Walk through any shipyard, desalination plant, or offshore platform and you will find the same family of tubing running through the pipe racks, coolers, and fire-fighting mains: a dull silver alloy that does not rust, does not pit, and quietly does its job for thirty years while everything around it gets replaced. That alloy is almost always a copper-nickel — usually 90/10, sometimes 70/30, occasionally a higher-nickel grade for the most aggressive service.

Despite being one of the oldest seawater-resistant alloys in continuous industrial use, copper nickel alloy is still routinely mis-specified. Buyers confuse 90/10 with 70/30, mistake the shipbuilding Cu-Ni spec for the heat-exchanger Cu-Ni spec, and pair the tube with carbon steel pipe fittings that quietly destroy the system through galvanic corrosion. This guide explains how to avoid those mistakes — and how to specify a Cu-Ni package that passes inspection on the first try.

1. Why Copper-Nickel Dominates Seawater Service

Three properties put copper-nickel ahead of every other alloy in raw seawater and brackish water service:

• Natural corrosion resistance in saline water. A stable, adherent, self-healing oxide film forms on the inner surface and protects the tube wall even under stagnant, low-velocity, or biologically active conditions. The film also resists macrofouling, which is why Cu-Ni is widely used for seawater piping without additional antifouling treatment.
• Resistance to stress corrosion cracking and pitting. Unlike stainless steels, Cu-Ni does not suffer chloride-induced SCC or the deep pitting that takes 316L out of service in warm seawater above about 40°C.
• Excellent fabricability. Cu-Ni is readily bent, flanged, brazed, and welded using standard shipyard procedures, and the welds retain most of the parent metal's corrosion resistance when filler and procedure are correct.

For these reasons, naval architects, classification societies, and offshore engineers default to copper-nickel for: shipbuilding seawater cooling and fire-fighting systems, desalination plant intake and brine piping, platform firewater deluge systems, and heat-exchanger tubes handling seawater or brackish cooling water.

2. The Two Grades You Will Actually Use

Of the full Cu-Ni family, two grades account for the overwhelming majority of industrial orders.

90/10 Copper-Nickel (UNS C70600, CN102, CuNi10Fe1Mn)

Composition is approximately 90% copper, 10% nickel, with 1–2% iron and 0.5–1% manganese added to improve corrosion resistance in flowing seawater and to stabilize the protective oxide film. 90/10 is the workhorse grade for shipbuilding seawater systems, platform fire mains, and brackish water cooling. It offers the best balance of cost, weldability, and seawater performance, and it is the first choice for piping where velocity is moderate (typically up to ~3.5 m/s) and biofouling resistance matters.

70/30 Copper-Nickel (UNS C71500, CN107, CuNi30Fe1Mn)

Composition is approximately 70% copper, 30% nickel, with 0.4–1% iron and 0.4–1% manganese. The higher nickel content gives better resistance to higher-velocity flow, slightly better corrosion resistance in polluted or sulfide-containing water, and improved strength at elevated temperature. 70/30 is the standard choice for heat-exchanger tubes in marine and offshore duty, refinery coolers handling saline cooling water, and the more demanding sections of naval piping where service conditions push the limits of 90/10.

3. Standards You Will See on Every Drawing

Cu-Ni tubes and pipes are bought and sold under a stack of overlapping international standards. The most commonly referenced ones are summarized below.

Dual certification across two of these standards (for example ASTM B466 plus EEMUA 234, or EN 12451 plus ASTM B111) is the most common way international EPC contractors de-risk procurement and ensure the same tube can be installed across multi-yard builds.

4. Specifying Size, Schedule, and Dimensional Tolerances

Cu-Ni tube is typically supplied in accordance with the outside diameter and wall-thickness tables in ASME B36.19M (stainless) or the project-specific pipe spec, with common sizes from 6 mm OD up to 610 mm OD (24 in) and wall thicknesses from 0.5 mm to 10 mm depending on the application. When the spec is written, the following items should be made explicit so there is no ambiguity at the receiving inspection:

• Outer diameter tolerance — usually per ASTM B466 or B111, with tighter limits available for heat-exchanger duty.
• Wall thickness tolerance — typically ±10% for seamless tube.
• Length — random single, double random, or cut-to-length to drawing, with end finish (plain end, beveled for welding, or expanded for tube-sheet rolling).
• Surface condition — mill finish, pickled, or bright annealed depending on duty. Heat-exchanger tubes are often required in a clean, pickled, and passivated condition.
• Hydrostatic or eddy current test — every tube should be tested in accordance with the standard's mandatory test method. Specify this explicitly in the purchase order.

5. Welding, Bending, and Joint Design

Cu-Ni is welded using TIG (GTAW) or shielded metal arc (SMAW) with matching filler metals — typically ERCuNi for 90/10 and ERCuNi for 70/30, or their SMAW electrode equivalents. The procedure is forgiving, but three points deserve attention:

• Joint cleanliness. Copper-nickel is sensitive to contamination by oil, sulfur, and lead. Cleaning before welding is not optional; it is the difference between a weld that passes radiographic test and one that does not.
• Heat input. Excessive heat input coarsens the grain and reduces corrosion resistance. A string-of-beads technique with controlled interpass temperature is the standard shipyard practice.
• Backing gas. For tube welds, internal backing gas (typically argon or argon/nitrogen mix) is essential to prevent oxide formation on the bore.

For piping, butt-welded joints with ASME B16.9 Cu-Ni pipe fittings are preferred. Brazed and flanged connections are also used, but they introduce dissimilar-metal interfaces that must be carefully managed with the right flange, gasket, and bolt materials to avoid galvanic corrosion.

6. Avoid These Five Common Mistakes

Most premature Cu-Ni failures on real projects come from a small number of recurring errors. The list below covers the ones that account for the majority of warranty claims.

1. Mixing Cu-Ni with carbon steel in the same system. Without proper isolation (insulating flanges, dielectric unions, or rubber-lined spool pieces), the carbon steel corrodes rapidly. Pair Cu-Ni tube with Cu-Ni or Cu-Ni/bronze fittings and use copper nickel flanges at all connection points.
2. Using 90/10 where 70/30 is required. High-velocity seawater, polluted harbor water, or refinery cooling water with high sulfide content will pit 90/10 much faster than 70/30. Confirm the water chemistry before finalizing the spec.
3. Skipping hydrostatic test at the mill. Always require the standard's mandatory hydrostatic or eddy-current test and an MTC (mill test certificate) per EN 10204 3.1. Do not accept a commercial test certificate in lieu.
4. Using the wrong gasket. Conventional graphite or compressed fiber gaskets can cause crevice corrosion on Cu-Ni flange faces. Use nitrile (NBR) or EPDM gaskets for seawater service, and isolate stainless steel bolts from the flange with insulating sleeves and washers.
5. Forgetting velocity limits. Sustained flow above 3.5 m/s for 90/10 or 4.5 m/s for 70/30 will erode the protective film and shorten service life. Confirm design velocity, not nominal line size, before the spec is locked.

7. Sourcing Cu-Ni From a Mill That Ships

The single biggest predictor of on-time delivery for a Cu-Ni package is whether the supplier actually stocks the grade and the size range your project needs. Cu-Ni is not a commodity that can be pulled from a general line-pipe catalog — production runs are scheduled months in advance, and the most common delays on shipbuilding and offshore projects are caused by a mill quoting a size it cannot actually produce in the required quantity.

When evaluating a Cu-Ni supplier, the practical checklist is short:

• Mill-direct or stockist? Mill-direct pricing and traceability is what makes a real difference on large orders.
• Standard coverage. ASTM B466, B467, B111, EEMUA 234, EN 12451, and GB/T 8890 should all be in active production, not just available on request.
• Project packaging. Ability to deliver tube, matching fittings, flanges, gaskets, and stud bolts as one bundled shipment reduces interface risk and clears customs once instead of five times.
• Documentation. EN 10204 3.1 / 3.2 certificates, classification society approval (DNV, LR, BV, ABS, CCS) for shipbuilding orders, and full traceability per heat number.

8. Where Copper-Nickel Fits in a Complete Piping System

A Cu-Ni line rarely stands alone. On a typical ship, a refinery, or a desalination plant, the same piping package also includes carbon steel or stainless steel headers, valve manifolds, and the connection hardware that ties the run into the rest of the system. The most reliable projects treat the Cu-Ni line as part of one engineered package — tube, fittings, gasket stud bolt nut sets, and valves, all sourced to a single specification and shipped together.

For the heat-exchanger side of the system, Cu-Ni tubes are normally paired with Cu-Ni or titanium tube sheets, and the bundle is fitted into a carbon steel shell using rolled-expansion joints. For the piping side, Cu-Ni tube is welded to Cu-Ni fittings and terminated in pipe flanges matched in grade and standard. The fewer dissimilar-metal interfaces in the run, the longer the system lasts.