Influence of Nd substitution on corrosion resistance of Pr-Nd-Fe-B sintered magnet

Taking the magnet weight loss after the HAST accelerated life test and the magnet dynamic potential polarization curve in different corrosive media as characterizations, the effect of the substitution of rare earth element Nd for Pr on the corrosion resistance of sintered Pr-Nd-Fe-B magnets is studied. The results show that: After Nd completely replaces Pr, the weight loss of the magnet is reduced, and the corrosion potential in 0.1 mol/L HCl solution, 3.0% NaOH solution, and 3.5% NaCl solution at room temperature shifts positively to varying degrees, indicating that the corrosion resistance of the magnet is improved. Through field emission Scanning electron microscope observation of the microstructure of the magnet after the HAST test found that after Nd replaced Pr, the grain boundary distribution became more continuous and clear, and the rare earth-rich phase content at the grain boundary junctions decreased, thereby delaying the grain boundary corrosion rate , Which improves the overall corrosion resistance of the magnet.

Neodymium iron boron (Nd-Fe-B) magnets have the advantages of high magnetic performance, strong anti-demagnetization ability, easy processing and molding, and easy realization of device miniaturization and energy saving. They are widely used in electronic communication, automatic control, new energy, and energy saving. Motors and many other fields [1-2]. Due to factors such as manufacturing costs and resource balance utilization, the industry generally uses neodymium praseodymium alloys instead of pure metal neodymium as raw materials to produce sintered Pr-Nd-Fe-B magnets. But Its corrosion resistance is poor [2-7], and it is prone to corrosion [2, 8], causing the magnet to fail and restricting the application of the magnet [1].
For this reason, researchers have used various methods and means to carry out a lot of research on its corrosion resistance behavior. ZHANG P et al. [9] used Cu and Nb powder as a modifier to improve the corrosion resistance of NdFeB magnets; literature [10-11] By adding Cu_(85)Sn_(15) and Cu/Al to improve the grain boundary to improve the corrosion resistance of the magnet; PAN M et al. [12] used Cu and Zr powder to improve the corrosion resistance of the magnet. ZHOU Q et al. [13] used refractory metals Nb and Ti instead of Fe to reduce the PCT weight loss of the magnet; in 2017, Wang Xiaoer [14] used Mg nanopowder to be added to the NdFeB magnet to form a Mg-Nd grain boundary Phase, thereby improving the corrosion resistance of the magnet. However, the use of rare earth element Nd instead of Pr research is very few.
Therefore, in combination with the actual production of the NdFeB industry, Nd is used to replace Pr-Nd to prepare sintered magnets, and the effect of Nd substitution on the HAST weight loss, corrosion potential and microstructure of the magnet is studied and analyzed. This is to improve the Pr-Nd-Fe-B sintered magnet Corrosion resistance provides a basis, which has certain practical significance.

Experimental materials and methods

Using industrial pure Nd, Pr-Nd alloy, pure Fe and B-Fe alloy with a B content of 19% as raw materials, according to the two composition ratios of Nd32Fe67.02B0.98 and Pr7Nd25Fe67.02B0.98. Use them to make them separately The alloy flakes with an average thickness of 0.3 mm are obtained by hydrogen crushing and jet milling to obtain magnetic powder with an average particle size of 3.2 μm. The magnetic powder is oriented and formed under a 2.1 T magnetic field. After 200 MPa cold isostatic pressing, it enters the vacuum sintering furnace for sintering and aging. The sintering process is 1 065 ℃×5.5 h, the first aging is 890 ℃×3 h, and the second aging is 480 ℃×4.5 h, and the sintered magnet is prepared. Then the size is 10 mm×10 by surface grinder, slicer and polishing mechanism mm×10 mm test sample magnet.
A high temperature accelerated aging test box (HAST, EHS-411M) is used to test the mass loss (weight loss) of the magnet under the conditions of 131 ℃, 100%RH, 0.202 MPa, and an electrochemical comprehensive tester (CHI660E) is used to test the magnet at room temperature at 0.1 Potential polarization curve in mol/L HCl solution, 3.0% NaOH solution, and 3.5% NaCl solution, using field emission scanning electron microscope (SEM, SU1510) to observe and analyze the microstructure of the magnet after HAST test.

Results and discussion

Impact on weightlessness

The weight loss curves of the two magnets with different compositions after HAST test for 24 h, 48 h, 72 h, 96 h, and 120 h are shown in Figure 1.

Fig. 1 Mass loss comparison of Nd-Fe-B and Pr-Nd-Fe-B magnets by HAST

After HAST aging for 120 h, the weight loss of the magnet (Pr-Nd-Fe-B magnet) prepared with Pr-Nd alloy as raw material reached 0.033 mg/cm2, which was significantly greater than that of the magnet using pure Nd (Nd-Fe-B magnet) 0.027 The weight loss of mg/cm2. This indicates that the corrosion resistance of the magnet is improved after the use of pure Nd to replace the Pr element. This is mainly due to the position of the Nd element in the periodic table of the chemical elements. The metal activity is weaker than that of Pr. Pr is less prone to oxidation reaction [15].

Influence on corrosion potential

Figure 2(a)~Figure 2(c) show the Nd-Fe-B magnets and Pr-Nd-Fe-B magnets in 0.1 mol/L HCl solution, 3.0% NaOH solution, and 3.5% NaCl solution at room temperature. It can be seen from Figure 2 that at room temperature, under the condition of 0.1 mol/L HCl solution as the corrosive medium, the corrosion potential of Nd-Fe-B magnets, Ecor(Nd-Fe-B, 0.1 mol/ L HCl) is -0.649 V, Pr-Nd-Fe-B magnet corrosion potential Ecor (Pr-Nd-Fe-B, 0.1 mol/L HCl) is -0.736 V; under the condition of 3.0% NaOH solution as the corrosive medium , Nd-Fe-B magnet corrosion potential Ecor (Nd-Fe-B, 3.0% NaOH) is -0.525 V, Pr-Nd-Fe-B magnet corrosion potential Ecor (Pr-Nd-Fe-B, 3.0% NaOH) -0.625 V; Under the condition of 3.5% NaCl solution as the corrosive medium, the corrosion potential of Nd-Fe-B magnets Ecor (Nd-Fe-B, 3.5% NaCl) is -0.739 V, Pr-Nd-Fe-B magnets Corrosion potential Ecor (Pr-Nd-Fe-B, 3.5% NaCl) is -0.94 V. It can be seen that after using pure Nd to replace Pr-Nd alloy as raw material, the corrosion potential of magnets under different corrosive media conditions are different The degree of positive shift indicates that the electrochemical corrosion resistance of the magnet becomes better [16-19].

Fig. 2 Potentiodynamic polarization curves of Nd-Fe-B and Pr-Nd-Fe-B magnets under different corrosive mediums

Figure 3 shows the SEM secondary electron images of two magnets with different compositions. The results show that the rare earth-rich phase of the Pr-Nd-Fe-B magnet is unevenly distributed (aggregated in a nebula shape, Figure 3(b)). The distribution is easy This causes rapid corrosion of the rare earth-rich phase in the local area, which makes the overall corrosion more serious. The rare earth-rich phase of the Nd-Fe-B magnet is less agglomerated (Figure 3(a)), which makes the corrosion more minor [20]. After replacing Pr, the corrosion resistance of the magnet is improved. This is consistent with the results described above.

Fig. 3 Secondary electron images of the magnets by SEM

The EDS point measurement results of the magnet in the different contrast areas in the secondary electron image (Table 1) show that the white area in Figure 3 is the grain boundary rare earth-rich phase, and the gray area is the main phase of the matrix.

Table 1 EDS of Nd-Fe-B and Pr-Nd-Fe-B magnets /%

Conclusion

• 1) After using Nd to completely replace Pr, the grain boundary phase distribution of the magnet is more uniform, the aggregation phenomenon is reduced, the HAST weight loss of the magnet is reduced, and the degree of corrosion is reduced.
• 2) After completely replacing Pr with Nd, the corrosion potential is positively shifted in 0.1 mol/L HCl solution, 3.0% NaOH solution, and 3.5% NaCl solution at room temperature, and the electrochemical corrosion resistance is enhanced.

Author: Zhi Qi Qi, Yu Xi, Du Junfeng, Wang Liang Liang, Liu Bin, Pang up again, Lee Festival, Wang Gongping

Source: China Permanent Magnet Manufacturer – www.rizinia.com

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