Q. Corrosion in Steam Condensate System?

Carbon dioxide and oxygen, either acting alone or in combination, account for the majority of steel corrosion problems in the condensate system.

The make-up water usually contains bicarbonate and carbonates. These species can accumulate and decompose in boilers at high temperatures to form carbon dioxide.
When the steam condenses, part of the carbon dioxide gas dissolves in the condensate (mostly in low pressure condensate) and forms carbonic acid.

The primary source of oxygen in the condensate system is air in-leakage at various locations. This includes threaded and other mechanical joints; packing glands of condenser return pumps, faulty steam traps, and areas under vacuum or areas where the system pressure is lower than the atmospheric pressure. Another source of oxygen is DO in the BFW due to improper oxygen scavenging process.

Simultaneous presence of oxygen and carbon dioxide in condensate can produce a synergistic interaction, resulting in severe corrosion rates.

Two forms of corrosion, general corrosion and pitting, can occur in the condensate system.General corrosion or thinning will occur at lower pH values, and pitting will occur at higher pH values.