George R. Engelhardt

Characterizing electrochemical systems in the frequency domain

Abstract The use of various impedance spectroscopies, including electrochemical impedance spectroscopy (EIS), photoelectrochemical impedance spectroscopy (PEIS), and those defined using various electrochemical, hydrodynamical, and mechanical conjugate variables, for studying physico-electrochemical systems is briefly reviewed with emphasis on identifying and defining a common theoretical basis. In particular, we explore the concepts of “conjugate properties” and “reciprocity”, the applicability of the Kramers-Kronig transforms, and the distinction between an impedance and a transfer function. We argue that the interpretation of impedance data in terms of electrical analogs and in terms of classical linear systems theory (LST) is possibly too restrictive, and that a more general basis is required to accommodate the broad range of “impedance spectroscopies” that are now being developed.

A simplified method for estimating corrosion cavity growth rates

A simplified method is proposed for calculating corrosion cavity propagation rates. This method is based on an assumption that if the rate of an electrode reaction depends (in an explicit form) only on the potential, the pit growth rate depends only on the concentration of those species that determine the potential distribution near the metal within the cavity. The advantage of this method is that it permits one to predict the rates of cavity propagation without knowing various parameters, such as the equilibrium constants of some chemical reactions and diffusion coefficients of species that are present at relatively low concentrations near the electrode surface. The analytical expressions for calculating propagation rates of cylindrical and hemispherical pits are compared with available experimental data. The influence of aggressive anions on the pit propagation rate has been investigated. It is shown that transport processes in the internal environment do, in the general case, influence the kinetics of metal corrosion.

New rate laws for the growth and reduction of passive films

A new rate law for the growth of anodic passive films on metal surfaces is derived from the point defect model (PDM). The model recognizes both the growth of the barrier oxide layer into the metal via the generation of oxygen vacancies at the metal/film interface and the dissolution of the harrier layer at the film/solution interface. The new rate law accounts for the existence of a steady state in film thickness, as well as for the transients in thickness and film growth current as the potential is stepped in the positive or negative direction from an initial steady state. The predictions of the PDM, with respect to the thickness of the barrier layer, are compared with those of the high field model. The latter cannot account for the existence of steady states or the decrease in barrier layer thickness on stepping the potential in the negative direction. The predicted transients in film thickness and growth current density are in good fidelity with the measured transients on tungsten in pH 1.5 phosphate buffer solution as the voltage is cycled between 10 V in reference to saturated calomel electrode (V SCE ) and 6 V SCE . Finally, the new rate law accounts for passive film thinning under negative potential step conditions as the pH is changed over the range of 1.5 to 5.1 and as the initial and final voltages are changed in a systematic manner, such that the voltage excursion is constant.

Importance of thermal diffusion in high temperature electrochemical cells

The significance of thermal diffusion in liquid systems with forced convection has been investigated over a wide range of temperature. Numerical calculations have been carried out for the case of dilute liquid solutions that flow into the entrance region of a tube or an annulus, and the theory has been developed for a first order chemical or electrochemical reaction that takes place on the electrode surface. As an example, dilute aqueous electrolyte solutions at sub-critical temperatures (up to 350°C) have been considered. As the Lewis number and the Soret coefficient increase, the impact of thermal diffusion on mass transport is predicted to become increasingly important at higher temperatures. Simple analytical relations for describing thermal diffusion effects in liquid solutions subject to forced convection have been derived, and good agreement between these analytical solutions and numerical results is shown. The use of an electrochemical cell with forced convection could be a promising experimental technique for determining Soret coefficients for electrolyte solutions and liquid mixtures at elevated temperature.

A Brief Review of Determinism in the Prediction of Localized Corrosion Damage

Abstract The great majority of models that have been developed to predict localized corrosion damage are empirical in nature. As such, they lack the ability to effectively predict damage outside of their immediate realm of calibration and generally can only “predict” what is already known. Furthermore, empirical models generally cannot predict new phenomena or relationships. It is shown that it is possible to describe propagation of corrosion damage deterministically, i.e. to describe how the systems evolves from the present state to the future state on the basis of natural laws [conservation of charge, mass-energy, and mass-charge equivalence (Faradays law), etc.], subject to constraint by the natural laws. It is clear that deterministic models have much more predictive power than do empirical models. However, in the general case, it is possible to describe the development of localized corrosion damage in terms of the propagation of an ensemble of corrosion events, rather than as individual cavities. Accordingly, the prediction can be made in terms of statistical terms, for example, probability that the deepest pit will exceed the critical dimension that defines failure (e.g., thickness of a pipe wall). In doing so, the statistical parameters (mean depth of the deepest pit with its standard deviation, etc.) can be calculated deterministically. In order to perform such calculations, we must possess deterministic models for every stage of cavity development (pit nucleation, propagation and repassivation, transition pit into crack, crack propagation, and so forth). Some of these models are outlined in the current review. The deterministic theory outlined here has been applied for the predation of localized corrosion damage in important, practical systems, including pitting in oil field components, in low pressure steam turbine blades and discs, and in condensing heat exchangers, to name but a few of the current and past applications, some of which are reviewed in this paper.

On the Development of a General Electrochemical Impedance Model

We wish to report upon our progress towards developing a general model for ascertaining electrochemical reaction mechanisms by electrochemical impedance spectroscopy. The model and associated computer codes generate theoretical electrochemical impedance spectroscopy (EIS) data for one-dimensional systems (homogeneous or heterogeneous) of arbitrary complexity based on a small periodic perturbation of the potential, E, applied to this system. Our goal is identify unknown mechanisms by optimization of the model on experimental EIS data with the most likely mechanism being selected by the code according to appropriate “goodness of fit” criteria. This process will also yield values for various model parameters. Even a quick search of the open literature reveals that many researchers tend to analyze electrochemical impedance data based on equivalent circuit analysis, rather than on a pure mechanistic model. Even when EIS data are interpreted in terms of reaction mechanisms, the mechanism that is often used is pre-conceived and hence may not be the most appropriate. Accordingly, the goal of the present work is to develop a general model comprising chemical and electrochemical elementary reactions, the combination of which (and hence the “reaction mechanism”) will be selected by an advanced optimization procedure. In order to describe a general model of electrochemical impedance, we began by first assuming that the system under study was within the framework of infinitely dilute solution theory such that migration could be neglected. We also assumed that the rates of each chemical reaction (homogeneous or heterogeneous) could be described by linear functions of the concentrations of each species in solution. Our current model of the electrochemical impedance for a one-dimensional system allows for concentrated solutions, based upon the work of Tribollet and Newman and we also allow for semi-infinite diffusion and forced convection. We begin below by outlining our model as it currently stands. We will describe both homogeneous and heterogeneous reactions by the single equation:

Importance of Thermal Diffusion in High Subcritical and Supercritical Aqueous Solutions

We have reviewed our experimental and theoretical studies of irreversible thermodynamics of non-isothermal aqueous systems, with particular emphasis on the investigation of thermal diffusion phenomena, via electrochemical methods. By employing Agar’s hydrodynamic theory, and by using the experimentally-derived standard entropies of transport,

Possible Distribution of Potential Inside Corroding Crevices

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Unification of the deterministic and statistical approaches for predicting localized corrosion damage. I. Theoretical foundation

, at temperatures from 25 to 125°C, we have calculated high temperature

Passivity breakdown on AISI Type 403 stainless steel in chloride-containing borate buffer solution

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