Study On The Zinc-nickel Alloy Electroplating In An Alkaline Bath

Zinc-nickel alloys are an important kind of protective coatings for steel. These alloys have a promising future since they demonstrate higher corrosion resistance and superior mechanical properties, as compared to other zinc alloy coatings. It is believed that the research of zinc-nickel alloys has a significant theoretical value.In this thesis, we prepared zinc-nickel alloy coatings from an alkaline electrolyte using direct current and studied the effects of principle technological parameters on the zinc-nickel films. Finally, we probe into the corresponding deposition mechanism in process of the electroplating.The mean contents are as follows:(1) The influences of principal technological parameters (nickel concentration, current density,temperature and the brightener concentration) on the zinc-nickel alloy coatings were studied using energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM), then we improved a set of operating conditions and conformed that the co-deposition of zinc and nickel behaved anomalously.(2) The microstructure of the optimized deposition was characterized by atomic force microscope (AFM), it could clearly seen that the film is compact and fine-grained. X-ray diffraction (XRD) measurements revealed that the alloy consisted mainly of the y-phase (NiZn3).The tafel polarization curves were measured to study the corrosion resistance of the film, the result showed the coating was an excellent anodic protective film exhibiting higher corrosion resistance than zinc.(3) The kinetics and mechanism for electrocrystallization of zinc-nickel alloys formed on steel substrate was studied using Cyclic Voltammetry (CV), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results showed that the coelectrodeposition of zinc and nickel occurred under mass transfer control, and the microstructure of the original state fitted the characteristic of 2D nucleation/growth mechanism, it is also found that at more positive potential there was an enrichment of nickel in the coatings. Finally, The kinetics and mechanism were further investigated with chronoamperometry (CHR) and electrochemical impedance spectroscopy (EIS) techniques. We got the model of the current density transients during the nucleation/growth of zinc-nickel alloys. We also found that with negative bias increasing, charge transfer resistance reduced, and the electrocrystallization gradually changed from heteroepitaxial growth to isoepitaxial growth.