What C68700 Aluminium Brass Is
C68700 is the UNS designation for aluminium brass, a copper-zinc alloy containing controlled additions of aluminium and arsenic. It was developed for condenser and heat exchanger tubing in seawater service, where it outperforms plain brass and admiralty brass at higher water velocities. ASTM B111 and ASME SB111 cover the seamless tube form, and the material is widely used for condenser tubes, evaporator tubes, ferrules and distiller tubing.
The aluminium addition is the key to its performance. In aerated seawater the alloy forms a thin, adherent aluminium-rich oxide film which is both corrosion resistant and tough, so it survives the impingement and turbulence that erode softer copper alloys.
Chemical Composition
| Element | Requirement, % |
|---|---|
| Copper (Cu) | 76.0 - 79.0 |
| Aluminium (Al) | 1.8 - 2.5 |
| Arsenic (As) | 0.02 - 0.06 |
| Lead (Pb) | 0.07 max |
| Iron (Fe) | 0.06 max |
| Zinc (Zn) | Remainder |
The arsenic addition, although very small, inhibits dezincification in the same way as in admiralty brass. The aluminium content must be kept inside the specified range: too little and the protective film is weak, too much and the alloy becomes difficult to hot work and cold draw.
Physical and Mechanical Properties
| Property | Value at 25 °C unless stated |
|---|---|
| Density | 8.33 g/cm³ |
| Elastic modulus | 117 GPa |
| Poisson ratio | 0.34 |
| Tensile strength, temper O61 | 345 MPa (50 ksi) min |
| Yield strength, temper O61 | 125 MPa (18 ksi) min |
Tube is normally supplied in the annealed O61 temper so that it can be rolled into tube sheets and bent into U-form without cracking. Typical manufacturing sizes run from roughly 3 mm to 70 mm outside diameter with wall thickness from 0.2 mm to 5 mm, with larger sizes available to order.
Corrosion Resistance in Seawater
C68700 performs well in clean, aerated seawater and in brackish water. It resists impingement attack and erosion-corrosion better than admiralty brass because the protective film reforms quickly after mechanical damage. It also tolerates the higher flow velocities used in modern condensers to keep tube bores clean.
Correct performance depends on a continuous, aerated water film; stagnant conditions and sulphide-polluted water reduce film stability.
Fouling deposits must be controlled, because attack begins under sludge and scale where the water film is interrupted.
Chlorination for biofouling control is normally acceptable at standard dosing rates; over-chlorination can disturb the film and should be avoided.
In severely polluted or high-sulphide seawater, copper-nickel grades C70600 and C71500 are preferred.
Applications
Condenser tubes in coastal power stations and industrial steam plants using seawater cooling.
Evaporator, distiller and brine heater tubing in multi-effect and multi-stage desalination plants.
Ferrules and inlet-end inserts that protect tube ends from erosion by high-velocity water.
Heat exchanger tubes in marine and offshore installations, shipbuilding and port equipment.
Refinery and petrochemical coolers handling brackish or saline cooling water.
Processing and Installation Guidance
Roll the tube into the tube sheet using a qualified procedure and stop expansion at the specified wall reduction; aluminium brass work-hardens and over-rolling thickens the risk of stress cracking.
Bend U-tubes with internal support and, where specified, stress-relieve after forming to restore ductility in the bend.
Keep the tube ends clean and free of scale before rolling so that the joint seals uniformly.
Avoid direct contact with less noble metals in the water box; use insulated joints or cathodic protection where the design requires it.
Annealing for bending or end forming is carried out in the range appropriate to the alloy, followed by a controlled cool, so that the aluminium stays in solid solution.
Inspection Points Before Acceptance
Verify the heat analysis against the C68700 limits for copper, aluminium and arsenic, and confirm that the tube is in the specified temper with tensile and yield strengths meeting ASTM B111. Eddy-current testing to ASTM E243 is the standard non-destructive check for longitudinal and transverse imperfections, and the expansion and flattening tests of ASTM B153 give a direct measure of ductility. Dimensional checks should cover outside diameter, wall thickness, eccentricity and length, and the bore should be examined for draw marks, inclusions and surface defects that could seed localised corrosion.
FAQ
Q: What is C68700 aluminium brass?
A copper-zinc alloy with about 2% aluminium and a small arsenic addition, used as seamless condenser and heat exchanger tube in seawater service.
Q: What are the C68700 composition limits?
76.0 - 79.0% copper, 1.8 - 2.5% aluminium, 0.02 - 0.06% arsenic, 0.07% max lead, 0.06% max iron and the balance zinc.
Q: What are its mechanical properties?
In the annealed O61 temper the minimum tensile strength is 345 MPa and the minimum yield strength is 125 MPa, with a density of 8.33 g/cm³.
Q: Why choose aluminium brass instead of admiralty brass?
Aluminium brass forms a tougher protective film and withstands higher seawater velocities and impingement, so it lasts longer where flow rates and turbulence are high.
Q: Where is C68700 tube used?
Seawater-cooled condensers, desalination evaporators and distillers, marine heat exchangers and condenser ferrules.
Q: Which standards cover the material?
ASTM B111 and ASME SB111 for seamless condenser tube, with equivalents CuZn20Al2 under European designations, CZ110 in the British tube series, C6870 under JIS H3300 and HAL77-2 under GB/T 8890.
Q: How is the tube tested?
Chemistry, tensile testing, eddy-current examination per ASTM E243, expansion and flattening tests per ASTM B153, and pressure testing where the order requires it.
