When a buyer asks "Will this grade survive seawater?" the most useful number you can give is the PREN — the pitting resistance equivalent number. It turns a stainless steel's chemistry into a single figure that predicts how well it resists chloride pitting. Suppliers who understand PREN can recommend grades confidently and avoid costly corrosion failures.
What is PREN?
PREN is a calculated index based on the elements that most improve resistance to pitting in chloride environments: chromium, molybdenum and nitrogen. The most common formula is:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Molybdenum is weighted 3.3 times and nitrogen 16 times because they are so effective against pitting. A higher PREN means better resistance to pitting and crevice corrosion.
Some standards use a variant that includes tungsten (for example in super duplex grades like UNS S32760), but the formula above is the one most buyers quote.
Typical PREN values
| Grade | Type | Approx. PREN |
|---|---|---|
| 304 / 304L | Austenitic | 18–20 |
| 316 / 316L | Austenitic | 23–26 |
| 317L | Austenitic | 28–30 |
| 904L | Super austenitic | 32–36 |
| 2205 (S31803/S32205) | Duplex | 34–36 |
| 2507 (S32750) | Super duplex | 40–43 |
| 6Mo (S31254) | Super austenitic | 42–45 |
Values vary with actual chemistry, which is why many oil and gas specifications set a minimum PREN — often 40 for super duplex — and require it to be calculated from the actual heat analysis on the mill certificate.
Types of corrosion buyers worry about
Pitting corrosion
Small, deep holes caused by local breakdown of the passive film, usually by chlorides. Pitting can perforate a pipe while the rest of the surface looks perfect. This is exactly what PREN predicts.
Crevice corrosion
Similar to pitting but happens in tight gaps — under gaskets, deposits, washers or in socket weld gaps. It occurs at lower temperatures than pitting, so a grade that resists pitting may still suffer crevice attack.
Stress corrosion cracking (SCC)
Cracking caused by tensile stress plus a corrosive environment, especially hot chlorides. Standard austenitic grades like 304 and 316 are susceptible above about 60°C. Duplex steels resist chloride SCC much better.
Intergranular corrosion
Attack along grain boundaries after sensitisation from welding. Low-carbon "L" grades prevent it — see SS 316 vs 316L.
General and galvanic corrosion
Uniform attack in strong acids, and corrosion when dissimilar metals are coupled in an electrolyte.
How to use PREN in grade selection
PREN is a quick screening tool. As a rough guide:
- PREN below 25: indoor, freshwater, mild environments — 304, 316L.
- PREN 25–35: brackish water, coastal atmospheres, many chemical services — 317L, 904L, duplex 2205.
- PREN 40 and above: seawater, hot chlorides, offshore — super duplex, 6Mo.
Temperature matters too. Laboratory tests such as ASTM G48 (ferric chloride test) measure the critical pitting temperature (CPT) and critical crevice temperature (CCT). Many offshore specifications require G48 testing alongside PREN.
For a detailed comparison of the most popular upgrade, read duplex 2205 vs 316L.
Limits of PREN
PREN is useful, but it is not the whole story:
- It ignores microstructure — poorly heat-treated duplex with harmful phases can fail despite good PREN.
- It does not cover acids, high temperatures or SCC directly.
- Surface condition matters: weld heat tint and iron contamination reduce resistance. See pickling and passivation.
- Crevices and deposits can cause attack at lower temperatures than PREN suggests.
When even super duplex is not enough, nickel alloys take over — see Inconel vs Monel vs Hastelloy. Always combine PREN with service experience, the project specification and, where needed, corrosion testing.
A worked PREN example
Take a 316L heat with 16.8% chromium, 2.05% molybdenum and 0.05% nitrogen:
PREN = 16.8 + (3.3 × 2.05) + (16 × 0.05) = 16.8 + 6.77 + 0.80 = 24.4
Now a super duplex 2507 heat with 25.2% chromium, 3.8% molybdenum and 0.27% nitrogen:
PREN = 25.2 + (3.3 × 3.8) + (16 × 0.27) = 25.2 + 12.54 + 4.32 = 42.1
Both heats meet their grade specification, but the super duplex value is almost double — which is why it survives in seawater while 316L may pit. Notice also that two heats of the same grade can give different PREN values. A 316L heat at the bottom of the chromium and molybdenum ranges can fall close to 23, while one at the top can exceed 26. For critical projects, buyers may ask you to select heats with a higher PREN, so keep heat-wise chemistry handy when you quote.
Showing PREN on certificates
Many buyers of duplex and super duplex pipes, fittings and flanges require the actual PREN value on the certificate, calculated from heat chemistry. If you prepare certificates in-house, a template-based tool such as MTCMitra helps you present these values consistently so buyers see compliance at a glance. Our EN 10204 3.1 vs 3.2 guide explains certificate types.
Frequently asked questions
What PREN is needed for seawater?
A PREN of 40 or higher is commonly specified for seawater service, which points to super duplex or 6Mo grades.
Is a higher PREN always better?
For chloride pitting, yes. But higher-PREN grades cost more and may be harder to fabricate, so choose the lowest grade that is safe for the service.
Does PREN apply to carbon steel?
No. PREN is meaningful only for stainless steels and similar corrosion-resistant alloys that rely on a passive film.
Why does nitrogen count 16 times?
Nitrogen strongly stabilises the passive film and austenite phase, so even small additions significantly improve pitting resistance.
Key takeaways
- PREN = %Cr + 3.3 × %Mo + 16 × %N.
- 304 ≈ 19, 316L ≈ 24, duplex 2205 ≈ 35, super duplex ≥ 40.
- PREN predicts chloride pitting, not every corrosion type.
- Use PREN with service conditions, testing and good surface finish.
When you can explain why a grade will last, buyers stop comparing you only on price. Quote the right grade clearly with QuoteMitra.
