Tafel cathodic slope

Figure 19.10a shows a theoretical plot of the right-hand side of equation 19.16 vs. AE in which the cathodic Tafel slope has been assumed to be constant at 120 mV and the anodic Tafel slope to have the arbitrary slopes of 40, 60 and 120 mV. It can be seen that linearity over a range of positive and negative potentials AE is achieved only when b = and that linearity is confined to AE 0 when b and b differ. 

The exact calculation of icorr for a given time requires simultaneous measurements of Rp and anodic and cathodic Tafel slopes (/> and be). Computer programs have been developed for the determination of precise values of /corr according to Eqs. (2) and (3). Experimental values of Rp (2p contain a contribution from the uncompensated solution resistance... 

In particular, Yeager and co-workers (42) have described a = a 4- j3 T for the reduction of oxygen on platinum in concentrated phosphoric acid in the temperature range 25—250°C with cathodic Tafel slopes nearly independent of temperature (a — 0.08 and j3 = 0.0012/K). The effect of electrode potential on the reaction kinetics is mainly through the entropy of activation [43]. 

Compare the form of these equations with the Tafel equation, eqn (1) the slope, b, of the Tafel line is thus (f T/( 1 - P)F) (anodic) and (RTj(PF) (cathodic). Note also that, when rj = 0, i = i0 and the exchange current density may be found from the intersection of the anodic and cathodic Tafel slopes (at tj = 0). This is one method of determining corrosion rates since,... 

One needs to know the values of the anodic and the cathodic Tafel slopes, to evaluate i from Eq. 32M or 33M. When these slopes are... 

On the other hand, as was shown recently,75 the value of jo can be estimated using yfi value extracted from the polarization curve and known value of the cathodic Tafel slope bc by (71)... 

The cathodic Tafel slope corresponding to reaction (19.33) in the reverse direction is close to —0.120 V per decade at 25°C. For both cathodic and anodic reactions, the interfacial capacity results in 30—35 xF cm. This figure is consistent with a low surface coverage by chlorine atoms. 

The rate expression for the multistep consecutive electron-transfer reaction of Scheme 1 [i.e., Eq. (31)] is able to relate complex consecutive electron-transfer reaction mechanisms to experimental potential vs. logarithmic current-density relations. When p is assumed to be 1/2, the Tafel slopes (1/a/) predicted by this relation can only have values less than or equal to 118 mV dec i (at 25 °C) for electron-transfer limited reactions, since electrons transferred in non-rds steps will add integers (to P) in the expected a values and therefore decrease the Tafel slope below 118 mV dec 1. For instance, the usual cathodic Tafel slope of 118 mV dec-i for a one- electron transfer over a synunetric harrier is decreased to 39 mV dec for one preceding quasi-equilibrium electron transfer and to 24 mV dec for two, etc., and the anodic Tafel slopes are similarly decreased for one and two following (where the reaction steps are still written as reductions, as in Scheme 1) electron transfers, respectively. It should be noted that the Tafel slopes that are determined hy a values involving y-i- P differ substantially and discontinuously from the value for a = P = 1/2, and therefore should be easily distinguishable. 

Similarly, the form of the cathodic Tafel equation and the expression for the cathodic Tafel slope are... 

Exchange enrrent densities and cathodic Tafel slopes for the hydrogen evolution reaction (2H+ + 2e = H2) at different electrodes and with different electrolytes are listed in Table 26.6 [3, 12-14]. 

Exchange Current Densities and Cathodic Tafel Slopes for the Hydrogen Evolution Reaction at 25 °C... 

Type 2 the inhibiting species takes part in the redox reaction, i.e. it is able to react at either cathodic or anodic surface sites to electroplate, precipitate or electropolymerize. Depending on its activation potential, the inhibitor affects the polarization curve by lowering the anodic or cathodic Tafel slope. 

When this choice criterion is adopted, the calculation of the 2modic and cathodic Tafel slopes is reduced to the solution of the equation of the second degree (x — exp(aAE))... 

The data are plotted in Figure 7. The anodic and cathodic Tafel slopes ("base 10") are 40 and 120 mV, respectively, and the exchange current density is 1.0 mA/cm". Therefore, the current density-electrode potential behavior of this activation controlled electrode reaction is described by... 

Bcath " cathodic Tafel slope then equation 10 describes the polarization behavior of inhibited electrodes in the same terms ... 

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. 

Fig. 17 Schematic representation of how the /R drop affects galvanic couple potentials (a) equal anodic and cathodic Tafel slopes and (b) unequal anodic and cathodic Tafel slopes.

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