Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex 2507 Corrosion Resistance
Super Duplex 2507 (UNS S32750, EN 1.4410) is selected wherever standard 2205 duplex and 6 percent molybdenum austenitic grades are insufficient and where the cost of nickel-base alloys cannot be justified. The corrosion-resistance package combines pitting resistance (PREN 41 or higher, CPT 50 deg C or higher per ASTM G48 Method E), crevice resistance (CCT 35 deg C or higher per ASTM G48 Method F), chloride stress-corrosion cracking (CSCC) resistance from the duplex microstructure, and sulfide stress cracking (SSC) compliance per NACE MR0175 / ISO 15156-3 with hardness restricted to 28 HRC maximum. The alloy performs across seawater, oil and gas (sweet and sour), desalination, dilute reducing acids, oxidising-chloride environments, pulp and paper bleach, and FGD service. This page summarises the principal corrosion mechanisms and the practical service envelopes derived from published mill data and project experience.
Pitting Corrosion (Chloride)
Pitting is localised corrosion at sites where the passive chromium-oxide film breaks down, normally under chloride attack. Resistance is quantified by PREN (calculated from chemistry) and confirmed by CPT (measured by ASTM G48 Method E). Detailed measurement methodology is on the ASTM G48 Method E page; the PREN calculation is on the PREN page.
| Property | Super Duplex 2507 | Acceptance Threshold |
|---|---|---|
| PREN (Cr + 3.3 Mo + 16 N) | 41 to 43 typical | 40 minimum (NORSOK M-630 super duplex) |
| CPT (ASTM G48 Method E) | 50 to 70 deg C typical | 50 deg C minimum per project specification |
| ASTM G48 Method A weight loss (35 deg C, 24 hr) | Below 1.0 g per square metre typical | 4.0 g per square metre maximum (NORSOK M-630) |
Crevice Corrosion
Crevice corrosion initiates in tight, restricted-flow regions where chloride concentrates and oxygen is depleted, locally driving a more aggressive environment than the bulk solution. Common geometries that create crevices: gasket seats on flanges, weld geometry at fillet roots, deposit accumulation on heat-exchanger tubes, marine biofouling on subsea hardware. Critical Crevice Temperature (CCT) is measured by ASTM G48 Method F using a PTFE crevice former.
| Property | Super Duplex 2507 | Acceptance Threshold |
|---|---|---|
| CCT (ASTM G48 Method F) | 35 to 50 deg C typical | 35 deg C minimum per project specification |
Crevice geometry control is part of the design and fabrication, not just material selection. Gasket selection, weld profile, surface finish, and avoidance of dead-leg crevices are essential to avoiding crevice attack even on a super duplex 2507 component.
Chloride Stress-Corrosion Cracking (CSCC)
Austenitic stainless steels (304L, 316L) are notoriously susceptible to CSCC in chloride environments above approximately 60 deg C, especially under tensile stress. The duplex microstructure of super duplex 2507 confers significantly higher CSCC resistance because the ferrite phase blocks crack propagation that nucleates in the austenite. The practical service envelope for super duplex 2507 in chloride-bearing service extends to approximately 250 deg C continuous, well above the limits of austenitic grades.
| Alloy | CSCC Service Limit (rough guide) |
|---|---|
| 304L austenitic | Below 60 deg C in chloride-bearing service |
| 316L austenitic | Below 60 to 80 deg C depending on chloride concentration |
| Duplex 2205 | Up to approximately 200 deg C |
| Super Duplex 2507 | Up to approximately 250 deg C continuous (sigma-phase risk above 300 deg C) |
| Alloy 625, Alloy 825 | Above 250 deg C; selected where super duplex temperature limit is exceeded |
Sulfide Stress Cracking (SSC) and Sour Service
Sulfide stress cracking is hydrogen-induced cracking that occurs in the presence of H2S and aqueous corrosion. NACE MR0175 / ISO 15156-3 qualifies super duplex 2507 for sour service with the principal restriction that hardness must not exceed 28 HRC (lower than the general 33 HRC maximum) and ferrite content must remain within 35 to 65 percent. Specific H2S partial pressure, chloride concentration, pH, and elemental-sulfur limits depend on the application zone classification under ISO 15156-3 Annex A. TorqBolt supplies super duplex 2507 fasteners and forgings with NACE compliance certification on request.
Service Environments
| Environment | Service Envelope | Notes |
|---|---|---|
| Seawater (chlorinated and natural) | Up to 6 m per second velocity, ambient to 60 deg C | Standard for North Sea topside hardware; crevice control mandatory under flange gaskets |
| Oil and gas, sweet (no H2S) | Sub-zero to 250 deg C; chloride to 30,000 ppm | Standard for subsea tubing, manifolds, christmas trees |
| Oil and gas, sour (H2S present) | Per NACE MR0175 / ISO 15156-3 zone classification | Hardness 28 HRC max; ferrite 35 to 65 percent |
| Desalination (MSF, MED, RO) | Brine to brine concentrate factor 2; ambient to 120 deg C | Pump shafts, brine heaters, flash chambers |
| Dilute hydrochloric acid (HCl) | Up to 1 percent at ambient; not recommended above 5 percent | Limited; alloy 625 or higher Ni-base preferred for stronger HCl |
| Dilute sulfuric acid (H2SO4) | Up to 50 percent at ambient; up to 30 percent at 60 deg C | Better resistance from Ferralium 255 (S32550) where copper helps |
| Hydrofluoric acid (HF) | Below 1 percent at ambient only | HF service is generally outside the super duplex envelope; alloy 400 (Monel) preferred |
| Pulp and paper, chlorine dioxide bleach | Standard service; ClO2 plus chloride at 50 to 80 deg C | D-stage and Eo-stage components |
| FGD absorber, scrubber, ducting | WFGD outlet plenum, demister, ducting | Aggressive oxidising chloride plus condensing acid; super duplex is the workhorse |
Service Environments Where Super Duplex 2507 Is Not Recommended
- Continuous service above 300 deg C: progressive sigma-phase precipitation reduces toughness and corrosion resistance over time. Use 904L, 254 SMO, or alloy 825 / 625 instead.
- Cryogenic below approximately minus 80 deg C: Charpy toughness drops below the typical 45 J threshold; austenitic 316L or 304L is preferred for liquid-nitrogen and LNG service.
- Concentrated reducing acids: HCl above 5 percent and H2SO4 above 50 percent at temperature exceed the chromium-oxide passive-film stability range. Nickel-base alloys (alloy 400 for HF, alloy 600 / 625 / Hastelloy for hot HCl) are preferred.
- Ammonia and ammonium chloride at elevated temperature: SCC risk for the austenite phase; alloy 825 (UNS N08825) is the standard alternative.
Related Super Duplex 2507 References
- UNS S32750 reference page
- PREN explainer
- ASTM G48 Method E (CPT)
- Ferrite content (NORSOK M-630)
- Sigma-phase embrittlement
- Seawater applications
- NORSOK M-630
Super Duplex 2507 Corrosion Resistance FAQ
What makes super duplex 2507 corrosion resistant?
Three factors. First, the high chromium (25 percent), molybdenum (3.5 to 4 percent), and nitrogen (0.24 to 0.32 percent) chemistry produces PREN above 41, qualifying the alloy for the most aggressive chloride environments. Second, the balanced 50 to 50 austenite-ferrite microstructure resists chloride stress-corrosion cracking that would attack a fully austenitic alloy. Third, hardness control to 28 HRC maximum (NACE MR0175) qualifies the alloy for sulfide stress cracking resistance in sour service.
What is the CPT of super duplex 2507?
Critical Pitting Temperature per ASTM G48 Method E (6 percent FeCl3) is normally 50 to 70 deg C for material that meets the chemistry and heat-treatment requirements. Mill heats from Sandvik, Industeel, and Outokumpu routinely return CPT in this band. CPT below 50 deg C signals sigma-phase contamination, ferrite imbalance, or out-of-spec chemistry.
Is super duplex 2507 acceptable for sour service?
Yes, qualified per NACE MR0175 / ISO 15156-3 with hardness restricted to 28 HRC maximum and ferrite content within 35 to 65 percent. Specific H2S partial pressure, chloride concentration, pH, and elemental-sulfur limits depend on the application zone classification under ISO 15156-3 Annex A. Sour-service certification on TorqBolt supply is provided on request.
Can super duplex 2507 be used in seawater?
Yes. Super duplex 2507 is the standard topside-hardware material for North Sea offshore platforms, with service experience covering pumps, valves, manifolds, jumpers, christmas trees, fasteners, and pipework. Service envelope is typically up to 6 metres per second velocity at ambient to 60 degrees Celsius. Crevice control under flange gaskets and at weld geometry is essential.
How does super duplex 2507 perform in dilute sulfuric acid?
Reasonable resistance up to about 50 percent H2SO4 at ambient temperature and up to 30 percent at 60 deg C. Stronger or hotter sulfuric acid is generally Ferralium 255 (S32550) territory, where the copper content (1.5 to 2.5 percent) actively suppresses corrosion in reducing acid environments. Super duplex 2507 with no intentional copper is not the optimum alloy for hot strong sulfuric acid.
Why does super duplex resist chloride stress-corrosion cracking better than 316L?
CSCC nucleates and propagates preferentially in austenite phase under chloride attack. In a fully austenitic alloy like 316L, there is no barrier to crack propagation. In duplex stainless, the ferrite phase blocks the propagating crack at every austenite-ferrite interface, dramatically slowing or stopping the crack. The practical service envelope shifts from below 60 deg C for 316L to approximately 250 deg C for super duplex 2507.
Where is super duplex 2507 not the right material choice?
Continuous service above 300 deg C (sigma-phase risk), cryogenic service below minus 80 deg C (Charpy toughness drop), concentrated reducing acids (HCl above 5 percent or H2SO4 above 50 percent at temperature), and ammonia plus ammonium chloride at elevated temperature. For these environments, austenitic 316L (cryogenic), nickel-base alloys (hot acids), or alloy 825 (ammonia) are normally preferred.
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