Tafel anodic 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. 

At 60 minutes only, dc potentiodynamic curves were determined from which the corrosion current was obtained by extrapolation of the anodic Tafel slope to the corrosion potential. The anodic Tafel slope b was generally between 70 to 80 mV whereas the cathodic curve continuously increased to a limiting diffusion current. The curves supported impedance data in indicating the presence of charge transfer and mass transfer control processes. The measurements at 60 minutes indicated a linear relationship between and 0 of slope 21mV. This confirmed that charge transfer impedance could be used to provide a measure of the corrosion rate at intermediate exposure times and these values are summarised in Table 1. 

It is known that double-layer effects are the most pronounced in the reaction of multivalent ions in a dilute solution. According to the calculation of Grahame, d°HP A3 potential region far from the pzc. Evaluate the cathodic and anodic Tafel slope values for the reaction... 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

The kinetics and mechanism of the chlorine evolution reaction in aqueous solutions have been studied on smooth, porous, and impregnated graphite [68, 69], The Tafel slope depends also on the nature and history of carbons. For HOPG and glassy carbon, the anodic Tafel slope is about 0.060 and 0.120 V per decade at 25°C, respectively, whereas for a graphite electrode consisting of a section parallel to the c-axis, three regions in the polarization curve with anodic Tafel slopes from 0.060 to 0.160 V per decade have been observed. 

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. 

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. 

The anodic and cathodic Tafel slopes are b and be- Assigning a positive sign for anodic Tafel slope, b, and negative sign for the cathodic Tafel slope, b, and rearranging Eqs. (5.10) and (5.11), one obtains ... 

Calculate the corrosion currents for the metals in Example 5.1, using the Stearn-Geary equation for cathodic and anodic Tafel slopes of h — 0.1 V and h — 0.1 V. Estimate the corrosion rates in mpy. 

The zinc (d=7.14 g/cm ) corrosion rate in a deaerated solution of pH 1 is 3785 mpy. Assuming the zinc surface acts as cathode, calculate the corrosion potential of zinc vs. hydrogen reference electrode. The activity of the metal in solution is 10 M. The value of the anodic Tafel slope, b, is 0.1 V per decade. The exchange current density for zinc is 10 A/cm. ... 

E5.6. Determine the corrosion potential, the corrosion current, and the corrosion rate in mpy for zinc (d=7.14g/cm ) in a solution of pH=l. The activity of the metal in the solution is 10 M. The values of both the cathodic and the anodic Tafel slopes are 0.1 V/decade. The exchange current densities for the anodic and cathodic reactions are 10 and 10 A/cm, respectively. 

In most cases discussed in this review, no firm conclusions could be reached with respect to the actual mechanism even for Pt, the most thoroughly investigated electrode. Instead, two or more mechanisms were cautiously indicated as consistent with experimentally found mechanistic criteria Tafel slope, reaction orders with respect to H", or OH and O2, and stoichiometric number. The use of the stoichiometric number as a mechanistic criterion for oxygen electrode reactions requires caution because its determination from the cathodic and anodic Tafel slopes involves the assumption that the same rate-controlling process is involved for both. This is questionable in view of the large differences in potentials and hence large difference in the state of the electrode surface between the cathodic and anodic branches of the polarization curves. 

Over- voltage range Anode material Electrolyte compo- sition Molarity (C/mol.dm ) Tempe- rature (T/°C) Anodic Tafel slope (fe/mV.logJ ) Exchange current densi decimal logarithm (log,j /A.cm" ) Overvoltage at 200 A.m (mV)... 

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