When microorganisms play a role in materials corrosion, it is known as microbiologically influenced corrosion (MIC). A most significant challenge faced by corrosion professionals in identifying MIC is connecting the microorganisms found in a system to the corrosion experienced by that system as the presence of microorganisms alone does not necessarily mean they are the cause of the localized corrosion.
There are usually multiple factors in any given environment that contribute to corrosion, and different corrosion mechanisms, including MIC, can result in the same corrosion structure. Therefore, it is critical to verify a clear relationship between the biofilm and corrosion.
To properly diagnose MIC, investigations should include a combination of chemical, metallurgical, and microbiological analyses.
The detection, testing, and evaluation of MIC on internal pipeline surfaces is outlined in NACE International Standard TM0212-2012, which also provides guidelines on sample collection, testing methods, corrosion monitoring, as well as how to relate results to MIC.
Below are the criteria for conducting sampling to diagnose MIC as prescribed in Standard TM0212-2012:
• Sampling programs generally collect information on operating conditions, corrosion rates, and microbiological conditions over a period of time in order to identify any trends in the findings.
• Also, due to normal statistical variations associated with sample collection and microbiological testing, more reliable data typically result from performing tests on samples representing a time range rather than any single sample.
• Data collection and analysis should focus on differentiating the effects of biotic and abiotic factors on the likelihood, location, cause, and severity of internal corrosion.
• Samples of bulk fluids from pipelines are often collected to identify and quantify levels of planktonic (moving) microorganisms.
• To ensure that test results are relevant to MIC, data from bulk fluids should be correlated with other pipeline data, including liquid composition, operational conditions, sessile (attached) microorganism counts, and corrosion data.
• Since both corrosion and microbiological activity may occur directly on the internal surfaces of a pipeline, samples that are representative of these surfaces may provide significant information and should be collected, whenever possible, in conjunction with each liquid sample retrieved.
Richard B. Eckert, senior principal engineer—corrosion management, Materials Advisory Services, with DNV GL—North America Oil & Gas and chair of NACE Task Group (TG) 254, which published the standard, notes that it can be very difficult to infer what is happening on the pipeline’s internal surface based solely on the composition of the bulk fluid because there isn’t always a clear relationship between what is found on the surface and what comprises the bulk fluid phase.
The microorganisms living in the bulk fluid (types and quantities of microbes), and even the chemical composition of the bulk fluid, can be far different than what is found on the internal pipe surface—that’s been well established in research.
Reference & Note: NACE TM0212-2012, “Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion on Internal Surfaces of Pipelines” (Houston, TX: NACE International, 2012). The methods presented in this standard /article represent the consensus of industry experts in pipeline corrosion and microbiology at the time this standard was published. See TM0212-2018-SG for 2018 revision.
Find microbiologically influenced corrosion resources here.