Crucial role of the REFe2 intergranular phase on corrosion resistance of Nd La Ce Fe B sintered magnets
Abstract Poor corrosion resistance is one major concern of RE2Fe14B type (RE, rare earth) sintered magnets due to the large electrode potential difference between the ferromagnetic matrix phase and the RE rich intergranular phase. The phase constituent within the intergranular region of high cost performance Nd La Ce Fe B magnets, however varies with the La/Ce substitution level, and extra REFe2 phase forms over a critical La/Ce content. In this work, we performed a comparative study of two magnets prepared by single main phase (SMP, sintering the Nd La Ce Fe B powders free of REFe2 phase) or by binary main phase (BMP, sintering the mixture of Nd Fe B and Nd La Ce Fe B powders containing REFe2 phase) approaches to reveal the role of REFe2 phase on the corrosion resistance. The results show that the BMP magnet with additional REFe2 intergranular phase exhibits much better corrosion resistance against both hot/humid and saline environments than the SMP one having merely conventional RE rich intergranular phase. It is attributed to that the REFe2 phase with higher electrode potential than RE rich phase stabilizes the intergranular regions and lowers the driving force for electrochemical corrosion. Besides, REFe2 phase that locates mainly at the triple junctions suppresses the corrosion propagation and blocks the subsequent dissolution of RE2Fe14B grains. The Nd La Ce Fe B BMP magnets with improved corrosion resistance as well as the formerly reported superior magnetic performance to SMP ones are appealing for practical applications.