by Grant Lobdell, President, Dyne Fire Protection Labs
Corrosion & MIC
Corrosion occurs in all fire protection sprinkler systems. Corrosion damage/products and mineral deposits can restrict water flow and impair mechanical operation, leaving facilities vulnerable. What’s more, microbiologically influenced corrosion (MIC), a specific type of corrosion caused by microorganisms, can cause localized pitting, pinhole leaks, and even blockages. For these reasons, the 2025 edition of NFPA 13, Standard for the Installation of Sprinkler Systems, requires the following during system installation:
5.1.4.1 Water supplies and environmental conditions shall be evaluated for the existence of microbes and conditions that contribute to microbiologically influenced corrosion (MIC).
The 2026 edition of NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, then requires an assessment of the internal condition of the piping at a minimum of every 5 years. As part of this assessment:
14.2.1.3 Tubercules or slime, if found, shall be tested for indications of microbiologically influenced corrosion (MIC).
No further guidance is given in regard to test methods or requirements in these NFPA standards. For that, we must look to other industry standards and white papers. Among them, FM Global’s Research Technical Report from 2014, Corrosion and Corrosion Mitigation in Fire Protection Systems, is one of the most in depth and fairly recent. According to this report:
The corrosivity of sprinkler water in [a fire protection system] is significantly influenced by water quality and chemistry parameters, such as concentration of dissolved gases (O2), dissolved anions (chloride, sulfate, etc.), pH, alkalinity (HCO3- and CO32-), solids (hardness and deposits), and microorganisms.
Diesel Fuel
Diesel fuel degrades over time due to oxidation, contamination (e.g. water), and microbial growth. As it degrades, insoluble particulates and sludge accumulate, which can clog fuel filters and foul diesel engine injectors leading to, at best, poor performance or, at worst, catastrophic fuel system damage and failure. For these reasons, the 2026 edition of NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, requires periodic testing of diesel fuel:
8.3.4.1 Diesel fuel shall be tested for degradation at least annually.
A.8.3.4.1 Biodiesel fuel might need to be tested for degradation more frequently.
The 2025 edition of NFPA 110, Standard for Emergency and Standby Power Systems, has a similar annual testing requirement:
8.3.7 A fuel quality test shall be performed at least annually using applicable ASTM standards or the manufacturer’s recommendations.
NFPA 25 further elaborates on the testing itself:
8.3.4.1.1 Fuel degradation testing shall include, at a minimum, fuel quality testing for stored fuel as specified in ASTM D975, Standard Specification for Diesel Fuel, or ASTM D6751, Standard Specification for Biodiesel Fuel Blendstock (B100) for Middle Distillate Fuels, depending on the fuel type, as approved by the engine manufacturer.
Water Mist Nozzles
According to the 2026 edition of NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems:
12.3.1.1 Where required by this section, sample automatic water mist nozzles shall be submitted to an approved testing laboratory acceptable to the authority having jurisdiction for functional testing.
It goes on to describe that these nozzles shall be tested 10 years (5 years if in a harsh environment) after installation and then every 5 years thereafter if they continue to be found acceptable.
NFPA 25 also describes test requirements for open nozzles as well:
12.3.2.3 Where the nature of the protected property is such that the water cannot be discharged annually…and not subject to the conditions in 12.3.2.2, open water mist nozzles that have been in service for 10 years shall be either replaced or representative samples shall be tested and then retested at 5-year intervals.
12.3.2.2 Where water mist nozzles are subjected to environments with residues, including corrosive atmosphere and corrosive water supplies, they shall be either replaced or representative samples tested on a 1-year basis.
The nozzles that pass the floor level visual inspection should then be tested for sensitivity (automatic nozzles only), functionality, and K-factor according to the annex of NFPA 25.
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