Electrodeposition Behavior of Zn–Ni Alloys from an Alkaline Zincate Solution Containing Ethylenediamine

Abstract

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 at 308 K in an unagitated zincate solution containing ethylenediamine (EDA), which forms a stable complex with Ni²⁺ ions. In the case of the TEA solution, the Zn–Ni alloy exhibited normal codeposition at low current densities, wherein electrochemically more noble Ni deposited preferentially, while it exhibited anomalous codeposition at high current densities, wherein less noble Zn deposited preferentially. In the EDA solution, the alloy exhibited anomalous codeposition at high current densities; on the other hand, even at low current densities, the Ni content in the deposit was almost identical with the composition reference line, showing a behavior similar to anomalous codeposition. In the EDA solution, Ni deposition and H₂ evolution were significantly suppressed over a 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 the current efficiency for alloy deposition on the current density was smaller in the EDA solution than in that containing TEA. In the TEA solution, the underpotential deposition of Zn apparently occurred with Ni, while in the EDA solution, 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 alloys in the EDA solution was better than that in the TEA solution.