Buried iron archaeological artefacts: Corrosion mechanisms related to the presence of Cl-containing phases

Deterioration after excavation of archaeological iron artefacts buried in soil is often associated with the presence of Cl-containing phases in corrosion products, leading to serious problems for conservation of metallic objects of cultural heritage. Thus, in order to better understand the corrosion process related to the presence of chlorine, some high-resolution techniques of materials characterisation are implemented. Particularly this paper shows the great utility of the combination of micro-X-ray diffraction, micro-Raman spectroscopy and micro-X-ray absorption spectroscopy. The analyses are realised on the cross sections of iron corroded objects excavated on archaeological sites dated from the 12th to the 16th century AD. In addition to the common oxyhydroxide containing chloride, akaganeite (β-FeOOH) often mentioned in the literature, a ferrous hydroxychloride β-Fe2(OH)3Cl was also found in the corrosion layers. In order to explain the corrosion system formed during burial, a corrosion mechanism including the presence of chlorine, is proposed.