| A nickel coating with nano-scale twins (NT) was synthesized by using pulsed electrodeposition technique. And pulsed electrodeposition technique was optimized by orthogonal test, with corrosion resistance in solutions containing chloride ions as inspecting criterion. The results indicated that the as-deposited nickel coating with the best corrosion resistance was synthesized by the optimized plating parameters:positive average pulse current density 1.5 A/dm2, duty cycle 20% and frequency 250 Hz. The effect of phytic acid addictive on the microstructure, micro-morphology and corrosion resistance of the coatings was observed by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and electrochemical impedance spectroscopy (EIS) techniques respectively. The results demonstrated that the addition of phytic acid was in favor of the growth of NT in the interior of grains and the typical morphology of pyramidal islands on the surface. And the corrosion resistance of samples was improved with the addition of phytic acid.The effects of NT structure on chemical component, chloride ions adsorption and nucleation and growth process of passive films formed on pure Ni were investigated by X-ray photoelectron spectroscopy (XPS), potential of zero free charge (pzfc) and current-time transient methods respectively. The results indicated that the passive film formed on NT coating was more integrated and compact and had weaker capability to adsorb chloride ions than that on industrial electrodeposited (IE) nickel; the nucleation process of passive films formed on both Ni samples was progressive nucleation, while the parallel growth rate of passive films formed on NT Ni was higher than that on IE Ni.The effects of NT structure on pitting corrosion resistance and semi conducting properties of passive films formed on pure Ni were investigated by the potentiodynamic polarization measurements and capacitance measurements. The results indicated that the passive films formed on NT coatings showed higher pitting corrosion resistance and a bi-layer semi conducting structure distribution (inner p-type, outer n-type), comparing with those formed on IE nickel. Vacancy density and vacancy diffusion coefficient in passive films were studied by using Mott-Schottky (M-S) relationship and Point Defect Model (PDM). It demonstrated that NT structure decreased vacancy diffusion velocity and developed the growth of thinner and more compact passive films consequently.Ultimately, the effects of NT structure on mechanical properties of Ni were investigated by Nano-Indenter technique. The results demonstrated that NT structure increased the hardness and elastic modulus of passive films formed on Ni, which maked the fracture of passive films difficult. As a consequentce, the pitting corrosion resistance was markedly enhanced according to the film breakdown mechanism. |
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