Influence of Post-Annealing on the Oxide Layer of AZ91 Mg Alloy Prepared by Plasma Electrolytic Oxidation

The influence of the post-annealing (PA) treatment on plasma electrolytic oxidation (PEO) treated AZ91 Mg alloy was investigated and the dependence of the dehydration reaction on the PA temperature was also studied. In this study, a series of PA treatments were carried out on the coated samples at four different temperatures, i.e. 373, 423, 473, and 523 K for 10 h. In contrast to the sample without PA treatment, the samples annealed at temperatures higher than 423 K exhibited a significant difference in terms of surface morphology since the MgO content in the oxide layer increased with increasing PA temperature. With increasing PA temperature, the dehydration of Mg(OH)(2) led to the increase in the relative amount of the MgO, which was a hard phase. From the nano-indentation results, the applied loads of the samples were seen to increase as PA temperatures increased. Moreover, the corrosion resistance of the sample annealed at 423 K was higher than that of the samples annealed at three different PA temperatures. [doi:10.2320/matertrans.M2009290]This work was supported by the National Research Foundation of Korea (Grant No. 2009-0079807). The authors further acknowledge the support from the Center for Advanced Materials Processing of the 21st Century Frontier R&D Program and the Korean Research Foundation (KRF-2007-357-d00136)