After pretreatment, water must be deaerated in the feedwater system. Mechanical deaeration removes gases, primarily oxygen, from the water, reducing them to an acceptable level (5 - 40 PPB, depending on the equipment used) so the water can be used in the boiler system. As much oxygen as possible is removed because of its highly corrosive properties.

Oxygen is a highly active element! In small amounts, it can cause serious corrosion in feedwater lines, economizers, boiler internals, steam-operated equipment and condensate return piping. The corrosion rate of oxygen dissolved in water doubles for each 18 Deg. F (10 Deg. C) increase in temperature. When oxygen and carbon dioxide are present together, the resulting corrosion rate may be 10 - 40% greater than the sum of the corrosion rates of the two gases acting separately. The combination of oxygen and ammonia is very corrosive to copper-based alloys.