Some observations concerning the Tafel equation and its relevance to charge transfer in corrosion

Following some biographic notes about Tafel, his famous equation is discussed. Particular attention is given to the limitations of applicability of this equation, the distinction between the symmetry factor and the transfer coefficient, and the theoretical basis for the former. The possible dependence of the symmetry factor on potential and temperature are discussed, and some highly accurate experimental examples are shown. The mechanism of charge transfer in metal dissolution, which can be one of the elementary steps in corrosion, is discussed. It is argued that in this case, as well as in metal deposition, charge is transferred across the interface by ions rather than by electrons. This could have significant ramifications concerning the mechanism. It is proposed that the often observed difference between anodic and cathodic Tafel slopes could result from values of the symmetry factor that are far from 0.5, rather than from a stepwise mechanism that is commonly assumed. Assuming that charge is carried across the interface by ions rather than by electrons eliminates the need to postulate the formation of unstable intermediates. (c) 2005 Elsevier Ltd. All rights reserved.