The Original 2205 UNS Designation — Dual Certification, Nitrogen Floor & PREN Explained

What is UNS S31803?

UNS S31803 was the first ASTM-listed chemistry for 2205 duplex stainless steel. Its specification allowed nitrogen from 0.08% to 0.20% — a wide range. In practice, some early heats ran nitrogen at the bottom of that range (0.08–0.10%), giving acceptable but inconsistent pitting resistance, with PREN values as low as ~30.

In 1996 ASTM introduced UNS S32205, raising the nitrogen floor to 0.14% and the molybdenum minimum to 3.0% to guarantee every heat reaches PREN ≥ 35. Because S32205’s chemistry sits entirely within the S31803 envelope, any heat that meets S32205 automatically meets S31803 — which is why today’s 2205 is almost always dual-certified. Specifying the UNS number and nitrogen content on a purchase order is the single most important step in getting the pitting resistance your service requires.

Chemical Composition (Weight %) — S31803 vs S32205

★ PREN (minimum, using floor values): S31803 ≈ 30.5 (21 + 3.3×2.5 + 16×0.08); S32205 ≈ 33.1 (21 + 3.3×3.0 + 16×0.14). Typical modern heats run PREN 35–36, safely above the seawater threshold of PREN > 34.

Mechanical Properties

The mechanical requirements for S31803 and S32205 are identical per ASTM A240 and A182: a minimum yield strength of 450 MPa (65 ksi) and tensile of 620 MPa (90 ksi). The nitrogen difference that drives PREN does not change the specified mechanical floor.

Elastic Constants — Modulus of Elasticity: 200 GPa; Shear Modulus: 77 GPa; Poisson’s Ratio: 0.30. S32205 heats tend to show slightly higher actual yield (480–520 MPa) because of their higher nitrogen; both fall under the same ASME B31.3 allowable-stress table.

Physical Properties

Corrosion Resistance

Pitting & Crevice — The Nitrogen Link

Pitting resistance tracks PREN, and PREN tracks nitrogen. An S31803 heat at the bottom of its nitrogen range (0.08%) sits near PREN 30.5 — below the PREN > 34 generally recommended for seawater. A dual-certified S31803/S32205 heat at N 0.16% reaches PREN ~36. For pitting-critical service, always confirm N ≥ 0.14% on the MTC.

Seawater & Marine Environments

Dual-certified S31803/S32205 is the standard material for offshore topside, seawater cooling, and marine structures, with immunity to chloride stress-corrosion cracking. S31803-only material at low nitrogen is marginal for seawater and should be avoided in immersed or crevice-prone service.

Sour Service (H₂S + Cl⁻)

For ISO 15156 sour service, the higher and guaranteed PREN of S32205 is normally specified. S31803-only material rarely meets the pitting-resistance requirement for sour chloride environments.

High-Temperature Limitation

Like all duplex, S31803 is limited to ~300°C continuous service. Between 250°C and 550°C it risks 475°C embrittlement and sigma-phase precipitation — see the 2205 material page for detail.

When to Specify S31803 vs S32205

Applicable Standards

HT PIPE Supply — UNS S31803 / S32205 Product Forms

HT PIPE stocks dual-certified 2205 (S31803/S32205) across the full product range and reports nitrogen content — not just chromium and molybdenum — on every mill certificate for critical service. For offshore and sour applications we deliver material with N 0.16–0.18%, well above the 0.14% floor.

Project Experience — Specifying UNS on Every 2205 Quote

The S31803-vs-S32205 question is the most common technical clarification HT PIPE handles on duplex inquiries. How we approach it:

Always write the UNS number explicitly on the purchase order. If a spec says only 2205 with no UNS, confirm whether S32205 is acceptable before accepting material — it usually is, and it is the same price.

FAQ — UNS S31803