Corrosion of Ti and Ti0.2Pd under high-level waste disposal conditions with microscopic investigation of passivity

Ti and its alloy Ti0.2Pd are considered as promising materials for the manufacture of high-level nuclear waste containers that could act as an engineered barrier in a rock salt repository. Due to an always present protective passive TiO2-layer on top of the metal surface, these materials show extremely high resistance to general corrosion in salt brines but localized corrosion might be critical. In the present review both, the effect of microscopic passive layer properties on corrosion in a disposal relevant salt brine, and the influence of gamma-radiation on the passive layer were investigated. Differently prepared samples (electropolished Ti welds and oxide covered ones) were exposed to salt brines up to 170 degrees C. Both before and after exposure, the passive layers were characterized at high local resolution applying a combination of new micro-electrochemical and optical methods. In general only passivation was observed without any signs of localized corrosion. The passive layers were stable during the corrosion test and general corrosion was negligibly small. In the case of anodically formed layers, a strong texture dependence of the film properties on substrate orientation was proved. This influence decreased after exposure, suggesting an even lower susceptibility to local phenomena. The effect of gamma-radiation on the TiO2 passive layer (gamma-photoeffect) was simulated by UV-laser irradiation of the metal surface. No direct dissolution of the passive layers, e.g. no laser induced pitting occurred. In contrast, laser induced oxide growth took place under anodic polarization, but even these layers were stable to corrosion. In view of these results, Ti and Ti0.2Pd are confirmed as promising container materials.