PNNL

Abstract

Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a scientific challenge with major implications in geoscience and industry. One of the main uncertainties limiting the development of predictive models lies in the formation of an amorphous surface layer – here called gel – that can in some circumstances control the reactivity of the buried interface. Here we report experimental and simulation results deciphering the mechanisms by which the gel layer becomes passivating. The study is conducted on a 6-oxide borosilicate glass altered at 90 °C, in a neutral pH and silica saturated solution. Contrary to predictions by current models of glass corrosion, it is shown that the gel forms by complete dissolution of highly soluble species and only partial dissolution of the glassy network forming species. This leads to a 3D porous structure in which a small fraction of water molecules diffuse quickly through open channels, whereas others are trapped in closed micropores formed by hydrolysis-condensation reactions. Gel reorganization is thus the key mechanism accounting for extremely low water diffusivity (here 10-21 m2·s-1), which is rate-limiting for the overall reaction. It is expected that the efficiency of the passivation effect is dependent on glass composition and environment. These findings could be used to improve kinetic models, and also inspire the development of new molecular sieve materials with tailored properties as well as highly durable glass for application in extreme environments.