Permanent magnet material is an object that can continuously provide magnetic energy without consuming electric energy. It has the function of energy conversion and is an important functional material. Ru-iron-boron NdFeB permanent magnet has made a sensation in the world with its extremely high "magnetic energy product". It is called the "magnet king" because of its excellent magnetism. It is currently the strongest permanent magnet in the world.
Although NdFeB has excellent magnetic properties, it has the disadvantage of poor corrosion resistance. It is very easy to form a galvanic cell in a humid environment and cause serious intergranular corrosion, which seriously affects the performance and life of NdFeB, and its chemical composition must be adjusted. And the use of surface treatment methods to improve it, in order to meet the requirements of practical applications.
Since the advent of NdFeB rare earth permanent magnet materials in the mid-1980s, they have been widely used in electronic communications with a series of unique advantages such as high magnetic energy product, high coercivity, high remanence, small size, and light weight. , Metallurgical manufacturing, geological prospecting, medical care, transportation, aerospace and many other fields, it can be said that it is everywhere.
1. The phase composition of NdFeB
The basic composition of the material affects the performance of the material. Sintered NdFeB permanent magnets are mainly produced by powder metallurgy, and at least the following four different phases exist simultaneously:
①Matrix phase (main phase): Nd2Fe14B phase. It is formed by peritectic reaction at around 1200°C and is the only magnetic phase in the alloy. The excellent magnetic properties of NdFeB magnets are mainly attributed to the high saturation magnetization (μMs=1.6T) and anisotropic field (7.3T) of the Nd2Fe14B phase;
②Nd-rich phase: its melting point is 650～700℃, which is the last solidified in the alloy, exists in thin layer and block shape, and is distributed at the intersection of grain boundaries or on the grain boundaries of Nd2Fe14B. Although it is a non-magnetic phase, due to its low melting point, it is dispersed around the main phase during sintering. It not only serves to densify the sintered body, but also inhibits the growth of crystal grains and promotes the increase in coercivity. Is essential.
③B-rich phase Nd1+εFe4B4: It is formed when the boron content in the alloy exceeds the normal composition of Nd2Fe14B. It does not contribute to the magnetic properties. Generally, the amount is very small and has little effect on the magnetic properties.
④α-Fe: Its melting point is 1520℃, which is the phase with the highest melting point in the alloy. It is the first to precipitate from the liquid alloy. a-Fe is a soft magnetic phase. Its existence leads to the decrease of the main phase and the increase of the neodymium-rich phase. It destroys the optimal ratio of the main phase and the neodymium-rich phase, damages the magnetic orientation of the main phase grains, and at the same time coarsens the grains in the local area during the sintering process, which not only deteriorates the magnetic properties, but also changes the structure of the electroplating layer. Bad, affecting the protective effect. Therefore, measures should be taken from the manufacturing process to minimize or eliminate the generation of α-Fe phase, such as sheet casting process and rapid quenching process.
2. Corrosion mechanism
On the one hand, the corrosion resistance of NdFeB permanent magnets is due to the fact that Nd is one of the most chemically active elements (its standard potential E0 (Nd3+/Nd) = -2.431V; on the other hand, the alloy is a multi-phase structure, The electrochemical phase difference between the phases is large, which easily causes electrochemical corrosion.
In addition, during the sintering process of NdFeB, defects such as micropores, loose structure, and rough surface are prone to appear in the interior and surface of the magnet, while the working environment of NdFeB permanent magnet materials in applications is often high temperature and high humidity. These defects are in high temperature and high humidity environments. The following provides convenient conditions for NdFeB corrosion. At the same time, the NdFeB manufacturing process easily contains impurity elements such as O, H, Cl and their compounds. The most corrosive elements are O and Cl. Magnets and O produce oxidative corrosion, and Cl and its compounds will accelerate the oxidation process of magnets .
The reasons why NdFeB is easy to corrode are mainly attributed to: working environment, material structure, and manufacturing process. Studies have shown that the corrosion of NdFeB magnets mainly occurs in the following three environments: warm and humid environment, electrochemical environment, and long-term high temperature environment (>250℃).
01 High temperature environment
In a dry environment, when the temperature is lower than 150°C, the oxidation rate of NdFeB magnets is very slow. However, at higher temperatures, the following reaction occurs in the Nd-rich zone: 4Nd + 3O2 = 2Nd2O3. Subsequently, the Nd2Fe14B phase will decompose to produce Fe and Nd2C3. Further oxidation, Fe2O3 and other products will appear.
02 Warm and humid environment
Under warm and humid conditions, the sensitive grain boundary phase of the NdFeB magnet surface layer first corrodes with the water vapor in the environment as follows: 3H2O+Nd=Nd(OH)3+3H. The H generated by the reaction penetrates into the grain boundary and further reacts with the Nd-rich phase: Nd+3H→NdH3, causing grain boundary corrosion. The formation of NdH3 will increase the grain boundary volume, cause grain boundary stress, cause grain boundary damage, and in severe cases, the grain boundary will fracture and cause magnet powder.
The influence of environmental humidity on the corrosion resistance of the magnet is much greater than that of temperature. This is because the corrosion product film formed by the magnet in a dry oxidizing environment is dense, which separates the magnet from the environment to a certain extent. The further oxidation of the magnet is prevented, and the hydroxide and hydrogen-containing compound generated in a humid environment are not dense and cannot prevent the further effect of H2O on it. Especially when the environmental humidity is too high, if there is liquid water on the surface of the magnet, electrochemical corrosion will occur.
03 Electrochemical environment
In an electrochemical environment, the electrochemical potential of each phase in the NdFeB magnet is different. Compared with Nd2Fe14B, the neodymium-rich phase and the boron-rich phase become anodes, and corrosion will occur preferentially to form localized corrosion microbatteries. This kind of micro-battery has the characteristics of large cathode and small anode. A small amount of neodymium-rich phase and boron-rich phase as the anode bear a large corrosion current density, and they are distributed on the grain boundary of the Nd2Fe14B phase, which will accelerate it. Grain boundary corrosion. When there is a metal coating on the surface of the magnet (such as electroplating Zn, Ni, etc.), once the coating has defects such as holes, cracks, etc., a corrosion battery effect will also be formed between the magnet and the metal coating.
Under normal circumstances, magnets are used as anodes and preferentially corrode, and metal coatings are used as cathodes. This is why the magnets with coatings often appear exploded. In addition, various plating solutions (such as electroplating, electroless plating, etc.) must be contacted during the process of surface treatment of the magnet, and the sintered NdFeB magnet has certain holes, so that in these processes, the acid or the plating solution will Into the hole, in the future use process will also cause electrochemical corrosion
Three, anti-corrosion technology
There are three main ways to prevent corrosion of sintered NdFeB magnets: First, improve the corrosion resistance of the magnet itself. By improving the microstructure of the magnet and adopting the hot pressing process, a magnet with high density and ultrafine grains can be obtained, which can greatly improve the corrosion resistance of the magnet itself. Second, adding some alloying elements to improve the corrosion resistance of the magnet. To improve the corrosion resistance of the magnet itself, it is necessary to add some alloying elements, but sometimes the magnetic properties are reduced, and the addition of alloys will increase the production cost. These factors limit the application of this method. Third, use an effective protective coating.
At present, the anti-corrosion of NdFeB magnets is mainly based on the surface coating protective coating, that is, the coating is used to improve the corrosion resistance of the magnet.
01 Electroplating coating
Electroplating is based on the redox reaction by using external charges to enrich and reduce the metal ions in the electroplating solution to form a metal coating. The period from 1985 to 1995 was the initial stage of electroplating of NdFeB permanent magnet materials. After nearly ten years of development, the electroplating technology of NdFeB permanent magnet materials has been relatively mature in 2006. The innovation and development stage of electroplating technology of magnetic materials.
At present, the electroplated layers of NdFeB permanent magnet materials mainly include: zinc plating, nickel plating, nickel-zinc alloy plating and other nickel alloys and composite coatings.
02 Chemical plating coating
Electroless plating is a process in which metal ions in the electroless plating solution are deposited on the surface of the substrate to form a plating layer with a certain function based on the oxidation-reduction reaction without an external current. Due to the autocatalytic effect of the substrate itself, the coating structure is dense and uniform, the porosity is low, and the equipment is simple and easy to operate. Relatively speaking, electroless plating has become more and more mature and has more and more applications. The method of electroless plating has been widely used in the world to provide a corrosion-resistant and wear-resistant protective film for the NdFeB matrix.
At present, the NdFeB electroless coating is mainly nickel-phosphorus electroless coating and other electroless coatings such as nickel copper phosphorous, nickel tungsten phosphorous and nickel copper phosphorous.
The plating solution used for electroless plating is also mainly divided into acidic and alkaline. High-phosphorus non-magnetic coatings are often formed in acidic environments, and low-phosphorus magnetic coatings are often formed in alkaline environments and have certain magnetic shielding properties. Because the hydrogen absorption effect is obvious in the acidic environment, which seriously affects the surface quality of the activated neodymium iron boron matrix, alkaline plating solutions are often used in industrial production.
03 Organic coating
Organic coating is one of the most widely used methods in metal protection methods. The organic coating methods used for NdFeB magnets are mainly resin and organic polymer materials. Among them, epoxy resin is the most used because epoxy resin has excellent Waterproof, chemical resistance, adhesiveness, and sufficient hardness, it is widely used in industry.
Electrophoresis coating of epoxy resin coating on electroplated zinc and nickel NdFeB, its anti-rust performance is far better than traditional zinc and nickel plating. In addition to epoxy resin, other resin materials include polyacrylate, polyamide, polyimide, etc., and a mixture of two or more of these resins is used as a coating, and some anti-rust coatings such as Red lead, chromium oxide, etc.
04 Physical vapor deposition coating
Physical vapor deposition is a new type of coating technology that is different from electroplating and electroless plating. The film prepared by this method can adhere well to the substrate, the film layer is denser, the surface is flat and smooth, and the porosity is less. Moreover, the electrolyte residue in the film layer during the electroplating process can be eliminated, and the residual liquid can avoid the film layer. Secondary damage reduces the possibility of coating embrittlement caused by hydrogen generated by the reaction of the magnet during electroless plating.
Common physical vapor deposition methods include vacuum evaporation coating, magnetron sputtering coating and multi-arc ion coating, etc. Commonly used film materials are Al, Ti/Al, Al/Al2O3, TiN, Ti and other physical vapor deposition methods. The film layer and the substrate combined film layer have excellent quality, excellent anti-corrosion performance, and no secondary pollution problems such as waste liquid and slag, which is an important direction for the development of current NdFeB anti-corrosion technology.