Objective: To establish the mechanism of failure and recommend corrective actions.
- The pictures show a failed sea cock from a motor boat.
- A sea cock is a through hull valve below the waterline that controls the flow of water between the vessels exterior and interior.
- The failure of a sea cock could be potentially disastrous for a vessel.
- The sea cock was manufactured from a leaded 60/40 brass.
- Failure had occurred at the first thread and the fracture surface exhibited a reddish colouration.
What we did
- Metallographic examination revealed a localised porous structure at the root of the threads as shown in the centre micrograph above.
- The reddish colouration and porous structure is indicative of plug dezincification.
- Chloride attack from the sea water selectively removes zinc from the alloy leaving behind a porous copper rich structure with little mechanical strength.
- A leaded 60/40 brass was an inadequate choice of material for sea water service.
- Dezincification resistant (DZR) brasses containing 0.03% arsenic are now readily available commercially.
- Technical advice was given to select a suitable alloy.
- All sea cocks on the vessel were replaced with DZR brass fittings.