Effect of Magnetic Field on Properties of Electrodeposited Ni Coating on NdFeB Surface
NdFeB is widely applied in many fields such as new energy vehicles,domestic appliances,electronics and so forth.However,the corrosion of NdFeB limits its service life in these fields.Thus,surface treatments are necessary for NdFeB,among which electrodeposition is one of the most useful techniques due to its simple process,convenient operation and low cost.In recent years,the technology of magnetic field electrodeposition(MFE)develops rapidly.Scientists find that the existence of magnetic field exhibits large effect on electrodeposition coatings.However,the MFE is rarely applied on NdFeB.Herein,the MFE technique was adopted to deposit a metallic Ni coating on the NdFeB surface.In this paper,the effect of magnetic field on the properties of the electrodeposited Ni coating on the NdFeB surface was studied systematically.Before deposition,the NdFeB was first decreased by 5 g/L sodium hydroxide,50 g/L anhydrous sodium carbonate,75 g/L anhydrous trisodium phosphate and 0.5 g/L OP emulsifier for 10 min at 70℃.Then,40 mL/L nitric acid was used for derusting the resultant NdFeB sample,which was afterwards activated by 30 mL/L hydrochloric acid at ambient temperature.At last,a Ni layer was electrodeposited on the sintered NdFeB surface by magnetic field electrodeposition with ultrasound assistance.It was worth noting that the current density was firstly set as 4 A/dm2 for 1 min to pre-deposit a fresh Ni layer and then immediately adjusted to 2.5 A/dm2 and kept for 30 min.The magnetic field direction was regulated by changing the direction of the sample while the magnetic field intensity was adjusted by adding NdFeB permanent magnet material.The surface morphology of coatings was investigated with a scanning electron microscope(SEM)while the component of coatings was analyzed with an equipped energy dispersive spectrometer(EDS).Besides,the microstructure of the coatings was characterized with an X-ray diffraction(XRD)from 10° to 80° with a scanning rate of 2°/min and the thickness of the coatings was measured with a thickness gauge.The corrosion resistance of the Ni coating was studied through an electrochemical workstation.The results showed that the surface morphology of the coating could be significantly changed by applying a magnetic field,and had slight effect on the thickness of the coating.More importantly,corrosion resistance of the coating was remarkable improved.The morphology of the coating was uniform and compact;the roughness of coatings was decreased under MFE;the thickness of the coating stated at 9-11 μm.The self-corrosion potential(Ecorr),self-corrosion current density(Jcorr)and impedance value of the Ni coating electrodeposited for 30 min under the parallel magnetic field with 0.07 T were-0.193 V,8.305×10-7 A·cm-2 and 4.050×104 Ω·cm2,respectively.It showed the best temperature resistance and corrosion resistance.As a whole,the parallel magnetic field shows a positive effect on the properties of the coating.A compact coating is obtained on the surface of NdFeB through the MFE,and the corrosion resistance of the sample prepared under the parallel magnetic field shows the best,followed by the sample under vertical magnetic field,which is better than that of the Ni coating prepared without magnetic field.
sintered NdFeBelectrodepositionmagnetic field intensitysurface topographycorrosion resistance
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