Chemical Durability of Fluoride Glasses: I, Reaction of Fluorozirconate Glasses with Water

Abstract

The chemical durability of five flouarozirconate glass compositions was studied. Measurements of leachant composition and solution chemistry by plasma emission spettroscopy, pH, and fluoride analysis are reported. Changes in surface structure were monitored by scanning electron microscopy, electron‐probe X‐ray analysis, and X‐ray diffraction. The modifer additives with high aqueous solubility (A1F3, NaF, LiF, PbF2) exhibited the highest leach rates with ZrF4, BaFs2, and LaF3 exhibiting lower rates. The leaching order of Zr > Ba > La was maintained for all samples. The leaching process consists of the dissolution of the glass matrix, leaving a thick and porous layer containing Zr, Ba, La, and molecular water. Hydroxylation of the Zr species to form a hydroxyfluoride complex in solution dominated a decrease in pH to moderately acidic values. The resulting increase in component solubility (particularly zirconium fluoride) in the acidic solutions accelerated the glass dissolution rate and demonstrated that, if no buffer is present to avert the pH drift, the chemical durability of these glasses in aqueous environments is extremely poor, being roughly equivalent to that of Na2O 2SiO2.