254 SMO (UNS S31254): Super Austenitic Stainless Steel for Seawater & Chemical Service
ASTM A240 / ASTM A312 · Published: 2026-06-01 · Updated: 2026-06-02
254 SMO (UNS S31254) is a super austenitic stainless steel with a Pitting Resistance Equivalent Number (PREN) of 43, placing it above duplex 2205 (PREN 34-36) and approaching super duplex 2507 (PREN 42+). The 6% molybdenum content is the key...
254 SMO (UNS S31254) is a super austenitic stainless steel with a Pitting Resistance Equivalent Number (PREN) of 43, placing it above duplex 2205 (PREN 34-36) and approaching super duplex 2507 (PREN 42+). The 6% molybdenum content is the key differentiator — it provides chloride pitting resistance sufficient for seawater service without the cost or welding complexity of nickel-base alloys like Inconel 625 (PREN 50+) or Hastelloy C276 (PREN 65+). 254 SMO is the most commonly specified material for seawater-cooled heat exchangers, offshore platform piping, and flue gas desulfurization (FGD) scrubbers where chloride concentrations preclude 316L and duplex 2205.
While 254 SMO is classified as stainless steel (iron-based, >50% Fe), its alloy content of nickel (17.5-18.5%), chromium (19.5-20.5%), molybdenum (6.0-6.5%), and nitrogen (0.18-0.22%) places it at the boundary between stainless steels and nickel alloys. This composition provides the seawater corrosion resistance of a nickel alloy at approximately 50-60% of the cost. Welding: matching filler metal (ERNiCrMo-3, Inconel 625 filler) is required to maintain corrosion resistance in the weld zone. Autogenous welding (without filler) depletes molybdenum in the weld metal and reduces PREN below acceptable levels. Post-weld pickling and passivation is recommended for maximum corrosion resistance.
Quick Facts
| Category | Stainless Steel |
| Standard | ASTM A240 / ASTM A312 |
| Density | 8000 kg/m³ |
| Yield Strength | 300 MPa |
| Tensile Strength | 650-850 MPa |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| UNS S31254 | Compare |
| EN 1.4547 X1CrNiMoCuN20-18-7 | Compare |
| 6Mo stainless steel | Compare |
Related Materials
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Frequently Asked Questions
What is the PREN formula and how is it calculated?
PREN = %Cr + 3.3 × %Mo + 16 × %N. For 254 SMO: 20 + 3.3×6.1 + 16×0.20 = 20 + 20.1 + 3.2 = 43.3. A PREN above 40 is considered suitable for seawater service. For comparison: 316L PREN = 17 + 3.3×2.1 + 16×0.05 = 24.7 (unsuitable for seawater); Duplex 2205 PREN = 22 + 3.3×3.0 + 16×0.17 = 34.6 (marginal for seawater); Super duplex 2507 PREN = 25 + 3.3×4.0 + 16×0.28 = 42.7 (suitable for seawater).
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu