Alloy Definition: CuZn22Al2 Aluminium Brass
UNS C68700 is an aluminium brass produced to ASTM B111/B111M, the specification for seamless copper and copper alloy condenser tubes and ferrule stock, and adopted in the ASME code as SB111. In the European designation system the same alloy is CuZn22Al2, standardised under EN 12451 as CuZn20Al2As with the material number CW702R. It also appears as HAl77-2 in GB/T 5231, as CZ110 in BS 2871 Part 3, as C6870 in JIS H3300 and as CuZn21Al2As in IS 1545.
The alloy is a copper-zinc base metal with aluminium as the key addition and arsenic as a deliberate inhibitor. Aluminium in solid solution forms a thin, strongly adherent oxide film that is continuously repaired by flowing aerated water, giving the material its notable resistance to impingement and erosion attack. Arsenic at 0.02-0.06 % prevents the selective leaching of zinc, which is the mechanism that destroys ordinary brasses in seawater. After hot working the alloy has a duplex alpha-beta structure, and it is normally supplied for condenser service in the annealed O61 temper so that tubes can be roller-expanded, flared and bent.
Chemical Composition
| Element | UNS C68700 limits (ASTM B111 / ASME SB111) |
|---|---|
| Copper (Cu) | 76.0 - 79.0 % |
| Aluminium (Al) | 1.8 - 2.5 % |
| Arsenic (As) | 0.02 - 0.06 % |
| Iron (Fe) | 0.06 % max |
| Lead (Pb) | 0.07 % max |
| Zinc (Zn) | remainder, nominally 19 - 22 % |
| System | Designation |
|---|---|
| ASTM B111 / ASME SB111 | C68700 |
| EN 12451 | CuZn20Al2As (CW702R) |
| BS 2871 Part 3 | CZ110 |
| JIS H3300 | C6870 |
| GB/T 5231 | HAl77-2 |
| NF A51-102 | CuZn22Al2 |
Physical Properties
| Property | Typical value for C68700 aluminium brass |
|---|---|
| Specific gravity | 8.33 |
| Melting range | approximately 930 - 970 degrees C |
| Modulus of elasticity in tension | about 110 GPa |
| Modulus of rigidity | about 41 GPa |
| Thermal conductivity | about 100 W/(m·K) |
| Coefficient of thermal expansion | about 18.5 × 10-6 per degree C over the range 20 - 300 degrees C |
| Specific heat capacity | about 0.38 kJ/(kg.K) |
| Electrical conductivity | about 23 % IACS |
Note that the excellent thermal conductivity of aluminium brass - high for a copper alloy - is the reason it was adopted for condensers and distillers in the first place. Designers accept lower corrosion resistance than a copper-nickel alloy in exchange for better heat transfer and lower cost.
Mechanical Behaviour in the Annealed Temper
Condenser tube supplied to ASTM B111 is ordered and certified in a defined temper, and mechanical properties must always be read against that temper. Annealed aluminium brass tube typically shows tensile strength in the region of 345-460 MPa, yield strength of roughly 125-250 MPa and elongation well above 30 %. Figures appreciably higher than this belong to cold-worked tempers, not to the soft O61 tube used where the tube must be expanded into a tubesheet; ordering a cold-worked tube for a condenser is a common and expensive mistake, because the residual cold work both embrittles the expansion joint and increases susceptibility to stress corrosion cracking.
Tensile strength, annealed: about 345 - 460 MPa.
Yield strength (0.2 % offset), annealed: about 125 - 250 MPa.
Elongation: at least 30 %, typically far more in the fully annealed condition.
Hardness: soft, suitable for roller expansion and bending.
Fatigue and thermal cycling: the annealed temper accommodates the differential expansion between tube and shell that condensers impose.
Seawater Service Behaviour
Aluminium brass has been the standard seawater condenser tube material for a century. Its strengths and its limits are well understood and both are relevant to selection.
Impingement and erosion: the aluminium-rich film resists high-velocity water and entrained air better than any plain brass.
Dezincification: the arsenic addition controls it; because the mechanism is a selective leaching process, resistance is assessed by dedicated copper alloy dezincification tests rather than by general salt spray testing.
Biofouling: copper ion release limits marine growth, reducing the frequency of mechanical cleaning.
Velocity and temperature: aluminium brass suits moderate velocities, roughly 1.5-2.5 m/s, and continuous service up to around 250-300 degrees C in seawater duty.
Polluted water: sulphide-bearing, heavily polluted or high-velocity water is the point at which 70/30 or 90/10 copper-nickel becomes the better choice.
Ammonia: all copper alloys are attacked by ammonia and ammonium compounds; steam-side ammonia must be controlled.
Fabrication and Installation
Cut tubes square, remove burrs and clean ends before expansion; a burr produces a scored hole and a leak path.
Roller-expand to about 3-5 % wall reduction, established on sample tubes before production expansion starts.
Stress-relief anneal in the region of 250-350 degrees C where severe cold forming has been carried out, and full soft anneal at roughly 500-650 degrees C in a protective atmosphere where the tube has been heavily worked.
Avoid arc welding thin-wall aluminium brass tube: zinc volatilisation causes porosity. Brazing and silver soldering are the practical joining methods for fittings and return bends.
Store dry and capped, and never leave standing water in tubes during storage or commissioning.
FAQ
Q: What is the difference between C68700 and C44300?
C68700 is an aluminium brass with aluminium as the principal addition; C44300 is an Admiralty brass based on tin with arsenic. Aluminium brass generally offers better resistance to impingement attack and higher thermal conductivity, while Admiralty brass is often chosen where good cold-forming behaviour and moderate cost are priorities.
Q: What does CuZn22Al2 mean?
It is the European short designation: a copper-zinc alloy containing approximately 22 % zinc and about 2 % aluminium. Because zinc is the balance element, small variations in it are expected from heat to heat.
Q: Why must the tube be annealed for condenser service?
Annealed tube has the ductility needed for roller expansion and bending, and lower residual stress, which reduces the risk of stress corrosion cracking during service.
Q: Is aluminium brass suitable for desalination plants?
Yes, it is widely used in thermal desalination condenser and distiller tube bundles, provided water velocity, temperature and pollution levels stay within the alloy's established limits.
Q: How is the arsenic content controlled?
Arsenic is added deliberately within the 0.02-0.06 % band and verified by the certified heat analysis on the material test certificate. Arsenic outside that band weakens dezincification resistance.
Q: What tube tests are performed before delivery?
Full-length eddy current examination, dimensional inspection, visual inspection of both surfaces, and certification of chemical and mechanical properties from the production heat with full traceability.
