Electrochemical characterization of galvanically coupled single phases and nanocrystalline NdFeB-based magnets in NaCl solutions

Abstract

Galvanic interaction between intermetallic phases occurring in NdFeB-based permanent magnets was investigated for the purpose of understanding the electrochemical behaviour of nanocrystalline hot pressed Nd14Fe80B6 and Nd12Dy2Fe73.3Co6.6Ga0.6B5.6 magnets in 0.5 M NaCl solution of pH 8.4. The microstructure of multiphase nanocrystalline magnets was characterized using XRD, SEM and EDX techniques. The electrochemical behaviour was assessed using zero resistance amperometry and potentiodynamic polarization measurements. From the electrochemical data obtained, it was concluded that the Nd-rich phase/ferromagnetic (φ)- phase couple generated the highest galvanic current density. The strength of the galvanic coupling effect was typically controlled by the oxidation rate of Nd-rich phase, which is the least polarizable. Pitting and intergranular corrosion were detected on the surface of nanocrystalline magnets and found to be dependent on each other, since intergranular corrosion processes nucleated on the pit walls and propagated from there. Crystallographic tunneling was found to be the way of both pit and intergranular corrosion propagation. Co and Ga additions generally increase localized corrosion resistance of NdFeB-based magnets due to the significant reduction in the strength of galvanic coupling effect between magnet phases.