A setup for electrochemical corrosion testing at elevated temperature and pressure

Journal Pre-proofs A setup for electrochemical corrosion testing at elevated temperature and pressure Ana Vallejo Vitaller, Ueli M. Angst, Bernhard Elsener PII: DOI: Reference:

S0263-2241(20)30074-9 https://doi.org/10.1016/j.measurement.2020.107537 MEASUR 107537

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Measurement

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22 November 2019 24 December 2019 20 January 2020

Please cite this article as: A. Vallejo Vitaller, U.M. Angst, B. Elsener, A setup for electrochemical corrosion testing at elevated temperature and pressure, Measurement (2020), doi: https://doi.org/10.1016/j.measurement. 2020.107537

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A setup for electrochemical corrosion testing at elevated temperature and pressure

Ana Vallejo Vitallera,*, Ueli M.Angst a, Bernhard Elsener a,b

a. Institute for Building Materials (IfB), ETH Zurich, Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland b. Department of Chemical and Geological Sciences, University of Cagliari, 09100 Monserrato (CA), Italy E-mail addresses: [email protected] [email protected] (B.E.)

(A.V.V.);

[email protected]

(U.M.A.);

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Abstract Laboratory corrosion and scaling testing of metallic materials exposed in high temperature and pressure environments generally involves complex, multi-instrument measurement setups. Here, we present a setup including an autoclave that is instrumented for in-situ electrochemical testing and that contains a ZrO2-based solid-state pH electrode and devices for temperature control and solution stirring. We show results highlighting the importance of adequate pre-calibration of the pH measurement, due to the hysteresis depending on temperature sweep. Additionally, we illustrate how interfacing the autoclave and the electrochemical cell to measuring and controlling instruments, using different data communication interfaces, can create ground loops. These ground loop interferences can introduce significant errors in the measurement, such as a potential shift of >100 mV. In complex, multi-instrument setups, a complete understanding of ground loops may often be difficult. Thus, we recommend systematic checks to identify the ground loops and we propose measures to avoid them.

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Keywords: autoclave, corrosion, pH electrode, data communication interface, potentiostat, ground loop

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1

Introduction

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Corrosion is defined as the degradation of a material due to the chemical or electrochemical interaction with its surrounding environment. Thus, both the material and the environmental conditions play an important role on the corrosion behaviour. An approach to prevent possible detrimental corrosion processes is to select appropriate materials based on the target durability specifications and on economic factors [1, 2]. To achieve this, laboratory experiments are often needed to assess the potential corrosiveness of the expected surrounding medium and the corrosion behavior of the candidate materials.

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In exposure environments involving high temperatures and pressures, autoclaves are typically used for material testing purposes. An example are corrosion tests for the oil & gas industry [3, 4, 5]. Experimental setups with similar characteristics are also used for testing geothermal environments [6, 7, 8, 9]. Depending on the specific testing purpose, the experimental setup presents distinct features. For instance, basic autoclaves with a flow-through system are used for continuously supplying fresh solution to the metal sample [10, 11], whereas autoclaves including electrochemical tools are used for performing electroanalytical measurements [5, 8]. The autoclave might feature other instruments for monitoring chemical parameters, such as pH and/or redox potential, which are relevant to characterize changes occurring in the electrolyte and to understand their relation to the corrosion of metals.

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With regard to geothermal energy, the cogeneration of electric power and direct heating from low and medium enthalpy resources is increasing worldwide [12, 13, 14, 15]. However, the development and use of this kind of geothermal energy systems face different technical and practical challenges. Among these, corrosion and scaling represent a major hazard for the long-term and cost-effective operation of geothermal facilities [16, 17, 18, 19]. In fact, the direct contact of metallic construction materials with corrosive fluids at high temperatures can lead to rapid deterioration of plant equipment, such as well casings, pumps, or heat exchangers [20]. Failures due to corrosion have been reported in many geothermal sites worldwide [8, 21, 22]. Furthermore, the selection of materials might be challenging for countries where no electrical energy from deep geothermal resources is yet produced and, thus, where limited experience in operational issues of power plants is available.

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The aim of this contribution is to present a setup used to reproduce a wide variety of environments and to analyse the corrosion and scaling behaviour of metallic materials at elevated temperatures and pressures. The main features of the measuring instruments implemented in the setup, such as the pH electrode, multimeters, and the potentiostat used for electrochemical measurements, are described. The setup is mainly designed for studying corrosion phenomena in geothermal installations and environments of low- and medium-enthalpy systems. This means that the temperature range of interest spans between 100 °C and 200 °C, as opposed to other fields, where also higher temperatures are studied [23, 24, 25]. This intermediate temperature range, particularly its lower end close to 100 °C, has implications, e.g. for the pH measurements, as will be discussed in this contribution.

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An additional challenge when interfacing the autoclave and the electrochemical cell to the numerous measuring and controlling instruments relates to the establishment of potential ground loops. These may significantly influence the experiment and introduce errors in the measurement and in the data communication between devices. Scholars working in this field may not always be fully aware of these potential problems. Moreover, even if one were aware of ground loop issues, a complete understanding may often be difficult, particularly in a complex setup involving a relatively large number of different commercial instruments and their interfaces. Here, we illustrate a number of different ground loop issues developed in a complex measurement setup. Additionally, we make recommendations for systematic checks to identify the ground loops and we propose measures to avoid them.

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2

Experimental setup

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The constituting material of the autoclave was Hastelloy C-276 (UNS N10276), which is a Ni-Mo-Cr wrought alloy that provides a high corrosion resistance in harsh environments [11, 26]. Any other metallic part inside of the vessel in contact with the aqueous solution was made of AISI 316 (UNS S31600) austenitic stainless steel. The lid was sealed with an O-ring gasket, and two needle valves at the bottom of the autoclave were used for draining the solution after the tests and for injecting gases into the autoclave (nitrogen, argon, carbon dioxide, and/or hydrogen sulfide).

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The experimental setup, shown in Figure 1, consisted of a 1-litre volume autoclave and several electrochemical instruments located on the lid and at the bottom of the main body. Figure 1 schematically depicts the electrochemical setup, as it is here recommended to be used and fully working.

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Figure 1: Schematic diagram of the experimental setup used for material testing purposes. The electrochemical cell is a 1-litre autoclave with integrated temperature controlling system connected to a Windows PC, gas supply, and measuring devices, including a benchtop multimeter (different devices tested) and a potentiostat. The connections between the autoclave and the used instruments (A-N) as well as the Protective Earth (P.E.) connections (PE1-PE5) are also schematically depicted. The connection G is explained in the text. We also tested different data communication interfaces (L) between the multimeter and the personal computer.

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The description of all the connections (A-N) between the electrochemical cell and the autoclave to the external measuring devices, in accordance with Figure 1, is shown in Table 1.

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Table 1: Description of all connections between the electrochemical cell and autoclave to the measuring and controlling instruments shown in Figure 1. Connection

Description

A

Temperature sensor

B

Pressure sensor

C

Heater

D

Magnetic stirrer

E

Counter electrode (potentiostat)

F

Working electrode (potentiostat)

G

Ground connector (potentiostat)

H

Reference electrode (potentiostat)

I

Working electrode (benchtop multimeter)

J

Reference electrode (benchtop multimeter)

K

pH sensor (benchtop multimeter)

L

Data communication interfaces

M

Windows computer – Temperature and pressure controller

N

Stirrer controller – Temperature and pressure controller

PE1

Protective Earth of Windows computer

PE2

Protective Earth of Stirrer controller

PE3

Protective Earth of Potentiostat through an isolation transformer

PE4

Protective Earth of Temperature and pressure controller

PE5

Protective Earth of Benchtop multimeter

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External measuring devices

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Figure 1 shows additional equipment connected to the autoclave, consisting primarily of a PGSTAT 302N potentiostat by Metrohm Autolab B.V. (Utrecht, The Netherlands), operated by the software NOVA 2.1.2 (also by Metrohm Autolab), a digital voltmeter with multiplexer module (models 2701 and 7702 by Keithley Instruments Inc., Cleveland, OH, USA), and an external Windows computer. Whereas the potentiostat was employed to perform electrochemical tests, the multimeter recorded the potentials of the working electrode and the pH electrode against the reference electrode. The computer was used for monitoring and controlling the parameters related to the environmental conditions of the testing solution (i.e., stirring rate, pressure, temperature). The potentiostat included an additional (optional) ground connector (connection G in Figure 1) that can be used to connect e.g. a Faraday cage to ground or the Hastelloy body of the autoclave to the internal ground of the potentiostat PGSTAT 302N.

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Generally, with (metallic) autoclaves, it is recommended to use potentiostats operating in floating mode. This is because the metallic body of the autoclave has to be connected to protective earth (PE) for safety reasons, and thus, if a potentiostat is used in normal mode (that is grounded to PE as well), ground loops can arise and disturb the measurements. The ground loop issue is discussed in more detail in section 4. Here, it shall be mentioned that we used a potentiostat operated in normal mode (PGSTAT 302N, Metrohm Autolab) that we could manually separate from the ground by means of an isolation transformer (Figure 1).

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Furthermore, ground loops can also be created through data communication interfaces, as often, these introduce ground connections from one device to another. Figure 1 shows that we recorded the data from the benchtop multimeter with the help of a computer. To illustrate the potential problems that may arise in such a configuration, especially in high impedance electrochemical systems, we tested different types of multimeters and data communication interfaces (RS-232/USB, RJ-45, GPIB). In addition, manual readings were taken from the display of the multimeter, thus with no physical connection to the computer.

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Temperature control

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The heating system of the cell was composed of a heating mantle that provided a power of approximately 2 to 3 kW and included a stainless-steel plate and an insulating ceramic wool. The temperature of the aqueous solution was measured inside the vessel with a resistance thermometer (Pt100 class A), which was enclosed in a protective tube of stainless steel (AISI 316 grade). While the resolution of the temperature measurement was 0.1 °C, the trueness declared by the manufacturing company was close to ±1 °C at high temperature. A separated K-type thermocouple attached to the external surface of the vessel was used to prevent overheating of the device.

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Stirring and pressure control

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The aqueous solution was continuously stirred by a RedVoy PRE 1771 magnetic stirrer, manufactured by Premex Reactor AG (Lengnau, Switzerland). The stirring minimized concentration and thermal gradients in the vessel. On the other hand, a pressure sensor working in the range of 0-100 bar controlled pressure changes with a resolution of 0.1 bar. Furthermore, the autoclave also included a venting valve to avoid overpressure.

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pH sensor

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Membrane-type pH electrodes are preferred over conventional glass sensors for pH measurements in high temperature aqueous media for their versatility and reliability. The main reasons for this include: (i) more reproducible potential response to pH changes [27], (ii) dry internal junction, and (iii) higher stability at high temperatures (≥ 150 °C) towards water and especially alkaline solutions [28].

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Among membrane pH electrodes, the most common ones used for testing high temperature hydrothermal fluids are primary pH electrodes with a zirconia membrane in contact with the testing fluid and a metal/metal oxide mixture acting as the internal electrode of the sensor (e.g. Hg/HgO/ZrO2 [29], Cu/Cu2O/ZrO2, or Ag/Ag2O/ZrO2 [30]). The outer zirconia ceramic membrane behaves as an ionic oxygen conductor owing to its stabilization with magnesium oxide, calcium oxide, or yttrium oxide [31]. The proton activity in the ceramic/liquid interface affects the oxygen vacancy concentration of the zirconia and, as a consequence, the equilibrium of the potential-determining redox species inside the membrane. Furthermore, the pressure dependence of the pH measurement is not significant at relatively low pressures (up to 140 bar) [32].

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In this study, a pH sensor electrode, manufactured by Corr Instruments LLC (San Antonio, TX, USA), was mounted at the bottom of the autoclave. This was a Ag/Ag2O/ZrO2 sensor, whose outer membrane is composed of yttria-stabilized zirconia (YSZ). The operation temperature of a regular pH electrode of this type is between 230 and 350 °C. However, the minimum operation temperature of the pH sensor installed in the autoclave was decreased to about 90 °C by doping the Ag2O powder of the electrode. The configuration of this pH electrode is schematically depicted in Figure 2. The pH sensor has a long lifetime without requiring any further service. The accuracy of the pH measurement is around ± 0.3 units.

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Figure 2: Schematic diagram of the tip of the Ag/Ag2O/ZrO2 pH electrode, where the outer membrane is an yttria-stabilized zirconium (YSZ) oxide and the Ag/Ag2O acts as internal electrode of the sensor.

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Reference, working, and counter electrodes

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The open circuit potential (OCP) is the potential of a metal sample developed spontaneously in an electrolyte in the absence of an external current [33]. At this potential, the net current passing the electrode surface is zero, i.e., the total anodic current is equal to the total cathodic current. The OCP is measured with a voltmeter and expressed versus a reference electrode that, for practical reasons, is commonly the saturated calomel electrode (SCE) or the silver-silver chloride (Ag/AgCl) reference electrode [34].

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The corrosion condition and behaviour of metallic materials may be evaluated and investigated by electroanalytical techniques, such as the linear polarization resistance method (LPR) [35, 36], electrochemical impedance spectroscopy (EIS), or cyclic voltammetry (CV). In the case of LPR, a small external DC voltage is applied to the working electrode, and the current flowing from the counter to the working electrode is recorded. From the EIS data, the ohmic resistance, polarization resistance, double layer capacity etc. describing the electrochemical interface metal/solution can be obtained. On the other hand, CV is often used to study redox processes, to evaluate the stability of reaction products, or the reversibility of a reaction [37, 38].

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In the autoclave, the wires of the counter and working electrodes were introduced into the vessel via wire feed-throughs that were tightly sealed with stretched polytetrafluoroethylene (PTFE) gaskets and can be used at temperatures up to 300 °C. The working electrodes were metal coupons with a cylindrical geometry, whereas the counter electrode was a platinum plate (5 × 20 mm2). A scheme of the working electrode, including the metal coupon and the sample holder, is shown in Figure 3. The coupon is cylindrical, with sharp machined edges at the top side that are pressed against the PEEK tube when mounted on the sample holder. This design was chosen to minimize the formation of crevice corrosion.

(a)

(b)

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Figure 3: Schematic diagram of the working electrode, including the metal coupon (a) and the sample holder (b) that consists of a poly(ether-ether-ketone) (PEEK) tube and a threaded metal bar. All units in mm.

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The reference electrode was a Ag/AgCl electrode manufactured by Cormet Oy (Vantaa, Finland) that is suitable for testing environments up to the maximum operating pressure and temperature of 160 bar and 300 °C, respectively. The inner part of this electrode was a silver rod covered with a silver chloride coating and was connected to the high temperature environment via a PTFE tube and a ceramic plug. A Ag/AgCl/Sat. KCl reference electrode (-43 mV vs. SCE) suitable for the required ranges of temperature and pressure was used for potentiometric measurements.

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3

Materials and methods

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3.1 Materials

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In all experiments, we tested a steel grade (API L80 Type 1) that is commonly used for the casing and tubing of deep geothermal wells. This is a Fe-based mild steel that is produced following the specification 5CT of the American Petroleum Institute (API) [39]. It has a tempered martensitic microstructure, and its chemical composition is shown in Table 2 .

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Table 2: Chemical composition of the steel grade API L80 Type 1, as reported by the producer. Steel sample

wt.% (and Fe bal.)

Composition

C

Si

Mn

Cr

Mo

S

L80 Type 1

0.25

0.19

1.02

0.45

0.16

0.004 0.014

P

Ni

Cu

Al

V

0.04

0.02

0.03

0.003

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The steel coupons (see Figure 3) had a total exposed surface of 1119 mm2. For the surface preparation of the cylinders, the horizontal and longitudinal surfaces were manually ground using SiC abrasive paper (up to 4000 grit). Finally, the samples were ultrasonically cleaned with ethanol and dried with compressed air.

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3.2 Test solutions

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Various buffer solutions were tested in the experiments. On one hand, commercial standard buffer solutions of pH values 1, 8, and 10 (purchased from Sigma-Aldrich Chemie GmbH, Buchs, Switzerland) were mainly used to verify the performance of the pH sensor. These buffer solutions are nominally named as pH 1, 8, and 10, but their actual pH values at 90 °C are 1, 7.75, and 9.49. On the

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other hand, high-temperature buffer solutions (S1, S2, and S3) were used for the performance verification and the calibration of the pH sensor as well as for performing electrochemical measurements and were prepared as suggested by [31, 40]. In Table 3, we report the composition of these high-temperature buffer solutions. Acetic acid (> 99.8%) was purchased from Fisher Scientific UK and sodium acetate (≥ 99.5%) from Sigma-Aldrich. The rest of chemicals (potassium dihydrogen phosphate (≥ 99%), sodium hydrogen phosphate (≥ 99.5%), and disodium tetraborate (99.0-103.0%)) were purchased from Sigma-Aldrich. Such buffer solutions present pH values in the acidic (S1), neutral (S2), and alkaline (S3) ranges. Table 4 shows the pH value of the three solutions as a function of temperature.

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Table 3: Chemical composition of high-temperature buffer solutions used for calibrating the pH electrode (S1, S2, and S3) [31, 40]. Solution

Chemical composition 0.01 mol kg−1 CH3COOH (acetic acid)

S1

0.01 mol kg−1 CH3COONa (sodium acetate) 0.025 mol kg−1 KH2PO4 (potassium dihydrogen phosphate)

S2

0.025 mol kg−1 Na2HPO4 (sodium hydrogen phosphate) 0.01 mol kg−1 Na2B4O7 (disodium tetraborate)

S3

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Table 4: pH values of the high-temperature buffer solutions as a function of temperature [31, 40]. The * indicates that the value has been interpolated from adjacent values because it is incorrectly reported in the original source (8.22). pH value

Temperature (°C) S1

S2

S3

20

4.72

6.86

9.22

100

4.82

6.88

8.80*

125

4.93

6.92

8.75

150

5.03

7.04

8.65

175

5.15

7.15

8.60

200

5.35

7.30

8.56

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3.3 Measurement procedures

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Before starting each experiment, the buffer solutions were deaerated by purging a stream of highpurity nitrogen gas (purity grade 5.0) for at least 1 h. This purging time is optimal for significantly reducing the residual concentration of dissolved oxygen in the fluid (to around 0.25 ppm) [41]. Afterwards, nitrogen was initially injected into the vessel with a pressure of 15 bar in order to prevent degassing of the solution when the temperature increased. During the tests, a change of pH could be caused by the dissolution of iron. However, the measurement of the pH before and after of each experiment at room temperature showed that the change was less than 0.02 pH units.

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The temperatures at which the tests were performed were either 90 °C (for verifying the performance

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of the pH sensor), 100 °C (for electrochemical measurements), or the range between 90 and 200 °C (for calibrating the pH sensor). The latter are in the order of temperatures that can be expected in geothermal systems: from the reservoir at around 200 °C to the heat exchanger on the surface power plant at around 90-100 °C.

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All tested solutions were continuously stirred with a speed of 500 rpm by means of the magnetic stirrer. This is particularly important when testing aqueous geothermal fluids as the conditions should represent those of real geothermal power plants. In such a case, the stirring speed (500 rpm) corresponds to a minimum flow rate of 100 l/s in a pipe of 50 cm of outer diameter and 3 cm of thickness, which is representative of the rate that could be achieved within the wells of a power plant.

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For all LPR measurements, a potentiodynamic sweep starting from the OCP was performed at a sweep rate of 10 mV/min. The scan was first applied in anodic direction and then reversed (OPC ± 10 mV). Prior to the LPR measurements, the OCP value was monitored during 30 min. No IR compensation was applied, as this was not deemed necessary for the purpose of this paper.

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4

Results and discussion

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4.1 Calibration of the Ag/Ag2O/ZrO2 pH electrode

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The performance of the ZrO2-based pH electrode installed in the autoclave was checked upon delivery from the supplier. The tests were carried out at ambient pressure and at a temperature near 90 °C with high-temperature buffer solutions (S1, S2, S3) and with commercial standard buffer solutions of nominal pH values of 1, 8, and 10. The pH values of the high-temperature buffer solutions (S1, S2, and S3) at 90 °C were interpolated from the values shown in Table 4, resulting in 4.76, 6.87, and 8.86, respectively. The potential of the pH probe was measured against the Ag/AgCl/Sat. KCl reference electrode with the Keithley 2701 multimeter. The relationship between the measured potentials and the actual pH values at 90 °C exhibits a linear behavior with a Nernstian slope of -68.3 mV/pH (Figure 4) in good agreement with the theoretical value (-71.9 mV/pH).

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Figure 4: Verification curve of the performance of the Ag/Ag2O/ZrO2 pH sensor. Note: The pH values used in this plot are those related to 90 °C.

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As no standard reduction potential values exist for the custom-made pH electrode and setup combination, the doped pH electrode needs to be calibrated in the expected pH range of the testing solutions [42]. Therefore, after verifying the correct functioning of the pH electrode at 90 °C (Figure 4), the calibration at higher pressures and temperatures (up to 200 °C) was performed with the hightemperature buffer solutions (S1, S2, and S3).

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The potential difference between the pH sensor and the Ag/AgCl/Sat. KCl reference electrode during the heating and cooling cycles (from room temperature to 200 °C and viceversa) were recorded by the Keithley 2701 voltmeter connected to the computer by means of an RJ-45 communication interface. These EpH sensor vs. temperature curves are shown in Figure 5 where the change in pH of the solutions S1, S2 and S3 results in the expected shift in potential. At temperatures below 150 °C, the EpH sensor measured during the heating cycles are different from the EpH sensor during the cooling cycles for all buffer solutions. This hysteresis effect appears in all three buffer solutions and is comparable.

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Figure 5: EpH sensor vs. temperature curves obtained during the pH sensor calibration with the Keithley multimeter 2701 and the high-temperature buffer solutions (S1, S2, and S3). Solid and dashed lines correspond to the heating and cooling cycles, respectively, with temperature scan rates of 50 °C/h for all measurements.

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Using the pH values of the buffer solutions S1, S2 and S3 (Table 4), the data may be plotted EpH sensor vs. pH curves (Figure 6) for the cooling and heating cycles. The experimentally determined slopes of the EpH sensor vs. pH curves (annotated in Figure 7) are similar to the theoretical values of -73.9 mV and -93.7 mV at 100°C and 200°C, respectively. As was already apparent in Figure 6, at 100 °C the EpH sensor vs. pH curves differ between heating and cooling, and the slope of the EpH sensor vs. pH curve at 100 °C is not affected by the direction of the temperature sweep.

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Figure 6: EpH sensor vs. pH curves obtained during the pH sensor calibration with the Keithley multimeter 2701 and the hightemperature buffer solutions (S1, S2, and S3). Solid and dashed lines correspond to the heating and cooling cycles, respectively, with temperature scan rates of 50 °C/h for all measurements. The slopes for curves at 100 °C and 200 °C are - 67.4 mV and - 87.9 mV, respectively.

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To further investigate whether the hysteresis can be explained by a delayed response of the pH electrode, two different temperature scan rates were tested. Figure 7 shows that the tested temperature

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scan rates of 25 °C/h and 50 °C/h did not influence the results. Faster scan rates were not tested because they are not recommended by the autoclave manufacturer for other reasons; slower scan rates were not tested because they are not considered practically feasible due to the extremely long experimental times resulting from them.

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Figure 7: EpH sensor vs. temperature curves obtained with the high-temperature buffer solution S3 and by applying slow (25 °C/h) and fast (50 °C/h) heating and cooling rates. Solid and dashed lines correspond to the heating and cooling cycles, respectively.

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According to the pH sensor manufacturer, the outer YSZ membrane is fragile, and thus, it might break or crack due to thermal shocks. To check this, we measured the AC impedance of the pH sensor when it was immersed in the high-temperature buffer solution S3 using the PGSTAT 302N potentiostat by Metrohm Autolab. Figure 8 shows the impedance of the pH sensor obtained versus the temperature during the heating and cooling cycles. The impedance increased markedly at temperatures lower than 150 °C, reaching a value of around 1000 MΩ at 90 °C, which is in agreement with the specifications of the supplier.

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Figure 8: Impedance (MΩ) of the pH sensor vs. temperature measured using the high-temperature buffer solution S3 and the PGSTAT302N potentiostat (Metrohm Autolab). The heating and cooling rates were 50 °C/h.

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The hysteresis shown in Figure 5 and Figure 7 upon heating and cooling implies an apparent difference in pH of around 0.6 units at low temperatures (100 °C). At higher temperatures (>150 °C), however, no significant difference in pH can be observed between heating and cooling. A similar type of hysteresis due to temperature change was also reported by [43]. This might be explained by the microstructure and composition of the ZrO2 membrane [44].

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Nevertheless, to obtain pH values as accurate as possible, we recommend a calibration procedure

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considering this hysteresis. More specifically, based on the results obtained in this work, a relation between the three variables EpH sensor - pH - temperature was established. Figure 9 shows a 3-D surface matrix that was interpolated from the calibration curves of the three buffer solutions (S1, S2, and S3) and that shows only the data corresponding to the heating cycle. This 3-D surface matrix enables the pH determination in the temperature range of 90-200 °C by measuring the potential of the pH sensor and taking into account the direction of the temperature scan.

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Figure 9. EpH sensor - pH - Temperature matrix (dark blue) obtained from the calibration curves (light blue) of the three hightemperature buffer solutions S1 (acidic), S2 (neutral), and S3 (alkaline) during the heating cycle.

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4.2 Ground loop interferences

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4.2.1 General remarks about ground loops

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Ground loops are physical loops in an electrical circuit that has multiple grounds or where a loop is otherwise created through ground connections (Figure 10). Grounds are generally low impedance, and therefore, present a negligible resistance against the flow of current through the ground path. Ground loops can arise if two or more devices interfaced with each other are connected to the safety ground (protective earth, PE). In this case, ground loops can lead to unwanted interference with the measurement and the data transmission. There are essentially two mechanisms through which such interferences arise.

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First of all, ground loops can act as antennas that, in the presence of magnetic fields, pick up noise and induce currents. In a laboratory environment, and especially in a system with many different devices in close proximity (as in the setup shown in Figure 1), magnetic fields cannot be avoided. One source are the numerous AC powered devices (heater, computer, etc.); other sources include the stirrer, but also lamps and other equipment.

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In addition to electromagnetic interference (noise), ground loops can allow DC currents to flow. A common example of this are autoclaves that contain electrochemical systems. This is because the autoclave body is often metallic, thus electrically conductive, and grounded to PE for safety reasons. As the autoclave is filled with an electrolyte (an ion conductor) and conductive itself, there are possible low impedance paths between the autoclave body and the electrodes in the inside (reference electrode, counter electrode, working electrode) – at least when some of these electrodes have an electrical connection to PE as well. Traditional potentiostats were operated in so-called normal mode,

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which means that they were grounded. This is advantageous to reduce noise. However, in these instruments, the working electrode is generally connected to PE as well and offers, therefore, a ground path, which results in polarization of the working electrode through DC ground loop currents flowing. For this reason, floating potentiostats were introduced in the 1990s to break the ground loops. Another common example for ground loops leading to DC currents is the situation in which multiple measuring devices, e.g. a potentiostat and a benchtop multimeter, both grounded to PE, are connected to electrodes situated in the same electrolyte.

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In the following two sections, we address two different problems related to ground loops that may potentially arise in a complex measurement setup such as the one depicted in Figure 1. With the results shown here, we aim to raise the awareness for such problems and propose simple measures to avoid them.

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Figure 10: Schematic illustration of a ground loop formed between two devices. The ground loop picks up noise from electromagnetic induction (purple). In addition, there is a DC interference (blue) that may be caused by a current source in the circuit (such as an electrochemical system) or by ground imbalance (if various PE are used). The electromagnetic interference primarily leads to noise. The DC interference causes polarization of the electrochemical system under test or adds a voltage to the transmitted signal.

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4.2.2 Pitfalls in data communication interfaces in complex measurement setups

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In this section, we discuss ground loops in data communication interfaces and how this can introduce errors in the measurement. In order to illustrate these effects, several multimeters with different data communication interfaces between the multimeter and the computer were used, as shown in Figure 1 (connection L). These devices and interfaces were selected due to their common use as test laboratory equipment, measurement accuracy, and their high input impedance, which is needed in order to allow for voltage measurement in high impedance systems such as with the present pH sensor. Table 5 shows the different devices and interfaces used.

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Table 5: Main characteristics of the multimeters and communication interfaces used for measuring the potential of the pH electrode vs. the Ag/AgCl/Sat. KCl reference electrode. Keithley multimeter

Input impedance (Ω)

Communication interface

617

>2·1014

GPIB

2000

>1010

RS-232 / USB

2001

>1010

Manual (read from display)

2701

>1010

RJ-45

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In the case of the multimeter Keithley 617, the data transmission was accomplished via a GPIB (general purpose interface bus). The interface bus used by the Keithley 2000 was a RS-232 serial-toUSB converter, and the one by the Keithley 2701 was a RJ-45 (Ethernet cable). The data acquisition from the multimeter Keithley 2001 was realized manually, i.e., by reading directly from the display, which means that there was no physical connection between the multimeter and the computer.

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Figure 11 shows the evolution of the EpH sensor measured with the different configurations when performing a temperature sweep (from 90 °C to 200 °C and viceversa). The voltage EpH sensor measured by manual recording (Keithley 2001) is considered the undisturbed response of the pH electrode. The multimeter Keithley 2701 (interface RJ-45) showed a response in good agreement with the one measured manually (note that the results presented in Figure 6 were recorded in this way). On the other hand, the EpH sensor vs. temperature curves obtained with the Keithley multimeter 617 with the GPIB interface as well as the Keithley multimeter 2000 with the RS-232/USB interface, clearly deviate from this behavior at temperatures below 150 °C, at some point by more than half a Volt.

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Figure 11 shows a systematic pattern between the temperature and the introduced error (due to the RS232/USB and the GPIB connectors). The temperature has, as was shown in Figure 8, a marked effect on the impedance of the pH sensor. At high temperatures the impedance of the pH sensor is comparatively low and the data shown here indicate that all multimeter/data communication interface combinations give comparable results in the high temperature range (Figure 11).

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At low temperatures where the impedance of the pH sensor is increasing and reaches 109 Ω at 90 °C, errors appear in the voltage measurement when using an RS232/USB interface (and less when using a GPIB interface). Therefore, there must be an interrelation between the high impedance of the electrochemical system, which is measured with a multimeter, and the susceptibility to ground loop interference of different data communication interfaces [45].

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Figure 11: Influence of measurement and data acquisition instrumentation (multimeter and data communication interface) on EpH sensor (measured vs. the Ag/AgCl/Sat. KCl reference electrode and using the acidic buffer solution (S1)).

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Thus, the possibility of ground loop formation has to be checked. Figure 12 shows this for the example of the RS-232 interface, but it does similarly apply also to other interfaces such as USB [46]. The multimeter measures, in principle, the voltage difference between the reference electrode (RE) and the pH sensor. If this measured system has a high impedance, possible influences of ground loop currents through the signal ground of the data communication interface or through device-internal grounds may affect the system measured. In our case, towards lower temperatures, the impedance of the pH sensor increases dramatically and thus, the voltage measured with the voltmeter may be a combination between pH sensor and autoclave body, both versus the RE.

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Figure 12: Schematic illustration of ground loop issues involving both data communication interfaces and high impedance electrochemical systems in grounded autoclave vessels. Possible ground loops shown in red.

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An a priori analysis of this complex and interrelated ground loop issue is, in our opinion, often impossible. Not only is the system composed of many different commercial products involved in the setup (including the multimeter, a potentially involved multiplexer, the data communication interface and related issues such as the exact version and specifications or the cable length and geometry) and the related impedances in the grounding situation in a laboratory are difficult to predict, but also the impedances and polarization behavior of the electrochemical system under study may change. Thus, to avoid problems of the kind shown in Figure 11, we recommend validating a setup including data communication interfaces against manual measurements (that is, reading the values from the display of the multimeter and in the absence of any data transferring connector). As becomes apparent from the results shown in this work, it is important that this validation is done within the entire temperature range that will later be used in experimental studies.

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4.2.3 Pitfalls in electrochemical measurements in grounded cells

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Figure 1 shows that the setup includes numerous devices and instruments connected to PE through the power ground and at the same time in connection with the different electrodes (WE, CE, RE) located in the autoclave body and lid. Additionally, also the autoclave body is grounded.

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We would first like to illustrate that a ground loop can significantly influence the measurements by establishing a galvanic element between the autoclave body (Hastelloy) and the WE, thereby causing a shift in the OCP of the WE. To illustrate this, the potentiostat was operated in normal mode (grounded to PE through the power supply). Figure 13a shows that as soon as the G cable (Figure 1, 13b) was connected to the autoclave body, the OCP of the WE was shifted anodically by more than 100 mV. Once the G cable was removed, the WE slowly depolarized. Figure 13b illustrates the ground loop currents that we measured with the help of a shunt resistor. It was found that in this case approximately 1 µA flew through the ground loop (IGNL). If the G cable was not connected to the autoclave body, the entire current flew through the WE connection F (IF); as soon as the G cable was connected, the current was split between cables F (approximately 80%) and G (approximately 20%). This demonstrates that in both cases (with G cable open or connected), the grounded potentiostat led to the formation of a galvanic cell between the autoclave body (Hastelloy, noble) and the WE (here carbon steel, less noble), polarizing the WE in anodic direction.

(a)

(b)

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Figure 13: (a) Influence of ground loop on OCP when the potentiostat was operated in normal mode and after connecting the ground connector to the autoclave body (indicated as G in Figure 1). For these measurements, the buffer solution S3 was used at 100 °C. (b) Schematic illustration showing the ground loop DC current that arises because of the low impedance ground path and the galvanic coupling between the autoclave body (Hastelloy, noble) and the working electrode (C-steel, less noble).

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Thus, it is essential to isolate the potentiostat from the ground. This can be easily achieved by either using a floating potentionstat or – in case of a normal potentiostat – by ensuring a galvanic separation of the power supply with an isolation transformer from PE. This is a relatively simple and low cost but effective solution to interrupt the ground loop. It may here be worth mentioning that this separation cannot be achieved simply by removing the third (PE, safety ground) pin of the plug of the power supply. The reason is that the conductors PE and neutral (N) are generally connected in the electrical mains of a building.

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Figure 14 shows examples of LPR measurements performed in different configurations. Figure 14a represents the situation with the potentiostat in floating mode and without the G cable connected to the autoclave body (that is connected to PE). This is the ideal situation, where no ground loops were detected to influence the measurement. In this case, the polarization resistance can reliably be determined from the slope of the potentiodynamic sweep (133 Ωm2). Figure 14b, again with the potentiostat in floating mode, but with the G cable connected, leads to comparable results. However, in agreement with the discussion above (Figure 13), the data in Figure 14c and Figure 14d clearly show the interference caused by the ground loop when the potentiostat was grounded. First, the OCP were shifted in anodic direction (here by approx. 50 mV) with respect to the cases with the floating potentiostat. Second, the slope of the potentiodynamic sweep was significantly affected because of the overlapping external (ground loop) current. It may also be mentioned that the potentiostat operated in normal mode, but with the G cable not connected resulted in significant noise (Figure 14c). This noise could be removed when the G cable was connected (Figure 14d), but as mentioned above, this configuration (potentiostat in normal mode) led to considerable DC interference at the WE and hence erroneous results. Thus, while grounding the autoclave body (connecting cable G) with a potentiostat operated in normal mode may seem an attractive solution to remove the noise (it may be mentioned that this solution is even proposed in the manual of the PGSTAT 302N potentiostat by Metrohm Autolab B.V.), this configuration significantly disturbs the experiment and presents an error source.

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Figure 14: Comparison of LPR measurements performed with the potentiostat in floating mode (a,b) and in normal mode (grounded) (c,d) as well as with and without connecting the G cable of the potentiostat. These measurements were performed in buffer solution S3 at 100 °C.

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4.2.4 Recommendations for systematic checks to avoid ground loop interferences

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On the basis of the observations reported above, we recommend to perform systematic checks in order to avoid ground loop interferences in a setup such as the one depicted in Figure 1. We consider this important because the complexity of such a setup, arising from the numerous devices interfaced with each other and with the electrochemical cell and the autoclave, makes it virtually impossible to a priori assess the risk for possible ground loop interferences. As our examples presented above illustrate, error sources may be present both in the measurement and in the communication of the measured data.

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In order to dismiss the influence of unwanted ground loop connections within the setup, we propose to systematically study the influence of removing or adding individual devices and instruments through their respective interfaces to the setup. An example of such a check of different configurations is shown in Figure 15. In this case, OCP values of the WE were compared to a reference value that was first measured when no external devices were connected to the autoclave. All OCP values were measured by using an external handheld multimeter of high input impedance (TRU-REF 1000, by Corrosion Electronics Pty. Ltd.), which was powered with a battery to avoid ground loops.

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Figure 15: Systematic check of the effects of different cable connections and grounding situations on the OCP of the WE (labels according to Figure 1, where the E connector corresponds to the counter electrode, F to the working electrode, and H to the reference electrode). The numbers are expressed with respect to the OCP of the WE in the autoclave with no other devices interfaced (in this case, the OCP was measured vs. the RE by means of a handheld, battery powered multimeter).

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From this figure, it can be deduced that the temperature and pressure controller, the PC, the stirrer controller, and the multimeter Keithley 2701 had no influence on the OCP measurement. On the contrary, the potentiostat, when operated in normal mode and when connected to the autoclave with the ground connector G significantly shifted the OCP. Thus, this systematic check allowed identifying the critical ground loop connection, and confirming that the other connections did not cause interference.

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In addition to influences on OCP, it can also be advisable to measure currents flowing in connectors, e.g. by introducing a shunt resistor. An example of such a test is shown in Figure 13b and the related text.

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Other external factors, such as the stirring or the heating of the solution, were also analysed. For instance, when the solution inside the vessel was agitated by means of the magnetic stirrer, it was observed that the noise in the OCP or LPR measurements increased. On the other hand, the electrical heating, which is needed to keep the required temperature of the autoclave stable, did not cause magnetic disturbances during the measurements of the OCP or LPR.

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5

Conclusions

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In laboratory tests, the reliable corrosion assessment of metallic materials exposed in high temperature and pressure environments generally involves complex multi-instrument measurement setups. Thus,

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all possible influencing factors and sources of interferences in the measurements should be evaluated carefully. In this work, the pH measurement of aqueous solutions with a solid-state electrode and the performance of electrochemical measurements with a potentiostat have been evaluated in detail. Additionally, the influence of different data communication interfaces with or without data ground for automated data-logging was studied. Although we used specific commercial devices and custom-made tools in our experiments, the following major conclusions from this study can be generalized and applied when using other multi-instrument measurement setups:

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

The ZrO2-based pH electrode was found to show a hysteresis in terms of potential vs. temperature, depending on the direction of the temperature sweep. While at higher temperatures (> 150 °C), there was no influence of the direction of the temperature sweep on the pH sensor’s potential, the potentials differed at temperatures below 150 °C, depending on the direction (heating vs. cooling). This difference, if not considered during the calibration, presents an error source in the pH measurement (in this case up to 0.6 pH units at 100 °C). Thus, we recommend that the hysteresis is taken into account in the calibration procedure.

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

The impedance of the ZrO2-based pH electrode was found to increase steeply when the temperature decreased below 150 °C, reaching 109 Ω at 90 °C. At T > 150 °C the impedance was low enough to ensure a stable measuring circuit reference electrode vs pH electrode irrespective of the data interface connection. With increasing impedance of the pH sensor at lower temperatures the voltage measurement progressively included also the metallic autoclave case in parallel to the pH electrode due to ground loops. This can be avoided either by recording the voltage measurements manually (no data communication interface) or by choosing an adequate data communication interface that does not introduce a ground loop.

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

Performing electrochemical measurements (OCP, Rp, EIS) in a metallic autoclave connected to protective earth (PE) with a potentiostat in the normal mode (grounded to PE) leads to a ground loop and DC current flow through the electrochemical cell resulting in uncontrolled polarization of the working electrode and thus erroneous results. This situation might not always be straightforward to detect as the readings of OCP, Rp, or EIS can be stable and noise-free. This problem can be solved by either using a floating potentionstat or – in case of a normal potentiostat – by ensuring a galvanic separation of the power supply with an isolation transformer from PE. The latter is a relatively simple and low cost but effective solution to interrupt the ground loop. It may here be worth mentioning that this separation cannot be achieved simply by removing the third (PE, safety ground) pin of the plug of the power supply. The reason is that the conductors PE and neutral (N) are generally connected in the electrical mains of a building.

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

Given the complexity of multi-instrument setups such as the one presented in this study, we recommend to perform systematic checks in order to avoid ground loop interferences. We consider this important because the complexity of such a setup, arising from the numerous devices interfaced with each other and with the electrochemical cell and the autoclave, makes it virtually impossible to a priori assess the risk for possible ground loop interferences.

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Funding: This work was supported by the Swiss Competence Center for Energy Research–Supply of Electricity (SCCER-SoE).

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6

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

A multi-instrument setup for corrosion testing of metals at high temperature is presented.



ZrO2-based pH electrodes showed hysteresis depending on temperature sweep direction.



Data communication interfaces and other unnoticed ground connectors can cause ground loops and introduce measurement errors.



Systematic checks are recommended to identify ground loops in complex, multiinstrument measurement setups.