Therapy against metal corrosion based on polystyrene films: implications in cultural heritage conservation

A large number of ancient artefacts are constituted by metals, in particular, by alloys, as bronze. It’s well known that the main form of deterioration of ancient bronze artefacts is due to chemical and electrochemical corrosion. This spontaneous process, whose mechanisms are not yet completely clarified, creates on the surface of the object a corroded layer, called patina. The corrosion process of artistic and archaeological objects principally interests the copper in the alloy and is generated by various factors: air, water and, when the objects are buried, the soil components, as sulphates, carbonates, and chlorides. There are mainly three typologies of corrosion, which result from different factor and generate various forms of patina, but in particular the most dangerous corrosion process for bronze is the production, for reaction with salts (as NaCl) and acids, of nantokite, CuCl; this process is the so-called “bronze cancer” or bronze disease. Despite the knowledge about the corrosion mechanism is sufficiently deep and well-understood measures and therapies for preventing and inhibiting corrosion phenomena still need new materials and improvement. In view of the aim of developing new materials, the idea has been to create a protective film, put onto the metallic surface, that has as the main characteristics low adhesion to the surface and strong internal cohesion. The result is a kind of protective "peeling off" or "disposable", which, once degraded, can be removed without problems and without leaving traces. This contribution reports the preparation, characterization and application on metallic objects of these protective “peeling off” films obtained from solutions of polystyrene, polymer which has the characteristics of a low adhesion to the surface. In particular, a meaningful test was performed on some samples of “bronzed” brass artificially degraded to simulate the corrosion process “bronze cancer”. Morphological and chemical analysis carried out using AFM and SEM/EDX showed promising results in terms of protection against corrosion. The results are discussed also in view of a possible innovative therapy for the so called “bronze cancer”.