Steam and return line corrosion control

Main Up Neutralizing Amines Filming / Neutralizing Amines Why treat condensates

There are three main chemical programs to control corrosion in the condensate system, being 

The most common method of addressing carbonic acid attack is through the use of neutralizing amines. These chemicals, such as morpholine and cyclohexylamine neutralize the carbon acid, and increase the pH of the condensate. Corrosion of mixed metallurgy condensate systems is minimized when the pH is maintained between 8.8 and 9.2. 

Due to high alkalinity in boiler feedwater elevating the pH to this level may not be economical. In this case the pH should be maintained at 8.3 or higher, or a filming amine applied. A filming amine, such as octyldecylamine, provides a non-wettable protective barrier against both carbonic acid and oxygen. 

When utilizing a filming amine, the pH is usually maintained between 6.5 and 7.5, so a neutralizing amine may still be required. Additionally, filming amines can be difficult to apply and generate fouling of the system, particularly in systems where they have not been applied previously.

In order to minimize oxygen pitting one can utilize a filming amine as previously mentioned, or a volatile oxygen scavenger such as DEHA (diethylhydroxyamine.) In the authorís opinion utilizing DEHA provides cost attractive, but sometimes unstable or even unpredictable results as it scavenges oxygen and passivates the condensate system, making it less susceptible to corrosion.

But the final protection is related to the oxygen presence, which can vary extremelly, and is related to the presence of neutralizing amine, as long as the final reaction products are strong organic acids, like acetic acid, which can extremelly dangerous in the condensate low alkalinity system.

Depending on the treatment method chosen, condensate monitoring can vary. In all cases the following tests should be performed.

  1. Soluble and insoluble iron levels.
  2. pH levels at various points in your steam condensate system. It is extremely important that pH measurements be made on cooled ("unflashed") samples. If the sample is taken hot, carbon dioxide will flash, which results in artificially high pH measurements.
  3. Condensate corrosion coupons.

If a filming amine is utilized, the residual should be measured. The same is true if DEHA is used as an oxygen scavenger. In the latter case, a residual of 100 to 150 ppb is usually targeted. Note that this may take time (as much as 3 to 6 months) since much of the DEHA will be consumed passivating the system. These are some general guidelines.

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