Cathodic inhibition and anomalous electrodeposition of Zn–Co alloys

The electrodeposition of Zn–Co alloys from chloride electrolytes was studied on steel substrate. Electrodeposition of Zn–Co alloys is usually divided into two potential regions, i.e. normal (positive to Zn deposition potential E Zn 0 = − 1.05   V versus SCE) and anomalous (negative to E Zn 0 ). In order to elucidate the deposition mechanism a complementary approach was used based on the combination of various electrochemical techniques. The morphology of the deposits and elemental composition analysis were determined by using SEM/EDX. It was found that the presence of Zn2+ in electrolyte inhibited Co2+ and H+ reductions in normal region. A critical potential was also noticed in the so-called normal deposition range above which a Co-enriched phase of Co–Zn alloy was favored and below that a severe mitigation to deposition occurred that was considered due to underpotential deposition (UPD) of Zn on the substrate and on active Co sites at either nucleation or growth stage. Beyond E Zn 0 the deposition is considered anomalous due to the fact that Zn deposits preferentially compared to the more noble Co. This anomalism was explained by the faster deposition kinetics of Zn as compared to Co on steel and could be overcome by either increasing the Co2+/Zn2+ ratios in the electrolyte or by carrying out the deposition at higher temperature.