There are few problems associated with steam generation equipment that are seen more often than oxygen corrosion. The damage caused by oxygen can be extensive and quite costly. This type of damage can be easily recognized by the characteristic pitting that results.
Oxygen is a natural component of virtually all water supplies and will be present as a dissolved gas at maximum saturation levels determined by water temperatures. At feedwater and boiler water temperatures, the corrosivity of even small concentrations of oxygen is greatly accelerated over what would be expected at ambient temperatures.
While no area of the preboiler, boiler or postboiler system is immune from potential oxygen attack, corrosion will typically be observed first in a few specific locations:
- The first point of observed attack will usually be at the economizer since there is a dramatic temperature increase in this unit. (Other sections such as the storage section of the deaerator and feedwater lines between the deaerator and the economizer would also likely be affected but these are difficult areas to inspect.)
- The next point of readily observed oxygen corrosion is in the steam drum at the steam/water interface. It is at this location, where all oxygen present is released, that maximum attack takes place. Any oxygen that is not reacted by this time flashes over with the steam. It is unlikely that oxygen pitting will take place anywhere below the level of the feedwater line in the boiler itself. Thus, boiler tubes and mud drums are not generally susceptible to this problem.
- Finally, oxygen that is carried through the system can be readily observed throughout the condensate lines. Any attack taking place here becomes much more severe in the presence of carbon dioxide, which is commonly found in most condensate return lines.
Note that while corrosion due to oxygen is easiest to spot in these locations, the potential for attack is similar throughout the system. Fortunately, there are options that can help control oxygen corrosion. They fall into 2 categories: mechanical deaeration and chemical deaeration. Check out our blog next week for that discussion.