Corrosion protection of sintered NdFeB magnets by phosphating

Phosphating of sintered NdFeB magnets has been studied by immersion in a solution of 0.15 M NaH 2PO 4 acidified to pH 3.7 under polarization. Cyclic polarization experiments indicated that phosphating could be assisted by polarization, with the current density decreasing as the number of polarization cycles increased. Auger electron spectroscopy (AES) and Energy Dispersive X-ray Analysis (EDX) of magnets exposed to the phosphating treatment confirmed the formation of the phosphate layer over both main phases of the specimen, namely, the magnetic (Φ) and the Nd-rich phase. Electrochemical impedance spectroscopy (EIS) measurements performed on treated and untreated magnets immersed in synthetic saliva showed the phosphate conversion layer to improve the corrosion resistance and provided evidence of its porous nature. The phosphating procedure adopted in the present investigation is a promising surface treatment for improving the corrosion resistance of sintered NdFeB magnets