Microbiologically influenced corrosion (MIC) is a type of corrosion caused by the metabolic activities of microorganisms such as bacteria, fungi, and algae. These microorganisms can produce corrosive by-products, including organic acids, hydrogen sulphide, and other metabolites. These by-products cause metal surfaces to corrode and degrade, leading to system failure, increased maintenance costs, and decreased efficiency.

How to prevent and remediate microbiologically influenced corrosion...

Regular Fluid Testing

Regular fluid testing is essential to prevent and detect MIC in closed loop heating and cooling systems. In addition, this testing can help identify the presence of microorganisms and the type and quantity of corrosive by-products they produce. 

This testing can help make an informed decision and determine the most appropriate course of action, such as:

  • In-line fluid remediation using biocides and other water treatment chemicals.
  • Full or partial system drain, pre-commission cleaning/flush and refill.


Biocides are chemicals used to control microorganisms' growth in closed loop heating and cooling systems, such as:

  • Tetrakis (Hydroxymethyl) phosphonium sulphate (THPS)
  • Isothiazolones
  • Bronopol

Tetrakis (hydroxymethyl) phosphonium sulphate (THPS) is a highly effective biocide that disrupts microorganisms' membrane structure, leading to their death. This biocide is typically used in low-temperature closed loop systems and is considered one of the most effective options for controlling the growth of bacteria, fungi, and algae.

Due to the acidic nature of THPS, it is also advised that a system is dosed with an appropriate yellow metal inhibitor. Azoles can be combined with THPS to provide a comprehensive approach to controlling MIC, with THPS effectively managing the growth of microorganisms and benzotriazole protecting against corrosion.

Isothiazolones are another effective group of biocides commonly used in closed loop heating and cooling systems because of their broad-spectrum antimicrobial activity and ability to control the growth of a wide range of microorganisms, even those resistant to other biocides.

In addition to its broad spectrum of activity, Isothiazolones are compatible with standard system construction materials and have a low toxicity profile.

Bronopol penetrates microorganisms' cell membranes and disrupts their integrity. The membrane of a microorganism acts as a barrier that protects the cell's interior from the external environment.

Disruption of this membrane affects essential cellular components, such as enzymes and metabolic pathways, leading to the microorganism's death. This biocide can also be used in conjunction with other water treatment chemicals to provide broad-spectrum control over biocontamination in closed loop heating and cooling systems.

Pre-Commission Cleaning

Pre-commission cleaning is essential in preventing MIC in closed loop heating and cooling systems. This process involves cleaning the system to remove any contaminants, including microorganisms, that may be present before the system is filled.

Pre-commission cleaning can be performed using various methods, including chemical and mechanical (via high-pressure water or process pigging). The most appropriate method will depend on the system's specific requirements and may involve using specialized equipment and chemicals. For most closed loop systems, a chemical cleaning policy is the least labour-intensive and cost-effective solution. 


Microbiologically influenced corrosion is a serious problem that can cause significant damage to closed loop heating and cooling systems. Regular fluid testing, the use of biocides, and pre-commission cleaning are all crucial steps in preventing system degradation, unscheduled downtime and costly repairs.

Hydratech offers a range of water treatment chemicals and services:

Alternatively, Hydratech's team of fluid engineers are always on hand to work with you to develop a customized solution that meets your specific needs and ensures your system's long-term performance, longevity and reliability.

Call on 01792 586800 or email info@hydratech.co.uk