ASTM B111 C68700 vs. C70600: Which Copper Alloy is Better for Your Heat Exchanger?

Choosing the right material for your heat exchanger or condenser is critical. Selecting the wrong alloy can lead to premature corrosion, system failure, and costly downtime. Two of the most popular options under the ASTM B111 standard are C68700 (Arsenical Aluminum Brass) and C70600 (Copper-Nickel).

So, which one should you choose? This comparison guide will help you decide.

The performance of each alloy is determined by its metal makeup. ASTM B111 C68700 includes arsenic to specifically inhibit dezincification corrosion, while C70600 relies on nickel for superior seawater resistance.

This is the most common question from engineers. ASTM B111 C68700 forms a protective aluminum-rich film in clean, high-velocity seawater. However, C70600 is better for polluted or low-flow conditions.

Key takeaway: For clean, fast-moving seawater, C68700 is a cost-effective choice. For slow, polluted, or sulfide-bearing water, you need C70600.

For heat exchanger applications, strength and hardness matter. Let's look at the numbers:

While ASTM B111 C68700 has slightly higher strength, both are suitable for standard heat exchanger pressures. The real difference is not in strength but in corrosion resistance.

ASTM B111 C68700: Good for bending and forming but requires proper heat treatment after fabrication to avoid stress corrosion cracking. Welding is difficult.

C70600: Excellent weldability and formability. This makes it a favorite for complex tube assemblies and repairs.

If your project involves extensive welding or tight bends, C70600 is clearly the winner.

This is a critical factor often overlooked. ASTM B111 C68700 requires a minimum flow velocity (> 1 m/s) to maintain its protective oxide film. If the flow is too low, deposits form, leading to under-deposit corrosion and pitting. C70600 has no such minimum requirement.

When Should You Absolutely Avoid C68700?

Avoid using ASTM B111 C68700 in:

Stagnant or low-flow seawater (< 1 m/s)

Environments with high ammonia (e.g., fertilizer plants)

Polluted harbors or brackish water with sulfides

Applications requiring extensive welding

For these challenging scenarios, C70600 is the safer choice. Alternatively, explore our full range of [ASTM B111 heat exchanger tubes] for other options.

Power plant (once-through cooling, clean seawater, high velocity): ASTM B111 C68700 is the industry standard. It offers excellent performance at a lower cost.

Desalination plant (low-flow areas, brine with impurities): C70600 is preferred due to its superior resistance to pitting and crevice corrosion.

1. What does C68700 stand for?

C68700 is the UNS (Unified Numbering System) designation for Arsenical Aluminum Brass. The "C" indicates it is a copper alloy, and the specific number identifies its unique chemical composition: approximately 76-79% copper, 1.8-2.5% aluminum, 0.02-0.06% arsenic, and the remainder zinc. This specific composition is defined under the ASTM B111 standard for seamless heat exchanger and condenser tubes.

2. Is C68700 suitable for seawater use?

Yes, but only under specific conditions. ASTM B111 C68700 performs excellently in clean, high-velocity seawater (typically > 1 m/s) because the aluminum content forms a protective oxide film. However, it is not recommended for stagnant, low-flow, or polluted seawater, as the protective film can break down, leading to pitting corrosion and under-deposit attack. For such harsh conditions, C70600 (Copper-Nickel) is a better choice.

3. What is the difference between C68700 and C44300 (Admiralty Brass)?

Both are used in heat exchangers, but C68700 offers superior resistance to impingement attack (erosion corrosion) caused by high-velocity water or entrained air bubbles. Admiralty Brass (C44300) contains tin and arsenic but no aluminum. The aluminum in C68700 helps form a more durable, self-healing protective film, making it the preferred choice for higher velocity applications like power plant cooling systems. C44300 is typically used in lower velocity applications such as marine diesel engine coolers.

4. Can C68700 be welded?

Welding C68700 is difficult and generally not recommended. The zinc content in aluminum brass tends to vaporize during welding, creating porosity and weakening the joint. Additionally, the heat from welding can cause stress corrosion cracking if not properly post-weld heat treated. For most heat exchanger applications, ASTM B111 C68700 tubes are joined using roller expansion into tubesheets or brazing, not welding. If your design requires extensive welding, consider C70600 or other copper-nickel alloys instead.

5. What is the maximum operating temperature for C68700?

In water service, ASTM B111 C68700 can typically operate up to 200°C (400°F). Above this temperature, the risk of dezincification (selective leaching of zinc) increases significantly, even with the arsenic inhibitor. For higher temperature applications, such as superheaters or high-pressure steam condensers, C70600 or stainless steel alloys may be more appropriate. Always consult your materials engineer for applications exceeding 150°C (300°F) for extended periods.

6. Does C68700 rust?

No, C68700 does not "rust" because it contains no iron. Rust refers specifically to iron oxide formation on ferrous metals. However, ASTM B111 C68700 can corrode in unsuitable environments. The most common forms of corrosion affecting aluminum brass are pitting (from low-flow conditions), impingement attack (from excessive turbulence or sand erosion), and in rare cases, dezincification if the arsenic content is not properly controlled or if operating conditions are severe.

7. How do I prevent dezincification in C68700?

The arsenic content in ASTM B111 C68700 is specifically added to prevent dezincification, and under normal clean water conditions, this protection is highly effective. To further prevent dezincification, you should: (1) maintain clean water chemistry with pH between 6.5 and 8.5, (2) avoid stagnant or low-flow conditions where deposits can form, (3) keep ammonia levels below 2 ppm, and (4) ensure the tube temperature stays below 200°C (400°F). With proper operation, C68700 tubes can last 20+ years without dezincification issues.

8. Is C68700 magnetic?

No, ASTM B111 C68700 is completely non-magnetic. Like all copper-based alloys, it contains no significant amount of iron, nickel (except trace levels), or cobalt that would produce magnetic properties. This non-magnetic characteristic is actually beneficial for certain applications, such as condensers near sensitive electronic equipment or compasses on marine vessels. If you need to sort C68700 from ferrous scrap, a magnet is a quick and effective tool - C68700 will not stick.

9. What industries commonly use ASTM B111 C68700 tubes?

Power generation is the largest user - specifically, fossil fuel and nuclear power plants use C68700 tubes in their main steam surface condensers. Other major industries include: chemical processing (for heat exchangers handling clean coolants), HVAC (for large chillers and evaporators), marine (for auxiliary coolers not exposed to polluted harbor water), and desalination (for pre-heaters before the brine heater). In each case, the common requirement is clean, flowing water - typically from rivers, cooling towers, or open seas with good circulation.

10. Can I replace C70600 (90/10 Copper-Nickel) with C68700?

Only after a thorough review of your operating conditions. While ASTM B111 C68700 is more economical (typically 20-30% cheaper), it cannot simply "drop in" for C70600. You must verify: (1) water velocity is consistently above 1 m/s (C70600 works at any velocity), (2) water is clean with no sulfides or pollution (C70600 tolerates polluted water), (3) no extended idle periods (C70600 handles stagnant water better), and (4) minimal welding requirements (C70600 is easily weldable). If all four conditions are met, replacement is feasible and cost-effective. Otherwise, stick with C70600 to avoid premature failure.

11. What is the standard delivery condition for ASTM B111 C68700 tubes?

Most ASTM B111 C68700 tubes are supplied in the annealed temper (O61). Annealing makes the tubes soft and ductile, allowing them to be easily roller-expanded into tubesheets without cracking. Some applications may require light drawn temper for better erosion resistance, but this reduces formability. Always specify your required temper when ordering - O61 is standard for new condensers, while H55 (light drawn) may be used for retubing jobs where tighter clearances are needed. Your supplier should provide a mill test report (MTR) confirming temper and chemistry.

12. Does ASTM B111 C68700 require any special maintenance?

Yes, maintaining minimum flow velocity is the most critical maintenance requirement. If flow drops below 1 m/s for extended periods, deposits can settle on the tube surface, leading to under-deposit corrosion and pitting. During plant shutdowns, you should either keep water circulating or flush the tubes with clean, dry air to remove moisture. Additionally, avoid prolonged exposure to stagnant seawater - if a unit is idle for more than a week, consider filling the water box with treated fresh water or drying the tubes completely. Regular eddy current testing (ECT) is recommended every 2-3 years to detect wall thinning before it leads to tube failure.

Every tube in this lot has passed third-party witnessed inspection per ASTM B111 standard for C68700 alloy. Below are actual photos from customer-onsite inspection, including eddy current testing and dimensional verification.

Inspection items verified:
• Eddy current testing (ECT) – no through-wall defects
• Outer diameter & wall thickness – within ±0.02mm tolerance
• Surface finish & temper (O61) – conforms to ASTM
• Hardness & chemical composition – certified.

After passing inspection, all tubes are packed according to export standards and customer-specific requirements. The packing process is documented below to ensure traceability and damage-free delivery.

Packing steps shown in video & images:
1. Tube cleaning & drying
2. Plastic end caps on both ends
3. VCI anti-rust paper wrapping
4. Bundle strapping with moisture barrier film
5. Plywood wooden case (ISPM-15 compliant) with foam padding
6. Labeling with ASTM grade, lot number, and inspection stamp

All ASTM C68700 tubes are produced and inspected on our in-house equipment, allowing full process control from billet casting to final packing.

Key equipment used for this lot:
• Induction melting furnace – precise alloying (Cu + Zn + Al + As)
• Horizontal continuous casting – uniform billet structure
• Extrusion press (800T / 1630T) – seamless tube forming
• Cold drawing bench (5–40m) – dimensional accuracy to ±0.02mm
• Online eddy current tester (FOERSTER / MAC) – 100% NDT
• Ultrasonic wall thickness gauge – real-time monitoring
• Annealing furnace (controlled atmosphere) – temper O61

In-house metrology: Micrometers, pin gauges, optical comparator, hardness tester (HV/HRB)

All equipment is calibrated quarterly. Production records are traceable by lot number.

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