Internal corrosion in pipelines is a complex and dynamic process influenced by chemical, physical, and biological factors. Among these, microbiologically influenced corrosion (MIC) is a significant threat, often responsible for up to 40% of internal corrosion pipeline failures. Two key groups of bacteria involved in MIC are Sulfate-Reducing Bacteria (SRB) and Acid-Producing Bacteria (APB). This article provides a detailed overview of what SRB and APB testing entails, how field sampling and testing are conducted, and how to interpret the results to support effective corrosion management.

These bacteria don't act in isolation; they interact with pipeline fluids, deposits, and operational conditions to accelerate metal corrosion. By knowing the basics of testing for these microorganisms, technicians can contribute to more accurate risk assessments and targeted mitigation strategies.

At its core, microbiologically influenced corrosion involves microorganisms that alter the local environment in ways that promote corrosion. SRB and APB are among the most common culprits in pipeline systems, particularly in environments where water accumulates or flows are low. Let's break down each group.

Sulfate-Reducing Bacteria (SRB) are anaerobic microorganisms, meaning they thrive in oxygen-deprived conditions. They reduce sulfate ions (SO₄²⁻) present in pipeline waters to hydrogen sulfide (H₂S), a highly corrosive gas. This process, known as sulfate reduction, follows the general reaction: SO₄²⁻ + 4H₂ → H₂S + 2H₂O + 2OH⁻. The H₂S produced can react with iron in the pipeline steel to form iron sulfide (FeS), which often appears as black, slimy deposits. These deposits not only protect the bacteria but also create localized anodic sites, leading to pitting corrosion. SRB are particularly problematic in stagnant areas, such as under biofilms, in dead legs, or beneath sediment layers where oxygen is excluded.

In practical terms, SRB contribute to "sour" corrosion, which is characterized by the presence of H₂S and can embrittle pipeline materials. Standards like NACE TM0194 (Field Monitoring of Bacterial Growth in Oil and Gas Systems) highlight SRB as a primary indicator of MIC, emphasizing their role in accelerating corrosion rates that might otherwise be minimal under purely chemical conditions.

Acid-Producing Bacteria (APB), on the other hand, are a broader group that includes both aerobic and anaerobic species. They metabolize organic compounds, such as hydrocarbons or nutrients in pipeline fluids, to produce organic acids like acetic acid (CH₃COOH), propionic acid, or butyric acid. This acidification lowers the local pH, often to levels below 5, which destabilizes protective oxide layers or scales on the pipeline interior. The reaction can be simplified as: Organic matter + O₂ (or anaerobic equivalents) → Organic acids + CO₂ + H₂O.

APB exacerbate corrosion by creating acidic microenvironments that dissolve passive films on steel surfaces, leading to uniform thinning or localized attacks. They are commonly found in water phases mixed with oils or in systems with fluctuating oxygen levels. Unlike SRB, APB can sometimes tolerate low oxygen, making them versatile in various pipeline segments.

Both SRB and APB often coexist in biofilms as complex communities of microorganisms adhered to pipeline walls. These biofilms act as diffusion barriers, concentrating corrosive byproducts and shielding bacteria from biocides.

Early detection of MIC can prevent failures that cost operators millions in repairs and lost production. According to industry reports, MIC accounts for a substantial portion of internal corrosion incidents, and ignoring microbial factors can lead to incomplete assessments.

First, testing enables early detection of MIC. Bacteria like SRB and APB can proliferate rapidly under favorable conditions, such as temperatures between 20-40°C (68-104°F), neutral to slightly acidic pH, and nutrient availability from pipeline fluids. By identifying their presence before visible damage occurs, technicians can flag areas for further inspection.

Second, it supports targeted mitigation. Not all biocides are effective against both SRB and APB; some target sulfate reducers specifically, while others address acid producers. For instance, glutaraldehyde based biocides are common for SRB control, but their effectiveness depends on knowing the microbial profile. Testing data guides the selection of chemical treatments, mechanical cleaning (pigging), or operational changes like increasing flow rates to disrupt biofilms.

Third, microbial testing enhances overall risk assessment. Combined with other parameters such as pH, total dissolved solids (TDS), sulfide levels, etc.; it provides a more comprehensive view.

While several options exist, the most popular method is by serial dilution using a prepared field testing kit. It is standardized by NACE TM0194 and involves inoculating a series of culture media vials with progressively diluted samples to determine the Most Probable Number (MPN) of bacteria per milliliter.

Other methods gaining popularity include ATP, qPCR, and EPA testing. We recently covered these in an article that appeared in our premium section. If you are a free subscriber to our newsletter site and this topic interests you, let me know in the comments and we'll grant a complimentary 30 day access - no strings attached.

Each method has it's advantages and it's important to understand these:

The process starts with careful sampling to capture representative microbial communities, followed by analysis methods that range from simple kits to advanced molecular tools.

Effective testing begins with proper sample collection, as mishandling can alter microbial viability or introduce contaminants. Sample types primarily include water-based liquids, which are the primary habitat for SRB and APB. These are collected from pig receivers during cleaning runs, drip pots or traps where water settles, separators in production facilities, or low points (with access) in the pipeline where gravity aids accumulation.

To ensure sample integrity, use clean, inert containers like sterile glass or plastic bottles pre-treated to avoid leaching. Avoid metal containers that could react with samples. For biofilms, use swabs or scrapers to collect surface deposits.

Documentation is critical: Record the sample location (milepost or facility ID), date and time, temperature, pressure, flow rate, and any operational notes like recent chemical treatments. This context helps interpret results later.

Many field kits offer semi-quantitative results without needing a lab. Culture based methods, such as serial dilution bottles (NACE TM0194 kits), involve inoculating samples into media vials with nutrients tailored for SRB (Postgate's medium) or APB (phenol red broth). Incubate at ambient or controlled temperatures for 14-28 days, observing color changes or blackening. Results are reported as most probable number (MPN) per mL, estimating bacterial counts.

Enzyme based testing provides faster insights. ATP luminometry measures adenosine triphosphate (ATP) as an indicator of total microbial activity, with results in minutes. For SRB-specific detection, adenosine-5'-phosphosulfate (APS) reductase assays detect the enzyme involved in sulfate reduction. Swab sampling for biofilms involves rubbing surfaces and transferring to test media.

Limitations include lower precision; field kits might overestimate or underestimate due to environmental variables; and they don't identify species. They're best for screening, triggering lab confirmation if counts exceed thresholds.

A lab can be used for additional detailed analysis. Specialized preservation of the sample is usually required for accuracy.

Complementary tests analyze water chemistry: pH, dissolved gases (O₂, CO₂, H₂S), and ions like sulfates or chlorides using ion chromatography.

Interpreting SRB and APB results requires context, as raw counts alone shouldn't dictate action. Presence of SRB or APB indicates active microbial populations capable of contributing to MIC, but severity depends on counts, trends, and corroborating data.

High counts—typically SRB >10⁵ CFU/mL or APB >10⁶ CFU/mL—often indicate significant MIC risk. This warrants immediate mitigation, such as injecting biocides, running cleaning pigs to remove biofilms, or adjusting inhibitors. For instance, persistent high SRB might point to inadequate sulfate control in injected waters.

Low or no detection may indicate low MIC risk, but interpret cautiously. Bacteria could be dormant or missed in sampling; always cross-check with chemical indicators like pH <5 (favoring APB) or H₂S >10 ppm (linked to SRB). Physical signs, such as black deposits or pitting in inspections, can confirm.

Trends over time are invaluable. Increasing counts from quarterly tests signal worsening conditions, perhaps due to nutrient influx from upstream changes. Decreasing trends validate mitigation success. Standards like NACE TM0212 (Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion on Internal Surfaces of Pipelines) provide thresholds.

To maximize the value of SRB and APB testing, technicians should adopt a systematic approach that minimizes errors and integrates data effectively.

Timely testing is essential: Analyze samples quickly to prevent die-off or overgrowth. If delays occur, use preservative methods.

Sample handling requires sterile techniques: Gloves, bottles, and tools such as spoons or scrapers. Maintain chain-of-custody documentation to ensure traceability, especially for regulatory compliance.

An integrated approach combines microbial data with chemical testing (immediate pH testing) and physical observations (solids analysis for biofilm indicators).

Documentation should be thorough: Include photos of sampling sites, data forms, and all notes. This aids in audits and trend analysis.

Common challenges include contamination from poor purging or variable field conditions. Overcome by standardizing procedures and calibrating equipment regularly. Misinterpretation happens by ignoring context. For example, high APB in neutral pH might not be an issue if flow disrupts acids. Always correlate with operational history.

Safety first: Handle samples as hazardous, ventilate areas, and dispose of waste properly.

SRB and APB testing is a vital component of internal corrosion monitoring programs, especially for detecting and managing microbiologically influenced corrosion. By comprehending the roles of these bacteria, knowing sampling techniques, and interpreting results in context; corrosion technicians can enhance pipeline integrity assessments and prevent failures. Combining microbial insights with chemical and physical data creates a framework for proactive management, ensuring safer and more reliable operations. As field professionals, applying these practices consistently will lead to better informed decisions and long term asset protection.

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

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