Micro-arc oxidation (MAO) coating was prepared on magnesium alloy AZ91D using an alternative square-wave power source. The effects of electric parameters, such as the applied voltage, the frequency and the duty cycle, on the thickness, surface morphology and structure of the MAO coating were investigated by orthogonal experiment. The results indicate that the thickness and porosity of MAO coating increase with the applied voltage and the duty cycle, but decrease with the frequency. The thickness and porosit...