Sodium sulfite:

Sodium sulfite reacts rapidly along with oxygen and is an extremely efficient scavenger.  Therefore, being a solid and the source of another solid (Na₂SO₄) which is generates during the reaction, sodium sulfite has the potential of fouling heat transfer surfaces.  A further problem linked along with the use of sodium sulfite is corrosion of secondary system elements resulting from its decomposition products.  At the temperatures present in the steam generators, sodium sulfite could decompose as displays.

H₂O + Na₂SO₃ →      2 NaOH +      SO₂                                                      (4-13)

Sulfur dioxide (SO2) is a gas and is carried over to the remainder of the steam facility.  With water (in the steam or in the feed/condensate system), the SO2 reacts in the following manner.

H₂O + SO₂     → H₂SO₃                                                                                                           (4-14)

    (acidic)

This acidic condition is corrosive to all components in the secondary system.

Because of the problems associated with sodium sulfites, many facilities use volatile chemistry control of the secondary steam system to control dissolved gases in conjunction with air removal systems. This control utilizes hydrazine (Reaction 4-11) and morpholine (Reaction 4-15) to eliminate oxygen and carbon dioxide, respectively.

N₂H₄ + O₂ → H₂O + N₂↑     (oxygen consumed)                                             (4-11)

2C₄H₉NO + CO₂+ 2H₂O → C₄H₉NO • C₄H₉COOH +    HNO₃ + H₂                  (4-15)

                                                                                                (carbon dioxide consumed)

 As could be seen through Reaction (4-11), no solids are formed; therefore, the tendency of fouling heat transfer surfaces is reduced. Further advantages results from the decomposition of hydrazine through the following reactions.

2 N₂H₄ → 2 NH₃ + N₂ + H₂                                                                  (4-16)

NH₃+ H₂O → NH₄OH                                                         (4-17)

These reactions give an output in an alkaline pH condition which decreases corrosion within the steam facility. As could be seen in Reaction (4-15), the consumption of CO₂ takes place.  There are two advantages result from this reaction; 1) the inventory of dissolved gases within the steam facility is decreased, and 2) is the reaction contributes to managing a higher pH through eliminating carbonic acid (H₂CO₃), therefore decreasing corrosion.