Tafel regions

This limit is called linear kinetics. On the other hand, if the surface overpotential is large, one of the exponential terms is negligible. This limit is called Tafel kinetics. The relationship was found empirically. In the anodic Tafel region... 

It is evident from these expressions that since in the Tafel region / (the current density actually determined) must be greater than /(, (the equilibrium exchange current density), the signs of the overpotentials will conform to equations 1.60 and 1.61, i.e. will be negative and will be positive. 

The corrosion current (it is also assumed that the area of the metal is 1 cm so that - ho ) occurs at a value within the Tafel region for the anodic and cathodic reaction, i.e. transport overpotential is negligible. 

It has been seen from the above simple examples that the concentration of the substrate has a profound effect on the rate of the electrode process. It must be remembered, however, that the process may show different reaction orders in the different potential regions of the i-E curve. Thus, electron transfer is commonly the slow step in the Tafel region and diffusion control in the plateau region and these processes may have different reaction orders. Even at one potential the reaction order may vary with the substrate concentration as, for example, in the case discussed above where the electrode reaction requires adsorption of the starting material. 

Fig. 11. Potentiostatic polarization curve on Co3P, measured in a droplet of DI water. The Tafel region is marked [125]. (Reprinted by permission of The Electrochemical Society).

Figure 7.12 Schematic Tafel plot of log / (as y ) against overpotential rj (as x ). The linear regions yield the Tafel slopes, from which the transfer coefficients a can be determined. The intersection between the two Tafel regions occurs on the y-axis at log /o. ...

The central Tafel portions of Figure 7.12 haven t been discussed in much detail yet. We sometimes call these linear Tafel regions the potentials under kinetic control , since the magnitude ofl g, is a function of the rate constant of electron transfer (itself a Junction of the overpotential r)). 

The current/voltage characteristic curves for the reduction of 2 on chelate catalysts mostly show a Tafel region with a slope of about 60 mV. This could... 

The electroanalytical chemist tends to use terms such as reversible and irreversible in a subtly different way, although the difference is more one of semantics rather than final understanding. Thus, the phrase totally irreversible is used by electroanalytical practitioners for reactions in the Tafel region [lT)l (RT/F)]. The physical electrochemist most certainly views such reactions as being thermodynamically irreversible, and distinctly so. However, it seems that the use of the term 101311/ irreversible may lead to misunderstanding, for it does not seem consistent with the ease of reversal in direction, which in practice can be made to occur even with the most (thermodynamically) irreversible reactions. 

Consider an Evans diagram in a general way. The anodic dissolution reaction is to be represented in the Tafel region the same applies for the cathodic partner reaction, (a) Draw the two Tafel lines and show the region of intersection Oanodic= Cathodic)- Indicate on the graph the corrosion rate and corrosion potential. 

Between the limiting current plateaux of a voltammogram and the linear region close to Eeq described by (6.38) there is a region of potential for irreversible reactions where j depends exponentially on potential. This is the Tafel region. Considering a system where there is only O in bulk solution, that is there is only reduction, from the equation... 

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. 

Figure 27 Example of real polarization data illustrating problems associated with analysis of limited or false Tafel regions.

The current-potential relationship for a reversible redox system (note the very high value of the exchange current density) is displayed in Fig. 3. As can be seen in Fig. 3(b) there is no -> Tafel region. On the other hand, in the case of an irreversible system, the Tafel region may spread over some hundreds of millivolts if the stirring rate is high enough (see there). 

Tafel region — Part of the current density vs. electrode... 

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