Improvement of protective coating on Nd–Fe–B magnet by pulse nickel plating

Abstract

Sintered Nd–Fe–B magnets were coated by pulse nickel plating at different plating conditions. Optimal pulse plating condition was established (average current density=1 A/dm², peak current density=6 A/dm² with T[sub on]:T[sub off]=1:2). In order to make a comparison, magnets with similar nickel coating thickness plated by dc were also prepared. The corrosion resistance of the coated magnets was evaluated by (i) Normal Salt Spray Test (5% NaCl, 35 °C) and (ii) potentiodynamic polarization measurement (3.5% NaCl solution). It was found that the corrosion resistance of the pulse nickel plated magnet was significantly improved as compared with that of the conventional dc plated ones, with negligible deterioration in magnetism. The microstructure of the coating was examined by optical microscopy and scanning electron microscopy. It was found that the porosity was much lower, and the grains much finer in the pulse-plated layer as compared with the dc plated ones.