Dissolved Gases, Oxygen Control
The most common source of corrosion in boiler systems is the amount of dissolved oxygen that remains in the boiler feedwater after treatment. Oxygen scavenging chemicals normally Sodium Sulphite, is added to the stored feedwater.
The whole ethos is based on a fixed feedwater temperature. Rarely if ever is the boiler feed tank temperature constant, with a fixed chemical feed rate there will be times when the water is under or over dosed.
Under dosing leads to corrosion attack: even small concentrations of dissolved oxygen can cause serious corrosion problems. Over dosing increases the Total dissolved Solids (TDS) level in the boiler resulting in higher blowdown rates and wasting energy.
Dissolved oxygen reacts with the iron in the boiler system to form oxides and so must be removed.
Unfortunately oxygen corrosion is not uniform across the entire metal surface, it is identified by well defined pits or a very pockmarked surface; the pits vary in shape but have very sharp edges at the surface.
An active oxygen pit has a cap of red/brown oxide which when removed reveals black iron oxide within the pit.
Because the pits penetrate very deep within the metal oxygen corrosion can result in rapid boiler failure.
Dissolved oxygen reacts with the iron in the boiler system to form oxides and so must be removed. Oxygen trapped in the feed water will eventually corrode the boiler and steam system.
With atmospheric feed tanks good oxygen control is achieved by preventing temperature stratification and maintaining a high water temperature: an ideal target temperature being 95°C ensuring a minimum of dissolved oxygen and low oxygen scavenger requirement. Once the majority of oxygen has been dealt with the remainder is removed by reacting with an oxygen scavenger, Sodium Sulphite being the most common for land based boilers.