PREN Calculator — Pitting Resistance of Stainless Steel
Rank a stainless steel's resistance to chloride pitting from its chromium, molybdenum and nitrogen. Pick the variant — 16, 30 or the tungsten-bearing form — or type a real grade and let its own specified composition fill the fields. A real GET form: computes correctly with JavaScript off.
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What PREN actually tells you
Chloride pitting starts where the passive chromium-oxide film breaks down locally. Chromium builds that film, molybdenum makes the breakdown harder to sustain, and nitrogen both stabilises the austenite and repassivates a pit that has already started. PREN weighs those three contributions into one number so that two grades can be ranked against each other without a corrosion test.
That is the whole ambition of the index: a ranking. PREN is an empirical weighting of chemistry, not a measurement, and it carries no units. It cannot tell you at what temperature or chloride concentration a given grade will actually pit — only that, all else equal, a higher number tends to resist pitting longer than a lower one.
Why the same steel has more than one PREN
The nitrogen coefficient is the fork. The classic fit uses 16 and is the form implied by a bare "PREN", conventional for the austenitic family. Duplex literature uses 30, because nitrogen inhibits pitting far more effectively in a ferritic–austenitic microstructure than the 16 fit credits it with. Tungsten-bearing super-duplex grades add a third form, PREN(W), in which tungsten enters at half the weight of molybdenum. For a 2205 duplex the 16 and 30 forms differ by roughly two points — enough to move a grade across a purchasing threshold, which is exactly why a PREN quoted without its variant is not an answer.
The thresholds people actually buy against
Two numbers do real work in specifications. PREN ≥ 32 is the customary dividing line for grades intended to face warm chloride service, and PREN ≥ 40 is the conventional entry point for the super-duplex and super-austenitic classes used in seawater handling. Below 32 the sensible language is comparative, not categorical: a 316-type grade around 25 clearly outranks a 304-type around 18, without either number promising a service life.
What PREN does not cover: crevice corrosion, stress-corrosion cracking, weld-zone and heat-affected-zone behaviour, surface finish, temperature and pH. Two grades with the same PREN can behave very differently in a real crevice. Treat the number as a first screen, then qualify the choice against the actual service conditions.
PREN — frequently asked
Which PREN variant should I use?
Whichever your specification, customer or datasheet names. Failing that: 16 for austenitic grades, 30 for duplex, and the tungsten-bearing form for super-duplex grades alloyed with tungsten. Always quote the variant alongside the number.
Where do the element values come from when I type a grade?
From our own composition table, one figure per element, with the source key and a link to the source shown beside it. The specification's original wording is displayed too — "16,5-18,2" rather than a bare 17.35 — so you can see exactly what was collapsed into the number.
What is the specification band?
A grade is specified as ranges, not as one analysis, so it has a range of PREN values rather than one. The band is PREN evaluated at the composition minima and at the maxima. It appears only when every element the chosen variant uses carries both bounds — where a specification states only a minimum, no upper case exists and no band is drawn.
What happens if a grade does not specify nitrogen or tungsten?
The composition table lists that element as not specified and it is taken as zero — visibly, not silently. That is also the physically right answer here: an element the specification does not guarantee cannot be credited with resisting pitting. It is exactly why the calculator reproduces the published PREN of grades like 304 and 316L.
Why does the calculator refuse chromium below 10.5 %?
Because 10.5 % chromium is the definition of a stainless steel — the minimum at which a continuous passive film forms at all. Below it there is no film for chlorides to break down, so PREN is not merely inaccurate, it describes nothing. The engine refuses rather than returning a number that would look usable.