The use and mis-use of the tafel equation in corrosion science

One of the fundamental equations of electrochemically-based corrosion science, the Tafel Equation has now been in use for over 110 years. It relates the overpotential to the logarithm of the current (or current density) and has been widely employed to determine corrosion rates, most often in the presence of corrosion inhibitors. It can also ascertain mechanistic information concerning pure metal and alloy dissolution processes. Ultimately derived from the Butler-(Erdey-Grúz)-Volmer phenomenological model of electrode kinetics and utilising the mixed potential theory of Wagner and Traud, its application is examined in corrosion science especially in the context of the Stern-Geary relationships. However more often than not alternative approaches should be undertaken to correctly ascertain corrosion rate and/or mechanistic information, such as mathematical modelling methods (simulations), Allen-Hickling plots and other techniques including Electrochemical Impedance Spectroscopy. The role of surface films, including semiconducting/passive films, concentration polarisation effects, the influence of adsorbed species and intermediates, the occurrence of back reactions and other situations (eg iR drop, effect of potential scan rate on non-faradaic currents and temporal surface roughness variations) are discussed with reference to corrosion of two cast aluminium alloys.