Raman spectroscopic studies of the corrosion of model iron electrodes in sodium chloride solution

We have explored the un-enhanced Raman spectra of both single and twin electrodes in 3.5% NaCl solution (at ambient temperatures) over a range of applied potentials (between 20 and 200 mV) and times (between 0 and 5 h). Under these conditions, we observed the initial formation of 'green rust' (hydroxychlorides and/or hydroxycarbonates), followed by the formation of magnetite (Fe3O4) and then a mixture of the alpha- and gamma-FeOOH (goethite and lepidocrocite, respectively). These data are consistent with a model for corrosion during which the initially formed magnetite is either covered, or replaced, by layers of the FeOOH oxidation products. Fitting of the data as a function of time and potential shows that, although the product range is independent of potential, the relative kinetics of formation of magnetite and its subsequent conversion to the gamma-FeOOH were potential and time dependent. Analysis by mapping of the dry corroded surface showed a variety of species, including green rust, some Fe(OH)(3), as well as the gamma-FeOOH, and possibly some beta-FeOOH. But no surface magnetite was found, indicating that this material had been either covered up or converted to FeOOH. We noted several complications during this work, including the interference of resonance effects (on the Raman intensities) and the heterogeneity of the corrosion process (and hence distribution of species on the corroded surface). However, we believe that the use of un-enhanced Raman methods has led to conclusions more likely to be relevant to 'real' corrosion processes. Copyright (c) 2007 John Wiley & Sons, Ltd.