Why do you inject oxygen scavenger in boiler feed water?

Oxygen is a highly reactive gaseous element. In the presence of steel, the corrosion rate of oxygen doubles for each 30°F rise in temperature. For example, in a boiler system operating at 400 psig and 444°F, the corrosion rate for oxygen is 256 times more reactive than at room temperature.

Sodium sulfite is a highly effective oxygen scavenger

Erythorbate, Diethyl hydroxylamine, Methyl Ethyl Ketoxime, Carbohydrazide and Hydroquinone are other alternatives.