Iron corrosion in Callovo-Oxfordian argilite : from experiments to thermodynamic/kinetic modelling
Résumé
Many designs for high-level nuclear waste deep geological disposal include steel waste canister and low-alloy steel overpacks. The container and overpack corrosion products may affect the alteration kinetics of nuclear waste glass and contaminant retention properties due to mineralogical transformation in the surrounding clay. To better quantify the effect of corrosion on the mineralogical alteration of the clay, the present study reports the corrosion of pure iron in raw Callovo–Oxfordian argilite. Batch experiments have been carried out at 90 °C, from one to six months, under oxygen-free atmosphere. Iron corrosion kinetics and secondary mineral formation have been studied with quantitative XRD measurements. Chemical analyses have been performed by ICP-AES, ICP-MS and ionic chromatography. Eh and pH have also been monitored along with the reaction progress.
The phases formed from the Fe release in solution are magnetite and Fe-rich silicate from the serpentine group (greenalite or cronstedtite) or chlorite. These phases are associated to the dissolution of quartz, illite and interstratified illite/smectite mixed layers. Solution analyses show that the Si, Fe, Mg and Al concentrations are controlled at a very low level by the precipitation of newly formed phases, although a noticeable pH increase (from 7 to 10 at 90 °C) is associated to iron corrosion. In the conditions of the experiments, the iron corrosion rate has been measured (R$_{iron}$ = 6 × 10$^{−9}$ mol/m$^2$/s equivalent to 1.4 μm/year) and is in good agreement with previous works.
The use of the geochemical code CHESS based on (i) solution analysis, (ii) mineral quantification and (iii) determination of kinetic data for iron corrosion allows to reproduce accurately this reaction-path. Fractionation of dissolved iron between iron silicate and magnetite can be correctly predicted, as well as the pH, Eh and other minerals stability.