Preparation and characterization of bio-based protective coatings for the indoor preservation of bronze works of art

The corrosion of metals involves irreversible electrochemical processes, which are promoted by the presence of water, oxygen and acidic species: long-term exposure of metallic objects to these environmental agents results in the formation of layers of corrosion patina, ultimately leading to the pulverization and deterioration of the material. The main products commercially available since decades to prevent metal corrosion are based on acrylic resins containing inhibitors such as benzotriazole and its derivatives, but their toxicity addressed the efforts of the scientific community in the field of cultural heritage preservation to the development of new green multi-functional formulations, based on biomaterials and nanoparticles. In this work, we investigate the use of three different bio-based polymers (i.e. alginate, chitosan, carboxymethyl cellulose) for the development of multifunctional coatings to provide long-term protection to bronze works of art from humidity and corrosion agents. As required in the field of art preservation, the composition of these formulations have been optimized to maintain the original appearance of the artworks, by producing adhesive, homogeneous and transparent films, and they are easy-to-use and removable using safe procedures. The characterization of the coatings’ morphology, thermal behavior and the properties of the substrate/coating interface is performed via Optical, Metallurgical and Scanning Electron Microscopy, UV-Vis spectrophotometry, colorimetry and vibrational spectroscopies (Fourier transform infrared and Raman), ageing under controlled humidity and temperature, removal tests. The efficacy of the coatings is assessed on bronze disks and mockups by means of accelerated corrosion tests with HCl vapors on bare and coated specimens.