Spark discharges during the micro-arc oxidation of AZ91D magnesium alloys

Spark discharge is the representative phenomenon of micro-arc oxidation (MAO) method. Accompanied with the spark discharge, the cell voltage presents oscillations within the range of several volts, and the surface morphologies, the element distribution as well as the surface potential distributions of the anodic layer are significantly changed. It was found that spark discharges resulted in a great deal of discharge pores in the oxide layer, accelerated the oxidation and the deposition of electrolytic particles. Due to the melting effect of the sparks, compact internal layer was formed underneath the outer loose layer, and the surface potential over the spark zone was greatly enhanced. In addition, it was found that oxygen played a key role in determining the intensity and the size of sparks, and abundant oxygen resulted in intensive and larger sparks.