Evaluation of Accelerated Corrosion Test Procedures 892583
Laboratory salt fog tests on E-coated steel often show greater creep from scribed marks when the underlying steel is galvanized than when it is not, while the scab test produces the more realistically greater corrosion on nongalvanized steel. To explain these observations, the corrosion mechanisms, kinetics, and products of both procedures were carefully examined using specimen weight loss, alternating current impedance techniques, and X-ray diffraction analysis. The laboratory tests showed that the salt fog exposure on zinc produced poorly protective oxide and chloride films with linear corrosion kinetics, while the scab test resulted in a more protective carbonate film with parabolic kinetics. Thus, the scab test compares more favorably with natural outdoor exposures of zinc, which generally produce carbonate films. The same tests on steel produced the opposite results, with protective films favored by the salt fog and nonprotective layers by the scab test. Thus the observed differences in the results of the two tests on galvanized and cold rolled steel lie in the nature of the respective corrosion products.
THE TRADITIONAL ACCELERATED corrosion test for coated sheet steel products, the salt fog exposure (ASTM-B117), often gives results that poorly correlate with service experience. Specifically, scribed painted specimens often show more edge creep and paint undercutting when the steel has been galvanized first than when there is no zinc layer between the steel and the paint. That observation is the opposite of service experience, which shows less creep from scribed marks on painted galvanized steel. A test based on cycles of salt water immersion, followed by drying and humid exposure, appears to more closely simulate service performance, although such a test has not yet been standardized.
The difference between the salt fog and the cyclic wetting and drying test procedures is generally thought to derive from the mechanism of passive film formation on zinc. The formation of stable, corrosion resistant surface films on zinc is believed to require drying. Thus, a continuously wet salt fog test never allows the passivation process to fully take effect. While this theory is supported by a number of anecdotal accounts, it has not been studied in great detail. This paper describes a research program to investigate the mechanisms of corrosion of painted sheet steel, with and without galvanizing, in these two test procedures. The purpose of the research was to determine the factors that cause the observed laboratory test results.