Flanges in seawater service live a tougher life than almost any other component in a process plant. The bolted joint is exposed to chloride attack from both faces, mechanical loading from the bolt circle, galvanic coupling to whatever dissimilar material sits on the pipe side, and — if the system is poorly designed — erosion from sand, silt or biofouling passing through the bore. A pipe flange chosen on the basis of price and nominal pressure class alone will not last a full dry-dock cycle in that environment. Selection has to be made against the actual corrosion mechanism, the design velocity, the operating temperature, and the mating standard of the connecting pipe.

This guide is built around the marine and offshore applications that the copper nickel flanges product family at EZ Steel Industrial is designed for. The recommendations are written for the engineer or buyer who has to defend each line of a flange schedule against a class surveyor, not against a catalog.

1. Why Copper-Nickel Is the Default for Clean Seawater

For ships, offshore platforms, coastal power stations and desalination intakes, copper-nickel alloys — primarily 90/10 (UNS C70600) and 70/30 (UNS C71500) — are the workhorse flange material. The reason is not just corrosion resistance; it is the combination of corrosion resistance, biofouling resistance, and good fabrication behavior. A 90/10 flange in a properly engineered seawater system routinely achieves more than twenty years of service before the first thickness inspection becomes a concern.

1.1 90/10 vs 70/30 — how to choose

Both alloys resist seawater, but they have different sweet spots. The rule of thumb in EEMUA Publication 234 and in naval architecture practice is straightforward: use 90/10 (C70600, CuNi 90/10) for the bulk of the seawater cooling, ballast and firemain piping; step up to 70/30 (C71500, CuNi 70/30) only where the design temperature, the design velocity, or the local corrosion risk is above the envelope of 90/10.

1.2 What about iron content and sulfur content?

For flange bodies and weld-end preparations, two chemistry ranges are worth tracking. Iron content in the 1.0–1.8% range improves the erosion-corrosion resistance of 90/10 by stabilizing the protective surface film; below 1.0% iron the film is too soft, above 2.0% the alloy becomes harder to hot-work. Sulfur is the other number to watch — it should sit below 0.02% in marine grades to avoid hot cracking during forging or weld-overlay repair. Reputable mills will provide these numbers on every MTR; if they are missing, the material is the wrong grade for marine service regardless of what the catalog page says.

2. When the Flange Has to Be Nickel Alloy Instead

Copper-nickel is not the answer everywhere. The moment the service moves away from clean seawater — into sour hydrocarbons, hot chlorides, strong acids, ammonia, or cryogenic LNG — the material family has to change. The copper nickel flanges range covers this transition: the same product family extends into Monel, Inconel, Hastelloy and pure nickel grades that are built to handle the conditions copper-nickel cannot.

3. Pressure Class, Facing and the Standard Behind the Drawing

A copper-nickel flange is only as good as the standard it is machined to. The two standards that govern marine and offshore work are ASME B16.24 (cast and malleable copper-alloy flanges) and ASME B16.5 (steel pipe flanges, into which nickel-alloy flanges also fall). The third, more specialized, is EEMUA Publication 234, which is the naval architecture reference for copper-nickel piping systems. Mixing these standards on a single piping system is the most common source of field leak paths — the bolt circle on a B16.24 Class 150 flange is not the same as the bolt circle on a B16.5 Class 150 flange of the same nominal size.

3.1 Pressure class selection

For most seawater cooling, ballast and firemain service, Class 150 (PN 20) is the working envelope. Step up to Class 300 (PN 50) on the suction side of large seawater lift pumps, on the discharge of firemain booster pumps, and on any line that sees a transient water-hammer spike above 19 bar. Class 600 and above is reserved for high-pressure RO desalination, for hydraulic power units, and for the offshore high-pressure firewater ring. Going one class up at the design stage costs a fraction of what a derate in the field costs — there is no reason to be conservative on the pressure class while staying aggressive on the material.

3.2 Facing finish

Raised face (RF) with a serrated finish is the default for copper-nickel Class 150 and Class 300 flanges. Flat face (FF) is acceptable for low-pressure copper-alloy flanges in the ASME B16.24 range, but it loses sealing margin as the class rises. Ring-type joint (RTJ) is not used on copper-nickel flanges — the groove geometry and the surface hardness required for an RTJ seal belong to the steel flange family. If the datasheet shows a copper-nickel RTJ flange, it is a substitution error somewhere upstream.

3.3 Bolting and gasket compatibility

Match the bolting and the gasket to the flange face and the service. For RF Class 150 copper-nickel flanges in seawater, a compressed non-asbestos fiber gasket with a nitrile binder is the standard, paired with ASTM A193 B8M (AISI 316) stud bolts and ASTM A194 8M nuts. Avoid galvanized bolting — the zinc coating is sacrificial in seawater and will be gone within the first inspection cycle. For nickel-alloy Class 150 and above, the gasket is typically spiral-wound with graphite or PTFE filler, paired with B8M or B16 stud bolts per the operating temperature.

4. Keeping the Flange Package Aligned with the Pipe and Fittings

Most flange failures in seawater service are not flange failures at all — they are transition failures where the flange was correctly specified but the connecting pipe or fitting was not. Galvanic mismatch, dimensional mismatch and metallurgical mismatch are the three recurring root causes.

4.1 Galvanic compatibility

A copper-nickel flange bolted to a carbon steel pipe in seawater will fail at the carbon steel side, not the copper-nickel side. The fix is to keep the run piping and the flange in the same alloy family — 90/10 tube to 90/10 flange, 70/30 tube to 70/30 flange — or to ｉｎｓｅｒｔ a short spool of insulation (a rubber-lined joint or a dielectric ｕｎｉｏｎ) at the transition. The copper nickel alloy tube and flange families at EZ Steel Industrial are matched on the same heat-treatment practice and the same iron-content window for exactly this reason.

4.2 Dimensional conformance across the spool

A copper-nickel flange must mate to a copper-nickel pipe with a copper-nickel weldolet or a pipe fitting that is dimensionally consistent with the bore. Schedule 10S 90/10 tube to a Class 150 90/10 flange is a routine match; Schedule 40S 90/10 tube to a Class 150 90/10 flange forces the bore to step, creating a local turbulence zone that accelerates erosion-corrosion on the tube side just upstream of the flange face. Keep the schedule consistent across the joint.

4.3 Sour service and the NACE question

If the flange sits in a sour hydrocarbon line — even briefly, during a future tie-in — the carbon-steel and standard-stainless options ｄｒｏｐ out and the choice narrows to NACE MR0175 / ISO 15156 compliant nickel alloys. Monel 400, Inconel 625, Inconel 718 and Hastelloy C276 all carry the certification, but the cost spread between them is wide. The cheap option is to specify a "for sour service" note on a standard Monel flange; the right option is to confirm the heat is dual-certified, the MTR carries the NACE statement, and the hardness is below the 35 HRC ceiling for sour service. That detail is what an inspection surveyor will check first.

5. Common Field Issues and How the Datasheet Should Pre-empt Them

A flange datasheet that captures the items below will eliminate most of the rework calls the field engineer has to make in the first year of service. Treat this as a working checklist when you release a flange schedule.

1. Fluid and full composition, including chloride, sulfide, ammonia and free oxygen content where relevant.
2. Design pressure, design temperature, and the maximum upset pressure-temperature pair.
3. ASME pressure class (150, 300, 600, …) and the relevant flange standard — B16.24, B16.5, B16.47, or EEMUA 234.
4. UNS alloy designation with iron-content and sulfur-content limits where the service is marine.
5. Face type (RF, FF) and the serration pitch if RF; reject RTJ on copper-nickel.
6. Bolting grade and gasket type, with reference to the same project bolting schedule.
7. NACE MR0175 / ISO 15156 statement for any flange in sour or potentially sour service.
8. Dimensional report on first article, hydrotest certificate, and MTR cross-referenced to the mating pipe or fitting.

6. The Sourcing Picture: One Mill, One Spool

The practical advantage of sourcing the flange together with the connecting tube and the matching pipe fittings is not just commercial. The metallurgical chain — chemistry, heat treatment, dimensional tolerance, surface finish, MTR — is held in one quality plan, on one production schedule, with one set of certificates at the receiving warehouse. The copper nickel flanges and the matching copper nickel alloy tube lines we manufacture are produced under the same iron-content window and the same EN 1254 / EEMUA 234 reference set, so the flange and the tube it bolts against come from the same metallurgical conversation rather than two unrelated catalogs.

This is the same logic that drives the rest of the product line — steel flanges with the matching structure works pipe, gaskets with the matching stud bolts, valves with the matching pipe fittings. For marine, offshore and desalination projects, that single-source discipline is the difference between a flange that passes the first dock survey and one that does not.