Effect of Applied Current Density on Anodic Oxidation Behavior of AZ31 Mg Alloy in NaOH Solution

This paper reports three different anodic oxidation behaviors of AZ31 Mg alloy in NaOH solutions, depending on the applied current density and concentration of NaOH between 0.1 M and 0.6 M. When applied current density is lower than about 30 mA/cm², dark anodic film was formed with a very low film formation voltage of less than 6 V and it was found to grow with anodizing time. If applied current density is higher than a critical value about 30 mA/cm², bright metallic surface appearance with small number of pits and/or trenches was obtained. The formation of the pits or trenches was attributed to the result of repeated formation and detachment of plasma electrolytic oxidation (PEO) films at the same site or neighboring places, as strongly supported by repeated micro-arc generations at the same surface site of the specimen during the anodic oxidation process. When applied current density exceeds about 80 mA/cm² in 0.6 M NaOH solution, micro-arcs were generated uniformly over the entire surface, resulting in grey surface appearance and the formation of PEO films with cracks and pores. Based on the experimental results obtained in this work, it is concluded that dark anodic films and grey colored PEO films can be formed on the AZ31 Mg alloy surface in NaOH solution if the applied current density is controlled to be lower than a critical value and if the applied current density and NaOH concentration are controlled to exceed critical values of about 80 mA/cm² and 0.6 M, respectively.