What causes MIC?
The presence of oxygen, trapped within air pockets in a fire sprinkler system and dissolved in the water itself, along with other dissolved nutrients and iron from the pipe walls create the perfect environment for Microbiologically Influenced Corrosion. A small amount of oxidation corrosion can be enough to allow several different types of bacteria to come together and create a colony and start the cycle of MIC.
Some of the bacteria thrive in an oxygen rich environment and are referred to as 'aerobic'. Some aerobic bacteria form gelatinous slimes and sticky polymers which attract and create attachment points for additional bacteria and nutrients.
Iron related bacteria can attach to slime and actually consume the iron of the pipe walls and create corrosion deposits on them. As iron is consumed, 'pits' are created and over time pits can develop into telltale 'pin-hole' leaks. Pinhole leaks are often the first (and most obvious) sign of MIC.
The deposits created by iron related bacteria can eventually grow to form rounded nodules called 'tubercles'. Tubercles decrease the diameter of the pipe and reduce flow rate, impeding proper performance of the sprinkler heads. Tubercles can also break free and create debris that can clog the heads.
Tubercles also create an environment for other bacteria that thrive without oxygen. These bacteria are 'anaerobic'.
Certain anerobic bacteria produce byproducts that lower the pH level of the entire system to acidic levels and contributes further damage to pipe walls through pitting and pin-hole leaks.
Sulfate reducing bacteria draw sulfates and hydrogen from the water and iron from the pipe walls to create sulfuric acid, iron sulfide and hydrogen sulfide gas as byproducts. Sulfuric acid furhter degrades pipe walls and iron sulfide, also known as 'black solids', creates deposits that add to water flow restriction. Hydrogen sulfide gas produces the distinctive 'rotten egg' smell associated with MIC.
The symbiotic relationship of the bacteria that make up a MIC colony creates a multi-pronged attack on fire sprinkler systems that can result in leaking pipes, restricted water flow and obstructed heads. The action of the colony can only be stopped if steps are taken to inhibit their formation and break their life cycle. MIC can return if a system is not properly monitored and maintained.