Marine Piping Procurement Guide

Copper Nickel Flanges in Seawater Service: Specifying, Sourcing, and Verifying for Project Buyers

For desalination plants, shipyards, offshore platforms, and coastal power stations, the flange at every joint is the leak point you cannot afford to get wrong. Here is how a project buyer should think about copper nickel flanges — from grade choice to weld prep to mill paperwork — before signing a procurement order.

Why copper nickel flanges dominate seawater service

Carbon steel and even 316 stainless steel struggle in raw, chloride-rich seawater. Within months, pitting and crevice corrosion attack gaskets and flange faces. Copper-nickel alloys (90/10 and 70/30) behave differently: in seawater they form a thin, adherent, protective oxide film that actually self-repairs when disturbed, giving decades of service life with minimal maintenance. That is why copper nickel alloy piping systems — including the matching flanges — are the default specification for hull cooling, ballast, firefighting, and desalination intake and outfall lines.

For buyers, this means the flange is not a generic commodity. It must match the pipe chemistry, the flange face geometry of the adjoining pipe fittings, and the bolt-up pattern of the upstream pipe flanges in the system. Specifying it as “a copper nickel flange” is the easy part; the harder work is in the details.

90/10 vs 70/30: which grade belongs on your project

Both alloys are accepted under EEMUA 144, EEMUA 234, ASME B16.5, and the major classification society rules, but they answer different service questions.

Item	Cu-Ni 90/10 (C70600)	Cu-Ni 70/30 (C71500)
Nominal composition	~90% Cu, 10% Ni, with Fe and Mn	~70% Cu, 30% Ni, with Fe and Mn
Typical service	Hull cooling, ballast, firefighting, shipbuilding	High-temperature seawater, desalination brine, chemical cargo
Max recommended velocity	~3.5 m/s for pipe, similar at flange face	~4.5 m/s, better resistance to erosion in higher-velocity sections
Relative cost	Lower nickel content, more economical	Higher nickel content, used where service demands it

If your MTO covers standard shipboard or coastal cooling lines, 90/10 is usually correct. Specify 70/30 only where the datasheet — or the classification society — calls for it, or where velocity, temperature, or water chemistry exceed 90/10 limits. Mixing grades on the same flange run is a common error and a known corrosion risk.

The standards you should reference in your RFQ

A clear specification saves weeks of back-and-forth. For a seawater service project, the flange RFQ should at minimum reference:

Material standard — EEMUA 144 (Cu-Ni 90/10) or EEMUA 234 (Cu-Ni 70/30), with UNS C70600 or C71500 named explicitly.
Dimensional standard — ASME B16.5 for sizes up to NPS 24, or ASME B16.47 Series A/B for larger diameters. Confirm the facing: flat face (FF) is the typical match for Cu-Ni screwed and socket-welded joints; raised face (RF) is the default for butt-welded Cu-Ni.
Weld preparation — matching the adjoining Cu-Ni pipe WPS. For Cu-Ni 90/10, this is usually TIG with Cu-Ni filler (e.g. ERCuNi); for 70/30, similar process with the matching 70/30 filler.
Testing — hydrostatic test on the flange body, PMI (positive material identification) on every piece, and dimensional check against the relevant ASME table.
Traceability — heat number, batch, and full Mill Test Certificate to EN 10204 3.1.

Galvanic risk at the flange joint — the part most buyers forget

The single most common reason a Cu-Ni flange joint fails prematurely is not the flange itself, but what touches it. In seawater, a copper-nickel flange bolted to a carbon-steel companion flange creates a galvanic couple, and corrosion will attack the less-noble metal — the carbon steel side. The standard mitigation is to isolate the joint electrically with a non-conductive gasket and insulating sleeves and washers on the gasket stud bolt nut assembly.

Field note for buyers

If your line transitions from Cu-Ni to carbon steel, do not rely on the flange chemistry alone. Spec the isolating kit (gasket, sleeves, washers) at the same time, from the same supplier, and confirm it ships in the same crate. Half-isolated joints are a common rework reason on commissioning day.

Sourcing checklist for project procurement

When you compare Cu-Ni flange suppliers, ask for the following up front, not after the PO is signed:

Mill origin and evidence of consistent EEMUA-grade feedstock (not generic “Cu-Ni”).
In-house PMI gun and hydrostatic test rig — not a sub-contracted third-party test.
Weld procedure qualification record (WPQR) for Cu-Ni to Cu-Ni and Cu-Ni to steel.
Ability to bundle with matching Cu-Ni pipes, butt-weld fittings, and steel flanges for hybrid sections, so the whole package ships on one BoL.
Documentation pack per shipment: MTC 3.1, PMI report, dimensional report, visual and dye-pen or radiographic records for welded flanges.

Common specification mistakes to avoid

Three errors appear over and over on seawater piping orders:

Specifying a forged carbon-steel flange with a Cu-Ni weld overlay. The overlay is thin and wears off; a solid Cu-Ni flange is the correct choice.
Mixing 90/10 and 70/30 within the same line. Pick one unless the process design explicitly transitions at a defined spool.
Ignoring flange face finish. Cu-Ni paired with a non-metallic gasket works best with a smooth stock finish (125–250 RMS); over-machining raises the cost without a service benefit.

Talk to a project-aware Cu-Ni flange supplier

EZ Steel Industrial supplies copper-nickel flanges, pipes, fittings, and isolating bolt kits from a single inventory pool, with EN 10204 3.1 certificates and in-house PMI on every heat. If you are scoping a seawater, ballast, or desalination project, send your MTO and datasheet to export@ezsteelpipe.com — we will return a quoted package matched to your line class and classification society rules, not a generic price list.