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Underground Corrosion - Can Bacteria Cause Corrosion?

03.19.19   Robert C. Rabeler, PE | More by this Author

Underground Corrosion - Can Bacteria Cause Corrosion?

Can bacteria in the soil actually cause corrosion of buried metals?

The answer is yes! Under certain situations and conditions, bacteria can cause buried metals to corrode.

The most common type of bacterial corrosion is due to the presence of sulfate-reducing bacteria. These bacteria exist only under anaerobic conditions (no oxygen) and in the pH range of about 5.5 to 8.5. This bacteria reduce sulfates to sulfides in the presence of hydrogen or organic matter. Iron may aid in this reduction by supplying hydrogen which is usually adsorbed on the metal surface (hydrogen ions are deposited at the cathode during electrochemical corrosion). In essence, the bacteria act as depolarizers, hindering the formation of protective film on the metal thereby increasing the corrosion rate. Sulfate-reducing bacteria occur in soils when moisture, sulfates, organic and mineral matter are present and oxygen is absent, such as swamps and lowlands.

Certain aerobic (with the presence of oxygen) bacteria are also capable of causing corrosion. These bacteria create a condition where elementary forms of sulfur are oxidized and produce sulfuric acid. These bacteria develop best under acidic conditions. Pipelines which use sulfur compounds for sealing joints (such as cast-iron pipe joints) may be subjected to corrosion problems near the joints because of the sulfur oxidation by bacteria. Also, aerobic bacteria feeding on organic matter may cause the oxide film to be removed from the metal surface. This surface, not being protected, will then corrode.

Several methods can be used to protect against bacterial corrosion. Bituminous coatings in conjunction with cathodic protection have been used successfully. However, the amount of required current for the cathodic protection will likely be many times the normal current. Certain organic corrosion inhibitors as well as a number of oxidizing agents have also been used successfully to counteract anaerobic bacteria.

The soil can be tested for soluble sulfates to determine if the conditions are suitable for bacterial action. Also, a Redox test can be used to tell if the soil is aerobic or anaerobic. Testing for Redox can be problematic since soil samples exposed to air will result in a change in the Redox potential. A sample of soil can be tested for its oxidation-reduction potential by using electronic meters in conjunction with special Redox electrodes.



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