Offshore Seawater Cooling Piping Material Selection: 2507 vs Titanium vs 90/10 Cu-Ni

This guide addresses continuously aerated raw seawater in open-loop cooling and utility piping on fixed offshore platforms, FPSOs, and coastal process facilities. It does not cover dry deluge systems, ballast and bilge service, deaerated water injection, produced-water reinjection, hydrocarbon flowlines, exchanger heat-transfer tubes, high-temperature concentrated brines, or chemical-injection lines. Those duties have different damage mechanisms and qualification requirements.

DAXUN manufactures Super Duplex 2507, titanium, and nickel-alloy pipe and tube and performs the agreed forming, welding, heat treatment, machining, surface preparation, marking, and export preparation in-house. Inspection is performed internally only within the capability and qualification scope confirmed for the order; external or independent facilities can be included when required. We do not select an alloy from the word “seawater” alone. A quotable solution starts with chemistry, temperature, velocity, solids, chlorination, shutdown behavior, joining, cathodic protection, inspection, and design life.

What should be defined before choosing a seawater piping material?

Define the system boundary first. Raw seawater at an intake, strainer, pump discharge, cooler inlet, bypass, drain, and return does not create one uniform environment. Temperature, pressure, dissolved oxygen, flow, sand, biofilm, disinfectant residual, deposits, and crevice geometry change along the route. A metal that performs in a continuously swept straight run may fail at a gasket face or stagnant branch.

ISO 21457 supplies a useful top-level materials-selection and corrosion-control process for hydrocarbon-production piping and equipment, including utility systems. Its public scope identifies corrosion mechanisms, evaluation parameters, material limitations, and corrosion control; it explicitly does not provide the detailed manufacturing and testing requirements contained in product standards.^[1] For applicable Norwegian offshore petroleum projects, NORSOK M-001:2025 can add a project framework; Standard Norge describes its current material-selection basis as a minimum 20-year design life.^[15] Applicability, contract edition, and any project-specific deviations must be confirmed. Neither document is a universal product specification for every marine or coastal facility.

At minimum, record:

Decision input	Questions the material review must answer	What goes wrong if it is missing
Water chemistry	Open ocean, harbor, estuary, polluted water, sulfides, chlorides, pH, dissolved oxygen, microbes?	A clean-water data sheet is applied to polluted or reducing water
Temperatur	Normal inlet/return, solar heat, exchanger return, idle temperature, upset duration?	Localized-corrosion margin is overstated
Hydraulics	Normal/minimum/maximum velocity, pump transients, solids, bends, reducers, control valves, dead legs?	Film damage, erosion-corrosion, deposits, or MIC occur outside straight-pipe calculations
Treatment	Continuous or intermittent chlorination, target residual, shock dose, cleaning acids, fluoride, biocide?	Oxidizing potential or incompatible cleaning chemistry is ignored
Mechanical system	Pressure, thermal movement, vibration, supports, fire requirement, weight, impact, connection method?	A corrosion-resistant material fails the installation or code requirement
Interfaces	Valves, pumps, strainers, exchanger water boxes, gaskets, fasteners, carbon steel, CP system?	Galvanic or hydrogen-related damage appears at connections
Operations	Commissioning water, initial film formation, shutdown duration, drainability, flushing, preservation?	The selected alloy experiences its worst condition when flow stops
Consequence	Leak detectability, access, production loss, fire/flooding consequence, design life?	Initial price is optimized while lifecycle risk is ignored

The material decision should follow the worst credible combination, not the annual average. A short high-temperature return condition, a few days of stagnant sulfide-bearing water, or a high-velocity elbow can dominate a long period of benign operation.

How do Super Duplex 2507, Titanium Grade 2, and 90/10 Cu-Ni compare?

All three are real seawater candidates, but they solve the problem differently. Super Duplex 2507 uses a high-alloy ferritic-austenitic microstructure and high strength. Titanium Grade 2 relies on a stable passive oxide in suitable oxidizing conditions. 90/10 Cu-Ni develops a protective surface film and has long marine service experience when hydraulics and commissioning are controlled.

Candidate	Main selection advantage	Conditions that can overturn the choice	Manufacturing and acceptance priority
Super Duplex 2507 / UNS S32750	High strength with strong resistance to chloride pitting, crevice corrosion, and chloride SCC in appropriate conditions	Tight crevices, high temperature/oxidizing potential, deposits, poor weld surface, harmful phases, CP-related HISC in relevant submerged stressed components	Exact UNS/product standard; solution condition; qualified WPS/PQR; heat input and interpass control; purge; weld cleanup; phase/intermetallic and corrosion acceptance when specified
Titanium Grade 2 / UNS R50400	Low density and very low corrosion rates in many aerated natural-seawater services, including high-flow duties	Hot acidic crevices, oxygen-depleted deposits, fluoride or reducing cleaning media, iron contamination, incomplete weld shielding, excessive cathodic polarization and hydrogen uptake	B861/B862 route; clean segregated fabrication; complete inert shielding; visual color acceptance; contamination control; NDE and pressure test
90/10 Cu-Ni / UNS C70600	Proven seawater piping option with favorable biofouling behavior and good fabrication when protective film is established	Sulfides/pollution, stagnant water, poor initial film formation, sand, excessive local velocity, turbulence, incompatible components and unfavorable galvanic area ratios	Correct alloy chemistry; ASTM B466/B466M-18 seamless or ASTM B467-14(2022) welded route, as applicable; qualified joining procedure; smooth geometry; commissioning and film-formation plan; interface review

This is not a performance ranking. A high PREN can help screen stainless steels, but it does not represent weld quality, crevice geometry, exposure potential, surface condition, or service life. Titanium cannot be assigned a stainless-steel PREN as if the mechanisms were identical. Cu-Ni performance should not be reduced to a single velocity number without geometry, water quality, pipe size, aeration, and commissioning.

When is Super Duplex 2507 a defensible choice?

Super Duplex 2507 becomes attractive when pressure, structural load, weight, space, and chloride resistance must be combined. ASTM A790/A790M-24 covers seamless and straight-seam welded ferritic/austenitic stainless-steel pipe for general corrosive service and emphasizes stress-corrosion-cracking resistance. It requires the applicable heat analysis and mechanical, hydrostatic, and nondestructive examinations within its scope.^[2] The product specification certifies the pipe; it does not certify the installed system.

The alloy’s advantage can be lost during manufacture. Too much or too little weld heat, excessive interpass temperature, inadequate root purge, uncontrolled reheating, or an incorrect final heat treatment can upset the ferrite/austenite balance or promote harmful intermetallic phases. The causal chain is:

uncontrolled thermal cycle → unfavorable phase balance or precipitation → lower toughness/localized-corrosion resistance → weld-region initiation → leakage despite an acceptable base-metal MTC.

ASTM A923-25 provides methods for detecting detrimental intermetallic phases in duplex stainless steels. Its scope also makes a vital point: chemistry and mechanical properties alone do not necessarily prove absence of harmful phases.^[3] Whether A923, ferrite measurement, ASTM G48, impact testing, macro examination, or additional production testing is required must be stated with sampling and acceptance criteria. Do not advertise every 2507 pipe as automatically passing every optional test.

Alleima’s current SAF 2507 product data describe resistance to chloride environments as well as fabrication limits, and warn about hydrogen-induced stress cracking for highly stressed duplex components under cathodic protection in relevant subsea exposure.^[4] DNV-RP-F112 addresses that particular design problem.^[5] It should not be misapplied to every topside cooling-water line, but it matters where piping or connected components are continuously submerged, highly stressed, and polarized.

Choose 2507 only after reviewing temperature, chlorination, crevices, stagnant periods, weld geometry, post-weld surface condition, and galvanic/CP interfaces. A statement such as “PREN above 40, therefore immune to seawater” is not an engineering acceptance criterion.

When is Titanium Grade 2 a defensible choice?

Grade 2 titanium is often a strong candidate for aerated seawater. It becomes attractive where its low density, corrosion performance in the defined water chemistry, and the project’s inspection and maintenance strategy justify the installed cost. This is a project lifecycle evaluation, not a universal low-maintenance guarantee. ASTM B861-24 covers titanium and titanium-alloy seamless pipe; ASTM B862-23 covers welded pipe made from annealed flat-rolled product.^[6][7] Exchanger tubing may use other product standards, but this article concerns piping, not the heat-transfer tube bundle.

Titanium performance depends on the passive film and local chemistry. TIMET’s corrosion manual explains that titanium benefits from oxidizing species in many aqueous environments, while hot chloride crevices, reducing conditions, and certain contaminants require separate evaluation.^[8] Grade 2 is not the universal titanium answer. Palladium-bearing or other grades may be required when the environment inside a crevice becomes hotter, more acidic, and less oxidizing.

Fabrication cleanliness is part of material performance. Iron pickup from shared tools, dirty work surfaces, or handling can produce surface contamination. Welding without full inert shielding can create unacceptable oxidation and embrittlement. A sound purchasing plan therefore controls material segregation, joint preparation, shielding on the face and root, interpass cleanliness, visual weld color, repair rules, NDE, pressure testing, and final cleaning.

Cathodic protection and galvanic connections also require system review. Coupling titanium to more active metals can accelerate attack of the active member, especially when a small anodic area supports a large titanium cathode. Excessively negative polarization can promote hydrogen evolution and uptake in susceptible conditions. Electrical isolation is a design tool, not a universal instruction: isolation pieces, bonding, fire safety, fault currents, CP continuity, and monitoring must be evaluated together.

When is 90/10 Cu-Ni a defensible choice?

UNS C70600 90/10 copper-nickel has extensive ship and seawater-system experience. ASTM B466/B466M-18 covers seamless copper-nickel pipe and tube, while ASTM B467-14(2022) covers the applicable welded copper-nickel pipe route.^[9][16] The alloy can be attractive for seawater cooling systems where pressure and mechanical requirements fit, because it combines fabrication experience, relatively low macrofouling tendency, and serviceable corrosion behavior when its protective film is allowed to form.

That film is the center of the selection logic. Commissioning with polluted, sulfide-bearing, or stagnant water can interfere with film development. Extended low flow can promote deposits and fouling. High local velocity or entrained sand can remove the film, especially at elbows, short-radius fittings, reducers, partially open valves, weld penetration, and pump discharge. A line-average velocity therefore cannot prove local acceptability.

Nickel Institute guidance comparing copper alloys and stainless steels for seawater cooling systems emphasizes that price, availability, fabrication, galvanic compatibility, and operating conditions all affect selection.^[10] Historical design guidance for 90/10 Cu-Ni also connects many field problems to overheating during joining, turbulence at fittings, and incompatible components.^[11] Those lessons should become purchase and commissioning controls:

1. specify C70600 chemistry and the governing product form;
2. design smooth transitions and avoid protruding weld roots;
3. qualify joining without overheating or contamination;
4. define acceptable water quality during initial film formation;
5. control low-flow, stagnant, and high-velocity operating periods;
6. isolate or otherwise engineer dissimilar-metal interfaces;
7. place inspection points at hydraulic and galvanic hot spots.

Cu-Ni should not be selected merely because it is “traditional,” nor rejected merely because 2507 has higher strength. The system pressure, wall design, weight, joining, water quality, maintenance strategy, and lifecycle cost decide.

Where does Alloy 625 fit in an offshore seawater system?

Alloy 625 is a conditional candidate for small-bore lines, local high-risk components, transition pieces, bellows, fasteners, weld overlay, or systems where seawater mixes with other aggressive process species. Its nickel-chromium-molybdenum-niobium composition provides broad corrosion resistance and useful strength.^[12] It is not automatically the economic or technical default for long runs of raw-seawater cooling pipe.

Special Metals’ aqueous-corrosion handbook warns that even Alloy 625 can suffer severe attack in particularly tight seawater crevices.^[13] That evidence is useful precisely because it prevents an absolute claim. For 625, as for 2507 and titanium, geometry and local chemistry can outrank a general alloy reputation.

If a project selects Alloy 625, the RFQ must state the actual product route and condition. ASTM B444-23 covers the applicable seamless pipe and tube route, ASTM B704-23 covers welded boiler, heat-exchanger, and condenser tube, and ASTM B705-24 covers welded pipe; these are different purchasing subjects.^[17][18][19] For the distinctions among these tubular routes, see Inconel 625 Tube and Pipe. Do not substitute an Alloy 625 sheet certificate or filler-metal certificate for pipe qualification.

How should galvanic connections, chlorination, and shutdowns change the design?

Interfaces can govern the corrosion risk even when the straight pipe alloy is suitable. A titanium line connected to carbon steel, a 2507 flange with an unsuitable gasket or fastener combination, or a Cu-Ni line with an exposed steel thermowell creates local electrochemical and geometric conditions that the straight pipe does not experience.

System event	Mechanism	Material-specific concern	Required project response
Dissimilar-metal connection	Galvanic current governed by potentials, polarization, electrolyte, and cathode/anode area	Active steel or Cu alloy can corrode rapidly next to large noble area	Map every wetted metal; calculate/review area ratios; define isolation, coatings, transition pieces, bonding and monitoring
Chlorination or electrochlorination	Higher oxidizing potential controls biofouling but can raise localized-corrosion demand	Stainless/duplex crevice margin can shrink; Cu-Ni film behavior changes; titanium usually benefits from oxidizing conditions but chemistry still matters	Supply normal, shock, residual, duration and injection-point data; qualify actual worst condition
Shutdown/stagnation	Deposits, oxygen concentration cells, microbial activity, sulfide generation, temperature change	Cu-Ni film and duplex crevices are vulnerable; titanium crevice chemistry can change	Design drainability; define fresh-water flush, preservation, restart and inspection
High local turbulence/solids	Film removal, impingement, erosion-corrosion	Cu-Ni and coatings can lose protection at fittings; any material may experience mechanical wear	Model/local-review bends and restrictions; set solids limits; inspect high-energy locations
Cathodic protection exposure	Polarization changes interface chemistry and hydrogen generation	Relevant highly stressed submerged duplex or titanium components need HISC/hydrogen review	Coordinate piping, structure and CP design; define material condition, stress, isolation and monitoring

The operating procedure is therefore part of corrosion control. A technically strong alloy can still fail if commissioning water is contaminated, a drained line remains half full, chlorination is applied at an unreviewed concentration, or a dead leg remains warm and stagnant.

What testing and documentation should the purchaser require?

Separate four questions: Is the alloy correct? Was the pipe manufactured correctly? Did fabrication preserve performance? Is the completed system leak-tight and traceable?

Evidence layer	Typical records	What it proves	What it does not prove
Base material	MTC, heat chemistry, mechanical results, heat treatment, dimensions, product NDE	Conformance of the represented pipe lot to the ordered product specification	Welded-system quality or service life
Fabrication	WPS/PQR, welder qualification, purge/heat records, filler traceability, repair map, visual/PT/RT/UT as specified	Conformance of joints and processing to the approved fabrication plan	Corrosion immunity in every field condition
Corrosion qualification	A923/G48 or project medium tests with method, specimen, surface, temperature, duration and criterion	Behavior under the stated test and sampling plan	Universal seawater ranking or propagation rate
Completed system	Dimensional report, pressure/leak test, flushing/cleanliness record, line list, punch closure	Installation and integrity under the stated acceptance test	Future operation outside the design envelope
Lifecycle control	Baseline UT, corrosion probes/coupons where appropriate, inspection map, shutdown procedure	A plan to detect change at credible hot spots	A guarantee that inspection can compensate for wrong material selection

ASTM G48-25 can support localized-corrosion screening of stainless steels and nickel alloys under defined ferric-chloride conditions. ASTM states that real-environment correlations have exceptions and that the methods do not determine propagation rate.^[14] A report that says only “G48 pass” is incomplete. The order needs method, test temperature, exposure time, specimen orientation and surface, crevice assembly, acceptance criterion, lot frequency, and retest rule.

For Super Duplex 2507, the project may add A923 and G48 controls, but the acceptance must match the actual product and governing specification. For titanium, cleanliness, shield quality, weld color, contamination prevention, and project-specified NDE are central. For Cu-Ni, alloy chemistry, joining quality, smooth internal geometry, commissioning water, and film formation may be more informative than imposing a stainless-steel corrosion test.

How does DAXUN turn the material decision into a quotable package?

DAXUN reviews the complete line duty before confirming a material route. For alloys and product forms within DAXUN’s confirmed written manufacturing scope, we manufacture the specified pipe or tube, perform the agreed in-house processing, retain heat and lot identity, inspect against the written acceptance plan, and assemble a document package linked to the shipped items. If the engineering shortlist includes 90/10 Cu-Ni, its manufacturing or supply scope must be confirmed separately in DAXUN’s written quotation. Third-party inspection or customer witnessing can be included at defined hold points without changing the identified manufacturer’s responsibility.

For a cross-material selection, review the actual Super Duplex 2507 pipe and tube und Grade 2 titanium tube and pipe routes as commercial product references. DAXUN website product pages do not replace ISO, ASTM, DNV, NORSOK, or the project specification as technical authority. The Testing page shows inspection categories; the purchase order must define the actual methods and criteria.

What should an offshore seawater piping RFQ include?

Send the following information before requesting a final material recommendation or price:

1. Facility type, system name, line list, P&ID, piping class, design code, and required life.
2. Full seawater analysis, source/season, temperature range, pressure, dissolved oxygen, solids, sand, sulfides, microbes, and pollution history.
3. Normal/minimum/maximum velocity, transient flow, pump duty, vibration, dead legs, drains, high points, and critical fittings.
4. Continuous/intermittent chlorination, normal and shock dose, residual, injection point, and all cleaning/preservation chemicals.
5. Proposed materials for pipe, fittings, flanges, valves, fasteners, strainers, pumps, exchanger water boxes, instruments, and gaskets.
6. Cathodic-protection exposure, electrical isolation/bonding philosophy, coatings, and transition details.
7. Exact UNS, product specification and edition, seamless/welded route, heat-treatment condition, dimensions, ends, and finish.
8. WPS/PQR, welder qualification, purge, heat input, interpass, repair, surface-cleaning, and contamination-control requirements.
9. MTC, PMI, NDE, pressure test, A923/G48 or service-medium test details, sampling, acceptance, witness, and document format.
10. Commissioning water quality, flushing, film formation where applicable, shutdown drainage/preservation, inspection locations, and baseline readings.
11. Quantity, spool or straight-length scope, packing, delivery location, Incoterm, and schedule.

Send the seawater analysis, line class, and drawings to DAXUN for a written manufacturing, inspection, and documentation proposal. If key operating values are unknown, mark them for technical review rather than allowing a supplier to assume benign conditions.

Frequently asked questions

What is the best pipe material for seawater?

There is no universal best material. Pressure, temperature, oxygen, chlorination, velocity, solids, stagnation, joining, fire requirements, galvanic interfaces, maintenance, and consequence of leakage determine whether 2507, titanium, Cu-Ni, another metal, lined steel, or a nonmetallic system is appropriate.

Is Super Duplex 2507 immune to seawater pitting?

No. It offers high localized-corrosion resistance in appropriate conditions, but tight crevices, deposits, elevated temperature and oxidizing potential, poor welding, harmful phases, or damaged surfaces can reduce the margin. PREN alone is not a service-life guarantee.

Is Titanium Grade 2 always better than Super Duplex 2507?

No. Grade 2 performs strongly in many aerated seawater conditions and is light, while 2507 offers much higher strength and established pressure-piping routes. Hot acidic crevices, reducing or fluoride cleaning media, hydrogen, weld cleanliness, pressure design, availability, and lifecycle cost can change the answer.

When should 90/10 Cu-Ni be selected?

It is a credible choice for many seawater cooling systems when pressure and mechanical requirements fit, water quality supports protective-film formation, local velocity and turbulence are controlled, sulfide exposure is managed, joining is qualified, and galvanic interfaces are designed.

Can 316L be used for offshore raw-seawater cooling pipe?

Do not select 316L from the word “marine.” Continuously aerated, warm, chlorinated seawater with crevices and stagnation creates substantial pitting and crevice-corrosion risk. A project-specific evaluation may identify limited uses, but long-term raw-seawater piping normally requires a more robust route or other controls.

Does an ASTM G48 pass prove seawater life?

No. It proves only the defined specimen met the stated criterion under the selected ferric-chloride method and conditions. It does not reproduce every natural seawater, chlorination, biofilm, crevice, flow, weld, or shutdown condition and does not predict propagation rate or years of life.

Is Alloy 625 the safest seawater piping choice?

Alloy 625 is valuable for specific high-risk components or mixed chemical duties, but it is not automatically the best bulk-pipe choice. Tight crevices can attack even 625, and a complete comparison must include product route, geometry, corrosion mechanism, fabrication, availability, pressure design, and lifecycle cost.

Technical Accuracy Statement

This article presents a decision framework, not a universal alloy ranking or piping design. Standard scopes, producer data, test results, and engineering judgment are kept separate. The approved design code, project specification, product standard edition, water analysis, fabrication procedures, and acceptance plan govern. An MTC or corrosion-test result does not certify the completed system or guarantee service life.

Last reviewed: September 10, 2026.

Technical Sources

1. ISO — ISO 21457:2010, Materials Selection and Corrosion Control for Oil and Gas Production Systems
2. ASTM International — ASTM A790/A790M-24, Seamless and Welded Duplex Stainless Steel Pipe
3. ASTM International — ASTM A923-25, Detecting Detrimental Intermetallic Phase in Duplex Stainless Steels
4. Alleima — SAF 2507 Seamless Tube and Pipe Data Sheet
5. DNV — DNV-RP-F112, Duplex Stainless Steel Design Against HISC
6. ASTM International — ASTM B861-24, Titanium and Titanium-Alloy Seamless Pipe
7. ASTM International — ASTM B862-23, Titanium and Titanium-Alloy Welded Pipe
8. TIMET — Titanium Corrosion Resistance Manual
9. ASTM International — ASTM B466/B466M-18, Seamless Copper-Nickel Pipe and Tube
10. Nickel Institute — Copper Alloys versus Stainless Steels for Seawater Cooling Systems
11. Nickel Institute — Design and Installation of 90/10 Copper-Nickel Seawater Piping Systems
12. Special Metals — INCONEL Alloy 625 Technical Bulletin
13. Special Metals — Aqueous Corrosion Handbook
14. ASTM International — ASTM G48-25, Pitting and Crevice Corrosion Test Methods
15. Standard Norge — NORSOK M-001:2025 Materials-Selection and Design-Life Context
16. ASTM International — ASTM B467-14(2022), Welded Copper-Nickel Pipe
17. ASTM International — ASTM B444-23, Nickel Alloy Seamless Pipe and Tube
18. ASTM International — ASTM B704-23, Welded Nickel Alloy Tubes
19. ASTM International — ASTM B705-24, Nickel Alloy Welded Pipe