Formation, structure and composition of anodic films on AM60 magnesium alloy obtained by DC plasma anodising

Anodising of AM60 magnesium alloy (6% Al + 0.27% Mn) was studied in a solution containing 1.5 M KOH + 0.5 M KF + 0.25 M Na₂HPO₄ · 12H₂O with addition of various NaAlO₂ concentrations. The experiments were carried out in DC current galvanostatic mode. Observations of phenomena occurring at the sample surface plus voltage monitoring revealed three stages: traditional anodising, followed by microarc anodising and finally arcing. The film was porous and cracked, with poor bonding to the substrate. It was composed of magnesium and aluminium oxide, and contained all the elements present in the electrolyte. The aluminium concentration in the film was dependent on the concentration of aluminate ions in the electrolyte. The transition from microarc to arcing stage took place when the alloy surface was completely covered by the anodic film.