Why is the corrosion resistance of sintered NdFeB poor?

Sintered NdFeB permanent magnet has superior magnetic properties, but it is prone to corrosion when exposed to air or humid environment. Therefore, the further improvement and expansion of the application field of sintered NdFeB magnet are limited. What are the main reasons for the poor corrosion resistance of sintered NdFeB permanent magnet? What is the root cause?

Analysis of several reasons for poor corrosion resistance of sintered NdFeB

• 1. The fundamental reason is that the chemical activity of Neodymium in sintered NdFeB permanent magnet is very high, and its standard electrode potential is E0 (Nd3 + / nd) = – 2.431 v. the sintered NdFeB permanent magnet itself is a multiphase structure, in which the chemical activity of Nd rich phase is the highest, and there is a large electrochemical potential difference between the phases in the material. In the electrochemical environment, it is easy to produce electrochemical corrosion. The grain boundary phase (nd rich phase) as anode will bear larger corrosion current, and the main phase (Nd2Fe14B phase) as cathode will bear smaller current. Finally, the corrosion characteristics of small anode and large cathode are formed, which accelerates the corrosion of intergranular phase and eventually leads to the destruction of the whole magnet due to corrosion.
• 2. Structure of sintered NdFeB magnet: the sintered NdFeB magnet made by powder metallurgy has low density, high porosity in the magnet, and can not form a dense oxide film on the surface of the magnet. Once oxidation occurs, the pores in the magnet will become a fast corrosion channel for the rapid diffusion of oxygen and other corrosive media, which can cause chain reaction in the magnet and cause damage to the magnet Oxidation and corrosion of itself;
• 3. Harmful impurities in magnets: due to the influence of production process, equipment and human factors, there may be h, O, N, s, Si, C, Cl, chloride and other harmful impurities in the magnets during the production of sintered NdFeB magnets. Among them, O, Cl and chloride are the most serious harmful impurities to the corrosion performance of magnets.
• 4. Influence of working environment: sintered NdFeB magnets are widely used in high and low temperature alternating, long-term maintenance free, extreme environment and other fields. Sintered NdFeB magnets are rarely used in room temperature or vacuum environment, so different working environments put forward higher requirements for the corrosion resistance of sintered NdFeB magnets.