Duplex 2205 vs Inconel 625 - Seawater Duty and Real Cost

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: Is Duplex 2205 Enough, or Do I Need Inconel 625?

Duplex 2205 (UNS S32205) has a PREN of about 35, a minimum yield strength of 450 MPa — slightly higher than annealed Inconel 625 — and costs roughly 25-35% of what 625 costs per kilogram. It handles flowing seawater, chloride-bearing process water and most desalination and marine piping duty. Inconel 625 (UNS N06625) has a PREN of about 51, has no practical chloride SCC limit, tolerates higher temperatures and is not subject to 475°C embrittlement, but cannot compete on price. Use 2205 for the bulk of the pipework; use 625 where chloride, temperature and H2S exceed duplex limits.

Key Takeaways

• 2205's yield strength matches or beats annealed 625 — 450 MPa minimum versus 414 MPa — so the strength argument alone does not justify the price gap.
• PREN is the real divider. About 35 for 2205, about 51 for 625. That gap governs pitting and crevice performance in chlorides.
• Temperature is duplex's hard limit. Around 300°C the duplex structure degrades; 625 has no equivalent constraint.
• Duplex is weldable but unforgiving. Heat input, interpass temperature and ferrite/austenite balance must be controlled, and the finished weld is checked for phase balance.
• On large projects the two are usually combined. 2205 for the bulk, 625 for the hot or most aggressive sections — that hybrid is the standard cost-optimised design.

What Are Duplex 2205 and Inconel 625?

Duplex 2205 is a two-phase stainless steel with roughly equal proportions of ferrite and austenite. The duplex microstructure is what gives it the combination of high strength and chloride resistance that neither phase delivers alone. It is commonly specified under UNS S32205 (the modern, nitrogen-controlled version of the older S31803) and is the workhorse of desalination, marine and chemical-tanker construction.

Inconel 625 is a nickel-chromium-molybdenum-niobium superalloy — a single-phase austenitic material with roughly three times the molybdenum of 2205 and a nickel content above 58%. It is the specification of last resort in chloride service, and it is also used as a weld overlay on cheaper substrates.

See our duplex & PH stainless and Inconel alloy supplier pages for stocked forms and sizes.

Composition Comparison (wt.%)

Table note: Values are standard composition limits from the listed ASTM specifications. Calculated PREN (Cr + 3.3 Mo + 16 N) is approximately 35 for 2205 and approximately 51 for 625 using nominal compositions. In a PMI check, nickel is the immediate discriminator: 4.5-6.5% for 2205 against 58% minimum for 625.

Mechanical Properties Comparison

Table note: Room-temperature values are standard minima. Note the counter-intuitive result: 2205 has a higher specified minimum yield strength than annealed 625. The trade-off for that strength is toughness and temperature capability, not strength.

Corrosion and Service Comparison

Table note: Ratings are qualitative engineering guidance from published data, NACE MR0175 / ISO 15156 sour-service limits and our own field experience; they are not ASTM G48 results for a specific heat. For sour service, the governing document is the standard, not a comparative table.

Where Does 2205 Stop and 625 Become Necessary?

The decision is almost always made on the combination of chloride concentration and temperature, with H2S as a decisive third factor:

1. Temperature above roughly 60-80°C with significant chloride. 2205's pitting and crevice resistance falls away as temperature rises, and where the fluid is not freely flowing, crevice attack under deposits becomes the controlling failure mode.
2. Any requirement above roughly 300°C. Duplex structures are not selected for hot service because of 475°C embrittlement and sigma-phase formation. 625 has no equivalent constraint.
3. Sour service above duplex limits. NACE MR0175 / ISO 15156 sets a narrower window for duplex grades than for 625, and operations outside that window require an upgrade.
4. Severely reducing acids. Molybdenum level decides this, and 2205 has less than half of 625's.

Note that on a purely mechanical basis 2205 is not the weaker material — it is the more brittle at high temperature material. Confusing strength with temperature capability is a common error in specification reviews.

Does 2205's Strength Really Close the Price Gap?

Partly, and it is worth calculating rather than assuming.

Table note: Price indications are 2026 EXW Shanghai reference ranges only and float with raw-material prices; strength comparison uses the standard minima from Table 2. The cost calculation must include fabrication quality control, not only material.

The honest conclusion for a large project: 2205 wins decisively on the bulk of the tonnage, and 625 is justified only in the sections that genuinely exceed duplex limits — hot brine circuits, aggressive acid contact, high-H2S wells, or overlay protection on cheaper steel. That hybrid design is standard practice in desalination and offshore projects.

Fabrication and Welding Notes

Duplex 2205 is weldable, but the shop has to be disciplined. Three controls matter:

1. Heat input is controlled within a narrow band. Too low and the weld freezes as almost fully ferrite, with poor toughness and corrosion resistance; too high and intermetallic phases such as sigma begin to form.
2. Interpass temperature is capped, typically well below 150°C in project specifications, to limit precipitation.
3. No post-weld heat treatment. Solution annealing after fabrication is not feasible for large structures, so the correct microstructure must be produced by the welding procedure itself. The finished weld is normally checked for ferrite content by a magnetic instrument and, on critical projects, by metallographic examination.

Qualification of duplex welding procedures against a specification such as NORSOK M-650 is common in oil and gas work precisely because the welding variables are so consequential.

Inconel 625 is considerably more forgiving and is routinely applied as an overlay or cladding on carbon steel and 316L. That gives a 625 corrosion surface with a cheap structural backing, which is often the highest-value way to use the alloy.

A failure case worth recording: a fabricated 2205 spool was annealed by a subcontractor at an incorrect temperature and quenched from the wrong point in the cycle. Hardness came back around 38 HRC — well above the expected level for correctly annealed 2205 — and Charpy values were roughly half of what the procedure qualification had produced. The spool was scrapped. Whenever duplex is thermally processed after fabrication, hardness and impact testing should be part of the release criteria, not an afterthought. Our alloy technical knowledge center covers the release checks we apply, and the related technical guides section has the accompanying duplex-versus-super-duplex comparison.

For the hottest and most aggressive chloride duties, where duplex reaches its practical limit, see the Inconel alloy supplier and Hastelloy C-276 plate & bar pages. For marine fasteners that must combine strength with chloride tolerance, Monel alloy round bar & tube is the family we normally verify against a hardness limit rather than substituting a high-strength stainless.

Which Alloy for Which Application?

Price Reference (2026, EXW Shanghai)

Table note: Reference range only — floats with LME nickel price. 2026, EXW Shanghai, USD/kg. For super duplex 2507, expect roughly 1.6-1.9 times the 2205 reference range for the same form.

Standard Index

How We Verify Duplex and Nickel Alloys Before Shipment

Duplex and nickel alloys are frequently confused in storage because both are non-magnetic-looking, matte-finished products. Our release sequence:

1. OES verification against the specified UNS number. For 2205 we read chromium (22-23%), nickel (4.5-6.5%), molybdenum (3.0-3.5%) and nitrogen; nitrogen is the element most often missing in material sold as S32205 but actually melted to the older S31803 or to a 2205-look-alike grade.
2. Phase-balance and hardness checks on duplex. A correctly annealed 2205 plate or bar sits in the expected hardness window; a reading well above it signals improper annealing, and the material is either re-annealed under control or rejected.
3. Ferrite check on welded assemblies by magnetic instrument, with metallographic confirmation on critical projects.
4. Certificate reconciliation against the stencilled heat number, with the proof photographed into the batch record.

Substitution cases are common enough in this family that we treat PMI as mandatory rather than optional: material sold as "duplex" has arrived with 4% nickel and no nitrogen, which analyses as a lean 316-type chemistry and would fail in seawater. Reading nitrogen and molybdenum is the fast, decisive check.

FAQ

Q1: Is duplex 2205 strong enough to replace Inconel 625?

At room temperature, yes — 2205 actually has a slightly higher minimum yield strength (450 MPa against 414 MPa for annealed 625). The replacement fails not on strength but on corrosion resistance in aggressive chlorides and on temperature capability above roughly 300°C.

Q2: What is the PREN of 2205 versus 625?

Approximately 35 for duplex 2205 and approximately 51 for Inconel 625, calculated as Cr + 3.3 Mo + 16 N using nominal compositions. The gap translates directly into pitting and crevice corrosion performance in chloride service.

Q3: Can duplex 2205 be used above 300°C?

No, not for engineering service. Duplex grades are subject to 475°C embrittlement and to sigma-phase formation at elevated temperature, and ASME pressure-vessel codes limit them to roughly 300°C. Verify the limit in the current code edition for your jurisdiction.

Q4: How much cheaper is 2205 than Inconel 625?

Typically 2205 costs about 25-35% of the 625 price per kilogram for the same product form in 2026 EXW Shanghai reference terms — a bar price of roughly \$12-20/kg against \$48-75/kg. That gap, not strength, is why 2205 dominates large seawater projects.

Q5: Is duplex 2205 suitable for sour service?

Only within the limits set by ISO 15156 / NACE MR0175, which are generally more restrictive for duplex grades than for 625. The acceptable H2S partial pressure depends on temperature, pH and chloride level, and the limits must be checked for each project rather than assumed.

Q6: Why does duplex welding need so much control?

The properties of duplex come from a near-equal ferrite/austenite balance in the microstructure. Welding heat input and interpass temperature determine whether that balance is achieved; too little heat leaves an over-ferritic weld with poor toughness, and too much heat allows sigma and other intermetallic phases to form.

Q7: Does duplex 2205 need post-weld heat treatment?

No — in fact it must not be post-weld heat treated in normal fabrication, because the required solution-annealing temperature cannot be applied to a large structure and sub-critical heating causes embrittlement. Correct microstructure must come from the welding procedure itself.

Q8: Can Inconel 625 be used as an overlay instead of solid duplex or solid 625?

Yes, and it is often the best value engineering choice. A 625 weld overlay on a carbon steel or 316L substrate places the corrosion-resistant alloy exactly where the process contacts it, at a fraction of the cost of solid 625 construction.

Q9: Which grade for seawater above 80°C?

Inconel 625, or another high-molybdenum nickel alloy. Above roughly 80°C in chlorinated or aerated seawater, 2205's crevice and pitting margins become inadequate for a safety-critical duty, and super duplex 2507 or a nickel alloy should be evaluated.

Q10: What UNS numbers apply?

Duplex 2205 is UNS S32205 (with the closely related, lower-nitrogen S31803 still appearing on older drawings). Inconel 625 is UNS N06625. Super duplex 2507 is UNS S32750. Always order by UNS number plus the governing ASTM specification.

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