Outdoor bronze corrosion: nanoscale XPS investigation of silane-based protective coatings

Application of a protective coating is the most widely used conservation treatment for outdoor bronzes (cast Cu-Sn-Zn-Pb alloys). However, improving coating protectiveness requires detailed knowledge of the coating/substrate chemical bonding. This is particularly the case for 3-mercapto-propyl-trimethoxy-silane (PropS-SH) applied on bronze, exhibiting a good protective behavior in outdoor simulated conditions. To this aim, Synchrotron Radiation (SR) XPS (X-Ray Photoemission Spectroscopy) as well as conventional XPS can be used as a very powerful tool for such investigations. In particular, scanning photoemission microscopy at ANTARES (Synchrotron SOLEIL) offers energy resolution and lateral resolution not available with conventional XPS. Hence, the present work deals with characterization of both the surface and the interface between PropS-SH and bronze, with sub-micrometric spatial resolution. The silane-based coating was applied both on bare bronze and on artificially aged bronze, so as to reproduce a representative application condition for real outdoor monuments. Coated bronze samples were analyzed both in free surface and in cross-sections (prepared by the FIB (Focused Ion Beam) lift-out technique). By compiling complementary spectroscopic and imaging information, a deeper insight in the interactions between the protective coating and bronze was achieved.