the ‘quasi-equilibrium’ solution conditions encountered and the formation of the dense protective gel layer. This is because glass is inherently, thermodynamically unstable by comparison to a collection of crystalline alteration products. As such it can be expected that glass will continue to undergo some form of reaction in perpetuity [78].
V - Resumption of alteration: This stage is not seen in all compositions of glass or under all test scenarios. This resumption is proposed to occur due to a precipitation of crystalline
phases which break up the passivating gel layer. As a result, the concentration of dissolved species in the surrounding solution is reduced, the affinity for reaction is increased and the diffusive barrier is destroyed. This is usually due to a precipitation of zeolites at a given pH or solution concentration level. There is the possibility of eventually returning to stable conditions where a steady residual state and dense gel layer are reformed, as such cycling between regime IV and regime V would occur [80, 103].
Variables Affecting the Aqueous Dissolution Behaviour of Glasses
The dissolution behaviour of glass in aqueous solution depends on a wide range of factors. A number of these factors and their effects on dissolution are summarised below. The terms in Equation 2.9 highlight the importance of composition (Ea), pH and temperature for the aqueous dissolution of glasses which will be discussed below.
Chemical Composition: While the glasses used in the nuclear industry have been developed to be resistant to aqueous dissolution, this is not true for all glasses. Sodium silicate glasses for example will dissolve readily in aqueous solution. The composition of a glass is extremely important in determining the dissolution rate of the material. Various ideas have been utilised to explain how composition affects aqueous durability.
The thermodynamic approach to dissolution of glasses is based on the ΔGhyd of glass components to their fully hydrated equivalent [104, 105]. This approach attempts to qualify which glass constituents have a positive effect on aqueous durability and which will be deleterious. It does this by comparing the free energy of hydration of their individual crystalline counterparts [79]. The summation of the components in a glass is weighted to provide a free energy of hydration for the entire glass. A good correlation has been found for a wide range of glass compositions between the free energy of hydration with experimental results for normalised elemental mass of silica, providing evidence supporting this approach.
It can therefore be considered a useful guide to which components will increase or decrease the aqueous durability of a glass. However, the effects of compositional change are not truly additive, therefore, a thermodynamic approach can only provide an estimate of glass stability in solution.
Effect of Modifier Cation Ratios: While sodium silicate glasses are readily soluble, soda-lime-silicate glasses form the basis for most modern glass production due to their superior performance in aqueous solutions [106]. The diffusion coefficients are lowered in glasses with more than one type of modifier, compared to a glass containing an individual modifier in the same molar proportions [107]. The effect is usually attributed to the blockage of ionic
diffusion channels in the glass structure, created by the need to accommodate cations of varying charge density. Equi-molar concentrations of numerous modifier cations tends to have a positive effect on aqueous durability and are favoured in the formulations of glasses for HLW immobilisation.
Precipitating Phases: Glass composition can also play a leading factor in the initiation of large scale crystalline precipitation. This can be detrimental to aqueous durability due to the breakdown of the dense passivating gel layer. This has been shown to occur in glasses containing high proportions of Al and Zr. Due to the high free energies of hydration for these components, they are often favoured in compositions due to the improved durability seen in the short term [86]. However, due to the low solubility of these elements in solution they are more liable to precipitate en-masse resulting in a stage V re-initiation of corrosion.
pH: The pH of aqueous solutions has a pronounced effect on the dissolution of glasses [82, 108]. It affects the dominant mechanism of corrosion as well as the rate. Between pH 5 - 9 silicate glasses typically show a rapid decrease in their rate of corrosion, when compared to more acidic or alkaline conditions, as shown in Figure 2.6 a). At low pHs the role of ion exchange within solution is dominant and a high rate of corrosion is observed.
At high pH values the dominant mechanism is hydrolysis. The effects are shown in Figure 2.6 b), where the equilibrium pH of SON68 glass has been shown to have a minimal dissolution rate between pH 9.5-10, this is suggested to relate to the formation of a more stable dense gel layer at the equilibrium pH of the glass [86].
Temperature: Higher temperatures provide the activation energy required for the dissolution mechanism to proceed. As such, an increased dissolution rate is seen at higher temperatures. This is useful for accelerating leaching experiments but consideration of potential changes in the mechanism of dissolution are required, especially when testing above 100 °C [78].
Surface Area to Volume Ratio (SA/V): The surface area to volume ratio affects the dissolution of silicate glasses significantly [109]. Higher SA/V ratios result in earlier formation of the passivating gel layer, pH equilibrium conditions and solution saturation. As such the alteration mechanism is accelerated to stage IV behaviour more rapidly with higher SA/V ratios. Therefore, methods for accelerated testing of glass durability, such as in the PCT method utilise a high SA/V ratio [110]. However lower SA/V ratios allow for a more in-depth study of the initial diffusion rate and the rate of hydrolysis in stage II. Agglomeration of powders are however known to cause issues in high SA/V test conditions.
Figure 2.6 – Plots illustrating a) the dissolution rate dependence of borosilicate glass on temperature and pH (Figure adapted from [108]) and b) the dependence of degree of borosilicate glass dissolution on pH over time (Figure adapted from [86]).