Corrosion mechanism of RE-Fe-Co-Cu-Ga-Al-B magnets

The corrosion stability of Nd-Fe-B magnets in hot and humid climates was in creased by about three orders of magnitude by simultaneous additions of Co, Cu, Al and Ga. Such magnets show almost no weight loss in a highly acceler ated stress test at 130 degreesC (95% humidity) even after 200 days of expo sure. In these magnets the very reactive Nd-rich intergranular constituents are replaced by Nd-3 (Co,Cu), Nd-5 (Co,Cu,Ga)(3) and Nd-6 (Fe,Co)(13) Ga c ompounds which are much more noble. The free electrochemical potentials of almost single phase samples of these compounds were determined to be about 800 mV higher compared to pure Nd. Therefore, the formation of hydroxides a nd thus the corrosion rate is much slower.