Carbon Dioxide Attack
Carbon dioxide exists in aqueous solutions as free carbon
dioxide and the combine forms of carbonate and bicarbonate ions.
Corrosion is the principal effect of dissolved carbon dioxide.
The gas will dissolve in water, producing corrosive carbonic
H+ + HCO3-
The low pH resulting from this reaction also enhances the
corrosive effect of oxygen.
In boiler systems, corrosion resulting from carbon dioxide is
most often encountered in the condensate system. Because feed
water deaeration normally removes carbon dioxide from the boiler
feed water, the presence of the gas in condensate is typically
due to carbonate and bicarbonate decomposition under boiler
conditions. For an approximation is estimated that feed water
with a total alkalinity of 100 mg/l as calcium carbonate could
be expected to generate a carbon dioxide level of 79 mg/l in the
steam (alkalinity multiplied by a factor 0.79). Such a high
carbon dioxide level would create a very corrosive condensate.
Carbon dioxide corrosion is frequently encountered in condensate
systems and less commonly in water distribution systems.