What Is Duplex Stainless Steel Pipe?

Quick Answer

Duplex stainless steel pipe has a two-phase microstructure of roughly equal parts austenite and ferrite. This combination delivers approximately twice the yield strength of standard austenitic grades (304, 316) with superior resistance to stress corrosion cracking and pitting, making it the material of choice for offshore oil and gas, chemical processing, and desalination.

The most common duplex grade is UNS S31803 / S32205 (2205), with 22% chromium and 5% nickel. Duplex pipes are covered by ASTM A790 (seamless and welded) and ASTM A928 (EFW with filler metal addition).

Properties Compared to Austenitic Stainless

Property	304L (Austenitic)	316L (Austenitic)	2205 Duplex	2507 Super Duplex
UNS	S30403	S31603	S31803/S32205	S32750
Cr (%)	18-20	16-18	21-23	24-26
Ni (%)	8-12	10-14	4.5-6.5	6-8
Mo (%)	-	2-3	2.5-3.5	3-5
N (%)	-	-	0.08-0.20	0.24-0.32
Yield (min)	170 MPa	170 MPa	450 MPa	550 MPa
Tensile (min)	485 MPa	485 MPa	620 MPa	795 MPa
PREN	~18	~24	~35	~42
SCC resistance	Low	Low-moderate	High	Very high

PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3(%Mo) + 16(%N). Duplex grades score significantly higher than austenitic, providing quantifiable corrosion resistance advantage.

Advantages of Duplex

• Higher strength: Min. yield 450 MPa (2205) vs. 170 MPa (316L) allows thinner walls and lighter piping
• Chloride SCC resistance: Far superior to 304/316 at temperatures up to ~250°C
• Pitting and crevice resistance: PREN >35 (2205) makes it suitable for seawater-contacted systems
• Lower nickel content: Less susceptible to nickel price volatility than austenitic grades
• Good fatigue resistance: Important for cyclic-loading environments (offshore, subsea)

Applications

• Offshore topside and subsea process piping
• Desalination plants (seawater reverse osmosis systems)
• Chemical processing (organic acids, chloride-containing media)
• Pulp and paper bleaching lines
• Oil and gas production facilities (produced water handling)
• FPSOs and semi-submersible platforms

Limitations

• Maximum continuous operating temperature ~300°C (above this, sigma phase embrittlement occurs)
• Requires careful welding: heat input must be controlled (typically 0.5-2.5 kJ/mm) to maintain 40-60% ferrite balance
• Welding filler metals typically over-alloyed with nickel (e.g., ER2209) to compensate for nitrogen loss
• Not suitable for cryogenic service (ductile-to-brittle transition above -40°C)

Common Mistake

Specifying duplex pipe without verifying the ferrite content in the weld zone. Too much ferrite (>70%) reduces toughness and corrosion resistance; too little (<30%) reduces SCC resistance. Always require ferrite measurement per ASTM E562 or magnetic methods in the pipe inspection plan.

For a comparison of standard duplex with super duplex, see duplex vs super duplex.

Read the full guide to pipe types