PREN Explained: Pitting Resistance Equivalent Number & Grade Ranking

What is PREN (Pitting Resistance Equivalent Number)?

The pitting resistance equivalent number, or PREN, is an index built from an alloy's composition, and it tells you how well a stainless steel or nickel-iron-chromium alloy stands up to chloride pitting. You get it by working through the weight percentages of the key passive-film elements in the metal. That means a first-pass ranking comes straight from the chemistry sheet, with no test rig needed.

PREN comes down to two things: what it measures, and what it doesn't. It ranks resistance to localized chloride pitting specifically, not general corrosion, not acid attack, not stress-corrosion cracking. A higher PREN simply means a grade is more likely to resist pitting in a given chloride service. Nothing more. It's an ordering tool, not a pass or fail guarantee. Specifiers use it that way: to rank candidate grades and decide when to step up, from 304 to 316, then 904L, then to a duplex or nickel alloy. For that purpose, PREN is the single most useful number a specifier has.

The PREN Formula: Cr + 3.3·Mo + 16·N

The standard (PREN16) formula, used here as the house convention, is:

PREN = %Cr + 3.3 × %Mo + 16 × %N

All terms are weight percent of the actual cast, meaning the heat chemistry, not a product category. Chromium gets a multiplier of 1 because it forms the passive oxide film. Molybdenum and nitrogen are weighted higher, since gram for gram they do more to resist and repair chloride breakdown of that film.

Why molybdenum is weighted 3.3 and nitrogen 16

The weightings here come from measured pitting behavior, not theory. A unit of molybdenum raises pitting resistance roughly 3.3 times as much as a unit of chromium, and a unit of nitrogen does it roughly 16 times as much. That's why a grade with only 2–3% molybdenum, like 316, pulls well ahead of 304. It's also why nitrogen-alloyed duplex grades reach high PREN values from relatively modest chromium. For more on what each element does, see the role of Cr, Mo and nitrogen.

PREN variants: PREN16, PREN30 and PRE(W) for tungsten grades

Several conventions get used here. PREN16 applies the 16× nitrogen factor, and it's the one used throughout this page. PREN30, by contrast, applies a 30× nitrogen factor and tends to show up for grades heavy on nitrogen alloying, which pushes their numbers higher. Check which factor a quoted value relies on before comparing figures across sources. Tungsten-bearing alloys bring in a third option: the PRE(W) / PREW variant, which adds a tungsten term (PREN = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N). More often than not, when two published PREN charts don't agree, it's because someone mixed these conventions.

How Pitting Works: the Mechanism PREN Predicts

Stainless steels and nickel-iron-chromium alloys resist corrosion because chromium builds a thin, self-healing passive film on the surface. Chloride ions go after that film in specific spots. Where they break it down faster than it can rebuild, a tiny anodic site opens up, the local chemistry turns acidic, and a pit forms that keeps eating into the metal on its own. Molybdenum and nitrogen work right at that site: they slow the breakdown and speed up re-passivation. That's why they carry so much weight in the PREN formula.

PREN predicts susceptibility to this one mechanism, nothing more. For the deeper mechanics of how and where pits initiate, see pitting corrosion. PREN is just the number you use to rank grades against it before committing to one.

What Drives PREN: the Alloying Elements

Chromium: the film former

Chromium is the baseline here. Below roughly 10.5% there's no passive film at all, and every stainless grade sits above that threshold, which is why chromium sets the floor of the PREN. Its multiplier is 1, so adding more chromium does help. But once the film has formed, it's the least efficient route to better pitting resistance.

Molybdenum: the 3.3 multiplier, the biggest lever

Molybdenum is where pitting resistance is actually bought. The 2–3% in 316 explains why it outlasts 304 against pitting, and the roughly 4.5% in 904L is what pushes it into genuine chloride-and-crevice territory. When a buyer upgrades a grade "for chlorides," they're almost always buying molybdenum.

Nitrogen: ×16, the cheap potent booster

Nitrogen is a controlled, cheap addition, and at the 16× weighting it hands you large PREN gains from tiny amounts. It's the defining lever in duplex and super-duplex grades, where 0.14–0.30% nitrogen does much of the work that would otherwise call for expensive molybdenum.

What does not count: nickel and copper

Nickel and copper don't show up in the PREN formula at all, though that doesn't mean they're worthless. Nickel stabilizes the austenitic structure and helps resist chloride stress-corrosion cracking. The roughly 1.5% copper in 904L is what gives it sulfuric-acid resistance. Neither one, though, does anything for chloride pitting, so neither counts toward PREN. People get tripped up by this constantly: a nickel-rich alloy isn't automatically a high-PREN alloy.

PREN Ranking of Stainless & Nickel-Alloy Grades

This is the orderable fact at the center of grade selection: the grades a chloride-service buyer chooses between, ordered by typical PREN. These are typical, nominal ranges. Real PREN comes from the actual cast, so a given heat can land a point or two to either side.

The ranking runs 304 (~18) up to 316 (~24), then 904L (~34) and duplex 2205 (~35) sit almost neck and neck, and super-duplex 2507 (~43) tops the list. Incoloy 825 (~31–40) lands in the same PREN territory as 904L, though it gets picked for different reasons entirely (see the caveat below). Each grade links out to its own page: 304 stainless, 316 stainless (PREN ~24) and 316L, the super-austenitic 904L grade, duplex stainless steel, and the Incoloy 825 Ni-Fe-Cr alloy.

PREN bands & what they mean for chloride service

As PREN climbs, so does the resistance needed to fight chloride damage, and this guide sketches where each grade tends to fit, it's not a spec. Below roughly 22, you're in mild territory: indoor settings or low-chloride duty, where 304 holds up fine. From about 22 to 30, things get tougher, moderate exposure, brackish water, occasional splash, and that's where 316 earns its keep. Push into the 30 to 38 range and you're dealing with high-chloride process streams or dilute seawater; 904L and duplex 2205 are built for that. Once you hit roughly 40 and above, you're looking at warm seawater and the harshest chlorides around, so super-duplex and 6-Mo alloys become the natural starting point.

For ambient seawater, the common rule of thumb lands at a minimum PREN near 40. That number isn't fixed, though. Temperature shifts it, crevices shift it, and flow conditions shift it too. PREN gives you a starting filter for narrowing down options. It was never meant to be the final word.

Worked Examples: Calculating PREN for 316, 904L and Duplex 2205

The formula works from any cast chemistry. Here are three examples at nominal compositions:

316 stainless (16.5% Cr, 2.1% Mo, 0.05% N):

1. Cr term: 16.5
2. Mo term: 3.3 × 2.1 = 6.93
3. N term: 16 × 0.05 = 0.8
4. PREN = 16.5 + 6.93 + 0.8 = ~24

904L (20% Cr, 4.3% Mo, 0.10% N):

1. Cr term: 20
2. Mo term: 3.3 × 4.3 = 14.19
3. N term: 16 × 0.10 = 1.6
4. PREN = 20 + 14.19 + 1.6 = ~36 (quoted nominally as ~34)

Duplex 2205 (22% Cr, 3.1% Mo, 0.17% N):

1. Cr term: 22
2. Mo term: 3.3 × 3.1 = 10.23
3. N term: 16 × 0.17 = 2.72
4. PREN = 22 + 10.23 + 2.72 = ~35

904L and duplex 2205 land close together in PREN, both around 35, but they get there from very different chemistries. 904L leans on molybdenum to do the work, while 2205 relies on nitrogen plus its duplex structure. That shared number is exactly where PREN starts to mislead you.

The Limits of PREN: What the Number Does NOT Tell You

PREN is a ranking aid, not a service guarantee, and its limits are worth stating plainly.

904L and duplex 2205 both sit near a PREN of 35, but that shared number hides real differences. Duplex delivers far higher strength and holds up better against chloride stress-corrosion cracking, while 904L wins out on sulfuric-acid resistance. The PREN figure alone won't tell you which one you need.

Crevice corrosion is another blind spot. Pits take hold more readily in crevices, gaskets, deposits, under-bolt gaps, than on an open surface, so a grade that looks fine on paper by PREN can still fail once it's tucked into a crevice. See crevice corrosion.

Temperature doesn't factor into the formula at all, yet pitting resistance drops as things heat up. A PREN value that looks more than adequate at room temperature might be barely sufficient at 60 °C.

Stress-corrosion cracking, finally, is its own failure mode entirely. Austenitic grades can resist pitting just fine according to their PREN, but they stay vulnerable to chloride stress-corrosion cracking. Duplex and high-nickel alloys, at the same PREN, hold up against it far better.

PREN vs a real pitting test: ASTM G48 & Critical Pitting Temperature (CPT)

Where pitting risk is real, PREN narrows the field, and a test confirms it. ASTM G48 exposes the material to a ferric-chloride solution to reveal whether it pits; the related critical pitting temperature (CPT) records the lowest temperature at which stable pits form, a measured, service-relevant number that PREN only estimates. A specifier uses PREN to rank candidates and a G48/CPT requirement to prove the chosen grade.

How to Use PREN When Specifying & Ordering

Turn the number into a defensible specification:

1. Set a minimum PREN on the spec. Base it on the chloride level, temperature and crevice risk of the actual service, not on a chart alone.
2. Compute PREN from the actual cast, not the nominal. The textbook value assumes a mid-range chemistry, but the as-delivered heat may differ. The full cast chemistry appears on the EN 10204 3.1 mill test certificate supplied with every order, so the PREN of the delivered heat can be worked out straight from that certificate. On request, our engineering team will confirm the computed PREN of the supplied heat from its MTC.
3. Confirm the right standard. The composition behind the PREN figure is certified against ASTM A240 / ASME SA-240 for the flat-product grades, tied to traceability through the EN 10204 3.1 mill test certificate.
4. Add a test clause where it matters, an ASTM G48 or CPT requirement for critical chloride service.

For an interactive route from chloride level and temperature to a candidate grade, use the grade selector; for applying PREN inside a written spec, see how to use PREN when specifying and the broader how to select a stainless grade guide.

PREN for the Focus Grades Nandini Stocks

Nandini Steel is an ISO 9001:2015-certified stockist and supplier, with certified specialist status in 904L, Alloy 28, Incoloy 800 and Hastelloy. The ranked grades sit ex-stock, ready for immediate dispatch. Every piece ships with an EN 10204 3.1/3.2 mill test certificate, sourced from genuine mill plate: Aperam, Outokumpu, Jindal, POSCO, Nippon Steel, JFE.

• 304 / 316 (austenitic, PREN ~18 / ~24). This pair handles most chloride-pitting problems. Reach for 304 in mild, indoor conditions, and move up to 316 once chlorides show up. What you're really paying for there is the molybdenum. See 304 vs 316.
• 904L (super-austenitic, PREN ~34). 904L, including the 904L coil option, handles high-chloride and sulfuric-acid environments that beat 316. It often costs less than jumping to a nickel alloy. See 904L vs 316L and 904L vs duplex 2205.
• Incoloy 825 (Ni-Fe-Cr, PREN ~31–40). Its PREN number lands close to 904L. But people specify Incoloy 825, also stocked as 825 coil, for its resistance to reducing and oxidizing acids and sour service, not for pitting resistance. Picking it on PREN alone misses the point of the alloy.

Where PREN earns its keep in the field, see marine / seawater service, oil & gas and chemical processing.

A note from the Nandini Steel Engineering Team: the costliest spec mistake we keep running into is over-specifying on PREN alone. A buyer picks a high-PREN grade to feel "safe," but the real threat in service turns out to be a tight crevice or chloride SCC. PREN never measured either one. A lower-PREN duplex would have done the job just as well, for less. Rank with PREN, sure. Then prove the grade against whatever mechanism actually threatens the part.

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