Corrosion rate determination electrochemical methods

It is evident from previous considerations (see Section 1.4) that the corrosion potential provides no information on the corrosion rate, and it is also evident that in the case of a corroding metal in which the anodic and cathodic sites are inseparable (c.f. bimetallic corrosion) it is not possible to determine by means of an ammeter. The conventional method of determining corrosion rates by mass-loss determinations is tedious and over the years attention has been directed to the possibility of using instantaneous electrochemical methods. Thus based on the Pearson derivation Schwerdtfeger, era/. have examined the logarithmic polarisation curves for potential breaks that can be used to evaluate the corrosion rate however, the method has not found general acceptance. 

The impossibility of a direct measurement of corrosion rate using electrochemical testing would seem to be discouraging. Application of mixed potential theory allows determination of the corrosion rate using a method known as Tafel extrapolation. 

A. A. Aksut, W. J. Lorenz, and F. Mansfeld, "The Determination of Corrosion Rates by Electrochemical DC and AC Methods II. Systems with Discontinuous Steady State Polarization Behavior," Corrosion Science, 22 (1982) 611-619. 

J. Jankowski, Electrochemical methods for corrosion rate determination under cathodic polarisation conditions—a review part I—DC methods, Corros. Rev. 20 (2002) 159—177. 

By the use of method a) with a noble metal like copper, the measurements are mainly expressing the wet time, while probes including a metal such as steel or zinc (also in method b) give results with a certain relation to corrosion rates in the actual environment. The ratio between corrosion rates determined with weight loss coupons and electrochemical probe, respectively, stays constant under varying conditions at a given site, but varies from one site to another [9.13],... 

Chemical corrosion tests focus primarily on resistance to surface and selective corrosion. In general, the effect of material and corrosive medium variables can be understood with these methods. The variable potential, on the other hand, is more or less undefined and can experience time changes depending on the properties of the various partial reactions involved in corrosion. The fluctuation range of the potentials found in practice cannot be taken into account in the immersion test. Often, therefore, the results obtained in chemical corrosion testing using electrochemical methods must be further differentiated to take account of the variable potential. If the corrosion rate determined in chemical corrosion testing depends heavily on the potential, and has little to offer, but if there is only a little potential dependence it is more reliable. 

Corrosion occurs at a rate determined by equilibrium between opposing electrochemical reactions. The rate of any given electrochemical process depends on the rates of two conjugate reactions proceeding at the surface of the metal. Transfer of metal atoms from the lattice to the solution (anodic reaction) with the liberation of electrons and consumption of these electrons by some depolarisers (cathodic reaction). When these two reactions are in equilibrium, the flow of electrons from each reaction of balanced and no net electron flow (current) occurs. Various methods are available for the determination of dissolution rate of metals in corrosive environments but electrochemical methods employing polarisation techniques are by far most widely used. The corrosion rate (CR) is evaluated by mass loss method considering uniform corrosion. The Corrosion rate is determined by the following formula as per standard [102]. 

Both harmonic and electrochemical frequency modulation (EFM) methods take advantage of nonlinearity in the E-I response of electrochemiced interfaces to determine corrosion rate [47-50]. A special application of harmonic methods involves harmonic impedance spectroscopy [5i]. The EFM method uses one or more a-c voltage perturbations in order to extract corrosion rate. The electrochemical frequency modulation method has been described in the literature [47-50] and has recently been reviewed [52]. In the most often used EFM method, a potential perturbation by two sine waves of different frequencies is applied across a corroding metal interface. The E-I behavior of corroding interfaces is typically nonlinear, so that such a potential perturbation in the form of a sine wave at one or more frequencies can result in a current response at the same and at other frequencies. The result of such a potential perturbation is various AC current responses at various frequencies such as zero, harmonic, and intermodulation. The magnitude of these current responses can be used to extract information on the corrosion rate of the electrochemical interface or conversely the reduction-oxidation rate of an interface dominated by redox reactions as well as the Tafel parameters. This is an advantage over LPR and EIS methods, which can provide the Z( ) and, at = 0, the polarization resistance of the corroding interface, but do not uniquely determine Tafel parameters in the same set of data. Separate erqreriments must be used to define Tafel parameters. A special extension of the method involves... 

Ijsseling, F. P., "Application of Electrochemical Methods of Corrosion Rate Determination to Systems Involving Corrosion Product Layers," Br. Corros. J., Vol. 21, No. 2, 1986, p. 95. 

The measurement of polarization resistance, Rp, from impedance data has become quite popular in the study of electrochemical corrosion [38-40]. The value of Rp determined from the Nyquist plot has been shown [40] to have the usual inverse proportionality to corrosion rate determined by conventional weight loss methods. A computerized curve-fitting procedure permits extrapolation of the relatively high frequency semicircular data to Rp at the zero-frequency limit. Deviation at the low frequency side of the semicircle may occur due to the... 

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. 

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

Although important contributions in the use of electrical measurements in testing have been made by numerous workers it is appropriate here to refer to the work of Stern and his co-workerswho have developed the important concept of linear polarisation, which led to a rapid electrochemical method for determining corrosion rates, both in the laboratory and in plant. Pourbaix and his co-workers on the basis of a purely thermodynamic approach to corrosion constructed potential-pH diagrams for the majority of metal-HjO systems, and by means of a combined thermodynamic and kinetic approach developed a method of predicting the conditions under which a metal will (a) corrode uniformly, (b) pit, (c) passivate or (d) remain immune. Laboratory tests for crevice corrosion and pitting, in which electrochemical measurements are used, are discussed later. 

That is, to determine the correct corrosion rates in pitting corrosion, as shown in Fig. 37, it is necessary to know the local corrosion currents on the electrode surface. The corrosion current observed is, however, obtained as the total current, which is collected by the lead wire of the electrode. From the usual electrochemical measurement, we can thus determine only an average corrosion current (i.e., the corrosion rate). Hence if we can find some way to relate such an average rate to each local corrosion rate, the local corrosion state can be determined even with the usual electrochemical method. 

Thus the key parameters influencing the corrosion rate under deposits will be the deposit porosity, which determines the available surface area of material for corrosion, and the deposit tortuosity, which along with porosity will modify the fluxes of diffusing species within pores. Readers interested in a more extensive discussion are referred to other sources (17). Here we concentrate on a brief discussion of electrochemical methods of investigating the properties of deposits as a basis for eventual modeling. 

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... 

A general scheme for the development of corrosion models based on electrochemical principles has been described, and a number of examples for active, passive, and localized corrosion has been given. This chapter is by no means comprehensive, and a search of the scientific and technical literature will unearth many additional examples. The value in using electrochemical methods both to develop understanding of the corrosion process and to measure the values of specific modeling parameters is obvious. However, their application alone would not provide all the elements and parameter values required for the development of corrosion models, so the use of supplementary techniques is necessary. It is necessary also to keep in mind that electrochemical techniques inevitably accelerate the corrosion process one is interested in. Consequently, the scaling of electrochemi-cally determined parameter values to the rates and time periods of interest in the corrosion process to be modeled should be undertaken carefully and with a full knowledge of the limitations involved. 

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. 

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