Mechanisms of the dechlorination of iron archaeological artefacts extracted from seawater

Treatments based on the immersion of iron artefacts in NaOH or KOH solutions with or without cathodic polarisation of the metal core, used for conservation purposes, were applied to Gallo-Roman iron ingots excavated from the Mediterranean Sea. The transformation of the initial layer of β-Fe2(OH)3Cl was followed with time. The resulting phases were characterised by a complete set of multi-scale complementary techniques. It could be demonstrated that the main process involved successively the transformation of β-Fe2(OH)3Cl into Fe(OH)2 and the oxidation of Fe(OH)2 into α-/δ-FeOOH and magnetite. Cathodic polarisation accelerated, via migration processes, the removal of chloride ions.

► Rust layers of archaeological artefacts were thoroughly characterised by a set of multi-scale complementary techniques.
► Evolution of rust layers of archaeological artefacts during dechlorination process was compared to that of laboratory models.
► The mechanism of the transformations occurring inside the rust layer during dechlorination were described and interpreted.
► Dechlorination treatments could be optimised in the light of the new understanding of the mechanisms.