Corrosion evaluation of metallic materials for long-lived HLW/spent fuel disposal containers. Final report 1996 - 1998

Extended corrosion studies were performed on preselected HLW/Spent Fuel container materials (carbon steel, stainless steels, Ti99.8-Pd, Hastelloy C4) under simulated disposal conditions in rock salt, granite and clay environments. The objectives of the studies were: To evaluate the effect of essential parameters on corrosion, to gain an improved understanding of corrosion mechanisms, and to provide more accurate data for a materials degradation model that can be used to predict the lifetime of containers. The investigations included long-term immersion tests, electrochemical studies, and stress corrosion cracking studies. Parameters investigated in salt environment were: pH, composition of brines, chemical species present in brines, gamma radiation, welding and slow strain rates at 25 C-170 C. In granitic water, the effect of slow strain rates on the stress corrosion cracking resistance of steels was examined at 90 C. Finally, in oxidizing clay water (aerobic conditions), the influence of temperature (16 C, 90 C), and content of Cl-, SO42- and S2O32- on corrosion was investigated. The results obtained confirm previous findings that the alloy Ti99.8-Pd is the strongest candidate for the realization of the corrosion-resistant container concept in the three geological formations rock salt, granite and clay. This alloy is under all test conditions resistant to pitting corrosion and stress corrosion cracking, and its general corrosion is negligibly low. The nickel base alloy Hastelloy C4 resists also pitting corrosion up to 90 C in oxidizing clay water. Therefore, this alloy is a further promising container material for disposal in clay. The stainless steels suffer from pitting corrosion in clay water at elevated Cl- concentrations. Also in granitic environment, pitting was observed on the stainless steel AISI 316L. Therefore, the use of stainless steels as container material could lead to long-term pitting corrosion problems. The TStE355 carbon steel is an actively corroding material in salt brines and clay water, and its corrosion rate is significantly higher than that of corrosion resistant materials such as Ti99.8-Pd or Hastelloy C4. However, the corrosion rates of this steel imply corrosion allowances acceptable for thick-walled containers. In granitic environment (90 C) some pits were detected on the carbon steel. Therefore, the kinetics of the pitting corrosion in this environment should be examined by long-term corrosion tests. Further investigations on steels, Hastelloy C4 and Ti99.8-Pd are in progress. (orig.)