Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

► The Zn-Ca coating has a flower-like morphology, which is different from Zn coating with a dry-riverbed-like morphology. The Zn-Ca coating is denser with almost no cracks than Zn coatings. ► The Zn coating contains Zn/ZnO, MgO, MgF2 and Zn3(PO4)2·4H2O. The Zn-Ca coating is mainly composed of Zn3(PO4)2·4H2O, and a small quantity of ZnO, Ca3(PO4)2 and CaHPO4. ► The Zn-Ca coating can reduce the corrosion current density more significantly than Zn coating. ► The formation mechanism of the Zn-Ca coating was discussed.