Fig. 4

Refined phenomenological model of aqueous glass corrosion. a In silica-undersaturated solution the corrosion starts with congruent dissolution of glass, resulting in the formation of etch pits with an initial or forward rate r₀. b Depending on the ratio of transport of dissolved silica into the bulk solution and silica release from glass dissolution, a concentration gradient will develop,⁴⁷ enabling localized saturation of aqueous silica triggering condensation and nucleation reactions.³⁵ c After silica-supersaturation has been reached in the solution boundary layer, amorphous silica spheres precipitate onto the dissolving glass surface, mimicking the initial glass surface. d Congruent glass dissolution continues, while dissolved species are transported through the developing SAL. Transport limitations (of water and dissolved species) lower the rate of congruent dissolution at the reaction interface (r₀ → r_r), which eventually is slow enough for solid state diffusion of protons and ion exchange with glass constituents to occur e.¹³ The occurrence of the inner dense layer in exp. QBG-90/150 is schematically outlined in f and g. f Quenching the sample before solution exchange causes the precipitation of silica from the interfacial (pore) solution. g During the following (isotope tracer) reaction step, this quenched layer potentially acts as transport barrier, influencing the composition of the inner (pore) solution