An inhibition mechanism of the black patina development on the surface of 4% Al-0.1% Mg-Zn alloy coated steel sheet by spraying aqueous solution of Co salt was investigated with various surface analytical methods.
Without Co treatment, the coating surface rapidly changed its color into black in a high temperature and high humidity environment. This phenomenon was due to the formation of thick oxide film which composed of Zn, Cr and Al. The blackening of the coating surface was effectively retarded by spraying aqueous solution of Co salt onto the molten coating surface just after hot dipping.
On the Co treated coating surface, a large number of Co3O4 deposits, of which structure was identified with HEED, were observed in TEM image and in IMA ion image. The thickness of this Co oxide was 0.1 0.2 μm as determined by a direct TEM observation of the ultramicrotomed cross section of the coating layer. SAM and ESCA depth-profiles showed that Co oxide retarded oxidation of the coating surface. This effect of Co oxide was attributed to cathodic inhibitive action or stable zinc hydroxide forming ability of Co ion which was eluted into the condensed water on the coating surface.
