Effect of additives on structure and corrosion resistance of plasma electrolytic oxidation coatings on AZ91D magnesium alloy in phosphate based electrolyte

Plasma electrolytic oxidation (PEO) coatings were prepared on magnesium alloy AZ91D in a phosphate based electrolyte with different additives such as sodium fluoride (NaF) and sodium aluminate (NaAlO2). The effects of F− and AlO2− additives on the morphology, phase structure and corrosion resistance of PEO coatings were examined by scanning electron microscopy (SEM/EDX), X-ray diffraction (XRD) and potentiodynamic polarization. The results show that PEO coatings produced in F− additive solution grow faster than that in AlO2− additive one. However, the growth rate decreases with treatment time in F− additive solution while it increases in AlO2− additive one. It is also found that the corrosion resistance is greatly improved after PEO treatment, and coatings produced in F− additive solution show lower corrosion current density than that in AlO2− additive solution.