Ind. Eng. Chem. P r d . Res. Dev. 1985, 2 4 , 353-357
Registry No. NaCl, 7647-14-5;HzO, 7732-18-5;Ni, 7440-02-0; polybutadiene, 9003-17-2.
Literature Cited Bacon, R. D.; Smith, J. J.; Rugg, F. M. Ind. Eng. Chem. Ind. Ed. 1948, 40, 161. Brasher, D. M.; Nurse, T. J. J . Appl. Chem. 1959, 9 , 96. Callow, L. M.: Scantlebury, J. D. J . OllColour Chem. ASSOC.19818, 84(2), 83. Caiiow, L. M.; Scantlebury, J. D. J . Oil Colour Chem. Assoc. 198lb, 64(2), 119. Callow, L. M.; Scantlebury, J. D. J . Oil Colour Chem. Assoc. 1981c, 64(2), 140. Epelboln, I.; Gabrieill, G.; Keddam, T.; Takenouti, H. In “Electrochemical and Corrosion Testing-ASTM”; MansfeM, F.; Bertoccl, U., Eds.; ASTM: Philadelphia, 1981; p 150. Haruyama, S.; Tsuru, T. I n ”Passivlty and Its Breakdown on Iron and Iron Base Alloys”; Staehle, R. W.; Okada, H., Eds.; NACE: Houston, 1976; p 41. Hubrecht, J.; Vereecken, J.; Piens, M. J . Electrochem. SOC. 1984, 131, 2010.
Coatings and Corrosion-Even
353
Kendig, M. W., Rockweii International, Thousand Oaks, CA, personal communication, 1983. Klnsella, E. M.; Mavne, J. E. 0.Br. Polvm. J . 1969. 1 . 173. Leldheiser, Henry, Jr.; Kendig, M. Corrosion (Houston) 1976, 3 2 , 69. Mansfeld, F.; Kendig. M.; Tsai, S. Corroslon (Houston) 1982, 3 8 , 478. Mayne, J. E. 0.;Mills, D. J. J . CVI Colour Chem. Assoc. 1975, 58(5), 155. Mayne, J. E. 0.;Scantiebury, J. D. 61.Polym. J . 1970, 2 , 240. Mikhailovskii, Y. N.; Strekaiov, P. V.; Baiandina, T. S. Zashch. Met. 1978,‘ 12, 513. O’Brlen, H. C. Ind. Eng. Chem. 1986, 58(6), 45. Piens, M.; Verbist, R. In “Corrosion Control by Organic Coatings”; Leidheiser, H., Jr., Ed.; NACE: Houston, 1981; p 32. Rothweil, G. W. J . Oil Colour Chem. Assoc. 1969, 52(3), 219. Scantiebury, J. 0.;Sussex, Graham A. M. I n “Corrosion Control by Organic Coatings”; Leidheiser, H., Jr., Ed.; NACE: Houston, 1981; p 51. Strivens, T. A.; Taylor, C. C. Mater. Chem. 1982, 7 , 199. Touhsaent, R. E.; Leidheiser, H., Jr. Corrosion (Houston) 1972, 28, 435. Wormweii, F.; Brasher, D. M. J . Iron Steel Inst. London 1950, 189, 141.
Received f o r reuiew November 8, 1984 Accepted April 3, 1985
the Best Fail
Elson G. Fernandes Clemmer Industries (1964) Ltd., Waterloo, Ontario, Canada N2J 4A 1
This nontechnical presentation correlates the failure of correctly selected, formulated, and applied protective coatings to the type of aggregate used in a dry-blasting metal preparation system. Field observations of protective coating performance indicate that cathodic disbonding tends to occur more extensively on metal surfaces that have been prepared by using steel grit as a blast aggregate than on those prepared by using silica sand. Protective coating failures directly traceable to blast aggregate inclusion are also found to be more prevalent on surfaces prepared with a steel grit blast aggregate. Visual examination of metal surfaces prepared by using four blast aggregates-aluminum oxide, silica sand, steel grit, and steel shot-indicates that physical surface parameters may be responsible for some of these failures. The continued use of current blast specifications without consideration for the particular blast aggregate can lead to premature failure of even the best protective coating application.
The protective coatings now available are certainly the best that can be produced. When they are correctly specified and applied to a properly prepared metal substrate, they have been made to perform admirably in the controlled conditions of coating and testing laboratories. Unfortunately, the move from testing laboratory to field application has not been favored with the same success. All protective coating applications are expected to have a limited life that, with inspection and proper maintenance scheduling, may be extended to the infinite stage on occasion. My concern is with those protective coatings that fail prematurely for reasons usually considered to be associated with their application procedure. The cathodic disbonding of protective coatings is one example of this type of premature protective coating failure. In late 1974 and early 1975 a few of the provinces of Canada adopted legislation for the corrosion protection of underground fuel storage tanks manufactured from hot rolled steel. The system, designated ULC-S-603.1, was developed in committee with representation from government, Underwriters’ Laboratories Canada (ULC), major oil companies, corrosion consultants, protective coating manufacturers, and steel tank manufacturers. In general the protective coating specifications of ULC-S-603.1 required that a coating be tested and approved by the ULC, 0196-4321/85/1224-0353$01.50/0
applied to a 16-mil dry film thickness on a prepared steel substrate equivalent to a CGSB-31GP404 (SSPC 6 or NACE #3) commercial blast, and designed to be used in conjunction with a sacrificial anode system consisting of single or multiple 17-lb,high-potential magnesium anodes. The control coating for the S-603.1 system was a 1GP184 coal tar epoxy that had been previously tested and approved by the Canadian Government Specifications Board (CGSB) for application to underground pipelines. This system, with its 48-h “dry to handle” time, was too slow for the tank manufacturers, and a 6-h room temperature “dry to handle” two-component coal tar epoxy system was developed. A short time later a two-component 10-min “dry to handle” polyurethane was approved for S603.1 service. Both the 6-h coal tar epoxy and the 10-min polyurethane required the use of two-component airless application equipment and had ULC S-603.1 approval with the inclusion of testing for resistance to cathodic disbonding, fuel immersion, impact, and dielectric properties. All of the initial test panels, including the 1GP184 control panels, were prepared and coated in the field by us. A commercial blast CGSB-31GP404 (SSPC6, NACE #3) on a hot rolled steel substrate, using #1Ottawa flint shot silica sand as the blast aggregate, was followed by one coat of the in0 1985 American Chemical Society
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Coating
0Saw
---
Cut to Bare Steel Immersion Level Fiwre 1. Coating layout and coupon condition after 2-year immenion in regular gas, no lead gas, and diesel fuel.
dividual protective coatings a t 16-mil dry film thickness. AU applications were made by airless spray equipmentthe control coating by single-component cold airless application and the remaining coatings by two-component heated airless application. A contingent agreement to the ULC-S-603.1 specifications allowed, at some future date, the original registered m e r s of 5603.1 tanks to remove those tanks from service and return them to an approved steel tank manufacturer where they would be inspected and refurbished to the then current 5603.1 specification. The refurbished tanks could only be reused by the original registered owner and could not be resold. In 1980 we inspected and refurbished a total of 14 tank, 2 of these were protected by pipeline tape and the remaining 12 by the 6-h coal tar epoxy. Of the 12 epoxycoated tanks 7 showed signs of cathodic disbonding; i.e., water-filled blisters appeared in the coating and the pH reading of the water was more alkaline than pH 8. I have since inspected 10 additional tanks in the field and found 6 of these showing evidence of cathodic disbonding. None of the disbonded tanks had been removed from service because of suspected coating failure. Observations of the 13 cathodically disbonded coating systems indicated that 8 of them were a coal tar epoxy coating and 5 were a polyurethane coating. They were placed underground for periods varying 3 to 5 years. The coatings, though blistered, were essentially intact, and removal of the blisters indicated no visible sign of metal corrosion in the area of the blister. Of the 13tanks 7 were extensively blistered, with 4 of these showing blisters on 50% of their total surface area. Coincidentally, these 4 tanks had been exposed to a gasoline spill; the blisters were found to contain a compound liquid that would ignite but was self-extinguishing,leaving a nonflammable liquid residue, presumably water, with a pH more alkaline than pH 8. Blistering of the protective mating by gasoline was a possibility; however, the coating appeared to be intactthere were unblistered areas adjacent to and surrounded by blistered ones. Immersion testing (Figure 1)instituted
on both protective coatings in hydrocarbon fuels, after 2-years immersion, indicated no changes in either of the protective coatings. Further observation indicated that the seven extensively disbonded coating systems were all subjected to a near white metal blast preparation SSPC-SP-10 using steel grit as an aggregate, suggestingthat a second conclusion based on blast aggregate was possible. Visual observations, utilizing a 2OX micromike, on hot rolled steel coupons prepared by blasting to a SSPC-SP-10 (NACE#2) near white metal blast using four different blast aggregates-24 grit aluminum oxide, 30-mesh silica sand, G-25 and G-40 steel grit, and S-230 steel shotindicate the following conditions exist: (1) Grit alumina oxide (#24) a t 75-psi working blast pressure with a 5/ls-in. venturi blast nozzle produces an evenly distributed valley to peak blast profile with little or no polished metal observed. The general appearance was that of a dull metallic nonreflecting surface. (2) Flint shot silica sand (30 mesh) at 95-psi working blast pressure with a 5/ls-in. venturi blast nozzle also produced an evenly distributed valley to peak profile but with a noticeable increase in polished metal reflective surface as compared to the results with aluminum oxide. The general appearance indicated a dull metallic finish with visible reflective points. (3) Steel grit (G-40)at 95-psi working blast pressure with a '/&. venturi blast nozzle produced no recognizable even peak to valley profile. There were visible areas of folded-over metal and minute cracks in the metal substrate. Polished metal appeared to account for more than 50% of the metal substrate. General appearance was that of a metallic surface with a high percentage of polished than reflective surface. Steel grit (G25. a coarser -ate G-40)used under the same conditions appeared to increase the destruction of the metal substrate. Folded-over metal was more visible and cracks in the metal substrate appeared to be larger and more frequent. Polished reflective areas also appeared to be larger. General appearance was that of a metallic surface with a very high percentage of polished metal visible.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 355
A
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(4) Steel shot (S-230) at 95-psi working blast pressure with a 5/ls-in. venturi blast nozzle produced circular indentations in the steel substrate with no visible folded-over metal or cracks in the metal substrate. There is more apparent surface polishing then with either of the steel grits. General appearance was that of a metallic substrate with craterlike polished depressions. There are obvious physical differences in the appearance of the blast-prepared metal traceable to a change in blast aggregate. The possible effect of these differences on coating adhesion is the most important concern in a study of premature coating failure. Protective coating adhesion as it is viewed by the coating industry does need clarification. The accepted test method for adhesive strength is commonly referred to as the stick on/pull off adhesion test; we are all familiar with the system. We are also aware that coatings do not fail by the square inch; they fail one molecule at a time. If, for example, we applied a protective coating to a polished metal bar (Figure 2), using a stick on/pull off test a reading for adhesion could be found. We take the bar, and without affecting the polished surface, we deform it to a threedimensional blast pattern and again apply a protective coating. Our stick on/pull off tester would indicate a higher reading, and we conclude the deformation of the metal surface provides better adhesion. This is not quite true. If molecule "X"on the bar is moved to the slope of the deformed surface, why would its adhesion to the surface change? The surface has not been changed. Mechanical molecular adhesion of a protective coating will increase by expanding the surface only if each newly created surface is also prepared in the process to permit good adhesion of each protective coating molecule, and, therefore, better cathodic disbonding resistance. From the physical differences presented by the individual blast aggregates it would appear that an aluminum oxide aggregate will provide a surface slightly better suited to mechanical molecular adhesion than will a silica sand aggregate. Steel grit and steel shot aggregates with their tendency to polish the metal substrate would provide much poorer surface conditions for mechanical molecular adhesion, tending to more readily disbond under cathodic forces. A second parameter in relation to protective coatings and cathodic disbonding is coating permeability. Since
there can be no cathodic current through the highly dielectric protective coating films unless there is an electrolyte path to the metal substrate, permeability of the coating is a prerequisite of cathodic disbonding. The electrolyte considered here is water, and the permeability of the protective coating film is dependent on two factors: (1) The permeability of the coating itself, primarily considered to be a factor of resin choice and formulation know how. In practice, however, it is a definite factor of application method. (2) The ability of the coating to flow out on the prepared metal substrate and displace all of the air in this complex surface. Total wetting out, of course, is also a necessary factor for good adhesion. The two coatings in question, coal tar epoxy and polyurethane, are made of resin materials that are good barriers to permeability and have high dielectric strength. On inspection of the 16-mil DFT films with a 20x micromike one finds that both resins contain trapped bubbles which appear to effectively reduce the dry film thickness (DFT) to somewhat less than 50% of the original 16-milDFT. In these instances the rate of permeation becomes a factor of film integrity between the trapped bubbles. At magnifications in the molecular range one would probably find connecting paths through the entire 16-mil film. The flow characteristic of a coating is also a factor of formulation; however, when working with fast-reacting 100% volume solids protective coatings, metal substrate profile becomes the major factor in protective coating flow out. The flow out of a highly viscous coating will only occur on a surface profile that provides even peak to valley configuration. A profile of bent-over peaks, cracked surfaces, etc. will not allow displacement and wet out by a fast curing highly viscous coating. Any voids left in the coating metal interface at the point of permeation will of course be immediately filled with water, promoting the chemical reactions necessary to form a blister. Here again the physical configuration presented by the use of aluminum oxide or silica sand aggregates should provide excellent flow out on a protective coating. The craterlike profie provided by steel shot, although providing excellent flow out characteristics, is defeated by the metal polishing effect on adhesion. The steel grits provide the worst surfaces for flow out characteristics.
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The following evidence supports the influence of blast aggregate on the cathodic disbondment of protective coatings: h i s k o and Schwab (1m)have concluded that in teating of coating bond strengths VB. type of blast aggregate used, in a salt-fog exposure for 8383 h, silica sand lost 62% of its original bond strength; G40 steel grit lost 85%; S-280 steel shot lost 83%; and Polygrit 80, a combination of 10-25% aluminum oxide, 36% silicon oxide, 2535% iron oxide, and l0-15% miscellaneous oxides, lost only 25%. Leidheiser et aL (1984), in a study based on the corrosion resistance of steel after blasting with alumina, have concluded that in cathodic disbonding tests metal surfaces blasted with steel grit as the blast aggregate always tend to disbond more readily than do those blasted with alumina as the blast aggregate. Bayliss and Bray (1981) have observed that in teats of polyurethane films very small voids or bubbles can be found in the fh, and they report that protective coatings applied by the airless spray method tend to have more than do those applied voids or bubbles trapped in the fh with conventional spray or applicator. Cathodic disbonding of a protective coating is not considered by many experta in the field to be a serious problem. The steel tank industry would probably concur. There was no evidence of corrosion under the blistered coating on any of the tank surfaces examined. A more serious premature coating failure d d y related to the physical differences in blast aggregate, in which instance corrosion of metal substrate is always apparent, is caused by the inclusion into the metal substrate of particular blast aggregate. Our first observation of blast aggregate inclusion occurred in the application of a 4-to 6-mil spray applied baked phenolic protective coating to the interior of a 28NBgal storage tank. Initial investigations into steel grit as a substitute blast aggregate for silica sand had included the consideration of the phenolic lining. We were mured that the change would have no effect on this particular application. We had been successfully performing this application by using a SSPC-SP-10 white metal blast with a silica sand blast aggregate. Our first attempt at using G40 steel grit as a blast aggregate resulted in the location of 18 holidays in the coating surface as found by a 68-V wet sponge holiday detector. We found that the holidays generally consisted of a sharp point that could be felt by running a hand over the coating surface. Assuming the points were probably steel grit missed in the vacuum cleaning proem, we tried removal by knife blade without success. Reblasting of the tank using silica sand as the blast aggregate followed by the same coating process produced a surface free of any holidays. We concluded that steel grit inclusion in the metal substrate was the problem. Consequent examination of random samples of G40 steel grit turned up slivers varying from 0.004 to 0.014 in. in length. We immediately ceased to use steel grit as a blast aggregate in any protective coating application that required exposure to harsh environments. We have succegsfuuy used G40 steel grit and S-230steel shot as blast aggregates for protective coating applications in atmospheric exposure. We obtained excellent results with a SSPC-SP6 commercial blast specification. We have, however, witnessed premature coating failure in atmospheric exposure when G-40 steel grit was used as the aggregate with an SSPC-SP10 white metal blast specification followed by one coat of chromate primer and two coats of white enamel. Rusting occurred as points in the coating surface after 8 mo exposure. Reblasting with a silica sand
Figure 3. Atmospheric corrosion due to blast aggregate inclusion, exposure 8 m a
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.
. -
. - * ' .
. .
Figure 4. Clms-up of m d d mea from Figure 3.
aggregate and recoating with the same system show no sign of failure after two years (Figures 3 and 4). Premature coating failure due to steel grit inclusion is always evidenced by corrosion initiation a t the point of inclusion. It is very prevalent in the construction industry where the immediate economic advantage through automation of the blast process is a major factor. The protective coating industry is not suffering from a lack of information but from a reluctance to put legitimate, well-documented, corrosion data into effect. We have, for instance, managed to transfer an accepted standard, intended to be a descriptive means of identifying a degree
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Ind. Eng. Chem. Prod. Res. Dev. 1085, 2 4 , 357-361
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
Chemical Society
