An electrochemical analysis of AZ91 Mg alloy processed by plasma electrolytic oxidation followed by static annealing

In this study, the effect of subsequent annealing on the electrochemical response of AZ91 Mg alloy coated via plasma electrolytic oxidation (PEO) was investigated. PEO coating was carried out on the Mg alloy under AC condition in an alkaline silicate electrolyte, and the PEO-coated samples underwent several subsequent annealing treatments at three different temperatures of 100, 150, and 200 degrees C. The surface morphologies of the coating layers were observed via a scanning electron microscope (SEM) and their constituent compounds were characterized by qualitative observation based on X-ray photoelectron spectroscopy (XPS). In addition, the corrosion protection properties of the PEO-coated sample were examined by electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl solution with a focus on exploring the effect of subsequent annealing on the electrochemical response in a quantitative manner. SEM and XPS observations evidenced that the subsequent annealing at temperatures higher than 150 degrees C resulted in significant morphological changes due to the dehydration reaction of Mg(OH)(2) to form MgO. Thus, it was found that the sample annealed at 150 degrees C exhibited a better corrosion resistance than the other samples, which were analyzed by taking an equivalent circuit model into account. (C) 2011 Elsevier B.V. All rights reserved