These three grades account for the majority of stainless steel specified in industrial, chemical, and infrastructure projects worldwide. Yet many procurement and engineering teams default to one without a clear technical reason — either over-engineering with expensive alloys or under-specifying and facing premature corrosion failures. This guide breaks down the differences so you can make an informed, defensible material selection.
Chemical Composition at a Glance
The performance of each grade is a direct result of its alloying additions. The key elements to compare are chromium (Cr), nickel (Ni), molybdenum (Mo), and nitrogen (N).
| Element | SS 304 | SS 316L | Duplex 2205 |
|---|---|---|---|
| Chromium (%) | 18–20 | 16–18 | 21–23 |
| Nickel (%) | 8–12 | 10–14 | 4.5–6.5 |
| Molybdenum (%) | — | 2–3 | 2.5–3.5 |
| Nitrogen (%) | — | — | 0.08–0.20 |
| Carbon (max %) | 0.08 | 0.03 | 0.03 |
| PREN (approx.) | 18–20 | 24–26 | 34–38 |
PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3×%Mo + 16×%N. It is the single most useful index for comparing chloride pitting resistance. A PREN above 25 is generally considered suitable for seawater splash zones; above 32 for continuous seawater immersion.
Corrosion Resistance
SS 304 — General Purpose Austenitic
With a PREN of ~18–20, SS 304 performs well in mildly corrosive environments: fresh water, food processing, architectural applications, and non-halide chemical exposures. It is unsuitable for chloride-containing environments — even moderately elevated chloride concentrations (above ~200 ppm at elevated temperatures) can initiate pitting and crevice corrosion.
SS 316L — Molybdenum-Enhanced Austenitic
The addition of 2–3% molybdenum raises the PREN to ~24–26 and substantially improves resistance to pitting and crevice corrosion in chloride media. 316L is the standard choice for pharmaceutical reactors, seawater cooled heat exchangers (at moderate temperatures), chemical storage tanks, and marine fabrications. The low carbon ("L" grade) reduces sensitization risk at heat-affected zones, making it easier to weld without post-weld heat treatment.
Duplex 2205 — Dual-Phase Microstructure
Duplex 2205 (UNS S31803 / S32205) has a microstructure that is approximately 50% austenite and 50% ferrite. This combination delivers a PREN of 34–38 — significantly higher than 316L. It is highly resistant to pitting corrosion, crevice corrosion, and — importantly — stress corrosion cracking (SCC). SCC is the failure mode that causes austenitic stainless steels to crack unexpectedly under tensile stress in chloride environments, often without visible corrosion beforehand. Duplex 2205 is virtually immune to chloride SCC in most industrial environments.
Mechanical Properties
| Property | SS 304 | SS 316L | Duplex 2205 |
|---|---|---|---|
| Tensile Strength (min) | 515 MPa | 485 MPa | 620 MPa |
| Yield Strength 0.2% (min) | 205 MPa | 170 MPa | 450 MPa |
| Elongation (min) | 40% | 40% | 25% |
| Hardness (max HRB) | 92 | 95 | 31 HRC |
Duplex 2205's yield strength is more than twice that of 316L. In practice, this means engineers can specify thinner wall sections and lighter structures while meeting the same pressure rating — reducing material weight and overall cost of fabricated assemblies such as pressure vessels, pipe spools and offshore structures.
Weldability
SS 304 and 316L are straightforward to weld using TIG (GTAW), MIG (GMAW), and SMAW processes. Low heat input is recommended to avoid sensitization in the heat-affected zone, where carbide precipitation can create chromium-depleted zones susceptible to intergranular corrosion. For 316L, the low carbon content already minimises this risk significantly.
Duplex 2205 requires more careful procedure control to maintain the correct austenite/ferrite phase balance in the weld and heat-affected zones. Use 2209 duplex filler metal, maintain interpass temperatures below 150°C, and ensure adequate heat input to allow austenite reformation. Properly welded 2205 does not require post-weld heat treatment in most applications.
Cost Comparison
Material cost is driven by nickel and molybdenum content. SS 304 is the baseline. SS 316L commands a 15–20% premium due to its molybdenum addition. Duplex 2205 is typically priced 10–20% above 316L on a per-kilogram basis — but because 2205's higher yield strength allows thinner sections, the total installed cost of fabricated components is often comparable to or lower than 316L for the same design pressure.
Application Selection Guide
| Application | Recommended Grade |
|---|---|
| Food processing equipment, kitchen sinks | SS 304 |
| Pharmaceutical reactors and vessels | SS 316L |
| Chemical storage tanks (mild acids) | SS 316L |
| Marine deck fittings and handrails | SS 316L or Duplex 2205 |
| Seawater cooling systems | Duplex 2205 |
| Offshore process piping | Duplex 2205 |
| Water treatment (chlorinated water) | Duplex 2205 |
| Pressure vessels (high pressure service) | Duplex 2205 |
| Architectural cladding and railings | SS 304 |
| Subsea flowlines | Super Duplex 2507 |
Summary
The decision framework is straightforward: if the environment is mildly corrosive and cost is the primary constraint, SS 304 is appropriate. For most industrial chemical and marine environments, SS 316L is the industry-standard choice. When chloride stress corrosion cracking is a risk, or where strength-to-weight ratio matters, Duplex 2205 is the better long-term value despite its higher unit price.
If your project involves continuous seawater immersion, highly acidic chloride media, or severe pitting risk, consider stepping up to Super Duplex 2507 (UNS S32750), which offers a PREN above 40.