What C70600 Copper Alloy Is
C70600 is the UNS number for the 90/10 copper-nickel alloy, also written CuNi 90/10 or CuNi10Fe1Mn. It is a copper base containing 9-11 % nickel together with controlled iron and manganese additions that give the alloy its seawater performance. Seamless condenser, evaporator and heat exchanger tube is supplied to ASTM B111; seamless pipe and tube in straight lengths to ASTM B466; plate, sheet, strip and rolled bar to ASTM B122; and rod and bar to ASTM B151. Tube plate for condenser tube sheets is ordered to ASTM B171.
One widespread misunderstanding should be cleared up first. C70600 is not a high-conductivity copper. Its electrical conductivity is only about 9 % IACS, roughly a tenth of that of ETP copper, because nickel held in solid solution scatters electrons. What the alloy offers instead is a rare combination of moderate strength, outstanding resistance to flowing seawater, natural resistance to biofouling and straightforward fabrication by cold drawing, bending, expansion and welding. Those are the properties that keep 90/10 tube on power station condensers, offshore pipework and desalination plant.
Chemical Composition and Physical Data
Composition limits for C70600 tube and pipe as specified in ASTM B111 and ASTM B466 are summarised below, in weight per cent, together with the function of each addition.
| Element | Limit (wt %) | Function |
|---|---|---|
| Copper | Balance | Base metal, ductility and thermal conduction |
| Nickel (including cobalt) | 9.0 - 11.0 | Solid-solution strengthening, seawater corrosion resistance |
| Iron | 1.0 - 1.8 | Protective oxide film, erosion and impingement resistance |
| Manganese | 1.0 max | Deoxidation and impurity control |
| Zinc | 1.0 max | Residual element |
| Lead | 0.05 max | Impurity control, weldability |
Typical physical values are a density of 8.94 g/cm³, a melting range of approximately 1100-1145 degrees C, thermal conductivity of about 45 W/(m·K) at 20 degrees C, electrical conductivity of approximately 9 % IACS, an elastic modulus near 138 GPa and a Poisson ratio of about 0.34.
Mechanical Properties by Temper
ASTM B111 fixes minimum mechanical properties by alloy and temper, and the same minima are recognised for the seamless product in ASTM B466. For C70600 in the annealed condition the tube must reach a tensile strength of at least 275 MPa (40 ksi), a 0.5 % offset yield strength of at least 105 MPa (15 ksi) and an elongation of at least 30 % in 50 mm.
| Condition | Development | Behaviour |
|---|---|---|
| Annealed (O61) | Fully softened, grain size controlled by the specification | Lowest strength, highest ductility, best for flaring and U-bend forming |
| Light drawn | Moderate cold reduction after annealing | Higher tensile minima than annealed, elongation reduced, still formable |
| Hard drawn | Heavy cold reduction | Highest strength of the family, low elongation, limited formability |
The exact tensile and yield minima for the drawn tempers are tabulated alloy by alloy in the standard rather than in a single figure, which is why a purchase description must state the temper as well as the alloy. Bending, flaring and U-bend forming are normally carried out on annealed or lightly drawn tube, and any cold forming is followed by a stress relief anneal where the specification for the assembly requires it.
Seawater Corrosion and Service Behaviour
The protective corrosion product film that forms on 90/10 copper-nickel in seawater is the key to its performance. Iron and manganese in the alloy stabilise that film, which is why the composition is not simply copper with nickel added. In clean, flowing seawater the uniform corrosion rate of good quality C70600 tube is measured in hundredths of a millimetre per year, and the alloy resists pitting and crevice attack far better than the brasses or unalloyed copper under the same conditions.
Two practical limits apply. The alloy is degraded by sulphide-polluted or stagnant seawater, where the protective film cannot be maintained and the tube becomes vulnerable to localised attack; and design velocities must be kept inside the range recognised for cupronickel tube in the relevant project standard, because excessive velocity causes impingement attack while very low velocity encourages deposits and under-deposit corrosion. Copper-nickel surfaces also release copper ions slowly, which discourages settlement of algae and shellfish and is the reason the grade is favoured for marine intakes and for tube bundles that are difficult to clean.
Electrical and Thermal Behaviour in Practice
Because C70600 is often compared with pure copper, its transport properties deserve a plain statement. Electrical conductivity is about 9 % IACS against approximately 100 % IACS for ETP copper, so the alloy is unsuitable for busbar and cable conductor duty. Thermal conductivity, at roughly 45 W/(m·K), is about a tenth of that of pure copper as well. In heat exchanger service that is not a handicap: the controlling resistance in a condenser tube is normally the steam-side or water-side film rather than the tube wall, and the wall resistance of a thin cupronickel tube stays small in absolute terms. Where maximum heat transfer is essential, the design answer is a thinner wall or an enhanced surface rather than a change of alloy.
Fabrication, Welding and Inspection Points
C70600 is readily cold worked, and it can be joined by brazing, soldering and fusion welding. Gas tungsten arc welding with a matching copper-nickel filler is the usual route for pipework, while tube-to-tube-sheet joints are commonly expanded, roller expanded and then seal welded where the duty demands it. Thermal cutting is not recommended; machining is gummy but manageable with sharp tooling and generous rake angles.
For tube, the acceptance tests specified under ASTM B111 include dimensional verification, tensile and hardness testing, flattening and expansion tests, a hydrostatic or pneumatic pressure test as ordered, and eddy-current examination of the finished tube to Practice E243, which is the standard non-destructive method for detecting longitudinal and transverse defects in copper alloy tube. Purchasers should confirm that eddy-current testing is included, that the temper requested matches the forming operation planned, and that the wall thickness allowance leaves material for the intended service life under the design corrosion rate.
FAQ
Q: What does the designation C70600 mean?
C70600 is the UNS number for the 90/10 copper-nickel alloy, a copper base with 9-11 % nickel plus iron and manganese. It is also written CuNi 90/10 or CuNi10Fe1Mn.
Q: Which standards govern C70600 products?
Seamless condenser and heat exchanger tube is ordered to ASTM B111, straight-length seamless pipe and tube to ASTM B466, plate sheet strip and rolled bar to ASTM B122, rod and bar to ASTM B151, and condenser tube plate to ASTM B171.
Q: What are the minimum mechanical properties of annealed C70600 tube?
ASTM B111 requires at least 275 MPa (40 ksi) tensile strength, at least 105 MPa (15 ksi) 0.5 % offset yield strength and at least 30 % elongation in 50 mm for the annealed temper.
Q: Is C70600 a high-conductivity copper?
No. Its electrical conductivity is about 9 % IACS, roughly a tenth of ETP copper, so it is chosen for corrosion and mechanical duty rather than for current carrying.
Q: Why does the alloy resist biofouling?
Copper ions released slowly from the surface create an environment that discourages the settlement of algae and shellfish, an effect that persists as long as the protective film is maintained.
Q: What service conditions damage 90/10 tube?
Sulphide-polluted, stagnant or heavily fouled seawater prevents the protective film from forming, and excessive flow velocity causes impingement attack, so both water chemistry and design velocity must be controlled.
