Product Overview: Rectangular Aluminium Brass Tube
C68700, often called aluminium brass, is a single-phase copper-zinc alloy to which aluminium has been added for corrosion resistance and arsenic for resistance to dezincification. It is a standard grade in ASME SB111, the ASME designation matching ASTM B111, and the same alloy appears in EN 12451 and in the GB/T 8890 designation HAL77-2.
Rectangular and flat sections are drawn from the same alloy as round condenser tube and are used where the coil designer needs a high heat-transfer surface in a shallow depth: the flat face carries the fin or the air stream while the bore carries the cooling water. Rectangular sections are produced to the customer's width, height and wall rather than to a single catalogue table, so the purchase order always states the section dimensions, the corner radius and the wall tolerance agreed with the mill.
Chemical Composition
| Element | Requirement (mass %) | Function |
|---|---|---|
| Copper (Cu) | 76.0 - 79.0 | Base metal |
| Zinc (Zn) | Remainder | Principal alloying element |
| Aluminium (Al) | 1.8 - 2.5 | Forms a protective surface film in seawater and brackish water |
| Arsenic (As) | 0.02 - 0.06 | Inhibits dezincification in aggressive waters |
| Iron (Fe) | 0.06 max | Residual limit, kept low to protect ductility |
| Lead (Pb) | 0.07 max | Residual limit |
| Manganese (Mn) | 0.06 max | Residual limit |
The aluminium content is the element that determines whether the tube survives in seawater service, and the arsenic addition is what keeps the alloy from losing zinc preferentially when the cooling water is brackish. Both are controlled within narrow bands, so the heat analysis is the first document a buyer should read on the mill certificate.
Corrosion Behaviour and Service Limits
Aluminium brass resists general attack in clean and brackish seawater and is widely used for condenser and cooler tubes on the water side.
Resistance to dezincification is measured by the ISO 6509 immersion test, which reports the maximum depth of dezincification in a polished specimen. A low maximum depth is the acceptance criterion for tube in polluted or chloride-rich cooling water.
The alloy resists chloride stress corrosion cracking, but it is sensitive to ammonia and ammonium compounds, which appear in polluted harbours and in some process streams; the copper-zinc family as a whole needs an ammonia review before it is selected.
Velocity limits for clean seawater are generally taken as about 1.5 to 2.5 m/s in the tube. Higher velocity strips the protective aluminium-rich film and leads to impingement attack at the inlet end.
Chlorine dosing for biofouling control is normally limited to low residual levels, because over-dosing accelerates attack on the protective film.
Where the cooling water is clean, aluminium brass gives excellent service at a lower material cost than titanium or high-nickel alloys. Where the water carries sand, sulphides or ammonia, the alloy is replaced by a copper-nickel grade or a higher-alloy alternative rather than protected by heavier walls.
Rectangular Section Characteristics
| Feature | Practical guidance |
|---|---|
| Section shape | Rectangle with rounded corners, or flat-oval where a coil core requires it |
| Wall thickness | Selected so that the reduced wall on the long flat face still meets the design pressure and the forming allowance |
| Aspect ratio | Kept within the mill's forming capability so that the long face does not collapse during drawing |
| Straightness and twist | Controlled because fins are stacked to a close pitch and twist causes fin damage during assembly |
| Surface condition | Supplied clean and dry, with the bore free of drawing lubricant and carbon film |
Forming, Finning and Inspection
Rectangular tube is bent in the plane of the short face wherever possible; the minimum bend radius follows the mill's forming data for the specific section and wall.
Finning is done by embedding or wrapping the fin material around the flat faces, and the fin material is selected to avoid a large galvanic couple with the brass.
Brazing with silver-based or copper-phosphorus filler is standard for coil headers; flux residues must be removed because they attack the alloy in service.
Tube ends are cleaned and de-burred before expansion into headers or tube plates so that the joint seals over its full length.
Eddy-current examination to ASTM E243 is applied to the straight length of the tube, and the finished coil is pressure tested with dry air or nitrogen before despatch.
Dezincification testing to ISO 6509 is ordered on a sample basis when the tube is destined for brackish or polluted water.
FAQ
Q: Why use a rectangular section instead of a round tube?
A flat face gives a larger contact area for the fin and a shallower coil depth for the same heat-transfer surface. That suits air-side applications such as chillers, cooling coils and condensers with limited installation space.
Q: Is C68700 the same as admiralty brass?
No. Both are copper-zinc alloys used in seawater, but C68700 contains about 2 % aluminium and is designated aluminium brass, while C44300 is the tin-bearing admiralty brass. They appear as separate grades in ASME SB111 and are not interchangeable without an engineering review.
Q: How is dezincification resistance demonstrated?
By the ISO 6509 immersion test on a sample of the finished tube, which measures the maximum depth of dezincification and is the accepted criterion for tubes that will see aggressive cooling water.
Q: Can the tube be welded?
It can be welded with matching filler for headers and repair work, but coil joints are usually brazed because the heat input of brazing is lower and easier to control on thin walls.
Q: What cooling water quality is acceptable?
Clean seawater, brackish water and fresh water with modest chloride content are all acceptable. Water containing ammonia, sulphides or high suspended solids requires a different alloy selection.
Q: Which information is needed for a quotation?
The section width, height, corner radius, wall thickness, length, temper, quantity and the acceptance standard, together with the cooling water analysis if the tube is intended for a marine or brackish duty.
