Metallic corrosion at the steel/bentonite interface under anoxic and water saturated conditions

The international community considers deep geological disposal to be the most sustainable solution for managing high-level nuclear waste (HLW), involving its storage in underground facilities within stable geological formations. The solution relies on a series of barriers both natural and engineered to isolate the waste from the biosphere, with the geological barrier (host rock), a buffer material (bentonite), and a container all playing vital roles in ensuring safety. Corrosion rates are needed to evaluate the container lifetime, and information on corrosion mechanism can be obtained knowing the nature of formed products. ... mehrFurthermore, anaerobic metallic corrosion at the steel/bentonite interface determines the performance of bentonite based high level radioactive waste barrier.
Iron-based and copper-based materials were mostly considered as candidate canister materials by many countries for deep geological repositories, where they serve as barriers to prevent radionuclide migration possibly for hundreds of thousands of years. This dissertation investigates the corrosion behavior of candidate canister materials for HLW disposal, focusing on three iron-based materials like low carbon steel (CS), spheroidal graphite iron (SGI) and spring steel (SS) which has high silicon content compared to carbon steel, as well as a copper based material like cupronickel alloy, under widely anoxic, water saturated conditions in contact with bentonite, a potential buffer material for radioactive waste disposal. This study simulates the corrosion processes in the initial, transient phase and does not look to the long-term processes under really oxygen free (anoxic) and reducing conditions. The SGI and CS are constituents of the reference POLLUX container for heat generating HLW in Germany developed for the previously considered repository at the Gorleben site. The cupronickel alloy and SS were selected as potential alternative or reference materials. The goal of this work was to fill the knowledge gap in the corrosion mechanisms and rates of these candidate canister materials, specifically focusing on the role of bentonite in the corrosion process, given the lack of studies on the potential bentonite based design for German radioactive waste repositories. A further aim is to identify secondary corrosion phases, which in case of container through corrosion may play a role for radionuclide retention.
Experiments have been performed either under static conditions in closed vessels or under dynamic conditions in vessels allowing imposing a low water flow rate. Setups have been developed and tested, then a series of controlled static and dynamic batch experiments were conducted over time periods of three, six, nine and maximum 12 months at room temperature (25°C) and elevated temperature (50°C) under anoxic and water saturated conditions, simulating repository environments in crystalline rock. Synthetic Grimsel groundwater and MX-80 bentonite were used to allow comparison with the in-situ MaCoTe experiment at the Grimsel Test Site (GTS), Switzerland. For cupronickel alloy, the effect of sulfide presence, mimicking the development of bacterial activity, was further investigated. For SGI, the effect of hematite presence, mimicking the presence of early oxic phase corrosion products, was also investigated.
For SS and CS, the effect of scratching the surface, as may possibly occur during canister handling, was investigated. Finally, for CS a series of experiments was performed with GMZ bentonite (from Gaomiaozi country in Inner Mongolia Autonomous Region, China) to identify a possible effect of bentonite composition on corrosion behavior. At the end of contact time, the systems were characterized. First pH and Eh of bentonite slurry/ground water were measured in situ and at room temperature, and then the collected slurry/groundwater was ultracentrifuged. The composition of the collected supernatant was determined by ICP-OES/MS and IC.
The coupon/bentonite interface was characterized by using various microscopy and spectroscopy techniques like SEM-EDX, XRD, XPS, XAS and corrosion rates were determined using weight loss measurement method. The findings of the study aim to fill knowledge gaps especially related to corrosion behavior, secondary corrosion phase formation and possibly bentonite alteration, because these may play an important role in the retention in the nearfield of radionuclide that will be released following breach of the canister.
The results reveal that all candidate materials exhibit varying corrosion behavior under the different experimental conditions, including temperature, time, presence of sulfide, hematite and mechanical scratches. Cupronickel alloy, which showed the lowest corrosion rates lying after 6 months in a range of (0.08±0.05 µm/a to 0.44±0.12 µm/a at 25°C) with a tendency of a temporal higher rates at elevated temperature of 50oC. Conversely, carbon steel showed the highest corrosion rates from all iron based materials ranging (3.84±1.87 µm/a to 5.07±1.60 µm/a at 25°C and up to ~ 17±10 µm/a at 50°C after 6 months). Scratching, addition of sulfide or hematite has a limited effect on corrosion rates and also the impact of increased temperature appears to decrease over time. These corrosion rates align closely with those measured in in-situ MaCoTe experiments, where observed corrosion rates of carbon steel (1-2 µm/a) and copper (0.1-0.3 µm/a) over similar periods of exposure.
The pH in static experiments remained stable for all materials, with a slight decrease observed at elevated temperature, whereas in dynamic experiments, the pH increased with time, tending towards that in the inlet groundwater (pH 9.8), indicating moderately alkaline conditions. The redox potential for all iron-based materials remained negative, indicating reducing conditions which develop due to formation of Fe(II) bearing compounds. For cupronickel, on the other hand, positive Eh maintained in static experiment, which shifted to negative values in dynamic experiments but remained in a range that corresponds to the stable metallic state for Cu, with low corrosion rates observed at room temperature. Note that the experimental setup does not reflect the real situation in a repository, where compacted bentonite is emplaced and advective flow normally is not expected. However, in the case of bentonite erosion, a situation as simulated in the experiments presented here, can develop.
Secondary phase analysis for cupronickel indicated the formation of copper oxide (Cu2O) in all static experiments and a mixture of nickel hydroxide Ni(OH)2, copper oxide (Cu2O), and copper sulfide (Cu2S) in dynamic experiments. Overall, the corrosion products observed in dynamic experiments were more complex, with notable contributions from sulfide, these findings are partly consistent with those observed in in-situ corrosion studies, such as MaCoTe, where copper corrosion products were predominantly oxides, with only minor sulfide compounds detected.
Iron-based candidate materials exhibited a wide variety of corrosion products, including iron oxides, iron silicates, iron sulfides, and green rust. Secondary phases such as berthierine (iron silicate) and green rust were observed at extended exposure times, particularly at elevated temperature, indicating an evolving corrosion process. The formation of iron silicate layers at the metal/bentonite interface, particularly at longer time period, slowed corrosion by slowing down further metal dissolution. These protective layers were more prominent in static conditions and at higher temperatures, in contrast to the MaCoTe in-situ experiments, where no such iron silicate layers were observed, likely due to differing environmental conditions (e.g., rock vs room temperature).
Surface characterization of corroded coupons confirmed the presence of the corrosion products, with significant differences in the composition and distribution of corrosion phases observed across the different materials and experimental conditions. The corrosion products detected on the iron-based materials in both static and dynamic conditions were influenced by temperature, exposure time, and the presence of added hematite. For example, in the presence of hematite (α-Fe2O3), the corrosion rate of SGI was elevated during the initial period, as hematite presumably acts as an oxidant and accelerates corrosion. Graphite inclusions in SGI also acted as cathodic sites, accelerating the anodic corrosion of ferrite, which ultimately led to the formation of mixed Fe(II)/Fe(III) corrosion products, such as magnetite. Spring steel exhibited similar corrosion behavior in both static and dynamic experiments, with the formation of iron (hydr)oxides and iron silicates observed at the metal/bentonite interface.
XPS and SEM-EDX results corroborated these findings, with iron silicate being identified as a major corrosion product. Carbon steel showed the formation of iron oxides, iron silicates, iron sulfides, and iron-rich carbonates (such as chukanovite evidenced by XANES) in the presence of MX-80 bentonite. In dynamic experiments, corrosion products formed more rapidly compared to static conditions, especially at elevated temperature, leading to more pronounced changes in corrosion behavior over time. For carbon steel, magnetite and iron silicate form predominantly in the presence of GMZ bentonite, and extended exposure periods (12 months) resulted in a more distinct morphology of corrosion products, including octahedral magnetite crystals.
The alteration of bentonite, as a result of the corrosion processes, was confirmed through XAS (XANES), and ICP-MS analysis following digestion of bentonite that was in contact with the metallic coupons which revealed changes in the elemental composition. For cupronickel, small amounts of copper and nickel were adsorbed onto the bentonite, with concentrations increasing at elevated temperatures in the presence of sulfide. The presence of secondary phases like berthierine (iron silicate), green rust, magnetite (iron oxide), and chukanovite (iron-rich carbonate) in altered bentonite was confirmed using XAS (XANES) analysis and these findings were corroborated by SEM-EDX analysis. These secondary phases could potentially act as a long-term sink for radionuclide retention, reducing the risk of their migration into the biosphere.
This study suggests that while the surface roughness caused by scratches during canister handling may not significantly affect the corrosion behavior of the candidate materials (carbon steel and spring steel), the formation of protective layers (such as iron silicates) and the alteration of bentonite play crucial roles in mitigating long-term corrosion. The short-term nature of this study does not fully capture the long-term corrosion behavior under real case geological conditions, where residual oxygen does not anymore play a role. Furthermore, microbially induced corrosion aspects are not investigated here. Future research should focus on predictive modeling of the long-term evolution of a repository nearfield taking corrosion processes and secondary phases forming in the short and long term into account.