Electrodeposition Behavior of Zn–Fe Alloy from Zincate Solution Containing Triethanolamine

Abstract

The electrodeposition behavior of Zn–Fe alloys has been investigated at 308 K 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. The content of Fe in the deposit changed significantly depending on the current density. At current densities lower than 20 A·m⁻², the content of Fe was approximately 90 mass%, that is, the Zn–Fe alloy exhibited normal co-deposition, whereby the electrochemically more noble Fe deposited preferentially. At current densities higher than 100 A·m⁻², however, anomalous co-deposition was observed, whereby the less noble Zn deposited preferentially. The current density at which the Fe content in the deposit changed significantly corresponded to that at which the cathode potential in the total polarization curve abruptly shifted to a less noble region than the equilibrium potential for Zn deposition. Iron deposition and H₂ evolution were significantly suppressed by the co-existence of Zn²⁺ ions in the region of anomalous co-deposition at higher current densities, showing the formation of an inhibitor to deposition arising from Zn²⁺ ions in the cathode layer. The current efficiency for alloy deposition was not close to zero even in the region of normal co-deposition, and the Fe content in the region of anomalous co-deposition was close to the composition reference line, which shows deposition behavior different from that in sulfate solutions. In the region of normal co-deposition at lower current densities, underpotential deposition of Zn occurred with Fe. TEM analysis revealed that Zn–Fe alloys deposited at lower current densities comprised the stable intermetallic compound Fe₅Zn₂₁. The activity coefficient of Zn in the deposit appears to remarkably decrease because of this formation of stable Fe₅Zn₂₁.