Corrosion Behaviors of Outdoor Bronze Sculptures in an Urban–Industrial Environment: Corrosion Experiment on Artificial Sulfide Patina

Abstract

Copper alloys interact with air pollutants to form corrosion products and, consequently, a patina on outdoor bronze sculptures. In this study, corrosion experiments were conducted to clarify the corrosion behaviors of artificial sulfide patina in an urban–industrial environment on a quaternary bronze alloy (Cu–Zn–Sn–Pb) with a composition and metallurgical properties similar to those of outdoor bronze sculptures. The correlation between the chromaticity and reflectance of the patina revealed increasing brochantite with the corrosion of the patina and an association between the chromaticity a* and patina growth. Cuprite and brochantite were distinguished, and the point at which brochantite covered the patina surface was determined. The quantitative changes in brochantite were mainly influenced by physical causes such as the crystal size and patina layer thickness as well as by Cu2+ ions working as color formation ions moving to the outermost layer. Atmospheric corrosion of the alloy resulted in reduced Cu and Zn contents and increased Sn and Pb contents. The patina consisted of brochantite in the outermost layer and cuprite and cassiterite in the inner layers. These findings should clarify corrosion characteristics such as the surface color, composition, and changes in corrosion products of outdoor bronze sculptures and contribute toward their preservation.