Cupronickel

Cupronickel or copper–nickel is an alloy of copper with nickel, usually along with small quantities of other metals added for strength, such as iron and manganese. The copper content typically varies from 60 to 90 percent.
Despite its high copper content, cupronickel is silver in colour. Cupronickel is highly resistant to corrosion by salt water, and is therefore used for piping, heat exchangers and condensers in seawater systems, as well as for marine hardware. It is sometimes used for the propellers, propeller shafts, and hulls of high-quality boats. Other uses include military equipment and chemical industry, petrochemical industry, and electrical industries.
In decorative use, a cupronickel alloy called nickel silver is common, although it contains additional zinc but no silver.
Another common 20th-century use of cupronickel was silver-coloured coins. For this use, the typical alloy has 3:1 copper to nickel ratio, with very small amounts of manganese. In the past, true silver coins were debased with cupronickel, such as coins of the pound sterling from 1947 onward having their content replaced.

Name

Cupronickel, as the German kupfernickel, originally referred to the mineral form of nickel arsenide; natural deposits had superficial similarities to copper ores, and local folklore blamed the sprite Nickel for the absence of usable copper and health issues from arsenic exposure. It was from a sample of this kupfernickel that Baron Axel Fredrik Cronstedt first isolated elemental nickel in 1751, naming the new metal for the sprite. The mineral was given its modern names, nickeline and niccolite, by the mid-19th century.
Aside from cupronickel and copper–nickel, several other terms have been used to describe the material: the tradenames Alpaka or Alpacca, Argentan Minargent, the registered French term cuivre blanc, Chinese silver, and the romanized Cantonese term Paktong, 白銅.
Cupronickel alloys containing zinc are referred to as nickel silver, also sometimes hotel silver, German silver, plata alemana.

Applications

Marine engineering

Cupronickel alloys are used for marine applications due to their resistance to seawater corrosion, good fabricability, and their effectiveness in lowering macrofouling levels. Alloys ranging in composition from 90% Cu–10% Ni to 70% Cu–30% Ni are commonly specified in heat exchanger or condenser tubes in a wide variety of marine applications.
Important marine applications for cupronickel include:

Shipbuilding and repair: hulls of boats and ships, seawater cooling, bilge and ballast, sanitary, fire fighting, inert gas, hydraulic and pneumatic chiller systems.
Desalination plants: brine heaters, heat rejection and recovery, and in evaporator tubing.
Offshore oil and gas platforms and processing and FPSO vessels: systems and splash zone sheathings.
Power generation: steam turbine condensers, oil coolers, auxiliary cooling systems and high pressure pre-heaters at nuclear and fossil fuel power plants.
Seawater system components: condenser and heat exchanger tubes, tube sheets, piping, high pressure systems, fittings, pumps, and water boxes.
Coinage

The successful use of cupronickel in coinage is due to its corrosion resistance, electrical conductivity, durability, malleability, low allergy risk, ease of stamping, antimicrobial properties and recyclability.
In Europe, Switzerland pioneered cupronickel-based billon coinage in 1850, with the addition of silver and zinc, for coins of 5, 10 and 20 Rappen. Starting in 1860/1861, Belgium issued 5, 10 and 20 Centimes in pure cupronickel, and Germany issued 5 and 10 Pfennig in the same 75:25 ratio from 1873/1874. In 1879, Switzerland, for 5 and 10 Rappen coins, also adopted that cheaper 75:25 copper to nickel ratio then being used in Belgium, the United States and Germany. From 1947 to 2012, all "silver" coinage in the UK was made from cupronickel. Moreover, when silver prices rose in the 1960s/1970s also some other European countries replaced remaining silver denominations by cupronickel, e.g. the 1/2 to 5 Swiss franc coins starting 1968 and German 5 Deutsche Mark 1975–2001. Since 1999, cupronickel is also used for the inner segment of the 1 euro coin and the outer segment of the 2 euro coin.
In part due to silver hoarding in the Civil War, the United States Mint first used cupronickel for circulating coinage in three-cent pieces starting in 1865, and then for five-cent pieces starting in 1866. Prior to these dates, both denominations had been made only in silver in the United States.
Cupronickel is the cladding on either side of United States half-dollars since 1971, and all quarters and dimes made after 1964. Currently, some circulating coins, such as the United States Jefferson nickel, the Swiss franc, and the South Korean 500 and 100 won are made of solid cupronickel.

Decorative housewares

Nickel silver cupronickels are used extensively as a substitute for silver in tableware and other decorative housewares. Nickel silver is also used as a base for silver plating, where the product is known as electro-plated nickel silver, or EPNS.

Other usage

A thermocouple junction is formed from a pair of thermocouple conductors such as iron-constantan, copper-constantan or nickel-chromium/nickel-aluminium. The junction may be protected within a sheath of copper, cupronickel or stainless steel.
Cupronickel is used in cryogenic applications. It retains high ductility and thermal conductivity at very low temperatures. Where other metals like steel or aluminum would shatter and become thermally inert, cupronickel's unusual thermal and mechanical performance at these low temperatures facilitate a number of niche uses. Machinery that must perform many duty cycles at continuously low-temperatures and heat exchangers at cryogenic plants are the main industrial destinations of cupronickel in cryogenic applications. Niche applications also exist, for example the alloy's high thermal conductivity at low temperatures has made cupronickel ubiquitous in freeze branding operations.
In the early 20th century, this material was used to make bullet jackets. But it was soon replaced by gilding metal to reduce metal contamination of the barrel bore.
Currently, cupronickel and nickel silver remain the basic material for silver-plated cutlery. It is commonly used for mechanical and electrical equipment, medical equipment, zippers, jewelry items, and both for strings for instruments in the violin family, and for guitar frets. Fender Musical Instruments used "CuNiFe" magnets in their "Wide Range Humbucker" pickup for various Telecaster and Starcaster guitars during the 1970s.
For high-quality cylinder locks and locking systems, cylinder cores are made from wear-resistant cupronickel.
Cupronickel has been used as an alternative to traditional steel hydraulic brake lines, as it does not rust. Since cupronickel is much softer than steel, it bends and flares more easily, and the same property allows it to form a better seal with hydraulic components.

Physical and mechanical properties

Cupronickel lacks a copper color due to nickel's high electronegativity, which causes a loss of one electron in copper's d-shell.
Important properties of cupronickel alloys include corrosion resistance, inherent resistance to macrofouling, good tensile strength, excellent ductility when annealed, thermal conductivity and expansion characteristics amenable for heat exchangers and condensers, good thermal conductivity and ductility at antimicrobial touch surface properties.

Alloy UNS No.	Common name	European spec	Ni	Fe	Mn	Cu	Density g/cm³	Melting point deg C	Spec heat J/	Thermal conductivity W/	TEC μm/	Electrical resistivity μOhm·cm	Elastic modulus GPa	Yield strength MPa	Tensile strength MPa	Linear expansion 10⁶/deg K	Modulus rigidity GPa
C70600	90–10	Cu₉₀Ni₁₀	9–11	1–1.8	1	Balance	8.9	1100–1145	377	40	17	19	135	105	275	17	50
C71500	70–30	Cu₇₀Ni₃₀	29–33	0.4–1.0	1	Balance	8.95	1170–1240	377	29	16	34	152	125	360	16	56
C71640	66–30–2–2	Cu₆₆Ni₃₀Fe₂Mn₂	29–32	1.7–2.3	1.5–2.5	Balance	8.86			25	15.5	50	156	170	435

Subtle differences in corrosion resistance and strength determine which alloy is selected. Descending the table, the maximum allowable flow rate in piping increases, as does the tensile strength.
In seawater, the alloys have excellent corrosion rates which remain low as long as the maximum design flow velocity is not exceeded. This velocity depends on geometry and pipe diameter. They have high resistance to crevice corrosion, stress corrosion cracking and hydrogen embrittlement that can be troublesome to other alloy systems. Copper–nickels naturally form a thin protective surface layer over the first several weeks of exposure to seawater and this provides its ongoing resistance. Additionally, they have a high inherent biofouling resistance to attachment by macrofoulers living in the seawater. To use this property to its full potential, the alloy needs to be free of the effects of, or insulated from, any form of cathodic protection.
However, Cu–Ni alloys can show high corrosion rates in polluted or stagnant seawater when sulfides or ammonia are present. It is important, therefore, to avoid exposure to such conditions, particularly during commissioning and refit while the surface films are maturing. Ferrous sulfate dosing to sea water systems can provide improved resistance.
As copper and nickel alloy with each other easily and have simple structures, the alloys are ductile and readily fabricated. Strength and hardness for each individual alloy is increased by cold working; they are not hardened by heat treatment. Joining of 90–10 and 70–30 is possible by both welding or brazing. They are both weldable by the majority of techniques, although autogenous or oxyacetylene methods are not recommended. The 70–30 rather than 90–10 weld consumables are normally preferred for both alloys and no after-welding heat treatment is required. They can also be welded directly to steel, providing a 65% nickel–copper weld consumable is used to avoid iron dilution effects. The C71640 alloy tends to be used as seamless tubing and expanded rather than welded into the tube plate. Brazing requires appropriate silver-base brazing alloys. However, great care must be taken to ensure that there are no stresses in the Cu–Ni being silver brazed, since any stress can cause intergranular penetration of the brazing material, and severe stress cracking. Thus, full annealing of any potential mechanical stress is necessary.
Applications for Cu–Ni alloys have withstood the test of time, as they are still widely used and range from seawater system piping, condensers and heat exchangers in naval vessels, commercial shipping, multiple-stage flash desalination and power stations. They have also been used as splash zone cladding on offshore structures and protective cladding on boat hulls, as well as for solid hulls themselves.