Fuel Right Australia · Fuel quality technical insight
Microbiologically influenced corrosion in diesel tanks
How microbial activity can contribute to localised corrosion, what evidence is needed to investigate it, and how fuel operators can reduce recurring water and contamination risk.
Technical guide · 8 November 2026
What is microbiologically influenced corrosion?
Microbiologically influenced corrosion (MIC) describes corrosion processes affected by the presence and activity of microorganisms. It is sometimes called microbiologically induced corrosion, but influenced is the more precise general term: microorganisms can alter the local chemistry and conditions at a metal surface without being the only cause of damage.
In fuel systems, MIC is a concern where water, microbial activity and susceptible metal surfaces coincide. The risk is not confined to fuel tanks: low points, water bottoms, pipework, fittings and other wetted components may require attention. Corrosion can be localised, so a tank that appears generally sound may still have concentrated damage.
Why water changes the risk
Fuel systems can contain free water at the bottom of a tank, fine droplets dispersed through fuel or moisture entering through deliveries, vents and damaged seals. Many microorganisms associated with fuel systems depend on available water for growth. The fuel–water interface and wetted surfaces can create conditions where microbial communities establish and produce deposits or biofilms.
These deposits may create chemically different micro-environments at the metal surface. Depending on the organisms, materials and conditions, microbial activity can contribute to changes in local acidity, oxygen availability and other corrosion-relevant chemistry. It is not technically sound to assume that every dark deposit is microbial, or that every microbial detection indicates active metal loss.
Free water
Accumulated water at tank bottoms and low points can sustain microbial activity and requires an appropriate physical removal and ingress-control plan.
Sludge and deposits
Deposits can contain corrosion products, dirt, degraded fuel and microbial material. Their appearance alone cannot establish composition or cause.
Biofilms
Surface-associated microbial communities may be missed by a single fuel-only sample. Surface or deposit sampling may be necessary during a specialist investigation.
Localised attack
Pitting and other concentrated damage require assessment of remaining wall thickness and integrity, not just treatment of the fuel.
Warning signs worth investigating
No individual sign is a definitive MIC diagnosis. Operators should escalate combinations of repeated water accumulation, persistent sludge, microbial test findings and corrosion-related defects, particularly where a tank has long storage periods or a history of poor housekeeping.
| Observation | What it may indicate | Next appropriate check |
|---|---|---|
| Recurring water at the tank bottom | Continuing ingress, condensation or ineffective water removal | Review deliveries, tank integrity, drainage, vents and low-point inspection |
| Dark deposits or repeated filter restriction | Solids, degradation products, microbial residues or corrosion products | Representative samples and suitable physical/chemical examination |
| Positive microbial field test | Target microorganisms detected under that test’s method | Interpret against sampling location, method and trend; investigate associated water |
| Pitting, leaks or abnormal wall loss | Potential integrity failure from one or more corrosion mechanisms | Competent corrosion and tank-integrity assessment; do not rely on fuel treatment alone |
How to investigate suspected MIC
- Make the system safe. Follow site isolation, hazardous-area, confined-space and fuel-handling procedures. Suspected leaks or structural damage require prompt specialist attention.
- Document the history. Record fuel type, tank material, age, deliveries, storage duration, water findings, maintenance, filtration issues and previous corrosion inspections.
- Sample deliberately. Consider tank-bottom water, the fuel–water interface, representative bulk fuel and relevant deposits or surfaces. Sampling location and handling can materially affect interpretation.
- Use tests for defined questions. Microbial field screening can help identify specified organisms; laboratory methods may be required to characterise deposits, water chemistry or organisms more fully.
- Assess the metal. A qualified inspector or corrosion specialist should evaluate pit morphology, wall thickness, corrosion products and relevant alternative causes.
- Correct the causes and verify. Remove water and deposits as appropriate, repair ingress pathways, implement a monitored maintenance programme and confirm that abnormal conditions do not recur.
For sampling principles, read the diesel fuel sampling and testing guide. For the relationship between water and microbial growth, see microbial contamination in diesel.
Prevention is a system programme, not a single additive
Effective control starts with tank condition and water management. A product may be useful within its defined application, but no fuel additive can restore lost metal thickness, seal a leak, certify a corroded tank or replace the removal of significant free water. Treatment selection must be based on verified fuel compatibility and current instructions.
1. Keep water out
Inspect fill points, seals, vents and delivery practices; investigate repeated water findings rather than repeatedly draining without identifying the source.
2. Remove what has accumulated
Drain substantial free water and assess deposits. Heavily contaminated tanks may require professional cleaning before other measures are effective.
3. Test where justified
FUELSTAT microbial testing may contribute to a contamination assessment. The result does not independently diagnose MIC or establish full fuel specification compliance.
4. Maintain integrity oversight
Schedule appropriate tank and pipework inspections, retain condition records and escalate abnormal corrosion findings to qualified specialists.
For stationary diesel-tank water management, review the Aquafighter technical information and applicable manufacturer instructions. For broader fuel maintenance options, see the Fuel Right product range. Neither is a substitute for integrity testing.
Frequently asked questions
Does a positive diesel bug test prove MIC?
No. It indicates the presence of microorganisms targeted by the test under the sampled conditions. MIC attribution requires corrosion and materials evidence as well as appropriate microbiological investigation.
Can MIC occur in an apparently clean diesel tank?
Potentially. Localised deposits, wetted surfaces and tank-bottom conditions may not be visible in a routine top-level fuel sample. The appropriate investigation depends on the tank design and history.
Will a biocide repair a tank affected by MIC?
No. Chemical microbial control, where suitable and authorised, does not reverse pitting or restore structural integrity. Any damage requires competent inspection and remediation.
What is the first action when water and corrosion are both found?
Address immediate safety and integrity risks first, then investigate the water source and obtain suitable samples. Do not enter or work on a fuel tank without the necessary controls and competent personnel.
Plan the next investigation step
For assistance selecting relevant microbial testing, fuel-quality information or water-management options, provide the fuel type, tank size, operating environment, water findings and available inspection records. Structural or corrosion-integrity decisions must be made by appropriately qualified specialists.
Discuss your fuel-quality requirements
Educational information only. Fuel handling, tank entry, corrosion assessment and any treatment must follow applicable legislation, site procedures and product instructions. Related resources: Fuel quality articles · Technical resources.