Electrochemical investigations of steel corrosion in saline media

Steel is planned to encapsulate the high-level nuclear waste under the chosen repository for long-term. One of the possible repository formations is Salt Rock. Even though genuine rock salt is largely impermeable, for the safety assessments contact of saline brines with the surface of the steel must be investigated in order to study corrosion behaviour and thus to decide on the suitability of the material as a technical barrier. For this purpose, AISI 309S and Spheroidal Graphite Iron (SGI)´s corrosion properties are examined with electrochemical methods. Microscopic and spectroscopic analytics are used for examining surface alteration and corrosion product formation. Anoxic conditions were ensured during corrosion experiments and surface analyses. Steels were cut into discs, and polished with carbide papers and diamond paste, followed by cleaning in an ultrasonic bath to be used as working electrode. Ag/AgCl (sat. KCl) and Pt are used for reference electrode and counter electrode, respectively, for a three-electrode cell. Prior to each experiment, electrolytes (variable ionic strengths (I) set by NaCl and MgCl2 concentrations) were deaerated with argon gas and samples were cathodically cleaned at -1.2V for 5 min. For corrosion potentials (Ecorr) and the tendency of passivation as well as of pit formations, potentiodynamic polarization tests were applied. Passivity and pit corrosion potentials are observed only on AISI 309S, while SGI showed uniform corrosion although different corrosion potentials were measured depending on I. The corrosion product of SGI mainly consists of Fe(OH)2, confirmed by XRD, with slight contribution from Fe+3 as was observed by XPS. SGI shows lower values for Ecorr than AISI 309S. It is clear from the polarization experiment results that the Ecorr decreases with increasing I. In case of AISI 309S, a thin passivation layer was detected at the surface at lower I and pit formation took place at more positive potentials. However at higher I, thicker passivation layers formed and pits formed at more negative potentials. Pits and the plane surface were analysed by XPS and SEM-EDX, displaying rather different types of corrosion layers. Secondary phases in pits enriched Cr+3, and the rest of the surface passive area are combination of Cr+3 and Fe+3. Cyclic voltammetry was applied to study dynamics of electrochemical processes. Results indicated that the first cycle does not show any significant current (due to passivity) until to the point where pits start to form. At each cycle, the observed current increase in the anodic region is due to the propagation of pit formation. This hints that aroused peaks at cathodic regions are the results of the pit formation and conversion of corrosion products formed mainly on the pits

Corrosion of steel in contact with bentonite under conditions relevant for nuclear waste disposal

Carbon steel is considered by many countries as a potential canister material to encapsulate high-level nuclear waste (HLW) before final disposal in a deep geological repository. Depending on the repository concept, compacted bentonite may be used as backfill material. Groundwater will eventually migrate through the barriers and induce steel corrosion. Information on corrosion rates (i.e., service lifetime of the container) and corrosion mechanisms are of high importance for the Nuclear Disposal Safety Case. A low-alloyed carbon steel and a spring steel of comparable composition, though with higher Si content, were selected in this study. The higher Si content of the spring steel is expected to reduce substantially the corrosion rate by forming of a layer of iron silicates better protecting the surface than for the carbon steel. This hypothesis was tested by performing corrosion experiments in closed vessels under anoxic and water saturated conditions at room temperature (RT) and at 50°C. Coupons were polished and the MX-80 bentonite was pre-equilibrated with synthetic Grimsel pore water prior to use. After 3 months of reaction time, and cooling down to RT where necessary, pH and redox potential (Eh) were measured in-situ and the composition of ultra-centrifuged pore water determined by ICP-OES and IC. Coupons were analyzed by various techniques and corrosion rates were determined from weight loss measurements. Relatively comparable pH values were measured for the carbon steel (pH 8.40) and the spring steel (pH 8.14) at RT, and lower values were measured in experiments conducted at 50°C. Eh values were not much affected by temperature, but were significantly lower for the carbon steel (around -380 mV). The composition of the pore water in all experiments were relatively comparable. The corrosion rate for the spring steel was larger than for the carbon steel and values increased with temperature. No presence of corrosion products on the surface of both steel coupons could be detected by XRD analysis. However, morphological changes could be seen at the surface of both coupons by SEM, and a change in chemical composition of the exposed surfaces was evidenced by SEM-EDX analysis. Elemental compositions point to the presence of a thin layer of Fe-silicate covering the coupons. Complementary information on elemental composition and oxidation state of the new-formed mineral was provided by surface sensitive XPS analysis. Overall, the investigated spring steel is less corrosive than carbon steel under elevated temperature conditions while at room temperature the investigated carbon steel has more corrosion resistance compared to spring steel. The increase of corrosion rate with temperature agrees with reported studies and is expected to decrease with reaction time (e.g., doi.org/10.5006/1.3287691) owing to the development of a more compact alteration layer at the surface of coupons. This assumption is being investigated in experiments with longer reaction time and the comparison of results from both materials will enable to conclude whether Si present in the steel has a significant impact on the corrosion resistance

Adsorption of Selenium and Strontium on Goethite: EXAFS Study and Surface Complexation Modeling of the Ternary Systems

Knowledge of the geochemical behavior of selenium and strontium is critical for the safe disposal of radioactive wastes. Goethite, as one of the most thermodynamically stable and commonly occurring natural iron oxy-hydroxides, promisingly retains these elements. This work comprehensively studies the adsorption of Se(IV) and Sr(II) on goethite. Starting from electrokinetic measurements, the binary and ternary adsorption systems are investigated and systematically compared via batch experiments, EXAFS analysis, and CD-MUSIC modeling. Se(IV) forms bidentate inner-sphere surface complexes, while Sr(II) is assumed to form outer-sphere complexes at low and intermediate pH and inner-sphere complexes at high pH. Instead of a direct interaction between Se(IV) and Sr(II), our results indicate an electrostatically driven mutual enhancement of adsorption. Adsorption of Sr(II) is promoted by an average factor of 5 within the typical groundwater pH range from 6 to 8 for the concentration range studied here. However, the interaction between Se(IV) and Sr(II) at the surface is two-sided, Se(IV) promotes Sr(II) outer-sphere adsorption, but competes for inner-sphere adsorption sites at high pH. The complexity of surfaces is highlighted by the inability of adsorption models to predict isoelectric points without additional constraints

Anaerobic corrosion of carbon steel in compacted bentonite exposed to natural opalinus clay porewater: bentonite alternation study

Carbon steel is a potential canister material for the disposal of high-level radioactive waste in deep geological repositories in clays and clay rocks. Bentonite is considered as a potential backfill material for those multi-barrier systems. To predict the long-term performance and for safety assessment the knowledge of canister corrosion behavior is important. The formed corrosion products and mineralogically altered bentonite at the canister/bentonite interface can potentially provide an additional barrier against radionuclide migration. In-situ corrosion experiments were performed at the Mt. Terri underground research laboratory. Coupons of carbon steel were embedded in Volclay MX-80 bentonite with controlled densities, installed in a borehole under simulated repository and anaerobic conditions and exposed to natural Opalinus Clay porewater for a period up to 5.5 years. The bentonite layer at the canister/bentonite interface was characterized by complementary microscopic and spectroscopic techniques (XPS, SEM-EDX, XRD) under anoxic conditions. The interface revealed reddish-brown staining up to 2 mm depth into the bentonite in the zone adjacent to the steel. The SEM-EDX analyses of the interface (embedded crosscut with steel removed) showed calcium and iron enrichment in the bentonite adjacent to the metal. µXRF analysis performed on the bentonite at the interface showed calcium enriched rim up to 100 µm into the bentonite, while µXANES analysis revealed formation of iron silicate compounds in the reacted reddish-brown zone. The steel coupon was removed prior embedding. A line scan from the edge towards bulk bentonite did not indicate any systematic gradient in the Fe2+ / Fe3+ ratio. The formation of mixed Fe2+/3+ silicate compounds appears to be heterogeneous. This work contributes to an increased understanding of steel corrosion mechanisms in clay, which can improve the robustness of canister lifetime predictions. We acknowledge the German Federal Ministry of Education and Research (BMBF) and the Helmholtz association for the financial support. We thank the Mont Terri IC-A Partners and Swisstopo for providing the samples. We acknowledge the provision of the beamtime at the KIT Light Source, KARA, Germany