INCREASES IN LEACH RATE DUE TO POSSIBLE CRACKING IN SILICATE GLASSES
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INCREASES IN LEACH RATE DUE TO POSSIBLE CRACKING IN SILICATE GLASSES
J.C. SANG,' A. BARKATT," I.G. TALMY,*" and M.K. NORR" *Department of Chemistry, The Catholic University of America, Washington, DC 20064 **Naval Surface Warfare Center, Silver Spring, MD 20910
ABSTRACT Comparative studies of two multi-component silicate glasses have confirmed the observation that glasses with a relatively low Si0 2 + A10 3/2 content may exhibit temporary increases in leach rate during the initial stages of their exposure to water. SEM studies of the leached glass surfaces strongly support the assumption that this phenomenon is due to cracking of the leached glass and a consequent increase of the exposed surface area.
INTRODUCTION Upon exposing certain multi-component borosilicate glasses to water in partialreplenishment tests, the leach rates fall off with time, but suddenly show a sharp increase by as much as a factor of 20 [1]. These leach rate excursions or spikes, which occur in certain cases at the end of periods as long as 20 months after the start of the exposure at 90'C, are reproducible. They affect the leach rates of all significantly soluble components of the glass, including B, P, Na, Li, Na and K, as well as Si. Following the excursion, the leach rates fall back to levels that are at least as low as the pre-excursion values. Only one such excursion has been observed in experiments lasting up to 4 years, which were carried out on such borosilicate glasses in de-ionized water or in dilute NaOH solutions [1]. In dilute NaHCO3 solutions, even this single excursion is suppressed. More generally, both the time at which a sharp increase in leach rate is observed and the extent of this increase depend on the leaching conditions, including the composition of the leachant, the ratio of exposed glass surface area to leachant volume (S/V ratio), the leach vessel material, and the temperature. In particular, the timing and magnitude of leach rate excursions are highly sensitive to relatively small variations in glass composition. The rise in leach rates cannot be associated with the attainment of a critical pH level, in view of the fact that pH increases observed during the excursion are modest, and the pH levels corresponding to the onset of the excursion are different from one glass to another. Indeed, at similar pH levels, one glass may exhibit a sharp rise in leach rate and another will not. Furthermore, as mentioned above, leach rate excursions are also observed in the cases of some glasses, upon substituting dilute NaOH for water as a leachant [1]. Another explanation, involving the formation of a more stable crystalline phase and corresponding release of minor species previously incorporated in the precursor surface phase, is also considered unlikely because the formation of the more stable alteration product is expected to result in a drop in the concentration of dissolved silica, contrary to the situation observed during a leach rate excursion. An alternative explanation for the delayed sharp rise in leach rates involv
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