Impedance technique

The apphcation of an impressed alternating current on a metal specimen can generate information on the state of the surface of the specimen. The corrosion behavior of the surface of an electrode is related to the way in which that surface responds to this electrochemical circmt. The AC impedance technique involves the application of a small sinusoidal voltage across this circuit. The frequency of that alternating signal is varied. The voltage and current response of the system are measured. 

The pulse-echo and resonance impedance techniques are capable of determining which layer contains a disbond, given an appropriate standard to compare against. Inspection standards are bond assemblies made to simulate specific areas of a part with disbonds purposefully placed in them. By comparing the signal response of a suspected void to that of a known disbond in the standard, the... 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

The use of a heavy arsenal of surface science (XPS, UPS, STM, AES, TPD) and electrochemical (cyclic voltammetry, AC Impedance) techniques (Chapter 5) showed that Equations (12.2) and (12.3) simply reflect the formation of an overall neutral backspillover formed double layer at the metal/gas interface. It thus became obvious that electrochemical promotion is just catalysis in presence of a controllable double layer which affects the bonding strength, Eb, of reactants and intermediates frequently in the simple form ... 

Deslouis C, Maurin G, Pebere N, Tribollet B (1988) Investigation of tellurium electrocrystal-Uzation by EHD impedance technique. J Appl Electrochem 18 745-750 Yagi I, Nakabayashi S, Uosaki K (1998) In situ optical second harmonic rotational anisotropy measurements of an Au(l 11) electrode during electrochemical deposition of tellurium. J Phys Chem B 102 2677-2683... 

Lincot D, Ortega-Borges R (1992) Chemical bath deposition of cadmium sulfide thin films. In situ growth and structural studies by Combined Quartz Crystal Microbalance and Electrochemical Impedance techniques. J Electrochem Soc 139 1880-1889... 

Among a few electrochemical techniques available for studying the adsorption, the AC impedance technique has been most widely employed. Its high sensitivity to the physico-... 

One of the most powerful methods for the investigation of electrochemical reactions is the use of alternating current (AC) impedance techniques. In order to understand the basis of this approach, however, it is helpful first to consider some simple electronic analogues which mimic closely the systems of electrochemical interest. 

This is the essence of AC impedance techniques the cell is replaced by a suitable model system, in which the properties of the interface and the electrolyte are represented by appropriate electrical analogues and the impedance of the cell is then measured over as wide a frequency range as possible. By comparing the measured results with values calculated from the model system, we can evaluate both the suitability of our model and the values of the parameters required. 

For the investigation of charge tranfer processes, one has the whole arsenal of techniques commonly used at one s disposal. As long as transport limitations do not play a role, cyclic voltammetry or potentiodynamic sweeps can be used. Otherwise, impedance techniques or pulse measurements can be employed. For a mass transport limitation of the reacting species from the electrolyte, the diffusion is usually not uniform and does not follow the common assumptions made in the analysis of current or potential transients. Experimental results referring to charge distribution and charge transfer reactions at the electrode-electrolyte interface will be discussed later. 

Only a small selection of the variants in the electrochemical literature can be mentioned here. Thus, impedance techniques (small amplitude sinusoidal perturbation at the electrode with observation of the system s response [22]) as well as polaro-graphic methods (at mercury electrodes) will not be described. Since the notion of a reaction mechanism requires consumption of substance, equilibrium techniques (such as potentiometry) will also not be discussed here. 

For an excellent introductory reading on ac impedance techniques for the purpose of ion conductivity measurement or study of interfacial properties, please see Linford, R. G. In Electrochemical Science and Technology of Polymers, 2nd ed. Linford, R. G., Ed. Elsevier Applied Science London, 1990 p 281. 

The separator resistance is usually characterized by cutting small pieces of separators from the finished material and then placing them between two blocking electrodes. The separators are completely saturated with the electrolyte. The resistance (Q) of the separator is measured at a certain frequency by ac impedance techniques. The frequency is chosen so that the separator impedance is equal to the separator resistance. To reduce the measurement error, it is best to do multiple measurements by adding extra layers. The average resistance of single layer is determined from multiple measurements. The specific resistivity, ps cm), of the separator saturated... 

Chemical capacitance. When the mechanism involves significant involvement of the bulk, accumulation of reactive intermediates not only involves surface species but oxidation and reduction of the bulk. This can be detected as an anomalously high effective capacitance, often referred to as a chemical (or pseudo) capacitance. This capacitance can be as large as 0.1 — 1 F/cm and thus easily detected by current-interruption or impedance techniques. Thus, capacitance is a strong indicator (independent of resistance) as to what degree the interface, surface, and/or bulk are playing in the... 

However, as mentioned in section 6, our awareness of this situation is not the same as being able to quantify the contributions of these various physical processes to the performance of a particular electrode under a specific set of conditions or in understanding all the factors that govern the rates of these processes. Unfortunately, due to the inherently convoluted nature of electrochemical and chemical processes, it has proven extremely difficult to isolate and study these processes individually in a complex system. We saw in sections 3—5 that impedance techniques can in some cases be used to isolate the linearized resistance of the interface from that of slower chemical steps via time scale. Various workers... 

Electrochemical Characterization Technloues. Since corrosion Is an electrochemical process, It Is not surprising that a considerable amount of work has been reported over the years on electrical and electrochemical techniques for the study of the corrosion process. Leldhelser Ql.) and Szauer (12.> 11) have provided good reviews of the principal techniques. Walter has recently provided a review of DC electrochemical tests for painted metals (14). Both AC and DC methods have been employed to study a variety of Issues related to corrosion and corrosion protection. DC techniques are especially useful for studying substrate processes, while AC impedance techniques are most useful for studying processes relating to coated substrates and the performance of coatings. 

The use of the impedance technique in the study of polymer coated steel, has been thoroughly described elsewhere. The present paper compares this technique with that of harmonic analysis, originally proposed by Meszaros ). The authors have presented preliminary data using the latter technique(3) wherein the early stages of polymer breakdown have been studied. The current paper extends this work to polymers which have been immersed for a considerable period of time. The harmonic method gives information not available from the impedance technique in the Tafel slopes and the corrosion current are directly measurable. A brief summary of the harmonic method and the equations used are given below. 

It can be seen that for severely degraded specimens, both the harmonic analysis and Impedance techniques are capable of detecting the presence of gross corrosion. The harmonics method provides a reasonable estimation of the corrosion rate when the Impedance data exhibits Warburg type behaviour. For less severely degraded specimens, especially those exhibiting blister attack, the Impedance method Is not as successful as the harmonic analysis technique. 

Where very little corrosion attack has occurred, neither method Is capable of providing reliable quantitative data. The non-appllcab-lllty In certain Instances of the Impedance technique as a monitoring tool has been reported previously. Further experience with the harmonic analysis technique may be capable of refining the results obtained. 

Simplicity and reliability of operation make AC impedance measurements attractive as a technique in the evaluation of coating integrity. As opposed to classical salt spray test, analysis times are shorter with the AC impedance technique and quantitative data are obtained permitting relevant mechanistic Information to be derived. Impedance test methods are likely to find many applications in the resolution of unsolved practical problems ( .) ... 

In the present work, steel surfaces polished, phosphated and painted are studied using AC Impedance technique In order to evaluate the protection efficiency of a commercial phosphatlng solution. The AC Impedance behavior of painted metal has been correlated with the Immersion time In the phosphatlng solution and with the desirability of a phospho-chromic rinse (18-23). A comparison of the Impedance behavior of two different types of commercial paints Is made for various durations of Immersion In sodium chloride solution at room temperature, and also for various temperatures at a given duration of Immersion. 

The investigations comprised zino pigmented coatings with the aim of testing the mechanism of their action and looking for ways of improving protective properties. With the use of impedance technique the meohanism for two periods of activity has been proven with these coatings. 

Another question is the proper technique for the evaluation of protective properties of zinc pigmented coatings. IMtll now, the most popular one is the test in a salt chamber (12.141. This technique does not allow a complete understanding of the complex phenomena oo-ouring in the coating. It seems that the impedance technique selected in this work provides more adequate and useful data. 

It was decided to carry out modifications of the highly zinc pigmented psdnt using the impedance technique to test the protective properties. Steel electrodes with the following coatings were tested ... 

Microbial metabolism results in an increase in both conductance and capacitance causing a decrease in impedance and a consequent increase in admittance. In the Rapid Automated Bacterial Impedance Technique (RABIT) system, the admittance was plotted against time to provide results (Bolton, 1990). The final electrical signal is frequency- and temperature dependent and it has a conductive and a capacitive component. At present, impedance instruments are able to detect 10 —10 bacteria/ml (Ivnitski et ah, 2000). Several commercially available systems are operated... 

Fehlhaber, K., and Kruger, G. (1998). The study of Salmonella enteritidis growth kinetics using rapid automated bacterial impedance technique. /. Appl. Microbiol. 84,945-949. 

In contrast to the chronocoulometric technique, the information obtained from the impedance technique concerns the derivatives with respect to potential of To, In /30 and d(ln (30)/dE. It is therefore needed to postulate an explicit isotherm in order to fit experimental data to the theoretical equations. However, the results, e.g. for (dT0/dE, can be integrated to give T0 as a function of potential within the faradaic region. As chronocoulometry is, by definition, applicable to potentials outside the faradaic region, the two methods may be considered as complementary tools to study adsorption phenomena that are hard to detect in the classical way, i.e. from double-layer data (e.g. the capacity Cd) that are influenced by the presence of electro-inactive absorbates. 

The fact of modulating the square root of Q was naturally supported by the results of the Levich theory in steady-state conditions [8]. With the increasing development of impedance techniques, aided by a sophisticated instrumentation [2], the authors of the present work promoted the use of impedance concept for this type of perturbation and introduced the so-called electrohydrodynamic (EHD) impedance [9, 10]. A parallel approach has been also investigated by use of velocity steps in both theoretical and experimental studies [5, 11, 12]. More recently, Schwartz et al. considered the case of hydrodynamic modulations of large amplitude for increasing the sensitivity of the current response and also for studying additional terms arisen with non linearities [13-15],... 

In some applications where adsorption is involved, the blocking ratio V can depend on the overpotential. The EHD impedance technique under potentiostatic conditions seems therefore the most appropriate technique. 

Except in special cases, the differential capacitance Cd is the most useful magnitude (see Fig. 1.9) since it is that measured with impedance techniques, although the integral capacitance is of interest as average data [13, 19]. 

---