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Rôle des espèces sulfures sur le comportement d’un acier non allié en milieu de stockage des déchets radioactifs de type C : interaction sulfures / produits de corrosion

Abstract : This PhD work deals with the nuclear waste disposal. In France, it is envisaged byAndra (French national radioactive waste management agency) that high-level radioactivewastes will be confined in a glass matrix, stored in a stainless steel canister, it self placed in a carbon steel overpack. The wastes will then be stored at a depth of ~500 m in a deep geological repositery, drilled in a very stiff (indurated) clay (argillite) formation. The kineticsof corrosion expected for the overpack in this disposal concept are low and will stay low if the somehow protective rust layer that will develop initially on the steel surface remains undamaged. Local changes of the physico-chemical conditions may however degrade this layer and induce accelerated kinetics of corrosion. In particular, the growth of sulphate reducing bacteria (SRB) close to the steel overpack cannot be excluded and the sulphid especies these micro-organisms produce may modify the corrosion process. The aim of this work was then to achieve a better understanding of the corrosion system constituted with steel, its rust layer mainly made of siderite FeCO3, and a sulphide-containing electrolyte.First, it proved necessary to characterise the iron sulphides involved in the corrosion processes by Raman micro-spectroscopy so as to study their formation and transformation mechanisms in various conditions of Fe(II) and S(-II) concentration, pH, temperature andaeration. It could be demonstrated that the Raman spectrum of mackinawite FeS, thecompound that precipitated in any case from dissolved Fe(II) and S(-II) species with the experimental conditions considered here, depended on the crystallinity and oxidation state.Moreover, the mechanisms of the oxidation of mackinawite into greigite Fe3S4 in acidicanoxic solutions at 80°C could be described. Finally, iron sulphides, often present on archaeological artefacts, could be identified using Raman micro-spectroscopy. The compounds present were mainly mackinawite and greigite.Secondly, to investigate the nature and properties of carbonated rust layers, carbonsteel electrodes were polarised anodically in NaHCO3 electrolytes continuously de-aerated byan argon flow. The experiments were performed at room temperature. The carbonated greenrust was observed to form at 0.003 and 0.1 mol L-1 NaHCO3 whereas FeCO3 was obtained atthe largest concentrations (0.5 and 1 mol L-1). Additional experiments were performed similarly in solutions of NaHCO3 and Na2SO4. Chukanovite, the Fe(II) hydroxycarbonate with formula Fe2(OH)2CO3, could be obtained in solutions containing 0.03 mol L-1 of eachsalt.Finally, interactions between sulphide species and corrosion products were studied.Siderite, goethite and lepidocrocite proved to be reactive towards sulphide. So, it seems clear that sulphide species produced by SRB should interact with the rust layer before to reach the metal underneath. Tests were performed with ferrous archaeological artefacts immersed 2months in anoxic sulphide-containing electrolytes to demonstrate it. The main effect of theimmersion was the formation of iron sulphide at the interface between the dense corrosion products layer, mainly constitute of siderite, and the transformed medium, where minerals ofthe soil are mixed with corrosion products. Sulphide species were not detected at the vicinityof the iron surface.
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Jacques-André Bourdoiseau. Rôle des espèces sulfures sur le comportement d’un acier non allié en milieu de stockage des déchets radioactifs de type C : interaction sulfures / produits de corrosion. Autre. Université de La Rochelle, 2011. Français. ⟨NNT : 2011LAROS328⟩. ⟨tel-00688136⟩

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