Polished Copper Nickel Tube: Purpose and Definition
UNS C70600 is the 90/10 copper-nickel alloy used for the majority of seawater-cooled condenser and heat exchanger tube. ASTM B111/B111M, adopted by ASME as SB111, is the governing specification for the seamless condenser tube and ferrule stock produced in this alloy. "Polished" describes the delivered surface condition: instead of the light oxide left by final annealing, the tube is mechanically polished or bright annealed to give a smooth, oxide-free surface with a controlled roughness.
Surface finish is not cosmetic in this application. A smoother bore means a thinner boundary layer and better heat transfer, less sites for deposit formation and biological fouling, easier cleaning, and better visibility in visual and eddy current inspection. For exchangers handling food-grade, pharmaceutical or high-purity process streams, a polished surface also reduces the risk of product contamination and of particulate retention.
Copper Nickel Grades and Their Chemistry
| Trade designation | UNS number | Composition highlights | Min. tensile | Min. yield | Elongation |
|---|---|---|---|---|---|
| CuNi 90/10 | C70600 | Cu 88.6 min, Ni 9-11, Fe 1.0-1.8, Mn 1.0 max, Pb 0.05 max, Zn 1.0 max | about 40 ksi (275 MPa) | about 15 ksi (103 MPa) | 30 % |
| CuNi 90/10 enhanced | C7060X | Cu remainder, Ni 10-11, Fe 1.5-2.0, Mn 0.5-1.0, Pb 0.01 max, Zn 0.2 max | about 40 ksi (275 MPa) | about 15 ksi (103 MPa) | 30 % |
| CuNi 70/30 | C71500 | Cu 65.0 min, Ni 29-33, Fe 0.40-1.00, Mn 1.0 max, Pb 0.05 max, Zn 1.0 max | about 52 ksi (360 MPa) | about 18 ksi (125 MPa) | 45 % |
| CuNi 70/30 modified | C71640 | Cu remainder, Ni 29-32, Fe 1.7-2.3, Mn 1.5-2.5 | about 63 ksi (435 MPa) | about 25 ksi (172 MPa) | as specified |
The enhanced 90/10 grade (C7060X) exists because piping and exchanger specifications written for seawater systems require tighter control of residual elements and a guaranteed iron range: iron is the element that builds the protective film which resists erosion corrosion in flowing seawater. Similar published minima apply across product forms, and the mechanical requirements actually applicable to a given order are those stated in the purchased standard and temper.
Surface Finish Options
Bright annealed: final anneal in a protective atmosphere, giving a clean, lightly reflective surface free from heavy oxide, while retaining the soft temper needed for expansion.
Pickled and passivated: oxide removed chemically, with a uniform matte-bright finish; standard for tube supplied to general exchanger duty.
Mechanically polished: abrasive polishing of the outside surface, and of the bore where required, to a specified roughness for cleanable, hygienic duty.
Mirror finish: high-roughness-reduction polishing for applications where fouling control or appearance is important.
Oiled or dry: tubes can be supplied dry or with a corrosion-inhibiting film for long sea transit, with the film removed before installation.
Whichever finish is chosen, the tube ends are deburred, the bore is cleaned of polishing residue, and the tubes are capped or wrapped so that the polished surface reaches site undamaged.
Heat Treatment, Heat Resistance and Physical Data
Copper-nickel tube is heat treated for two purposes only: soft annealing and stress relief. Recrystallisation annealing is carried out over a wide temperature band of roughly 620 to 900 degrees C, selected according to the degree of cold work, the nickel content and the minor alloying elements; high temperatures are paired with short continuous furnace times and lower temperatures with longer static times. Stress-relief annealing is performed in the region of 280 to 500 degrees C, and an oxygen-free or slightly reducing atmosphere avoids the need for heavy pickling afterwards.
Hot strength improves with nickel and with the iron and manganese additions. The 90/10 grade is used in pressure equipment up to about 300 degrees C and the 70/30 grade up to about 350 degrees C; above these limits creep strength and creep strain limits fall away quickly. The 90/10 alloy has a density of about 8.94 g/cm³ and thermal conductivity of roughly 40 W/(m·K), an order of magnitude below pure copper - the property exchanger designers trade against the alloy's corrosion resistance.
Corrosion Behaviour and Service Conditions
Seawater and brackish water: excellent resistance to general corrosion, pitting and crevice corrosion; the alloy is not susceptible to dezincification because it contains no zinc beyond a residual maximum.
Stress corrosion cracking: high resistance in chloride environments, but cold-formed tube that is subsequently stressed in service should be stress-relieved.
Velocity: keep design velocity in the region of 1.5-3 m/s for 90/10 tube; higher velocities call for 70/30 or for inlet ferrules.
Fouling: copper ion release limits marine growth, reducing cleaning frequency; mechanical cleaning must not scratch through the protective film.
Ammonia and ammonium compounds: aggressive to all copper alloys, so steam-side carry-over and cleaning chemicals must be checked.
Inspection and Quality Points
Eddy current examination of the full length for seam, wall and surface defects; hydrostatic test where the order requires it.
Surface roughness verification on polished tube against the agreed roughness value, measured at defined positions along the length.
Dimensional check of outside diameter, wall, length and straightness, plus end squareness.
Visual inspection of the bore with a borescope on sampled tubes to confirm that polishing residue and drawing marks have been removed.
Certified chemistry and mechanical properties, with heat number traceability, on the material test certificate.
FAQ
Q: What does "polished" add to a copper-nickel tube?
A polished surface reduces fouling and deposit formation, improves heat transfer slightly, and makes cleaning and inspection more effective. It is specified for hygienic, high-purity and fouling-sensitive exchangers.
Q: Does polishing change the mechanical properties?
No. Polishing is a surface operation. The chemistry and the tensile, yield and elongation requirements remain those of the ordered alloy and temper under ASTM B111 or ASME SB111.
Q: Which copper-nickel grade should be selected for seawater?
90/10 (C70600) suits moderate velocity seawater duty at reasonable cost, while 70/30 (C71500) is chosen for higher velocities, polluted water and higher temperatures.
Q: Is stress relief needed after bending?
For moderate bends in annealed tube it is usually unnecessary. Where forming has produced significant cold work and the tube will see warm seawater, stress-relief annealing in the 280-500 degrees C range is good practice.
Q: Can the polished tube be welded?
Yes. Copper-nickel alloys weld well with matching filler metal, and welded forms of the alloy are covered by their own specifications. The polished surface should be cleaned locally before welding.
Q: How should polished tube be stored on site?
Dry, indoors where possible, with end caps in place and bundles supported to prevent bowing. Standing water and grit are the main causes of surface damage before installation.
