Review on Oxidation of Nd–Fe–B Magnets: Mechanisms, Kinetics, and Implications for Neodymium Recovery

A systematic understanding of the oxidation behavior of Nd–Fe–B permanent magnets is crucial for correlating oxidation-induced phases and microstructural transformations with magnetic degradation, corrosion susceptibility, and the efficiency of oxidation-based neodymium recovery during end-of-life magnet recycling. Despite extensive research, the underlying mechanisms, kinetics, and thermodynamic boundaries are frequently examined in isolation. This review presents a comprehensive analysis of oxidation behavior in Nd–Fe–B magnets, emphasizing the relationships among microstructural evolution, rate-controlling mechanisms, and phase stability covering a broad temperature spectrum. In bulk magnets, oxidation predominantly occurs via internal oxidation, characterized by preferential oxidation of neodymium, dissociation of the Nd2Fe14B phase, and the formation of distinct multilayered oxidation zones at elevated temperatures. Conversely, powdered magnets undergo accelerated oxidation at lower temperatures due to greater oxygen accessibility and shorter diffusion distances, resulting in distinct oxidation kinetics. The review critically assesses the applicability and limitations of classical solid-state kinetic models, such as the Wagner, Jander, and Ginstling–Brounshtein formulations, using experimental data to underscore the regime-dependent nature of diffusion control and advancing reaction fronts. The review further demonstrates how oxidation progressively degrades magnetic performance through the oxidation of the Nd-rich grain-boundary phase and decomposition of the Nd2Fe14B matrix, leading to losses in coercivity, remanence, and maximum energy product. Thermodynamic analysis, incorporating Gibbs free-energy calculations and Ellingham-type diagrams, clarifies the pronounced preferential oxidation of neodymium compared to iron and boron, as well as the conditions that promote the formation of refractory mixed oxides, including NdFeO3 and NdBO3.