Glass corrosion mechanisms are different depending on the pH of the solution. Vials from the same lot of material were exposed to both low (acidic) and high (alkaline) pHs. Figures 3a and 3b show depth profiles of vials exposed to a low pH solution for varying times. An 80 nm thick B and Na enriched layer, after initial forming, (Figure 2) becomes a 50-100 nm depleted layer after exposure to an acidic solution for 18 days (Figure 3a). Not surprisingly, increasing time of exposure leads to a thicker depletion layer (Figure 3b). ICP-MS (not shown) confirmed elevated levels of Na and B in the stored solution, compared to levels in the solution at the start.
At high pH, the mechanism of corrosion in silicate glasses is fundamentally different than at low pH. Rather than removal of modifying species, the Si-O-Si structure of the glass dissolves. The surface of a vial exposed to high pH contains a 75 nm Al2O3–rich layer. The Al2O3 concentration (>10 mole%) was found to be significantly higher than in the bulk (Figure 4).
In conclusion, there are a number of processes in both the forming and use of Type I glass vials that can result in significant chemical gradients. These gradients may lead to mechanical or other stresses that ultimately result in the production of microscopic flakes under certain circumstances. A suite of analytical tools can be used to track changes in the surface chemistry, solution chemistry and defects. These include: SIMS, XPS, SEM, optical microscopy and ICP-MS, which can all provide valuable information regarding the effect of a stored solution on the glass storage medium.