Improving the corrosion resistance of NdFeB rare earth permanent magnet materials and improving their surface protection technology have become the key to break through the engineering application of rare earth permanent magnet materials, which is mainly related to the continuous improvement of market quality requirements and diversified demands for materials. For this reason, there have been many articles reporting the corrosion mechanism and surface protection of NdFeB materials in the past 20 years, involving material corrosion, environmental corrosion and surface science. It can be seen from these documents that based on the corrosion sensitivity of NdFeB materials, on the one hand, alloying means are used to control the material composition and structure, and improve the corrosion resistance of the magnet itself. For example, adding metal elements M1 (such as: Cu , Al, Zn, Ga, Ge, Sn, etc.), forming Nd-M1 or Nd-Fe-M1 intermetallic compounds, or adding metal elements M2 (such as Co, Ti, Nb, Zr, V, Mo, W, etc.), Formation of M2-B or Fe-M2-B intermetallic compounds, the intergranular formed by these compounds has a higher corrosion potential than the Nd-rich and B-rich phases, reducing the electrochemical difference with the main phase (Nd2Fe14B) , thereby weakening the driving force of interphase corrosion; on the other hand, coatings are applied on the surface of magnets to protect them, such as chemical conversion, electroplating, electroless plating, electrophoresis, physical vapor deposition, organic coatings and other composite coatings, etc. The method forms a protective layer on the surface of the material, which can prevent the corrosive medium (such as O2, H2O or Cl-) from penetrating from the surface to the substrate, thereby protecting the substrate from corrosion. Driven by the market, with the continuous expansion of the application field of permanent magnets, various technologies are gradually put into practice, and the advantages and disadvantages of each method have also been evaluated successively.

Although the vast number of researchers have done a lot of work in improving the corrosion sensitivity of rare earth permanent magnet materials, the corrosion mechanism of NdFeB materials still needs to be further analyzed, and its surface protection technology lags behind the development of materials. Not often. This is mainly because many factors affect the analysis of its corrosion and protection mechanism, including material matrix, corrosion medium, test environment and technical solutions, etc. These factors will change the intermediate reaction process, thereby affecting the composition, structure and protection characteristics of corrosion products. Due to the complexity of characterizing corrosion product films, it is very difficult to unambiguously explain specific mechanisms and develop effective protective measures. Nevertheless, according to the above literature reviews, a lot of valuable information can be extracted to analyze the corrosion and protection mechanism of NdFeB materials. Based on the contents of the above literature, this paper summarizes the factors affecting the corrosion of sintered NdFeB materials, expounds the causes of corrosion, and makes a comprehensive review of the research on the alloying process to improve the corrosion resistance of magnets and the surface protective coating technology in recent years. In the future, material design and protection strategies will be developed to provide reference for rare earth permanent magnet materials to better meet the needs of social development, which will help breakthrough key technologies in the production of rare earth permanent magnet materials, develop corrosion-resistant magnetic materials and establish corresponding surface protection Strategy matters.

1 Factors Affecting Material Corrosion

 To master the factors affecting material corrosion and analyze the causes of corrosion is to seek ways to improve the corrosion resistance of permanent magnets to improve product reliability.

1.1 Material factors

The material factor is the internal cause of corrosion, which mainly refers to the corrosion caused by the difference in electrochemical properties between phases. Figure 1 is a schematic diagram of the internal structure of NdFeB material. The main phase (Nd2Fe14B) is polygonal and is a magnetic phase; the Nd-rich phase (Nd4Fe) is distributed along the grain boundaries or grain boundaries of the main phase in a thin layer or granular form, and surrounds the grains of the main phase; the B-rich phase (Nd1 +xFe4B4) is massive or granular, and exists in the grain boundary in a metastable state, and the volume fraction of each phase is about 84%, 14% and 2%.

   The research results show that the electrochemical potentials of Nd-rich phase and B-rich phase in the same main phase are significantly different, and the electrochemical potentials from low to high are B-rich phase, Nd-rich phase, and Nd2Fe14B phase. Nd phase and B-rich phase will be corroded preferentially. It is not difficult to know from the above analysis that this kind of corrosion battery has the characteristics of "small anode-big cathode", and the Nd-rich phase and B-rich phase that occupy a small volume fraction in the magnet will accelerate corrosion along the grain boundaries of the main phase, resulting in the surrounding main phase grains fall off, and the resulting bulky corrosion products will cause magnet powdering and performance degradation. Figure 2 shows the magnetic field distribution of NdFeB magnets before and after corrosion. It can be seen that the corrosion of the magnet leads to obvious changes in the magnetic field distribution; at the same time, the uneven distribution of the magnetic field further confirms that the corrosion degree of each part of the magnet is different.

According to the latest research results of the team of Academician Li Wei of the General Iron and Steel Research Institute, the addition of high-abundance rare earth element cerium (Ce) is beneficial to improve the corrosion resistance of the magnet. The main reason is that after part of the Nd in the structure is replaced by Ce, the grain boundary. The Nd content of Nd is significantly reduced, which makes the rare earth-rich phase thinner and more uniformly distributed. At the same time, the grain size of the main phase becomes smaller and the structure is tighter. The corrosion products are easy to block the transmission channel of the medium, which can retard corrosion.

 For this reason, on the one hand, reduce the content of Nd element to avoid the accumulation of Nd-rich phase at the grain boundary; on the other hand, adjust the intergranular structure to induce the Nd-rich phase to diffuse and distribute in the triangular grain boundary of the main phase, so as to narrow the diffusion channel of the corrosion medium. Inhibit the corrosion process to improve the corrosion resistance of the magnet. On this basis, adjusting the crystal structure of the main phase, ensuring the uniform distribution of each phase, and reducing the driving force of corrosion reaction is also one of the efforts to improve the corrosion resistance of magnets.

1.2 Environmental factors

 Environmental factors are external causes of corrosion, including ambient temperature, humidity, and corrosive media. As far as the ambient temperature is concerned, when the temperature exceeds 150 °C, the oxidation rate of Nd element at the grain boundary increases significantly, and its main reaction is shown in formula (1); and as time goes on, the main phase will also oxidize.

Humidity is one of the main factors leading to corrosion failure of permanent magnets. To this end, the researchers used high-temperature and high-pressure accelerated corrosion experiments to study the effect of humidity on magnet corrosion with the help of high-pressure reactors. The results show that magnet corrosion is more sensitive in humid environments than in high-temperature environments, and the presence of water vapor in the environment is a magnet. A necessary condition for corrosion to occur. And the study found that the weight loss of high-abundance rare earth permanent magnets (Ce15Nd85)30FebalB1M or (Ce20Nd80)31FebalB1M) is lower than that of conventional rare earth permanent magnets (Nd2Fe14B) in a hot and humid environment. The enrichment of phase edge and controlling the content of rare earth-rich phase are the key to improve the corrosion resistance of rare earth permanent magnet materials.

In addition, the research results show that the corrosion behavior of rare earth permanent magnet materials is closely related to the corrosion medium. For example: the corrosion rate of NdFeB magnets in HCl and H2SO4 is relatively high; HCl solution has a serious corrosion on the Nd-rich phase, so after being corroded by HCl solution, the maximum magnetic energy product of the magnet is significantly reduced, while HNO3 solution has a greater impact on the main phase , so it will cause the intrinsic coercive force of the magnet to decrease; the surface of the magnet can be passivated in H3PO4 and H2C2O4, and sometimes the oxide layer or dirt on the surface of the magnet cannot be completely removed. The research results provide a theoretical basis for revealing the correlation between the lifetime of NdFeB magnets and the environment.

It is worth noting that the development of NdFeB permanent magnet materials has strongly promoted the progress of magnetic surgery in the medical field. With the wide application of rare earth permanent magnet materials in medical devices, in order to improve the reliability of devices, the corrosion behavior of magnets in body fluids (such as saliva, gastric juice, bile, etc.) has gradually attracted attention. It can be seen from this that the corrosion problem of permanent magnets used in special fields, like many common problems, is an urgent problem to be solved in the development of NdFeB materials, and the diverse needs bring great challenges to the development of NdFeB materials.

1.3 Magnetization state

     Studies have shown that the corrosion behavior of rare earth permanent magnets is significantly different in different magnetization states, and the corrosion tendency of magnets in the magnetization state is greater, especially in the direction of magnetization. The corrosion rate is faster. Costa et al. believe that this is due to the enhanced migration driving force of paramagnetic oxygen molecules in the magnetic field; Sueptitz et al. believe that under the action of Lorentz force, the ion convection movement in the electrolyte promotes material transport and increases the electrochemical reaction rate, which is the cause The main reason for the accelerated corrosion rate; Zheng Jingwu et al. proposed that the surface residual magnetic field will affect the electric double layer structure, resulting in a large magneto-induced overpotential, which will lead to the intensification of the corrosion reaction.


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