Composition and morphology of the film formed on a lead alloy under conditions typical of the electro-winning of copper

The composition and morphology of the anodic film formed on a lead alloy under conditions typical of copper electro-winning was studied with the aim of establishing the mechanism by which cobalt ions decrease the oxidation rate of lead. The film was characterized by scanning electron microscopy, X-Ray Diffraction (XRD) and X-Ray Photon Electron Spectroscopy (XPS). The anodic film formed in the presence of cobalt ions was thin and compact whereas the film was thick and porous without cobalt ions. No crystalline cobalt compounds were detected by XRD and no cobalt peaks were detected by XPS. In the absence of cobalt ions in the solution, initial oxidation produces three layers on the lead surface, namely PbSO4, α-PbO2 and β-PbO2 in contact with the solution. In steady state, there are two layers: α-PbO2 and β-PbO2. In the presence of a moderate cobalt ion concentration of up to and including 100 ppm, in steady state there remain the three layers: PbSO4, α-PbO2 and β-PbO2. The results indicate that the presence of cobalt ions increases the imperviousness of the α-PbO2 layer in hindering the oxidation of metallic lead to α-PbO2 and also in hindering the oxidation of PbSO4 to β-PbO2.