The IVR Association has published an information document explaining the hidden risks of biological corrosion and providing additional information on how to avoid bacteria that damages ship bilges. The technical leaflet Biological Corrosion focuses on a number of cases where the issue of leaking engine rooms seems to be more and more common as a result of this type of corrosion.
In recent years, an increasing number of cases of leaking engine rooms, caused by very local perforations of the ship’s plating have been noted. This happened despite the recent class renewal in which bottom inspection and thickness measurements showed that the plating was sound. In these cases, investigation reveals that the water ingress is by very local corrosion perforations.
The release of this documentation coincides with information published by IIMS on the subject of biological corrosion, something of a less well known phenomenon. Veteran marine surveyor, Jeffrey Casciani-Wood, has studied this subject over a long period and has published a short free guide on microbial corrosion or bug attack as he calls it. He feels that many surveyors are not as familiar with form of corrosion as they should be, not least because of its aggressive properties. The 16 page booklet is available to download in pdf format here.
His lecture on the subject is available for viewing on YouTube (see below).
Biological corrosion is an aggressive corrosion that starts at the inner side of the plating and proceeds to the outside until a perforation occurs. Typically, these perforations are mainly crater shape.
While corrosion is a chemical process, in these cases, it is caused by microbiological activity. This form of corrosion is often found in engine rooms because bacteria flourishes in carbonaceous bilge water. It is because they flourish in bilges that they are also difficult to detect. Bilges are hidden below the engine room floor plates and are mostly dirty.
The phenomenon known as biological corrosion is commonly seen in somewhat older ships, although not exclusively in older vessels. This kind of corrosion, starting from the inside, cannot be detected during a bottom survey from the outside. Finding one of these craters during random thickness measurements would therefore be pure coincidence. In the case of class renewals, it is also not common practice to clean the bilges thoroughly for inspection.
It is because of their hidden character that they are so dangerous. The development of these perforations is sudden and unexpected and can lead to high consequential damage (a flooded engine room for example).
It is advisable that on board of older ships, the floor plates in the engine room periodically are removed (advisably once every 20 years) in order to allow the cleaning of the bilges with the pressure washer and to preserve the inside of the plates.