Atmospheric corrosion of Silicon Bronze for contemporary art

Artistic foundries are increasingly requested to select new bronze compositions, with improved aesthetic features and resistance toward the different corrosive agents. The development of industrial copper-based alloys led to less toxic, lead-free bronzes, such as Cu-Si-Mn bronze. Due to its excellent corrosion resistance, good weldability, remarkable castability and golden colour, this silicon bronze is now widely used by artistic foundries for bronze monuments [1]. This work aims to study mechanisms of atmospheric corrosion and durability of an as-cast Si- Mn bronze. The bronze, supplied by Livartis Foundry d.o.o, Slovenia, corresponds to the copper casting alloy UNS C87300 [2] and contains 3.1±0.4 Si and 0.9±0.1 Mn (wt%) as main alloying elements; Sn and Zn (< 0.1 wt%) as well as P (0.02 wt%) are also present as trace elements. As-cast plates (5 mm thick) were obtained by sand casting. Metallographic analyses were performed by optical (OM) and electron (SEM and FEG) microscopy. The corrosion behaviour was studied through both electrochemical and accelerated ageing tests simulating outdoor conditions. Electrochemical impedance spectroscopy (EIS)was used to monitor the evolution of corrosion process during 10 days of exposure to synthetic acidic rain and potentiodynamic polarization tests were carried out at the end of exposure. Surface analyses (OM, SEM-EDS, FEG, TEM, Raman and FT-IR spectroscopy, profilometric measurements) were performed, together with analyses of Cu, Si and Mn released in the ageing environment (MP-AES). Corrosion rates were determined. Metallographic examination revealed a typical dendritic microstructure with Si coring and a few shrinkage cavities within interdendritic spaces. Coring also led to localised precipitation of Si-, Mn- and P-containing phases in interdendritic spaces. The role of microstructural constituents was examined during accelerated tests. Both electrochemical and accelerated ageing tests showed that modern and traditional quaternary bronzes exhibited similar corrosion rates at the end of exposure, however relevant differences were observed in the metal release trends and in the corrosion products formations, suggesting different corrosion mechanisms.