Micro-characterisation of innovative organic coatings applied for the protection of outdoor bronze monuments

Application of protective coating is, to date, the best treatment for the protection of outdoor bronzes. In the present work, new protective organic coatings were developed taking into consideration low environmental impact, inhibiting efficiency and the nature of the patina according to the exposure geometry. The research presented in this study is a part of B-IMPACT project and aims at assessing the protectiveness of innovative organic coatings for historical and modern bronze monuments. Within this context, this work focuses on the characterisation of protectivecoating/ patina/bronze systems. The investigated system is related to an organic coating on an artificially patinated bronze, simulating natural patina. A quaternary as-cast alloy (Cu-Sn-Zn- Pb), as usually used for artworks, was artificially aged by simulating natural outdoor exposures, and then coated by an optimised fluorosilane coating applied by brushing. In order to understand the protective properties of this coating/patina/bronze system, the investigation was performed before and after the application of a representative accelerated ageing test. As the coated artificial patinas are a few μms thick, a fine characterisation sequence was applied for understanding of these multi-layer systems. A multi-analytical approach was performed, including conventional analytical methods such as microscopy coupled with elemental analysis (FEG-SEM with EDS) as well as X-ray Photoelectron Spectroscopy (XPS). In order to characterise the interaction between the protective coating and the patinated substrate down to the nanometric scale, structural analysis of the coated system was also investigated from cross-sections, obtained by Focused Ion Beam (FIB) coupled with FEG-SEM. Achieved information on structural homogeneity, such as (micro- and nano-) porosities and elemental distribution within the different layers of the systems, as well as the coating/patina interface cohesion are here reported.