The atmospheric corrosion of statue bronzes exposed to SO2 and NO2

Abstract

We report on the atmospheric corrosion of different cast statue bronzes in humid air containing ppb levels of SO2 and SO2+NO2. In addition, copper, tin, zinc, and lead samples were studied in these environments. The samples were exposed to synthetic atmospheres with careful control of pollutant concentrations, relative humidity and flow conditions. Deposition of SO2 was studied using on-line gas analysis. The weight gain was registered after four weeks exposure, and corrosion products were analysed by Electron Probe Micro Analyser (EPMA), X-ray diffraction (XRD), and Fourier Transform Infrared spectroscopy (FTIR). The synergistic effect of SO2 and NO2 was remarkable on all bronze materials examined. The weight gain was correlated to alloy composition. Thus, high zinc and low lead content resulted in the greatest weight gain, while high tin content favoured a low weight increase. Lead exhibited a rapid deposition of SO2 followed by zinc and copper, while tin was unreactive towards SO2. There was no measurable indication that microstructure influenced corrosion. The corrosion product morphology found in SO2+NO2 environment indicated a localised type of attack. The anodic sites were covered by a tin-rich corrosion product close to the metal. Oxidation of soluble divalent tin by O2 at the anodic sites to form insoluble SnO2 · xH2O, is suggested to explain the corrosion protection afforded by alloying with tin. Tin was enriched in the corrosion products while no lead was found. The zinc/copper relation was higher in the corrosion products than in the alloy composition.