Corrosion of Construction Materials in Clay Environments
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CORROSION OF CONSTRUCTION MATERIALS IN CLAY ENVIRONMENTS*
JOZEF DRESSELAERS, FLORENT CASTEELS, HUGO TAS SCK/CEN Studiecentrum voor Kernenergie, Boeretang 200, B-2400 Belgium
ABSTRACT The corrosion rate and mechanism of construction materials of galleries and casings for boreholes (possibly to be used as consumable overpack of HLW canisters) have been evaluated after a first campaign of corrosion experiments in direct contact with clay, in a humid clay atmosphere and in a gas corrosion chamber. The envisaged construction material is ductile iron (grade 60) and has been tested with and without metallic and organic coatings. The corrosion mechanism is governed by the formation of a rust scale composed of Fe 3 0 4 and C. Preferential attack underneath the surface reaction layer is associated with the progress of a reaction front at the interface between the graphic nodules or flakes and the ferritic or pearlitic structure. Post-corrosion analyses of old gray iron coal mine shafts exposed for 60 and 90 years in well defined conditions are included to validate the use of the results of the short-time tests for very long exposure times. INTRODUCTION Disposal of conditioned waste of high and medium activity and of a tearing waste in stable geological formations is considered as one of the most relimethods for the isolation of the radioactive waste from the ecosphere for long periods of time. In Belgium, the use of a clay formation ("Boom" clay) for the isolation of radioactive waste is studied (1). The lifetime of canister materials and of the linings of galleries and pits for the disposal of conditioned radioactive waste is being established after metallurgical compatibility studies carried out in different environments which may occur during geologic isolation. These environments are: direct contact with clay, interstitial clay water and humid atmospheres loaded with corrosive products escaping from the clay under influence of thermal effects and in the presence of air. The subsurface portion of the repository will consist of a certain number of galleries and storage holes excavated in a tertiary clay formation. Nonmetallic and metallic coatings applied on the ductile iron segments can be used to ensure a structural stability during a period of 50 to 100 years. For most of the sub-surface structure ductile iron segments are chosen as construction materials, i.e. for the head gallery and the secondary galleries. Corrosion rates of cast iron will not only be derived from our own experiments but also by the post-corrosion analysis of underground structures of cast iron in operation for 60 to 90 years under similar conditions. EXPERIMENTAL The corrosion resistance of construction materials of galleries and pits for the storage of radioactive waste of medium and high activity in holes and galleries excavated in a geological clay formation, has been evaluated after *Work performed under contract with the commission of the European Communities, Brussels
312 tests in direct contact with clay at 16*C (temperature o
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