Study On Relationship Of Chemical Stability Properties With Composition And Microstructure For NdFeB Permanent Magnet

The scope of application by sintered neodymium-iron-born?S-NdFeB?is expanding with the further development of society and the scientific technological innovation,.But since the poor corrosion of S-NdFeB,it is limited its application in humid,high temperature and other complex environments.In this dissertation,the corrosion behavior and mechanism of different commercial S-NdFeB magnets in different environments were systematically studied,including neutral salt spray?NSS?corrosion and electrochemical corrosion in NaCl,NaOH and tap-water medium.The difference and process of the different magnets' corrosion in different environments were be compared.The electrochemical corrosion behavior of commercial S-NdFeB magnets in NaCl solution were studied in this dissertation by measurementing polarization curves and electrochemical impedance spectroscopy?EIS?and corrosion microstructures.The study found:The electrochemical corrosion process of S-NdFeB in NaCl solution is controlled by the mixing of cathodic reaction and anodic reaction.The cathodic reaction includes a hydrogen evolution corrosion reaction and an oxygen absorption corrosion reaction.The anodic reaction is mainly the process of active dissolution of neodymium-rich?Nd-rich?phase and Fe in the grain boundary.The corrosion product,Nd?OH?₃ and Fe?OH?2,mainly adsorbs or deposits the surface of the electrode with Nd?OH?₃ and Fe?OH?2,resulting in a passive state of the magnets.Corrosion product film covered by the magnet surface was destroried by the Cl-ions in the solution,and the corrosion reaction is accelerated.Obtained from the corrosion morphology,Nd-rich phase in grain boundary volume expands and cracks on the magnet surface.While the grain boundary phase of the magnet is dissolved,grains separation,the Nd2Fe14B grain and the other phase within the grain formation corrosion micro-cell,While the other phases are mainly distributed in grains or grain boundaries,anode dissolution occurred in these phase showing granular.So that pitting corrosion begins to occur in the grains of the Nd2Fe14B.The presence of corrosion micro-cells exacerbates the rate of corrosion.In aqueous NaCl solution in electrochemical corrosion,48SH grade corrosion current density is maximum,Maybe smaller of the grain boundary phase occupied area and anode dissolution phase area is smaller to get a tendency of increasing the corrosion current density.While the formation of less corrosion products can not effectively cover the entire magnet surface to hinder the further progress of the corrosion,so the corrosion rate is large.Other high-grade magnets are more resistant to corrosion,the element of Dy and Tb addition have a tendency to increase the corrosion resistance of magnets in NaCl solutions.In NSS,the corrosion products of magnets looking like the red-brown rust are mainly oxides by Fe and Nd.The surface along the grain boundary phase was network-like cracking,and the Nd-rich phase is dissolved.When the Nd-rich phase between the Nd2Fe14B grains is completely dissolved,the corners of the Nd2Fe14B grains begin to corrode,resulting in that the Nd2Fe14B grains are spherical.The electrochemical corrosion process of S-NdFeB in NaOH solution is controlled by the mixing of cathodic reaction and anodic reaction.The cathodic reaction includes a hydrogen evolution corrosion reaction and an oxygen absorption corrosion reaction,The anodic reaction is mainly the process of active dissolution of Nd-rich phase and the formation of Nd?OH?₃ passivation film.The electrochemical reaction of S-NdFeB in NaOH solution has a large charge transfer resistance,indicating a certain thickness and dense protective film has been formed to showing a passive state.The passivation film formed on the surface significantly reduced the rate of corrosion reaction.The electrochemical corrosion process of S-NdFeB in NaOH solution,35AH grade magnet the worst corrosion resistance.The addition of heavy rare earth elements such as Dy and Tb has a tendency to decrease the corrosion resistance of the magnet in NaOH solution.The electrochemical corrosion process of S-NdFeB in tap water is mainly controlled by anodic reaction.The Icorr of is high-grade high coercivity magnets too high,exhibiting a worse corrosion resistance.EIS in tap water is divided into high frequency capacity reactance arc,medium frequency capacity reactance arc and low-frequency Warburg impedance caused by diffusion.