The anaerobic corrosion of the carbon steel overpack under anoxic alkaline conditions representing the Belgian supercontainer concept

Abstract. The Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) is responsible for the management of all radioactive materials (currently existing and future waste) generated in Belgium. Disposal in deep stable geological formations is, at present, accepted at an international level to be the most promising option for the long-term management of high-level and/or long-lived radioactive waste. ONDRAF/NIRAS currently recommends underground disposal in a geological stable and poorly indurated clay formation as the reference management option to steer their RD&D research programmes. The supercontainer concept is currently being considered as the reference design for the final disposal of vitrified high-level radioactive waste (VHLW) and spent fuel (SF) in Belgium. It comprises a prefabricated Portland cement-based buffer that completely surrounds a carbon steel overpack. In this highly alkaline environment (pH ∼ 13.6), and under normal conditions (i.e. without the ingress of aggressive species), the carbon steel overpack will be protected by a passive oxide film, which is expected to result in very low uniform corrosion rates. The main goal of the ongoing R&D corrosion studies performed at ONDRAF/NIRAS is to provide confidence that the integrity of the carbon steel overpack will not be jeopardized at least for the duration of the thermal phase. Considering the long timescales applicable to waste disposal, determining accurate and reliable estimates of the uniform corrosion rate under anoxic conditions forms a very important part of the R&D methodology of ONDRAF/NIRAS' corrosion programme. On the other hand, the occurrence of localized forms of corrosion cannot be neglected, and therefore it is also crucial to demonstrate that no other form of corrosion apart from uniform corrosion can take place. This paper gives an overview of the status of the research performed at
ONDRAF/NIRAS with respect to the uniform corrosion, pitting corrosion and
stress corrosion cracking behaviour of the carbon steel overpack of the
supercontainer.



Glass cell Autoclave
Results -hydrogen sensor

Oxide film characterization
The outer surface of the film changed from mixed maghemite/ magnetite to pure magnetite from 100 to 1000 hours exposure  ▪ SSRT tests at OCP and at applied potential:  OCP: no increased SCC susceptibility, typical stress-strain behaviour of a ductile material. Applied potential: so far, the potential does not seem to have a significant effect on the SCC susceptibility and the stress-strain behaviour of carbon steel in high pH solutions.

Stress corrosion cracking (welds)
Typical results

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Modelling of the results and long-term prediction by means of the Point Defect Model and the Mixed Potential Model.
mg/l S2-, pre-corroded, 25 °C After approximately one year's exposure the corrosion rate had dropped to less than 0.03 µm/year (still decreasing) Currently transferring all glass cell experiments (operating >10 years) to autoclaves.detection: <1 nm/year)

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Detection limit: ~0.01 nm/y) composition microstructure form Effect of temperature on passivation?• 50°C → unexpectedly high corrosion rate • Exactly same conditions as a previous cell at 80 °C, corroding at 10 nm/yr.• Temperature change to 80°C → lower LT corrosion rate ▪ In the long-term corrosion products predominantly composed of magnetite ▪ Thickness corrosion product layer ~ 2-3 µm (after ~ 5 years)

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More complex composition than the corrosion products in YCW: outer and inner layer are composed of calcium, silicon, iron and oxygen (and traces of aluminum).

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Cl -<1M, no passivity breakdown occurs on carbon steel in SCPS.▪ Cl ->2M, pitting corrosion occurs.The E b decreases with increasing chloride concentration.▪ When the Cl-of SCPS is 1Experimental confirmation that, even if pits nucleate in the oxic period, they cannot maintain differential aeration during the anoxic period → no further pit activation ▪ The pit cavity becomes polarity inverted, → hydrogen evolution occurs within the cavity →(metal oxidation) occurs on the external surface ▪ The solution within the cavity is expected to become more alkaline than the external environment and ▪ Cations (e.g., Na + ) are preferentially transported into the pit and chloride ejected from the pit.▪ Deterministic Monte Carlo Simulation: keep track of each stable pit that nucleates, propagates, and repassivates on the metal surface.→Simulation of the following processes →Pit nucleation rate →Pit propagation rate →Probability of pit repassivation Long-Term modelling approach Assumption: Pit nucleation occurs immediately after metal exposure, during the high temperature, oxic period.No repassivation considered Repassivation considered Average depth of the deepest pit as a function of time Pitting represents no threat to the integrity of the overpack (if corrosion allowance = 15 mm) ▪ SSRT tests were conducted on base and welded samples:  Base and welded samples (SAW, MIG/MAG, RPEB)  Anoxic conditions  T = 140°C  Open circuit potential/applied potential  YCW (pH 13.5) (in the presence or absence of agressive species)

Selection of candidate materials ▪
List of candidate materials was studied (pitting and uniform corrosion behaviour