Electrodeposition behavior of Zn-Co alloy from alkaline zincate solution containing triethanolamine

Synopsis: Electrodeposition behavior of Zn-Co alloys was investigated at current densities of 2-500 A·m^-2 and a charge of 5 × 10⁴ C·m^-2 in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Co²⁺ ions at 308 K. At low current densities below 5 A·m^-2, the Zn-Co alloy exhibited normal codeposition, wherein electrochemically more noble Co deposited preferentially, while at high current densities above 6 A·m^-2, it exhibited anomalous codeposition, wherein less noble Zn deposited preferentially. The current efficiency for Zn-Co alloy deposition was low to be about 20% in the region of nomal codeposition at low current densities, while it was 95% in the region of anomalous codeposition at high current densities. In the region of anomalous codeposition at high current densities, the partial polarization curves for Co deposition and H2 evolution were significantly shifted to less noble direction by coexisting of Zn ²⁺ ions, showing the formation of an inhibitor for deposition, which results from Zn²⁺ ions in the cathode layer. In contrast, in the region of normal codeposition at low current densities below 5 A·m ^-2, the underpotential deposition of Zn apparently occurred with Co. Because Zn-Co alloys are composed of the stable intermetallic compounds of CoZn₁₃ and Co₅Zn₂₁, the activity coefficient of Zn in the deposit appears to decrease remarkably.