| As the third-generation rare earth permanent magnet materials,sintered NdFeB magnets have been widely used in various fields due to their high magnetic properties.However,the polyphase structure and large electrochemical difference between phases,result in the electrochemical corrosion,which affects the service life of NdFeB magnets and limits the further development.Therefore,how to improve the corrosion resistance of sintered NdFeB magnets has become the focus of attention in this field.Metallic coating is widely used in the field of surface protection of NdFeB magnets.However,Zn,Al and other active metals will react with water and oxygen when exposed in the air,which limits the protection ability.The chromates,including hexavalent chromium and trivalent chromium,are mostly used to form a passivation layer on the galvanized layer surface due to their simple operation,low production cost and high corrosion resistance.However,the aerosol and the wastewater generated during the passivation process possess serious harm to the organism and the environment.Therefore,it is of considerable significance to develop new environmentally friendly chromium-free passivation processes.In this paper,the green chromium-free passivation of metallic coatings on the surface of sintered NdFeB magnets was carried out.And the effects of different passivation processes on the surface composition,structure and corrosion resistance of the metallic coatings were analyzed.The mechanism of passivation layer to improve the corrosion resistance of metallic coatings was studied.The main research is as follows:1.A phosphating passivation is conducted on the surface of Al coated NdFeB magnets by a gaseous process,and the formation mechanism and corrosion resistance of the passivated layer were studied.The surface microstructure,structure and morphology of the samples were characterized by X-ray photoelectron spectroscopy,X-ray diffraction and scanning electron microscopy.The anti-corrosion performance and anti-corrosion mechanism of the samples were analyzed by neutral salt spray test and weight gain test.The corrosion rate of the sample was analyzed by potentiometric polarization curve and electrochemical impedance spectroscopy measurements.The corrosion process and corrosion mechanism of the samples were studied by analyzing the coating morphologies,potentiodynamic polarization curves and surface elemental compositions during the immersion test in 3.5 mass% Na Cl solution for different time.The results show that the corrosion resistance of Al coated NdFeB can be improved effectively after phosphating passivation by gas phase phosphating method.The optimized sample obtained with phosphating temperature at 300? achieves the improvement of NSS time from 96 h to288 h.The synergistic effect of phosphating passivation and high temperature densification leads to the improvement of anti-corrosion performance.2.Cerium passivation is conducted on Zn coated NdFeB magnets by liquid phase method and the effect of cerium concentration on the corrosion resistance of Zn/NdFeB magnets was studied.Compositions,microstructures and morphologies of different passivated coatings were studied by X-ray photoelectron spectroscopy,X-ray diffraction and scanning electron microscopy,respectively.The formation mechanism of cerium passivation layer was analyzed and the corrosion behavior was evaluated by potentiometric polarization curve,electrochemical impedance,neutral salt spray test and weight gain test.The results show that the cerium passivation is mainly composed of oxide and hydroxide of Ce,which can obviously enhance the corrosion resistance of Zn/NdFeB magnets.The optimized Zn(Ce-5)/NdFeB magnets can achieve the longest neutral spray test time of 360 h,which is 144 h longer than that of the pristine sample.This passivation layer enhances the corrosion resistance of Zn/NdFeB magnets.3.The anti-corrosion performances of cerium passivation and trivalent chromate passivation on Zn coated NdFeB magnets was compared and the corrosion mechanism was studied.Composition,morphology and microstructure were characterized by X-ray photoelectron spectroscopy,scanning electron microscopy and X-ray diffraction,respectively.The corrosion resistance was investigated by polarization curve,electrochemical impedance spectroscopy,neutral salt spray test and weight gain test.The corrosion process and corrosion mechanism of the samples were studied by analyzing the surface morphology,composition and potentiodynamic polarization curves after neutral salt spray test for different time.The results show that the corrosion performance of the cerium passivation(432 h)is slightly weaker than that of the trivalent chromate passivation(528 h),much higher than that of the unpassivated Zn/NdFeB magnets.4.A silane-cerium composite passivation was conducted on the surface of Zn coated NdFeB magnets by chemical reaction method and the effect of silane content on the microstructure of passivation layer was analyzed.Composition,morphology and microstructure of the passivated Zn/NdFeB magnets were characterized by X-ray photoelectron spectroscopy,scanning electron microscopy and X-ray diffraction,respectively.The corrosion resistance was investigated by polarization curve,electrochemical impedance spectroscopy,neutral salt spray test and weight gain test.The corrosion process and corrosion mechanism of the samples were studied by analyzing the surface morphology,composition and potentiodynamic polarization curves after neutral salt spray test for different time.The results show that the composite passivation film with high concentration of Ce can be formed by adding an appropriate amount of silane,and the neutral salt spray test time can reach 840 h,which is 2 times of that of cerium passivation,indicating the further improvement of the magnets' corrosion resistance. |
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