Tafel coefficient

Hence, under the stated conditions, a plot of the varying overpotential T (, during decay against In t gives the Tafel coefficient a. The intercept will yield i0 if CDL is known from an independent source, e.g., a galvanostatic charging curve. 

The cathodic reaction takes place with an activation overpotential of-0.021 V, and the anodic reaction takes place according to the Tafel coefficients a = 0.06... 

Table 7. Tafel coefficients as a function of the temperature of aluminum deposition from alkylalumi-num electrolyte (EL II/T).

The effect of dc potential and Tafel coefficient on the harmonic response is shown in Figure 4.4.8. The power of the current response normalized by the free corrosion current (fc), shown on a log-linear scale, parallels that of the dc current response with constant ratio between the harmonics. For the symmetric case, shown... 

Precise measurements of the current responses at each of a number of harmonics can be used, in conjunction with Eq. (16), to evaluate the unknown terms in Eq. (11). A complete description of the faradic current-voltage response is sufficient to define the anodic partial current (the corrosion rate) at any potential the parameters needed are the forward and reverse Tafel coefficients, the free corrosion current, and the free corrosion potential. 

We can now bring the overvoltage r = E - rev explicitly into (4.32) and define the anodic Tafel coefficient... 

The values of the Tafel coefficients and j8c depend on the mechanism of the electrode reactions, which often consist of several elementary steps (Section 5.1). It is however not necessary to know the mechanism in order to apply the Butler-Volmer equation. Indeed, equation (4.36) describes the charge-transfer kinetics in a global, mechanism-independent fashion, making reference to three easily measured quantities t o, nd The formulae (4.37) and (4.38) then define the anodic and cathodic Tafel coefficients ... 

The extrapolation of an experimental polarization curve, measured from the Tafel region to the reversible potential, reveals the exchange current density (Figure 4.9). The reciprocal of the slopes of the straight lines yield the Tafel coefficients and P. ... 

Value of the exchange current density and of the Tafel coefficients... 

In contrast to the exchange current density, the Tafel coefficients and Pc vary relatively little between different reactions their values are typically in the range of 20-50 mV. 

Thus, if the Tafel coefficients are known, the measurement of the polarization resistance at the corrosion potential allows us to determine /cor. 

In practice, the exact values of )3a M nd )3c3 arc often not needed, particularly if one is more interested in the variation of corrosion rate with time than in its precise magnitude. Indeed, Tafel coefficients for different electrode reactions lie typically in the range of 20-50 mV. As a consequence, when estimating the corrosion rate from polarization resistance the uncertainty in the rate of corrosion will hardly exceed a factor of two, even if one does not know the actual values of Pc B ... 

To obtain an analytical expression for the anodic Tafel coefficient we apply the Butler-Volmer equation to each step. Their respective rates are Vj and Vn [mol m s ]. To simplify the equations, we use the abbreviation ... 

This allows us to obtain the anodic Tafel coefficient. 

By analogous reasoning, the cathodic Tafel coefficient is given by the relation ... 

In order to calculate the anodic Tafel coefficient for the Bockris mechanism, we use the quasi equilibrium approach and write the rate expression for the ratedetermining step (II) ... 

If the first step would limit the reaction rate, the cathodic Tafel coefficient would be identical to that of the Volmer-Heyrovsky mechanism (the anodic Tafel coefficient would be different though). On the other hand, if the second step is rate determining the cathodic Tafel slope differs. The rate of the reaction determining step, Vn, is given by... 

According to this equation the cathodic Tafel coefficient is equal to RT... 

Calculate the cathodic Tafel coefficient, and the exchange current density, ig. 

The anodic and cathodic Tafel coefficients are equal to jSg = 30 mV, = 50 mV. Calculate (a) the polarization resistance at the corrosion potential and (b) the corrosion current density. 

The corrosion potential of a zinc electrode in a de-aerated solution at pH 3.0 is equal to Fjor = 500 mV and the corrosion current density is = 10 A cm . Upon addition of hydrochloric acid, the pH decreases to 1.0 and the corrosion current density increases to j cor = 10 A cm. What is the new corrosion potential For this calculation, we assume that the cathodic reaction is first order with respect to H" ions. The cathodic Tafel coefficient is equal to = 0.053 V. 

Calculate (a) the ohmic resistance between the Luggin capillary and the working electrode, (b) the charge transfer resistance, and (c) the value of the Tafel coefficient... 

Calculate the anodic and cathodic Tafel coefficients for the special case where the exchange current density of the first step, iqj is significantly larger than that of the second step ipjj tpi ipn- For this calculation, we assume that the transfer coefficients for the two steps are equal aj = an = a. In addition, the concentration of the Cu ions at the electrode surface remains constant. 

A steel tank having a surface of 9 is eathodieally protected with a magnesium anode. The polarization resistanee of the tank is equal to 4 x 10" Q cm, the anodic Tafel coefficient is j8a = 20 mV. The eathodie partial reaction is the reduction of oxygen at the limiting eurrent. The proteetion must last 6 years. Because of different losses, only 50% of the anode mass ean be used for the protection of the tank. Caleulate the mass of the anode that is required. 

---