Corrosion resistance of oxide layers formed on AZ91 Mg alloy in KMnO4 electrolyte by plasma electrolytic oxidation

The plasma electrolytic oxidation (PEO) process in AZ91 Mg alloy is studied using a solution containing KOH + KF + Na2SiO3 both with and without potassium permanganate (KMnO4). The addition of potassium permanganate to the electrolyte influences coating thickness, surface morphology and the microstructure of oxide layers obtained by the PEO process. Oxide layers formed on AZ91 Mg alloy by the electrolyte containing KMnO4 consists of MgO, MgF2, Mg2SiO4 and Mn2O3. The corrosion resistance of the sample processed in bath containing KMnO4 was superior to that of the sample processed in the bath without KMnO4. It is suggested that enhancement of the corrosion resistance of AZ91 Mg alloy depends strongly on the presence of manganese oxide in the oxide layer.