Electrodeposition behavior of a Zn-Ni alloy in an alkaline zincate solution

Electrodeposition behavior of Zn-Ni alloys was investigated at current densities of 10-500 A·m ^-2 and a charge of 5 × 10 ⁴C·m ^-2 in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Ni ²⁺ ions at 308K. At low current densities, the Zn-Ni alloy exhibited normal codeposition, wherein electrochemically more noble Ni deposited preferentially, while at high current densities, it exhibited anomalous codeposition, wherein less noble Zn deposited preferentially. Ni deposition and H ₂ evolution were significantly suppressed in the region of anomalous codeposition at higher current densities, showing the formation of an inhibitor for deposition, which results from Zn ²⁺ ions in the cathode layer. In contrast, in the region of normal codeposition at lower current densities, the underpotential deposition of Zn apparently occurred with Ni. Because Zn-Ni alloys are composed of a stable intermetallic compound of Ni ₅Zn ₂₁, the activity coefficient of Zn in the deposit appears to decrease remarkably.