An unusual and perhaps unexpected corrosion problem can be caused by bacteria. As one of the oldest groups of organisms on the planet, bacteria have evolved to survive even in extreme environments.

Bacterial corrosion can occur in fuel tanks, for example (Fig1): fuel oil contaminates bilge water on tankers, and bacteria then grow profusely in the mixture.

The bacteria feed on the organic oil, releasing mild organic acids and depleting the oxygen content of the water. The acids will accelerate corrosion of the steel container, but a more serious stage can develop when certain species known as sulphate-reducing bacteria take over. These reduce the oxygen content of the sulphates commonly present in dirty fuel oils to produce hydrogen sulphide, or H2S. This compound is potent at corroding steel and can also enhance hydrogen embrittlement (which is a form of stress corrosion cracking), attack usually occurring as pits in the metal close to or under the bacterial colonies. Such colonies are perhaps better known for the ‘rusticles’ they produce – as were present on the wreck of the Titanic (Figure 2). The colonies of bacteria live on the rust, and promote further rusting through chemical attack of the underlying steel.

Such bacterial attack can also cause disasters directly, as in the gas explosion near Carlsbad in New Mexico, USA on 19 August 2000. The natural gas was carried in a 760 mm diameter steel pipe across a river via a suspension bridge. The pipe fractured suddenly, releasing gas that ignited into a fireball, engulfing the bridge and killing 12 people. It left a large crater, at the base of which were found the ends of the pipe; the missing pieces were ejected by the explosion (Figure 3).

Analysis of sludge found in the pipe showed evidence of extensive microbial attack in the form of deep pits in the pipe wall, and the presence of various contaminants including chlorides, hydrogen sulphide and sulphates. The fracture had occurred at a deeply corroded section of the 7.6 mm thick wall, where the wall thickness had been reduced to less than 2.5 mm. The rupture took the form of a 525 mm long crack along the axis of the pipe, which was under an internal pressure of 4.65 MPa. Better inspection procedures were recommended after the accident, including the use of cleaning ‘pigs’, which travel within pipes, both monitoring internal problems and cleaning debris away.
From- http://openlearn.open.ac.uk/