Electrodeposition Behavior of a Zn–Ni Alloy in an Alkaline Zincate Solution

Abstract

Electrodeposition behavior of Zn–Ni alloys was investigated at current densities of 10–500 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 Ni²⁺ ions at 308 K. 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 H₂ 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 Zn²⁺ 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 Ni₅Zn₂₁, the activity coefficient of Zn in the deposit appears to decrease remarkably.