What Are Copper-Nickel (CuNi) Alloys?
Copper-nickel alloys, also known as cupronickel, are copper-based alloys in which nickel is the principal alloying addition and the zinc content may not exceed 1%. Nickel is completely soluble in copper, so the commercial CuNi grades are single-phase solid solutions; that structure underpins their combination of toughness, thermal stability and resistance to flowing seawater. Iron and manganese are deliberate additions rather than impurities: iron markedly improves resistance to erosion-corrosion in fast-flowing seawater and other aggressive waters, while manganese supports hot workability and helps stabilise the protective corrosion-product film.
The internationally recognised designations for this family are the UNS numbers C70600, C70620, C7060X, C71500, C71520 and C71640. They correspond to the traditional trade descriptions CuNi 90/10 and CuNi 70/30 and to the iron- and manganese-modified 70/30 grade, and they are supplied as seamless and welded tube, pipe, plate, sheet, rod, bar and forgings.
Grade Range, Standards and Chemical Composition
| Trade name | UNS | Principal specifications | Ni % | Fe % | Mn % | Other | Cu % |
|---|---|---|---|---|---|---|---|
| CuNi 90/10 | C70600, C70620 | ASTM B111, B122, B151, B171, B359, B395, B432, B466, B467, B543, B608; ASME SB111, SB171, SB359, SB395, SB466, SB467, SB543; DIN 2.0872 | 9.0-11.0 | 1.8 max | 1.0 max | Pb 0.05 max, Zn 1.0 max | 88.6 min |
| CuNi 90/10 (modified) | C7060X | 90/10 iron-modified grades to ASTM and equivalent national specifications | 10.0-11.0 | 1.5-2.0 | 0.5-1.0 | Pb 0.01 max, Zn 0.2 max, P 0.2 max, S 0.2 max | Remainder |
| CuNi 70/30 | C71500, C71520 | ASTM B111, B122, B151, B171, B359, B395, B432, B467, B543, B552, B608, F467, F468; ASME SB111, SB171, SB359, SB395, SB466, SB467, SB543; DIN 2.0882 | 29.0-33.0 | 0.4-1.0 | 1.0 max | Pb 0.05 max, Zn 1.0 max | 65.0 min |
| CuNi 70/30 (iron-manganese modified) | C71640 | ASTM B111, B543, B552 | 29.0-32.0 | 1.7-2.3 | 1.5-2.5 | Pb 0.05 max, Zn 1.0 max | Remainder |
One provision is worth agreeing at the enquiry stage. When the product is intended for subsequent welding and is so specified by the purchaser, zinc shall be 0.50% max., lead 0.02% max., phosphorus 0.02% max., sulfur 0.02% max. and carbon 0.05% max. These tighter residuals protect weld soundness in the heat-affected zone and are the reason welding-grade C70620 and C71520 are ordered instead of the base grades.
Mechanical Properties of the Annealed Grades
| UNS | Condition | Min tensile strength, ksi (MPa) | Min yield strength, ksi (MPa) | Min elongation, % |
|---|---|---|---|---|
| C70600 / C70620 | Annealed | 38 (262) | 15 (103) | 30 |
| C7060X | Annealed | 40 (276) | 15 (103) | 30 |
| C71500 / C71520 | Annealed | 52 (359) | 18 (124) | 45 |
| C71640 | Annealed | 63 (434) | 25 (172) | Reported by test |
The figures above are minimum values for the soft (annealed) condition as published for these grades and product forms; the metric values are straight conversions at 1 ksi = 6.895 MPa and are rounded. Actual properties vary with product form, temper and section size, so the governing product standard - for example ASTM B111/B111M for condenser tube or ASTM B171/B171M for plate - always takes precedence over a general comparison table such as this one.
Seawater Corrosion, Heat Resistance and Heat Treatment
Copper-nickel alloys resist seawater, brine, organic compounds, salts and dilute non-oxidising acids. CuNi 90/10 (C70600 and C7060X) is a copper-nickel-iron alloy with a small manganese content that gives excellent resistance to sea water and brackish water. Of the commercial copper alloys, CuNi 70/30 (C71500, C71520) offers the best corrosion resistance in watery environments: it resists acid solutions, is highly resistant to stress-corrosion cracking and impingement attack, and retains greater strength at higher pressures. The iron- and manganese-modified C71640 is selected where the highest resistance to impingement and sand erosion at high seawater velocity is required.
Heat resistance follows the same pattern. The hot strength of copper rises with even a small nickel addition, and iron additions improve mechanical properties at room temperature and at elevated temperature alike. CuNi10Fe1Mn (C70600) can be used in pressure vessel construction up to 300 °C and CuNi30Mn1Fe (C71500) up to 350 °C; above these limits strength drops markedly, particularly creep strength and the creep strain limit.
Heat treatment is normally confined to soft annealing and stress relief. Recrystallisation annealing temperatures for CuNi alloys vary over a wide range, roughly 620 °C to 900 °C, depending on the degree of cold work, nickel content and additional constituents. High temperatures are used with short continuous annealing times and low temperatures with long static annealing cycles. Stress-relief annealing is carried out at 280 °C to 500 °C. Annealing under a protective or slightly reducing atmosphere is advisable, because an air-containing atmosphere makes pickling necessary afterwards.
Applications
Desalination plants: brine heaters, evaporator and condenser tube bundles, seawater intake and reject lines.
Seawater piping systems, shipboard cooling water circuits and offshore platform pipework and fittings.
Power generation condensers and heat exchangers, including nuclear plant condensers.
Heat exchanger and condenser tube to ASTM B111/B111M and ASME SB111, plate and sheet to ASTM B171/B171M, welded pipe to ASTM B466/B466M and ASTM B467, and forged valve and pump parts to ASTM F467 and F468.
Chemical equipment such as distillation towers, reactors and exchanger sleeves, where austenitic stainless steel would be at risk of chloride stress-corrosion cracking.
Selection, Fabrication and Inspection Points
Selection should follow the water chemistry and the flow velocity rather than the price alone. CuNi 90/10 suits clean seawater at moderate velocity; CuNi 70/30 and C71640 are chosen for higher velocity, higher sand loading or polluted and sulphide-bearing water, and for higher design pressures where their additional strength is useful. For welded fabrication, order the low-lead, low-zinc, low-sulfur variant and state the welding requirement on the purchase order.
Typical inspection points for incoming material are: verification of the heat number against the mill test certificate; tensile, yield and elongation results; chemical analysis of nickel, iron and manganese and the residual limits; eddy-current or hydrostatic testing as required by the product standard for tube; dimensional and wall-thickness checks; and a surface free from scale, drawing lubricant and embedded iron. Tubes intended for roller expansion into tubesheets should also be checked for end cleanliness, ovality and hardness consistency.
FAQ
Q: What is the difference between C70600 and C70620?
C70620 is the welding-grade version of 90/10 copper-nickel. It carries the same nominal composition but tighter limits on lead, zinc, phosphorus, sulfur and carbon so that welds remain sound; C70600 is the standard grade used for tube, plate and pipe in non-welded or readily weldable service.
Q: Is C71500 the same as CuNi 70/30?
Yes. C71500 is the UNS designation for the 70/30 copper-nickel alloy with about 30% nickel; C71520 is its welding-grade counterpart with tighter residual limits. CuNi 70/30 is the traditional trade name for the same material family.
Q: Which copper-nickel grade should be used for seawater coolers?
For clean seawater at moderate velocity, 90/10 (C70600 or C7060X) is the economical choice. Where velocity, sand content or pollution is higher, or where higher design pressure applies, 70/30 (C71500/C71520) is preferred, and C71640 is used for the most severe impingement and erosion conditions.
Q: Can copper-nickel tubes and pipes be welded?
Yes. All grades in this family are readily welded by gas tungsten arc and gas metal arc processes with matching or nickel-copper filler metal. Joint preparation must be clean and dry, heat input should be controlled, and the welding-grade residual limits should be specified for critical seawater pipework.
Q: What is the maximum service temperature of these alloys?
CuNi10Fe1Mn (C70600) is used up to about 300 °C in pressure vessel construction and CuNi30Mn1Fe (C71500) up to about 350 °C. Above these temperatures creep strength falls sharply, so high-temperature designs should be verified against the actual stress and life requirement.
Q: What temper is normally supplied for heat exchanger tube?
Condenser and heat exchanger tube is normally supplied in the annealed (soft) condition to allow roller expansion into tubesheets, with hardness and grain size controlled by the product standard. Some plate, sheet and bar products are supplied cold worked or as rolled, and strip for fabricated components may be ordered in intermediate tempers.
