Growth process and corrosion resistance of ceramic coatings of micro-arc oxidation on Mg-Gd-Y magnesium alloys

The regulation of ceramic coating formed by micro-arc oxidation on Mg-11Gd-1Y-0.5Zn (wt.%) magnesium alloys was investigated by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The relation of phase structure and corrosion resistance of MgO coating formed by micro-arc oxidation in different growth stages was analyzed. The results showed that the growth of coating accorded with linear regularity in the initial stage of micro-arc oxidation, which was the stage of anodic oxidation controlled by electrochemical polarization. With elongated treated time and increased thickness of the coating, the growth of coating accorded with parabolic regularity, which was the stage of micro-arc oxidation. In contrast to the stage of anodization, the growth rate of micro-arc was slower than that in anodization. In the stage of local arc light, the slope of parabola and thickness of loose coating were increased so that the growth rate was enlarged. The phase structure of loose coating was mainly composed of MgSiO3 and the phase structure of compact ceramic coating was mainly composed of MgO. From the stage of micro-arc oxidation to local arc light, corrosion resistance of coating was firstly increased and then decreased. The satisfied corrosion resistance corresponded to the coating time ranging from 7 to 12 min. The addition of rare earth elements in the magnesium alloy reduced the amount of smooth areas on ceramic surface. So the ceramic coating became more compact and smoother. The rare earth elements did not form independent phases in ceramic coating but affected the relative proportion of constitution phases, resulting in the reduction of intermixed magnesium phase and the increase of the MgO and MgSi2O4 phases.