Vitreous silica corrosion rates

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Table 3. Corrosion Rates of Vitreous Silica in Aqueous Media ...

As H+ ions replace Me" ions, a surface film is produced, resembling vitreous silica or silica gel, and having properties different from the parent glass [28]. This film swells, acts as a barrier to further reaction, and decreases diffusion rates into and out of the surfaces, thereby inhibiting further attack [8,29,30]. This is the mechanism of corrosion in neutral and acidic media. 

The fracture behaviour of glasses is also influenced by humidity. An increase of the relative humidity results in an increase of fracture velocity. Wiederhorn [562] provided an explanation of the mechanism of the impact of moisture on the fracture behaviour. Michalske and Freiman [356] discussed the molecular mechanism of stress corrosion in vitreous silica, which depends on the environmental conditions. The increase in the fracture velocity in glasses with increase in environmental moisture content is due to the chemical reaction between glass with water at the crack tip. The transport rate of water to the crack tip influences the fracture behaviour. These effects are of particular importance for the mechanical shaping of glasses in the presence of aqueous lubricants. 

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