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of metal cleanliness that can be obtained from a blasting process, into a protective coating performance system. There is an increasing trend in the steel tank industry to achieve the specification of coating applications by following the coating manufacturers’ suggested procedures. I have been in the coating industry for 24 years and have never seen a coating manufacturers’ specification sheet that specifically addresses the type of blast aggregate that is best suited to his particular coating. The economics of blast aggregate selection are a limitation; they can, however, be overcome. As coating applicators, we have managed to design and install a metal preparation system, based on compressed air blasting, that enables us to select any one of four blast aggregates-black beauty, aluminum oxide, steel grit, or steel shot-and at the same time keep our blast aggregate costs below that of silica sand on a one time basis. The coating industry is being constantly reminded that
losses due to corrosion are in the billions of dollars per annum. We already have the coatings; all that is really needed to reduce these losses is to insist that they be applied to a properly prepared surface. The blast aggregate must be considered if premature failure, of even the best coating, is to be prevented.
Literature Cited Bayliss, D. A,; Bray, H. ”Permeability Tests on Moc!ern Coatings”; presented at Corrosion/81, National Association of Corrosion Engineers -Annual Meeting, Toronto, Canada, April 1981; Paper 84. Drisko, R. W.; Schwab, L. K. “Reiation of Steel Surface Profile to Coating Performance”; presented at CorrosionlBO, National Association of Corrosion Englneers Annual Meeting, Chicago, IL, March 1980, Paper 116. Lekiheiser, H., Jr.; Music, S.; McIntyre, J. F. “Improved Corrosbn Resistance of Steel in Miid Media After Abrasive Blasting with Alumina”; Department of Chemistry and Center for Surface and Coating Research, Lehigh University: Bethelem. PA, 1984.
Received for review November 27, 1984 Accepted April 30,1985
Mechanisms Associated with Underfilm Corrosion of Painted Cold Rolled Steel John V. Standish The BFooodrch Company, Brecksviile, Ohio 44 14 1
Painted coid rolled steel panels were exposed in the atmosphere as well a s to laboratory corrosion tests. Scanning electron microscopy was used to observe samples cut from panels after corrosion occurred beneath the paint films. Energydispersive X-ray analysis was used to characterize the corrosion products. Results show that the corrosion process which occurs in the traditionally used laboratory salt-spray test Is not representative of that which occurs during the atmospheric exposure of painted steel. An important mechanism in atmospheric corrosion is the mechanical damage caused to the paint film and the painthubstrate interface by the formation of solid corrosion products beneath the paint film. Such corrosion products do not form in the saltspray test. A laboratory corrosion test involving alternate exposure of panels to humid and dry conditions does allow corrosion products to form. The results also provide information concerning the influence of chloride ion, zinc phosphate pretreatments, and steel surface roughness on the corrosion process.
Introduction This report will present and discuss results that have been obtained by using scanning electron microscopy to observe the spread of corrosion beneath paint films on cold rolled steel. Painted panels were exposed to corrosive environments either in the laboratory or in the atmosphere prior to observation with the microscope. Samples were also characterized by energy-dispersive X-ray analysis. Substantial differences were found between the appearance of samples exposed to laboratory salt spray and that of samples exposed to atmospheric environments. The microscopic observations help provide greater understanding of the mechanisms associated with underfilm corrosion of painted cold rolled steel during atmospheric exposure. The results also provide at least a partial explanation for the poor correlation between atmospheric and salt-spray corrosion tests for painted steel found in other studies, for example, Westberg and Borjesson (1980) and Franks and Nowak (1983). Experimental Section Sample Preparation. Results have been obtained with different coatings applied to drawing quality low carbon 0196-4321/85/1224-0357$01.50/0
cold rolled steel. One group of coatings was prepared to simulate automotive exterior finishing. Steel panels were zinc phosphatized and coated with a primer, sealer, and top coat of paint. A second group of panels was prepared with an iron phosphate pretreatment. The primers used were either a thermosetting latex coating or a thermosetting epoxy coating, typical of those used in the coil coating industry. An acrylic top coat was applied over the primers. The pretreatments and coatings used are proprietary formulations, and details of their composition are not available. Prior to exposure to a corrosive environment a sharp carbide-tipped tool and a straight edge were used to create a linear defect in each paint film extending to the base metal. These defects, referred to as scribes, were typically about 8 cm long. Corrosive Environments. (1) Laboratory salt spray as described in ASTM B117. (2) Laboratory cyclic exposure. One day was required for one cycle of exposure that involved (a) 15-min immersion in a 5% NaCl solution, (b) 11/4h in the ambient laboratory atmosphere, typically 55% relative humidity and 22 O C and, (c) 221/2h in a cabinet maintained at 100% 0 1985 American
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I
7/ Figure 1. Cross section through a sine phosphatized and painted accelerated atmospheric exposure.
steel sample after 9 months of
relative humidity and 38 OC. This cycle was repeated daily except for weekends when the panels were kept only in the humidity cabinet. The purpose of this cyclic test was to simulate the wet-dry cycles that occur in atmospheric exposure. The test used is similar to that used by Nowak et al. (1982)and HosDadaruk et al. (1978). (3) Atmospheric exbsure in Middletown, OH. Panels were exposed at an angle of 30' to the ground. Middletown is considered to have a mild industrial atmosphere. (4) Accelerated atmospheric exposure in Homestead, FL. Samples were exposed at 45' to the ground and were sprayed each week day with a 5% NaCl solution. Homestead is located about I-mi inland from the ocean in southern Florida. The zinc phosphatized panels were exposed to the salt spray, cycle, and accelerated atmospheric environments. The iron phosphatized panels were exposed to the Middletown atmospheric environment only. Preparation of Samples for Microscopy. Sample pieces containing the scribed defect were cut from ex& panels. The samples were mounted on edge in an epoxy potting compound (Buehler 20-8130-128) and polished so that the steel substrate, coating, and corrosion products within and adjacent to the scribe could be viewed in cross section. Samples were polished by using standard metallographic techniques to a I-pm diamond finish. Samples were examined by using an AMR Model loo0 scanning electron microscope operated at 20 kV. Electron micrographs were obtained by using either secondary or backscatter electron imaging. In many cases backscatter imaging provided micrographs with better contrast than that obtained with secondary electron imaging. Qualitative energy dispersive X-ray analysis was also used to identify the elemental composition of samples or portions of samples. Spot X.ray analysis of small features was obtained by holding the electron beam in a single location. Spot analysis was considered to analyze a volume of about 1 pm'. Analysis of relatively larger areas was obtained by isolating the particular field of interest beneath the electrnn beam. Results and Discussion Accelerated Atmospheric Exposure. Figure 1 shows a cross section through a sample after 9 months of exposure. The steel substrate is labeled and runs horizontally across the micrograph. Arrows point to the paint layers. a break in the paint that is the scribe defect, and the solid corrosion product that has formed beneath the paint. The corrosion products did not adhere strongly to the substrate and became detached during sample preparation. The sample is surrounded by the epoxy potting compound.
Figure 2. steel/MrroSiOn product interfacebeneath the paint film shown in Figure 1.
e 4
1
CORROSION
/
~.~
2 0 pm
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Figure 3. Paint/substrate interface to the right of the field shown in Figure 1. Some c o m i o n products have formed on the outside paint surface. Several breaks are seen in the paint film above the corrosion products. These breaks are mechanical damage caused by the corrosion products. The only element detected in the large volume of corrosion products by X-ray analysis was iron. Figure 2 shows a cross section of the steel/corrosion product interface in an area below the detached corrosion products. The dark corrosion products shown by the arrow were found to contain chloride. Chloride was not detected in the adjacent corrosion products. This observation indicates that the chloride ion is concentrated at specific sites along the steel/corrosion product interface. Cleary (1984) used an electron microprobe to observe corrosion products in cross section on steel samples taken from automobiles exposed to chloride environments and also found that chloride was concentrated at the steel/corrosion product interface. Cleary identified the presence of small amounts of chloride in the bulk of the corrosion products; an electron microprobe is likely to be more sensitive than an energy-dispersive X-ray attachment on a scanning electron microscope. In pitting and crevice corrosion, locations having high chloride concentrations are associated with the anodic corrosion reaction. Perhaps in a similar manner, sites like those in Figure - 2 are the location of the anodic corrosion reaction. Figure 3 shows the paint/substrate interface to the right of the field shown in Figure 1. A thin layer of corrosion products is seen between the paint and the steel in the left-hand side of Figure 3. Figure 4 shows the underlined area of Figure 3 at higher magnification. From the righthand side of the figure, the arrows show (A) a zinc phosphate crystal/steel interface,
Ind. Eng. C h " Rod. Res. Dev.. Vol. 24. NO. 3, 1985 359
D
E E ' STELL
I
I
t. Figure 4. The underlined area shown in Figure 2.
4
CORROSION
STEEL
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-
4 'CRACK
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Figure 5. A zinc phosphatized panel after 28 days of laboratory
STEEL
4
CORROSION
2 0 pl
cyclic exposure.
Figure 6. A crack at the paint/subatrate interface adjacent to the corrosion products shown in Figure 5.
(B)a zinc phosphate crystal/paint interface, and (C) an interface where the paint is in contact with a surface that has no visible zinc phosphate crystals. The size of these features is large enough that they were readily identified by X-ray analysis. On the lefthand side of Figure 4 a crack is seen. This crack runs at point D through the base of a phosphate crystal. At point E the crack appears to run through the bulk of a phosphate crystal. A t point F the crack is running through the paint, and a paint residue appears to remain on the substrate surface. In the accelerated atmospheric exposure, as well as in all other exposures discussed below, corrosion occurred on panels only in areas adjacent to the scribe defect. Cyclic Laboratory Exposure. Figure 5 shows a eample after 28 days of cyclic exposure; the scribe is out of view to the right of the field shown. Corrosion products are present, lifting the paint film from the substrate. Epoxy potting compound is also seen below the paint film, indicating that the corrosion products are porous. Figure 6 shows the left-hand side of Figure 5 at higher magnification. A crack is seen at the paint/substrate interface. Paint and zinc phosphate residue remain on the steel surface. The results shown so far suggest that in the accelerated atmospheric and cyclic tests an important corrosion mechanism involves the formation and buildup of solid corrosion products beneath the paint film adjacent to the scribe. These corrosion products act as a wedge that lifts or peels the paint from the substrate. The rate of corrosion is then influenced by the amount of mechanical damage caused to the paint film and paint/substrate interface by the buildup of corrosion products. Also, the amount of paint and phosphate residue that remain on the steel surface after crack formation at the paint/substrate in-
Figure 7. A zinc phosphatized and painted panel after 14 days of exposure to salt spray.
terface will influence the rate of corrosion product formation on freshly exposed surfaces. One concern is that shrinking of the epoxy mounting compound during curing of the compound may have pulled the paint film from the substrate and caused the cracks that were observed. While this concern cannot be completely eliminated, there are a few reasons to believe that the cracks are in fact mechanical damage caused primarily, if not entirely, by the buildup of solid corrosion products. First is that the literature shows that the peel strength of adhesives to metals is low (Bolger, 1973), about 30 Ib/in. of width a t best and often as low as 5 Ib/in. Next, Pickering et al. (1962) have shown that the force exerted by the formation of corrosion products in confined spaces can be several thousand psi. Also, the only location where
360 Ind. Eng. chsm.Rod. Res. Dev.. Vol. 24, No. 3. 1985
Figure 8. A latex primed panel after 18 months of atmospheric exposure.
cracks were o h w e d a t the paint/substrate interfa- was adjacent to corrosion produds, cracks were not observed at locations some distance from the corrosion products. Finally, in any event, these cracks show the area just ahead of the c o m i o n products to be very susceptible to damage. Salt-Spray Exposure. Figure 7 shows a sample after 14 days of salt-spray exposure. The steel substrate has dissolved from beneath the paint film at area A. While corrosion products exist on the paint surface, area B,only a thin layer of corrosion products exists beneath the paint fdm at area C, and no corrosion products exist to the right of area C, although the paint film is delaminated from the substrate. The features observed in this micrograph are consistent with the mechanism for corrosion in neutral saline environments that has been presented earlier by others (for example, Dickie and Smith, 1980; Funke, 1983). There is metal dissolution only a t the scribed defect where the anodic reaction is localized. Corrosion products do not form beneath the paint film since this area is the location of the cathodic corrosion reaction believed to involve the reduction of water and oxygen to form hydroxide ions. The salt-spray sample did not show the large buildup of corrosion products beneath the paint film that was obtained during the accelerated atmospheric or cyclic testing. This absence of corrosion product formation and the associated mechanical damage that corrosion products can cause to paint films and paint/substrate interfaces probably is an important reason for the poor correlation found between salt-spray and atmospheric testing of painted steels shown in papers by Franks and Nowak (1983) and Westberg and Borjesson (1980). Atmospheric Exposure. Figure 8 shows an iron phosphate treated and latex primed panel after 18months
of atmospheric exposure. Corrosion products have lifted the paint film from the substrate. There are crack networks in the c o m i o n product. X-ray analysis showed the bulk of the products contain only iron. The corrosion products at point A, however, contained sulfur, potansium, and perhaps phosphorus. These elements could not be detected in either the substrate or the paint film by X-ray analysis. At least the sulfur and potassium are believed to be from the atmosphere, and they perhaps reached the steel substrate through cracks in the corrosion products. The phosphorus may be from the iron phosphate pretretment. This localized concentration of foreign species is similar to the local concentration of chloride found in the accelerated Florida exposure. Point B shows an area where the top coat has delaminated from the primer. Figure 9 shows the paint/substrate interface of an e p oxy-primed sample after 18 months of atmospheric exposure. Arrow A shows the corrosion products that have spread furthest from the scribe. Arrow B shows a crack in the paint film that precedes the corrosion products. Considerable paint residue remains on the steel surface. The amount of residue may be related to the steel surface roughness. The iron phosphate layer is so thin it cannot be seen in cross section. Although the iron and zinc phosphatized panels involved different paint coatings and atmospheric exposure locations, they showed many features in common. These similarities included a volume of corrosion products lifting the paint film from the substrate, mechanical damage in the paint film and at the paint/substrate interface, and ionic species concentrated a t the corrosion product/substrate interface. These s i m i i t i e a indicate that the general mechanisms associated with the atmospheric corrosion of painted steel may be similar for a variety of coatings and exposure conditions.
Conclusions Scanning electron microscopy has been used to observe painted steel samples after exposure to various corrosive environments. Microscopy revealed that solid corrosion products form beneath the paint film adjacent to defects in the paint film during atmospheric and laboratory cyclic exposure. The corrosion products cause mechanical damage to the coating and to the coating/substrate interface. The absence of solid corrosion product formation during the salt-spray test helps explain the poor correlation often found between salt-spray and atmospheric tests. Two important variables are thought to influence the performance of painted steel in atmospheric exposure. One is the effectiveness of a pretreatment as a corrosion inhibitor, which will help reduce the rate of corrosion product formation. A second is the resistance of a paint film and
Figure 9. An epoxy primed panel after 18 months of atmospheric exposure.
Ind. Eng. Chem. Prod. Res, Dev. 1985, 24, 361-369
film/substrate interface to mechanical damage caused by the corrosion products. Acknowledgment
The contributions of T. R. Roberts and G. W. Whelan to the development of this paper are gratefully acknowledged. Registry No. Steel, 12597-69-2. L i t e r a t u r e Cited
361
Dickie, R. A.; Smlth. A. G. CHEMTECH 1980, 10, 31. Franks, L. L.; Nowak, E. T. Society of Automotive Engineers Technical Paper 831 819, Detroit, MI, 1883. Funke, W. J. Coat. Techno/. 1083, 55(705), 31. Hospadaruk, V.; Huff, J.; Zuriila, R. W.; Greenwood, H. T. Society of Automotive Engineers Technical Paper 780186, Detroit, MI, 1978. Nowak, E. T.; Franks, L. L.; Froman, G. W. Society of Automotive Engineers Technical Paper 820427, Detrolt, MI, 1982. Pickering, H. W.; Beck, F. H.; Fontana, M. G. Corrosion (Houston) 1962, 13, 2301. Westberg, J.; Borjesson, L. National Association of Corrosion Engineers Technical Paper 278, Chicago, IL, 1980.
Bolger, J. C. "Treatise on Adhesion and Adhesives"; Patrick. R. L., Ed.; Marcel Dekker: New York, 1973; Voi. 3. Cieary, H. J. Corrosion (Houston) 1084, 40, 606.
Receiued for review November 27, 1984 Accepted April 29, 1985
Interface Chemistry of Stoved Organic Coatings James E. Castle and John F. Watts' Department of Materlals Science and Engineering, University of Surrey, Guildford, Surrey GU2 5XH, United KinHom
The failure, by mechanical delamination and by cathodic disbonding (both at rest potential and at cathodic polarization), of epoxy-phenolic, epoxy-acrylic, and styrenated alkyd coated mild steel has been characterized by X-ray photoelectron spectroscopy. I n all cases failure occurs close to, but not at, the substrate/coating interface. By matching the C 1s spectra to the sum of their components' peaks, it has proved possible to elucidate subtle differences in surface functionality. I n the case of the epoxy resins, epoxy residues are displaced from the metal oxide surface by aqueous solution in accord with thermodynamic principles. The substrate and coating C 1s spectra following mechanical failure are, however, similar, as are those of the styrenated alkyd resin following corrosion-induced failure. These resutts are considered In the light of current theories of cathodic disbondment and other published data; it is concluded there is no universally applicable mechanism for this phenomenon.
Introduction
The interface between a metallic substrate and an organic adherate, be it a surface coating or an adhesive, is of crucial importance in determining the performance of that system. Such performance may be assessed in terms of load-bearing ability, resistance to failure in the presence of aggressive aqueous media (high humidity, salt-spray, salt solutions), or resistance to attack in a combination of such deleterious environments. A major advance in the field of adhesion science and technology in recent years has been the chemical definition of such interfacial regions. This has become possible by the application of surface analysis methods to adhesion investigations. In particular X-ray photoelectron spectroscopy (XPS) has been found to be an ideal technique for the study of polymer-to-metal adhesion phenomena. Recent progress in this area has been the subject of overviews by Dickie (1983) and Watts (1984a). Work carried out in this laboratory using XPS has shown that the interface between a polybutadiene coating and a mild steel substrate is characterized by a chemically reacted zone identified by the presence of divalent iron in the XPS spectrum (Castle and Watts, 1981; Watts and Castle, 1983). The formation of this zone is brought about as a result of the oxidative curing of the polybutadiene resin, which in turn a c b as a reducing agent for the iron oxide. By carrying out model experiments on bulk iron oxide substrates, stoving the organic coating in an inert atmosphere, and removing the uncured resin with a solvent, Watts and Castle (1983) observed such interfacial modification directly by XPS. Further confirmatory evidence of such a reaction has been provided by emission Mossbauer spectroscopy (Leidheiser et al., 1982). More 0196-4321 /85/ 1224-0361$01.50/0
recently FT-IRS studies of thin polybutadiene films on a variety of metallic substrates have led to the same conclusions (Dickie et al., 1984). This interfacial region of the polybutadiene/mild steel couple is thus better described in terms of a three-dimensional interphase zone rather than a two-dimensional interface. Figure 1 presents an idealized view of this scheme. Such a view of a chemically reacted interphase contrasts markedly with the general view that adhesion phenomena are generally a result of van der Waals or hydrogen bonding [see, e.g., the review by Kinloch (1980)l. The formation of such a zone may also be important in the design of new adhesive or coating systems where superior properties are required. To see if the occurrence of an interface zone is a general consequence of stoving, three commercially available resin systems were investigated: epoxy-phenolic, epoxy-acrylic, and styrenated alkyd, which are all normally stoved after application. We report the failure mode of these coatings, as determined by XPS, when subjected to three different tests: mechanical failure, exposure to NaCl solution, and cathodic polarization in the salt solution. Experimental Procedure Surface Coatings. The coatings were applied to thin
steel test panels (Gold Seal Type, Pyrene Chemical Services Ltd.) purchased in either the cold rolled or polished condition. In some cases they were conditioned by heating in an air oven at 150 OC for 1h following solvent cleaning, prior to coating. The surface composition was determined by XPS for both types of panels, before and after such a conditioning process, although most of the tests reported here used the bare steel panels. The coatings were applied with a wire-wound bar coater to a 0 1985 American
Chemical Society
