Nature of anode passivation in Zn-Ni electroplating baths

The behaviour of zinc anodes in two zinc-nickel electroplating baths (ammon ium-containing and ammonium-free) was studied by recording changes in the a node potential and by analyzing the surface films formed at open-circuit an d anodic polarization. The surface films were analyzed using XRD, SEM and X PS. The two baths gave films of similar composition but there were differen ces in their thickness and the zinc dissolution products. After various cyc les of anodic polarization, the film formed in the ammonium bath contained nickel and zinc hydroxide and, although the oxidation process was more diff icult, it was not completely hindered; in the ammonium-free bath the film w as thicker and more compact and it was composed of nickel and the zinc hydr oxichloride (simonkolleite), located in such a way that the zinc anode oxid ation was completely blocked. (C) 2000 Elsevier Science Ltd. All rights res erved.