Pitting Resistance Equivalent (PREN) Calculator
The Pitting Resistance Equivalent Number (PREN) is a widely used index for comparing the pitting corrosion resistance of stainless steels in chloride environments such as seawater, food processing brines, and chemical plant effluents. It is calculated from the chromium, molybdenum, and nitrogen content of the alloy. PREN = %Cr + 3.3*%Mo + 16*%N. For example, Type 316L stainless steel (Cr = 16.5%, Mo = 2.1%, N = 0.03%) has PREN = 16.5 + 3.3*2.1 + 16*0.03 = 24.00. Super-duplex grades target PREN above 40. Enter the alloy composition to compute PREN and see the corrosion resistance category.
PREN formula
PREN = %Cr + 3.3 * %Mo + 16 * %N
Where %Cr, %Mo, and %N are the weight percent chromium, molybdenum, and nitrogen in the stainless steel. Higher PREN indicates better resistance to pitting in chloride environments.
PREN reference values for common grades
Type 304/304L: PREN approximately 19 to 21 (no molybdenum, low nitrogen). Type 316/316L: PREN approximately 23 to 26. Type 2205 duplex: PREN approximately 34 to 38. Type 2507 super-duplex: PREN typically above 42. Alloy 6Mo (254 SMO, AL-6XN): PREN above 42. Materials with PREN above 40 are generally considered suitable for seawater service without risk of pitting at ambient temperature.
PREN: frequently asked questions
What is the PREN number?
The Pitting Resistance Equivalent Number (PREN) is an empirical index of a stainless steel's resistance to pitting corrosion in chloride-containing environments. Higher PREN indicates better pitting resistance. It is calculated as: PREN = %Cr + 3.3*%Mo + 16*%N.
What PREN values indicate good corrosion resistance?
General guidance: PREN less than 25 (ferritic and martensitic grades, e.g., 430, 410) is suitable for mild environments. PREN 25 to 40 (austenitic grades 304, 316) for moderate environments. PREN above 40 (duplex and super-austenitic grades) for severe chloride service such as seawater.
What is PRE_W and when is it used?
PRE_W includes a tungsten contribution: PREN_W = %Cr + 3.3*(%Mo + 0.5*%W) + 16*%N. Tungsten is added to some super-duplex grades (e.g., Zeron 100) for improved corrosion resistance. This calculator uses the standard PREN without tungsten.
Why does nitrogen improve pitting resistance?
Nitrogen stabilizes the austenite phase, increases strength, and importantly enhances pitting resistance by concentrating at the pit initiation site and raising the local pH through nitrate/ammonium chemistry, inhibiting the autocatalytic pit growth cycle.
Is PREN the only factor in corrosion resistance selection?
No. PREN is a first-order screening tool. Actual corrosion resistance also depends on microstructure (phase balance in duplex grades), surface finish (smoother surfaces resist pitting), heat treatment (sensitization from carbide precipitation), weld quality, and the specific chloride concentration, temperature, and pH of the service environment.
Official sources
- ASTM A276/A276M, "Standard Specification for Stainless Steel Bars and Shapes": astm.org.
- NIST, "Corrosion of Stainless Steels": nist.gov.
- ASM International, "ASM Handbook Vol. 13: Corrosion": asminternational.org.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.