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Index >> Applications of Microbial Interactions >> Pitting Corrosion

Pitting Corrosion

	Pitting Corrosion Pitting corrosion is a localised form of corrosion; the bulk of the surface remains unattacked. Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films. Localised pitting attack is found where these passive films have broken down.
Pitting attack induced by microbial activity, such as sulphate reducing bacteria (SRB) also deserves special mention. Within the pits, an extremely corrosive micro-environment tends to be established, which may bear little resemblance to the bulk corrosive environment. For example, in the pitting of stainless steels in chloride-containing water, a micro-environment essentially representing hydrochloric acid may be established within the pits. The pH within the pits tends to be lowered significantly, together with an increase in chloride ion concentration, as a result of the electrochemical pitting mechanism reactions in such systems.
The detection and meaningful monitoring of pitting corrosion usually represents a major challenge. Pitting failures can occur unexpectedly, and with minimal overall metal loss. Furthermore, the pits may be hidden under surface deposits, and or corrosion products. Monitoring pitting corrosion can be further complicated by a distinction between the initiation and propagation phases of pitting processes. The highly sensitive electrochemical noise technique may provide early warning of imminent damage by characteristic signals in the pit initiation phase. Show the extent of pitting corrosion.
Pipe failures resulting from microbiologically influenced corrosion (MIC) have been widely recognised in petrochemical, gas and nuclear power industries, but only recently has .this phenomenon been associated with failures in fire protection systems (FPS). MIC results in mechanical blockages of piping and sprinkler heads, as well as through-wall penetration of ferrous and non-ferrous metals. FPS 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.
	Pitting corrosion occurring under deposits in FPS 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, FPS make-up waters are typically stagnant, soft (relatively low in hardness), acidic and devoid ot 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 FPS is mechanical blockage of piping and sprinkler heads. MIC-associated organisms can attach to the metallic surfaces of FPS, 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. Localised pitting-type attack can also occur underneath tubercles, resulting in through-wall penetration. The resulting acid production, hydrogen sulphide 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.

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