Prediction of corrosion behaviour of metallic materials over several 100 years could be assessed through the study of ancient archaeological artefacts ^[1]. The observation and characterisation of corrosion layers at a microscopic scale provides information helping formulating hypothesis for long term mechanisms. Raman micro-spectroscopy is a versatile tool for structural identification of iron corrosion products, thanks to its easy implementation, spatial resolution and ability to detect even low crystallised phases.

Various environment of iron corrosion were considered ^[2-6]: soil, atmosphere and concrete. For each case, Raman spectroscopy imaging had allowed to propose a schematic organisation of the corrosion layer, with phase identification and organisation. Some unexpected facts were encountered. For instance for buried artefacts, in the specific situation of the identification of chlorinated iron phases which are especially involved in active corrosion processes, we underlined the importance of a newly identified ferrous hydroxychloride (β-FeOOH). About atmospheric corrosion Raman spectroscopy emphasised the large presence of poorly crystallised phases as Maghemite, Ferrihydrites and Ferroxyhite.

Because each phase does not present the same activity in these corrosion mechanisms, it is not only needed to recognise and localise, but furthermore to quantify their proportions. But even at a microscopic scale Raman spectra display phase mixes, so that spectral decomposition is required to quantify the different phases. For that linear combination of reference spectra is used to extract “quantitative” structural composition. Thus normalised compositional images are deduced and by scanning a representative part of each sample “global” phase proportions are obtained. The final aim is one hand to provide data for alteration diagnosis and on the other hand to confront the results to existing predictive corrosion models.

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