Electrochemical nucleation and growth of Zn-Ni alloys from chloride citrate-based electrolyte

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In this work, the effects of deposition potential, [Ni2+]/[Zn2+] molar ratio, pH and bath temperature on the electrocrystallization mechanism of ZnNi alloy from an aqueous solution containing zinc chloride (ZnCl2), nickel chloride (NiCl2) and tri‑sodium citrate as complexing agent were studied. The Zn–Ni alloys were deposited from solutions with different molar ratios (0.25; 1; 2; 4), pHs (1.5; 3; 4) and bath temperatures (20; 40; 60°C). Scharifker and Hills model, used to analyze the potentiostatic current transients, revealed that in a plating bath containing low [Ni2+]/[Zn2+] molar ratios (R≤1), the nucleation mechanism was instantaneous with a typical three-dimensional (3D) growth process, regardless of the bath pH and temperature. In Ni-rich electrolytes (R≥2), the nucleation becomes more consistent with the theoretical progressive nucleation type. The nuclei number density increased with increasing the applied cathodic potential and bath pH and decreases for both of high [Ni2+]/[Zn2+] molar ratios in solution and elevated plating temperatures. The morphological, compositional and structural characterizations of the deposits were carried out using Field-Emission Scanning Electron Microscopy (FESEM), Wavelength Dispersive X-ray Fluorescence Analysis (WDXRF) and X-ray diffraction (XRD) respectively. The obtained results revealed that nickel content of the coatings was increased by increasing cathodic potential and [Ni2+]/[Zn2+] molar ratio and ranged between 5.82 and 29.3wt% (6.44–31.59at.%). ZnNi alloys coated at room temperature from an equimolar ([Ni2+]/[Zn2+]=1) bath were composed of a mixture of two major phases: the δ-phase (Ni3Zn22) with a monoclinic structure and the γ-phase (NiZn3) with an orthorhombic structure, while for deposits obtained in Ni-rich bath, the γ-NiZn3 phase was particularly prevailing. A mechanism of anomalous co-deposition of zinc with nickel is proposed and discussed.