TECHNICAL TOOL
PREN calculator
Pitting resistance, read by grade or calculated from a heat analysis.
Result
- PREN
- 25,0
Position on the scale
Fresh water, food processing, moderate chemistry
How it works
In grade mode, PREN is read from the grade's datasheet — never recalculated. In composition mode, three formulas are offered from the heat chemistry (EN 10204 3.1 certificate):
- PREN₁₆ = %Cr + 3.3 × %Mo + 16 × %N — the most common
- PREN₃₀ = %Cr + 3.3 × %Mo + 30 × %N — usual for duplex grades
- PREW = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N — tungsten alloys
Reading scale
| PREN range | Reading |
|---|---|
| < 20 | Non-chlorinated environments, indoor atmosphere |
| 20 – 26 | Fresh water, food processing, moderate chemistry |
| 26 – 35 | Brackish water, moderately chlorinated environments |
| 35 – 40 | Severe chlorinated environments, duplex |
| > 40 | Seawater (with reservations on crevice corrosion) |
PREN benchmarks by grade
Published ranges for the 26 grades on this site — to be recalculated heat by heat from the 3.1 certificate.
| Grade | Common designation / ASTM | Family | PREN |
|---|---|---|---|
| Inox 304L | 1.4307 / 304L | Austénitiques | 18–20 ¹ |
| Inox 316L | 1.4404 / 316L | Austénitiques | 24–26 ¹ |
| Inox 316L à molybdène élevé | 1.4435 / 316L | Austénitiques | 26–28 ¹ |
| Inox 316Ti stabilisé titane | 1.4571 / 316Ti | Austénitiques | 24–26 ¹ |
| Inox 321 stabilisé titane | 1.4541 / 321 | Austénitiques | 18–20 ¹ |
| Duplex 2205 | 1.4462 / 2205 | Duplex & super duplex | 34–36 ¹ |
| Super duplex 2507 | 1.4410 / 2507 | Duplex & super duplex | 42–43 ¹ |
| Inox 904L | 1.4539 / 904L | Super austénitiques | 34–36 ¹ |
| Super austénitique 254 SMO | 1.4547 / 254 SMO | Super austénitiques | 43–45 ¹ |
| Alliage 625 | 2.4856 / Alloy 625 | Alliages nickel | 50–54 ¹ |
| Titane grade 2 | 3.7035 / Grade 2 | Titane | does not apply to this family |
| Cupronickel 90/10 | CW352H / 90/10 | Alliages de cuivre | does not apply to this family |
| Cupronickel 70/30 | CW354H / 70/30 | Alliages de cuivre | does not apply to this family |
| 310S — réfractaire austénitique 25-20 | 1.4845 / 310S | Réfractaires | 24–28 † |
| 253 MA — réfractaire austénitique au silicium et cérium | 1.4835 / 253 MA | Réfractaires | 22–25 † |
| 317L — austénitique à haute teneur en molybdène | ≈ 1.4438 / TP317L | Austénitiques | 28–33 |
| 347H — austénitique stabilisé au niobium, tenue au fluage | no exact equivalent / TP347H | Nuances H et stabilisées | 17–19 † |
| 2101 — duplex lean, faible teneur en nickel | 1.4162 / 2101 | Duplex & super duplex | 24–29 |
| 2304 — duplex sans molybdène | 1.4362 / 2304 | Duplex & super duplex | 24–30 |
| 430 — ferritique au chrome, sans nickel | 1.4016 / TP430 | Ferritiques & martensitiques | 16–18 |
| 444 — ferritique au molybdène, stabilisé | 1.4521 / 444 / 18Cr-2Mo | Ferritiques & martensitiques | 23–28 |
| Alliage 825 — nickel-fer-chrome au molybdène, cuivre et titane | 2.4858 / Alloy 825 | Alliages nickel | 28–35 † |
| Alliage 600 — nickel-chrome à 72 % de nickel minimum | 2.4816 / Alloy 600 | Alliages nickel | 14–17 † |
| Alliage 800 — nickel-fer-chrome pour service à chaud | 1.4876 / Alloy 800 | Alliages nickel | 19–23 † |
| C-276 — nickel-chrome-molybdène au tungstène | 2.4819 / C-276 | Alliages nickel | 75 (PREW) † |
| Titane grade 7 — titane non allié au palladium | 3.7235 / Ti Grade 7 | Titane | does not apply to this family |
† PREN calculable but not the deciding factor for this grade — see the caveat below · ¹ legacy value, not cross-checked against a product standard.
PREN caveats (†)
- 310S — réfractaire austénitique 25-20
- ⚠️ PREN is NOT the relevant criterion for a heat-resistant grade. 310S is designed for hot oxidation resistance, not aqueous corrosion: without molybdenum, it isn't suited to chlorinated environments.
- 253 MA — réfractaire austénitique au silicium et cérium
- ⚠️ As with 310S, PREN is not the relevant criterion for a heat-resistant grade: 253 MA is designed for hot oxidation resistance, not aqueous corrosion, and its pitting resistance in aqueous environments is lower than standard austenitics.
- 347H — austénitique stabilisé au niobium, tenue au fluage
- ⚠️ PREN is not the deciding factor for this grade. 347H is chosen for creep resistance and stabilisation, not aqueous corrosion — its pitting resistance is at the level of a 304.
- Alliage 825 — nickel-fer-chrome au molybdène, cuivre et titane
- Indicative only. On an alloy with 38–46% nickel, performance isn't captured by PREN alone: nickel provides resistance to chloride stress corrosion cracking, and the molybdenum-copper pair provides resistance in reducing environments.
- Alliage 600 — nickel-chrome à 72 % de nickel minimum
- ⚠️ A misleading indicator on this alloy. A PREN of 15 is lower than a 304L's, yet Alloy 600's performance comes from its 72% nickel content: resistance to chloride stress corrosion cracking, high-purity water and caustic environments.
- Alliage 800 — nickel-fer-chrome pour service à chaud
- ⚠️ Not relevant: Alloy 800 is chosen for hot oxidation, carburisation and nitriding resistance, not aqueous corrosion.
- C-276 — nickel-chrome-molybdène au tungstène
- Tungsten requires the PREW formula, not PREN₁₆. For reference: 2507 at 42–43, 254 SMO at 43–45.
PREN notes
- 317L — austénitique à haute teneur en molybdène
- Notably higher than 316L (24–26) thanks to molybdenum raised to 3–4%. That's the reason this grade exists.
- 2101 — duplex lean, faible teneur en nickel
- Comparable to a 316L, with nickel reduced to 1.35–1.70% versus 10–14%. That's the whole logic of lean duplex: performance comes from nitrogen and manganese, not nickel.
- 2304 — duplex sans molybdène
- Equivalent to 2101 in PREN, but via a different route: 2304 relies on chromium at 21.5–24.5% where 2101 relies on nitrogen and manganese.
- 430 — ferritique au chrome, sans nickel
- Lower than 304L. 430 is not a corrosion-resistant grade for aggressive environments.
- 444 — ferritique au molybdène, stabilisé
- Comparable to 316L, without a single gram of nickel. That's the whole logic of this grade.
Limits of this calculation
Frequently asked questions
- Which PREN formula should I use?
- PREN₁₆ (Cr + 3.3 Mo + 16 N) is the most common and serves as the default reference. PREN₃₀ (with a nitrogen coefficient of 30) is the usual choice for duplex grades. PREW adds tungsten for alloys that contain it.
- Why doesn't the grade mode PREN match my calculation from the median composition?
- Because grade mode shows the value published on the technical datasheet, not a recalculation. Recalculating from the EN 10088 median composition can give a slightly different figure (25.80 instead of 25 for 316L, for example) without either being "wrong" — they are two different methods.
- Why do titanium and cupronickels have no PREN?
- PREN measures the resistance provided by a Cr-Mo-N alloy (the passive film of stainless and duplex grades). Titanium and cupronickels resist corrosion through a different mechanism — a TiO₂ or cuprous oxide film — to which this index does not apply.
- Is a higher PREN always better?
- No: it is a comparative index, not a rating. A grade with PREN 42 is not automatically better than a grade with PREN 40 for your application — see the Limits section below.
- Is PREN enough on its own to choose a grade for seawater?
- No. It ignores crevice corrosion, service temperature and surface condition. For seawater, a PREN ≥ 40 for duplex and ≥ 45 for austenitic grades is commonly used as a rule of thumb, but a case-by-case review is still needed.