Electrochemical characterization of organic coatings for protection of historic steel artifacts

Electrochemical techniques are mainly known in the field of cultural heritage conservation as a tool for the elimination of corrosion layers or the removal of chlorides. However, these techniques are also a valuable tool for assessing the anti-corrosive efficiency of protective coatings. The aim of this study was to evaluate the performance of different coatings for their use in metallic heritage conservation using polarization resistance (Rp) and electrochemical impedance spectroscopy (EIS). Carbon steel samples were prepared to simulate the surface composition and morphology of historic steel artefacts, and coated by a conservator-restorer following the common practices in conservation treatments. Three commercial organic coatings have been studied: a microcrystalline wax (Renaissance) and a methyl acrylate/ethyl methacrylate copolymer resin (Paraloid B-72) dissolved in acetone –both them commonly used in conservation and restoration treatments– and a ethylene copolymer wax emulsion in water (Poligen ES91009), that has not been used so far for this purposes. Four commercial corrosion inhibitor additives were added to the Paraloid B-72 resin and Poligen ES-91009 wax. The additives were commercial preparations with the following known active components: a blend of triazoles (M435), an ammonium salt of tricarboxylic acid (M370), a calcium sulphonate (M109), and a bis-oxazoline (Alkaterge-T). Rp and EIS results showed that the best protection of the steel specimens was afforded by Poligen ES-91009 when applied in thick layers. None of the additives have shown a clear improvement of the protection properties of the coatings, and one of them impaired the barrier effect of the coating.