Corrosion prevention of sintered Nd–Fe–B magnet by a phosphate chemical conversion treatment

Abstract

The corrosion-resistant coating formed on the surface of sintered Nd–Fe–B magnet by a phosphate chemical conversion (PCC) treatment was studied. The morphology, phase composition and thickness of the coating were investigated by field emission scanning electron microscopy (FE-SEM), energy-dispersive spectrometer (EDS), Fourier transform infrared (FTIR) spectrometer and coating thickness gauge. The corrosion behaviour of the phosphated magnet was evaluated by copper sulphate spot test, neutral salt spray test and electrochemical potentiodynamic polarization experiment. The magnetic properties of the phosphated magnet were also tested. The experimental results show that the phosphate coating has such characteristics as dense granular growth, uniform distribution and thickness range of 10–18 μm. The corrosion resistance of the magnet is significantly improved by phosphate coating without losing magnetic properties. Therefore, this highly efficient PCC was a good way for increasing the corrosion resistance of the sintered Nd–Fe–B magnets.