The presence of carbon dioxide and oxygen in the condensate are a serious problem.
The gasses enter the system from either from leaks or thermal decomposition of carbonates and bi-carbonates in the boiler water and the carry over from dissolved oxygen in the feed.

The heat and pressure of the boiler break down the alkalinity in the boiler water to form carbon dioxide gas CO₂. Leaving the boiler with the steam it travels throughout the plant supply system. When the steam condenses, the carbon dioxide dissolves in it to form carbonic acid. This reaction is chemically expressed as:

H₂O + CO₂  =   H₂CO₃

which is a weak acid and drops the pH of the condensate.

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.

Fe(HCO3)2 can react with oxygen to form iron oxides thus freeing the carbon dioxide for further attack.

This acid depresses the condensates pH and causes corrosion to take place.  This corrosion appears as grooving or gouging in the bottom of steam headers or condensate return lines.  Most often it weakens pipe walls at threaded joints and the resultant metal loss can lead to large amounts of copper and/or iron being returned to the boiler to cause troublesome deposits.  

Oxygen, as in the boiler system, can cause localized attack in the form of pitting when present in the condensate system.  This type of corrosion can generally cause equipment to fail more quickly than the generalized corrosion caused by carbonic acid attack due to it concentrating in a small area. Oxygen can infiltrate the system from open condensate receivers, poor deaeration or leaky siphons.

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