Why 90/10 Copper-Nickel Is Used in Oil and Gas Service
Copper nickel 90/10, UNS C70600, contains roughly ten percent nickel with controlled additions of iron and manganese in a copper base. It is selected where seawater, brine or brackish water is handled: cooling circuits, fire water systems, hydraulic and instrumentation lines, splash zone pipework and heat exchangers on platforms, floating production units, ships and shore terminals. The alloy resists seawater corrosion far better than plain copper or brass, it tolerates reasonably high flow velocities, and it forms a protective surface film that limits the settlement of marine organisms, which reduces fouling and the associated loss of heat transfer.
Chemical Composition of C70600
| Element | Composition, % |
|---|---|
| Copper, Cu | Balance, 86.5 min |
| Nickel, Ni including Co | 9.0 to 11.0 |
| Iron, Fe | 1.0 to 1.8 |
| Manganese, Mn | 1.0 max |
| Zinc, Zn | 1.0 max |
| Lead, Pb | 0.05 max |
The iron addition is the key to the alloy performance. With about 1.0% to 1.8% iron the protective film becomes more resistant to erosion, and the alloy performs well in flowing seawater at a considerably lower cost than the 70/30 copper-nickel grade.
Mechanical and Physical Properties
| Property | Value for C70600 annealed |
|---|---|
| Tensile strength, min | 275 MPa / 40 ksi |
| Yield strength, min, 0.5% extension | 105 MPa / 15 ksi |
| Elongation, min | 30% |
| Density | About 8.94 g/cm³ |
| Melting range | About 1100 C to 1145 C |
| Thermal conductivity | About 45 W/m.K |
| Electrical conductivity | About 9% IACS |
The alloy is essentially non magnetic, and it keeps useful strength and ductility at moderate service temperatures, which allows it to be used in both cold worked and annealed conditions. Drawn tempers give higher strength with reduced elongation, and the limits for those tempers are taken from the product standard on the order.
Corrosion Behaviour and Service Limits
In flowing seawater the alloy develops a thin, adherent film enriched in iron and nickel that slows general corrosion and protects the surface from erosion.
Resistance to chloride stress corrosion cracking is far better than that of austenitic stainless steels, which is one reason the alloy is used in wet, chlorinated marine environments.
Copper-nickel should not be used where ammonia or ammonium compounds are present, because the alloy is susceptible to ammonia stress corrosion cracking.
Hydrogen sulphide, low pH and sulphate reducing bacteria conditions require verification against the project specification and, where sour service is specified, against NACE MR0175 or ISO 15156 before the material is accepted.
Flow velocity limits are set by the project specification. Velocity, turbulence, suspended solids and sand content all affect the erosion corrosion allowance, and the design should avoid sharp changes of direction that create local impingement.
Seamless Tube Manufacture and Testing
Seamless tube is produced by extrusion and cold drawing, then annealed, straightened and cut. Copper-nickel pipe and tube are ordered to ASTM B466 for seamless pipe, to ASTM B467 for welded pipe and to ASTM B111 for condenser and heat exchanger tube, with ASME equivalents used for pressure equipment. Routine testing covers chemical analysis, tension test, hardness, expansion or flaring test, flattening test and dimensional check. For heat exchanger and critical service an eddy current test to ASTM E243 and a hydrostatic test are added, and grain size is checked on annealed material. Tube is supplied with capped ends, and each bundle is marked with heat number and the test certificate reference.
Fabrication and Installation
Bending is carried out cold for ordinary radii, and the tube is annealed after severe cold work or after a tight radius bend to restore ductility.
Tube ends are deburred and degreased before they are rolled into a tubesheet, and a seal weld is added for high integrity joints.
Gas tungsten arc welding uses a matching copper-nickel filler, clean joint faces and a low heat input. Dilution from the parent metal should be limited so that the weld keeps the composition and corrosion resistance of the alloy.
Brazing with silver bearing filler is used for small diameter lines and instrument connections.
Copper-nickel should be isolated from carbon steel and stainless steel in seawater systems to avoid galvanic effects on the less noble material; use an insulating gasket or a suitable transition joint.
Do not use chloride bearing wrapping or insulation products in contact with the tube, and keep the surfaces clean during storage and erection.
FAQ
Q: What is the difference between 90/10 and 70/30 copper-nickel?
70/30, UNS C71500, contains about 30% nickel and tolerates higher flow velocity and more aggressive seawater than 90/10, but it costs more. 90/10 covers most marine cooling and utility duty at lower cost.
Q: Where should copper-nickel not be used?
Avoid ammonia and ammonium bearing environments because the alloy is susceptible to ammonia stress corrosion cracking, and verify hydrogen sulphide and low pH service against the project specification.
Q: Which standard covers C70600 pipe and tube?
ASTM B466 covers seamless copper-nickel pipe, ASTM B467 covers welded pipe and ASTM B111 covers condenser and heat exchanger tube, with ASME equivalents for pressure equipment.
Q: What is the minimum tensile strength of the annealed grade?
Annealed C70600 tube is specified with a minimum tensile strength of 275 MPa and a minimum yield strength of 105 MPa at 0.5% extension, with a minimum elongation of 30%.
Q: How is the tube joined to other materials?
Roller expansion into a tubesheet with an optional seal weld is standard for exchangers, and welded or brazed joints are used for pipework. Isolate the alloy from steel in seawater circuits to avoid galvanic corrosion.
Q: Why does 90/10 copper-nickel resist fouling?
The protective surface film that forms in seawater makes it difficult for marine organisms to attach, so biological fouling and the resulting loss of heat transfer are reduced.
