The primary cause of internal corrosion in fire sprinkler systems is oxygen attack of the iron and zinc (in galvanized pipe) materials that are used to fabricate fire sprinkler system piping. While it is true that microbiologically influenced corrosion (MIC) is often a contributing factor to the overall corrosion picture, it is not the primary cause of internal corrosion. The solid byproducts that are produced by oxidation of the metal piping actually produce conditions that favor the proliferation of bacteria in the system.
When oxygen dissolves in any water that contacts the fire sprinkler system piping, it becomes available to react with the metal pipe. In dry and preaction fire sprinkler systems, oxygen corrosion is limited to areas of the pipe that are wet with liquid moisture. Unlike wet fire sprinkler systems where the supply of trapped oxygen is limited, oxygen for the corrosion reaction is in great excess in dry and preaction fire sprinkler systems and it will continue to react with the iron in the pipe. This is why oxygen corrosion more aggressive in dry and preaction systems.