Micro-arc oxidation of AZ91 Mg alloy: An in-situ electrochemical study

•The dielectric breakdown of layers grown on Mg in different electrolytic media occurs for a specific capacitance value.•The corrosion resistance of micro-arc anodized coatings is mainly due to the resisting properties of the inner layer.•Fluorides or silicates are necessary to stabilize the inner layer during the classical anodizing step.

The growth of coatings by micro-arc oxidation is based on the formation of spark discharges appearing at the metal/electrolyte interface over the dielectric breakdown potential. On Mg and Mg alloys, this anodizing process is usually performed in alkaline electrolytic bathes in which fluorides or silicates are added. In concentrated KOH electrolytes, in-situ electrochemical impedance spectroscopy measurements are performed at high voltage (up to 80 V) and show that the dielectric breakdown occurs for a threshold capacitance at the electrochemical interface. The main positive role of fluorides and silicates in both the reinforcement of the resisting properties of the coating and its further growth under spark effect is highlighted.