Tafel polarization corrosion rate determination

In this expression, bd and bc refer to the appropriate anodic and cathodic Tafel constants. Comparison of weight loss data collected as a function of exposure time determined from R , Rf from EIS, and gravimetric measurements of mild steel exposure to 0.5 M H2S04 are often within a factor of two. This suggests that use of Rn in the Stern-Geary equation may be appropriate for the estimation of corrosion rate (147-150). However, Rn measurements may underestimate corrosion rates. / p is often measured at effective frequencies of 1(T2 Hz or less in linear polarization or EIS measurements, while Rn is measured at 1 Hz or greater. An example of this is provided in Fig. 57, which shows the corrosion rate of carbon steel in 3% NaCl solution as a function of exposure time determined by EIS, linear polarization, noise resistance, and direct current measurement with a ZRA. Among these data, the corrosion rates determined by noise resistance are consistently the lowest. 

The corrosion rate determined by the fit for the polarization curve shown in Fig. 3(b) is 1.4 X 10 A cm , and the fitted anodic and cathodic Tafel slopes are 47 and 98 mV decade" respectively. The corrosion rate is between the values determined by extrapolation of the anodic and cathodic Tafel regions, and the Tafel slopes are different than those determined manually. 

The use of the inflection point of the polarization curve has also been suggested for corrosion-rate determination. " As the inflection point usually occurs near the corrosion potential, measurements at small current densities are sufficient, and it has been claimed that independent measurement of the Tafel slopes is not needed to obtain an approximate value of the corrosion rate. 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

Once Rp is determined by the EIS method, icorr is evaluated in the same way as with the polarization-resistance method (i.e., with Eq 6.28). Therefore, the Tafel constants still must be experimentally determined. The intrinsic value of the EIS method lies in the fact that extensive information is extracted (i.e., Rp, Rs, and C are all determined) and, ideally, interpreted to not only determine the corrosion rate but also the rate-controlling mechanisms at the material surface and within the electrolyte. 

Generally, the difference in form between ideal and real responses, which is exclusively due to the experimental procedures adopted for performing the polarization curve, affects the determination of the values of the corrosion current density and the Tafel slopes. The practical impossibility of a well-defined determination of these quantities, which, with reference to the law (2), characterize the behaviour of the system under examination, may result in an unsatisfactory formulation of the reaction mechanisms and an incorrect evaluation of the corrosion rate. 

The numerical methods [32, 33, 34, 35, 36, 37] developed for the analysis of experimental polarization curves described by the current-voltage characteristic (2) with the aim of determining the electrochemical parameters a, /3 and h, are of considerable importance in the field of basic research as well as in corrosion rate monitoring. They permit, in fact, a more objective evaluation of these quantities with reference to a given potential difference interval AE, removing the degree of subjectivity that is inherent in the graphic determination of the Tafel slopes. 

It is important to remember that some assumptions have been made in the derivation of Fig.l. First, the equations given there are applicable only if the electron transfer is the rate determining step in the partial corrosion reactions. This is important with respect to the calculation of the Tafel slope (RT/anF) or the interpretation of an experimental one. It is further assumed that during the polarization of the test electrode (corroding piece of metal) the composition of the solution in the vicinity of the electrode remains... 

It was shown above that the corrosion rate can be determined experimentally from the extrapolation of the linear portions of the polarization curves plotted in semilogarithmic space back to the corrosion potential. In order to perform Tafel extrapolation, it is necessary to polarize the electrode to large potentials on either side of the corrosion potential. It is also possible to determine corrosion rate experimentally using much smaller polarization from the corrosion potential, as is shown in this section. 

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. 

Unlike the cathodic portion of the polarization curve, the anodic portion of the curve in Fig. 3(b) does not exhibit clear Tafel-type behavior. The mechanism for Fe dissolution in acids is quite complex. A line can be drawn in the region just above the corrosion potential, giving a Tafel slope of 34 mV decade k Extrapolation of this line intersects the zero-current potential at 7 X 10 A cm , a considerably different value than the extrapolation of the cathodic portion of the curve. This is not uncommon in practice. When this happens, it is usually considered that the anodic portion of the curve is affected by changes on the electrode surface, that is, surface roughening or film formation. The corrosion rate is typically determined from the extrapolated cathodic Tafel region. 

H2SO4 in which the potential was scanned from —20 to -1-20 mV relative to the OCP. A relatively linear response is observed in this potential range. From Fig. 4, the polarization resistance can be found to be 80 cm. Using the Tafel slopes for this system determined from the potentiodynamic polarization curve given in Fig. 3 and the polarization resistance taken from Fig. 4, the corrosion rate is found to he 1.4 x 10 A cm , which is close to the value determined hy the Tafel extrapolation technique. 

It is possible to use the data from a standard potentiodynamic polarization curve to determine polarization resistance. So, even though the scan was performed over a wide potential range, the data near the zeroTafel slopes are determined from the potentials further away from the zerocorrosion rate is determined from... 

For the data in Fig. 6, the ohmic and polarization resistances can be determined to be about 0.3 and just under 100 cm, respectively. The value of Rp is slightly higher than that determined by linear polarization (Fig. 4) in a measurement that just preceded the EIS experiment on the same electrode. The double layer capacitance is seen to be 1/3000 SI cm = 333 pF cm . The polarization resistance determined by EIS can be used to determine the corrosion rate with the Stern-Geary equation, just as was described above for polarization resistance determined by linear polarization. EIS data provide no estimation of the Tafel slopes, which are required in the Stem-Geary equation. 

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