Carbon dioxide attack in boilers

Galvanic corrosion

Caustic corrosion

Acidic corrosion

Hydrogen embrittlement

Oxygen attack

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 acid:

H₂O + CO₂ çè H₂CO₃ çè H⁺ + HCO^3-

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.

References
‘Water treatment handbook’ Vol. 1-2, Degremont, 1991
‘Industrial water conditioning’, BeltsDearborn, 1991
http://www.thermidaire.on.ca/boiler-feed.html