What Aluminum Brass C68700 Is
C68700 is aluminum brass, a copper-zinc alloy in which a controlled addition of aluminium, supported by a small amount of arsenic as an inhibitor, produces a tube material with a useful combination of strength, thermal conductivity and resistance to moving water. It belongs to the same seamless tube family as admiralty brass and the copper-nickel grades, but it is selected for a different duty: C68700 is the alloy of choice where the cooling water flows quickly or carries a light load of suspended solids that would erode a softer material.
Aluminum brass is often confused with admiralty brass in catalogue descriptions. The two alloys are not the same. Admiralty brass is C44300, with tin and arsenic additions, and aluminum brass is C68700, with aluminium and arsenic. Because the design limits on water velocity, on solids content and on corrosion allowance differ between them, the alloy designation on the drawing must always be checked against the designation on the tube certificate.
Chemistry and Standard Requirements
C68700 seamless tube is specified under ASTM B111 and its ASME counterpart ASME SB111, the specifications that cover copper and copper alloy seamless condenser tubes and ferrule stock. The composition ranges below are those applied to this alloy.
| Element | Range for C68700 | Function |
|---|---|---|
| Copper | 76.0 - 79.0 % | Base metal, thermal conductivity |
| Aluminium | 1.8 - 2.5 % | Strengthening and protective surface film |
| Arsenic | 0.02 - 0.06 % | Inhibitor against dezincification |
| Lead | 0.07 % maximum | Residual element |
| Iron | 0.06 % maximum | Residual element |
| Zinc | Remainder | Principal alloying element |
The aluminium addition forms a thin, self-repairing protective film on the wetted surface, and that film is the reason the alloy tolerates higher water velocities than plain brass. Arsenic performs the same function as it does in admiralty brass, suppressing the loss of zinc that would otherwise leave a porous, weak tube. The mechanical property requirements, including minimum tensile strength, minimum yield strength and minimum elongation, are set out for each alloy and temper in ASTM B111 and ASME SB111, and the tube is ordered against those tables.
Corrosion Performance in Seawater and Process Water
Sulphide-free seawater: aluminum brass resists attack well and is widely used for coastal power station condensers and marine cooling systems.
High water velocity: the protective aluminium-rich film makes the alloy resistant to impingement and erosion, so it suits tubes where the cooling water is pumped at speed.
Suspended solids: aluminum brass is more tolerant of sand and silt than admiralty brass, which is why it is favoured in estuarine and river cooling water.
Sulphides and ammonia: these are the enemy of all copper-zinc alloys. Hydrogen sulphide in polluted water or ammonia carried over on the steam side attacks the tube, so water chemistry must be known before the alloy is fixed.
Stagnation: deposits and standing water can drive pitting and under-deposit attack, so flushing and cleaning procedures form part of the operating regime.
Applications
Tubes in C68700 are recommended for marine and land-based power stations in which cooling water velocity is high, and more generally for any heat transfer duty where a copper alloy tube is required and the water is aggressive.
Steam surface condensers in coastal and river power stations and in process plants.
Shell-and-tube heat exchangers, including oil coolers and cooling water exchangers in oil, gas and petrochemical plant.
Desalination and water treatment plant condensers and evaporator bundles.
Marine engine and auxiliary heat exchangers, and shipboard cooling systems.
Refrigeration and air-conditioning condensers using water as the cooling medium.
Selection, Fabrication and Inspection Guidance
The tube is normally supplied in the annealed condition so that it can be roller expanded into a tube sheet, and the expansion should be controlled to keep the reduction of wall in the joint within the limits set by the exchanger designer. Tubes are supported by baffles at the spacing the design specifies, because an unsupported span vibrates in service and frets through where it touches the baffle. Where the tube is bent into a U, the bend radius must suit the temper, and the bend must be checked for ovality. On delivery, tube certificates should be matched against the order for alloy, temper, dimensions and test results, and the bundle should be examined for transit damage before it is installed, since a dented or ovalised tube will not roll into a tube sheet correctly.
Common Pitfalls and Inspection Points
Accepting a tube described loosely as brass without confirming that the certificate reads C68700.
Using aluminum brass where the water carries sulphides, which attacks copper-zinc alloys regardless of inhibitor content.
Choosing a stronger temper for a U-bend bundle and then finding that the tube cannot be formed or expanded without cracking.
Neglecting inlet-end protection, where the water jet first strikes the tube sheet and impingement is most severe.
Leaving the bundle wet and fouled during a shutdown, which allows deposits and pitting to start at the tube surface.
FAQ
Q: Is C68700 the same as admiralty brass?
No. C68700 is aluminum brass, alloyed with aluminium and arsenic, while admiralty brass is C44300, alloyed with tin and arsenic. The two are specified separately and are selected for different water conditions.
Q: Why is C68700 used for condensers?
It combines high thermal conductivity with a protective aluminium-rich surface film, so it tolerates fast-flowing cooling water and light suspended solids while transferring heat efficiently.
Q: Which standard covers C68700 seamless tube?
ASTM B111 and the ASME equivalent ASME SB111 cover copper and copper alloy seamless condenser tubes and ferrule stock, including C68700.
Q: What limits the life of an aluminum brass tube?
Water chemistry and operating practice decide tube life: sulphides, ammonia, deposits, stagnant water and excessive velocity are the usual causes of early failure, not the alloy itself.
Q: Can aluminum brass tubes be welded or brazed?
The alloy can be joined by common brazing and welding processes, but in condensers the tubes are normally roller expanded into the tube sheet, and any thermal joint must be cleaned of flux afterwards.
Q: How should aluminum brass tubes be stored before installation?
Keep the bundle dry and under cover, support it along its length, and protect the tube ends; a wet, stacked bundle is the starting point for surface staining and under-deposit corrosion.
