Copper-nickel alloys have excellent resistance to seawater corrosion, anti bio-fouling properties and ability to withstand corrosion and erosion attack. Hence copper-nickel alloys are widely used for marine offshore applications.
Typical applications include:
- Fire protection systems
- Deluge systems
- Sanitary plumbing services
- Process cooling lines
- Seawater piping
- Splash zone sheathing
Corrosion resistance of Copper-Nickel Alloys
The corrosion resistance of CuNi alloys is due to the presence of protective surface film of copper nickel oxide which is firmly bonded to the metal and which is almost insoluble in water. This oxide film protects the piping from further attack. The protective oxide film takes about three months to stabilize. Hence it is recommended not to expose newly fabricated copper-nickel piping to aggressive environments. The corrosion rate in static seawater is approximately 0.1 mm/year. Once the protective film is formed the rate of corrosion is reduced to 0.01 mm/year. The corrosion rate dramatically increases in presence of sour environment such as H2S due to breaking of protective oxide film by sulphide ions.
In systems which are in continuous service, too high fluid velocities can break the protective film due to erosion. The copper oxide film can get destroyed by erosion at fluid velocities of about 3.5 m/s. To limit the erosional velocities, copper-nickel piping systems could have larger diameters than equivalent steel piping systems.
Commonly used copper-nickel alloys
The following two alloys of copper-nickel are most commonly used in the industry.
- 90-10 Alloy (UNS C70600) containing 10% nickel also called Alloy 706
- 70-30 Alloy (UNS C71000) containing 30% nickel also called Alloy 710
90-10 copper-nickel is more widely used in the industry due its better fouling resistance than 70-30 copper-nickel.
Applicable Industry Codes and Standard
- EEMUA 144 - 90/10 Copper-Nickel Alloy Piping for Offshore Applications Specification: Tubes Seamless and Welded
- EEMUA 145 - 90/10 Copper-Nickel Alloy Piping for Offshore Applications Specification: Flange Composite and Solid
- EEMUA 146 - 90/10 Copper-Nickel Alloy Piping for Offshore Applications Specification: Fittings