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 increased 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 accelerated stress test at 130°C (95% humidity) even after 200 days of exposure. In these magnets the very reactive Nd-rich intergranular constituents are replaced by Nd₃ (Co,Cu), Nd₅(Co,Cu,Ga)₃ and Nd₆(Fe,Co)₁₃Ga compounds 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 and thus the corrosion rate is much slower