Pipe failures resulting from Microbially Influenced Corrosion (MIC) have been widely recognized in petrochemical, gas and nuclear power industries. Recently this phenomenon has been associated with failures in fire protection systems (FPS).

Microbially Influenced Corrosion results in mechanical blockages of piping and sprinkler heads, as well as through-wall penetration of ferrous and non-ferrous metals. Fire protection systems are designed for the life of the structures in which they reside; however, reports of new systems developing MIC-associated through-wall leaks within months of installation are becoming more prevalent.

Fire protection systems can represent a particularly complex challenge for biological fouling prevention and control. Mechanical blockages and MIC have been implicated as significant problems in fire protection systems, both in carbon steel and copper-based systems, because these water distribution systems are usually composed of carbon steel of decreasing diameters. Fluid flow is nearly always stagnant, and the piping conduits are not designed to facilitate routine cleaning operations. As a result, the presence of reduced corrosion products coupled with long residence times diminishes available chlorine, which would otherwise control biological fouling activities.

Pitting corrosion occurring under deposits in fire protection systems can be initiated or propagated by these microbial activities. Through-wall penetration of carbon steel and copper has been reported within months after a new pipeline has been brought into service. This extensive tuberculation can cause occlusion of pipelines, sometimes completely blocking flow in six-inch diameter pipelines. These problems become more critical as pipe diameter decreases, posing a potential threat to proper sprinkler head mechanical functioning.

In addition, fire protection systems make-up waters are typically stagnant, soft (relatively low in hardness), acidic and devoid of antimicrobial agents such as the sodium hypochlorite that is used for microbial control in potable waters. These characteristics predispose FPS to biological fouling and MIC. Regulatory requirements that dictate periodic testing can also contribute to development of MIC in FPS when make-up waters are replaced with oxygenated and nutrient-rich waters. MIC-associated microorganisms can use these nutrients as growth sources, leading to fouling of affected systems.

The most serious consequence of MIC in fire protection systems is mechanical blockage of piping and sprinkler heads. MIC-associated organisms can attach to the metallic surfaces of fire protection systems, forming corrosion deposits that are termed tubercles.

Tubercles can completely occlude pipes, and more significantly, these deposits can break off and block sprinkler head flow channels. Localized pitting-type attack can also occur underneath tubercles, resulting in through-wall penetration. The resulting acid production, hydrogen sulfide generation and development of differential aeration cells can lead to the loss of essential metallic properties of mild steel, copper, stainless steel and other ferrous and non-ferrous metals.