Microbial Induced Corrosion (MIC) in Pipes

So how important is your water treatment in your chilled water and condenser pipes? Very important indeed. All cooling tower maintenance and operation shall be required to comply with AS/NZS3666.2 Air-handling and water systems of building – Microbial control Part 2: Operation and maintenance or AS/NZS3666.3 Air-handling and water systems of building – Microbial control Part 3: Performance-based maintenance of cooling water systems. It’s not only we have to ensure there is no risks of legionellae in our cooling towers and boilers, we have to also control the microorganisms that may be making our pipes their home colony.

The fundamental objectives for water treatment is to maintain operating efficiency and asset protection. Water treatment is always carried out by specialist contractors. Different specialist water treatment contractors may use different chemical composition and type, but the approach remained similar to control and minimise scale, fouling, corrosion and microbiological growth.

microbial induced corrosion — General view of the inner wall of the pipe.
Thick layer of biofilm more likely a combination of calcium, iron oxide and microorganism on the top layer.

microbacteria induced corrosion — General view of the inner wall of the pipe in another section.
Thick layer of biofilm more likely a combination of calcium, iron oxide and microorganism on the top layer.

microorganisms induced corrosion — Underneath the top layer.
A black localized layer was observed, a product of sulphate reducing bacteria; bacteria reaction and iron sulfide corrosion products.

microbial influenced corrosion — General view of the pipe after cleaning.
Corrosion here was not severe, and pitting are not aggressive yet.

microorganisms influenced corrosion — Corrosion pitting in the sample.
They lacked in any significant depth; however, there was still potential for this to develop and become more severe.

microbial influenced pitting corrosion — Cross section of the pit reveals the saucer shape cross section which is a typical microorganisms induced corrosion (MIC) pit shape.

microbial influenced corrosion pit — Microscopic view of a corrosion pit and corrosion product shows a strong bond, however a porous structure in the corrosion products.

microbial influenced corrosion microstructure — Microscopic view of the pipe microstructure in higher resolution(X400).

microbial induced pitting corrosion radiography — Radiography image of the pipe shows the distribution of corrosion pits. As it’s obvious they are not very dark, so their depths are not severe.

microbial induced pitting corrosion distribution radiography — Radiography image of the pipe shows the distribution of corrosion pits.

All effective corrosion controls requires cleaning and passivation of the pipe reticulate then maintaining the appropriate alkalinity with maintenance dosage of chemical corrosion inhibitors and biocides. When the initial cleaning is not carried out appropriately, it will compromise the passivation that protects the base metal, the overall effectiveness of the initial high dosage of inhibitors at start up will be compromised. It is because the chemical corrosion inhibitors will not be able to forms a complete protective film on the metal surfaces. It will then be more likely to allow microorganism induced corrosion to develop in the years that follows; even with the recommended dosage of corrosion inhibitors, scale inhibitors and biocides during the monthly maintenance.

What does it likely to mean when you see black particulate that leaks from the Victaulic joints? It may be that the condenser pipe works and mechanical joints (Victaulic) were compromised by MIC. When MIC started below the gaskets in the joints it could not be stopped. You will need to verify the following; If the Victaulic seals is compatible for use with the chemical treatment regime undertaken, incompatibility may be evident by the black particles on the pipe external due to the breakdown of the seals by water treatment and pipes aggravated by MIC.

How to mitigate and reduce MIC in existing pipeworks

Suitable chemical etch to clean the pipe internal surfaces, pressure clean to remove scale and biofilms, epoxy coated then passivise the pipes with high dosage of corrosion inhibitors and biocide. Record completed work with pipe camera visual inspections. Before any works is carried out, care and due diligent will need to be taken into account for the following 1. Not to compromise and deteriorate the Victaulic seals. 2. Not to degrade any pipes and equipment 3. Not to compromise or clog coils by fugitive debris created during the cleaning 4. Comply with statutory requirements on all corrective works. Note: It’s important to identify, assess, control and review risks associated with high pressure cleaning techniques. Alternatively; replace the entire condenser water reticulate if it has a more favourable and sustained cost benefits outcome compared with proposals mentioned above. Ultimately it pays to do it right the first time to avoid costly premature failures.

What is MIC

MIC or microorganisms induced corrosion is due to heterotrophic microorganisms such as sulfide reducing bacteria (SRB), iron reducing bacteria (IRB) and acid producing bacteria (APB) in the water. MIC is metal deterioration as a result of the metabolic activity of various microorganisms. This type of corrosion applies to non-metallic objects as well as metals. Microbiologically influenced corrosion has previously been referred to as biological corrosion and microbial corrosion.

MIC can take place on almost any metallic surface exposed to non-sterile systems, induced by the direct or indirect activities of microbiological organisms. Several bacteria are implicated in MIC such as sulfate-reducing bacteria (SRB), acid-producing bacteria (APB), iron-reducing bacteria (IRB) etc. These microorganisms and their metabolic products can form a biofilm layer on the steel. Moreover, their metabolic activities can facilitate the formation of an environment leading to accelerate the corrosion process of metals.

SRB are commonly considered the main culprits associated with anaerobic MIC due to the wide availability of sulfate (SO42−) in the aquatic environment. SRB’s are those bacteria that can live by oxidizing organic compounds or molecular hydrogen (H2) while reducing sulfate (SO42−) to hydrogen sulfide (H2S). In a sense, these bacteria breathe sulfate rather than oxygen in a form of anaerobic respiration. SRB’s, or their characteristic corrosion product FeS, are ubiquitously found on anaerobically corroded steel.

MIC Corrosion could be slowed by water treatment, adding biocides and corrosion inhibitors in the cooling water. However if there is a thick layer of biofilm in the pipe work, it is important to thoroughly remove the biofilms first and clean the pipe work otherwise the problems will persist.

Biofilm and deposits also increase friction resistance against the water which leads to increase energy consumption to operate the circulating pumps. These biofilms also protect underlying bacteria from biocides and biocide treatment. It is important to remember, when the biocide is become depleted and biofilm is removed, the system will become re-contaminated from the bacteria in the system. So the next step should be the bacterial removal process. It is important to re-test after the corrective action is carried out and then every few years to ensure the effectiveness of the chemical treatment and presence of bacteria in the water.

It is also important to review the maintenance program with the water treatment specialists and schedule specially the actions required when there is no circulation of water. When there is no water circulation, there is more chance of MIC formation. SRB, IRB and APBs can be detected by water analysis.

Microbial Induced Corrosion (MIC) in Pipes