Decuprification phenomenon of Cu-Sn alloys in aqueous solution in nearly neutral pH conditions

Natural patinas of ancient or historic bronzes are characterized by a selective dissolution of copper. A recent model to explain the formation of bronze patinas has been developed on the basis of a decuprification phenomenon. In order to check this point, experiments have been carried out on a synthetic single-phase Cu-13 Sn alloy (wt per cent) in a 0.01MN Na2SO4 solution. Both electrochemical (voltametric and chronoamperometric curves) and surface characterisation methods (SEM, EDSX and infrared spectrometry) have been utilized. The results clearly demonstrate that bronze corrosion proceeds by decuprification and internal oxidation of the alloy. Patinas artificially obtained correspond to a compact corrosion layer, strongly enriched in poorly crystallized tin compounds, which maintains the shape of the surface alloy. They are similar to naturally formed ones observed on bronze artifacts.