TECH REPORT: Corrosion and etching of glass

MICHAEL OLSEN
Research Glassblower
Colorado State University
Department of Chemistry
Ft. Collins CO 80523-1872
(970) 491-5229 (voice)
(970) 491-1801 (FAX; attn. M Olsen)



Q: What is the difference between glass etching and glass corrosion?

> I have observed that after treatment the glass appears more stable to a
> certain degree (i.e. the rate of corrosion decreases) but the treatment
> seems to cause an amount of corrosion by itself.

I assume that you are using borosilicate glass and STP conditions? In my experience, neither dilute HCl nor NH3 should 'corrode' the glass to any appreciable extent. After all, both reagents are sold in glass containers. Have you determined that corrosion has occurred by measuring some loss in weight of the apparatus, or by detecting the presence of leachates? If, after a run, the apparatus looks 'scuzzy', it could be that reactants/products are adhering (H-bonded?) to the glass, and the underlying glass is actually not being affected.

In general, acids tend to leach the alkaline metal 'fluxes' out of glass, whereas alkalai (especially OH-) attacks the Si-O bonds (true corrosion). Both of these will result in grungy-looking glass, but acid attacked glass tends to appear frosted when dry, and alkalai attacked glass tends to look greasy. Note though, that the acid-frosted glass will appear similar to undamaged but encrusted glass, except that acid attack is subtractive and encrusting is additive to your system. Might this be possible to determine by careful weighing before and after a run?

If acid attack is occuring, consider using a 'harder', flux-free glass: fused quartz. If alkalai attack is the mechanism, you may be at a dead end. The only idea that pops to mind is to apply some sort of resistant coating to the apparatus, or use plastic instead of glass.

The following is derived from 'The Properties of Glass Surfaces' by L. Holland, Chapman and Hall, 1966, pp 130-134:

  • The rate of degradation from acid attack decreases with time as aflux-depleted layer is formed. Alkalai degradation rates are, in general,constant.
  • Corning 7280 glass is somewhat alkalai resistant, but becomes opaque with severe attack.
  • Weight loss over 6 hours at 100 C in 5% NaOH (in mg/cm2) for Corning 7900 96% silica quartz glass is 0.9; for 7740 borosilicate is 1.4; for 7280 is 0.09.
  • Weight loss over 24 hours at 100 C in 5% HCl (in mg/cm2) for 7900 is 0.0004; 7740 is 0.005; 7280 is 0.01.
  • SO2 treatment at >500 C will result in an alkalai depleted surface on soda-lime glass (and, although not mentioned, possibly of borosilicate too?), thus making it resistant to acid and water attack. The cited potential reactions are, 2Na+ -glass + SO2 + 1/2 O2 + H2O = 2H+ -glass + Na2SO4, or, Na2O + SO2 + 1/2 O2 = Na2SO4 Holland notes that this SO2 reaction occurs to some extent during 'normal'lehr processing with certain open-fired manufacturing processes.
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Last edited 02-18-03