Tafel behavior

Fig. 5. Currrent—potential behavior of an electrode reaction based on equation 37. Tafel behavior is noted at high currents for two different values of d.

Figure 6.20 Extrapolated current density at t = 0 obtained from chronoamperometric experiments for Pt(l 11), Pt(lOO), and Pt(l 10) electrodes in 0.2 M HCOOH + 0.5 M H2SO4 on electrode. The straight lines show the regions where the Tafel behavior is observed. (Data taken from Herrero et al. [1994].)...

On mercury and gold the Volmer reaction is rate determining Tafel behavior is observed, but the apparent transfer coefficients depend on temperature [1],... 

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. 

It is useful now to describe the origins of the shape of the anodic and cathodic E-log i behaviors shown in Fig. 2. Note that the anodic reaction is linear on the E-log i plot because it is charge transfer controlled and follows Tafel behavior discussed in Chapter 2. The cathodic reaction is under mixed mass transport control (charge transfer control at low overpotential and mass transport control at high overpotential) and can be described by Eq. (1), which... 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

In a Tafel plot, the logarithm of the current is plotted against t], as illustrated in Figure 1. Note that the slope is equal to —ccnF/2.2RT and the intercept corresponds to logj o. From these values, k° can be determined with Eq. 11. Tafel plots are often employed in corrosion studies, since k° is usually small and the condition Co(0, t) Si Cq can be accomplished by simply stirring the solution. Deviations from the idealized Tafel behavior are seen at large t], where Cq(0,/) becomes significantly smaller than Cq. 

The impedance behavior of electrode reactions is often complex but can be conveniently simulated by computer calculations, especially in the case of the method based on kinetic equations (108, 113). The forms of the frequency response represented in terms of the Z versus Z" complex-plane plots and by relations of Z or phase angle to frequency ai or log (o (Bode plots) are often characteristic of the reaction mechanism and involvement of one or more adsorbed intermediates, and they thus provide diagnostic bases for mechanism determination complementary to those based on dc, steady-state rate versus potential responses. The variations of Z versus Z" plots with dc -level potential, in controlled-potential experiments, also give rise to useful diagnostic information related to the dc Tafel behavior. 

Equations (5.21) and (5.22) are examples of Tafel equations in which the current is an exponential function of potential. The Tafel behavior is illustrated in Figure 5.5. The intersection of the extrapolated lines for anodic and cathodic currents yields the equilibrium potential and the exchange current density. 

Solution For a system that follows Tafel behavior, the current density response to a potential perturbation... 

Solution The formal dependence of equation (11.30) on potential can be obtained under the assumption of a Tafel behavior, e.g.. 

At high anodic overpotentials, methanol oxidation reaction exhibits strongly non-Tafel behavior owing to finite and potential-independent rate of methanol adsorption on catalyst surface [244]. The equations of Section 8.2.3 can be modified to take into account the non-Tafel kinetics of methanol oxidation. The results reveal an interesting regime of the anode catalyst layer operation featuring a variable thickness of the current-generating domain [245]. The experimental verification of this effect, however, has not yet been performed. 

Note that lezyl should not be too small—the Tafel law holds only beyond a bias that satisfies kz > k[jT. When rj —> 0 the net current which results from the balance between the direct and reverse reactions, must vanish like rj. This imphes that the Tafel behavior is always preceded by a low bias Ohmic regime. 

Bockris and Wass [28] used a photocatalytic reduction. A 103 increase in rate was found in the photoreduction on a cadmium catalyst if 18 crown ether in dimethyl formamide 5% water was present. NR4 ions and the appropriate crown ether were essential to the reaction, which fitted the requirements of the Tafel behavior. 

This new parameter is now able to compensate the instability produced by the first term through the surface energy that is always negative. The definition of the situation arises in the third term that comprises the modified expression of the Tafel behavior due to ohmic drop and mass transport effects ... 

Fig. 3.1 Polarization curves illustrating charge-transfer polarization (Tafel ° behavior) for a single half-cell reaction, (a) Anodic polarization,...

In Chapter 4, analysis of the kinetics of coupled half-cell reactions shows how the corrosion potential and corrosion current density depend on the positions of the anodic and cathodic polarization curves. The anodic polarization curves are generally represented as showing linear or Tafel behavior, and the cathodic curves are shown with both Tafel and... 

Fig. 5.20 Representative anodic polarization curves for indicated pure metals ini N H2S04, pH = 0.56. Linear sections at lower potentials are representative of Tafel behavior. Redrawn from Ref 5,10-14...

Interpretation of an experimentally determined polarization curve, including an understanding of the information derivable therefrom, is based on the form of the polarization curve, which results from the polarization curves for the individual anodic and cathodic half-cell reactions occurring on the metal surface. These individual polarization curves, assuming Tafel behavior in all cases, are shown in Fig. 6.2 (dashed curves) with Ecorr and the corrosion current, Icorr, identified. It is assumed that over the potential range of concern, the Iox x and Ired M contributions to the sum-anodic and sum-cathodic curves are negligible consequently, Uox = Iox M and Ured = Ired x. At any potential of the... 

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