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

• Cu and Nb powders were co-added as intergranular modifiers to improve corrosion resistance of NdFeB magnets.
• Corrosion resistance is significantly improved; Mass loss by PCT test reduces 80%.
• High magnetic properties are retained, with Br = 13.6 kGs, Hcj = 11.4 kOe, and (BH)max = 46.2 MGOe.

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 Ecorr 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 Br = 13.6 kGs, Hcj = 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.