What Is C68700 Aluminium Brass?
C68700 is a copper-zinc alloy containing about 2 % aluminium and a controlled arsenic addition. In the unified numbering system it is UNS C68700; in European standards the corresponding material is CuZn20Al2As, designated CW702R in EN 12451 and commonly described as aluminium brass. The aluminium addition forms a thin, tenacious oxide film on the tube surface, and it is this film rather than the bulk alloy that carries the corrosion resistance in seawater. Arsenic is added to inhibit dezincification, which is otherwise the classic failure mode of high-zinc brasses in chloride-bearing water.
The alloy is supplied as seamless tube produced by extrusion and cold drawing, with no longitudinal weld. It is one of the three classic seawater condenser materials, alongside admiralty brass and copper-nickel, and is generally selected where the cooling water is more aggressive than admiralty brass can tolerate.
Chemical Composition
The values below follow the C68700 requirements of ASTM B111/B111M and the closely corresponding CuZn20Al2As composition of EN 12451.
| Element | Requirement, % | Function |
|---|---|---|
| Copper (Cu) | 76.0-79.0 | Matrix of the alpha brass |
| Aluminium (Al) | 1.80-2.50 | Forms the protective oxide film; increases strength |
| Arsenic (As) | 0.02-0.06 | Inhibits dezincification |
| Iron (Fe) | 0.06 max | Residual impurity |
| Lead (Pb) | 0.07 max | Residual impurity |
| Manganese (Mn) | 0.10 max | Residual impurity |
| Zinc (Zn) | Remainder | Balance of the alloy |
| Copper plus named elements | 99.6 min | Sum requirement of the specification |
Iron is deliberately kept low in condenser tube, because high iron contents reduce the adhesion and continuity of the protective film, which is the one property the alloy is chosen for.
Mechanical Requirements and Typical Physical Properties
Mechanical requirements are specified by alloy and temper in the property table of ASTM B111/B111M; annealed tube is the normal delivery condition for condensers and is verified by tension test, expansion or flare test and, when ordered, eddy-current examination. Because the aluminium brass film is sensitive to residual stress, tube is supplied in a stress-relieved condition for U-bend and expanded-joint service.
| Test or property | Basis of requirement |
|---|---|
| Tensile strength, yield strength, elongation | Property table for C68700 and the ordered temper in ASTM B111/B111M |
| Grain size | Metallographic examination against the grain-size limits of the specification |
| Residual stress | Ammonia or mercurous nitrate test as ordered, to demonstrate freedom from stress-corrosion cracking |
| Eddy-current test | To ASTM E243 acceptance limits when specified for condenser tube |
| Typical physical property | Value, typical at 20 C |
|---|---|
| Density | 8.33 g/cm³ |
| Thermal conductivity | about 100 W/(m K) |
| Modulus of elasticity | about 106 GPa |
| Melting range | about 950-990 C |
Comparison With Other Seawater Condenser Tubes
Material selection for seawater service is a comparison of three families. The table summarises the practical differences rather than replacing a water-chemistry assessment.
| Material | Designation | Seawater performance | Typical use |
|---|---|---|---|
| Admiralty brass | C44300, CuZn28Sn1 | Good in clean seawater; sensitive to velocity and pollution | Water coolers, light-duty condensers |
| Aluminium brass | C68700, CuZn20Al2As | Very good; tolerant of higher velocity and polluted water | Power station condensers, desalination |
| Copper-nickel 90/10 | C70600, CuNi10Fe1Mn | Best in high-velocity and sand-laden water | Marine, offshore, high-velocity circuits |
Aluminium brass occupies the middle ground. It handles higher water velocities and more polluted water than admiralty brass at lower cost than copper-nickel, and it remains one of the most widely installed condenser tube materials in thermal power plants and desalination trains.
Applications
Surface condensers and auxiliary exchangers in power generation.
Multi-stage flash and multi-effect desalination plants, including brine heaters and preheaters.
Marine heat exchangers, coolers for main and auxiliary engines, and shipboard condensers.
Seawater-cooled oil coolers, after-coolers and central cooling systems.
Process industry exchangers using brackish or estuarine cooling water.
Tube is normally ordered in the annealed condition in sizes from about 12 mm to 38 mm outside diameter with walls from 0.7 mm to 2.0 mm, as straight lengths or U-bent. The bent leg length, bend radius and leg alignment are controlled to the exchanger drawing, because a mismatched U-bend cannot be corrected after the final stress relief.
Selection, Processing and Service Guidance
Confirm the water chemistry first: chloride content, dissolved oxygen, ammonia, sulphide, suspended solids and design velocity together decide whether aluminium brass or copper-nickel is the correct choice.
Limit water velocity to the recommended band for aluminium brass and control inlet turbulence with ferrules or inlet-end protection, since the inlet is where impingement attack starts.
Order tube in a stress-relieved temper and keep U-bends within the minimum bend radius for the diameter and wall, so that the protective film is not cracked by deformation.
Roller-expand into the tube sheet to the specified expansion percentage; over-expansion damages the film and creates a crevice at the joint.
Avoid ammoniacal environments on the steam side, and prevent stagnant water during shutdowns by draining and drying the exchanger.
Clean by brushing or by a chemical method approved for aluminium brass; strong mineral acids strip the protective film and should be avoided.
Welding is normally avoided, and tube is joined by expansion or brazing. Where a fusion weld is required it is carried out with a copper-based filler under a qualified procedure, and the joint is stress relieved afterwards.
FAQ
Q: Why is arsenic added to C68700 aluminium brass?
Arsenic is a dezincification inhibitor. Without it, the high-zinc brass would progressively lose zinc in chloride-bearing water and become porous. The addition is small, in the range of 0.02-0.06 %, but it is essential to tube life.
Q: Is C68700 the same material as CuZn20Al2As?
They are equivalent designations for the same family of aluminium-arsenic brass. C68700 is the UNS designation, CuZn20Al2As is the European composition designation, and CW702R is the number used for it in EN 12451. Grade numbers should always be quoted together with the standard.
Q: How does C68700 compare with admiralty brass C44300?
Aluminium brass tolerates higher water velocities, more suspended solids and more polluted water, and is the preferred material for large seawater condensers. Admiralty brass is lower in cost and adequate for clean, low-velocity cooling water. Both depend on a protective surface film and on staying inside the recommended velocity range.
Q: What causes premature failure of aluminium brass tube?
Most failures trace back to loss of the protective film: excessive velocity or turbulence, sand and debris abrasion, deposits that create oxygen concentration cells, ammonia on the steam side, or residual stress from bending that was not relieved. Correct design velocity, filtration and stress relief remove most of the risk.
Q: Can aluminium brass tube be used in fresh water and process water?
Yes. The alloy performs well in most natural fresh waters and in treated cooling water, and it is often used where a single tube material must cover both fresh and brackish conditions during plant life.
Q: What documentation is supplied with the tube?
A mill certificate showing the chemical analysis, temper, mechanical test results and any non-destructive examination, traced to heat number and delivery lot, so that the material can be verified against the purchase specification.
