The Pitting Resistance Equivalent Number (PREN) is the primary metric for ranking the resistance of stainless steels and nickel alloys to chloride-induced pitting and crevice corrosion. This calculator computes both PREN (standard formula) and PRENw (extended formula including tungsten — required for super duplex and some super austenitic grades) from alloy composition.
PREN Calculator — Pitting Resistance Equivalent Number
PREN = %Cr + 3.3×%Mo + 16×%N (ISO 10272, ASTM A923 Annex)
PRENw = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N (includes tungsten contribution)
Critical Pitting Temperature (CPT) in 6% FeCl₃ ≈ PREN − 20°C (approximate)
PREN Reference Values for Common Grades
| Grade (UNS) | Cr% | Mo% | N% | PREN | Service Limit |
|---|---|---|---|---|---|
| 304L (S30403) | 18.5 | — | 0.06 | ~19 | Mild, non-chloride |
| 316L (S31603) | 17.5 | 2.2 | 0.06 | ~25 | Cooling water, mild Cl⁻ |
| 317L (S31703) | 19.0 | 3.3 | 0.06 | ~30 | Chemical plant |
| 2205 Duplex (S31803) | 22.0 | 3.1 | 0.17 | ~35 | Seawater ambient |
| SAF 2507 (S32750) | 25.0 | 4.0 | 0.28 | ~42 | Hot seawater to 40°C |
| 254 SMO (S31254) | 20.0 | 6.1 | 0.20 | ~43 | Seawater heat exchangers |
| Alloy 625 (N06625) | 21.5 | 9.0 | 0.02 | ~51 | Aggressive acids and brine |
References
- ISO 10272-3 Corrosion of metals and alloys — Pitting resistance equivalent numbers.
- ASTM A923 Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels.
- Sedriks, A.J., Corrosion of Stainless Steels. 2nd ed. Wiley-Interscience, 1996.
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