Cupronickel

Cupronickelorcopper–nickel(CuNi) is analloyofcopperwithnickel,usually along with small quantities of other elements added for strength, such asironandmanganese.The copper content typically varies from 60 to 90 percent. (Monelis a nickel–copper alloy that contains a minimum of 52 percent nickel.)

Two stacks ofHalf dollars.The coins in the stack on the right are composed of copper with cupronickelcladding,and can be distinguished from thesilverhalf dollars on the left by their visiblecoppercores.

Despite its high copper content, cupronickel is silver in colour. Cupronickel is highly resistant to corrosion bysalt water,and is therefore used for piping, heat exchangers and condensers inseawatersystems, as well as for marine hardware. It is sometimes used for thepropellers,propeller shafts,andhullsof high-qualityboats.Other uses include military equipment and chemical, petrochemical, and electrical industries.

Another common 20th-century use of cupronickel was silver-colouredcoins.For this use, the typical alloy has 3:1 copper to nickel ratio, with very small amounts of manganese.

In the past, truesilvercoins weredebasedwith cupronickel, such as coins of thepound sterlingfrom 1947 onward having their content replaced.

Name

Aside fromcupronickelandcopper–nickel,several other terms have been used to describe the material: the tradenamesAlpakaorAlpacca,Argentan Minargent,the registeredFrenchtermcuivre blanc,and the romanizedCantonesetermPaktong,Bạch đồng (the French and Cantonese terms both meaning "white copper" ); cupronickel is also occasionally referred to ashotel silver,plata alemana(Spanishfor "German silver" ),German silver,andChinese silver.

Applications

Marine engineering

Cupronickel alloys are used for marine applicationsdue to their resistance to seawatercorrosion,good fabricability, and their effectiveness in loweringmacrofoulinglevels. Alloys ranging in composition from 90% Cu–10% Ni to 70% Cu–30% Ni are commonly specified inheat exchangeror condenser tubes in a wide variety of marine applications.

Important marine applications for cupronickel include:

• Shipbuildingand repair:hullsof 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 oiland gas platforms and processing andFPSOvessels: 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

Five Swiss francs

Five Indian rupees, commemoratingILO

Twenty five Indonesian rupiah

The successful use of cupronickel in coinage is due to itscorrosion resistance,electrical conductivity,durability,malleability,lowallergyrisk, ease ofstamping,antimicrobial propertiesandrecyclability.

In Europe,Switzerlandpioneered cupronickel-basedbilloncoinage in 1850, with the addition of silver and zinc, for coins of 5, 10 and 20 Rappen.Starting in 1860/1861,Belgiumissued 5, 10 and 20 Centimes in pure cupronickel (75% copper, 25% nickel, without additional silver and zinc),andGermanyissued 5 and 10 Pfennig in the same 75:25 ratio from 1873/1874 (until 1915/1916).In 1879, Switzerland, for 5 and 10 Rappen coins, also adopted that cheaper 75:25 copper to nickel ratiothen being used in Belgium, theUnited Statesand Germany. From 1947 to 2012, all "silver"coinage in the UKwas made from cupronickel (but from 2012 onwards the two smallest UK cupronickel denominations were replaced with lower-cost nickel-plated steel coins). Moreover, whensilver pricesrose in the 1960s/1970s also some other European countries replaced remaining silver denominations by cupronickel, e.g. the 1/2 to (pictured) 5Swiss franccoins starting 1968and German 5Deutsche Mark1975-2001. Since 1999, cupronickel is also used for the inner segment of the1 euro coinand the outer segment of the2 euro coin.

In part due to silver hoarding in the Civil War, the United States Mint first used cupronickel for circulating coinage inthree-centpieces starting in 1865, and then forfive-centpieces starting in 1866. Prior to these dates, both denominations had been made only in silver in the United States. Cupronickel is thecladdingon either side of United Stateshalf-dollars(50¢) since 1971, and allquarters(25¢) anddimes(10¢) made after 1964. Currently, some circulating coins, such as the United StatesJefferson nickel(5¢),theSwiss franc,and theSouth Korean500 and 100wonare made of solid cupronickel (75:25 ratio).

Other usage

Athermocouplejunction 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 incryogenicapplications. 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 infreeze brandingoperations.

Beginning around the turn of the 20th century,bulletjackets were commonly made from this material. It was soon replaced withgilding metalto reduce metal fouling in thebore.

Currently, cupronickel andnickel silverremain 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 Instrumentsused "CuNiFe" magnets in their "Wide RangeHumbucker"pickup for variousTelecasterandStarcasterguitarsduring the 1970s.

For high-qualitycylinder locksand locking systems, cylinder cores are made from wear-resistant cupronickel.

Cupronickel has been used as an alternative to traditional steelhydraulic brakelines (the pipes containing thebrake fluid), as it does not rust. Since cupronickel is much softer than steel, it bends andflaresmore easily, and the same property allows it to form a better seal with hydraulic components.

Properties

Cupronickel lacks a copper color due to nickel's high electronegativity, which causes a loss of one electron in copper's d-shell (leaving 9 electrons in the d-shell versus pure copper's typical 10 electrons).

Important properties of cupronickel alloys includecorrosion resistance,inherent resistance tomacrofouling,goodtensile strength,excellentductilitywhenannealed,thermal conductivityandexpansioncharacteristics amenable forheat exchangersandcondensers,good thermal conductivity and ductility atcryogenictemperatures and beneficialantimicrobial touch surfaceproperties.

Properties of some Cu–Ni alloys

Alloy UNS No.	Common name	European spec	Ni	Fe	Mn	Cu	Density g/cm3	Thermal conductivity W/(m·K)	TEC μm/(m·K)	Electrical resistivity μOhm·cm	Elastic modulus GPa	Yield strength MPa	Tensile strength MPa
C70600	90–10	Cu90Ni10	9–11	1–1.8	1	Balance	8.9	40	17	19	135	105	275
C71500	70–30	Cu70Ni30	29–33	0.4–1.0	1	Balance	8.95	29	16	34	152	125	360
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 incorrosionresistance 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 flowvelocityis not exceeded. This velocity depends on geometry and pipe diameter. They have high resistance tocrevice corrosion,stress corrosion crackingandhydrogen embrittlementthat 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 inherentbiofoulingresistance to attachment by macrofoulers (e.g.seagrassesandmolluscs) 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 ofcathodic protection.

However, Cu–Ni alloys can show high corrosion rates in polluted or stagnant seawater whensulfidesorammoniaare present. It is important, therefore, to avoid exposure to such conditions, particularly during commissioning and refit while the surface films are maturing.Ferrous sulfatedosingto sea water systems can provide improved resistance.

Crack in 90–10 Cu–Ni metal plate due to stresses during silver brazing

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 bycold working;they are not hardened byheat treatment.Joining of 90–10 (C70600) and 70–30 (C71500) is possible by bothweldingorbrazing.They are both weldable by the majority of techniques, although autogenous (welding without weld consumables) oroxyacetylenemethods 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 (see image). 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 andheat exchangersin naval vessels, commercial shipping, multiple-stage flash desalination and power stations. They have also been used assplash zonecladding onoffshore structuresand protective cladding on boat hulls, as well as for solid hulls themselves.

Fabrication

Due to itsductility,cupronickel alloys can be readily fabricated in a wide variety of product formsand fittings. Cupronickel tubing can be readily expanded intotube sheetsfor the manufacturing of shell and tubeheat exchangers.

Details of fabrication procedures, including general handling, cutting and machining, forming, heat treatment, preparing for welding, weld preparations, tack welding, welding consumables, welding processes, painting, mechanical properties of welds, and tube and pipe bending are available.

Standards

ASTM,EN,andISOstandards exist for ordering wrought and cast forms of cupronickel.

Thermocouplesandresistorswhose resistance is stable across changes in temperature contain alloyconstantan,which consists of 55% copper and 45% nickel.

History

Chinese history

Cupronickel alloys were known as "white copper" to theChinesesince about the third century BC. Some weapons made during theWarring States periodwere made with Cu-Ni alloys.The theory of Chinese origins ofBactriancupronickel was suggested in 1868 by Flight, who found that the coins considered the oldest cupronickel coins yet discovered were of a very similar alloy to Chinesepaktong.

The author-scholar, Ho Wei, precisely described the process of making cupronickel in about 1095 AD. Thepaktongalloy was described as being made by adding small pills of naturally occurringyunnanore to a bath of molten copper. When a crust ofslagformed,saltpeterwas added, the alloy was stirred and theingotwas immediatelycast.Zinc is mentioned as an ingredient but there are no details about when it was added. The ore used is noted as solely available fromYunnan,according to the story:

"San Mao Chun were at Tanyang during a famine year when many people died, so taking certain chemicals, Ying projected them onto silver, turning it into gold, and he also transmuted iron into silver – thus enabling the lives of many to be saved [through purchasing grain through this fake silver and gold] Thereafter all those who prepared chemical powders by heating and transmuting copper by projection called their methods" Tanyang techniques ".

The late Ming and Qing literature have very little information aboutpaktong.However, it is first mentioned specifically by name in theThien Kung Khai Wuofcirca1637:

"Whenlu kan shih(zinc carbonate,calamine) orwo chhein(zinc metal) is mixed and combined withchih thung(copper), one gets 'yellow bronze' (ordinary brass). Whenphi shangand other arsenic substances are heated with it, one gets 'white bronze' or white copper:pai thong.Whenalumandniterand other chemicals are mixed together one getsching thung:green bronze. "

Ko Hung stated in 300 AD: "The Tanyang copper was created by throwing a mercuric elixir into Tanyang copper and heated- gold will be formed." However, thePha Phu Tsuand theShen I Chingdescribing a statue in the Western provinces as being of silver, tin, lead and Tanyang copper – which looked like gold, and could be forged for plating and inlaying vessels and swords.

Joseph Needhamet al.argue that cupronickel was at least known as a unique alloy by the Chinese during the reign of Liu An in 120 BC in Yunnan. Moreover, the Yunnanese State of Tien was founded in 334 BC as a colony of the Chu. Most likely, modernpaktongwas unknown to Chinese of the day – but the naturally occurring Yunnan ore cupronickel alloy was likely a valuable internal trade commodity.

Greco-Bactrian coinage

A Cupro-nickel coin of kingEuthydemus II,c. 185–180 BC. Notice the coin looks more corroded than the usual silver coins.

In 1868, W. Flight discovered aGreco-Bactriancoin comprising 20% nickel that dated from 180 to 170 BCE with the bust ofEuthydemus IIon the obverse. Coins of a similar alloy with busts of his younger brothers,PantaleonandAgathocles,were minted around 170 BCE. The composition of the coins was later verified using the traditional wet method and X-ray fluorescence spectrometry.Cunningham in 1873 proposed the "Bactrian nickel theory," which suggested that the coins must have been the result of overland trade from China through India to Greece. Cunningham's theory was supported by scholars such as W. W. Tarn, Sir John Marshall, and J. Newton Friend, but was criticized by E. R. Caley and S. van R. Cammann.

In 1973, Cheng and Schwitter in their new analyses suggested that the Bactrian alloys (copper, lead, iron, nickel and cobalt) were closely similar to the Chinesepaktong,and of nine known Asian nickel deposits, only those in China could provide the identical chemical compositions.Cammann criticized Cheng and Schwitter's paper, arguing that the decline of cupronickel currency should not have coincided with the opening of the Silk Road. If the Bactrian nickel theory were true, according to Cammann, theSilk Roadwould have increased the supply of cupronickel. However, the end of Greco-Bactrian cupronickel currency could be attributed to other factors such as the end of theHouse of Euthydemus.

European history

The alloy seems to have been rediscovered by the West duringalchemyexperiments. Notably,Andreas Libavius,in hisAlchemiaof 1597, mentions a surface-whitened copperaes albumby mercury or silver. But inDe Natura MetalloruminSingalarumPart 1, published in 1599, the same term was applied to "tin" from the East Indies (modern-dayIndonesiaand thePhilippines) and given the Spanish name,tintinaso.

Richard Watsonof Cambridge appears to be the first to discover that cupronickel was an alloy of three metals. In attempting to rediscover the secret of white-copper, Watson critiquedJean-Baptiste Du Halde'sHistory of China(1688) as confusing the termpaktong'., He noted the Chinese of his day did not form it as an alloy but rathersmeltedreadily available unprocessed ore:

"...appeared from a vast series of experiments made at Peking- that it occurred naturally as an ore mined at the region, the most extraordinary copper ispe-tongor white copper: it is white when dug out of the mine and even more white within than without. It appears, by a vast number of experiments made at Peking, that its colour is owing to no mixture; on the contrary, all mixtures diminish its beauty, for, when it is rightly managed it looks exactly like silver and were there not a necessity of mixing a little tutenag or such metal to soften it, it would be so much more the extraordinary as this sort of copper is found no where but in China and that only in the Province of Yunnan ". Notwithstanding what is here said, of the colour of the copper being owing to no mixture, it is certain the Chinese white copper as brought to us, is a mixt [sic: mixed] metal; so that the ore from which it was extracted must consist of various metallic substances; and from such ore that the naturalorichalcumif it ever existed, was made. "

During the peak European importation of Chinese white-copper from 1750 to 1800, increased attention was made to its discovering its constituents. Peat and Cookson found that "the darkest proved to contain 7.7% nickel and the lightest said to be indistinguishable from silver with a characteristic bell-like resonance when struck and considerable resistance to corrosion, 11.1%".

Another trial byAndrew Fyfeestimated the nickel content at 31.6%. Guesswork ended whenJames Dinwiddieof the Macartney Embassy brought back in 1793, at considerable personal risk (smuggling ofpaktongore was a capital crime by the Chinese Emperor), some of the ore from whichpaktongwas made.Cupronickel became widely understood, as published by E. Thomason, in 1823, in a submission, later rejected for not being new knowledge, to theRoyal Society of Arts.

Efforts in Europe to exactly duplicate the Chinesepaktongfailed due to a general lack of requisite complex cobalt–nickel–arsenic naturally occurring ore. However, theSchneebergdistrict ofGermany,where the famousBlaufarbenwerkemadecobalt blueand other pigments, solely held the requisite complex cobalt–nickel–arsenic ores in Europe.

At the same time, thePrussianVerein zur Beförderung des Gewerbefleißes(Society for the Improvement of Business Diligence/Industriousness) offered a prize for the mastery of the process. Unsurprisingly, Dr E.A. Geitner and J.R. von Gersdoff of Schneeberg won the prize and launched their "German silver"brand under the trade namesArgentanandNeusilber(new silver).

In 1829, Percival Norton Johnston persuaded Dr. Geitner to establish afoundryin Bow Common behind Regents' Park Canal in London, and obtained ingots of nickel-silver with the composition 18% Ni, 55% Cu and 27% Zn.

Between 1829 and 1833, Percival Norton Johnson was the first person to refine cupronickel on the British Isles. He became a wealthy man, producing in excess of 16.5 tonnes per year. The alloy was mainly made intocutleryby theBirminghamfirm William Hutton and sold under the trade-name "Argentine".

Johnsons' most serious competitors, Charles Askin and Brok Evans, under the brilliant chemist Dr. EW Benson, devised greatly improved methods of cobalt and nickel suspension and marketed their own brand of nickel-silver, called "British Plate".

By the 1920s, a 70–30 copper–nickel grade was developed fornaval condensers.Soon afterwards, a 2% manganese and 2% iron alloy now known as alloy C71640 was introduced for a UK power station which needed better erosion resistance because the levels of entrained sand in the seawater. A 90–10 alloy first became available in the 1950s, initially for seawater piping, and is now the more widely used alloy.

See also

• Brass(copper alloyed withzinc)
• Bronze(copper alloyed withtin)
• Copper alloys in aquaculture
• Freeze branding

External links

• Copper-Nickel Alloys
• Copper-Nickel-Alloys: Properties, Processing, Applications(Source: German Copper Institute (DKI))]
• Copper-Nickel Alloys for Seawater Corrosion Resistance and Antifouling - A State of the Art Review(C.A. Powell and H.T. Michels; Corrosion 2000, NACE March 2000 ( NACE))