What the number is for
PREN turns alloy selection into a first-pass screening: for a chloride-bearing service, alloys can be shortlisted by whether their PREN — and the critical pitting temperature that correlates with it — offers margin over the service conditions. Indicative values: 304 ≈ 18-20, 316 ≈ 23-28, duplex 2205 ≈ 33-35, super duplex and 6Mo grades ≥ 40. The reason 316 outperforms 304 in chloride service is visible in the formula: its molybdenum.
What the number is not
PREN describes the alloy, not the installation. It assumes a clean, passive, properly finished surface in the stated bulk chemistry — and pitting in real equipment is dominated by everything that assumption excludes: crevices, deposits, stagnation, weld heat tint, embedded iron, evaporative concentration. A duplex under a wet deposit can lose to a well-passivated 316. Use PREN to shortlist, then let the service factors decide.
Where the fight actually happens
For the mechanism the number screens against — the self-accelerating pit and its defenses — see the parent article linked below.