Cupronickel, or copper-nickel, is a copper alloy composed of nickel. The two main copper-nickel alloy grades that are used in marine service are copper base alloys with 10% or 30% nickel, referred to as 90-10 and 70-30 copper-nickel, respectively. The alloys have small iron and manganese additions that are significant in that they add a combination of corrosion resistance and resistance to flowing seawater. Individually, copper and nickel both have characteristics that make them desirable for a variety of applications. Copper is ductile, non-reactive, and highly conductive, whereas nickel is notably corrosive resistant. Due to the similar atomic structures of both metals, the elements blend extremely well and the mechanical properties of 90-10 copper-nickel and 70-30 copper-nickel are useful for everything from machinery to flanges and other connective methods for piping within the Navy’s saltwater systems.

A seawater environment makes metals susceptible to a range of corrosion, however, as 90-10 and 70-30 copper-nickel were designed for naval condensers and piping, they have a notable corrosion resistance. A large reason for copper-nickel’s corrosion resistance stems from the alloy’s ability to form a complex surface film in response to the presence of seawater. The surface film protects the metal below, and gives the alloy a corrosion rate in seawater that is between 0.025 and 0.0025 mm/yr. Copper-nickels also have great resistance to seawater flow velocity as long as the flow rates are kept bellow recommended design velocity to reduce the risk of erosion corrosion. Copper-nickels are resistant to crevice corrosion, stress corrosion cracking, chloride pitting, and ammonia stress corrosion cracking, making the metal useful for machinery, piping, flanges, and other pieces in a seawater environment.

Copper-nickel is also useful for Naval saltwater systems for its resistance against biofouling, as the adhesion of sea life on marine structures can have a profoundly negative effect on performance and is costly to clean. In the case of saltwater systems, biofouling can block the inside of piping and condensers and effect connecting pieces such as flanges. Like copper-nickel’s resistance to several types of corrosion, its resistance to biofouling is linked to the protective films that form on its surfaces. Organisms that do manage to attach themselves to copper-nickel, even in quiet conditions that promote sea life colonization, can only attach themselves loosely and can be easily removed.

While both 90-10 and 70-30 copper-nickel are used for suitable for marine application, the 90-10 variant is more widespread due to its high level of performance at a lower cost than 70-30 copper-nickel. 70-30 copper-nickel is used in a variety of Naval saltwater systems, despite its cost, because it has slightly improved strength when compared to 90-10. This means that it is suitable for more demanding applications. For example, 70-30 copper-nickel’s corrosion resistance is improved in high-velocity and polluted seawater, both of which are experienced by Naval vessels out in the field.