Corrosion Properties of NdFeB Magnets Coated by a Ni/Cu/Ni Layer in Chloride and Sulfide Environments

In this study, the corrosion behavior of a permanent magnetic alloy, Nd-Fe-B, coated by a triple layer of nickel-copper-nickel was studied. The corrosion resistance was investigated by (i) flux tests, carried out at 90 °C in an atmosphere containing chlorides or at 70 °C containing sulfides, (ii) electrochemical impedance spectroscopy (EIS), and (iii) potentiodynamic polarization tests, carried out in solution at different temperatures (0-90 °C) containing chlorides or sulfides. The morphology and the composition of the samples and the corrosion products were analysed by optical microscopy and scanning electron microscopy, equipped with an energy dispersive x-ray detector. The magnetic properties were characterized by magnetic flow measurements. The flux tests indicated that the triple layer of coating provided a greater corrosion resistance in atmosphere containing chlorides than the one with sulfides. The potentiodynamic and the EIS tests showed that the corrosion rate increased with temperature. The magnetic properties of the sample remained unchanged after exposure to the aggressive environment.