From microbial corrosion rates to cathodic protection strategy
MICBUSTERS presented a harmonised review of corrosion rates associated with different microbial functions and discussed what these differences mean for the management of cathodically protected buried pipelines.
During the 70th edition of the CEOCOR International Congress and Technical Exhibition, held from 2 to 5 June 2026 aboard the historic ss Rotterdam, MICBUSTERS presented its literature review on microbiologically influenced corrosion, or MIC.
The presentation addressed a central question: should MIC be treated as a separate corrosion entity within corrosion management and mitigation?
To explore this question, we compared published corrosion rates associated with different microbial groups and functions. We then translated those findings into practical considerations for cathodic protection, corrosion monitoring and field sampling on buried pipelines.
CEOCOR provides an important platform for specialists in external corrosion, coatings, electrical interference and cathodic protection. More information about the organisation, its commissions and its technical activities can be found on the official CEOCOR website.
Why harmonising reported corrosion rates matters
Published MIC studies use different exposure periods, materials, media, temperatures, flow conditions, microorganisms and methods for calculating corrosion rates. Some studies report average mass loss, while others focus on the deepest pit or another localised feature.
Direct comparison is consequently difficult. To make the literature more interpretable, the reported results were organised using the corrosion-rate classifications from AMPP SP0775.
Average corrosion rate
Usually calculated from coupon mass loss and converted into an average metal thickness loss per year.
Localised corrosion rate
Based on the deepest measured pit or localised feature divided by the exposure period.
Separate interpretation
Average and localised rates must be classified independently. Severe pitting can coexist with moderate average metal loss.
| SP0775 classification | Average coupon rate | Maximum pitting or localised rate |
|---|---|---|
| Low | <0.025 mm/year | <0.13 mm/year |
| Moderate | 0.025–0.12 mm/year | 0.13–0.20 mm/year |
| High | 0.13–0.25 mm/year | 0.21–0.38 mm/year |
| Severe | >0.25 mm/year | >0.38 mm/year |
These classifications are useful for organising coupon and experimental data. They are not, by themselves, fitness-for-service criteria or pipeline design values.
Different microbial functions produce different risk profiles
The review demonstrated that the generic label “MIC organisms” hides important functional differences. Some groups mainly contribute to deposit formation, chemical cycling or biofilm development. Other organisms have metabolic or extracellular electron-transfer mechanisms that can directly accelerate metal oxidation.
| Microbial function or group | Representative literature picture | Practical interpretation |
|---|---|---|
| Organotrophic sulfate-reducing bacteria | Reported average rates range from negligible to severe, with literature examples reaching approximately 0.4 mm/year. | The presence of SRB alone does not establish a corrosion rate. Electron donors, sulfide chemistry, deposits, biofilm structure and local conditions remain decisive. |
| Acid-producing and fermentative microorganisms | Representative average rates fall mainly within moderate to high classifications. | Acid generation and biofilm metabolism can create highly aggressive local conditions, even where bulk-fluid measurements appear acceptable. |
| Thiosulfate-reducing microorganisms | High average corrosion and severe localised attack have been reported in specific experimental systems. | Sulfur intermediates should not be assessed only through conventional sulfate-reducer monitoring. |
| Lithotrophic, micC-associated sulfate reducers | Reported average rates extend from low to severe, with severe localised corrosion in several studies. | Functional markers related to direct extracellular electron transfer can provide more relevant information than broad taxonomic detection alone. |
| micH-associated methanogenic archaea | Severe average and localised rates have been reported in selected laboratory and biofilm systems. | Methanogens should not automatically be considered harmless. Mechanistic biomarkers are needed to distinguish potentially corrosive populations from general methanogenic presence. |
| Sulfur-oxidising nitrate-reducing bacteria | Moderate to severe average corrosion and severe localised attack have been reported. | Nitrate treatment can change microbial competition and sulfur cycling. Its effect should be verified rather than assumed. |
| Iron- and manganese-cycling biofilms | Deposit formation, ennoblement and localised corrosion are documented, although comparable group-wide datasets remain limited. | Deposits can create differential aeration cells and interfere with the electrochemical and microbiological conditions at coating defects. |
The reported figures are representative results from heterogeneous literature. Short-duration pit-depth equivalents can be useful severity indicators, but they should not be extrapolated directly as long-term pipeline penetration rates.
What does this mean for cathodic protection?
Cathodic protection remains one of the principal measures for controlling external corrosion at coating defects. However, the literature review indicates that MIC should be incorporated more explicitly into the way CP performance is assessed, particularly where biofilms, deposits and groundwater create locally variable conditions.
1. There is no universal “MIC CP potential”
The presence of a microbial group does not automatically justify changing a CP setpoint. Protection criteria still need to be evaluated using recognised electrochemical principles and applicable standards. Microbiological information should strengthen the risk assessment—not replace CP measurements.
2. CP effectiveness must be considered at the coating defect
Bulk pipe-to-soil measurements may not fully represent the conditions beneath deposits, within disbonded coatings or at individual coating defects. Biofilm growth, soil resistance, local chemistry and deposit accumulation can all influence current distribution and the interpretation of potential measurements.
3. Localised corrosion must not be averaged away
MIC is frequently associated with localised attack. A relatively modest average coupon rate can therefore coexist with a much more serious pit-growth rate. CP monitoring programmes should be integrated with coating-defect information, inspection data, corrosion morphology and representative microbiological samples.
4. Microbial functions are more informative than generic counts
Total bacterial numbers or a general SRB result do not explain which corrosion mechanism is active. Functional targets associated with sulfate reduction, sulfur oxidation, acid production, methanogenesis and extracellular electron transfer can provide a more focused assessment.
Our article From mechanisms to field practice: micH and micC biomarkers explains how mechanistic biomarkers can help distinguish microbial presence from a potentially corrosive function.
5. CP and microbiology should be trended together
A stronger monitoring strategy combines CP data with coating condition, soil or groundwater chemistry, corrosion measurements and microbial-function data. This makes it possible to investigate whether changes in microbial populations coincide with CP interruptions, seasonal groundwater changes, excavation activities or changing conditions at coating defects.
Sampling remains the critical first step
Even the most advanced analytical method cannot compensate for an inappropriate sample. For buried pipelines, this means that the exact sampling location, excavation conditions, coating defect, corrosion product, biofilm layer, soil interface, groundwater and sample-preservation procedure all matter.
A sample of nearby bulk soil or water may describe the wider environment, but it may not represent the microorganisms active directly at the metal surface. Samples should therefore be collected in a way that preserves the spatial relationship between the pipeline, the coating defect, the corrosion product and the surrounding environment.
More background on this multiple-lines-of-evidence approach is available in:
Related MICBUSTERS articles
Thank you to the CEOCOR community
We would like to thank the organisers, technical committees, speakers, moderators and volunteers for the considerable effort required to organise the congress. The setting in Rotterdam created valuable opportunities for open technical discussion between CP specialists, pipeline operators, researchers, suppliers and microbiology experts.
MICBUSTERS was pleased to contribute to the technical programme and to support the broader environment in which this exchange of knowledge could take place. The quality of the discussions demonstrated why collaboration between corrosion engineering, electrochemistry and microbiology is essential for improving pipeline integrity management.
Would you like to assess MIC alongside your CP data?
MICBUSTERS combines field sampling, functional qPCR biomarkers and corrosion interpretation to help asset owners identify relevant microbial processes and prioritise further investigation.
Discuss your pipeline applicationThis article is intended for informational and educational purposes only and does not replace project- or site-specific engineering or scientific assessment. MICBUSTERS has a commercial interest in MIC monitoring solutions, including an on-site qPCR kit.
MICBUSTERS specializes in measuring microbiological processes that lead to the degradation of metals.
Corrosion rates reported in scientific studies depend on experimental conditions, materials, exposure periods, calculation methods and microbial communities. They should not be used directly as pipeline design rates, remaining-life calculations or cathodic-protection criteria. Implementation of mitigation or CP measures remains the responsibility of the asset owner and their appointed corrosion specialists.