Correlation between KOH concentration and surface properties of AZ91 magnesium alloy coated by plasma electrolytic oxidation

A study was made to investigate the surface properties of an AZ91 magnesium alloy coated by plasma electrolytic oxidation (PEO) using electrolytes with different potassium hydroxide (KOH) concentrations. We also evaluated the corrosion resistance of the PEO-treated AZ91 magnesium alloy via electrochemical analysis. During the PEO coating, the ignition voltage for the sample coated in the electrolyte with 0.27 M KOH was lower than the one that was coated in the electrolyte with 0.09 M KOH due to its low electrolyte conductivity. This was in line with the present XPS measurement where the intensity of the MgO spectrum increased as the KOH concentration increased. When the PEO-treated samples were exposed to a corrosive environment with Cl− ions, the oxide layer clearly exhibited higher corrosion resistance as the KOH concentration increased in the electrolyte. This was attributed to the fact that the relative amount of MgO in the coating layer, which played a role in enhancing the corrosion resistance of AZ91 magnesium alloy, increased as the KOH concentration increased. The electrochemical mechanism underlying the high corrosion resistance of the PEO-treated AZ91 magnesium alloy was also revealed by impedance tests and the accompanying interpretation related to the equivalent circuit model.