Electrochemical Mechanical Impedance

The use of impedance electrochemical techniques to study corrosion mechanisms and to determine corrosion rates is an emerging technology. Elec trode impedance measurements have not been widely used, largely because of the sophisticated electrical equipment required to make these measurements. Recent advantages in micro-elec tronics and computers has moved this technique almost overnight from being an academic experimental investigation of the concept itself to one of shelf-item commercial hardware and computer software, available to industrial corrosion laboratories. 

Paints used for protecting the bottoms of ships encounter conditions not met by structural steelwork. The corrosion of steel immersed in sea-water with an ample supply of dissolved oxygen proceeds by an electrochemical mechanism whereby excess hydroxyl ions are formed at the cathodic areas. Consequently, paints for use on steel immersed in sea-water (pH 8-0-8-2) must resist alkaline conditions, i.e. media such as linseed oil which are readily saponified must not be used. In addition, the paint films should have a high electrical resistance to impede the flow of corrosion currents between the metal and the water. Paints used on structural steelwork ashore do not meet these requirements. It should be particularly noted that the well-known structural steel priming paint, i.e. red lead in linseed oil, is not suitable for use on ships bottoms. Conventional protective paints are based on phenolic media, pitches and bitumens, but in recent years high performance paints based on the newer types of non-saponifiable resins such as epoxies. 

After this step, the understanding of microwave electrochemical mechanisms deepened rapidly. G. Schlichthorl went to the laboratory of L. Peter to combine potential-modulated microwave measurements with impedance measurements, while our efforts focused on laser pulse-induced microwave transients under electrochemical conditions. It is hoped that the still relatively modest knowledge provided will stimulate other groups to participate in the development of microwave photoelectrochemistry. 

The fact that chemical potentials are not experimentally handled, being neither directly measurable nor subject to control, prevents one from using impedance techniques to characterize chemical reactions occurring alone, that is, not participating in a mechanism with controllable steps. (For instance, in an electrochemical mechanism the electron transfer provides means for controlling the flow of chemical reactions in series.)... 

It is possible to write the impedance for each electrochemical mechanism which is described by a series of chemical/electrochemical reactions. In this chapter a general method will be presented using matrix notation, which simplifies the task. An example of a reaction mechanism containing two diffusing, A and C, and one adsorbed species B, described by Eq. (6.1), will be presented ... 

Electrochemical impedance spectroscopy is a mature technique, and its fundamental mathematical problems are well understood. Impedances can be written for any electrochemical mechanism using standard procedures. Modem electrochemical equipment makes it possible to acquire data in a wide range of frequencies and with various impedance values. The validity of experimental data can be verified by standard procedures involving Kramers-Kronig transforms. Several programs either allow for the use of predefined simple and distributed elements in the construction of electrical equivalent circuits or directly fit data to equations (which should be defined by the user). 

In this section, we review the application of impedance spectroscopy to the study of corrosion phenomena. Emphasis is placed on iUustrating how the method is applied to identify the different processes that occur at a corroding interface. We also review the use of impedance measurements for measuring corrosion rate, since this was the initial application of the technique in corrosion science and engineering. The use of impedance spectroscopy to analyze other cause and effect phenomena of interest in corrosion science, including electrochemical-hydrodynamic, fracture, and electrochemical-mechanical processes, is also discussed. 

Figure 4A.53. CT specimen geometry for determining the electrochemical/mechanical impedance for the propagation of a crack.

Without going through the algebra, for the equivalent circuit shown in Figure 4.4.54 we can obtain an expression for the electrochemical-mechanical impedance (Zenj) for this system, defined as the ratio of the ac voltage that appears at the reference electrode to the crack opening displacement due to the sinusoidal load variation under dc galvanostatic conditions ... 

Figure 4.4.55 presents the real vs. imaginary components of the electrochemical-mechanical impedance response measured for an FIY80 steel specimen of geometry shown in Figure 4.4.53 immersed in 3.5 wt % NaCl. Due to the form of Eq. (177), these plots are somewhat more complex than a conventional Nyquist plot. Nevertheless, these data are amenable to standard methods of electrical... 

Emery, S.B., Hubbley, J.L., Darhng, M.A., et al., 2005. Chemical factors for chemical-mechanical and electrochemical—mechanical planarization of silver examined using potentiodynamic and impedance measurements. Mater. Chem. Phys. 89, 345—353. 

The electrochemical impedance spectroscopy (EIS) method is very useful in characterizing an electrode corrosion behavior. The electrode characterization includes the determination of the polarization resistance (/J ), corrosion rate (Cfl), and electrochemical mechanism [1,4,6,19-28]. The usefulness of this method permits the analysis of the alternating current (AC) impedance data, which is based on modeling a corrosion process by an electrical circuit. Several review papers address the electrochemical impedance technique based on the AC circuit theory [22-24,29-30]. 

Below, the model for DMFC cathode impedance is presented, assuming the electrochemical mechanism of MOR on the cathode side (Kulikovsky, 2012b). In this section, the nonstationary version of the DMFC cathode performance model (the section Cathode Catalyst Layer in a DMFC ) is used to calculate the cathode impedance. As discussed in the section Cathode Catalyst Layer in a DMFC, the model takes into account spatial distribution of the MOR and ORR, through the cathode thickness. It is shown below that the spatial separation of MOR and ORR, discussed in the section Cathode Catalyst Layer in a DMFC, leads to the formation of a separate semicircle in the impedance spectrum. 

Using impedance data of TBN+ adsorption and back-integration,259,588 a more reliable value of <7 0 was found for a pc-Cu electrode574,576 (Table 11). Therefore, differences between the various EffM) values are caused by the different chemical states and surface structures of pc-Cu electrodes prepared by different methods (electrochemical or chemical polishing, mechanical cutting). Naumov etal,585 have observed these differences in the pzc of electroplated Cu films prepared in different ways. 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Electrochemical techniques have been utilized for many years to study metal corrosion. Two of these techniques, linear polarization (LP) and cyclic voltammetry (CV), complement each other, LP providing corrosion rates under conditions where the surface is minimally altered and CV furnishing information about the corrosion mechanism. With the advent of impedance spectroscopy (IS), both kinds of information can be gleaned simultaneously and more rapidly, while leaving the surface almost intact. In this paper, we discuss the application of IS to the study of rapid steel corrosion and describe a study we undertook to elucidate the roles played by adsorption and film formation in the inhibition mechanisms of the above-named compounds. For comparison, we also investigated two quaternary nitrogen salts, which appear to adsorb electrostatically and presumably do not form macroscopic films (8). 

Itagaki, M. Fukushima, H. Inoue, H. Watanabe, K. Electrochemical impedance spectroscopy study on the solvent extraction mechanism of Ni(II) at the water 1,2-dichloroethane interface. J. Electroanal. Chem. 2001, 504, 96-103. 

In this paper we present results from independent studies on the stage 2 to stage 1 transition area that show some unexpected features (anomalies). The results are obtained by electrochemical impedance spectroscopy (EIS), entropy measurements (AS(x)) and in situ x-ray diffractometry (XRD). The aim is to understand the mechanism of stage transition dealing with the observed anomalies. 

With the aid of steady-state polarization and electrochemical impedance measurements in an ammonia containing solution maintained at pH = 9-10.5, Touhami et al. proposed the following mechanism for the H2PO2 reaction utilizing an intermediate adion, Ni,[ds [72] generated in the two step reduction of Ni2+ ... 

Even refined electrochemical methods cannot alone provide full information about the molecular structure of the metal/ solution interface. Hence, many nonelectrochemical techniques have been developed in the past few decades to study the double layer. They include spectroscopic, microscopic, radiochemical, microgravimetric, and other methods. A combination of electrochemical (chronovoltammetry, chronocoulometry, impedance spectroscopy, etc.) and nonelectrochemical methods is often used in studying mechanisms of the electrode process. 

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