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of repeat units bound to the surface is not too surprising when the energy of bonding per repeat unit is this strong (-9 kT). Conclusions The FeOH surface sites of ferric oxides are acidic and adsorb nitrogen and oxygen bases with appreciable heats of adsorption, which can be quantitatively correlated and predicted with the Drago E and C equation. The measured heats of adsorption of test bases of different C I E ratios predict that the stronger acidic surface sites have the following Drago constants: CA = 0.8 f 0.2 and EA = 4.5 f 1.1 (kcal/mol)1/2. These constants are similar to those already measured for the SiOH sites of silica and the TiOH sites of titania, all of which are hard acid sites similar to, but about twice as strongly acidic as, alcohol groups. Poly(viny1pyridine) adsorbed rapidly onto ferric oxide from a dilute solution in benzene, with 52% of the repeat units bonded to acid sites and with a heat of adsorption of -9 kT per site. Flow calorimetry with a UV solute concentration detector downstream of the calorimeter bed can give accurate distributions of heats of adsorption. Moisture levels of solvents, solutes, and adsorbents strongly influence heats of adsorption. In the flow microcalorimeter, control of the moisture levels was attained by pumping down with a vacuum pump with liquid nitrogen trap while monitoring evaporative cooling and pressure of the bed. Karl Fischer analyses of water contents of solvents and solutions was necessary to achieve proper dryness.
A laboratory microcomputer to collect and correlate data from the heat and concentration sensors was an important part of this project. Acknowledgment We acknowledge with thanks the support of this project by the Office of Naval Research, Agreement N00014-79C-0731 (H. L. Leidheiser, Jr., P. I.). We also thank George Salensky of Union Carbide for valuable suggestions and Prof. D. W. Dwight of Virginia Polytechnic Institute and State University for ESCA analyses of our oxides. Registry No. THF, 109-99-9; DMF, 68-12-2; PMMA, 901114-7; Me2S0,67-68-5; ferric oxide, 1309-37-1;pyridine, 110-86-1; triethylamine, 121-44-8; ethyl acetate, 141-78-6; poly(viny1pyridine), 9003-47-8; ethylamine, 75-04-7;diethylamine, 109-89-7; acetone, 67-64-1; ethyl ether, 60-29-7; diethyl sulfide, 352-93-2; benzene, 71-43-2; mesitylene, 108-67-8;steel, 12597-69-2.
Literature Cited Drago, R. S.; Vogel, G. C., Needham, T. E. J . Am. Chem. SOC.1871, 93, 6014.
Drago, R. S.; Parr, L.
B.; Chamberlain,C. S. J . Am. Chem. SOC. 1977, 99,
3203.
Fowkes, F. M. Rubber Chem. Techno/. 1984, 5 7 , 328-343. Fowkes, F. M.; Sun, C.-Y.; Joslln, S. J. In "Corroslon Control by Organic Coatings";Leidheiser, H., Jr., Ed. NACE: Houston, TX, 1981; p 1. Fowkes, F. M.; Tischler, D. 0.; Wolfe, J. A,; Lannlgan, L. A,; AdemuJohn, C. M.; Halliwell, M. J. J . Poly" Scl. Po/ym. Chem. Ed. 1984, 22, 547-566. Fowkes, F. M.; McCarthy, D. C.; Tischler, D. 0.J . Polym. Sci. Polym. Chem. Ed., in press. Silberberg, A,, Plenary Address, 58th Colloid and Surface Science Symposi' um, Pittsburgh, PA, June 1984. Received for review January 9, 1985 Accepted April 11, 1985
Reactions on Painted Steel under the Influence of Sulfur Dioxide, Sodium Chloride, and Combinations Thereof Lars Igetofl ASEA Research & Innovation, S-721 83 Vaster& Sweden
Corrosion on a painted steel surface generally starts at defects in the coating, where ionic species can reach the metal surface and stimulate the anodic dissolution of metal. The degree of spreading of the corrosion reactions around a defect depends on several factors, e.g., type of surface preparation, coating materlal and environment. A first step in this spread is a loss of coating adhesion through cathodic delamination. Sodium chloride and sulfur dioxide are both common corrosion stimulators, but they Influence painted steel differently. Exposure in sodium chloride environment tends to give an alkaline subcoating liquid whereas exposure in sulfur dioxide environment tends to give a neutral to acid subcoating liquid. Depending on the properties of the bonds between vehicle and substrate, different effects may be expected. This difference should be considered in both laboratory and outdoor testing.
Introduction Sulfur dioxide and sodium chloride are two common corrosion stimulators that can be found in the natural environment. They are also used in accelerated laboratory testing of paint coatings. In areas with high concentrations of industry and dense population the air is strongly polluted with sulfur dioxide, which is also spread over long distances. The sulfur dioxide is not only a danger from a biological point of view but has also a strong corrosive action. Typical concentrations and deposition rates are given in Table I. Much is known about the action of sulfur dioxide on unpainted 0196-4321/85/1224-0375$01.50/0
Table I. Occurrence of Atmospheric Sulfur Compounds according to IS0 N43E deposition concn in air rate SOz, mg.m-z.day-l ~ g . m - ~ ppm (vol) type of atmosphere 0-20 0-30 0-0.010 clean, rural 20-60 30-75 0.010.025 urban 60-110 0.025-0.044 industrial 75-130 110-250 130-290 0.044-0.099 heavily polluted
steel. Sulfur dioxide is absorbed to nearly 100% in a humid rust layer and readily oxidized to sulfate, which is really the component that is active in the corrosion process. 0 1985 American Chemical Society
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376 Steel weight loss (9.m-Z)
IO00
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Figure 1. Integral corrosion vs. sulfur dioxide deposition rate (Knotkova et al., 1984).
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Figure 2. Integral corrosion vs. sodium chloride deposition rate (Ambler and Bain, 1955).
Table 11. Occurrence of Airborne Salinity according to I S 0 N53E deposition rate NaCl, mgm-2.day-' type of atmosphere 0-50 clean, rural >1000-2000 m from sea 50-100 maritime >200-300 m from sea 100-500 marine, outside splash zone 500-1500 splash zone
When iron sulfate is oxidized to iron oxide, Fe203,the sulfate ion is released to react with more iron. A high correlation between integral corrosion and sulfur dioxide deposition rate has been found by Knotkova et al. (1984) (Figure 1). A t SOz concentrations higher than -1 ppm other reactions occur that are not representative for the natural environment. In some cases, for example, protective sulfides are formed. Accelerated tests in the laboratory where the sulfur dioxide concentration is raised above this value are therefore unreliable (Barton, 1981). It should be mentioned that the initial concentration of SOz in the German Kesternich test (DIN 50018) is 1500 or 15000 ppm of SOz,depending on the cycle used. Despite its great economical importance, very little is published about the effects of SO2 on painted steel surfaces. Sodium chloride is in the natural environment found only in coastal areas and in connection with road salting, i.e., in rather limited areas. Usually no increase in salt deposition is found a few kilometers from the sea. In spite of this there is a surprisingly great interest in testing of organic coatings in sodium chloride containing environments, both natural, e.g., Kure Beach, and in the laboratory. Typical outdoor deposition rates are given in Table 11. These rates can be compared with the deposition rate in the standard salt-spray test (ASTM B 117-73), which is at least 150 000 mg.m-2.day-1. The stimulating action of sodium chloride on corrosion is due to the fact that the iron chlorides are soluble and hygroscopic, that they increase the surface conductivity, and that the chlorides actively prohibit passivation. In outdoor exposure there is a close connection between the integral corrosion and the deposition rate of sodium chloride in the absence of air pollution as found by Ambler and Bain (1955) (Figure 2).
Figure 3. Summary of cyclic exposure. X = 0, 2,8,24,48 h. Y = 0, 8, 24, 48 h.
Rust-protective paints can be made today with a very high quality. With a good surface preparation and a sufficient film thickness, a life of 15-20 years can be expected. Corrosion in a shorter time is generally limited to pores, mechanical damages, and areas where the film thickness is low, e.g., at edges. Such defects can usually not be totally avoided. It is therefore of great importance that the paint has the ability to protect the surface from the spread of rust around a defect.
Experimental Section Two experiments will be discussed, one cyclic exposure and one immersion test. Test Panels. Cold rolled steel sheet, 0.5 mm thick, was cut to 55 mm X 55 mm test panels. These were wet-ground with Carborundum 180 grinding paper and washed with distilled water and acetone. The panels for the cyclic test were painted with one layer of zinc phosphate alkyd primer and one layer of alkyd top coat to a total film thickness of 70-100 pm. The alkyd paints were dried at room temperature. The panels used for the immersion test were coated with one layer of unpigmented polybutadiene paint and cured 20 min at 200 OC to a film thickness of 25-30 pm. A 1-mmz-circular defect was made with a needle in the center of each test panel. Cyclic Exposure. The panels were exposed to nine repeated 1-week cycles as described in Figure 3. Twenty different programs with combinations of sodium chloride and sulfur dioxide were included. During each week from Monday 9 a.m. to Friday 4 p.m. the panels were kept in solution or in humid atmosphere, and during the weekends
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 377
ri
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Figure 5. Delaminated area (mm2)in immersion exposure vs. salt solution composition.
defect. The corrosion was limited to the original defect area, and the delaminated surface was clean and shiny. Measured delaminated areas are shown in Figure 5.
Discussion 1 48 jO,/wek
i 46 hLNaC11 reek
Figure 4. Delaminated area in cyclic exposure vs. environmental conditions, average and 95% confidence interval.
they were kept dry in the normal laboratory atmosphere. The portion of wet time was therefore about 60% for all panels. During the wet period the panels were exposed first to 0.5 M sodium chloride solution and then in a climate chamber with >95% relative humidity and 0.7 ppm sulfur dioxide. During the rest of the wet period the panels were kept in a humid atmosphere in a plastic box, the bottom of which was covered with water. Each test was performed with four replicate samples. Immersion Exposure. The panels were totally immersed for 127 h in solutions of potassium chloride and ammonium chloride acidified to pH 3.5 with s u l h r i c acid. Sixteen different combinations of the two salts were included. Each test was performed with one panel only. Evaluation. At the end of the exposure adhesive tape was used to remove loose coatings. Loss of adhesion occurred only around the defect. The surface area of the uncovered steel was measured and, after subtraction of the original defect area, reported as delaminated area.
Results Alkyd-coated panels exposed in the cyclic test to humid atmosphere with or without sulfur dioxide had small blisters at the edge of the defect, whereas panels also exposed to sodium chloride had smaller or larger ellipitical areas around the defect within which the paint had lost its adhesion. The loose paint layer showed blisters, particularly a t the periphery, i.e., at the borderline between good and bad adhesion. For panels exposed to humid atmosphere and sulfur dioxide, the uncovered steel surface was dark gray and matte. Panels also exposed to sodium chloride had a dark spot around the defect surrounded by a clean and shiny metal surface. The delaminated surface on panels exposed to sodium chloride only was shiny and showed no dark spot around the defect. Measured delaminated areas are shown in Figure 4. Polybutadiene-coated panels exposed in salt solutions showed delamination within an elliptical area around the
Both sodium chloride and sulfur dioxide stimulated the anodic dissolution of metal at the defect. But their effects on the painted steel were completely different, and the result of the combined exposure was dependent on their relative doses. Due to the presence of sodium ions the cathodic delamination was strongly stimulated. The delaminated area increased with time in the sodium chloride solution. Sulfur dioxide did not stimulate cathodic delamination at all in this experiment but caused a slight blistering at the edge of the defect. In fact, a closer look at the results for panels exposed 2,8, and 24 h in sodium chloride solution and which have 0 and 48 h of SOz exposure reveals that the SO2 decreased the delamination. Possible explanations for this might be that the pH of the delaminating front was reduced by the addition of SOz or that the increased ionic concentration in the subcoating liquid reduced the amount of sodium ions transported to the delaminating front. A similar reduction effect was clearly demonstrated in the addition of ammonium sulfate to potassium chloride solutions in the immersion exposure of the polybutadiene-coated panels. The delamination decreased with increased amount of added ammonium sulfate.
Summary The situation may be summarized in the following way: Corrosion on painted steel starts on places where ions can be transported to the metal surface, such as pores, damages, and thin spots. SO2 and NaCl stimulate the corrosion reactions at the defect in the same way as on unpainted steel, Le., by forming soluble iron salts that are later on oxidized to insoluble rust. Spread of the corrosion reaction to the steel surface under the coating adjacent to the defect requires loss of paint adhesion. This loss of paint adhesion is connected with development of a cathodic area under the coating adjacent to the defect. Through the action of the electrochemical cell, cations will be transported to the boundary between adhesion and nonadhesion. In a pure SO2 enviroment the only cations available are hydrogen ions. The combination of hydrogen ions and the hydroxide ions produced in the cathodic reaction will give water of a neutral pH. For some substrate-coating com-
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binations this may give cathodic delamination. The alkyd system studied in this investigation did not show this kind of adhesion loss. If instead active cations are available, e.g., sodium, a solution with increased pH will form due to the formation of sodium hydroxide. This will, for many substratecoating combinations, increase the amount of cathodic delamination. The alkyd system in the cyclic exposure showed pronounced cathodic delamination upon exposure to sodium chloride solution. If other ions, which do not cause cathodic delamination, are present in the subcoating liquid and partly carry the electric current in the cell, the delamination rate will decrease. It is questionable whether the inhibiting action of SO2 on sodium chloride induced cathodic delamination found in this investigation has any practical significance, since outdoor concentrations of SO2 are at least 10 times lower. Very little is known about the relation between paint life and SOz concentrations in outdoor exposure. Some papers have reported very short paint life in heavily polluted areas necessitating frequent repainting. This might in some cases very well be due not to SO2influence on the coating but due to a bad surface preparation that left iron sulfates and other salts on the surface to be painted.
It is remarkable that in spite of the fact that SO2is the most common corrosion stimulator, most testing of organic coatings is done in sodium chloride environments-either in severe marine atmosphere or, e.g., in the salt-spray test, where the chloride deposition is at least 100 times higher that what can be found at the most severe marine test sites. Registry No. NaCl, 7647-14-5;SOz, 7446-09-5; steel, 1259769-2.
Literature Cited Ambler, H. R.; Baln, A. J. J. Appl. Chem. 1955, 5 , 437. Barton, L. I n "Air Pollution Control, Part IV"; Bragg, G. M.; Strauss, W., Eds.; Wiley: New York, 1981; pp 125-185. Knotkova, D.; Gullman, J.; Holler, P.; Kucera, V. "Proceedings, 9th International Congress on Metallic Corrosion", Toronto, June 1984; National Research Council of Canada: Ottawa, 1984; Vol. 3, pp 198-205. International organization for Standardlzatlon; TC 156 Corrosion of Metals; WG4 Classification of Corrosivity of Atmospheres; Worklng Document N43E, 1982. International Organization of Standardization; TC 156 Corrosion of Metals; WG4 Classlfication of Corroslvity of Atmospheres; Working Document N53E; 1982. Deutsches Instltut fur Normung; DIN 50018; Testing of Corrosion, Methods of Test in Condensation Water Altering Atmosphere Containing Sulphur Dioxide; May 1978. American Society for Testlng and Materials; ASTM B 117-73; Standard Method of Salt-Spray (Fog) Testing; 1983.
Received for review November 27, 1984 Accepted M a y 6 , 1985
Accelerated Corrosion Tests of Precoated Sheet Steels for Automobiles Michael R. Lambert,' Herbert E. Townsend, Robert 0. Hart, and Daniel J. Frydrych Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, Pennsylvania 180 16
Precoated steel sheet is utilized extensively by the automobile industry in the production of vehicles with increased protection against corrosion. Laboratory accelerated corrosion tests such as the satt-spray test and the cyclic tests, which involve atternating cycles of saltwater immersion and humidity-chamber exposure, are widely used to make preliminary evaluations of the various precoated products. This paper reports the results of salt-spray and cyclic tests conducted on a variety of precoated sheet products. The precoated sheets were tested both with and without additional applicatlon of cathodically deposited electrophoretic primer. Results are compared to those for ordinary sheet steel coated only with primer and are discussed in terms of the degree of protection provided by barrier and galvanic mechanisms, as determined in separate electrochemical measurements.
Introduction The manufacture of vehicles with improved resistance to road-salt-induced corrosion is currently the focus of much activity on the part of U.S.automakers and their suppliers (VandeWalle, 1981; Baboian, 1981; Neville, 1978). Vehicle corrosion can be minimized by proper design, application of protective coatings and sealants, and the use of corrosion-resistantmaterials. Precoated steel sheet, that is, steel sheet supplied by a steelmaker with a uniform coating applied, is a cost-effectivematerial with a favorable combination of the required properties, namely, high strength and toughness, formability, weldability, paintability, and corrosion resistance. The variety of precoated sheet steel products currently available and under development has been previously reviewed (Baboian, 1981). Among the property requirements mentioned above, corrosion resistance is the most difficult to evaluate. 0196-4321/85/1224-0378$01.50/0
Although service tests of actual vehicles driven on roads is generally believed to be the only completely reliable tests, this approach requires long test duration, typically several years, and is difficult to control. Test-track exposure reduces the time period to several months. Laboratory accelerated tests further reduce the testing time to several weeks. Unfortunately, as the test duration and sample complexity decrease, so does confidence in the results. The purpose of this paper is to present and examine test results obtained with a variety of precoated steel sheet products in two widely used laboratory accelerated test environments, namely, salt-spray and cyclic exposure. An explanation for material performance in these tests in terms of galvanic and barrier corrosion protection mechanisms is also proposed. The salt-spray, or salt-fog, corrosion test involves ex0 1985 American
Chemlcal Society
