What Is Microbiologically Influenced Corrosion (MIC)?

WHAT IS MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)?

Microbiologically Influenced Corrosion (MIC) is corrosion that is initiated, accelerated, or influenced by microorganisms.

These microorganisms can form biofilms on surfaces, creating localized environments that promote corrosion.

Unlike traditional corrosion, MIC often develops beneath microbial growth where damage may not be immediately visible.

As a result, corrosion can progress for extended periods before being detected.

HOW DOES MIC OCCUR?

Microorganisms attach to surfaces and form biofilms.

Within these biofilms, bacteria may:

• Produce acids
• Alter oxygen concentrations
• Create corrosive by-products
• Influence electrochemical reactions
• Trap moisture and nutrients

These conditions can accelerate corrosion rates and create localized damage.

WHERE DOES MIC OCCUR?

MIC is commonly found in:

Cooling Water Systems
Cooling towers and recirculating water systems.

Boiler Systems
Areas where microbial growth may occur during shutdowns or low-flow conditions.

Industrial Water Systems
Process water systems and manufacturing operations.

Pipelines
Water distribution systems and industrial pipelines.

Storage Tanks
Water storage facilities and process tanks.

Wastewater Systems
Collection systems, treatment facilities, and associated infrastructure.

Municipal Water Infrastructure
Distribution systems, reservoirs, and storage facilities.

MICROORGANISMS COMMONLY ASSOCIATED WITH MIC

Sulfate Reducing Bacteria (SRB)

Sulfate Reducing Bacteria are among the most commonly discussed microorganisms associated with MIC.

These bacteria reduce sulfate and generate hydrogen sulfide as a by-product.

Potential Effects

Accelerated corrosion

Pitting corrosion

Infrastructure deterioration

Why They Matter

SRB are frequently identified in industrial water systems, pipelines, cooling systems, and wastewater infrastructure.

Acid Producing Bacteria (APB)

Acid Producing Bacteria generate acidic compounds during growth.

Potential Effects

Reduced surface pH

Increased corrosion rates

Localized damage

Why They Matter

APB can contribute to corrosion in industrial water systems and process equipment.

Iron Related Bacteria

Iron Related Bacteria interact with iron compounds within water systems.

Potential Effects

Deposits and fouling

Flow restrictions

Corrosion under deposits

Why They Matter

Iron bacteria can contribute to operational and maintenance challenges in water infrastructure.

WHAT ARE BIOFILMS?

Biofilms are communities of microorganisms attached to surfaces.

They often develop on:

Pipes

Tanks

Heat exchangers

Cooling systems

Water infrastructure

Biofilms provide protection for microorganisms and create localized environments where corrosion can occur.

Many MIC investigations focus on understanding biofilm development and activity.

COMMON SIGNS OF MIC

MIC is often difficult to identify without testing.

Potential indicators include:

Unexplained Corrosion
Corrosion occurring despite treatment programs.

Localized Pitting
Small but deep corrosion pits.

Repeated Equipment Failures
Recurring corrosion-related maintenance issues.

Slime Formation
Visible microbial growth within water systems.

Reduced System Performance
Fouling and deposit accumulation affecting operations.

Accelerated Infrastructure Deterioration
Corrosion progressing faster than expected.

HOW IS MIC TESTING PERFORMED?

MIC investigations often involve testing for microorganisms associated with corrosion.

Common testing includes:

Sulfate Reducing Bacteria (SRB) Testing

Acid Producing Bacteria (APB) Testing

Iron Related Bacteria Testing

Water Quality Analysis

Corrosion Monitoring Programs

Biofilm Assessments

Testing helps determine whether microbial activity may be contributing to observed corrosion.

WHAT DOES A POSITIVE MIC TEST RESULT MEAN?

A positive result indicates that microorganisms associated with MIC were detected.

However, detection alone does not automatically confirm active corrosion.

Results should be evaluated alongside:

Corrosion observations

System conditions

Water chemistry

Operational history

Asset performance

The goal is to understand whether microbial activity may be contributing to corrosion risk.

WHAT DECISIONS DOES MIC TESTING SUPPORT?

Corrosion Investigations
Determine whether microorganisms may be contributing to asset deterioration.

Treatment Program Reviews
Evaluate water treatment effectiveness.

Infrastructure Management
Support maintenance planning and asset protection.

Risk Assessments
Identify potential corrosion risks before failures occur.

Operational Improvements
Reduce conditions that promote microbial growth.

COMMON MISCONCEPTIONS ABOUT MIC

"All Corrosion Is Caused By Chemistry"
False. Microorganisms may significantly influence corrosion under certain conditions.

"Finding SRB Means Corrosion Is Occurring"
Not necessarily. Results should be interpreted together with corrosion observations and system conditions.

"MIC Only Occurs In Industrial Facilities"
False. MIC can occur in municipal, commercial, industrial, and environmental water systems.

"Corrosion Inhibitors Eliminate MIC"
Not always. Microbial growth and biofilms may still develop if underlying conditions remain favorable.

WHY MIC TESTING MATTERS

MIC can contribute to:

Asset failures

Increased maintenance costs

Reduced equipment life

Operational disruptions

Infrastructure replacement costs

Testing helps organizations understand whether microorganisms may be contributing to corrosion risks and supports informed asset management decisions.