The iron artefacts of the Amiens Cathedral: in situ EIS measuring campaign for the assessment of the corrosion behaviour

Conservation of ferrous artefacts in heritage buildings requires the characterization of the corrosion products in order to understand the atmospheric corrosion mechanisms and to develop tailored, long lasting conservation methodologies. In this study, the corrosion products grown on several iron artefacts exposed to over 500 years in the Amiens Cathedral have been studied by means of electrochemical impedance spectroscopy (EIS). The artefacts, subjected to long time indoor atmos-pheric corrosion, are coated by corrosion products constituted by iron oxides and oxy-hydroxides, such as goethite, lepidocrocite, maghemite, akaganeite, organised in a very complex structure. In situ EIS has been employed to investigate the phenom-ena involved at the electrochemical interfaces among the various corrosion products and to assess and predict their corrosion behaviour. The measurements have been performed by means of a portable EIS analyser specifically designed to work as a stand-alone device, all over a frequency range from 0.001 Hz to 40 kHz and for impedances in the range of 1 k - 10 G. Adhesive and flexible probes with conduc-tive gel have been employed to carry out the impedance measurements without af-fecting the rust layers and the metal surface. The EIS data have been compared with the microchemical and microstructural characterisations of the layers.