Dissolution of sodium silicate glasses for the production of water glass – Part II: Dependence of corrosion process on pH in basic aqueous media

One basic process for the production of liquid sodium water glasses is the dissolution of sodium silicate glasses which can be regarded as an extreme case of glass corrosion. The dissolution processes occur at high pH values and achieve highly concentrated liquid sodium water glasses. To better understand the dissolution process, the methods of glass corrosion studies were adapted and applied. 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. The corrosion temperatures were 30 and 50°C. The investigated glass compositions are close to those used for water glass production. The corrosion of sodium silicate glasses generally follows linear time laws at short corrosion times. The release rates of Na₂O and SiO₂ are correlated with the glass composition and to the thermodynamics of glass hydrolysis. Two distinct pH regimes exist. Between pH 7 and a certain pH value between 11 and 13 (depends on glass composition and corrosion temperature) the corrosion rate increases slightly with pH and the glasses develop extensive reaction layers. Generally, published corrosion models predict such a rate increase, but at a higher extent than observed here. Above a pH of 11 to 13 the corrosion rates decline with pH by a factor up to 100 and the glasses develop almost no reaction layer. A possible explanation of the rate decrease at higher pH is the shielding of the silicate surface by alkalis. The shielding effect increases with alkali concentration. The pH dependence of reaction layer formation is discussed in terms of solubility or diffusion through a boundary layer with pH and silica concentration gradients in the leachate near the glass surface.