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).