The Study On The Corrosion Resistance Of (Pr,Nd,MM)-Fe-B Magnets Prepared By Various Processes

Nd-Fe-B magnets have been widely applied in various fields due to their excellent magnetic properties,such as hybrid vehicles,wind power generation and magnetic separators etc.,since they were discovered.It causes the demand of the closely-relied Pr and Nd metals increasing.However,the abundant La and Ce that are associated with Pr and Nd in rare earth ore are seriously overstock.In order to balance the comprehensive utilization of rare earth resources and reduce the environmental pollution originating from the rare earth separation process.Therefore,in this thesis,misch metal（MM for short）from Bayan Ebo ore are used as the main raw material,and single main phase and double main phase processes are applied to prepare RE-Fe-B magnets with different MM contents.Moreover,the magnetic properties and microstructure of the contained MM RE-Fe-B magnets（RE is the general term of rare earths）are analyzed,and the corrosion resistance of these magnets under different environments is compared to reveal the mechanism.Magnets with nominal composition of[（Pr,Nd）_1-x,MM_x]_30.3（Fe,Co）_balM_0.73B_0.98（wt.%,x=0,0.1,0.3,0.5,0.7,and 0.9）are prepared by single main phase（SMP）and double main phase（DMP）processes to investigate the variation of their magnetic properties.It was found that the magnetic properties of the two-type magnets with the same composition decrease with increasing MM content.However,for x=0.5,the RE-Fe-B magnets remains possessing relative high magnetic properties,and the magnetic properties of the magnets prepared by the single main phase process are B_r=12.93 k Gs,H_cj=9.383 k Oe,（BH）_max=39.70 MGOe,while the magnetic properties of the magnets prepared by the double main phase process are B_r=13.25k Gs,H_cj=6.941 k Oe,（BH）_max=37.87 MGOe.Furthermore,the comparision of the magnetic performance for SMP and DMP magnets is performed.As a result,the remanence of the DMP magnets is higher than that of the SMP magnets,but the coercivity is more lower.Compared to the single main phase process,the remanence enhancement is contributed to the stronger exchange coupling between rich-La Ce grains and lean-La Ce grains.And the coercivity decrement may be as a result of more lower the anisotropy in the third main phese originating from the diffusion between rich-La Ce grains and lean-La Ce grains,where are easy to form the reversal magnetic domain.For practical applications,the weight loss behavior of[（Pr,Nd）_1-x,MM_x]_30.3（Fe,Co）_balM_0.73B_0.98（x=0,0.1,0.3,0.5,0.7 and 0.9）magnets prepared by SMP and DMP processes is compared under120°C,2 bar pressure and 100%relative humidity atmosphere.When the time is placed in hot and humid environment for the two-type magnets at 24 h and 48 h,for x=0.1,0.3 and 0.5,the mass loss rate is between 0.13 mg/cm²and 0.16 mg/cm²,which are close to the value of 0.14 mg/cm²for free-MM magnets.But when x>0.5,for the two-type magnets fabricated by SMP and DMP processes,the value of mass loss rate comes to between0.31mg/cm²and 0.35 mg/cm².In addition,the corrosion resistance of DMP magnets is more superior to that of SMP,due to the separation of La and Ce elements with relative high chemical activity in the grain boundary triple junctions.Comparing the electrochemical behavior of MM-contained[（Pr,Nd）_1-x,MM_x]_30.3（Fe,Co）_balM_0.73B_0.98（x=0,0.1,0.3,0.5,0.7 and 0.9）magnets prepared by SMP and DMP processes in 3.5wt.%Na Cl solution.The E_corrfor DMP magnets is higher than that for SMP magnets,but I_corris diametrically opposite.Because the improved distribution of the rich-RE grain boundary acting as the anode compared to the main phase as the cathode makes the corrosion channel and driving force of corrosion decreased.Therefore,magnets with different Misch metal contents can be prepared by different preparation processes of single and double main phases,which can fill the blank area between ferrite materials and traditional sintered Nd-Fe-B permanent magnet materials.At the same time,in the same Misch metal content of magnets,double main phase magnets have higher corrosion resistance,which can be used in more complex environments.