Duplex 2205 costs more than SS 316L — but specifying 316L where duplex is needed leads to failures. Here is how to make the right call.
SS 316L is a fully austenitic stainless steel. Duplex S31803 has a two-phase microstructure — approximately 50% austenite and 50% ferrite. This dual-phase structure gives duplex its defining advantages: approximately twice the yield strength of 316L, and significantly better resistance to chloride-induced stress corrosion cracking (SCC) — the most common failure mode for austenitic stainless steel in marine and offshore environments.
| Property | Duplex S31803 (2205) | SS 316L |
|---|---|---|
| Microstructure | Austenite + Ferrite (50/50) | Fully Austenitic |
| Yield Strength (min) | 450 MPa | 170 MPa |
| PREN | >35 | ~24 |
| Chloride SCC Resistance | Excellent | Susceptible above ~60°C |
| Pitting Resistance in Seawater | Good to ~25°C | Prone to pitting in stagnant SW |
| Max Service Temperature | 300°C (sigma phase) | 870°C |
| Wall Thickness vs 316L | ~35% thinner possible | Baseline |
| Cost vs 316L | Higher (~30–50% premium) | Lower |
| Weldability | Good — requires care | Excellent |
Austenitic SS 316L becomes susceptible to stress corrosion cracking in chloride solutions at temperatures above 60°C. Duplex resists SCC due to its ferritic phase. Any hot seawater or brine service above 60°C should use duplex.
Seawater contains ~19,000 ppm chloride. Duplex's PREN >35 provides adequate pitting resistance. SS 316L (PREN ~24) is marginal and has failed in stagnant seawater service.
Where wall thickness must be minimized — weight-critical offshore structures, high-pressure pipework — duplex's 2× yield strength allows thinner walls, offsetting some of the material cost premium.
For cold or ambient temperature service in chloride environments without sustained tensile stress, 316L is often adequate. The SCC risk is temperature-dependent.
Duplex is limited to 300°C due to sigma phase embrittlement risk. For elevated temperature service, SS 316L (or 321/347 for higher temperatures) is the correct choice.
When corrosion assessment confirms 316L is adequate for the specific environment and temperature, the 30–50% cost saving versus duplex is significant for large projects.
We stock both grades. Tell us your service conditions and we can help you select the correct grade.