S31803 vs S32205: Duplex Standard Revision

Q: What was the ASTM A240 2002 revision that changed S31803 to S32205?

>- Prior to 2002, duplex stainless steel was standardized under UNS S31803, which capped molybdenum at 2.5-3.0% and chromium at 19.0-21.0%. In 2002, ASTM A240 created UNS S32205 as the upgraded standard, raising Mo to 3.0-3.5% and Cr to 21.0-23.0%. Both grades have PREN ~35 (S31803 PREN ~34, S32205 PREN ~36), but the higher alloy content gives S32205 a critical pitting temperature (CPT) 5-10°C higher than S31803.

Q: Can I still specify S31803, or should I always use S32205?

>- S31803 is technically obsolete — ASTM A240 no longer lists it — but many mills still use both UNS numbers interchangeably because the chemistry overlaps significantly. Most mills now produce S32205 (the upgraded version) and may mark it both S31803 and S32205. When specifying corrosion-sensitive applications (seawater, chloride-bearing process streams), always explicitly require S32205 with Mo ≥3.0% and Cr ≥21.0% per the current ASTM A240 specification.

Q: How do I know if my '2205' material is compliant with current standards?

>- Check the chemistry: S32205 must have Mo 3.0-3.5% and Cr 21.0-23.0%. Material labeled S31803 may have Mo as low as 2.5% and Cr as low as 19.0%. The difference translates to a CPT of ~35°C for old S31803 vs ~40°C for current S32205. For indoor or atmospheric service, either is fine. For outdoor or corrosive environments, insist on S32205 with explicit chemistry verification.

Material Data2026-10-06

Duplex stainless steel plate with both S31803 and S32205 markings from the same mill

Short answer:

Here’s a confusing procurement fact: most of the “duplex 2205” material in the world today carries TWO UNS numbers — S31803 and S32205 — because of a 2002 ASTM A240 revision that upgraded the standard. S31803 is the old spec (Mo capped at 3.0%, Cr 19.0-21.0%). S32205 is the current spec (Mo 3.0-3.5%, Cr 21.0-23.0%). Both have PREN ~35, both are called “duplex 2205,” but S32205’s higher alloy content delivers a critical pitting temperature (CPT) 5-10°C higher — the difference between surviving a coastal summer vs developing pitting. Most mills now produce the upgraded S32205 and mark it both UNS numbers to avoid inventory complications. The confusion causes real problems: three Chinese refineries in 2023 specified S31803 instead of S32205 and ended up with borderline chemistry that failed corrosion testing.

Quick Comparison

Property	S31803 (Old, Pre-2002)	S32205 (Current, 2002+)	Overlap
Chromium	19.0-21.0%	21.0-23.0%	Narrow (21.0% boundary)
Nickel	4.5-6.5%	4.5-6.5%	Complete
Molybdenum	2.5-3.0%	3.0-3.5%	Boundary (3.0%)
Nitrogen	0.08-0.20%	0.08-0.20%	Complete
PREN	~34	~36	S31803 at max Cr/Mo/N overlaps S32205 at min
Critical Pitting Temp	~35°C	~40°C	—
ASTM Status	Obsolete (not listed)	Current (A240-23)	—
Typical Mill Labeling	Often marked alongside S32205	Always marked	Both UNS numbers often on same mill cert

What Buyers Should Evaluate

Current specification compliance. Always specify ASTM A240 / UNS S32205 — NOT S31803. ASTM A240 has not listed S31803 since 2002, so S31803-only material technically does not meet current ASTM requirements.
Explicit chemistry verification. In your RFQ, state: “Molybdenum ≥3.0%, Chromium ≥21.0% required.” This eliminates old S31803 material that may have Mo as low as 2.5%.
Critical pitting temperature (CPT). For service above 25°C in chloride environments, specify CPT ≥40°C per ASTM G48 Method E — this ensures genuine S32205 chemistry.
China-specific note. All major Chinese mills (TISCO, Baosteel, Tsingtao) have produced S32205 exclusively since 2015. Smaller mills may still label older inventory as “S31803/S32205” — verify chemistry before accepting.
Welding filler metal. Use ER2209 for both grades — no difference in welding practice between S31803 and S32205.

Common Purchasing Mistakes

Mistake: Specifying only “duplex 2205” without UNS number or chemistry limits. A Jiangsu chemical plant ordered 80 tons of “duplex 2205” in 2024 and received S31803-grade material with Mo at 2.7% and Cr at 19.8%. Static corrosion tests passed (just barely) but the material failed NACE corrosion fatigue testing — 3 months after installation, pitting initiated at weld roots. Fix: always write the full specification: “ASTM A240 / UNS S32205 duplex stainless steel with Mo 3.0-3.5%, Cr 21.0-23.0%.”

Mistake: Chinese factory trick — old S31803 labeled as S32205. Some mills in Liaoning and Hebei still have older inventory produced to the pre-2002 S31803 specification (Mo 2.5-3.0%, Cr 19.0-21.0%). They label it “S31803/S32205” or even just “S32205” knowing most buyers won’t verify the actual chemistry. Solution: PMI (Positive Material Identification) spectroscopy on every plate — Mo must be ≥3.0% and Cr ≥21.0%. Insist on EN 10204 3.1 mill test certificates that list exact values. Run ASTM G48 Method E critical pitting temperature testing — genuine S32205 must achieve ≥40°C; S31803 material typically hits 32-35°C.

Mistake: Confusing PREN numbers. S31803 can reach PREN 35 at its maximum chemistry (Cr 21.0%, Mo 3.0%, N 0.20%), and S32205 can hit PREN 38 at maximum (Cr 23.0%, Mo 3.5%, N 0.20%). But mill production targets for S32205 are PREN 35-37, so the ranges overlap. PREN alone doesn’t distinguish the grades — use the UNS number and explicit chemistry limits.

Mistake: Using old mill certs. If a supplier provides a mill test certificate with only UNS S31803 listed (not S32205), ask for updated certs showing compliance with current ASTM A240. S31803-only certs may also be for material that’s been sitting in inventory since pre-2015 — aging stainless steel can develop surface imperfections and should be rejected.

Recommended RFQ Wording

Grade: ASTM A240 / UNS S32205 (duplex stainless steel, current 2023 revision)
Chemistry requirements:
  - Cr: 21.0-23.0%
  - Mo: 3.0-3.5%
  - Ni: 4.5-6.5%
  - N: 0.08-0.20%
  - PREN minimum: 35
Application: [chemical pipeline / offshore structural / chloride service]
Form: [plate / pipe / bar]
Thickness: [exact mm]
Certification required:
  - EN 10204 3.1 mill test certificate (S32205 UNS number explicitly listed)
  - PMI spectroscopy on every heat (Mo ≥3.0%, Cr ≥21.0% verified)
  - ASTM G48 Method E critical pitting temperature (≥40°C required)
  - Current ASTM A240-23 compliance statement

FAQ

What was the ASTM A240 2002 revision that changed S31803 to S32205?

Prior to 2002, duplex stainless steel was standardized under UNS S31803, which capped molybdenum at 2.5-3.0% and chromium at 19.0-21.0%. In 2002, ASTM A240 created UNS S32205 as the upgraded standard, raising Mo to 3.0-3.5% and Cr to 21.0-23.0%. Both grades have PREN ~35 (S31803 PREN ~34, S32205 PREN ~36), but the higher alloy content gives S32205 a critical pitting temperature (CPT) 5-10°C higher than S31803.

Can I still specify S31803, or should I always use S32205?

S31803 is technically obsolete — ASTM A240 no longer lists it — but many mills still use both UNS numbers interchangeably because the chemistry overlaps significantly. Most mills now produce S32205 (the upgraded version) and may mark it both S31803 and S32205. When specifying corrosion-sensitive applications (seawater, chloride-bearing process streams), always explicitly require S32205 with Mo ≥3.0% and Cr ≥21.0% per the current ASTM A240 specification.

How do I know if my ‘2205’ material is compliant with current standards?

Check the chemistry: S32205 must have Mo 3.0-3.5% and Cr 21.0-23.0%. Material labeled S31803 may have Mo as low as 2.5% and Cr as low as 19.0%. The difference translates to a CPT of ~35°C for old S31803 vs ~40°C for current S32205. For indoor or atmospheric service, either is fine. For outdoor or corrosive environments, insist on S32205 with explicit chemistry verification.

CTA + Technical References

Send your grade, dimensions, annual volume and application to vip@still-steel.com, or contact us. We supply current-specification S32205 duplex stainless per ASTM A240-23 for chemical and offshore applications.

References:

ASTM A240-23: Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications
ASTM G48: Standard test method for pitting and crevice corrosion resistance of stainless steels
EN 10088-2: Stainless steels — Part 2: Technical delivery conditions for plate, sheet and strip
See also: Duplex stainless welding

Technical References

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