Oxygen Injection

The oxygen-treatment (OT) is based on an idea that by using oxygen gas and volatile alkaline agents, or using only oxygen gas in high-purity water, hardly-soluble corrosion products are formed on the iron surface uniformly and the subsequent steel corrosion and the discharge of corrosion products into water can be controlled.
The metal oxides formed on the surface of iron-based materials in the OT forms the two-layer structure of hematite and magnetite from the water side.

Under the coexistence of dissolved oxygen, the particles of hematite bury the holes of porous magnetite and the crystal diameter of magnetite is smaller than that in the all volatile treatment (AVT), the corrosion of materials is controlled by the very stable protective film.



However, this requires the high-purity boiler feed water and boiler water in which impurities are hardly contained. If anions such as chloride ions and sulfate ions increase, the above-mentioned protective film will be destroyed and corrosion will progress.
For this reason, the control value of conductivity (cation-pass) shall be set at severer value than that of AVT.





Fig. 2 shows under the AVT condition, the stable iron oxide is magnetite (Fe3O4), and under the OT (oxygen treatment) hematite (Fe2O3) is stable.

Question?
In the case that the conductivity (cation pass) at the boiler feed water exceeds > 0.2 μS/cm, oxygen injection is stopped under the oxygen treatment.
What kind of problems will occur, if oxygen injection into boiler feed water continues in the case of the conductivity (cation pass) is higher than > 0.2 μS/cm?

Answer
The case in which conductivity (cation pass) at the boiler feed water is higher than > 0.2μS/cm indicates the ingress of contaminants, it means the amount of anions such as chloride and sulfate ion increases. In this case, the iron oxide protective film is destroyed and corrosion will progresses.
Because the coexistence environment of anions and high concentration oxygen is prone to progress corrosion, oxygen injection is stopped to create an environment in which the corrosion does not progress easily.