food grade product line by Water Services Ltd

Attack of condensate and steam lines

Causes of condensate corrosion.

Effects of condensate corrosion

The effects of condensate corrosion are either leaks or pipe blockage.
Threaded joints are most susceptible to leaks as the metal has been thinned and stressed as a result of thread cutting.
Horizontal pipes are more susceptible to attack than vertical ones.

Carbon Dioxide
 

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.

Dissimilar metals

Where dissimilar metals are in contact with one another galvanic corrosion will occur forming deposits downstream of the corrosion site, this leads to restricted flows in feed lines economizers and other equipment.
If the condensate becomes contaminated with metallic compounds as a result of corrosion in the condensate corrosion will occur.
Failure of boiler tubes can result if this deposition takes place on heat transfer surfaces.

On / off operation

 Considered a process steam consumption operating under on/off conditions. During normal operation (on) presents a corrosion rate of 2 mpy and produces dissolved iron as corrosion result of 0.05 ppm.

If operation turns to off, steam is condenced, pressure drops, and oxygen enters to the system as to brake the void.

Corrosion rate increases extremelly, and when system turns again to on extremelly high iron return is noticed.

During the off operation the deterioration of the system is very fast.