Organosilane coatings applied on bronze: Influence of UV radiation and thermal cycles on the protectiveness

•Corrosion protection of outdoor bronzes in urban atmosphere.•Coating with 3-mercapto-propyl-trimethoxy-silane (with/without CeO2, TiO2, La2O3).•Characterization of coated surfaces after natural and accelerated ageing tests.•Role of UV in the degradation process of studied coatings.•Influence of oxide particles on UV action. 3-Mercapto-propyl-trimethoxy-silane (PropS-SH) coatings, with or without the addition of three types of oxide particles (CeO2 (nano-sized), TiO2 (nano-sized) and La2O3 (micro-sized)), were applied on quaternary bronze (Cu-Sn-Zn-Pb) and tested through natural and artificial exposures. In particular, uncoated and coated samples were exposed both in the coastal town of Rimini (Italy) under sheltered/unsheltered conditions and in a climatic chamber where the samples underwent temperature/UV cycles, at constant humidity. The sample surfaces were periodically characterized by SEM, EDS, micro-Raman, FTIR and XRD techniques. Moreover, during natural exposures, runoff rainwater was collected monthly, in order to evaluate the release of the alloying metals from bronze by atomic absorption spectrometry (AAS). The results showed that micro-scale cracking and spalling phenomena took place during natural exposure, partly limiting the protective efficiency of the organosilane coating. In fact, UV radiation has a strong influence on coating performance, inducing structural modifications of the polymer. Oxide particle- and especially TiO2-charged coatings proved to be more resistant to UV degradation.