Why C68700 Aluminum Brass Is Chosen for Condenser Tubes
Aluminum brass UNS C68700, with roughly 22% zinc and 2% aluminium, is one of the workhorse alloys for tubes in condensers, evaporators and heat exchangers. The aluminium addition forms a thin, self-repairing protective oxide film on the water side, so the tube tolerates flowing seawater and brackish cooling water far better than plain copper or unmodified brass, while still offering thermal conductivity high enough for efficient heat transfer. It is a single-phase alloy, which means it can be drawn to thin walls and supplied annealed for rolling into tube sheets, and its resistance to impingement and erosion-corrosion suits it to steam condensers, turbine oil coolers and distiller tubing.
Grade Identification and International Equivalents
The same alloy appears under several national designations, and a purchase specification should state both the UNS number and the equivalent so that mill certificates can be checked without ambiguity.
| System | Designation |
|---|---|
| UNS (ASTM) | C68700 |
| CDA | 687 |
| BS | CZ110 |
| DIN | 2.0460, CuZn22Al2 |
| JIS | C6870 |
| ISO designation | CuZn22Al2 |
| GB wrought designation | HAl77-2 |
| ASTM specification | B111, B395 |
| ASME specification | SB111, SB395 |
Note that the alloy is aluminium brass, not admiralty brass; the two are frequently confused because both are used in the same condensers. Admiralty brass is an inhibited Cu-Zn-Sn alloy, whereas C68700 contains aluminium as the passivating element and no deliberate tin.
Chemical Composition and Temper
| Element | Requirement |
|---|---|
| Cu | 76.0 - 79.0% |
| Al | 1.8 - 2.5% |
| As | 0.02 - 0.06% |
| Fe | 0.06% max |
| Pb | 0.07% max |
| Zn | remainder |
The arsenic addition is an inhibitor that suppresses dezincification, and it is the reason the arsenic range is specified rather than simply capped. Tubes are normally supplied annealed, with O61 annealed temper as the standard condition, while all tempers are available and special tempers can be produced on request.
Dimensions, Surface and Supply Condition
Outside diameter: 5 mm to 350 mm for general supply, with the commonly ordered condenser range at 5 mm to 219 mm.
Wall thickness: 0.5 mm to 50 mm depending on diameter, with the common condenser range at 0.5 mm to 15 mm.
Length: 1000 mm to 23000 mm, straight lengths, with length and end tolerances agreed to the buyer decision.
Form: seamless round tube, supplied in the annealed condition with good straightness and a clean surface inside and outside.
Tubes are produced seamless rather than welded so that there is no longitudinal seam to act as a corrosion initiation site in seawater service, and both the bore and the outside surface are kept clean because deposits and scale are the usual starting points for pitting.
Alloy Comparison for Seawater Service
| Alloy family | European designation | ASTM B111 grade | Service note |
|---|---|---|---|
| Copper, deoxidised | EN 12449, Cu-DHP | ASTM B111 / ASTM B75 C12200 | Fresh water and general service |
| Admiralty brass | EN 12451, CuZn28Sn1As | ASTM B111 C44300 | Clean seawater, inhibited against dezincification |
| Aluminum brass | EN 12451, CuZn20Al2As | ASTM B111 C68700 | Seawater and brackish water, good erosion resistance |
| Copper nickel 90/10 | EN 12451, CuNi10Fe1Mn | ASTM B111 C70600 | Higher velocity seawater, sand and silt bearing water |
| Copper nickel 70/30 | EN 12451, CuNi30Fe1Mn | ASTM B111 C71500 | Most aggressive seawater and high velocity duty |
Typical applications for C68700 tube include condenser, evaporator and heat exchanger tubing, distiller tubing, water evaporators, boiler blowdown heat exchangers, air coolers, gland steam condensers, steam ejectors, turbine oil coolers, fuel oil heaters, compressed air intercoolers and aftercoolers, ferrules and oil well pump liners.
FAQ
Q: What is the difference between admiralty brass and aluminum brass?
Admiralty brass C44300 is an inhibited Cu-Zn-Sn alloy, while aluminum brass C68700 relies on about 2% aluminium to form a protective surface film; both are used in condensers but they are not interchangeable in specification.
Q: Which standard covers C68700 condenser tubes?
ASTM B111 and ASME SB111 cover the seamless tube, with ASTM B395 and ASME SB395 applicable to related product forms.
Q: Why is arsenic included in C68700?
The arsenic range of 0.02-0.06% acts as an inhibitor that suppresses dezincification, so the range is specified rather than left open.
Q: What temper are the tubes supplied in?
All tempers are available, with the standard supply condition being O61 annealed and soft annealed; special tempers can be produced on request.
Q: What sizes can be supplied?
Outside diameters from 5 mm to 350 mm with wall thicknesses from 0.5 mm to 50 mm, or agreed to requirements, with the condenser range typically 5-219 mm outside diameter and 0.5-15 mm wall.
Q: When should copper nickel be selected instead?
When cooling water is heavily silt laden or velocities are high enough to threaten impingement attack, the copper nickel grades C70600 and C71500 offer better erosion-corrosion resistance than aluminum brass.
