2205 or 2507: choosing a duplex for seawater duty

Both are duplex stainless steels with roughly double the yield of 316L. The choice between them is set by chloride level, temperature, flow and how much welding the fabrication carries.

Duplex grades solve a specific problem: austenitic stainless steels are strong enough in clean water and far too weak against chloride, while nickel alloys solve the corrosion but cost a great deal. Duplex sits between them, and of the two grades that dominate procurement, 2205 covers most of the market and 2507 covers the difficult end.

	2205	2507
UNS	S31803 / S32205	S32750
Chromium	22.0 – 23.0%	24.0 – 26.0%
Nickel	4.5 – 6.5%	6.0 – 8.0%
Molybdenum	3.0 – 3.5%	3.0 – 5.0%
Nitrogen	0.14 – 0.20%	0.24 – 0.32%
PREN	about 34	about 42 – 43
Yield strength	about twice 316L	higher again, and tougher

2205 is the default

For brackish water, cooling water with moderate chloride, process piping and most structural applications near the coast, 2205 does the job at a price that projects can live with. It welds predictably, is available in every form we roll, and has twenty years of service history behind it.

2507 is for the edges of the envelope

• Chloride levels and temperatures beyond what 2205 tolerates — hot seawater, brine, produced water.
• Erosion-corrosion duty: 2507's higher chromium and nitrogen content holds up better where flow velocity is high.
• Thick sections and pressure-retaining parts that need the extra strength.

The price premium is substantial, so the specification should say why 2507 is required. "Because it is better" is not a reason a project can afford; "because 2205 pitted at 65 °C in 30,000 ppm chloride" is.

The fabrication issue both grades share

Duplex is not austenitic stainless. Welding needs controlled heat input and interpass temperature, the ferrite balance has to be preserved, and post-weld cleaning matters more because the surface is what resists the chloride. A 2507 fabrication welded with 316L procedure habits will underperform a properly welded 2205 job.

We supply both grades as plate, bar, seamless tube and forgings, and we will say plainly when a drawing calls for 2507 where 2205 would have done.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.