MIC: A Positive Vial Is Not a Diagnosis

You pulled the samples off the receiver a month ago. Fluids in sterile bottles, solids scraped into their own container, everything inoculated into the kit vials in the truck, ready for the lab.

The phenol red vials you counted two weeks ago. Three of them went yellow. Now it is day 28, the SRB vials are finally readable, and four of those came back black. You write the counts on the form and send it up the ladder. And next time you hear about it somebody has typed one word into the corrosion cause field.

MIC.

The standard that tells you how to grow those bacteria also tells you, in the second paragraph of its own scope, that growing them settles nothing on its own. NACE TM0194 §1.1.2: the presence of bacteria in a system does not necessarily indicate that they are causing a problem, and populations causing problems in one system may be harmless in another. Bacterial population determinations are one more diagnostic tool useful in assessing problems.

One more tool. Not the verdict.

Internal corrosion sampling has a way of falling on people who do not do it every week. External corrosion is more familiar work. The pig receiver call comes in now and then, and whoever takes it is often sorting out the order of operations on the drive there. Which is why the interpretation rules deserve more attention than they usually get. The field procedure is the easy half.

Both MIC standards open their interpretation sections with nearly the same sentence, and it is worth reading closely.

TM0212 §9.1.1 states that because microorganisms are ubiquitous, their presence does not necessarily indicate a causal relationship with the internal corrosion observed in a pipeline. It goes further: microorganisms may nearly always be cultured from natural environments, so merely detecting viable organisms in samples associated with corrosion does not necessarily prove that MIC has occurred. The external-surfaces standard says the same thing about the outside of the pipe.

So a positive result is close to the expected result.

Be just as careful with the opposite reading. TM0194 §4.1.1 lists several reasons a culture could come back negative that has nothing to do with the condition of the pipe. A vial or medium only grows the organisms that can use its nutrients. Oxygen picked up during sampling hampers anaerobes. Only a small percentage of the viable bacteria in a sample can be recovered by any single medium. And some bacteria cannot be grown in culture media at all. No growth may be a fact about your medium rather than a fact about the system.

Eckert’s Field Guide to Internal Corrosion Mitigation and Monitoring for Pipelines makes the point with the bluntest line in the literature. Its second MIC rule reads: bacteria tests are NOT MIC tests. The chapter is direct about why it needs saying, noting how often someone points at positive culture media and declares MIC the cause of every corrosion problem in a line.

The first rule in that set is the constructive version. MIC has to be understood using both microbiological and corrosion data. TM0212 says the same thing: MIC cannot be evaluated, assayed, or diagnosed solely with microbiological data. And when it comes to actually determining a cause, §9.1.3 and §9.1.4 shift into mandatory language. All chemical, microbiological, metallurgical and operational data about the pipeline must be examined, integrated and analyzed.

This is the part to know, because it is the actual test and almost nobody discusses it.

TM0212 §9.5.2 states that to validate MIC as the cause of internal corrosion, three conditions must be met. TM0106 §8.2.2 lists the same three for external corrosion. Not two. Not a high count plus a hunch. Three.

Condition 1 — Elevated organisms relative to somewhere else. Absent known or suspected outside contamination, you have to demonstrate increased levels of specific types of viable organisms associated with the corrosion, compared against samples taken outside the corroded area.

Read what that requires. A number from the corroded area, by itself, satisfies nothing. The clause is comparative, so you need samples from outside the corroded area to measure against. Good practice supplies the rest: same handling, same media vendor and formulation, same incubation.

Condition 2 — Chemical indicators that support the biology. Chemical indicators in the corroded area have to corroborate the microbiological evidence. The standard’s examples are elevated sulfide or sulfur in pit deposits for sulfate reducers, and organic acids for acid producers. The chemistry has to agree with what the microbiology says.

Condition 3 — Biotic factors identified as the primary contributor. This is the one that does the work, and the standard details it well. The objective is to establish that biotic conditions were the predominant contributor to the corrosion in front of you. Abiotic factors must be considered in every case. And the nature of the damage should be consistent with the organisms found, with their by-products, or with their physical influence on the formation of corrosion cells.

The standard is candid about why that step is hard. It is often difficult to discern between the relative contributions of biotic and abiotic factors in localized corrosion. Underdeposit corrosion, a concentration cell under those same solids, low pH from dissolved CO₂: any of these produces pitting in the same place bacteria are living. A line can have more than one mechanism running at once, and where concurrent threats exist TM0212 asks you to establish the relative contribution of each so the mitigation matches the problem. Deciding which predominates is a judgment built on integrated data. It is not a vial count.

Start with the SRB vials, because TM0194 §4.3.4.2 carries two details that change how you read them.

A vial that blackens in the first two hours is not a positive. Those are discounted, because the blackening comes from sulfide already present in the water rather than from anything growing. If they are the only vials that darken after full incubation, the procedure calls for subcultures into fresh medium as a check.

A negative is not a negative until day 28. SRB vials shall not be scored as negative until 28 days, and shall is mandatory language in a NACE standard. An early read is an incomplete read.

The phenol red vials run on a different clock. Under §4.2.5.2 they are scored positive for general heterotrophs when they turn turbid between 1 and 14 days, positive for acid producers on the color change from red to yellow, and they may be discarded after 14 days. Two windows, one kit.

Then there is the accuracy of the number itself. Using the interpretation table is simplistic. Estimating populations by serial dilution is properly a subject for statistical analysis, and the ranges the table prints are actually too narrow for the testing the standard prescribes. Growth media also underestimate populations inherently. The values are accepted by convention for oilfield and pipeline use.

So the count you write on the form is an order-of-magnitude estimate, from a method that reads low, in bands the standard itself calls too tight. Useful. Not precise. Never meant to carry a diagnosis alone.

The third MIC rule in the Field Guide is about location: surface samples are far more meaningful than bulk liquid samples when evaluating MIC.

The reasoning is straightforward. Free-floating organisms in the bulk fluid have little effect on the corrosion of a surface, and the numbers and types in the liquid are seldom representative of what is living in the biofilm on the wall. Corrosion happens on surfaces. Biofilms form on surfaces. The organisms that matter are the attached ones, sitting under deposits where they can occlude the surface, fix anodic sites, concentrate acidic species and shift the local redox condition.

That is not an argument against liquid samples. It is an argument about what each sample can answer. Pigging returns, coupons and removable components exposed to the stream are the practical routes to a surface sample, and TM0194 is firm on the handling: testing on sessile samples must be performed immediately upon removal from the system.

There is one more reason a bulk reading can mislead. Organisms flourish in microniches. A line can run clean across the great majority of its length and still be under attack in one short section, because the environment that supports the attack exists only there.

This question comes up every time, so here is what the standards do and do not give you.

Neither TM0194 nor TM0212 publishes a universal count above which MIC is established. TM0212 §9.4.1 states that key performance indicators are usually considered system-specific, and that trending data over time alongside documented operational changes is usually a more dependable basis for monitoring than absolute values. A threshold also depends on the technique that produced it. Values are not interchangeable between methods, so a figure derived for serial-dilution SRB counts is likely different from one built on qPCR.

Counts and trends tell you where to look, and whether mitigation is moving the needle. They do not close the question, which is why the three conditions exist.

And TM0212 §9.6.4 closes the loop at the other end: simply reducing the number of viable microorganisms in a system, with a biocide for instance, should not be equated with controlling internal corrosion in that system.

A short illustration, with realistic numbers.

An ILI call brings you to a dig. The cut-out shows pitting under a band of black solids along the bottom of the pipe. You collect from the corroded area: solids from inside the pit deposit, a scrape of the film, and fluid. Then you move along the same joint to clean wall with no corrosion and collect the same three sample types, the same way, into vials from the same lot.

Identical. Condition 1 is not met, and the MIC diagnosis stops there no matter how healthy those counts look in isolation. What you have is a system with bacteria in it and localized corrosion that needs a different explanation. The solids band and the pit location under it point toward deposits, and the next questions are about water accumulation, flow and cleaning rather than about biocide.

Now change one number. The corroded area comes back at 10⁶ per mL against 10³ in the reference. Condition 1 is met. You go to the deposit chemistry looking for sulfide or sulfur in the pit material, and to the pit morphology for damage consistent with what sulfate reducers do. If the sulfide is there and the morphology fits, you have Conditions 2 and 3 in hand and a call you can defend.

Same field work either way. The reference sample is what turns it into evidence.

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.

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