Failure and protection of rare earth permanent magnet materials
Since its invention, rare earth permanent magnet materials have been rapidly applied and promoted due to their high magnetic energy density. They are essential functional materials for modern technology. China mines and provides more than half of the rare earth resources to the world every year, and more than half of these are used as raw materials for manufacturing rare earth permanent magnet materials.
Rare-earth permanent magnet materials will corrode and fail due to environmental influences in different applications, and some protection technologies used in rare-earth permanent magnet materials themselves will also bring potential failure risks to the magnet.
The crystal structure of SmCo5, Sm2Co17 main phase and Nd2Fe14B main phase
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The external chemical environment such as gas, aqueous solution, organic solution, and the external physical environment such as pressure, vibration, high temperature, radiation, etc. may cause the failure of rare earth permanent magnet materials. From the energy point of view, all matter spontaneously transform into a low-energy state. The energy state of the oxide formed by iron, neodymium, samarium, etc. combined with oxidation is lower than that of iron, neodymium, samarium, and oxygen. The rare earth permanent magnet material will oxidize in the use environment, and the oxidation process will change from the surface to the surface. The magnet expands inside, causing the magnetic material to fail.
We call the phenomenon of material destruction under the action of physical and chemical factors of environmental media as corrosion. Mainly divided into three categories: chemical corrosion: corrosion caused by the direct reaction between the metal matrix and non-electrolyte, the oxidation of neodymium iron boron and samarium cobalt magnets in dry air and the hydrogen absorption of neodymium iron boron and samarium cobalt magnets at high temperatures, etc. It belongs to chemical corrosion; electrochemical corrosion: oxidation caused by the electrochemical interaction between the metal matrix and the ion-conducting medium; physical corrosion: structural changes due to physical dissolution. Since there are many applications of magnetic materials, their failures are mostly caused by chemical reasons. Here, chemical corrosion and electrochemical corrosion are mainly considered.
Chemical protection technology of NdFeB magnet
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 coating, conversion film for preparing ceramic coating, and spraying and electrophoresis of organic coating. In the production, the electroplating process is most commonly used to prepare the metal protective coating on the surface of the NdFeB magnet workpiece.
Electroplating is the process of using a magnet workpiece as a cathode and using external current to reduce the metal cations in the electroplating solution on the surface of the magnet to form a metal coating. The electroplating protection of sintered NdFeB magnet is mainly to improve the corrosion resistance of the magnet, and it also has the functions of improving the surface mechanical properties and decoration. The advantages of electroplating include: relatively simple process, fast film forming speed, and easy mass production. Most of the electroplated metal coatings used for the protection of steel and non-ferrous metal workpieces can be used for neodymium iron boron magnets. The main plating species used for the protection of NdFeB magnets 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 higher corrosion resistance requirements. Generally speaking, multi-layer composite coatings can provide more effective protection for the surface of the magnet. Currently widely used are electro galvanizing, electroplating Ni-Cu-Ni, electroplating Ni-Cu-Ni+Ag, electroplating Ni-Cu-Ni+Au, electroplating Ni-Cu-Ni+ electrophoretic epoxy, etc.
Zinc has no magnetism and has little effect on the magnetic properties of the magnet as a protective coating. Compared with nickel and copper, the price of zinc plating is relatively low. The hardness of zinc is low, and the internal stress of the coating is small, so it is not suitable for the protection of NdFeB magnet parts that are easy to wear. It is reported in the literature that the zinc plating layer can be used to protect the substrate by sacrificial anode when forming the galvanic cell. The standard electrode potential of the zinc coating is -0.762V. After studying the electrode potentials of the various phases of the neodymium iron boron magnet, it can be basically concluded that the zinc coating does not provide complete anode protection. From the actual use effect, the sacrificial protection effect of the zinc coating on the NdFeB magnet is not obvious. If the zinc coating is not treated, it will become dark in the air, so passivation treatment is required after galvanizing.
The standard electrode potential of nickel plating is -0.25V, which is more positive than neodymium iron boron magnets. It is a cathodic plating. Once the external electrolyte penetrates into the plating, it will cause accelerated corrosion of the substrate, resulting in poor bonding between the plating and the substrate, and the coating appears Defects such as delamination and blistering require very high density of nickel plating in applications. Electroplating nickel on the surface of neodymium iron boron magnets usually adopt a multilayer system such as Ni-Cu-Ni 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 electroplating zinc, but it is favored by users because of its high temperature resistance, oxidation resistance, corrosion resistance, decorative properties, and mechanical properties.
The elemental metal plating layer that can be directly used for the protection of NdFeB magnets is copper, tin, gold, silver, etc. At the same time, there are quite a lot of alloy plating techniques that can also be used for the protection of NdFeB magnets, such as nickel-phosphorus alloys and nickel. Boron alloy, zinc-iron alloy, zinc-nickel alloy, 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 [[iii]], it is shown that when the Ni content is about 13%, Zn -Ni coating is a single-phase intermetallic compound of γ phase, which has high thermodynamic stability and corrosion resistance.
After years of production and use, the shortcomings of the NdFeB magnet electroplating protective coating are quite obvious: the porosity of the coating is large, the coating is not dense, and the shape is dependent. The corners of the workpiece will be thickened due to the concentrated coating of the power line during the electroplating process. Chamfering the edges and corners of the magnet, it is impossible to plate the deep hole samples; the electroplating process has a damage effect on the magnet matrix. In some severe situations, the electroplating layer will crack, peel off and easily fall off after a long time use of the electroplating layer. The problem is that the protective performance is declining; with the increasing awareness of environmental protection in my country, the proportion of the cost of the three wastes of electroplating in the total cost of the magnet has increased sharply.
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 applying an external current, and the metal ions are reduced and deposited under the catalysis of the workpiece surface. Compared with electroplating, the electroless plating process equipment is simple, does not require power supply and auxiliary electrodes, and the thickness of the plating layer is uniform. It is especially suitable for surface plating of complex-shaped workpieces, deep-hole parts, and inner walls of pipe fittings. The plating layer has higher density and hardness. Electroless plating also has some shortcomings, the thickness of the coating does not go up, there are not many varieties that can be plated, the process requirements are relatively high, and the maintenance of the plating solution is more complicated. Electroless plating types mainly include nickel plating, copper plating and silver plating. At present, chemical plating nickel-phosphorus alloys are used in the protection process of neodymium iron boron magnets, and are mostly used as additional protection for electroplating coatings. Because a large amount of hydrogen is precipitated during the electroless nickel plating process, it causes greater damage to the neodymium iron boron magnet matrix, and at the same time causes the coating to have a higher stress, and the coating is prone to cracking and skinning during use.
The use of conversion coatings such as phosphating, passivation and other technologies is common in steel. Using traditional phosphating on the surface of the NdFeB magnet can also form a dense protective layer on the surface. After phosphating, the NdFeB magnet can increase the protection during transportation and at the same time improve the bonding force of the glue.
There are many types of organic coatings, and most of them can be coated by spraying, brushing and electrophoresis. The organic coating is densely formed and has a good barrier effect on salt spray and water vapor. The organic coating can be used in combination with the NdFeB magnet electroplating technology to further improve the protection performance of the magnet.
Preparation of protective film of samarium cobalt
Samarium cobalt magnets will have an aging layer under high temperature conditions, and the formation of the aging layer is related to the diffusion of oxygen. If the magnets used at high temperatures are protected by surface treatment to isolate the diffusion of oxygen into the magnets, the attenuation of the magnetic properties of the samarium cobalt magnets can be greatly delayed. In addition, the mechanical properties of samarium-cobalt magnets are relatively brittle, and the brittleness of Sm2Co17 is more serious, and cracks and corners are prone to occur during machining and use. By means of electroplating, physical vapor deposition, etc., the brittleness of samarium cobalt can be improved and the reliability of use can be improved.
Source: China Permanent Magnet Manufacturer – www.rizinia.com