Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys

Natural patinas on archaeological bronzes (Cu-Sn alloys) have been classifi ed and characterized in order to get a deeper insight into their formation mechanisms. From examinations of cross-sections on archaeological artefacts , two classes of corrosion structures were defined (Type I and Type II), us ing both optical and electron microscopies, EDSX, XRD, IRS and a statistica l treatment of data (Principal Components Analysis). A Type I structure ('e ven' surface) is defined as a two-layer passivating deposit due to an inter nal oxidation with a decuprification process (i.e. selective dissolution of copper). A Type II structure ('coarse' surface) corresponds to more severe attacks, such as pitting but also general uneven corrosion; it is modelled by a three-layer structure, characterized by the presence of cuprous oxide and by an increase in the chloride content at the internal layer/alloy int erface related with selective dissolution of copper. A phenomenological mod el to explain the formation of bronze patinas is developed on the basis of a decuprification phenomenon. (C) 1998 Elsevier Science Ltd. All rights res erved.