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Cupronickel is an alloy of copper, nickel and strengthening impurities, such as iron and manganese. Cupronickel does not corrode in seawater, because its electrode potential is adjusted to be neutral with regard to seawater. Because of this it is used for marine hardware, and sometimes for the propellers, crankshafts and hulls of premium tugboats, fishing boats and other working boats.

A common use is in many silver-coloured modern circulation coins. A typical mix is 75% copper, 25% nickel, and a trace amount of manganese. In the past true silver coins were debased with cupronickel.

It is used in thermocouples, and a 55% copper/45% nickel alloy is used to make very accurate resistors.

Monel metal is a copper-nickel alloy, containing up to 65% nickel.

See also bronze (copper alloyed with tin), brass (copper alloyed with zinc), and nickel silver (another group of copper-nickel alloys).

Cupronickel coin of king Pantaleon c. 170 BCE.
Obv: Bust of Dionysos with a wreath of leaves.
Rev: Panther with a small bell around the neck, touching a vine with the left leg. Greek legend: BASILEOS PANTALEONTOS "King Pantaleon".

The cupronickel alloy technology has been known by the Chinese since the 3rd century BC under the name "white copper" (some weapons from the Warring States Period were in copper-nickel alloy ^[1]).

The Greco-Bactrian kings Agathocles and Pantaleon were the first in the world to issue copper-nickel (75/25 ratio) coins ^[2] around 170 BC, suggesting that exchanges of the metallic alloy, or possibly exchanges of technicians, were happening at the time between China and the region of Bactria. The practice of exporting Chinese metals, in particular iron, for trade is attested around that period.

Cupro-nickel was not used again in coinage until the 19th century. Cupro-nickel is the cladding on either side of United States Half Dollars (50¢) since 1971, and all quarters (25¢) and dimes (10¢) made after 1965. The United States Jefferson Nickel (5¢) coin is solid cupro-nickel (75/25 ratio).^[3]

Beginning around the turn of the 20th century, bullet jackets were commonly made from this material. It was soon replaced with gilding metal to reduce metal fouling in the bore.

1. ^ Ancient Chinese weapons and A halberd of copper-nickel alloy, from the Warring States Period.
2. ^ Copper-Nickel coinage in Greco-Bactria.
3. ^ About the United States Mint: Coin Specifications

• www.coppernickel.org
• Corrosion resistance - Copper Development Association UK
• National Pollutant Inventory - Copper and compounds fact sheet
• National Pollutant Inventory - Nickel and compounds fact sheet