C70600 Copper Nickel vs C68700 Aluminum Brass

C68700 aluminum brass was popular decades ago. C70600 copper nickel replaced it in most seawater applications. However, C68700 still appears in freshwater and some brackish water systems. Buyers need to know the difference.

Dezincification is selective removal of zinc from brass. C70600 has no zinc, so it never dezincifies.

In C68700, zinc corrodes preferentially in certain water conditions. The zinc dissolves away, leaving behind porous, weak copper. The tube loses strength and develops pinhole leaks.

Conditions that cause dezincification in C68700:

Stagnant or low flow water (below 0.5 m/s)

High water temperature (above 60°C)

Low pH (below 6.0) or high pH (above 9.0)

Chlorides present (seawater or brackish water)

What dezincification looks like on C68700:

Tube surface turns dark brown or reddish

Soft, chalky appearance when scratched with a knife

Loss of metallic ring when tapped

Pinhole leaks without visible wall thinning

Prevention for C68700:

Maintain flow above 1.0 m/s

Keep water temperature below 60°C

Use arsenic-inhibited C68700 (modern standard)

But in seawater – even inhibited C68700 fails within 5 years

C70600 handles seawater. C68700 does not. This is the single biggest difference between these two alloys.

Real world data:

A coastal power plant switched from C68700 to C70600. Tube life increased from 6 years to 25+ years.

A desalination plant using C68700 retubed every 4 years. After switching to C70600, no retube was needed for 15 years.

Conclusion for seawater: Use C70600. C68700 is not suitable for any permanent seawater installation.

C68700 is acceptable for freshwater and some brackish water applications. It should never be used in seawater.

Good applications for C68700:

Freshwater cooling towers

River or lake water heat exchangers

HVAC condensers on the freshwater side

Low chloride industrial cooling water (below 500 ppm Cl)

Low temperature service (below 60°C)

Bad applications for C68700 (use C70600 instead):

Any seawater contact

Brackish water (estuaries, coastal rivers, 500–5,000 ppm Cl)

High temperature cooling water (above 60°C)

Stagnant water systems

Water with ammonia or sewage contamination

C70600 has better erosion resistance. C68700 erodes at lower flow velocities.

Why C70600 wins:
The nickel-iron matrix in C70600 is harder and more resistant to mechanical wear. C68700 has high zinc content (20–23%), which makes it softer and easier to erode.

If your system has any of these conditions, choose C70600:

High flow pumps delivering over 2.5 m/s

Sand or debris particles in the water

Frequent startups and shutdowns (transient velocity spikes)

Long pipe runs with turbulence at elbows

C70600 costs more upfront but lasts longer. C68700 is cheaper initially but requires frequent replacement.

Assumptions: 19mm OD x 1.24mm wall tube, seawater service, 20 year operation, no other major failures.

Winner on cost: C70600. Higher upfront price, but lower lifetime total cost.

Most reliable identification method:
PMI with an XRF gun. Takes 10 seconds. Confirms nickel (C70600) or zinc and aluminum (C68700).

Can C68700 be used in seawater for a short time?

Short term (a few months) maybe. Long term (years) no. Even 12 months of continuous seawater exposure causes significant dezincification on C68700. For any permanent seawater installation, use C70600.

What is the advantage of C68700 over C70600?

Lower upfront material cost and higher thermal conductivity (100 vs 45 W/m·K). For freshwater applications with no corrosion risk, C68700 works well and costs less. For seawater, that cost advantage disappears after the first retube.

Does C70600 ever suffer from dezincification?

No. C70600 contains no zinc. Dezincification is chemically impossible. This is why copper nickel replaced brass alloys in seawater applications.

Why do some old power plants still use C68700?

Because they were designed and built 30–50 years ago before C70600 became widely available. Many have since retubed with C70600. Those still using C68700 typically retube every 5–8 years.

What is the maximum water temperature for C68700?

60°C for continuous service. Above 60°C, dezincification accelerates rapidly. C70600 handles up to 315°C. For hot water or steam condensate cooling, C70600 is the clear choice.

Can I weld C68700 tubes like C70600 tubes?

C68700 is weldable but less common. Use aluminum bronze filler metal. C70600 welding is more straightforward with standard ERCuNi filler. For field repairs, C70600 is easier and more reliable.

What does the aluminum in aluminum brass actually do?

Aluminum helps form a protective oxide film on the tube surface. This improves corrosion resistance over regular brass (like C44300 Admiralty brass). Arsenic is also added to inhibit dezincification.

How do I convert an existing C68700 system to C70600?

Direct replacement is possible. Same OD and wall thickness works. C70600 is slightly stronger, so tube sheet expansion may need adjustment. Thermal performance changes – C70600 has lower conductivity, so more tubes or larger shell may be needed for the same heat duty.

Testing methods

Eddy current test (ECT) to ASTM E243 – 100% of tubes

Hydrostatic test up to 20 MPa – 100% of tubes

PMI (XRF) for alloy verification – every heat

Tensile and hardness test – per heat

Flattening and expansion test – per heat

Microscopic grain examination – per heat

Packaging standards

Plastic end caps on both ends

Individual polybag wrapping

Wooden crate (ISPM15 fumigated) for export

Moisture-proof paper + desiccant

Label with heat number, size, quantity

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