How does NdFeB prevent corrosion?

Permanent magnet material is a kind of object that can continuously provide magnetic energy without consuming electric energy. It has the function of energy conversion and is an important functional material. The neodymium iron boron permanent magnet is world-famous for its extremely high “magnetic energy product”. It is called “magnetic king” due to 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 easy to form serious intercrystal corrosion of the primary battery in a humid environment, thus seriously affecting the performance and service life of NdFeB, it must be improved by adjusting its chemical composition and adopting surface treatment method to meet the requirements of practical application.
Since the advent of neodymium iron boron rare earth permanent magnet material in the middle of the 1980’s, it has been widely used in electronic communication with a series of unique advantages such as high magnetic energy product, high coercive force, high remanent density, small volume and light weight, in many fields such as metallurgical manufacturing, geological exploration, medical care, transportation and aerospace, it can be said that it is everywhere.

The downstream consumption distribution of global NdFeB in 2019 (source: Prospective Industry Research Institute) remarks: NdFeB permanent magnet has sintered NdFeB, bonded NdFeB, and hot pressed NdFeB, according to the data of China Rare Earth Industry Association in 2019, the output of sintered NdFeB blank was 170,000 tons, accounting for 94.3% of the total NdFeB magnetic materials in that year, and 4.4% of the bonded NdFeB.

Look at the composition of NdFeB

The basic composition of the material affects the properties of the material. The sintered NdFeB permanent magnet is mainly produced by Powder Metallurgy. It has at least the following four different phases:

• ① matrix phase (main phase): Nd2Fe14B phase. It is formed by inclusion reaction at about 1200℃ and is the only magnetic phase in the alloy. The excellent magnetic properties of NdFeB magnets are mainly due to the high saturation magnetization (μms = 1.6T) and Anisotropy Field (7.3T) of Nd2Fe14B phase;
• ② Nd-rich phase: its melting point is 650~700 ℃, which is finally solidified in the alloy and exists in a thin layer and a block, distributed at the junction of grain boundaries or in the crystal world of Nd 2Fe14B. Although it is non-magnetic phase, due to its low melting point characteristics, it distributes around the main phase during sintering, which not only plays the role of densifying the sintered body, but also suppresses the grain growth, promoting the improvement of coercive force is therefore essential.
• ③ B- rich phase Nd1 +εFe4B4: it is formed when the boron content in the alloy exceeds the normal composition of Nd2Fe14B. It has no contribution to magnetic energy, and the general quantity is extremely small, which has little effect on magnetic energy.
• ④ α-Fe: its melting point is ℃, which is the phase with the highest melting point in the alloy. It is first precipitated from the liquid alloy, and a-Fe is the soft magnetic phase, its existence leads to the decrease of the main phase and the increase of the neodymium-rich phase, destroys the optimal ratio of the main phase and the neodymium-rich phase, and damages the magnetic orientation of the grains of the main phase, at the same time, it also makes the grain coarse in the local area of the sintering process, which not only makes the magnetic energy worse, but also makes the structure of the electroplating layer worse, affecting the protection effect. Therefore, take measures in manufacturing process to reduce or eliminate the generation of α-Fe phase as far as possible, such as piece casting process and rapid quenching process.

Classification of neodymium iron boron

Due to different manufacturing methods and use requirements, NdFeB permanent magnet can be divided into three categories:

• (1) Bonded NdFeB: NdFeB bonding magnet is to obtain microcrystalline powder by chilling, and each powder contains multiple Nd-Fe-B microcrystalline grain, then use polymer or other binder to bond the powder by mixing and pressing, and squeeze or roll to form a plastic-like permanent magnet. Therefore, the usual NdFeB bonded magnet is a non-dense isotropic magnet. Generally, the magnetic properties of NdFeB bonded magnets are much lower than those of NdFeB sintered magnets, but NdFeB bonded magnets have many advantages that NdFeB sintered magnets cannot be replaced: high processing precision, high yield rate, high precision, excellent magnetic properties, good corrosion resistance, good temperature stability; In addition, the Nd-Fe-B-bonded magnet is also convenient for magnetizing in any direction, and can be convenient for the manufacture of multipole and even countless whole magnets.
• (2) Sintered NdFeB: Sintered NdFeB permanent magnets are made by Powder Metallurgy. The main processes are: alloying (smelting) → coarse crushing → fine crushing → grinding into 3~5.0μm fine powder → magnetic field orientation suppress→ vacuum sintering tempering → inspection → processing → finished product. Sintered NdFeB permanent magnet has a high coercive force value and good mechanical properties. It can cut and process different shapes and holes, but it is easy to cause corrosion. Therefore, different coating treatments must be carried out on the surface according to different requirements. And it is very hard and brittle, with high anti-demagnetization, and not suitable for high working temperatures.
• (3) Injection-molded NdFeB: it has extremely high accuracy and is easy to be made into thin-walled rings or thin magnets with complex anisotropic shapes.

Corrosion mechanism of neodymium iron boron permanent magnet

NdFeB permanent magnets are easy to corrode because Nd is one of the elements with the highest chemical activity (its standard potential E0(Nd3+/Nd)=-2.431V; On the other hand, the alloy is a multi-phase structure, and the electrochemical bits of each phase are quite different, which is easy to cause electrochemical corrosion.

In addition, in the process of NdFeB sintering, the interior and surface of the magnet are prone to have defects such as micropore, loose structure and rough surface, while the working environment of NdFeB permanent magnet material in application is usually high temperature and high humidity, these defects provide convenient conditions for NdFeB corrosion in high temperature and high humidity environment. Meanwhile, impurity elements such as O,H and Cl and their compounds are easy to be contained in the manufacturing process of NdFeB, O and Cl elements have the greatest impact on corrosion, cl and its compounds will accelerate the oxidation process of magnets.
The reason why NdFeB is easy to corrode is mainly attributed to: working environment, material structure and manufacturing process. Research shows that the corrosion of NdFeB magnets mainly occurs in the following three environments: warm and humidity environment, electrochemical environment and high temperature environment for a long time (>250℃).

High temperature environment

In a dry environment, the oxidation rate of NdFeB magnets is very slow when the temperature is lower than 150℃. However, at higher temperature, the rich Nd Region will react as follows: 4Nd +3O2=2Nd2O3. Subsequently, Nd2 Fe 14B phase decomposition Fe and Nd2C3. Further oxidation, there will also be products such as devotes.

Warm and humid environment

Under warm and wet conditions, the fuming grain boundary phase on the surface of the NdFeB magnet first corrodes with the water vapor in the environment by pressing the type: 3H2O + Nd=Nd (OH)3+3H. The H generated by the reaction penetrates into the crystal boundary and further reacts with the Nd-rich phase: Nd +3H → NdH3, causing grain boundary corrosion. The generation of NdH3 will increase the volume of the grain boundary, cause the stress of the grain boundary and the damage of the grain boundary. If serious, it will break the grain boundary and cause magnet pulverization. The influence of environment humidity on the corrosion resistance of magnets is much bigger than that of temperature, which is because the corrosion product film of the magnet is relatively dense under the dry oxidation environment, separating the magnet from the environment to some extent, the further oxidation of the magnet is prevented. However, the hydrogen oxide and hydrogen-containing compound produced in the humid environment are not dense, the further action of H2O on it cannot be prevented. Especially when the humidity of the environment is too high, electrochemical corrosion will occur if there is liquid water on the surface of the magnet.

Electrochemical environment

In electrochemical environment, the electrochemical potentials of each phase in NdFeB magnet are different. Compared with Nd2Fe14B, neodymium-rich phase and boron-rich phase become anode, which will give priority to corrosion and form partially corroded micro-cells. 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 bear a large corrosion current density as electrodes, and they are distributed in the crystal world of Nd2Fe14B phase, this will accelerate the corrosion of its grain boundary. When there is a metal plating layer (such as electroplating Zn, Ni, etc.) on the magnet surface, once the plating layer has defects such as holes and cracks, corrosion battery action will also be formed between the magnet and the metal plating layer. Under normal circumstances, the magnet as the anode and priority corrosion, metal coating as the cathode, this is the reason why the magnet with coating often appear skin phenomenon. In addition, in the process of surface treatment of the magnet, various plating solutions (such as electroplating and electroless plating) are needed, and the sintered NdFeB magnet has certain holes. In this way, acid solution or plating bath will enter the hole, which will also cause electrochemical corrosion in the future use process.

The anti-corrosion technology of neodymium iron boron permanent magnet

There are three main ways to prevent corrosion of sintered NdFeB magnets: first, improve the corrosion resistance of magnets themselves. The magnet with high density and ultra-fine grain can be obtained by improving the magnet microstructure through hot pressing process, 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 alloy elements need to be added, but sometimes the magnetic properties are reduced, and the addition of alloy will increase the production cost. These factors limit the application of this method. Third, the effective protection coating is adopted.
At present, the anti-corrosion of NdFeB magnets mainly focuses on the surface coating, that is, the coating is used to improve the corrosion resistance of magnets.

Electroplate coating

Electroplating is based on the oxidation-reduction reaction with external charge to enrich the metal ion cathode in the electroplating solution to form a metal coating. From 1985 to 1990, it was the initial stage of Nd-Fe permanent magnet material electroplating. After nearly ten years of development, the electroplating technology of ND-fe permanent magnet material has been relatively mature up to 2016. From 2006 till now, the innovation and development stage of electroplating technology for neodymium iron boron permanent magnet materials.
At present, neodymium iron boron permanent magnet material electroplating layer mainly includes galvanized, nickel plating, nickel plating zinc alloy and other nickel alloy and composite coating.

Electroless coating

Electroless plating is a process method that, under the condition of no applied current, according to the redox reaction, the metal ions in the chemical plating bath are deposited on the surface of the substrate to form a coating with certain functions. Due to the self-catalytic 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. In contrast, electroless plating has become more and more mature and widely used. Internationally, electroless plating has been widely used to provide corrosion-resistant and wear-resistant protective film for NdFeB substrate.
At present, neodymium iron boron chemical coating is mainly nickel phosphorus chemical coating and nickel copper phosphorus, nickel tungsten phosphorus and nickel copper phosphorus and other chemical coating.
The plating bath used for chemical plating is also mainly divided into acidic and alkaline. High phosphorus non-magnetic coatings are often generated in acidic environment, and low phosphorus magnetic coatings are often generated in alkaline environment and have certain magnetic shielding. Due to the obvious hydrogen absorption in acidic environment, the surface quality of activated NdFeB substrate is seriously affected, so alkaline plating solution is mostly used in industrial production.

Organic coating

Organic coating is one of the most widely used means in metal protection methods. The organic coating methods for NdFeB magnets are 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 adhesion, and has enough hardness, it is widely used in industry. The epoxy resin coating is coated on the NdFeB of galvanized and nickel, and its antirust property is far better than the traditional galvanized and nickel coating. In addition to epoxy resin, other resin materials include polyacrylate, polyamide, polyimide, etc. There are also mixtures using two or more of these resins as coatings, at the same time, add some anti-rust coatings such as hongdan, chromium oxide, etc.

Physical vapor deposition coating

Physical vapor deposition is a new coating technology different from electroplating and electroless plating. The film prepared by this method can stick well to the base, the film layer is relatively compact, the surface is smooth and the porosity is relatively small, and the residual of electrolyte in the film layer during the electroplating process can be eliminated, avoid the secondary damage to the film layer caused by residual fluid, and reduce the possibility that the coating is brittle caused by hydrogen generated by magnet reaction during electroless plating.
Common physical vapor deposition methods include vacuum evaporation coating, magnetron sputtering coating and multi-arc ion plating, etc. Common film materials include Al ,Ti/ Al, Al/Al2O3, TiN, the membrane coating and the substrate bonding membrane coated by Ti and other physical vapor deposition methods have excellent quality, good corrosion resistance, and no secondary pollution problems such as waste liquid and waste residue, which are the important direction of NdFeB anti-corrosion technology development at present.

Source: China Permanent Magnet Manufacturer – www.rizinia.com