Marine & Process Piping Buyer's Guide

Copper Nickel Alloy in Seawater and Refinery Service: A 2026 Buyer's Guide to 90/10, 70/30, and Bundled Pressure-Boundary Procurement

Copper nickel alloy has quietly held the seawater side of shipbuilding, offshore platforms, and refinery cooling systems for more than five decades. This guide brings that track record up to 2026: what the 90/10 and 70/30 grades actually do, when to upgrade the alloy, and how a one-stop pressure boundary saves money on the joint that everyone forgets.

Why Copper-Nickel Is Still the Default Marine Piping Material

Most process piping decisions are made against pressure and temperature. Seawater decisions are made against corrosion, biofouling, and galvanic compatibility — and that is where copper nickel alloy has earned its place. The protective surface film that forms on a 90/10 or 70/30 Cu-Ni surface in clean seawater is thin, adherent, and self-healing. Once matured, it holds general corrosion rates in the 0.02 to 0.002 mm/yr range for the rest of the piping's service life.

The second, less-discussed benefit is biofouling resistance. Cu-Ni surfaces discourage macrofouling naturally, which lowers the maintenance burden on chlorination systems and reduces the cleaning frequency of shipboard seawater lines. For a shipyard or a coastal power plant, the cost of dry-docking a single corroded line — including off-hire time, dock fees, and the labor of cutting out a flanged joint below the waterline — is typically 8 to 12 times the original pipe purchase price. The alloy pays for itself long before the first vessel leaves port.

Step 1: Lock the Service Envelope Before Opening a Catalogue

Before discussing alloy choice, the engineering team must close out the service envelope. Five parameters drive almost every decision downstream:

• Water chemistry: natural seawater, brackish estuary water, treated ballast with biocide injection, or concentrated refinery brine. Chlorinity, dissolved oxygen, and sulfide content all shift the alloy answer.
• Design temperature: tropical surface service (above 30 °C) accelerates biofouling; cold North Sea or Arctic service (below 10 °C) requires impact-tested body materials and slower commissioning.
• Velocity: seawater velocities above roughly 3.5 m/s in Cu-Ni systems cause erosion-corrosion. The local velocity at the seat, elbow, or tee outlet matters as much as the line velocity.
• Operating duty: continuous circulation, intermittent fire-main use with short bursts to 12-15 m/s, or once-through refinery cooling with seasonal turnover. Each duty rules out different alloy and joint combinations.
• Installation environment: above-deck exposed, below-deck sheltered, submerged hull penetrations, or offshore platform splash zone. Coating, cathodic protection, and alloy selection all move with this answer.

These five numbers belong on a one-page datasheet before any vendor conversation begins. Without them, every quote received is a guess dressed up as a price.

Step 2: Read the Alloy Designation Like a Spec Sheet

The two main copper nickel alloy grades used in marine and refinery service are 90/10 and 70/30. Both contain small but important additions of iron and manganese that improve resistance to flowing seawater. The 70/30 grade is stronger and tolerates higher seawater velocity, but the 90/10 grade is more widely used because it covers most service cases at a lower material cost.

Both 90/10 and 70/30 are readily welded with the 70/30 consumable (AWS A5.6 ECuNi or A5.7 ERCuNi). When joining Cu-Ni to steel, switch to a 65% nickel-copper consumable (ECuNi-7 / ERNiCu-7) to tolerate iron dilution. A small titanium addition in the consumable counteracts atmospheric nitrogen pickup during welding.

Step 3: Match Velocity and Sand Loading to the Grade

Velocity is the single most under-engineered parameter in marine procurement. The protective film on Cu-Ni tolerates a wide velocity range, but exceeding the design value strips it and exposes fresh metal to erosion. British Standard BS MA18 and EEMUA 234 publish conservative values that have worked for half a century:

• 90/10 in piping ≥100 mm: 3.5 m/s steady state, with short fire-main peaks up to 12-15 m/s acceptable.
• 70/30 in piping ≥100 mm: 4.0 m/s steady state; preferred for higher-flow cooling loops.
• Modified 70/30 (C71640): 2 m/s in MSF desalination heat-rejection section, where local turbulence is hard to control.
• Minimum velocity: stay above 1 m/s wherever practical to prevent sediment and biofilm buildup.

Sand loading is the second hidden variable. Fine sand below 0.05 mm is tolerable up to 1,000 ppm; coarser sand between 200 and 1,000 ppm starts to damage the film on 90/10 and is the original use case for the modified 70/30 grade developed at Uskmouth Power Station. Specifying Cu-Ni without naming both the velocity window and the expected sand loading leaves the supplier guessing.

Step 4: Pick a Filler Metal and Welding Procedure Before You Sign the PO

Cu-Ni welds are not forgiving of contamination. Sulfur, phosphorus, lead, and atmospheric nitrogen all embrittle the weld pool or crack the HAZ. The first defect on a new marine Cu-Ni line is almost always tied to a missed cleaning step, not a wrong alloy choice.

• Use a 70/30 consumable (AWS A5.6 ECuNi, A5.7 ERCuNi) for both 90/10 and 70/30 base metals. The 70/30 weld metal is more resistant to iron dilution from the bevel.
• For Cu-Ni to steel transitions, use a 65% nickel-copper consumable (ECuNi-7, ERNiCu-7) to absorb iron dilution without cracking.
• Clean the joint within 50 mm of the bevel with a dedicated stainless wire brush — never reuse a brush that has touched carbon steel.
• Shield the back of the joint with forming gas (90% Ar / 10% H₂ or pure Ar) when welding thin-wall tube and pipe.
• Hydrotest the line with clean, sulfide-free water. Sulfide-contaminated hydrotest water is the most common cause of early pitting.

Step 5: Treat the Joint as a System, Not a Connection

A copper-nickel line is only as good as the joint it sits in. Three adjacent items deserve equal engineering attention: the pipe flanges on either side, the gasket that seals them, and the stud bolts and nuts that clamp the assembly. In seawater service, mismatches in this trio cause the majority of dry-dock findings.

For ASME B16.5 / B16.47 flanges, the gasket style, bolt grade, and facing finish must be specified together. A spiral-wound gasket on a Class 150 flat face is a leak waiting to happen; the same gasket on a raised face is correctly clamped. The same principle applies to copper nickel flanges, where the gasket and bolt selection also has to survive galvanic compatibility with the Cu-Ni body — typically by using aluminum-bronze or monel bolting rather than standard B7 carbon steel.

• Match flange rating (150#, 300#, 600#) to the design pressure and temperature — not to the pipe schedule alone.
• Use the same facing type (RF, FF, RTJ) on both sides of the valve or fitting. A mixed-facing joint will not seal evenly under cyclic thermal load.
• Specify ASTM A193 B7 / A194 2H for general service, A320 L7 / A194 4 for low-temperature, and B8M / 8M for stainless bolting. Avoid mixing carbon and alloy bolting in the same joint.
• For ESG-sensitive or class-approved service, add ISO 15848 Class AH or API 624 stem sealing requirements on the mating industrial valves.

Step 6: Commissioning Steps That Protect the Film

The first 90 days of a new Cu-Ni system are when the protective film matures. Three commissioning habits make the difference between a 30-year line and a 3-year replacement:

• Flush with clean seawater or potable water before first start-up. Remove debris, grease, and oil left over from fabrication.
• Avoid stagnant lay-up periods longer than two weeks. If lay-up is unavoidable, drain the line and keep it dry, or fill it with clean seawater treated with ferrous sulfate at 1-2 ppm to feed the film.
• Hold ammonia and sulfide levels low during commissioning. Ammonia from decaying marine debris or hydrotest water has caused the rare denickelification and hot-spot attacks reported in 70/30 refinery condensers.

Step 7: Standards a Marine Cu-Ni Data Sheet Must Show

A traceable Cu-Ni data sheet lists the standards the pipe, fittings, and flanges were designed, tested, and certified against. If any of these references is missing, the document is incomplete and the material should not be released for service.

Step 8: Tests That Cannot Be Skipped

Reputable Cu-Ni suppliers run a standard test sequence on every heat and add optional tests when the project demands them. The minimum set a specifier should require on every Cu-Ni certificate:

• Chemical composition: spectrometer analysis on a sample from every heat, reported against the UNS alloy limits.
• Tensile and hardness test: confirms the 90/10 or 70/30 strength band and rules out over-aged material.
• Hydrostatic test: 1.5× the design pressure held long enough to inspect every joint.
• Eddy-current or ultrasonic test: for seamless and welded tube to surface defects per ASTM E243 or equivalent.
• Material certificate (MTC): EN 10204 3.1 mill test report, traceable to heat number, with chemical composition and mechanical values.
• Dye-penetrant or radiographic test on welds: on fabricated spools and headers, before delivery.

For LNG, ammonia, or cryogenic service, add cryogenic testing per BS 6364. For refinery sour service, add NACE MR0175 / ISO 15156 compliance on the trim material.

Step 9: Bundle the Pressure Boundary to Cut Interface Risk

Most marine Cu-Ni leaks on commissioning are not the alloy's fault. They are caused by flange, gasket, and bolt combinations sourced from three or four different vendors, each holding the other responsible. Bundling the copper nickel alloy pipe, the matching copper nickel flanges, the gasket stud bolt and nut kit, and the industrial valves from a single supplier collapses the interface into one technical contact.

EZ Steel Industrial has supplied this kind of pressure-boundary bundle for over three decades — since 1994 — out of a single manufacturing and inventory base in Changsha, China. With eight product lines covering carbon, stainless, copper-nickel, and heat-efficiency tubes alongside fittings, flanges, gaskets, and valves, an annual capacity above 480,000 units, and certifications covering API, EN, ASME, AWS, and ISO 9001, the company can hold the entire marine pressure boundary under one quality file, one MTR trail, and one point of accountability.

A Short Field Checklist Before You Sign the PO

Use this list on every marine Cu-Ni purchase order, regardless of vessel size:

• Service envelope (water chemistry, temperature, velocity, sand loading, duty) documented on the datasheet.
• Alloy grade and UNS designation called out, with EEMUA or ASTM reference noted.
• Filler metal and welding procedure qualified before fabrication starts.
• End connection type and facing matched to the mating flange (RF, FF, RTJ).
• Gasket, stud bolt, and nut grade matched to the flange class and the alloy family.
• Hydrotest fluid and water chemistry specified (no sulfides, low ammonia).
• Commissioning plan covers first-90-day film maturation and lay-up rules.
• MTC traceability and chemical composition on file for every heat number.