 Copper
nickel alloys have been specified for sea water applications for over
50 years. These alloys have a remarkable combination of resistance
to both corrosion and biofouling in sea water. Since they can be easily
welded and fabricated, copper-nickel alloys are an excellent choice
for structures engineered for marine use, such as pipe systems, heat
exchangers, and boat hulls.
Many of the world's navies and merchant ships rely on copper nickel
alloys to get long lasting life from piping and condenser systems.
The alloys are also used in desalination, power plants, offshore fire
water systems, and sheathed protection of oil and gas platform legs
and risers.
The two main grades of copper nickel alloys for marine use are 90Cu-10Ni
and 70Cu-30Ni. The 70-30 alloy is stronger and has greater resistance
to sea water flow. The 90-10 alloy is a less expensive alternative.
This alloy provides good service for most marine applications and
is more widely used. Both alloys contain small amounts of iron and
manganese which help to provide the best combination of resistance
to flowing sea water and to overall corrosion.
The resistance to sea water corrosion results from a thin protective
surface film which forms naturally and quickly when exposed to clean
sea water. The initial film forms fairly quickly over the first several
days but takes up to three months to fully mature. Once a good surface
film forms, the corrosion rate will continue to decrease over a period
of several years.
Copper nickels are stronger than copper. Their ductility, toughness,
and formability are excellent attributes to construction for the marine
environment. They do not embrittle at low temperatures, as certain
steels do.
Painting copper nickel alloys is unnecessary, since they inherently
resist corrosion and biofouling. Painting may, however, be desirable
in special cases where there is a risk of galvanic corrosion, or for
aesthetic reasons.
Copper nickel alloys for boat hulls: Growth of marine organisms
on the steel hulls of marine vessels increases drag and has a significant
detrimental effect on their performance. It is customary to inhibit
growth with a biocide coating, replaced from time to time in dry dock.
When copper nickel alloy sheet are sheathed over a steel hull, a smooth
hull surface can be maintained. Alternatively, the hull can be protected
if it is constructed from solid copper nickel alloy plate or if it
is constructed from roll-bonded clad plate.
Sheathing off-shore structures: The corrosion rate of steel
in the splash zone falls between 0.5-1.5 mm/yr, depending on location
and season. Corrosion rates increase dramatically at the higher surface
temperatures found in hot riser pipes which are constructed from the
sea floor to the platform. At 900C, corrosion rates can reach 8 mm/yr.
Splash zone sheathing with a copper nickel alloy should span at least
from below mean tide level to well into the atmospheric zone. Galvanic
corrosion on the adjacent steel can be addressed by painting the top
section.
Desalination plants: Large heat exchangers producing up to
57,000 cubic meters of water per day are generated by the multi-stage
flash process. Copper nickel alloys are widely used to fabricate piping,
waterboxes, evaporator shells, and tube plates. The 90-10 nickel alloy
is usually used, although a 70-30 copper nickel with 2% iron and 2%
manganese is also widely used for heat exchanger tubing. |
