Corrosion Characteristics of Plasma Electrolytic Oxidation Treated AZ31 Magnesium Alloy with an Increase of the Coating Thickness

The growth of the PEO coating with processing time on an AZ31B magnesium alloy in a dilute alkaline electrolyte containing silicate and the corrosion characteristics according to its thickness were investigated. The oxide film grew a porous outer layer mainly consisting of silicate with the spark discharge phenomenon from the beginning of the reaction and reached a certain limit thickness later in the reaction. This reaction was followed by the inner layer, mainly consisting of MgO, growing rapidly and densifying. Based on the results of the potentiodynamic polarization and chronoamperometric test, it was found that within 1 minute of the processing time, due to the contribution of the dielectric layer and MgF2 phase layer formed at the interface between the oxide and the substrate, the corrosion resistance was relatively high, but its stability against corrosion was not enough. Moreover, when the thickness of the oxide coating was grown mainly with the silicate outer layer, the increase in the pore size, which acts as a channel of spark discharge, was the leading cause of the decrease in the corrosion resistance. The superior corrosion resistance and the stability of the PEO coating in the later reaction time of the processing are due to the growth and densification of the inner layer of MgO.