Classification of NdFeB
Due to different manufacturing methods and use requirements, NdFeB permanent magnets can be divided into three categories:
(1) Bonded NdFeB (Bonded NdFeB): NdFeB bonded magnets are obtained by chilling microcrystalline powder, each powder contains multiple Nd-Fe-B microcrystalline grains, and then polymer or other the binder mixes and bonds the powder and then presses, extrudes or calenders to form a plastic permanent magnet, so the usual NdFeB bonded magnet is a non-dense isotropic magnet.
The magnetic properties of ordinary NdFeB bonded magnets are much lower than those of NdFeB sintered magnets, but NdFeB bonded magnets have many irreplaceable advantages of NdFeB sintered magnets: high processing accuracy, high yield, high precision, excellent magnetic properties, corrosion resistance Good performance and good temperature stability; in addition, Nd-Fe-B bonded magnets are also easy to magnetize in any direction, and can easily make multi-pole or even infinite pole overall magnets.
(2) Sintered NdFeB (Sintered NdFeB):
Sintered NdFeB permanent magnets are made by powder metallurgy, the main process is: alloying (smelting) → coarse crushing → fine crushing → grinding into 3 ~ 5.0μm fine powder → Magnetic field orientation pressing→vacuum sintering and tempering→inspection→processing→finished product. Sintered NdFeB permanent magnet has a high coercive force value and good mechanical properties. It can be cut and processed into different shapes and drill holes, but it is easy to cause corrosion, so the surface must be treated with different coatings according to different requirements. And very hard and brittle, with high resistance to demagnetization, not suitable for high working temperature.
(3) Injection molding NdFeB (Zhusu NdFeB):
It has extremely high precision and is easy to make thin-walled rings or thin magnets with complex anisotropic shapes.
Corrosion mechanism
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 multiphase structure, The electrochemical potential difference between the phases is large, which is easy to cause electrochemical corrosion.
In addition, during the sintering process of NdFeB, defects such as micropores, loose structure, and rough surface are prone to appear inside and on the surface of the magnet, and the working environment of NdFeB permanent magnet materials in applications is often high temperature and high humidity. Provides convenient conditions for NdFeB corrosion. At the same time, the NdFeB manufacturing process is easy to contain impurity elements such as O, H, Cl and their compounds. The elements that have the greatest impact on corrosion are O and Cl elements. The magnet and O will be oxidized and corroded, and Cl and its compounds will accelerate the oxidation process of the magnet.
The reason why NdFeB is easy to corrode mainly comes down 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°C).
High temperature environment
In a dry environment, when the temperature is lower than 150 °C, the oxidation rate of NdFeB magnets is very slow. But at a higher temperature, the Nd-rich region will react as follows: 4Nd+3O2=2Nd2O3. Subsequently, the Nd2Fe14B phase will decompose to generate Fe and Nd2C3. Further oxidation, there will be products such as Fe2O3.
Warm and humid environment
Under warm and humid conditions, the sensitive-rich grain boundary phase on the surface of NdFeB magnets first undergoes a corrosion reaction with water vapor in the environment according to the following formula: 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 generation of NdH3 will increase the grain boundary volume, cause grain boundary stress, and cause grain boundary damage. In severe cases, the grain boundary will break and cause magnet powdering.
The impact of ambient humidity on the corrosion resistance of magnets is much greater than that of temperature. This is because the corrosion product film formed by the magnet in a dry oxidizing environment is relatively dense, which separates the magnet from the environment to a certain extent. Further oxidation of the magnet is prevented, and the hydroxides and hydrogen-containing compounds generated in a humid environment are not dense and cannot prevent the further action of H2O on them. Especially when the ambient humidity is too high, if there is liquid water on the surface of the magnet, electrochemical corrosion will occur.
Electrochemical environment
In an electrochemical environment, the electrochemical potentials of each phase in a NdFeB magnet are different. The neodymium-rich phase and the boron-rich phase become anodes relative to Nd2Fe14B, and corrosion will occur preferentially to form a locally corroded micro-battery. This micro-battery has the characteristics of a large cathode and a small anode. A small amount of neodymium-rich phase and boron-rich phase are used as anodes to bear a large corrosion current density, and they are distributed on the grain boundaries of the Nd2Fe14B phase, which will accelerate its corrosion. grain boundary corrosion. When there is a metal coating on the surface of the magnet (such as electroplating Zn, Ni, etc.), once there are holes, cracks and other defects in the coating, a corrosion battery will also form between the magnet and the metal coating. In general, the magnet acts as the anode and corrodes preferentially, and the metal coating acts as the cathode, which is why the magnet with the plating layer often appears the reason for the phenomenon of skinning. In addition, in the process of surface treatment of magnets, various plating solutions (such as electroplating, chemical plating, etc.) are required, and sintered NdFeB magnets have certain holes, so in these processes, acid or plating solutions will Entering the hole will also cause electrochemical corrosion during later use.
Protection technology
The protection technology of NdFeB magnets is simply divided into two categories: chemical protection technology and physical protection technology.
Chemical protection technology mainly includes electroplating and electroless plating for preparing metal coatings, conversion coating for preparing ceramic coatings, spraying and electrophoresis of organic coatings, etc. In production, the electroplating process is most commonly used to prepare metal protective coatings on the surface of NdFeB magnet workpieces.
Electroplating is a process in which the magnet workpiece is used as a cathode, and the metal cations in the electroplating solution are reduced on the surface of the magnet by using an external current to form a metal coating. The electroplating protection of sintered NdFeB magnets is mainly to improve the corrosion resistance of the magnets, and at the same time, it also has the functions of improving the surface mechanical properties and decoration.
The advantages of electroplating include: the process is relatively simple, the film forming speed is fast, and it is easy to mass produce. Most of the electroplated metal layer plating types used for the protection of steel and non-ferrous metal workpieces can be used for NdFeB magnets. The main plating types used for NdFeB magnet protection are Zn, Ni, Cu, Cr, Sn, Au, Ag, etc. Due to the porous structure and active chemical properties of NdFeB magnets, single-layer coatings often cannot meet high corrosion resistance requirements. Generally speaking, multi-layer composite plating can provide more effective protection for the surface of magnets. At present, electroplating zinc, electroplating Ni-Cu-Ni, electroplating Ni-Cu-Ni+Ag, electroplating Ni-Cu-Ni+Au, electroplating Ni-Cu-Ni+electrophoretic epoxy, etc. are widely used.
Zinc is not magnetic, and as a protective coating, it has little effect on the magnetic properties of the magnet. Compared with nickel and copper, the price of galvanizing is relatively low. The hardness of zinc is low, and the internal stress of the coating is small, so it is not suitable for protecting NdFeB magnet workpieces that are easy to wear. It has been reported in the literature that when the zinc coating is used for the protection of NdFeB magnets to form a primary battery, the substrate can be protected by sacrificial anodes. The standard electrode potential of the zinc coating is -0.762V. After studying the electrode potential of each component phase of the NdFeB magnet, it can basically be concluded that the zinc coating cannot provide complete anode protection. In terms of actual use effect, the effect of zinc coating on the sacrificial protection of NdFeB magnets is not obvious. If the zinc coating is not treated, it will darken in the air, so passivation treatment is required after galvanizing.
The standard electrode potential of the nickel coating is -0.25V, which is more positive than that of the NdFeB magnet. It is a cathodic coating. Once the external electrolyte penetrates into the coating, it will cause accelerated corrosion of the substrate, resulting in poor bonding between the coating and the substrate, and the appearance of the coating. Delamination, blistering and other defects, the requirements for the density of nickel coating are very high in the application. The surface of the NdFeB magnet is electroplated with nickel, and a multilayer system such as Ni-Cu-Ni is usually used to reduce the porosity of the coating and improve the corrosion resistance of the coating. Relatively speaking, the cost of electroplating Ni-Cu-Ni is higher than that of electro-galvanizing, but it is favored by users because of its high temperature resistance, oxidation resistance, corrosion resistance, decorative performance and mechanical properties.
The single metal plating layer that can be directly used for the protection of NdFeB magnets includes copper, tin, gold, silver, etc. At the same time, there are quite a few alloy plating technologies that can also be used for NdFeB magnet protection, such as nickel phosphorus alloy, nickel Boron alloys, zinc-iron alloys, zinc-nickel alloys, etc. For NdFeB magnets, the Zn-Ni alloy coating is a cathode type coating. After studying the stable potential of Zn-Ni alloy coatings with different compositions, it is shown that when the Ni content is about 13%, the Zn-Ni coating is γ Phase single-phase intermetallic compound, it has high thermodynamic stability and corrosion resistance.
After years of production and use, the shortcomings of the NdFeB magnet electroplating protective coating are also quite obvious: the coating has a large porosity, the coating is not dense, and has shape dependence. The corners of the workpiece will be thickened due to the concentration of power lines during the electroplating process. The corners of the magnet are chamfered, and the deep hole samples cannot be plated; the electroplating process has a damaging effect on the magnet substrate. In some severe occasions, the electroplated coating will crack, peel off, and fall off after a long period of use. The problem is that the protection performance is reduced; with the increasing awareness of environmental protection in our country, the cost of electroplating three wastes treatment has increased sharply in proportion to the total cost of magnets.
The electroless nickel plating technology refers to the process in which the metal salt and the reducing agent in the plating solution undergo an oxidation-reduction reaction without an external current, and under the catalytic action of the workpiece surface, the metal ion is reduced and deposited. Compared with electroplating, the electroless plating process has simple equipment, does not require power supply and auxiliary electrodes, and has uniform coating thickness. It is especially suitable for plating on the surface of complex-shaped workpieces, deep-hole parts, and inner walls of pipe fittings. The density and hardness of the coating are higher. Electroless plating also has some disadvantages. The thickness of the plating layer cannot be increased, there are not many varieties that can be plated, the process requirements are relatively high, and the maintenance of the plating solution is relatively complicated. Electroless plating mainly includes nickel plating, copper plating and silver plating. At present, electroless nickel-phosphorus alloys are used in the protection process of NdFeB magnets to a certain extent, and are mostly used as additional protection for electroplating coatings. Due to the large amount of hydrogen gas precipitation during the electroless nickel plating process, it will cause great damage to the NdFeB magnet substrate, and at the same time make the coating have high stress, and the coating is prone to cracking and peeling during use.
The use of conversion coatings such as phosphating, passivation and other technologies is very common in steel. Using traditional phosphating on the surface of NdFeB magnets can also form a dense protective layer on the surface. Phosphating NdFeB magnets can increase the protection during transportation and improve the bonding force of viscose at the same time.
There are many types of organic coatings, most of which can be coated by spraying, brushing and electrophoresis. The organic coating forms a dense film and has a good barrier effect on salt spray and water vapor. Organic coatings can be used in combination with NdFeB magnet electroplating technology to further improve the protection performance of magnets.