The characteristics of a true Tafel slope

2005 is the 100th anniversary of the two original publications of the Tafel equation [1,2]. The international corrosion community is currently celebrating [3] the use of the corresponding Tafel slope (?), which is one of the most frequently used parameters in electrochemical corrosion. Even now, with the use of modern research and testing techniques, both electrochemists and corrosion engineers are frequently encountering this ‘Tafel constant’ in the technical literature and in instrumentation manuals (you can’t use an LPR meter without assuming beta values). Unfortunately, Tafel slopes are commonly misused and measured completely out of context. So, what is a Tafel slope? Perhaps most importantly, when is an apparent ‘Tafel’ slope a true Tafel slope?