What is Cupronickel?

This copper-nickel alloy has many uses

Cupronickel bars
••• Image copyright Gurudev Metal

Cupronickel (also referred to as "cupernickel" or copper-nickel alloy) refers to a group of copper-nickel alloys that are used in saltwater environments due to their corrosion resistant properties.

The most common cupronickel alloys are: 90/10 Cupro-nickel (copper-nickel-iron) or 70/30 Cupro-nickel (copper-nickel-iron)

These alloys have good working properties, are readily weldable and considered insensitive to stress corrosion.

Cupronickel is also resistant to biofouling, crevice corrosion, stress corrosion cracking and hydrogen embrittlement.

Slight differences in corrosion resistance and strength generally determine which alloy grade is used for a particular application.

History of Cupronickel

Cupronickel has been made and used for over a thousand years. Its first known use was in China in about 300 BCE. Chinese records describe the process for making "white copper," which involved heating and mixing copper, nickel, and saltpeter.

Cupronickel was also used to make Greek coins. Later European "rediscovery" of cupronickel involved alchemical experiments.

The alloy was used by the U.S. Mint to make three-cent pieces and five-cent pieces in the period after the Civil War. The coins had previously been made of silver, which became scarce during the war. For the past several decades, the cladding or coating on American 50-cent pieces, quarters and dimes has been made of cupronickel.

There are many coins in circulation, if not in current use, that either use cupronickel or are made of cupronickel. This includes the Swiss franc, the 500 and 100 won pieces in South Korea and the American Jefferson nickel. 

Corrosion Resistance of Cupronickel

Cupronickel is naturally resistant to corrosion in seawater, making it a valuable metal for marine use.

This alloy is able to resist corrosion in seawater because its electrode potential is essentially neutral in such environments. Consequently, it will not form electrolytic cells when placed in close proximity to other metals within an electrolyte, which is the main cause of galvanic corrosion.

Copper also naturally forms a protective oxide layer on its surface when exposed to seawater, which protects the metal from deterioration.

Applications for Cupronickel

Cupronickel has a wide array of uses. In some cases, it is valued for its strength and corrosion-resistance. In other cases, it is valued for its silver color and rust-free shine. Some examples of cupronickel's uses include:

  • tubes for light-duty condensers, feedwater heaters, and evaporators used in power stations and desalination plants
  • pipes carrying seawater to fire mains, cooling water systems and ship sanitary systems
  • sheathing for wooden piles
  • underwater fencing
  • cabled tubes for hydraulic and pneumatic lines
  • fasteners, crankshafts, hulls and other marine hardware used in boats
  • silver-colored circulation coins
  • silver-plated cutlery
  • medical equipment
  • automobile parts
  • jewelry
  • cylinder cores in high-quality locks

Cupronickel has a wide array of applications in cryogenics since it has a good thermal conductivity at extremely low temperatures.

The material also used to be used to coat the jackets of bullets in the late 19th century, but caused some metal fouling in the bore, and was later replaced.

Standard Cupronickel Compositions (Wt. %)

Cupronickel AlloyAlloy UNS No.CopperNickelIronManganese
90/10 CupronickelC70600Balance9.0-11.01.0-2.00.3-1.0
70/30 CupronickelC71500Balance29.0-32.00.5-1.50.4-1.0