Oxygen attack in boilers

Galvanic corrosion

Caustic corrosion

Acidic corrosion

Hydrogen embrittlement

Oxygen attack

Carbon dioxide attack

Without proper mechanical and chemical deaeration, oxygen in the feed water enters the boiler. Much is flashed off with the steam; the remainder can attack boiler metal. Oxygen in water produces pitting that is very severe because of its localized nature. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper bearing alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability.

Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems: iron oxide generated by the corrosion can produce iron deposits in the boiler. Oxygen corrosion may be highly localized or may cover an extensive area. Oxygen attack is an electrochemical process that can be described by the following reactions:

Anode: Fe è Fe²⁺ + 2e^-

Cathode: ½ O₂ + H₂O + 2e^- è 2 OH^-

Overall: Fe + ½ O₂ + H₂O è Fe(OH)₂

In this reaction a temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in a rapid and severe corrosion.

The acceptable dissolved oxygen level for any system depends on may factors, such as feed water temperature, pH, flow rate, dissolved solids content, and the metallurgy and physical condition of the system. In general, the limit value of oxygen in make up water can be stared 0.10 mg/kg

For a complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration. Membrane contractors are also a possibility.

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