Abstract
Because the atmospheric corrosion products that form on the surface of Al, Zn, and AlZn hot-dip coatings on sheet steel are difficult to characterize, they have not been extensively studied in the past. In this study, the composition and structure of products formed in industrial and marine environments were identified, and the effects of time, side exposed, and surface chromate treatment were described. Amorphous aluminum sulfate hydrate is the most abundant corrosion product on Al and AlZn coatings in both environments; it is well formed after only three years of exposure and is changed little after nine years. Al hydroxides were also present on aluminum-containing coatings exposed to the marine atmosphere, but not on those in the industrial atmosphere. The groundward side of the AlZn-coated panel exposed 250 m from the Atlantic Ocean had roughly as much AlZn sulfate hydrate as the skyward side of a panel exposed 25 m from the ocean. This finding discounts sea spray as the source of sulfur and indicates an atmospheric source. The effect of chromate treatment was to reduce the overall corrosion rate and, particularly, to reduce the loss of zinc. In conclusion, it is evident that sulfur is a significant factor in the atmospheric corrosion of metallic coatings, even in a marine environment; however, further work is necessary to explore the properties of the sulfate and to ascertain whether it might even be protective in some cases.
