PREN in Stainless Steel: Calculation and Published Grade Values

The pitting resistance equivalent number (PREN) is an empirical index that ranks the resistance of a stainless steel to pitting corrosion in chloride environments, such as seawater, brine, swimming pools, chlorinated process water, coastal air and road salt. Pitting in chlorides is the corrosion mode most likely to destroy stainless steel in service. Published tables commonly assign one PREN value per grade, calculated from a typical or nominal composition. A grade specification, however, defines a composition window rather than a single chemistry, and every composition inside the window is compliant. Calculated from specification limits, each grade therefore has a PREN band rather than a single value. This article gives the bands for the commonly traded stainless grades, calculated from the composition windows of the 167 stainless grades in the Laxcon grade reference.

The formula and its variants

The common form of the index is PREN = %Cr + 3.3 x %Mo + 16 x %N. Chromium builds the passive film, molybdenum repairs the film where chloride ions have broken through, and nitrogen stabilises the film and slows the local acidification inside a pit once one has started. The coefficients are empirical, fitted to critical pitting temperature data, and are not derived from theory.

Two variants are in circulation, and quoted figures cannot be compared without knowing which was used:

• 16 x N against 30 x N. The coefficient of 16 is the general form. A coefficient of 30 is often applied to duplex grades, in which nitrogen partitions into the austenite phase and contributes more strongly. Substituting 30 for 16 adds about 4 points to a duplex with 0.30 percent nitrogen, so a duplex figure quoted without its coefficient is not interpretable.
• PREW, which adds tungsten. PREW = %Cr + 3.3 x (%Mo + 0.5 x %W) + 16 x %N. In common trade this affects one grade, S32760, which specifies tungsten up to 1.0 percent. At the top of its window the grade reads 44.0 as PREN and 45.6 as PREW, a difference of 1.6 points on the same steel.

Every figure in this article uses the 16 x N form, applied to the composition limits published in the grade reference. Where a specification gives no minimum for an element, the minimum is taken as zero, which is what the specification permits.

PREN bands across the register

The grade reference carries 547 grades, of which 167 are stainless: 71 austenitic, 46 martensitic, 26 ferritic, 15 duplex and 9 precipitation hardening. Of the 167, 65 specify molybdenum, 61 specify nitrogen, and 78 specify neither, so that their PREN equals their chromium content. The following table gives the band each commonly traded grade allows, calculated from its own composition window. Each grade name links to its full chemistry.

Several of the bands overlap. The overlap is not visible when each grade is reduced to a single nominal value, and it is the basis of the comparisons in the sections that follow.

The width of the 316 band

Grade 316 permits a PREN anywhere from 22.6 to 29.5, and both ends are fully compliant. The gap is 6.9 points, or 31 percent of the lower figure. The lean end corresponds to chromium 16.0, molybdenum 2.0 and no nitrogen; the rich end to chromium 18.0, molybdenum 3.0 and nitrogen 0.10. The grade name does not distinguish between them, so two certified 316 heats can differ materially in chloride resistance.

The width has practical consequences. A common specification threshold for seawater-splash or chlorinated service is PREN 25. A 316 heat at 22.6 falls below that threshold; a 316 heat at 29.5 exceeds it. Where a service condition requires PREN 25, the grade name alone does not establish compliance; the certified analysis of the individual heat does. The same applies, less strongly, to grade 304, whose band of 18.0 to 21.6 arises from its 2.0 percent chromium range plus the nitrogen the specification tolerates but does not require.

Limitations of the index

The 200 series scores higher than its service performance

AISI 202 permits a PREN of 17.0 to 23.0 and AISI 201 permits 16.0 to 22.0. The upper end of both bands lies above the lower end of the 304 band, which starts at 18.0, so on the formula alone a nitrogen-rich 202 heat outranks a lean 304 heat.

The comparison is misleading because the formula contains no term for nickel. The 200-series grades reach their PREN values on nitrogen, with manganese substituting for nickel, yet nickel is what stabilises the austenite and largely governs resistance to stress corrosion cracking and to general corrosion in acids. Grade 202 carries 4.0 to 6.0 percent nickel against 8.0 to 10.5 percent in 304, a difference the formula cannot register. Substituting a 200-series grade for 304 on a PREN comparison is the most common misuse of the number in the Indian market, and it is usually a cost decision presented as a technical one.

A grade with no molybdenum and no nickel can exceed 316

AISI 446 is a ferritic grade with chromium 23.0 to 27.0, no molybdenum and nitrogen tolerated to 0.25 percent. Its PREN band of 23.0 to 31.0 straddles the 316 band entirely and tops out above it, at a fraction of the alloy cost, because it contains no nickel. Grade 446 is nevertheless not a general substitute for 316: ferritic grades lose toughness at low temperature, are harder to weld in section, and cannot be used where a design assumes austenitic ductility. The index is silent on all three properties.

The index is a ranking, not a service life

The number has no units and predicts nothing on its own. It does not account for surface condition, although a pickled and passivated surface resists pitting far better than a ground or as-welded one at identical chemistry. It says nothing about crevice corrosion, which initiates at lower temperatures than pitting under gaskets and flange faces. It says nothing about welds, where the heat-affected zone can be the weakest chemistry on a component. It does not address sensitisation, since carbon is absent from the formula; sensitisation is the reason grades 304 L and 316 L exist. The index is suited to sorting candidate grades; qualification of a specific grade for a specific service requires critical pitting temperature testing.

The 2507 designation and the super duplex threshold

By trade convention, a duplex grade qualifies as super duplex at a PREN of 40 or above. Calculated from its own specification limits, grade 2507 (S32750) has a band of 37.7 to 47.6, at 9.9 points the widest of any commonly traded grade in the register. The lower bound of 37.7 corresponds to chromium 24.0, molybdenum 3.0 and nitrogen 0.24, all legitimate specification minima. A heat at that composition is compliant S32750 and is not super duplex by the PREN 40 convention, so the grade designation does not itself carry the guarantee often assumed of it.

Where a project specification requires a minimum PREN of 40, the requirement is effective only as a certified minimum on the heat analysis in the purchase order, not as a grade name. Specifications with this requirement generally state it explicitly. NORSOK M-650 qualification, which Laxcon holds through TUV India, exists because grade names are insufficient at this end of the range.

Overlap between 904 L and 2205

A 904 L heat can rank below or above a 2205 heat, depending on where each composition falls inside its window.

Grade 2205 has the narrower window because its specification is tighter at both ends: chromium is held to a single percentage point and nitrogen has a true minimum of 0.14 rather than being merely tolerated. A narrow window is itself a purchasable property, since successive heats fall closer to the ordered composition. Grade 904 L costs considerably more than 2205 on nickel content alone, at 23.0 to 28.0 percent nickel against 4.5 to 6.5, and a 904 L heat delivered at PREN 32.2 carries that premium without the expected pitting resistance, an outcome the grade name does not signal.

Specification practice

Four elements of a purchase order make a PREN requirement enforceable where a grade name alone is not:

1. The grade named against one standard. A wording such as "316 or equivalent" is indeterminate: AISI 316 and DIN 1.4401 do not have identical windows, and neither do their equivalents in JIS or UNS. The equivalents tables exist to read a designation across standards on a drawing, not to make substitutions automatic.
2. The required PREN minimum, with the formula stated. For example, "PREN 25 minimum, calculated as Cr + 3.3Mo + 16N on the certified ladle analysis." Without the formula, a duplex supplier using 30 x N and a buyer using 16 x N are not describing the same steel.
3. The certified analysis, not the grade declaration. At Laxcon, every heat is sampled from the ladle during pouring and analysed on optical emission spectrometers, and the analysis is issued on the test certificate for that heat; PREN follows from it in one line of arithmetic. The PREN of a specific heat can be requested from sales@laxconsteels.com with the heat number. Grade confirmation on the finished piece is a separate step, performed with handheld spectrometers for positive material identification.
4. A stated consequence for non-compliance. A PREN minimum with no consequence attached is a preference rather than a specification.

The Laxcon laboratory at Sanand is NABL accredited and assessed to ISO/IEC 17025:2017, so the chemistry on the certificate is traceable and the accreditation is independently held rather than self-declared. The full certification list is on the quality page.

Frequently asked questions

Is a higher PREN always better?

A higher PREN is better only against chloride pitting, and only when nothing else changes. Higher PREN grades usually cost more, are harder to machine and weld, and in the duplex family carry fabrication rules on heat input and interpass temperature that carbon-steel practice violates. In dry indoor service a PREN 18 grade lasts as long as a PREN 40 grade at a fraction of the cost.

Which PREN do I need for seawater?

No single PREN figure covers seawater, because temperature governs. Ambient-temperature seawater is commonly handled at PREN 40 and above, which is the application that super duplex and the 6 percent molybdenum austenitics serve. Warm or stagnant seawater defeats even those grades, and the qualifying measure is a critical pitting temperature test to ASTM G48 or an equivalent method, not a calculated number. PREN produces the shortlist; the test produces the decision.

Can I calculate PREN from my mill test certificate?

PREN can be calculated from a mill test certificate, and this is the only calculation that describes the steel actually supplied. The chromium, molybdenum and nitrogen values from the certified heat analysis, entered into Cr + 3.3Mo + 16N, give the figure for that heat rather than for the grade. Where nitrogen is not reported, it is treated as zero and the result is a lower bound.

Why do published PREN values differ between sources?

Published PREN values differ between sources for three reasons. The nitrogen coefficient may be 16 or 30. The source may use a typical or nominal composition rather than the specification limits. And the grade may be specified differently in the standard the source used, since the AISI, EN, JIS and UNS windows do not coincide.

Does PREN apply to martensitic and precipitation hardening grades?

PREN can be calculated for martensitic and precipitation hardening grades, but it is rarely the deciding number for either family. Martensitic grades in the register run from 11.5 to 17.1 and are selected for hardness and wear resistance rather than corrosion resistance. The precipitation hardening grades run from 11.0 to 25.9 and are selected for strength. In both families the heat treatment condition affects corrosion behaviour more than the small differences in PREN do.

What is the highest PREN grade in your register?

The highest PREN grade in the register is 654 SMO, with a band of 54.3 to 60.2. It reaches that level on 7 to 8 percent molybdenum and 0.45 to 0.55 percent nitrogen, with 21 to 23 percent nickel holding the structure austenitic. It is a specialised alloy and priced accordingly.

Sources

Composition windows for all 547 grades, including the 167 stainless grades used here, are published in full in the Laxcon grade reference, with cross-standard pairings in the equivalents tables. The PREN bands in this article are calculated from those windows using PREN = Cr + 3.3Mo + 16N, taking the specification minimum and maximum for each element and treating an unspecified minimum as zero. Laboratory accreditation, spectrometry and inspection facilities are listed on the quality page, and product size ranges, tolerances and supply conditions on the individual product pages. PREN figures for a specific heat are available from Laxcon against the heat number at sales@laxconsteels.com.