A4 Chemical Aging and Corrosion of Glass (Leaching)

Topic

Water, or an aqueous solution, hydrolytically attacks the silica network of this glass, and this hydrolytic attack is the phenomenon described by this model: water molecules react with the silica network, breaking its bonds and displacing the network-modifying ions. Because water attacks the silica network in this hydrolytic manner, the attack manifests—within this model—in two distinct ways, depending on the environmental conditions.

The first of these two ways is the selective leaching of the silica network-modifying ions: these ions, which are not part of the silica backbone itself, diffuse out of the glass and are replaced by hydrogen ions from the solution, leaving behind a surface layer depleted of these modifying ions. Since this leaching occurs via a diffusion-controlled process, its kinetics are analogous to those of another model in this set—the one describing the growth of an oxide layer on a ceramic: as the leached layer thickens, it interposes itself between the solution and the unleached glass, causing the leaching process to slow down progressively as the layer grows, in the same self-limiting manner observed in the growth of that oxide layer.

In a basic medium, however, a second mechanism is added to this leaching process: the dissolution of the silica network itself, rather than just its modifying ions. The basic nature of the medium causes the silica network itself—and not merely the ions that modify it—to dissolve directly into the solution. The fact that this dissolution of the network itself—unlike leaching—proceeds under a linear regime means it does not become self-limiting over time; there is no protective layer intervening between the solution and the glass to inhibit the process, so it advances at a constant rate as long as the medium remains basic.

Consequently, in this model—which begins with the hydrolytic attack on the silica network—the solution's pH modulates the rates of both mechanisms simultaneously: a more basic pH causes the non-self-limiting dissolution of the network itself to become more significant than the self-limiting leaching of modifier ions, thereby accelerating the overall attack on the glass; conversely, if the pH remains neutral or acidic, the self-limiting leaching process—with kinetics analogous to oxide layer growth—dominates the attack on the glass.

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