PREN calculator
PREN tells how well stainless steels, duplex and nickel alloys resist pitting in chlorides. Pick the material from the catalog or enter the composition from the certificate.
1You enter (wt. %)
Typical composition = middle of the standard's ranges, taken from each material's data sheet. Use the certificate composition for the actual value.
PREN of the catalog materials
| Material | Group | Minimum PREN | Typical PREN |
|---|---|---|---|
| Hastelloy C-276 | Nickel alloys | 68.9 | 74.5 (with W) |
| Hastelloy C-22 | Nickel alloys | 65.4 | 70.8 (with W) |
| Hastelloy C-4 | Nickel alloys | 60.2 | 67.2 |
| Inconel 625 / Alloy 625 | Nickel alloys | 46.4 | 51.2 |
| 254 SMO / F44 | Stainless steels | 42.2 | 44.1 |
| Super duplex 2507 / F53 / S32750 | Duplex | 37.7 | 42.7 |
| Super duplex Zeron 100 / F55 / S32760 | Duplex | 37.9 | 41.8 (with W) required ≥ 40 (NORSOK M-630 and manufacturer) |
| Ferralium 255 / F61 | Duplex | 35.2 | 39.5 |
| Alloy 28 | Stainless steels | 35.9 | 38.6 |
| 904L stainless steel | Stainless steels | 32.2 | 36.7 |
| Duplex 2205 / F60 / S32205 | Duplex | 34.1 | 35.9 |
| Nitronic 50 (XM-19) | Stainless steels | 28.7 | 34.2 |
| Duplex F51 / S31803 | Duplex | 30.5 | 34.1 |
| Incoloy 825 | Nickel alloys | 27.8 | 31.4 |
| 317L stainless steel | Stainless steels | 27.9 | 31.3 |
| Incoloy 925 | Nickel alloys | 27.8 | 30.9 |
| 329 stainless steel | Duplex | 26.3 | 30.5 |
| Inconel 718 | Nickel alloys | 26.2 | 29.1 |
| Alloy 20 (Carpenter 20) | Stainless steels | 25.6 | 28.3 |
| AISI 446 | Stainless steels | 24.6 | 27.8 |
| F138 stainless steel (316LVM) | Stainless steels | 24.4 | 27.5 required ≥ 26 (Cr + 3.3 Mo, ASTM F138) |
| 316Ti stainless steel | Stainless steels | 22.6 | 26.1 |
| 316 / 316L stainless | Stainless steels | 22.6 | 25.3 |
| 310S stainless steel | Stainless steels | 24.0 | 25.0 |
| Inconel 601 | Nickel alloys | 21.0 | 23.0 |
| Incoloy 800HT | Nickel alloys | 19.0 | 21.0 |
| Nitronic 60 | Stainless steels | 17.3 | 20.3 |
| 304L stainless steel | Stainless steels | 18.0 | 19.8 |
| A-286 / Alloy 660 | Stainless steels | 16.8 | 18.9 |
| 321 stainless steel | Stainless steels | 17.0 | 18.8 |
| 347 stainless steel | Stainless steels | 17.0 | 18.0 |
| 17-7PH – UNS S17700 (Type 631) | Stainless steels | 16.0 | 17.0 |
| 17-4PH stainless steel – UNS S17400 (ASTM A564 Type 630) | Stainless steels | 15.0 | 16.3 |
| AISI 431 | Stainless steels | 15.0 | 16.0 |
| Inconel 600 | Nickel alloys | 14.0 | 15.5 |
| ASTM A182 F6NM | Stainless steels | 13.2 | 15.2 |
| 15-5PH – UNS S15500 (XM-12) | Stainless steels | 14.0 | 14.8 |
| AISI 416 | Stainless steels | 12.0 | 14.0 |
| AISI 403 | Stainless steels | 11.5 | 12.3 |
Minimum = lowest Cr, Mo, N (and W) contents of the standard; typical = middle of the ranges. Materials without chromium (Monel, nickel 200, Hastelloy B-2, copper, titanium) do not use PREN.
How to interpret
| PREN | Typical use |
|---|---|
| below 20 | Atmosphere and fresh water without chlorides |
| 20 to 30 | Water with little chloride, mild chemicals |
| 30 to 40 | Moderate chlorides, cold brine |
| 40 or more | Seawater, hot chlorides |
To choose the alloy by medium, temperature and concentration, see steel by corrosive medium. For carbon and low-alloy steel, use the carbon equivalent calculator.
Frequently asked questions
What is PREN?
PREN (Pitting Resistance Equivalent Number) estimates the resistance of stainless steels and nickel alloys to pitting corrosion in chlorides: PREN = %Cr + 3.3 × %Mo + 16 × %N. With tungsten, PRENw = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N is used. The higher, the better.
What PREN is needed for seawater?
For natural seawater the usual reference is a PREN of 40 or more, such as super duplex, 6Mo and nickel alloys. Below that there is a risk of pitting and crevice corrosion, especially above 20 to 30 °C.
Minimum or typical PREN: which one to use?
The minimum comes from the lowest Cr, Mo and N contents of the standard and is what the material guarantees. The typical value uses the middle of the ranges. When the project requires a minimum PREN (for example ≥ 40 for super duplex), the value must be calculated from the heat analysis and stated on the certificate.
Does PREN apply to carbon steel?
No. PREN only makes sense for passive alloys containing chromium (stainless, duplex and nickel alloys with Cr). For carbon and low-alloy steel the useful index is the carbon equivalent, which measures weldability.
