Tafel-Plot Technique

Double-Layer Effect (i) Tafel-Plot Technique... 

The error of the Tafel-plot technique due to the neglect of the double-layer effect was reported to be unpredictable. The doublelayer effect distorts both the anodic and cathodic polarization curves, not necessarily to the same degree this results in possible cancellation or enhancement of the error caused by the doublelayer effect. Furthermore, because the Tafel lines are distorted and may not even have a truly linear portion in any potential range, the results strongly depend on the rather arbitrary drawing of a straight line through the data points. 

The effect of the uncompensated solution resistance was discussed qualitatively for the Tafel-plot technique by Stern and Geary and Tomashov, and for the potentiodynamic experimental technique by Mansfeld, " but a quantative error analysis has not been reported for this technique. 

Of the four electrochemical techniques discussed in this chapter, the errors of the Tafel-plot technique have been the least evaluated, in the sense that every reported error analysis is only qualitative. Consequently, it is difficult to compare this technique to the others. The expected errors may be larger, under some conditions, than for the other techniques, but this is, at least partially, compensated by the fact that the error will often be accompanied by a distortion of the Tafel line (deviation from linearity) that will alert the user to the problem. Considering all the disadvantages enumerated in Section II.2, it is probably the least desirable alternative among the four techniques discussed. 

The problem with redox reactions of this type is that their rate constants are usually too large for regular steady-state techniques to be reliably applied, a or p then have to be determined through the reaction order or by some method such as Faradaic rectification. Usually, such methods require evaluation of the double-layer behavior in order to make double-layer corrections. This is often an unsatisfactory business, especially when corrections would be required over a range of temperatures. We conclude that for this important class of electrochemical reactions more data for examination of b T) or a T) are required. However, for certain ionic redox reactions that are sufficiently slow, Weaver has been able to evaluate a as /( T) from Tafel plots over a range of 0.3 ... 

Throughout this book, Tafel plots are drawn and discussed with log / as the y axis and rj as the x axis. This reflects the way such data are usually recorded (i.e. using a controlled potential technique and measuring the current). A consequence is that the units of the Tafel slope are (mV)" Tafel originally recorded l-rj data using a controlled-current technique with measurement of overpotential. Hence, he plotted log / as the x axis and overpotential as the y axis. This is a convention still followed by some technologists (e.g. many corrosion scientists) and as a result they commonly consider the units of the Tafel slope to be mV. 

Further techniques that are exploited for full BES and individual electrode studies include electrochemical impedance spectroscopy [77], current interrupt method [78], Tafel plots [79], and others. 

The intermediate case, when neither Eq. (1) nor Eq. (39) is applicable, results in curved Tafel plots. Consequently, the extrapolation technique for the determination of the corrosion current density can give erroneous results. Quantitative analysis of this error possibility has not been published, but qualitative discussions of the mixed-control Tafel plots have been given by Stern. "... 

When E > Ecorr, the first exponential term is greater than the second exponential term and Iex is positive. Plotted as E versus log Iex, Eq 6.5 plots as the upper solid curve in Fig. 6.2. For E < Ecorr, Iex is negative, and a plot of E versus log Iex plots as the lower solid curve in Fig. 6.2. These equations will be used in establishing relationships for the analysis of corrosion rates by the experimental techniques of Tafel-curve modeling and polarization resistance. 

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