| With the continuous development of advanced technologies such as communication,automobile and medical equipment,higher requirements have been put forward for the magnetic properties and corrosion resistance of sintered NdFeB magnets.Traditional NdFeB magnet surface plating protection technology due to plating solution difficult to handle,environmental protection pressure,it is in urgent need of economic,efficient,green surface protection technology to replace.More and more attention has been paid to the preparation of aluminum coatings with excellent corrosion resistance on the surface of NdFeB magnets by spraying technology.In this paper,aluminum protective coating was prepared on the surface of NdFeB magnets by flame spraying technology.Four kinds of pretreatment processes for surface spraying of NdFeB magnets,namely polishing,sandblasting,pickling and sandblasting+pickling,were compared and studied.The process conditions of preheating temperature and spraying distance were optimized.The phase structure,morphology and element distribution of the coating were analyzed by means of X-ray diffraction（XRD）,scanning electron microscope（SEM）and electron energy spectrometer（EDS）.The corrosion performance of NdFeB magnets and aluminum protective coating in different environments was investigated by salt spray test and acid and alkali immersion test.The polarization curves of NdFeB magnets and aluminum protective coatings in corrosion environment were tested by an electrochemical workstation.The main research results are as follows:The results show that polishing,sandblasting,acid pickling and sandblasting+acid pickling all affect the surface roughness of sintered NdFeB magnet and the bonding strength between magnet and aluminum coating.The surface roughness of NdFeB magnet was 12.84μm,20.36μm,24.01μm and 28.34μm after polishing,sandblasting,pickling and sandblasting+pickling,respectively.The bonding strength of NdFeB magnet and aluminum coating was 4.9 MPa,6.83 MPa,9.79 MPa and 11.2 MPa.The higher the surface roughness of the magnet,the higher the bonding strength of the magnet and the aluminum coating.After polishing,the surface of sintered NdFeB magnet is smooth but has obvious pits;after sand blasting,the surface of magnet is uneven and broken;after acid pickling,the surface of magnet is regular and neat with small pores;after sand blasting+acid pickling,the surface flatness of magnet is significantly improved without obvious pores.The study of sintering spray technology shows that the aluminum coating on NdFeB magnet is composed of face centered cubic structure single al phase,and there are a few pores on the surface of the coating.With the increase of preheating temperature and spraying distance,the porosity of aluminum coating decreases first and then increases.According to the porosity of aluminum coating,the process conditions of coating preparation were optimized as follows:preheating temperature 100°C,spraying distance150 mm.In this process,the thickness of aluminum coating on NdFeB magnet is moderate,the surface is flat,the bonding strength of NdFeB magnet and aluminum coating is the highest,which is 12.8 MPa,the hardness of coating is 53.24 HV,and the porosity is the lowest,which is 1.1%.The corrosion test of NdFeB magnet in 5%Na Cl neutral solution showed that corrosion rust appeared on the surface of NdFeB magnet after 24 hours,and the surface of NdFeB magnet was completely corroded after 48 hours;however,no obvious rust was found on the surface of NdFeB magnet after 120 hours of continuous salt spray corrosion.After 30 minutes of immersion corrosion in 3%HNO3 solution,the weight loss of NdFeB magnets with aluminum coating was 0.09%and 4%respectively compared with that without coating;after 30 minutes of immersion corrosion in 3%Na OH solution,the weight loss of NdFeB magnets with aluminum coating was 0.037%and 0.013%respectively compared with that without coating.The corrosion voltage of NdFeB magnet is-0.8208V,magnet after aluminum coating is-0.7710 V,and the corrosion current density is 6.71×10-6A·cm-2.The corrosion voltage is shifted to 0.0498V,and the corrosion current density isAfter spraying aluminum coating on the sintered NdFeB magnet,the remanence and magnetic energy product of the magnet decreased by 0.3%and 0.12%respectively,and the coercive force decreased by 0.28%,but the corrosion resistance of the magnet surface was significantly improved.The preparation of aluminum coating on the surface of sintered NdFeB magnet by thermal spraying technology can be used as a reserve technology of magnet surface protection,which has a prospect of industrial application. |
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