Anne Michelin

Investigation at the nanometre scale on the corrosion mechanisms of archaeological ferrous artefacts by STXM

For the first time, corrosion products of a 450 year old archaeological iron nail were investigated at the nanometer level using STXM. NEXAFS acquisitions at the Fe L-edge were performed on a thin film taken of the metal–corrosion products including the interface. Comparison with Fe L-edge reference spectra gathered on maghemite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3), chukanovite (Fe2(OH)2CO3) and metallic iron (Fe) showed the presence of an interfacial layer of about 100 nm at the metal–corrosion product interface consisting of maghemite and magnetite. Further from this interface, corrosion products are mainly constituted of Fe-carbonates, as well as smaller quantities of iron oxides, probably maghemite. These results support the hypothesis of the presence of a nanolayer controlling the corrosion processes at the metal–corrosion product interface, proposed during former studies at the macroscopic and microscopic levels. They also bring important new insights for the prediction of very long term corrosion of steels, especially into the fields of cultural heritage conservation and storage of nuclear wastes.

Silicate glass alteration enhanced by iron: origin and long-term implications.

Silicate glasses are used as containment matrices for deep geological disposal of nuclear waste arising from spent fuel reprocessing. Understanding the dissolution mechanisms of glasses in contact with iron, an element present in large amounts in the immediate environment (overpack, claystone, etc.) would be a major breakthrough toward predicting radionuclide release in the geosphere after disposal. Two different reacted glass-iron interfaces-a short-term nuclear system and a long-term archeological system-were examined using a multiscale and multianalytical approach including, for the first time on samples of this type, STXM under synchrotron radiation. Comparisons revealed remarkable similarities between the two systems and shed light on Fe-Si interactions, including migration of iron within a porous gel layer and precipitation of Fe-silicates that locally increase short-term glass alteration and are sustainable over the long-term.