'While corrosion is a chemical process, in some cases it is caused, or reinforced by, microbiological activity, this is a type of local corrosion where bacteria show a fast development, producing chemical agents that accelerate the corrosion process.
This highly underestimated phenomenon is known as MIC (microbiologically influenced corrosion).
In recent years, BMT Surveys has seen an increasing number of cases of leaking engine rooms caused by very local perforations of the ship’s plating. And this happened despite the recent Class renewal in which bottom inspections and thickness measurements showed that the plating was sound.
Typically, these perforations were mainly crater shape with sloping sides while the plating surrounding the craters still measured at its optimal, sound thickness.
This form of corrosion is mostly found in engine rooms because these bacteria flourish in carbonaceous bilge water, which of course, makes these little steel eaters very difficult to detect. They can be found under floor plated where dirt is often present in abundance, particularly infecting the places most difficult to reach or clean.
Salt water vessels – being exposed to high levels of sodium chloride – are for this reason very susceptible to this form of corrosion, unless they are, for example, fitted with a protective coating.
This is why this phenomenon is mostly seen in somewhat older ships. The hidden character of these old ships is what makes them so risky. The development of these perforations is sudden and unexpected and the consequential damage of flooded engine rooms can be significant when you think of engine failure, towage costs etc. Exterior bottom inspections do not expose these perforations because the source is found on the inside. Finding one of these craters during a random thickness measurements would therefore be pure coincidence.
Furthermore, cleaning bilges for a thorough inspection of the inside of the plating is not general practice in the case of Class renewals.
Consequently, it is important – once a ship is 15 years old – to remove the floor plating in the engine room, to clean the bilges with a pressure washer an preserve the inside of the plates. Prevention is better than cure in this case as well, because it is not a question of whether these MIC problems will arise, it is only a matter of time before they will.'
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