Understanding PRE — Pitting Resistance Equivalent and When It Is Not Enough
PRE is a single number that summarises an alloy's resistance to chloride pitting. It is one of the most useful tools in corrosion-resistant material selection — and one of the most misapplied.
The PRE Formula
PRE = %Cr + 3.3×%Mo + 16×%N
The coefficients reflect relative effectiveness: molybdenum is 3.3× more effective than chromium per percent at blocking pit initiation; nitrogen in austenitic and duplex steels is 16× more effective. For duplex grades, the formula sometimes includes tungsten: PRE = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N.
PRE Hierarchy (typical values)
- 1.4307 (304L): PRE ~18
- 1.4404 (316L): PRE ~24
- 1.4462 (Duplex 2205): PRE ≥35
- 1.4410 (Super Duplex 2507): PRE ≥40
- 1.4547 (254 SMO): PRE ≥43
- 2.4856 (Inconel 625): PRE ~51
- 2.4819 (Hastelloy C-276): PRE ~73
When PRE Is Not Enough
PRE predicts behaviour in bulk, unstirred, ambient-temperature chloride — typically the ASTM G48 test condition. Real process environments deviate in several ways:
- Temperature: Critical pitting temperature (CPT) drops as temperature rises. Super Duplex 2507 (PRE ≥40) has a CPT of approximately 50°C in seawater — above that temperature even a PRE ≥40 alloy can pit.
- Crevice geometry: Crevice corrosion occurs at lower chloride concentrations and temperatures than open-surface pitting. A flange face, threaded connection, or under-deposit crevice can corrode an alloy that would be stable in a bulk test.
- Gulf seawater: The combination of warm temperature (~30–35°C ambient seawater), high salinity, and crevices means Duplex 2205 (PRE ≥35) is marginal for Gulf seawater pump and valve bodies. Super Duplex 2507 is the minimum reliable choice.
Use PRE as a first filter, not a final answer. For chloride-critical applications, verify against critical pitting temperature data in the specific environment.