Studies on the enhanced properties of nanocrystalline Zn-Ni coatings from a new alkaline bath due to electrolyte additives

The effective composite additives coumarin (CA) and vanillin (VL) were used to electrodeposit nanocrystalline Zn-Ni alloys in a novel alkaline bath containing 5,5'-dimethylhydantoin (DMH) as the complexing agent. Similarly to the cyanide bath, an excellent mirror-like bright Zn-Ni alloy deposit with smooth, compact morphology and good leveling capability can be obtained from the newly developed bath with the composite additives. One cathodic peak of cyclic voltammogram (CV) curves indicates a single step two-electron transfer mechanism of Zn2+ and Ni2+. The inhibition effect of VL is more pronounced than that of CA in the electrolyte through preferential adsorption on the cathode. Larger cathodic polarization and finer grains are observed with the increase of VL than that of CA, indicating that VL is the main brightening agent. However, CA has a more prominent effect on the structure of Zn-Ni alloys compared to VL, which can be considered as a brightening promoter. The mechanisms of the decrease in grain size are considered in the bath with the addition of the composite additives. No oxidized zinc exists in the bulk of deposits by X-ray photoelectron spectroscopy (XPS) analysis. Moreover, mechanical, wear and corrosion resistance properties of Zn-Ni coatings were directly dependent on Ni content in deposits. The lower Ni content of bright coatings with smoother appearance is obtained with composite additives and the bright coatings display superior wear and corrosion resistance relative to the dull coatings. The composite additives have a synergistic effect to significantly improve the properties of dull Zn-Ni alloys. Thus, the introduced new alkaline bath is a promising replacement for the conventional cyanide Zn-Ni alloy bath.