Protection of bronze covered with patina by innoxious organic substances

Cultural bronze artefacts are exposed in indoor or outdoor environment. They often suffer of a substantial alteration due to an increasing atmospheric pollution. In this work, we propose the use of some innoxious compounds as corrosion inhibitors of bronze objects covered with patina. The bronze used was Cu-6Sn (in wt.%). This composition was selected after a preliminary work on several archaeological bronzes found in Transylvania, Romania, dated from the Late Neolithic to Roman periods. First, an artificial patina was formed on Cu-6Sn bronze under potential regulation, in 0.2 g L⁻¹ NaHCO₃ + 0.2 g L⁻¹ Na₂SO₄ aqueous solution (pH 8), during 4 days. A pale blue to green patina was obtained and characterized using EDS and Raman spectroscopy. Then, four innoxious organic substances were examined as corrosion inhibitors: 5 mM 4-methyl-1-(p-tolyl)-imidazole (TMI), 10 mM 1-phenyl 4-methyl-imidazole (PMI), 1 mM 2-mercapto 5-R-acetylamino-1,3,4-thiadiazole (MAcT), 1 mM 2-mercapto 5-R-amino-1,3,4-thiadiazole (MAT), and for comparison 1 mM benzotriazole (BTA). The impedance spectra collected showed, for all of them, three depressed capacitive loops. On the basis of these capacitance values, these loops were allocated to the surface film with ionic conduction, the double layer capacitance with the charge transfer resistance, and the oxidation-reduction process involving the surface patina. TMI and MAcT were found to be efficient inhibitors though their performances are significantly lower than that of BTA.