What is PREN and what PREN do I need for seawater?

PREN is a single number calculated from composition that lets you compare pitting resistance across grades: PREN = %Cr + 3.3 x %Mo + 16 x %N.

Typical values: 304L about 19, 316L about 24, 904L about 34, duplex 2205 about 35, super duplex 2507 about 42, and Alloy 625 well above 50.

Pitting is driven by chloride concentration and temperature together, so PREN alone is not a specification — it is a screening tool. The critical pitting temperature (CPT) test to ASTM G48 is the measured answer, and for seawater service most project specifications call for G48 Method A results on the actual heat.

For Gulf seawater, which is warmer and more saline than North Sea water, the common selections are: 316L only for splash-zone and non-critical duty, 2205 for brackish water, produced water and intermittent seawater, 2507 or 6Mo for continuous seawater service and firewater ring mains, and 90/10 or 70/30 cupro nickel for seawater piping and condenser tubes where biofouling resistance also matters.

Crevice corrosion is always worse than open-surface pitting — gaskets, tube-to-tubesheet joints and under-deposit areas need a grade one step above what the bulk water would suggest.