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Corrosion of Outdoor Bronzes: the influence of leaching/stagnant acid rain

Constituent Materials, Microstructure and Inner Structure

University of Bologna Team Leader: Martini Carla - Assistant Prof. - Dep. of Sciences of Metals, Electrochemistry and Chemical Techniques.

University of Bologna Research Group: Martini Carla; Chiavari Cristina, Morselli Luciano, Bernardi Elena, Di Lonardo Gianfranco , Ospitali Francesca.

Partner/Collaborations: Université de Toulouse - CNRS, UMR 5608 TRACES, Toulouse, France; Department of Industrial Chemistry and Materials, University of Bologna, Italy; Department of Physical and Inorganic Chemistry, University of Bologna, Italy; Department of Metals Science, Electrochemistry and Chemical Techniques, University of Bologna, Italy.

Context and objectives

Many parameters influence the decay of outdoor bronze monuments. A key role is played by cyclic rain events, in particular by the geometry of exposure to rain: different patinas are formed in "sheltered" or "unsheltered" areas. In fact, sheltered areas are exposed to stagnant humid deposition, whereas unsheltered areas undergo the leaching action of rainwater. Therefore, natural patinas formed on these different regions have different structures and compositions. This work aims to study both the influence of different exposure conditions (unsheltered/ sheltered) and the role of each metal component on the corrosion behaviour of quaternary bronzes exposed to acid rain. The results obtained by accelerated ageing are compared to results obtained from real outdoor bronzes, so as to optimize simulation parameters and to contribute to full understanding of corrosion processes, with a view to the design of effective conservation strategies.

Methodologies and equipment

Accelerated outdoor exposure tests in artificial acid rain are carried out by two different testing devices: (i) alternate immersion (wet&dry), simulating stagnant rain in sheltered areas and (ii) runoff (dropping) tests, simulating leaching rain in unsheltered areas. During exposure, parallel monitoring of both dissolved metal ions (from the collected leaching water) and surface features of bronze samples is carried out, so as to investigate the corrosion process as a whole. In particular, for both artificially aged samples and real outdoor bronzes, investigations are carried out by topographic analysis (by stylus profilometry and atomic force microscopy (AFM)) as well as by morphological and compositional analysis both on the free surface and in cross-section (by variable pressure scanning electron microscope (VP-SEM) with energy dispersive spectrometry microprobe (EDS) and micro-Raman probe for the identification of phase composition in the micrometric scale).

Results

Bronze patinas, artificially obtained in conditions close to those of unsheltered areas of outdoor sculpture, are significantly different from those observed in sheltered areas and mainly consist of tin compounds. This result has significant implications for conservation purposes, since the remarkable depletion of copper and enrichment of tin observed in unsheltered areas of sculpture requires the use of specifically tailored inhibitors. In fact, in the case of outdoor bronze statues, also after restoration, the application of corrosion inhibitors is done on the already corroded surface. Therefore, the support where the inhibitors should be adsorbed is not the original alloy, but the previously formed patina. Due to the different reactivity of alloying metal oxides towards inhibitors, corrosion inhibitors offer different performances: tin oxides, for examples, differently from copper (I) oxides, show a very weak reactivity towards BTA-based inhibitors, typically used for the conservation of bronzes. This explains the lower efficiency of these inhibitors on bronzes with respect to copper.

More details can be found in:

  • E. Bernardi, C. Chiavari, B. Lenza, C. Martini, L. Morselli, F. Ospitali, L. Robbiola, Corrosion Science 51 (2009) 159-170.
  • C. Chiavari, E. Bernardi, C. Martini, F.Passarini, F. Ospitali, L. Robbiola, Corrosion Science (2010) in press.