The susceptibility of Nd-Fe-B magnet powders to oxidation and corrosion is well known and presents problems in processing and in service. Although the oxidation of Nd-Fe-B magnet powders can lead to severe irreversible losses, such losses can be stemmed by the application of antidegradation coatings. The effects of oxidation on Nd-Fe-B magnet powders and magnets at elevated temperatures have been examined with respect to time, particle size, and treatment, using isothermal thermogravimetric analysis and long-term oven aging. The magnetic properties of each sample were then evaluated by hysteresis testing. The effects of corrosive agents were also examined, and although the oxidation and corrosion of Nd-Fe-B magnet powders may yield similar weight gains and losses in magnetic properties, each process exhibits a different magnetic hysteresis "signature." The correlations between the weight gain and magnetic measurements have proven quite reliable. The application of coatings to magnet powders can reduce the weight gain due to degradation by more than 98%, while the losses in magnetic properties are reduced from over 75% (for uncoated powders) to less than 10% (for coated powders).
