Anodic Tafel line

Average from cathodic and anodic Tafel lines. 

Fig. 5. Comparison of anodic Tafel lines during oxygen evolution in 1 N H2S04 on (1) Pt, (2) Pb02 and (3) Sb (4 mol%) doped Sn02 [82, 84]...

Fig. 12.21. The cathodic and anodic Tafel lines at two different electrode reactions. At l/corr there is a steady state. No net current passes, but there is a steady production of A (e.g., H2) and B+ (the metal, corroding) in a net electrochemical reaction that appears to be chemical (no net current flows to an outside circuit). Note the difference in these diagrams from the Evans-Hoar diagrams, which show corrosion is spontaneous and drives itself. The difference is similar to that between an electrochemical reaction and a fuel cell. (Reprinted from J. O M. Bockris and S. N. M. Kahn, Surface Electro Chemistry, Fig. 8.1, p. 747 Plenum, 1993).

The experimentally determined cathodic and anodic Tafel lines for the electrodeposition of copper in acid copper sulfate solution are given in Fig. 4. Data presented in Fig. 4 can be used to evaluate the Tafel... 

All of the curves in Fig. 5.6 start in the active dissolution potential range and hence do not show the complete polarization curve for the iron extending to the equilibrium half-cell potential as was done in Fig. 5. 4. This extension was shown as dashed lines and the equilibrium potential was taken as -620 mV for Fe2+ = 10 6. Qualitatively, the basis for estimating how the active regions of the curves in Fig. 5.6 would be extrapolated to the equilibrium potential can be seen by reference to Fig. 4.16. There, the corrosion potential is represented as the intersection of the anodic Tafel curve and the cathodic polarization curve for hydrogen-ion reduction at several pH values. It is pointed out that careful measurements have shown that the anodic Tafel line shifts with pH (Ref 6), this shift being attributed to an effect of the hydrogen ion on the intermediate steps of the iron dissolution. 

There have been relatively few studies on the mechanism of the hydrogen dissolution reaction. In these cases, it was found that the cathodic and anodic Tafel lines intersect at the reversible potential (77). Thus, in these Tafel regions, the mechanism of the anodic is identical with that of the cathodic reaction. Until the present time, the anodic reaction has been investigated only on the noble metals. It is necessary to extend the studies of the anodic reaction to other metals to check if there is any change in mechanism from the cathodic reaction. 

Figure 17 shows how the system determines what the galvanic current, ohmic potential drop, and electrode potentials will be. In Fig. 17(a), the anodic Tafel line for an active metal N and the hydrogen evolution Tafel line on metal M are given with the assumption that the Tafel slopes are equal. Also shown are curves representing 7 X for three different values of R, the solution resistance between the electrodes. The line I x R has the shape of an exponential curve in the semilogarithmic space. The position at which the 7x7 curve for a given resistance intersects the Tafel lines... 

From the plots, it is seen that the corrosion current increases as the angular velocity increases from 0 to 30. However, when the velocity is increased further, the reduction of oxygen becomes activation controlled. As a result, the corrosion rate becomes independent of velocity. Table 3.5 summarizes the corrosion current and the corrosion potential as a frmction of electrolyte angular velocity. The data were taken from Fig. 3.18. (b) To estimate the corrosion current and the corrosion potential when sacrificial Zn-Mn alloy is short circuited with iron, the anodic Tafel line for Zn-Mn sacrificial alloy is plotted in Fig. 3.19. The point of intersection of the Zn-Mn dissolution line and cathodic oxygen Tafel line provides the value of the galvanic current (GC) or impressed current (IC) and the corrosion potential. 

Another possibility is, of course, determination of a and in the cathodic and anodic regions, respectively, but it is then a prerequisite that these constants are indeed confirmed to be constant, independent of the electrode potential. Unfortunately, this is not necessarily expected in the case of the HER where a variation with electrode potential of the symmetry factor p is noticeable in some cases (cf. Figure 11 below), and more seriously, the anodic Tafel line does not exhibit any reasonable linearity. 

Cathodic and anodic Tafel lines showing such good symmetry have never been reported for the overall HER on any electrode metals (including Pd itself). This observation is, hence, a strong indication that those asymmetric 97 vs. log/ lines usually observed on HER are not due to any peculiar characteristics of the proton/water discharge, but to an interference between the Tafel recombination process and the Volmer process. This observation hence indicates that the system lacks a unique rds. 

We see that a +2 may approach zero when 6 does so under anodic polarization. Namely, there is a general tendency that the anodic Tafel line for 172 readily bends up in the anodic region. The prediction is, indeed, substantiated by experiments (Figure 13). 

The anodic Tafel line should, therefore, become very steep and show a limiting current behavior when 0 approaches zero, or a +2 approaches zero, as indeed is observed on Pd (Figure 13). It seems likely that the same concept also applies to analogous behaviors on some other metals, ° although similar experiments would no longer be possible for obtaining a +2. 

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 corrosion rate data published in the literature are generally values averaged over a period of time. They are based on measurements of the loss of material (the weight loss method), of the ferrous ions present in the solution (photocolorimetric method), and of the pH change in the solution and on volumetric methods (collection of evolved hydrogen). Electrochemical procedures have frequently been used. Thus, corrosion current has been determined at the intersection of the initial or steady-state cathodic and anodic Tafel lines, or any of these lines with E=Another way in which information has been acquired is through Stem-Geary linear polarization and impedance analysis. 

Figure 5.14 shows some typical polarization curves, both anodic and cathodic, obtained for solutions with different concentrations of Cl" and Cl2. A peculiar feature of these curves is the low slope of the anodic Tafel line (30-33 mV) and the presence of the limiting current followed by a new increase in current in the cathodic branch (under certain conditions, the region of the limiting current becomes narrow and degenerates into a point of inflection on the cathodic curve). 

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