Hollandite, [BaxCSy][Ti, Al)3+2x+y Ti4+8−2x+y]O16, has been subjected to aqueous chemical attack at 75 and 150 °C. The resultant surface alterations have been monitored by solution analysis, SEM/TEM investigations, and surface analysis. The evolution of the solid/solution system can be described by the following mechanisms: There is a near-instantaneous release of Cs and Ba from reactive surface sites, followed by relatively rapid dissolution of the first few monolayers. The release of Al drives the pH to low values as a result of Al-OH hydroxylation and precipitation of AIO(OH) and/or AI(OH)3 species. The rate of dissolution is reduced by an order of magnitude or more by the formation of a thin and continuous Al(Ti)-rich surface layer. The layer remains stable and protective even under extreme hydrothermal conditions. This description has implications for the use of hollandite as a ceramic phase for disposal of high level nuclear wastes.

The severity of degradation of YBa2Cu3O7−x from exposure to an aqueous phase has the following dependence: wet steam (condensed thin film) > bulk fluid phase > dry steam, other variables being constant. Also, the surface reactivity increases with oxygen deficiency. It is found that the early stages of degradation are associated with surface hydroxylation and preferential extraction of Ba. Subsequently, dissolution takes place, followed by solution saturation and sequential precipitation, nucleation, and growth of secondary phases. These observations form the basis of a model which incorporates surface hydroxylation and OH− penetration of the bulk, formation of planar defects and extraction of Ba, lattice dissolution, and mobility and solubility constraints.

Perovskite (CaTiO3 )and hollandite ([BaxCsy,][(Ti,Al) 3+ Ti4 + ]016) 2x+y 8-2x-y are two of the three major phases of the Synroc titanate mineral assemblage. The chemical durabilities of these two phases have been investigated by solution analysis, surface analysis and electron microscopy. While a detailed model of the durability of perovskite has evolved from previous work, much less is known about the mechanisms which affect the durability of hollandite. The present investigations show that, for single-phase hollandite, selective extraction by ion exchange of Cs+ and Ba 2+ does not play a significant role. On the other hand, total dissolution of the network does seem to occur as a result of base-catalysed hydrolysis. Also, complexation and precipitation of Al and TI species in solution has the effect of driving the system to low pH. These results can be correlated with the more complete model for dissolution of perovskite in order to infer the overall effects of aqueous attack on the Synroc HLW solid.