Formation mechanism of oxide layer on AZ31 Mg alloy subjected to micro-arc oxidation considering surface roughness

Abstract This study investigated the formation mechanism of the oxide layer generated by micro-arc oxidation (MAO) considering surface roughness of AZ31 Mg alloy. For this purpose, a series of MAO treatments under alternating current in a silicate-based electrolyte were performed on two present samples with grooved and flat surfaces. The time-voltage response presented that the grooved sample reached the breakdown voltage earlier than the flat sample. This indicated the appearance of micro-discharges on the grooved sample occurred earlier than that of the flat sample since the oxide fragments nucleated preferentially at the ridge areas where electrons were concentrated severely under high electrical field. The oxide layer of the grooved sample was relatively less dense than that of the flat sample. This was attributed to the fact that the oxide layer of the grooved sample was likely to grow with ease on pre-existing coating layer whereas the oxide layer of the flat sample developed conformably throughout the whole areas. Thus, the growth rate of the grooved sample was higher than that of the flat sample, which agreed well with micro-discharge activities. Such formation mechanism was proposed in relation to nucleation and growth of the oxide layer formed on different surface structures.