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What does PREN mean?
Short answer
PREN, the Pitting Resistance Equivalent Number, ranks how well a stainless steel or nickel alloy resists chloride pitting. It is calculated as Cr + 3.3 × Mo + 16 × N. SS 304 scores about 19, SS 316L about 24, duplex 2205 about 35 and super duplex 2507 over 40, the usual threshold for seawater.
The formula
The most common form is PREN = %Cr + 3.3 × %Mo + 16 × %N. Some versions add tungsten (Cr + 3.3(Mo + 0.5W) + 16N) or use 30 × N for duplex grades. Always state which formula is being used. Try it in the PREN calculator.
What PREN tells you and what it doesn't
Higher PREN generally means higher resistance to pitting and crevice corrosion in chloride environments. A PREN of 40 or more is the usual threshold for seawater service. PREN doesn't account for temperature, crevice geometry, microstructure (sigma, inclusions), surface finish or weld quality, so it's a ranking tool, not a design guarantee.
Typical values
Values vary with actual heat chemistry. The table shows typical ranges.
Typical PREN values
| Grade | Typical PREN |
|---|---|
| 304 / SS 304L | 18 – 20 |
| 316 / SS 316L | 23 – 25 |
| SS 317L | 28 – 30 |
| SS 904L | 32 – 36 |
| Duplex 2205 | 34 – 35 |
| Super Duplex 2507 | ≥ 40 |
| 254 SMO (6Mo) | 42 – 45 |
| Inconel 625 | ≈ 51 |
| Hastelloy C276 | ≈ 68 – 76 |
Worked example
For 2205 with 22.5% Cr, 3.2% Mo and 0.17% N: 22.5 + 3.3 × 3.2 + 16 × 0.17 = 22.5 + 10.6 + 2.7 ≈ 35.8. Try your own heat chemistry in the PREN calculator.
Why the minimum chemistry matters
PREN calculated from a grade's minimum limits can be several points lower than from its nominal chemistry. Specifications for seawater duplex often state a minimum PREN on the actual heat, for example 40 or 42.5 for super duplex, so the certificate must show it.
Related guides, comparisons and articles
Frequently asked questions
Is a higher PREN always better?
For pitting resistance, yes, but PREN ignores stress-corrosion cracking, temperature limits and fabrication quality. Use it to rank grades, not as a design value.
Do nickel alloys have a PREN?
Yes, the same formula is often applied, and some versions add tungsten as 1.65 × W.
What PREN is needed for seawater?
A PREN of 40 or above is generally used as the threshold for ambient seawater, subject to temperature, chlorination and crevice design.
Are corrosion indices enough to select a material?
No. Indices such as PREN rank alloys, but temperature, crevices, flow, contaminants and fabrication quality all matter. Use service experience, corrosion data or testing for critical applications.
Does Bright Tubes supply the materials in this guide?
Yes. The grades discussed and related products are supplied from stock or mill-sourced, with EN 10204 certificates.