Achieving excellent corrosion resistance in Nd-Ce-Fe-B magnets via oxygen partial pressure regulation

Abstract

Beyond the primary challenge of low coercivity, poor corrosion resistance presents another critical limitation for the widespread application of Nd-Ce-Fe-B magnets. This work demonstrates that rapid thermal oxidation at 350 °C for 0.5 h under a controlled oxygen partial pressure (PO₂) synergistically enhances the anti-corrosion and mechanical performance of Ce50 (Ce/RE = 50 wt.%, RE = rare earth) magnets, accompanied with a high coercivity above 12 kOe. A systematic survey of wide-ranged PO₂ (0–21%) identifies 1% as the optimal condition to form a thin and dense oxide layer with minimal microcracks. Microstructural analysis indicates that the REFe2 intergranular phase oxidizes faster than the RE2Fe14B main phase, leading to an uneven oxidation front. Furthermore, controllable PO₂ with limited oxygen source restricts the complete oxidation, resulting in the oxide layer composed of an outer region dominated by iron oxides, and an inner nanocomposite of α-Fe nanocrystals dispersed in amorphous α-Fe and RE2O3 matrix. These findings provide fundamental insights into the oxidation mechanism under controllable PO₂, and present a viable route for designing high-performance Nd-Ce-Fe-B magnets through surface engineering.