Pitting resistance

What is the pitting resistance equivalent number?

The pitting resistance equivalent number is an empirical relationship between a stainless steel's composition and its relative resistance to pitting in chloride-bearing solutions. Chromium, molybdenum and nitrogen are the elements that contribute. Tungsten also contributes and is treated as about half as effective as molybdenum on a weight percent basis.

Every figure on this page is computed as PREN = Cr + 3.3Mo + 16N, where each symbol is that element's content in weight percent. The relationship is published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014) in its tungsten-bearing form, PREW = Cr + 3.3(Mo + 0.5W) + 16N, which is the same arithmetic for the great majority of grades because they specify no tungsten at all. The two grades in this reference that do specify tungsten carry both figures, the second labelled PREW, because a supplier quoting one and a buyer reading the other are looking at different numbers for the same steel.

The published source gives the relationship for austenitic and duplex stainless steels. The table below covers all 167 stainless grades in the reference, across five families, which extends it past the scope its publisher claims. The same source says the number is “useful for ranking grades within a single family of steels”, so read the table that way: down a family, not across the whole list.

What the figures are computed at, and why it matters

A grade is a composition window, not a composition. Computing the number at the maximum permitted chromium, molybdenum and nitrogen flatters every grade and describes a heat a buyer has no right to expect. Both columns are given: the first uses the minimum specified value of each element, so it is the floor for any conforming heat, and the second uses the maxima, so the pair is the width of the band one compliant grade allows.

An element the grade does not specify contributes nothing, and an element specified only as a maximum contributes nothing to the floor. Neither is a substituted guess: a grade with no molybdenum requirement can be supplied with no molybdenum in it, and a grade with no chromium requirement at all gets no figure rather than a zero.

The gap is not academic. AISI 316 spans 22.6 to 29.5 within one compliant grade, and the low end of that band sits below where most people assume 316 sits. The newsroom article PREN: what your grade guarantees works that case through in full.

What this number does not predict

It ranks resistance to chloride pitting. It does not predict crevice corrosion, stress corrosion cracking, general corrosion, oxidation, weld behaviour or service life, carbon does not appear in it at all, and it is not a substitute for a critical pitting temperature test in the actual medium.

The published source is explicit about the limits of its own relationship. It states that the coefficient for nitrogen “varies among investigators and 16, 22, and 30 are commonly used”, that the number “is useful for ranking grades within a single family of steels” but that “care must be taken to avoid inappropriate over-reliance on this relationship”, and that the relationships “are not linear” and ignore cross relationships such as the synergies of chromium and molybdenum. It also warns that “a composition modified to meet a specific PREN does not necessarily lead to correct metallurgical balance”, because the formula implies that chromium and molybdenum are substitutable with nitrogen while metallurgically they do opposite things to the structure.

A high number on a lean grade is not a better grade

AISI 202 computes to 23.0 at the maxima against AISI 304's 21.6, because 202 is allowed nitrogen up to a quarter of a percent and the formula pays 16 times the weight for it. At the minima the order reverses: 202 floors at 17.0 and 304 at 18.0. Acting on the first pair would be a mistake. The 200-series grades reach those numbers on nitrogen with manganese standing in for nickel, and the formula has no term for nickel at all.

Super duplex 2507 does not clear 40 at its minima

The convention that a super duplex grade carries a pitting resistance equivalent number of 40 or more comes from the same source, which describes super duplex as “approximately 25% Cr and 3% Mo, with PREN of 40 to 45, such as 2507”. Computed at the specified minima, 2507 in this reference starts at 37.7 and reaches 47.6 at the maxima. Both statements are true and they are not in conflict: the convention describes the grade as it is normally made, and 37.7 at specified minima is what the specification actually guarantees. A project that needs 40 has to put it on the purchase order as a certified minimum on the heat analysis.

The bands overlap, so ranking by a single figure misleads

AISI 904 L runs 32.2 to 39.5 and duplex 2205 runs 34.1 to 37.8. The two overlap across most of their range, so which one scores higher depends entirely on which end of each window the heats actually land in. They are also different materials in every other respect: 2205 has roughly twice the yield strength and far more resistance to chloride stress corrosion cracking, neither of which this number describes.

Pitting resistance of every stainless grade in the reference

Computed 2026-08-18 by Laxcon Steels from its own grade reference of 500+ grades. Formula from International Molybdenum Association, Practical Guidelines for the Fabrication of Duplex Stainless Steels, third edition, 2014. This table is part of the open grade dataset.