Potentiodynamic Tafel slopes

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. 

Consider the following information on the electrochemical oxidation of methanol on platinum It is known that upon adsorption of CH3OH upon platinum, dehydrogenation occurs. The final product is C02. Several analyses made on the basis of potentiodynamic sweep data suggest the presence on the electrode surface of c -OH. However, further spectroscopic work, particularly that in potentiostatic work in steady state finds linear CO, i.e., S=0, the intermediate. The Tafel slope is 60 mV at low current densities and 120 mV at higher values of the current. Potentiodynamic profiles show that the overall number of electrons in the oxidation is 1.2—1.5 (i.e., two kinds of CO radicals of somewhat different character as to their adsorption are on the surface). 

Figure 7.56 Determination of the anodic (pa) and cathodic (Pc) Tafel slopes from the potential versus current density plot obtained by potentiodynamic polarisation testing.

H2SO4 in which the potential was scanned from —20 to -1-20 mV relative to the OCP. A relatively linear response is observed in this potential range. From Fig. 4, the polarization resistance can be found to be 80 cm. Using the Tafel slopes for this system determined from the potentiodynamic polarization curve given in Fig. 3 and the polarization resistance taken from Fig. 4, the corrosion rate is found to he 1.4 x 10 A cm , which is close to the value determined hy the Tafel extrapolation technique. 

It is possible to use the data from a standard potentiodynamic polarization curve to determine polarization resistance. So, even though the scan was performed over a wide potential range, the data near the zeroTafel slopes are determined from the potentials further away from the zerocorrosion rate is determined from... 

Mansfeld has described a method to determine Tafel slopes from the nonlinearity found in small magnitude linear polarization measurements [15]. The approach is similar to the fitting method described earlier for fitting potentiodynamic polarization curves to the Wagner-Traud equation, Eq. (3). Mansfeld showed that the fit can be made with only two parameters, the Tafel slopes, instead of four. The values of Ecorr and Ep can be determined, and icon- can be eliminated from Eq. (3) by insertion of Eq. (4)... 

Potentiodynamic curves at Tafel line with a slope of 83-93 mV ... 

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