Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Free Carbon Dioxide

Last updated: June 22, 2018

What Does Free Carbon Dioxide Mean?

Free carbon dioxide is carbon dioxide that exists in the environment. It is present in water in the form of a dissolved gas. Surface water normally contains less than 10 ppm of free carbon dioxide, while some ground waters may exceed that concentration. Free carbon dioxide is a greenhouse gas believed to be responsible for global warming.

Carbon dioxide is found soluble in water. Corrosion is the principal effect of dissolved carbon dioxide. Carbon dioxide corrosion is frequently encountered in condensate systems and less commonly in water distribution systems. Carbon dioxide one of the main corroding agents in oil and gas production systems.


Corrosionpedia Explains Free Carbon Dioxide

Free carbon dioxide (CO2) in water exists naturally in varying amounts, but a large amount of carbon dioxide in water creates acidic water conditions. When carbon dioxide dissolves in water it yields carbonic acid (H2CO3).

Various mechanisms have been postulated for the CO2 corrosion process, but all involve either carbonic acid or the bicarbonate ion formed in the dissolution of CO2 in water as:

  • H2O + CO2 <====> H2CO3 <====> H+ + HCO3

The low pH resulting from this reaction also enhances the corrosive effect of oxygen.

Corrosion is the principal difficulty caused by carbon dioxide. With a decrease in pH, corrosive characteristics are induced in water, resulting in severe corrosion of heat exchangers, pipes, valves, etc.

Similarly, the corrosion rate of copper also increases along with the concentration of free carbon dioxide. The free carbon dioxide continuously provides water with H+ ions if the CO2 gas exists in the atmosphere. The production of H+ can assist in the reduction of dissolved oxygen, leading to the oxidation of copper.

Free CO2 in water can be easily dissipated by aeration. A two-column deionizer (consisting of a hydrogen-form strong-acid cation and a hydroxide-form strong-base anion) can also remove carbon dioxide from drinking water. The cation exchanger adds the hydrogen ion (H+), which shifts the above equation to the left in favor of water and carbon dioxide release. The anion resin removes the carbon dioxide by actually removing the bicarbonate ion.


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