Talk about the anti-corrosion work of "Permanent Magnet King Ferrite Boron".

Permanent magnet material is an object that does not need to consume electricity to continuously provide magnetic energy, it has the function of energy conversion, is an important functional material. Ferrer boron NdFeB permanent magnet with its very high "magnetic energy accumulation" sensational, because of its excellent magnetism and known as the "magnetic king", is currently the world's strongest permanent magnet. Although NdFeB has excellent magnetic properties, but there is a disadvantage of poor corrosion resistance, it is easy to form the original battery in a humid environment to produce serious intercrystalline corrosion, which seriously affects the performance and life of ferrite boron, must adjust its chemical composition and surface treatment methods to improve it, in order to achieve the practical application of
Since the mid-1980s, ferrite boron rare earth permanent magnet material has come out, it has been widely used in electronic communications, metallurgical manufacturing, geological exploration, health care, transportation and aerospace and many other fields, it can be said that there is its figure.
figure above: Global downstream consumption distribution of ferrite boron in 2019 (Source: Prospective Industrial Research Institute) Note: Ferrite boron permanent magnet has sintered ferrite boron, bonded ferrite boron, hot-pressed ferrite boron, according to the 2019 data of the China Rare Earth Industry Association, sintered ferrite boron billet production of 170,000 tons, accounting for 94.3% of the total amount of ferrite boron magnets in that year, 4.4%.
I. Look at the phase composition of NdFeB
material composition affects the performance of the material, sintering ferrite boron permanent magnet is mainly produced by powder metallurgy, it has at least four different phases at the same time:
(1) substrate phase (main phase): Nd2Fe14B phase. It is formed by a wrap crystal reaction at about 1200 degrees C and is the only magnetic phase in the alloy. The excellent magnetic properties of the NdFeB magnets are mainly attributable to the high saturation magnetization strength of the Nd2Fe14B phase (s.Ms=1.6T) and anosynchronous field
(7.3T); 2) Rich Nd phase: its melting point is 650 to 700 degrees C, is the last solidified alloy, with thin layer and block-like presence, distributed at the intersection of the crystal boundary or Nd2Fe14B crystal boundary. Although it is non-magnetic phase, but because of its low melting point characteristics, dispersion in sintering around the main phase, not only play a role in making the sintering body dense, but also make the grain grow inhibited, promote the improvement of stubbornness, so it is essential.
(3) Rich B-phase Nd1-Fe4B4: When the boron content in the alloy exceeds the normal composition of Nd2Fe14B, it does not contribute to magnetic performance, the general number is very small, the effect on magnetic performance is not great.
(4) α-Fe: Its melting point is 1520 degrees C, is the highest melting point phase in the alloy, first from the liquid alloy, a-Fe is a soft magnetic phase, its existence led to the reduction of the main phase and The increase of fuchsium phase destroys the optimal ratio of the main phase and the rich phase, damages the magnetic orientation of the main phase grain, and also makes the grain coarse in the local area of the sintering process, which not only worsens the magnetic properties, but also makes the plating layer tissue worse and affects the protective effect. Therefore, measures are taken from the manufacturing process to minimize or remove α-Fe phase generation, such as die-casting and fast-quenching processes.
, corrosion
. On the one hand, the susceptibleness of NdFeB permanent magnet is due to the fact that Nd is one of the most chemically active elements (its standard potential E0 (Nd3 s/Nd) is -2.431V;
In addition, during the sintering process of NdFeB, the interior and surface of the magnet are prone to micropores, loose structure, rough surface and other defects, while NdFeB permanent magnet materials in the application of the working environment is often high temperature, high humidity, these defects in high temperature, high humidity environment for NdFeB corrosion provides a convenient condition. At the same time, NdFeB manufacturing process easily contains O, H, Cl and other impurities and their compounds, the most corrosive impact on O and Cl elements, magnets and O oxidation corrosion, and Cl and its compounds will accelerate the oxidation process of magnets.
NdFeB corrosion is mainly due to: working environment, material structure, production process. The research shows that the corrosion of NdFeB magnets mainly occurs in the following three environments: warm and humid environment, electrochemical environment, long-term high temperature environment (>250 degrees C).
01, high temperature
in a dry environment, when the temperature is below 150 degrees C, NdFeB magnet oxidation speed is very slow. However, at higher temperatures, the fund region reacts as follows: 4Nd plus 3O2 plus 2Nd2O3. The Nd2Fe14B phase then decomposes to generate Fe and Nd2C3. Further oxidation, fe2O3 and other products will also appear.
02, warm and humid environment
Under warm and humid conditions, the sensitive crystal boundary phase of the surface of the NdFeB magnet first reacts with the water vapor press in the environment: 3H2O-Nd-Nd(OH) 3-3H. In the H-permeable crystal boundary generated by the reaction, a further reaction occurs with the rich Nd phase: Nd-3H → NdH3, resulting in crystalline corrosion. The generation of NdH3 will increase the volume of the crystal boundary, causing stress in the crystal boundary, causing the crystal boundary to be destroyed, and in severe cases causing the crystal boundary to break and causing magnet powdering. The effect of ambient humidity on magnet corrosion resistance is much greater than the effect of temperature, because the magnet in the dry oxidation environment, the formation of corrosion product film is denser, to a certain extent, the magnet and the environment separated, preventing further oxidation of the magnet, and in the humid environment generated by hydroxides and hydrogen-containing compounds are not dense, can not prevent H2O to its further effect. Especially when the ambient humidity is too high, electrochemical corrosion occurs if liquid water is present on the surface of the magnet.
03, electrochemical environment
in electrochemical environment, NdFeB magnets in different phases of electrochemical potential. The boron-rich phase and boron-rich phase become an anodes relative to Nd2Fe14B, which will give priority to corrosion, resulting in locally corroded micro-cells. This microcable has the characteristics of a large cathode anode, a small amount of boron-rich phase and boron-rich phase as an anode bear a large corrosion current density, and they are distributed in the Nd2Fe14B phase of the crystal boundary, which will accelerate its crystal boundary corrosion. When the surface of the magnet has a metal coating (e.g. electroplating Zn, Ni, etc.), but the coating appears holes, cracks and other defects, between the magnet and the metal coating will also form a corrosive battery action. In general, magnets corrode as anodes and metal plating as cathodes, which is why magnets with coatings tend to have a violent skin phenomenon. In addition, in the process of surface treatment of magnets to contact a variety of plating fluids
(such as plating, chemical plating, etc.), and sintering NdFeB magnets have a certain hole, so that in these processes, acid or plating liquid will enter the hole, in the later use of the process will also cause electrochemical corrosion
By improving the microstructure of the magnet and using the thermal pressure process, the magnet of the highly dense super-fine grain can be obtained, which can greatly improve the corrosion resistance of the magnet itself. Second, add some alloy elements to improve the corrosion resistance of magnets. To improve the corrosion resistance of the magnet itself, some alloying elements need to be added, but sometimes the magnetic properties are reduced, and adding alloys increases production costs, which limits the application of this method. Third, the use of effective protective coating.
, the anti-corrosion of NdFeB magnets is mainly based on surface coating protection coating, that is, the use of coatings to improve the corrosion resistance of magnets.
1, electroplating coating
electroplating is the use of external charge based on redox reaction, so that the metal ion cathode in the plating solution is very rich reduction to form a metal coating. From 1985 to 1995, it was the initial stage of electroplating of ferrite boron permanent magnet material, and after nearly ten years of development, the electroplating technology of ferrite boron permanent magnet material has been relatively mature, and from 2006 to the present, the electroplating technology of ferrite boron permanent magnet material has been relatively mature.
, the electroplating layer of ferrite boron permanent magnet material mainly includes: galvanized, nickel-plated, nickel-plated zinc alloy and other nickel alloys and composite coatings.
2, chemical coating
chemical plating is in the absence of an applied current, according to the redox reaction, so that the metal ions deposited in the chemical plating liquid to the surface of the substrate to form a certain functional coating process. Due to the self-catalytic effect of the substrate itself, the coating structure is dense and uniform, the porosity rate is low and the equipment is simple and easy to operate. Relatively speaking, chemical plating has become more and more mature, more and more widely used in the world, the method of chemical plating for ferrite boron substrate to provide corrosion-resistant and wear-resistant protective film.
present, the chemical coating of ferrite boron is mainly nickel-phosphorus chemical coating and other chemical coatings such as nickel-copper phosphorus, nickel-tungsten phosphorus and nickel-copper phosphorus.
for chemical plating of plating is also mainly divided into acidic and alkaline, in the acidic environment often produce high phosphorus non-magnetic coating, in alkaline environment often produce a low phosphorus magnetic coating and has a certain magnetic shielding. Because hydrogen absorption in acidic environment is obvious, which seriously affects the surface quality of the biothermed ferrite boron substrate, alkaline plating is used in industrial production.
3, organic coating
organic coating is one of the most widely used means of metal protection methods, the organic coating method used in NdFeB magnets is mainly resin and organic polymer materials, of which epoxy resin is the most used, because epoxy resin has excellent water resistance, chemical corrosion resistance and bonding, and has sufficient hardness, widely used in industry. Electrophoresis is coated with epoxy resin coating on the NdFeB of galvanizing and nickel, and its anti-rust performance is much better than that of the traditional galvanized and nickel-plated layer. In addition to epoxy resins, other resin materials include polyacrylates, polyamides, polyamides, etc., there are two or more of these resins as a mixture as a coating, while adding some rust-resistant coatings such as red dan, chromium oxide and so on.
4, physical vapor deposition coating
physical vapor deposition is a different from electroplating and chemical plating of a new coating technology. The film prepared by this method can be well bonded with the substrate, the film layer is denser, the surface is smooth, the porosity is less, and can eliminate the residue of electrolyte in the film layer during the plating process, avoid the residual liquid caused secondary damage to the film layer, reduce the possibility that the magnetic reaction during the chemical plating process produces hydrogen causing the coating to crack.
Common physical vapor deposition methods have vacuum evaporation coating, magnetically controlled sputter coating and multi-arc ion coating, etc. , commonly used film layer materials are Al, Ti/Al, Al/Al2O3, TiN, Ti, Ti and other physical vapor deposition methods coating and substrate binding film layer quality is excellent, good corrosion performance, and no waste liquid residue and other secondary pollution problems, is the current Development of NdFeB anti-corrosion technology.
Refringer
1, corrosion pathology and surface protection technology of sintered ferrite boron permanent magnet materials; Taiyuan University of Technology Surface Engineering Research Institute, Su Yong'an; Taiyuan Haihai High Materials Surface Technology Co., Ltd., Wan Panshun, Guo Huiming, etc.
2, progress in corrosion mechanism and protection of ferrite boron permanent magnet materials; (1) Shenyang Zhongbei Tong magnetic technology Co., Ltd., (2) Northeastern University School of Mechanical Engineering and Automation; Deng Wenyu (1), Wang Pengyang (2) ), Qi Lijun (1), Du Yongli (1), Sun Baoyu (1), Trillion (2), Zhang Weijie (2), Xie Yuanhua (2), Du Guangxuan (2), Liu Kun (2).
3, ferrite boron permanent magnetic material corrosion process and surface protection technology status quo, Chinalco Guangxi non-colored gold source rare earth Co., Ltd., Li Turn, Zeng Yangqing, Gan Jiayi, Tang Shenglong, Huang Weichao and so on.
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