Electrodeposition Behavior of Zn-Co Alloy from 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 x 10(4) C.m(-2) in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Co2+ 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 Zn2+ ions, showing the formation of an inhibitor for deposition, which results from Zn2+ 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 CoZn13 and Co5Zn21, the activity coefficient of Zn in the deposit appears to decrease remarkably.