PREN Explained: The One Number That Predicts Pitting Resistance (and Its Limits)
Every datasheet in the chloride world quotes PREN, and most buyers use it exactly right: as a fast ranking tool. A few use it as a guarantee — and that ends in crevice corrosion. Here is the number, honestly explained.
The formula
PREN = %Cr + 3.3×%Mo + 16×%N (some versions add 30×%W). It compresses the three pitting-fighting elements into one score. Chromium maintains the passive film; molybdenum (weighted 3.3× because it works far above its percentage) blocks pit growth; nitrogen strengthens both effects.
The reference points worth memorizing
- 316L stainless: PREN ≈ 24 — marginal in seawater, fails in warm chloride service.
- PREN ≥ 40: the traditional "seawater-worthy" threshold (super duplex just crosses it).
- Alloy 625: ≈ 51 — comfortably immune to seawater pitting at ambient temperature.
- C-276: ≈ 68 — near the top of the practical scale.
What PREN ignores — and why it matters
- Crevice corrosion starts at lower thresholds than pitting and is the more common field failure. A PREN-51 alloy that never pits can still crevice-attack under a gasket.
- Temperature. PREN is computed from chemistry; it knows nothing about your 80 °C process stream. Pitting resistance falls as temperature rises.
- pH and oxidizing species. The formula assumes chloride attack. Hot reducing acids are a molybdenum game with different rules.
- Microstructure. PREN uses nominal chemistry. Segregation, sigma phase or a bad weld HAZ can drag the local PREN far below the certificate number — one more reason heat input control matters in fabrication.
How to use it properly
Use PREN to build a shortlist, never to close the decision. Rank the candidates, then overlay crevice geometry, temperature, pH and weld condition. For seawater at ambient temperature with tight crevices, we start the conversation at PREN 50+ — and for anything warmer or more aggressive, we move to corrosion-rate data in the actual medium, which we maintain for the grades we sell.