XANES at the Cl K-edge as a relevant technique to reveal the iron archaeological artefact dechlorination treatments

Abstract

Degradation state evaluation, corrosion diagnosis methods' reliability, and also the development and improvement of conservation strategies are dramatically important to preserve iron archaeological objects. In this way, dechlorination processes for iron artefacts are traditionally used in conservation workshops to stabilize objects retrieved from archaeological excavations. This paper aims at presenting unpublished results but also to give an overview of the last 10 years study of stabilization processes using synchrotron X-ray absorption spectroscopy at the Cl K-edge as a key approach: first to decipher the corrosion patterns involving Cl ions on iron artefacts, and second to understand, mostly by in situ observation, the dechlorination processes. Wrought iron bars found on Gallo-Roman shipwrecks were studied. The difference in reactivity involved in the corrosion processes make it essential to distinguish and locate the phases in the heterogeneous corrosion layers. XANES at the Cl K-edge is a relevant technique to determine the Cl chemical form, distribution and its role in each phase's stability. Chloride ions may be trapped inside the structure of iron oxyhydroxide as akaganeite β-FeO_1−x(OH)_1+xCl_x and β- or γ-ferrous hydroxychlorides β-Fe₂(OH)₃Cl. Some Cl may also be adsorbed at the surface of the grains of the various phases present, i.e. not only the previsouly cited Cl-containing phases but also other oxyhydroxydes as lepidocrocite and goethite as shown by the XANES spectra.