Duplex F51 vs Super Duplex F53 Valves for Chloride Service
Technical guide · Supreme Valves India · Updated 2026-10-24
Duplex and super duplex stainless steels are the usual step up from 316 stainless when chlorides, seawater or high-chloride produced water are involved. Both have a mixed austenite and ferrite structure that gives them higher strength than 316 and much better resistance to chloride stress corrosion cracking. The choice between them depends mainly on how aggressive the chloride exposure is.
Grades and equivalents
| Duplex (22Cr) | Super duplex (25Cr) | |
|---|---|---|
| UNS | S31803 / S32205 | S32750 (also S32760) |
| Forged (ASTM A182) | F51 / F60 | F53 (S32760: F55) |
| Cast (ASTM A995 / A890) | 4A (CD3MN) | 5A (CE3MN); 6A (CD3MWCuN) |
| Nominal Cr / Ni / Mo / N | 22 / 5 / 3 / 0.17 | 25 / 7 / 4 / 0.27 |
| Typical PREN | 34 to 36 | Over 40 |
For comparison, 316 stainless has a PREN of about 24. PREN (%Cr + 3.3 × %Mo + 16 × %N) is a rough ranking of resistance to pitting in chloride solutions, not a design value.
Where each grade fits
Duplex (F51, 4A)
- Brackish and cooling water, produced water with moderate chlorides.
- Chemical service where 316 suffers chloride stress corrosion cracking.
- Higher-strength valves where the yield strength (roughly double that of 316) allows thinner walls.
Super duplex (F53, 5A, 6A)
- Seawater systems: firewater, seawater cooling and injection, desalination intakes, offshore utilities.
- High-chloride produced water and brines.
- Crevice-prone locations such as flange faces and valve seats in stagnant seawater.
Even super duplex has temperature limits in chlorinated seawater because crevice corrosion risk rises with temperature. Project material selection reports set these limits; follow them rather than a generic rule.
Temperature range
Duplex steels form brittle phases after long exposure at intermediate temperatures, often called 475 °C embrittlement, with sigma phase forming at higher temperatures. Design codes therefore cap their use, typically around 250 to 300 °C depending on the code and grade. At the low end, they are usually impact tested at -46 °C, and their toughness falls off below that. They are not a replacement for austenitic grades in cryogenic service.
Why testing is stricter than for 316
The properties of duplex depend on the right phase balance and on avoiding intermetallic phases during casting, heat treatment and welding. Project specifications commonly require:
- Ferrite content measured by point count, typically within a band around 35 to 55 percent.
- Charpy impact testing at low temperature.
- Corrosion testing to ASTM G48 Method A at a specified temperature, with weight loss and pitting limits.
- Microstructure examination for intermetallic phases.
- Positive material identification on finished parts.
Operator standards such as NORSOK M-630 and M-650 set these requirements in detail for offshore projects, and they often call for EN 10204 3.2 certificates.
Sour service
Both grades can be used in H2S service within the limits of NACE MR0175 / ISO 15156-3, which restrict hardness, H2S partial pressure, chloride content, pH and temperature. Check the actual conditions against the standard; duplex is not automatically suitable for every sour service.
Trim, bolting and seals
Match trim to the body: super duplex stems and balls in a super duplex valve, or a qualified alternative such as alloy 625 overlay. Bolting in seawater service is often super duplex or alloy 625 instead of B7 with coating. Seals and seats must suit the temperature and medium.
How to choose
- Moderate chlorides, no seawater, temperature under the code limit: duplex F51 / 4A.
- Seawater, brine, high-chloride produced water, or crevice-prone service: super duplex F53 / 5A / 6A.
- Seawater with a cost constraint: consider nickel aluminium bronze for valves, if the system allows.
- Hot or high-temperature service: choose another alloy.
Send the medium, chloride content, temperature and applicable material specification, and we will confirm the grade and test requirements.
Frequently asked questions
What is the difference between F51 and F53?
F51 is forged 22 percent chromium duplex stainless steel (UNS S31803 or S32205), with a PREN of about 34 to 36. F53 is forged 25 percent chromium super duplex (UNS S32750), with a PREN above 40. The higher chromium, molybdenum and nitrogen give super duplex much better resistance to pitting and crevice corrosion in seawater.
What is the maximum temperature for duplex valves?
Duplex and super duplex steels lose toughness after long exposure in roughly the 300 to 500 degrees C range, so design codes limit their use, typically to about 250 to 300 degrees C depending on the code. They are not used for hot steam or high-temperature service.
Why do duplex valve specifications ask for ferrite and G48 testing?
Duplex steels rely on a balance of roughly equal austenite and ferrite. Incorrect heat treatment or welding can upset that balance or form harmful intermetallic phases that reduce toughness and corrosion resistance. Ferrite measurement, impact testing and ASTM G48 pitting tests verify the material was processed correctly.
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