Study of the kinetics of glass alteration by small-angle X-ray scattering

The alteration in water of homogeneous sodium borosilicate glasses containing different concentrations of zirconium has been studied. The dissolution kinetics were followed by chemical analysis, and the morphology of the interfacial altered layer was studied by small-angle X-ray scattering (SAXS). Two types of dissolution behaviors have been observed, depending on the initial concentration of zirconium in the glass. Below 2% ZrO2, the glass dissolves with a constant thickness of the altered layer (i.e., the leaching of the elements of the glass causes a shrinkage of the particle size). Above 2% ZrO2, the skeleton of the altered layer becomes less soluble; consequently, the alteration is isovolumic with an altered layer thickness increasing with time. Whatever the zirconium content, the altered layer initially presents a large porosity associated with a high specific surface area. Then, the dissolution and recondensation of silica cause a ripening of the pores with a decrease of the specific surface area. Zirconium strongly modifies the dissolution kinetics and also the morphology of the altered layer. It increases the specific area of the altered layer, which delays the formation of a passivating layer at the glass-water interface. As a result, small amounts of zirconium lead to larger alteration depths.