Surface characterisation of corrosion inhibitors on bronzes for artistic casting

The corrosion behaviour of outdoor bronze artefacts depends not only on environmental factors but also on the alloy characteristics. An increasing requirement of the artistic foundries is the selection of new bronze compositions, with improved aesthetic features and resistance toward different corrosive agents. In this frame, surfaces of cultural bronzes (traditionally used Cu-5Sn-5Zn-5Pb and new Si-containing Cu-8Sn-3Si) have been characterised by means of X-Ray Photoelectron Spectroscopy (XPS) before and after immersion in solutions of corrosion inhibitor, in order to investigate the interaction of the inhibitor with the bronze surfaces. For comparison, the pure elements were also analysed in the same experimental conditions. The corrosion inhibitor, in aqueous solution, was a mixture of 5-hexyl-1,2,3-benzotriazole (C6-BTA) with 1,2,3-benzotriazole (BTA). The results obtained on pure copper indicate that, after mechanical polishing, the copper surface is covered by a thin layer of Cu2O (3 to 6 Å). After immersion in the inhibitor solution, the Cu LMM Auger lines are modified, and a signal at a binding energy of 400.0± 0.2 eV appears, both characteristic of the bonding of the inhibitor. By comparison with data already published on the adsorption of pure BTA on copper, there is the formation of a Cu(I)-inhibitor complex, on top of the Cu2O layer, still detected by XPS. The XPS analyses performed on the other pure elements, before and after immersion in the same inhibitor solution, showed that whereas no adsorption was observed for Sn, Pb and Si, the inhibitor reacts with the zinc sample, covered by a thin oxide layer. As regards the bronze samples, before immersion, the XPS analyses indicate that the bronze surface is composed of different copper, tin, lead and zinc oxides on Cu-5Sn-5Zn-5Pb, and of copper, tin and silicon oxides, with contamination of lead and zinc oxides, on Cu-8Sn-3Si. In both cases, it is possible to detect the signals from the metallic substrate. The heterogeneous distribution of the different oxides on the alloy surfaces is confirmed by EDS mapping. After immersion in the inhibitor solution, the same Cu(I)-inhibitor complex as on pure copper is detected, while the signals corresponding to the presence of tin oxide has disappeared. The signals from lead and silicon oxides are still present on the surface, and unchanged. From these observations, it can be proposed that the protection efficiency is not as good on the new bronze and on the traditionally used one. Indeed, electrochemical data (polarisation curves and EIS) and accelerated corrosion tests indicate that the Si-containing bronze alloy is more corroded than the Cu-5Sn-5Zn-5Pb alloy, after treatment in the inhibitor solution under identical conditions.