Effect of γ-radiation on the passive layers of Ti and Ti0.2Pd container-materials for high-level waste disposal

Ti and its alloy TiO.2Pd are considered as promising materials for the manufacture of high-level nuclear waste (HLW) containers which could act as an engineered barrier in a rock salt repository. Despite their extremely high resistance to general corrosion in salt brines, :localized corrosion might be critical. In the present work, pitting in hot q-brine as well as the effect of gamma-radiation on the passive layers were investigated. The gamma-photoeffect was simulated by uv-laser irradiation. All measurements were carried out on single Ti grains of known orientation to allow for a correlation of layer properties with the substrate texture. Without radiation, pitting occurred only at potentials much higher than the free corrosion potentials, ie no pitting on any grain orientation has to be expected under conditions relevant for HLW disposal. Furthermore, uv-laser ilumination of the sample surface in q-brine did not result in photo-induced pitting. On the contrary, a laser-induced oxide growth took place and even these layers were stable to corrosion. Anisotropy Micro-Ellipsometry (AME) experiments revealed a photo-induced formation of a two-layer system. The initial anodic layers which were amorphous before, showed a recrystallization to anatase after uv-illumination. The photo-induced oxides on top of the recrystallized ones were amorphous with a lower density. This two layer system was not affected by exposure to hot q-brine in an autoclave, ie both the unmodified original anodic layers as well as the uv-laser modified layers were stable under HLW relevant conditions on all grains. (C) 1997 Elsevier Science Ltd.