Study On The Corrosion Characteristics And Protection Of Nd-fe-b Sintered Magnets

Nd-Fe-B alloy is widely used in electromotor industry and apparatus industry etc.as the third era permanent magnet.It has the advantage of distinguished magnetic,properties and comparatively low price.But it is prone to be oxidized in common atmosphere,especially be heavily electrochemical eroded at humidity environment.Corrsion becomes the obstruction to broaden applications of Nd-Fe-B.In this thesis,the corrosion characters of Nd-Fe-B sintered magnets in the common medium of acid-base solutions has been investigated. Phosphating coating and Cu/Ni composite electroplating coating on the surface of NdFeB sintered magnets were employed to enhance the corrosion resistances,based on the corrosion mechanism and anticorrosion treatment research of the papers in recent years.Various analytical instruments and methods such as scanning electron microscopy, electrochemical methods,the corrosion weight-loss method,energy dispersive spectrometer,X-ray diffraction and neutral salt spray test were used in this study.The corrosion mechanism of Nd-Fe-B sintered magnets in common medium of acid and alkali,formation process and mechanism of phosphating film were studied systematically.The effects of electroplating system,electroplating methods and electroplating layer structure on corrosion resistance of Nd-Fe-B sintered magnets were further investigated.The results indicated that corrosion of Nd-Fe-B sintered magnets without remanence state in hydrochloric and sulphuric acid was most serious among all acid,and Nd-Fe-B sintered magnets was passivated in phosphoric and oxalic acid.Every surface of magnet corroded by above acid became coarse.Because of the effect of clean the surface better and the corrosion rate moderate,nitric acid was most suitable for Nd-Fe-B sintered magnets pre-rust agent.The electrochemical reaction mechanism of Nd-Fe-B sintered magnets in different solutions was not changed by the remanence state.But corrosion of the magnets was accelerated in H₂SO₄ and NaCl solutions,owing to the formation of an occluded corrosion cell with self-catalyzing effect.Contrarily,Nd-Fe-B sintered magnets were passivated and inhibited corrosion in H₃PO₄ and NaOH solutions.Remanence state changed the electric double layer structure on the magnet/electrolyte interface,which was responsible for the production of the magneto overpotential and the change of electrochemical corrosion of Nd-Fe-B sintered magnets.Direct Mid-temperature phosphating treatment the surface of Nd-Fe-B sintered magnets,the results showed that a significant increase in the corrosion resistance of the magnet due to the phosphate film which cover with the magnet surface and block the pores.The phosphate film coating on the crystalline intergranular phases and main crystal phases was different in element composition:the crystalline intergranular phases include more NdPO₄,and has larger thickness.HR pre-pulse copper electroplating process for the optimal level was 15%duty cycle,5000HZ pulse frequency.The electroplating copper coating-obtained atthis process had better corrosion resistance and high (111) crystal plane preferred orientation.Because of the multiple anodic expense effect and tile better cover each other performance,the corrosion resistance of Nd-Fe-B sintered magnets which coated with Pulse Cu / dark Ni / DC Cu / dark Ni / bright Ni electroplating coating achieved the best:the self-corrosion voltage was improved nearly 1200mV,neutral salt spray corrosion resistance time reached to 386 hours.