A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art
Introduction
This paper deals with the conservation state of metallic artefacts of archaeological and historical interest, exposed outdoors, which are affected by severe degradation, due to exposure to aggressive environmental conditions, in particular in most modern cities. Different corrosion product layers and patinas can grown on an artifact surface, depending on alloy composition and environmental conditions, and these can affect the corrosion behaviour of the metallic substrate. For example, patinas formed on Cu-based alloys can have a partially protective effect, while the atmospheric corrosion phenomena that occur on iron artifacts can lead to the formation of active corrosion product phases, whose contact with metal can accelerate degradation.
The development of suitable preventive conservation strategies for this important evidence of our culture requires their corrosion behaviour to be investigated in order to understand the electrochemical phenomena that occur on the metallic surface [1]. In this context, the possibility of performing in situ measurements, which allow an artefact to be characterized in its environment, and long-lasting monitoring campaigns to be set up to assess its stability, as a function of the exposure time, is of utmost importance [2], [3].
Moreover, it should be taken into account that large artefacts, such as bronze statues, iron structures and reinforcements in historical buildings and archaeological sites cannot be moved easily to a laboratory; therefore, the assessment of their conservation state requires a completely different approach. Microsampling carried out to obtain small specimens for the analysis by means of different analytical techniques can only be performed in particular conditions, and for these reasons it is important to find solutions that are suitable for in situ measurements and which respect the integrity and aesthetic appearance of the artefact [4].
This paper describes a possible solution that relies on Electrochemical Impedance Spectroscopy (EIS) measurements, and which is based on a simple portable instrument, and on some polymeric measuring probes, specifically designed and developed for non-invasive in situ measurements.
EIS is generally used to investigate the protective properties of organic coatings [5], [6], [7], but can also be employed to study the corrosion mechanisms that occur on metallic surfaces, and to estimate the corrosion rates [8], [9]. EIS measurements are capable of discriminating between different kinds of corrosion behaviour when coupled with a suitable data analysis. They can be proposed as a simple solution for the monitoring of the conservation state of an artifact to help restorers and conservators develop tailored conservation strategies.
Section snippets
Electrochemical impedance spectroscopy
Electrochemical Impedance Spectroscopy (EIS) consists of the measurement of the amplitude and phase of the surface impedance at different frequencies. The measurement is performed by using an electrolytic solution, so that, if the metallic surface is coated by a corrosion product layer, the current though it is due to both an electronic and an ionic contribution. Since corrosion products, such as metallic oxides and hydroxides, have low electronic conductivity, the ionic contribution can become
A low-cost EIS instrument, based on an Arduino board and logarithmic amplifier
Several portable devices for EIS in situ measurements are already on the market. These systems are generally quite large and expensive, because they need to be equipped with complex input front-ends, that are either ADC with more than 24 bits, or input amplifiers with a variable gain which spans more than one thousand, in order to be able to measure over a wide impedance range.
The proposed simple low cost solution is based on an architecture that takes advantage of the inherent compressing
The proposed system in the field
The complete EIS solution, composed of the logarithmic-based instrument and of the cell, has been used in several measuring campaigns on exposed outdoor artefacts in different Italian cities.
One of the monitoring campaigns on a Agapito Miniucchi’s masterpiece called Reditus ad origines is still in progress. The sculpture is exposed at the Scientific and Technological Pole of the University of Ferrara, and is an interesting example of the architectural recovery of an industrial area (Fig. 6).
The
Conclusions
The monitoring of the corrosion processes of artefacts exposed to the environment for very long times is a problem of utmost importance, both as far as the stability of the artefacts and their aesthetic appearance are concerned. Unfortunately, most of the analytical techniques generally used in laboratories require an artefact sampling and therefore cannot be used routinely.
From this point of view, EIS could be an interesting approach, provided that a comprehensive solution is available for in
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