Improvement of corrosion resistance of Cu and Nb co-added Nd-Fe-B sintered magnets

Cu and Nb powders are co-added as intergranular modifiers to improve the corrosion resistance of Nd-Fe-B sintered magnets. For the magnet co-added with 0.2 wt.% Cu and 0.8 wt.% Nb, mass loss of accelerated corrosion test in 120 °C, 2 bar and 100% relative humid atmosphere for 96 h drops from 2.47 mg/cm² to 0.49 mg/cm² in comparison with the Cu/Nb free one. The corrosion potential E_corr in 3.5% NaCl aqueous solution increases from -1.115 V to -0.799 V, which indicates the better resistance against electrochemical corrosion. The improved corrosion resistance is ascribed to the enhanced stability of the intergranular phase by forming high electrode potential Cu-containing phase and reduced Nd-rich phase at triple junctions. Besides, the distribution of (Pr, Nd)-rich phases along the grain boundaries becomes more clear and continuous through Cu/Nb co-addition, maintaining fairly good magnetic properties of B_r = 13.6 kGs, H_cj = 11.4 kOe, (BH)_max = 46.2 MGOe. Further investigation demonstrates that Nb is effective to refine the grains of hard magnetic Nd₂Fe₁₄B phase and Cu is beneficial for optimizing the distribution of the intergranular phases.