The Early-Stage Propagation of Localized Corrosion of a Sintered NdFeB Alloy in Aqueous Environments

To capture the initial corrosion of sintered NdFeB alloys, a quasi-in situ investigation of the localized corrosion behavior was conducted in low-conductive aqueous solutions. It was clearly observed that the corrosion pits were started from the active sites in matrix phase adjacent to the phase boundaries and the oxide rings were simultaneously deposited some distance from the pits. The spatial separation of the anode and cathode areas during the corrosion could be explained by the potential-pH diagram of Fe and the transportation of hydroxide and ferrous ions in the solution. Based on the variation of oxide rings with environmental factors, it was deduced that the ring-shaped deposition of corrosion products was cooperatively determined by the ferrous cations from pits and the hydroxide anion generation at the cathodic regions limited by the diffusion of oxygen from the bulk solution. This observation gives new insights into the galvanic corrosion of alloys in aqueous media.