Duplex 2205 vs 316L:
Which for Seawater Service?
316L has been the "default" stainless for decades. But in seawater — especially above 25°C, with crevices, or under cathodic protection — 316L fails. Duplex 2205 costs 20–30% more per kilogram, but its corrosion resistance and strength often make it the lower total-cost choice.
Why 316L Fails in Seawater
316L has a PREN (Pitting Resistance Equivalent Number) of 22–25. Seawater typically requires PREN > 35 for reliable service. The result:
• Pitting: Starts within weeks in warm seawater (>25°C)
• Crevice corrosion: Under gaskets, flanges, and deposits
• Stress corrosion cracking (SCC): Above 60°C with tensile stress — catastrophic failure mode
• MIC (Microbiologically influenced corrosion): SRB colonies thrive on 316L surfaces
Pitting Resistance: The Numbers
| Property | 316L | Duplex 2205 | Advantage |
|---|---|---|---|
| PREN | 22–25 | 34–38 | 2205: +55% |
| CPT (°C, 6% FeCl₃) | 15–20 | 40–50 | 2205: +150% |
| CCPT (°C) | <5 | 25–35 | 2205: critical for flanges |
| SCC threshold (°C) | 60 | >150 | 2205: essentially immune |
Mechanical Strength
| Property | 316L | Duplex 2205 |
|---|---|---|
| Yield Strength (MPa) | 170 | 450 |
| Tensile Strength (MPa) | 485 | 620 |
| Design Stress (ASME, MPa) | 115 | 260 |
Duplex 2205 has 2.6x the yield strength of 316L. This means you can use thinner wall sections for the same design pressure, often offsetting the higher per-kg cost entirely.
Cost Analysis: Real Projects
| Component | 316L Cost | 2205 Cost | Notes |
|---|---|---|---|
| Pipe 6" SCH10 (per meter) | $85 | $95 | 2205: thinner wall possible → net savings |
| Heat exchanger tube bundle | $12,000 | $14,500 | 2205: 3x longer service life |
| Pump casing (cast) | $3,200 | $4,100 | 2205: no pitting replacement cost |
| Flange ANSI 300# 6" | $280 | $340 | 2205: no crevice corrosion at gasket |
Bottom line: When you factor in reduced wall thickness (weight savings), longer service intervals, and avoided failure costs, Duplex 2205 is typically 15–30% cheaper on a lifecycle basis for seawater applications.
When to Still Use 316L
316L remains appropriate for:
• Cold seawater (<15°C) with continuous flow (no stagnation)
• Intermittent splash zone exposure with regular washdown
• Budget-constrained projects where replacement is acceptable
• Non-critical, low-stress components
When Duplex 2205 Is Mandatory
• Seawater above 25°C (tropical/middle east)
• Creviced joints (flanges, gaskets, under deposits)
• Service temperatures above 60°C (SCC risk zone for 316L)
• Desalination plants (MED/MSF/SWRO)
• Offshore platforms (NORSOK M-001 requires PREN > 40 for seawater)
• Firewater systems (stagnant + chlorinated)
Super Duplex vs Duplex
For the most aggressive seawater conditions (hot, high-chloride, high-H₂S), consider Super Duplex 2507 (PREN 40–43) or 6Mo grades like 254SMO (PREN 43+). These are the next step up when 2205 is borderline.
Need Seawater Grade Material?
We stock Duplex 2205 and Super Duplex 2507 in plate, pipe, and bar. Send your project spec for same-day pricing.
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