Influence of glass composition on the kinetics of glass etching and frosting in concentrated HF solutions

During etching of a multicomponent glass in concentrated hydrofluoric acid (HF) solutions, a crust can gradually appear on the glass surface, depending on the chemical etching parameters. This crust deposition is the result of the precipitation of hexafluorosilicate anions released by the glass dissolution reaction, with cations coming either from the etching solution or from the glass (like Na+, K+ or Ca2+). To understand the impact of this crust on the overall kinetics of both the etching and frosting process, we have studied the dissolution of four types of commercial glass substrates in etching solutions containing various concentrations of HF. The kinetics of the frosting process was investigated by two independent methods: on the one hand the chemical analysis by ICP-OES of the amount of Si dissolved from the glass as a function of time, and on the other hand the measurement of the glass weight loss with time. These two methods showed that the glass etching rate decreases with time as a result of crust formation which gradually becomes more protective. Both methods also indicated that increasing the amount of HF in the etching solution increases the etching rate in a non-linear way, and that the etching rate increases with the alumina content of the glass. A separate chemical analysis of the amount of Si present in the crust also revealed that the amount of HF in the etching solution has an impact on the amount of crust deposited on the glass surface. Finally, we have been able to rationalize all these kinetic data based on a semi-empirical quantitative etching and frosting model, allowing to extract a characteristic dissolution and precipitation constant (Kdiss and Kdep), respectively.