Effects of Zn2+ concentration and pH on the zinc phosphate conversion coatings on AZ31 magnesium alloy

• Effects of Zn2+ concentration and pH level on zinc phosphate conversion coatings are reported.
• The coatings on AZ31 through zinc phosphating conversion coating solution are characterized as double layer structure.
• The method for optimization of Zn2+ concentration and pH based on chemical equilibrium diagram is provided.
• Zinc phosphating conversion coatings formed at [Zn2+] = 0.068 M and pH 3.07 showed the best corrosion resistance.

The effects of various Zn2+ concentrations and pH levels (nine conditions in total) on the formation of zinc phosphate conversion coatings (ZPCCs) on AZ31 magnesium alloy were investigated, and corrosion resistances of the coated samples were evaluated by immersion test and potentiodynamic polarization experiment. The corrosion resistance of the coated AZ31 samples was found to increase with increasing Zn2+ concentration and the lowest corrosion rate was obtained for the samples coated at pH of 3.07, independent of Zn2+ concentration. The best coatings on AZ31 were obtained at [Zn2+] = 0.068 M and pH 3.07.