Electrodeposition behavior of Zn-Co alloys from an alkaline zincate solution containing triethanolamine

Electrodeposition behavior of Zn-Co alloys was investigated at current densities of 2-500 A·m^?2 and a charge of 5 × 104 C·m^?2 at 308 K in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Co²⁺ ions. At current densities lower than 5 A·m^?2, the Zn-Co alloys exhibited normal co-deposition behavior, with the electrochemically more noble Co being preferentially deposited. By contrast, at current densities higher than 6 A·m^?2, they exhibited anomalous co-deposition behavior, with the electrochemically less noble Zn being preferentially deposited. The current efficiency for Zn-Co alloy deposition was low (about 20%) in the normal co-deposition region, while it was 95% in the anomalous co-deposition region. Also, in the anomalous co-deposition region, the partial polarization curves for Co deposition and H² evolution were significantly shifted to the less noble direction by the coexistence of Zn²⁺ ions, suggesting the formation of an inhibitor species that results from the presence of Zn ²⁺ ions in the cathode layer. On the other hand, in the normal co-deposition region, the underpotential deposition of Zn apparently occured simultaneously with Co deposition. Zn-Co alloys are composed of stable intermetallic compounds CoZn₁₃ and Co₅Zn₂₁; therefore, the activity coefficient of Zn in the deposits appears to decrease remarkably.