THE ATMOSPHERIC CORROSION OF STATUE BRONZES EXPOSED TO SO2 AND NO2

We report on the atmospheric corrosion of different cast statue bronze s in humid air containing ppb levels of SO2 and SP2 + NO2. in addition , copper, tin, zinc, and lead samples were studied in these environmen ts. The samples were exposed to synthetic atmospheres with careful con trol of pollutant concentrations, relative humidity and flow condition s. Deposition of SO2 was studied using on-line gas analysis. The weigh t gain was registered after four weeks exposure, and corrosion product s were analysed by Electron Probe Micro Analyser (EPMA), X-ray diffrac tion (XRD), and Fourier Transform Infrared spectroscopy (FTIR). The sy nergistic 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, w hile high tin content favoured a low weight increase. Lead exhibited a rapid deposition of SO2 followed by zinc and copper, while tin was un reactive towards SO2. There was no measurable indication that microstr ucture 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 O-2 at the anodic sites to form insoluble SnO2.xH(2)O, is suggested to explain the corrosion protectio n afforded by alloying with tin. Tin was enriched in the corrosion pro ducts while no lead was found. The zinc/copper relation was higher in the corrosion products than in the alloy composition.