Electrochemical impedance spectroscopy and finite element analysis modeling of a 4-electrode humidity sensor for natural gas transportation pipelines

Abstract

Reliable corrosion monitoring of natural gas transmission lines is a major tool providing a foundation for safe management of natural gas infrastructures. Through the development of membrane-based electrochemical sensors which are able to function in low-conductivity gas environments, corrosion monitoring practices can be further strengthened by real-time monitoring of key risk factors such as relative humidity and corrosion rates of corrodible structures. In this work, we demonstrate and validate how a 4-electrode conductivity sensor can provide a means to monitor relative humidity in gases via electrochemical impedance spectroscopy through finite element analysis (FEA). For a relative humidity range of 5%–55%, the impedance response varied from 1 kΩ to 66 kΩ, showing a high sensitivity for gas humidity. To confirm that the measured impedance values reliably interpreted relative humidity, it was found that precise estimation of the sensor’s cell constant was needed. FEA was used to assess how the cell constant depended on the electrode geometry, membrane geometry, and electrode placement within the sensor. Through this approach, assumptions about the characteristic area and length were validated using electrolyte equipotential and current density vector mapping. This reduced possible cell constant uncertainties by 70%. With a cell constant of 14.84 cm−1, obtained via FEA, membrane conductivity values were in good agreement with published data.

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“…This sensor makes use of the galvanic current in a coupled multi-electrode array [73,74] (Figure 3) [75] to detect the localized corrosion. With the help of an Advanced Electrochemical Sensor (AES), the amount of water and the rate of corrosion in simulated natural gas were measured (Figure 3) [76,77].…”

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“…( a ) Commercial 2-electrode linear polarization resistance (LPR) corrosion sensors [30] (Courtesy of Rohrback Cosasco Systems); ( b ) Coupled multi-electrode array corrosion sensors [33] (Courtesy of Corr Instruments, LLC); ( c ) Ion-conducting membrane-based advanced electrochemical sensor (AES) for simultaneous humidity and corrosion rate monitoring (Reprinted from Reference [35] with the permission of AIP Publishing).…”