Structural evidence for the desalination of akaganeite in the preservation of iron archaeological objects, using synchrotron X-ray powder diffraction and absorption spectroscopy

Corrosions that are associated to the presence of chloride-containing phases are one of the main causes of the decay of metallic materials of historic value. In order to improve the currently used desalination treatments, the processes occurring have to be understood. The purpose of the present study is based on the preparation and desalination of pure akaganeite β-FeOOH powder, which is one of the main chloride-containing compounds present in the corrosion layers of ancient iron objects. The combination of X-ray Absorption Spectroscopy (XAS) and X-ray Diffraction (XRD) is a powerful tool, which revealed a correlation between the chloride content and the β-FeOOH structural parameters. We show that akaganeite contains different forms of chloride. These latter could be trapped inside the crystalline structure or adsorbed at the surface of the compound. In addition, we prove that dechlorination treatments, even in pure water, can easily remove the adsorbed chloride, while the chloride located within the channels of the β-FeOOH structure is also extracted but with less efficiency. Such structural evidence for the desalination process of akaganeite provides fundamental results for the treatment optimisation used in the context of the preservation of iron archaeological artefacts.