Nature of anode passivation in Zn-Ni electroplating baths

The behaviour of zinc anodes in two zinc-nickel electroplating baths (ammonium-containing and ammonium-free) was studied by recording changes in the anode potential and by analyzing the surface films formed at open-circuit and anodic polarization. The surface films were analyzed using XRD, SEM and XPS. The two baths gave films of similar composition but there were differences in their thickness and the zinc dissolution products. After various cycles of anodic polarization, the film formed in the ammonium bath contained nickel and zinc hydroxide and, although the oxidation process was more difficult, it was not completely hindered; in the ammonium-free bath the film was thicker and more compact and it was composed of nickel and the zinc hydroxichloride (simonkolleite), located in such a way that the zinc anode oxidation was completely blocked. (C) 2000 Elsevier Science Ltd. All rights reserved.