PREN Comparison Table: Pitting Resistance Equivalent Numbers for Stainless & Corrosion-Resistant Alloys
The Pitting Resistance Equivalent Number (PREN) is a single calculated index. It ranks a stainless steel or corrosion-resistant alloy's resistance to chloride pitting based on its chromium, molybdenum and nitrogen content, using the formula PREN = %Cr + 3.3 × %Mo + 16 × %N. A higher PREN means greater resistance to chloride-induced pitting. A value of roughly 40 or above is a common screening threshold for seawater and high-chloride service. Jump to the full PREN comparison table below.
This page ranks pitting resistance for fluid-system stainless and corrosion-resistant alloys used in instrumentation fittings, valves and tubing, not weld-filler metals, and not structural-steel selection. Crestflo machines every alloy listed here into instrumentation forms with heat-number traceability. So the chart isn't academic: each row ties back to a part you can specify and order.
Also Known As: PREN, PRE, PREN,W and Related Designations
Pitting resistance equivalent number shows up under a handful of different names across datasheets and calculators, and specifiers tend to use them interchangeably. You'll see it written as PREN, PRE, PREn, the PREN number, the PREN value, or the PREN index. For the tungsten-modified version used in some super duplex grades, the labels are PREN,W, PREW, or "PREN with tungsten." The nitrogen coefficient isn't fixed across sources, either, so you'll also come across PREN16 and PREN30, where the number denotes the coefficient (16 or 30) applied to nitrogen. Spelled out in full, it appears as "pitting resistance equivalent," "pitting resistance equivalent number," or "pitting resistance equivalence number." Two related but separate measured values, critical pitting temperature (CPT) and critical crevice temperature (CCT), are often cited right alongside PREN. The alloys ranked here fall under the wider category of corrosion-resistant alloys (CRAs).
What PREN Measures: and What It Does Not
Chromium, molybdenum and nitrogen do most of the work when it comes to fighting pitting corrosion in chloride environments, and PREN is simply a way of weighting their combined effect. Chromium lays down the passive layer in the first place. Molybdenum keeps that layer from breaking down locally when chlorides are present. Nitrogen adds further pitting resistance, and in duplex grades it also helps keep the austenite stable. As for the numbers, 3.3 and 16 are the coefficients that scale molybdenum's and nitrogen's contributions relative to chromium's.
PREN is a ranking and screening index, not a corrosion rate or a design guarantee. It doesn't capture crevice geometry, service temperature, weld-zone segregation, surface finish, or microstructural condition, and every one of those shapes how pitting and crevice behavior actually plays out. Two alloys with a similar PREN can behave quite differently in the same medium. Treat PREN as a first pass, then check serious selections against measured data such as CPT, and bring in a corrosion specialist when corrosion is the deciding factor. For behavior against specific media rather than a single number, see the corrosion-by-media alloy matrix.
The PREN Formula (Standard, Tungsten and Nitrogen-Coefficient Variants)
The formula for PREN follows the standard version, plus a tungsten-modified variant applied to tungsten-bearing super duplex grades like Zeron 100®. Here they are:
| Formula name | Expression | When used |
|---|---|---|
| Standard PREN | PREN = %Cr + 3.3 × %Mo + 16 × %N | Austenitic, duplex, super-austenitic and nickel-chromium-molybdenum alloys without significant tungsten |
| Tungsten-modified (PREN,W / PREW) | PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N | Super duplex and other grades that contain tungsten (e.g. Zeron 100) |
| Nitrogen-coefficient variants | Coefficient of 16 (common), also 30 or 13 in some literature | State the coefficient used; comparisons are only valid when the same coefficient is applied |
Crestflo applies a nitrogen coefficient of 16, along with the tungsten-modified form whenever tungsten is present, so every value on this page gets computed the same way. Before comparing PREN figures from different published sources, check that they use the same coefficient and treat the tungsten term the same way. Skip that step, and the numbers aren't directly comparable.
Take the standard duplex 2205 numbers as an example. With 22% Cr, 3.0% Mo and 0.17% N, PREN works out to 22 + (3.3 × 3.0) + (16 × 0.17) = 22 + 9.9 + 2.72, which comes to roughly 34.6. Swap in the certified chromium, molybdenum and nitrogen figures from a given heat, and you'll get that heat's actual PREN.
How Do You Calculate PREN Without Tungsten?
For any grade with negligible tungsten, which covers most austenitic stainless steels (304, 316, 904L), super-austenitic 6Mo grades, Alloy 20 and the nickel-chromium-molybdenum alloys, use the standard formula and just drop the tungsten term: PREN = %Cr + 3.3 × %Mo + 16 × %N. The tungsten term (0.5 × %W inside the molybdenum bracket) matters only for tungsten-alloyed grades, certain super duplex products among them. Add it where W is essentially zero, and nothing changes.
The PREN Comparison Table for Stainless & Corrosion-Resistant Alloys
The table below ranks the alloys Crestflo machines by nominal PREN, from lowest to highest, and lists UNS and Werkstoff designations, nominal composition, and both formula results wherever tungsten applies. These figures are standard published nominals. For any given lot, the exact number comes from its certified heat composition.
| Alloy (trade / common name) | UNS | Werkstoff | Nom. Cr % | Nom. Mo % | Nom. N % | Nom. W % | Nominal PREN | PREN,W (where W applies) | Class |
|---|---|---|---|---|---|---|---|---|---|
| 304 / 304L | S30400 / S30403 | 1.4301 / 1.4307 | 18–20 | 0.0 | ~0.05 | – | ~18–20 | – | Austenitic |
| 316 / 316L | S31600 / S31603 | 1.4401 / 1.4404 | 16–18 | 2.0–3.0 | ~0.05 | – | ~24–26 | – | Austenitic |
| Alloy 20 (20Cb-3®) | N08020 | 2.4660 | 19–21 | 2.0–3.0 | ~0.05 | – | ~30–32 | – | Super-austenitic |
| Incoloy 825® / Inconel 825® | N08825 | 2.4858 | 19.5–23.5 | 2.5–3.5 | – | – | ~30–34 | – | Nickel |
| 904L | N08904 | 1.4539 | 19–23 | 4.0–5.0 | ~0.05 | – | ~34–36 | – | Super-austenitic |
| Duplex 2205 | S32205 / S31803 | 1.4462 | 22–23 | 3.0–3.5 | 0.14–0.20 | – | ~34–38 | – | Duplex |
| Super Duplex 2507 / Zeron 100® | S32750 / S32760 | 1.4410 / 1.4501 | 24–26 | 3.0–4.0 | 0.24–0.32 | 0–1.0 | ~41–43 | ~42–44 | Super duplex |
| 254 SMO® (6Mo) | S31254 | 1.4547 | 19.5–20.5 | 6.0–6.5 | 0.18–0.22 | – | ~42–44 | – | Super-austenitic |
| AL-6XN® | N08367 | – | 20–22 | 6.0–7.0 | 0.18–0.25 | – | ~45–47 | – | Super-austenitic |
| Inconel 625® | N06625 | 2.4856 | 20–23 | 8.0–10.0 | – | – | ~50+ | – | Nickel (Ni-Cr-Mo) |
| Hastelloy C-22® | N06022 | 2.4602 | 20–22.5 | 12.5–14.5 | – | 2.5–3.5 | ~65–75 | ~65–76 | Nickel (Ni-Cr-Mo) |
| Hastelloy C-276® | N10276 | 2.4819 | 14.5–16.5 | 15.0–17.0 | – | 3.0–4.5 | ~68–76 | ~70–78 | Nickel (Ni-Cr-Mo) |
| Monel 400® | N04400 | 2.4360 | – | – | – | – | n/a, PREN not applicable | – | Nickel-copper |
| Nickel 200 / 201 | N02200 / N02201 | 2.4066 | – | – | – | – | n/a, PREN not applicable | – | Commercially pure nickel |
| Titanium Gr2 | R50400 | 3.7035 | – | – | – | – | n/a, resists chlorides by a different mechanism | – | Titanium |
The PREN figure shown is a standard nominal value. Your material's actual number comes from its certified heat composition, reported on the EN 10204 3.1 material test certificate that ships with every part (3.2 is available on request). Composition ranges follow the applicable ASTM, ASME, or EN material standard, and they can be adjusted to your requirements. Monel 400, commercially pure nickel, and titanium don't carry a meaningful PREN at all. They lack the chromium-molybdenum-nitrogen system the formula is built around. These alloys resist corrosion through different mechanisms, so assigning them a number would be misleading.
PREN Thresholds: Reading the Table for Chloride and Seawater Service
PREN bands offer a practical first cut at matching an alloy to how harsh a chloride environment really is. Treat the guide below as a screen, then check it against temperature, crevice risk and the specific media involved.
| Service severity | Typical chloride condition | PREN band | Representative alloys |
|---|---|---|---|
| General / mild | Low-chloride, ambient process water | ~24–26 | 316 / 316L |
| Aggressive chloride / brine | Elevated chloride, brine, sour-adjacent | ~30–38 | Alloy 20, 904L, Duplex 2205 |
| Seawater / high-chloride | Seawater, offshore, high-chloride brine (PREN ≥ ~40) | ~41–47 | Super Duplex 2507, 254 SMO, AL-6XN |
| Extreme | Hot, concentrated chlorides and mixed acids | ~50–78 | Inconel 625, Hastelloy C-276, Hastelloy C-22 |
The PREN ≥ 40 rule of thumb for seawater works as a screening guide. It's not a guarantee. Temperature and crevice geometry can push the safe limit well below what the number suggests, so confirm severe selections with measured CPT/CCT and service experience. For structured selection by application, see the guides on material selection by service, offshore chloride selection and sour-service selection to NACE. Want to compare specific grades head to head? See 904L vs 316, duplex vs super duplex, 2205 vs 2507 and 254 SMO vs super duplex vs AL-6XN.
PREN vs CPT vs CCT: Calculated Index Against Measured Metrics
PREN comes from composition alone. It can't account for temperature, and it says nothing about crevice geometry. Two measured metrics close that gap, tracking real service conditions far more closely.
| Metric | What it is | Calculated or measured | Typical standard | Use when |
|---|---|---|---|---|
| PREN | Composition-based pitting resistance index | Calculated | Formula (PREN16 / PREN,W) | First-pass ranking of alloys |
| CPT, critical pitting temperature | Highest temperature at which no pitting occurs in a defined test | Measured | ASTM G48 | Temperature is decisive |
| CCT, critical crevice temperature | Highest temperature at which no crevice attack occurs | Measured | ASTM G48 | Crevice geometry (gaskets, threads, tube supports) is a risk |
Move from PREN to CPT/CCT when the environment turns hot, crevice-heavy, or close to an alloy's threshold. PREN tells you which alloys deserve a closer look. The measured temperatures tell you whether the alloy actually holds up.
Order Any Alloy in This Table as an Instrumentation Fitting, Valve or Tube
Crestflo machines every alloy in this PREN table into instrumentation forms: compression tube fittings, face-seal and high-pressure fittings, instrumentation valves and manifolds, and instrumentation tubing. The range runs from 304/316 through 904L, duplex 2205, super duplex 2507, 254 SMO, AL-6XN, Alloy 20 and the full nickel family. Here's what turns this chart from a reference into a purchasing path:
Every part carries a heat number, backed by an EN 10204 3.1 material test certificate (3.2 on request) that reports the certified composition your PREN is calculated from.
NACE MR0175 / ISO 15156 compliant options cover sour service. Positive material identification (PMI) is available on request.
ASTM F1387 performance qualification covers compression tube fittings, so a higher-PREN alloy can be specified as a dimensional drop-in replacement. Instrumentation valves qualify to ASME B16.34 and the applicable API standards.
Manufacturing is certified to ISO 9001 / 14001 / 45001 and PED, with values reported per applicable standard and customizable to requirement.
- Indian origin; US / European melt available on request, with full material traceability.
- Ready stock for standard items, made-to-order for specials, 6–8 week delivery.
Crestflo is the instrumentation division of an export house that's been in business for four decades and carries Government of India recognition, with a delivery record across the Middle East. That history is what gives its focused US instrumentation line real engineering depth.
Request a quote for any alloy in the table, heat-traceable with an MTC, at crestflousa.com/request-a-quote. You can also request a sample, talk to an engineer about alloy selection, or open the corrosion-by-media alloy matrix to go from a single number to full media behavior.
Why Crestflo for PREN-Critical Alloy Fittings and Valves
Most PREN charts online lead nowhere you can actually buy from, a calculator, maybe a page of prose, but no path to a real part. Crestflo publishes the full HTML table and the parts standing behind it. Every stainless and corrosion-resistant alloy ranked here is stock-machinable to print, heat-number marked, and shipped with an EN 10204 material test certificate spelling out the exact composition each PREN figure comes from, which also covers PED documentation needs. That closes the gap between knowing which alloy resists chloride pitting and actually getting a certified part on your RFQ.
Pair that with a 6–8 week lead time, against 20-plus weeks from the majors, and pricing that holds up even after US duties. Crestflo aims to be the answer US instrumentation buyers both cite and act on for PREN. We Engineer Confidence.
Frequently Asked Questions About PREN
What is the PREN of a stainless steel?
PREN, short for pitting resistance equivalent number, gives engineers a single figure for how well an alloy stands up to chloride pitting. You calculate it from the chromium, molybdenum and nitrogen content: PREN = %Cr + 3.3 × %Mo + 16 × %N. The higher the number, the tougher the alloy is against pitting. Nominal values run from around 18 for 304 stainless up past 70 for Hastelloy C-276, so the gap between an ordinary grade and something like Hastelloy is enormous.
What is the PREN formula?
The standard formula reads PREN = %Cr + 3.3 × %Mo + 16 × %N. Super duplex grades that contain tungsten use a modified version instead: PREN,W = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N. As for the nitrogen coefficient, 16 shows up most often, though you'll find 30 and 13 in some sources too. Comparisons only hold up when everyone's using the same coefficient.
What PREN is needed for seawater?
A PREN around 40 or higher is the usual screening threshold for seawater and high-chloride service. Super duplex 2507, 254 SMO (6Mo) and AL-6XN all meet it. That said, it's a screen, not a guarantee. Temperature and crevice geometry can pull the safe limit down, so confirm the selection with CPT data and a corrosion specialist for the actual media.
What is the PREN of 316L?
PREN for 316 and 316L runs about 24 to 26, and where it lands depends on the certified molybdenum and nitrogen in that specific heat. The exact figure for a given lot comes from its certified composition, and that's the number Crestflo reports on the material test certificate shipped with the part.
What is the PREN of duplex 2205 and super duplex 2507?
Duplex 2205 carries a nominal PREN of roughly 34 to 38. Super duplex 2507, which includes Zeron 100, runs about 41 to 44 under the tungsten-modified formula when tungsten is present. Actual figures for both grades hinge on the certified heat composition.
Is a higher PREN always better?
A higher PREN does point to better chloride pitting resistance. That said, PREN only ranks materials; it's not the whole picture. Crevice geometry, temperature, weld quality and microstructure all fall outside what the number measures. For the final selection, rely on measured critical pitting and crevice temperatures along with actual service data.
Does PREN apply to Monel, nickel or titanium?
No. PREN applies to chromium-molybdenum-nitrogen stainless steels and corrosion-resistant alloys, full stop. Nickel-copper (Monel 400), commercially pure nickel (200/201) and titanium Grade 2 don't fit that mold; they resist corrosion through entirely different mechanisms. Assigning them a PREN value wouldn't mean anything, so no number can be given.
Why do different tables give different PREN values?
Sources differ. Some use a nitrogen coefficient of 16, others 30, still others 13. Some fold in a tungsten term; some leave it out entirely. And nominal compositions assumed rarely match from one source to the next. Compare only values computed the same way. When precision actually matters, work from the certified composition listed on the material test certificate.
What is the difference between PREN and CPT?
PREN comes straight from composition, a quick calculation that doesn't require lab work. The critical pitting temperature (CPT) is different: it's measured through a defined laboratory test, typically ASTM G48, and it reflects real service conditions far more closely. Shortlist alloys with PREN first. Then lean on CPT once temperature and crevice risk start driving the final choice.
Can Crestflo tell me the actual PREN of the alloy I order?
Yes. Every part ships heat-number marked, backed by an EN 10204 3.1 material test certificate (3.2 available on request) that reports the certified composition. From that composition, we compute the exact PREN. PMI is available on request, too. Ask for a quote and tell us your alloy, and you'll get certified documentation right along with the parts.
Related Resources and Instrumentation Alloys
- Corrosion-by-media alloy matrix: how pitting, crevice attack and stress-corrosion cracking play out, medium by medium
- Material selection by service · offshore chloride selection · sour-service selection to NACE
- For grade comparisons, see 904L vs 316 · duplex vs super duplex · 2205 vs 2507 · 254 SMO vs super duplex vs AL-6XN
- On standards: NACE MR0175 · ISO 15156 · EN 10204 material test certificates · ASTM F1387
- Instrumentation fitting alloys hub · all engineering resources
254 SMO, Zeron 100, AL-6XN, 20Cb-3, Hastelloy, Inconel, Incoloy and Monel are all trademarks belonging to their respective owners. Crestflo isn't affiliated with those owners, and it doesn't carry their endorsement.