Causes of Corrosion in the Condensate Return Line and Monitoring of Recovery Water

How Does Carbonic Acid Corrosion Develop?

The corrosion that develops in the condensate return line is caused not only from inside the boiler, but also from the bicarbonate/carbonate alkalinity in the feed water breaking down at high temperatures and carrying carbon dioxide (CO₂) with the steam. When the steam condenses, CO₂ passes back into the water phase, forming carbonic acid and rapidly lowering the condensate pH value. This acidic environment creates grooving, especially at the bottom of the pipe, thinning at threaded joints, and metal loss in low-flow pockets. Iron oxide (rust) particles mixed into the water as a result of corrosion in the line can be carried to the boiler with the return water and form dangerous iron scale on the hot boiler pipes.

Don't Evaluate Recycled Water Only by Its Quantity

High rate of condensate recovery is very valuable in terms of energy and water savings; However, it should not be assumed that all returning water is directly suitable for the boiler. Hardness that may come from the process exchangers, oil or process chemical leaks, and iron transport resulting from line corrosion should be monitored regularly. Elif Çevre Kimya's boiler chemicals page includes neutralizing volatile amines for carbonic acid corrosion in condensate lines and film-forming amines (octadecylamine-based) that create a hydrophobic barrier on the metal surface. Which approach or amine distribution ratio to choose; It depends on line length, steam pressure stages, return rate and analysis results.

Root Cause Investigation at the Time of Deviation

When a sudden change in condensate pH or conductivity is observed, it is not right to simply increase the chemical dosage without finding the source. The alkalinity load in the pre-treatment, heat exchanger sealing, steam trap malfunctions and air suction points should be checked together. Exact dosage is not guaranteed without seeing water and condensate analysis.

Field Control and Monitoring List

Chemical dosage and system capacity decisions should be determined by analysis and facility conditions; There is no guarantee of fixed dosage or performance.

• pH and conductivity measurement by taking cooled samples from condensate return collectors at different distances
• Monitoring total iron (Fe), hardness leakage and oil/turbidity in condensate water
• Reducing the potential for CO₂ carried into steam by controlling the feed water M-alkalinity
• Selection of amines with vapor-liquid distribution ratio suitable for short and long distance lines
• Periodic maintenance of traps and vacuum breakers against air leakage