Corrosion resistance and microstructure characteristics of plasma electrolytic oxidation coatings formed on AZ31 magnesium alloy

Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy was investigated in an alkaline electrolyte containing 100 g L⁻¹ sodium hydroxide, 60 g L⁻¹ sodium silicate, 20 g L⁻¹ sodium tetraborate and 50 g L⁻¹ sodium citrate. Different oxide coatings were prepared on AZ31 substrate by changing the oxidation time or current density. The corrosion resistance of PEO coatings was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarisation techniques. The microstructures and composition of the PEO coatings were examined by scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction. The results indicate that the coating prepared at 20 mA cm⁻² for 40 min has good corrosion resistance, and its average thickness is 48·1 μm. Similar to other PEO coatings, the coating is porous and mainly composed of MgO and Mg₂SiO₄. The elements of O, Mg and Si are distributed uniformly across the coating.