Improving the corrosion resistance of NdFeB magnets: an electrochemical and surface analytical study

This study reports the results of work carried out to improve the corrosion resistance of NdFeB magnets by a direct phosphate treatment of the NdFeB magnet surface. The growth of a phosphate film on a NdFeB based magnet in a solution of 0.15M NaH2PO4 acidified to pH 3.8, was followed by electrochemical impedance spectroscopy (EIS) and open circuit potential variation as a function of time. After removal from the phosphating solution, the magnet surface was analysed by Auger electron spectroscopy (AES) and energy dispersive X-ray analysis (EDX). Phosphate formation on the surface was confirmed and a significant variation in the Fe/Nd ratio in the phosphate between the Fe and Nd rich phases of the magnet was found. The corrosion resistance of the phosphated magnets was evaluated in a normal enclosed atmospheric environment and in Na2SO4 solution and synthetic saliva by EIS measurements. The corrosion resistance was found to be substantially improved by the presence of a phosphate layer.