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Marine corrosion diagnosis

Corrosion detection in ships' hulls using the T-scan ultrasonic system has recently attracted keen attention. The interest follows requests for help with corrosion diagnosis in coastal vessels by former TWI Industrial Member Ocean Fleets.

When the conditions creating corrosion cannot be avoided or changed, it is important to monitor corrosion progress so that corroded parts can be replaced before they fail. Work at Abington has involved computerised ultrasonic systems. The T-scan system adopted is able to image the corroded surface and thus detect isolated corrosion pits that could easily be missed when taking individual thickness measurements with a digital ultrasonic thickness gauge.

TWI's inspection of corrosion in coastal vessels has revealed pitting on the inside surface near the keel caused by the action of micro-organisms in sediments accumulating beneath the engines. Access to view the corroded surfaces from inside the ship is limited and inspection has to be carried out from the outside while the vessel is in dry dock.

The T-scan ultrasonic system is used to produce an image of the corroded internal surface of a ship hull
The T-scan ultrasonic system is used to produce an image of the corroded internal surface of a ship hull

The T-scan provides an image of the corroded surface that can be compared directly with what is observed visually. The spatial distribution of corrosion pits can be related to features such as bulkheads, which helps in diagnosis of the corrosion problem. Moreover, scans can be repeated at intervals and the images compared to monitor the progress of corrosion.

So far, the results of the inspection have enabled the owners and operators of the vessels to quantify the extent of corrosion in susceptible regions of the hulls. This in turn has provided operator confidence in the ability of the vessels to continue in service without major problems and has allowed repair scheduling to be more accurate.

For more information please email contactus@twi.co.uk.

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