Boiler corrosion oxygen, dissolved, effect

Water Treatment. Sodium sulfite is an agent in the reduction of chlorine or oxygen in water. Dissolved oxygen in boiler water tends to enhance pitting and other types of corrosion. In boilers operated at below 4.82 MPa (700 psi), a residual concentration of 30 ppm of sodium sulfite is generally effective. Catalytic amounts of cobalt are often added to accelerate the reaction of oxygen with sulfite (321,322) (see Water, industrial water treatment). 

Kimmel, A. L., Holingshand, W. R. and Shea, E. P., Effect of Dissolved Oxygen on Pitting of Boiler Tubes , Am. Soc. Mech. Engrs., Paper No. 61-WA-285 Colborne, G. F., Allen, A. R. and Thunaes, A., Crash Repair Program to Control Serious Pitting Corrosion on Process Tanks , Corrosion, 17, 20 (1961)... 

The potential for corrosion as a result of the reactions of noncondensable gases present in steam-water circuits is a major area of risk. The dissolved oxygen (DO) content of MU water is recognized as a primary source of gas entering a boiler system, and effective deaeration of MU and FW is therefore critical. 

Oxygen is almost always a contributing factor to corrosion mechanisms therefore, the effective removal of dissolved oxygen (DO) is of paramount importance in controlling the rate of boiler system corrosion, irrespective of the size, design, or pressure of the plant. 

The softened water is now in its most corrosive state, being still saturated with DO and having no hardness to form a protective scale. For reasons of economy, the removal of DO is usually first effected in part by thermomechanical deaeration. The boiler feedwater is preheated, then flashed in a deaerator to remove any free carbon dioxide and most of the dissolved oxygen. Then the last traces of DO are chemically scavenged with sodium sulfite (Na SO ) that reacts with oxygen to form sodium sulfate (Na SO ) as described in Eq. (8.22) ... 

Inhibitors are substances that, when added in relatively low concentrations to the environment, decrease its corrosiveness. The specific inhibitor depends on both the alloy and the corrosive environment. Several mechanisms may account for the effectiveness of inhibitors. Some react with and virtually eliminate a chemically active species in the solution (such as dissolved oxygen). Other inhibitor molecules attach themselves to the corroding surface and interfere with either the oxidation or the reduction reaction or form a very thin protective coating. Inhibitors are normally used in closed systems such as automobile radiators and steam boilers. 

Uses Oxygen scavenger for use in boiler systems that operate with high purity feedwater, where dissolved solids must be limited protects entire system from corrosion particularly effective under wet standby conditions Properties Tan to brn. liq. sp.gr. 1.00 dens. 8.3 Ib/gal fp. 25 F flash pt. (TCC)... 

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