Characterisation of microdischarge evolution and coating morphology transition in plasma electrolytic oxidation of magnesium alloy

The present paper studied the appearance evolution of microdischarges, the phase composition and the morphology transition of the coating formed by plasma electrolytic oxidation on AZ91D magnesium alloy. The appearance of microdischarges population experienced apparent changes in size, spatial density and colour, which related with the changes of the type and quantity of the disintegrated gas bubbles generated in the interface between the electrolyte and substrate. Correspondingly, the diameter of micropores together with netlike fine microcracks increased when the higher voltage was employed. The coating composed of MgO, MgAl2O4 and there existed a fluoride enriched zone of about 1-2 mu m at the film/substrate interface, which may be closely related with the decomposition of the hexafluorinealuminate ion and the initiation, damping of the microdischarges.