Pitting Corrosion and Critical Pitting Temperature: What PRE Doesn't Tell You
PRE (Pitting Resistance Equivalent) is the single most cited metric for comparing stainless steel and duplex alloys in chloride-containing environments. It is a useful screening tool — but it is not the whole story. Two grades with identical PRE can have completely different real-world performance depending on temperature, chloride concentration, crevice geometry, and weld quality. This article explains what PRE measures, where it fails, and how Critical Pitting Temperature (CPT) completes the picture.
PRE — What It Calculates
PRE = %Cr + 3.3×%Mo + 16×%N
The formula weights molybdenum 3.3× chromium because Mo has a disproportionately strong effect on repassivation kinetics — once a pit initiates, Mo-rich grades heal the pit wall more effectively, interrupting propagation. Nitrogen (×16) strongly suppresses pit initiation and is the reason super duplex 2507 and 254 SMO achieve PRE ≥40+ despite having Cr contents not much higher than 2205.
| Grade | Cr% | Mo% | N% | PRE (typical) |
|---|---|---|---|---|
| 316L (1.4404) | 17 | 2.1 | 0.05 | ~24 |
| 904L (1.4539) | 20 | 4.3 | 0.05 | ~35 |
| Duplex 2205 (1.4462) | 22 | 3.1 | 0.17 | ~35 |
| 254 SMO (1.4547) | 20 | 6.1 | 0.20 | ~43 |
| Super Duplex 2507 (1.4410) | 25 | 3.8 | 0.27 | ≥42 |
| Inconel 625 (2.4856) | 22 | 9.0 | — | >50 (PRE formula not directly applicable to Ni alloys) |
Critical Pitting Temperature (CPT)
CPT is the temperature above which a grade will pit in a specified test solution (typically 6% FeCl₃ per ASTM G48 Method C, or 1M NaCl per ASTM G150). It is the more operationally useful metric because it translates PRE into an actual temperature threshold. A grade with PRE 35 may pit at 20°C in stagnant Gulf seawater at 40°C — but the same grade may perform for years in flowing North Sea water at 10°C.
| Grade | CPT in 6% FeCl₃ (ASTM G48C) | Practical seawater limit (approx.) |
|---|---|---|
| 316L | ~0–5°C | Not suitable above ambient in seawater |
| Duplex 2205 | ~20–25°C | Suitable for North Sea; marginal in Gulf above 25°C |
| Super Duplex 2507 | ~40–50°C | Suitable for Gulf seawater to ~35°C |
| 254 SMO | ~45–55°C | Slightly higher CPT than 2507 in hot seawater |
| Inconel 625 | >85°C | No pitting observed in seawater service |
Where PRE Misleads
Crevice geometry: Crevice corrosion initiates at lower temperatures than pitting — the ASTM G48 crevice temperature (CCT) is typically 15–25°C below the CPT. A grade selected on PRE alone for an open-surface application may fail in a crevice (at a threaded connection, under a gasket, or beneath a weld root mismatch). Always design out crevices in chloride service — or account for CCT rather than CPT in material selection.
Weld quality: The weld HAZ of a duplex fitting in the wrong condition (excessive ferrite, sigma phase) has a lower effective PRE than the base metal. ASTM G48 testing on weld samples — not just parent material — is the only way to confirm actual pitting resistance of a welded fitting.
Temperature: PRE is calculated from chemistry; it does not include temperature. The jump from 2205 (PRE ~35) to 2507 (PRE ≥42) is driven not by bulk corrosion rate improvement at ambient temperature (both perform well at 10°C) but by the CPT shift from ~20°C to ~45°C — which matters enormously for tropical offshore and desalination service.
Specifying CPT on the Purchase Order
For duplex and super duplex fittings in seawater or high-chloride service: specify ASTM G48 Method C or E testing at the agreed acceptance temperature — typically 22°C for 2205 and 40°C for 2507 — on production weld samples, not just on base material. Include the G48 report in the EN 10204 3.1 certification package. Without G48 production testing, weld quality-related CPT degradation will not be caught before installation.