Preparation Of Ni Based W Alloy On Mild Steel And Its Evaluation Of Hardness And Corrosion Resistance

In this study, an attempt had been made to prepare Ni based tungsten alloycoatings for improving the corrosion resistance and hardness of mild steel. A series ofworks have been carried out as follows: Firstly, the influences of pH, current densityand sodium tungstate content on electro-deposition of Ni-W alloy coating had beenstudied. And a mapping relation model between Ni-W alloy electrodepositiontechnology parameters and its performance had been established by the BP neuralnetwork prediction system. Secondly, the electrodeposition mechanism of nano Ni-Walloy coating had been studied by various electrochemical method. Thirdly, throughheat treatment or passivation, the performance of coating was promoted. Moreover, byaddition of Cu2+or nano particles(Al2O3、SiO2、Cr2O3)，excellent performance Nibased tungsten alloy such as Ni-W-Cu ternary alloy and Ni-W-nano composite coatingwith were preparation separately in this dissertation.ByAnalysis of the morphology, composition and structure of Ni-W alloy coating,theinfluence of electrodeposition parameterson Ni-Walloydeposition process, surfaceperformance such as coating roughness, hardness and the electrochemical corrosionhad been studied. Results indicated that, not all of W determined by EDX was presentas a solute in the Ni-W fcc phase, but also as tungstate (at lower pH) or citrate (at higherpH) exists in the coating. The structure of coating prepared in acid solution wasnanocrystalline structure with ultra hardness (794.4HV); Amorphous coating preparedin alkaline solution would be passivated in3.5%NaCl solution, and corrosionresistance of the coating can be improved greatly. Finally, a visual form operation ofartificial neural network software was developed by Visual C#language, theapplication of artificial neural network software can establish mapping relation modelbetween Ni-W electro-deposition technology parameters and its performance.Cyclic voltammetry curves (CV), electrochemical impedance spectroscopy (EIS)and chronoamperometry(CA) in conjunction with the Scharifker-Hills Model (SH) were used to research the initial stage of Ni-W alloy electro-deposition in acidicsulphate. The results indicated that the electro-deposition of the Ni-W alloy ontoelectrode consistedof nucleationand growth,the crystal nucleationofplatingimprovedas the concentration of WO2-4increased and its reduction intermediate products couldbe considered as favorable sites of the Ni-W alloy. According to the analysis ofScharifker-Hills model, the pure Ni without W nucleation process follows threedimension-instantaneous mechanism, while the Ni-W alloy is progressive nucleationmechanism at a lower overpotential. For this reason, it would innhibit crystal grew upand form a needlelike nanostructures Ni-W coating.The influence of heat treatment on of Ni-W alloycoating has been studied. Theresults indicated that NiW, Ni4W metal mesophase was formed after annealed at500°C under hydrogen atmosphere, and W was formed at1000°C. While WC wasformed annealed at500°C and Ni4W、Ni6W6C was formed at1000°C under argonatmosphere. Hardness test results indicated that heat treatment can enhance thesurface hardness of the coating. The samples annealed at500°C under argonatmosphere shows a maximum hardness (1280HV).Ni-W alloy exhibited a better passive property of than pure Ni coating or mildsteel in boric acid solution (pH=8.4). The results showed that passive film of mild steeldisplayed n-type semiconductor, while the pure Ni coating and Ni-W coatings werep-type semiconductor. The Ndof Ni-W coatings are about~1.12×1019cm-3. And thevariation of Dois small (approximately5.5×10-14cm-2) according to PDM model.Hence, the key factor of semiconductor performanceis carrier density of the passivefilm formed on the sample surface. In all specimens, amorphous Ni-W alloy coatingsamples (specimen D) shows the best passivation performance, which can provideexcellent protection for mild steel substrate.Ni-W-Cu ternary coating with micro/nanobinary surface architectures wasprepared in this paper. Amazingly, it is found that the electrodeposited Ni-W-Cucoating deliver the superhydrophobic surfaces without any further modifications by low surface-energy materials, which might be attributed to the vigorousmicro/nanobinary architectures and the surface chemical composition. Theelectrochemical measurements reveal that the superhydrophobic Ni-W-Cu coatingcould provide enhanced corrosion resistance for the substrate in3.5wt.%solutions.In addition, Ni-W-nano composite coatings were electrodeposited in this paper.Results show that amorphous Ni-W coatings electrodeposited in the bath without nanoparticle, while crystalline composite coatings would be electrodeposited in the bathcontained nano particle(Al2O3, SiO2, Cr2O3), which exhibit a eminent hardness. In allcomposite coatings, Ni-W-Al2O3electrodeposited in the bath contained20g/L Al2O3exhibit a maximum hardness (894HV), and minimum corrosion current density inconjunction with a maximum impedance, which can provide an protection for mildsteel.