Studies of the corrosion mechanisms related to the presence of chlorine onthe archaeological ferrous artefactsContribution of the local and structural characterisation

摘要

During burial of iron archaeological artefacts, when water of the soil contains a certainrnamount of chlorine, Cl- ions migrate in the corrosion products conducting to highrnconcentrations in pores or fractures at the metal/oxide interface. After excavation, the suddenrnsupply of oxygen and the release of Cl- seems to lead to the development of specific phases.rnThe formation mechanisms of these phases are not understood up to now, but is often noticedrnthat the precipitation of such phases accelerates the deterioration of archaeological objects.rnThus, it is essential to apprehend the corrosion mechanism related to the presence of chlorine.rnFor this reason we are interested to the characterisation of the corrosion products of somernarchaeological artefacts coming from for different archaeological site. The compositionrnanalyses are performed by Energy Dispersive Spectroscopy (EDS) coupled to ScanningrnElectron Microscope (SEM) and Electron Probe Micro Analyses (EPMA). Respectively, EDSrnobservations show that chlorine seems to be mainly localised at the metal/oxide interface andrncomposition EPMA analyses allow evaluating the local absolute of these high-Cl zones andrnsuggest the presence of two different phases: the first one has a Cl amount about 3 to 8 mass %,rnand about 14 to 20 mass% for the second one. Moreover, for a precise local and structuralrnanalysis of the corrosion products, some fine techniques of materials characterisation arernimplemented: the micro diffraction under synchrotron radiation (μXRD at D15, LURE) andrnthe micro spectroscopy Raman (μRaman). The two phases identified are akaganeite ?β-rnFeOOH, and a ferrous hydroxychloride β-Fe2(OH)3Cl. These results are particularlyrninteresting because the only phase reported in literature is akaganeite. To our knowledge, thernβ-Fe2(OH)3Cl was never identified as archaeological artefacts corrosion products. Thesernresults will be discussed regarding the burial environment of the artefacts.