Electrodeposition behavior of Zn-Ni alloy from alkaline zincate solution containing ethylenediamine

Electrodeposition behavior of Zn-Ni alloys was investigated at current densities of 5-500 A·m^-2 and a charge of 5×104 C·m^-2 in an unagitated zincate solution containing ethylenediamine, which forms a stable complex with Ni2+ ions at 308 K, and was compared with that from the solution containing triethanolamine. In a solution containing triethanolamine, the Zn-Ni alloy exhibited normal codeposition at low current densities, wherein electrochemically more noble Ni deposited preferentially and it exhibited anomalous codeposition at high current densities, wherein less noble Zn deposited preferentially, while in a solution containing ethylenediamine, it exhibited anomalous codeposition at high current densities, however, even at low current densities, the Ni content in deposit was almost identical with the composition reference line, showing the behavior close to anomalous codeposition. In a solution containing ethylenediamine, Ni deposition and H2 evolution were significantly suppressed in the larger region of current densities, showing the formation of an inhibitor for deposition, which results from Zn2+ ions in the cathode layer. The dependence of current efficiency for alloy deposition on the current density was smaller in a solution containing ethylenediamine than triethanolamine. In a solution containing triethanolamine, the underpotential deposition of Zn apparently occurred with Ni, while in a solution containing ethylenediamine, the underpotential deposition of Zn never occurred because Ni deposition was suppressed by the coexistence of Zn2+ ions even at low current densities. The throwing power of Zn-Ni alloy was better in a solution containing ethylenediamine than triethanolamine.