April 2009: Corrosion in Sprinkler Systems
Wet and dry sprinkler systems consist of metal pipe, water and trapped or compressed air. Any environment that places oxygen, metal and untreated water in prolonged contact with each other is subject to corrosion. The presence of air gaps in wet systems or undrained water in dry systems can be a contributing factor to promote corrosion.
Corrosion is a natural phenomenon that gradually destroys metal by a chemical or electrochemical reaction with its environment. The environment inside the pipe normally contains water, which is conductive. When the water comes in contact with the metal pipe, it promotes electrochemical reactions between itself and the metal. In steel pipe, these reactions result in the creation of iron oxide or rust.
The most common forms of corrosion found in sprinkler systems are crevice corrosion, uniform corrosion and pitting. Crevice corrosion is a localized form of corrosion that occurs in crevices (beneath gaskets, in holes, beneath surface deposits, in thread and groove joints) exposed to a stagnant corrosive environment. Uniform corrosion is a general thinning involving the regular loss of a small quantity of metal evenly distributed over a large surface area of pipe. The increased use of thin wall pipe has made uniform corrosion more prevalent than in the past. Pitting is a localized form of corrosion that results in covered or open holes or cavities in the metal.
Corrosion can result in increased friction loss in sprinkler pipe, which can affect the output of a sprinkler system. In addition, if deposits are significant, they can completely block the orifice of a sprinkler head.
Another form or corrosion that can accelerate previously occurring corrosion is Microbiologically influenced corrosion (MIC). MIC is an electrochemical process, which involves bacteria that can accelerate previously occurring corrosion in both wet and dry pipe sprinkler systems. MIC is caused by several different types of microorganisms that live on nutrients in water and react with the products of other microorganism reactions and with piping material.
MIC generally results in the growth of biofilms and nodules within the piping. Biofilms, usually appearing as a black slime, can be flushed out. Nodules, however, are solid and will adhere to the pipe interior and will not flush. While MIC is not new, its association with fire protection systems is relatively new. Unlike many other industries, the water in a sprinkler system in generally dormant and there has not been established corrosion control practices in place.
Beyond obvious external visual observations, leaks or obstructions can also be potential indicators of corrosion. Other indicators can be a noted reduced flow from Inspectors Test Connections or 2 in. drains during normal sprinkler system testing.
NFPA 13-2007, "Standard for the Installation of Sprinkler Systems"
indicates that sprinkler systems shall be maintained in accordance with
NFPA 25, "Standard for the Inspection, Testing, and Maintenance of
Water-Based Fire Protection Systems," as follows: