Surface Treatments
Certifications
- ISO 9001:2015
- NORSOK M-650
- NACE MR0175
- ISO 15156-3
- PED 2014/68/EC
- EN 10204 type 3.2
- AD 2000 W2/W7/W10
PREN: Pitting Resistance Equivalent Number Calculation for Super Duplex Stainless Steel
PREN (Pitting Resistance Equivalent Number) is the standard scalar predictor of pitting and crevice corrosion resistance for stainless steels in chloride-bearing environments. The widely used formula is:
PREN = %Cr + 3.3 · %Mo + 16 · %N
PREN ≥ 40 is the threshold defining super duplex stainless steel grades; PREN 30–38 covers standard duplex; PREN 20–25 covers most austenitic stainless steels (304/316L). Higher PREN correlates closely with Critical Pitting Temperature (CPT) measured per ASTM G48 method A.
Q: What is the PREN formula, and how are Cr, Mo, and N weighted?
PREN Formula and Element Weights
The element weights (Cr × 1, Mo × 3.3, N × 16) reflect each element's contribution to passive-film stability in chloride solution per electrochemical and field-corrosion data accumulated over decades. Nitrogen is the most powerful per-percent contributor, which is why super-duplex grades push N up to 0.20–0.32% (limited by solubility and weld-zone porosity concerns above 0.32%). Molybdenum is the second most powerful per-percent contributor; super-duplex grades push Mo up to 3–5% (limited by sigma-phase precipitation susceptibility above 4%).
PREW for W-Bearing Super Duplex (Zeron 100)
Tungsten contributes to pitting resistance similarly to molybdenum but with about half the per-percent effectiveness. The modified PREW formula captures this:
PREW = %Cr + 3.3(%Mo + 0.5 · %W) + 16 · %N
For Zeron 100 (UNS S32760, with ~0.7% W): PREW ≈ 25 + 3.3(3.6 + 0.35) + 4.0 ≈ 42 typical per heat. Without the W contribution, the simpler PREN formula would underpredict Zeron 100's actual pitting resistance.
Q: What PREN values do common stainless steel grades have?
PREN by Stainless Steel Grade
| Grade | UNS | Composition (Cr, Mo, N, W) | Nominal PREN | Class |
|---|---|---|---|---|
| 304L | S30403 | 18.5, 0, 0.05, 0 | ~19 | Austenitic |
| 316L | S31603 | 17, 2.2, 0.06, 0 | ~25 | Austenitic |
| 317L | S31703 | 19, 3.2, 0.08, 0 | ~31 | Austenitic |
| 904L | N08904 | 21, 4.5, 0.05, 0 | ~37 | Super austenitic |
| Duplex 2304 | S32304 | 23, 0.4, 0.10, 0 | ~26 | Lean duplex |
| Duplex 2205 | S31803/S32205 | 22, 3.2, 0.16, 0 | ~35 | Standard duplex |
| Ferralium 255 | S32550 | 25, 3.4, 0.20, 0 | ~40 | Super duplex |
| Super Duplex 2507 | S32750 | 25, 4.0, 0.27, 0 | ~42 | Super duplex |
| Zeron 100 | S32760 | 25, 3.6, 0.25, 0.7 | ~42 (PREW) | Super duplex |
| Hyper duplex S32707 | S32707 | 27, 5.0, 0.40, 0 | ~49 | Hyper duplex |
| 254 SMO | S31254 | 20, 6.1, 0.20, 0 | ~43 | Super austenitic |
| AL-6XN | N08367 | 21, 6.5, 0.22, 0 | ~46 | Super austenitic |
Q: How does PREN correlate with Critical Pitting Temperature (CPT) per ASTM G48?
PREN vs CPT (Critical Pitting Temperature) Correlation
PREN is a calculated value from composition. Critical Pitting Temperature (CPT) is the experimentally measured temperature in 6% FeCl3 solution per ASTM G48 method A. The two metrics correlate closely. Approximate relationship:
- PREN 25 (316L): CPT ~5 °C
- PREN 35 (Duplex 2205): CPT ~25 °C
- PREN 40 (Ferralium 255): CPT ≥ 35 °C
- PREN 42 (Super Duplex 2507 / Zeron 100): CPT ~40 °C
- PREN 49 (Hyper duplex S32707): CPT ~55 °C
For applications above the CPT, pitting initiation and propagation become likely. NORSOK M-650 QTRs require CPT ≥ 35 °C measurement per ASTM G48 method A for Ferralium 255. Per-heat CPT verification documented on EN 10204 type 3.2 MTC.
Q: What does PREN NOT predict, and where does it break down?
PREN Limits and What PREN Doesn't Predict
PREN is a useful first-cut metric but doesn't capture: (1) crevice corrosion (CCT, Critical Crevice Temperature, is a separate test); (2) reducing-acid resistance, Ferralium 255's Cu advantage in HCl and H2SO4 isn't reflected in PREN; (3) sigma-phase embrittlement risk, high-Mo super-duplex grades have higher sigma-phase precipitation tendency; (4) sulphide stress cracking susceptibility, addressed separately by NACE MR0175 hardness and microstructure limits. PREN is necessary but not sufficient for material selection.
Need super duplex stainless steel with PREN >= 40? Email info@torqbolt.com or WhatsApp +91-22-66157017 with form, dimensions, quantity, ASTM/EN spec, MTC level (3.1 / 3.2). NACE MR0175 sour-service stamp on every heat. NORSOK M-650 QTR on request. Per-heat EN 10204 type 3.2 MTC included with every consignment.
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