Characterisation of thermally affected corrosion products originating on steel-bentonite interface

Abstract

The Czech national concept of the deep geological repository (DGR) is based on the Swedish KBS-3 concept including an engineered barrier system (EBS) situated in the crystalline host rock. The EBS comprises the double-walled waste disposal package (WDP) embedded in compacted bentonite in vertical drill holes. The double-walled WDP comprises the inner carbon steel package and the outer stainless steel package. Regarding the concept, there was a designed laboratory experiment simulating the thermal and irradiation loading of stainless steel coupons embedded in saturated compacted bentonite. Four experimental setups were conducted using compacted BCV and MX80 bentonites under anoxic conditions differing in initial saturation level (15-20 wt. %), heating temperature (ambient temperature, 90°C and 150°C), and irradiation (0.4 Gy/h). The analysis of steel samples included: visual inspection, scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD), Raman spectroscopy, and profilometry. Corrosion rate was calculated from the mass loss. The analysis of bentonite samples included: analysis of chemical (X-ray fluorescence microscopy, XRF) and mineralogical (XRD) composition, cation exchange capacity (CEC), and SEM-EDS analysis. The steel samples embedded in the BCV bentonite heated up at 150 °C indicate a lower corrosion rate when irradiated compared to unirradiated samples. A combination of 150 °C and irradiation leads to surface corrosion indicating an almost constant corrosion rate for the whole testing period. Unirradiated samples heated up to 150 °C showed the highest corrosion rate after 6 months with decreasing tendency when the loading period was prolonged up to 18 months. The decreasing corrosion rate was observed in both irradiated and unirradiated steel samples heated up at 90 °C correlating with increasing loading period. A minimum corrosion rate was found in steel samples embedded in water-saturated BCV bentonite stored under laboratory temperature without irradiation. The inhibiting effect of irradiation on steel corrosion was observed when the steel samples were embedded in MX80 bentonite heated up at 150 °C. Almost no effect of irradiation was observed when the MX80 bentonite was heated up at 90 °C.  Hematite and Fe-rich carbonates (chukanovite, siderite) were confirmed to form corrosion layers on the steel surface. The thickness of the corrosion layer varied, ranging from 10 to 45 µm, and was directly correlated with the loading duration. Steel samples that remained unirradiated and were heated up at 90 °C exhibited corrosion layers up to 45 µm in thickness after 12 months of loading. In contrast, irradiation and heating up at 150 °C led to the formation of thinner corrosion layers, typically ranging from 10 to 20 µm. The corrosion layer composed of Fe-Si-O was identified only on the surface of steel heated up to 150 °C. The layer was identified only by SEM-EDS indicating amorphous or poorly crystalline structure. The origin of Fi-Si-rich corrosion products needs to be confirmed by future research.