XANES at Cl K-edge as relevant technique to reveal the iron archaeological artefact dechlorination treatments
Résumé
The degradation state evaluation, the corrosion diagnosis methods reliability, but also the development and improvement of conservation strategies, are dramatically important to preserve iron archaeological objects. In that way, dechlorination processes of iron artefacts are traditionally used in conservation workshops to stabilize objects from archaeological excavations. This paper aims to present unpublished results but also to give an overview of the last 10 years study of stabilization processes using synchrotron X ray absorption spectroscopy at Cl K-edge as a key approach: first to decipher the corrosion patterns involving Cl ions on iron artefacts, second to understand, mostly by in-situ observation, the dechlorination processes. Wrought iron bars found on Gallo-Roman shipwrecks were studied. The difference of reactivity involved in the corrosion processes make essential to distinguish and locate the phases in the heterogeneous corrosion layers. XANES at Cl K-edge is relevant to determine the Cl chemical form, distribution and role on phase’s stability. Chloride ions may be trapped inside the structure of iron oxyhydroxide as akaganeite and β- or γ- hydroxychlorides. Some Cl may also be adsorbed at the surface of the grains of the various phases in presence, i.e. not only the previsouly cited Cl containing phases but also other oxyhydroxydes as lepidocrocite and goethite as shown by the XANES spectra.