Copper Nickel alloys have been the standard material for seawater systems, desalination plants, and marine applications for over a century. The two primary alloys – 90/10 (C70600) and 70/30 (C71500) – offer excellent corrosion resistance in seawater, but their performance characteristics differ significantly. Understanding these differences is essential for optimizing system design, cost, and service life.
Chemical Composition
Copper Nickel 90/10 (C70600 / CW352H)
- Copper: Balance (≈65-70% with Ni+Co)
- Nickel + Cobalt: 9.0-11.0%
- Iron: 1.0-1.8%
- Manganese: 0.5-1.0%
- Zinc: ≤0.5%
- Lead: ≤0.02%
Copper Nickel 70/30 (C71500 / CW354H)
- Copper: Balance (≈55-60% with Ni+Co)
- Nickel + Cobalt: 29.0-33.0%
- Iron: 0.4-1.0%
- Manganese: 0.5-1.5%
- Zinc: ≤0.25%
- Lead: ≤0.02%
The significantly higher nickel content in 70/30 is the primary driver of its enhanced corrosion resistance and mechanical properties.
Corrosion Resistance in Seawater
General Corrosion
Both alloys develop a protective oxide film when exposed to clean, flowing seawater. The corrosion rate in clean seawater is:
- 90/10: 0.025-0.050 mm/year (1-2 mils/year) in flowing seawater
- 70/30: 0.012-0.025 mm/year (0.5-1 mil/year) in flowing seawater
The 70/30 alloy shows approximately 50% lower corrosion rate, particularly significant in long-service installations.
Impingement Attack
When seawater velocity exceeds the protective film’s stability, impingement (erosion-corrosion) attack occurs:
- 90/10: Maximum recommended continuous flow velocity of 2.4 m/s (8 ft/s); susceptible to impingement above 3 m/s
- 70/30: Maximum recommended continuous flow velocity of 3.7 m/s (12 ft/s); significantly more resistant to impingement attack
For high-velocity systems (fire water pumps, ballast systems), 70/30 is strongly preferred.
Stagnant Seawater (Pitting and Crevice Corrosion)
In stagnant or low-flow conditions, both alloys can experience localized attack:
- 90/10: Maximum recommended stagnation period of 24-72 hours before flushing; susceptible to pitting in warm stagnant seawater above 30°C
- 70/30: Can tolerate stagnation up to 30 days in moderate temperatures; much more resistant to pitting and crevice corrosion in stagnant conditions
Sulfide Pollution Resistance
In harbors and estuaries where sulfide pollution occurs:
- 90/10: Susceptible to accelerated corrosion when sulfide films form and are then exposed to aerated water
- 70/30: Significantly more resistant to sulfide pollution effects
Desalination Plant Performance
Multi-Stage Flash (MSF) Distillation
For MSF desalination plants, tube material selection depends on brine temperature:
- Top brine temperature below 70°C: Either 90/10 or 70/30 acceptable; 90/10 is cost-effective choice
- Top brine temperature 70-100°C: 70/30 strongly preferred due to superior high-temperature corrosion resistance
- Top brine temperature above 100°C: 70/30 or titanium required; 90/10 not recommended
Reverse Osmosis (RO) Systems
For RO intake and outfall piping, seawater cooling systems, and post-treatment:
- 90/10: Widely used for intake screens, low-pressure piping, and heat rejection systems
- 70/30: Specified for high-pressure seawater lines, brine heaters, and critical heat exchangers
Median Corrosion Rates in MSF Plants
| Location | 90/10 Rate | 70/30 Rate |
|---|---|---|
| Heat rejection section | 0.025 mm/y | 0.012 mm/y |
| Heat input section | 0.050 mm/y | 0.020 mm/y |
| Brine heater tubes | 0.075 mm/y | 0.025 mm/y |
Mechanical Properties
| Property | 90/10 (C70600) | 70/30 (C71500) |
|---|---|---|
| Tensile Strength (MPa) | 345-415 | 370-485 |
| Yield Strength (MPa) | ≥110 | ≥150 |
| Elongation (%) | ≥30 | ≥30 |
| Hardness (HV) | 70-100 | 90-130 |
| Maximum Service Temp | ~260°C | ~315°C |
The 70/30 alloy offers approximately 30-40% higher strength, allowing thinner-wall tube designs that partially offset the higher material cost.
Biofouling Resistance
Both alloys release copper ions that inhibit marine biofouling:
- 90/10: Higher copper content provides slightly better anti-fouling performance
- 70/30: Still effective against biofouling, though marginally less than 90/10
In practice, the difference in biofouling resistance between the two alloys is minimal for most applications. Both significantly outperform titanium, stainless steel, and aluminum-bronze in biofouling resistance.
Cost Comparison
- Material cost: 70/30 is typically 30-50% more expensive than 90/10 per kilogram, reflecting the higher nickel content
- Fabrication cost: Similar for both alloys – both are readily weldable and formable
- Wall thickness savings: 70/30’s higher strength can allow 10-15% thinner walls, partially offsetting cost difference
- Service life benefit: In aggressive environments, 70/30’s superior corrosion resistance can double or triple service life, providing excellent lifecycle economics
Selection Guidelines
Choose 90/10 When:
- Clean, flowing seawater below 30°C
- MSF plants with top brine temperature below 70°C
- Velocity below 2.4 m/s
- Budget is a primary constraint
- Standard marine piping (non-critical systems)
Choose 70/30 When:
- Seawater temperature exceeds 30°C
- MSF plants with top brine temperature above 70°C
- High-velocity systems (>2.4 m/s)
- Intermittent or stagnant flow conditions
- Sulfide-polluted water
- Critical heat exchangers where failure is unacceptable
- Desalination brine heaters and evaporators
CoreMetal Supply for Marine and Desalination Applications
CoreMetal Steel supplies Copper Nickel 90/10 (C70600) and 70/30 (C71500) in tube, pipe, plate, and fitting form for marine and desalination projects. Our CuNi inventory includes seamless tube per ASTM B466/B467, welded pipe per ASTM B467, and plate per ASTM B122, all with full mill certification.
Contact CoreMetal Steel for competitive pricing on Copper Nickel products, with technical support for alloy selection based on your specific seawater system design parameters.