Monte Carlo simulations of the corrosion of aluminoborosilicate glasses

Aluminum is one of the most common components included in nuclear waste glasses. Therefore, Monte Carlo (MC) simulations were carried out to investigate the influence of aluminum on the rate and mechanism of dissolution of sodium borosilicate glasses in static conditions. The glasses studied were in the compositional range (70 - 2x)% SiO₂ x% Al₂O₃ 15% B₂O₃ (15 + x)% Na₂O, where 0 ≤ x ≤ 15%. The simulation results show that increasing amounts of aluminum in the pristine glasses slow down the initial rate of dissolution as determined from the rate of boron release. However, the extent of corrosion - as measured by the total amount of boron release - initially increases with addition of Al ₂O₃, up to 5 mol% Al₂O₃, but subsequently decreases with further Al₂O₃ addition. The MC simulations reveal that this behavior is due to the interplay between two opposing mechanisms: (1) aluminum slows down the kinetics of hydrolysis/condensation reactions that drive the reorganization of the glass surface and eventual formation of a blocking layer; and (2) aluminum strengthens the glass thereby increasing the lifetime of the upper part of its surface and allowing for more rapid formation of a blocking layer. Additional MC simulations were performed whereby a process representing the formation of a secondary aluminosilicate phase was included. Secondary phase formation draws dissolved glass components out of the aqueous solution, thereby diminishing the rate of condensation and delaying the formation of a blocking layer. As a result, the extent of corrosion is found to increase continuously with increasing Al ₂O₃ content, as observed experimentally. For Al ₂O₃ < 10 mol%, the MC simulations also indicate that, because the secondary phase solubility eventually controls the aluminum content in the part of the altered layer in contact with the bulk aqueous solution, the dissolved aluminum and silicon concentrations at steady state are not dependent on the Al₂O₃ content of the pristine aluminoborosilicate glass.