You might wonder if SRB is the same as MIC. The short answer is: No, these terms do not refer to the same concept. Especially in oil and gas operations, the terms Sulfate-reducing bacteria (SRB) andMicrobially Influenced Corrosion (MIC) are used interchangeable. This gives you the idea that both terms refer to the same thing. However, SRB stands for the group of sulfate-reducing microorganisms, which is a broad and scientifically very well studied group of microorganisms. The term MIC on the other hand, refers to the broader concept of Microbially Influenced Corrosion. This term MIC is the abbreviation and definition of all mechanisms by microorganisms that lead to corrosion. SRBs represent only one group under this definition. There are many more organisms that can directly or indirectly accelerate corrosion processes. Below are some examples of groups of organisms that can cause or accelerate corrosion:
SOB Sulfide or Sulpher oxidizing bacteria (SOB)
This is a group of microorganisms that uses sulphide (S2−) or elemental sulfur (S) as their source of energy, thereby releasing sulfate (SO42-) into their environment. Within the sulfur cycle, thiosulfate (S2O32-) can play an important role as well, which can be oxidized by numerous sulfur oxidizing microorganisms. Both these intermediate acids such as Thiosulfate and end products such as sulfuric acid (H2SO4) can have a large impact on the corrosion process. They are capable to survive in acidic environments with a low pH. It’s important to realize that SRB can form a synergistic relationship with SOB; SOB uses sulfide that is produced by SRB, SRB use sulfate that is produced by SOB.
Iron reducing bacteria (IRB)
This group of bacteria uses oxidized iron (Fe3+) as a source of energy, thereby releasing dissolved (or reduced) iron (Fe2+) into their environment. Usually, these organisms are not a key driver of corrosion, asFe3+ is in many cases a corrosion product, but related to makeFe2+ available to other groups of organisms such as iron oxidizing bacteria, also in this case they can form a synergistic relationship. Through their microbial activity, they can enhance other corrosion mechanisms, by removingFe3+, they are basically removing corrosion products and accelerate the transfer of electrons.
Iron oxidizing bacteria (IOB)
Iron oxidizing bacteria use dissolved iron (Fe2+) as their primary source of energy, and they are releasing oxidized iron (Fe3+) into their environment. Similar to Iron reducing activity, it can be seen as a secondary process. It is required to have another source to let Fe0 dissolve into the solution. This source can be microbiological, like SRB’s, but this is not strictly required.
In order to convert Fe2+ intoFe3+, they require low concentrations of oxygen. By removingFe2+ from the metal surface, the electrochemical equilibrium at the surface becomes disturbed. As a consequence, more iron will come into solution. Metaphorically speaking, the IOB’s are pulling at the surface, enhancing other co-existing corrosion mechanisms. The oxidized iron (Fe3+) precipitates and is frequently noticeable as rust-colored particles (often in a slimy layer of microorganisms). Iron oxidizers are well known for their capabilities to cause precipitates and discoloration of water. These precipitates can result into blockages of pipelines, filter houses and pumps.
This group of microorganisms uses a variation of molecules such as carbon dioxide (CO2), hydrogen (H2) and acetate as a source of energy. As their name refers to, they are producing methane (CH4). These organisms are what they call strict anaerobes, which implies that they can only live under conditions without any oxygen.
Within scientific literature, the role of methanogens in the corrosion process has been debated for quite a bit. Within environmental systems, the byproducts of methanogenic activity can slow down corrosion processes. They have also been described to disturb the equilibrium at the cathode, thereby increasing the rate of corrosion. More recently, it has been proven that methanogens can utilize iron electrons directly from the surface of the metal, by extracellular processes. Recent studies have shown that a combination of both SRB and methanogens can result into very high corrosion rates in oil and gas related pipelines.
How to measure micro-organisms related to MIC?
Culture based methods have (for a long) time been the golden standard for the measurement of micro-organisms, related to MIC. Especially SRB’s culture kits are well known among the industry and are easy to use and do not require difficult sample logistics. It can be used by non-specialist at every location. The big disadvantage is that it takes a few days before you retrieve the results.
In those situations where you want to have fast results, the culture based method is not the most preferred method. For these instances, ATP is often used. ATP stands for Adenine triphosphate assays. Adenine triphosphate is a general energy carrier of life. You will measure generic biological activity, as both micro-organisms and higher organisms use this energy carrier. ATP assays are a common method to measure microbial activity. This method is quick and the sampling protocol is relatively straightforward, and can be used to measure general microbial activity at different sample sources like water and surfaces.
ATP is a reliable measurement assay for applications such as evaluation of biocide effectiveness. The disadvantage is that it does only provide a generic biological estimate and does not discriminate harmful and non-harmful MIC related organisms.
qPCR-Polymerase Chain reaction
The polymerase chain reaction measurement has become quite popular over the last years. It is a method that targets specific fragments of DNA and can thereby measure very specifically. Within the MIC mitigation masterclass, we have dedicated a lesson specifically about these more advanced molecular microbiological methods.
Around the world there are many different highly specialized laboratories that are capable to isolate the DNA from industrial samples and measure them with qPCR (or other) molecular tools. Usually these laboratories process your sample in 5 to 10 working days.
Another new possibility that has become available since last year, is a field detection kit. With this kit, you can measure DNA onsite with qPCR assays. This enables you to have enhanced accuracy over other field methods on the one hand and flexibility and speed on the other hand.
A perfect solution? We are providing our first opinion about the test kit at our MMM masterclass. You can subscribe for FREE and view this lesson.
The testkit that we evaulate during this masterclass session, is the MTEST-qPCR. You can find it here.