Current density effects on plasma emission during plasma electrolytic oxidation (PEO) on AZ91D-magnesium alloy

The effect of bipolar pulse mode current ratio (anodic to cathodic) on plasma emission behavior was investigated in Plasma Electrolytic Oxidation (PEO) on AZ91D Mg-Alloy. Two cases of current ratio as 1.20 and 0.88 were applied to the sample. Plasma emission behavior was studied using plasma images and plasma emission measured by photodetector and Intensified Charged-Couple Device (ICCD) camera. The current ratio of greater than 1 shows the continuous increase and then stabilization in emission intensity with a gradual increase in voltage throughout the PEO process. In contrast, the current ratio of less than 1, a sudden drop in plasma emission intensity with voltage was found after 786 s. Therefore, PEO process can be divided into two regimes, arc regime and soft regime, before and after voltage drop, respectively. Results of measured spectra show that a soft regime does not have atomic or ionic excitation during PEO process. It is demonstrated that the growth of porous layer during PEO can be controlled, which is benefit for the protective oxide coating of sample.