Corrosion studies on selected packaging materials for disposal of high level wastes

In order to qualify corrosion resistant materials for high level waste (HLW) packagings acting as a long-term barrier in a rock salt repository, the corrosion behaviour of the preselected materials Ti 99.8-Pd, Hastelloy C4 and two unalloyed steels was investigated. The resistance of the materials to general corrosion, local corrosion and stress corrosion cracking was examined under postulated accident conditions in the repository by long-term immersion tests of up to 4 years duration and electrochemical methods. The parameters investigated were different salt brines, temperatures of 90 deg. C, 170 deg. C and 200 deg. C as well as a gamma radiation field of 103Gy/h (105rad/h). Among the materials studied, Ti 99.8-Pd exhibited the highest corrosion resistance. This material corroded at a very low rate (<lμm/a) both with and without gamma radiation, and proved to be resistant to local corrosion and stress corrosion cracking. Hastelloy C4 exhibited a good general corrosion behaviour (≤2μm/a) but was susceptible to crevice corrosion, and pitting corrosion occured at 200 deg. C. Under gamma irradiation i.e. in the presence of strong oxidants (e.g. H2O2, C103-bar) the susceptibility of Hastelloy C4 to local corrosion increased and strong pitting and crevice corrosion was observed at 90 deg. C. The electrochemical studies confirmed the tendency of this material to local corrosion. They show that the stability of the protective passive layer is greatly reduced with increasing temperature. The two unalloyed steels (fine-grained steel and cast steel) were resistant to local and stress corrosion cracking in the absence of gamma radiation. However, the steels must be sufficiently protected against gamma radiation. An unalloyed steel seems to be the most promising material for a long-term HLW-packaging