Influence of alloying elements and phase composition in the corrosion behaviour of bronze protected by organosilane coatings

The conservation of the artistic heritage is of great importance because it allows the preservation of the historical and cultural identity of a society for the benefit of future generations. The bronze artworks are an important part of this cultural heritage so a deep understanding of the corrosion process is necessary to improve the restoration practice and to develop optimized conservation approaches. These alloys can show a quite complex and defective microstructure: due to metallurgical processes, bronzes undergo dendritic segregation with formation of a cored microstructure, with a tin-rich interdendritic phase and secondary phases separation (like Pb globules formation at the grain junctions or δ phases precipitation). This can induce micro-galvanic effects and preferential dissolution of the less noble phases. This work aims at investigating the role of alloying elements, secondary phase separation and segregation in the mechanism of corrosion and corrosion protection of a quaternary bronze (Cu-Sn-Pb-Zn), reproducing the composition and microstructure of a Renaissance bronze. The corrosion behaviour was monitored by Electrochemical Impedance Spectroscopy (EIS) during 7 days of immersion in an artificial acidic rain and the potentiodynamic polarization curves were recoded after 1 hour and at the end of exposure in the aggressive solution. The protectiveness of 3-mercaptopropyl-trimetoxy silane (PropS-SH) coating against the corrosion of these substrates was also investigated. The studied bronze (fig. 1a) showed a dendritic microstructure, typical of as cast bronzes. The EDS microanalysis carried out on the representative points of its microstructure (fig. 1b) showed that the core of the dendrite has a higher content of Cu and a lower of Sn with respect to the average composition of the alloy, whereas in the interdendritic zone a noticeable increase in Sn and Sb content was detected (table 1). The corrosion behaviour of this alloy was interpreted on the basis of the behaviour afforded by pure alloying elements and the alloys reproducing the composition of the dendritic and interdendritic phases. The results show that the bronze corrosion behaviour is not the weighted average of the behaviour of the alloying metals. Pure Cu and Cu8%wtSn can provide a good description of the behaviour of the dendrite core and the interdendritic zone. The galvanic corrosion measurements confirm that the core of dendrite have an anodic behaviour with respect the interdendritic zone, likely protected by a tin-rich oxides/hydroxide film. The PropS-SH coating does not alter the practical nobility of the dendrite and interdendrite (interdendrite is still nobler than dendrite), but the galvanic activity is greatly reduced.