Why C68700 Aluminium Brass Is the Default Condenser Tube
UNS C68700, known as arsenical aluminium brass and as aluminium brass B in the ASTM B111 grade list, is the alloy most often specified for seawater cooled condensers and heat exchangers. It is a copper-zinc alloy containing 76.0-79.0 % copper with about 2 % aluminium and a controlled arsenic addition. The aluminium content is what separates it from admiralty brass: it forms a tough, self-repairing aluminium-rich oxide film that stands up to flowing seawater far better than the film that forms on tin brass.
The grade is supplied as seamless condenser tube and ferrule stock to ASTM B111/B111M and is also available under the equivalent European, Japanese and Chinese designations. Its reputation rests on three properties that appear together in very few alloys: good thermal conductivity, high resistance to impingement and dezincification, and enough ductility to be U-bent and rolled into a tube sheet without cracking.
Chemical Composition
ASTM B111 specifies the following composition limits for C68700, in mass percent.
| Element | Standard range | Typical value | Function |
|---|---|---|---|
| Copper (Cu) | 76.0-79.0 % | 77.5 % | Matrix; thermal and electrical conduction |
| Aluminium (Al) | 1.8-2.5 % | 2.2 % | Strength and protective film formation |
| Arsenic (As) | 0.02-0.06 % | 0.06 % | Dezincification inhibitor |
| Iron (Fe) | 0.06 % max | 0.03 % | Grain refinement |
| Lead (Pb) | 0.07 % max | 0.02 % | Residual element |
| Zinc (Zn) | Remainder | About 20 % | Principal alloying element |
| Total other impurities | 0.30 % max | 0.15 % | Tin and antimony tightly limited |
Tin and antimony are held to very low levels because both interfere with the aluminium-rich film. This is the reason a heat of C68700 cannot simply be made by blending admiralty brass scrap, and why chemistry verification matters more for this grade than for most copper alloys.
Physical Properties and Temper Requirements
| Property | Value | Condition |
|---|---|---|
| Density | 8.33 g/cm³ (0.301 lb/in³) | 20 C |
| Melting range | Approximately 900-930 C | - |
| Thermal conductivity | About 100 W/(m K) | 20 C |
| Electrical conductivity | About 23-28 % IACS | Annealed |
| Elastic modulus | 117 GPa | Room temperature |
Condenser tube is normally ordered in the annealed O61 temper, for which ASTM B111 requires a minimum tensile strength of 345 MPa and a minimum yield strength of 125 MPa. That balance of strength and ductility is what allows the tube to be rolled into tight U-bends and then expanded into a tube sheet. Light drawn and hard drawn tempers are available for straight tubes that must resist higher internal pressure, at the cost of bendability. The standard also requires expansion and flattening tests and, when specified, an eddy current examination of the full length.
Seawater Corrosion and Service Limits
The alloy performs best in clean, aerated seawater at moderate velocity, which is the normal operating condition of a power station condenser. The aluminium-rich film resists impingement attack and the arsenic inhibitor blocks dezincification, so tube life of many years is routine in well designed plants. Aluminium brass is also less prone to fouling than the copper-nickel grades because the copper released at the surface limits the settlement of marine organisms.
Its limits should be respected. Waters carrying large amounts of suspended sand, high sulphide content or ammonia call for a different alloy, and stagnant conditions prevent the protective film from repairing itself. Velocity is the single most important design variable: too low a velocity allows deposits and pitting, while excessive velocity or repeated air bubbles cause impingement attack on the inlet end. Field experience is that the inlet length of the tube is where most failures begin, which is why inlet end protection and correct flow distribution matter as much as the alloy itself.
Applications and Selection Guidance
Surface condensers and auxiliary coolers in thermal and nuclear power plants
Seawater cooled heat exchangers in refineries, chemical plants and steel mills
Desalination and multi-effect distillation plant tube bundles
Shipboard condensers, coolers and evaporators
Ferrules and tube sheet fittings for the equipment above
When choosing between the common condenser tube alloys, the usual logic is straightforward. Admiralty brass such as C44300 suits clean water at moderate velocity and lower cost. Aluminium brass C68700 handles higher seawater velocities and more aggressive conditions. Copper-nickel grades such as C70600 and C71500 are chosen for polluted water, high velocities and higher temperatures, at a higher price. The final selection should be based on water chemistry, velocity, temperature and cleaning method rather than on alloy cost alone.
Fabrication, Installation and Inspection
Tube is cold drawn and annealed to size, straightened and cut, then either shipped straight or bent to the required U-shape. Bending is done in the annealed temper with a mandrel, and the legs are kept free of scratches because surface damage becomes a corrosion initiation site. Tubes are fixed in the tube sheet by rolling, a process that must be controlled to the correct expansion rather than over-rolled, since excessive expansion leaves residual tensile stress that promotes stress corrosion cracking. Tube sheets, baffles and water boxes should be of compatible materials to avoid galvanic attack.
Three handling rules prevent most premature failures: never mark the tube with ammonia-bearing or graphite markers, keep tubes dry and ventilated in storage, and clean the water side only with inhibited chemicals or mechanical methods. Incoming inspection covers chemical analysis, tensile and hardness testing, dimensional and surface checks, grain size, hydrostatic pressure testing, expansion and flattening tests, and eddy current examination where specified.
FAQ
Q: What does the arsenic addition do in C68700?
Arsenic at 0.02-0.06 % inhibits dezincification, so the alloy loses zinc selectively only in very aggressive waters rather than suffering premature wall thinning.
Q: What are the O61 temper requirements for C68700 tube?
ASTM B111 sets a minimum tensile strength of 345 MPa and a minimum yield strength of 125 MPa for the annealed O61 condition used for condenser tubing.
Q: Is C68700 suitable for potable water?
No, the alloy is intended for industrial cooling and seawater systems; for potable water lines a phosphorus deoxidised copper tube such as C12200 is the correct material.
Q: How do I choose between C68700 and C70600?
C68700 is the lower cost choice for clean seawater at moderate velocity, while C70600 copper-nickel is preferred for polluted water, higher velocities and higher temperatures.
Q: Why is tin limited in aluminium brass?
Even small amounts of tin interfere with the formation of the protective aluminium-rich film, so the impurity limits are enforced tightly in this grade.
Q: How should C68700 tube be stored before installation?
Tubes should be kept dry and ventilated, away from ammonia sources and from contact with dissimilar metals, so that no corrosive residue forms on the surface before the bundle is assembled.
