Technology And Properties Of Electrophoretic Coatings On Sintered NdFeB Permanent Magnets

Sintered NdFeB permanent magnetic material with high magnetic properties has dominated the world of permanent magnet.But its poor corrosion resistance limits its application to further expand.The epoxy coating prepared by cathode electrophoresis with great corrosion resistance and little environmental harm has become a important and effective method for surface protection of sintered NdFeB magnets.In this paper,the process of cathodic electrophoresis mode and the properties of epoxy resin coatings was studied and optimized,and the carbon nanotube/epoxy resin composite coating was prepared and performed a better corrosion resistance and mechanical properties than conventional epoxy resin coatings.The main summary was obtained as followings:The temperature of bath solution and the electrophoretic voltage and the time affected the quality of the epoxy coatings directly.As the temperature of bath solution increased,the thickness,the corrosion resistance and the adhesion of the coating were improved.But a too high temperature would make the coating rough and instable.The compactness of the coating could be improved by increasing the electrophoretic voltage,which also improved the corrosion resistance and the adhesion of the coating.But a too high voltage would decrease the compactness and increase the porosity.Otherwise sufficient electrophoretic time was beneficial to obtain a more uniform and dense coating.But a too long electrophoretic time might lead to the redissolution of the coating and made the coating rough,which reduced the strength of the coating itself.So electrophoresis for 120 s under 30 ? and 120 V would obtain the epoxy resin coatings with a higher quality which performed a corrosion current density of 1.041×10-8A/cm,600 h for neutral salt spray test and 21.9 MPa bonding strength with the substrate.Adding 0.4 g/L SDBS into the epoxy bath solution with 90 min ultrasonic time proved to be the most available and effective for a smooth,compact and uniform carbon nanotube/epoxy resin composite coating.The corrosion rate of the composite coating decreased by two orders of magnitude in the 3.5 wt.%NaCl solution compared to that of conventional epoxy coatings,which means that a better corrosion resistance was obtained.The bonding strength between the composite coating and the substrate improved to 23.0 MPa.The carbon nanotubes greatly improved the hardness of the coating.The pencil hardness of the composite coating performed 3 more levels than that of the conventional coating and reached to the maximum—5H by adding 4%carbon nanotubes into the epoxy coating.Both the conventional coating and the carbon nanotube/epoxy resin coating modified the surface of sintered NdFeB magnets well and improved the corrosion resistance of the magnets effectively without influence on the magnetic properties of the magnets.And the carbon nanotube/epoxy resin coating performed a better corrosion resistance and mechanical properties than the convential coating,which provided a broader application prospect for sintered NdFeB permanent magnets.