Copper-nickel pipe (90/10 and 70/30 CuNi) is the standard material for seawater piping systems on ships, offshore platforms, and coastal power plants. It welds with ERCuNi filler on DCEN TIG, doesn’t need preheat, and is actually the easiest copper alloy to weld because its low thermal conductivity holds heat in the joint. The critical requirements are keeping interpass temperature under 350F, running a clean argon back purge on the root side, and removing every trace of surface oxide before welding.
CuNi Alloy Overview
The two common grades are 90/10 (90% copper, 10% nickel) and 70/30 (70% copper, 30% nickel). Both contain small amounts of iron and manganese for additional corrosion resistance and strength.
| Property | 90/10 CuNi (C70600) | 70/30 CuNi (C71500) |
|---|---|---|
| Tensile strength (ksi) | 44-52 | 50-60 |
| Yield strength (ksi) | 16-20 | 18-25 |
| Melting range (F) | 2010-2100 | 2140-2260 |
| Thermal conductivity (BTU/hr-ft-F) | 26 | 17 |
| Max seawater velocity (ft/s) | 10-12 | 12-15 |
| UNS designation | C70600 | C71500 |
90/10 is the workhorse for standard seawater cooling lines, fire mains, and condensate piping. 70/30 handles higher-velocity seawater and more aggressive conditions (desalination plants, high-temperature heat exchangers). Both alloys develop a protective oxide film in seawater that improves corrosion resistance over time.
Filler Metal Selection
ERCuNi (AWS A5.7) is the standard TIG filler for both 90/10 and 70/30 base metal. It contains approximately 70% copper and 30% nickel, which is an intentional over-alloying on the nickel side. The higher nickel content in the weld deposit improves corrosion resistance and ensures the weld is at least as noble as the base metal in the galvanic series.
| Base Metal | Filler (TIG/MIG) | Filler (Stick) | Notes |
|---|---|---|---|
| 90/10 CuNi to 90/10 CuNi | ERCuNi | ECuNi | Standard; over-alloys nickel for corrosion resistance |
| 70/30 CuNi to 70/30 CuNi | ERCuNi | ECuNi | Same filler; weld chemistry close to 70/30 base |
| 90/10 CuNi to 70/30 CuNi | ERCuNi | ECuNi | ERCuNi handles dissimilar CuNi combinations |
| CuNi to carbon steel | ERNiCu-7 or ERNiCr-3 | ENiCrFe-3 | Nickel-based filler required for dissimilar joint |
| CuNi to stainless steel | ERNiCr-3 | ENiCrFe-3 | Nickel filler compatible with both metals |
For CuNi pipe joined to steel flanges or steel pipe (common at system transitions), use a nickel-based filler like ERNiCr-3 (Inconel 82) or ERNiCu-7 (Monel). These fillers are compatible with both the copper-nickel and the steel without creating brittle intermetallic phases.
TIG Welding Procedure
TIG (GTAW) on DCEN is the primary process for CuNi pipe. Use a 2% thoriated or 2% lanthanated tungsten.
Shielding Gas
100% argon at 15-20 CFH through a standard gas cup or gas lens. Argon/helium blends (75Ar/25He) increase arc voltage and heat input for thicker pipe walls where penetration is a concern. Helium doesn’t clean oxide like the EP cycle does on AC aluminum, but CuNi doesn’t have the same oxide problem, so DCEN works fine.
Root Pass
The root pass is the most critical weld on CuNi pipe. Use a consumable insert (ER CuNi ring) or an open root with a 1/16 to 3/32 inch gap and a feather edge (no land, or 1/32 inch max land). Feed filler from the start; don’t autogenous-weld the root.
| Pipe Wall Thickness | Root Gap | Filler Dia. | Amps (DCEN) | Tungsten |
|---|---|---|---|---|
| 0.065 in (Sch 5) | 1/16 in | 1/16 in | 40-60 | 3/32 in |
| 0.083 in (Sch 10) | 1/16 in | 3/32 in | 50-80 | 3/32 in |
| 0.109 in (Sch 40, 2") | 3/32 in | 3/32 in | 70-100 | 3/32 in |
| 0.154 in (Sch 40, 3") | 3/32 in | 3/32 in | 90-120 | 1/8 in |
| 0.237 in (Sch 40, 6") | 3/32 in | 1/8 in | 100-140 | 1/8 in |
Fill and Cap Passes
Stringer beads or narrow weave patterns work best. Keep bead width under 3 times the filler rod diameter to minimize heat input per pass. CuNi is susceptible to hot cracking from sulfur and phosphorus, and wide weave beads concentrate impurities at the bead centerline.
Between passes, check the interpass temperature with a contact pyrometer or temp stick. If it exceeds 350F, stop welding and let the pipe cool naturally. Don’t quench CuNi with water; it can cause stress corrosion cracking in some conditions.
Back Purging
Back purging with argon is required for all CuNi pipe welding. The root side oxidizes during welding and produces a porous, weak root bead without gas coverage.
Purge procedure:
- Seal the pipe ends with water-soluble purge dams, tape, or foam plugs. Position dams 4-6 inches from each side of the joint.
- Flow argon at 10-15 CFH into the sealed pipe volume. Use a purge outlet on the opposite side to displace air.
- Purge until the oxygen content drops below 0.5% (5,000 ppm). An inline oxygen analyzer is the only reliable way to verify this. Color inspection is unreliable on CuNi because the oxides aren’t as visually distinct as on stainless steel or titanium.
- Maintain purge flow during the root pass and at least one fill pass. Reduce flow after the second pass if the root is fully sealed.
Insufficient purging causes root-side porosity and oxide inclusions that weaken the joint and create crevices where corrosion initiates in seawater service.
Surface Preparation and Cleanliness
CuNi develops a tenacious surface oxide that traps moisture and contaminants. This oxide is the primary source of porosity in CuNi welds.
Cleaning procedure:
- Degrease with acetone or isopropyl alcohol. Remove all cutting fluid, marking ink, and fingerprint oils.
- Mechanically clean with a stainless steel brush, 80-120 grit abrasive, or flap disc. Clean a 1 inch band on each side of the joint. Do not use carbon steel brushes (iron contamination causes pitting in seawater).
- Bevel preparation by machining or grinding. Standard V-groove for wall thicknesses over 3/16 inch: 60-75 degree included angle, 1/32 inch land, 3/32 inch root gap.
- Weld within 2 hours of cleaning. If the pipe sits longer, re-clean before welding.
- Filler rod must be cleaned too. Wipe with acetone and handle with clean gloves.
Sulfur contamination is the hidden enemy on CuNi. Sources include cutting fluids, anti-seize compounds, marking materials, and even ambient shop air near diesel exhaust. Sulfur forms low-melting-point copper sulfide films at grain boundaries that cause hot cracking. Keep everything scrupulously clean.
Heat Input and Interpass Temperature Control
CuNi doesn’t need preheat, but it does need controlled heat input. Excessive heat causes:
- Grain growth in the HAZ, reducing mechanical properties
- Increased hot cracking risk
- Reduced corrosion resistance from changes in grain boundary chemistry
The interpass temperature limit of 350F (175C) per AWS D10.18 (Recommended Practices for Welding Austenitic Stainless Steel Piping and Tubing) and CDA (Copper Development Association) guidelines keeps these problems in check.
| Parameter | Guideline |
|---|---|
| Preheat | None (min 50F ambient) |
| Max interpass temperature | 350F (175C) |
| Heat input (max) | 35 kJ/in for 90/10; 30 kJ/in for 70/30 |
| PWHT | Not required; not recommended |
| Bead width | Max 3x filler rod diameter |
On multi-pass joints, plan your welding sequence to balance heat distribution. On larger pipe (6 inches and up), alternate sides of the joint (1 o’clock, then 7 o’clock) rather than running continuous passes all the way around.
Post-Weld Treatment
CuNi pipe doesn’t need PWHT. In fact, extended high-temperature exposure after welding can reduce corrosion resistance. Clean the finished weld with a stainless steel brush to remove surface oxides, then let the seawater service develop the natural protective film.
For piping systems, pressure testing follows before commissioning. CuNi’s excellent ductility means it’s forgiving during hydrostatic testing. Typical test pressure is 1.5 times design pressure per the applicable piping code (ASME B31.1 or B31.3).
Common Defects
Porosity is the most frequent problem and almost always traces back to insufficient purging or contaminated surfaces. If you’re seeing scattered or clustered porosity, check your purge gas flow rate, verify oxygen levels at the root, and re-clean the joint.
Hot cracking along the weld centerline indicates sulfur contamination. Review all materials that contacted the joint (markers, tape, anti-seize, cutting fluid). Switch to sulfur-free products and re-clean thoroughly.
Lack of fusion on the root pass happens when amperage is too low or the root gap is too tight. CuNi’s thermal conductivity is lower than pure copper but still higher than steel, so you need slightly higher amps than you’d expect for the wall thickness.
Root concavity (suck-back) occurs when purge pressure is too high. Reduce back purge flow rate or use a smaller purge dam spacing to control internal pressure.
Incomplete penetration results from insufficient root gap or excessive root face. CuNi flows less freely than steel weld metal, so the root opening needs to be on the wider end of the range (3/32 inch) to ensure the filler reaches the back side. On thin-wall Schedule 5 and Schedule 10 pipe, a feather edge with no root face provides the best results.
NDE Requirements for Marine Piping
Marine CuNi piping systems follow classification society NDE requirements:
- Visual inspection on 100% of welds (all classes)
- Radiography on butt welds in critical systems (seawater cooling main, fire main): typically 10-20% random plus 100% of welder qualification joints
- Dye penetrant testing on socket welds and branch connections where radiography isn’t practical
Keep weld repair records. Most classification societies limit the number of repair attempts per joint (typically two repairs maximum before the joint must be cut out and replaced).
For more on copper alloy welding fundamentals, see the copper and brass welding overview. For information on brazing copper alloys, see the brazing brass fittings guide. For filler selection across all copper alloys, check the deoxidized copper filler guide.
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