microbiological corrosionReading time:
For some time, microbiological corrosion has been the subject of in-depth research. It can be defined as the interaction between micro-organisms and materials causing the latter to corrode or, more frequently, an acceleration of existing corrosion mechanisms. It is often seen as a rapid localised attack and can cause premature component breakage.
Microbiological corrosion can be seen in a number of cases. The most common of these are described below.
sulphur reducing bacterias
Corrosion by sulphur reducing bacteria constitutes one of the best known forms of microbiological corrosion found in industrial cooling systems and in wastewater transport networks. The class of bacteria responsible for this corrosion includes Desulfovibrio, Desulfomonas and Desulfomaculum. The special feature of anaerobic bacteria is that they metabolise sulphates and sulphites to form sulphides.
There is disagreement on the precise mechanism behind the action exercised by this corrosion. However, it is thought that it involves depolarisation (acceleration) by eliminating hydrogen from cathodic sites. The presence of sulphide ions in a corrosion site is revealed by their odour (acidification). This presence is an indicator of the role played by sulphur reducing bacteria in the corrosions found.
A great many bacteria produce mineral or organic acids as part of their metabolism. The ensuing reduced pH brings into play chemical processes that accelerate corrosion. The Thiobacillus, Thiooxidans and Clostridium genuses have often been linked to microbiological corrosion in steel. Thiobacillus and Thiooxidans have the property of oxidising sulphur compounds to form sulphuric acid whereas Clostridium produces organic acids (lowering pH and complexing action on iron).
iron and manganese bacteria
This type of bacteria includes Gallionella, Sphaerotilus, Crenothrix and Leptothrix. These bacteria oxidise endogenous dissolved ferrous ions (Fe2+) to the ferric state (Fe3+). These bacteria contribute to corrosion by encouraging corrosion beneath deposits. Nevertheless, it is important to remember that many other factors, such as chlorination and dissolved oxygen, can cause or promote the oxidation of reduced metal ions.
bacteria forming biofilms
Many bacteria species form biofilms. The latter are mainly formed by water bonded to extracellular polymers. These biofilms can grow rapidly by trapping other material: colloids and other debris.
Corrosion develops beneath these biofilms :
- as it would under any deposit through differential aeration;
- by combining with other corrosive anaerobic bacteria;
- the stainless steel potential can increase until it reaches the transpassivity zone.
Algae only proliferate when sufficient light is present. Under favourable conditions, algae can form dense, fibrous mats that block channels and create conditions encouraging anaerobic bacteria to develop on the underside of the mat. These thick biomasses may be extremely difficult to kill with biocides. As the dead algae rot, they produce corrosive organic acids. When an algae mat dies, it may break down into large fragments, causing downstream blockages.
The simplest and most effective way of combating algae proliferation consists in designing equipment in such a way that there are no light sources available to the algae.
Nitrifying bacteria represent a group of micro-organisms that, through their metabolism, can convert ammonia (NH3) or nitrites (NO2-) into nitrates (NO3-). The best known of these bacteria are Nitrosomonas and Nitrobacter.
These organisms contribute to corrosion by lowering the pH associated with the conversion of ammonia into nitric acid.
It can be extremely difficult to attribute, with certainty, damage or breakage to microbiological corrosion. The mere presence of potentially corrosive bacteria does not prove that they are responsible for the corrosion found. Any conclusive diagnostic must be based on the presence of four factors :
- presence of micro-organisms or their by-products;
- specific morphologies of microbiological corrosion;
- specific deposits and corrosion products;
- compatible environment.
For additional information on this topic, please refer to the article by H.M. Herro and R.D. Port, quoting numerous examples, and to be found in The Nalco Guide to Cooling Water Systems Failure Analysis.