Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
PREN, Pitting Resistance Equivalent Number for Super Duplex 2507
The Pitting Resistance Equivalent Number (PREN) is the standard single-number metric used to rank stainless steels for chloride pitting resistance. The formula is PREN = Cr + 3.3 times Mo + 16 times N, where each element is the weight percent from the mill test certificate. NORSOK M-630 sets PREN of 40 or higher as the threshold for super duplex classification. The most heavily traded super duplex grade, 2507 (UNS S32750), sits at PREN 41 to 43.
PREN is calculated from the actual heat chemistry, not from the ASTM minimum or maximum. Mill test certificates show the heat composition, and the PREN value on the certificate is the basis for project compliance under NORSOK M-630, NACE MR0175 / ISO 15156-3 and oil major project specifications worldwide.
PREN Formula
The standard formula, defined in ASTM G48 and adopted by NORSOK, NACE and the oil and gas project specifications, is:
PREN = %Cr + 3.3 x %Mo + 16 x %N
Each element is in weight percent from the heat chemistry. The 3.3 weighting on molybdenum and the 16 weighting on nitrogen are empirical constants derived from chloride-pitting tests on duplex and super duplex stainless steels. For tungsten-bearing grades like Zeron 100, the alternative PREW formula adds tungsten:
PREW = %Cr + 3.3 x (%Mo + 0.5 x %W) + 16 x %N
Worked Example: Super Duplex 2507
A typical 2507 mill heat certified to ASTM A479 / EN 10088-3 with heat chemistry as below:
| Element | Heat Chemistry (%) | Contribution to PREN |
|---|---|---|
| Chromium (Cr) | 25.0 | 25.0 |
| Molybdenum (Mo) | 3.8 | 3.3 x 3.8 = 12.54 |
| Nitrogen (N) | 0.28 | 16 x 0.28 = 4.48 |
| PREN | 25.0 + 12.54 + 4.48 = 42.02 | |
PREN of 42.0 places this heat comfortably above the NORSOK M-630 super duplex threshold of 40. A heat at the low end of the chemistry window (24.0 percent Cr, 3.0 percent Mo, 0.24 percent N) would yield PREN of 24.0 + 9.9 + 3.84 = 37.74, below the super duplex threshold. This is why mills target the centre of the window for consistent compliance.
PREN Thresholds Across Stainless Steel Families
| Class | Example Grade | PREN Range | Designation |
|---|---|---|---|
| Austenitic (300 series) | 304L (UNS S30403) | 17 to 19 | Low chloride resistance |
| Austenitic (Mo-bearing) | 316L (UNS S31603) | 23 to 28 | Moderate chloride |
| Lean duplex | 2304 (UNS S32304) | 23 to 28 | Lean duplex |
| Standard duplex | 2205 (UNS S32205) | 34 to 38 | Standard duplex |
| 6Mo austenitic | 254 SMO (UNS S31254) | 40 to 44 | Super austenitic |
| Super duplex | 2507 (UNS S32750) | 41 to 43 | Super duplex (NORSOK M-630) |
| Super duplex (Cu+W) | Zeron 100 (UNS S32760) | 41 to 44 | Super duplex |
| Hyper duplex | SAF 2707 HD (UNS S32707) | 49 to 50 | Hyper duplex |
PREN and Critical Pitting Temperature
PREN correlates with Critical Pitting Temperature (CPT) measured per ASTM G48 Method E in 6 percent ferric chloride. Each PREN point above 30 adds approximately 5 to 8 degrees Celsius to the CPT for duplex and super duplex grades. The relationship is empirical and varies modestly with chemistry detail and surface condition.
| Grade | PREN (typical) | CPT (degrees Celsius, ASTM G48 Method E) |
|---|---|---|
| 316L | 23 to 28 | 10 to 20 |
| 2205 | 34 to 38 | 30 to 40 |
| 2507 | 41 to 43 | 50 to 70 |
| Zeron 100 | 41 to 44 | 55 to 75 |
NORSOK M-630 PREN Requirement
NORSOK M-630 requires PREN of 40 or higher for super duplex classification. The PREN must be calculated from the actual heat chemistry on the mill test certificate, and the calculated value must appear on the certificate. NORSOK MDS D55 and D57 (super duplex pipe and bar) require PREN of 40 minimum; the 2 PREN-point margin (achieved by mill targeting at PREN 42) provides design conservatism.
PREN in Project Design
Project specifications use PREN as a design metric in three ways:
- Material selection: minimum PREN required for the service environment (chloride concentration, temperature, presence of H2S)
- Acceptance: calculated PREN on the mill test certificate must meet or exceed the project minimum, typically 40 for super duplex
- Welding qualification: weld metal PREN must equal or exceed the base metal PREN; this is why ER2594 (overmatching) is the standard filler for 2507 rather than ER2553 (matching)
Limitations of PREN
PREN is a useful single-number metric but has limitations. It does not capture crevice geometry sensitivity, surface condition (passive film integrity), galvanic coupling, microstructural defects (sigma phase, intermetallic precipitates), or weld-metal segregation effects. For service in dilute reducing acids (HCl, H2SO4), PREN is a poor predictor; copper-bearing grades (Ferralium 255) outperform their PREN in such media. PREN is best used as a screening number, not as the sole basis for material selection.
Standards Citing PREN
| Standard | Use of PREN |
|---|---|
| ASTM G48 | Pitting and crevice corrosion test methods (defines test conditions) |
| NORSOK M-630 | Super duplex classification (PREN 40 minimum) |
| NORSOK M-650 | Manufacturer qualification (QTR), PREN reporting required |
| NACE MR0175 / ISO 15156-3 | Sour service envelope (PREN cited for selection) |
| ASTM A182 F53 | Forging grade for UNS S32750 (PREN 40 minimum implicit) |
| API 6A / 17D | Wellhead and subsea materials (PREN cited) |
PREN Frequently Asked Questions
What is PREN?
PREN is the Pitting Resistance Equivalent Number, a single-number metric used to rank stainless steels for chloride pitting resistance. The formula is Cr + 3.3 times Mo + 16 times N, with each element in weight percent from the mill heat chemistry. NORSOK M-630 sets PREN of 40 or higher as the threshold for super duplex classification.
What is the PREN of super duplex 2507?
Typical 2507 mill heats sit at PREN 41 to 43. A heat with 25 percent Cr, 3.8 percent Mo and 0.28 percent N yields PREN of approximately 42.0. NORSOK M-630 requires PREN of 40 minimum for super duplex; mill targeting at the centre of the chemistry window provides a 2 PREN-point design margin.
How is PREN calculated?
PREN equals %Cr + 3.3 x %Mo + 16 x %N, with each element taken from the mill heat chemistry on the test certificate. The 3.3 and 16 weightings are empirical constants derived from chloride-pitting tests. For tungsten-bearing grades, PREW = %Cr + 3.3 x (%Mo + 0.5 x %W) + 16 x %N is sometimes used.
What is the difference between PREN and PREW?
PREN uses chromium, molybdenum and nitrogen only. PREW adds tungsten with a 0.5 weighting on the molybdenum-equivalent term. PREW is used for tungsten-bearing super duplex grades like Zeron 100 (UNS S32760) and DP3W (UNS S39274). For tungsten-free grades like 2507 (UNS S32750) and Ferralium 255 (UNS S32550), PREN and PREW are identical.
What is the minimum PREN for super duplex stainless steel?
PREN of 40 or higher is the super duplex threshold per NORSOK M-630. UNS S32750 (2507) typically delivers PREN 41 to 43 from a centred mill heat. UNS S32760 (Zeron 100) typically delivers PREN 41 to 44. UNS S32550 (Ferralium 255) sits at PREN 38 to 42, just inside the super duplex window.
Where does PREN appear on a mill test certificate?
EN 10204 Type 3.1 and 3.2 mill test certificates for duplex and super duplex stainless steels typically show the calculated PREN value alongside the heat chemistry. Some mills report PREN in a dedicated row; others note it as a remark. Project acceptance under NORSOK M-630 requires PREN to be present on the certificate and at or above the project minimum.
Is PREN a good predictor of crevice corrosion?
PREN correlates with crevice corrosion resistance but is not a perfect predictor. The Critical Crevice Temperature (CCT) per ASTM G48 Method F is typically 15 to 25 degrees Celsius below the Critical Pitting Temperature (CPT) for the same grade. Crevice geometry, surface finish, and contact material all affect the actual crevice corrosion threshold. PREN is best used as a screening number, supplemented by ASTM G48 Method F testing for critical service.
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