| Studies on the corrosion behavior of sintered NdFeB magnets have been extensively investigated, but few concentrated on that of nanocrystalline magnets, especially on their corrosion behavior in atmosphere. Considering that the real application environment of nanocrystalline NdFeB magnets is atmospheric environment, in this work, corrosion behavior of hot-pressed nanocrystalline NdFeB magnet in a marine atmosphere was discussed in view of both external and internal factors. The objectives of this study were to investigate the effects of chloride on the atmospheric corrosion of nanocrystalline NdFeB magnet, and to elucidate the failure process and corrosion mechanism of this newly developed material. SEM/EDS results indicated that chloride greatly enhanced the initiation and development of the corrosion of nanocrystalline NdFeB magnet, and the corrosion resistance of the Nd-rich phase of nanocrystalline NdFeB magnet was much stronger than that of the commercial NdFeB magnet (sintered NdFeB magnet). XPS results suggested that the corrosion product scales of nanocrystalline NdFeB magnet were rich in neodymium, indicating the preferential corrosion of this element. The deposition of NaCl particles on the surface of NdFeB slightly affected the corrosion rate of iron and boron, but significantly accelerated the corrosion rate of neodymium. In addition, NaCl facilitated the formation of Nd(OH)3. Nd(OH)3was unstable in the system, and gradually decomposed into Nd2O3.Anistropic nanocrystralline NdFeB magnet with enhanced remanence and larger energy product can be obtained by subsequent hot-deformation of the hot-pressed nanocrystralline NdFeB magnet. Hot-pressed/hot-deformed nanocrystralline NdFeB magnet possesses preferential oriented crystal structure with crystallographic c-axis perpendicular to the pressure direction. For anistropic materials, the corrosion behaviors of surfaces with different crystal orientations of can be different due to their differences in microstructure. Consequently, in this study, the effects of crystal orientation on the corrosion behavior of hot-pressed/hot-deformed nanocrystralline NdFeB magnet were investigated by electrochemical measurements and materials analysis methods. The results indicated that the hot-pressed/hot-deformed nanocrystralline NdFeB samples with different crystal orientations followed similar corrosion mechanism, and both of the surface (⊥) and surface (||) showed preferential corrosion of neodymium. Compared with surface(⊥), surface (||) possessed a higher dissolution rate and a higher degree of oxidation when exposed to3.5wt%NaCl solution. The slight differences in the corrosion behavior of the surface (⊥) and surface (||) were probably ascribed to their differences in the Nd-rich phase proportions and grain boundary quantities.In order to prevent the corrosion of NdFeB magnets during manufacturing, the efficiency of cyclohexylamine carbonate (CHC) as vapor phase inhibitor for sintered NdFeB magnet and its corrosion inhibition mechanism were investigated in this work. Results indicated that CHC was an effective vapor phase inhibitor for NdFeB in the simulated atmospheric environments as well as under thin electrolyte layers, and effectively protected the Nd-rich phase also. CHC acted as an anodic inhibitor for NdFeB, and the inhibition efficiency attained a relatively high value. The inhibition of CHC was achieved mainly by physisorption on the NdFeB surface. Since the surface of Nd-Fe-B carried excess negative charges at free corrosion potential, protonated amine groups could be absorbed on the negatively charged electrode surface via electrostatic attraction. The high inhibition efficiency of CHC for NdFeB was attributed to the extremely large proportion of protonated amine groups existed in the electrolyte. |
