MIC in Fire Sprinkler Systems
While it is true that microbiologically influenced corrosion (MIC) can be a contributing factor to the overall corrosion picture, it is not the primary cause of internal corrosion in fire sprinkler systems. Unfortunately, many within the fire sprinkler industry have over-emphasized the role that MIC in fire sprinkler systems plays in causing corrosion while under-emphasizing the predominant role that oxygen plays in these systems. In many cases, MIC has become synonymous with all corrosion activity.
MIC, an uncommon cause of leaks
While ECS agrees that bacteria are almost always present in fire sprinkler systems and that they can influence the location and severity of pitting, they play a very secondary role in the cause of corrosion. Further, data from hundreds of systems indicates that it is the debris from oxygen corrosion that stimulates and sustains the microorganism populations that exist in these systems. The most common type of bacteria found in fire sprinkler systems are Iron Related Bacteria (IRB) that utilize iron oxide as a nutrient source. Based on examined samples, less than 5% of corrosion-related failures were attributable to microbial activity in these systems.
Most bacteria produce slimes as part of their metabolic processes. These slimes act to “capture” nutrients for the colonies of bacteria that form. It is precisely these adherent deposits that contribute to under-deposit corrosion mechanisms, but analysis of the deposits in a typical fire sprinkler system reveals that the primary constituent of any deposit found is always inorganic iron oxide and not organic microbial slime. These oxides are formed by the action of oxygen on the iron in the steel pipe, a natural process that will occur in any environment where steel is in contact with oxygenated water. By removing the oxygen from the system, iron oxide deposits will not form and there will be no resulting corrosion.
Evidence Does not Support the MIC Myth
One strong piece of evidence supporting the fact that bacteria are not necessary to initiate corrosion can be found in galvanized steel pipe systems. Based on water samples analyzed from galvanized steel system piping there is a lack of microbial contamination in the water and deposit samples from the corroding galvanized systems which indicates that MIC is not a viable explanation for the corrosion that is taking place. Soluble zinc can be biocidal when levels in the water exceed 800 ppm. Field data from galvanized dry and preaction systems indicates that it is very common to find zinc levels in the water at levels over 1000 ppm. All of the evidence strongly supports oxygen corrosion as the predominant mechanism that leads to rapid pitting and metal loss in these systems.
Additional supporting evidence that points to oxygen as the primary driver of corrosion in sprinkler systems is apparent in the hundreds of wet pipe systems that ECS has video scoped. The formation of deposits, and the significant metal loss associated with it, is isolated to any location in the system that contained trapped air. This trapped air provides a source of oxygen to drive the corrosion reaction at the air/water interface where iron oxides are formed and result in significant under-deposit pitting, eventually resulting in leaks. From the hundreds of assessments performed by ECS, there has been no correlation between the number of bacteria found in water samples and the frequency of corrosion-related leaks in the system. Corrosion only occurs where oxygen is present in the system to drive the corrosion reaction. The bacteria found in fire sprinkler systems are almost always a by-product of oxygen corrosion. If oxygen corrosion is controlled, bacteria present no risk to fire sprinkler systems.
The ECS Solution
Understanding that oxygen corrosion is the primary cause of leaks in fire sprinkler systems, ECS developed Wet Pipe Nitrogen Inerting (WPNI) and Dry Pipe Nitrogen Inerting (DPNI) processes to completely remove oxygen from the piping network and prevent any future ingress of fresh oxygen. Both processes utilize a “fill and purge” breathing method to push nitrogen to the most remote parts of the system. Removing the oxygen from the piping reduces the corrosion rate in several ways:
- Removal of oxygen prevents the formation of additional solid deposits in system piping.
- By preventing the iron oxide deposits from forming, the nutrient source of many of the bacteria has been eliminated.
- Without oxygen, even if any deposits remain in the system, under-deposit corrosion mechanisms will not take place since there are no electron receptors available to complete the corrosion reaction.
Chemicals are not the answer
Biocides and chemical corrosion inhibitors do not provide an acceptable solution to the oxygen corrosion problem found in fire sprinkler systems. In other industries, it is common to use “filming amine” type chemical corrosion inhibitors to control oxygen corrosion in flowing environments. This approach achieves corrosion control through continuous low dosage injection of the inhibitor into the flowing stream of water. A thin inhibitor film protects the metal surface by forming a barrier which inhibits the action of oxygen on the metal. Any rupture or breach of the inhibitor layer on the metal surface is “repaired” by maintaining a threshold level of chemical in the water stream. The list of chemical corrosion inhibitors that provide corrosion control in fresh water flowing environments are not effective in controlling corrosion in fundamentally different stagnant fire sprinkler systems.
In addition to the ineffectiveness of chemically treating a stagnant system, chemical additives present several other challenges:
- The potential cost to upgrade sprinkler system backflow preventer to RPZ type.
- Chemical toxicity for maintenance personnel handling chemicals or first responders exposed to discharged treated water.
- A requirement to capture discharged water during testing procedures to prevent chemical runoff to a sanitary sewer.
- Lack of compatibility with all sprinkler system components, i.e. mechanical fitting gaskets, sprinkler seats, valve clappers, etc.