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.
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%).
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.
| 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 |
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:
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.
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.
