Effect of Ca~(2+) on phosphate conversion coating on magnesium alloy

Ca2+ was added into the phosphate conversion solution of magnesium alloy as an additive. Roughness tester and electrochemical test were used to analyze the effect of Ca2+ on the roughness and corrosion resistance of coating; SEM was used to observe effect of Ca2+ on the morphology of coating. XPS was used to investigate the composition of coating and what kind of Ca salts existing in the coating. The results show that the surface of coating turns to smooth and the crack of the coating becomes narrow when the concentration of Ca(NO3)2 in conversion solution exceeds 2 g/L. The corrosion resistance of coating is the best at 5 g/L of Ca(NO3)2. XPS results show that the coating is composed of Mn, P, O, Mg, Ca and Al, and Ca2+ exists in the coating in the form of CaCO3, CaO and a kind of amorphous calcium phosphate salt.