Removal of non-magnetic impurities on the surface of NdFeB magnets

NdFeB rare earth permanent magnets are widely applied in numerous fields including consumer electronics, industrial motors and new energy vehicles. Sintered NdFeB magnets are prepared by pressing and high-temperature sintering, followed by machining into desired shapes with anisotropism and high magnetic properties. During industrial production process, the appearance of non-magnetic impurities adhering on NdFeB magnet surface is a critical problem, which takes bad influences to not only magnetic properties of NdFeB, but also the qualification rate of subsequent assembled parts. There are two forms of impurities, one is gelatinous and the other is filamentary. In this paper, non-magnetic impurities on the surface of NdFeB magnets were characterized and analyzed by FTIR, EDS and SEM, and the methods to remove impurities were studied. The results showed that gelatinous impurities are stearamide derivative, which can be dissolved by xylenes under heating conditions. Filamentous impurities are cellulose derivative that can be removed by economical sodium hydroxide-urea system instead of expensive AmimCl, without damaging magnet plating and magnetic properties.