Electrochemical Corrosion-Rate Determination Techniques

Electrochemical impedance, weight loss, and potentiodyne techniques can be used to determine the corrosion rates of carbon steel and the activities of both sulfate-reducing bacteria and acid-producing bacteria in a water injection field test. A study revealed that the corrosion rates determined by the potentiodyne technique did not correlate with the bacterial activity, but those obtained by electrochemical impedance spectroscopy (EIS) were comparable with the rates obtained by weight loss measurements [545]. 

The results of the error analysis indicate that the error of the corrosion-rate determination can be considerable, even for small values of /corrA/j if Ih ratio is high. That is, the often used assumption that the mass-transport effect is negligible when the corrosion current density is a small fraction of the limiting current density (//// 1) is not justifiable for the general case. However, at low b jb ratios the conventional data evaluation methods can be used with acceptable errors for any value of i corr A/ At bjb = 0.25, the error for the corrosion current density is less than about 5 %, and at balb = Q.5, the maximum error is 25%. As discussed in Section IV.l(ii), the b jb ratio of many corrosion reactions is small therefore, this classical electrochemical technique may be applicable, without correction for the mass-transport effect, for many practical systems even when the system is near or under cathodic mass-transport control. 

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. 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

The measurement of corrosion current has provided, as is well known, a quite useful electrochemical technique for determining corrosion rates. However, contrary to homogeneous corrosion, pitting corrosion is a typical heterogeneous reaction on a metal surface, so that it is difficult to estimate the actual corrosion state from the usual corrosion current data. 

A simplification of the polarization resistance technique is the linear polarization technique in which it is assumed that the relationship between E and i is linear in a narrow range around E . Usually only two points ( , 0 are measured and B is assumed to have a constant value of about 20 mV. This approach is used in field tests and forms the basis of commercial corrosion rate monitors. Rp can also be determined as the dc limit of the electrochemical impedance. Mansfeld et al. used the linear polarization technique to determine Rp for mild steel sensors embedded in concrete exposed to a sewer environment for about 9 months. One sensor was periodically flushed with sewage in an attempt to remove the sulfuric acid produced by sulfur-oxidizing bacteria within a biofilm another sensor was used as a control. A data logging system collected Rp at 10-min intervals simultaneously for the two corrosion sensors and two pH electrodes placed at the concrete surface. Figure 2 shows the cumulative corrosion loss (Z INT) obtained by integration of the MRp time curves as ... 

However, since this corrosion reaction is short-circuited on the corroding surface, no current will flow in any external measuring circuit. Consequently, a direct electrochemical measurement of the corrosion current (convertible to corrosion rate by the application of Faraday s law) cannot be made. Despite this limitation, electrochemical techniques can be used to decouple the two half-reactions, thereby enabling each to be separately and quantitatively studied. This involves the determination of the current-potential relationships for each half-reaction. Subsequently, the behavior under electrochemically unperturbed (open-circuit or natural corrosion) conditions can be reconstructed by extrapolation of these relationships to Ecorr-... 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

In the determination of corrosion rates by electrochemical techniques the corrosion current density /corr in pa/cm2 is measured which is written as ... 

In another study [35], the electrochemical emission spectroscopy (electrochemical noise) was implemented at temperatures up to 390 °C. It is well known that the electrochemical systems demonstrate apparently random fluctuations in current and potential around their open-circuit values, and these current and potential noise signals contain valuable electrochemical kinetics information. The value of this technique lies in its simplicity and, therefore, it can be considered for high-temperature implementation. The approach requires no reference electrode but instead employs two identical electrodes of the metal or alloy under study. Also, in the same study electrochemical noise sensors have been shown in Ref. 35 to measure electrochemical kinetics and corrosion rates in subcritical and supercritical hydrothermal systems. Moreover, the instrument shown in Fig. 5 has been tested in flowing aqueous solutions at temperatures ranging from 150 to 390 °C and pressure of 25 M Pa. It turns out that the rate of the electrochemical reaction, in principle, can be estimated in hydrothermal systems by simultaneously measuring the coupled electrochemical noise potential and current. Although the electrochemical noise analysis has yet to be rendered quantitative, in the sense that a determination relationship between the experimentally measured noise and the rate of the electrochemical reaction has not been finally established, the results obtained thus far [35] demonstrate that this method is an effective tool for... 

Other techniques to determine the corrosion rate use instead of DC biasing, an AC approach (electrochemical impedance spectroscopy). From the impedance spectra, the polarization resistance R ) of the system can be determined. The polarization resistance is indirectly proportional to An advantage of an AC method is given by the fact that a small AC amplitude applied to a sample at the corrosion potential essentially does not remove the system from equilibrium. 

S. Feliu, J. A. Gonzales, C. Andrade, Electrochemical methods for onsite determinations of corrosion rates of rebars , in Techniques to Assess the Corrosion Activity of Steel Reinforced Concrete Structures,... 

The information about the tendenqf for corrosion to occur that can be obtained from thermodynamic calculations is important and useful. However, most of the science and engineering aspects in the field of corrosion focus on knowing and reducing the rate of corrosion. The rate of corrosion is not addressed by thermodynamics it falls instead within the purview of kinetics. So the kinetics of electrochemical reactions in general, and corrosion reactions specifically, are at the heart of the subject of corrosion. This chapter will introduce electrochemical kinetics at a simple level, with sufficient detail to develop the concept of mixed potential theory. The interested reader is referred to other volumes of this encyclopedia and to textbooks in corrosion [1-9] for a more detailed description. The kinetic underpinnings of some of the electrochemical techniques for determination of corrosion rate will also be presented. The influence of transport on the rates of electrochemical reactions will be discussed in the next chapter (see Chapter 1.4). 

Electrochemical techniques for determination of corrosion rate generate a value in units of A cm , which is current density. 

Many different electrochemical and non-electrochemical techniques exist for the study of corrosion and many factors should be considered when selecting a technique. Corrosion rate can be determined by Tafel extrapolation from a potentiodynamic polarization curve. Corrosion rate can also be determined using the Stem-Geary equation from the polarization resistance derived from a linear polarization or an electrochemical impedance spectroscopy (EIS) experiment. Techniques have recently been developed to use electrochemical noise for the determination ofcorrosion rate. Suscephbility to localized corrosion is often assessed by the determination of a breakdown potenhal. Other techniques exist for the determinahon of localized corrosion propagahon rates. The various electrochemical techniques will be addressed in the next section, followed by a discussion of some nonelectrochemical techniques. 

This section will discuss a number of different electrochemical techniques used for determination of corrosion resistance or corrosion rate. It will be assumed that a modem, computerexperiment control and data collection, as well as powerful... 

The electrochemical techniques described in Sects. 7.3.1 and 7.3.2 measure the response of the system to an externally applied perturbation. It is possible to use the inherent noise in the system as a stimulus and measme the system response [27-36]. The fundamental basis to use noise analysis as a determination of corrosion rate is not as strong as that for the other techniques. However, the advantage of noise analysis is that there is no need to apply any external signal, and the system is not polarized at all away from its natural... 

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