In France, it is envisaged to dispose of high and intermediate level long lived radioactive waste at a depth of ~500 m in a deep geological disposal, drilled in a very stiff clay (Callovo-Oxfordian claystone - COx) formation. A carbon steel casing will be inserted inside disposal cells which are horizontal tunnels drilled in the COx claystone. A specific low-pH cement slurry (pH ~10.7±0.1 at 20°C) will be injected between the carbon steel casing and the claystone. Its role is to neutralize the acidity due to the oxidation of sulphur-containing species (e.g. pyrite FeS2) present in the claystone. A maximum temperature of 90°C is expected at the steel surface.

To study the behaviour of the carbon steel casing, various experiments, with durations from up to 6 months, were realized in this specific cement slurry in aerated or deaerated solutions at 80°C. The first electrolyte used simulates the short-term behaviour of the environment and corresponds to the pore water of the fresh cement slurry. The second electrolyte simulates the long-term behaviour and corresponds to the pore water of the cement slurry finally modified by the COx. The third electrolyte, already used in previous study [1, 2], is similar in terms of pH, carbonate and chloride concentrations to the pore water of the claystone formation.

At the end of the experiments, corrosion product layers were analysed by µ-Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS). This characterisation revealed that the corrosion product layers were mainly composed of magnetite and iron sulphides, in particular greigite and mackinawite. However, another corrosion product was frequently observed. Its Raman signature does not correspond to any spectrum listed in the literature. On the corresponding XRD patterns, several peaks could not be attributed and more likely correspond to the unknown compound also identified by Raman spectroscopy. The µ-Raman analysis suggests some similarity between this unknown compound and FeS2 phases, but neither marcasite nor pyrite were identified by XRD. SEM/EDS confirmed that the unknown compound contained Fe and S elements.

These results show that the corrosion product layers formed on carbon steel in the specific cement slurry, in aerated or deaerated conditions, contain iron sulphides. Among these compounds, a new phase was observed. It could correspond to an intermediate phase between mackinawite (FeS) and greigite (Fe3S4) or pyrite (FeS2).

References:

1.         Romaine, A., et al., Corrosion processes of carbon steel in argillite: Galvanic effects associated with the heterogeneity of the corrosion product layer. Electrochimica Acta, 2015. 182: p. 1019-1028.

2.         Romaine, A., et al., Electrochemical synthesis and characterization of corrosion products on carbon steel under argillite layers in carbonated media at 80 °C. Electrochimica Acta, 2013. 114: p. 152-158.