Duplex vs Super Duplex: Key Differences

Duplex (22% Cr) and super duplex (25% Cr) stainless steels share the same dual-phase austenite-ferrite microstructure but differ in alloy content and performance. Super duplex contains more chromium, molybdenum, and nitrogen, which push the PREN above 40 and deliver significantly better resistance to pitting, crevice corrosion, and chloride stress corrosion cracking.

The trade-off: super duplex costs 30-50% more, is harder to weld, and has more limited mill availability.

Comparison Table

Property	Duplex 2205	Super Duplex 2507
UNS	S31803/S32205	S32750
ASTM pipe spec	A790	A790
Cr (%)	21-23	24-26
Ni (%)	4.5-6.5	6-8
Mo (%)	2.5-3.5	3-5
N (%)	0.08-0.20	0.24-0.32
PREN	~35	~42
Yield strength (min)	450 MPa (65 ksi)	550 MPa (80 ksi)
Tensile strength (min)	620 MPa (90 ksi)	795 MPa (116 ksi)
Critical pitting temp (CPT)	~35°C (in 6% FeCl3)	~65°C (in 6% FeCl3)
Max. service temp	~300°C	~300°C
Cost relative to 316L	1.5-2x	2-3x
Weldability	Good (controlled)	More difficult

When Duplex 2205 Is Sufficient

Duplex 2205 handles the majority of applications where 316L falls short:

• Seawater-cooled heat exchangers (non-contact side)
• Produced water handling with moderate chloride levels (<50,000 ppm at moderate temperatures)
• Chemical processing with organic acids and mild chlorides
• Structural applications requiring high strength-to-weight ratio
• Process piping on offshore platforms (topside)

When Super Duplex Is Required

Super duplex is specified when the environment exceeds duplex 2205 capabilities:

• Direct seawater service at elevated temperatures (>20°C)
• High-chloride produced water (>50,000 ppm Cl-) at temperatures above 80°C
• Subsea manifolds, trees, and flowline components
• Sour service with chlorides per NACE MR0175 / ISO 15156
• Umbilical and control system tubing in subsea applications
• Desalination plant high-pressure piping

Pro Tip

Use the Critical Pitting Temperature (CPT) test per ASTM G48 Method C to compare duplex and super duplex for a specific environment. If the operating temperature is within 10°C of the CPT, upgrade to the next alloy level. A 2205 duplex with a CPT of 35°C should not be used above 25°C in that environment.

Welding Considerations

Parameter	Duplex 2205	Super Duplex 2507
Filler metal	ER2209	ER2594 or equivalent
Heat input	0.5-2.5 kJ/mm	0.5-2.0 kJ/mm
Max. interpass temp	150°C	150°C
Shielding gas	Ar + 2% N2	Ar + 2% N2
Back purge	Ar + N2	Ar + N2
PWHT	Not required	Not required
Ferrite target	40-60%	40-60%

Both grades require nitrogen-supplemented shielding gas to maintain the austenite-ferrite balance in the weld zone. Super duplex welds are more sensitive to intermetallic phase formation (sigma, chi) and require tighter heat input control.

For the governing pipe specification, see ASTM A790.

Read the full guide to pipe types