Dissolution of sodium silicate glasses for the production of water glass – Part III: Dependence of corrosion process on silica concentration of leachate

The corrosion and dissolution of Na₂O.xSiO₂ glasses (x=2·0, 2·5, and 3·3) was investigated by static and dynamic corrosion tests at pH values between 7 and 14 with a focus on saturation effects. The corrosion temperatures were 30 and 50°C. The investigated glass compositions are close to those used for water glass production. Two types of leachants with varied concentrations have been applied to study saturation effects. The first type was deionized H₂O enriched with up to 2·5 mM SiO₂ by dispersing and equilibrating pyrogenic silica. The second type of leachants were liquid sodium water glasses with increasing concentrations and a SiO₂:Na₂O ratio similar to the dissolving glasses forming dilution series. Two different saturation effects which influence corrosion kinetics were recognised: first, the enrichment of deionised H₂O with pyrogenic silica reduced the dissolution rate of reaction layers. An extrapolation of mass loss rates yielded silica saturation levels of 2·8 to 3·8 mM SiO₂. Second, the dilution series of sodium water glasses demonstrated the shielding of glass surfaces when pH conditions hindered reaction layer formation. The corrosion rates of a sodium silicate glass with a molar ratio SiO₂:Na₂O=2·0 were reduced but the respective corrosion rates of a sodium silicate glass with a molar ratio SiO₂:Na₂O=3·3 were increased with increasing sodium water glass concentration of the initial leachants. The sodium silicate glass with a molar ratio SiO₂:Na₂O=2·5 showed first an increase and at higher sodium water glass concentrations a decrease of the corrosion rate with increasing concentration of the liquid sodium water glass and thus displayed an intermediate dependence on water glass concentration.