Standard Duplex Stainless Steel (50% Austenite / 50% Ferrite) for Offshore, Chemical & Seawater Service

What is Duplex 2205?

Duplex 2205 is a second-generation duplex stainless steel that balances the best of both ferrite and austenite: the ferritic portion provides high strength and resistance to chloride stress-corrosion cracking, while the austenitic portion contributes toughness, ductility, and weldability. The result is a material with a minimum yield strength of 450 MPa (65 ksi) — roughly 2.2× that of 304/316 austenitic stainless — at roughly 30–45% lower cost than super duplex 2507.

First standardized in the 1980s as UNS S31803 and refined in 1996 as UNS S32205 (tighter nitrogen and molybdenum floors), 2205 is today one of the most widely specified stainless grades for demanding industrial piping. Its PREN of 34–36 places it well above 316L (PREN ~25) for pitting resistance, yet below the super duplex grades needed for warm, aggressive seawater.

Chemical Composition (Weight %)

★ S32205 requires nitrogen ≥ 0.14% and molybdenum ≥ 3.0% (vs older S31803: N 0.08% min, Mo 2.5% min), guaranteeing PREN ≥ 35. Most modern 2205 is dual-certified S31803/S32205; S32205 automatically meets S31803.

Mechanical Properties

2205 is strengthened by its two-phase microstructure rather than by heat treatment. The values below are for the solution-annealed condition per ASTM A182 (F60) and ASTM A790 (pipe).

Elastic Constants — Modulus of Elasticity: 200 GPa (at 21°C); Shear Modulus: 77 GPa; Poisson’s Ratio: 0.30. The high yield strength allows wall-thickness reductions of 30–50% versus 316L in pressure systems, offsetting the material premium through lower weight.

Physical Properties

Corrosion Resistance

Seawater & Marine Environments

With PREN 34–36, 2205 is immune to chloride stress-corrosion cracking (SCC) — the failure mode that cracks 316L in months — and is the standard material for offshore topside piping, seawater cooling, and marine structures. It is the most common duplex grade in North Sea and Middle East platforms.

Pitting & Crevice Corrosion

The 3% molybdenum plus nitrogen give 2205 a Critical Pitting Temperature (CPT) of about 35°C per ASTM G48. It performs well in ambient-temperature seawater with flow, but crevice corrosion risk rises in warm or stagnant seawater above ~35°C — the point where super duplex 2507 (CPT ~55°C) becomes the safer choice.

Acid Environments

2205 resists dilute organic acids and many process streams, but it is not a substitute for nickel alloys in strong reducing acids (hydrochloric, sulfuric). For those environments, consider Alloy 20, 904L, or a nickel alloy.

High-Temperature Limitation

All duplex grades share a critical weakness: exposure between roughly 250°C and 550°C precipitates brittle intermetallic phases (sigma, chi) and alpha-prime (475°C embrittlement). Continuous service is therefore limited to ~300°C per NORSOK M-001; for higher temperatures use 316L, 310S, or a nickel alloy.

Duplex 2205 vs Similar Grades — Selection Guide

Applicable Standards

HT PIPE Supply — Duplex 2205 Product Forms

All 2205 products from HT PIPE are supplied in the solution-annealed condition with EN 10204 3.1/3.2 MTRs. We stock dual-certified S31803/S32205 material with nitrogen ≥ 0.14% — the S32205 floor required for reliable seawater and sour service.

Project Experience — Duplex 2205

HT PIPE supplies 2205 pipe, fittings, and flanges to offshore, desalination, and chemical projects worldwide. Representative cases:

Our technical team provides material selection, welding procedure specification (WPS), and PMI / ferrite verification for 2205 in seawater and sour service.

FAQ — Duplex 2205