The Atmospheric corrosion of statue bronzes exposed to SO 2 and NO 2

We report on the atmospheric corrosion of different cast statue bronzes in humid air containing ppb levels of SO 2 and SO 2 +NO 2 . 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 SO 2 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 SO 2 and NO 2 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 SO 2 followed by zinc and copper, while tin was unreactive towards SO 2 . There was no measurable indication that microstructure influenced corrosion. The corrosion product morphology found in SO 2 +NO 2 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 SnO 2 – xH 2 O, 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. Die atmosphärische Korrosion von Statuenbronzen bei Anwesenheit von SO 2 und NO 2 Es wird berichtet über die atmosphärische Korrosion von verschiedenen Gußbronzen für Statuen in feuchter Luft mit ppb‐Gehalten an SO 2 und SO 2 +NO 2 . Zusätzlich wurden Kupfer‐, Zinn‐, Zink‐ und Blei‐Proben in diesen Umgebungen untersucht. Die Proben wurden den synthetischen Atmosphären bei sorgfältiger Kontrolle der Schadgaskonzentrationen, der relativen Feuchtigkeit und der Strömungsbedingungen ausgesetzt. Die Abscheidung von SO 2 wurde mittels on‐line Gasanalyse untersucht. Die Gewichtszunahme wurde nach vier Wochen Auslagerung registriert. Die Korrosionsprodukte wurden mit der Elektronenstrahlmikroanalyse (EPMA), der Röntgenbeugung (XRD) und der Fourier‐Transformations Infrarot‐Spektroskopie (FTIR) analysiert. Der synergistische Effekt von SO 2 und NO 2 war auf allen untersuchten Bronzen bemer