| Condition | C71500 (70/30) | C70600 (90/10) | C68700 (Al Brass) |
|---|---|---|---|
| Seawater, velocity >3 m/s | Yes | No | No |
| Seawater, velocity 1.5–3 m/s | Yes | Yes | No |
| Brackish water, low velocity | Yes | Yes | Yes |
| Fresh water, any velocity | Yes | Yes | Yes |
| Temperature >80°C | Yes | Marginal | No |
| Sand/silt present | Yes (with erosion allowance) | No | No |
| Ammonia exposure | No | No | No |
| Sulfide-polluted seawater | Yes | Marginal | No |
Composition
| Element (wt%) | C71500 | C70600 | C68700 |
|---|---|---|---|
| Nickel | 29.0–33.0 | 9.0–11.0 | - |
| Aluminum | - | - | 1.8–2.5 |
| Copper | 65.0–70.0 | 86.5 min | 76.0–79.0 |
| Iron | 0.4–1.0 | 1.0–1.8 | 0.06 max |
| Zinc | - | - | Remainder |
C68700 contains zinc (up to 22%). In high-temperature seawater, dezincification occurs. C68700 is not recommended for seawater heat exchangers.
Mechanical Properties (ASTM B111 minimums)
| Property | C71500 | C70600 | C68700 |
|---|---|---|---|
| Tensile strength (MPa) | 380 | 303 | 324 |
| Yield strength 0.5% ext (MPa) | 125 | 105 | 105 |
| Elongation (%) | 30 | 30 | 15 |
C71500 provides 25% higher tensile strength than C70600. For high-pressure systems or vibration-prone installations, C71500 is the correct choice.
Corrosion Resistance Comparison
| Environment | C71500 | C70600 | C68700 |
|---|---|---|---|
| Clean seawater, <3 m/s | Excellent | Excellent | Good |
| Clean seawater, >3 m/s | Excellent | Poor (erosion) | Poor |
| Seawater + sand | Fair (needs allowance) | Poor | Poor |
| Sulfide-polluted seawater | Good | Fair | Poor |
| Brackish water | Excellent | Excellent | Fair |
| Ammonia (any concentration) | Poor | Poor | Poor |
| High-velocity inlet (6 m/s) | Acceptable | Failure | Failure |
In sulfide-polluted harbors, C70600 and C68700 both underperform. C71500 retains corrosion resistance due to higher nickel content.
For ASTM B111 C70600 copper nickel pipe in clean seawater, maximum recommended velocity is 3 m/s. Above that, protective film fails within months.
Temperature Limits
| Grade | Max continuous (°C) | Max intermittent (°C) |
|---|---|---|
| C71500 | 120 | 150 |
| C70600 | 80 | 100 |
| C68700 | 65 | 80 |
For steam condensers or hot brine (>90°C), C71500 is the only viable option among these three grades.
Weldability
| Grade | Filler metal | Back purge required? | Preheat |
|---|---|---|---|
| C71500 | RN-67 (70/30 Cu-Ni) | Yes | 150°C max |
| C70600 | RN-67 or RN-60 | Yes | None |
| C68700 | RAlBr (aluminum bronze) | No | None |
C68700 welds are prone to porosity. For seawater systems requiring welded joints, avoid C68700. Use C71500 or C70600 instead.
Cost Hierarchy (relative to C70600)
| Grade | Cost multiplier | Typical premium |
|---|---|---|
| C70600 | 1.0x | Baseline |
| C71500 | 1.15 – 1.20x | +15–20% |
| C68700 | 0.85 – 0.90x | -10–15% |
C68700 is cheaper upfront but has higher failure risk in seawater. The apparent cost saving often disappears within 2–3 years of operation.
Application
Marine / Shipbuilding
C71500: Propeller shaft liners, sea chests, firewater mains
C70600: Low-pressure cooling lines, ballast systems
C68700: Not recommended
Desalination / Power plants
C71500: High-velocity brine, hot sections
C70600: Lower-temperature stages
C68700: Freshwater preheaters only
Refineries / Chemical plants
C71500: Seawater cooling (closed loop OK)
C70600: Treated cooling water
C68700: Air fin coolers (freshwater side)
FAQ
Q1: Can C68700 replace C71500 in seawater?
No. C68700 suffers from dezincification in warm seawater. Failure usually occurs within 2 years.
Q2: Which grade is easiest to weld?
C70600. Lower nickel content means less cracking risk. C71500 requires stricter purge and temperature control.
Q3: Does C68700 cost less than copper nickel?
Yes, approximately 10–15% less. But total lifecycle cost is often higher due to shorter service life.
Q4: Is ASTM B111 C70600 copper nickel pipe suitable for firewater systems?
Yes, for land-based systems. For offshore platforms with high pump discharge pressure, use C71500 instead.
Q5: Can I mix C71500 and C70600 in the same heat exchanger?
Yes, dimensionally they are identical. Differential thermal expansion is minimal. Use RN-67 filler for all tube-to-tubesheet welds.
Q6: Which grade resists sand erosion best?
C71500. But even C71500 requires one schedule or BWG extra thickness as erosion allowance if sand >0.2 wt%.
Q7: Does C68700 require cathodic protection?
No, but it does not harm it either. C68700 is galvanically similar to C70600. Isolation from stainless steel is still required.
Q8: What is the maximum OD available for C68700 per ASTM B111?
3 inches (76.2 mm) seamless. Larger diameters are not covered by the standard. For larger sizes, use C70600 or C71500.
Q9: Which grade is standard for U-tube heat exchangers?
C71500. Higher strength allows tighter bend radii without wall thinning issues.
Q10: How do I verify I received the correct grade?
Positive Material Identification (PMI). C68700 contains no nickel; C70600 has 9–11% nickel; C71500 has 29–33% nickel. A handheld XRF gun identifies all three in 10 seconds.
