富钕相对烧结NdFeB磁体耐腐蚀性的影响

The Effect of Nd-Rich Phase for Sintered NdFeB Corrosion Resistance

摘要:

研究了Nd2Fe14B单晶、传统烧结NdFeB磁体和放电等离子烧结（简称SPS）NdFeB磁体在电解液溶液中的电化学特性。采用扫描电子显微镜和电子能谱分析了磁体的微观组织成分。结果表明在3．5％NaCI溶液的极化曲线中，Nd2Fe14B单晶具有最高的电化学腐蚀电位，放电等离子烧结NdFeB磁体的腐蚀电位高于传统烧结NdFeB磁体。与传统烧结NdFeB磁体相比，放电等离子烧结NdFeB磁体富Nd相具有独特的分布形态，主相Nd2Fe14B晶粒细小、均匀，富钕相在主相晶粒边界上分布较少，主要集中在三角晶界处。这种组织结构有效地抑制了磁体沿富钕相发生晶间腐蚀的过程，磁体因此具有良好的耐腐蚀性能。此外，从不同稀土含量的烧结NdFeB磁体的高压加速实验中可以看出磁体的腐蚀速度随稀土含量的增加而增大。以上结果表明富Nd相的化学特性及其分布状态和含量是决定合金耐蚀性能的关键，它在合金中以网络状分布在主相晶粒边界上，并决定了烧结NdFeB易于发生选择性晶间腐蚀，从而导致耐蚀性差。

Abstract:

The electrochemical properties of Nd2Fe14B single crystal grain,conventional sintered NdFeB and SPS NdFeB in electrolyte were studied. The microstructures and compositions of the magnets were investigated with scanning electron microscope (SEM) and energy dispersive X-ray detector (EDX). Polarization curves in 3.5% NaCl solution show that Nd2Fe14B single crystal grain has displayed most positive corrosion potential and the SPS NdFeB magnet has more noble potentials compared with the conventional sintered one. Compared with conventional sintered NdFeB magnet,SPS processed magnet has original microstructure i.e. the grain size of the Nd2Fe14B main phase is fine and uniform,and the fine Nd-rich phase does not form along the grain boundaries of main phase,but agglomerates into the triple junctions. The pathways for corrosion propagation are restricted and the inter-granular corrosion process though Nd-rich phase in the magnet is suppressed effectively. Therefore,the SPS NdFeB magnet possesses excellent corrosion resistance. Autoclaves test indicates that a strong correlation has been established between an increase in the total rare earth level,which corresponds to the amount of Nd-rich phase,and an increase in the corrosion rate. The results show that chemical character; microstructure and the amount of Nd-rich phase determine the corrosion resistance of magnets and indicate an intergranular corrosion leads to poor corrosion resistance.