Analysis of the influence of the electrochemical cell setup for corrosion measurements on metallic cultural heritage

In the field of metallic cultural heritage (CH), electrochemical techniques such as corrosion potential measurements, polarization resistance (Rp) or electrochemical impedance spectroscopy (EIS) have been used occasionally to evaluate corrosion or coatings performance. More recently the interest on the application of these techniques, especially EIS, in cultural heritage has been growing. Considering that sculptures and monuments cannot be brought into the laboratory, and the often complex surfaces of metallic CH objects, measurements have to be carried out in situ . To address the main constraints related to in situ measurements, especially handling the electrolyte, we considered two approaches: contact probe CP with a commercial cloth soaked with the electrolyte and G-PE cell with agar gelled electrolyte. In CH, the variety of situations and the heterogeneity of surfaces can make the interpretation of results a major issue. On the other hand the use of low conductivity electrolytes in order to not affect CH surface can make the cell impedance not negligible. For these reasons it is crucial to deepen the knowledge on possible parasitic contributions in order to fully exploit the electrochemical information from in situ measurements. With this purpose, and to a better understanding of the differences, advantages and disadvantages of different approaches, a series of EIS and Rp measurements have been done conjointly with the two cells and a traditional one. To minimize other possible contributions, measurements have been performed in the same laboratory under same conditions. AISI316 stainless steel has been chosen as reference material for coupons, as a stable and well characterised working electrode (WE) making its choice quite effective in order to focus on possible contributions of the different elements of the experimental setup. Laboratory test were complemented with in situ measurements on a stainless steel sculpture, Mediterranea III , by Martin Chirino (Leganes Sculpture Museum). From a qualitative point of view, some differences in uncompensated resistance and time constants between different setups have been observed. The identification of an appropriate model to describe and fit the measured data is under way. This will allow getting the information necessary to separate the electrochemical response of the systems of interest from the contribution of the measuring setup. Results from this joint work will contribute to a better understanding of in-situ corrosion measurements on metallic CH, and hence to tackle conservation problems on metal CH.