Thank you to our guest contributor, Lucas Kirn, with Engineered Corrosion Solutions (ECS). Stay tuned for more posts on the role of corrosion in sprinkler systems and also check out ECS’s Website http://ecscorrosion.com/ and Blog: http://ecscorrosion.com/category/blog/
Picture on left: Wet System with Black Steel Pipe, Accumulation of Solids and Sludge due to Oxygen
Picture on right: Dry System with Galvanized Pipe, Oxygen Corrosion Has Breached Zinc Layer and Attacked Base Black Steel
The Role of MIC in Fire Sprinkler System Corrosion
You may have heard the term “MIC” used in reference to corrosion problems in fire sprinkler systems, as it has become synonymous with all general corrosion activity in sprinkler systems. MIC, which stands for microbiologically influenced corrosion, is a very specific type of corrosion caused by bacteria. Over the past several years many practitioners in the fire sprinkler industry have over-emphasized the role that bacteria play in causing corrosion in a fire sprinkler system while under-emphasizing the predominant role that oxygen plays in the corrosion that occurs in these systems.
The simple fact is that the primary cause of internal corrosion (metal loss) in fire sprinkler systems is oxygen attack of the iron and zinc (in the case of galvanized pipe) that are used to fabricate fire sprinkler system piping. While it is true that MIC is often a contributing factor to the overall corrosion picture, it is not the primary cause of internal corrosion. Moreover, the solids that are produced by the action of oxygen on the metal piping actually produce conditions that favor the proliferation of bacteria in the system.
Oxygen is present as a component in the air trapped within the fire sprinkler piping system at a level of about 21%. When oxygen dissolves in any water that contacts the fire sprinkler piping, it becomes available to react with the metal pipe. This corrosion reaction produces solids that are trapped within the fire sprinkler system piping that tend to settle at low points because they are more dense than water. Solids provide an ideal environment for bacteria to colonize. In addition the solids may be nutrient sources for bacteria. For example, iron oxide solids provide a fundamental nutrient for iron related bacteria (IRB), which is the most common form of bacteria found in fire sprinkler systems.
The main point of confusion for most people is whether the presence of bacteria is necessary to initiate pitting underneath corrosion by-product solids in a fire sprinkler system. One strong piece of evidence supporting the fact that bacteria are not necessary to initiate pitting 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. Additionally, soluble zinc can be biocidal when levels in the water exceed 800 ppm, which means the environment will not support the growth of bacteria. Data collected from galvanized dry and preaction systems indicates that it is very common to find zinc levels in the water at levels between 1000 ppm and 2000 ppm. All of the evidence strongly supports oxygen corrosion as the predominant mechanism that leads to rapid pitting and metal loss in these systems.
Corrosion control programs for fire sprinkler systems that seek to slow down or stop the leaks and metal loss by controlling or killing bacteria without addressing oxygen as the primary cause of corrosion WILL NOT be successful. The most effective corrosion control strategy must include provisions to prevent attack of the fire sprinkler system piping by oxygen that is dissolved in the water. This is true for wet, dry and preaction fire protection systems.
Removing the oxygen from sprinkler system piping reduces the corrosion rate in multiple ways:
- Removal of oxygen prevents the formation of additional solid deposits in system piping.
- By preventing the iron oxide solid deposits from forming, the nutrient source of many of the bacteria has been eliminated.
- Without oxygen, even if any deposits remain in the system, secondary corrosion mechanisms will not take place since there is no corrosive gas available to complete the corrosion reaction.
Once oxygen corrosion is controlled, controlling or preventing microbiologically influenced corrosion (MIC) becomes much easier primarily because the system remains free of the solids that support the growth and proliferation of bacteria.
AIE Inspection Services has considerable experience working with corrosion in fire sprinkler systems. Whether you’re tired of dealing with sprinkler system leaks, you’re actively replacing piping, or you want to understand how extensive your corrosion problem really is, we can provide you with answers. Along with our partner, Engineered Corrosion Solutions (ECS), we will provide you with a customized solution for your property that minimizes downtime and costs while ensuring peace of mind.