Oxygen Attack in Boilers

Oxygen Attack refers to the corrosive action of dissolved action on the boiler.  Dissolved oxygen causes pitting on the boiler surface.  Oxygen enters the boiler through the feed water.  Though, the deaerators remove a large amount of oxygen, the oxygen that remains can cause corrosion. 
When the feed water is heated, the oxygen becomes even more aggressive resulting in severe corrosion.

If the water contains ammonia, this results in corrosion of components containing copper and copper alloys such as bearings. 

Corrosion also results in deposits on the heat transfer surfaces which affect efficiency. 

Corrosion caused by oxygen is usually localized.  Oxygen Corrosion can also be extensive. 
Oxygen Attack is not monitored and prevented can result in failure of the boiler components. 
Heating the feed water reduces its solubility and reduces the dissolved oxygen.  Mechanical deaerators can further reduce the dissolved oxygen level.  Finally, chemical deaerators such as sodium sulphite can scavenge the remaining oxygen ions.