Corrosion Behaviors Of Nanocrystalline Nickel And Nickel-chromium Composite Coatings Synthesized By Jet Electrodeposition

Nanocrystalline nickel coatings were synthesized by jet electrodeposition technique,using direct current and pulse current, respectively. Their microstructures were characterizedby TEM. The effects of current types on corrosion resistance of the coatings, thesemiconductor properties and chloride ions adsorption properties of passive films formed onthe samples were investigated by potentiodynamic polarization measurements andelectrochemical impedance spectroscopy (EIS) techniques, Mott-Schottky analysis, potentialof zero free charge (PZFC) methods, respectively. The results demonstrated that the pulsecurrent can fine the grain and prefer the formation of twins. Although the pulse current didn’tchange the type of the semiconductor of the passive film (p-type) on the sample, it candecrease the sacceptor concentration (NA) and defect diffusion coefficient (D0) in the passivefilm. The existence of twins decreased the adsorption of chlorine ions on the surface of thesample, which led to the enhancement of the corrosion resistance.Ultrafine nanocrystalline nickel coatings were obtained with adding bright agent(saccharin and2-butyne-1,4-diol) in the bath by using pulse jet electrodeposition technique.SEM observation indicated that the surface morphology of coating was fairly fine and smooth.TEM analysis demonstrated that the average grain size of the coating was approximately10nm and the twins were disappeared compared with the samle (average grain size80nm withtwins) obtained from the bath without bright agent. The decrease of grain size deteriorated thecorrosion resistance of the sample. In order to investigate such abnormal “grain size effect” ofcorrosion property, samples with various microstructures were obtained by a series ofisothermal annealing treatment on the coatings synthesized from the bath without bright agent.Their microstructures and corrosion properties were investigated by TEM, potentiodynamicpolarization measurements and electrochemical impedance spectroscopy (EIS) techniques,respectively. The results indicated that the effect of the grain refinement on the corrosionbehavior of the sample was much less than that of twins structure. The corrosion resistance ofthe sample was markedly enhanced by high density of twins.Nanocrystalline nickel-chromium composite coatings were prepared in the bathcontaining10g/L chromium nanoparticles (80nm) by pulse jet electrodeposition technique.The effect of chromium nanoparticles on the morphology, microstructure and corrosion resistance of the composite coatings were discussed by SEM, TEM and a series ofelectrochemical measurements. The results demonstrated that chromium nanoparticles madethe surface morphology of coatings transform from pyramid-like for nanocrystalline nickelcoatings to cauliflower-like for composite coatings. Chromium nanoparticles extraordinaryuniformly distributed in nanocrystalline nickel matrix (2.24at.%), which notably improvedthe corrosion resistance of composite coatings.