Preparation And Properties Of Surface Functional Film Of NdfeB Permanent Magnet

As having high magnetic energy and high ratio of performance to price,NdFeB permanent magnetic materials have used widespreadly in aviation,aerospace,maglev trains and magnetic medical technology.In China,rare earth resources proven reserves accounted for about 80%of the world’s total reserves,in which the rare earth permanent magnet field competition is advantage.However,the corrosion resistance of NdFeB permanent magnetic materials is poor.The selective corrosion happens easily along the grain boundary in magnet because of its special structure,and the NdFeB permanent magnetic materials need to be protected when used.In this work,the chemical and electrochemical methods were used to obtain phosphating film.The effect on the different process,parameters REE and sodium tungstate treatment on the corrosion resistance were discussed.The influence of porosity and morphology of phosphating coating were also studied in the work.Chemical phosphating formula of chemical phosphating is as follow:Zn（NO₃）₃ 70 g/L,Zn（H₂PO₄）₂·2H₂O 90 g/L,tartaric acid 1g/L,triethanolamine2g/L.Through single factor test,the time of chemical phosphating,and temperature and pH were analyzed.According to the 3D laser microscope measurement scanning electron microscope（SEM）,energy spectrum（EDS）,electrochemical test,copper sulfate intravenous drip,and the influence of the porosity.When phosphating time is 3 min,temperature is70℃,pH=2.0,the phosphating film full crystal,copper sulfate intravenous drip experiment the longest and best corrosion resistance.When nitrate cerium adding quantity of 1 g/L,the phosphating film was full crystal,best corrosion resistance.And the phosphating film is more meticulous,better corrosion resistance,lower porosity after adding rare earth elements.The compositions of electrochemical phosphating are as follows:Zn（NO₃）₃ 70 g/L,Zn（H₂PO₄）₂·2H₂O 90 g/L.Through single factor test,the current density of Electrochemical phosphating,time,temperature and pH value were analyzed.According to the measure methods,which consisted of 3D laser microscope measurement,scanning electron microscope（SEM）,energy spectrum（EDS）,electrochemical test,copper sulfate intravenous drip and the influence of the porosity,it was shown that when phosphating time is 3 min,temperature is 70℃,pH=2.0,the phosphating film had rystalline integrity,and it can obtained the the longest time for copper sulfate intravenous drip experiment,and the the best corrosion resistance can obtained.the phosphating film is more meticulous,better corrosion resistance and lower porosity after adding rare earth elements.Comparing NdFeB chemical phosphating with electrochemical phosphating,it was shown that the crystallization of electrochemical phosphating coating was bigger than that of chemical phosphating.The growth pattern of the crystallization for chemical phosphate was point growth,while the electrochemical phosphating was line growth.The corrosion resistance of electrochemical phosphating coatingwas better than that of electrochemical phosphating,and the porosity of electrochemical phosphating coating was lower.Both the chemical phosphating coating and the electrochemical phosphating coating main ingredients were Zn₃（PO₄）₂（H₂O）₄.Comparing with REE chemical phosphating and REE electrochemical phosphating,the rsults showed that the the REE electrochemical phosphating had bigger crystallization of phosphating,better corrosion resistance and lower porosity.From the mesurement results,it was shown that the Na₂WO₄ happened hydrolysis reaction when the phosphating film was treated by Na₂WO₄.It can be explained that it would become polymerization to adsorb on the phosphating film surface and pore,thus to protect the phosphating film,and leading to the better corrosion resistance,lower porosity.