Tafel corroding system

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

The effect of mass transfer on electrode kinetics is shown in Fig. 3.12. Many useful kinetic rate expressions based on Tafel conditions, mass transport limitations can be developed from Eq. (3.59). Prediction of mass transfer effects may be useful in corrosion systems depending on the system s corrosion conditions. The mass transport limitations in corrosion systems may alter the mixed potential of a corroding system. Under Tafel conditions (anodic or cathodic), Eq. (3.59) can be written as ... 

The nonlinearity of the current-voltage relationship in corroding systems provides an opportunity to determine corrosion rates without the need to measure independently the Tafel constants. The reason is that the electrical perturbation, which is imposed on the system at a frequency of /, in a nonlinear system results in a response at 2/, 3/, 4/, and so on, in addition to a dc component (McKubre [1983], Morring and Kies [1977], McKubre and Macdonald [1984], Bertocci [1979], Bertocci and Mullen [1981], Kruger [1903]). Neither the fundamental response (fo) nor the total power response can be analyzed to determine uniquely... 

FIG. 1— Application of mixed potential theory showing the electrochemical potential-current relationship for a corroding system consisting of a single charge transfer controlled cathodic reaction and single charge transfer controlled anodic electrochemical reaction. p and p, are Tafel slopes. 

In corroding systems the detailed mechanisms of the partial electrode reactions are frequently not known. Therefore, it has been found usefiil to introduce the empirical Tafel coefficients P and P defined by... 

It is worth noting that the error is so small at small 6 /6 values that, for many practical corrosion measurement situations, it can be considered negligible. The maximum error is only 20% at Z /6 = 0.25, and 33% at 6 /6 = 0.5. While a complete generalization cannot be made, the Tafel slope of the anodic reaction in a corroding system is very often considerably smaller than the Tafel slope of the cathodic reaction. Typically, the Tafel slope of an anodic metal dissolution reaction is in the range of 0.03 to 0.06 while common cathodic reactions occurring during... 

The use of Tafel plots for the analysis of metal corrosion systems indicates how dissolved O2 in solution and the subsequent O2 reduction (which is under kinetic control) accelerates metal corrosion. Due to the high value of E for O2 reduction (+1.23 V vs. SHE), the intersection of the oxygen reduction and metal dissolution Tafel lines occurs at high values of E and When the reduction of both H+ and O2 drives metal corrosion (with the reduction reactions under kinetic control), one simply adds together the current-voltage Tafel lines for the two reduction reactions. A new line is then drawn for the sum of the cathodic currents on the corroding metal. The intersection of this new line with the metal oxidation Tafel line gives E and... 

The anode and cathode corrosion currents, fcorr.A and fcorr,B. respectively, are estimated at the intersection of the cathode and anode polarization of uncoupled metals A and B. Conventional electrochemical cells as well as the polarization systems described in Chapter 5 are used to measure electrochemical kinetic parameters in galvanic couples. Galvanic corrosion rates are determined from galvanic currents at the anode. The rates are controlled by electrochemical kinetic parameters like hydrogen evolution exchange current density on the noble and active metal, exchange current density of the corroding metal, Tafel slopes, relative electroactive area, electrolyte composition, and temperature. 

In Fig. 19.106 it has been assumed that the Tafel slopes are equal, i.e. 6, = b = b and the modified expression for the right-hand side of equation 19.16 has been plotted against A for different values of 6(30, 60 and 120 mV). Comparison of Fig. 19.10a and 19.106 shows how the curvature of the plots differs at cathodic potentials, i.e. A < 0. Thus the kinetic behaviour of a corroding metal, as expressed by different combinations of Tafel slopes, can be organised by this method of plotting curves. This theoretical approach has been confirmed experimentally by Mansfeld for the system Fe/H2S04. 

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