Fire sprinkler system corrosion can cause serious problems before anyone sees them. Microbiologically influenced corrosion, or MIC, is a form of internal corrosion caused by microorganisms that live inside sprinkler piping. These problems can leave a building less protected and lead to costly repairs.
MIC can occur in wet-pipe, dry-pipe and preaction sprinkler systems. It can affect both new and older systems and has been identified in systems less than two years old.
Types of organisms linked to MIC
Microbiologically influenced corrosion is caused or sped up by microorganisms that live inside a fire sprinkler system.
- Sulfate-reducing bacteria
- Iron-related bacteria
- Acid-producing bacteria
These microorganisms can create conditions that damage steel pipes and other metal parts. This often causes small areas of deep corrosion, called pitting. Pitting can quickly wear through a pipe wall.
Common warning signs
-
Pinhole leaks in pipe or fittings
- Blistering or peeling paint on sprinkler pipe
- Black, reddish-brown or slimy material in drained water
- A sulfur or “rotten egg” smell
- Slime, tubercles or unusual corrosion inside the pipe
- Foreign material found during an inspection or test
These signs do not prove that fire sprinkler corrosion is present. They do show that the system may need more testing. Contact a qualified fire protection professional when warning signs appear.
How can microbiological corrosion affect a fire sprinkler system?
- Pinhole leaks can release water and damage building materials
- Corrosion products can build up and block water flow
- Damaged pipes can fail before or during a fire
- Blockages and leaks can reduce sprinkler performance
- Repairs may require parts of the system to be taken out of service
Sprinkler system corrosion often develops without clear signs on the outside of the pipe. A leak or an internal inspection may be the first sign of a problem. This is why regular fire sprinkler maintenance is important.
NFPA 25 inspections and internal assessments
NFPA 25 is the standard for the inspection, testing and maintenance of water-based fire protection systems. It includes requirements for checking the inside of fire sprinkler piping for corrosion, foreign material and obstructions that could affect system performance.
NFPA 25 requires internal pipe assessments at intervals no longer than five years. The exact timing can depend on the type of system, its condition and past inspection results.
If an assessment finds tubercles, slime or other biological deposits, the material should be tested for signs of corrosion. If heavy deposits or obstructions are found, an investigation may be needed. This helps determine how much of the system is affected and what repairs or other actions are needed.
How to prevent and manage MIC
- Complete NFPA 25 inspections and required internal pipe assessments
- Test water quality and check for microbial activity when needed
- Reduce water stagnation inside the system
- Flush the system to remove sediment and loose deposits
- Investigate leaks, odors, deposits and other corrosion signs promptly
- Keep clear records of inspections, tests, findings and repairs
- Work with qualified fire protection and corrosion professionals when fire sprinkler corrosion is suspected
Flushing can remove loose debris, but it does not stop active MIC. It also cannot repair pipes that are already damaged. If corrosion is severe, a damaged pipe may need to be repaired or replaced.
Additional corrosion-control methods may be appropriate depending on the type of fire sprinkler system and the severity of corrosion. For dry-pipe and preaction sprinkler systems, nitrogen inerting is a commonly used strategy. By reducing the amount of oxygen inside the piping, nitrogen inerting can slow corrosion and help reduce the likelihood of MIC-related damage.
Other measures may include water-quality controls, corrosion-control equipment, biocide treatment and closer monitoring. A qualified professional should help select the right approach for the system.
These measures can support a corrosion-management program, but they do not replace required NFPA 25 inspections, testing or internal pipe assessments.
Key takeaways
MIC is a hidden form of fire sprinkler system corrosion that can lead to leaks, blockages, pipe failure and impaired sprinkler performance.
Early action can help protect the system. Building owners and managers should follow NFPA 25 requirements, complete regular fire sprinkler maintenance, investigate warning signs and use qualified professionals to test and repair the system.
A sprinkler system must be ready when it is needed most. Finding and managing corrosion early can help keep water flowing and the system working as designed.
- National Fire Sprinkler Association. (2023). Assessing the internals: NFPA 25 internal assessments. https://nfsa.org/2023/07/19/nfpa-25-internal-assessment/
- National Fire Protection Association. (2023). NFPA 25: Standard for the inspection, testing, and maintenance of water-based fire protection systems (2026 ed.). National Fire Protection Association.
- National Fire Sprinkler Association. (2025, March 21). Assessing the internals: Understanding NFPA 25 Chapter 14: Internal assessments and obstruction investigations. https://nfsa.org/2025/03/21/internal-assessments-fire-sprinkler-obstructions-nfpa-25/