduction reactions: +

1/2 0

2

+ 2 H 0

1/2 0

2

+ H 0 + 2 e" —> 2 OH" (neutral/basic solutions)

3

+ 2 e " —> 3 H 0 (acid solutions) 2

2

In general, the pH increases at cathodic reaction s i t e s . As the pH increases, the hydrogen e v o l u t i o n reaction becomes k i n e t i c a l l y more d i f f i c u l t , and the oxygen reduction reaction more important. Oxygen t r a n s p o r t i s o f t e n r a t e l i m i t i n g , e s p e c i a l l y at h i g h r a t e s of corrosion i n aqueous s o l u t i o n s . The many corrosion e q u i l i b r i a i n v o l v i n g i r o n can be conveniently summarized i n an e q u i l i b r i u m or Pourbaix diagram (15). These diagrams map r e g i o n s of s t a b i l i t y f o r v a r i o u s i o n i c and s o l i d s p e c i e s as f u n c t i o n s of pH and E° on the b a s i s of e q u i l i b r i u m p o t e n t i a l s . A s i m p l i f i e d e q u i l i b r i u m diagram i s shown i n Figure 2. At s u f f i c i e n t l y n e g a t i v e p o t e n t i a l , i r o n i s t h e r m o d y n a m i c a l l y s t a b l e , and c o r r o s i o n cannot o c c u r . The domain of s t a b i l i t y of water i s defined by the dashed l i n e s a and b; below a, H 0 + e" —> 1/2 H + 0H~ 2

2

and above b, H 0 —> 1/2 0 2

2

+ 2 H+ + 2 e"

Because there i s no o v e r l a p of the domains of s t a b i l i t y f o r water and i r o n , iron i s normally susceptible to corrosion i n the presence of water and oxygen. At s u f f i c i e n t l y p o s i t i v e p o t e n t i a l and h i g h pH, passivation can be achieved by formation of a p r o t e c t i v e oxide l a y e r (anodic protection). Immunity from corrosion can be achieved


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32.

An introduction to Corrosion Control by Organic Coatings779

4

7

10

13

pH Figure 2.

S i m p l i f i e d equilibrium diagram for i r o n i n water. The dashed l i n e s a and b denote the l i m i t s of s t a b i l i t y of water. (Reproduced with permission from Ref. 15. Copyright 1966 Pergamon)



780


by l o w e r i n g the p o t e n t i a l ( c a t h o d i c p r o t e c t i o n ) — f o r example, by c o u p l i n g to a more r e a c t i v e m e t a l . A l t h o u g h the e q u i l i b r i u m diagrams are useful for q u a l i t a t i v e considerations of the corrosion of metals, they g i v e no i n d i c a t i o n of the rates of corrosion. They are v a l i d o n l y f o r the environment f o r which they have been c o n s t r u c t e d ; the diagram reproduced i n F i g u r e 2 does not, f o r example, take i n t o account the e f f e c t s of complexing agents or e l e c t r o l y t e s on the corrosion of i r o n . The description of corrosion k i n e t i c s i n electrochemical terms i s based on t h e use o f p o t e n t i a l - c u r r e n t d i a g r a m s and a c o n s i d e r a t i o n of p o l a r i z a t i o n e f f e c t s . The e q u i l i b r i u m or r e v e r s i b l e p o t e n t i a l s i n v o l v e d i n the construction of e q u i l i b r i u m diagrams assume that there i s no net transfer of charge (the anodic and c a t h o d i c c u r r e n t s are a p p r o x i m a t e l y z e r o ) . When the c u r r e n t f l o w i s not z e r o , the anodic and c a t h o d i c p o t e n t i a l s of the c o r r o s i o n c e l l d i f f e r from t h e i r e q u i l i b r i u m v a l u e s ; the anodic p o t e n t i a l becomes, more p o s i t i v e , and the cathodic p o t e n t i a l becomes more negative. The voltage difference, or p o l a r i z a t i o n , can be due to c e l l resistance (resistance p o l a r i z a t i o n ) ; to the depletion of a r e a c t a n t or the b u i l d - u p of a product at an e l e c t r o d e s u r f a c e ( c o n c e n t r a t i o n p o l a r i z a t i o n ) ; or to a s l o w step i n an e l e c t r o d e reaction ( a c t i v a t i o n p o l a r i z a t i o n ) . P o t e n t i a l c u r r e n t diagrams f o r the anodic and c a t h o d i c r e a c t i o n s i n v o l v e d i n a t y p i c a l metal d i s s o l u t i o n process are schematically i l l u s t r a t e d i n Figure 3A. As the imposed current i s i n c r e a s e d , the d i f f e r e n c e between the observed and e q u i l i b r i u m p o t e n t i a l s (the electrochemical p o l a r i z a t i o n ) becomes progressively l a r g e r . In the absence of resistance and concentration effects, the observed p o l a r i z a t i o n , termed a c t i v a t i o n p o l a r i z a t i o n , i s due to a slow step i n an electrode process. A c t i v a t i o n p o l a r i z a t i o n i s an i n t r i n s i c f e a t u r e of e l e c t r o d e processes at nonzero current flow. B e c a u s e t h e r e i s no net a c c u m u l a t i o n of c h a r g e d u r i n g an e l e c t r o c h e m i c a l r e a c t i o n , the t o t a l anodic and c a t h o d i c c u r r e n t s must be equal. In Figure 3A, t h i s s i t u a t i o n occurs at the point at which the anodic and c a t h o d i c c u r v e s c r o s s ; the magnitude of the c u r r e n t at t h i s p o i n t p r o v i d e s a measure of the r a t e of c o r r o s i o n . I f the medium has e l e c t r i c a l resistance, R, the anodic and cathodic p o t e n t i a l s w i l l d i f f e r by IR, and the c u r r e n t ( i . e . , the r a t e of c o r r o s i o n ) w i l l be reduced, as i l l u s t r a t e d i n F i g u r e 3B. I f the a v a i l a b i l i t y of, for example, the cathodic reactant i s l i m i t e d by d i f f u s i o n ( c o n c e n t r a t i o n p o l a r i z a t i o n ) , as often occurs i n the aqueous c o r r o s i o n of m e t a l s , a r e d u c t i o n i n c u r r e n t i s a l s o observed. In s o l u t i o n , the c o n c e n t r a t i o n p o l a r i z a t i o n can be reduced, and the corrosion current increased, by s t i r r i n g . Thus, i n F i g u r e 3C, under d i f f u s i o n - c o n t r o l l e d c o n d i t i o n s , the c o r r o s i o n current i s I; with s t i r r i n g the current increases, for example, to I. The maximum current achievable by suppression of concentration p o l a r i z a t i o n i s I". Concentration p o l a r i z a t i o n can be c o n t r a s t e d w i t h the e f f e c t of a change i n a c t i v i t y : i f the e q u i l i b r i u m p o t e n t i a l i s i n c r e a s e d (e.g., by employing oxygen i n s t e a d of a i r ) , the corrosion rate w i l l a l s o increase (Figure 3D). This effect i s , however, thermodynamic rather than k i n e t i c . F i n a l l y , the question of g a l v a n i c corrosion a l s o needs to be addressed. For two metals Ν and M, as i l l u s t r a t e d i n F i g u r e 4 (adapted from Ref. 16), the i n d i v i d u a l corrosion rates are given by I(M) and I(N); M i s the more 1


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32.

Figure 3.

Idealized potential-log(current) diagrams i l l u s t r a t i n g c e l l p o l a r i z a t i o n effects. (Reproduced with permission from Ref. 10. Copyright 1975 Pergamon)


782



LOG (CURRENT) Figure 4.

Idealized potential-log(current) diagram for a galvanic couple. (Reproduced with permission from Ref. 16. Copyright 1978 Society of Automotive Engineers)



32.

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a c t i v e of the two m e t a l s . When c o u p l e d , the system assumes p o t e n t i a l E(M:N), and the i n d i v i d u a l metal c o r r o s i o n r a t e s s h i f t such t h a t Ν c o r r o d e s at a r a t e I(M:N_) < I(N), and M corrodes at a r a t e I(M:N) > I(M). G e n e r a l l y , the more a c t i v e metal becomes the anode, and t h e l e s s a c t i v e one becomes t h e c a t h o d e , w i t h corresponding changes i n t h e i r corrosion rates. Some of these concepts of the k i n e t i c s of metal c o r r o s i o n are f u r t h e r i l l u s t r a t e d i n F i g u r e 5 f o r i r o n i n an aqueous s a l i n e medium. At pH 6.5 i n the presence of oxygen, the dominant cathodic r e a c t i o n i s the r e d u c t i o n of oxygen to form h y d r o x i d e . The e s s e n t i a l l y v e r t i c a l cathodic curve indicates that the reaction i s diffusion c o n t r o l l e d . At higher oxygen concentration and with good s t i r r i n g , the rate of corrosion would be s u b s t a n t i a l l y higher than shown. I f , however, the s t e e l e l e c t r o d e were to be h e l d a t p o t e n t i a l s below about -800 mV (vs. SCE), no i r o n d i s s o l u t i o n would occur. At high pH, the s i t u a t i o n i s very different, even though the dominant c a t h o d i c r e a c t i o n remains the formation of h y d r o x i d e . Below about -1000 mV ( v s . SCE), the exact v a l u e depending on pH, t h e r e i s no i r o n d i s s o l u t i o n ; t h i s s i t u a t i o n corresponds to the immune r e g i o n of the Pourbaix diagram. Between a p p r o x i m a t e l y 600 and -700 mV (vs. SCE), the rate of corrosion i s very low because of the f o r m a t i o n of a p a s s i v e o x i d e f i l m ; t h i s r e g i o n corresponds to the p a s s i v e r e g i o n of the Pourbaix diagram. During n a t u r a l corrosion, anodic and cathodic reactions are p o l a r i z e d to the same p o t e n t i a l , save f o r r e s i s t a n c e e f f e c t s . When anodic and c a t h o d i c s i t e s become separated on the s u r f a c e , the pH of c a t h o d i c s i t e s i n c r e a s e s , and the c a t h o d i c areas become p a s s i v a t e d (no v i s i b l e c o r r o s i o n occurs i n these a r e a s ) . I r o n d i s s o l u t i o n c o n t i n u e s at anodic s i t e s which tend, as noted before, to become a c i d i c . Organic coatings c o n t r o l corrosion by resistance i n h i b i t i o n , by s u p p r e s s i o n of the anodic r e a c t i o n , and by s u p p r e s s i o n of the c a t h o d i c r e a c t i o n . I s o l a t i o n of the r e a c t i v e s u b s t r a t e from i t s environment by a h i g h - r e s i s t a n c e b a r r i e r reduces the r a t e of the corrosion reactions and i s probably the most general mechanism of corrosion protection afforded by paint f i l m s . Resistance i n h i b i t i o n i s a f f e c t e d by the presence of e l e c t r o l y t e s i n the p a i n t f i l m , or beneath i t , by p e n e t r a t i o n by water, and by the adhesion and mechanical i n t e g r i t y of the f i l m . Organic c o a t i n g s a l s o a c t as r e s e r v o i r s for a c t i v e metal pigments and for corrosion i n h i b i t o r s . Suppression of the anodic r e a c t i o n i s a c h i e v e d by p i g m e n t a t i o n i n some coatings, for example, i n paints that supply electrons, such as z i n c - r i c h paints, and i n paints that help maintain a passive oxide f i l m , for example chromate-pigraent-containing p a i n t s . Suppression of the cathodic reaction i s achieved by i s o l a t i n g the substrate from the c a t h o d i c r e a c t a n t s . I n o r g a n i c c o n v e r s i o n c o a t i n g s a c t to c o n t r o l the r a t e of c o r r o s i o n i n p a r t by s u p p r e s s i n g the c a t h o d i c reduction of oxygen. Types of Corrosion Phenomena. The major categories of phenomena (5) i n c l u d e uniform, l o c a l i z e d , and p i t t i n g c o r r o s i o n ; s e l e c t i v e d i s s o l u t i o n ; and c o r r o s i o n a c t i n g together w i t h a mechanical phenomenon. In uniform c o r r o s i o n , a l l areas corrode at the same rate. Examples of uniform corrosion include tarnishing and a c t i v e d i s s o l u t i o n of metals i n acids. In l o c a l i z e d corrosion some areas corrode more r e a d i l y than o t h e r s ; c r e v i c e c o r r o s i o n and f i l i f o r m



Figure 5.

Comparison of i r o n corrosion k i n e t i c s at neutral and a l k a l i n e pH. (Reproduced with permission from Ref. 19. Copyright 1968 Journal of Coatings Technology)



32. DICKIE


corrosion are examples. P i t t i n g i s severe l o c a l i z e d corrosion, and may be exemplified by the p i t t i n g of s t a i n l e s s s t e e l i n the presence of c h l o r i d e i o n . S e l e c t i v e d i s s o l u t i o n i s t y p i c a l l y a problem encountered with a l l o y s , for example, the d e z i n c i f i c a t i o n of brass. F i n a l l y , the combined action of corrosion and a mechanical factor, as i n stress corrosion cracking, may i n v o l v e accelerated l o c a l i z e d c o r r o s i v e a t t a c k or mechanical f r a c t u r e due to the s y n e r g i s t i c a c t i o n of c o r r o s i o n and a p p l i e d s t r e s s . A l t h o u g h a l l of these c a t e g o r i e s are of t e c h n o l o g i c a l importance, and a l l may be a l l e v i a t e d i n some measure by p r o p e r l y s e l e c t e d and a p p l i e d c o a t i n g s , l o c a l i z e d c o r r o s i o n phenomena a r e o f p a r t i c u l a r importance. Furthermore, an understanding of l o c a l i z e d corrosion phenomena and the formation of separated anodic and cathodic s i t e s on metal surfaces i s c e n t r a l to understanding paint performance i n corrosion p r o t e c t i o n . Important f a c t o r s i n f l u e n c i n g l o c a l i z e d c o r r o s i o n phenomena include the formation and/or presence of s o l i d corrosion products, and the effect of such products on oxygen a v a i l a b i l i t y ; changes i n pH at anodic and cathodic reaction s i t e s ; and the r a t i o of cathodic to anodic reaction s i t e area. At the onset of corrosion, the anodic and cathodic reaction s i t e s are l i k e l y to be c l o s e together, and may occur at the same s i t e s s e q u e n t i a l l y ; as a d e p o s i t of c o r r o s i o n products forms, the a v a i l a b i l i t y of oxygen for the cathodic reaction may be l o c a l l y r e s t r i c t e d , and thus the anodic r e a c t i o n w i l l be f a v o r e d . With subsequent changes i n pH and an i n c r e a s e i n the concentration of s o l u b l e corrosion products i n anodic areas, oxygen a v a i l a b i l i t y can become more r e s t r i c t e d and cause further separation and l o c a l i z a t i o n of reaction s i t e s . Figure 6 (adapted from Ref. 17) i l l u s t r a t e s a c l a s s i c experiment t h a t demonstrates the s e p a r a t i o n and l o c a l i z a t i o n of anodic and cathodic corrosion s i t e s . A drop of s a l t water containing s m a l l amounts of phenolphthalein and potassium ferricyanide as i n d i c a t o r s i s placed on an i r o n surface. At f i r s t , there are random spots of blue (anodic s i t e s : ferricyanide i n the presence of ferrous ion) and pink (cathodic s i t e s : phenolphthalein i n the presence of h y d r o x i d e ) . I f a l l o w e d to remain u n d i s t u r b e d , rings of c o l o r e v e n t u a l l y form, blue at the center, then rust, and f i n a l l y pink at the perimeter. The drop i s an oxygen concentration c e l l , the a v a i l a b i l i t y of oxygen f o r the c a t h o d i c r e a c t i o n being s l i g h t l y greater at the edges than i n the center. The c o r r o s i o n phenomena commonly observed on p a i n t e d m e t a l s include cathodic d e l a m i n a t i o n , anodic u n d e r c u t t i n g , and f i l i f o r m corrosion. Cathodic delamination r e s u l t s when the a l k a l i produced by the c a t h o d i c c o r r o s i o n r e a c t i o n d i s r u p t s the p a i n t - m e t a l interface. This phenomenon has long been observed on c a t h o d i c a l l y p r o t e c t e d p a i n t e d s t e e l (18) and has a l s o been demonstrated to be r e s p o n s i b l e f o r the l o s s of p a i n t adhesion t h a t often occurs adjacent to corrosion s i t e s on painted s t e e l (19). The l o c a l i z a t i o n and s e p a r a t i o n of anodic and c a t h o d i c s i t e s a s s o c i a t e d w i t h c o r r o s i o n at a break i n a p a i n t f i l m on s t e e l are s c h e m a t i c a l l y i l l u s t r a t e d i n Figure 7. Anodic u n d e r c u t t i n g does not s i g n i f i c a n t l y c o n t r i b u t e to the l o s s of p a i n t adhesion from s t e e l under normal aqueous c o r r o s i o n c o n d i t i o n s ; e v e n when p a i n t e d s t e e l has been s u b j e c t e d t o s u b s t a n t i a l anodic c u r r e n t s , l i t t l e or no adhesion l o s s has been observed (19-21). Anodic u n d e r c u t t i n g has been r e p o r t e d to be of



786


CATHODE Figure 6.

Figure 7.

ANODE

CATHODE

Schematic i l l u s t r a t i o n of corrosion i n a drop of s a l t water. (Reproduced with permission from Ref. 17. Copyright 1979 Elsevier-North Holland)

I l l u s t r a t i o n of d e l a m i n a t i o n mechanism a t a c o r r o s i o n s i t e on painted s t e e l .



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importance f o r aluminum (22). L e i d h e i s e r (14) c i t e s as an outstanding example the d i s s o l u t i o n of the t i n coating between the o r g a n i c c o a t i n g and the s t e e l i n a food c o n t a i n e r ; as the t i n d i s s o l v e s , the coating separates from the substrate and loses i t s p r o t e c t i v e character. F i l i f o r m c o r r o s i o n i s c h a r a c t e r i z e d by formation of i n t e r c o n necting filaments of corrosion under a paint f i l m upon exposure to a humid environment. F i l i f o r m corrosion t y p i c a l l y occurs only when the r e l a t i v e h u m i d i t y exceeds about 65%. The mechanism i s complicated and has been the subject of considerable discussion i n the l i t e r a t u r e (_5, 14, 21_, 23, 24). Basically, a localized c o r r o s i o n mechanism i s r e s p o n s i b l e ( F i g u r e 8). The head of the growing filament i s anodic, and as a r e s u l t the f i l i f o r m corrosion process has been termed a s p e c i a l i z e d form of anodic undermining (14). N e v e r t h e l e s s , the i n i t i a l d i s r u p t i o n of p a i n t adhesion i s most l i k e l y a c a t h o d i c d e l a m i n a t i o n (21). Perhaps the most i n t e r e s t i n g question—the o r i g i n of the f i l a m e n t a r y form—has not been answered with c e r t a i n t y . Methods of Studying Corrosion Control G o a l s of T e s t i n g and C l a s s i f i c a t i o n of Test Methods. The goals of corrosion testing are twofold: f i r s t , to evaluate the performance (and predict s e r v i c e h i s t o r y ) of e x i s t i n g materials, and second, to d i r e c t the development of improved p a i n t m a t e r i a l s and processes. The e v a l u a t i o n of performance g e n e r a l l y r e l i e s on t e s t s that compare the performance of materials i n simulated and accelerated s e r v i c e environments. The i d e a l t e s t environment i s a r e a l i s t i c one, with a l l r e l e v a n t r e a c t i o n s and p r o c e s s e s e q u a l l y a c c e l e r a t e d . L a b o r a t o r y t e s t s o f t e n do not l i v e up to t h i s i d e a l , however, and c o n s i d e r a b l e c a u t i o n must be e x e r c i s e d i n the i n t e r p r e t a t i o n of r e s u l t s . Performance t e s t r e s u l t s are g e n e r a l l y evaluated by type ( q u a l i t a t i v e ) and extent ( q u a n t i t a t i v e ) of f a i l u r e . To d i r e c t development of improved paint materials and processes, t e s t s based on an understanding of basic mechanisms of corrosion protection and f a i l u r e are arguably more u s e f u l . Test environments i n t h i s case may be chosen to i s o l a t e i n d i v i d u a l f a c t o r s , and the t e s t r e s u l t s are e v a l u a t e d by comparison w i t h standards and/or t h e o r e t i c a l models. Test methods have been d i s c u s s e d e x t e n s i v e l y i n the l i t e r a t u r e ; general reviews have been given by s e v e r a l authors (13, 25-27); electrochemical test methods have received s p e c i a l attention (28-31). Nonelectrochemical Methods. Nonelectrochemical methods of studying c o r r o s i o n i n c l u d e exposure t e s t s of performance and primary f i l m property measurements. Standard exposure tests include s a l t water immersion (3-5% aq. N a C l , u s u a l l y at room temperature, sometimes oxygen s a t u r a t e d ) ; c y c l i c immersion (e.g., s a l t water immersion alternated with drying periods); s a l t fog or spray (5% aq. NaCl fog, e.g., a c c o r d i n g to ASTM B117); the CASS t e s t (5% aq. NaCl fog w i t h CuClo and a c e t i c a c i d , according to ASTM B368); the Kesternich t e s t (100% r e l a t i v e h u m i d i t y to which S 0 i s added; c f . DIN 50018); and S 0 modified s a l t fog (ASTM B117 with S 0 added). C y c l i c exposure t e s t s t h a t i n c o r p o r a t e freeze-thaw and d i r t exposure have been proposed; these t e s t s are more c o m p l i c a t e d than c o n v e n t i o n a l 2

2

2



788


laboratory corrosion t e s t s , but have been reported to g i v e better agreement w i t h c o r r o s i o n performance i n s e r v i c e (32). Useful p r e l i m i n a r y t e s t s are water or humidity r e s i s t a n c e (e.g., by water immersion, s t a t i c h u m i d i t y , or condensing humidity exposure) and, for many a p p l i c a t i o n s , p h y s i c a l resistance tests such as abrasion, stone peck, mandrel bend, and impact t e s t s . Standard methods f o r most of these t e s t s are d e s c r i b e d i n the Annual Book of ASTM Standards (33). The r e s u l t s of exposure t e s t s are t y p i c a l l y the product of many p h y s i c a l and c h e m i c a l i n t e r a c t i o n s w i t h the t e s t environment and r e f l e c t combinations of f i l m p r o p e r t i e s , so t h a t i n t e r p r e t a t i o n of test r e s u l t s i n molecular or mechanistic terms i s at best d i f f i c u l t . Primary f i l m property measurements include measures of adhesion, of p e r m e a b i l i t y , of m e c h a n i c a l p r o p e r t i e s , and of c h e m i c a l resistance. C o a t i n g adhesion i s a p r e r e q u i s i t e f o r c o r r o s i o n protection, and i s assessed by a v a r i e t y of methods i n c l u d i n g tape adhesion (ASTM D3359), scrape and p a r a l l e l groove adhesion (ASTM D2197), and d i r e c t p u l l - o f f . T e s t i n g adhesion w i t h a d h e s i v e tape p r o v i d e s a q u a l i t a t i v e measure of p a i n t adhesion; the method i s o f t e n employed at the end of an exposure t e s t or a f t e r c o n t r o l l e d mechanical damage to the f i l m (e.g., s c r i b i n g a g r i d , or subjecting the p a i n t e d specimen to an impact or stone impingement t e s t ) . The d i r e c t p u l l - o f f method i n v o l v e s adhesive bonding of a test button to the sample; a s p e c i a l t o o l or a conventional t e n s i l e t e s t i n g machine w i t h a s p e c i a l f i x t u r e may then be used to measure the f o r c e r e q u i r e d to remove the t e s t b u t t o n . Adhesion of p a i n t f i l m s when dry i s not, however, g e n e r a l l y a r e l i a b l e i n d i c a t o r of adhesion upon exposure to a humid environment (34). Both the tape adhesion and d i r e c t p u l l - o f f methods have drawbacks i n a p p l i c a t i o n to measurement of the adhesion of f i l m s during or immediately f o l l o w i n g exposure to water; a comparative a b s o r p t i o n method has been proposed by Funke and coworkers (25-26, 35). Many other t e c h n i q u e s — s c r a t c h or s t y l u s , u l t r a s o n i c , u l t r a c e n t r i f u g a l , b l i s t e r , and abrasion—are d e s c r i b e d i n the l i t e r a t u r e (36, 37). A c o u s t i c e m i s s i o n has been used to study the e f f e c t s of water immersion on c o a t i n g s and to d i s t i n g u i s h between f a i l u r e mechanisms (38-40). Permeability of paint f i l m s to water, oxygen, and ions has been s t u d i e d by many workers w i t h a v a r i e t y of methods i n c l u d i n g v o l u m e t r i c , g r a v i m e t r i c , and c a p a c i t a n c e methods f o r water; v o l u m e t r i c , b a r o m e t r i c , and oxygen e l e c t r o d e based methods f o r oxygen; and r a d i o a c t i v e t r a c e r , c o n d u c t i v i t y , and i o n s e n s i t i v e e l e c t r o d e s f o r i o n s (25, 41, 42; see a l s o 14, 13, 43, and the references c i t e d therein). E l e c t r o c h e m i c a l and E l e c t r i c a l Methods. E l e c t r o c h e m i c a l and e l e c t r i c a l methods f o r s t u d y i n g f i l m p r o p e r t i e s and c o r r o s i o n phenomena have been e x t e n s i v e l y reviewed (29-31). Comparisons of c o r r o s i o n t e s t r e s u l t s w i t h d i r e c t c u r r e n t measurements o f conductivity suggest that v i s i b l e corrosion i s associated with f i l m resistance l e s s than about 1 Mohm/cm » but t h i s condition may w e l l correspond with the occurrence of v i r t u a l pores i n the f i l m a l l o w i n g development of l o c a l c o n d u c t i v e pathways. In s t u d i e s of the e q u i v a l e n t a l t e r n a t i n g c u r r e n t r e s i s t a n c e as a f u n c t i o n of frequency, Kendig and Leidheiser (44) found that the development of a region of slope -1 on a l o g p e r m i t t i v i t y versus l o g frequency p l o t



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( i n d i c a t i v e of l o c a l p e n e t r a t i o n by c o n d u c t i n g e l e c t r o l y t e ) corresponded with the onset of v i s i b l e corrosion. In studies of the capacitance of coating f i l m s , Touhsaent and Leidheiser (45) observed increases i n capacitance due to the uptake of water; i n t e r p r e t a t i o n of c a p a c i t a n c e data r e l i e s on somewhat a r b i t r a r y models f o r the d i s t r i b u t i o n of water, and i s t h e r e f o r e n e c e s s a r i l y somewhat u n c e r t a i n . L e i d h e i s e r has d i s c u s s e d the t o p i c at l e n g t h (14, 29). Impedance measurements have been d i s c u s s e d (31); the p r o g r e s s i v e d e t e r i o r a t i o n of p a i n t f i l m s can be monitored by s t u d y i n g the changes i n the frequency dependence of impedance. Corrosion p o t e n t i a l measurements g i v e an i n d i c a t i o n of the changes i n the r a t i o of anodic to c a t h o d i c s u r f a c e a r e a ; movement i n the a c t i v e d i r e c t i o n suggests an increase i n the anode/cathode r a t i o , with an increase i n the rate of metal d i s s o l u t i o n (46). P o l a r i z a t i o n curves have been determined for coated metals, but the flow of current i s l i k e l y to affect the properties of the coating-metal interface (29). B a r r i e r C h a r a c t e r i s t i c s of Coatings Adhesion of Organic Coatings. Although i t i s i n p r i n c i p l e p o s s i b l e for a nonadherent f i l m to provide good b a r r i e r properties, p r a c t i c a l p a i n t systems must n o r m a l l y remain adherent to the m a t e r i a l s to which they are a p p l i e d f o r e f f e c t i v e c o r r o s i o n p r o t e c t i o n to be m a i n t a i n e d . The i n i t i a l s t r e n g t h of the a d h e s i v e bond between c o a t i n g and s u b s t r a t e need not be p a r t i c u l a r l y h i g h . What i s important i s that after environmental exposure the coating should s t i l l be able to withstand the forces exerted on i t i n i t s intended s e r v i c e a p p l i c a t i o n . The adhesion of v i r t u a l l y a l l c o a t i n g s i s a d v e r s e l y a f f e c t e d by exposure to water or humid environments. Walker (34) found t y p i c a l o r g a n i c c o a t i n g s to have i n i t i a l dry adhesion of 20 to 40 MPa as measured i n a d i r e c t p u l l - o f f t e s t ; a f t e r more or l e s s p r o l o n g e d exposure to humid environments, the adhesion was observed to drop to 5 to 15 MPa. Exposure at 65% r e l a t i v e humidity or l e s s was found to have l i t t l e or no effect on adhesion. Funke and coworkers (35) have compared the moisture absorption c h a r a c t e r i s t i c s of free and supported p a i n t f i l m s , and have found that f i l m s that l o s e adhesion upon exposure to humid environments tend to absorb more water as supported f i l m s than as unsupported ones. These r e s u l t s are i n t e r p r e t e d i n terras of a c c u m u l a t i o n of moisture within an i n t e r f a c i a l region. I t should be noted that the moisture absorption r e s u l t s are subject to pronounced scatter, and r e l a t i v e l y l i t t l e weight can be attached to i s o l a t e d measurements. The scatter i n the r e s u l t s appears to be a property of v a r i a t i o n s i n f i l m properties, rather than an a r t i f a c t of the experimental method (47). T r a n s p o r t P r o p e r t i e s of C o a t i n g s . There has been s u b s t a n t i a l disagreement o v e r the years as to the t r a n s p o r t of water, oxygen, and i o n i c s p e c i e s through p a i n t f i l m s . I t has been s t a t e d t h a t under normal c o n d i t i o n s , p a i n t f i l m s are s a t u r a t e d w i t h water f o r about h a l f t h e i r u s e f u l l i f e , and f o r the remainder the water content corresponds with an atmosphere of high humidity. The water permeation r a t e of c o a t i n g s i s s u f f i c i e n t l y h i g h t h a t c o r r o s i o n c o u l d proceed on s t e e l s u r f a c e s as f a s t as i f the s t e e l were not


790


coated were there no other l i m i t i n g factors. Mayne (41) estimated t h a t f r e e l y c o r r o d i n g unpainted s t e e l consumes water at a r a t e of 0.4 to 6 χ 1 0 ~ g/cm^d. The water t r a n s m i s s i o n r a t e s of o r g a n i c f i l m s 0.1 mm t h i c k t y p i c a l l y range from 0.5 to 5 χ 10~" g / c m d . Oxygen transmission rates are approximately two orders of magnitude lower. Freely corroding unpainted s t e e l consumes oxygen at a rate about 1 to 15 χ 10~ g / c m d . Thus, oxygen t r a n s m i s s i o n and consumption rates are nearly comparable, and i t has been argued that the rate of oxygen transport i s rate l i m i t i n g for the corrosion of p a i n t e d s t e e l (35). Other c a l c u l a t i o n s (48) suggest, however, that—at l e a s t for t h i n (15 pm) paint films—there i s ample oxygen a v a i l a b l e f o r the observed r a t e s of c o r r o s i o n , and t h a t other l i m i t i n g factors (e.g., the d i f f u s i o n of cations to cathodic s i t e s ) must be operable. One other factor i s l i k e l y the high i o n i c resistance of organic c o a t i n g s , which tends to p r e v e n t the f o r m a t i o n of a complete e l e c t r o l y t i c c e l l ( c . f . the p r e v i o u s d i s c u s s i o n of r e s i s t a n c e inhibition). Conduction i n polymer f i l m s i s i o n i c and can be increased by the presence of underfilm e l e c t r o l y t e s ; by ionogenic groups i n the coating; and by water and e l e c t r o l y t e s external to the p a i n t f i l m (41). U n d e r f i l m e l e c t r o l y t e s tend to promote osmotic b l i s t e r i n g and thus r e s u l t i n underfilm corrosion and l o s s of paint adhesion. P a i n t i n g of r u s t y s u r f a c e s i s bad p r a c t i c e i n p a r t because of the e l e c t r o l y t e s embedded i n r u s t d e p o s i t s . Ionogenic groups i n c l u d e s o l u b l e pigment components and i o n i z a b l e r e s i n f u n c t i o n a l groups. These groups can r e s u l t i n i o n exchange processes and e v e n t u a l l y reduce f i l m r e s i s t a n c e and promote transport of e l e c t r o l y t e s through the f i l m (42, 49). A l t h o u g h both water and e l e c t r o l y t e can p e n e t r a t e f i l m s , e l e c t r o l y t e tends to penetrate f i l m s only l o c a l l y , as evidenced by s t u d i e s of the e l e c t r o l y t i c r e s i s t a n c e of free f i l m s (41, 50, 51). The regions of d i r e c t e l e c t r o l y t e penetration have been shown to be s m a l l , l o c a l i z e d , and randomly d i s t r i b u t e d . Furthermore, these r e g i o n s have been demonstrated to correspond to i n i t i a l s i t e s of corrosion. The m o l e c u l a r o r i g i n of the d i r e c t e l e c t r o l y t e p e n e t r a t i o n r e g i o n s i s not e n t i r e l y c l e a r , but i n some cases they have been associated with inhomogeneous c r o s s - l i n k i n g . P i g m e n t a t i o n can a l s o have a profound e f f e c t on the t r a n s p o r t properties of organic coatings. Schematically i l l u s t r a t e d i n Figure 9 i s the effect of pigment volume concentration (PVC) on b l i s t e r i n g , r u s t i n g , and p e r m e a b i l i t y . At low PVC, pigment i s f u l l y wet by r e s i n , and at h i g h PVC, t h e r e remain u n f i l l e d i n t e r s t i c e s between pigment p a r t i c l e s . The t r a n s i t i o n between these regimes i s centered on the c r i t i c a l pigment volume c o n c e n t r a t i o n (CPVC). The CPVC i s the formula pigment volume concentration above which a s i g n i f i c a n t v o i d f r a c t i o n i s present i n the f i l m because of a d e f i c i e n c y of binder r e s i n . Many f i l m properties i n addition to those i l l u s t r a t e d i n F i g u r e 9 change s u b s t a n t i a l l y at the CPVC, i n c l u d i n g t e n s i l e strength, s t a i n penetration, e x t e r i o r d u r a b i l i t y , enamel hold-out, hiding power, and g l o s s (52-54). Despite these manifold changes i n p r o p e r t i e s , d e t e r m i n a t i o n of the CPVC by o b s e r v a t i o n of the v a r i a t i o n i n f i l m performance tends to be u n c e r t a i n . Methods of determining CPVC based on density determinations have been described (53, 54). B a r r i e r and i n h i b i t i v e primers are best formulated at PVC < CPVC, i n t h e r a n g e P V C / C P V C = 0.8 t o 0.9 ( 5 5 ) . Moisture 5

J


5

2


z


32. DICKIE

An Introduction to Corrosion Control by Organic Coatings791

(CROSS SECTION)

Figure 8.

Schematic i l l u s t r a t i o n of f i l i f o r m c o r r o s i o n . (Repro duced with permission from Ref. 4. Copyright 1971 Wiley)

Ο >

RUSTING y* PERMEABILITY

Figure 9.

Dependence of paint f i l m properties on pigment volume concentration. (Reproduced from Ref. 52. Copyright 1949 American Chemical Society)



792


a d s o r p t i o n or a b s o r p t i o n by pigments, pigment p a r t i c l e s i z e d i s t r i b u t i o n and p a r t i c l e aggregation, pigment p a r t i c l e shape, and reactions of pigments to form products of different s o l u b i l i t y or volume are complicating factors. M o i s t u r e e x c l u s i o n , a d s o r p t i o n , and permeation are s t r o n g l y affected by pigment-binder and pigment-water i n t e r a c t i o n . Strong pigment-binder i n t e r a c t i o n tends to r e s u l t i n r e s t r i c t e d molecular m o b i l i t y , reduced s w e l l i n g , and reduced water transmission through the pigmented f i l m . Strong water-pigment i n t e r a c t i o n tends to r e s u l t i n i n c r e a s e d water update by the f i l m and i n c r e a s e d f i l m p e r m e a b i l i t y , e s p e c i a l l y at h i g h PVC. Below the CPVC, s t r o n g pigment-binder i n t e r a c t i o n e f f e c t i v e l y increases the volume f r a c t i o n i n a c c e s s i b l e to water. Strong water-pigment i n t e r a c t i o n eases the passage of water (the d i f f u s i o n c o e f f i c i e n t i s o f t e n s t r o n g l y c o n c e n t r a t i o n dependent). Above the CPVC, u n f i l l e d i n t e r s t i c e s a l l o w passage through the f i l m (56). S i z e d i s t r i b u t i o n and p a r t i c l e shape w i l l a f f e c t the way t h a t pigment p a r t i c l e s can pack, and hence w i l l affect the CPVC. Large aggregates t h a t are not broken up and wet out d u r i n g pigment grinding can r e s u l t i n the presence of pores i n the pigmented f i l m to mimic the e f f e c t of p i g m e n t a t i o n above the CPVC. F l a k e or micaceous pigments properly employed can s i g n i f i c a n t l y reduce the p e r m e a b i l i t y of the p a i n t f i l m , as can the presence of c h e m i c a l l y r e a c t i v e pigments (such as zinc dust) that form voluminous corrosion products (41, 56). Other F i l m Properties. The mechanical properties of polymer f i l m s are dominated by the g l a s s t r a n s i t i o n phenomenon. The g l a s s t r a n s i t i o n temperature, Τ , i s t h a t temperature at which the behavior of a polymer changes from g l a s s l i k e (high modulus, often b r i t t l e ) to r u b b e r l i k e (low modulus, f l e x i b l e ) . At temperatures below Tg, the m o l e c u l a r c h a i n s are i m m o b i l e , and at temperatures above Τ , the molecular chains become i n c r e a s i n g l y mobile u n t i l ( i n the absence of c r o s s - l i n k s ) flow begins. An increase i n the T of a coating f i l m tends to reduce i t s permeability, but may r e s u l t i n the f i l m having a higher l e v e l of i n t e r n a l stress and being more prone to cracking and fracture. Coatings can, of course, suffer many forms of degradation other than c o r r o s i o n and mechanical damage. P h o t o o x i d a t i v e processes, t y p i c a l l y most s e v e r e upon exposure to UV l i g h t (290-400 nm) can r e s u l t i n bond r u p t u r e and thereby l e a d to a l o s s of g l o s s and e v e n t u a l l y a l o s s of f i l m i n t e g r i t y . Control of these processes i s a c h i e v e d by proper c h o i c e of r e s i n pigment, and p h o t o s t a b i l i z e r s . Many r e s i n s a r e prone t o h y d r o l y s i s , e s p e c i a l l y a t h i g h temperatures. The r e s u l t can be an increase i n r e s i n p o l a r i t y and permeability to e l e c t r o l y t e . Control i s p r i n c i p a l l y by r e s i n design for g r e a t e r c h e m i c a l r e s i s t a n c e . B i o l o g i c a l processes (e.g., mildew) are a l s o of concern; c o n t r o l i s achieved p r i n c i p a l l y by the use of b i o l o g i c a l l y a c t i v e a d d i t i v e s ( b i o c i d e s and b i o s t a t s ) . Miscellaneous environmental factors i n c l u d i n g i n d u s t r i a l p o l l u t i o n , high temperature, and unvented fumes may play a s i g n i f i c a n t r o l e i n coating degradation i n s p e c i f i c a p p l i c a t i o n s . g


32. DICKIE



Corrosion Resistance of Painted Metals Metal Surface Preparation and Conversion Coatings. I t i s axiomatic that only with good preparation of the substrate surface can optimal p a i n t performance and c o r r o s i o n p r o t e c t i o n be o b t a i n e d . For m e t a l l i c s u b s t r a t e s , adequate p r e p a r a t i o n may range from g r i t b l a s t i n g or abrasive cleaning with subsequent a p p l i c a t i o n of wash primers or chemical conversion coatings to much milder chemical or physical treatments to remove gross surface contamination (57). In i n d u s t r i a l c o a t i n g a p p l i c a t i o n s , i n o r g a n i c p h o s p h a t e c o n v e r s i o n c o a t i n g s are a commonly used s u r f a c e pretreatraent f o r m e t a l s . The o b j e c t i v e of such treatments i s the d e p o s i t i o n of an i n s o l u b l e metal phosphate on the surface to be painted. Bender et (58) and Gabe (9) have reviewed the l i t e r a t u r e and g i v e r e f e r e n c e s to e a r l i e r r e v i e w s and compendia on the s u b j e c t . Secondary phosphates of d i v a l e n t i r o n , manganese, and z i n c are only sparingly s o l u b l e . In the phosphating process, the metal substrate i s attacked, the pH r i s e s at the metal surface, and amorphous metal phosphate i s precipitated on the metal surface. C r y s t a l l i z a t i o n and c r y s t a l growth of the phosphate then occur. F i n a l l y , a process of c r y s t a l d i s s o l u t i o n and r e o r g a n i z a t i o n sets i n which modifies the p o r o s i t y of the phosphate l a y e r and decreases the exposed metal surface area. O v e r a l l , the reaction of primary zinc phosphate with s t e e l may be written as: Fe + 3 Z n ( H P 0 ) -> Z n ( P 0 ) + FeHP0 + 3 H P 0 + H 2

4

2

3

4

2

4

3

4

2

In practice, depolarizers are added to the bath so that the cathodic reaction becomes the oxygen (or o x i d i z i n g agent) reduction reaction rather than the hydrogen e v o l u t i o n reaction. Bath compositions are p r o p r i e t a r y , the r e a c t i o n s i n v o l v e d are very complex, and the performance of the c o a t i n g s obtained are h i g h l y dependent on bath c o m p o s i t i o n and d e p o s i t i o n c o n d i t i o n s as w e l l as on the i n i t i a l condition of the substrate (32, 59-62). T y p i c a l l y the coatings have a mixed crystal/amorphous structure; "zinc phosphate" i s p r i m a r i l y t e r t i a r y z i n c phosphate t e t r a h y d r a t e and d i h y d r a t e , w i t h s m a l l mounts of z i n c - i r o n phosphate ( p h o s p h o p h y l l i t e ) . Coating weights are t y p i c a l l y 0.1 to 1 mg/cm ; c o a t i n g t h i c k n e s s i s i n the range 1 to 10 ym. Oxide and chromate conversion coatings (used p r i m a r i l y for z i n c , aluminum, t i n , and magnesium) improve p a i n t adhesion r e l a t i v e to t h a t which i s observed on u n t r e a t e d m e t a l . These c o a t i n g s are applied by a v a r i e t y of proprietary processes (9, 57, 58). Corrosion Control by Pigments. Pigmentation for p r o t e c t i v e coatings has been reviewed b r i e f l y by v a r i o u s a u t h o r s (13, 41); P a t t o n s Pigment Handbook (63) p r o v i d e s more comprehensive i n f o r m a t i o n . Chromate pigments have been reviewed by Wormald (64), and lead based pigments have been reviewed by Dunn (65). Recent developments—many aimed at e l i m i n a t i o n of chromâtes and lead from coatings—have been d i s c u s s e d by Wienand and O s t e r l a n d (66). The i n f l u e n c e of i n e r t pigments on c o r r o s i o n p r o t e c t i v e p r o p e r t i e s has been reviewed by Kresse (56). The requirements for an i d e a l corrosion i n h i b i t o r , as summarized by Leidheiser (67), are that the i n h i b i t o r be e f f e c t i v e at pH v a l u e s i n the range of 4 to 10, and i d e a l l y i n the range of 2 f


794


to 12; r e a c t w i t h the m e t a l s u r f a c e such t h a t a product i s formed with much lower s o l u b i l i t y than the unreacted i n h i b i t o r ; have a low but s u f f i c i e n t s o l u b i l i t y ; form a f i l m at the c o a t i n g / s u b s t r a t e i n t e r f a c e t h a t does not reduce adhesion; be e f f e c t i v e both as an anodic and c a t h o d i c i n h i b i t o r ; and i n h i b i t the two important c a t h o d i c r e a c t i o n s , H 0 + 1/2 0 + 2 e ' = 2 OH"* and 2 H + 2 e " = H · Corrosion c o n t r o l by pigments r e l i e s on well-known p r i n c i p l e s of c o r r o s i o n i n h i b i t i o n . I r o n and s t e e l exposed to a i r are q u i c k l y covered by an o x i d e f i l m ; aqueous e l e c t r o l y t e s tend to break down t h i s f i l m , and further oxidation of the metal surface ensues. The r o l e of anodic c o r r o s i o n i n h i b i t o r s i s to supplement or to a i d i n the r e p a i r of the s u r f a c e o x i d e f i l m . B a s i c pigments may form soaps, f o r example, w i t h l i n s e e d o i l ; a u t o x i d a t i o n of these soaps may y i e l d s o l u b l e i n h i b i t o r s i n the f i l m . Some other pigments of l i m i t e d s o l u b i l i t y a c t d i r e c t l y as i n h i b i t o r s . A c t i v e m e t a l pigments supply electrons to the i r o n substrate and thus lower i t s p o t e n t i a l and prevent metal d i s s o l u t i o n . T y p i c a l basic pigments i n c l u d e basic lead carbonate, basic lead s u l f a t e , red l e a d , and z i n c o x i d e . The soaps formed when these materials i n t e r a c t , for example, with l i n s e e d o i l , are oxidized i n the presence of water and oxygen to form mono- and d i b a s i c s t r a i g h t c h a i n Cy to Cg a c i d s . M a t e r i a l s of t h i s type (e.g., sodium and c a l c i u m a z e l a t e and p e l a r g o n a t e ) are known to i n h i b i t c o r r o s i o n . I n h i b i t i o n i s associated with formation of complex f e r r i c s a l t s that r e i n f o r c e the o x i d e f i l m . Lead s a l t s a c t at lower c o n c e n t r a t i o n than the sodium or calcium s a l t s (3, 41). Anodic p a s s i v a t i o n of s t e e l surfaces can be e f f i c i e n t l y achieved by m e t a l c h r o m â t e s . Chromâtes of i n t e r m e d i a t e s o l u b i l i t y (e.g., z i n c chromate and s t r o n t i u m chromate) a l l o w a compromise between m o b i l i t y i n the f i l m and l e a c h i n g from the f i l m to be a c h i e v e d . C h r o m â t e s i n h i b i t c o r r o s i o n i n aqueous systems by f o r m a t i o n of a passivating oxide f i l m . The effectiveness of chromate i n h i b i t o r s i n aqueous systems depends on the concentration of other i o n i c species i n s o l u t i o n , for example, c h l o r i d e . Synthetic r e s i n composition can a l s o s i g n i f i c a n t l y influence the effectiveness of chromate pigments. The effect appears to be r e l a t e d to the p o l a r i t y of the r e s i n (20); chromate pigments appear to be l e s s e f f e c t i v e i n r e s i n s of low polarity. Corrosion c o n t r o l by the incorporation of a c t i v e metal pigments i s a form of c a t h o d i c c o r r o s i o n p r o t e c t i o n . The pigment must be l e s s noble than the substrate; furthermore, the pigment p a r t i c l e s must be i n m e t a l l i c ( e l e c t r o n i c ) c o n t a c t w i t h each other and w i t h the substrate. These requirements d i c t a t e the use of high l e v e l s of z i n c (90 to 95% by weight i n z i n c - r i c h p r i m e r s , 75 to 80% by weight i n the older conventional zinc-dust paints) and thorough cleaning of the substrate. For protection of s t e e l substrates, Zn, A l , and Mg appear to be a t t r a c t i v e candidates as p r o t e c t i v e metal pigments. Of these, however, only Zn meets the requirement of e l e c t r o n i c contact, as both A l and Mg are covered with a low c o n d u c t i v i t y oxide. Part of the effectiveness of the zinc pigment may be due to the formation of z i n c h y d r o x i d e and other z i n c c o r r o s i o n products t h a t tend to c l o g the pores of the c o a t i n g f i l m (13, 41, and r e f e r e n c e s c i t e d t h e r e i n ) ; t h e r e i s , however, good e v i d e n c e f o r t r u e c a t h o d i c p r o t e c t i o n w i t h these m a t e r i a l s (68). When z i n c - r i c h primers are +

2

2


2


32. DICKIE


coated with paints having high e l e c t r i c a l resistance, the e f f e c t i v e s a c r i f i c i a l area may be r e s t r i c t e d to a s m a l l zone around the damaged area. The primer may a i d i n h e a l i n g the damaged area and i n preventing the spread of corrosion (68). A v a r i e t y of binders, both o r g a n i c and i n o r g a n i c , have been used; i n o r g a n i c b i n d e r s , f o r example, e t h y l s i l i c a t e , are reported to g i v e the best performance


(u»

20).

E f f e c t s of Resin Structure on Corrosion C o n t r o l . Resin composition plays a r o l e i n many different aspects of corrosion protection. The e f f e c t s of r e s i n p e r m e a b i l i t y and of adhesion i n the presence of water have been discussed i n previous sections. Resin composition a l s o influences the chemistry of the interface between coating and substrate during corrosion. From performance tests based on simple e l e c t r o c h e m i c a l p r i n c i p l e s , i t i s known t h a t the d e l a m i n a t i o n of otherwise strongly adherent organic coatings that often accompanies corrosion i s a t t r i b u t a b l e to the action of cathodic a l k a l i (19, 20). The d e t a i l s of the chemical changes that occur at the interface have been e x t e n s i v e l y studied (71-81); i t i s evident that degradation of the o r g a n i c r e s i n can p l a y a major r o l e i n the l o s s of adhesion of c o a t i n g s t h a t are not r e a d i l y d i s p l a c e d by water (72-77). The degradation mechanism may i n v o l v e h y d r o l y s i s of base l a b i l e bonds such as e s t e r s , urethanes, or ureas by h y d r o x i d e . Use of p a i n t r e s i n s t h a t do not have a l k a l i s e n s i t i v e bonds has been shown to r e s u l t i n s i g n i f i c a n t improvements i n p a i n t performance provided that other important paint performance c r i t e r i a are a l s o met (e.g., resistance to humidity) (73, 75). The degradation of paint r e s i n to form i o n i c products p r o b a b l y g r e a t l y i n c r e a s e s the i o n i c conduc t i v i t y of the i n t e r f a c i a l region, and thus expands the cathodic zone of reaction and accelerates the o v e r a l l corrosion process. I t has a l s o been found that the oxide l a y e r tends to increase i n thickness under the i n t a c t f i l m (79-81), and i t has been argued (24, 81) t h a t the i n i t i a l step of d e l a m i n a t i o n i n v o l v e s d i s s o l u t i o n of the i n t e r f a c i a l i r o n oxide. This argument does not, however, take i n t o account the l a r g e decrease i n r a t e of d e l a m i n a t i o n observed w i t h changes i n polymer structure designed to improve resistance of the organic coating to a l k a l i degradation (73, 745, 82). The mechanism of c o r r o s i o n - i n d u c e d adhesion f a i l u r e can range from s i m p l e displacement by water to a process i n v o l v i n g s u b s t a n t i a l chemical changes i n the c o a t i n g , the s u b s t r a t e or both (77); the mechanism that occurs i n any given case i s e v i d e n t l y a complicated function of t e s t environment, substrate and coating composition, and d e t a i l s of i n t e r f a c i a l structure and composition. Summary The factors that influence the corrosion p r o t e c t i v e performance of organic coatings can be broken i n t o three groups: environmental conditions, b a r r i e r properties of the coating system, and i n h i b i t i v e p r o p e r t i e s of the c o a t i n g system. T y p i c a l l y , the c o r r o s i v e environment comprises water, oxygen, and an e l e c t r o l y t e . The s e v e r i t y of the environment depends e s p e c i a l l y on the a c t i v i t y of oxygen and e l e c t r o l y t e , and on pH. The i n i t i a t i o n of corrosion i s i n f l u e n c e d by the c h o i c e and p r e p a r a t i o n of the s u b s t r a t e , the transport and permeability c h a r a c t e r i s t i c s of the organic coating,



796


pigmentation ( e s p e c i a l l y as i t a f f e c t s f i l m transport c h a r a c t e r i s t i c s ) , mechanical i n t e g r i t y of the coating, and coating adhesion ( e s p e c i a l l y under c o n d i t i o n s of the a n t i c i p a t e d s e r v i c e environment). The propagation of corrosion from an i n i t i a l s i t e i s a f f e c t e d by s u b s t r a t e p r e p a r a t i o n , pigmentation ( e s p e c i a l l y the presence of a c t i v e pigments), and the adhesion of the c o a t i n g adjacent to corroding s i t e s ( i n t h i s respect, the composition of the coating i n the i n t e r f a c i a l region can play a s i g n i f i c a n t r o l e ) . An understanding of the electrochemistry of corrosion processes, of the t r a n s p o r t processes i n c o a t i n g f i l m s , and of the chemistry and p h y s i c s of the c o a t i n g - s u b s t r a t e i n t e r f a c e can be of s u b s t a n t i a l assistance i n r e s o l v i n g corrosion r e l a t e d coating problems.

Literature Cited 1. NBS Special Publication 511-1, "Economic Effects of Metallic Corrosion in the United States," SD Stock No. SN-003-01926-7, 1978; NBS Special Publication 511-2, "Economic EFfect of Metallic Corrosion in the United States," Appendix B, SD Stock No. SN-003-01927-5, 1978, 2. Smith, C. A. Mod. Paint Coat. 1978, 68(3), 59. 3. Evans, U. R. "The Corrosion and Oxidation of Metals"; St. Martins Press: New York, 1960; Ibid., 1st Supplementary Volume, St. Martins Press: New York, 1968; Ibid., 2nd Supplementary Volume, Edward Arnold: London, 1976. 4. Uhlig, H. H. "Corrosion and Corrosion Control," 2nd ed.; Wiley: New York, 1971. 5. "Corrosion," 2nd ed.; Shreir, L. L., Ed.; Newnes-Butterworths: London, 1976. 6. Symposium on Interfacial Phenomena in Corrosion Protection Preprints, Am. Chem. Soc. Div. Org. Coat. Plast. Chem., 37(1), 1977; Symposium on Advances in Coating Metals for Corrosion Protection, Preprints, Am. Chem. Soc. Div. Org. Coat. Plast. Chem., 43, 1980; "Corrosion Control by Coatings"; Leidheiser, H., Jr., Ed.; Science Press: Princeton, 1979; "Corrosion Control by Organic Coatings"; Leidheiser, J., Jr., Ed.; National Association of Corrosion Engineers: Houston, 1981; "Automotive Corrosion by De-icing Salts"; Baboian, R., Ed.; National Association of Corrosion Engineers: Houston, 1981; "Adhesion Aspects of Polymeric Coatings"; Mittal, K. L., Eds.; Plenum: New York, 1983. 7. Of particular interest are The Journal of Coatings Technology, Progress in Organic Coatings, The Journal of the Oil and Colour Chemists Association, Farbe und Lack, Corrosion-NACE, Corrosion Science, and I&EC Product R&D. 8. Evans, U. R. "An Introduction to Metallic Corrosion," 2nd ed.; Edward Arnold: London, 1972. 9. Gabe, D. R. "Principles of Metal Surface Treatment and Protection," 2nd ed.; Pergamon: Oxford, 1978. 10. Scully, J. C. "The Fundamentals of Corrosion," 2nd ed.; Pergamon: Oxford, 1975. 11. West, J. M. "Electrodeposition and Corrosion Processes," 2nd ed.; Van Nostrand: London, 1972.


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12. Bennynk, P. Janssen; Piens, M. Prog. Org. Coat. 1979, 7, 11339. 13. Von Fraunhofer, J. Α.; Boxall, J. "Protective Paint Coatings for Metals"; Portcullis Press: Redhill, Surrey, 1976. 14. Leidheiser, H. Corrosion NACE, 1982, 38(7), 374. 15. Pourbaix, M. "Atlas of Electrochemical Equilibria in Aqueous Solutions"; Pergamon Press: New York, 1966; and "Lectures on Electrochemical Corrosion"; Plenum: New York, 1971. 16. Hospadaruk, V. Paper No. 780909, SAE Proceedings P78 1978, 123. 17. Banfield, T. A. "The Protective Aspects of Marine Paints," Prog. Org. Coat. 1979, 7, 253. 18. Anderton, W. A. Off. Dig. 1964, 36, 1210. 19. Wiggle, R. R.; Smith, A. G.; Petrocelli, J . V. J. Paint Technol. 1968, 40(519), 174. 20. Smith, A. G.; Dickie, R. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 42. 21. Funke, W. Prog. Org. Coat. 1981, 9, 29. 22. Koehler, E. L. In "Localized Corrosion"; Staehle, R. W.; Brown, B. F.; Kruger, J.; Agarwal, Α., Eds.; National Association of Corrosion Engineers: Houston, 1974; p. 117. 23. Hoch, G. M. In "Localized Corrosion"; Staehle, R. W.; Brown, B. F.; Kruger, J.; Agarwal, Α., Eds.; National Association of Corrosion Engineers: Houston, 1974; p. 134. 24. van der Berg, W.; van Laar, J. A. W.; Suurmond, J. In "Advances in Organic Coatings Science and Technology"; Parfitt, G. D.; Patsis, Α. V., Eds.; Technomic: Westport, Conn., 1979; Vol. 1, p. 188. 25. Funke, W. J. Oil Colour Chem. Assoc. 1979, 62 63-67. 26. Funke, W. Farbe und Lack 1978, 84, 380; Funke, W.; Machunsky, E.; Handloser, G. Ibid., 1979, 84, 498; Funke, W.; Zatloukal, H. Ibid., 1979, 84, 584. 27. Rowe, L. C.; Chance, R. L. In "Automotive Corrosion by De-icing Salts"; Baboian, R., Ed.; National Association of Corrosion Engineers: Houston, 1981; p. 133. 28. Baboian, R. "Electrochemical Techniques for Corrosion"; National Association of Corrosion Engineers: Katy, Tex., 1977. 29. Leidheiser, J., Jr. Prog. Org. Coat. 1979, 7, 79. 30. Sato, Y. Prog. Org. Coat. 1981, 9, 85. 31. Szauer, T. Prog. Org. Coat. 1982, 10, 171. 32. Hospadaruk, V.; Huff, J.; Z u r i l l a , R. W.; Greenwood, H. T. Paper No. 780186, Trans. SAE 1978, 87, 755. 33. "Annual Book of ASTM Standards"; Part 27, "Paint--Tests for Formulated Products and Applied Coatings"; American Society for Testing and Materials: Philadelphia, 1982. 34. Walker, P. Paint Tech. 1967, 31(8), 22; Ibid. 1967, 31(9), 15. 35. Funke, W.; Haagen, H. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 50. 36. M i t t a l , K. L. In "Adhesion Aspects of Polymeric Coatings"; M i t t a l , K. L., Ed.; Plenum: New York, 1983.



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