A gel electrochemical cell for in situ assessment of patinas and protective coatings for metals

Abstract

Over the past few years a big effort has been done in developing non-invasive techniques and portable instrumentation for cultural heritage studies. Besides classical analytical techniques, giving information on materials composition, other techniques assessing the conservation status of artefacts have acquired an increasingly important role in conservation science. Nevertheless, these techniques are less known and still far from being routinely, despite the relevance of having diagnostic tools in order to a better design of conservation treatments. When speaking about metals, Electrochemical Impedance Spectroscopy (EIS) is a technique which provides information on electrochemical properties and behaviour of materials. EIS has been widely used in the study of corrosion mechanisms and coatings performance for industrial applications, and in the last two or three decades some researchers have been working in developing its application for cultural heritage. The application of EIS for in situ evaluation of the preservation status of metals sculptures and monuments is not an easy task. How to hold the electrolyte in contact with the irregular surface of the object, or how to interpret the results according to the complexity of the substrate and the often-limited quality of measurements due to low conductivity of the electrolyte or environmental noise- are the main constraints. To overcome all these difficulties, we have been working in the development of a portable electrochemical cell with a gelled electrolyte (G-PE cell), especially designed for in situ measurements on cultural heritage. All design parameters such as influence of the gelled electrolyte, optimal concentration, selection of electrodes, cell geometry, etc. have been carefully studied to optimize the cell design. The design of the cell and a selection of case studies are presented to show the potential of this diagnostic tool for conservation assessment of metals and evaluation of coatings performance.