Influence factors of corrosion weight loss of sintered NdFeB magnets and improvement of corrosion resistance

A series of sintered NdFeB magnets are prepared by powder metallurgy. The microstructure composition of the magnet was analyzed by scanning electron microscope and electron energy spectroscopy, and the alloy composition (including Dy content, Pr content, total rare earth content and trace additive elements) and the influence of process methods on the corrosion weight loss of the magnet were studied, and the correlation was analyzed. mechanism. The experimental results show that the control of Pr content, Dy content and total amount of rare earth can improve the weight loss of the sintered NdFeB magnet and improve the corrosion resistance. On this basis, a magnet with low weight loss was prepared by optimizing the composition of the magnet and the preparation process. The improved weight loss performance of the magnet has reached the level of less than 3 mg/cm2 in 30 days. Through the compound addition of elements, the magnetic properties and corrosion resistance of the magnet are improved, so that it can be widely used in corrosive environments.

In 1983, Japan’s Sumitomo Special Metals Corporation crushed and sintered the alloy of Nd15Fe77B8 (atomic fraction) and then annealed it to obtain a square intermetallic compound whose main phase was in the form of Nd₂Fe₁₄B, that is, the third-generation rare earth permanent magnet material: NdFeB magnets. After more than 30 years of development, this high-performance magnet is widely used in electric motors, generators, computers, new energy vehicles, intelligent manufacturing, robots, and communication equipment, and has become indispensable for modern industrial production, national life, and aerospace technology. Missing important materials. However, neodymium iron boron magnets are easily corroded in the working environment [1-2], causing deterioration of the magnetic properties of the magnets and structural failure, which become the main obstacles and focus of its application. The poor corrosion resistance of sintered NdFeB magnets can be attributed to the following three factors:

• (1) The intrinsic structure of the material. Sintered NdFeB alloy has a multi-phase structure, and the corrosion resistance of each phase is different. The neodymium-rich and boron-rich phases distributed at the grain boundary are prone to oxidation and form intergranular corrosion [3]. The NdFeB alloy is prepared by powder metallurgy. The voids and looseness in the magnet make it difficult to form an effective oxide protective film on the surface of the magnet; once it is oxidized, it will cause a chain reaction and accelerate the oxidation.
• (2) Impurity elements present in the alloy. The impurity elements that may exist in sintered NdFeB permanent magnet alloys are mainly O, H, N, C, Si, Cl and chlorides, among which oxygen, chlorine and chlorides are the most harmful to corrosion performance. The corrosion of the magnet is mainly manifested in the oxidation process, and chlorine and chloride will accelerate the oxidation process of the magnet.
• (3) Working environment. Ambient temperature, medium conditions, humidity and pressure have a great influence on the corrosion behavior of magnets [4-6]. The neodymium iron boron magnet is composed of the main phase Nd2Fe14B, boron-rich phase and neodymium-rich phase. The neodymium-rich phase surrounds the main phase as the grain boundary phase, and most of the boron-rich phase also exists in the grain boundary and the intersection of the grain boundary. This is an area prone to corrosion. Sintered NdFeB magnets are prone to corrosion. On the one hand, the element Nd is one of the metal elements with higher chemical activity [7], and its standard potential (Nd3+/Nd) = -2.431V; on the other hand, the multi-phase electricity of the magnet The chemical potential difference is large [8]. The electrode potential of the neodymium-rich grain boundary phase on the surface of the magnet is the most negative and becomes the anode in the galvanic cell, while the main phase becomes the cathode of the galvanic cell, and the neodymium-rich phase is corroded preferentially.

The source of intergranular corrosion of NdFeB material comes from the chemical electromotive force difference between the main phase and the Nd-rich phase and the boron-rich phase. Therefore, the potential difference between different phases can be reduced as much as possible to avoid or weaken the intergranular corrosion. By changing the composition of the intergranular phase, increasing its resistance and reducing the corrosion current, the corrosion resistance of the magnet can be improved. At the same time, the requirements of the magnetic properties of the magnet should be taken into account, and the melting point of the intergranular phase should be kept low and the wettability is good. It is evenly and continuously coated outside the main phase to weaken the effects of magnetic exchange coupling. The main way to change the intergranular phase is to adopt the alloying method. The addition of alloying elements has a great influence on the magnetic properties, and the remanence Br, the magnetic energy product (BH) max and the coercive force may all decrease. Therefore, how to design the alloy composition to ensure that the magnetic properties of the magnet are not greatly reduced, but also to improve its corrosion resistance has become an important research topic. ASKim’s research shows [9] that adding a certain amount of Dy, Co, and Cu to NdFeB can improve the corrosion resistance without reducing the coercivity of the magnet, and the irreversible magnetic flux loss of the magnet becomes very high. Small; microstructure analysis shows that stable intermetallic compounds are formed at the grain boundaries, which surround the main phase. Nd3Co, NdCu and Nd1+εFe4B4 phase compounds are not found in the grain boundaries. The content of the neodymium-rich phase is also very small. The existence of a large number of stable intermetallic compounds is the main reason for the improvement of its corrosion resistance; however, the addition of Dy and Co will increase the cost of the material. In addition, although the anti-corrosion of NdFeB magnets can also be treated with surface coatings, such as Ni, Zn plating on the magnet surface, cathodic electrophoretic coating or epoxy resin spraying [10-11], this will increase Cost, on the other hand, may also reduce the magnetic properties of the magnet.
In this paper, by designing the composition and modulating the production process, the influencing factors of the weight loss performance of the sintered NdFeB magnet are studied. The weight loss here means that the mass loss of the sample is used as a sign of the corrosion rate under given conditions; through the preparation of the alloy composition Method and adjustment of microalloying elements, analysis of the influence of preparation methods and alloying elements on weight loss performance, to prepare sintered NdFeB permanent magnet products with weightless performance, excellent magnetic properties and moderate cost under constant temperature, humidity and pressure conditions, to meet customer requirements The requirements of weightlessness performance; to ensure that the magnet has stable shape, good performance and controllable weightlessness during long-term operation under specific conditions; thus laying the foundation for expanding the application of NdFeB.

Experimental method

Using 99.5% purity neodymium, praseodymium, iron, boron-iron alloy, aluminum, dysprosium and copper as raw materials, they are smelted into alloy ingots in a vacuum induction furnace. After the ingots are peeled and roughly crushed, they are made by jet milling under N2 protection Powder (or using chip casting SC technology, crushed by hydrogen explosion and airflow milling), compressed into a shape after being oriented by a magnetic field. The green body is placed in a high vacuum sintering furnace, sintered at 1050°C for 200 minutes, and then tempered at 630°C for 150 minutes to prepare sintered NdFeB magnets of 30EH, 35UH, 38SH, 42H and 48M. The Dy content is: 10.0, 8.0, 6.0, 3.0 and 1.0wt%. Inductively coupled plasma spectrometer XSP is used to determine the composition of the magnet.
The weightlessness experiment was carried out on the 10×10×10mm sample in the WGD/SJ-5001 pressure vaporization tester at 130℃ and 0.2MPa pressure, and the self-made rare earth permanent magnet non-destructive magnetization characteristic measuring instrument was used to measure the magnetic properties of the magnet, using JSM-6010LA The morphology of the magnet is analyzed by a type scanning electron microscope, and the metallographic structure is observed with the OLYMPUS-MX5O digital optical microscope.

Results and discussion

Alloy composition improvement

Dy content

Because the anisotropy field of Dy2Fe14B (μ0Ha=5.0T) is much larger than that of Nd2Fe14B (7.6T), Dy is often added to increase the coercivity of sintered NdFeB, it is necessary to study the influence of Dy content on the weight loss of the magnet Performance impact. The weightlessness experiment test was carried out on the magnets of various grades of SC process under 120℃, 0.2MPa constant temperature and pressure conditions. The test results are shown in Figure 1.
It can be seen from Figure 1 that the 30EH magnet has the smallest mass loss, with a weight loss of 2.88mg/cm2 after 12 days (288h), followed by 35UH magnets, with a weight loss of 2.98mg/cm2 after 10 days (240h); 38SH magnets, 5 days (120h) The weight loss of) is 3.87mg/cm2; the weight loss of 42H magnet is 2.4mg/cm2 in 2 days (48h); and the mass loss of 48M magnet is the largest, which is 18.54mg/cm2 after 2 days (48h). through
After 240h weightlessness experiment, the test result shows the weightlessness performance of the magnet more clearly

Figure 1 The weight loss curve of a magnet at 120°C and 0.2MPa (SC process)
The relationship with Dy content is shown in Figure 2. It can be seen from the figure that as the content of heavy rare earth element Dy gradually increases from 1wt% to 10wt%, the weight loss of the magnet gradually decreases from 180mg/cm2 to 2.4mg/cm2 after 240h constant temperature and pressure experiment. Moreover, the downward trend has gradually slowed down. However, it is not that the higher the Dy content, the smaller the weight loss of the magnet, but there is a critical value beyond which the addition of more Dy has little effect on the weight loss performance of the magnet. For example, 42H increased 2wt% of Dy element compared with 48M, and the mass loss decreased from 180.36mg/cm2 to 45.53mg/cm2, while 30EH increased 2wt% of Dy element compared with 35UH, and the mass loss only decreased from 2.98mg/cm2 To 2.4mg/cm2. This should be related to the distribution of Dy element in the matrix and the neodymium-rich phase. When the Dy content reaches a certain value, Dy has reached saturation at the grain boundary. With the increase of the heavy rare earth element Dy content, the weight loss performance of the sintered NdFeB magnet is improved. The observation of the microstructure and the analysis of the main phase and the Nd-rich phase further confirm this.

Figure 2 The relationship between the weight loss of the 120℃/0.2MPa/240h test magnet and the Dy content (SC process)

Figure 3 Backscattered electron image of sintered NdFeB magnet (35UH)
Figure 3 is the backscattered electron image of the 35UH sample. It can be seen that most of the gray area is the main phase, and the main intersection is the neodymium-rich phase and the boron-rich phase. The neodymium-rich phase is not uniformly distributed and is in agglomerated state. Further component analysis of each phase of the sample (SC process), the data obtained is shown in Table 1. It can be seen from Table 1 that the heavy rare earth element Dy is distributed in the main phase and the neodymium-rich phase, with more in the neodymium-rich phase; the oxygen element is mainly distributed in the neodymium-rich phase, with little in the main phase; the heavy rare earth element Dy is enhanced The corrosion resistance of the magnet is mainly due to the large amount of Dy present in the neodymium-rich phase of neodymium iron boron, and forms a complex Nd-O-Fe-Dy compound with the oxygen element in it, which reduces the electrochemical potential of the neodymium-rich phase , Enhance the oxidation resistance of the neodymium-rich phase [12].

Figure 4 The relationship between magnet weight loss and Pr content (SC process)

Table 1 Composition of main phase and neodymium-rich phase of different grades of magnets (SC process)

Element	30EH		35UH		38SH		42H		48M
	Main phase	Neodymium-rich phase	Main phase	Neodymium-rich phase	Main phase	Neodymium-rich phase	Main phase	Neodymium-rich phase	Main phase	Neodymium-rich phase
C	4.02	4.6	7.55	13.85	7.55	12.07	4.58	5.78	7.13	7.97
O	2.67	12.49	1.72	7.08	1.74	10.06	2.08	12.51	1.91	12.41
Dy	8.27	16.96	9.5	14.11	3.56	8.27	0	4.98	1.44	1.13
Pr	4.28	16.55	3.77	15.18	2.26	4.66	6.48	20.58	4.17	12.53
Nd	15.5	46.75	13.98	47.85	22.61	50.98	18.81	54.48	23.35	59.64
Fe	63.37	2.04	62.56	1.46	62.28	13.9	66.58	1.39	62	5.99
Co	1.89	0.61	0.92	0.47	0	0.06	1.47	0.28	0	0.33

Pr content

By controlling the content and distribution of Dy, the best weight loss performance of the 35UH and 30EH magnets can reach 10 mg/cm2 for 10 days, which is still far from the customer’s demand for the weight loss performance of the magnet. Therefore, it is necessary to study the influence of other elements on the weight loss performance to further reduce the weight loss of the magnet.
In the sintered magnets studied, apart from Dy and Nd, there is another rare earth element, Pr, whose content is about 7%. Will Pr also have a significant effect on weight loss? For this reason, 4 kinds of magnets with different Pr content (7%, 4%, 2% and 0% respectively) were prepared under the same process. The results of the weightlessness experiment are shown in Figure 4. It can be clearly seen from Figure 4 that the weight loss performance of the magnet increases with the decrease of the Pr content. The Pr content in the magnet decreased from 7% to 2%. The 5-day weight loss experiment showed that the weight loss was improved from 43.58mg/cm2 to 19.16mg/cm2. When the magnet does not contain Pr element, the 5-day weight loss is 12.03mg/cm2. The weight loss performance of the magnet is approximately linear with the decrease of the Pr content. The decrease of the Pr content has a more obvious effect on the long-term weight loss performance. The lower the Pr content, the better the weight loss performance of the magnet. It can be seen from Table 1 that a large amount of Pr element exists in the neodymium-rich phase; and the activity of Pr is greater than that of Nd; therefore, it is easier to form oxides in a humid environment, causing corrosion and peeling of the neodymium-rich phase, and long-term environmental corrosion It may even cause the main phase to fall off, resulting in large area corrosion and block loss.

Total rare earth elements

The weight loss of the magnet is improved with the increase of the content of the heavy rare earth Dy element, and it is increased with the decrease of the Pr content; then, what effect does the total rare earth content have on the weight loss performance? A series of experimental test results show that the total amount of rare earths also has an impact on weight loss. The results are shown in Figure 5. When the total amount of rare earths in the alloy is 32.8%, 32.6%, 32.4, and 32.2%, the weight loss of the magnet is 29.4mg, 11.8mg, 3.19mg and 1.09mg after 7 days of weight loss experiment. When the total amount of rare earths dropped from 32.8% to 32.6%, the weight loss was reduced by 60% (from 29.4mg to 11.8mg). After further reducing the total amount of rare earths, the 30-day weight loss of the magnet with 32.4% of the total rare earths was 8.53 mg; the 30-day weight loss of the magnet with 32.2% of the total rare earths was 2.39 mg. Subsequently,
The effect of the change in the total amount of rare earths in the 35UH and 30EH magnets on the weight loss achieved similar results. It can be preliminarily considered that the lower the total amount of rare earths, the lower the weight loss of the magnet. Pr and Nd are both active elements, and they are abundant in neodymium-rich phase and boron-rich phase; in a humid and hot environment, the agglomeration area of these elements will form an anode relative to the main phase Nd2Fe14B, and corrosion will occur preferentially. The neodymium-rich phase is distributed in a network in the structure, and the corrosion rate is very fast; its corrosion will cause the bonding interface between the main phase Nd2Fe14B grains to disappear, and the grains will fall off, which will eventually lead to the overall corrosion of the alloy. Therefore, under the premise of not affecting the magnetic properties of the magnet, the total amount of rare earths in the alloy should be reduced as much as possible. However, the reduction of the total amount of rare earths will inevitably affect the magnetic properties of the magnet; the method of adding alloy elements can improve the magnetic properties of the magnet [13] In order to compensate for the decrease in magnetic properties caused by the decrease in the total amount of rare earths, it is necessary to add trace elements in the raw materials for the preparation of magnets.

Adding trace elements

Adding Al, Cu and other trace elements can inhibit the growth of crystal grains, refine the crystal grains, and reduce the probability of neodymium-rich phase agglomeration. In addition, after adding Al and other trace elements, the coercivity of the magnet is also improved [13]. Table 2 shows the performance changes of magnets with and without trace elements. The 38SH sample shown in the table has a weight loss of less than 3 mg after 30 days of weight loss experiment. After adding trace elements, the magnetic properties of the sintered magnet remain basically unchanged, while the total rare earth content and heavy rare earth content have decreased by 3% and 10%. The decrease in the amount of rare earth added, especially the decrease in the amount of heavy rare earth element Dy, will greatly reduce the production cost.

Table 2 Effect of trace element changes on the magnetic properties of 38SH sintered magnet

Trace element addition	Dy/wt%	Total rare earth content/wt%	Hcj/Oe	Br/T
0 wt% Cu+ 0 wt % Al	6.2～6.8	32.8～33.3	≥20000	1.25～1.28
0.05 wt% Cu+ 0.1 wt % Al	6～6.5	32.5～33	≥22000	1.22～1.25
0.15 wt % Cu+ 0.3 wt % Al	5～5.5	32～32.5	≥22000	1.22～1.25

Process method

Not only the addition of trace elements will have an impact on the grain size of the magnet and the degree of agglomeration of the grain boundary phases, but it has also been found in actual industrial production that through appropriate technological measures, the grains can also be further refined and the microstructure of the material improved. Compared with the traditional ingot casting process and the sheet casting (SC) process, the weight loss of the magnet is also significantly different. Figure 6 shows the comparison test results of the traditional ingot casting process (indicated by “A”) and SC process (indicated by “C”). It can be clearly seen from Fig. 6 that the magnets corresponding to the SC process are far less important than those using the traditional ingot casting process. The weight loss of the magnet using the ingot process is 2-18mg/cm2 in 24h; after 48h, the weight loss is more than 21mg/cm2. After 120h, the weight loss is more than 105mg/cm2, and most of the magnets have been broken or even powdered. Obviously, as time goes by, the mass loss of the magnet increases rapidly. However, the weight loss of the magnet using SC technology decreases slowly, and the weight loss remains below 10mg/cm after 14h (except for 48MC containing very low 2Dy). Moreover, as time goes by, the increase in mass loss decreases and changes The gradient is slow. The SC process uses hydrogen blasting (HD) technology to crush the ingots. After crushing, the powder is uniform and small in size, while the ingot casting process uses coarse and medium crushing methods. Without HD treatment, the size of the powder is larger. This affects the final grain size of the magnet. The photomicrographs of the magnets obtained by the two processes confirm the above knowledge. Figure 7 shows the microstructure of the conventional ingot and SC process magnet. Figure 7a shows that the main phase of the traditional process magnet is large and the size difference is large. The maximum crystal grain size is greater than 10μm, most of which are distributed in the range of 6-10μm. The neodymium-rich phase between the main phases is coarse; thick and connected into a network. The neodymium-rich phase structure forms a rapid corrosion path, resulting in poor corrosion resistance. At the same time, due to the presence of a large amount of highly chemically active Nd in the neodymium-rich phase, the stability is poor and oxidation is prone to occur, which is also the main reason for the poor corrosion resistance of the magnet; when the magnet is in a high temperature environment, the neodymium-rich phase will be oxidized to Nd2O3[8]; In an electrochemical environment, the active neodymium-rich phase will also form an anode relative to the main phase Nd2Fe14B, and corrosion will occur preferentially. As the anode metal, it will bear a large corrosion current density. Because the neodymium-rich phase is distributed on the grain boundary of the main phase in a network, the corrosion form of the magnet has typical intergranular corrosion characteristics, which accelerates the oxidation and corrosion rate of the magnet. In the microstructure of the magnet prepared by the SC process, the main phase crystal grains are uniform and the size is in the range of 4~6μm; the neodymium-rich phase is basically distributed at the triangular grain boundary of the main phase, in a refined or discontinuous state (Figure 7b) This kind of distribution makes the diffusion channel of magnet intergranular corrosion narrow or even disappear, thereby effectively inhibiting the oxidation and corrosion process of the magnet, and improving the corrosion resistance of the magnet. In addition, the density of magnets produced by the ingot casting process is between 7.3 and 7.5 g/cm3, while the density of magnets produced by the SC process is between 7.5 and 7.7 g/cm3. The increase in density is also conducive to the improvement of corrosion resistance. The SC process increases the compactness of the magnet and improves the microstructure of the main phase and the neodymium-rich phase of the magnet. These improvements reduce the weight loss of the magnet and improve the corrosion resistance of the magnet.
Through the above-mentioned experimental research, the alloying process of Pr, Dy, total rare earth elements and trace elements was optimized, and the SC+HD process was adopted to obtain products that meet the weight loss performance and realize mass production. Figure 8 shows the changes in the weight loss of the product before and after the improvement. The hollow legend in the figure is the product after the improvement, which has reached the level of 30-day weight loss less than 3mg/cm2, which meets the requirements of customers.

Figure 6 Comparison of weight loss between traditional process and SC process magnet

Figure 7 Photomicrograph of sintered NdFeB magnet: (a) ingot casting process, (b) SC process

Figure 8 Comparison of magnet weight loss before and after improvement

Conclusion

Through the research on the corrosion weight loss of sintered magnets, a clear understanding of the basic mechanism of weight loss of sintered NdFeB magnets has been obtained. The weight loss performance of the magnets can be effectively improved through the improvement of alloy control and preparation technology. The following understandings are obtained:

• (1) The chemical properties, distribution and content of the neodymium-rich phase have a significant effect on the weight loss of the sintered magnet; when the neodymium-rich phase is thick and connected into a network, the magnet loses weight and is easily corroded. The refinement or discontinuity of the neodymium-rich phase can reduce or inhibit the rate of intergranular corrosion, thereby reducing the weight loss of the magnet and improving the corrosion resistance.
• (2) Through the control of Pr content, Dy content and the total amount of rare earths, the appropriate increase of trace elements can not only effectively reduce the corrosion weight loss of the magnet, but also ensure that the magnetic performance of the magnet does not decrease. The SC+HD process can refine the main phase grains of the magnet, and the neodymium-rich phase is more evenly distributed, forming a denser microstructure, thereby improving the corrosion resistance of the magnet.

Author: ZHOU Lei, LIU Tao, CHENG Xing-hua, HE Jun, YU Xiao-jun, LI Bo

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