What is MIC?
Microbiologically Influenced Corrosion (MIC) is a result of the presence or activity of microorganisms on the surface of a material that is susceptible to corrosion. There are multiple mechanisms that can be categorized as MIC. Depending on the type of bacteria that contaminate the system, the corrosion rate can be accelerated by the following mechanisms:
Slime formation – under‐deposit pitting corrosion
Acid production – acidic (H+) attack and pitting corrosion
Sulfide (S=) anion production – cathodic depolarization of the corrosion cell resulting in pitting corrosion
It is well known that bacteria can almost always be found in water and deposit samples that are collected from fire sprinkler systems for evaluation, but that is not the primary cause of corrosion, or metal loss, in the system. One can’t deny the fact that MIC is sometimes a contributing factor to the overall corrosion taking place, but the act of oxygen dissolving into the moisture and then reacting with the metal in the pipe, is the dominant aspect that drives corrosion based leaks in fire sprinkler systems.
What is Oxygen Corrosion?
For years, misinformation regarding oxygen corrosion of steel sprinkler piping has circulated within the fire sprinkler industry. Oxygen is present as a component of the air within the fire sprinkler piping system at a level of about 21%, and when it dissolves in any water that contacts the fire sprinkler piping, it becomes available to react with the metal pipe.
There are three physical attributes of oxygen gas that greatly affect the corrosion reaction with steel pipe:
- Oxygen gas exhibits very low solubility in water. It does not participate in the corrosion reaction until it has dissolved into the water because it is the combination of the water and oxygen that reacts with the steel.
- Once the oxygen and water mixture contact the steel pipe, the reaction between the oxygen and the iron occurs very quickly. A common example of this reaction occurs on the brake rotors of your vehicle after a light rain. After contact with the warm oxygen saturated water for only a few minutes, rust forms on the surface.
- Oxygen molecules that are dissolved in water have very poor mobility in stagnant water. As a result, the corrosion reaction generally occurs in very close proximity to the location of trapped water in dry systems and to the location of trapped air in wet systems. It occurs in other locations as well, but these are the primary locations where oxygen corrosion will occur.
All the reactions that take place due to the presence of oxygen produce solids that are trapped within the fire sprinkler system piping and will impede water delivery in the case of a fire related emergency.
How Do We Know?
ECS has been assisting clients in understanding and mitigating risks associated with corrosion in fire sprinkler systems for over a decade, and our main approach is to dissect and analyze failed sprinkler pipes. We have found that each type of corrosion exhibits characteristic metal loss patterns that are distinguishable from each other, and after analyzing thousands of samples, less than 5% of the leaks that occurred were caused by bacteria. As you can tell from Figure 1, the difference in pitting between MIC and Oxygen corrosion is fairly obvious.
Figure 1. Microbiologically Influenced Corrosion (MIC) Pit (left) vs. Oxygen Corrosion Pit (right)
We have also found that there is absolutely no correlation between the amount of bacteria in a fire sprinkler system and the number of leaks that the system experiences; and there is a direct correlation between how often oxygen rich air is introduced to a fire sprinkler system and the number of leaks that occur in that system. This correlation allows us to identify the root cause of the problem and provide a solution.
Dry Pipe Nitrogen Inerting
The reason that dry and preaction fire sprinkler systems develop corrosion related leaks is as follows:
- Water gets trapped in the piping from hydrostatic testing, trip testing or condensate from the air compressor – corrosion cannot occur without liquid water
- The air compressor provides what is essentially an unlimited supply of oxygen gas (21% of the air)
- Oxygen gas quickly dissolves into the water at the air/water interface
- The oxygen water mixture reacts with the iron or zinc at the pipe wall adjacent to the air/water interface – this reaction takes place in minutes
- The oxygen corrosion reaction produces two (2) results:
- Pit in the pipe wall where metal is removed
- Corrosion by-product deposit containing iron or zinc
The goal of Dry Pipe Nitrogen Inerting (DPNI) in dry and preaction fire sprinkler systems is to first purge the oxygen rich air from the piping and second to eliminate the future introduction of oxygen gas into the system piping, thus stopping the corrosion process all together.
When inerting a dry or preaction fire sprinkler system with nitrogen, there are three essential components that are required:
- A continuous source of nitrogen gas at 98%+ purity – nitrogen generator.
- An oxygen removal vent for removal of corrosive oxygen from the dry system piping.
- A “Fill and Purge” breathing protocol to facilitate the removal of all corrosive oxygen.
Wet Pipe Nitrogen Inerting
The reason that wet fire sprinkler systems develop corrosion related leaks is as follows:
- As the system is filled with water, the air that was originally in the pipe gets trapped and compressed at the high points in the system.
- Because the trapped air is in direct contact with the water, the oxygen molecules will dissolve into the fluid until it reaches its saturation limit. The remaining oxygen in the trapped air remains as a gas.
- The dissolved oxygen molecules in the water react with the iron or zinc molecules in the pipe wall at the air/water interface, creating iron oxide, or rust. This iron metal is removed from the pipe wall and creates a pit, thus exposing more metal for oxygen attack.
- This process continues each time new air is introduced to the system (every time it is drained).
If all the oxygen in the wet pipe fire sprinkler system piping is displaced with nitrogen gas before the system is filled with water, oxygen corrosion can be completely stopped. This is at the heart of the Wet Pipe Nitrogen Inerting (WPNI) process.
When inerting a wet fire sprinkler system with nitrogen, there are three essential components that are required:
- An integral venting device to remove oxygen installed at a remote accessible high point
- A nitrogen injection port installed at the sprinkler riser
- A source of nitrogen gas - either nitrogen cylinders or a nitrogen generator
Once these protocols have been successfully implemented into a fire sprinkler system, full corrosion control has been achieved. By removing Oxygen from the system all together, the corrosion process stops completely.