The atmospheric corrosion of quaternary bronzes: The leaching action of acid rain E. Bernardi a , C. Chiavari b, * , B. Lenza b , C. Martini b , L. Morselli a , F. Ospitali c , L. Robbiola d a Dipartimento di Chimica Industriale e dei Materiali, Università di Bologna – Via Risorgimento 4, 40136 Bologna, Italy b Dipartimento di Scienze dei Metalli, Elettrochimica e Tecniche Chimiche, Università di Bologna – Via Risorgimento 4, 40136 Bologna, Italy c Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna – Via Risorgimento 4, 40136, Bologna, Italy d Service des Microscopies Electroniques, LECA, UMR 7575 CNRS, ENSCP, Université Paris 6, 11 rue P&M Curie, F-75231 Paris Cedex 05, France article info Article history: Received 17 July 2008 Accepted 8 October 2008 Available online 1 November 2008 Keywords: A. Alloy A. Copper B. SEM B. Raman spectroscopy C. Atmospheric corrosion abstract The effect of leaching rain on the corrosion behaviour of bronze UNSC83600 was investigated as to the influence of alloying elements (Cu, Sn, Zn, Pb) through dropping tests simulating a severe runoff condition with a solution reproducing natural acid rain. Corrosion was followed with time monitoring both samples and leaching solutions (up to 30 days) by SEM, EDS, Raman spectroscopy, XRD, AAS. The bronze patina behaves as a porous layer enriched in stable tin compounds allowing uniform dissolution of Cu, Zn and partly of Pb. Laboratory results are in good agreement with field studies of outdoor bronzes in unsheltered condition. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Outdoor bronzes naturally form corrosion layers called patina, partially protecting the metallic substrate. During the last century, the phenomenon of acid rain induced a strong change in the nature and properties of copper based patinas [1–7]. In urban atmosphere, these patinas are unstable and partially leached away by rainwa- ter, as demonstrated by the green streaking on bronze and copper monuments in metropolitan areas [2,3,8]. The action of the envi- ronment on outdoor bronzes has been the subject of several stud- ies. However, the corrosion mechanism is still not completely clarified [1,2,7,9–12]. The corrosion mechanism of bronze has been usually assimilated to that of copper, even if the role of other alloy- ing elements has been increasingly considered [7,9,10,13–17]. In particular, several studies focused on the role of tin and its insolu- ble oxides in the mechanism of formation of patinas, clearly indi- cating that alloys and pure metals behave very differently when exposed to the environment [13–21]. Furthermore, the influence of the geometry of exposure on the corrosion behaviour has been taken into account [6,14,22,23]. In particular, the cyclic action of acid rain was found responsible for the different patinas formed in ‘‘sheltered” or ‘‘unsheltered” areas on a given bronze monument. Patinas grown on sheltered and on unsheltered regions strongly differ in electrochemical, morphological and compositional fea- tures [6,7,14,15]. Actually, unsheltered areas undergo the leaching action of the rain (runoff). For example, in unsheltered areas on Rodin’s bronzes exposed in outdoor conditions, a significant loss of thickness has been observed sometimes reaching several hun- dreds of lm [6]. In the last decade, the action of runoff conditions on metals has been extensively studied, through laboratory and field studies [20–27], with particular attention to those metals (copper and zinc) widely used for roofing. Concerning bronze alloys, the metal runoff also appears to play a determinant role as commonly ob- served in the bronze statuary exposed to the direct action of the rain. Accordingly, the characterisation of patinas, i.e. the support where to apply corrosion inhibitors or protective coatings, be- comes of primary importance [28,29]: a clear comprehension of the mechanism of formation of patinas on outdoor bronzes is nec- essary in order to define a proper conservation strategy. Thus, it is of utmost importance to complete our understanding of the influ- ence of acid rain on the corrosion of quaternary bronze in unshel- tered condition. Within this context, the present work focused on the study of the cause-effect correlation between the corrosion evolution and runoff condition, as well as on the influence of each alloying ele- ment on corrosion behaviour. To this aim, a multidisciplinary work, inclusive of several mon- itoring techniques, has been set up. This work has been carried out within a research activity on atmospheric corrosion of the quater- nary bronze alloy UNS C83600. In the first steps of this research, this alloy was submitted to cyclic wet–dry tests, in both natural and synthetic rain solution [15,16]. The comparison of results ob- tained with the different weathering solutions gave useful indica- tions for the formulation of artificial rains. In order to complete the 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.10.008 * Corresponding author. Tel.: +39 0512093462; fax: +39 0512093467. E-mail address: cristina.chiavari@unibo.it (C. Chiavari). Corrosion Science 51 (2009) 159–170 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci