Artificial patina formation onto bronze alloys: chloride and sulphate induced corrosion processes

Copper-based alloys are affect by degradation processes leading to the formation of surface patinas, whose composition depends on several parameters such as the alloy composition, metallurgical features and environmental conditions. Naturally grown patinas are typically detected onto the surface of modern bronze works of art [1] and strictly affect the surface reactivity. The production of representative patinas onto bronze substrates is a key issues in order to obtain model systems which can be used for the development and validation of appropriate conservation strategies. In this study, we have prepared different artificial representative patinas by using a quaternary copper-based alloy (Cu-Sn-Zn-Pb) with chemical composition and metallurgical features similar to those of valuable modern works of art [1]. In order to produce degradation products commonly observed onto their surface, chloride and sulphate species were used to induce corrosion processes. Different patinas were produced by changing some experimental parameters, as concentration of corrosive species, temperature and ageing time. The composition and structural properties of the patinas were investigated by attenuated total reflectance Fourier transform infrared spectroscopy, X-ray diffraction, optical microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy. The results allow to identify degradation products and to distinguish polymorph species. In the case of chloride induced patina formation, red Cu2O (cuprite) and green Cu2(OH)3Cl polymorphs were mainly detected. When the patina formation is induced by sulphate species, it mainly consists of blue Cu4SO4(OH)6 polymorphs. [1] D.E. Couture-Rigert, P.J. Sirois, E.A. Moffatt, Studies in Conservation, 57, 142 (2012).