Tafel analysis reversible

Tafel Analysis Electrochemically Reversible Processes Problem... 

Problem 2.6 demonstrated that for the Tafel analysis of a multi-electron system we expect to measure a gradient which is proportional to n + otrosj where n is the number of electrons transferred prior to the rate-determining step and ctros is the transfer coefficient for the rate-determining step, note that if all electron transfers are reversible and highly driven then the gradient is proportional to n (the number of electrons). 

Tafel analysis of the voltammetric wave will provide the same information. From Problems 2.3 and 2.4, we know that for a one-electron irreversible wave, Tafel analysis yields a line of gradient equal to aP/RT, whereas for a reversible one-electron wave the gradient is equal to F/RT, such that the exponential portion (Tafel region) of the voltammogram is steeper in the reversible case. 

Fig. 9. Analysis of the experimental steady-state current-potential and impedance-potential data from E = - 1300 mV to E = -600 mV for a titanium rotating-disc electrode (45 Hz) in a solution of 0.02 M nitric acid, (a) Standard rate constant-potential curve calculated for the hydrogen evolution reaction on titanium assuming that Z)A = 7.5 x 10 5cms 1 and E° = - 246 mV. The Tafel slope bc = 211 mV and the measured ohmic resistance was 1.0 ohm cm2. The potentials are the "true potentials, (b) Steady-state current-potential curve. The potentials are the "true potentials. The squared symbols refer to the calculated "reversible curve, (c) High-frequency double layer capacity-potential curve. The potentials are the "true potentials.

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

Hence, for a reversible system, the well-known linear relation is obtained between the potential E and log (/iim -///). Other equations have been derived for those reversible systems that involve semiquinone formation, dimerization, or the formation of complex compounds with mercury. Logarithmic analysis of the polarographic wave is often the only proof of reversibility which is considered but recently several authors, in particular Zuman and Delahay, " have pointed out that it is inadequate to assume that an electrode process is reversible on this evidence alone. For a reversible reaction, plots of E vs. In (/lim - ///) give the electron number z from the slope of the plot, RT/zF, A clearer indication of irreversibility is the evaluation of slopes of log i-E curves for higher concentrations (for i < /lim). Irreversible processes will give Tafel behavior. 

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