AQ-11: A Design of Stainless Nd-based Permanent Magnets without Coatings and the Corrosion Mechanism Research

Nd-Fe-B magnets have become the most consumed permanent magnet due to their excellent magnetic properties. However, the susceptibility to corrosion is a main disadvantage[1,2]. It is important to improve the corrosion resistance of uncoated sintered Nd-Fe-B magnets. In this work, the sintered Nd-Ni-Fe-Cr-B magnet was designed and fabricated. This sintered Nd-Ni-Fe-Cr-B magnet maintains a typical 2:14:1 tetragonal phase, and the metallurgical behavior of Ni and Cr is characterized by the distribution of Ni at the grain boundary and Cr in the main phase. To evaluate the effect of the co-addition of Ni and Cr on corrosion resistance, sintered Nd-Fe-B magnets with the same Nd content were also prepared. The results of the corrosion tests are encouraging: the mass loss of sintered Nd-Fe-B magnet (5.00 mg/cm2) is ~11.40 times that of sintered Nd-Ni-Fe-Cr-B magnet (0.44 mg/cm2) after corroded in accelerated corrosion test for 480 hours; The corrosion current of sintered Nd-Fe-B magnet (191.00 μA/cm2) is ~17.36 times that of sintered Nd-Ni-Fe-Cr-B magnet (11.00 μA/cm2) in 3.5 wt% NaCl solution. The impedance of NdNiFeCrB magnet (Rct0 is 2800.00 Ω) is greatly increased considerably by an order of magnitude compared to Nd-Fe-B magnet (Rct0 is 238.80 Ω). The metallurgical behavior of Ni and Cr, diminishing the potential difference between the grain boundary phase and the main phase, gives a huge improvement in the corrosion resistance of sintered Nd-Fe-B magnet. This work provides a direction for future research on magnets without coatings. Fig. 1 (a) The mass loss curves of samples; (b) The open circuit potential curves; (c) Potentiodynamic polarization plots. Fig. 2 In 3.5 wt% NaCl solution:(a) Nyquist plots of samples. The inset is the enlarged view of Nyquist plots of NdFeB magnet. (b) The equivalent circuit of NdNiFeCrB magnet. (c) The equivalent circuit of NdFeB magnet.References: [1] Edgley DS, Le Breton JM, Steyaert S, et al. Teillet. Characterisation of high temperature oxidation of Nd-Fe-B magnet. J Magn Magn Mater 1997;173(1e2):29e42. [2] Lemarchand D, Delamare J, Vigier P. Thermomagnetic study of the high-temperature oxidation behavior of Nd-Fe-B permanent magnets and powders. J Appl Phys 1992;72:1996e2000.