Sintered RE-(Fe, Co, Ni)-B magnets in a regime of RE~Co-B phase diagram were prepared by a powder metallurgical method, and their corrosion behavior as well as magnetic properties were studied in comparison with those of conventional rare earth magnets. Nd-Fe-B magnets substituted with Co and Ni for Fe exhibited no corrosion at all for 1000 h at 70 °C in air with a relative humidity of 95%. They also be resistant to spray salt to the same extent as Sm-Co magnets. The potentiostatic anode polarization curves of the magnets exhibited a remarkable effect of Ni in elevating the corrosion potential in acid- and neutral solutions and in yielding passivation in an alkali solution. Magnets singly substituted with Co displayed only a slight improvement of corrosion over Nd-Fe-B magnets. Microscopic observations of the magnets with analytical TEM and SEM indicated an important role of a Nd(Ni, Co) phase present at the grain boundaries in realizing the good magnetic properties and suitable corrosion resistance at the same time. The influences of replacement of Nd by La, Pr and Dy in the Nd-(Fe, Co, Ni)-B magnets on the corrosion behavior were also studied.
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- 化学 (全般)