Self-repairing capability of magnesium alloy during the plasma electrolytic oxidation process

Abstract

During the growth process of a plasma electrolytic oxide (PEO) coating on an AZ-31 Mg alloy, electric breakdown inevitably occurs and causes local disintegration of the coating, thus forming breakdown pores that result in an increased coating porosity and suppressed corrosion resistance. Here, we report for the first time that the self-repairing capability of the PEO process can restore the coating after breakdown damage by decreasing the porosity, tuning the components and improving the corrosion resistance via the deposition of electrolyte anions into the breakdown pores. Based on detailed scanning electron microscopy and transmission electron microscopy characterizations of the PEO coatings processed in alkaline silicate electrolytes, it was found that the pores were repaired by silica fillings formed by the plasma-induced deposition of anions (SiO32−) in the electrolyte. The plasma-deposited silica possesses short SiO bond lengths that are manifested by an Si-L2,3 edge shift by ∼1.3 eV to higher energies. In addition, the presence of some amount of Mg together with a minor admixture of K indicates the contribution of ionic migration and plasma collisions to the self-repairing process in the PEO coating.