Why Aluminium Brass Tubes Are Used in Seawater Exchangers
ASTM B111 C68700 aluminium brass seamless tubes are cold-drawn copper-zinc tubes containing 1.8 to 2.5% aluminium with a small arsenic addition. The aluminium forms a protective surface film that resists moving seawater, which is why the grade became the default tube material for surface condensers, evaporators and heat exchangers in power stations, refineries and desalination plants. The tube is covered by ASTM B111 for diameters up to 3 1/8 in (80 mm) and is ordered worldwide as HAL77-2, CZ110, C6870 or CuZn20Al2.
Compared with admiralty brass, aluminium brass tolerates higher water velocity and dirtier cooling water, a difference that directly extends the interval between bundle retubing.
Chemistry and Equivalent Grades
| 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 |
| Standard | Corresponding designation |
|---|---|
| ASTM B111 | C68700 |
| GB/T 8890 | HAL77-2 |
| BS 2871 | CZ110 |
| JIS H3300 | C6870 |
| DIN 1785 | CuZn20Al2 |
The aluminium range is the controlling figure. Too little aluminium weakens the protective film and the tube reverts to ordinary brass behaviour in seawater; too much aluminium makes hot working and drawing difficult, so the 1.8 to 2.5% window is held tightly in production.
Mechanical Properties and Temper
| Property | Value |
|---|---|
| Temper | O61 annealed |
| Tensile strength, min. | 50 ksi / 345 MPa |
| Yield strength, min. | 18 ksi / 125 MPa |
The annealed temper gives a minimum yield strength of 125 MPa. That level is high enough to resist collapse during rolling into a tube sheet and low enough to allow the tube end to expand without cracking, which is the balance a condenser bundle needs.
Size Range and Manufacturing Route
| Item | Range |
|---|---|
| Manufacture | Seamless, cold drawn |
| Outside diameter | 5 mm to 80 mm, up to 3 1/8 in |
| Wall thickness | 0.5 mm to 12.7 mm |
| Length | 6 m or specified length |
| Ends | Plain, bevelled or threaded |
Tubes are hot or cold worked to finished size and then annealed to meet the specified temper. Straightening, trimming and surface cleaning follow, and tubes are normally varnished for transport protection. Diameter tolerance decreases as the tube gets smaller:
| Outside diameter, mm | Diameter tolerance, plus and minus, mm |
|---|---|
| Up to 12 | 0.064 - 0.076 |
| Over 12 to 18 | 0.076 - 0.10 |
| Over 18 to 25 | 0.10 - 0.15 |
| Over 25 to 35 | 0.13 - 0.20 |
| Over 35 to 50 | 0.15 |
| Over 50 to 80 | 0.17 |
Eddy-current test: applied to every tube to detect wall defects, with hydrostatic or pneumatic testing available as the specified alternative.
Flattening test: specimens are inspected to the applicable flattening test method and the flattened area must be free of defects that would affect the intended application.
Grain size control: the annealed structure is checked so that tube ends roll and bend predictably in the field.
Workmanship: roundness, straightness and wall uniformity must suit the application, and cut ends are deburred.
Marking and packing: standard, grade, size, heat number and lot number are marked on the tube, and bundles are supplied in hexagonal bundles, wooden boxes or steel and wooden crates.
Application and Alloy Selection
Aluminium brass tube is used in surface condensers, evaporators, coolers and heat exchangers in power generation, refineries and chemical plants, in desalination and distillation trains, and in shipboard seawater systems. Its aluminium-rich film resists moving and aerated water better than plain admiralty brass. Where chloride levels are very high, where the water carries entrained sand or where the tube must also resist ammonia-bearing condensate, copper-nickel C70600 or C71500 is the stronger choice, at a higher cost and with lower thermal conductivity.
FAQ
Q: What is ASTM B111 C68700 aluminium brass?
It is a seamless cold-drawn brass tube containing 76.0 to 79.0% copper, 1.8 to 2.5% aluminium and 0.02 to 0.06% arsenic, used mainly in seawater condensers, evaporators and heat exchangers.
Q: How does it differ from admiralty brass C44300?
C68700 relies on aluminium instead of tin for its protective film, giving better resistance to faster and more aerated seawater, while C44300 is used in fresh and low-velocity cooling water.
Q: What sizes of aluminium brass tube are available?
Seamless tube runs from 5 mm to 80 mm outside diameter, equivalent to about 3 1/8 in, with 0.5 mm to 12.7 mm wall and 6 m standard length.
Q: What are the minimum mechanical properties in the annealed temper?
The O61 annealed temper provides a minimum tensile strength of 345 MPa (50 ksi) and a minimum yield strength of 125 MPa (18 ksi).
Q: Why is arsenic added to this alloy?
Arsenic acts as an inhibitor against dezincification, protecting the zinc-rich phase so that the aluminium film can do its work on the water side of the tube.
