In Situ Electrochemical Impedance Spectroscopy Measurements and their Interpretation for the Diagnostic of Metallic Cultural Heritage: A Review

Abstract

Protecting metallic cultural heritage from corrosion is a major challenge for metal conservators. Electrochemical techniques offer a great potential for monitoring the conservation condition of this type of heritage, providing a powerful tool for designing conservation strategies. For this reason, the application of techniques such as electrochemical impedance spectroscopy (EIS) has aroused increasing interest in recent years, although some difficulties in application and interpretation of the results still keep it from being routine practice. This Review aims to picture the state-of-the-art of using electrochemical impedance spectroscopy (EIS) for the in situ monitoring of metallic cultural heritage, focusing on spectra interpretation and different approaches that have been proposed to perform field measurements since the 1990's. In the first part of the Review, different cell designs based on the use of liquid or solid electrolytes for in situ measurements are reviewed. In the second part, a few general equivalent circuits are proposed as a basis for interpreting results on different metal surfaces after discussing different models proposed in literature. This overview intends to summarize and clarify the key points in the use of EIS, and encourage the use of this and other electrochemical techniques in the field of conservation science, for a better preservation of our cultural heritage.

Agar versus Agarose Gelled Electrolyte for In Situ Corrosion Studies on Metallic Cultural Heritage

“…In order to elucidate the contribution of agarose to the EIS spectra at other frequencies, results were fitted to the general equivalent circuit presented in Figure . The use of this circuit has been discussed in a previous paper . Rs is the resistance of the electrolyte, CPE1‐R1 represent the resistance and capacitance of the patina or rust layer, CPE2‐R2 the capacitance of the double layer at the metal‐electrolyte interface and charge transfer resistance of the redox reactions and W represents diffusive effects related to copper ions diffusion in the patina for bronze and to oxygen diffusion through the pores in the rust layer, in the case of steel .…”

“…Development of new and improved diagnostic tools is a relevant subject in conservation science, and the application of electrochemical techniques has become an outstanding topic in recent years for the assessment of patinas and protective coatings for metallic heritage . The unique nature of the objects, their irreplaceable value, the long‐term evolution of patinas and corrosion products (which cannot be reproduced in the laboratory) and the requirement of conservators‐restorers to have scientific evidence to support decisions on conservation treatments make necessary to carry out in‐situ scientific analysis on the real heritage artefacts.…”

A Novel Application of EIS for Quantitative Coating Quality Assessment During Neutral Salt Spray Testing of High-Durability Coatings

“…The overall system is characterized by high electrical resistance and low capacitance which may quickly and easily be quantitatively measured, especially, after the development of advanced EIS instruments having: pA current measurement range, high input impedance up to 1 TΩ, variable gain amplifiers, variable filters input front-ends and a floating ground. 2,3 Over the last few decades, electrochemical methods have found widespread use for characterization of anticorrosive coatings and have been commonly employed to assess the performance and durability Mrđa Lalić and Martinez: A Novel Application of EIS for Quantitative Coating ... of anticorrosive coatings in the laboratory. 4 By making periodic EIS measurements during the exposure to the corrosive environment, the level of coating damage can be estimated.…”

“…14 Impedance greater or equal to 10 9 Ω corresponds to excellent protecting properties of coatings which provide almost purely capacitive behaviour over long exposure periods. 15 The described EIS model may be applied to various industrial coating types and thicknesses providing a universal tool for coating quality and protective ability comparison [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] .…”

“…EIS has been measured periodically during the NSS test by shortly stopping the exposure to salt mist and performing measurements with the flexible gel electrodes which enabled capturing of the momentary coating state. 2,3,[25][26][27] For comparison, measurements have also been done by EIS according to EN ISO 16773 standard and by FTIR, DSC and TGA methods.…”