Surface analysis of spangles on batch hot-dipped Zn-0.05Al-0.2Sb alloy coatings

The surface morphology and segregation of spangles on a batch hot-dipped Zn-0.05Al-0.2Sb alloy coating were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS). It is found that the coating surface usually exhibits three kinds of spangles:shiny, feathery and dull spangle, with the surface roughness and segregation of Al and Sb increasing in turn. Electrochemical impedance spectroscopy (EIS) tests show that their corrosion-resistances decrease in turn. On the coating surface, acicular β-Sb₃Zn₄ precipitates from the segregated Sb, and Al mainly exists as Al₂O₃ in the oxide film of the Zn coating. The above phenomena were discussed by a crystal growth model of spangles and the Zn-Sb phase diagram.