Glass Corrosion
Glass Corrosion
Glass Corrosion
Page 1 of 8
PPG Industries, Inc.
Glass Technical Document TD-105
The reason solution pH increases can be adverse effects on surface quality being
explained by basic chemistry principles. observed, so long as increases in solution
Pure water is a weak electrolyte that pH levels are controlled. However,
spontaneously breaks down partially into unrestrained State 1 corrosion can lead, as
hydrogen ions (H+) and hydroxide ions previously mentioned, to highly alkaline
(OH-) according to the simplified conditions.
expression: H2O H+ + OH-. At 25° C
(77° F), this equilibrium condition is Once solution pH levels reach 9.0 or greater,
described by an ionization constant, Kw, as the second important reaction in the glass
given by the equation Kw = 1 X 10-14 = (H+) corrosion process - Stage 2 - begins. At this
(OH-). point, hydroxide ion concentration is
sufficient to begin attack of the silicate
In other words, at a given temperature, the network. As shown in the following
product of the hydrogen ion and hydroxide equation, the main reaction is the severing of
ion concentrations remains constant. For silicon-oxygen bonds (the glass itself is
water in contact with a glass surface, slowly dissolved): Si-O-Si (glass) + OH-
however, the exchange process tends to (solution) Si-OH (glass) + O-Si
deplete the supply of hydrogen ions in (dissolved glass; sodium and calcium
solution, causing an imbalance in the silicates).
equilibrium represented by the equation just
given. During the beginning stages of this reaction,
microscopic pitting of the surface occurs. If
In order to reestablish equilibrium, more the reaction is allowed to continue, surface
water molecules dissociate, producing damage will become more apparent, and the
additional H+ and OH- species. As the glass may have a widespread iridescence or
original quantity of OH- was not affected by a dense, translucent haze like that shown in
the exchange process, further dissociation of Fig. 1.
H2O results in an increase in hydroxide ion
concentration and a commensurate rise in
pH.
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PPG Industries, Inc.
Glass Technical Document TD-105
silicates work together to affect the can be used in less critical applications
appearance of the glass. Carbon dioxide in where no coatings are involved.
the atmosphere can also react with moisture
on the glass surface to form additional When lightly corroded glass is successfully
surface residues, typically of sodium and restored, the sodium and calcium silicates
calcium carbonates. are removed as well as any carbonate
residues that may have precipitated on the
Restoring the pristine surface quality to glass surface as by-products of the Stage 2
severely corroded glass is at best a reaction. The actual network damage
formidable task. Grinding and polishing affecting optical quality is so slight that it is
could, with much time and effort, restore the of no consequence in less critical
optical characteristics of glass corroded like applications.
that shown in Fig. 2. However, this solution
is not practical nor economical; it is simply Conditions for Corrosion
easier to discard all heavily corroded glass.
Once the glass has been properly cleaned
and dried, the corrosion process is
suppressed. It is unlikely that it will begin in
the finished glass product.
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PPG Industries, Inc.
Glass Technical Document TD-105
Interleaving Systems
Page 4 of 8
PPG Industries, Inc.
Glass Technical Document TD-105
Page 5 of 8
PPG Industries, Inc.
Glass Technical Document TD-105
How does a powdered interleaving system leaves the glass highly susceptible to being
actually prevent glass staining as well as scratched.
mechanical damage, such as surface
scratches? In order to understand, it may be What happens when a neutral, non-reactive
helpful to know what happens if interleaving separation medium, such as PMMA beads,
is not used, a question asked by some buyers is applied between stacked glass lights as
of primary glass. If no interleaving is used, shown in Fig. 5. Notice how the beads act as
the stage is set for development of the most tiny ball bearings that prevent sliding
severe glass corrosion environment friction between the glass. The distance
imaginable. between adjacent surfaces has been
increased to 0.01 inch; a 0.1-milliliter water
Without interleaving materials, the distance droplet would cover only 1.6 square inches
between non-contacting points on adjacent of glass surface, tenth that for non-
glass surfaces can be as little as 0.0007 to interleaved glass. In other words, it would
0.0009 inch, which is about a third of the take an entire milliliter of water to achieve
thickness of a sheet of newspaper. Fig. 4 intimate contact with 16 square inches of
shows a unique cross-sectional view of non- glass surface area.
interleaved glass.
Fig. 5
Fig. 4
However, this "spacing" effect doesFig. not4
Under these conditions, the quantity of significantly act to prevent glass staining.
water found in a small droplet -0.1 milliliter, The additional space between the glass
or about the amount of water in two lights permits more condensate to form on a
raindrops - could come into intimate contact given surface area. As a result, the by-
with more than 16 square inches of glass products of Stage 1 corrosion, when
surface. In a very short time (several days at evolved, accumulate in a greater volume of
room temperature), pH levels would reach moisture than they would when non
the critical stage -pH of 9.0- and the glass interleaving is used. The net result is that it
would soon suffer irreparable corrosive takes slightly longer for critical pH levels to
damage. The absence of interleaving also be reached.
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PPG Industries, Inc.
Glass Technical Document TD-105
Fig. 6
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PPG Industries, Inc.
Glass Technical Document TD-105
HISTORY TABLE
ITEM DATE DESCRIPTION
Original Publication 11/15/1986
Revision #1 1/17/2002 Transferred to TD-105
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PPG Industries, Inc.