PREN explained for stainless and duplex steels
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What the pitting resistance equivalent number calculates, the common Cr-Mo-N formula and tungsten variant, and why PREN is only a screening index rather than a corrosion guarantee.
PREN stands for Pitting Resistance Equivalent Number. You may also see the shorter term PRE.
It is a composition-based index used to compare the expected resistance of stainless steels to pitting corrosion, especially in chloride-containing environments.
The key word is compare.
PREN is useful for screening alloys, but it is not a corrosion rate, a guaranteed service temperature or a certificate of suitability for seawater, H2S or any specific process fluid.
The most common formula
A widely used form is:
PREN = %Cr + 3.3 × %Mo + 16 × %N
where Cr, Mo and N are the mass percentages of chromium, molybdenum and nitrogen.
Outokumpu uses this formula in its duplex stainless-steel technical literature and explicitly describes PRE as a rough material-comparison tool.
What do the alloying elements represent?
The formula gives weight to three elements that strongly influence localized corrosion resistance:
- chromium supports formation and repair of the passive film;
- molybdenum improves resistance to localized attack in many chloride environments;
- nitrogen strongly improves pitting resistance and is particularly important in duplex stainless steels.
The coefficients are empirical weighting factors. PREN should not be interpreted as a fundamental physical property such as density or yield strength.
What about tungsten?
For tungsten-bearing grades, a modified expression is often used:
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N
Other formulas also exist, including versions with different nitrogen multipliers.
This means two sources can produce different PREN values for the same alloy if they use different formulas.
Always state the formula when PREN is a contractual requirement.
Typical comparative values
Using typical producer chemistry, Outokumpu publishes approximate PRE values such as:
| Grade family | Typical PRE |
|---|---|
| 304L / 1.4307 | about 18 |
| 316L / 1.4404 | about 24 |
| 2205 / 1.4462 | about 35 |
| 2507 / 1.4410 | about 43 |
These are useful for understanding the ranking. They should not be copied into a purchase specification as guaranteed heat values unless the specification explicitly defines how PREN is to be calculated and what chemistry basis is to be used.
Actual heat chemistry vs nominal chemistry
A PREN calculation can use:
- typical/nominal producer chemistry;
- minimum specified chemistry;
- maximum specified chemistry;
- the actual heat analysis from the MTC.
Those methods can produce different numbers.
If a project says “PREN ≥ 40,” the buyer should determine whether the requirement applies to:
- the actual heat analysis;
- a specified formula;
- base metal only or also weld metal;
- minimum values for individual elements;
- a particular product standard or project MDS.
Do not assume.
PREN does not measure every corrosion mechanism
PREN is mainly a localized-corrosion screening index. It does not directly predict:
- uniform corrosion in a particular acid;
- crevice geometry effects;
- galvanic corrosion;
- stress-corrosion cracking;
- microbiologically influenced corrosion;
- erosion-corrosion;
- sour-service cracking limits.
For oil-and-gas sour service, see NACE MR0175 / ISO 15156 applicability.
PREN is not the same as CPT or CCT
A more direct corrosion comparison may use laboratory tests such as:
- CPT — critical pitting temperature;
- CCT — critical crevice temperature.
Outokumpu notes that CPT testing can be more reliable than PREN for ranking actual pitting behaviour because it tests the material rather than only calculating from bulk chemistry.
ASTM G48 and ASTM G150 are examples of standards used in localized-corrosion testing, depending on the specified method.
Why welds can complicate PREN
A base-metal heat analysis does not automatically describe the local chemistry and microstructure of a weld or heat-affected zone.
Weld filler composition, nitrogen loss, dilution, heat input and phase balance can all affect actual corrosion performance.
Therefore a project may combine minimum PREN with welding qualification and corrosion testing rather than relying on the number alone.
Examples within MetalMate Knowledge
For related alloy families, see:
MetalMate takeaway
PREN is a useful composition-based ranking tool for pitting resistance, not a corrosion guarantee. State the formula, define whether actual heat chemistry is required, and use project-specific corrosion tests and service limits where the application demands more than a screening number.
Related Knowledge
References
- Outokumpu, Forta duplex datasheet — PRE formula, limitations and typical PRE values: https://www.outokumpu.com/de-de/industries/-/media/files/products/forta/outokumpu-forta-range-datasheet.pdf
- British Stainless Steel Association, Calculation of pitting resistance equivalent numbers (PREN): https://bssa.org.uk/bssa_articles/calculation-of-pitting-resistance-equivalent-numbers-pren/
- Alleima, SAF 3007 technical data — tungsten-modified PRE formula example: https://www.alleima.com/contentassets/37f3cde16c2e403ab17667d128f11fad/datasheet-saf-3007-for-umbilicals-en-v2026-02-17-1757-version-1.pdf/download
Standards referenced
- ASTM G48 context
- ASTM G150 context
- API/ISO project formulas may vary
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PREN explained for stainless and duplex steels
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