How Organic Coating Systems Protect Against Corrosion - ACS

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch020

20 How Organic Coating Systems Protect Against Corrosion Werner Funke Forschungsinstitut für Pigmente und Lacke, Allmandring 37, D-7000 Stuttgart 80, Federal Republic of Germany

The electrochemical, physicochemical and adhesional aspects of corrosion protection by organic coatings are shortly discussed. Attention i s drawn to some inconsistancies in the interpretation of protective mechanisms and suggestions are given how protective principles may be optimally realized in practical systems. There are e s s e n t i a l l y three important mechanisms by which organic coating systems protect against metal corrosion: The electrochemical, the phy s iœchemical and the adhesional mechanism. In order t o obtain optimum protection, i t i s commonly proposed to incorporate as many as possible of these mechanisms i n a coating system. I t w i l l be discussed how f a r t h i s strategy i s tenable i n p r a c t i c a l paint formulation and whether i t i s reasonable i n the l i g h t of a c r i t i c a l judgement. For t h i s purpose i t i s h e l p f u l to r e c a l l how these mechanisms work and what the requirements are f o r t h e i r operat i o n . Correlations of permeability with anticorrosive action i n corrosion protection by organic coatings have been recently d i s cussed (J_). The Electrochemical Mechanism The elec±rochemical mechanism i s generally connected with the presence of a c t i v e anticorrosive pigments, l i k e red lead o r zinc chromate, and occasionally also t o corrosion i n h i b i t o r s , which are added to the base coat of the system. I t i s a wide-spread opinion that such anticorrosive pigments are almost indispensable f o r a s a t i s f a c t o r y corrosion protection because "no organic coating i s impermeable to water" (2) . Therefore anticorrosive pigments are considered t o be an ultimate l i n e of defense f o r corrosion protection. Active a n t i c o r rosive agents act only i n presence of water, which dissolves a small f r a c t i o n of them and makes them a v a i l a b l e at the coating/metal i n t e r This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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

20.

FUNKE

How Organic Coating Systems Protect Against Corrosion

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face ( 3 , 4 ) . Passivation o r corrosion i n h i b i t i o n of metal surfaces i s mostly achieved by supporting the i n s i t u formation of t h i n l a y ers of insoluble corrosion products ( 4 , 5 ) , which cover corroding areas and s t i f l e the action of the corrosion elements. For a continued corrosion i n h i b i t i o n the anticorrosive solution must keep steady contact with the metal surface. Substitution by normal water usually i n i t i a t e s corrosion again. To allow d i f f u s i o n o f the dissolved anticorrosive agent to the coating/metal i n t e r f a c e , the binder should be permeable t o water. This requirement c l e a r l y contradicts the other requirement of corrosion protective coatings, namely to prevent the access of water as a corrosive agent to the metal surface. Frequently binders used i n p r a c t i c a l corrosion protective coating systems scarcely swell by wat e r and are only s l i g h t l y permeable to i t . Accordingly the protective agent i s locked i n the binder and i s not a v a i l a b l e i n s u f f i c i e n t concentration at the metal surface. Good protective properties claimed i n these cases are rather due to other protective mechanisms than to the electrochemical one. -

2-

Unfortunately some corrosion stimulants, l i k e C l , S0^ or NO.. , strongly oppose i n h i b i t i o n by anticorrosive pigments and i n h i b i t o r s (6). Steel corrodes i n saturated aqueous solutions o f an anticorrosive pigment i n presence of small amounts of these stimul a n t s , e.g. 1% w/w NaCl i s s u f f i c i e n t to make a saturated aqueous extract o f zinc chromate corrosive (1). Therefore, i r r e s p e c t i v e of environmental requirements, the usefulness of a c t i v e anticorrosive pigments and i n h i b i t o r s as well has become questionable. The Physicochemical

mechanism

The physicochemical mechanism consists i n blocking up d i f f u s i o n of corrosive agents, l i k e water and oxygen, and of corrosion stimulants. This b a r r i e r a c t i o n of organic coatings may be enhanced s i g n i f i c a n t l y by pigments, f i l l e r s or extenders which, due to a f l a k y o r p l a t e l i k e geometrical shape, greatly increase the length of d i f f u s i o n a l pathways through the cross section of the coating f i l m . In order to avoid d i f f u s i o n i n the pigment/binder i n t e r f a c e , i n t e r f a c i a l bonds between both phases should be as water-resistant as possible. I f permeability i s taken as a measure, properly formul a t e d b a r r i e r coatings may compare i n corrosion protective e f f i c i e n c y with normal coatings, the thickness of which i s two o r three times as high. The binder contributes a l s o to the b a r r i e r e f f e c t of a coating system. Permeability of the binder depends on the r i g i d i t y and pol a r i t y of i t s macromolecular structure and also on the density of the molecular packing. Accordingly permeability decreases by decreasing the chain mobility, e.g. by c r o s s l i n k i n g , by decreasing the hydrophilic character of the macromolecules and by increasing the density of molecular packing up to c r y s t a l l i n e o r c r y s t a l l i n e - l i k e structures.


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POLYMERIC MATERIALS FOR CORROSION CONTROL


Considering the measures to be taken i n binders f o r ensuring an optimum b a r r i e r e f f e c t , they obviously oppose the requirements f o r the a n t i c o r r o s i v e function o f pigments, which need water-permeable and swellable binders. One may argue, however, that concurrently with improving the b a r r i e r properties of a coating system the a n t i corrosive function of pigments o r i n h i b i t o r s i s l e s s challenged. Furthermore the r i g i d i t y of the macromolecular structure of the binder may oppose the demand f o r mechanical strength and shock r e sistance o f the coating f i l m . Increasing the r i g i d i t y by c r o s s l i n k ing leads to i n t e r n a l stresses, which accumulate with increasing f i l m thickness. A way out of t h i s dilemma may be the use of very t h i n but highly cros s i inked base coats (8). The Adhesional Mechanism The adhesional mechanism up to now has not yet received s u f f i c i e n t attention i n corrosion protection by organic coatings. As long as adhesion o f the base coat t o the metal surface i s unchanged no corrosion can take place below a coating. Too much emphasis has been given t o adhesion under dry cond i t i o n s . However, corrosion i s only possible i f enough water i s present i n the coating/metal i n t e r f a c e to provide the e l e c t r o l y t e f o r the corrosion elements to operate. This condition i s hardly imaginable without a previous s i g n i f i c a n t reduction o r even the l o s s of adhesion. Therefore "wet adhesion" i s considered to be o f c r u c i a l importance to corrosion protection by organic coatings (9). I t i s generally agreed that due t o the polar nature of o x i d i c metal surfaces good dry adhesion i s only possible by incorporating polar groups i n the binder molecules. However, these polar groups may e f f e c t water s e n s i v i t y of the coating/metal i n t e r f a c e thus causing poor wet adhesion. That water accumulates at the i n t e r f a c e coating/metal s u b s t a n t i a l l y , has been shown by comparing water absorption o f f r e e and supported f i l m s (10). One way to make waters e n s i t i v e interfaces r e s i s t a n t against water i s to adsorb polar groups which are attached to r i g i d polymer backbone chains. I t i s s t i l l not known f o r c e r t a i n , whether on exposure to water adhesion i s uniformly reduced over the exposed area o r only l o c a l l y l o s t at channels providing the e l e c t r o l y t i c pathways between anodic and cathodic areas of the metal surface. In choosing binders with good adhesion, again protective prop e r t i e s are encountered, which exclude each other, e.g. non-polar macromolecules with low permeability would be benif i c i a l t o the b a r r i e r e f f e c t but objective to good dry as well as wet adhesion i . e . t o the adhesional mechanism. On the other hand p o l a r groups supporting dry adhesion are required despite o f t h e i r weakness i n presence of water. The question remains how the adhesional i n t e r action may be s t a b i l i z e d t o r e s i s t the attack of water. For the sake of good adhesion a metal surface should be clean and f r e e of water-soluble substances. On the other hand, f o r the


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protective action of anticorrosive pigments a soluble f r a c t i o n , i . e . an "impurity", must be present at the i n t e r f a c e . I t i s hard to r e concile t h i s requirement with good wet adhesion of the base coat. Corrosion protective base coats with hydrophilic binders, such as water-borne coatings drying a t ambient temperatures, usually exhibit, high water permeability and poor wet adhesion. In these cases a c t i v e anticorrosive pigments are needed. Protective properties can be improved by delaying the access of water to the coating /metal interface, e.g. by increasing f i l m thickness, incorporating b a r r i e r pigments o r applying b a r r i e r top coats (Figure 1). However, even then these systems are l a t e n t l y weak and may f a i l on prolonged exposure to water o r high humidity, e s p e c i a l l y i f mechanical stressess simultaneously act on the coatings and place excessive demands on t h e i r adhesion. The Combination Of D i f f e r e n t Protective Mechanisms The combination o f d i f f e r e n t protective mechanisms i n one coat o r one coating system i s frequently recommended f o r optimal r e s u l t s i n c o r rosion protection. However, ways and measures t o optimize protective mechanisms may be d i f f e r e n t and sometimes even exclude each other. For example t r y i n g to combine good wet adhesion and corrosion i n h i b i t i o n by an a c t i v e anticorrosive pigment i n the same base coat does not make much sense, despite being frequently postulated t o explain protective properties of commercial paint systems (Figure 2). Binders with good wet adhesion lock i n the anticorrosive pigment and therefore dindnish o r even prevent i t s corrosion i n h i b i t i n g e f f e c t . Sometimes i t i s claimed that p r a c t i c a l experience disproves t h i s statement, but i t cannot be excluded i n these cases that protection i s mostly due to good b a r r i e r properties and/or good wet adhesion. On the other hand i t i s advantageous to choose a primer e x h i b i t i n g optimal wet adhesion and simultaneously optimal b a r r i e r properties. The b a r r i e r mechanism not only decreases water permeating to the coating/metal i n t e r f a c e but likewise retards the release o f solvents from a coating. In order t o avoid delayed f i l m formation f o r t h i s combination solventless paint systems are most s u i t a b l e . In two-layer coating systems the best choice i s to endow both layers with the b a r r i e r e f f e c t and choose a binder having good wet adhesion. Other combinations are l e s s e f f e c t i v e or even not reasonable (Figure 3 ) . The use o f a c t i v e anticorrosive pigments i s only j u s t i f i e d t o prevent corrosion at scratches, pinholes o r s i m i l a r coating defects and even then only i n absence o f v i r t u a l amounts of corrosion stimulants. I t i s commonly assumed by paint technologists that prot e c t i v e e f f e c t s incorporated i n each l a y e r of a coating system add together i n preventing corrosion at the coated metal surface. However, i n coating systems, which base coats protect by an e l e c t r o chemical mechanism (Figure 4 ) , the successive layers including the top layer rather should prevent any rhysicochemical or e l e c t r o chemical reaction a t the base coat/metal i n t e r f a c e . Considering the


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POLYMERIC MATERIALS FOR CORROSION CONTROL


VARIATIONS

COMBINATIONS

BARRIER PIGMENT

Figure 1 : V a r i a t i o n and combination f o r iirproving corrosion prot e c t i o n by organic coatings composed o f non-barrier binders

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Figure 2: V a r i a t i o n f o r improving corrosion protection by organic coatings composed o f b a r r i e r binders


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Figure 4: Two-layer system f o r corrosion protection a t coating defects (Top coat protects base coat, base coat protects metal support at defects!)


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POLYMERIC MATERIALS FOR CORROSION C O N T R O L


high demands on the metal surface pretreatment to achieve good adhesion, i t i s not conceivable to allow this interface being kind of a reaction vessel. Layers succeeding to electrochemical protecting base coats should prevent any reaction in the base-coat and especially at i t s interface to the metal surface. The base coat should only come into action at coating defects extending down to the metal surface. Likewise coating systems suitable for cathodic protection should prevent any reaction at the metal surface below the intact coating system. Otherwise cathodic delamination i s unavoidable. In a l l these cases we actually have not a "twofold-protection" but interdependent as well as œmplementary mechanisms. The intact coating system protect by i t s barrier action and, possible, by good wet adhesion, whereas the electrochemical mechanism must be restricted to coating defects. Literature Cited 1.

F . L . Floyd, R.G. Groseclose, C.M. Frey, J. O i l Col. Chem. Assoz., 1983, 329 2. J.E.O. Mayne, Pigment Handbook Vol. III, Edited by T.C Patton, Wiley Interscience Publ. 1973, p. 459 3. J.E.O. Mayne, E.H. Ramshaw, J. Appl. Chem. 13, 1969, 553 4. H. Leidheiser, J. Coatings Technol., 53 No. 678, 1981, 29 5. J.E.O. Mayne, Pigment Handbook Vol. III, Edited by T.C. Patton, Wiley Interscience Publ. 1973, p. 457-464 6. L.A. Buckowiecke, Schweizer Archiv f. Wissenschaft u. Technik (6), 1954, 1 7. W. Funke, unpublished results 8. W. Funke, J. O i l Col. Chem. Assoz., 1985, 229 9. W. Funke, J. Coatings Technol., 55 No. 705, 1983, 31 10. W. Funke, Fette, Seifen, Anstrichmittel, 64, 1962, 714 RECEIVED March 5, 1986


How Organic Coating Systems Protect Against Corrosion - ACS

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