2205 (S32205/S31803) and 2507 (S32750) are the common duplex stainless steels. Selection depends on how severe the corrosion is and the required strength:

| Item | 2205 | 2507 | |---|---|---| | PREN | about 34–36 | about 42–43 | | Yield strength | ≥450 MPa | ≥550 MPa | | Chloride/crevice resistance | good | better | | Typical use | chemical tanks, FGD, seawater piping | desalination, oil & gas, deep sea | | Processing/welding | mature | narrower window, qualification needed |

Guidance:

1) For general high-chloride service (cooling water, FGD), 2205 is usually sufficient. 2) For high temperature + high chloride, severe crevices or longer life, choose 2507. 3) 2507 demands tighter welding and forming control, with WPS/PQR and ferrite content control. 4) Do not move to 2507 blindly on cost-sensitive projects; assess measured medium data first.

The key with duplex is the ferrite/austenite balance: cooling rate and heat input after welding affect life more than the grade selection alone.