Corrosion-resistant Treatment Process And Film Properties Of NdFeB Magnetic Materials

Silanization has been widely used in the field of surface protection due to its advantages of green and environmental protection.To cope with the poor corrosion resistance of sintered NdFeB magnets,silane films were prepared on the surface of magnets by using dip-coating technique.The silanization process was explored,and the corrosion resistance of silane film was further improved by doping modification.The hydrolysis and film forming process of KH550 and BTMSE silane films were optimized by the single factor test and orthogonal test.The optimum process of KH550 was determined by the conductivity tests and polarization curve tests and showed that the best ratio of silane: ethanol: deionized water,pH,hydrolysis time,dipping time,curing temperature and curing time were 10:35:65,pH=9,8 h,2 min,140 ℃ and 40 min,respectively.Similarly,the optimum process of BTMSE was 7:70:30,pH=4,24 h,2 min,140 ℃ and 40 min,respectively,compared with the results of KH550.The corrosion resistance of silane films prepared under the optimum conditions was studied by SEM,EDS,polarization curve,electrochemical test,copper sulfate drop,contact Angle test and Fourier transform infrared spectroscopy(FT-IR)and other testing methods.The results showed that the silane films prepared by KH550 and BTMSE were both uniformly coated on the surface of the magnets and the protective coatings were obtained.The contact Angle tests showed that the hydrophobic BTMSE-based silane film had stronger protective ability to the magnet than the hydrophilic KH550-based silane film.The both silanization-magnets showed much better corrosion-resistance than the original magnet during the salt water immersion tests.More specifically,the less corrosion products on the surface of the BTMSE-based silane film mean stronger barrier effect on the corrosive medium,resulting much weaker corrosion.The KH550-based silane films were doped with rare-earth salt R or titanium salt T to improve the corrosion resistance.The polarization curve results indicated that the doping concentration of rare-earth salt R affected the corrosion resistance of the composite silane film.Through a single factor test,the optimal doping amount of rare-earth salt R was 6 g/L.The contact Angle of the R-doped silane film prepared under the optimal conditions increased from74.6° to 92.8°,and the film appeared to be hydrophobic.SEM images indicated that many nanoparticles were uniformly dispersed on the surface of the film and EDS spectra proved that there were abundant Si and Ce elements in the KH550-based silane film.There were no characteristic absorption peaks of rare-earth salt R in the FT-IR,indicating that rare-earth salt R was only doped in the silane film by physical filling.But rare-earth salt R could promote the hydrolysis of silane and the condensation between silanol,so the composite silane film was denser and the degree of cross-linking was also increased.As a result,a more uniform silane film with higher roughness was obtained.The polarization curve results showed that the self-corrosion current density of the R-doped silane film decreased about two orders of magnitude,compared with the original silane film.Meanwhile,the much better results during the salt water immersion and copper sulfate dripping experiments indicated that the corrosion resistance of the magnet was dramatically enhanced by the R-doped silane film.SEM images showed that many particles were uniformly distributed on the surface of the titanium salt T-doped silane film and EDS spectra proved that the major composition elements of the film were Si,Ti,F and O.As shown in the FT-IR of the Ti-doped silane film,the Ti-O-Si bond was found,indicating that the titanium salt T was involved into the silane film formation process,and the density of silane film was greatly improved.As a result,the infiltration of corrosive media was effectively prevented by the dense silane film and the corrosion resistance of the modified magnet was improved,as proved by the results of the electrochemical tests and copper sulfate dripping experiments.