简介概要

Enhancement of corrosion resistance in sintered Nd-Fe-B permanent magnet doping with different CuZn₅ contents

来源期刊：Rare Metals2017年第10期

论文作者：Zhuang Wang Wei-Qiang Liu Dong-Tao Zhang Ming Yue Xiu-Lian Huang Xu-Liang Li

文章页码：812 - 815

摘要：Nd-Fe-B permanent magnets doped with CuZn₅ powders were prepared via conventional sintered method. The effects of CuZn₅ contents on magnetic properties and corrosion resistance of the magnets were systematically studied. It shows that the remanence,coercivity, and maximum energy product decrease gradually with the increase in CuZn₅ doping content. The magnet’s corrosion kinetics in autoclaves environment and its electrochemical properties in electrolytes were also examined. It is interesting to see that the weight loss of3.5 wt% and 4.5 wt% CuZn5 powders doping magnets is only 1 and 0 mg·cm^-2 after autoclaves test at 121℃,2 ×10⁵ Pa for 500 h, respectively, which is much lower than that of the magnets without CuZn₅ doping. Electrochemical results show that the CuZn₅ powders doping magnets display more positive corrosion potential(E_corr)and lower corrosion current density(I_corr) than those of the original magnets without CuZn₅ doping in sulphuric acid electrolyte and distilled water. It is, therefore, concluded that doping CuZn₅ powders is a promising way to enhance the corrosion resistance of sintered Nd-Fe-B magnets.

稀有金属(英文版) 2017,36(10),812-815

Enhancement of corrosion resistance in sintered Nd-Fe-B permanent magnet doping with different CuZn₅ contents

Zhuang Wang Wei-Qiang Liu Dong-Tao Zhang Ming Yue Xiu-Lian Huang Xu-Liang Li

College of Materials Science and Engineering, Beijing University of Technology

The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology

State Key Laboratory of Rare Earth Permanent Magnetic Materials

Anhui Earth-Panda Advance Magnetic Material Co.,Ltd

收稿日期：27 January 2015

基金：financially supported by the National High Technology Research and Development Program of China (No. 2012AA063201);the National Natural Science Foundation of China (Nos. 51001002 and 51371002);the International S&T Cooperation Program of China (No.2015DFG52020);the Natural Science Foundation of Anhui Province (No.1408085MKL72);the 2011 Cooperative Innovation Center of Beijing University of Technology;

Enhancement of corrosion resistance in sintered Nd-Fe-B permanent magnet doping with different CuZn₅ contents

Zhuang Wang Wei-Qiang Liu Dong-Tao Zhang Ming Yue Xiu-Lian Huang Xu-Liang Li

College of Materials Science and Engineering, Beijing University of Technology

The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology

State Key Laboratory of Rare Earth Permanent Magnetic Materials

Anhui Earth-Panda Advance Magnetic Material Co.,Ltd

Abstract：

Nd-Fe-B permanent magnets doped with CuZn₅ powders were prepared via conventional sintered method. The effects of CuZn₅ contents on magnetic properties and corrosion resistance of the magnets were systematically studied. It shows that the remanence,coercivity, and maximum energy product decrease gradually with the increase in CuZn₅ doping content. The magnet's corrosion kinetics in autoclaves environment and its electrochemical properties in electrolytes were also examined. It is interesting to see that the weight loss of3.5 wt% and 4.5 wt% CuZn5 powders doping magnets is only 1 and 0 mg·cm^-2 after autoclaves test at 121℃,2 ×10⁵ Pa for 500 h, respectively, which is much lower than that of the magnets without CuZn₅ doping. Electrochemical results show that the CuZn₅ powders doping magnets display more positive corrosion potential(E_corr)and lower corrosion current density(I_corr) than those of the original magnets without CuZn₅ doping in sulphuric acid electrolyte and distilled water. It is, therefore, concluded that doping CuZn₅ powders is a promising way to enhance the corrosion resistance of sintered Nd-Fe-B magnets.

Keyword：

Sintered Nd-Fe-B magnet; Corrosion resistance; CuZn₅ powders; Autoclave; Polarization characteristics;

Author： Wei-Qiang Liu,e-mail: liuweiqiang77@hotmail.com;

Received： 27 January 2015

1 Introduction

Nd-Fe-B sintered magnets have widely commercial and functional applications because of their superior magnetic properties mainly due to their compositions and micro structure [ 1] .Up to now,however,their undesirable chemical stability is still a major concern for some practical application such as acidic and humidity/hot environments [ 2, 3, 4] .Many investigations show that the poor corrosion resistance of sintered Nd-Fe-B magnets results from their complex microstructure;in particular,the size and distribution of the Nd-rich grain boundary phase [ 5, 6, 7] .In electrochemical corrosion environment,the Nd-rich phase distributed on the grain boundaries of Nd₂Fe₁₄B phase shows more negative potential compared to the Nd₂Fe₁₄B phase,and is priority to corrode,which will lead to disconnection of the matrix phase grain and deterioration of the magnetic properties [ 8, 9] .In order to improve the corrosion resistance,addition of alloying elements to sintered Nd-Fe-B magnets during alloy melting process has been explored widely to improve their corrosion resistance [ 10, 11] .Unfortunately this method was always accompanied by the deterioration of the magnetic properties of the magnets.Recently,many efforts have been focused on the micro structural modification by additive elements,especially in Nd-rich phase,which is an effective way to improve the corrosion resistance of sintered Nd-Fe-B [ 12, 13, 14, 15, 16, 17, 18, 19] .During the sintering process,these additive elements play an important role in modification of grain boundary microstructure for the magnets.Based on our previous research results,doping a small amount of CuZn₅ powders(wt%) could enhance the corrosion resistance of sintered NdFe-B magnet [ 20] .It is therefore expected that doping more CuZn₅ powders could lead to better corrosion resistance in the magnet.Considering the above theory,more CuZn₅ powders were doped into sintered Nd-Fe-B magnets and the chemical stability of the magnets was systematically investigated in this paper.

2 Experimental

Alloy ingots with nominal composition of Nd_22.5Pr_6.6Feba1B_1.0 and CuZn₅ were prepared by induction melting.The ingots were then prepared into flakes via strip casting technique.Subsequent hydrogen decrepitating and jet milling were applied to obtain Nd-Fe-B powders with even size distribution from the flakes.The CuZn₅ ribbons were crushed into powders by ball milling in Ar.The values of particle size of jet milling NdFeB powder and CuZn₅powder are 3-5 and 10-20μm,respectively.The Nd-Fe-B powders and the CuZn₅ powders were mixed homogeneously and pressed together with rubber isotropic pressing under a pulse magnetic field of 5 T.The amounts of the CuZn₅ powders were set as 0 wt%,3.5 wt%,4.5 wt%and6.0 wt%of the Nd-Fe-B powders.The green compacts were then sintered at 1040℃for 3 h and subsequently annealed at 900 and 480℃for 2 and 1 h,respectively.The microstructure and magnetic properties of the magnets were investigated by scanning electron microscope (SEM,FEI Nova Nano 200) and B-H (magnetic induction-magnetic field) loop-line instrument (NIM 500 C),respectively.The corrosion kinetics of the magnets were examined by autoclave test at 121℃,2×10⁵ Pa,up to 500 h,and the corrosion rates of magnets were evaluated by weight loss.Potentiodynamic polarization experiments were carried out with Princeton 4000 potentiostat.All experiments were performed in a standard three-electrode cell consisting of Nd-Fe-B working electrode,saturated calomel reference electrode,and Pt counter electrode.The open-circuit potential was measured with the scanning rate of 1 mV·s^-1to obtain the anode polarization curve.Experiments were carried out at room temperature in 0.1 vol%sulphuric acid solution and distilled water electrolytes without stirring.

3 Results and discussion

In previous research,the magnets of good corrosion doping with 0.5 wt%-3.0 wt%CuZn₅ were prepared [ 20] .In order to achieve better corrosion resistance,the magnets doping with CuZn₅ up to 6.0 wt%were fabricated.Figure 1 shows the magnetic properties of sintered magnets as a function of CuZn₅ content.It is observed that the remanence (B_r),coercivity (H_ci),and the maximum energy product((BH)_max) of the magnets decrease gradually with the CuZn₅ content increasing.The B_r,H_ci and (BH)_max of the magnet doping with 3.5 wt%CuZn₅ drop by 21.49 kA·m^-1(1.89%),0.087 T (7.32%) and 50.37 kJ·m^-3 (14.90%),respectively.Although the magnetic properties of the magnets doping with 4.5 wt%and 6.0 wt%CuZn₅decrease remarkably,it is worth expecting that doping CuZn₅ could improve the corrosion resistance of the magnets.

Fig.1 Magnetic properties of sintered magnets as a function of CuZn₅ powders doping content

Figure 2 manifests the corrosion kinetics curves of sintered magnets doping with different CuZn₅ contents in autoclave test at 121℃,2×10⁵ Pa.The weight loss of the magnets per area during corrosion was plotted as a function of time.It is observed that the weight loss of the CuZn₅ free magnet increases substantially with the corrosion time prolonging,while the weight loss of CuZn₅powders-doped magnets almost keeps invariable.Table 1shows that the corrosion rate of the magnets doping with different CuZn₅ contents in autoclave test at 121℃,2×10⁵ Pa for 500 h.Results show that the weight loss of the CuZn₅ free magnet after autoclave test for 500 h can reach 106.14 mg cm^-2,which is far greater than that of CuZn₅ powders-doped magnets.In detail,the weight loss of the magnet doping with 3.5 wt%CuZn₅ powders is only1 mg·cm^-2 and there is no weight loss for the magnets doping with 4.5 wt%and 6.0 wt%CuZn₅ powders.It is therefore concluded from above experimental results that Nd-Fe-B magnets doping with CuZn₅ powders possess much better corrosion resistance than that of the CuZn₅free magnet.

Fig.2 Corrosion kinetics curves of sintered Nd-Fe-B magnets doping with different CuZn₅ contents in autoclave at 121℃,2×10⁵ Pa

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Table 1 Corrosion rate of magnets doping with different CuZn₅contents in autoclave test at 121℃,2×10⁵ Pa for 500 h

Potentiodynamic polarization curves for sintered NdFe-B magnets doping with 0 wt%,3.5 wt%and 6.0 wt%CuZn₅ in 0.1 vol%sulphuric acid solution and distilled water are shown in Fig.3.Potentiodynamic polarization curves show a typical behaviour of activation polarization.The corrosion potential (E_corr) and corrosion current density (I_corr) of sintered Nd-Fe-B magnets doping with0 wt%,3.5 wt%and 6.0 wt%CuZn₅ in 0.1 vol%sulphuric acid solution and distilled water were obtained from the potentiodynamic polarization curves by the extrapolation method and are summarized in Table 2.It is observed that the corrosion potential and corrosion current densities of the magnets decrease gradually with the CuZn₅ content increasing.The more positive corrosion potential and lower corrosion current densities indicate that the magnets are more stable to electrochemical corrosion.It is therefore concluded that doping CuZn₅ is an effective way to improve the corrosion resistance of sintered Nd-Fe-B magnets.

In previous research,the distributions of the doped Cu and Zn element in the Nd-Fe-B magnet were examined [ 20] .It is observed that both Zn and Cu enrich mainly in the Nd-rich phase and form a new compound with Nd,which has more positive electrochemical potential,and their uniform distribution could effectively restrain the aggressive inter-granular corrosion.Such micros true tural modification could effectively restrain the aggressive intergranular corrosion.

Fig.3 Potentiodynamic polarization curves of sintered Nd-Fe-B magnets doping with 0 wt%,3.5 wt%and 6.0 wt%CuZn₅ in a distilled water and b 0.1 vol%sulphuric acid solution

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Table 2 Corrosion potentials (E_corr) and corrosion current densities (I_corr) of sintered Nd-Fe-B magnets doping with different CuZn₅ contents in different solutions

4 Conclusion

In summary,the magnetic properties and corrosion resistance of sintered Nd-Fe-B magnets with CuZn₅ powders doping were systematically investigated.With the increase of CuZn₅ doping content,the remanence,coercivity,and maximum energy product decrease gradually.However,the CuZn₅ doping magnets show greater anti-corrosion resistance,i.e.,there is no weight loss for the magnet doping with 4.5 wt%and 6.0 wt%CuZn₅ powders in autoclave test,and the CuZn₅-doped magnets have more positive corrosion potential (E_corr) and much lower corrosion current density (I_corr) than the magnets without CuZn₅doping.As a result,the CuZn₅-doped magnet possesses excellent corrosion resistance.