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A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art
Accepted Manuscript A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art S. Grassini, S. Corbellini, M. Parvis, E. Angelini, F. Zucchi PII: DOI: Reference:
S0263-2241(16)30373-6 http://dx.doi.org/10.1016/j.measurement.2016.07.014 MEASUR 4201
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
20 March 2016 4 July 2016 5 July 2016
Please cite this article as: S. Grassini, S. Corbellini, M. Parvis, E. Angelini, F. Zucchi, A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art, Measurement (2016), doi: http://dx.doi.org/10.1016/ j.measurement.2016.07.014
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A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art S. Grassinia , S. Corbellinib , M. Parvisb,1,∗, E. Angelinia , F. Zucchic a Dipartimento
di Scienza Applicata e Tecnologia Politecnico di Torino di Elettronica e Telecomunicazioni Politecnico di Torino, Corso Duca degli Abruzzi, 24 - 10129 Torino c Corrosion Study Center “A. Dacc` o”, TekneHub, Universit` a di Ferrara, Italy b Dipartimento
Abstract Metallic artifacts of archaeological and historical interest, exposed to outdoor environmental conditions, can be affected to a great extent by degradation, due to the presence of aggressive compounds in the surrounding environment.The development of suitable preventive conservation strategies for these important signs of our culture, requires their corrosion behavior to be investigated in order to understand the electrochemical phenomena that occur on the metallic surface. This investigation can easily be performed in a laboratory by means of different chemico-physical techniques, which provide information on the composition, microstructure and morphology of the corrosion products, as well as by means of electrochemical measurements, which allow the corrosion resistance of the metal to be estimated. Unfortunately, these measurements are often invasive and require microsampling. Electrochemical Impedance Spectroscopy (EIS), thanks to the its high degree of sensitivity, and to the small perturbation applied to the corroding system, can be considered a useful non-destructive testing method to obtain valuable information on the conservation state of metallic works of art. This paper describes an innovative, portable, low-cost and user-friendly solution for EIS in situ measurements, based on the widely diffused Arduino board, and on specifically designed measuring probes. The proposed solution cannot provide the detailed knowledge of the corrosion mechanisms that occur on metallic surface, but it can be an extremely useful tool for conservators and art historians for the assessment and long-time monitoring of the stability of the artifacts. Keywords: EIS, Metallic artefacts, Corrosion measurement, Cultural Heritage, Portable instrument
∗ Corresponing
author: [email protected]
Preprint submitted to Elsevier
July 4, 2016
1. 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. 2. 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 2
Reference electrode
ICW
Counter electrode
VAC Electrolytic solution VWR
Corrosion layer
VDC
RS Cp1
R1
Cp2
R2
Metallic artifact (working electrode)
Figure 1: EIS measuring principle: an electrolytic solution is put in contact with the sample and the impedance is measured between the artefact, which is usually referred to as “working electrode”, and a “counter electrode” usually made of a noble metal, such as Pt. A third electrode referred to as reference electrode is employed to measure the so called open corrosion potential, however the impedance spectrum can be obtained also in the absence of the reference electrode.
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 important. Therefore the measurement of the impedance at different frequencies allows studying the electrochemical reactions that occur on the metallic surface and permits to highlight the presence of cracks and porosity on the surface layer, which can act as starting points for localized corrosion attacks. Fig. 1 shows the measurement setup generally used for EIS measurements in a laboratory. EIS measurements are performed by employing a conventional electrochemical cell filled with an electrolytic solution. The cell consists of three electrodes: a “counter electrode”, which is usually made of a noble metal, such as platinum; a “reference electrode”, which has a stable and know electrochemical potential (Ag/AgCl, SCE - saturated calomel electrode, etc...); the metallic artefact, whose surface impedance has to be measured, which is part of the cell, and is referred to as the “working electrode”. A difference in the electrochemical potential between working and reference electrodes occurs in the presence of an electrolytic solution. This electrochemical potential, the so-called open circuit potential (E OCP ), can be monitored in order to follow its drift over time, and to collect further information on the degradation mechanism that is affecting the metallic sample. The working and counter electrodes are connected to the two poles of a signal generator. In order to minimize the effect of the measurements on the corrosion behaviour of the metallic artefact, the signal generator applies a stimulus, which is composed of a DC component and an AC component. The DC component is set to the voltage that occurs naturally between the working and counter electrodes, due to their different electrochemical potentials; the AC component 3
is limited to a few millivolts, usually no more than 10 mV, to avoid stimulating any non linearity of the system under investigation and accelerating the corrosion process. The EIS system measures the ICW current that flows through the metallic surface, i.e. between the counter and working electrodes, and computes the surface impedance as: Z=
VWR ICW
(1)
where VWR is the alternating voltage between the working and reference electrodes. All the values in eqn. 1 are shown in boldface as they are complex numbers. If the impedance between the counter and reference electrodes is negligible with respect to the impedance of the corrosion layer, VWR can be measured between the counter and working electrodes, thus avoiding the necessity of using of the reference electrode. The development of a two-electrode EIS measuring approach simplifies the procedure used to perform in situ measurements and, consequently, should encourage the employment of the EIS technique in the cultural heritage field, where simple to use and fast response methodologies are required. When EIS measurements have to be performed on historical and archaeological works of art to asses their conservation state, non aggressive electrolytic solutions should be used, in order to minimize their effect on the degradation process. In principle, the measurements can be performed in mineral water, but its very low conductivity has to be taken into account, in particular if the metallic artefact is coated with a very thin corrosion product layer. In this case, low aggressive electrolytic solutions, such as a sodium sulphate solution (N a2 SO4 ), can be employed. However, it should be considered that it is not possible to measure the solution resistance with a two-electrode cell, thus, since this may affect the measurement, such a resistance has to be estimated on the basis of the electrode/cell geometry, as well as the electrolyte chemical composition and conductivity. In principle, the measurement can be performed on any surface area, but the impedance values are generally scaled to an exposed area of 1 cm2 in order to enable easy comparisons. The impedance is measured for frequencies in the range of 0.01 Hz to 100 kHz. The impedance measured values obviously depend on the nature of the surface layer and can vary from a few tens of ohms per square centimetre, for bare metal, up to several gigaohms for a metallic substrate coated with a thick, compact and non-conductive layer. The EIS spectra are then analysed to determine simple electrical components, such as resistors, capacitors, and inductors, of the equivalent circuit models that represent the coated metal/solution interface. These circuits reproduce the electrical properties of the system under investigation, and the different elements are assigned to different physical elements, but there is not a unique correspondence between the EIS data and the equivalent circuit, and different circuits can be employed to model the same impedance spectrum.
4
Several approaches can be followed for this identification (see for example [10, 11]) but, due to the low electronic conductivity and thickness of the corrosion product layers that are usually present on the surface of metallic archaeological and historical artefacts, the impedance spectra tend to have a mainly capacitivelike behaviour, at least at mid frequencies. However, the actual behaviour over a large frequency range, e.g. spanning 7 decades, requires equivalent circuit models with several electrical components, in order to take into account the electrode kinetics, diffusion phenomena and the heterogeneous nature of the corrosion product layer. Electrochemical processes often exhibit complex behaviour and ideal capacitors are not able to model all the EIS spectra, as for example, in the case of a non-uniform surface layer, surface roughness or an inhomogeneous current distribution; thus, some specialized electrical elements such as the Warburg and Constant Phase Element (CPE) have been introduced to overcome this shortcoming and to develop equivalent circuit models that provide useful and meaningful results. The CPE has the following impedance: Z=
1 Q(jω)α
(2)
where j is the imaginary unit, Q is the CPE value and α is an exponent in the range of 0 to 1 which characterizes the CPE. The CPE can model a resistor (α = 0), a capacitor (α = 1), a pure diffusion phenomenon (α = 0.5) and/or mixed conditions when α assumes intermediate values. As an example, fig. 1 shows a typical circuit, based on the circuits proposed by Randles in 1947 [12]. The circuit contains a resistor that models the ohmic resistance of the electrolytic solution, and two cells, composed of a resistor and a CPE, which model the phenomena that occur inside the corrosion product layer and at the interface between the corrosion product layer and the solution, respectively. This circuit can be employed, for example, to model the electrochemical behaviour of an iron artefact exposed to outdoor atmospheric corrosion. It relies on the flexibility of the CPE to model the presence of a heterogeneous surface, and the diffusion processes through the porous material in the corrosion product layer. This circuit has 7 parameters (i.e. 4 parameters for the 2 CPEs, plus 3 parameters for the resistances) which have to be adjusted to fit the experimental data [13]. In this case, the data fitting is not easy, and the values obtained for the circuit components are often questionable since, as already mentioned before, the impedance values can span 6-8 decades and a fitting procedure that is simply devoted to minimizing the sum of squares would therefore mainly weight on the high impedance values, i.e. on the low frequency values: X 2 min |Ze − Zm | (3)
5
where Ze are the measured experimental points and Zm are the impedance values obtained by the model. Several approaches can be followed to minimize this effect such as the ones described in [11]. Another simpler possibility is the use of a different metric, such as: min
X |Ze − Zm |2 2
|Ze |
(4)
which weights the experimental data regardless of their actual values, and is used in the examples considered in this paper. It is important to observe that the presence of noise on the measured data, which mainly arises from the extremely small current amplitudes, makes the fitting problematic: for example, when the impedance is of the order of 109 Ω, with a voltage amplitude of 10 mV, the current is of the order of 10 pA and the noise contribution can therefore become quite important. All these problems are usually tackled by designing costly instruments, equipped with complex input front-ends [14], [15] based on variable gain amplifiers and variable filters. Such a solution is effective, but limits the possibility of arranging multiple measuring systems that can used in parallel to reduce the measuring time. In fact, since the lower frequency used for the measurement is usually 0.01 Hz, which corresponds to a period of 100 s, a measurement session generally requires no less than 20 min. 3. The low-cost EIS system for on-site measurements 3.1. 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 capability of devices based on logarithmic amplifiers [16], coupled with the processing capability of an Arduino Due board, which is equipped with a simple 12 bit converter. Fig. 2 shows the basic idea of the logarithmic process. An example of two input current signals with an amplitude ratio equal to one hundred is shown on the left. The signal it would be possible to obtain with the conventional linear processing and a direct 8-bit conversion is shown in the lower part of the picture: it is easy to observe how the smaller signal, which has an amplitude of only a few less significant bits, get heavily corrupted by the quantization noise. The upper section of the figure shows the logarithmic process. It can be observed that the signals are initially DC shifted to become positive and are then sent to the logarithmic converter which compresses the dynamic range. The 6
Logarithmic processing
In (µA)
Signals with amplitude ratio 1:100
DC shifter
Log Amplifier ADC
AntiLog & DC rem.
Digitized signals scaled to the same amplitude
Out (V)
Conventional linear processing
ADC
Figure 2: Principle of the EIS system based on the logarithmic converter.
compressed signal is digitally converted and the obtained values are eventually passed through a digital anti-log function to recreate the original signal. It is easy to see how in this case both signals can be recreated making the minimal quantization noise. The overall process obviously heavily relies on the correct choice of the DC shift [16] and the proposed solution therefore offers real advantages when sine signals are used; nevertheless, in the proposed application, it permits quite interesting results to be obtained. The block diagram of the proposed instrument is shown in fig. 3. The system is composed of two boards: one board is an off-the-shelf Arduino Due microcontroller, while the other board is arranged as a shield that has to be connected to Arduino Due. Arduino Due is based on a 32 bit microcontroller, which contains a 12 bit ADC and two 12 bit DACs. A complete EIS solution which employs the internal DACs and ADC of the Arduino board can be arranged [16], with good results. However, the system flexibility can be increased if the ADC embedded inside the Arduino microcontroller is used to sample the signals, and a separate DDS, allocated on the shield, is employed to generate the stimulus. The DDS is paced by the Arduino clock to ensure a synchronous sampling with the generated signal and to enable an equivalent time acquisition that also permits several samples to be acquired at the maximum measuring frequency, thus overpassing the limit connected to the maximum sampling frequency of the ADC, which is of 1 MHz. Since the complete system works in synchronous mode, the frequency of each signal is known in advance and the acquired samples can therefore be processed by using a simple conventional 3 parameter sine fit. The bias that is required to keep the input signal positive and to allow the logarithmic converter to work correctly, is obtained by using a 16bit DAC, 7
Shield Arduino
Buffer DDS
Switch DA
Cell
DA LogAmp Arduino Due
512kB Flash
Digital I/O
ADC
Clock 96kB SRAM
USB PC
Figure 3: Block diagram of the proposed instrument.
installed on the shield. The DAC output is fed into a diode, so that it is easy to obtain the very large range of bias currents that are required to cope with the large range of input currents. The relationship between the DAC output voltage and the bias current is of course not know and it depends on the actual temperature. The bias is therefore continuously updated, by means of a feedback loop, which keeps the input signal in the correct logarithmic amplifier interval. This bias can be easily removed from the sampled values by means of the sine-fit algorithm [16]. A digital switch has to be used to disconnect the signal generator output from the artifact, thus permitting the open corrosion potential (E OCP ) to be monitored: when a measurement is started, the instrument measures the E OCP , sets the the value of the DC component to such value, closes the switch, and then starts the measurements. Fig. 4 shows the proposed system, along with a small box, which is used to enclose the Arduino Due board and the shield that contains the analogue circuits. Table 1 shows the most important specifications of the proposed system. The impedance magnitude and phase uncertainty have been measured by using standard components thus in the absence of varying corrosion potentials, and by adjusting the parameters of the logarithmic amplifier which is the most critical component so that worse values can be expected on less favourable conditions. 3.2. Simple cells for in situ EIS measurements In order to perform in-situ measurements, in addition to the EIS portable instrumentation, a simple electrochemical cell is required. The cell has to be suitable for measurements on tilted and vertical surfaces and it had to be specifically designed in order to measure the impedance, without damaging the artefact surface. 8
Arduino Shield
Arduino Due board
Instrument enclosure
Figure 4: Picture of a prototype of Arduino Due based instrument.
Table 1: System Specifications
Impedance range* Frequency range Stimulus amplitude Current consumption Magnitude uncertainty Phase uncertainty Meas. Time+ (0.01 Hz − 0.1 Hz) Meas. Time+ (0.1 Hz − 1 Hz) Meas. Time+ (1 Hz − 10 Hz) Meas. Time+ (10 Hz − 1 kHz) Meas. Time+ (1 kHz − 100 kHz) * only at low frequencies + with 15 points per decade
9
100 Ω − 10 GΩ 0.01 Hz − 100 kHz 10 mVpp − 2 Vpp 100 mA better than 5% better than 3◦ 900 s 180 s 80 s 100 s 30 s
Some commercially available probes that exploit a fixing solution are based on a magnetic ring. However, these devices can only be used on ferromagnetic materials. In addition, in order to have an acceptable magnetic force, these probes have to have a diameter of the order of 70 mm, so that they only can be used on almost completely flat surfaces. Fig. 5 shows the measuring cell specifically designed to be fixed onto complex shaped metallic structures by means of a removable double-side bonding tape. The cell is made of acrylonitrile butadiene styrene (ABS) and has an external diameter of about 30 mm, and a thickness of about 20 mm. The cell has a measuring chamber with a diameter of about 8 mm, and, therefore an exposed surface area of about 0.5 cm2 . The cell can be filled with the electrolytic solution by injecting it into the inlet pipe; the air in the cell flows out from the outlet pipe shown in fig. 5, where a section of the electrode is depicted. The cell is equipped with a 2 − 3 mm thick adhesive closed-cell polyurethane disk which makes it possible for the cell to be used on non-perfectly flat and/or rough surfaces. This pad is fixed to the artefact by means of a 0.3 mm thick bi-adhesive disk. The selection of the adhesive disk is of utmost importance since the glue must be strong enough to prevent the electrolytic solution from pouring out, but not too strong to prevent the cell from being removed after the measurement. Outlet tube Inlet tube
Pt counter
Measuring chamber
Figure 5: The cell specifically designed for EIS in-situ measurements.
A platinum wire is used as the counter-electrode. The wire is positioned inside a specific circular slot, at about 1 mm from the cell bottom, and is thus at 3 − 4 mm from the artefact surface, depending on the thickness of the employed foam pad. When this configuration and a 0.1 M solution of N a2 SO4 , are adopted, the solution resistance that appears in series to the measurements is negligible and the current distribution on the measured surface is uniform, within 30%, even in the presence of a very low surface impedance.
10
Reditus ad origines 5
|Z| (Ω)
10 Thick corrosion layer on slanted surface 2
10 Thin corrosion layer on vertical surface
θ (deg)
50
0 −2
10
−1
10
0
1
2
3
4
5
10
10
10
10
10
10
Frequency (Hz) Figure 6: EIS spectra collected on the Reditus ad origines weathering steel sculpture, exposed to outdoor environmental conditions in Ferrara, Italy.
4. 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 sculpture which is made of weathering steel, better known as CORTENTM is composed of two blocks, having sizes 11 × 5.7 × 5.2 m and 2.5 × 6 × 6 m respectively where the basic geometric shapes, square, rectangle, triangle, semicircle, evokes the symbols of our common heritage, from the plane tuning fork to the impending beam, to the dangerous spear. Some COR-TEN sheets are positioned vertically and some are positioned at different descending degrees down to about 30◦ . A visual inspection has highlighted different surface conditions: the vertical surfaces, which undergo a fast drying after rainy periods are coated with a thin orange-red corrosion product layer, while a thicker red-brown corrosion product layer is present on the other surfaces. Fig. 6 shows the EIS spectra collected on three different areas. The impedance is shown as Bode plots, and all the values have been scaled to an equivalent 11
area of 1 cm2 . The impedance modulus, |Z| value, indicates the extent of the corrosion phenomenon at the metal/corrosion product layer interface. The higher the impedance value, the higher the electrochemical stability of the rust layer and, consequently, the higher the protective effectiveness against further corrosion. The EIS spectra recorded on the red-brown area (black and red lines in the Bode plot) highlight the higher stability of the corrosion product layer, a result that is confirmed by the higher impedance value, while the orange-red layer (pink line in the Bode plot) shows different electrochemical behaviour, and a lower impedance value. Moreover, it should be noted that the black line has a clearly different phase trend from that of the red and pink plot traces. This can be explained by considering the different porosity of the surface layers, which can easily be observed by looking at the small insets, which show the artefact surface where the plots have been obtained. Fig. 7 shows the results of the data fitting, performed using the two-cell model described in fig. 1, which accounts for the impedance of the two interfaces: metal/corrosion product layer and corrosion product layer/solution, respectively. The three pictures show the impedance modulus of the three points, using the same colours as fig. 6 (i.e. black, red, and magenta). In addition, the impedance values of the two cells for each plot are shown in green and blue, and the values of the corresponding parameters are reported on the right. It is easy to see how the CPE cell, whose contribution is reported in blue, is responsible for the impedance at low frequency, while the other CPE cells are mainly responsible for the high frequency behaviour. As expected the CPE that accounts for the behaviour at low frequencies has an exponent which makes it almost appearing like a capacitor thus accounting for a double layer capacitance. Such a capacitance is lower for the sloped surfaces, as expected, due to the greater thickness, and the polarization resistance varies accordingly. A real time constant cannot be determined for these CPE cells since the reactive component is not a capacitor, nevertheless values of the order of 35 − 55 s are obtained in all cases. The second CPE cell, which accounts for the corrosion layer morphology, only appears to be important in the case of the black line: the parallel resistance, which is usually interpreted as a charge transfer resistance, has a non-negligible value in this case. Therefore, the Impedance Spectrum highlights that, only in this case, the corrosion layer reaches a consolidated state, which makes it protective with respect to the corrosion process. In the other two cases, either due to the vertical orientation, which does not permit the formation of a thicker corrosion layer, or due to a still under-developed layer, the corrosion process can still proceed. 5. 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 12
5
R1=3.5MΩ Cp1= 16µF α= 0.9
|Z| (Ω)
10 2
R2=12kΩ Cp2= 110µF α= 0.4
10 −1
10 5
R1=2.0MΩ Cp1= 16µF α= 0.84
|Z| (Ω)
10 2
R2=0.5kΩ Cp2= 500µF α= 0.2
10 −1
10
5
R1=0.5MΩ Cp1= 65µF α= 0.84
|Z| (Ω)
10 2
R2=0.1kΩ Cp2= 150µF α= 0.7
10 −1
10 −2
10
−1
10
0
10
1
10
2
10
3
10
4
10
5
10
Frequency (Hz) Figure 7: Result of EIS data fitting using the the two-cell equivalent model of fig. 1.
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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-situ use, directly on the artefacts. This paper proposes the use of a cheap and simple Arduino board, coupled with a shield to set-up a simple portable EIS instrument, and an adhesive electrode, which permits measurements to be obtained in the field in an almost non-invasive way. The cost of the instrument is of the order of e 150 while the electrodes have a negligible cost. The system does not require a dedicated power supply and just needs to be connected to a normal PC equipped with a USB port to work. [1] Y. Waseda S. Suzuki (Eds.) Characterization of Corrosion Products on Steel Surfaces, Advances in Material Reasearch, 2006 [2] A. J. Davenport - In situ corrosion studies, Electrochem Soc Interface 7:28, 1998 [3] Y. Takahashi et. al - In-situ X-ray Diffraction of Corrosion Products Formed on Iron Surfaces, Materials Transactions, Vol. 46, No. 3 pp. 637 to 642, 2005 [4] P. Dillmann et al (eds). - Corrosion and Conservation of Cultural Heritage Metallic Artefacts, Woodhead Publishing, 2013 [5] P. Singh and M.A. Quraishi - Corrosion inhibition of mild steel using Novel Bis Schiffs Bases as corrosion inhibitors: Electrochemical and Surface measurement, Measurement, Vol. 86, pp. 114-124, 2016 [6] M. M. Solomon and S. A. Umoren - Electrochemical and gravimetric measurements of inhibition of aluminum corrosion by poly (methacrylic acid) in H2SO4 solution and synergistic effect of iodide ions, Measurement, Vol. 76, pp. 104-116, 2015 [7] P.M. Dasami, K. Parameswari, S. Chitra - Corrosion inhibition of mild steel in 1MH2SO4 by thiadiazole Schiff bases, Measurement, Vol. 69, pp. 195-201, 2015 [8] S. Grassini, E. Angelini, M. Parvis, M. Bouchar, P. Dillmann, D.Neff - An in situ corrosion study of Middle Ages wrought iron bar chains in the Amiens Cathedral, 2013 APPLIED PHYSICS. A, MATERIALS SCIENCE & PROCESSING, vol. 113 n. 4, pp. 971-979. [9] E. Angelini, D. Assante, S. Grassini, M. Parvis - EIS measurements for the assessment of the conservation state of metallic works of art, 2014. INTERNATIONAL JOURNAL OF CIRCUITS, SYSTEMS AND SIGNAL PROCESSING, vol. 8, pp. 240-245
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[10] G. Qiao, Y. Hong, J. Ou - Corrosion monitoring of the RC structures in time domain: Part I. Response analysis of the electrochemical transfer function based on complex function approximation - Measurement, Vol. 67, pp. 78-83, 2015 [11] G. Qiao, Y. Hong, J. Ou, X. Guan - Corrosion monitoring of the RC structures in time domain: Part II. Recognition algorithm based on fractional derivative theory - Measurement, Vol 67, pp. 84-91, 2015 [12] J.E. B. Randles Kinetics of rapid electrode reactions Discussions of the Faraday Society, vol. 1, no 11, 1947 [13] R. A. Latham - Algorithm Development for Electrochemical Impedance Spectroscopy Diagnostics in PEM Fuel Cells, BSME, Lake Superior State University, 2001, available at https://www.uvic.ca/research/centres/iesvic/assets/docs/dissertations/ Dissertation-Latham.pdf (last checked 2016-03-18) [14] A. Carullo, F. Ferraris, M. Parvis, A. Vallan, E. Angelini, P. Spinelli, Lowcost electrochemical impedance spectroscopy system for corrosion monitoring of metallic antiquities and works of art, IEEE Tr. on Instrumentation and Measurement, 2000, Vol: 49, no. 2, pp. 371 - 375 [15] E. Angelini, A. Carullo, S. Corbellini, F. Ferraris, V. Gallone, S. Grassini, M. Parvis, A. Vallan, A.Handheld-impedance-measurement system with seven-decade capability and potentiostatic function, IEEE Tr. on Instrumentation and Measurement, 2006, Vol. 55, no. 2, pp. 436 - 441 [16] S. Grassini, S. Corbellini, E. Angelini, F. Ferraris, M. Parvis, Low-Cost Impedance Spectroscopy System Based on a Logarithmic Amplifier, IEEE Tr. on Instrumentation and Measurement 2015, Vol: 64, No 5, pp. 1110 1117
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S0263-2241(16)30373-6 http://dx.doi.org/10.1016/j.measurement.2016.07.014 MEASUR 4201
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
20 March 2016 4 July 2016 5 July 2016
Please cite this article as: S. Grassini, S. Corbellini, M. Parvis, E. Angelini, F. Zucchi, A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art, Measurement (2016), doi: http://dx.doi.org/10.1016/ j.measurement.2016.07.014
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A simple Arduino-based EIS system for in situ corrosion monitoring of metallic works of art S. Grassinia , S. Corbellinib , M. Parvisb,1,∗, E. Angelinia , F. Zucchic a Dipartimento
di Scienza Applicata e Tecnologia Politecnico di Torino di Elettronica e Telecomunicazioni Politecnico di Torino, Corso Duca degli Abruzzi, 24 - 10129 Torino c Corrosion Study Center “A. Dacc` o”, TekneHub, Universit` a di Ferrara, Italy b Dipartimento
Abstract Metallic artifacts of archaeological and historical interest, exposed to outdoor environmental conditions, can be affected to a great extent by degradation, due to the presence of aggressive compounds in the surrounding environment.The development of suitable preventive conservation strategies for these important signs of our culture, requires their corrosion behavior to be investigated in order to understand the electrochemical phenomena that occur on the metallic surface. This investigation can easily be performed in a laboratory by means of different chemico-physical techniques, which provide information on the composition, microstructure and morphology of the corrosion products, as well as by means of electrochemical measurements, which allow the corrosion resistance of the metal to be estimated. Unfortunately, these measurements are often invasive and require microsampling. Electrochemical Impedance Spectroscopy (EIS), thanks to the its high degree of sensitivity, and to the small perturbation applied to the corroding system, can be considered a useful non-destructive testing method to obtain valuable information on the conservation state of metallic works of art. This paper describes an innovative, portable, low-cost and user-friendly solution for EIS in situ measurements, based on the widely diffused Arduino board, and on specifically designed measuring probes. The proposed solution cannot provide the detailed knowledge of the corrosion mechanisms that occur on metallic surface, but it can be an extremely useful tool for conservators and art historians for the assessment and long-time monitoring of the stability of the artifacts. Keywords: EIS, Metallic artefacts, Corrosion measurement, Cultural Heritage, Portable instrument
∗ Corresponing
author: [email protected]
Preprint submitted to Elsevier
July 4, 2016
1. 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. 2. 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 2
Reference electrode
ICW
Counter electrode
VAC Electrolytic solution VWR
Corrosion layer
VDC
RS Cp1
R1
Cp2
R2
Metallic artifact (working electrode)
Figure 1: EIS measuring principle: an electrolytic solution is put in contact with the sample and the impedance is measured between the artefact, which is usually referred to as “working electrode”, and a “counter electrode” usually made of a noble metal, such as Pt. A third electrode referred to as reference electrode is employed to measure the so called open corrosion potential, however the impedance spectrum can be obtained also in the absence of the reference electrode.
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 important. Therefore the measurement of the impedance at different frequencies allows studying the electrochemical reactions that occur on the metallic surface and permits to highlight the presence of cracks and porosity on the surface layer, which can act as starting points for localized corrosion attacks. Fig. 1 shows the measurement setup generally used for EIS measurements in a laboratory. EIS measurements are performed by employing a conventional electrochemical cell filled with an electrolytic solution. The cell consists of three electrodes: a “counter electrode”, which is usually made of a noble metal, such as platinum; a “reference electrode”, which has a stable and know electrochemical potential (Ag/AgCl, SCE - saturated calomel electrode, etc...); the metallic artefact, whose surface impedance has to be measured, which is part of the cell, and is referred to as the “working electrode”. A difference in the electrochemical potential between working and reference electrodes occurs in the presence of an electrolytic solution. This electrochemical potential, the so-called open circuit potential (E OCP ), can be monitored in order to follow its drift over time, and to collect further information on the degradation mechanism that is affecting the metallic sample. The working and counter electrodes are connected to the two poles of a signal generator. In order to minimize the effect of the measurements on the corrosion behaviour of the metallic artefact, the signal generator applies a stimulus, which is composed of a DC component and an AC component. The DC component is set to the voltage that occurs naturally between the working and counter electrodes, due to their different electrochemical potentials; the AC component 3
is limited to a few millivolts, usually no more than 10 mV, to avoid stimulating any non linearity of the system under investigation and accelerating the corrosion process. The EIS system measures the ICW current that flows through the metallic surface, i.e. between the counter and working electrodes, and computes the surface impedance as: Z=
VWR ICW
(1)
where VWR is the alternating voltage between the working and reference electrodes. All the values in eqn. 1 are shown in boldface as they are complex numbers. If the impedance between the counter and reference electrodes is negligible with respect to the impedance of the corrosion layer, VWR can be measured between the counter and working electrodes, thus avoiding the necessity of using of the reference electrode. The development of a two-electrode EIS measuring approach simplifies the procedure used to perform in situ measurements and, consequently, should encourage the employment of the EIS technique in the cultural heritage field, where simple to use and fast response methodologies are required. When EIS measurements have to be performed on historical and archaeological works of art to asses their conservation state, non aggressive electrolytic solutions should be used, in order to minimize their effect on the degradation process. In principle, the measurements can be performed in mineral water, but its very low conductivity has to be taken into account, in particular if the metallic artefact is coated with a very thin corrosion product layer. In this case, low aggressive electrolytic solutions, such as a sodium sulphate solution (N a2 SO4 ), can be employed. However, it should be considered that it is not possible to measure the solution resistance with a two-electrode cell, thus, since this may affect the measurement, such a resistance has to be estimated on the basis of the electrode/cell geometry, as well as the electrolyte chemical composition and conductivity. In principle, the measurement can be performed on any surface area, but the impedance values are generally scaled to an exposed area of 1 cm2 in order to enable easy comparisons. The impedance is measured for frequencies in the range of 0.01 Hz to 100 kHz. The impedance measured values obviously depend on the nature of the surface layer and can vary from a few tens of ohms per square centimetre, for bare metal, up to several gigaohms for a metallic substrate coated with a thick, compact and non-conductive layer. The EIS spectra are then analysed to determine simple electrical components, such as resistors, capacitors, and inductors, of the equivalent circuit models that represent the coated metal/solution interface. These circuits reproduce the electrical properties of the system under investigation, and the different elements are assigned to different physical elements, but there is not a unique correspondence between the EIS data and the equivalent circuit, and different circuits can be employed to model the same impedance spectrum.
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Several approaches can be followed for this identification (see for example [10, 11]) but, due to the low electronic conductivity and thickness of the corrosion product layers that are usually present on the surface of metallic archaeological and historical artefacts, the impedance spectra tend to have a mainly capacitivelike behaviour, at least at mid frequencies. However, the actual behaviour over a large frequency range, e.g. spanning 7 decades, requires equivalent circuit models with several electrical components, in order to take into account the electrode kinetics, diffusion phenomena and the heterogeneous nature of the corrosion product layer. Electrochemical processes often exhibit complex behaviour and ideal capacitors are not able to model all the EIS spectra, as for example, in the case of a non-uniform surface layer, surface roughness or an inhomogeneous current distribution; thus, some specialized electrical elements such as the Warburg and Constant Phase Element (CPE) have been introduced to overcome this shortcoming and to develop equivalent circuit models that provide useful and meaningful results. The CPE has the following impedance: Z=
1 Q(jω)α
(2)
where j is the imaginary unit, Q is the CPE value and α is an exponent in the range of 0 to 1 which characterizes the CPE. The CPE can model a resistor (α = 0), a capacitor (α = 1), a pure diffusion phenomenon (α = 0.5) and/or mixed conditions when α assumes intermediate values. As an example, fig. 1 shows a typical circuit, based on the circuits proposed by Randles in 1947 [12]. The circuit contains a resistor that models the ohmic resistance of the electrolytic solution, and two cells, composed of a resistor and a CPE, which model the phenomena that occur inside the corrosion product layer and at the interface between the corrosion product layer and the solution, respectively. This circuit can be employed, for example, to model the electrochemical behaviour of an iron artefact exposed to outdoor atmospheric corrosion. It relies on the flexibility of the CPE to model the presence of a heterogeneous surface, and the diffusion processes through the porous material in the corrosion product layer. This circuit has 7 parameters (i.e. 4 parameters for the 2 CPEs, plus 3 parameters for the resistances) which have to be adjusted to fit the experimental data [13]. In this case, the data fitting is not easy, and the values obtained for the circuit components are often questionable since, as already mentioned before, the impedance values can span 6-8 decades and a fitting procedure that is simply devoted to minimizing the sum of squares would therefore mainly weight on the high impedance values, i.e. on the low frequency values: X 2 min |Ze − Zm | (3)
5
where Ze are the measured experimental points and Zm are the impedance values obtained by the model. Several approaches can be followed to minimize this effect such as the ones described in [11]. Another simpler possibility is the use of a different metric, such as: min
X |Ze − Zm |2 2
|Ze |
(4)
which weights the experimental data regardless of their actual values, and is used in the examples considered in this paper. It is important to observe that the presence of noise on the measured data, which mainly arises from the extremely small current amplitudes, makes the fitting problematic: for example, when the impedance is of the order of 109 Ω, with a voltage amplitude of 10 mV, the current is of the order of 10 pA and the noise contribution can therefore become quite important. All these problems are usually tackled by designing costly instruments, equipped with complex input front-ends [14], [15] based on variable gain amplifiers and variable filters. Such a solution is effective, but limits the possibility of arranging multiple measuring systems that can used in parallel to reduce the measuring time. In fact, since the lower frequency used for the measurement is usually 0.01 Hz, which corresponds to a period of 100 s, a measurement session generally requires no less than 20 min. 3. The low-cost EIS system for on-site measurements 3.1. 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 capability of devices based on logarithmic amplifiers [16], coupled with the processing capability of an Arduino Due board, which is equipped with a simple 12 bit converter. Fig. 2 shows the basic idea of the logarithmic process. An example of two input current signals with an amplitude ratio equal to one hundred is shown on the left. The signal it would be possible to obtain with the conventional linear processing and a direct 8-bit conversion is shown in the lower part of the picture: it is easy to observe how the smaller signal, which has an amplitude of only a few less significant bits, get heavily corrupted by the quantization noise. The upper section of the figure shows the logarithmic process. It can be observed that the signals are initially DC shifted to become positive and are then sent to the logarithmic converter which compresses the dynamic range. The 6
Logarithmic processing
In (µA)
Signals with amplitude ratio 1:100
DC shifter
Log Amplifier ADC
AntiLog & DC rem.
Digitized signals scaled to the same amplitude
Out (V)
Conventional linear processing
ADC
Figure 2: Principle of the EIS system based on the logarithmic converter.
compressed signal is digitally converted and the obtained values are eventually passed through a digital anti-log function to recreate the original signal. It is easy to see how in this case both signals can be recreated making the minimal quantization noise. The overall process obviously heavily relies on the correct choice of the DC shift [16] and the proposed solution therefore offers real advantages when sine signals are used; nevertheless, in the proposed application, it permits quite interesting results to be obtained. The block diagram of the proposed instrument is shown in fig. 3. The system is composed of two boards: one board is an off-the-shelf Arduino Due microcontroller, while the other board is arranged as a shield that has to be connected to Arduino Due. Arduino Due is based on a 32 bit microcontroller, which contains a 12 bit ADC and two 12 bit DACs. A complete EIS solution which employs the internal DACs and ADC of the Arduino board can be arranged [16], with good results. However, the system flexibility can be increased if the ADC embedded inside the Arduino microcontroller is used to sample the signals, and a separate DDS, allocated on the shield, is employed to generate the stimulus. The DDS is paced by the Arduino clock to ensure a synchronous sampling with the generated signal and to enable an equivalent time acquisition that also permits several samples to be acquired at the maximum measuring frequency, thus overpassing the limit connected to the maximum sampling frequency of the ADC, which is of 1 MHz. Since the complete system works in synchronous mode, the frequency of each signal is known in advance and the acquired samples can therefore be processed by using a simple conventional 3 parameter sine fit. The bias that is required to keep the input signal positive and to allow the logarithmic converter to work correctly, is obtained by using a 16bit DAC, 7
Shield Arduino
Buffer DDS
Switch DA
Cell
DA LogAmp Arduino Due
512kB Flash
Digital I/O
ADC
Clock 96kB SRAM
USB PC
Figure 3: Block diagram of the proposed instrument.
installed on the shield. The DAC output is fed into a diode, so that it is easy to obtain the very large range of bias currents that are required to cope with the large range of input currents. The relationship between the DAC output voltage and the bias current is of course not know and it depends on the actual temperature. The bias is therefore continuously updated, by means of a feedback loop, which keeps the input signal in the correct logarithmic amplifier interval. This bias can be easily removed from the sampled values by means of the sine-fit algorithm [16]. A digital switch has to be used to disconnect the signal generator output from the artifact, thus permitting the open corrosion potential (E OCP ) to be monitored: when a measurement is started, the instrument measures the E OCP , sets the the value of the DC component to such value, closes the switch, and then starts the measurements. Fig. 4 shows the proposed system, along with a small box, which is used to enclose the Arduino Due board and the shield that contains the analogue circuits. Table 1 shows the most important specifications of the proposed system. The impedance magnitude and phase uncertainty have been measured by using standard components thus in the absence of varying corrosion potentials, and by adjusting the parameters of the logarithmic amplifier which is the most critical component so that worse values can be expected on less favourable conditions. 3.2. Simple cells for in situ EIS measurements In order to perform in-situ measurements, in addition to the EIS portable instrumentation, a simple electrochemical cell is required. The cell has to be suitable for measurements on tilted and vertical surfaces and it had to be specifically designed in order to measure the impedance, without damaging the artefact surface. 8
Arduino Shield
Arduino Due board
Instrument enclosure
Figure 4: Picture of a prototype of Arduino Due based instrument.
Table 1: System Specifications
Impedance range* Frequency range Stimulus amplitude Current consumption Magnitude uncertainty Phase uncertainty Meas. Time+ (0.01 Hz − 0.1 Hz) Meas. Time+ (0.1 Hz − 1 Hz) Meas. Time+ (1 Hz − 10 Hz) Meas. Time+ (10 Hz − 1 kHz) Meas. Time+ (1 kHz − 100 kHz) * only at low frequencies + with 15 points per decade
9
100 Ω − 10 GΩ 0.01 Hz − 100 kHz 10 mVpp − 2 Vpp 100 mA better than 5% better than 3◦ 900 s 180 s 80 s 100 s 30 s
Some commercially available probes that exploit a fixing solution are based on a magnetic ring. However, these devices can only be used on ferromagnetic materials. In addition, in order to have an acceptable magnetic force, these probes have to have a diameter of the order of 70 mm, so that they only can be used on almost completely flat surfaces. Fig. 5 shows the measuring cell specifically designed to be fixed onto complex shaped metallic structures by means of a removable double-side bonding tape. The cell is made of acrylonitrile butadiene styrene (ABS) and has an external diameter of about 30 mm, and a thickness of about 20 mm. The cell has a measuring chamber with a diameter of about 8 mm, and, therefore an exposed surface area of about 0.5 cm2 . The cell can be filled with the electrolytic solution by injecting it into the inlet pipe; the air in the cell flows out from the outlet pipe shown in fig. 5, where a section of the electrode is depicted. The cell is equipped with a 2 − 3 mm thick adhesive closed-cell polyurethane disk which makes it possible for the cell to be used on non-perfectly flat and/or rough surfaces. This pad is fixed to the artefact by means of a 0.3 mm thick bi-adhesive disk. The selection of the adhesive disk is of utmost importance since the glue must be strong enough to prevent the electrolytic solution from pouring out, but not too strong to prevent the cell from being removed after the measurement. Outlet tube Inlet tube
Pt counter
Measuring chamber
Figure 5: The cell specifically designed for EIS in-situ measurements.
A platinum wire is used as the counter-electrode. The wire is positioned inside a specific circular slot, at about 1 mm from the cell bottom, and is thus at 3 − 4 mm from the artefact surface, depending on the thickness of the employed foam pad. When this configuration and a 0.1 M solution of N a2 SO4 , are adopted, the solution resistance that appears in series to the measurements is negligible and the current distribution on the measured surface is uniform, within 30%, even in the presence of a very low surface impedance.
10
Reditus ad origines 5
|Z| (Ω)
10 Thick corrosion layer on slanted surface 2
10 Thin corrosion layer on vertical surface
θ (deg)
50
0 −2
10
−1
10
0
1
2
3
4
5
10
10
10
10
10
10
Frequency (Hz) Figure 6: EIS spectra collected on the Reditus ad origines weathering steel sculpture, exposed to outdoor environmental conditions in Ferrara, Italy.
4. 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 sculpture which is made of weathering steel, better known as CORTENTM is composed of two blocks, having sizes 11 × 5.7 × 5.2 m and 2.5 × 6 × 6 m respectively where the basic geometric shapes, square, rectangle, triangle, semicircle, evokes the symbols of our common heritage, from the plane tuning fork to the impending beam, to the dangerous spear. Some COR-TEN sheets are positioned vertically and some are positioned at different descending degrees down to about 30◦ . A visual inspection has highlighted different surface conditions: the vertical surfaces, which undergo a fast drying after rainy periods are coated with a thin orange-red corrosion product layer, while a thicker red-brown corrosion product layer is present on the other surfaces. Fig. 6 shows the EIS spectra collected on three different areas. The impedance is shown as Bode plots, and all the values have been scaled to an equivalent 11
area of 1 cm2 . The impedance modulus, |Z| value, indicates the extent of the corrosion phenomenon at the metal/corrosion product layer interface. The higher the impedance value, the higher the electrochemical stability of the rust layer and, consequently, the higher the protective effectiveness against further corrosion. The EIS spectra recorded on the red-brown area (black and red lines in the Bode plot) highlight the higher stability of the corrosion product layer, a result that is confirmed by the higher impedance value, while the orange-red layer (pink line in the Bode plot) shows different electrochemical behaviour, and a lower impedance value. Moreover, it should be noted that the black line has a clearly different phase trend from that of the red and pink plot traces. This can be explained by considering the different porosity of the surface layers, which can easily be observed by looking at the small insets, which show the artefact surface where the plots have been obtained. Fig. 7 shows the results of the data fitting, performed using the two-cell model described in fig. 1, which accounts for the impedance of the two interfaces: metal/corrosion product layer and corrosion product layer/solution, respectively. The three pictures show the impedance modulus of the three points, using the same colours as fig. 6 (i.e. black, red, and magenta). In addition, the impedance values of the two cells for each plot are shown in green and blue, and the values of the corresponding parameters are reported on the right. It is easy to see how the CPE cell, whose contribution is reported in blue, is responsible for the impedance at low frequency, while the other CPE cells are mainly responsible for the high frequency behaviour. As expected the CPE that accounts for the behaviour at low frequencies has an exponent which makes it almost appearing like a capacitor thus accounting for a double layer capacitance. Such a capacitance is lower for the sloped surfaces, as expected, due to the greater thickness, and the polarization resistance varies accordingly. A real time constant cannot be determined for these CPE cells since the reactive component is not a capacitor, nevertheless values of the order of 35 − 55 s are obtained in all cases. The second CPE cell, which accounts for the corrosion layer morphology, only appears to be important in the case of the black line: the parallel resistance, which is usually interpreted as a charge transfer resistance, has a non-negligible value in this case. Therefore, the Impedance Spectrum highlights that, only in this case, the corrosion layer reaches a consolidated state, which makes it protective with respect to the corrosion process. In the other two cases, either due to the vertical orientation, which does not permit the formation of a thicker corrosion layer, or due to a still under-developed layer, the corrosion process can still proceed. 5. 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 12
5
R1=3.5MΩ Cp1= 16µF α= 0.9
|Z| (Ω)
10 2
R2=12kΩ Cp2= 110µF α= 0.4
10 −1
10 5
R1=2.0MΩ Cp1= 16µF α= 0.84
|Z| (Ω)
10 2
R2=0.5kΩ Cp2= 500µF α= 0.2
10 −1
10
5
R1=0.5MΩ Cp1= 65µF α= 0.84
|Z| (Ω)
10 2
R2=0.1kΩ Cp2= 150µF α= 0.7
10 −1
10 −2
10
−1
10
0
10
1
10
2
10
3
10
4
10
5
10
Frequency (Hz) Figure 7: Result of EIS data fitting using the the two-cell equivalent model of fig. 1.
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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-situ use, directly on the artefacts. This paper proposes the use of a cheap and simple Arduino board, coupled with a shield to set-up a simple portable EIS instrument, and an adhesive electrode, which permits measurements to be obtained in the field in an almost non-invasive way. The cost of the instrument is of the order of e 150 while the electrodes have a negligible cost. The system does not require a dedicated power supply and just needs to be connected to a normal PC equipped with a USB port to work. [1] Y. Waseda S. Suzuki (Eds.) Characterization of Corrosion Products on Steel Surfaces, Advances in Material Reasearch, 2006 [2] A. J. Davenport - In situ corrosion studies, Electrochem Soc Interface 7:28, 1998 [3] Y. Takahashi et. al - In-situ X-ray Diffraction of Corrosion Products Formed on Iron Surfaces, Materials Transactions, Vol. 46, No. 3 pp. 637 to 642, 2005 [4] P. Dillmann et al (eds). - Corrosion and Conservation of Cultural Heritage Metallic Artefacts, Woodhead Publishing, 2013 [5] P. Singh and M.A. Quraishi - Corrosion inhibition of mild steel using Novel Bis Schiffs Bases as corrosion inhibitors: Electrochemical and Surface measurement, Measurement, Vol. 86, pp. 114-124, 2016 [6] M. M. Solomon and S. A. Umoren - Electrochemical and gravimetric measurements of inhibition of aluminum corrosion by poly (methacrylic acid) in H2SO4 solution and synergistic effect of iodide ions, Measurement, Vol. 76, pp. 104-116, 2015 [7] P.M. Dasami, K. Parameswari, S. Chitra - Corrosion inhibition of mild steel in 1MH2SO4 by thiadiazole Schiff bases, Measurement, Vol. 69, pp. 195-201, 2015 [8] S. Grassini, E. Angelini, M. Parvis, M. Bouchar, P. Dillmann, D.Neff - An in situ corrosion study of Middle Ages wrought iron bar chains in the Amiens Cathedral, 2013 APPLIED PHYSICS. A, MATERIALS SCIENCE & PROCESSING, vol. 113 n. 4, pp. 971-979. [9] E. Angelini, D. Assante, S. Grassini, M. Parvis - EIS measurements for the assessment of the conservation state of metallic works of art, 2014. INTERNATIONAL JOURNAL OF CIRCUITS, SYSTEMS AND SIGNAL PROCESSING, vol. 8, pp. 240-245
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[10] G. Qiao, Y. Hong, J. Ou - Corrosion monitoring of the RC structures in time domain: Part I. Response analysis of the electrochemical transfer function based on complex function approximation - Measurement, Vol. 67, pp. 78-83, 2015 [11] G. Qiao, Y. Hong, J. Ou, X. Guan - Corrosion monitoring of the RC structures in time domain: Part II. Recognition algorithm based on fractional derivative theory - Measurement, Vol 67, pp. 84-91, 2015 [12] J.E. B. Randles Kinetics of rapid electrode reactions Discussions of the Faraday Society, vol. 1, no 11, 1947 [13] R. A. Latham - Algorithm Development for Electrochemical Impedance Spectroscopy Diagnostics in PEM Fuel Cells, BSME, Lake Superior State University, 2001, available at https://www.uvic.ca/research/centres/iesvic/assets/docs/dissertations/ Dissertation-Latham.pdf (last checked 2016-03-18) [14] A. Carullo, F. Ferraris, M. Parvis, A. Vallan, E. Angelini, P. Spinelli, Lowcost electrochemical impedance spectroscopy system for corrosion monitoring of metallic antiquities and works of art, IEEE Tr. on Instrumentation and Measurement, 2000, Vol: 49, no. 2, pp. 371 - 375 [15] E. Angelini, A. Carullo, S. Corbellini, F. Ferraris, V. Gallone, S. Grassini, M. Parvis, A. Vallan, A.Handheld-impedance-measurement system with seven-decade capability and potentiostatic function, IEEE Tr. on Instrumentation and Measurement, 2006, Vol. 55, no. 2, pp. 436 - 441 [16] S. Grassini, S. Corbellini, E. Angelini, F. Ferraris, M. Parvis, Low-Cost Impedance Spectroscopy System Based on a Logarithmic Amplifier, IEEE Tr. on Instrumentation and Measurement 2015, Vol: 64, No 5, pp. 1110 1117
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