Analysis of oxide formed on Ti during exposure in bentonite clay. Pt. 2

Titanium owes its excellent corrosion properties due to a passive oxide layer on its surface. In a long term perspective a change of the oxide structure may influence these properties. This is an important question when titanium is used as an overpack material for storage of spent nuclear fuel. The structure of the oxide formed on titanium during exposure in ground-water saturated bentonite clay at 95deg C up to six years is reported. The initially formed oxide is amorphous. Exposures for 3 to 6 years causes crystallization of the oxide. The microcrystals (rutile) are about 50-100 A and coexist with the amorphous phase. The crystallinity increases with exposure time. The reason for crystallization is a driving force toward the thermodynamically most stable state. A consequence of the crystallization is an increasing oxidation rate where the grain-boundaries provide a short diffusion path for the ions. A dedicated investigation of the influence of the sample preparation technique and the electron beam irradiation was performed since it was feared that these could create structural artifacts. These experiments showed that it was possible to analyze the true structure of titanium oxides if certain precautions were taken. (orig.)