Failure analysis of magnets in a servomotor by Mössbauer and X-ray diffraction

The failure of a servo motor with Nd–Fe–B magnets under harsh atmospheric conditions is analysed. The magnets presented corrosion both on the side glued to the rotor (magnet-bottom) and on the opposite side (magnet-top), and eventually came off the rotor shaft. The corrosion products were studied by X-ray diffraction and Mössbauer spectroscopy and compared to the uncorroded magnet material. The observed products, Nd(OH)₃ and iron oxyhydroxides, evolve from the magnet-bottom to the top, and show that the atmospheric environment was wet with a high concentration of salts. Finally, some paramagnetic powder was found away from the magnet set, as a consequence of rotor rotation, and it represents a further oxidation process of the magnet products. The surface protection of the magnets has shown to be insufficient for the atmospheric working conditions.