Condensate Corrosion

The two primary corrosive agents in condensate systems are carbon dioxide and dissolved oxygen. The first of these, carbonic acid attack, will be discussed in this technical tip. Oxygen pitting of the condensate system will be discussed in subsequent technical tips.

The most common cause of corrosion in the condensate system is due to Carbon dioxide (CO2). Natural alkalinity enters the boiler water in the boiler feedwater. The thermal breakdown of carbonate alkalinity in the boiler produces CO2 and hydroxide alkalinity. The reactions are as follows:

2NaHCO3 + Heat => Na2CO3 + CO2 + H2O (1)

Na2CO3 + H2O + Heat => 2 NaOH + CO2 (2)

The NaHCO3 in water is the bicarbonate alkalinity and is a major portion of the M-Alkalinity test. The first reaction nearly goes to completion. The carbon dioxide leaves with the steam in the boiler, because it is extremely volatile. At various points in the condensate system the carbon dioxide becomes soluble forming carbonic acid.

CO2 + H2O => H2CO3 => H+ + HCO3-

It requires very little carbon dioxide to lower the pH in the condensate. This is because of the high purity of the water, which means low buffering capacity of the forming acid. The following is a table that shows how much carbon dioxide is needed to reduce the pH in pure water:

As you can see by the table 1 ppm of CO2 will reduce the pH below neutral.

Once the carbonic acid has formed it becomes aggressive to iron and copper in the condensate system. The corrosion reaction for iron is shown below:

2H2CO3 + Fe => Fe(HCO3)2 +H2

The resulting Fe(HCO3)2 is soluble and as such is removed by the condensate leaving behind nothing to protect the metal surface. Carbonic acid reveals itself as a general loss of metal. This takes the form of thinning of the metal on the lower diameter of the pipe. A corrosion problem in the condensate system usually first shows up as thinning of the pipe at threaded fittings and the downstream side of steam traps where abrupt pressure changes are present.

Neutralizing Amine

The most common way of treating a condensate system is through the use of Neutralizing Amines. Neutralizing amines are volatile and enter the steam system in the same manner that steam does. There are two ways that neutralizing amines reduce corrosion in a steam system. First it neutralizes the acids and second elevates the pH into the basic range. This helps to stabilize and protect the magnetite layer on the steel surface.

The pH control range in a softened makeup boiler system is from 7.5 to 8.5; although a pH above 8.3 is recommended. In high purity water systems with a mixed metallurgy a pH range of 8.8 to 9.2 is generally recommended.

There are several physical properties that determine the effectiveness of a neutralizing amine. These include:

• Neutralizing capacity
• Basicity
• Distribution ratio
• Thermal stability

The neutralizing capacity of the amine is the ability to reduce acids in the condensate system. A lower value for the neutralizing capcity indicates a greater ability to neutralize acids, thus reducing the amount of amine required. Basicity is the base strength of an amine and determines how well the amine will raise the pH of the condensate after all acids have been neutralized. The amines with larger base strength’s have a greater ability to raise pH. 

The distribution ratio of a neutralizing amine determines where in the condensate system the amine will be effective. It is the simple relationship between the amount of amine in the steam phase and the condensate phase.

 

Distribution Ratio

=

ppm Amine in Steam ppm Amine in Water

An amine with a low distribution will tend to drop out at the initial condensate sites, where as a high distribution will remain in the steam phase further out into the steam system.

In most cases a blend of amines is the best approach. Choosing the correct amines to use will be dependent upon the size of the condensate system. For very small boiler systems a product with only a low distribution ratio maybe all that is necessary. For a large condensate system that may spread across many buildings or processes may require a blended product that has a greater percentage of an amine with a high distribution ratio.

Thermal stability is an important consideration especially when considering what the molecule will breakdown into. Some amines may breakdown into ammonia. If significant levels of ammonia build up in a condensate system that has extensive copper or bronze, rapid corrosion will result of both iron and copper. The copper migrates to the iron and forms galvanic corrosion cells which liberates iron.