The Effect of MB40 Addition on the Electrochemical Corrosion Resistance of an NdFeB Magnet Recycled from Electronic Waste

Abstract

A Ni-Cr alloy powder, designated MB40, with an average size of 30.5 µm, was used as a grain boundary modifier in a recycled neodymium-iron-boron (Nd-Fe-B) matrix to improve the corrosion resistance, coercivity and other properties of the rare-earth sintered magnet. Nd-Fe-B powder with an average grain size of 95.1 µm was obtained by milling. The effects of doping with different amounts of MB40 (0, 0.5 and 1 wt.%) on the properties of the recycled magnet compared with an initial magnet were investigated. X-Ray Diffraction analysis revealed the formation of the Fe₁Nd₁Ni₄ and NiCrFe phases, after sintering, as well as the tetragonal phase Nd₂Fe₁₄B matrix phase in the magnet doped with 1 wt.% of MB40. Scanning Electron Microscopy images coupled with energy-dispersive x-ray spectroscopy demonstrated that in the sample doped with 1 wt.% MB40, Cr-Ni diffuses into the grain boundaries, thus improving the microstructure of the magnet. Electrochemical Impedance Spectroscopy and potentiodynamic polarization were employed for corrosion characterization. The magnet doped with 1wt.% MB40 exhibited the highest corrosion potential (E_corr = − 696 mV), polarization resistance (R_P = 1151.2 Ω·cm²) and a low corrosion rate (V_corr) of 4.34 µm·year^-1 compared to the starting (initial) sample. The optimum characteristics of recycled NdFeB were obtained by blending milled NdFeB powder with 1 wt.% MB40, achieving the best values for product energy (BH_max), coercivity (H_C) and hardness, respectively 48.03 MGOe, 10.8 KOe and 587 HV.