Comparison of calcium phosphate coatings on Mg–Al and Mg–Ca alloys and their corrosion behavior in Hank's solution

Calcium phosphate coatings were prepared on AZ31 and Mg–1.0Ca magnesium alloys by electrochemical deposition. Hydrogen evolution testing and potentiodynamic electrochemical technique were employed to investigate the corrosion behavior of the coated alloys in Hank's solution. The results show that the deposited coatings mainly consist of flaky brushite (DCPD, CaHPO4·2H2O) crystallites. The obvious increase in free corrosion potential (open circuit potential) and decrease in corrosion current density of the coated samples indicate that the coating significantly enhanced the corrosion resistance of the substrates. The hydrogen evolution rates of the coated samples were much lower than that of the substrates during initial immersion in Hank's solution. However, the corrosion rate of the coated samples increased rapidly once the samples suffered pit corrosion. Furthermore, the corrosion properties of the coated samples have much to do with the corrosion properties of their substrates.