Environmental degradation of NdFeB magnets

A mechanism for pitting of NdFeB magnet because of differential-aeration beneath a water droplet is proposed and observations of the localised corrosions are presented. NdFeB magnets exhibit general corrosion along the grain boundaries when etched in Viella’s reagent.1 However, localised corrosion of these magnets results in a crater-like feature when corrosion is produced in an environmental chamber, e.g. when Nd16Fe76B8 magnets are corroded in the environmental chamber at 85 °C, relative humidity (RH): 80%. This is attributed to the condensation of water droplets on the surface of samples and the concentration gradient of oxygen dissolved in the droplets then influencing the corrosion process. It is thought that during the process of pitting, the high concentration of H+ in the center of the pit accelerates the pit development; meanwhile, the cathodic Nd2Fe14B matrix phase absorbs the nascent hydrogen atoms. It is believed that pits start at the Nd-rich phase and then propagate along the grain boundaries.