Electrodeposition behavior of Zn-Fe alloy from zincate solution containing triethanolamine

Electrodeposition behavior of Zn-Fe 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 Fe²⁺ ions at 308 K. The content of Fe in deposit changed significantly depending on the current density. At lower current densities than 20 A·m^-2, the content of Fe was approximately 90 mass%, that is, the Zn-Fe alloy exhibited normal codeposition, wherein electrochemically more noble Fe deposited preferentially, while at higher current densities than 100 A·m^-2, it exhibited anomalous codeposition, wherein less noble Zn deposited preferentially. The current density, at which the content of Fe in deposit changed significantly, corresponded to the one, at which the cathode potential in total polarization curve abruptly shifted to the less noble region than the equilibrium potential for Zn deposition. Iron 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. The current efficiency for alloy deposition was not close to zero even in the region of normal codeposition, and the content of Fe in the region of anomalous codeposition was close to the composition reference line, which shows the different deposition behavior from the sulfate solutions. In the region of normal codeposition at lower current densities, the underpotential deposition of Zn occurred with Fe. TEM analysis revealed that Zn-Fe alloys deposited at lower current densities were composed of a stable intermetallic compound of Fe₅Zn₂₁. The activity coefficient of Zn in the deposit appears to decrease remarkably because of the formation of a stable Fe₅Zn₂₁.