Use of EIS for the evaluation of the protective properties of coatings for metallic cultural heritage: a review

The best way to reduce the degradation of metallic cultural heritage is through preventive conservation measures but, in many cases, it is not possible to obtain adequate environmental conditions, and it is necessary to apply coatings to the artefacts in order to protect them against corrosion. There is a continuous search in the metal conservation community for new and improved coatings that provide a better protection to the objects, while respect the special requirements of the conservation-restoration ethics. While electrochemical techniques have a long tradition in conservation-restoration treatments for metallic cultural heritage, the evaluation of protective coatings using electrochemical impedance spectroscopy (EIS) has only been used very recently. EIS is a very well established method to investigate metal coatings for general purposes, and have many advantages that make it especially suitable for testing coatings for metallic works of art. This paper makes a review of the use of EIS for testing coatings for metallic cultural heritage, from the first publications in the mid nineties of the last century to the last papers. The experimental setup used, the types of coatings and metals investigated and the interpretation of the results is reviewed and compared with the use of EIS for testing general purpose anticorrosive coatings.