Boiler Oxygen Control

The main responsible for boiler corrosion is oxygen . If only pure, oxygen- free water enters a boiler, an initial corrosion reaction between water and iron takes place resulting in the formation of a microscopically thin layer of protective magnetic iron oxide (Fe3O4, magnetite) as shown below:

3 Fe(s) + 4 H2O(l) = = > Fe 3O4(s) + 4 H2(g) 

The Fe 3O4  is durable and impermeable either from water, either from ions, so the coated surface resists to further attack.

After completion of this initial reaction, the metal is passivated and further corrosion is minimal. Any chemical or mechanical damage to the magnetic layer allows corrosion to advance until the protective layer repairs itself or until the tube fails.

Dissolved oxygen reacts with the passivating film creating iron oxide, which is not protective, promotes galvanic corrosion and further attack of the boiler steel from the water.

2 Fe 3O4 + ¹/2 O2 -->  3 Fe2O3

Unfortunately oxygen corrosion is not uniform across the entire metal surface its occurrence depends on boiler design, it generally takes place at the feed water distribution holes, in the steam drum at the waterline and in the downcomer tubes.

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.

Oxygen Damage

The curve shows the amount of dissolved oxygen in water with varying temperatures, under atmospheric pressure.

Obviously the higher the feed temperature the less the oxygen being fed to the boiler.

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