Chromate conversion coatings with hexavalent chromium are currently being replaced by eco-friendly surface treatments due to environmental legislation. A novel non-chromate passivating coating based on vanadium compound in an acidic system has been successfully synthesized. This new coating for galvanized steel meets the general standards such as those for corrosion resistance, black tarnish resistance, and adherence. The corrosion resistance of the new coating was evaluated by salt spray test and polarization curves. The property of black tarnish resistance was improved by the addition of aluminum ion. The distribution of the elemental composition of the new coating was studied by glow discharge spectrometer (GDS) and X-ray photoelectron spectroscopy (XPS) to investigate the microstructure. The superiority of the new coating can be characterized by the following aspects: (a) vanadyl-phosphate complexes in the coating can enhance the barrier effect which suppresses diffusion of corrosive elements and improve corrosion resistance; and (b) P-O-Zn and Si-O-Zn bonds formed in the coating/zinc interface enhance the adherence of the coating.
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