Corrosion in Double Bottom in Engine Room

Water ingress to the engine room of a bulk carrier was discovered during a voyage. The leakage was found to come through the short sounding pipe of the bilge water holding tank in the double bottom. After the visual inspection of the tank, the sea water ingress was found to come through an approximately 45 mm diameter area of serious pitting/hole in bottom plate (see figure below).

2013.11.22 - Corrosion in Double Bottom in Engine Room Figure 1

The hole was temporarily tightened using 10 mm thick round steel plate of 200 mm diameter with rubber gasket, and properly secured. An area of approximately 0.50 m2 was found severely corroded and was to be repaired by an insert plate.

The probable cause of the corrosion was that waste chemicals had accumulated at the bottom of the bilge water tank, and had not been possible to drain properly during normal operation, (see figure below).

2013.11.22 - Corrosion in Double Bottom in Engine Room Figure 2

During investigation by the crew it was found that a very corrosive chemical, hydrochloric acid, had been used for de-scaling of the fresh water generator and then later drained into the bilge water holding tank.

It was also found that the sounding pipe cover had been removed from the pipe for easy sounding!

It should be noted that this damage could have caused flooding of the engine room and hampered the safety of the ship if not discovered in time.

2013.11.22 - Corrosion in Double Bottom in Engine Room Figure 3

Lessons to be learned

  • Be careful in use of corrosive chemicals in general.
  • Waste chemicals should in general be stored in separate containers of plastic or glass, or in other spaces adequately protected against attack from the chemicals.
  • Regular draining and inspection of tanks containing bilge water should be provided. Such tanks should be included in the maintenance programme on board.
  • Put the sounding pipe cover back in place after sounding!

Source: DNV

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