By Jacob “Jake” Ashabraner – ASSE 12080 Certified — Legionella (Certificate #64392) – California Pesticide Applicators # 137045 – Oregon Pesticide Applicator # AG-L1086507CPA — Glacier Laboratories, Inc.
Key Takeaways
- MIC (microbiologically influenced corrosion) is commonly associated with biofilm accumulation and under-deposit conditions in cooling water systems. Some industry references estimate that MIC contributes to a meaningful share of corrosion damage, but the exact percentage varies by industry and study.
- Sulfate-reducing bacteria (SRB), acid-producing bacteria (APB), iron-oxidizing bacteria, and other biofilm-forming organisms can contribute to MIC depending on water chemistry, metallurgy, flow, deposits, and operating conditions.
- Pitting under deposits or biofilm patches is a common field warning sign. On carbon steel, technicians may see localized pits with dark corrosion products, but confirmation requires more than visual inspection alone.
- Chemistry alone does not fix MIC; you also need mechanical removal (cleaning, side-stream filtration) and monitoring (dip slides, ATP, coupons).
- Prevention usually costs less than failure response. A monitored program can help identify early warning signs before corrosion progresses to leaks, tube failures, or equipment damage.
What is MIC?
Microbiologically influenced corrosion is corrosion affected by the presence or activity of microorganisms, usually within biofilms or deposits on a metal surface. The organisms are not simply “eating the metal.” More often, their activity changes local chemistry, creates concentration cells, produces corrosive byproducts, or supports under-deposit conditions that accelerate metal loss. MIC has been documented in cooling water, process water, oil and gas, and other industrial systems, although the reported contribution to total corrosion damage varies by source and application.
The mechanism is usually local, not uniform. Biofilm or deposits form on the metal surface. Under those deposits, oxygen transfer, pH, chloride concentration, sulfide production, and other local conditions can differ from the surrounding bulk water. That difference can create electrochemical corrosion cells. In many field cases, the result is localized pitting that progresses faster than the general corrosion rate would suggest.
Our companion article on biofilm’s impact on cooling tower performance covers the heat-transfer side of the story. MIC is the corrosion side — same root cause, different symptom.
The three bacterial actors
Not every microbe drives MIC, and bulk-water test results do not always reflect what is happening under a deposit. In cooling water systems, these functional groups are often part of the discussion:
- Sulfate-reducing bacteria (SRB) — Typically associated with low-oxygen or anaerobic zones under deposits, tubercles, or stagnant areas. SRB can reduce sulfate to sulfide, which may react with iron and contribute to dark iron-sulfide deposits and localized corrosion.
- Acid-producing bacteria (APB) — Can produce organic acids and lower the pH at the metal surface. That local chemistry may attack protective films and increase corrosion risk, especially where deposits, crevices, or low-flow conditions are present.
- Iron-oxidizing bacteria (IOB) — Often associated with oxygenated water and iron deposits. Their activity can contribute to tubercles or rust deposits that shelter other organisms and create under-deposit corrosion conditions.
Manganese-oxidizing bacteria, slime-forming organisms, algae, and general aerobic heterotrophs can also play supporting roles, particularly where makeup water quality, nutrients, sunlight, warm temperatures, or stagnant areas encourage biological growth.
How to spot MIC in the field
MIC has patterns that experienced technicians learn to recognize, but field indicators should be treated as warning signs rather than proof by themselves:
- Localized pitting, rather than uniform corrosion. Pits may be clustered, sharp-edged, or hidden below deposits.
- Black or dark green deposits at the pit sites — iron sulfide from SRB activity.
- Odor or sulfide indicators when deposits, stagnant sections, or tubercles are opened. Odor can be a clue, but it is not a diagnostic test.
- Under-deposit corrosion — you find the pits only after mechanical cleaning removes the tubercle above them.
- Preferential attack at welds, heat-affected zones, crevices, and stagnant areas, where metallurgy, flow, deposits, and local chemistry can differ from the rest of the system.
On stainless steel, suspected MIC often appears as pitting or crevice corrosion near weldments, gaskets, low-flow branches, standby equipment, or other areas where deposits and stagnant water can persist.
Diagnosis: how to know for sure
Visual inspection can tell you where corrosion is occurring. Confirming MIC as a contributing cause usually takes multiple lines of evidence:
- Corrosion coupons or probes — installed and reviewed on a defined schedule, then evaluated for corrosion rate, pitting, deposit character, and metallurgy-specific concerns.
- Dip slides, BART tests, or other culture-based methods — useful for trending selected microbial groups, while recognizing that culture tests are semi-quantitative and may not capture all organisms present in biofilm.
- ATP (adenosine triphosphate) testing — a rapid indicator of total biological activity. ATP is useful for trending, but results should be interpreted with system history, residuals, deposits, and sampling location.
- Molecular methods such as qPCR — can identify target organisms or genetic markers when a deeper investigation is needed. These results support diagnosis but should not be treated as stand-alone proof of MIC.
- Deposit and metallurgical analysis — laboratory review of deposits, corrosion products, and the affected metal surface can help connect microbial activity, local chemistry, corrosion morphology, and material condition.
Industry guidance, including AMPP/NACE methods used in MIC investigations, supports a consistent sampling and monitoring approach rather than relying on one test result. The strongest diagnosis combines operating history, water chemistry, microbiology, deposit analysis, corrosion morphology, and corrosion-rate data.
Treatment strategy: chemistry plus mechanical plus monitoring
MIC is one of the corrosion mechanisms where chemistry alone is rarely enough. Biofilm and deposits can limit biocide contact and create localized conditions at the metal surface. A practical control program usually combines three areas: treatment chemistry, mechanical removal or solids control, and scheduled monitoring.
1. Chemistry
- Oxidizing biocide baseline — Chlorine, bromine, chlorine dioxide, stabilized halogens, or other EPA-registered products may be appropriate depending on system conditions, product label directions, metallurgy, pH, temperature, organic load, discharge limitations, and the site water management plan.
- Non-oxidizing biocide rotation or supplemental feed — Products such as DBNPA, isothiazolinones, glutaraldehyde, or other registered chemistries may be used when compatible with the system and allowed by label directions. Selection and frequency should be based on water quality, biological trends, discharge limits, and site-specific operating conditions.
- Corrosion inhibitor program — Inhibitor selection should match the metallurgy, cycles of concentration, makeup water quality, temperature, deposit potential, and discharge requirements. Molybdate, phosphonate, azole, phosphate, polymer, and other program components each have limitations and compatibility considerations.
- Bio–dispersant or deposit-control aid — A compatible dispersant may help loosen biological deposits and improve contact between treatment chemistry and fouled surfaces, but it should be selected with attention to foaming, discharge, product compatibility, and system operating conditions.
2. Mechanical removal
- Side-stream filtration or solids control to reduce suspended solids, corrosion products, and detached biological material that can settle and support under-deposit corrosion.
- Routine tower cleaning of basins, strainers, fill, distribution decks, and accessible heat exchanger areas where deposits accumulate. Cleaning frequency should follow the site water management plan, manufacturer guidance, operating history, and applicable regulatory requirements.
- Flow management — eliminate dead legs and stagnant branches wherever design permits.
3. Monitoring
- Corrosion coupons or probes on a defined schedule appropriate for the system, often 30–90 days depending on the monitoring objective and operating conditions.
- Microbiological trending using dip slides, ATP, BART tests, or laboratory methods at a frequency matched to the risk profile, season, water management plan, and prior results.
- Disinfectant residual monitoring at a frequency appropriate to the system. Automated monitoring and control may be appropriate for higher-risk or larger cooling tower systems.
- Trend the results. Rising biological activity, increasing corrosion rates, deposit accumulation, declining residual control, or new pitting should trigger follow-up before the issue becomes a failure.
The economics
The cost of MIC varies widely by equipment type, metallurgy, severity, downtime exposure, and whether a failure affects production or occupant comfort. A perforated condenser tube, damaged heat exchanger, or fouled tower can create repair costs, lost capacity, emergency labor, and operational disruption that exceed the cost of routine monitoring and cleaning.
By comparison, a monitored prevention program — routine inspection, corrosion monitoring, microbiological trending, deposit control, biocide program review, and mechanical cleaning — is usually easier to budget and manage than an emergency repair. The practical value is not just lower cost; it is fewer surprises.
Bottom line
If you have biofilm or deposits in a cooling water system, you have conditions that can increase MIC risk. That does not mean every deposit is MIC, and it does not mean one test result tells the whole story. The best approach is practical and repeatable: keep the system clean, maintain the treatment program within its intended control range, monitor corrosion and biological activity, and investigate changes before they become failures.
Glacier Laboratories helps facility teams evaluate MIC risk as part of a complete cooling water program, including system inspection, coupon review, microbiological trending, deposit evaluation, and treatment-program adjustments based on site conditions. If you see rising corrosion rates, unexplained heat-transfer loss, stubborn deposits, or localized pitting, that is the time to review the system before the damage progresses. Contact Glacier Laboratories at glacierlabs.com/contact/ or call (800) 637-6132.
Professional note: Always consult with your water treatment company and a state-licensed pesticide applicator before selecting, applying, or changing a biocide program. Any quality water treatment company should have access to properly licensed applicators who understand product labels, site conditions, safety requirements, and state-specific pesticide regulations.
Educational disclaimer: This article is intended for educational purposes only. Treatment selection, feed rates, and application practices should always follow EPA-registered product labels, Safety Data Sheets (SDS), site-specific water management plans, manufacturer recommendations, applicable regulations, discharge limitations, and sound engineering judgment.
