Anaerobic Corrosion Of Carbon Steel Research Articles

Anaerobic corrosion of carbon steel in compacted bentonite exposed to natural Opalinus clay porewater: Bentonite alteration study

Abstract. 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 corrosion products formed 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 Mont 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 (Smart et al., 2017). In the present study, the bentonite layer at the canister/bentonite interface was characterized by complementary microscopic and spectroscopic techniques (XPS, SEM-EDX, µXANES) under anoxic conditions. The interface revealed reddish-brown staining up to 2 mm depth into the bentonite in the zone adjacent to the steel in all three obtained samples. The XPS analysis revealed formation of sulfides at the interface consisting of iron and other trace metals present in the steel. The SEM-EDX analyses of the interface (embedded cross-cut with steel removed) showed different degrees of calcium enrichment in the bentonite adjacent to the metal for various samples. The µXRF analysis performed on the bentonite at the interface showed a scarce or distinct calcium-enriched rim up to 100 µm into the bentonite and iron-enriched rim depending on the sample (one sample in Fig. 1), while µXANES analysis revealed formation of iron silicate compounds in the reacted reddish-brown zone. The iron appears to displace calcium from the interlayer sites in montmorillonite. The calcium then precipitates at the interface as calcite. The extent of this process seems to be strongly related to the bentonite density. The steel coupon was removed prior to embedding, with the location marked as resin in Fig. 1. A line scan from the edge towards the bulk bentonite did not indicate any systematic gradient in the Fe2+/3+ ratio. The formation of mixed Fe2+/3+ silicate compounds appears to be heterogeneous. This work contributes to an increasing understanding of steel corrosion mechanisms in clay, which can improve the robustness of canister lifetime predictions.

Further studies on the anaerobic corrosion of carbon steel in alkaline media in support of the Belgian supercontainer concept

AbstractIn the Belgian supercontainer concept, a carbon steel overpack surrounds high‐level radioactive waste and spent fuel containers and is encased in a cementitious buffer material. Research has been carried out to characterise the anaerobic corrosion of carbon steel in an anoxic artificial alkaline porewater that simulates the cementitious buffer material and also in solid cement matrices, in the presence of chloride and the sulphur species expected in the groundwater. The current study focuses on recent results. Specifically, it provides an update on the long‐term corrosion rates obtained from gas generation measurements (including new data from higher‐sensitivity autoclaves) and weight loss measurements and provides results of postmortem analysis of dismantled cells. The behaviour of the system is discussed, focusing on the effects of the solution chemistry, precorrosion treatments and γ radiation on corrosion rates, the electrochemical response of the steel when irradiated, and the morphology of the corrosion product. The corrosion rate was lower in cement than in an aqueous system. Radiation affected the corrosion potential and the morphology of the corrosion product.

Further results on the in situ anaerobic corrosion of carbon steel and copper in compacted bentonite exposed to natural Opalinus Clay porewater containing native microbial populations

AbstractSince 2012, a long‐term in situ corrosion experiment (IC‐A) is being conducted in the Mont Terri Underground Research Laboratory in Switzerland to investigate the corrosion behaviour of candidate canister materials in conditions representative of the Swiss concept for the disposal of high‐level waste and spent nuclear fuel. To date, carbon steel and various types of copper coatings have been retrieved after different exposure periods of up to 3 years, and characterised to establish the composition of the corrosion product, the morphology of the corroded surface, the nature of the interaction between the metal and the surrounding bentonite, and the microbial populations in the bentonite and surrounding porewater. For carbon steel specimens, a complex corrosion product was identified, consisting predominantly of magnetite. Much less alteration on either the metal or the bentonite was observed in the case of copper samples. Low average anaerobic corrosion rates were measured for carbon steel and a very modest amount of alteration was identified on copper. The density and the initial form of the bentonite had a small influence on the rate of corrosion, across all materials. This paper summarises the results of the experimental programme obtained to date and discusses the relationship observed between exposure time and the evolution of the metal–bentonite interface for both carbon steel and copper.

Summary of studies on the anaerobic corrosion of carbon steel in alkaline media in support of the Belgian supercontainer concept

ABSTRACTIn the Belgian supercontainer concept, a carbon steel overpack will surround high-level waste and spent fuel containers and be encased in a cementitious buffer material. A programme of research has been carried out to investigate and measure the rate of anaerobic corrosion of carbon steel in an artificial alkaline porewater that simulates the aqueous phase in the cementitious buffer material and also in solid cement matrices. This paper highlights the main features of the programme and presents some recent results from the analysis of several samples, including two 20-year-old steel in cement samples, which showed that under anoxic conditions there is no localised corrosion or incorporation of iron into the surrounding cement, even in the presence of 20 000 ppm chloride in the porewater.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

The anaerobic corrosion of carbon steel in saturated compacted bentonite in the Swiss repository concept

ABSTRACTCarbon steel is a candidate disposal canister material for high-level radioactive waste and spent fuel in the Swiss deep geological repository concept and so a long-term laboratory-based corrosion experiment was undertaken to measure the anaerobic corrosion behaviour and corrosion rate of carbon steel in compacted bentonite under simulated anoxic repository conditions. The corrosion rate was measured using barometric gas cells over a period of more than 5 years, until quasi-steady-state conditions were reached. The measured corrosion rates are in agreement with weight loss measurements and with previous tests under similar conditions. The carbon steel and surrounding bentonite removed from one cell after 3 years were analysed with a range of analytical techniques, weight loss measurements and mineralogical characterisation. The results provide an insight into the prevailing corrosion mechanisms under repository conditions.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

The anaerobic corrosion of carbon steel in compacted bentonite exposed to natural Opalinus Clay porewater containing native microbial populations

ABSTRACTA long-term in situ corrosion experiment is ongoing in the Mont Terri Underground Research Laboratory in Switzerland to (i) measure the in situ corrosion behaviour of carbon steel in compacted bentonite under simulated repository conditions, (ii) study the effect of the bentonite buffer density on microbial activity and microbially influenced corrosion and (iii) study the effect of welding on the corrosion rate. Carbon steel corrosion coupons, with and without welds, were surrounded by compacted bentonite with a range of dry densities and mounted in modules allowing free exchange with the local anoxic groundwater. After about 20 months of exposure, corrosion coupons and bentonite were sampled. A complex corrosion product was identified, consisting predominantly of magnetite. The bentonite adjacent to the metal was finer grained, more dispersed and enriched in iron. Aerobic, anaerobic and sulphate-reducing bacteria were identified both in the porewater surrounding the modules and in the bentonite.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Effect of sulphur species on anaerobic corrosion of carbon steel in alkaline media

In the Belgian Supercontainer concept for disposal of vitrified high level radioactive waste and spent fuel, a carbon steel overpack will surround the waste containers and be encased in a cementitious buffer material. In previous work the effects of radiation, temperature and chloride concentration on the anaerobic corrosion rate of carbon steel in an artificial alkaline porewater were measured and recently the effects of sulphur containing species, particularly sulphide and thiosulphate, on the anaerobic corrosion rate of aerobically precorroded carbon steel have been studied. Corrosion rates were measured by monitoring hydrogen evolution, by applying electrochemical methods and by gravimetric analysis. Corrosion product layers formed after several years’ exposure were analysed using focused ion bombardment (FIB) and SEM/EDX analysis. This paper summarises recent results, for experiments with and without sulphur species present. No increase in the anaerobic corrosion rate of pre-corroded carbon steel in the presence of sulphide or thiosulphate was measured.

The anaerobic corrosion of carbon steel in alkaline media – Phase 2 results

In the Belgian Supercontainer concept a carbon steel overpack will surround high-level waste and spent fuel containers and be encased in a cementitious buffer material. A programme of research was carried out to investigate and measure the rate of anaerobic corrosion of carbon steel in an artificial alkaline porewater that simulates the aqueous phase in the cementitious buffer material. The corrosion rates were measured by monitoring hydrogen evolution using a manometric gas cell technique and by applying electrochemical methods. Phase 2 of the programme has repeated and extended previous Phase 1 measurements of the effects of radiation, temperature and chloride concentration of the anaerobic corrosion rate. This paper provides an update on the results from Phase 2 of the programme. The results confirm previous conclusions that the long-term corrosion rate of carbon steel in alkaline simulated porewater is determined by the formation of a thin barrier layer and a thicker outer layer composed of magnetite. Anaerobic corrosion of steel in cement requires an external supply of water.

Further studies on the effect of irradiation on the corrosion of carbon steel in alkaline media

This paper summarises recent experimental results from a programme that was carried out to investigate the effects of radiation on the anaerobic corrosion of carbon steel, in relation to the overpack that will be used in the Belgian supercontainer concept for radioactive waste disposal. Anaerobic corrosion rates of carbon steel were measured by monitoring hydrogen evolution using manometric gas cells and an autoclave and the corresponding electrochemical behaviour was investigated by measuring open circuit potential, linear polarisation resistance and alternating current impedance. The test medium was alkaline simulated pore water, at γ-irradiation dose rates of 0 and 25 Gy h−1, temperatures of 25 and 80°C and chloride concentrations of 0 and 100 mg L−1. The anaerobic corrosion rates exhibited a slow decline due to the formation of magnetite. Good agreement was found with weight loss measurements. A dose rate of 25 Gy h−1 had a negligible effect on gas generation rates.

Experimental studies of the effect of irradiation on the anaerobic corrosion of carbon steel in relation to the Belgian supercontainer concept

This paper describes recent results from an investigation of the effects of -radiation on the anaerobic corrosion of carbon steel in cement, in relation to the Belgian Supercontainer Concept for radioactive waste disposal. Anaerobic corrosion rates were measured by monitoring hydrogen evolution and the corresponding electrochemical behaviour was investigated by measuring open circuit potential and linear polarisation resistance. The test medium was alkaline simulated porewater, at -irradiation dose rates of 0 and 25 Gy hr −1 , temperatures of 25 ◦ C and 80 ◦ C and chloride concentrations of 0 and 100 mg/l. The effects of radiation on the corrosion behaviour were found to be small. The reference design for the Belgium Supercontainer for disposal of high-level nuclear waste and spent fuel consists of a 30 mm thick carbon steel cylindrical vessel ('the overpack') surrounding 309 grade stainless steel waste canisters. The overpack will be surrounded by a cementitious buffer material, which may or may not be encased in a stainless steel vessel ('the envelope'). This paper is concerned with the anaerobic corrosion processes that may affect the carbon steel overpack within the Supercontainer. The carbon steel will be exposed to alkaline porewater in the cementitious buffer or in the alkaline material which may be used as a filler in the annulus between the cementitious buffer and the overpack. After a relatively short period the interface between the steel and alkaline porewater will become anoxic as residual oxygen in the repository is consumed by aerobic corrosion and microbial activity. Low concentrations of chloride (typically 100 ppm) from the groundwater may penetrate the buffer and reach the surface of the overpack. Following a review of the corrosion issues affecting the Supercontainer design an experimental programme was initiated to investigate (i) the possible effect of gamma radiation on the anaerobic corrosion rate of carbon steel in cementitious porewaters and on the nature of the corrosion products formed, and (ii) the effect of radiation on the electrochemical and corrosion behaviour of carbon steel corroding anaerobically in simulated concrete porewater conditions. Previous publications have reported earlier results from this programme (1, 2) for both gas generation and electrochemical measurements. The aim of this paper is to provide an update on this work programme, with the main focus being on the results of the electrochemical measurements, although the results from the gas generation experiments are also shown for comparison. In essence the work programme involved setting up test cells containing representative corrosion systems and exposing them to controlled gamma radiation

Nominally Anaerobic Corrosion of Carbon Steel in Near-Neutral pH Saline Environments

Abstract Gas transmission pipeline corrosion commences when coatings disbond, exposing the steel to groundwater. When this occurs, a number of anaerobic and aerobic corrosion scenarios can be envisaged. The initial nominally anaerobic corrosion period has been investigated by applying a combination of electrochemical methods (i.e., corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy [EIS] measurements) and surface analytical techniques (scanning electron microscopy, energy-dispersive x-ray spectroscopy, and Raman spectroscopy). An evolution in film properties was observed and attributed to the entry of adventitious oxygen into faults within the preformed film. This leads to an increase in overall corrosion and a change in properties of the film as detected by EIS and Raman analysis. This article describes the mechanism involved in this transition, and provides a basis for a more extensive study of the corrosion process encountered on switching between anaerobic ...

Corrosion of carbon steel in compacted bentonite and its effect on neptunium diffusion under reducing condition

The corrosion of carbon steel and its effect on neptunium diffusion were studied by corrosion tests of carbon steel and neptunium diffusion tests under conditions designed with consideration of constituents of a repository for High-level Radioactive Waste disposal. The major constituents of the conditions were carbon steel, corrosion products pre-formed under aerobic conditions, compacted bentonite, reductant and a low-oxygen glove box. The corrosion tests were performed in the presence of water-saturated compacted bentonite. The neptunium diffusion tests were performed with and without carbon steel. The carbon steel and the bentonite were observed by taking photographs and analyzed by X-ray diffraction. The color of the corroded surface of the carbon steel changed from red/orange/black under aerobic conditions to a dark-green under reducing conditions. Portions of the bentonite changed their original color, pale-gray, to a brownish color after the test. Gas-induced cracks were also observed in some bentonite specimens. These changes were discussed combining thermodynamic predictions under the present experimental conditions. The results revealed that anaerobic corrosion of carbon steel occurred in the bentonite. The corrosion processes were also discussed. The corrosion rate of carbon steel was estimated to be ∼0.1 μm/year from iron profiles in the bentonite. This value was much lower than the conservative setting value 20 μm/year in the current performance assessment. The significant restriction in corrosion could be attributed to the presence of bentonite and corrosion products. Oxidation states of neptunium in the bentonite were investigated by thenoyltrifluoroacetone in xylene extraction technique. It was found that tetravalent neptunium accounted for 99% of the total neptunium in the presence of carbon steel and only 63% without. The results revealed that anaerobic corrosion of carbon steel could maintain strong reducing conditions to keep most tetravalent neptunium from being oxidized to pentavalent neptunium, thus effectively restraining migration of neptunium in the bentonite. The apparent diffusions were estimated to be in the order of 10 −15–10 −14 m 2/s for tetravalent neptunium and 10 −13–10 −12 m 2/s for the pentavalent. No significant effect of the corrosion products on neptunium sorption was found through comparing the apparent diffusion coefficients obtained in the diffusion with and without carbon steel. Corrosion of carbon steel under conditions relevant to the repository is essential for further studies, providing a confident and defensible value of corrosion rate for repository design. The effects of corrosion on radionuclide migration, especially redox-sensitive radionuclides in the buffer, should be considered for a reliable safety assessment of geological disposal.

Anaerobic Corrosion of Carbon Steel and Cast Iron in Artificial Groundwaters: Part 2—Gas Generation

Abstract In Sweden, high-level radioactive waste will be disposed of in a canister with a copper outer container and a cast iron or carbon steel insert. If the iron insert comes into contact with a...

Anaerobic Corrosion of Carbon Steel and Cast Iron in Artificial Groundwaters: Part 1—Electrochemical Aspects

Abstract In Sweden, high-level radioactive waste will be disposed of in a canister with a copper outer container and a cast iron or carbon steel insert. If the iron insert comes into contact with anoxic geological water, anaerobic corrosion will occur. This paper presents a study of the electrochemical aspects of anaerobic corrosion of carbon steel in artificial Swedish granitic groundwaters. The electrochemical measurements confirm that anaerobic corrosion will lead to the generation of hydrogen, but the rate of corrosion will be anodically limited by the formation of a corrosion product film. The corrosion rate was unaffected by hydrogen pressures up to 100 atm.