Alloy

Cupronickel

Copper and nickel, which mix in any proportion — the metal of modern coins, condenser tubing and anything that has to sit in seawater.

Currency metal

Copper and nickel are unusual among metal pairs in being completely soluble in one another: any ratio from pure copper to pure nickel forms a single continuous solid solution, with no intermediate compounds and no eutectic. That makes cupronickel a family rather than a recipe, and it makes its properties adjustable by simply changing the proportion.

What the alloy is prized for is behaviour in seawater. It resists corrosion far better than copper alone, and it resists the marine fouling that colonises almost everything else immersed in the sea — copper ions leaching slowly from the surface make it an unattractive place to settle.

Uses

Condenser and heat-exchanger tubing in ships and power stations, seawater piping, desalination plant, and boat hull sheathing where the anti-fouling behaviour is worth the cost.

And coins. Most silver-coloured circulating coinage in the world is cupronickel, typically three parts copper to one of nickel, chosen because it looks like silver, wears extremely well, and has an electrical signature that vending machines can be built to recognise. That last property is not incidental — a coin has to be machine-readable and hard to counterfeit, and the alloy is picked partly for its conductivity.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is composed of

Copper — element · 60–95% · the majority in every common grade — 90/10 for tubing, 75/25 for coinage
Nickel — element · 5–40% · the addition that buys the seawater resistance and the silver colour, in any proportion the designer wants — the two metals are completely soluble in one another

is produced by

Alloying and melting — process · straightforward to make, the two metals being completely soluble in one another at every ratio

is an alternative to

Silver — element · in coinage, which is what replaced silver in circulating money almost everywhere in the twentieth century
Sterling silver — alloy · in tableware, where cupronickel is the cheap substitute that looks similar and was sold as such

is used in

Shipbuilding — industry · seawater piping and condenser tubes, where its resistance to biofouling matters as much as its corrosion resistance

Sources

Wikidata
Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
Material World
Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Cupronickel back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Cupronickel → is composed of (the majority in every common grade — 90/10 for tubing, 75/25 for coinage) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
Cupronickel → is composed of (the addition that buys the seawater resistance and the silver colour, in any proportion the designer wants — the two metals are completely soluble in one another) → Nickel → is produced by (reduced from roasted pentlandite concentrate, with the platinum-group metals following into the refinery) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
Cupronickel → is produced by (straightforward to make, the two metals being completely soluble in one another at every ratio) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
Cupronickel → is composed of (the majority in every common grade — 90/10 for tubing, 75/25 for coinage) → Copper → is extracted from (the principal copper ore worldwide) → Chalcopyrite
Cupronickel → is composed of (the majority in every common grade — 90/10 for tubing, 75/25 for coinage) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
Cupronickel → is composed of (the majority in every common grade — 90/10 for tubing, 75/25 for coinage) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite

These are the most distinct paths back. Cupronickel can be traced through others besides.

Downstream — what it becomes

Cupronickel → is used in (seawater piping and condenser tubes, where its resistance to biofouling matters as much as its corrosion resistance) → Shipbuilding