Dissolved Oxygen and Corrosion

Dissolved oxygen can destroy the protective hydrogen film that can form of many metals and oxidize dissolved ions into insoluble forms. Deposits of rust in a plumbing system is such an example of differential aeration cells and accelerate corrosion.

Dissolved oxygen (DO) refers to the volume of oxygen that is contained in water. Oxygen enters the water by photosynthesis of aquatic biota and by the transfer of oxygen across the air-water interface. The amount of oxygen that can be held by the water depends on the water temperature, salinity, and pressure. Gas solubility increases with decreasing temperature (colder water holds more oxygen). Gas solubility increases with decreasing salinity (freshwater holds more oxygen than does saltwater). Both the partial pressure and the degree of saturation of oxygen will change with altitude . Finally, gas solubility decreases as pressure decreases. Thus, the amount of oxygen absorbed in water decreases as altitude increases because of the decrease in relative pressure.

In modern boiler systems, dissolved oxygen is handled by first mechanically removing most of the dissolved oxygen and then chemically scavenging the remainder. The mechanical degasification is typically carried out with vacuum degasifiers that reduce oxygen levels to less than 0.5-1.0 mg/L or with deaerating heaters that reduce oxygen concentration to the range of 0.005-0.010 mg/L. Even this small amount of oxygen is corrosive at boiler system temperatures and pressures.

Removal of the last traces of oxygen is accomplished by treating the water with a reducing agent that serves as an oxygen scavenger. Hydrazine and sulfite have been widely used for this purpose, but they have some shortcomings. Sodium sulfite, although an effective scavenger, is not recommended for use in systems operating above 1,000 psi because breakdown occurs to form corrosive hydrogen sulfide and sulfur dioxide. Also, sodium sulfite increases the amount of dissolved solids, as well as the conductivity, in the boiler water.

Hydrazine efficiently eliminates the residual oxygen by reacting with the oxygen to give water and gaseous nitrogen. Unfortunately, however, it has become widely recognized that hydrazine is an extremely toxic chemical. It is therefore desirable to provide alternate boiler water treatment chemicals which are generally free of the dangers inherent in the use of hydrazine, but which effectively scavenge oxygen and passivate steel surfaces under typical boiler conditions.

Erythorbic acid and its sodium salt are replacing sulfite and hydrazine as oxygen scavengers in boiler water treatment. Based upon the stoichiometric relationship, it should take about 13 parts of sodium erythorbate to react with one part of dissolved oxygen. Actual lab and field test data show that much less erythorbate is actually needed than theoretical to scavenge the oxygen. This result occurs because the erythorbate breakdown products accomplish further oxygen scavenging. Field trials in large utility boilers show the intermediate breakdown products to be lactic and glycolic acids. The ultimate breakdown product is CO₂.

Dissolved Oxygen and Corrosion

Consult the Water Glossary, Water Cooling Towers