Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 343-347
343
SYMPOSIA SECTION
I.
Symposium on "Anticorrosion Barriers: Chemist ry and A ppIications" Henry Leidheiser, Jr., Chairman 188th National Meeting of the American Chemical Society Philadelphia, Pennsylvania, August 1984
Toward a Unified View of the Mechanism Responsible for Paint Defects by Metallic Corrosion Werner Funke Forschungsinstitut fur Pigmente und lacke e. V. Stuttgart und I I , Institut fur Technische Chemie, Universlt Stuttgart, 7000 Stuttgart SO, Federal Republic of Germany
Paint defects, which are related to metal corrosion, are delamination or undercutting, blister formation, underrusting, and filiform corrosion. Despite well-known fundamentals (Figure 1) these defects appear to be rather confusing phenomena due to a lack of information on their mechanism, interrelations, and influencing parameters. The following discussion will be confined to corrosion of steel under atmospheric conditions; i.e., the underlying electrochemical reactions are essentially of the oxygen-consuming type. These paint failures are explained on the basis of the common electrochemical mechanism that in the special cases of failure is controlled by the parameters involved and their dominancy.
Loss of Adhesion and Blistering. Osmosis is considered to be the most important mechanism responsible for blister formation of organic coatings on steel surfaces. Osmotic pressures may be expected to range between 2500 and 3000 kPa. On the other hand, mechanical resistance of organic coatings to deformational forces is much lower, ranging from 6 to 40 kPa (Bullett, 1961; Bullett and Rudram, 1961; van der Meer-Lerk and Heertjes, 1975). It is therefore easy to understand that blisters may develop if adhesion is lost over the respective area. For blister formation, however, the organic coating should still adhere to the support beyond the periphery of the base area of the blister. When the adhesive tape test was applied to samples of various organic coatings after exposure to liquid water at ambient or elevated temperatures over different times, it was shown that adhesion of the wet coating has always substantially decreased before any blistering is visible. However, blisters grow even at this low residual adhesion (Figures 2-4). Obviously the interfacial forces are still sufficient to keep the film on the support at the area surrounding a growing blister but are too weak to resist the mechanical tearing force applied in the adhesive tape test. As will be discussed later this result has an interesting implication for diffusion processes involved in paint failures due to corrosion. It is feasible to suppose that weakened coating/metal interfaces allow a direct electrolytic connection of anodes and cathodes. As in electrochemical corrosion testing of organic coatings, the electrolytic connection always traverses the coating section, and as this section differs considerably from the interfacial path by diffusional and
electrical properties, the usefulness of electrochemical tests becomes questionable. Alkaline Blistering. In electrochemical corrosion, oxygen depolarizes cathodic areas with production of hydroxyl anions. For the balance of the ionic charges of cathodic and anodic primary corrosion products, the corresponding counterions must be available. In the presence of salts, like NaC1, as electrolytes, cations may migrate to cathodic areas and form NaOH, which is responsible for the strong alkaline reaction of the aqueous solution present in these blisters. Migration of cations to cathodic areas may take place through the coating (Meyer and Schwenk, 1979) or along the coating/metal interface. Diffusion experiments with 22 Na cations indicated very low diffusion rates (Lonsdale et al., 1965; Matsui, 1975),even with films known for being relatively permeable (Figure 5). Moreover, the initial concentration of osmotically active substances in the film/support interface is probably lower than that of the external aqueous solution of NaCl used in salt-spray or immersion tests. Therefore cation diffusion through the coating to cathodic areas of the'metal surface will increase the osmotic pressure instead of decreasing it. In view of this inconsistency Leidheiser et al. (1983) have proposed a migration of sodium ions from paint film defects to cathodic areas liable to blistering. They found that alkaline blisters developed only if pores or similar paint defects extending to the metal surface were exposed to an aqueous solution of NaCl (Figure 6). These results were confirmed in our laboratory. As adhesion decreases markedly on exposure to water before blistering starts, the weakened organic coating/ metal surface interface greatly facilitates ion migration.
0 1985 American Chemical Society 0196-4321/85/1224-Q343!§O1.50/0
344
Ind. Eng. Chem. Rod. Res. Dev.. Vol. 24. No. 3. 1985 DEFECIS
FUNDAMENTALS
DE LAM1NAT ION IUNOERCUTTINGI BLISTERING UNDERRUSTING FILIFORM CORROSION
LOCAL CORROSION ELEMENTS DIFFERENTIAL AERATION POLAR ISAT ION osrmsis AND ELECTRO osnosis MEMBRANE PROPERllES tPERMEATION. ELECTRICAL CHARGE1 I N l i R F A C l A L INTEPACTIONS
Figure 1. Paint defects related to metallic corrosion and fundamentals involved.
Figure 2. Loss of adhesion and blistering at various times of esposure in aqueous NaCI. 3% (w/w) porefixed. Ti0,-pigmented two-component EP paint on steel, thickness 66 pm.
I!? NO OELAHINAIION
O~lNAIION
a m (Ka Ka OSM
SOME BLISIERS DELAHIMION
p%,6. Experiments of Leidheiser et al. to prove the way m which K* diffuse to blisters.
-
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--
llllillllllllllllllllllllllllll Figure 7. Electrochemical procews on internal and external POlarization.
Figure 3. Loss of adhesion and blistering at various times of exposure in aqueous NaCI, 3% (w/w) porefixed, Tiorpigmented CR paint on steel. thickneea 50 rm.
Figure 4. Loss of adhesion and blistering at various times of esposure in aqumua NaCI. 3% (w/wu) pore-fixed. Ti02PUR paint on steel. thickness 53 pm.
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Figure 6. Diffuion conshnta of polymer film for Na* ions.
This weakened interface supporta a lateral diffusion of sodium cations via the interface as proposed by Leidheiser in explaining the mechanism of ionic charge balance. In conformance with these results, it could be shown that the pH value of blisters observed with defect-free organic coatings on exposure to dilute aqueous solution of NaCl is weakly acid to neutral. It may be concluded, therefore, that in the absence of an external applied electric potential, cations, like Na+, migrate to cathodic areas via coating defects like pores or scratches along the coating/metal interface. An increase of osmotic pressure in the blister area may be compensated rapidly by dilution with water and the consecutive growth of the blister. Mechanism of Undercutting and Underrusting. Atmospheric corrosion of steel at unprotected areas of a coating system starts with numerous small corrosion elements, which areas are estimated to be ca. 0.01 mm2 (German Standards, 1951). For the operation of these elements Fe is dissolved at local anodes and oxygen depolarizes local cathodes with formation of OH- anions. However, secondary corrosion reactions, involving the oxidation of Fe(I1) to Fe(II1) compounds, also compete for the oxygen supplied by the corrosive environment. Therefore local cathodes are increasingly lacking oxygen and become anodically polarized (Figure 7, top). The growing anodic character of the initial coating defect area stimulates the formation of cathodic areas adjacent to it. On exposure to aqueous salt solutions, like NaCl solutions, blisters develop at cathodic areas (Figure 8) and contain strongly alkaline aqueous solutions. Undercutting is usually considered to take place in a lateral direction starting at coating defects like a scribe. When an external electrical potential was applied to coated steel panels, however, electrochemical delamination adjacent to a scribe was found to occur only on cathodic polarization (Funke, 1981; Leidheiser et al., 1983; Schwenk, 1977). On anodic polarization heavy pitting takes place at the unprotected
I d . EW. than. Rod. Res. Dev.. Vd. 24. No. 3, 1985 945
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Figure 8. Blisters developed at cathodic area8 on expasure to aqueolu, salt solution.
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Figure 9. Pitting on extend anodic polarition of coated steel. no electrochemicaldelamination!
area (Figure 9), and physicochemical delamination over the whole panel is usuaUy observed only after prolonged exposure. If electrochemical delamination is cathodic hy nature and no external applied potential is involved, delamination should start at cathodic blisters and not at the paint defect causing them. This process may repeat until, finally, general physicochemical delamination has taken place. Accordingly, undercutting and underrusting must he stepwise processes (Figure 10). Thii electrochemical delamination prcxesa is facilitated by the signifcant reduction of adhesion due to the previous interaction of water at the coating/metal interface. As mentioned before, the electrical resistance between cathodic and anodic areas is much lower along the weakened section. On the other hand, interface than through the fh the residual adhesion of the coating, persisting for some time after exposure to water, is still sufficient to allow the film deformation involved in blistering. Depending on the location of blisters relative to a coating defect, alkaline blisters as well as neutral blisters may develop on the same sample if it is exposed to salt spray or diluted solutions of alkali salts. With increasing salt concentration, however, neutral blistering decreases for osmotic reasons. Neutral Blistering. On exposure of intact coatings on steel to water or even to diluted aqueous solutions of salts, like NaCI, bliiters may develop, having contents that renct weakly acid to neutral. Neutral blistering may be also observed hy exposing intact or defective coatings systems to condensed water at ambient or elevated temperatures or to high humidity. Undoubtedly the reduction of adhesion due to the interfacial interaction of water is also in this case the first step of the failure mechanism. The appearance of corrosion products a t the coating/metal interface and especially at the dome of the blisters indicates that again electrochemical processes are involved. However, in this case no alkali cations can be involved.
Figure 11. Mechanism of neutral blistering.
In order to explain the formation of neutral blisters some aeaumptions are nec8888ry. Evans (1945) has shown that for corrosion of steel under a droplet the marginal area becomes cathodic, since there oxygen can be readily replenished. A similar differential aeration may also be responsible for neutral blistering. Consecutive to the reduction of adhesion by water, I d corrosion elements are formed at the steel surface under the film. Impurities such as sulfates or ionizable components from the coating probably serve as electrolytes. As their concentration normally is low compared with concentrations found in alkaline blistering, the corrosion rate is also low. Polarization takes place due to different availabilities of oxygen at the steel surface. As previously discussed, again secondary corrosion reactions may be involved, which assist differential aeration. The center of the blister becomes anodic, and the peripheral zone becomes cathodic. Electroneutrality may be achieved by the dissociation of water, involving a proton migration along the interface coating/metal surface (Figure 11). According to this mechanism, neutral blistering is expected to be slower than cathodic blistering, which is actually the case. Superimposed to this process may be osmosis and electrosmosis (Grubitsch and Heckel, 1960; Kittelberger and Elm, 1947), which contribute to the water contents of the blisters and, most importantly, are responsible for the deformation pressure in blister formation. Localization and Termination of Blistering. When the mechanism of blistering is considered, it has to be
-
340 I d . Eng. Chem. Rod. Res. Dev.. Vd. 24, No. 3. 1985 m n w Iolnn
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I
A
Figure 14. Limitation of blister growth. Figure 12. Blister formation due to solvent entrapment at the film/support interface. LBAliR
PAIN1
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E l A L SUFWRT
POINTS. UAlER AND OXYGEN ARRIA1 VES EARLIER A1 THE M A L SURFACE LOCAL LOSS OF lOHESlOW A33 F O W l I O W OF LMAL CDRKWON ELEElilS k C ~ P A l l MOF RUST AT THE WIE OF 1M
BLISTER OR M D I C FQLYRlUllOW OF THE W A U FUINl. AND lNDUCTlOW OF CATHODIC BLISTERS BESIDES THE I(1y W I N l .
Rigare 1% Blister formation at "weak points" of a coating.
explained why blisters develop a t certain locations on the coating and why no general delamination takes place. It was shown earlier ( M e , 1981)that phase separation and local retention of hydrophilic, high boiling point solvents during f h formation may lead to blistering if coatings are ex& to water or high humidity shortly after application and f h formation (Figure 12). Blisters may also develop at weak spots of coatings with a low film thickness (Figure 13). In this case differential aeration and polarization of local corrosion elements at the weak spot area may be the reason for the localized delamination involved in bliitering. Last but not least the specific structure of the metal surface may explain localized delamination. The influence of metal surface pretreatment on bliiter formation reflects the importance of the metal surface structure. Unfortunately, current knowledge about the relation between the chemical and morphological structure of metal surfaces and the location of blisters is rather scanty. Probably blisters appear only after polarization of the metal surface at the weakened coating/metal interface. Polarization creates larger cathodic areas, which are sufficiently active to accumulated water and thereby build up the internal preasure required for the local deformation of the film. It is therefore difficult to trace blistering back to the original surface structures, which may cover much smaller areas than do the blisters. Blisters do not increase indefinitely but rather stop growing after some time of exposure. The termination of blister growth may be due to defects a r i s i i from the f h deformation involved in blistering. Termination can also be due to attaining an equilibrium between osmotic and hydrostatic pressure. However, most probably, blistering is terminated by a general physicochemical delamination affected by water a t the coating/metal interface (Figure 14).
Figure 16. M e e h a n i of filiform growth.
Filiform Corrosion. It is generally agreed that in filiform corrosion the head of a filiform is anodic and that differential aeration is involved. A question not yet resolved is the mechanism of disbonding. As the head is anodic, an anodic disbonding is suggested. However, in blistering, delamination occurs cathodically, and on anodic polarization pitting rather than delamination of the adjacent film is observed. It is emphasized by Ruggeri and Beck (1983) that oxygen and water are supplied to the corroding area of the filiform head by diffusion through the porous tail only. In this case, however, it is difficult to understand why filiforms become broader with increasing thickness and finally are replaced by blistering. Moreover, filiorms grow periodically, as indicated by their segmented structure, and the growth again depends on thickness. In accord with the mechanism of blistering a cathodic mechanism of disbonding seems to be realistic and would also explain the stepwise growth (Figure 15). [Experimental evidence for the existence of a cathodic area in front of the anodic head was given by Maeda et al. (1977).] An important role is played by the membrane separating the filiform head from ita tail. This membrane probably consists of polymeric iron oxide hydrates of colloidal character (Flynn, 1984). This membrane-forming ability of polymeric oxidation products of iron is also important in blistering caused by micropores.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 347
Relations between Phenomenologically Different Coating Failures. The basic principles underlying the mechanisms of coating failures by metal corrosion are well-known. Local corrosion elements, polarization, differential aeration, and membrane properties are everywhere involved. For the different failure phenomena mainly the rates, intensities, and the interplay of various parameters are responsible. An indispensable requirement for all these coating failures is a substantial decrease of adhesion on exposure to corrosive conditions. Water penetrates the coating film and interferes with the interaction of the coating with the metal surface. On exposure of defective coatings to salt spray, aqueous solutions of alkali salts, like NaCl cause cathodic delamination, starting at cathodic blisters that grow in size and finally meet the corroding anodic area. Accordingly, delamination occurs stepwise. Local cathodes of the corroding area are deprived of oxygen by secondary corrosion reactions involving the oxidation of Fe(I1) to Fe(II1). Therefore this area becomes mostly anodic, and the oxygen needed for depolarization of cathodic areas adjacent to the anodic defect has to be supplied by diffusion through the intact film. Increase of film thickness finally decreases oxygen permeability to a level that is too low for the cathodic reaction to take place at a significant rate. Reduction in oxygen permeability causes a transition from the faster process of blistering and delamination to the slower process of delamination without blistering. In the latter case delamination proceeds, if at all, from the defect into the coatinglmetal interface similar to the delamination process related to cathodic protection (Figure 7, bottom). Therefore, this transition has some interesting implications for combining cathodic protection with organic protective coatings. For such systems permeability must be low enough and film thickness must be high enough to prevent blister formation at cathodic areas by effectively cutting off the oxygen supply. In agreement with practical experience thick-film coating systems are recommended for cathodic protection because then both protective measures are compatible with each other. Blistering on exposure to salt water, e.g., in salt-spray testing, differs significantly from blistering on exposure to pure water, e.g., as used in the humidity cabinet test. In the first case blisters usually appear earlier, their aqueous contents react alkaline, and the presence of coating defects, like pores or scribes, is essential. In the second case the contents of blisters react neutral to weakly acid, formation is slower, especially if caused by the structure of the metal surface, and typical corrosion products are deposited at the dome of the blisters. Formation of such neutral blisters is probably governed by differential aeration. The water transport may be explained by electroosmosis, eventually combined with osmosis, and the electroneutrality at cathodic and anodic areas is probably achieved by hydrogen and hydroxyl ions from the ionization of water. Alkaline and neutral blistering are thus caused by different exposure conditions, which are both met in practical application of organic coating systems. On defective coatings both types of blisters may develop, depending on
the proximity to the defect area. The dependence of the blistering type on exposure conditions sheds some light on the controversy about the usefulness of the salbspray test for evaluating the corrosion protective property of organic coating systems. Filiform corrosion as well as alkaline blistering obviously are periodic processes. An essential requirement is that the humidity of exposure be above the level needed for the liquefaction of salts contained in the filiform head. When the humidity is increased to the level of a condensed aqueous phase adjoining the coating, e.g., in salt-spray testing, filiform corrosion passes over to alkaline blistering that is cathodic by nature. When the film thickness is increased, the width of filiforms increases and finally filiform corrosion ceases. Paint failures by metal corrosion are processes that involve more than one mechanism and many parameters. The phenomena of these failures appear to be confusing, not so much because fundamentals are unknown but rather because it is difficult to correlate the influencing factors and estimate their importance in each case. The picture drawn in this lecture of a unified view of the mechanism of coating failures by metal corrosion is not yet complete. Some influences, as that of membrane charges which were studied by Maitland and Mayne (1962) and Mayne (1957), have not been included in this discussion. Moreover, some explanations still have more hypothetical character. However, various experimental results fit too well together to deny the connections discussed. It is hoped that work going in several laboratories in Europe as well as in the U.S.A. will provide the missing links to complete our understanding of this industrially important field of coatings technology. Acknowledgment We gratefully acknowledge the support of the Bundesministerium fuer Wirtschaft via the Arbeitsgemeinschaft fuer Industrielle Forschung e.V. (AIF). Literature Cited Bullett, T. R. J . Oil Colour Chem. Assoc. 1961, 4 4 , 807. Bullett, T. R.; Rudram, A. T. S.J . Oil Colour Chem. Assoc. 1961, 4 4 , 787. Evans, U. R. “Metallic Corrosion Passivity and Protection”; E. Arnold: London, 1945; p 268. Flynn, C. M., Jr. Chem. Rev. 1984, 8 4 , 31. Funke, W. Rag. Org. Coat. 1961, 9 , 29. German Standards DIN 50900 (ed. 1951) Definitions. Grubitsch, H.; Heckel, K. farbe Lack 1960, 66, 22. Kittelberger, W. W.; Elm, A. C. I n d . Eng. Chem. 1947, 39, 876. Leidheiser, H., Jr.; Wang, W.; Igetott, L. hog. Org. Coat. 1963, 1 1 , 19. Lonsdaie, H. K.; Merten, U.; Riley, R. L. J . Appl. Polym. Sci. 1965, 9 , 2341. Maeda, S.; Hayashi, T.; Yamamoto, K.; Tanaka, T. Proc. 56th Met. finish. Conf. Jpn. 1977, 102. Maitland, C. C.; Mayne, J. E. 0. Off. Dig. f e d . SOC. Paint Techno/. 1962, 3 4 , 972. Matsui, E. S. Technical Report N 1373, Civil Engineering Laboratory, Port Hueme, CA, Feb 1975. Mayne, J. E. 0. J . Oil Colour Chem. Assoc. 1957, 4 0 , 183. Meyer, W.; Schwenk, W. farbe Lack 1979, 85, 179. Ruggeri, R. T.; Beck, T. R. Corrosion (Houston) 1983, 3 9 , 452. Schwenk, W. GWf Gas Wasserfach: GaslErdgas 1977, 118, 7. van der Meer-Lerk, L. A.; Heertjes, P. M. J . Oil Colour Chem. Assoc. 1975, 58, 79.
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Received for review November 8, 1984 Revised manuscript received April 2 , 1985 Accepted May 14, 1985
