Introduction
This article explains copper nickel CuNi 70/30 heat exchanger tubes. The 70/30 copper-nickel family, principally C71500, contains about 70 percent copper and 30 percent nickel. It offers the highest strength and corrosion resistance of the copper-nickel alloys and is specified for demanding seawater, chemical and high-temperature service.
Composition
C71500 contains 29.0 to 33.0 percent nickel, 0.40 to 1.0 percent iron and up to 1.0 percent manganese with the balance copper. The higher nickel content raises strength and improves the stability of the protective film compared with 90/10 alloy, which is why 70/30 is chosen where flow velocities are higher and the water is more aggressive.
Corrosion Behavior
The nickel-rich film gives excellent resistance to general corrosion, pitting and crevice attack in seawater, and the alloy withstands flow velocities above the limits of 90/10 material. It also resists stress-corrosion cracking and performs well in polluted water conditions that would accelerate attack on many other alloys.
| Property | Typical Value |
|---|---|
| Nickel | 29.0 - 33.0% |
| Iron | 0.40 - 1.0% |
| Tensile strength | 380 - 450 MPa annealed |
| Thermal conductivity | 29 - 31 W/(m·K) |
Properties
70/30 cupronickel reaches tensile strengths of about 380 to 450 MPa annealed with elongation above 30 percent, and thermal conductivity of roughly 29 to 31 W/(m·K). The alloy keeps good mechanical properties at elevated temperature, which makes it useful for steam and hot-water systems as well as cryogenic service.
Applications and Standards
Typical uses are high-pressure seawater lines, heat exchanger and condenser tubes, feedwater heaters, evaporators, submarine and ship systems, and offshore process equipment. Products are covered by ASTM B111, B122, B151, B171 and B466, with ASME pressure-service counterparts.
Verification
Confirm the nickel and iron content, temper, dimensions and pressure test results on the mill certificate. For marine classification service, verify that the grade and test documentation meet the classification society requirements.
