Tafel extrapolation

With regard to eq. (3.32), can be neglected due to its small contribution to the ceU potential. However, the electrolyte conductivity is of significance in determining the governing current expression. For instance, when IRx 77J. the electrolyte has a high conductivity and if IRx IR the electrolyte has a low conductivity. Hence, from eq. (3.33) the governing current expressions are 

Nowadays, sophisticated instrumentation, such as a potentiostat/galvanostat is commercially available for conducting electrochemical experiments for characterizing the electrochemical behavior a metal or an alloy in a few minutes. Nevertheless, a polarization diagram or curve is a potent control technique. This curve can experimentally be obtained statically or dynamically. The latter approach requires a linear potential scan rate to be applied over a desired potential range in order to measure the current response. 

On the other hand, a galvanostat can be used as the current cmtrrd source for determining the potential response on a electrode surface. However, the potential control approach is common for characterizing electrochemical behavior of metallic materials. The potential can be plied uniformly or in a stepwise manner using a waveform The former case generates a steady-state current response, while the latter provides a transient current response. 

One important fact is that the solution must be continuously stirred to keep a uniform concentration of the species in solution otherwise, the concentration 

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Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

While the specific corrosion rate number determined by Tafel extrapolation is seldom accurate, the method remains a good confirmation tool. 

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

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... 

Tafel Extrapolation Corrosion is an electrochemical reaction of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While... 

The primary use of this laboratoiy technique today is as a quick check to determine the order of magnitude of a corrosion reaction. Sometimes the calculated rate from an immersion test does not Took correct when compared to the visual appearance of the metal coupon. While the specific corrosion rate number determined by Tafel extrapolation is seldom accurate, the method remains a good confirmation tool. 

Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

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. 

The Evans lines in Fig. 26 are key to the method of Tafel extrapolation. At potentials well away from the corrosion potential, the applied current density... 

The logarithmic nature of the current density axis amplifies errors in extrapolation. A poor selection of the slope to be used can change the corrosion current density calculated by a factor of 5 to 10. Two rules of thumb should be applied when using Tafel extrapolation. For an accurate extrapolation, at least one of the branches of the polarization curve should exhibit Tafel (i.e., linear on semiloga-rithmic scale) over at least one decade of current density. In addition, the extrapolation should start at least 50 to 100 mV away from Ec[Pg.45]

There are several factors that can lead to non-Tafel behavior. Diffusion limitations on a reaction have already been introduced and can be seen in the cathodic portion of Fig. 27. Ohmic losses in solution can lead to a curvature of the Tafel region, leading to erroneously high estimations of corrosion rate if not compensated for properly. The effects of the presence of a buffer in solution can also lead to odd-looking polarization behavior that does not lend itself to direct Tafel extrapolation. 

Tafel Extrapolation. The most fundamental procedure for experimentally evaluating Icorr is by Tafel extrapolation. This method requires the presence of a linear or Tafel section in the E versus log Iex curve. A potential scan of 300 mV about Ecorr is generally required to determine whether a linear section of at least one decade of current is present such that a reasonably accurate extrapolation can be made to the Ecorr potential. Such linear sections are illustrated for the cathodic polarization curves in Fig. 6.2 to 6.5. The current value at the Ecorr intersection is the corrosion current, Icorr, as shown in Fig. 6.10. Assuming uniform corrosion, the corrosion current density is obtained by dividing Icorr by the specimen area (i.e., icorr = Icorr/A). Anodic polarization curves are not often used in this method because of the absence of linear regions over... 

The time required to determine Icorr by Tafel extrapolation is approximately 3 h, which corresponds to the approximate time required for experimental setup and generation of a cathodic polarization curve at a commonly employed, slow scan rate of 600 mV/h. In comparison, a comparable gravimetric evaluation (mass-loss measurement) on a corrosion-resistant metal or alloy could take months, or longer. A limitation of the Tafel extrapolation method is the rather large potential excursion away from Ecorr, which tends to modify the WE surface, such that if the measurement is to be repeated, the sample should be re-prepared following initial procedures and again allowed to stabilize in the electrolyte until a steady-state Ecorr is reached. Consequently, the Tafel extrapolation method is not amenable to studies requiring faster, or even continuous, measurements of Icorr. 

In contrast to the EIS method, the Tafel-extrapolation, Tafel-curve-modeling and polarization-resistance methods are conducted under essentially dc conditions. In these cases, in generating the appropriate Eexp versus log iex or iex curve, the potentiodynamic potential scan rate is very slow, or the time between potentiostatic potential steps is very long. The common practice is a potential scan rate of 600 mV/h or an equivalent step rate of 50 mV every 5 min. Underthese conditions, it is assumed that a steady-state, extemal-current-density results at every discrete potential. Consequently, every element in the electrical circuit is purely resistive in nature, and therefore, the applied potential and resultant extemal-current-density are exactly in phase. Since the impedance (normalized with respect to specimen area) is dEexp/diex, under these conditions, the impedance, Z, at Ecorr is given by (see Eq 6.29) ... 

In the Tafel-extrapolation method for evaluation of Icorr, why is the cathodic polarization curve generally analyzed rather than the anodic polarization curve ... 

Why should the corrosion-specimen surface be re-prepared after a Tafel-extrapolation corrosion-rate measurement before conducting a subsequent measurement ... 

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