Anodic Tafel constant

Although the primary objective of Tafel analysis based on experimental measurements is the determination of the corrosion current density, icorr, the measurements also can give values for the cathodic and anodic Tafel constants, Pred x and Pox M, and estimates of the exchange current densities, i0 x and i0 M. The values of these parameters can provide information on the kinetic mechanisms of the electrochemical reactions,... 

It would be preferable to implement CP criteria based on the actual corrosion rate of the protected metal - that is, by lowering the corrosion rate using the anodic Tafel constant to some value that is adequate. However, this may be impractical because, in practice, the actual corrosion rate of the structure may not be available. A workable alternative would be to specify the potential change necessary to reduce corrosion by a given percentage. The anodic Tafel constant provides a reasonable guide or criterion for cathodic protection and enables a better understanding of how and why the cathodic protection is effective. However, determination of an accurate anodic Tafel constant for the protected structure is not an easy task. 

By other measurements it is shown that the cathodic reaction at this time is limited by oxygen diffusion, with a diffusion-limiting current density iL = 30 pm/cm on both materials, and that the anodic Tafel constants are... 

By converting to base-10 logarithms and defining the anodic Tafel constants and, we obtain the Tafel equation of a simple anodic reaction, also called the anodic Tafel line-. 

The values of h, and b, i.e. The Tafel constants of the anodic and cathodic polarisation curves, first have to be measured directly in the laboratory or deduced by correlating values of AE/Ai measured on the plant with values deduced from corrosion coupons. The criticism is that the K value is likely to be inaccurate and/or to change markedly as conditions in the process stream change, i.e. the introduction of an impurity into a process stream could not only alter i but also the K factor which is used to calculate it. 

The controversy that arises owing to the uncertainty of the exact values of and b and their variation with environmental conditions, partial control of the anodic reaction by transport, etc. may be avoided by substituting an empirical constant for (b + b /b b ) in equation 19.1, which is evaluated by the conventional mass-loss method. This approach has been used by Makrides who monitors the polarisation resistance continuously, and then uses a single mass-loss determination at the end of the test to obtain the constant. Once the constant has been determined it can be used throughout the tests, providing that there is no significant change in the nature of the solution that would lead to markedly different values of the Tafel constants. 

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. 

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

Figure 9—4 shows the polarization curves observed for the transfer reaction of cadmium ions (Cd Cd ) at a metallic cadmium electrode in a sulfuric acid solution. It has been proposed in the literature that the transfer of cadmium ions is a single elemental step involving divalent cadmium ions [Conway-Bockris, 1968]. The Tafel constant, a, obtained from the observed polarization curves in Fig. 9-4 agrees well with that derived for a single transfer step of divalent ions the Tafel constant is = (1- P) 1 in the anodic transfer and is a = z p = 1 in the cathodic transfer. 

Fig. 9-4. Anodic and cathodic polarization curves measured for transfer of divalent cadmium ions (dissolution-deposition) at a metallic cadmium electrode in a sulfate solution (0.005MCd + 0.4MS04 ) i (i )= anodic (cathodic) reaction current a = Tafel constant (transfer coefficient). [From Lorenz, 1954.]...

If the anodic anion transfer (anionic adsorption, Eqn. 9-13a) to form an adsorbed metallic ion complex is the rate-determining step, the Tafel constant, a = 1 - p, win be obtained from Eqn. 9-14. If the anodic transfer of the adsorbed metallic ion complex (desorption of complexes, Eqn. 9-13b) is the rate-determining step, the Tafel constant, a = 2 - p, will be obtained from Eqns. 9-16 and 9-17. Similarly, if the cathodic anion transfer (anionic desorption, Eqn. 9-13a) is determining the rate, the Tafel constant in the cathodic reaction, a = 1 p, will be obtained from Eqns. 9-15 and 9-16 and if the cathodic transfer of a metallic ion complex (adsorption of complexes, Eqn. 9-13b) is determining the rate, the Tafel constant, a-sp, will be obtained from Eqn. 9-18. In this discussion we have assumed Pi = Ps P then, Eqns. 9-19 and 9-20 follow ... 

When the cathodic reaction is the reduction of oi n molecules for which the equilibrium potential is relatively high (much more anodic than the corrosion potential), the corrosion current is frequently controlled by the diffusion of hydrated o Q en molecules towards the corroding metal electrode thus, the corrosion ciurent equals the diffusion current of o en molecules as shown in Fig. 11-8. For this mode of diffusion-controlled corrosion of metals the cathodic Tafel constant is... 

For the restricted conditions of unstressed iron in 0 < pH < 6, there are two main diagnostic results that suggest what is the most well-known mechanism for iron dissolution. Thus, under the conditions stated, the Tafel constant, banodic, is found to be 2RT/3Fmd the cathodic3 slope, bCithodicis-2RT/F. Surprisingly, the reaction orders with respect to a0H- are 1 for both the anodic (Fe —> Fe2+ + 2e) and the reverse cathodic reactions. For the latter, the actual experimental reaction order found was 0.8, but it is usually taken as 1. A mechanism that fits these facts is... 

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. 

In the Tafel equations j8a and [lc are known as the anodic and cathodic Tafel constants. Tafel plots are useful in obtaining corrosion rates. Consider a sample of metal polarized 300 mV anodically and 300 mV cathodically from the corrosion potential Econ. The potential scan rate may be 0.1-1.0mV/s. The resulting current is plotted on a logarithmic scale. The plot is shown in Figure 1.24. The corrosion current icort is obtained from the... 

The Tafel constant was b = 0.20 V decade-1 for iron electrodes [55] and b = 0.20 V decade-1 for austenitic stainless steels [54] in acid solution. It is noticed that these Tafel constants are greater than those (0.03-0.1 V) usually observed with general dissolution of metals in acid solution. The other mode of localized corrosion is the active mode of corrosion that prevails in the potential range less positive (more cathodic) than the passivation potential, EP, in which potential range the localized corrosion is mainly controlled by the acidity of the occluded pit solution. In the potential range of active metal dissolution, the anodic dissolution current density is also an exponential function of the electrode potential, except for diffusion-controlled dissolution. 

As previously stated, once Rp is determined, calculation of icorr requires knowledge of the Tafel constants. These constants can be determined from experimental anodic and cathodic polarization curves, or by Tafel-curve modeling, forthe material and solution of interest as discussed earlier. In the absence of these values, an approximation is sometimes used. 

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. 

Comparison of the anodic and cathodic Tafel constants shows that when a = 0.5, a = — a -, b = — b. Tables 3.2 and 3.3 list values of the Tafel constants for cathodic hydrogen evolution at T= 20 it 2 ° C on different metals and the effect of electrode materials and solution composition on oxygen overpotential [21]. The Tafel equation has been confirmed for numerous cathodic and anodic reactions, and its use is illustrated in the examples and case studies that follow. 

The Steam-Geary equation requires both the anodic and cathodic slopes to remain constant. For accurate measurements, it is necessary for the Tafel constants, and be, to be determined independendy using the Tafel technique. The slope of a linear polarization curve is controlled mainly by Ieo - Assuming that b and b are in the range of 120 mV, Eq. (5.24) reduces to ... 

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