From protection to failure: Oxidation of pure Mg and rare-earth-enhanced Mg alloys at elevated temperature

• Pure Mg exhibits a transient protective MgO layer before catastrophic oxidation failure. • Failure of pure Mg is driven by Mg vapor pressure and stress-induced crack networks. • Rare earth elements promote selective RE oxide formation with thermodynamic-kinetic synergy. • Dense multi-layered MgO-RE 2 O 3 scales suppress Mg 2+ outward diffusion and enhance protection. • Comparative multi-scale characterization clarifies protection-to-failure transitions in Mg. Although magnesium holds great promise as a lightweight structural material, its oxidation failure mechanisms at elevated temperatures remain insufficiently understood. This study systematically investigates the oxidation process of pure Mg at 500 °C through multi-scale characterization, with Mg-Gd-Y-Al alloy serving as a comparative system. The results reveal that during the initial oxidation stage (0–20 h), pure Mg exhibits oxidation kinetics similar to Mg-RE alloys, with SEM observations confirming the absence of visible cracks in its oxide layer, indicating its protective nature at this stage. However, during the accelerated oxidation period (20–300 h), the transformation from protective to non-protective behavior occurs due to the formation of microcrack networks and the rupture of oxidation blisters caused by Mg vapor, ultimately leading to catastrophic oxidation failure of pure Mg. In contrast, the Mg-RE alloy forms a dense, multi-layered oxide structure through the thermodynamic advantages of rare earth elements during oxidation, likely act as barriers to Mg outward diffusion and thereby enhancing oxidation resistance. This study provides new insights into the failure mechanisms of pure Mg and the protective role of rare earth elements in Mg alloys.