Organic Corrosion Inhibitors for copper-based artefacts: investigating the mechanism of interaction with an historical corrosion layer

摘要

Many studies aim at understanding the mechanism of action of organic inhibitors. Usually, suchstudies refer to uncorroded metallic surfaces, which need to preserve their functionality whenexposed to specific conditions, such the immersion in an aggressive medium. Different is the casewhen inhibitors are employed for the conservation of metallic artefacts and especially when acorrosion layer, the patina, is to be preserved. In this case, the role and the efficiency of suchinhibitors is not always clear and other issues such as toxicity of the compounds arose a greatdebate in the last decades. Benzotriazole (BTAH), despite being classified in many studies ascarcinogenic, is one of the most diffused inhibitors for copper in the industrial field [1] while inconservation controversial opinions exist on its efficacy, although it is also employed for botharchaeological and historical objects.Our study aims at clarifying the role of benzotriazole and similar organic inhibitors: 5-carboxybenzotriazole(5-CBT), benzimidazole, 2-mercaptobenzothiazole, trimesic acid. Especially, theformation, characteristics and role of coordination compounds resulting from the interactionbetween inhibitor and copper phases is investigated, considering that such compounds will be partof the modified corrosion layer.The methodology followed involves on one side the interaction of organic compound and (solubleand insoluble) copper phases typically present on a corroded copper artefact [2]. The interaction ofBTAH and soluble copper sulfate phase chalcanthite gives the characteristic XRD pattern and IRspectrum corresponding to the coordination compound Cu(BTA)2 [3]. In presence of anothersoluble copper species CuCl2·2H_2O, a difference in crystallinity of the resulting compound isobserved, thus suggesting a certain role of chlorine when the treatment is applied and the possibleconnection between BTAH treatment efficacy and chlorine in the corrosion layer.In parallel, we study an historical corrosion layer and its chemico-physical and morphologicalmodifications after interaction with the treatment solutions. A global evaluation of the differenttreatments is initially carried out by determining the polarization resistance (Rp) for corroded anduncorroded copper in the different inhibitors’ solutions. Additionally, a multi-analytical and multiscaleseries of complementary techniques is put in place in order to assess and relate parameterssuch as color change, surface tension (drop test), penetration of the treatment, changes in porosityand permeability. First results show how BTAH exhibits a better penetration and increased surfacetension compared to 5-CBT while both compounds determine a shift towards darker green color ofthe brochantite surface layer.