CORROSION-RESISTANCE OF ND-FE-B SINTERED MAGNETS

The corrosion mechanism of Nd-Fe-B sintered magnets has been investigated. A correlation between the lifetime of epoxy-coated magnets in a humidity test (80-degrees-C, 90% RH) and the rate of weight loss (DELTA-W/DELTA-t) of the bare magnets in the pressure cooker test (PCT: 125-degrees-C; 85% RH; 2 atm) has been established. In this system, corrosion is attributed to the selective oxidation of grain boundaries (the Nd-rich phase). The value of DELTA-W/DELTA-t is closely related to the concentration of neodymium and it decreases when cobalt or nickel is added. Consistency of corrosion resistance of the surface-coated Nd-Fe-B sintered magnets can be achieved by controlling DELTA-W/DELTA-t below a certain critical value.