In chloride-saturated environments (seawater injection, desalination), flange material selection is critical. Lab tests and field data confirm UNS S32750 (2507 Super Duplex) outperforms standard 2205 at 10,000ppm Cl⁻, despite a 60-80% cost premium. Below, we dissect the science with lab video evidence and real-world cost analysis.
1. Core Difference: PREN = Power
| Parameter | Duplex 2205 (S32205) | Super Duplex S32750 |
|---|---|---|
| PREN | 34-38 | 42-45 |
| Chemistry | 22Cr-5Ni-3Mo-0.17N | 25Cr-7Ni-4Mo-0.28N |
| Key Advantage | Cost-effective | Chloride immunity |
PREN (Pitting Resistance Equivalent):
PREN = %Cr + 3.3×%Mo + 16×%N
Higher PREN = Superior chloride resistance
2. Lab Video Proof: 10,000ppm NaCl at 60°C
| Test | 2205 Result | S32750 Result | Video Time |
|---|---|---|---|
| ASTM G48 Method A (72h) | Pitting at 35°C | No pitting at 75°C | 00:45-01:20 |
| Crevice Corrosion (ASTM G78) | 0.8mm attack | 0.0mm attack | 02:15-03:05 |
| Electrochemical | Breakdown at 450mV | Passive to 950mV | 04:30-05:10 |
Key Observations:
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2205: Severe crevice corrosion under Teflon blocks (video 02:35).
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S32750: Mirror-like surface after 168h exposure (video 05:45).
3. Real-World Performance: North Sea Seawater Injection
| Metric | 2205 Flanges | S32750 Flanges |
|---|---|---|
| Environment | 32,000ppm Cl⁻, 40°C | Identical |
| Inspection (24mo) | 0.5mm pitting, 2 leaks | Zero corrosion |
| Maintenance Cost | $18,200/flange | $0 |
| MTBF | 4 years | 25+ years |
4. When to Upgrade from 2205 to S32750
Switch to S32750 if:
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Chlorides >5,000ppm AND Temperature >40°C
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H₂S presence (>50ppm)
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Critical systems (subsea manifolds, injection pumps)
Cost-Benefit Threshold: S32750 pays back in 3-5 years if chloride-induced downtime exceeds $100K/year.
5. Procurement & Fabrication Alerts
Material Verification
- RED FLAG: Suppliers claiming "S32750" with Cr<24.5% or N<0.24% + REQUIRE: Mill certs showing ACTUAL chemistry (Cr 24-26%, Mo 3-4.5%, N 0.24-0.32%)
Welding Protocol
| Step | 2205 Procedure | S32750 Critical Fix |
|---|---|---|
| Filler Metal | ER2209 | ER2594 (Ni-alloyed) |
| Interpass Temperature | ≤150°C | ≤100°C |
| Post-Weld Treatment | Optional | Solution Anneal + Quench |
-
*00:22*: Sigma phase in S32750 HAZ (improper cooling).
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*01:15*: Proper microstructure after solution annealing.
6. Total Cost of Ownership (10-Year)
| Cost Factor | 2205 | S32750 |
|---|---|---|
| Flange (DN200, 900LB) | $1,800 | $3,200 |
| Installation | $1,200 | $1,200 |
| Repairs (3x) | $24,000 | $0 |
| Downtime Penalties | $180,000 | $0 |
| TOTAL | $207,000 | $4,400 |
| *S32750 saves $202,600/flange in chloride service.* |
Conclusion: The Chloride Imperative
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<5,000ppm Cl⁻ at <40°C: 2205 is sufficient.
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>5,000ppm Cl⁻ OR >40°C: S32750 is non-negotiable.
Act Now:
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Specify UNS S32750 on POs for chloride service.
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Demand ASTM G48 test reports with ≥45°C CPT.
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Use ER2594 filler and solution annealing.
“In the Arabian Gulf, using 2205 instead of S32750 is a $2M mistake waiting to happen.”
– Materials Engineer, Saudi Aramco Seawater Injection
