Corrosion of Sintered NdFeB Permanent Magnets

Abstract

The green transition to reduce reliance on fossil fuel energy sources and minimise global warming is accelerating the need for NdFeB permanent magnet materials. Sintered NdFeB magnets cover the largest segment percentage within the permanent magnet type group, with a share of over 50%. The microstructure of NdFeB magnets includes grains of the Nd 1+ ε Fe 4 B 4 (B-rich phase), which form grain boundaries with both the Nd-rich (Nd 4 Fe) and matrix (Nd 2 Fe 14 B) phases. NdFeB magnets are prone to degradation in harsh environments due to the low corrosion resistance of both iron and neodymium. Consequently, magnets require corrosion protection because their magnetic properties would be jeopardised due to the degradation caused by the corrosion process. Understanding magnets’ electrochemical and corrosion properties is crucial for developing their corrosion protection and thus prolonging their smooth operation in numerous industrial applications. This review aims to present the fundamental corrosion processes on sintered NdFeB magnets, provide an overview of the contemporary magnet production processes of NdFeB magnets and their impact on the corrosion resistance, and summarise the corrosion protection of NdFeB magnets. Prospects are presented, especially regarding the use of magnets in the transportation (hybrid and electric vehicles) industry and the development of alternative types of coatings.