PREN (Pitting Resistance Equivalent Number) ranks our alloys' pitting-corrosion resistance on one scale, using chromium, molybdenum and nitrogen content. A higher PREN means greater resistance to pitting in chloride environments. As a benchmark, 316L stainless steel has a PREN of about 25; our range spans grades from around 30 up to 45.
PREN = %Cr + 3.3 x %Mo + 16 x %N
Work out the PREN for any composition with our PREN calculator.
PREN compares grades against each other on a single scale; it does not predict absolute performance in a specific service condition. For how the number is derived and used, see Understanding PREN.
PREN 30 to 40
These grades offer a clear step up from standard stainless steels.
| Grade | Designation | PREN | Notes |
|---|---|---|---|
| Alloy 2205 | S32205/S31803, F60/F51, 1.4462 | ~34 | A 22% chromium duplex with around twice the strength of 316L. |
| Fermonic® 50 | S20910, XM-19, Nitronic® 50 | ~34 | A high-strength austenitic stainless that stays tough at cryogenic temperatures and is non-magnetic. |
| Alloy 825 | N08825, Incoloy® 825 | ~31 | An entry-level nickel alloy usable across a wide temperature range. |
| Alloy 925 | N09925, Incoloy 925 | ~31 | A higher-strength development of 825. |
| Alloy 718 | N07718, Inconel® 718 | ~31 | A precipitation-hardened nickel alloy that retains its strength to around 700°C and resists sulphide stress cracking. |
PREN above 40
Super duplex and high-performance nickel alloys sit at the top of the scale.
| Grade | Designation | PREN | Notes |
|---|---|---|---|
| Ferralium® 255 | S32550, F61, 1.4507 | >40 | The original super duplex stainless steel, developed by Langley Alloys in the 1960s. Its ~2% copper addition forms a copper-sulphide layer that improves resistance in acidic and chloride conditions. |
| Alloy 32750 | S32750, F53, 1.4410 | >40 | A super duplex grade developed in the 1980s by Alleima. |
| Alloy 32760 | S32760, F55, 1.4501, Zeron 100® | >40 | A super duplex with up to 1% copper and 1% tungsten. |
| Alloy 625 | N06625, Inconel 625 | ~45 | A versatile corrosion- and heat-resistant nickel alloy. |
| Alloy 725 | N07725 | ~45 | A high-strength, age-hardenable development of 625. |
The role of copper
Small additions of copper improve the corrosion resistance of several of these grades, particularly in acidic and reducing conditions. We look at this in more detail in our technical papers TP44 and TP49.
Calculate PREN for your own composition with our PREN calculator.