Microbiologically Influenced Corrosion (MIC) is the corrosion of metals as a result of the metabolic activity of microorganisms. When bacteria are introduced, the already initiated corrosion will be accelerated due to the influence of the microorganisms. The bacteria that participate in MIC can be generalized into two types: aerobic (requires oxygen) and anaerobic (does not require oxygen). Some of the well-known bacteria that participate in MIC include acid-producing bacteria (APB) and sulfate reducing bacteria (SRB). SRB and ARB are anaerobic and two types of bacteria commonly found in oil and gas pipelines. MIC is one form of corrosion that contributes to substantial economic losses, and it is recognized as one of the major problems in the field and industries. Some of the affected industries include: chemical process industries, underground pipeline industries, on shore & off shore oil and gas industries, and water treatment industries, etc. It is clear that MIC is an important issue that requires intensive studying, and investigating effective ways to prevent MIC is crucial.
Since MIC are just a type of corrosion, general corrosion prevention techniques are a good starting point for the prevention of MIC. [1] There are also many corrosion prevention techniques that are designed specifically for MIC.
Physical Methods
Cleaning and Maintenance
One of the easiest, most straightforward methods to prevent MIC is to ensure the cleanliness of the pipeline. [1] Keeping the pipe system clean can effectively reduce the likelihood of bacterial growth and accumulation. Therefore, companies should conduct regular mechanical cleaning and maintenance of the pipeline. Flushing is a common method that is often used to clean pipelines. During flushing, water is injected into the pipelines at a high rate and is then allowed to be discharged freely at the pipe exit. Any particulate matter within the system will be removed from the pipe wall and exit with the flush water. Flushing will get rid of the biofilms accumulated as well as the fluid impurities which provide nutrients to the bacteria. Therefore, flushing can reduce bacterial growth and thus indirectly reduce the MIC corrosion rate. Due to the limited efficiency of flushing with water, flushing is often done with the use of fluids that have cleaning power or chemical agents that have the power to remove biofilm on the pipe wall. Although flushing is a straightforward and fairly effective technique, it requires that the pipeline cease operation for a period of time, which may be uneconomic. [1]
Filters
The use of liquid particulate and gas particulate filters within pipe systems is also common. As mentioned earlier, contaminants within pipe fluids provide nutrients for bacteria to populate. The introduction of bacteria will increase the rate of corrosion. Therefore, using filters to remove traces of dirt and other contaminants from the processed fluid will stunt the growth of bacteria and thus inhibit corrosion. Figures 1 and 2 below provide examples of liquid and gas pipeline filters used in industry. [1] Although filters reduce the rate at which contaminants enter the system, they are an added expense to the pipeline and cannot operate indefinitely. Eventually, filters will require replacement to properly function.
Figure1. Example of Liquid Particulate Filters, LL-142 [9]
Figure 2. Example of a gas particulate filter GP-146 [9]
Pipe Material
Using a special pipe material that is resistant to MIC is another prevention method. [1] For example, titanium [2] is such a metal that is highly resistant to MIC due to its relative chemical inertness compared to other potential pipe metals such as iron. However, the high cost, roughly 4.5 times the cost of a schedule 40 carbon-steel pipe, and the requirement of special fabrication methods are disadvantages of using titanium pipes. [3] An alternative non-metal that can be used is Polyvinyl Chloride (PVC). PVC also significantly limits the rate of MIC and has a lower cost compared to titanium, costing roughly 0.7 times as much as a schedule 40 carbon-steel pipe. [3] However, PVC pipes are structurally weaker than metal pipes and cannot be used in operations requiring extremely high pressure.
The figures below show examples of titanium and PVC piping.
Figure 3. Example of titanium pipes [10]
Figure 4. Example of PVC pipes [11]
Other Physical Methods
In addition to the techniques discussed above, some general corrosion prevention methods include applying protective pipe coatings and reducing pipe crevices, welds and joints. [1] Protective coatings apply the same principles as using a resistant pipe material. The coating is less expensive, but also less durable. Bacterial growth is more likely to occur at welds and joints, as these structures may contain areas of low fluid velocity. Thus, reducing the number of welds and joints therefore reduces the likelihood of bacterial growth.
Chemical Methods
Biocide
The use of biocide, chemicals targeted to kill biological organisms, to combat bio-corrosion is very common. In general, biocides are used to control the activity of bacteria in a system. The biocide treatment approach is recommended whenever bacteria are found to be present within a production system, since a single identified bacterium is a strong indication of large colonies of bacteria that already exist.
There are generally two types of biocides: oxidizing and non-oxidizing biocides. Oxidizing biocides oxidize protein groups within cells. This can deactivate the cell membrane and also disrupt cell metabolic processes, killing the microbe. The most commonly used oxidizing biocide is chlorine, due to its effectiveness on nearly all micro-organisms. Bromine and ozone are also commonly used in industry. Non-oxidizing biocides cover all biocides which operate through a different mechanism; some of these mechanisms include enzyme poisons which deactivate enzymes in a microbe, and toxicants which reduce the permeability of the cell membrane or cytoplasm (fluid that fills the cell) to stunt movement of nutrients and wastes in and out of the cell. Metals and metal complexes are examples of enzyme poisons, while ammonium and amine compounds are examples of toxicants. [6] Non-oxidizing biocides are reported to be more effective than oxidizing biocides for overall control of bacteria as they are more persistent and many of them are pH independent. [5] Combination of biocides are often used to maximize their effects. In addition, oxidizing and non-oxidizing biocides may be injected in alternating fashion to provide more complete coverage of targeted microorganisms and reduce biocide dosage. [6]
Listed in the table below are some specific examples of biocide compounds used in industry, their properties and usual concentrations [4]
New developments in biocides are due to a push to find greener, more environmentally friendly biocides that are still highly effective. These changes are due to new changes in legislation which aim to reduce environmental impact. Some traditional biocides such as chlorine have a significant negative environmental impact, and have now been restricted in their use. [5]
Although the use of biocide is a common MIC prevention technique, it is also the most misapplied chemical technique in the petroleum industry since they are used to combat a problem that is very delicate and difficult to detect. [7] Therefore, mistake such as overdosing and choosing the wrong biocide are common. To solve the problem concerning misuse of biocides, various oil and gas companies offer services to evaluate and test certain biocides. For example, DTI Oil&Gas Technical Team offer services to evaluate biocide performance. Their evaluation procedure includes: best in class tests, biocide efficiency assay, biocide regime optimization, as well as field test monitoring. The biocide treatment is optimized with the use of a monitoring program to monitor both the effectiveness of the biocide as well as the MIC rate. [8]
Biocompetitive Exclusion
Biocompetitive exclusion is a fairly new chemical treatment method targeted at stopping microbially influenced corrosion. In this process, chemicals are added to the pipeline fluid that promote the growth of benign bacteria. These benign bacteria compete for resources with corrosion-causing bacteria, and an increase in growth can displace the harmful bacteria. Injected chemicals are usually nutrients that can only be used by the benign microbes. In this process, it is necessary to identify the specific microbe species, both benign and malicious, present in the pipeline in order for this technique to be effective; otherwise, the injected chemical may have no effect or a negative effect on the bacterial species in the system. [5] Thus, biocompetitive exclusion also requires the use of an effective and accurate monitoring program. [8]
Monitoring Programs [7]
One important note is that initial and continuous care of the system is better than later prevention of MIC. Therefore, developing a simple monitoring program is also an effective technique to prevent MIC in the long run. A simple monitoring program includes but is not limited to the following aspects [7]:
Conduct periodic chemical analysis on the water/oil/gas that is transported through the pipeline
Sample and identify the microbial population in the pipe fluid
Identify the microbes that are attached to the tube wall
Detect low velocity and stagnation points in the system
Quantify the nutrient level of the pipeline fluid
Treat the pipeline with specific biocides at specific concentrations that are targeted to the highest population of bacteria to ensure the most efficient result
Monitor the bacteria population after every injection of the biocide
Periodically monitor the rate of MIC using electrochemical probes.
Monitoring programs are being enhanced through new methods of identifying microbes in biofilms. Identification of microorganisms through the use of molecular techniques examining bacterial RNA and DNA have been recent incorporated into the oil and gas industry. These molecular methods involve extracting DNA from the biofilm microbes, copying and sequencing the DNA, then comparing these DNA samples to database DNA sample obtained from known microbes grown in a controlled environment. DNA probes that bind to specific part of a target microbe's DNA can also be used to identify microbes that are not in the database. These new techniques allow for more accurate identification and quantification of bacterial species in the pipeline, allowing for the application of treatment methods more specific to the instance of corrosion. [5]
Prevention and Treatment
Microbiologically Influenced Corrosion (MIC) is the corrosion of metals as a result of the metabolic activity of microorganisms. When bacteria are introduced, the already initiated corrosion will be accelerated due to the influence of the microorganisms. The bacteria that participate in MIC can be generalized into two types: aerobic (requires oxygen) and anaerobic (does not require oxygen). Some of the well-known bacteria that participate in MIC include acid-producing bacteria (APB) and sulfate reducing bacteria (SRB). SRB and ARB are anaerobic and two types of bacteria commonly found in oil and gas pipelines. MIC is one form of corrosion that contributes to substantial economic losses, and it is recognized as one of the major problems in the field and industries. Some of the affected industries include: chemical process industries, underground pipeline industries, on shore & off shore oil and gas industries, and water treatment industries, etc. It is clear that MIC is an important issue that requires intensive studying, and investigating effective ways to prevent MIC is crucial.
Since MIC are just a type of corrosion, general corrosion prevention techniques are a good starting point for the prevention of MIC. [1] There are also many corrosion prevention techniques that are designed specifically for MIC.
Physical Methods
Cleaning and Maintenance
One of the easiest, most straightforward methods to prevent MIC is to ensure the cleanliness of the pipeline. [1] Keeping the pipe system clean can effectively reduce the likelihood of bacterial growth and accumulation. Therefore, companies should conduct regular mechanical cleaning and maintenance of the pipeline. Flushing is a common method that is often used to clean pipelines. During flushing, water is injected into the pipelines at a high rate and is then allowed to be discharged freely at the pipe exit. Any particulate matter within the system will be removed from the pipe wall and exit with the flush water. Flushing will get rid of the biofilms accumulated as well as the fluid impurities which provide nutrients to the bacteria. Therefore, flushing can reduce bacterial growth and thus indirectly reduce the MIC corrosion rate. Due to the limited efficiency of flushing with water, flushing is often done with the use of fluids that have cleaning power or chemical agents that have the power to remove biofilm on the pipe wall. Although flushing is a straightforward and fairly effective technique, it requires that the pipeline cease operation for a period of time, which may be uneconomic. [1]
Filters
The use of liquid particulate and gas particulate filters within pipe systems is also common. As mentioned earlier, contaminants within pipe fluids provide nutrients for bacteria to populate. The introduction of bacteria will increase the rate of corrosion. Therefore, using filters to remove traces of dirt and other contaminants from the processed fluid will stunt the growth of bacteria and thus inhibit corrosion. Figures 1 and 2 below provide examples of liquid and gas pipeline filters used in industry. [1] Although filters reduce the rate at which contaminants enter the system, they are an added expense to the pipeline and cannot operate indefinitely. Eventually, filters will require replacement to properly function.
Pipe Material
Using a special pipe material that is resistant to MIC is another prevention method. [1] For example, titanium [2] is such a metal that is highly resistant to MIC due to its relative chemical inertness compared to other potential pipe metals such as iron. However, the high cost, roughly 4.5 times the cost of a schedule 40 carbon-steel pipe, and the requirement of special fabrication methods are disadvantages of using titanium pipes. [3] An alternative non-metal that can be used is Polyvinyl Chloride (PVC). PVC also significantly limits the rate of MIC and has a lower cost compared to titanium, costing roughly 0.7 times as much as a schedule 40 carbon-steel pipe. [3] However, PVC pipes are structurally weaker than metal pipes and cannot be used in operations requiring extremely high pressure.
The figures below show examples of titanium and PVC piping.
Other Physical Methods
In addition to the techniques discussed above, some general corrosion prevention methods include applying protective pipe coatings and reducing pipe crevices, welds and joints. [1] Protective coatings apply the same principles as using a resistant pipe material. The coating is less expensive, but also less durable. Bacterial growth is more likely to occur at welds and joints, as these structures may contain areas of low fluid velocity. Thus, reducing the number of welds and joints therefore reduces the likelihood of bacterial growth.
Chemical Methods
Biocide
The use of biocide, chemicals targeted to kill biological organisms, to combat bio-corrosion is very common. In general, biocides are used to control the activity of bacteria in a system. The biocide treatment approach is recommended whenever bacteria are found to be present within a production system, since a single identified bacterium is a strong indication of large colonies of bacteria that already exist.
There are generally two types of biocides: oxidizing and non-oxidizing biocides. Oxidizing biocides oxidize protein groups within cells. This can deactivate the cell membrane and also disrupt cell metabolic processes, killing the microbe. The most commonly used oxidizing biocide is chlorine, due to its effectiveness on nearly all micro-organisms. Bromine and ozone are also commonly used in industry. Non-oxidizing biocides cover all biocides which operate through a different mechanism; some of these mechanisms include enzyme poisons which deactivate enzymes in a microbe, and toxicants which reduce the permeability of the cell membrane or cytoplasm (fluid that fills the cell) to stunt movement of nutrients and wastes in and out of the cell. Metals and metal complexes are examples of enzyme poisons, while ammonium and amine compounds are examples of toxicants. [6] Non-oxidizing biocides are reported to be more effective than oxidizing biocides for overall control of bacteria as they are more persistent and many of them are pH independent. [5] Combination of biocides are often used to maximize their effects. In addition, oxidizing and non-oxidizing biocides may be injected in alternating fashion to provide more complete coverage of targeted microorganisms and reduce biocide dosage. [6]
Listed in the table below are some specific examples of biocide compounds used in industry, their properties and usual concentrations [4]
New developments in biocides are due to a push to find greener, more environmentally friendly biocides that are still highly effective. These changes are due to new changes in legislation which aim to reduce environmental impact. Some traditional biocides such as chlorine have a significant negative environmental impact, and have now been restricted in their use. [5]
Although the use of biocide is a common MIC prevention technique, it is also the most misapplied chemical technique in the petroleum industry since they are used to combat a problem that is very delicate and difficult to detect. [7] Therefore, mistake such as overdosing and choosing the wrong biocide are common. To solve the problem concerning misuse of biocides, various oil and gas companies offer services to evaluate and test certain biocides. For example, DTI Oil&Gas Technical Team offer services to evaluate biocide performance. Their evaluation procedure includes: best in class tests, biocide efficiency assay, biocide regime optimization, as well as field test monitoring. The biocide treatment is optimized with the use of a monitoring program to monitor both the effectiveness of the biocide as well as the MIC rate. [8]
Biocompetitive Exclusion
Biocompetitive exclusion is a fairly new chemical treatment method targeted at stopping microbially influenced corrosion. In this process, chemicals are added to the pipeline fluid that promote the growth of benign bacteria. These benign bacteria compete for resources with corrosion-causing bacteria, and an increase in growth can displace the harmful bacteria. Injected chemicals are usually nutrients that can only be used by the benign microbes. In this process, it is necessary to identify the specific microbe species, both benign and malicious, present in the pipeline in order for this technique to be effective; otherwise, the injected chemical may have no effect or a negative effect on the bacterial species in the system. [5] Thus, biocompetitive exclusion also requires the use of an effective and accurate monitoring program. [8]
Monitoring Programs [7]
One important note is that initial and continuous care of the system is better than later prevention of MIC. Therefore, developing a simple monitoring program is also an effective technique to prevent MIC in the long run. A simple monitoring program includes but is not limited to the following aspects [7]:
Conduct periodic chemical analysis on the water/oil/gas that is transported through the pipeline
Sample and identify the microbial population in the pipe fluid
Identify the microbes that are attached to the tube wall
Detect low velocity and stagnation points in the system
Quantify the nutrient level of the pipeline fluid
Treat the pipeline with specific biocides at specific concentrations that are targeted to the highest population of bacteria to ensure the most efficient result
Monitor the bacteria population after every injection of the biocide
Periodically monitor the rate of MIC using electrochemical probes.
Monitoring programs are being enhanced through new methods of identifying microbes in biofilms. Identification of microorganisms through the use of molecular techniques examining bacterial RNA and DNA have been recent incorporated into the oil and gas industry. These molecular methods involve extracting DNA from the biofilm microbes, copying and sequencing the DNA, then comparing these DNA samples to database DNA sample obtained from known microbes grown in a controlled environment. DNA probes that bind to specific part of a target microbe's DNA can also be used to identify microbes that are not in the database. These new techniques allow for more accurate identification and quantification of bacterial species in the pipeline, allowing for the application of treatment methods more specific to the instance of corrosion. [5]
[1] Rob. (2010, June), Microbiologically induced corrosion prevention and analysis [Online]. Available:http://failure-analysis.info/2010/06/microbiologically-induced-corrosion-prevention-and-analysis/
[2] J. A. Mountford. (2002) Titanium - properties, advantages and applications solving the corrosion problems in marine service [Online]. Available: http://www.ticotitanium.com/wp-content/uploads/2009/11/paper02170.pdf
[3] The Engineering Toolbox. Piping Materials Cost Ratios [Online]. Available: http://www.engineeringtoolbox.com/piping-materials-cost-ratios-d_864.html
[4] M.E.Akpan, (2011), Inhibition and Control of Microbiologically Influenced Corrosion in Oilfield Materials [Online]. Available: http://www.medwelljournals.com/fulltext/?doi=erj.2011.59.65
[5] H.A. Videla and L.K. Herrera. (2005). Microbiologically influenced corrosion: looking into the future [Online]. Available: http://scielo.isciii.es/pdf/im/v8n3/04%20Videla.pdf
[6] ChemTreat. Biological Control [Online]. Available: http://www.chemtreat.com/solutions/chemical-treatment-programs/cooling-tower-water-chemicals/biological-control/
[7] N.Muhukumar. (2013, Sept), Microbilogically Influenced Corrosion In Petroleum Product Pipelines-A Review [Online]. Available: http://nopr.niscair.res.in/bitstream/123456789/17162/1/IJEB%2041%289%29%201012-1022.pdf
[8] T. Lundgaar. Evaluation of Biocide Performance [Online]. Available: http://www.dti.dk/services/microbiology-management/products/23602,4
[9] Clean Energy System Co.Ltd. Process Filteration [Online]. Available:http://www.cleanenergy.co.th/Process%20Filtration.html
[10] Shree steel India. Titanium Seamless Welded Pipe [Online]. Available:http://www.shreesteelindia.com/material-stock-available/pipes-seamless-welded-ibr-erw-pipe-tubes/titanium_pipes.html
[11] Allegro Central Vacuum System. 2 inch PVC central vacuum pipe [Online]. Available:https://www.allegrovacuums.com/white-2-inch-central-vacuum-pvc-pipe-box-of-80-foot-zic005-80.html