The effect of processing parameters and substrate composition on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloys

Magnesium alloys are considered one of the more promising materials for future use in many engineering applications. However, due to their high chemical and electrochemical reactivity, magnesium alloys have poor corrosion resistance in aqueous environments. Improving their corrosion resistance by coating can greatly extend their application. One promising coating method is plasma electrolytic oxidation (PEO). The nature of the coating formed, and the ultimate corrosion performance depends on the both the processing parameters (electrolyte, current density, current mode, processing time) and specific Mg-alloy substrate. In the present study, PEO coatings were produced on three different Mg-alloys (AJ62, AM60B and AZ91D) using different processing parameters. Scanning electron microscopy was used to characterize the coatings. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) in an aqueous 3.5% NaCl solution. Relationships are drawn between PEO processing parameters, substrate composition and corrosion performance. Electrochemical impedance spectroscopy data indicate that the bipolar PEO coated AZ91D Mg alloy demonstrates a higher corrosion resistance when compared to coated AM60B, AJ62 and pure Mg.