Duplex 2205 vs Super Duplex 2507 — Understanding the Difference

Composition and PREN Value

The Pitting Resistance Equivalent Number (PREN) determines chloride pitting resistance: PREN = %Cr + 3.3×%Mo + 16×%N.

Duplex 2205 (UNS S31803/S32205): ~22% Cr, 3% Mo, 5.5% Ni, 0.15% N → PREN ≈ 35 Super Duplex 2507 (UNS S32750): ~25% Cr, 4% Mo, 7% Ni, 0.27% N → PREN ≈ 42

A PREN above 40 is required for seawater service. This makes 2507 the correct choice for offshore and subsea applications.

Corrosion Resistance

Both grades resist chloride stress corrosion cracking (SCC) far better than austenitic 316L. Duplex 2205 is suitable for mildly corrosive chloride environments — coastal structures, desalination, chemical processing. Super Duplex 2507 is designed for severe chloride service — offshore platforms, subsea umbilicals, seawater cooling systems.

Mechanical Strength

2205: UTS ≈ 620 MPa, Yield ≈ 450 MPa 2507: UTS ≈ 795 MPa, Yield ≈ 550 MPa

Both are approximately twice as strong as austenitic grades, allowing thinner wall sections and lighter weight designs.

Weldability and Fabrication

Both grades require careful heat input control during welding to maintain the austenite-ferrite phase balance. Intermetallic phases (sigma phase) can precipitate at 700–1000°C, causing embrittlement. Post-weld solution annealing is recommended for critical service. Super duplex is more sensitive and requires tighter process control.

Applications

Duplex 2205: Chemical plant piping, heat exchangers, pressure vessels, FGD scrubbers, paper and pulp digesters, general offshore structural.

Super Duplex 2507: Seawater handling, subsea equipment, high-pressure injection lines, desalination high-pressure sections, oil & gas umbilicals.

Cost Comparison

Super Duplex 2507 typically costs 30–50% more than Duplex 2205, reflecting its higher alloy content (particularly molybdenum and nickel). Specify 2507 only where the PREN > 40 requirement or the extreme corrosion resistance is genuinely needed.