The electrochemical properties and mechanism of formation of anodic oxide films on Mg-Al alloys

The electrochemical properties and the mechanism of formation of anodic oxide films on Mg alloys containing 0-15 mass% Al, when anodized in NaOH solution, were investigated by focusing on the effects of anodizing potential, Al content, and anodizing time. The intensity ratio of Mg(OH)(2) in the XRD analysis decreased with increasing applied potential, while that of M(TO increased. Mg(OH)(2) was barely detected at 80 V, while MgO was readily detected. The anti-corrosion properties of anodized specimens at each constant potential were better than those of non-anodized specimens. The specimen anodized at an applied potential of 3 V had the best anti-corrosion property. The intensity ratio of the P phase increased with aluminum content in Mg-Al alloys. During anodizing, the active dissolution reaction occurred preferentially in beta phase until about 4 min, and then the Current density increased gradually until 7 rnin. The dissolution reaction progressed in a phase, which had a lower At content. In the anodic polarization test in 0.017 mol(.)dm(-3) NaCl and 0.1 mol-dm(-3) Na2SO4 at 298 K, the current density of Mg-15 mass% At alloy anodized for 10 ruin increased, since the anodic film that forms on the a phase is a non-compacted film. The anodic film on the a phase at 30 min was a compact film as compared with that at 10 min.