Microbiological Influenced Corrosion (MIC) Testing

MIC Pipe Corrosion Can Affect the Efficiency of Your Cooling Towers and Closed Loop Systems

One of the most potentially dangerous forms of corrosion in open recirculating waters and closed loops, is often overlooked; Microbiologically Influenced Corrosion (MIC). Most treatment programs focus on scale and corrosion control related to dissolved oxygen and carbon dioxide, but MIC must not be neglected. It has been said in the water treatment industry that, “You can’t stop Mother Nature, but you can slow her down.” That is the job of those in the industry, to identify and control threats to water systems, whether that is dissolved gasses or microscopic living organisms.

Corrosion costs the United States nearly ~2.8 trillion dollars a year in both direct and indirect costs.  Corrosion can potentially have long-term and costly impacts on your facility.  Efficiency loss and potential repair costs of damaged systems can be avoided by closely monitoring the microbial conditions of the systems in your facility.  Some of the major cost impacts of MIC are:

  • Equipment malfunction or failure
  • Increased capital costs to repair or replace failed equipment
  • Operational challenges from loss of services from failed equipment
  • Scale build-up in pipes and on mechanical parts
  • Increased operation cost due to loss of efficiency
  • Accelerated wear and reduced operational life of equipment

It is important to understand that multiple types and numbers of microorganisms will be present in a system (whether open recirculating or closed loops).  Some of these microorganisms can form biofilms that are damaging to systems and offer protection from biocides.  Controlling these microorganisms and potential biofilm formation is an important factor in operating a system at peak efficiency.  In attempting to control MIC many building operators ask what are the standards for control? Unfortunately, numerous factors can make it difficult to set action limits.  Agencies such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Cooling Tower Institute (CTI) have developed the following guidelines for various microbiological levels:

SpeciesOpen CoolingSourceClosed LoopsSource
Heterotrophic Bacteria (CFU/mL)*10,000ASHRAE/CTI1000Master Spec
Biofilm (colonies/in2)100,000CTINo Data found 
Total Anaerobic Plate Count No Data found100Master Spec
Sulfate reducing bacteria (CFU/mL)**0Master Spec0Master Spec
Denitrifying bacteria (CFU/mL)***100Master Spec100Master Spec
Fungi, Yeasts and Molds (CFU/mL)0Master Spec0Master Spec

*  According to CTI, the Total “Planktonic Counts” in open cooling systems should not exceed 10,000 CFU/ml. This also includes Heterotrophs.

** This is included on the Master Spec.  (SECTION 232500 – HVAC WATER TREATMENT).

*** See note above on the Master Spec.

Sampling for MIC is indicative of a point-in-time, and the samples are generally taken from the bulk water, where the real danger of MIC exists primarily in the biofilm.  Thus, it is very important to sample on a regular basis so that a history can be established, maintained, and action can be taken when sample results vary form that system’s baseline.

EST provides monitoring services to help detect corrosion-causing bacteria before they become detrimental to your facility’s operations. Our laboratory and our staff are fully equipped in the interpretation of microbiological species using the Biological Activity Reaction Test (BART) and metagenomic methods. By monitoring these types of bacteria, you can reduce the impact of corrosion on your equipment.  Assessing the presence/absence and approximate count (aggressiveness) of corrosion-causing bacteria in water systems, a disinfection plan can be developed for your facility’s water system. For persistent contamination of a system, Metagenomic testing is also available for a more comprehensive analysis.  Our microbial corrosion screening will assess the presence and level of activity of the following:


  • Acid Producing Bacteria
    • Monitor production of acids that attack metal surfaces
    • Turnaround: 8 days
  • Denitrifying Bacteria
    • Track ammonia producers that attack copper alloys
    • Turnaround: 4 days
  • Slime Forming Bacteria
    • Evaluate slime & sludge forming bacteria
    • Turnaround: 8 days
  • Sulfate Reducing Bacteria
    • Monitor corrosion of metal surfaces
    • Turnaround: 8 days
  • Nitrifying Bacteria
    • Detect corrosion on copper & steel
    • Turnaround: 5 days
  • Iron Related Bacteria
    • Track iron deposits causing plugging & metal deterioration
    • Turnaround: 8 days
  • Algae
    • Detect algae that can absorb and inhibit biocides causing pitting of metal
    • Turnaround: 8days
  • Fluorescent Pseudomonads
    • Monitor biofilm that may supply nutrients for Legionella
    • Turnaround: 8 days


  • Metagenomic Bacterial Testing
    • Detailed report identifying the type and relative quantity of specific bacteria
    • Turnaround: 7 days
  • Metagenomic Fungal Testing
    • Detailed report identifying the type and relative quantity of specific fungi
    • Turnaround: 7 days

General Guidance on MIC control

EST recommends testing open recirculating systems and chilled loops to get a baseline sample. Prior to commissioning of a system, the system piping should be cleaned and sanitized. Care must be taken to passivate the system piping, and routine monitoring of both chemical and bacterial levels should be prioritized.

If a potential issue is found, EST also recommends that a follow up sample should be done shortly after remedial action is taken. Since these numbers represent a point-in-time of testing, it would be prudent to establishing testing on a routine basis to identify trends and potentially prevent a corrosion issue that may be evolving.

For additional reading on MIC, click here.

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