Calculate PREN from chromium, molybdenum, tungsten and nitrogen, compare 316L, 904L, 2205 and 2507, and plan ASTM G48 or ISO 15156 review.
Calculate the pitting resistance equivalent number for austenitic, duplex and super duplex stainless steels, compare common UNS grades, and see when a project needs ASTM G48 testing or an ISO 15156 sour-service review. The result is a screening index, not a corrosion guarantee.
Change the inputs to match the project. The result updates immediately and is not stored.
Values are mass percent. A mill certificate heat analysis is more reliable than a grade midpoint. Tungsten is zero for the listed stainless grades.
ASTM A789 / A790 duplex grade; verify ferrite-austenite balance and heat treatment. The preset is a shortlisting aid, not a purchase specification.
Pitting resistance equivalent number
35.9
Formula: Cr + 3.3 x (Mo + 0.5 x W) + 16 x N
At this composition, PREN is a screen for chloride pitting resistance. It does not include crevice geometry, welding, surface finish, temperature, pH or H2S.
| Grade | UNS | Midpoint PREN |
|---|---|---|
| 316L stainless steel | S31603 | 26.1 |
| 904L super austenitic | N08904 | 36.6 |
| 254SMO super austenitic | S31254 | 43.8 |
| 2205 duplex | S32205 / S31803 | 35.9 |
| 2507 super duplex | S32750 | 42.7 |
| Inconel 625 | N06625 | 51.2 |
PREN is a shortlisting number, not a corrosion test. Use it to compare candidate grades, then verify the actual heat and the service environment.
Pick a grade midpoint or enter the heat analysis from the mill certificate. Molybdenum and nitrogen have the largest effect on the result.
Compare the calculated number with the project corrosion allowance. A higher PREN is not automatically the right choice where stress corrosion cracking, sour service or reducing acids govern.
Confirm the test basis. Pitting potential, critical pitting temperature and crevice corrosion are measured differently; the project may call for ASTM G48, ASTM G150 or a specific client test.
The expression Cr + 3.3(Mo + 0.5W) + 16N is an empirical pitting-resistance index used across stainless-steel literature. It is not a single harmonised acceptance test. Different documents may use a slightly different coefficient or treat tungsten differently, so state the formula and the composition basis when the number is part of a specification review.
Using typical midpoint compositions, 2205 (UNS S32205) calculates to about 35 and 2507 (UNS S32750) to about 43. Actual values depend on the heat chemistry, especially molybdenum and nitrogen, so calculate from the mill certificate when the number matters. A PREN above 40 is often used as a shorthand for super duplex, but that label alone does not establish fitness for a stream.
No. PREN ranks resistance to pitting in a simplified way and does not account for crevices, welding, surface finish, temperature, pH, oxygen, chloride concentration or elemental sulfur. A higher-PREN alloy can still fail in a severe crevice or sour environment. Use the index for shortlisting, then verify with ASTM G48 testing, welding procedure review and the project corrosion assessment.
No. ISO 15156 evaluates the specific material, environment and service condition, including H2S partial pressure, in-situ pH, chloride concentration and temperature. PREN is not a substitute for the standard, nor is it a material-selection approval. For sour service, start with the H2S partial-pressure screen and then review the applicable ISO 15156-3 table and limits.
Send the grade, dimensions, quantity, standard and inspection scope. Junda reviews the requirement and returns a technical quotation.