Microstructure Evolution During Plasma Electrolytic Oxidation and Its Effects on the Electrochemical Properties of AZ91D Mg Alloy

Abstract

The microstructure development during plasma electrolytic oxidation (PEO) and its influence on the electrochemical corrosion properties of AZ91D Mg alloy were investigated. PEO was conducted in a KF-NaAlO2 electrolyte, and the corrosion properties of PEO coatings were evaluated by potentiodynamic and potentiostatic tests and electrochemical impedance spectroscopy (EIS). At the breakdown voltage, a non-porous, fluoride-containing barrier-type layer with less-crystalline MgAl2O4 and MgF2 was formed. With the intense sparking, a porous layer appeared at the surface. The outer porous layer was a nearly amorphous oxide. The inner dense layer was composed of nano-crystalline MgAl2O4 and MgF2, and its fluoride content and crystallinity increased with PEO time. The existence of MgF2 phase in the innermost layer was clearly demonstrated by high resolution transmission electron microscopy (HRTEM). The potentiodynamic tests indicated that the corrosion potential (Ecorr) remained relatively constant, but the corrosion current density (icorr) significantly decreased with PEO time. The EIS results revealed that ∼2 orders of magnitude higher resistance of inner dense layer mainly contributed to the improved corrosion resistance of AZ91D Mg alloy. The enhanced corrosion resistance with the progress of PEO was discussed in terms of thickness, fluoride content, and crystallinity of inner dense layer.