| The sintered NdFeB magnets were the third generation rare earth permanent magnet materials.And the mangets has a high energy storage density and can realize the storage,conversion,transmission and detection of information and energy which greatly promoted the development of integration,lightweight and miniaturization of production equipment.However,due to its unique phase structure and preparation process,the corrosion resistance of the sintered magnets is very weak,and the corrosion of the magnets will result in the sharp deterioration of magnetic properties and the increase of operating cost of magnets.In this paper,pulse current（PC）electroplating technology was used to prepare mental and nano-mental coatings on the magnets.The surface quality of the coating was optimized with the regulation of pulse parameters to improve the corrosion resistance of the magnets.Compared with direct current（DC）,the PC electroplating can significantly improve the surface quality of the coatings.At present,the internal relationship between PC parameters,properties and microstructure are not comprehensive,and study on the corrosion mechanism of coatings is also valuable.Secondly,the variety of nanoparticles and the concentration on the corrosion resistance of composite coatings were studied.The corrosion resistance of the composite coatings increase significantly,and high-performance composite coatings preparation and its corrosion mechanism research has great research value.（1）Firstly,the changing rule betweenthe the pulse parameters and corrosion resistance of Ni coatings were obtained by adjusting the duty cycle,frequency and current density.The influence of parameters on the surface quality and corrosion resistance of Ni coatings were studied combining with the microstructure and structure characterization.When the duty cycle is 60%,the frequency is 1000 Hz,and the current density is 6 A·dm-2,the the Ni coatings obtain the best corrosion resistance.（2）Scendly,the thickness of the coating was adjusted by change coating time.The effect of coatings’thickness on corrosion resistance under different coating time was analyzed:the corrosion resistance of the coating was slightly improved with the increase of the coating time,the magnetic property of the magnets had a large decreases after coating,which was mainly due to the magnetic shielding effect of the Ni coatings.Therefore,the appropriate coating time was determined to ensure the stabler corrosion resistance and the smaller magnetic property loss of the Ni coatings.（3）The strengthening mechanism of corrosion resistance of Ni coating by PC was analyzed by electrochemistry property test,microstructure characterization and theoretical calculation.It was found that pulse current can make the Ni coatings grow preferentially on（111）crystal plane and increase the density of the coatings.In addition,the diffusion energy barrier of O on different crystal planes were calculated by first-principles calculation.It was found that the diffusion of O on（111）crystal plane was more likely to result in higher oxygen concentration on（111）,the（111）and（100）crystal planes were used as cathode and anode respectively in the micro-electrochemical corrosion process.The number of anodes in the Ni coatings were reduced,and the electrochemical corrosion spots were reduced,the Ni coatings by PC had better corrosion resistance.（4）Finally,Ni-Y2O3 and Ni-graphene composite coatings were successfully prepared.The effects of different kinds and concentrations of nanoparticles on the corrosion resistance of the composite coatings were studied,and the influence of nano-particles doping on the quality of the composite coatings and the strengthening mechanism were further investigated.When the concentration of nano-Y2O3/graphene are 0.60 g/L and 0.05 g/L,respectively,the Ni-Y2O3and Ni-graphene composite coatings got the best corrosion resistance.However,with the increase of the concentration of nano-particles in the coatings,the high surface activity nano-particles agglomerate leads to the abnormal growth of grains and poor corrosion resistance of the composite coatings. |
