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Electrodeposition behavior of Zn-Co alloys was investigated at current densities of 2-500 A·m −2 and a charge of 5 × 10 4 C·m −2 at 308 K in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Co 2+ 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 Zn 2+ 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.

緒言Zn 実験方法Synopsis : Electrodeposition behavior of Zn-Co alloys was investigated at current densities of 2-500 A·m −2 and a charge of 5 × 10 4 C·m −2 in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Co 2+ ions at 308 K. At low current densities below 5 A·m -2 , the Zn-Co alloy exhibited normal codeposition, wherein electrochemically more noble Co deposited preferentially, while at high current densities above 6 A·m -2 , it exhibited anomalous codeposition, wherein less noble Zn deposited preferentially. The current efficiency for Zn-Co alloy deposition was low to be about 20% in the region of nomal codeposition at low current densities, while it was 95% in the region of anomalous codeposition at high current densities. In the region of anomalous codeposition at high current densities, the partial polarization curves for Co deposition and H 2 evolution were significantly shifted to less noble direction by coexisting of Zn 2+ ions, showing the formation of an inhibitor for deposition, which results from Zn 2+ ions in the cathode layer. In contrast, in the region of normal codeposition at low current densities below 5 A·m -2 , the underpotential deposition of Zn apparently occurred with Co. Because Zn-Co alloys are composed of the stable intermetallic compounds of CoZn 13 and Co 5 Zn 21 , the activity coefficient of Zn in the deposit appears to decrease remarkably.