Plasma Electrolytic Oxidation of Magnesium Alloy AZ31B in Electrolyte Containing Al₂O₃ Sol as Additives

Plasma electrolytic oxidation (PEO) coatings were produced on AZ31B magnesium alloys in alkaline electrolytes with the addition of various concentrations of Al₂O₃ sols. Effects of Al₂O₃ sol concentrations on the microstructure, phase composition, corrosion resistance and hardness of PEO coatings were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness testing and potentiodynamic polarization measurements, respectively. It was revealed that the Al₂O₃ sol mostly participated in the formation of the ceramic coating and transferred into the MgAl₂O₄ phase. With the increase of the Al₂O₃ sol concentration in the range of 0⁻6 vol%, the coating performance in terms of the microstructure, diffraction peak intensity of the MgAl₂O₄ phase, corrosion resistance and microhardness was improved. Further increase of Al₂O₃ sol addition did not generate better results. This indicated that 6 vol% might be the proper Al₂O₃ sol concentration for the formation of PEO coatings.