Corrosion of borosilicate glass under unsaturated conditions: temperature-dependent evolution of secondary phases and surface alteration layer

: Predicting the long-term performance of borosilicate glass for nuclear waste immobilization under unsaturated repository conditions requires a fundamental understanding of the evolution of the surface alteration layer (SAL) and secondary phases during corrosion. In this study, long-term static leaching experiments (up to 760 d, 50-90 °C) were conducted in pre-equilibrated solutions to investigate the corrosion behavior of borosilicate glass under unsaturated conditions. The evolution of aqueous chemistry, secondary phases, and the SAL was characterized using solution analysis combined with XRD, SEM, and STEM techniques. The results show that B release increases continuously with temperature and reaction time, whereas dissolved Si initially increases and subsequently exhibits slight variations associated with SAL evolution and secondary phase precipitation. XRD and SEM analyses reveal the progressive formation of secondary phases on the glass surface, including zeolites, analcime, calcium silicate, smectite, and barite. STEM-EDS observations further demonstrate that the SAL develops a multilayer structure consisting of a relatively dense inner alteration layer, a porous outer alteration region, and a compositional gradient region near the glass-SAL interface. These findings provide new insights into the coupled processes controlling borosilicate glass durability and improve the basis for performance assessment of vitrified nuclear waste in geological disposal environments.