In high level waste disposal concepts, metallic containers are used to prevent the release of radionuclides into the disposal environment. The aim of our corrosion studies is to determine the lifetime of these containers, using laboratory and in situ experiments. Among the investigated container materials are carbon steel, stainless steels, nickel alloys and titanium.
Corrosion of carbon steel in cementitious environments
Currently, our studies are mainly focused on the Belgian supercontainer (SC) concept. The SC concept aims at establishing and preserving a favourable chemical environment around the carbon steel overpack, so that it will be exposed to essentially unchanged, benign conditions for at least the duration of the thermal phase. This is achieved by using a high pH cement buffer, in which the carbon steel overpack is covered by a protective film.
The lifetime predicition of the overpack is based on knowledge of the corrosion evolutionary path, which describes the evolution of the cement buffer surrounding the overpack. For each of the phases in the lifetime of the overpack, a best estimate for uniform corrosion is determined. Integration of these corrosion rates over the corrosion evolutionary path yields the lifetime of the overpack. This approach is valid only when localised corrosion phenomenae can be excluded.
Corrosion of container materials in clay environments
We investigated the corrosion behaviour of container materials in contact with bentonite and Boom clay. We focused in particular on stainless steel containers but our studies also extended to carbon steel, nickel alloys and titanium. In a clayey environment, stainless steel containers may be prone to localised corrosion, in particular pitting corrosion.
We applied a twofold electrochemical approach to assess the risk of pitting corrosion, as illustrated in the figure:
- With potentiodynamic polarization measurements we determined the critical potentials for pitting: the pitting potential E NP, and the repassivation potential E PP
- Long-term monitoring of the corrosion potential E CORR then allowed us to predict the risk of pitting corrosion: this will occur when E CORR exceeds E NP.
In addition we used immersion tests to simulate in situ conditions as realistically as possible and at the same time offering the possibility of changing individual parameters, e.g. chloride concentration. Finally, in situ tests in the Boom Clay formation yield realistic information on the long-term corrosion behaviour of HLW container materials.
Contact: Druyts Frank