Article
Pitting is a localised form of corrosion that drills small holes into stainless steel, mainly in chloride environments such as seawater, brine and some process fluids. PREN gives a quick way to compare how well different grades resist it.
The formula
PREN = %Cr + 3.3 × %Mo + 16 × %N
Chromium, molybdenum and nitrogen are the elements that most improve pitting resistance. Some formulas for duplex grades use 30 × %N, and some add tungsten, but the version above is the most widely quoted.
Typical values
| Grade | Type | Typical PREN |
|---|---|---|
| 304 / 304L | Austenitic | 18–20 |
| 316 / 316L | Austenitic | 23–26 |
| 317L | Austenitic | 28–30 |
| 2205 (F51) | Duplex | 34–36 |
| 904L | Super austenitic | 34–36 |
| 6Mo (254 SMO) | Super austenitic | 42–44 |
| 2507 (F53) | Super duplex | > 40 |
Values depend on the actual chemistry of each heat, which can be calculated from the MTC.
How PREN is used
- As a ranking tool when comparing grades for chloride service.
- As a minimum requirement in specifications, for example "PREN ≥ 40" for super duplex in seawater systems.
- To check that a heat is at the rich end of a grade's range when service is borderline.
Limits of PREN
PREN does not account for temperature, pH, oxygen, crevices, surface finish, welding or microstructure. Two materials with the same PREN can behave differently. For important decisions, critical pitting temperature (CPT) tests per ASTM G48 or G150, and experience in similar service, should be considered alongside PREN.
Frequently asked questions
What is a good PREN for seawater?
Grades with PREN of 40 or more, such as super duplex 2507 and 6Mo, are typically selected for seawater service.
Does nitrogen really matter in PREN?
Yes. Nitrogen has a strong effect, which is why it carries a factor of 16 (or 30 in some duplex formulas).
Can PREN be calculated from an MTC?
Yes. Use the actual Cr, Mo and N values reported on the certificate.
Key takeaways
- PREN = Cr + 3.3 Mo + 16 N.
- 316L is around 24; duplex 2205 around 35; super duplex above 40.
- Use PREN to rank grades, not as the only selection criterion.
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