Case Studies - Plug Dezincification of a Brass Seacock

Objective: To establish the mechanism of failure and recommend corrective actions.

Situation

Seacock 1

  • 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

Stud 2

  • 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.

Impact

Stud 3

  • 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.