Comparative studies of chemical and electrochemical preparation of artificial bronze patinas and their protection by corrosion inhibitor

The bronze artefacts of cultural heritage are often covered with patina, a layer of corrosion products, which confers their aesthetic and also protects the substrate bronze. Due to the increasing atmospheric pollution these layers are often dissolving when exposed in urban environment. In this work we propose the use of an innoxious imidazole compound as a corrosion inhibitor for patinated bronze. On a Cu-6Sn (wt%) bronze, three types of patinas were synthesized: two by chemical methods (in a sulphate solution and a chloride one) and one by an electrochemical process (in a sulphate/carbonate solution). A blue-green patina was obtained in all three cases, and their morphological and structural characterization was performed by SEM, EDS and Raman spectroscopy. It was found that the sulphate patina is composed essentially of brochantite, the chloride patina of atacamite, and the electrochemical patina of malachite. All three patinas have also a smooth part of surface consisted of cuprite. As corrosion inhibitor 4-methyl-1-(p-tolyl) imidazole was used on all patinas, in a solution of 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ acidified to pH 5 which simulates acid rain in urban environment. The results have shown that the inhibitor improves the stability of all three kinds of patinas and can be recommended for protection of works of art.