Study On Boronizing Process Of Pure Nickel By Molten Salt Electrolysis

Nickel is widely used in military manufacturing,civil machinery manufacturing and electroplating industries due to its high strength,acid resistance,alkali resistance and high temperature resistance.However,the surface hardness of nickel is not high,and the wear resistance is low,which limits the further expansion of its application field.Hence surface treatment needs to be carried out on nickel in order to improve its performance properties.In this thesis,molten salt electrolysis and powder-pack methods were used for the boronizing of pure nickel.Firstly,85%Na₂B₄O₇·10H₂O-15%Na₂CO₃was used as the electrolyte in molten salt electrolysis boronizing process and 96%B₄C-4%KBF₄was used as the boronizing agent in the powder-pack method.The effects of different process parameters on the microstructure and composition of the layer were studied by single factor experiment.Scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS)were used to analyze the cross-section morphology and element content of boronized samples.X-ray diffraction(XRD)was used to analyze the surface of boronized samples.The results showed that during the boronizing process of molten salt electrolysis,the current density(250¹500A/m²),electrolysis time(15²40min)and electrolysis temperature(880⁹70?)have significant effects on the structure and thickness of the layer.When the electrolysis temperature is 950?,the electrolysis time is 60min and the current density is 1500A/m²,the thickness of the nickel borides layer is about 197.92?m.In the process of boronizing by powder-pack method,with the increase of boronizing temperature(850⁹50?)and boronizing time(4¹0h),the thickness of the borides layer has a significant growth trend.The thickness of the borides layer is about176.35?m when the boronizing temperature is 950? and the boronizing time is 10h.Under different process parameters,the borides layer consisting of Ni₂B and Ni₃B was formed on the surface of pure nickel by the above methods.Secondly,the microhardness tester was used to measure the surface hardness and cross-section microhardness of the boronized sample.The corrosion resistance of the boronized sample was analyzed by electrochemical workstation.The results show that for the boronized sample prepared by the above two methods,the greater the thickness of the layer is,the higher the surface hardness value will be,with the maximum value being up to 1763HK;the microhardness value for the corss section of boronized sample is 1008HK,which is more higher than the hardness of the substrate(199HK).The electrochemical corrosion behavior was studied in 3.5%NaCl corrosive medium.Through electrochemical impedance spectroscopy and polarization curve analysis,the corrosion resistance of pure nickel after boronizing was significantly improved.Finally,the thermodynamic data for the boronizing reactions were calculated.The growing kinetics of the borides layer was analyzed.The diffusion constant K₀ of boron atom through the nickle matrix was calculated to be 1.12×10^-2,and the diffusion activation energy Q is 230.714 kJ/mol.