# Glass Corrosion

Glass is generally seen as a highly corrosion resistant material, but despite being good material choice for severe environments, glass can undergo corrosion, known as “glass disease.” Seen here, spot corrosion of the optical glass surface develops following mold attack, aka ‘bio-pitting’. Via cvma.co.uk

Glass is generally seen as a highly corrosion resistant material and is many times a good material choice for severe environments. Typically, glass undergoes two types of corrosion, both of which are known as “glass disease”. The first is known as diffusion-controlled leaching and because it involves ion exchange, the rate of this form of corrosion decreases as pH increases. In contrast, the second mechanism of glass corrosion is hydrolytic dissolution of the glass’s network and occurs faster as pH increases.

There are many ways to quantifiably measure the corrosion resistance (also known as the chemical durability) of a glass in environments with widely varying pH values. In addition, there are many mathematical models that represent the rate of corrosion of glasses. These models use a normalized corrosion rate for each element (i) that is present in the formula of the glass (NRi). These normalized corrosion rates are calculated using the total amount of species present in the water (Mi), the contact surface area (S), the time of contact (t) and the weight fraction of each element in the glass (fi).

\$\$NR_i = {M_i over Sf_it}\$\$

Each mechanism (ion leaching and dissolution) contributes to the overall corrosion rate (NRi = NRxi + NRh).

The corrosion rates for the leaching mechanism measure the rate of replacement of alkali ions in the glass by a hydronium ion (H3O+) from the aqueous solution. Because this corrosion mechanism is related to the diffusion rate, it gives an inverse square root dependence with exposure time. In the below equation, ρ is the density of the glass, Di is the i-th cation effective diffusion coefficient, and t is the exposure time.

\$\$NRx_i = 2rho sqrt {D_i over pi t}\$\$

In contrast, the hydrolytic dissolution generally happens after the leaching mechanism during corrosion. This type of corrosion is time-independent when the surrounding solution is dilute. In the below equation, the normalized rate of corrosion due to dissolution is calculated from the density of the glass (ρ), and the stationary dissolution rate of the glass (rh).

\$\$NRh = rho r_h\$\$

When the system is closed, the leaching phase of corrosion happens quite quickly. As the available protons are consumed in this manner, the pH increases and the dissolution phase of corrosion begins not long after. However, once the solution becomes saturated with silica the dissolution process is impeded and ionic leaching (or diffusion-controlled) becomes the primary mechanism of corrosion once again.

While glasses do not generally have the mechanical properties required for high-stress applications, they can sometimes serve an important purpose in situations where a high degree of corrosion resistance is required. As always, it is important to understand the degree of their chemical durability in the environment in which they will be placed.