Comparative corrosion mechanism of ThF 4 and ZrF 4 based fluoride glasses in aqueous solutions

Abstract

The chemical durability of ThF 4-based glasses and Th-containing ZrF 4-based glasses in aqueous solutions was studied. ThF 4-based glasses are especially stable in basic and neutral solutions. In water, fluorine leach rates from glasses prepared under different conditions were compared. The F −/OH − ion exchange between glass and solution causes, after long soak times, devitrification of the glass surface thereby increasing the leach rate. The porous surface layer consists of small filament-like ThF 4·nH 2O crystals. The dissolution of ZrF 4-based glasses in water at different temperatures was measured through weight loss data, solution analysis and IR-transmission spectra. After an initial adjustment period the corrosion process follows a time 1 2 relationship. After an immersion time of less than one hour a heavily hydrated thorium-enriched surface layer of several microns showed crack formation. Only after longer leaching times, well formed needle-shaped hydrated fluoride crystals containing Ba and Zr as metal ions, were deposited on the surface. The dissolution rate of the Th-containing ZrF 4-based glass in cold water is in the range of 3.3×10 −4 g cm −2 h −1 which is about 100–1000 times faster than that for ThF 4-based glasses.