The corrosion susceptibility of NdFeB permanent magnets is due to the fact that Nd is one of the most chemically active elements. On the other hand, the alloy is a multiphase structure with large electrochemical phase differences between phases, which can easily 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. The working environment of NdFeB permanent magnet materials in applications is often high temperature and high humidity. These defects provide convenient conditions for NdFeB corrosion in high temperature and high humidity environments.
(1)High temperature environment
In a dry environment, when the temperature is below 150°C, the oxidation rate of NdFeB permanent magnet is very slow. However, at higher temperatures, the following reaction will occur in the Nd-rich zone: 4Nd + 3O2 = 2Nd2O3. Subsequently, the Nd2Fe14B phase will decompose to form Fe and Nd2C3. Further oxidation will also produce products such as Fe2O3.
(2)Warm and humid environment
Under warm and humid conditions, the sensitive grain boundary phase on the surface of the NdFeB permanent magnet first reacts with the water vapor in the environment according to the following formula. The H generated by the reaction penetrates into the grain boundary and further reacts with the Nd-rich phase, causing grain boundary corrosion. The generation of NdH3 will increase the volume of the grain boundary, causing grain boundary stress and grain boundary damage. In severe cases, the grain boundary will break and cause the magnet to pulverize.
The effect of environmental 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 to a certain extent separates the magnet from the environment and prevents further oxidation of the magnet.
