Preparation of ceramic conversion layers containing Ca and P on AZ91D Mg alloys by plasma electrolytic oxidation

Oxide based ceramic conversion layers containing Ca and P were prepared on AZ91D Mg alloy by plasma electrolytic oxidation technique in Na₂SiO₃ and NaOH systems respectively. The conversion layers prepared in both systems were porous and grey white. The doping of Ca decreased the porosity of the conversion layers in Na₂SiO₃ system, while increasing the porosity of and the microcracks of the conversion layers in NaOH system. The conversion layer prepared in NaOH system was composed of MgO, and the one prepared in Na₂SiO₃ system was composed of Mg₂SiO₄ and MgO; P and Ca only existed in the form of amorphous state in the conversion layer. The doping of Ca decreased the relative content of P while increasing the relative content of Ca in both conversion layers. The ceramic conversion layers in both systems improved the corrosion resistance of the AZ91D Mg alloy in 0·9%NaCl solution by one or two orders of magnitude.