Study On Densification And Corrosion Resistance Of Metal Coatings On Sintered NdFeB Magnets

Sintered NdFeB permanent magnetic material has outstanding magnetic properties,including excellent coercivity,magnetic energy product and other properties.This material has been widely used in various fields,such as electronics,acoustics,communications,automation,magnetic resonance imaging and biomedical applications.However,the multiple phases in the matrix introduce poor corrosion resistance and thermal stability,further limiting the loading conditions.The surface protection technology,which effectively enlarges the barrier between the corrosion medium and magnet,obviously improves the corrosion resistance.Electroplating,chemical plating and vacuum evaporation have been used to introduce the metal coatings on NdFeB magnets to improve the protective effects.Whereas,the density of the magnets is not good as well.Therefore,densificate coatings have to be made to improve their protection effects.In this paper,high energy ball milling and gas phosphating technologies were used to densify the Al and Ni metal coatings,and further improving the corrosion resistance.The research results are shown as follows:(1)The Al coated NdFeB magnets,obtained by vacuum evaporation technique,were densified by high energy ball milling method to study the corrosion resistance of densified Al coating.The composition,microstructure and surface morphology of the coatings were characterized by X-ray photoelectron spectrometer,X-ray diffractometer and scanning electron microscope,respectively.The bonding force between the NdFeB matrix and the coating was measured by tensile testing machine.Potentiodynamic polarization curves,neutral salt spray test and static immersion corrosion test were used to characterize the corrosion resistance of samples,which can analyze the effects of different milling times on the microstructure and corrosion resistance of Al/NdFeB.Meanwhile,the anticorrosion mechanism was deeply studied.The results show that Al coating obviously densificated due to the ball milling treatment.The binding force and corrosion resistance of the coating are improved as well.The binding force of optimized Al-D30/NdFeB reaches 28.46 MPa,and the time of salt spray resistance increases from 72 to 144 h.(2)The Ni coated NdFeB magnets were densified by high energy ball milling method.The composition,microstructure and surface morphology of the coatings were characterized by X-ray photoelectron spectrometer,X-ray diffractometer and scanning electron microscope,respectively.The mechanical properties were tested by tensile testing machine and microhardness tester.Potentiodynamic polarization curves and neutral salt spray tests were used to analyze the corrosion resistance of samples to study the effects of different ball milling times on the microstructure and corrosion resistance of Ni/NdFeB magnets.Besides,the corrosion process of the magnet was analyzed by static immersion corrosion test to study the corrosion resistance mechanism.According to the results,the cellular grains preferentially grown on the surface of Ni coating,which tends to be smooth and dense under the impact of high energy abrasive particles,leading to the obvious densification of the Ni coating.Therefore,the coating achieves better adhesion and corrosion resistance.As the result,the salt spray resistance time of NiD24/NdFeB magnets increase from 312 h to 480 h,which shows the best corrosion resistance.(3)Ni coated NdFeB magnets were phosphated by thermal decomposition method with NaH2PO2·H2O as phosphorus source.The composition,microstructure and surface morphology of the coatings were characterized by X-ray photoelectron spectrometer,Xray diffractometer and scanning electron microscope,respectively.Potentiodynamic polarization curves,Nyquist plots and neutral salt spray tests were used to investigate the corrosion resistance of NdFeB magnets,which study the effects of different phosphating temperatures on the microstructure and corrosion resistance of the samples.Besides,the corrosion process of the magnet was analyzed by static immersion corrosion test to study the corrosion resistance mechanism.The results show that at high temperature,a dense phosphate film can be obtained on the surface of Ni coating,which can close the grain gap and enhance the densification of the coating.Meanwhile,coating was improved by prolonging the immersion channel of the corrosion medium.Compared with Ni/NdFeB,the salt spray resistance time of optimized Ni(P)-300/NdFeB increases from 312 h to 648 h.