Research status and challenges of corrosion and protection of sintered NdFeB permanent magnet materials
The corrosion sensitivity of sintered NdFeB permanent magnet materials limits its application in complex working conditions. Improving the corrosion resistance of magnets and developing excellent protective coatings are the key directions in the field of development. Although a lot of work has been done on the exploration of long-life NdFeB magnets, there are still few systematic studies on the corrosion of NdFeB magnets from technical process to basic theory. On the one hand, the basic research on material corrosion and protection lags behind the research on magnetic properties. On the other hand, it has a great deal to do with the continuous improvement of market requirements for material quality and diversified demand Big deal. In this paper, the latest research results of corrosion-resistant sintered NdFeB permanent magnet materials are reviewed, including the factors affecting corrosion, the basic theories and methods to improve the corrosion resistance of magnets, the basic framework of surface protection strategy and the key technologies in engineering application. Finally, the future prospects and challenges are prospected, and it is expected to point out the direction for the future development.
Improving the corrosion resistance and surface protection technology of NdFeB rare earth permanent magnetic materials has become the key to break through the engineering application of rare earth permanent magnetic materials , which is directly related to the continuous improvement of material quality requirements and diversified demand in the market. For this reason, there have been many articles about the corrosion mechanism and surface protection of NdFeB materials in recent 20 years, including material corrosion [2,3], environmental corrosion [4,5] and surface science [6 ~ 11]. It can be seen from these literatures that, based on the corrosion sensitivity of NdFeB materials, on the one hand, alloying is used to regulate the composition and microstructure of materials and improve the corrosion resistance of magnets. For example, metal element M1 (such as Cu, Al, Zn, GA, Ge, Sn, etc.) is added to magnets to form nd-m1 or nd-fe-m1 intermetallic compounds, or metal element M2 is added (such as CO, Ti, Nb, Zr, V, Mo, W, etc.) to form m2-b or fe-m2-b intermetallic compounds. Compared with Nd rich and B-rich phases, the intergranular compounds formed by these compounds have higher corrosion potential, which reduces the electrochemical difference with the main phase (Nd2Fe14B), thus weakening the driving force of interphase corrosion. On the other hand, they are protected by coating on the surface of magnets, such as by chemical conversion and electrochemical oxidation A protective layer is formed on the surface of the material by plating, electroless plating, electrophoresis, physical vapor deposition, organic coating and other composite coating methods. The protective layer can prevent the penetration of corrosive media (such as O2, H2O or Cl -) from the surface to the substrate, so as to protect the substrate from erosion. Driven by the market, with the continuous expansion of the application field of permanent magnet, various technologies are gradually put into practice, and the advantages and disadvantages of each method have been evaluated.
Although the majority of researchers have done a lot of work to improve the corrosion sensitivity of rare earth permanent magnet materials, the corrosion mechanism of NdFeB materials still needs to be further analyzed, and its surface protection technology lags behind the development of materials. So far, some innovative achievements are rare. This is mainly due to many factors affecting its corrosion and protection mechanism analysis, including material matrix, corrosion medium, test environment and technical scheme. These factors will change the intermediate reaction process, thus affecting the composition, structure and protection characteristics of corrosion products. Because of the complexity of characterizing the corrosion product film, it is very difficult to clearly explain the specific mechanism and then formulate effective protection measures. However, according to the above review literature, a lot of valuable information can be extracted to analyze the corrosion and protection mechanism of NdFeB materials. Based on the above literature, this paper summarizes the influencing factors of corrosion of sintered NdFeB materials, expounds the causes of corrosion, and makes a comprehensive review on the recent research related to alloying process to improve the corrosion resistance of magnets and surface protective coating technology, so as to carry out material design and formulate protection strategies in the future, so as to make rare earth permanent magnet materials better meet the needs of social development It is of great significance to break through the key technology in the production of rare earth permanent magnetic materials, develop corrosion-resistant magnetic materials and establish the corresponding surface protection strategy.
Factors affecting material corrosion
In order to improve the corrosion resistance of permanent magnets and improve the reliability of products, we should master the factors that affect the corrosion of materials and analyze the causes of corrosion.
Material factor is the internal cause of corrosion, which mainly refers to the corrosion caused by the difference of electrochemical properties between phases. Figure 1 shows the internal structure of NdFeB material. The main phase (Nd2Fe14B) is polygonal and magnetic; the nd rich phase (nd4fe) is distributed along the grain boundary or grain boundary corner of the main phase in thin layer or granular form, which surrounds the main phase grains; the B-rich phase (ND1 + xfe4b4) is massive or granular, which exists in the grain boundary in metastable form, and the volume fraction of each phase is about 84%, 14% and 2% .
Fig.1 Schematic of the microstructure of NdFeB materials
The results  show that the electrochemical potentials of the main nd rich and B-rich phases are obviously different. The electrochemical potentials from low to high are in the order of B-rich phase, nd rich phase and Nd2Fe14B phase. In the environment, the nd rich and B-rich phases at the grain boundary will be preferentially corroded. It is not difficult to know from the above analysis that this kind of corrosion battery has the characteristics of “small anode large cathode”. The nd rich phase and B-rich phase with small volume fraction in the magnet will accelerate the corrosion along the grain boundary of the main phase, causing the surrounded main phase grains to fall off, and the generated bulky corrosion products will lead to the pulverization of the magnet and the performance degradation . Fig. 2  shows the magnetic field distribution of NdFeB magnet before and after corrosion. It can be seen that the corrosion of the magnet leads to the obvious change of the magnetic field distribution, and the uneven distribution of the magnetic field further proves that the corrosion degree of each part of the magnet is different.
According to the latest research results of Academician Li Wei’s team [14, 15] The main reason is that the content of Nd in the grain boundary is significantly reduced after the Nd in the structure is replaced by CE, which makes the rare earth rich phase transition thinner and more evenly distributed. At the same time, the grain size of the main phase becomes smaller and the structure is more compact. The corrosion products are easy to block the transmission channel of the medium, which can block the corrosion.
Therefore, on the one hand, the content of Nd element is reduced to avoid the accumulation of Nd rich phase at the grain boundary; on the other hand, the intergranular structure is regulated to induce the dispersion of Nd rich phase at the triangular grain boundary of the main phase, which narrows the diffusion channel of the corrosion medium and inhibits the corrosion process, so as to improve the corrosion resistance of the magnet [16,17]. On this basis, adjusting the crystal structure of the main phase, ensuring the uniform distribution of each phase and reducing the driving force of corrosion reaction are also one of the efforts to improve the anti-corrosion ability of magnets .
Environmental factors are the external causes of corrosion, including environmental temperature, humidity and corrosive media . As far as the ambient temperature is concerned, when the temperature exceeds 150 ℃, the oxidation rate of Nd element at the grain boundary increases significantly, and the main reaction is shown in equation (1); and with the extension of time, the main phase will also be oxidized.
Humidity is one of the main factors leading to permanent magnet corrosion failure. Therefore, researchers [13,19] studied the influence of humidity on the corrosion of magnets by means of high temperature and high pressure accelerated corrosion experiments with the help of autoclave. The results show that the corrosion of magnets in humid environment is more sensitive than that in high temperature environment, and the existence of water vapor in the environment is a necessary condition for the corrosion of magnets. In addition, research  found that the corrosion weight loss of high abundance rare earth permanent magnet ((ce15nd85) 30febalb1m or (ce20nd80) 31febalb1m) in hot and humid environment is lower than that of conventional rare earth permanent magnet (Nd2Fe14B). This result fully shows that reducing the enrichment of rare earth elements at the edge of the main phase and controlling the content of rare earth rich phase are the key to improve the corrosion resistance of rare earth permanent magnet materials.
In addition, the research results [20 ~ 22] show that the corrosion behavior of rare earth permanent magnet materials is closely related to the corrosion medium. For example, the corrosion rate of NdFeB magnet in HCl and H2SO4 is relatively high ; the corrosion of Nd rich phase in HCl solution is relatively serious, so the maximum magnetic energy product of the magnet decreases significantly after being corroded by HCl solution, while the influence of HNO3 solution on the main phase is relatively large, so the intrinsic coercivity of the magnet decreases ; the surface of the magnet can be passivated in H3PO4 and H2C2O4, and sometimes it can not be completely removed Oxide layer or dirt on the surface of magnet [23,25,26]. The results provide a theoretical basis for revealing the correlation between NdFeB magnet life and environment.
It is worth noting that the development of NdFeB permanent magnet materials has strongly promoted the progress of magnetic surgery technology in the medical field . With the wide application of rare earth permanent magnet materials in medical devices, in order to improve the reliability of the equipment, the corrosion behavior of magnets in body fluids (such as saliva , gastric juice , bile  and so on) has been gradually concerned. It can be seen that the corrosion problem of permanent magnets used in special fields, like many common problems, is an urgent problem to be solved in the development of NdFeB materials, and the diversified needs bring great challenges to the development of NdFeB materials.
Research  shows that the corrosion behavior of rare earth permanent magnets in different magnetization States is significantly different, and the corrosion tendency of magnets in magnetization state is greater, especially in magnetization direction, as shown in Fig. 3 . Costa et al.  think that this is caused by the enhanced migration driving force of paramagnetic oxygen molecules in the magnetic field; sueptitz et al.  think that under the action of Lorentz force, the ion convection movement in the electrolyte promotes the material transport and improves the electrochemical reaction rate, which is the main reason for the accelerated corrosion rate; Zheng Jingwu et al.  put forward that the residual magnetic field on the surface will affect the structure of the electric double layer, A large magnetic overpotential is produced, which leads to the aggravation of corrosion reaction.
(a) unmagnetized sample(b) axile magnetized sample(c) radial magnetized sample
Fig.3 Schematic representations for effects of remanence on degradation process of NdFeB materials
The porous and uneven structure of NdFeB creates conditions for the erosion of oxygen, water and other media. Figure 4  shows the typical microstructure of NdFeB. Yan et al.  have proved that the corrosion resistance of materials with low magnet density is weak by weight loss experiment, hydrogen absorption and oxygen absorption. Therefore, by improving the powder preparation method, adopting advanced molding and sintering process, the microstructure of the magnet is improved, the density of the magnet is increased, and the material with low weight loss rate and corrosion resistance is obtained. The experiment  also proved that by adding appropriate amount of heavy rare earth elements such as PR and Dy, controlling the total amount of rare earth, and adopting the technology of sheet casting (SC) + hydrogen crushing (hd), the uniform and dense structure can be obtained, so as to improve the corrosion resistance of the magnet.
Fig.4 Typical morphology of NdFeB microstructure
Research progress of improving corrosion resistance of NdFeB magnets by alloying
At present, alloying is the main method to prepare corrosion-resistant rare earth permanent magnetic materials, that is, adding alloying elements to the structure, these elements exist in the form of simple substance or form intermetallic compounds. Because the grain boundary in the magnet structure is more sensitive to corrosion, most of the alloying methods are to improve the corrosion resistance of the magnet by regulating the phase composition, structure and distribution of the intergranular phase in the magnet . The basic idea of improving the corrosion resistance of magnets by alloying can be summarized into two aspects: reducing the proportion of Nd rich phase in the structure, optimizing the structure, promoting the formation of uniform and fine grain structure and continuous grain boundary phase distribution, increasing the density of magnets; increasing the electrochemical potential of intergranular phase, reducing the potential difference between the main phase and the intergranular phase, weakening the driving force of corrosion reaction. Based on the dual alloy technology route, the corrosion resistance of the magnet was improved by preparing multi-element alloy powder, adopting heat treatment process, grain boundary reconstruction or grain boundary diffusion .
Research progress on improving corrosion resistance of magnets
The alloying elements added in NdFeB permanent magnet materials can be divided into three categories: first, rare earth metal elements represented by Dy, TB, CE, La and Y; second, transition or main group metal elements represented by Cu, Al and GA; third, transition metal elements represented by CO, Ti, Nb and Zr. The three types of alloying elements and their functions are shown in Table 1.
(1) Class I alloying elements
The production process of adding heavy rare earth elements such as Dy and TB to NdFeB materials has been more than 20 years, and it has been mature at present. At first, the main purpose is to improve the coercivity of magnets and enhance their thermal stability [37 ~ 41]. Later studies [42 ~ 45] found that after sintering, Dy or TB and other elements in the magnet enter the rare earth rich phase at the grain boundary, which not only reduces the nd rich phase, but also disperses it at the edge of the main phase, and the distribution of each phase is more uniform, thus reducing the corrosion sensitivity of the material. With the gradual maturity of “strip throwing + hydrogen explosion” technology and dual alloy process, the addition of multi-element alloy powder has become the mainstream process. For example, dy69ni31 [46, Then, according to the production needs, appropriate amount of auxiliary alloy powder is directly mixed into the main alloy powder, and then the subsequent processes such as pressing and sintering are carried out. The biggest advantage of this process is that it can not only improve the utilization rate of heavy rare earth elements, reduce its content, but also improve the production efficiency.
Since 2015, mass production of high abundance rare earth elements (CE, LA) permanent magnet materials has started. According to the report , in 2018, Ningbo Institute of materials technology and engineering, Chinese Academy of Sciences also made great breakthroughs in the R & D and industrialization of high abundance yttrium (y) mixed rare earth permanent magnet materials. Although high abundance rare earth elements (CE, La, y) replace some nd elements, the cost of developing magnets with the same performance is 30% ~ 40% lower than that of ordinary magnets , however, the contribution of adding high abundance rare earth elements to the corrosion resistance of materials still needs to be further evaluated, which is mainly due to the fact that a complete process system has not yet been established, and the basic research work on the corrosion of such materials has been carried out It is less . In the future, the development of rare earth permanent magnets with high corrosion resistance and high abundance will be one of the hot spots in the research of permanent magnet materials [3,21].
(2) The second kind of alloy elements
The technology of adding Cu and Al elements to NdFeB permanent magnet materials has been developed in the early 21st century. Research  shows that Cu element exists in the form of Cu rich phase besides intermetallic compound (such as ndcu, etc.) at the grain boundary. This discrete element is very disadvantageous to improve the corrosion resistance of the material. Therefore, researchers [56 ~ 58] tried to add Nano Cu and Al particles to the structure, and found that some elements failed to form stable intermetallic compounds. For this reason, researchers first prepared copper or aluminum alloys, such as cu60zn40 , al100 xcux  or la70al10cu20 , and then added Cu and Al elements into the material structure by double alloy process, thus eliminating the discrete Cu rich phase and Al rich phase in the structure. The formation of stable nd6fe13sn compound in the structure is the main reason for adding Sn element to improve the corrosion resistance of the material ; the effect of adding GA element on the corrosion resistance of rare earth permanent magnet is still controversial . In addition, according to the latest research results [63, The results show that the corrosion resistance of the magnet can be enhanced by adding mg / MgO nano powder, which is attributed to the low melting point of metal Mg, which improves the wettability of the liquid phase at the grain boundary. The nd-o-fe-mg intermetallic compound formed during the sintering process of the magnet is easy to form a uniform and continuous rare earth rich phase around the main phase grains to make up for the grain boundary defects By changing the microstructure and increasing the density of the magnet, the corrosion resistance of the material is enhanced.
(3) The third kind of alloy elements
The third kind of alloy elements mainly refer to the metal elements with high standard electrode potential such as CO, Nb, Zr and Ti. At the beginning of the 21st century, researchers [65 ~ 67] found that Co element formed intermetallic compounds with high chemical stability at the grain boundary of the magnet structure, such as nd3co  or nd64co36 , which had a significant effect on reducing the weight loss of the material and improving the corrosion resistance of the magnet. However, the price of metal co is high, and it is not often used in industrial production except for magnets used in special fields. NB element is also helpful to enhance the corrosion resistance of rare earth permanent magnets. The analysis [69,70] shows that the main reason is that Nb element can improve the thermal stability of permanent magnets, thus improving the corrosion resistance of materials. The research  shows that the growth of the main phase grain and the rare earth rich phase at the grain boundary is inhibited by the influence of ZrB2, which makes the structure distribution more uniform, so the corrosion resistance of the magnet is enhanced.
It is worth noting that the successful implementation of alloying process is closely related to the current advanced technical means and supporting equipment, such as rapid solidification sheet smelting, hydrogen explosion crushing, spark plasma sintering, etc. The above measures can not only improve the density of the magnet, but also optimize the microstructure of the magnet and improve the distribution of rare earth rich phase at the grain boundary, so as to enhance the corrosion resistance of the magnet. Although the addition of trace elements and the use of dual alloy process have played an important role in improving the corrosion sensitivity of NdFeB permanent magnets, there is still a long way to go to fundamentally solve the corrosion problem of rare earth permanent magnets. This is because the addition of alloy elements can improve the corrosion resistance of the magnet to a certain extent, but the effect is very limited, sometimes it will reduce the magnetic performance, and often increase the manufacturing cost of the material . These factors must be considered carefully when designing alloying process.
Research progress of surface protection technology
The development of NdFeB permanent magnet materials is closely related to the level of its surface protection technology. Under the situation of limited corrosion resistance of permanent magnet, the development of its surface protection coating technology has become another hot topic in the research of rare earth permanent magnet materials [73 ~ 75]. Therefore, there are many reports on the surface protection coating of NdFeB materials, and the main surface treatment methods are as follows Including: chemical conversion, electroplating, electroless plating, electrophoresis, physical vapor deposition, spraying and micro arc oxidation.
Chemical conversion coating
Chemical conversion is an economical surface treatment method. Chemical conversion coating is mostly used for temporary protection or as the bottom layer of organic coating to enhance the adhesion between surface coating and substrate . There are many literatures on chemical conversion coatings [11,20,77,78]. The characteristics of typical conversion coatings have been characterized in detail, including composition, structure, hardness, wear resistance, adhesion, color adsorption capacity, high and low temperature resistance, corrosion resistance and protection performance. The protective ability of conversion film is closely related to its thickness and density. The biggest challenge in conversion film technology is to obtain uniform and defect free film . In the past 20 years, the chemical conversion coating technology on NdFeB surface has not achieved substantial breakthrough. This is mainly because the formation and protection performance of the conversion coating are affected by many factors, such as matrix material, pretreatment process, solution composition, post-treatment process (baking temperature and time), conversion process (temperature, time, pH value and stirring), etc. these factors directly affect the intermediate reaction The composition, structure and properties of the film are determined by the process and final product [80,81]. In addition, the thickness of the conversion film is usually only a few nanometers , and the characterization of its growth process, composition and structure is very complex, so it is very difficult to analyze its formation and protection mechanism.
Electroplating, electroless plating and electrophoretic coating
At present, electroplating, electroless plating and electrophoresis are still the most widely used surface protection methods in the rare earth permanent magnet industry [83,84]. The coatings prepared by these three processes have been widely recognized by customers. In the past 10 years, with the gradual improvement of environmental protection laws and regulations, China has established large electroplating parks in Ningbo, Baotou and Ganzhou, and also equipped with mature automatic processing lines, which has laid a good foundation for ensuring the quality of coating . With the continuous expansion of the application field of rare earth permanent magnet materials, customers put forward many new requirements for the metal coating on the magnet surface, so it is urgent to seek new breakthroughs on the basis of these traditional processes . For example, in order to make up for the negative effect of shielding effect of Ni Cu Ni coating on the performance of magnet, researchers are working hard to develop direct copper plating technology on the surface of magnet ; in order to reduce the thickness of coating, Zn Ni alloy coating with high corrosion resistance is developed; in order to reduce the corrosion of solution on magnet, organic solution electroplating [87, In order to meet the needs of welding, Zn Sn alloy coating was developed ; in order to improve the appearance of decoration, black nickel coating was developed, and so on. In the process of plating with this kind of traditional processing method, the erosion of the solution to the magnet is inevitable, and the treatment before and after plating has an important influence on the quality of the coating.
Although these traditional processing technologies have been very mature for iron and steel materials, there are still many problems that are difficult to solve when using these technologies for surface treatment of NdFeB materials due to the high chemical activity of NdFeB materials and the porous structure of magnet surface. As far as NdFeB permanent magnet materials are concerned, it is the key of surface treatment to provide clean, uniform, dense and blunt substrate surface for pre plating coating before plating. Therefore, the development of advanced pre-treatment technology even has a brighter development prospect than coating technology.
Physical vapor deposition coating
The physical vapor deposition (PVD) technology avoids the corrosion of the solution on the magnet, and can obtain the coating with uniform structure and firm adhesion without environmental pollution. Therefore, this method has attracted much attention in the field of surface protection of rare earth permanent magnet materials . In recent years, aluminum, titanium and zirconium based coatings have been prepared on the surface of NdFeB magnets by evaporation [90 ~ 92], magnetron sputtering [93 ~ 95] and ion plating , and the protective properties of these coatings on magnets have been studied.
Mao et al, 97] found that the magnetron sputtered Al film crystals on the surface of NdFeB magnets grow preferentially along the (111) crystal plane, showing columnar structure, and there are many pores in the film. Therefore, Hu Fang et al.  prepared multilayer Al films by cyclic Ar + bombardment method, which broke the columnar structure, and finally obtained stacking fault structure, which extended the transmission channel of corrosion medium, thus improving the protection of vacuum Al coating Performance.
However, the PVD process has not been widely used in the industry due to the limitations of NdFeB product diversity and size specifications, as well as cost considerations. In the future, according to the characteristics of NdFeB magnets, designing special fixtures to improve the uniformity of coating, the utilization rate of target elements and production efficiency is the key point of PVD technology in the field of surface protection of rare earth permanent magnet materials.
In addition to the above several coatings, there are also some protective layers which are of great concern. For example, thin organic coating is far better than chemical conversion film and metal coating in corrosion resistance, water resistance and adhesion , and adding some additives can give the coating surface a variety of functions, such as anti ultraviolet radiation, fingerprint resistance, self-cleaning, etc., which can meet the diverse needs. In addition, high molecular polymer has the characteristics of diversity, which can give the coating a variety of colors, because of its unique characteristics Therefore, it is expected that ultra-thin and high corrosion-resistant functional organic coatings can make new breakthroughs in the field of NdFeB in the future.
In addition, with the successful application of 3D technology in the manufacture of magnetic materials [99 ~ 101], iron-based coatings (such as Fe Si , Fe Co  and Fe Ni ) can be prepared on the surface of magnets for large size magnets used in military and other special fields by additive manufacturing method, and the gradient structure of composition and structure can be obtained [105, In order to meet the mechanical and corrosion resistance requirements of the coating, as shown in Figure 5 , other coatings can also be applied on the surface of the iron-based coating according to the requirements.
Fig.5 Schematics of functionally graded coatings (a, b) discontinuous and continuous curves, respectively(c-e) schematics of discontinuous coatings that contain interfaces with gradual change in composition, grain orientation, and volume fractions of two types of second-phase particles, respectively(f-h) schematics of continuous coatings in absence of interfaces and with gradual change in grain size, fiber orientation, and volume fraction of second-phase particles, respectively
Summary and Prospect
After 20 years of rapid development, the rare earth permanent magnet material industry pays more attention to material quality and production cost control, so the research and development of corrosion-resistant rare earth permanent magnet materials and its surface protective coating technology usher in a good development opportunity. Under the guidance of the market, the multi polarization development trend of NdFeB materials is more and more obvious, which can be divided into four categories: low-cost magnets, high-performance / high coercivity magnets, magnets for special fields, and emerging high abundance rare earth permanent magnets. Because the composition and microstructure of each type of magnet are quite different, it is necessary to treat them differently when using alloying methods to improve the corrosion resistance of magnets. In addition to considering the high-quality utilization and balanced utilization of rare earth resources, the manufacturing cost of materials should also be considered to meet the diversified needs of the market. Increase investment in basic research on Corrosion of NdFeB materials, incorporate corrosion and protection engineering into the project of rare earth permanent magnetic materials genetic engineering, carry out research on corrosion mechanism of different structures, structures and intergranular, accumulate material corrosion and environmental corrosion data, analyze the corrosion rule and phase structure evolution, and the correlation between composition, structure, magnetic properties and corrosion resistance In order to improve the accuracy of scientific research and shorten the development cycle, and accelerate the industrialization of corrosion-resistant rare earth permanent magnetic materials, the database of material corrosion is established.
The multi-phase structure and corrosion sensitivity of sintered NdFeB material bring great challenges to its surface protection. In the past 20 years, the coating technology of NdFeB magnet surface has not achieved substantial breakthrough. As far as surface coating technology is concerned, it is the key to develop a reliable protective coating based on the requirement oriented, service environment and coating design requirements and material surface characteristics. Surface protection technology has become a comprehensive technology integrating material manufacturing, coating technology and efficient equipment development. For example, surface technologies such as electroplating, electrophoresis, spraying and sputtering have been successfully used to produce low-cost and high coercivity permanent magnets. With the help of these methods, continuous, uniform and controllable thickness coatings are deposited on the surface of magnets. Through grain boundary diffusion treatment, the coercivity of magnets can be accurately controlled, and the problem of poor consistency of sheet products can be solved. It can be seen that although the research and industrial development of surface protection technology of rare earth permanent magnet materials started late due to the lack of core technology and equipment and low overall manufacturing level, the key technology of protective coating also has important strategic significance and significance for the development of resource-saving high-performance rare earth permanent magnet materials and the maintenance of China’s strategic advantage in the rare earth industry Huge economic and social benefits.
Author: WU Yucheng, GAO Zhiqiang, XU Guangqing, LIU Jiaqin, XUAN Haicheng, LIU Youhao, YI Xiaofei, CHEN Jingwu, HAN Peide (https://www.ams.org.cn/CN/Y2021/V57/I2/171)
Source: China Permanent Magnet Manufacturer – www.rizinia.com
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