25 May 2005
Condensate Corrosion and Control
Causes of Condensate Corrosion. Oxygen and carbon dioxide are
common steam condensate impurities that promote condensate corrosion. Less
common are process contaminants, each of which has corrosive properties dependent
upon the nature of the contaminant and on the materials that may be corroded. The
piping found in steam condensate systems is commonly constructed of mild steel,
whereas heat exchangers are usually copper or mild steel.
Air. Air is the source of oxygen in a condensate system. Condensate lines
that are used intermittently are particularly susceptible to condensate corrosion because
the cooling of a heated condensate line creates a vacuum that can draw air into the
system piping through valve seals or condensate sumps. Additionally, any dissolved
oxygen in the feedwater that is not completely removed, either chemically or
mechanically, can enter the boiler and pass with the steam into the condensate lines
where it is consumed in a pitting corrosion attack on the metal surfaces. Oxygen
corrosion in a condensate system is evidenced by pitting and by the presence of
corrosion product deposits called "tubercles."
Carbon Dioxide. Carbon dioxide (CO2) can enter the boiler system if it is
dissolved in the feedwater. However, CO2 can be removed effectively through the
deaerator. The most common source of carbon dioxide in steam condensate is the
carbonate (CO32-) and bicarbonate (HCO3-) ions (alkalinity) in the feedwater. Under the
influence of heat and pressure in the boiler, the carbonate (CO32-) and bicarbonate
(HCO3-) ions (alkalinity) break down into hydroxyl (OH-) ions (alkalinity) and CO2 gas.
This free CO2 is volatile and leaves the boiler with the steam. When the steam
subsequently condenses, the CO2 dissolves in the condensate and forms carbonic acid
(H2CO3), which lowers the pH of the condensate and is corrosive to most metals,
particularly to mild steel (see Figure 3-13). You can recognize carbon dioxide corrosion
by the thinning of condensate line walls, particularly at the condensate liquid level within
the pipe and at pipe threads where the condensate flows. Prevent corrosion in steam
boiler systems by neutralizing the CO2 with volatile amines.