Influence of nanoparticles on the inhibiting efficiency of organosilane coatings on bronze. Part 2: metal release in accelerated ageing tests

In the past, different chemicals have been tested as corrosion inhibitors for outdoor bronzes, with the purpose to protect the bare metal against atmospheric corrosion. Nevertheless, benzotriazole (BTA) and its derivatives are still the most common compounds used for this application, even if, it has been demonstrated that BTA is efficient when applied on bare copper but not as much efficient when applied on bare bronze. Although copper is the main alloying element in bronze, the corrosion behaviours of copper and bronze are different because, in the case of bronze, each alloying element plays a different role, leading to a decuprification phenomenon with relative tin enrichment on the bronze surfaces. Furthermore, in real outdoor bronze monuments, the corrosion behaviour is strongly influenced by the exposure geometry: sheltered and unsheltered areas (with respect to the rainfall) may produce patinas with significantly different features. This must be taken into account when designing conservation procedures, since the patina is in most cases the support on which corrosion inhibitors are applied. With the aim to obtain reliable results, the inhibitors should be applied on supports representative of real surfaces and their efficiency should be tested under conditions which reproduce as much as possible the complexity of outdoor exposure. In this work, which is part of a wider project concerning the conservation methods of bronze and gilded bronzes, the protective efficiency of a silane-based inhibitor (3-mercapto-propyl-trimethoxy-silane, henceforward PropS-SH) with or without addition of ceria (CeO2) nanoparticles has been evaluated towards both bare and aged (pre-patinated) bronze. These inhibitors were tuned by previous tests in chloride solutions and the results are presented in Part 1 of the work (see correlated work by Balbo et al.). The efficiency of PropS-SH and PropS-SH+CeO2 was preliminarily tested by total immersion tests on pre-filmed of bare bronze specimens. Subsequently, in order to produce pre-patinated bronze surfaces to be used as a representative substrate for the application of the inhibitors, a wet & dry accelerated ageing device (simulating the action of stagnant rain in sheltered areas) was used both to age the surfaces (1st phase) and subsequently to test the inhibitor efficiency on the aged surfaces (2nd phase). The evolution of the corroding surfaces and the metal release in the ageing solution were analyzed through the use of a wide range of spectroscopic techniques. The results showed the high protective efficiency of the tested inhibitors, which further increased when the inhibitors were applied on pre-patinated bronze.