Commercially, the 10 and 30% nickel (described as 90-10 and 70-30) alloys are the most commonly used. They have small but important addition of iron and manganese to optimise their corrosion resistance. These very ductile alloys can only be strengthened by cold work, the 70-30 alloy being the stronger of the two and capable of handling higher flow rates. Good thermal conductivity is also beneficial for heat exchangers and condensers, particularly in the 10% alloy. These alloys are readily fabricated and welded and can also be successfully welded to steel.
Other wrought alloys include a 2% Mn and 2% Fe grade, (66-30-2-2), only available as tubing, which can be used at higher flow velocities and in the presence of abrasive particles. There is also a more recent development for even higher flow rates, which contains a chromium addition (Cu-30Ni-Cr).
Cast copper-nickel alloys are available, although often nickel aluminium bronzes are used for pumps and valves in copper-nickel systems. A cast Cu-30Ni-Cr alloy has been developed as an alternative to nickel aluminium bronze for UK Royal Naval use.
High-strength copper-nickel alloys fall into two categories; those strengthened by age hardening (Cu-Ni-Al alloys) and those that can be spinodally strengthened by means of a thermally-induced sub-microscopic chemical composition fluctuation (Cu-Ni-Sn and Cu-30Ni-Cr). Hardnesses approaching those of high strength steel can be obtained in some of these alloys, although their anti-galling and non-sparking properties are often important requirements.

Corrosion resistance

As with other nickel-containing alloys, 90-10 and 70-30 copper-nickel alloys rely on a protective surface film to maintain their corrosion resistance. However, they differ because the protective films result from a reaction with the seawater itself, rather than the oxide film formed in air, and are a complex and layered mixture of oxides, chlorides and hydroxy-chlorides. These protective surfaces initially form rapidly but will continue to develop over the months and years, providing low corrosion rates. This means that short-term corrosion rate results are misleading. It is also important to ensure the alloys encounter the appropriate seawater conditions during initial exposure, particularly during commissioning and hydrotesting. This will ensure the surface films offer suitable protection.
Copper-nickel alloys are not susceptible to chloride-induced pitting, crevice corrosion or stress corrosion cracking, freeing them from the temperature limitations associated with these types of corrosion in stainless steels. These alloys are similar to other copper alloys, in that sulphides and ammonia can affect the surface films. Sulphide stress corrosion and hydrogen embrittlement are not problematic in these alloys; however, sulphides may alter the nature of the protective film, leading to pitting and higher corrosion rates. Therefore, extended exposure to polluted seawater containing sulphides or, in calm conditions, deposits containing sulphate-reducing bacteria (SRBs) should be avoided. Unlike brass alloys, copper-nickel alloys display high resistance to ammonia stress cracking, which is not an issue in seawater although the presence of ammonia can cause higher corrosion rates.

The surface film can lose its tenacity when exposed to high velocities and turbulent areas, and erosion-corrosion can be experienced. However, the process is well understood and occurs at higher flow rates in copper-nickel alloys than in other copper alloys. It is important to adhere to the appropriate guidelines. Good design and operational practices should avoid circumstances that increase velocity, such as partially-throttled valves, tight-angled bends and obstructions in piping systems.
Copper-nickel alloys sit midway in the galvanic series and are generally compatible with other copper alloys. They are more noble than steel and aluminium but may corrode preferentially when connected to passive stainless steels, high-nickel alloys and titanium.

Attachment by Marine Organisms

Adhesion of marine organisms on surfaces can cause various problems, including extra energy consumption as well as cleaning and maintenance costs. Applications affected include seawater intake systems, piping, aquaculture cages, boat hulls and offshore sheathing.

Although copper-nickel alloys can harbour biofilms (slimes), macro-organism attachments - such as marine grasses and hard-shell organisms are impaired. If these do become attached under calm conditions, adherence is poor and they can be easily removed. To minimise attachment, the alloys need to be freely exposed and should not be influenced by cathodic or other means of galvanic protection.

An overview of the properties of these alloys is provided in the sub-sections given at the top of this page. For more detailed information see the Literature sub-section which includes relevant Nickel Institute publications, selected literature from other sources plus an Ask US request to look at data, info and experience from our comprehensive archive. Information is also still currently available from www.coppernickel.org.