Passivity and its breakdown of sintered NdFeB-based magnets in chloride containing solution

Abstract Passivity and its breakdown of sintered Nd 11.6 Dy 3.6 Fe 79 B 5.78 and Nd 11.3 Dy 2.3 Fe 77 Co 3 Cu 0.1 Ga 0.5 B 5.8 magnets were investigated in 0.5 M NaCl solution of pH 8.4, using polarization and electrochemical noise measurements. The microstructure of the magnets was characterized using XRD, SEM and EDX techniques. Both magnets were spontaneously passive and showed metastable pitting at freely corroding conditions as well as intergranular corrosion propagation above the pitting potential. Characteristic charge ( Q ) and frequency ( F ) of the events constituting the corrosion process at freely corroding conditions were estimated from the potential and current power spectra of the noise signals and presented as useful probes for metastable pitting characterization. The time dependence of the characteristic charge was described by Q ( t )= Q (0)exp( at ) while that of the characteristic frequency was well described by F ( t )= F (0)exp( bt ), 0⩽ t τ and F ( t )= F ( τ )exp(− ct ), t ⩾ τ . Crystallographic tunneling was found to be the way for both pitting and intergranular corrosion propagation. The partial replacement of Fe with minor amount of Co, Cu and Ga beneficially enhanced the passivity and overall localized corrosion resistance of the magnets. The beneficial effect of alloying addition was attributed to the reduction in the strength of galvanic coupling effect between ferromagnetic matrix and Nd-rich intergranular phases and subsequently in the lateral heterogeneity on the magnet surface.