Page 200 - Corrosion Engineering Principles and Practice

174 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 175

contact with the glass will be greatly affected by this ionic exchange
due to the production of hydroxyl ions (OH ).
−
−
+
2H O + Na SiO 3(glass) → 2 Na + 2OH + H SiO 3(surface) (6.2)
2
2
2
e
The second stage of corrosion is a process of destruction of the
leached surface layers of glass. Glass is resistant to most acids but
is highly susceptible to attack by alkaline materials, especially a
concentration of OH ions giving a pH greater than 9.0. The result
-
is an attack of the network forming silica-oxygen (Si-O) bonds,
leading to dissolution and reprecipitation of the glass components
resulting in complete “frosting” of the normally transparent glass
material (Fig. 6.30).
6.3.3 Galvanic Corrosion
Galvanic corrosion (also called “dissimilar metal corrosion” or
wrongly “electrolysis”) refers to corrosion damage induced when
two dissimilar materials are coupled in a corrosive electrolyte. In a
bimetallic couple, the less noble material becomes the anode and
tends to corrode at an accelerated rate, compared with the uncoupled
condition and the more noble material will act as the cathode in the
corrosion cell.

FIGURE 6.30 Double-pane window completely “frosted” due to a broken seal
and subsequent corrosion of the glass material.