Characterization of oxide films formed on magnesium alloys using bipolar pulse microarc oxidation in phosphate solutions

The surface morphology and chemical composition of the oxide films formed on pure magnesium and AZ91D alloy in aqueous electrolytes which contained sodium hexafluorinealuminate(Na3 AlF6 ), potassium hydroxide (KOH), sodium hexametahposphate ((NaPO3)6 ), and triethanolamine were investigated by X-ray diffraction (XRD), scanning electron microscope(SEM) and energy dispersive spectroscopy(EDX). The results show that the input of the negative pulse has great influences on the quantity and the appearance of the microdischarges. Three types of pores can be distinguished on the surface of the oxide film and their size ranges are 0.5- 1μm, 1 - 2μm and 4- 7μm, respectively. A few microcracks are seen around the large pores. There exists a remarkable fluorideenriched zone of about 4 - 6μm for pure magnesium and 3 - 5μm for AZ91D alloy at the coating/substrate interface.