Analog Actuator For Multimodal Environmental Corrosion Measurements

J. A. Gutierrez-Gnecchi, M. Larios-López, A. Téllez-Anguiano, J. C. González-Hernández

Departamento de Ingeniería Electrónica Instituto Tecnológico de Morelia

Morelia, Michoacán, México e-mail: biodsprocessing@aol.com

J. L. Ramírez-Reyes Instituto de Ingeniería

Universidad Veracruzana Boca del Río, Veracruz, México

e-mail: luiramirez@uv.mx

Abstract—The degradation of metallic materials due to the environment is a matter of strategic interest, because it is considered the number one enemy of the national infrastructure. For the Mexican industry is essential to know the degree of environmental corrosion in situ to take preventive action in order to extend the lifetime of the facilities, consider the type of materials used for construction, refurbish existing infrastructure and plan preventive maintenance services. To quantify the degree of atmospheric corrosion is necessary to develop a tool that allows in situ monitoring and measuring the level of corrosion using different methods for comparison. In this paper, the authors present the design and construction of an analogue circuit that can operate as a potentiostat / galvanostat and allow electrochemical noise measurements intended for a multimodal portable corrosion measurement system for remote monitoring. The actuator section was simulated and tested in the laboratory. The experimental results agree with simulations and suggest that it is possible to build a portable, multimodal equipment for in-situ monitoring of environmental corrosion levels.

Keywords-electrochemical noise, potentiostat, galvanostat, corrosion, athmospheric corrosion.

I. INTRODUCTION Atmospheric corrosion impacts directly on the economy

of any society and the structural integrity of the national infrastructure [1]. In some industrialized countries, the losses caused by corrosion effects range from 3 to 5% of the gross domestic product (GDP). For example in the United States, the corrosion damages cost up to 276 billion/year [2]. In particular for Mexico, corrosion affects industry severely. Environmental factors such as humidity, salinity, and pollution emissions largely determine corrosion rates. The Gulf of Mexico is the second most corrosive region of the world [3], just below the North Sea.

In turn, the state of Veracruz, ranks second in atmospheric corrosion nationwide. The high relative humidity levels in Veracruz (daily average of 75 to 80%), chlorides emanating from the sea and airborne sulfate and sulfur from combustion processes largely increase the corrosive properties of the environment [4]. Mexico’s national power company (Spanish: Comisión Federal de Electricidad, CFE) is one of the main companies that are

greatly affected by the effects of corrosion. Mexico’s corrosion effect on CFE infrastructure costs per year amount to 4% of the gross domestic product which incidentally corresponds to 15% of the annual budget spent by the CFE [5].

The consequences caused by corrosion to CFE infrastructure, lay beyond the deterioration of facilities and are difficult to quantify. Severe failure of CFE infrastructure could have catastrophic consequences for the end user and hence the national economy. To determine the effects of corrosive environments it is necessary to develop tools that allow continuous monitoring or measurement of corrosion levels at which metals are exposed [6]. In general, researchers studying the effects of atmospheric corrosion may use more than one measurement method for validation purposes [7] [8]. Knowledge of the degree of atmospheric corrosion in industrial or aggressive environments in situ, derived from a multimodal equipment can help prevention and minimization of damages caused by the deterioration of the metal, and may contribute to ensure the integrity of the structures and process equipment. However, desktop and laboratory corrosion test equipments are bulky [9] and are not suitable for field measurements. Thus the use of desktop corrosion measurement equipment is confined to the laboratory.

It has been proposed that the use of operational transconductance amplifiers in the current/voltage signal excitation section of the instrument, can lead to the construction of a portable, multimodal tool [10] capable of operating as galvanostat / potentiostat, and also allow electrochemical noise measurements over a wide bandwidth in the field, in line, in real time and be operated by telemetry.

A. Block diagram of the proposed circuit. The control block (Figure 1.I) uses a Proportional +

Integral controller (Figure 1.II), and allows selection of the closed-loop operation in voltage (Figure 1.III) or current modes (Figure 1.IV) (Figure 1.V). The scheme also allows electrochemical noise measurements (Figure 1.VI). Although the analog section of the instrumentation system has a wide bandwidth (> 50 MHz), it is proposed that a maximum sampling rate of 50,000 samples per second (to set a measurement bandwidth of 10 kHz) can be used for electrochemical noise measurements.

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DOI 10.1109/CERMA.2011.57

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II. CIRCUIT DESIGN AND TESTING The design was performed using OrCAD Capture © and

was simulated using the PSPICE option (Figure 2). The circuit uses operational transconductance amplifiers (diamond transistors from Texas Instruments: OPA860). Diamond transistor application circuits operate in a similar fashion to bipolar transistors in the sense that they are voltage controlled current sources. To function as a current source, the load (electrochemical cell) is placed between an array of two transconductance amplifiers in closed-loop balanced form. When the circuit works as potentiostat, the load is connected with respect to ground to form the closed loop control system.

The proposed circuit can provide a current of ± 75mA @ ± 5V to function as a galvanostat / potentiostat. For electrochemical noise measurements, the signals obtained from the electrodes are measured using buffer and differential amplifiers.

A. Working as galvanostat The circuit was set (in simulation and experimentation)

to produce a current signal in the cell of 1mA peak-to-peak corresponding to an input voltage of 1V peak-to-peak. For simulations, the circuit was tested using input signals with frequencies in the range of DC-20 MHz. For the experiments the excitation signals varied in the range of DC-16 MHz, using a Tektronix AFG310 arbitrary function generator.

Figure 3A shows simulated results and Figure 3B shows the Fourier transform of the measured values, for the circuit operating as galvanostat with simulated cell resistances of 100 Ω in parallel with a capacitor of 100 pF.

B. Working as potentiostat The circuit gain was adjusted to 1 to operate as

potentiostat in voltage mode. The input voltage is a triangular wave -20mV to +20 mV at a frequency of 0.125 Hz (Figure 4).

C. Electrochemical noise measurements For electrochemical noise measurements, samples of

galvanized steel and stainless steel-grade 316 were cut to construct 2.5 cm x 1.4 cm electrodes. To accelerate the corrosion process, samples were introduced into a solution of 2% H2SO4. The electrodes were treated with a potential of 1.34 Volts and 100 mA for 30 minutes. Electrochemical noise values were measured using pairs of intact electrodes and the results were compared to measurements obtained with treated electrodes (intact electrode-treated electrode), exposed to the environment, in the shade at 3 meters above the ground.

Figure 1. Block diagram of the proposed circuit

Figure 2. Equivalent diagrams of the proposed circuit operating as A) galvanostat and B) potentiostat

Figure 3. A) Circuit simulation operating as galvanostat (input voltage signal: 1 Volt pp at 1MHz, output signal: 1 mA pp) into a cell (resistances of 100 Ω in parallel with 0.01 nF capacitor). B)

Fourier Transform of measured signals. Applied voltage signal [I] and measured current signal [II].

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Figure 5 shows the summary of results of electrochemical noise measurements for the electrodes of galvanized steel. The noise values were obtained by averaging 512 measurements taken over a period of 1 ms. In a similar manner, Figure 6 shows the summary of results of electrochemical noise measurements using stainless steel electrodes (intact electrode and treated electrode).

III. RESULTS AND DISCUSSION When the circuit was set to work as galvanostat, the

simulation indicated that the current signal is maintained over a wide frequency range. The measured values agree with the simulations. For a 1 MHz signal, the resulting measured values have less than 0.5 dB attenuation and phase shift less than 3 dB. The harmonic content is consistent in both cases where the floor noise is probably related to the digitization process and windowing function used

When the circuit was adjusted to operate as potentiostat, qualitatively, the measured output signal follows the input signal closely. Quantitatively, the input signal has a magnitude of 11.60 mV RMS and average value of -0.92 mV, due to adjustment of the signal generator. The measured RMS value of the output signal is 11.59 mV, and average value of -0.90 mV. Thus, the experimental results agree with simulated values. For electrochemical noise measurements using galvanized steel electrodes, measurements with the pair of intact electrodes resulted in an RMS noise value of 134 mV. When one of the electrodes was replaced with an electrode treated with the H2SO4 solution, the measurements

Figure 4. A) Simulation of the circuit operating as potentiostat. and B) Measurements taken with a digital oscilloscope Tektronix TDS 3000

series, exported to Excel ©.

Figure 5. Row A: Images of galvanized steel electrodes. Row B: Chemical composition of each electrode. Row C: electrochemical noise measured. Column 1: original galvanized steel. Column 2:

electrode exposed to H2SO4.

Figure 6. Row A: Electron microscope Images of stainless steel electrodes. Row B: Chemical composition. Row C: Electrochemical noise measured. Column 1: original stainless steel. Column 2: one

electrode treated with H2SO4.

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increased by 18 mV (152 mV) compared to measurements obtained with intact electrodes under similar weather conditions (~ 10:30 AM, 40% Relative Humidity, 24 oC, in the shade).

For intact stainless steel electrodes the electrochemical noise measurement obtained was 52 mV RMS. When one of the intact electrodes was replaced with a treated electrode, the measurement increased 6 mV RMS (58 mV) for similar weather conditions (~ 11:30 AM, 39% RH, 27 ° C in the shade). Although the levels of noise obtained are smaller that those obtained with galvanized electrodes it is possible to detect useful levels of noise that can be related to material degradation.

IV. CONCLUSIONS AND FUTURE WORK This paper presents the design and construction of an

analogue circuit intended as the control stage of a multimodal equipment (potentiostat / galvanostat - electrochemical noise measurement) for conducting measurements related to atmospheric corrosion. The laboratory tests are in agreement with simulations using the circuit operating as a potentiostat / galvanostat using passive loads, built according to the simulations. In the case of electrochemical noise measurements, the electrodes treated with a solution of H2SO4 caused detectable electrochemical noise measurement levels with respect to intact electrodes intact under similar weather conditions. The results suggest that the proposed circuit configuration can be used to build a multimodal computer that allows measurements of atmospheric corrosion using different methods. The next step is the integration of systems to perform field measurements. The circuit design show agreement between simulated and measured data when set as galavanostat and potentiostat. In the case of electrochemical noise measurements, the results show that it is possible to detect changes in the material based on electrochemical noise levels. Although noise levels are lower for stainless steel to galvanized steel, it is possible to measure useful changes of measured noise levels which in turn can related to material degradation due to the effect of atmospheric corrosion. The circuit operates with a ± 5 voltage source so that it is possible to use it as part of a dedicated corrosion measurement system. Current work is dedicated to conclude full systems integration to carry out tests near power lines, in locations around the Gulf of Mexico.

V. ACKNOWLEDGEMENT The authors acknowledge the financial support from

the Public Education Secretariat (SEP) through the General Directorate of Technological Education (DGEST) under grant 4330.11-P to carry out this work. The authors are also grateful to M. C. Lourdes Mondragón from the Metallurgy Graduate Program at Morelia Institute of Technology, for her assistance for metallographic analysis of samples.

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