Formation and Corrosion Resistance of Micro-Arc Oxidation Coating on Equal-Channel Angular Pressed AZ91D Mg Alloy

A commercial AZ91DMg alloy, after bulk grain refinement by various passes of equal-channel angular pressing (ECAP), was selected for micro-arc oxidation (MAO) in silicate electrolyte, corrosion testing in 3.5 wt % NaCl solution and morphology analyses. The results showed that a large number of ECAP passes resulted in the homogeneous ultrafine-grained (UFG) Mg substrate with broken second-phases. The high-energy defects in the ECAPed samples lowered the anodizing potential of the MAO process, but the partial discharge was severe for those samples below eight passes. Increasing the ECAP pass, the compactness and thickness of the MAO coating first decreased and then increased. Due to the compact coating and the existence of Mg2SiO4, the coated alloy with 16 ECAP passes has a lower corrosion rate and a larger R-t value. Besides the well-known strengthening-toughening effect, grain refinement via multi-pass ECAP can improve surface protection of the MAO coating on the UFG Mg alloy.