Novel Eco-friendly Al-based A Nticorrosive Coating For Rare Earth Permanent Magnet And Its Corrosion Resistance Mechanism

The sintered NdFeB rare earth permanent magnets are important magnetic functional material and have been widely used in many fields due to their high remane nce,high coercivity and high magnetic energy product.However,the poor corrosion resistance of NdFeB rare earth permanent magnet materials severely limits its application.At present,electroplating anti-corrosion technology is the commercial method for improving the anti-corrosion performance of NdFeB magnets.However,the corrosion resistance provided by this method still cannot meet the requirements of the applications.Moreover,the waste liquid generated by electroplating pollutes the environment and harms to human health.Therefore,a new anti-corrosion technology has become an important issue in this field.In view of the above background,a new type of Al-based anti-corrosion coatingis proposed in this research,which is obtained by coating,curing and sintering processes.The preparation of anti-corrosion coating is environmentally friendly,and the small amount of stabilizer chromic anhydride in the raw material could be fully converted to harmless Cr3+ at high temperature.For the preparation of the anti-corrosion coating,magnesium dihydrogen phosphate was introduced as a binder.The introduced phosphate would form a macro molecular network structure at a high temperature of 500?,and a close anticorrosive coating can be obtained by combining with the Al powder.This study proposes a new path for improving the corrosion resistance of rare earth permanent magnets and developing new green environmental protection coatings.In this study,the formulation of the new coating was designedon the basis of experiments,and the Al-based anti-corrosion coating was successfully prepared.The coating as prepared was resistant to neutral salt spray(NSS)for 500 h and high temperature aging test(PCT)for 240 h.The corrosion resistance far exceeds the existing plating coating.The paper has studied the effect of preparation process on coating properties.The results show that the particle size of aluminum powder and the coating processes have significant influence on the coating properties.In the condition of the particle size of aluminum powder ?10?m,the coating obtained by secondary spraying process performs a better corrosion resistance.The anti-corrosion performance and electrochemical performance of the coatings prepared at different curing temperatures and different sintering temperatures were compared.It was found that the anti-corrosion performance increased first and then decreased with the increase of curing temperature,and the effect of sintering temperature on corrosion resistance also displays the same trend.In this experiment,the coating prepared by curing temperature of 320?,the sintering of 500? for 1 h exhibits the best corrosion resistance.The corrosion process and corrosion resistance mechanism of the coating were studied via structure and microscopic analysis.It is well kwon that there are pinholes on the surface of the coating due to the large number of pores in the NdFeB magnet matrix,which are the major defects in the coating and prone to corrosion here.In view of the shortcomings of the existing coatings,the preparation process of the coating is optimized.A sealing process was employed.The sintered coating is sealed,and the pinholes on the surface of the coating are blocked to obtain better corrosion resistance.After the sealing treatment,the final Al-based coating achieved NSS and PCT resistance of 600 h and 300 h,respectively,far exceeding the NSS(72 h)and PCT(24 h)time of conventional electroplating coatings.Therefore,it is expected to replace the traditional NdFeB anti-corrosion plating process.