Influence of plasma electrolytic oxidation treatment on corrosion behaviour of MRI 230D and AZ91D magnesium alloys

The corrosion behaviour of MRI 230D and AZ91D magnesium alloys with and without Plasma Electrolytic Oxidation (PEO) treatment was investigated by different techniques in 3.0 wt% NaCl sat. with Mg(OH)₂ solution. PEO of the alloys was accomplished in a silicate-base electrolyte with KF addition using an AC power source. The corrosion process of both pristine and oxidized samples was evaluated by open circuit potential measurements, electrochemical impedance spectroscopy, linear polarization tests, Tafel extrapolation and chemical methods such as mass loss and hydrogen evolution. For each technique corrosion rates were calculated. The corrosion of both alloys is more localized for oxidized than for pristine samples, so that corrosion pits are developed on an oxidized specimen surfaces. For both alloys, PEO treatment shifts the corrosion potential to a more noble value and decreases the corrosion rate. However, the important factors in corrosion stability of PEO coatings were morphology and porosity of the oxide layer, and not coatings thickness. The AZ91D alloy disclosed higher corrosion resistance than MRI 230D alloy, probably due to the formation of a more compact coating. Yet, the localized character of corrosion causes some deviations in the results obtained even by the same method and deviations of corrosion rates are observed for all the methods.