Corrosion

Corrosion occurs in systems where non-condensables are allowed to combine with retained condensate. The most familiar form of corrosion results from the oxidation of iron in the presence of water to form ferric acid. You probably know it as common rust.

Rust In a steam heating system, differences in temperature increase electron flow. Because water is an electrolyte, it conducts electrons from the anode to the cathode, polarizing them chemically and electrically. The ions of the water combine with the ions of the metal at its surface. Ferrous oxide is formed at the anode, then combines with atmospheric oxygen to form ferric acid, or rust.

Pitting and Scaling Corrosion can occur in several ways, depending on the oxygen content, alkalinity, temperature and other factors of the water. The damage caused by corrosion can range from pitting in certain areas to scaling, in which the entire surface area becomes affected. Large pieces of scale can flake off, drifting downstream until they become lodged in small orifices and cause blockage.

Carbonic Acid An even more damaging form of corrosion results from the creation of a highly corrosive compound called carbonic acid. As condensate cools within the system, a chemical reaction combines it with carbon dioxide, forming carbonic acid. Carbonic acid within a steam system can completely dissolve piping and equipment.

( CO₂ + H₂O = H₂CO₃ )

As long as the condensate remains hot, carbonic acid is not formed. The carbon dioxide behaves like any other non-condensable.