| With excellent magnetic properties,in recent years,sintered Nd Fe B magnets have become the most popular rare earth permanent magnet materials,which have been widely used in electronics,medical,aerospace and other fields.However,the poor corrosion resistance strongly limits their application potential,thus improvement of Nd Fe B corrosion resistance is urgent at present stage.In this paper,cathodic electrochemical deposition of Ce O2films and magnetron sputtering of Al/Cr bilayer films were respectively carried out,and the relationship between the film preparation process parameters and the overall properties of the film was systematically studied.The main research work is as follows:(1)In electrodeposition,Ce O2 films were prepared by Chronopotentiometry Parameters of electrochemical workstation.First of all,through the control of current size,electrolyte Ce(NO3)3solution concentration,electrolyte solution and additive CH3COONH4 solution volume ratio,deposition time and other four deposition parameters,a complete Ce O2 film was prepared on Nd Fe B substrate.Next,the whole electroplating environment was heated up,and the temperature contrast experiment was designed to prepare Ce O2 film under different temperature conditions on Nd Fe B substrate surface.The results of contact angle test and electrochemical polarization corrosion test show that the film has superhydrophobic properties when the electroplating temperature is 75?,and the contact angle is 163.9°.When the electroplating temperature is 60?,the sample has the best corrosion resistance,the corrosion current density is 4.01×10-7 A/cm2,which is about 2 order of magnitude lower than that of bare Nd Fe B,and the peak electrochemical impedance is 1200?,which is 700?higher than that of Nd Fe B magnet.The corrosion growth mechanism of the coated samples was analyzed by impregnation experiment.With the corrosion reaction,hydrogen evolution reaction occurred on the surface of the substrate,and the release of H2enlarged the defects on the surface of the film,resulting in corrosion of the substrate.Finally,the influence of electrolyte solution concentration and volume ratio of electrolyte solution to additive on electroplating experiment was explored at a constant temperature of 60?.The results of SEM,XRD and electrochemical corrosion test show that the density and corrosion resistance of Ce O2film prepared on Nd Fe B surface are relatively optimal when the concentration of Electrolyte Ce(NO3)3 solution is 0.025 M and the volume ratio of electrolyte solution to additive is 100:1.(2)In terms of magnetron sputtering,DC(direct current)magnetron sputtering was used to prepare Al/Cr double-layer film on the surface of sintered Nd Fe B.Among them,Al film acts as an intermediate layer to strengthen the bonding force between the substrate and Cr layer,and Cr film plays a leading role in the overall corrosion protection.In this experiment,Al/Cr bilayer films with better corrosion resistance were prepared by adjusting the sputtering power of Cr films.The experimental results show that when the sputtering power of Cr film is 40 W,the corrosion resistance of the sample film reaches the best,the corrosion current density is 6.17×10-8 A/cm2,which is about 3 orders of magnitude lower than bare Nd Fe B,and the peak electrochemical impedance is 1300?,which is 800?higher than bare Nd Fe B.The corrosion growth mechanism of the coated samples was analyzed by in-situ impregnation corrosion experiment.With the corrosion reaction,the substrate surface passivation reaction occurred first,and then hydrogen evolution reaction occurred on the substrate surface,and the defects on the surface of the film expanded,resulting in the corrosion of the substrate.By nano indentation test and magnetic properties test,the hardness and elastic modulus of the coated sample are 2.1 GPa and 39.5 GPa higher than bare Nd Fe B,and the coercivity is increased by about 2.5 k Oe.XPS test results show that the outermost layer of Cr film exposed to air is easy to oxidize into Cr2O3,providing a good corrosion protection effect for Nd Fe B substrate. |
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