Research Progress on surface protection technology of NdFeB permanent magnet materials
The corrosion mechanism and four kinds of surface protection technologies of NdFeB permanent magnet materials were introduced, including electroplating, electroless plating, organic coating and composite coating. The current application status and problems of these four kinds of surface protection coatings were analyzed, and the future development direction of NdFeB surface protection coatings was expounded.
Permanent magnet material is a new type of magnetic material, also known as hard magnetic material. The difference between permanent magnet material and soft magnetic material is that it can maintain constant magnetism after magnetization under the action of external magnetic field. Since the advent of the third generation NdFeB permanent magnet in the 1980s, it has been favored by people for its high cost performance and rich resource reserve. Intelligent development direction, and strong promotion Modern science and technology and the development of information industry. At present, the material has been widely used in computer, network information, aerospace, communications, transportation, household appliances, health care and other high-tech fields [2-3]. However, there are still some disadvantages in the application of NdFeB permanent magnet, that is, the corrosion resistance of NdFeB permanent magnet is poor, which is due to its multiphase structure and chemical characteristics of each phase Therefore, to solve the corrosion problem of NdFeB has become the focus of current research. The fundamental measure of NdFeB surface protection is to use coating to block the corrosion of oxygen, water or other substances in the air to achieve the purpose of protection.
Corrosion mechanism of NdFeB
Table of Contents
The NdFeB permanent magnet consists of three phases: Main Nd2Fe14B phase, nd rich phase and B rich phase. The potential of each phase in contact with each other is different, which will inevitably lead to electrochemical reaction in humid environment, that is, the formation of corrosion cell and intergranular corrosion. The results show that the corrosion of NdFeB permanent magnet mainly occurs in three environments, namely long-time high temperature environment (> 150 ℃), warm humid environment and electrochemical ring .
High temperature environment
The oxidation rate of NdFeB permanent magnetic materials is very slow in the dry environment below 150 ℃, but when the temperature is higher than 150 ℃, even in the dry environment, the following reactions will occur in the nd rich region: 4Nd + 3O2 = 2Nd203.Nd2Fe14B phase will decompose to form Fe and Nd2O3, and Fe will further oxidize to form Fe2O3 and other products, which will reduce its magnetic properties.
Warm and humid environment
In the warm and humid environment (θ = 30 ~ 50 ℃, humidity 50% ~ 75%), the nd rich grain boundary phase on the surface of NdFeB permanent magnet first reacts with the water vapor in the environment: Nd + 3H2O = Nd (OH) 3 + 3h, the generated h penetrates into the grain boundary, and further reacts with the nd rich phase: Nd + 3H → ndh3, causing the grain boundary corrosion. The formation of ndh3 will lead to the increase of the grain boundary volume, the formation of grain boundary stress, and then damage the grain boundary, In severe cases, the grain boundary will break and the permanent magnet will be powdered.
In the electrochemical environment, there is an obvious potential difference between the contacting phases in NdFeB magnets. Compared with Nd2Fe14B, the nd rich phase and B-rich phase become anodes, which lead to preferential corrosion and form local corrosion micro cell, resulting in intergranular corrosion in the corrosive medium and poor corrosion resistance. Surface protection treatment process of 2ndfeb and existing problems The main corrosion coatings are metal coatings, organic coatings and composite coatings. The following is a brief introduction of these three coatings.
In general, Ni, Zn, Al, Ni-Fe, Ni-P, Ni-Cu-P, Ni-Co-P, Cu, Cr, CD, tin, ZrN and other metals or alloys and compounds are coated on the surface of magnet by electroplating, electroless plating or physical vapor deposition.
At present, the application of alloy electroplating technology in the surface treatment of NdFeB has attracted much attention. Because the pulse electroplating technology has significant advantages for alloy electroplating, the application of this technology in the surface treatment of NdFeB can also be an effective way to improve the corrosion resistance of NdFeB. At present, the most typical and effective method is electroplating Ni and Zn, among which Ni plating is the most widely used.
Zhao Qing et al.  discussed the nickel plating process on the surface of NdFeB permanent magnet, and studied the semi bright nickel / bright nickel and neutral pre nickel / semi bright nickel bright nickel systems respectively. The results show that the design of nickel plating layer is related to porosity and corrosion resistance, and the neutral pre nickel plating has better performance.
The effects of current density, temperature, pH value and phosphite content on the composition and microstructure of Electrodeposited Ni-P alloy were investigated by MACB et al. . The results show that the amorphous Ni-P alloy obtained under the optimized conditions still has no corrosion spot after 180 h of 3.0% NaCl salt spray test.
Zhang Lei et al.  tested the properties of NdFeB materials after surface treatment with a new process, which showed that the process greatly improved the corrosion resistance of NdFeB materials after surface treatment and reduced the magnetic loss. Li Jian et al.  prepared three kinds of sintered NdFeB magnets by different casting processes and crushing processes, and compared the corrosion resistance of three kinds of magnets and their nickel plating. The results show that the anti-corrosion performance of nickel plated magnet prepared by rapid solidification strip combined with hydrogen crushing process is the best.
Songzl et al.  prepared Ni-Al2O3 composite coating by composite electrodeposition method. Immersion test in 3.5% NaCl solution for 288h proved that the composite coating has excellent corrosion protection performance for permanent magnet. Electroplating Ni and its alloy or composite coating has the advantages of good corrosion protection performance and easy operation, and is widely used in corrosion protection of NdFeB permanent magnet. Wang Xin et al.  The effect of different pretreatment processes on the properties of sintered Nd-Fe-B electro galvanized coating was studied. The bright, low porosity, strong adhesion and corrosion resistance coating was obtained by testing the porosity, adhesion and corrosion resistance of the coating. However, in practical application, the poor protection performance of the coating is common, because it is difficult to avoid water, acid, alkali and corrosion in the electroplating process When the bath infiltrates into the magnet material and the magnet is used as the cathode, there is hydrogen precipitation, which leads to hydrogen embrittlement, whitening and bubbling of the permanent magnet material after plating and affects the corrosion resistance.
In order to overcome the shortcomings of electroplating, electroless plating method has been studied in recent years to provide corrosion protection for magnets.
Yang Peiyan et al.  introduced ultrasonic technology into electroless plating to protect the surface of NdFeB permanent magnet materials. The porosity, morphology, adhesion and corrosion resistance of the plating hole were analyzed by porosity test, scanning electron microscope, thermal shock test, electrochemical test and salt spray test. The results show that the ultrasonic technology and the secondary coating have good corrosion resistance The combination of electroless plating and Ni-P alloy plating on the surface of permanent magnetic materials can effectively improve the adhesion and corrosion resistance of the coating.
Zhang TS et al.  studied the process of electroless nickel plating on the surface of NdFeB magnets. The results show that the amorphous Ni-P alloy coating obtained by electroless nickel plating has high corrosion resistance, high stability and low magnetic shielding property, which can be used as one of the ideal coatings for NdFeB permanent magnets.
In this paper, the effect of Electroless Ni-P alloy plating on the corrosion resistance of NdFeB magnet was studied by using ultrasonic electroless plating method. The effects of ultrasonic power on the deposition rate and phosphorus content of the coating were measured. The results show that compared with electroless Ni-P alloy plating without ultrasonic field, electroless Ni-P alloy plating under ultrasonic condition has more fine structure and better corrosion resistance The columns are more compact and have better corrosion resistance.
Ying Huagen et al.  improved the corrosion resistance of sintered NdFeB permanent magnet by electroless plating Ni-Cu-P alloy coating. The results showed that the corrosion resistance of electroless plating Ni-Cu-P ternary alloy coating was better than that of Ni-P binary alloy coating under the same conditions. The corrosion results showed that the neutral salt spray test time of Ni-Cu-P ternary alloy coating was 268h, which could be used for magnet Provide good corrosion protection.
However, electroless plating also has obvious defects: the cost of electroless plating is much higher than that of electroplating, the stability of plating solution is poor, there is still hydrogen precipitation during the plating of permanent magnetic materials, which causes hydrogen absorption and embrittlement of permanent magnetic materials, and there are holes in the coating.
Organic coatings are generally used for magnets used in severe corrosive environment or when the surface of magnets is required to be electrically insulated in some service environments. In recent years, cathodic electrophoretic coating is the most widely studied coating method for NdFeB material surface protection. Li Jun et al.  and Chen Gang et al.  have studied the cathodic electrophoretic process of bonded NdFeB magnets, and the optimum process conditions have been determined The anti-corrosion performance of bonded NdFeB magnets was improved significantly.
Xiao xiangding et al.  studied the cathodic electrophoretic coating process of NdFeB magnets. The results show that the high corrosion resistance coating can be obtained by using the initial electrophoretic voltage of 80-90v, deposited voltage of 140-160v, controlled electrophoretic θ of 28-30 ℃, electrophoretic t of 150-180s and baking process of 70-80 ℃ for 20-30min.
Jiang Liqiang et al.  studied the effects of matrix properties of composite permanent magnet and process parameters of cathodic electrophoretic epoxy resin on the electrophoretic coating quality of composite NdFeB / ferrite bonded magnet. The results show that when the mass fraction of strontium ferrite is less than 30%, the surface of the magnet with eed-060m epoxy electrophoretic coating is under the condition of u 120-180v, paint solution θ 28-32 ℃ and curing θ 150-170 ℃ The surface of NdFeB magnet can be coated with a dense, smooth coating with excellent corrosion resistance. Because the hardness of organic coating is slightly poor, the surface wear resistance and scratch resistance of NdFeB magnet are not ideal. The sintered permanent magnet with organic coating is not suitable for use at high temperature and the corrosion resistance time is not long.
In order to enhance the corrosion resistance of NdFeB permanent magnet materials in harsh environment, the combination of the above coatings is adopted according to different environmental conditions to form gradient protection [23-24]. Zhao Chunying  combines the technology of high temperature baking and degreasing → boiling water sealing → degreasing → rust removal → activation → electro galvanizing → phosphating → passivation → cathodic electrophoresis → water washing → drying and solidification. The corrosion resistance of the obtained coating is 4-5 times higher than that of the general plating layer, and the service life of NdFeB is long. An organic coating is added to the metal coating such as Ni, Zn or Al ion plating. The electroless Ni-P alloy coating is used as the intermediate layer, and the TiO2 film is formed on the surface of the material by sol gel process, so as to improve the corrosion resistance of the permanent magnetic material [26-28].
Generally speaking, the anti-corrosion performance of composite coating is the superposition of each coating, but the coating used in the surface protection of permanent magnet materials, its process is complex, the cost is correspondingly high, in practical application, it is often difficult to achieve the anti-corrosion effect of two kinds of coating composite.
After years of research and development, the domestic surface protection technology of NdFeB permanent magnet materials has made great progress, but the anti-corrosion problem of NdFeB permanent magnet materials has not been fundamentally solved. The poor sales of domestic NdFeB permanent magnet products is largely due to its poor protective coating. Therefore, to solve the problem of surface protection of NdFeB materials is to expand its production capacity In the future, the key to improve the corrosion resistance of NdFeB permanent magnet is to develop new surface coating technology (such as nano modified cathodic electrophoretic coating technology) to improve the corrosion resistance of NdFeB permanent magnet surface without affecting its magnetic properties; to develop new pre-treatment technology such as sealing technology; to improve the corrosion resistance of NdFeB permanent magnet surface; To improve the matching of organic coating and metal coating on the surface of NdFeB magnet, the corrosion resistance can reach the comprehensive performance of the two.
Author: Yan Fen Ying, Zhao Chun Ying, Zhang Lin (1001-3849 (2012) 08-0022-04)
Source: China Permanent Magnet Manufacturer – www.ymagnet.com
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