Linearity Assessment for Electrochemical Impedance of 625 AM Inconel in Aqueous Sodium Chloride Solutions
Authors: Baturina Olga A , Martin Farrel J 
Published: February 2020

Electrochemical impedance spectroscopy (EIS) is frequently used for assessment of corrosion rates and mechanisms of submerged metal structures in the marine environment. This technique is commonly used in linear approximation, when small oscillary perturbation of voltage (or current) at a certain frequency is used to generate a current (or voltage) fluctuation at the same frequency. This approximation works well for low amplitude signals. Increase in amplitude leads to broadening of potential region covered by ac signal and violation of linearity assumption. As a result, EIS spectra become amplitude-dependent (non-linear), with multiple harmonics generated by one excitation frequency. Non-linear EIS (NLEIS) also can be used for analysis of reactions kinetics and mechanisms [1], and in fact, it offers advantage of a better interpretation and quantification of charge transfer, adsorption and diffusion phenomena [2]. Recently, NLEIS has been increasingly used for analysis of non-linear practical systems, such as corrosion [3], fuel cells [4], batteries [5], and dielectrics [6]. This presentation explores application of NLEIS to a corrosion-related system. The aim of this work is to assess linearity of the impedance spectra of 625 Inconel alloy in aqueous sodium chloride solutions at high perturbation amplitudes and different applied potentials.
Electrochemical experiments were conducted in a three-electrode flat electrochemical cell (PAR, Inc). Freshly polished coupons of 625 Inconel alloys served as working electrodes (WEs), while platinum mesh and Ag/AgCl in 3 M NaCl (BioLogic, Inc) were used as a counter and reference electrodes, respectively. An oscillatory voltage signal with an amplitude of 10, 50, 100 and 200 mV was imposed on WEs held at potentials of -1, -0.2, 0.4 and 1 V vs Ag/AgCl reference electrode. Experiment was controlled by Virtual Front Panel software (Gamry potentiostat). Amplitudes and frequencies of generated currents were extracted using FFT software built in Origin 8.5.
Fig.1 shows current amplitudes at fundamental frequencies of 0.1, 1, 10 and 100 Hz, and those of additional harmonics for 625 Inconel alloy held at -1V. Impedance is dominated by the oxygen reduction reaction at the WE surface at this potential. With increase in amplitude, more additional harmonics are generated. However, the ratio of current amplitudes at fundamental frequencies to those ones at additional harmonics indicate that non-linearity effects are more pronounced at low frequencies of 0.1 and 1 Hz.
The presentation will discuss the reasons behind the differences observed at different electrode potentials, and compare mechanistic insights obtained from non-linear and traditional EIS.
References
[1] T.J. McDonald, S. Adler, Theory and Application of Nonlinear Electrochemical Impedance Spectroscopy, in: M.B. Mogensen, T.M. Gur, X.D. Zhou, T. Armstrong, H. Yokokawa (Eds.) Ionic and Mixed Conducting Ceramics 8, vol. 45, 2012, pp. 429.
[2] V.F. Lvovich, M.F. Smiechowski, Electrochimica Acta, 53 (2008) 7375.
[3] K. Darowicki, Corros Sci, 37 (1995) 913.
[4] K. Sedghisigarchi, A. Feliachi, Ieee Transactions on Energy Conversion, 19 (2004) 423.
[5] S. Abu-Sharkh, D. Doerffel, Journal of Power Sources, 130 (2004) 266.
[6] M. Kollosche, J. Zhu, Z.G. Suo, G. Kofod, Physical Review E, 85 (2012).

Figure 1