Evaluation of the Raman spectroscopy technique for in situ corrosion studies.

Laser Raman spectroscopy has been widely used for corrosion studies but it appeared that, in most of the cases, the sample under investigation was removed from the media in which it has been corroded prior the Raman investigation. The necessity of running 'ex situ' analysis was often due to the poor Raman response due to the absorption of the scattered light by the solution. A large number of studies have previously been conducted after increasing the amount of scattered light by using surface enhanced Raman spectroscopy. Unfortunately, removing the sample from the media in which it has been corroded, or applying metal islands onto the sample under investigation may have a significant effect on the sample's electrochemical reactions. Therefore, the objective of the current work was to evaluate the possibility of using conventional Raman spectroscopy for in situ corrosion studies.The first stage of the research was to identify the potential advantages and the limitations of the technique itself. Once these parameters were identified, it was necessary to assess the technique through different experimental tests. Two model systems, both based on pure iron wires, were exposed to 3.5 % NaCl solution. From both systems, we showed that first a layer of lepidocrocite (gamma-FeOOH) was developing with time at open circuit potential and second that the anodic reaction started with the formation of magnetite which later oxidised to lepidocrocite. Further experiments on iron, performed in three different buffer solutions (borate, carbonate and phosphate) permitted us to observe different oxidation reactions. Although no Raman data could be obtained from the iron sample immersed in borate solution, it was possible firstly to identify the reactions associated to the anodic / cathodic waves visible on the cyclic voltammograms and second to follow the development of the oxide layers and propose a model representation of the passive film. The passive film developed in phosphate buffer solution was identified as a mixture of Fe[3](PO[4])[2.]8H[2]O and FePO[4].xH[2]O. Further pitting experimental tests examined the electrochemical resistance of the passive film developed in such solution. It has therefore been shown through the series of experiments describe here that conventional Raman spectroscopy is an appropriate tool to study, in situ, corrosion processes.