Chemical Composition Comparison
| Element | C71500 (70/30) | C70600 (90/10) | C68700 (Al Brass) |
|---|---|---|---|
| Copper (Cu) | 65.0-70.0% | 86.5% min | 76.0-79.0% |
| Nickel (Ni) | 29.0-33.0% | 9.0-11.0% | - |
| Aluminum (Al) | - | - | 1.8-2.5% |
| Iron (Fe) | 0.4-1.0% | 1.0-1.8% | 0.06% max |
| Manganese (Mn) | 1.0% max | 1.0% max | - |
| Zinc (Zn) | - | - | Remainder |
| Lead (Pb) | 0.02% max | 0.02% max | 0.09% max |
Mechanical Properties Comparison (ASTM B111 Minimums)
| Property | C71500 | C70600 | C68700 |
|---|---|---|---|
| Tensile Strength (MPa) | 380 | 303 | 324 |
| Tensile Strength (ksi) | 55 | 44 | 47 |
| Yield Strength 0.5% ext (MPa) | 125 | 105 | 105 |
| Yield Strength 0.5% ext (ksi) | 18 | 15 | 15 |
| Elongation (%) | 30 | 30 | 15 |
Physical Properties Comparison
| Property | C71500 | C70600 | C68700 |
|---|---|---|---|
| Density (g/cm³) | 8.94 | 8.91 | 8.33 |
| Density (lb/in³) | 0.323 | 0.322 | 0.301 |
| Melting Point (°C) | 1170-1240 | 1100-1145 | 899-932 |
| Thermal Conductivity (W/m·K) | 29 | 40 | 100 |
| Electrical Resistivity (µΩ·m) | 0.038 | 0.028 | 0.006 |
C68700 has better thermal conductivity (100 vs 29 for C71500). That is why it is used in heat exchangers with clean water. But that is its only advantage.
Corrosion Resistance in Seawater
| Condition | C71500 | C70600 | C68700 |
|---|---|---|---|
| General seawater corrosion | Excellent | Excellent | Fair to Good |
| High velocity (>3 m/s) | Good | Poor (film erodes) | Poor |
| Stagnant / low flow | Good | Fair (sediment risk) | Poor (dezincification) |
| Sand / debris present | Fair | Poor | Poor |
| Biofouling (barnacles, algae) | Excellent | Excellent | Good |
| Crevice corrosion | Resistant | Resistant | Susceptible |
| Dezincification | Not applicable | Not applicable | Yes – major failure mode |
| Ammonia attack | Susceptible | Susceptible | Susceptible |
| Sulphide-polluted seawater | Good | Fair | Poor |
The most important difference: C68700 dezincifies in seawater. Zinc leaches out. The pipe becomes porous copper and fails. C71500 and C70600 have no zinc – no dezincification.
Temperature and Velocity Limits
| Limit | C71500 | C70600 | C68700 |
|---|---|---|---|
| Max continuous temperature (°C) | 120 | 80 | 65 |
| Max clean water velocity (m/s) | 4-6 | 3 | 2-3 |
| Max velocity with sand (m/s) | 2-3 | 1.5 | Not recommended |
Price Comparison
| Grade | Relative price (per ton) | Typical application |
|---|---|---|
| C70600 (90/10) | Baseline (1.0x) | General seawater, low velocity |
| C71500 (70/30) | 1.15 – 1.20x | High velocity, high temperature |
| C68700 (Al Brass) | 0.85 – 0.90x | Fresh or brackish water only |
Application
| Application | C71500 | C70600 | C68700 |
|---|---|---|---|
| Ship seawater piping (firewater, cooling) | Yes (standard) | Yes (low pressure only) | No |
| Offshore platform firewater mains | Yes | No | No |
| Power plant condenser (seawater) | Yes | Yes (lower temp stages) | No |
| Desalination brine heater | Yes | No | No |
| Desalination recovery section | Yes | Yes | No |
| Heat exchanger with clean seawater | Yes | Yes | Marginal |
| Heat exchanger with brackish water | Yes | Yes | Yes (short term) |
| Fresh water cooling | Yes | Yes | Yes |
| Tropical seawater (warm, biofouling) | Yes (best) | Yes | No |
When to Choose Each Grade
Choose C71500 if:
Velocity >3 m/s (up to 6 m/s)
Temperature >80°C (up to 120°C)
Sand or debris present
Offshore or ship firewater systems
Long service life (20+ years) in aggressive water
Budget allows 15-20% premium over C70600
Choose C70600 if:
Velocity <3 m/s
Temperature <80°C
Clean seawater, no sand
Budget is the main constraint
Service life 10-15 years is acceptable
Choose C68700 if:
Fresh or brackish water only
Temperature <65°C
Clean, no debris
Low cost is critical
Short service life (3-7 years) is acceptable
You are not using full seawater
FAQ
Q1: Which grade is best for seawater?
C71500. Highest nickel content gives best corrosion resistance, highest velocity limit, and highest temperature limit.
Q2: Can I use C70600 instead of C71500 to save money?
Yes, but check your conditions. If velocity is under 3 m/s, temperature under 80°C, and water is clean, C70600 works fine. If any of those conditions are exceeded, use C71500.
Q3: Is C68700 acceptable for seawater?
No. Not for long-term. It dezincifies. Some old systems still run it, but they fail earlier and require more maintenance. Do not specify it for new seawater projects.
Q4: Why is C68700 cheaper than copper nickel?
Because it contains zinc (cheap) instead of nickel (expensive). You get what you pay for.
Q5: Which grade is easiest to weld?
C70600. Lower nickel content. C71500 requires more care (back purge, temperature control). C68700 is weldable but prone to porosity.
Q6: Does C68700 have any advantage over copper nickel?
Better thermal conductivity (100 vs 29 W/m·K). For heat exchangers with clean, cool water, that matters. For seawater, the corrosion risk outweighs the thermal benefit.
Q7: Can I mix C71500 and C70600 in the same system?
Yes. Same dimensions, compatible alloys. Use RN-67 filler for welding. No galvanic issues.
Q8: Can I mix C68700 with copper nickel in the same system?
Not recommended. Different corrosion potentials. Also, C68700 dezincification products can contaminate the water and affect copper nickel.
Q9: What is the most common failure mode for each grade?
C71500: Erosion if velocity >6 m/s with sand
C70600: Erosion if velocity >3 m/s
C68700: Dezincification in any seawater
Q10: What do you recommend for a new desalination plant?
C71500 for brine heaters and high-temperature sections. C70600 for lower-temperature recovery stages. Do not use C68700 anywhere in seawater service.
