EFFECTS OF AQUEOUS CORROSION ON BOROSILICATE GLASS AS REVEALED BY VERTICAL SCANNING INTERFEROMETRY (VSI)

Vertical Scanning Interferometry (VSI) has emerged as a premier tool to observe and quantify surface retreat of mineral surfaces that develop during dissolution experiments. To date, however, few investigations have been carried out on glass surfaces subjected to aqueous corrosion. Typically, dissolution rates of glass are determined by analyzing effluent solution, but near silica saturation, release of elements is slow such that element concentrations are near detection limits. On the other hand, VSI techniques are not limited by such considerations and provide a powerful complement to established methods for obtaining rates. Polished coupons of borosilicate glass were observed by VSI in the pristine state and then heat-resistant glue was applied to small areas on the coupons. The purpose of the glue is to ensure that a pristine surface is preserved during the course of the experiments, which one can then compare to the altered surface. Glass chemical compositions varied in molar Na/(Al+B) from >>1 to 0.9. We reacted the suite of glass compositions in Single-Pass Flow-Through (SPFT) experiments at T=90oC and pH=10 under both highly undersaturated and near silica-saturated conditions for 1 hour to 10 days. Chemically complex glass specimens manifested extensive randomly oriented cracking after ~20 hours of reaction time. Cracking appeared to emanate from surface imperfections in the glass and showed signs of curling at edges, similar to mud cracks. We attribute this cracking to hydration and subsequent swelling of the glass during reaction. Experiments near silica saturation displayed the same characteristics, yet the extent of global surface retreat was less (by ~10X) than that observed for coupons at undersaturated conditions. This result is consistent with rates obtained by analysis of the chemistry of the effluent solution. In contrast, a simple sodium borosilicate glass composition displayed small (10 to 20 nanometer) elliptical pits after only 1 hour of reaction. With longer reaction times the pits developed along sub-parallel lines that coincide with flow banding structures in the glass. The origin of the pits is unclear, but may be related to phase separation in the glass. Our results demonstrate the efficacy of using VSI techniques to aid development of models for the reactivity of glass.