Raman imaging of ancient rust scales on archaeological iron artefacts for long term atmospheric corrosion mechanisms study

Iron archaeological artefacts were studied to understand long-term corrosion by the atmosphere. Indeed, these samples collected on the construction elements of ancient monuments present ancient rust layers formed during their exposure over centuries to the indoor atmosphere. Thanks to Raman spectroscopy and the acquisition of hyperspectral images of the corrosion scales, several zones of the samples observed on cross sections could be characterised. It has been shown on six dated samples that the main phase is goethite (α-FeOOH). Lepidocrocite and akaganeite (γ-FeOOH and β-FeOOH) occur locally in the corroded products, often correlated with cracks. A less crystallised phase, a hydrated oxy-hydroxide, has been identified abundantly in more or less extended zones inside the layer. This phase could play an important role in atmospheric corrosion mechanisms. Copyright © 2006 John Wiley & Sons, Ltd.