Composition and electrochemical properties of natural patinas of outdoor bronze monuments

The present study concerns the chemical–physical and electrochemical characterizations of the corrosion compounds formed on outdoor bronzes exposed in urban conditions. Here, we focused on the pale green patinas of surfaces directly exposed to the rainfall leaching. The investigation has been performed from scraped-off patina products of a well representative outdoor bronze monument (Monument to Francis Garnier, 1898) situated in Paris (France). The composition of the patina was analysed by energy dispersive spectrometry (EDS), coupled with SEM, and Raman spectroscopy. Structural characterization was conducted through high-resolution TEM (EDS coupled). The electrochemical reactivity of patina was investigated by cyclic voltammetry and EIS with a microcavity electrode in sulphate solution at pH 2 and 5.6 with time of immersion (up to 10 h). The bronze patina corresponds to a two-layer structure. The external layer is characterised by a marked selective dissolution of Cu and Zn of the alloy, which has been determined from the calculation of dissolution factors. The same phenomenon with the same amplitude has been evidenced on other bronze monuments used for comparison. The pale green patina is a complex mixture of crystalline copper hydroxisulphate and of amorphous/nanocrystal Sn-containing compounds. This patina is stable at pH 5.6 and reactive at pH 2 in relation with the dissolution of copper sulphate compounds in the acidic solution.