Statistical analysis of the voltage-time response produced during PEO coating of AZ31B magnesium alloy

Plasma-electrolytic oxidation (PEO) is a corrosion prevention technique which produces a protective oxide layer on a metal surface by high-voltage oxidation to increase its corrosion resistance. Traditionally, PEO processing on a magnesium alloy surface has been investigated via the voltage-time response measured during the treatment. Thus far, the information extractable from voltage-time measurements has been limited due to the sporadic nature of coating evolution. The current study presents a new approach to examine the mechanisms governing PEO coating formation and the subsequent influence of process parameters. The instantaneous voltage-time response has been recorded and analyzed using a statistical approach. Combining the voltage-time measurements with structural analysis (XRD/SEM/EDX), a multi-step coating formation mechanism has been proposed. The first stage of the treatment process was similar to traditional anodization. When the voltage was sufficiently high to induce dielectric breakdown of the existing oxide material, plasma discharging occurred and a silica containing layer was formed. Two predominant types of discharge governed the oxidation process; discharge through deep pores, and discharge through micro-voids. Increasing the current density proportionally accelerated the rate of oxide development. Increasing the temperature of the electrolyte resulted in thinner coatings due to an increased oxide dissolution rate. From an application point of view, the PEO coating process can be accelerated by choosing a high current density, but the problem of localized heating by the discharges must be addressed to avoid severe dissolution of coating.