Abstract
A structure of the black patina formed on the surface of 4%Al-0.1%Mg-Zn alloy coated steel during storage as a coil was investigated by various surface analyses.
After chromating, Al-rich phase of the coating surface preferentially changed its color into black and then β-Zn phase gradually turned into the same color as Al-rich phase, which was due to the difference in growth rate of each phase. It was recognized that specific double-layered oxide film was formed on both phases : an outer layer mainly of Zn and Cr, and an inner layer mainly of Al and Zn. The black patina was related primarily to the outer amorphous oxide film which was composed with the mixture of non-stoichiometric zinc oxide and chromium oxide. The patina formation after chromate treatment was accelerated by Al and Mg in the coating. It was considered that these two elements diffused to the surface and were oxidized preferentially to form the inner layer, which accelerated the formation of non-stoichiometric zinc oxide of the outer layer.
