Accurate assessment of instantaneous corrosion rates

Slow potentiodynamic polarisation curves are often utilised for determination of corrosion rates, through identification of corrosion current densities. As this electrochemical approach is relatively easy to perform, this classic technique can be carried out at various stages of a corrosion investigation (albeit on identical samples) to ascertain instantaneous corrosion rates occurring during different stages of a corrosion process. Assuming that the corrosion is uniform, (which is in fact most often not the case, especially during the initial stages of corrosion) the corrosion current obtained from such scans is then typically converted to a corrosion rate. Most often the Tafel extrapolation method is used to estimate Tafel slopes in conjunction with a Stern-Geary treatment to ascertain corrosion current densities and hence evaluate an alloy's corrosion rate in a selected environment. Problems encountered with this commonly used method, which is strictly only applicable to irreversible reactions for corrosion rate determination (using estimates of Tafel slopes) are discussed. This approach however is not suitable for many alloy/environment combinations, such as aluminium alloys in neutral saline environments where it is extremely difficult to determine accurate Tafel slope values. Following discussion of electrode kinetics in electrochemistry and corrosion studies, an alternative but little used method based on the Allen-Hickling plot approach to elucidate corrosion rates is outlined. After showing its derivation, potentiodynamic polarisation curves of two cast aluminium alloys, LM24/A380 and LM25/A356, that are not amenable to the standard Tafel extrapolation method are presented and discussed. The Allen-Hickling method overcomes many of the inherent issues associated with the classic Tafel extrapolation and Stern-Geary polarisation resistance treatments. Allen-Hickling plots minimise problems associated with contributions from concentration polarisation and the inability of the standard electrochemical methods to deal with corrosion processes that involve significant contributions from back reactions, commonly seen in the corrosion of aluminium and other metal alloys. Finally a discussion of other approaches based on polarisation curve analysis is given which indicates that the Allen-Hickling methodology deserves more attention from the corrosion science community.