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 x 10(4) 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 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 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. [doi:10.2320/matertrans.M2012241]