Why is steam condensate corrosive?

A lot of times we hear a customer say “the steam condensate is pure water from condensation, there is no reason to treat this water”.

What is our reply to this statement? Why do we need to treat the steam condensate?

The main reason for treating the steam condensate is to prevent corrosion. Corrosion will destroy condensate pipes and will generate corrosion products which will negatively affect the operation of the boiler system, especially if part of the condensate is recovered to be used as boiler feedwater.

Why is  condensate corrosive?

Make-up water and subsequently boiler feedwater contain alkalinity (here we are referring to low-medium pressure boilers that use softened water).

Alkalinity is the acid neutralizing ability of the water and it mainly consists of carbonate (CO32-) and bicarbonate (HCO3) ions. Under the influence of heat and pressure in the boiler, the alkalinity ions  break down into hydroxyl (OH) ions and CO2 gas..

2 NaHCO2  à Na2 CO3  +   CO2     +  H2O
Na2 CO3  +   H2O à 2 NaOH  +  CO2

This free carbon dioxide gas (CO2 ) is volatile and leaves the boiler with the steam and reacts with condensed water to form carbonic acid

CO +   H2O à H2 CO3

Carbonic acid is a weak acid, but as the condensate water is pure, this weak acid reduces the pH significantly, often in the range of 5-6. At this low pH, the condensate water is very corrosive to carbon steel piping.

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.

The cure for this type of corrosion is the neutralization of carbonic acid with volatile amines or ammonia (more information on the use of volatile amines for controlling the corrosion in condensate systems will appear in another post).

You can confirm the above by taking a sample of condensate water and measuring its pH and the concentration of iron in the sample.

Other issues that affect condensate water quality are related to the contamination of the condensate by the process.

Points to remember

  • Explain to the customer the mechanism of alkalinity breakdown in the boiler and the subsequent carbonic acid formation in the condensate
  • Take a sample of condensate and check the pH and iron concentration
  • In case part of the condensate is recovered, to be used as boiler feedwater, also check the iron concentration in the boiler feedwater. The iron in the condensate is likely to appear in the boiler feedwater analysis