Analysis of Polarization Curves Under Mixed Activation-Diffusion Control: An Algorithm to Minimize Human Factors

Abstract

A substantial part of corrosion research relies on the analysis of polarization curves to obtain corrosion currents, Tafel slopes and other parameters, such as exchange current densities. This, often manual analysis remains highly subjective, which hampers the reproducibility of corrosion research and makes comparison of reported Tafel slopes, corrosion rates, or exchange current densities from different literature sources difficult. One reason is that the analysis is strongly influenced by the selected range of the measured data. To improve this, we developed a Python library for the reliable analysis of polarization curves. A particular novelty is an algorithm designed to fit polarization curves under mixed activation-diffusion control, which is a situation often encountered in corrosion research. This algorithm reduces the subjectivity related to the measured or selected potential range. Moreover, the algorithm offers the possibility to diagnose and quantify the accuracy of the fit. We use experimentally measured polarization curves to test the proposed approach, and show that for curves without a clear, purely activation controlled Tafel region in the cathodic branch, accurate and consistent analysis is only possible by applying the mixed activation-diffusion control technique. Re-evaluation of literature data shows that by applying the library, the variability in reported Tafel slopes can be greatly reduced. Thus, the here proposed approach and the related open access Python library for the analysis of polarization curves may foster reproducibility and enhance comparability of data measured in corrosion research.

Corrosion Electrochemistry of Chromium in Molten FLiNaK Salt at 600 °C

“…The kinetic properties of each of these redox reactions can be interrogated by electrochemical methods. 11,72 However, the complexity of interpreting experimental polarization curves (i.e., LSV) arises from the presence of multiple oxidation paths and rateregulating phenomena (e.g., salt film formation, MT limitation, etc.) that are dependent upon potential, experiment conditions and salt chemistry.…”

Tafel slopes and exchange current densities of oxygen reduction and hydrogen evolution on steel

“…To analyse the measured polarisation curves and to evaluate the Tafel slopes and exchange current densities, the python library PolCurveFit was applied. 20 This library was specifically developed to fit a theoretical curve, derived from the Butler-Volmer equation (Equation 4), to the measured data, assuming the measured currents are either purely activation Table 1. Chemical composition (wt%) of the carbon steel in accordance with the supplier information and the measured composition (wt%) of the stainless steel using energy-dispersive X-ray spectroscopy (EDS).…”

“…More details can be found in earlier work. 20 The evaluation of the polarisation curves measured in the de-aerated electrolyte, aiming to study the HER kinetics, was performed assuming purely activation controlled kinetics. The fitted range included data starting at −200 mV versus OCP up to +50 mV versus OCP, which includes a cathodic Tafel region of almost two decades of current.…”

“…[15][16][17][18][19] However, even documented kinetic parameters for steel in solution vary greatly in the literature, as highlighted by the authors in earlier work. 20 Both the data analysis as well as the measurement methodology can influence the determined values.…”

Modeling and Predicting Reduction Reaction Kinetics for a Stainless-Steel in NaCl Solutions

“…The data files and MATLAB code to analyze and model cathodic polarization curves are available to download on the Open Science Framework web application. a The algorithm we have outlined here differs slightly from aninteresting method recently suggested by van Ede and Angst 41 to extract kinetics data from polarization curves. Their implementation 42 requires both a user-specified potential window over which the fitting method operates and a selection of governing equation: either Tafel extrapolation, activation kinetics, or mixed activation and diffusion control kinetics, to start the model.…”