Application of Copper-Nickel Alloys in Ships and Marine Engineering

Application of Copper-Nickel Alloys in Ships and Marine Engineering

Copper-nickel alloys are most widely used in seawater piping systems, including heat exchangers and condensers. They are also applied in coinage, wire mesh, and the paper-making industry. In seawater piping systems, two grades are commonly used: B10 and B30, which contain 10% and 30% nickel, respectively. These correspond to ASTM standards UNS C70600 and UNS C71500. Generally, B30 copper-nickel alloy, due to its higher nickel content, offers better resistance to erosion-corrosion in high-velocity seawater and has higher strength. However, the more economical and widely used alloy is B10, with a lower nickel content (10%).

Another grade corresponding to the domestic B30 alloy is UNS C71640, a modified copper-nickel alloy containing 30% nickel, 2% iron, and 2% manganese. The commercialized C71640 alloy is available only in tubing and is primarily used in multi-stage flash desalination units, where its superior erosion-corrosion resistance is essential for heat exchange systems.

In the 1970s, the development of high-speed naval ships and submarines raised higher demands for the erosion-corrosion resistance of certain copper-nickel alloy components. This led to the emergence of a new wrought copper-nickel alloy, C72200, which contains 16% nickel and 0.5% chromium. Research indicates that the critical velocity of C71640 is higher than that of C71500 but lower than that of C72200. The erosion-corrosion resistance of C71640 and C72200 is superior to that of C71500 and C70600. Regarding resistance to sand particle erosion, the ranking is C71640 > C72200 > C71500 > C70600.

Currently, C70600 remains the primary material for condensers and heat exchangers. However, under harsh conditions such as high flow velocity, turbulence, and sand content, materials like C71640, C71500, and C72200 must be considered for these applications.

Development of High-Strength Copper-Nickel Alloys

Modern ships and marine instruments require copper materials with both excellent seawater corrosion resistance and high strength. The emergence of "high-strength copper-nickel alloys" is based on the precipitation hardening of aluminum in the Cu-Ni binary alloy system. This phenomenon was recognized as early as the 1930s. Later, based on this Cu-Ni-Al system, several grades of copper-nickel alloys were developed.

The first commercially available aluminum-containing copper-nickel alloy was Cu-15Ni-3Al, later designated as UNS C72400. Developed in the 1940s, this alloy was created to meet the naval demand for high-strength corrosion-resistant materials. The fundamental strengthening mechanism of Cu-Ni-Al alloys is the formation of nanometer-scale Ni₃Al (γ') precipitates.

Subsequent studies found that adding iron, manganese, and niobium to the alloy enhances the strengthening effect of the Ni₃Al phase on mechanical properties. As a result, high-strength copper-nickel alloys were developed in various compositions, including Cu-Ni-Fe-Mn-Al, Cu-Ni-Fe-Mn-Al-Cr, and Cu-Ni-Fe-Mn-Al-Cr-Nb systems. These precipitation-hardened high-strength copper-nickel alloys exhibit excellent "all-round" corrosion resistance, providing immunity to hydrogen embrittlement, seawater erosion-corrosion, stress corrosion cracking, and abrasion. Additionally, they possess high elastic modulus, low magnetic permeability, biofouling resistance, and good machinability.

Cost and Application Considerations

As mentioned earlier, although B30 copper-nickel alloys (C71500 and C71640) and high-strength copper-nickel alloys (C72200, C72400, C72420, etc.) offer superior strength and resistance to high-velocity seawater erosion-corrosion, their application is limited by cost considerations. Currently, they are mainly used for high-strength components in marine equipment such as ship shafts, fasteners, valve stems, and flanges on naval vessels and offshore drilling platforms.

Meanwhile, B10 copper-nickel alloy remains the dominant material for seawater piping and other marine systems due to its balanced advantages in cost and performance.