Characterisation of anodic layers on Cu–10Sn bronze (RDE) in aerated NaCl solution

The anodic surfaces formed on Cu–10Sn (wt.%) alloy (α-bronze) are investigated in aerated 0.1 M aqueous chloride solution, using electrochemical reduction and characterisation methods such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the whole anodic domain, investigations performed on a bronze rotating disk electrode (RDE) reveal the systematic formation of a uniform oxidation layer. It is evidenced that the chemical composition of the layer varies with the applied anodic potential, but also that the latter always exhibits a poorly crystallised (probably nanocrystalline) hydrated and hydroxylated nature. Close to Eoc, the compounds are mainly (hydroxide) oxides of tin and copper, incorporating very low amounts of chlorides. At intermediate oxidation potentials corresponding to the active–passive transition, the first oxidation peak corresponds to the formation of hydrated tin oxyhydroxide chloride species which transforms in a more stable one – probably related to the Sn(II) → Sn(IV) oxidation. At higher anodic potential, on the current plateau, the layer contains hydrated tin (IV) oxyhydroxide and copper chloride (mainly CuCl). However, XRD and XPS results reveal that the barrier layer has a complex nature, including unidentified products and different spatially distributed charged surface zones. The corrosion mechanism involves an internal oxidation of the alloy linked to a preferential dissolution of copper, namely a decuprification. A decuprification factor fCu is defined and calculated. Both fCu and the layer thickness increase with the applied potential. We show unambiguously that the tin compounds remain in the corrosion layer, acting as stabilizing species. It is suggested that the tin species promote the formation of a network as for tin oxide xerogel, through which copper ions and anions migrate. Both the layer microstructure and the decuprification factor (fCu) are in agreement with those found in Type I patina of ancient bronzes.