Study On The Pulse Electroplating Process And Corrosion Resistance Of Ni-Cu Alloy

Due to nickel and nickel-based alloys with high hardness and strong corrosion resistance,they have been gradually concerned to place chromium alloys in the field of corrosion and protection of metal.In order to meet the requirements of dense structure,low porosity,and strong corrosion resistance,the technology of pulse electroplating about nickel or nickel alloy is being studied extensively.Ni-Cu alloys possess many outstanding advantages,such as good electrical conductivity,strong corrosion resistance,good flexibility and shape memory property,which promote them to be great potential candidate of corrosion-resistant coatings.In this thesis,Ni-Cu alloy coating was deposited on 27SiMn substrate by pulse electroplating in citrate bath system.The orthogonal design was used to optimize the pulse electroplating parameters,including average current density(im),pH of the plating solution and temperature.The microstructure of the coating was observed by scanning electron microscopy(SEM).The electrochemical polarization curves and electrochemical impedance spectra of the coatings were analyzed by electrochemical workingstation.The corrosion resistance of the Ni-Cu alloy coating was evaluated by the experimental results showing the corrosion resistance of Ni-Cu alloy coating with high performance when the experimental conditions are average current density at 7.5A/dm2,pH at 4 and the plating temperature at 65?.The effects of boric acid concentration were studied on the microtopography,grain size,composition and corrosion resistance of Ni-Cu alloy coating,via scanning electron microscopy,X-ray diffractometer,laser scanning confocal microscopy and Vickers hardness tester.The results show that the alloy coating prepared by adding boric acid is smooth and compact without obvious voids and low surface roughness,which indicates that the boric acid plays a significant role in the grain refinement of the alloy coating.As increasing the concentration of boric acid,the grain size decreases first and then increases.At the boric acid concentration of 15 g/L,the grain size is about 14.1 nm,and the Vickers hardness of the coating is the highest.After the corrosion-resistant test,the results showed that the minimum current density(ip)of the coating eletroplated by adding 15 g/L boric acid was about 1.01 ?A cm-2.The range between the pitting potential and the protective potential is the smallest(0.0845V).After the characterizations of microstructure observation,grain size analysis,hardness and corrosion resistance test analysis,it was found that the optimum addition of boric acid was 15g/L.The corrosion mechanism of Ni-Cu alloy coating was studied by XPS,Mott-Schottky theory,point defect model(PDM)and zero charge potential(PZFC)technique.The result drew that the passive film contains Ni,Cu,O element,almost without Cl element.Ni and Cu coexist in the passive film in the form of NiO and Cu2O,respectively and with a less amount of Cu2O.The results demonstrated that the feature of the n-type semiconductor is Cu2O at low potential and the feature of the p-type semiconductor is NiO at high potential,and the number of hole carriers in p-type semiconductor is reduced by Cu+ doping into Ni2+,thus the rate of oxidation-reduction reaction decreases.When the amount of Cu2O in the passive film is low,Cu+ is hard to convert into Cu2+.The passive film has the lowest carrier density and the lowest point diffusion coefficient,and the adsorption capacity to Cl" is the weakest,indicating that the sample has good corrosion resistance.The results show that the stronger the compressive ability and the greater the elastic modulus of the passive film,which indicats that the passive film has less cracks and defects.Therefore,the performance of passive film is better.