Cupronickel or copper-nickel is a copper alloy that contains nickel and reinforcing elements such as iron and manganese. Copper Nickel is highly resistant to corrosion in seawater because its electrode potential is adjusted to be neutral with respect to salt water.
Around 1920, a 70-30 copper-nickel alloy was developed for capacitors in the marine industry. Soon after, an alloy of 2% manganese and 2% ferroalloy was also developed for a UK power station that needed better erosion resistance, due to the drag sand levels in the seawater.
The 90-10 alloy became available in 1950, initially for seawater piping, and is now the most widely used alloy.
In seawater, alloys have excellent corrosion rates that remain low as long as the design flow velocity is not exceeded. This speed depends on the geometry and diameter of the tube.
They have high resistance to corrosion, corrosion cracking and hydrogen embrittlement which can be problematic for other alloy systems. Copper Nickel naturally forms a thin protective layer over the first few weeks of exposure to seawater and this provides its ongoing resistance.

In addition, this alloy has a high biofouling resistance inherent to attachment by “macrofoulers” (eg seaweed and molluscs) that live in seawater. To use this property to its full potential, the alloy must be free from isolated effects or any form of cathodic protection.
However, copper nickels 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 return as surface films are maturing. Ferrous sulfate for administration of seawater systems may offer greater strength.
As copper and nickel alloys with each other easily and have simple structures, the alloys are ductile and easy to manufacture. Strength and hardness for each individual alloy is increased by cold working, not hardened by heat treatment. Joining 90-10 (C70600) and 70-30 (C71500) is possible either by soldering or brazing. Both are weldable by most techniques, but autogenous (no solder welding consumables) or oxy-acetylene methods are not recommended. 70-30 instead of 90-10 welding consumables are normally preferred for both alloys and no post weld heat treatment is required. They can also be welded directly to steel providing a nickel-consumption 65% copper solder is used to avoid the dilution effects of iron. Brazing requires appropriate silver-based solder alloys. Alloy C71640 tends to be used as seamless and extended tubing rather than welded to the tube plate.