PREN is the single most important number when specifying duplex or super duplex fittings for corrosive service. Here is what it means, how it is calculated, and what value to specify.
PREN stands for Pitting Resistance Equivalent Number. It is a calculated index that predicts a stainless steel or duplex alloy's resistance to chloride-induced pitting corrosion. Higher PREN = better pitting resistance. The formula is:
Molybdenum is 3.3× more effective than chromium at improving pitting resistance. Nitrogen is 16× more effective — which is why modern duplex grades are nitrogen-alloyed.
| Grade | Cr % | Mo % | N % | PREN | Recommended Service |
|---|---|---|---|---|---|
| SS 316L | 17 | 2.5 | 0.05 | ~24 | General industrial, low-chloride |
| SS 317L | 19 | 3.5 | 0.05 | ~31 | Moderate chloride, pulp & paper |
| Duplex 2205 (S31803) | 22 | 3.1 | 0.17 | >35 | Offshore, seawater, chemical |
| SS 904L | 21 | 4.5 | 0.05 | ~36 | Sulfuric acid, reducing acids |
| Super Duplex 2507 (S32750) | 25 | 4.0 | 0.28 | >42 | Deepwater offshore, aggressive chloride |
| Super Duplex S32760 | 25 | 3.5 | 0.22 | >40 | North Sea, mixed acid service |
Minimum for most offshore process piping. Duplex 2205 meets this threshold. Required by NORSOK and most major oil company specifications for process piping in contact with seawater or production fluids.
Required for high-chloride or high-temperature seawater service. Super duplex S32760 meets this. Specified when the standard duplex 2205 threshold is marginal for the service conditions.
Super duplex 2507 (S32750) exceeds PREN 42 — required for the most aggressive offshore environments, deepwater production, and FPSOs. This is the threshold where pitting does not initiate in seawater at ambient temperature.
Tell us your PREN requirement or service conditions and we'll recommend the right grade.