Surface enrichment in hot-dipped metallic coatings investigated by Auger electron spectroscopy

The treatment, appearance, and corrosion resistance of metallic coatings are largely governed by the chemical composition of the surface. Auger electron spectroscopy shows that the surfaces of hot-dipped metallic coatings differ markedly from the bulk compositions of the coatings. For example, the surfaces of terne coatings, lead-tin alloys, contain little lead. The conventional galvanized coating, which is more than 99% zinc, has a predominantly aluminium oxide surface. Typical surface compositions of a range of hot-dipped metallic coatings are provided. A qualitative prediction of the dominant metallic species present on the surface of each of these coatings is presented in terms of the relative oxygen affinities of the metals. Theoretical equations for various mechanisms, such as atomic size mismatch, solubility, and oxidation, which could lead to surface segregation are considered, in order to place the experimental observations on a more quantitative basis.