What Is C72200 Copper?
C72200 is a copper-nickel alloy of the 90/10 and 70/30 family, but with its own distinct chemistry. Nickel is held at about 15 percent, iron and manganese are added for strength and corrosion control, and chromium is present at a deliberate level that is the defining feature of the grade. Because of the chromium addition the alloy is also described as CuNi 15Cr.
The grade was developed to solve a specific problem in seawater cooling systems. The standard 90/10 copper-nickel alloy performs well in clean, moderately fast water, but where flow is turbulent or contains entrained sand the protective surface film can be stripped away and impingement attack begins. Chromium strengthens that film, so C72200 tolerates higher water velocities and dirtier cooling water than the chromium-free copper-nickel grades.
It remains a fully solid-solution-strengthened, non-hardenable alloy. There is no heat treatment that can raise its strength in the way that a precipitation-hardening alloy can be treated, and its properties are obtained by composition control and cold work.
Chemical Composition of C72200
The composition limits below correspond to the copper-nickel alloy grades specified for tube and pipe in ASTM B466 and B467 and for tube in ASTM B111. Cobalt is counted with nickel for the purposes of the nickel determination.
| Element | Minimum (wt.%) | Maximum (wt.%) | Function |
|---|---|---|---|
| Copper (Cu) | Remainder | Balance | Base of the alloy |
| Nickel plus cobalt (Ni + Co) | 14.0 | 16.0 | Primary alloying element, gives strength and seawater corrosion resistance |
| Iron (Fe) | 0.25 | 0.75 | Improves resistance to impingement and erosion, raises strength |
| Manganese (Mn) | 0.20 | 0.80 | Deoxidiser, improves castability and workability |
| Chromium (Cr) | 0.30 | 0.70 | Key addition, reinforces the protective surface film against impingement attack |
| Lead (Pb) | - | 0.01 | Impurity, limited to avoid hot shortness |
| Zinc (Zn) | - | 0.20 | Impurity |
| Carbon (C) | - | 0.05 | Impurity |
| Phosphorus (P) | - | 0.01 | Impurity |
| Sulphur (S) | - | 0.01 | Impurity |
| Other elements, total | - | 0.30 | Sum of all unnamed elements |
The chromium range is narrow for a good reason. Below it, the impingement benefit is not achieved; above it, chromium-rich phases can form and the alloy loses ductility and weldability. For this reason C72200 should always be purchased to a specification that states the chromium range explicitly, and a mill certificate without a chromium analysis should not be accepted for seawater service.
Mechanical and Physical Properties
C72200 is supplied in annealed, light drawn and hard drawn tempers, and the mechanical requirements are set out in the applicable ASTM product standard for each form. Annealed tube is the normal starting condition for heat exchanger service, where the tube is expanded into tube sheets and must therefore retain ductility. Cold drawing raises tensile and yield strength while reducing elongation, and the temper should be specified along with the mechanical values rather than by name alone.
| Property | General character |
|---|---|
| Structure | Single-phase solid solution, face-centred cubic |
| Strengthening mechanism | Solid solution and cold work, non-heat-treatable |
| Strength level | Higher than the 90/10 copper-nickel grade, similar order to the 70/30 grade |
| Thermal conductivity | Moderate, lower than pure copper, adequate for heat exchanger duty |
| Magnetic behaviour | Non-magnetic |
| Service temperature | Suited to ambient and moderate temperature water systems |
Exact tensile strength, yield strength and elongation values are governed by the temper tables of the relevant specification and should be taken from the standard edition quoted on the order.
Corrosion Behaviour in Seawater
The alloy's whole commercial case rests on its behaviour in moving seawater. Copper-nickel alloys protect themselves with a thin, adherent oxide film that is enriched in nickel and iron. Chromium strengthens the film so that it re-forms quickly after mechanical damage, which is what gives C72200 its resistance to impingement attack and to erosion-corrosion in tube inlets, bends and areas downstream of restrictions.
It also resists general corrosion, stress corrosion cracking and ammonia attack far better than the brasses, and it is not susceptible to dezincification because it contains essentially no zinc. Where chlorination is used to control biological fouling, the alloy tolerates normal residual chlorine levels used in cooling water treatment. As with all copper alloys, sulphide contamination in the water is the main threat: sulphides interfere with film formation and can raise corrosion rates sharply, so sulphide levels should be monitored and the alloy should not be used in heavily polluted, oxygen-deficient water without additional protection.
Applications
Because the grade was designed for fast-flowing seawater, its applications cluster around marine and power generation cooling systems.
Condenser and heat exchanger tubing in power stations, desalination plants and on board ships.
Seawater piping, sea water service lines and splash zone systems.
Tube bundles, ferrules and components in systems where water velocity is high or sand is entrained.
Offshore platform cooling and fire fighting water systems.
Shipbuilding service systems, including sanitary and ballast lines in some designs.
Desalination multi-stage flash and reverse osmosis plant tubing and pipework.
Fabrication and Joining Guidance
C72200 is readily cold formed, bent and expanded, and it can also be hot worked within the temperature range recommended by the supplier. It is normally joined by roller expansion into tube sheets, by brazing, or by welding. Fusion welding with matching copper-nickel filler metal is feasible and is commonly used for pipe systems; the chromium addition means that heat input and interpass temperature should be controlled to avoid segregation and loss of corrosion resistance in the weld zone.
Machining is similar to that of other copper-nickel alloys and requires sharp tooling and attention to work hardening. Because the material work hardens during drawing and forming, stress relief annealing may be required before severe forming operations or before service in ammonia-bearing environments. Cleaning after fabrication also matters: iron contamination from carbon steel tools or grinding dust must be removed, since embedded iron particles can become initiation sites for localised attack in seawater.
FAQ
Q: What is C72200 copper?
C72200 is a copper-nickel alloy with 14 to 16 percent nickel plus cobalt, 0.25 to 0.75 percent iron, 0.20 to 0.80 percent manganese and 0.30 to 0.70 percent chromium, with copper as the balance.
Q: Why does C72200 contain chromium?
The chromium addition reinforces the protective oxide film on the alloy surface, which improves resistance to impingement attack and erosion-corrosion in fast-flowing or sand-laden seawater.
Q: Is C72200 a 90/10 or 70/30 copper-nickel?
Neither. It is a separate 15 percent nickel grade with chromium, positioned above the 90/10 grade in strength and impingement resistance and roughly comparable to the 70/30 grade in general seawater performance.
Q: Which standards cover C72200 products?
The alloy is covered as copper-nickel tube by ASTM B111, as seamless pipe and tube by ASTM B466, as welded tube by ASTM B467, and in plate, sheet and strip or rod and bar forms by the corresponding ASTM copper-nickel product specifications.
Q: Can C72200 be heat treated to increase strength?
No. It is a solid-solution alloy and is not hardenable by heat treatment. Strength is obtained by composition control and cold work, so temper selection is the main lever available to a designer.
Q: How does C72200 compare with 90/10 copper-nickel in service?
It offers higher strength and significantly better resistance to impingement and erosion, which permits use at higher seawater velocities. The trade-off is a slightly lower thermal conductivity and a higher alloy cost.
