Sulfate Reducing Bacteria
Sulfate reducing bacteria are responsible for significant corrosion of equipment. The two most common species of these bacteria, known scientifically as Desulfovibrio desulfuricans or Desulfotomaculum nigrificans, can be the cause of severe pitting corrosion – even on stainless steel components.
The growth of these organisms is highly favored by an anaerobic (oxygen free) environment. Desulfovibrio is the most common sulfate-reducing organism that can obtain its energy predominantly from the anaerobic reduction of sulfates. These adaptable organisms can be found in fresh water, brackish water, polluted water and in soils that are waterlogged and rich in organic materials. Often the first signs of the presence of this organism is a blackening of the water by the sulfides formed and perhaps the “rotton egg” smell of hydrogen sulfide. Desulfovibrio may also use sulfite, thiosulfite or tetrathionates as an alternative nutrient source to sulfates.
Another sulfate reducing bacterium, Desulfotomaculum nigrificans, is a spore forming, thermophilic (heat loving) microorganism also capable of reducing sulfate under anaerobic conditions. As spore forming organisms, these bacteria are capable of survival as dormant spores through very unfavorable environmental conditions. This microorganism is common in the canning industry where it may cause spoilage of canned goods.
Even though the growth of sulfate-reducing bacteria is favored by anaerobic environments, it is possible for them to flourish in strongly aerated systems by growing within or under deposits or other growths in this microenvironment. Because of this, problems with sulfate-reducing bacteria are accentuated when there are concurrent fouling problems (algae or oil, for example).
In nature many kinds of microorganisms grow in association with sulfate reducers. This association exists for their mutual benefit. The sulfate reducers depend upon the activities or death of the other organisms for necessary organic materials. Other organisms can create conditions facilitating anaerobic growth or, as with some aerobic bacteria, may provide simple organic acids that will promote the growth of sulfate-reducing bacteria.
The Corrosion Reaction
When sulfate-reducing bacteria are present in a system the anode of a galvanic cell is formed:
2H++ + SO4– + 4H2 Bacteria> H2S+4H2O
Fe++ + H2S Æ FeS + 2H+
Fe° Fe++ + 2e-
The surface of metal, free of sulfate-reducing bacteria, becomes the cathode of the galvanic cell:
2H2O Æ 2H+ + 2OH-
2H+ + 2e- Æ H2
The free electrons thus formed, migrate to the cathode and react with the hydrogen ions to form gaseous hydrogen. The free ferrous ions then react with the H2S formed at the anode to form free hydrogen ions and ferric sulfide.
Hydrogen ions thus formed may either migrate to the cathode or react with hydroxyl groups to form water. The free ferrous ions must react since they cannot remain in the free state. Ferric sulfide formed at the anode will act cathodically and release more ferrous ions.
Many field analyses of deposits may not show the presence of sulfides but will show other materials such as metal oxides.
Detection of Sulfate Reducers:
If a system is suspected of having a microbiological corrosion problem, samples of deposits and slimes should be collected and analyzed for the presence of sulfate-reducers. Since these organisms are associated with anaerobic environments, samples should be kept in full containers having no free air space.
There are several methods for detection and quantification of the infection of a system by this organism. However, the number of sulfate-reducing bacteria detected by culture techniques appears to show little correlation with the amount of damage that may be caused by these organisms. This localized damage is caused by pockets of these organisms concentrated under other foulants, in very small areas, on the surface of equipment. The number of bacteria that are freed from these foulants to the flowing water is random and therefore, the analysis of the water may have no correlation to the actual infection and impending damage to a system.
Control of Sulfate Reducers
Control of sulfate reducers requires not only a complete biocide program but also the addition of antifoulants to reduce the fouling in a system, thereby eliminating the under-deposit anaerobic conditions that so favor these organisms. Dosage and types of biocides and antifoulants will be determined by your system’s condition and the quality of the make-up water. The treatments must be monitored continually to ensure the proper feed rate and that they are maintaining cleanliness of your system.
All equipment should be given regular visual inspections for cleanliness and routine microbiological analysis should be performed to stop the damage that can be done by this organism, before it happens.