Mécanismes de la corrosion atmosphérique multiséculaire des alliages ferreux : Le cas des renforts de la cathédrale de Metz.

Marie Bouchar 1
1 LAPA - UMR 3685 - Laboratoire Archéomatériaux et Prévision de l'Altération
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
Abstract : The study of the mechanisms of the multisecular atmospheric corrosion of ferrous alloys has various applications, from the preservation and restoration of cultural heritage metals, to the evaluation of their longterm behaviour, specifically when they are used for the storage containers surrounding nuclear wastes. The study of the corrosion product layers (CPL) developed during 5 centuries on the Metz cathedral reinforcements brings new results for a better understanding of the complex processes involved in the formation of the atmospheric CPL. The phases and chemical elements constituting the CPL of these reinforcements were characterized at the micrometric scale (μDRX, Raman μ-spectroscopy (μRS), SEM-EDS). Results specifically showed that these CPL differ from other multisecular systems previously studied by their very high content in ferrihydrite (5Fe2O3, 9H2O). This very reactive phase is distributed in the whole CPL and mixed at the microscopic scale with goethite (a-FeOOH) and lepidocrocite (g-FeOOH). Diffusion experiments of bromide ions followed by in situ X-ray μ-fluorescence allowed a better understanding of the transport of dissolved species in the porous network of the CPL. Furthermore, a test of the corrosion system behavior in conditions simulating the wetting stage of the RH cycle of atmospheric corrosion, also followed in situ by μRS, highlighted the reduction of ferrihydrite at the metal/CPL interface. These results allowed to verify for the first time a fundamental hypothesis about the mechanisms of very long term atmospheric corrosion. Finally, recorrosion experiments of the corrosion system were monitored in a climatic chamber simulating accelerated atmospheric cycles in an 18O-labelled environment. Then the detection of the 18O isotope linked to the precipitated phases, by nuclear reaction analysis using a nuclear microprobe, allowed to localise the formation sites of the new corrosion products. All these results improve the understanding of the long-term corrosion mechanisms and bring fundamental data for future modelling of corrosion processes.
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Marie Bouchar. Mécanismes de la corrosion atmosphérique multiséculaire des alliages ferreux : Le cas des renforts de la cathédrale de Metz.. Science des matériaux [cond-mat.mtrl-sci]. Ecole Polytechnique, 2015. Français. ⟨tel-01186439⟩

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