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When alkalinity is present in feedwater in the form of bicarbonate (HCO3-) and carbonate (CO3--) and enters the boiler, it breaks down to form carbon dioxide (CO2). The carbon dioxide travels with the steam and condenses throughout the system. When carbon dioxide condenses in the condensate system, it forms carbonic acid (H2CO3) which reduces the pH and causes corrosion.
Condensate corrosion can also occur when poorly de-aerated feedwater enters the boiler and oxygen flashes with the steam. Since carbon dioxide has a high vapor liquid distribution ratio, most of it condenses in the farthest points of the condensate system. Vented condensate receivers assist in removing some of the carbon dioxide to reduce the amount of acid formed. Condensate corrosion causes severe metal loss and damage to equipment. The metal ions lost to corrosion also return to the boiler where they can cause deposition, efficiency loss, and failure.
Chemical treatment programs which utilize neutralizing, filming agents, and oxygen scavengers minimize the effects of condensate corrosion. The most effective way to minimize the costs of treatment is to reduce the alkalinity in the feedwater by demineralization, dealkalization, or reverse osmosis.