Effects of Direct Fluorination on the High-Temperature Oxidation Resistance of AZ31 Magnesium Alloy

Fluorination has been proposed as an effective surface modification method for magnesium. The high-temperature oxidation behavior and protective mechanism of fluorinated AZ31 magnesium alloys, especially under prolonged isothermal conditions, have not been systematically investigated. In this study, an efficient and safe surface fluorination method that requires no post-treatment was developed to directly fluorinate the surface of AZ31 machining chips using F2 gas. By adjusting the fluorination parameters, including fluorine gas pressure, temperature, and reaction time, the content and uniformity of the surface MgF2 layer were effectively improved. High-temperature isothermal oxidation tests demonstrated a remarkable enhancement in oxidation resistance after fluorination; specifically, the weight change of the fluorinated samples decreased from 64.65% for the untreated alloy to 0.68% after oxidation at 450 °C for 12 h. To verify the formation of the MgF2 layer and its protective mechanism, all samples were systematically characterized before and after heat treatment using XPS, SEM/EDS, and XRD. The results confirm that direct fluorination with F2 is an effective approach for improving the high-temperature stability of AZ31 magnesium alloy.