--- title: "Pitting Resistance Equivalent Number Calculations" description: "Calculate PREN from chromium, molybdenum, and nitrogen, compare variants, and understand its limits versus ASTM G48 testing." canonical: https://steelequivalents.com/wiki/calculated-values/pitting-resistance-equivalent-number-calculations language: en robots: index, follow og_type: article type: wiki-article language_alternates: en: https://steelequivalents.com/wiki/calculated-values/pitting-resistance-equivalent-number-calculations.md cs: https://steelequivalents.com/cs/wiki/vypočítané-hodnoty/výpočty-ekvivalentního-čísla-odolnosti-proti-důlkové-korozi.md de: https://steelequivalents.com/de/wiki/berechnete-kennwerte/berechnungen-der-lochfraßbeständigkeitszahl.md es: https://steelequivalents.com/es/wiki/valores-calculados/cálculos-del-número-equivalente-de-resistencia-a-la-picadura.md fr: https://steelequivalents.com/fr/wiki/valeurs-calculées/calculs-du-nombre-équivalent-de-résistance-à-la-piqûration.md hu: https://steelequivalents.com/hu/wiki/számított-értékek/a-lyukkorróziós-ellenállási-egyenértékszám-számításai.md it: https://steelequivalents.com/it/wiki/valori-calcolati/calcoli-del-numero-equivalente-di-resistenza-alla-vaiolatura.md nl: https://steelequivalents.com/nl/wiki/berekende-waarden/berekeningen-van-het-pitting-resistance-equivalent-number.md pl: https://steelequivalents.com/pl/wiki/wartości-obliczane/obliczenia-liczby-równoważnej-odporności-na-wżery.md pt: https://steelequivalents.com/pt/wiki/valores-calculados/cálculos-do-número-equivalente-de-resistência-à-corrosão-por-picadas.md ru: https://steelequivalents.com/ru/вики/расчётные-величины/расчёт-эквивалентного-числа-стойкости-к-питтингу.md tr: https://steelequivalents.com/tr/wiki/hesaplanan-değerler/çukurcuklanma-direnci-eşdeğer-sayısı-hesaplamaları.md zh: https://steelequivalents.com/zh/维基/计算得出的数值/点蚀抗力当量数计算.md ja: https://steelequivalents.com/ja/ウィキ/計算で求める値/耐孔食当量数-pren-の計算.md ar: https://steelequivalents.com/ar/ويكي/القيم-المحسوبة/حسابات-رقم-المقاومة-المكافئة-للتنقر.md ko: https://steelequivalents.com/ko/위키/계산으로-얻는-값/공식-저항-당량-수-계산.md pt-br: https://steelequivalents.com/pt-br/wiki/valores-calculados/cálculos-do-número-equivalente-de-resistência-à-corrosão-por-pites.md hi: https://steelequivalents.com/hi/विकी/परिकलित-मान/पिटिंग-प्रतिरोध-समतुल्य-संख्या-की-गणना.md content_hash: sha256:b316aaba39bd836649a94bf42916e6ec97b962b6b522d7a71e99bf9964161926 generated: 2026-09-24 --- > **Machine-readable Markdown rendering.** This is a Markdown representation of a web page, > served for AI agents and crawlers as a token-efficient alternative to the full HTML. > Full interactive HTML version: https://steelequivalents.com/wiki/calculated-values/pitting-resistance-equivalent-number-calculations > Canonical URL: https://steelequivalents.com/wiki/calculated-values/pitting-resistance-equivalent-number-calculations > Available languages: en, cs, de, es, fr, hu, it, nl, pl, pt, ru, tr, zh, ja, ar, ko, pt-br, hi > Change detection: `content_hash` is the SHA-256 of this document's Markdown body — everything > below this block, whitespace-trimmed, excluding the front-matter. Compare it to detect a real > content change; `generated` moves on every render and is not a change signal. ![Pitting Resistance Equivalent Number Calculations](/images/uploads/958e5c85-7eea-4256-8b87-69f73223d656/wiki-hero-a-stainless-steel-surface-showing-a-chromium-rich-passive-film-chloride-ions-rea-1920x823.jpg) Calculated Values # Pitting Resistance Equivalent Number Calculations Calculate PREN from chromium, molybdenum, and nitrogen, compare variants, and understand its limits versus ASTM G48 testing. ![Portrait of Anders Bergström, steel industry reporter](/images/uploads/a7d8e5bc-7cdf-409b-913b-211e63e8c441/anders-bergstr-m-1920x1920.jpg) **[Anders Bergström](/news/author/anders-bergstrom "Anders Bergström")** Calculated Values 60+ min read Updated Aug 14, 2026 Evidence-reviewed On this pageOn this page - [What the Pitting Resistance Equivalent Number Measures](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#what-the-pitting-resistance-equivalent-number-measures "What the Pitting Resistance Equivalent Number Measures") - [The Common PREN Formula](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#the-common-pren-formula "The Common PREN Formula") - [How to Calculate PREN Step by Step](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#how-to-calculate-pren-step-by-step "How to Calculate PREN Step by Step") - [Why Chromium, Molybdenum and Nitrogen Matter](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#why-chromium-molybdenum-and-nitrogen-matter "Why Chromium, Molybdenum and Nitrogen Matter") - [PRENW: The Tungsten-Containing Variant](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#prenw-the-tungsten-containing-variant "PRENW: The Tungsten-Containing Variant") - [PRENMn and the Treatment of Manganese](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#prenmn-and-the-treatment-of-manganese "PRENMn and the Treatment of Manganese") - [PREN Across Ferritic, Duplex and Other Stainless Steels](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#pren-across-ferritic-duplex-and-other-stainless-steels "PREN Across Ferritic, Duplex and Other Stainless Steels") - [Nickel, Alloy Design and a Common Misreading](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#nickel-alloy-design-and-a-common-misreading "Nickel, Alloy Design and a Common Misreading") - [PREN Versus ASTM G48 Testing](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#pren-versus-astm-g48-testing "PREN Versus ASTM G48 Testing") - [Limits of PREN in Real Service Environments](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#limits-of-pren-in-real-service-environments "Limits of PREN in Real Service Environments") - [How to Report a PREN Calculation Responsibly](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#how-to-report-a-pren-calculation-responsibly "How to Report a PREN Calculation Responsibly") ## What the Pitting Resistance Equivalent Number Measures ### PREN as an empirical comparison index #### Interpret the number narrowly A higher PREN generally supports a higher resistance ranking under comparable metallurgical and environmental conditions. It is not a pass/fail certificate, guaranteed service limit or measured corrosion rate. The Pitting Resistance Equivalent Number (PREN) is a composition-based, semi-quantitative index for comparing the resistance of [stainless steels](/categories/stainless-steels "stainless steels") to localized corrosion, particularly pitting in chloride-containing environments. It is not a corrosion rating on a universal scale. A higher number generally indicates a greater tendency to resist pit initiation under comparable conditions, but the number does not establish a guaranteed service limit, a fixed pitting potential, or a predicted operating life. The commonly used expression is: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) PREN **PREN** Pitting Resistance Equivalent Number: an empirical, composition-based index used to compare stainless steels for resistance to localized corrosion, especially pitting in chloride-containing environments. The alloying contents are weight percentages. Chromium, molybdenum and nitrogen are the terms in this expression, and each is assigned an empirical coefficient based on its relative contribution to localized-corrosion resistance. The British Stainless Steel Association published this form in its 2025 explanation of PREN calculations. For example, an alloy containing 22% chromium, 3% molybdenum and 0.18% nitrogen would have: 22+3.3⁢(3)+16⁢(0.18)=34.78 That result is a calculated index, not a measured corrosion rate. It allows the composition of one stainless steel to be compared with another on a common basis. A nominally higher value usually places the alloy in a higher resistance category, provided the materials are compared under similar metallurgical and environmental conditions. The equation also explains why grade comparisons require more than reading a product designation. Type 304, commonly represented by UNS S30400, contains little or no molybdenum and therefore normally has a lower PREN than Type 316L, UNS S31603, whose composition includes molybdenum. Duplex stainless steels such as UNS S32205 and UNS S31803 contain substantial chromium, molybdenum and nitrogen, producing a higher calculated value than many austenitic grades. Superduplex grades such as UNS S32750 and UNS S32760 generally calculate higher still. Ferritic grades can also have high chromium and molybdenum contents, so PREN is not limited to one stainless-steel family. The number does not include every element that affects corrosion. Nickel is not included in the common equation, as the Society of Petroleum Engineers stated in 2025. That omission does not mean nickel has no metallurgical or corrosion significance. Nickel affects phase balance, passive-film behavior, toughness and resistance to some reducing environments, but its contribution is not represented by the standard PREN coefficients. Carbon, silicon, copper, manganese and other elements can also influence performance without appearing in the basic expression. PREN variants must be labelled because their numerical results are not interchangeable. | Convention | Expression | When to identify it | |---|---|---| | Common PREN | %Cr + 3.3(%Mo) + 16(%N) | General composition comparison | | PRENW | %Cr + 3.3(%Mo + 0.5%W) + 16(%N) | Tungsten-bearing alloys | | PRENMn | %Cr + 3.3(%Mo) + 16(%N) − %Mn | Manganese-adjusted comparison | \[1\] \[1\] [**Duplex Stainless Steels**](https://worldstainless.org/wp-content/uploads/2025/02/ISSF_Duplex_Stainless_Steels.pdf). International Stainless Steel Forum. International Stainless Steel Forum publication, 2025.Alternative equations are used for particular alloy families or calculation conventions. The International Stainless Steel Forum reported the tungsten-bearing expression in 2025 as: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) Here, tungsten receives half the molybdenum term inside the parenthesis. The same ISSF publication gives a manganese-adjusted expression: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn These values should not be mixed casually. A reported PREN must identify the equation used, particularly when comparing tungsten-bearing duplex stainless steels or applying the manganese adjustment. Calling every result simply “PREN” can conceal a meaningful difference in calculation method. PREN is therefore useful as a screening and ranking tool. It helps engineers identify compositional differences that may matter in chloride service and provides a quick comparison before testing or detailed design review. It does not replace the material certificate, heat-treatment history, weld assessment or corrosion test. ![Schematic comparing a surface pit with crevice corrosion beneath a gasket.](/images/uploads/e8313d63-f0ed-4692-b16a-71fbc5d2c702/wiki-inline-a-stainless-steel-surface-comparing-open-surface-pitting-with-crevice-corrosion-1520x1920.jpg)[](/images/uploads/e8313d63-f0ed-4692-b16a-71fbc5d2c702/wiki-inline-a-stainless-steel-surface-comparing-open-surface-pitting-with-crevice-corrosion-2027x2560.avif "Enlarge image — Schematic comparing a surface pit with crevice corrosion beneath a gasket.")Pitting and crevice corrosion are related localized attacks but develop under different conditions. ### Pitting, crevice corrosion and chloride exposure Pitting is a localized breakdown of the passive film. Stainless steel normally protects itself through a thin chromium-rich oxide film, but chloride ions can concentrate at defects or weak points in that film. Once a pit begins, its interior can become acidic and enriched in chloride, sustaining an electrochemical environment that differs sharply from the surrounding surface. A small surface opening can then conceal significant depth. Molybdenum and nitrogen improve resistance to this form of attack through several interacting effects on passivity and pit chemistry. Chromium supports formation and repair of the passive film. The PREN equation treats the effects as additive, but actual alloy behavior depends on how the elements are distributed in the microstructure and whether they remain available in the surrounding matrix. Crevice corrosion is related but not identical. It develops in shielded gaps, such as beneath gaskets, at lap joints, under deposits or inside poorly flushed connections. Oxygen depletion inside the crevice changes the electrochemical conditions, while chloride and acidic species can concentrate there. A steel may resist open-surface pitting yet perform poorly in a tight crevice, especially when the crevice geometry permits stagnant solution. Higher PREN generally corresponds to greater resistance to localized attack in chloride-containing solutions. Limited evidence \[2\] \[2\] [**Localized corrosion resistance and PREN relationship**](https://www.mdpi.com/2075-4701/11/5/836). Metals authors. Metals, 2021.This distinction matters because the name PREN is often used too broadly. The equation is principally associated with pitting resistance, although higher values often correlate with improved resistance to other forms of localized attack. The 2021 Metals paper reported that higher PREN corresponds to greater resistance to localized attack in chloride-containing solutions. That is a comparative relationship, not a promise that all localized-corrosion mechanisms will respond in the same way. #### Variables outside the equation - Chloride concentration and temperature can change pit initiation and growth. - Acidity, oxygen availability and flow alter electrochemical conditions. - Deposits, crevices and surface condition create local environments not represented by bulk chemistry. - Exposure time and fabrication history can change observed performance. Chloride concentration, temperature, acidity, oxygen availability, flow, deposits and exposure time can change the result substantially. Seawater, a hot chloride process solution and a ferric-chloride laboratory bath are not interchangeable environments. Chloride-bearing soils add further variables, including moisture retention, differential aeration and contamination. The ISSF stated in 2025 that a higher value indicates semi-quantitatively greater resistance to localized corrosion in chloride-bearing soils; “semi-quantitatively” is important. It signals a ranking tendency rather than a field-life formula. ASTM G48 provides defined ferric-chloride laboratory methods for comparing resistance to pitting and crevice-corrosion initiation. Strong evidence ASTM G48 provides a way to test the distinction between calculation and validation. ASTM G48:2011 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Depending on the selected method, results can include a critical pitting temperature, a critical crevice temperature or mass loss after exposure. Such tests impose controlled conditions and make comparisons possible, but ferric chloride is deliberately severe and does not reproduce every plant, marine or soil environment. #### Do not qualify a weld from parent-metal PREN alone A parent-metal calculation cannot establish the corrosion behavior of weld metal or the heat-affected zone. Assess phase balance, heat tint, dilution, surface treatment and the applicable weld procedure. Welding adds another complication. A welded joint can contain heat-affected zones, altered phase balance, oxide scale, residual stress and chromium- or molybdenum-depleted regions. Calculating PREN from the parent-metal specification cannot establish the corrosion behavior of the weld metal or heat-affected zone. Duplex stainless steels require particular attention to ferrite-austenite balance and nitrogen retention; a nominal chemistry alone does not verify that the finished weld has the intended resistance. ### What a PREN value cannot predict A PREN value cannot predict a direct service life. It does not say that an alloy with a value of 40 will last twice as long as one with a value of 20, nor does it define a universal boundary such as “above this number, pitting cannot occur.” No single PREN threshold applies to every chloride concentration, temperature, surface condition, geometry or fabrication route. It also cannot predict an exact pitting potential. Pitting potential is measured electrochemically and varies with test solution, temperature, scan method, surface preparation and specimen condition. PREN may help explain why one composition tends to show a higher pitting resistance than another, but it does not produce the measured voltage. Nor can PREN establish resistance to general corrosion. Uniform corrosion, galvanic corrosion, erosion-corrosion, stress-corrosion cracking and corrosion fatigue involve different controlling factors. An alloy may have a favorable PREN and still suffer unacceptable general attack in a strong acid, cracking in a hot chloride environment or accelerated damage where velocity removes the passive film. Nickel’s absence from the equation is especially relevant when evaluating environments in which phase stability or reducing-acid resistance matters. The calculation also cannot account fully for inclusions, segregation, surface contamination, roughness, iron contamination, heat tint, crevice dimensions or deposits. These features can create local conditions that dominate the nominal bulk composition. A polished laboratory specimen and an oxidized weld root made from the same heat of steel do not present the same corrosion surface. For that reason, PREN should be reported with the equation, composition basis and alloy designation. It is a valuable empirical comparison index for chloride-related localized-corrosion resistance. It is not a pass/fail certificate, a universal corrosion rating or a substitute for ASTM G48 testing, site-specific evaluation and sound fabrication control. ## The Common PREN Formula The commonly used Pitting Resistance Equivalent Number calculation is: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) Here, %⁢Cr, %⁢Mo, and %⁢N mean the chromium, molybdenum, and nitrogen contents of the alloy, each expressed as a percentage by mass. The result is a dimensionless number. It is not a percentage, a corrosion rate, or a guaranteed service-life value. This expression is an empirical composition-based index. It is used to rank or compare stainless steels for resistance to localized corrosion, especially pitting, in chloride-containing environments. The British Stainless Steel Association reproduced the equation in this form in 2025, while Outokumpu describes PRE or PREN as an index for comparing stainless steels rather than a direct measurement of corrosion performance. A higher value generally indicates greater resistance to chloride-induced localized attack, but the comparison has meaning only when the alloys and test or service conditions are considered together. ![Diagram showing chromium, molybdenum and nitrogen in a stainless-steel microstructure and passive film.](/images/uploads/b2e69fcf-757a-495a-bb0d-8a47dd7a81da/wiki-inline-a-stainless-steel-alloy-cross-section-showing-chromium-molybdenum-and-nitrogen-c-1920x1094.jpg)[](/images/uploads/b2e69fcf-757a-495a-bb0d-8a47dd7a81da/wiki-inline-a-stainless-steel-alloy-cross-section-showing-chromium-molybdenum-and-nitrogen-c-1920x1094.avif "Enlarge image — Diagram showing chromium, molybdenum and nitrogen in a stainless-steel microstructure and passive film.")The common PREN expression weights chromium, molybdenum and nitrogen as separate composition terms. ### The chromium, molybdenum and nitrogen terms Chromium supplies the basic corrosion-resisting character of stainless steel. In air or an oxidizing aqueous environment, chromium supports formation of a thin chromium-rich passive film on the metal surface. That film reduces the rate at which iron enters solution. Stainless steel standards commonly require at least about 10.5% chromium for the stainless designation, but PREN calculations are mainly used to distinguish among alloys containing substantially more chromium than that minimum. Molybdenum receives a multiplier of 3.3 in the common equation. Its contribution is associated particularly with improved resistance to the development and propagation of pits in chloride-bearing solutions. Chromium and molybdenum do not perform interchangeable functions, and a steel with more chromium is not automatically equivalent to one with more molybdenum. The formula assigns each element a separate term because their effects on localized corrosion are different. Nitrogen has the largest coefficient: 16. In suitable stainless-steel microstructures, nitrogen can raise resistance to pitting while also strengthening austenite and affecting phase balance. Its metallurgical role is especially important in duplex stainless steels, where nitrogen helps support the austenitic phase alongside ferrite. Nitrogen must remain in solution or in appropriate alloy phases for the intended benefit; a nominal chemical analysis alone does not describe every effect of processing, heat treatment, welding, or segregation. PREN contribution The common equation assigns different empirical weights to chromium, molybdenum and nitrogen. The numerical weighting is easy to see. One additional percentage point of chromium adds 1 PREN unit. One additional percentage point of molybdenum adds 3.3 units, and one additional percentage point of nitrogen adds 16 units. Thus, 2% Mo contributes 6.6 units, while 0.20% N contributes 3.2 units. Those values do not mean that nitrogen is physically “16 times better” than chromium in every corrosion situation. They express the scale selected for this particular empirical correlation. Nickel is not included in this common equation. That omission does not mean nickel has no metallurgical or corrosion-related importance. Nickel stabilizes austenite, influences phase balance, affects passivity and repassivation, and is present in many austenitic and duplex grades. It simply is not one of the composition terms in the standard three-element PREN expression. The Society of Petroleum Engineers made this distinction explicitly in its 2025 discussion of PREN-based material selection. For the same reason, PREN does not account directly for carbon, silicon, manganese, copper, tungsten, sulfur, phase distribution, inclusion population, surface condition, or weld-metal dilution in its basic form. These factors can affect actual localized-corrosion behavior even when two steels have similar calculated PREN values. A simple comparison illustrates the calculation. Consider an alloy containing 22.0% Cr, 3.0% Mo, and 0.18% N: PREN=22.0+3.3⁢(3.0)+16⁢(0.18) PREN=22.0+9.9+2.88=34.78 The result is conventionally reported as PREN 34.8, or sometimes rounded to PREN 35. The rounding should not be mistaken for a precision level that the underlying corrosion behavior can support. ### Why the coefficients are 3.3 and 16 The coefficients 3.3 and 16 are empirical weighting factors. They were developed from observed relationships between stainless-steel composition and resistance to localized corrosion, rather than derived from a fundamental thermodynamic equation for pit initiation. The formula condenses several experimental trends into a convenient numerical index. Chromium is assigned a coefficient of 1, establishing the reference scale. Molybdenum is given a coefficient of 3.3 because changes in molybdenum content were found to have a stronger effect on localized-corrosion resistance than equal changes in chromium content within the alloy ranges used to establish the correlation. Nitrogen is assigned 16 because relatively small changes in nitrogen were associated with a comparatively large effect in the relevant stainless-steel data. Nitrogen contribution The nitrogen term rises linearly because nitrogen is multiplied by 16 in the common equation. That is the practical reason a small nitrogen addition can materially change a calculated value. Increasing nitrogen from 0.10% to 0.20% adds 16×0.10=1.6 PREN units. Increasing chromium by the same numerical amount, 0.10 percentage points, adds only 0.10 unit. The arithmetic is clear; the physical interpretation must remain limited. The coefficient does not establish a universal conversion between alloying elements, and it does not predict identical electrochemical behavior across ferritic, austenitic, martensitic, precipitation-hardening, and duplex stainless steels. The equation is therefore best treated as a ranking convention within a relevant group of alloys. The 2021 *Metals* paper identified the usual relationship that a higher PREN corresponds to greater resistance to localized attack in chloride-containing solutions, but that relationship is semi-quantitative. It does not say that every alloy with PREN 40 will outperform every alloy with PREN 35 in every environment. Processing can break a simple composition ranking. A weld heat-affected zone may have a different phase balance from the parent plate. Intermetallic phases, chromium nitrides, sulfide inclusions, surface contamination, crevices, and poor heat treatment can all create sites for pit initiation. Chloride concentration, temperature, acidity, oxygen availability, flow, deposits, and electrochemical potential also influence the result. PREN cannot encode those service variables. \[3\] \[3\] [**Standard test methods for pitting and crevice corrosion resistance of stainless steels**](https://store.astm.org/g0048-11.html). ASTM International. ASTM G48, 2011. Validation requires testing or field-specific corrosion assessment. ASTM G48 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to the initiation of pitting and crevice corrosion under defined ferric-chloride conditions. An ASTM G48 result is not the same thing as a PREN value. PREN is calculated from composition; ASTM G48 produces a measured response under a specified test procedure. Neither one, on its own, reproduces every service environment. ### Weight percentages and notation Weight percentage **Weight percentage** The mass percentage of an element in the alloy. PREN inputs are not atomic percentages, volume fractions or decimal mass fractions. The percent signs in the formula refer to weight percentages, also called mass percentages. They do not refer to atomic percentages, volume fractions, or the percentage of an element remaining in the passive film. If a mill certificate reports 20.00% Cr, 2.50% Mo, and 0.10% N, those reported mass percentages are inserted directly: PREN=20.00+3.3⁢(2.50)+16⁢(0.10) PREN=20.00+8.25+1.60=29.85 The notation %⁢Cr is shorthand for “chromium content in weight percent.” Parentheses show that the coefficient multiplies the complete elemental term. It is not correct to read 3.3⁢(%⁢Mo) as 3.3% molybdenum, or to multiply the entire PREN result by 3.3. The formula also has variants for alloy systems containing tungsten or for comparisons where manganese is treated as a reducing factor. The International Stainless Steel Forum reported the tungsten-bearing expression in 2025 as: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) The 0.5⁢%⁢W notation means one-half of the tungsten weight percentage is added to the molybdenum term before multiplication by 3.3. ISSF also reported: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn These are not interchangeable labels for the basic equation. The chosen expression must be identified when a value is reported, particularly for tungsten-bearing duplex grades or technical comparisons that apply the manganese adjustment. For a meaningful calculation, use the composition of the actual product or weld zone being assessed, keep all inputs in the same weight-percent basis, and state the equation used. Chemical ranges in a grade specification can produce a range of possible PREN values rather than one exact number. A calculated value is a useful screening and comparison tool. It is not a universal corrosion rating. ## How to Calculate PREN Step by Step The commonly used Pitting Resistance Equivalent Number is calculated as: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) Here, chromium, molybdenum and nitrogen are reported as weight percentages, not fractions, atomic percentages or mass concentrations in a solution. The result is a dimensionless index. It is not a percentage and does not carry units. This equation is the form quoted by the British Stainless Steel Association in 2025. PREN is an empirical, composition-based index for ranking and comparing stainless steels in relation to localized corrosion, particularly pitting, in chloride-containing environments. A larger calculated value generally indicates greater resistance under comparable conditions. It does not directly predict service life, guarantee a particular pitting potential or replace corrosion testing. #### Calculation workflow 1. **1. Identify the material** Use the grade, product form and applicable specification. 2. **2. Select the chemistry basis** Distinguish measured heat analysis, nominal composition and specification limits. 3. **3. Confirm the units** Use chromium, molybdenum and nitrogen as weight percentages. 4. **4. Select the equation** State whether the result is PREN, PRENW or PRENMn. 5. **5. Substitute and calculate** Show each elemental contribution before summing. 6. **6. Report and qualify** State rounding, source and the limits of the comparison. The calculation is simple. Selecting the correct composition basis is not. ### Reading composition data from a material specification Begin with the material document, not a generic grade table. A mill test certificate, heat analysis, product specification or standard may present several different kinds of chemical information. The document may give a nominal composition, a specified range, a maximum permitted value, a minimum value, or the measured analysis of a particular heat. These values are not interchangeable. For a hypothetical stainless-steel heat analysis, suppose the certificate reports: - Cr: 22.10 wt% - Mo: 3.05 wt% - N: 0.185 wt% These figures are invented for calculation practice only. They do not represent a standard composition, a registered grade or product data. The relevant entries are the measured chromium, molybdenum and nitrogen contents. Nickel does not appear in the common PREN equation. The Society of Petroleum Engineers stated this point directly in 2025: nickel is not included in the equation. That omission does not mean nickel has no metallurgical effect; it means the selected empirical expression assigns no separate nickel term. Read the symbols carefully. “Cr” means chromium, “Mo” means molybdenum and “N” means nitrogen. Do not substitute the total alloy content, the balance designation or a value from a different heat. If a certificate lists “\<0.010” for nitrogen, that is not a measured value of 0.010%; it is a reporting limit or upper bound. A calculation using 0.010% would be an assumption and must be identified as such. The source basis should be written beside the result. For example: “PREN calculated from reported heat analysis” is materially different from “PREN calculated from nominal composition.” If the document gives only specification limits, report that fact rather than presenting the result as the alloy’s measured PREN. Consider a second hypothetical case based on a specification range: - Cr: 21.00–23.00 wt% - Mo: 2.50–3.50 wt% - N: 0.10–0.20 wt% This range does not produce one unique composition. It produces a range of possible calculated PREN values if the terms are varied independently. Calculated PREN The same hypothetical specification range can produce different calculated values depending on the composition basis. Using the lower listed values: %⁢Cr=21.00 3.3⁢(%⁢Mo)=3.3⁢(2.50)=8.25 16⁢(%⁢N)=16⁢(0.10)=1.60 PRENlower=21.00+8.25+1.60=30.85 Using the upper listed values: %⁢Cr=23.00 3.3⁢(%⁢Mo)=3.3⁢(3.50)=11.55 16⁢(%⁢N)=16⁢(0.20)=3.20 PRENupper=23.00+11.55+3.20=37.75 The resulting interval, 30.85 to 37.75, is a mathematical envelope based on the stated limits. It is not necessarily the actual range of heats, because alloying elements may not reach all limits simultaneously and the specification may impose additional constraints. Still, it shows why a single figure drawn from the maximum values can overstate the value associated with a particular material. Nominal values create a third result. If the hypothetical nominal composition is Cr 22.00%, Mo 3.00% and N 0.15%, then: Chromium contribution=22.00 Molybdenum contribution=3.3⁢(3.00)=9.90 Nitrogen contribution=16⁢(0.15)=2.40 PREN=22.00+9.90+2.40=34.30 That 34.30 is a nominal-composition calculation, not a measurement of every heat sold or produced to that composition range. ### Substituting values without changing units #### Unit warning The stated equation expects values such as 22.10, 3.05 and 0.185 for wt%. Do not insert 0.2210, 0.0305 and 0.00185 without first converting the equation. The coefficients 3.3 and 16 are paired with weight-percent inputs. Entering percentages as decimal fractions changes the answer by a factor of 100 and is a common error. Using the hypothetical heat analysis above: \\\[ \\%Cr=22.10,\\quad \\%Mo=3.05,\\quad \\%N=0.185 \\\] Calculate each contribution separately: Chromium contribution=22.10 Molybdenum contribution=3.3⁢(3.05)=10.065 Nitrogen contribution=16⁢(0.185)=2.960 Then sum them: PREN=22.10+10.065+2.960=35.125 Before reporting, choose a rounding rule and apply it consistently. The same calculation using decimal mass fractions would require a different form of the equation. For example, 22.10 wt% is 0.2210 as a fraction, but inserting 0.2210 directly into the stated equation gives: 0.2210+3.3⁢(0.0305)+16⁢(0.00185)=0.35125 That is not the PREN convention expressed in the source formula. The equation expects 22.10, 3.05 and 0.185, not 0.2210, 0.0305 and 0.00185. Unit conversion is also important when a laboratory report uses parts per million. Nitrogen at 1,850 ppm by mass equals 0.185 wt%, because: 1850ppm÷10,000=0.185⁢% Only after this conversion should 0.185 be substituted into the equation. A report giving 0.0185% nitrogen would instead correspond to 185 ppm and would produce a different nitrogen contribution. For a tungsten-bearing stainless steel, the International Stainless Steel Forum gives the following expression: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) The tungsten term must be placed inside the molybdenum bracket. For a clearly hypothetical composition of Cr 24.00%, Mo 3.00%, W 2.00% and N 0.20%: Chromium contribution=24.00 Modified Mo term=3.3⁢(3.00+0.5⁢(2.00))=3.3⁢(4.00)=13.20 Nitrogen contribution=16⁢(0.20)=3.20 PRENW=24.00+13.20+3.20=40.40 Do not silently replace ordinary PREN with PRENW. State which expression was used. The ISSF also reports a manganese-adjusted form: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn For hypothetical values Cr 22.00%, Mo 3.00%, N 0.15% and Mn 1.00%, the separate terms are 22.00, 9.90, 2.40 and −1.00. Therefore: PRENMn=22.00+9.90+2.40−1.00=33.30 The formula name and composition basis belong in the report. ### Rounding, significant figures and reporting Keep extra digits during the arithmetic, then round the final result. With the hypothetical heat analysis, the unrounded result is 35.125. Reporting PREN 35.13 preserves two decimal places; reporting PREN 35.1 is more restrained when the composition data do not justify hundredths. The number of reported digits should reflect the input precision. If chromium is reported only as 22%, molybdenum as 3% and nitrogen as 0.2%, a result such as 35.127 is false precision. A sensible statement would be “calculated PREN ≈35,” with the nominal or rounded inputs identified. Do not round each contribution too early. In the heat-analysis example, rounding the molybdenum contribution from 10.065 to 10.1 and the nitrogen contribution from 2.960 to 3.0 before summing gives 35.1, whereas retaining the values gives 35.125. The difference is small here, but borderline comparisons can be affected. #### Minimum reporting fields - Grade designation and product form - Measured, nominal or specification-limit composition basis - Cr, Mo, N and any W or Mn values in wt% - Full equation and variant label - Substitution arithmetic and rounding rule - ASTM G48 method or other validation evidence, where applicable - Relevant chloride, temperature, surface and fabrication conditions A complete report should identify the equation, composition source and rounding convention: “PREN = 35.13, calculated using PREN = %Cr + 3.3(%Mo) + 16(%N) from the reported heat analysis; inputs in wt%; rounded to two decimal places.” If nominal values, specification minima or maxima were used, say so explicitly. Finally, calculation is not validation. ASTM G48 specifies laboratory methods for comparing stainless steels and related alloys for resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Actual performance also depends on microstructure, inclusions, welding, surface condition, temperature, chloride concentration, oxygen availability and crevice geometry. PREN helps compare composition-based potential; it does not reproduce every service environment. ## Why Chromium, Molybdenum and Nitrogen Matter The commonly used Pitting Resistance Equivalent Number is calculated as: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) The alloying contents are weight percentages. This arithmetic gives chromium, molybdenum and nitrogen different coefficients because each element has been associated, through empirical comparison, with the resistance of stainless steels to chloride-induced localized corrosion. The expression is not a mechanistic law and does not convert composition directly into a service-life prediction. It is a composition-based index for ranking or comparing alloys. The British Stainless Steel Association gave the expression above in 2025, while Outokumpu describes “PRE or PREN” as a composition-based index used to rank and compare stainless steels for resistance to pitting corrosion. The direction of the comparison is useful: a higher calculated value generally corresponds to greater resistance to localized attack in chloride-containing solutions. The number is still only one part of the assessment. Microstructure, heat treatment, surface condition, inclusions, welding, crevice geometry, temperature, chloride concentration, pH, oxidizing conditions and flow can change actual performance. ### Chromium and passive-film formation Chromium is the first term in the equation because it provides the chemical basis for stainless-steel passivity. When an iron-chromium alloy is exposed to air or an aqueous environment, chromium at the surface promotes formation of a thin, adherent oxide-rich passive film. This film sharply reduces the rate at which the underlying alloy dissolves compared with unalloyed iron or ordinary carbon steel. Stainless steel is therefore not corrosion-proof; its useful behavior depends on the film forming, remaining attached and repairing itself after minor damage. The familiar threshold of approximately 10.5% chromium is associated with the definition of stainless steel in many technical contexts, although the precise behavior depends on alloy chemistry and environment. Increasing chromium generally strengthens the capacity to form and maintain the passive film, which is why chromium receives a coefficient of 1 in the basic PREN expression. That coefficient should not be read as proof that chromium is less important than molybdenum or nitrogen in every circumstance. Chromium establishes the passive condition on which the other alloying effects act. Localized corrosion begins when a small area of the passive surface becomes unstable. Chloride ions can concentrate at flaws, inclusions, crevices or damaged areas, and the resulting electrochemical cell may cause a pit to grow even while most of the surface remains passive. Higher chromium can improve the alloy’s resistance to this transition, but the total chromium content does not describe its entire distribution. Chromium tied up in chromium nitrides or carbides is not equivalent, at the affected surface, to chromium available in the surrounding matrix. This is one reason sensitization, poor welding practice or unsuitable heat treatment can produce localized corrosion despite a respectable nominal PREN. Ferritic grades such as EN 1.4016, commonly designated X6Cr17, depend strongly on chromium for passivity but contain little or no molybdenum and nitrogen compared with higher-alloyed grades. Austenitic grades such as EN 1.4301, [X5CrNi18-10](/materials/material-no/1.4301 " — composition, equivalents and standards"), and EN 1.4404, [X2CrNiMo17-12-2](/materials/material-no/1.4404 " — composition, equivalents and standards"), show why chromium must be considered with the rest of the composition. The latter contains molybdenum as well as chromium, so its calculated PREN is higher than that of a typical 304-type alloy. The comparison is informative, not absolute. ### Molybdenum and localized breakdown resistance Molybdenum is assigned a coefficient of 3.3 in the common expression: 3.3⁢(%⁢Mo) This weighting reflects its strong empirical association with resistance to pit initiation and propagation in chloride-bearing environments. Molybdenum does not simply make the passive film “thicker.” Its effect is generally discussed in terms of improving the stability and repair of the passive state at sites where chloride has begun to attack, and of reducing the severity of localized dissolution. The exact mechanism varies with alloy structure, potential and environment, so the coefficient should be treated as an empirical comparison factor rather than a universal measure of molybdenum’s physical action. The distinction matters when comparing grades. EN 1.4404, X2CrNiMo17-12-2, contains approximately 2% molybdenum within its specified composition range, whereas EN 1.4301, X5CrNi18-10, does not intentionally rely on molybdenum for its corrosion resistance. A nominal calculation using representative chemistry gives the molybdenum-bearing grade a higher PREN. That result is consistent with its usual advantage in chloride-containing environments, but it does not guarantee immunity to pitting, particularly in warm, concentrated chloride solutions or under deposits and crevices. Molybdenum also appears in ferritic, austenitic and duplex stainless steels. In duplex grades, the element contributes to the high localized-corrosion resistance associated with compositions such as EN 1.4462, [X2CrNiMoN22-5-3](/materials/material-no/1.4462 " — composition, equivalents and standards"). Yet the measured behavior of a duplex weld or heat-affected zone can differ from that of correctly solution-annealed base metal. Phase balance, intermetallic phases, nitrogen distribution and chromium-molybdenum partitioning all matter. A bulk chemistry calculation cannot detect those local conditions. The International Stainless Steel Forum reports a tungsten-bearing form: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) Here tungsten is given half the contribution of molybdenum inside the molybdenum-related term. ISSF also reports a manganese-adjusted expression: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn These variants show that PREN conventions are not interchangeable labels. The selected expression should be identified, especially for tungsten-bearing or high-manganese alloys. ### Nitrogen in stainless-steel alloy design Nitrogen has the largest coefficient in the common equation, 16: 16⁢(%⁢N) That large coefficient does not mean nitrogen is sixteen times as important as chromium in every metallurgical event. It records an empirical contribution per unit weight percent in the alloys and test data from which the expression was developed. Nitrogen can improve resistance to localized corrosion, strengthen austenite, and influence the phase balance of duplex stainless steels. These effects make it particularly significant in modern alloy design. In duplex stainless steels, nitrogen helps stabilize the austenitic phase while chromium and molybdenum support ferritic-phase corrosion resistance. The target is not simply a high nitrogen number. The alloy must retain a suitable ferrite-austenite balance after fabrication, welding and cooling. Nitrogen loss or redistribution during welding can affect the weld metal and heat-affected zone, while excessive thermal exposure may promote unwanted precipitates that consume chromium or molybdenum from the surrounding matrix. Consequently, a duplex grade with a high calculated PREN can still suffer localized attack if its processed microstructure is incorrect. Nitrogen also contributes to the corrosion resistance and strength of high-alloy austenitic grades. In an alloy such as EN 1.4410, [X2CrNiMoN25-7-4](/materials/material-no/1.4410 " — composition, equivalents and standards"), nitrogen is part of a carefully balanced composition containing substantial chromium and molybdenum. Its role is combined with those elements, not separable from them in practical service. The same nominal nitrogen addition may produce different results depending on whether it remains dissolved in the matrix, forms nitrides, or is affected by segregated chemistry. Nickel is not included in the basic PREN equation, as the Society of Petroleum Engineers stated in 2025. That omission does not make nickel metallurgically unimportant. Nickel affects austenite stability, phase balance, transformation behavior and general corrosion performance, but the basic index was constructed around the principal composition terms used to compare localized-corrosion resistance. Nickel therefore cannot be added to the formula merely because it is an important stainless-steel alloying element. PREN calculation and corrosion validation are separate tasks. ASTM G48 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Those tests can distinguish materials under controlled conditions; they do not reproduce every chloride solution, deposit, weld, surface finish or operating temperature. PREN helps organize alloy comparisons. It does not replace ASTM G48 testing, field evidence or an assessment of the actual service environment. ## PRENW: The Tungsten-Containing Variant ### The PRENW expression PRENW **PRENW** A tungsten-bearing PREN convention in which one-half of the tungsten weight percentage is added to the molybdenum term before multiplication by 3.3. The tungsten-containing form of the pitting resistance equivalent number is: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) All composition values are weight percentages. The expression is reported by the British Stainless Steel Association (BSSA), the International Stainless Steel Forum (ISSF), and the cited 2021 *Metals* paper as a variant for comparing stainless steels that contain tungsten. It is not a replacement that should be applied automatically to every stainless steel composition. The ordinary expression discussed elsewhere in this article is: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) The difference is confined to the treatment of tungsten. Chromium, molybdenum and nitrogen remain the active terms, while nickel remains outside both expressions. The Society of Petroleum Engineers stated in 2025 that nickel is not included in the PREN equation. That omission does not mean nickel has no metallurgical importance. Nickel affects phase balance, austenite formation, mechanical properties and the corrosion behaviour of particular microstructures. It means only that nickel is not assigned a coefficient in this empirical index. PRENW is therefore a composition-based comparison tool, not a universal corrosion rating. Outokumpu describes PRE, or PREN, as an index used to rank and compare stainless steels for pitting resistance. The 2021 *Metals* paper similarly reports that a higher PREN generally corresponds to greater resistance to localized attack in chloride-containing solutions. “Generally” matters. The value ranks compositions under the convention used; it does not directly calculate a service lifetime, a guaranteed pitting potential or a safe chloride concentration. The tungsten-bearing convention is especially relevant when comparing stainless-steel families in which tungsten is a deliberate alloy addition. Such comparisons can include duplex and ferritic grades, as well as other high-alloy stainless steels. The calculation should use the certified or specified composition for the grade and the same convention for every alloy being compared. Mixing a standard PREN value for one steel with a PRENW value for another can create an apparent difference that comes from the formula rather than from the material. For example, consider a hypothetical composition containing 25.0% Cr, 3.0% Mo, 0.7% W and 0.25% N. Under the ordinary formula, which ignores tungsten, the result is: 25.0+3.3⁢(3.0)+16⁢(0.25) =25.0+9.9+4.0=38.9 Under PRENW, the same composition gives: 25.0+3.3⁢(3.0+0.5⁢(0.7))+16⁢(0.25) =25.0+3.3⁢(3.35)+4.0 =25.0+11.055+4.0=40.055 The difference is 1.155 index units. That difference is the contribution assigned to 0.7% tungsten under this convention, not a measured increase in critical pitting temperature or a direct prediction of field performance. ### The 0.5W contribution inside the molybdenum term The placement of 0.5⁢%⁢W inside the parentheses is easy to misread. It does not mean that tungsten contributes 0.5 PREN units for each weight percent. The entire parenthetical term is multiplied by 3.3. Expanding the equation algebraically makes the weighting clear: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) PRENW=%⁢Cr+3.3⁢%⁢Mo+3.3⁢(0.5⁢%⁢W)+16⁢%⁢N PRENW=%⁢Cr+3.3⁢%⁢Mo+1.65⁢%⁢W+16⁢%⁢N Thus, within this convention, one additional percentage point of tungsten contributes 1.65 PREN units. A composition containing 0.5% W receives a tungsten term of: 1.65⁢(0.5)=0.825 A composition containing 1.0% W receives 1.65 units. At 2.0% W, the contribution is 3.30 units. The coefficient is a convention for comparative calculation; it should not be read as proof that tungsten and molybdenum produce identical effects in every alloy, phase balance or chloride environment. This distinction matters because the formula compresses several metallurgical effects into one number. Tungsten can influence the stability and chemistry of passive films, particularly in combination with chromium and molybdenum, but the response also depends on segregation, inclusions, heat treatment, phase distribution, surface condition and the local environment. In duplex stainless steels, for example, ferrite and austenite may not have identical alloy contents. A bulk composition inserted into PRENW cannot show whether one phase is locally depleted in chromium, molybdenum or nitrogen. The expression also does not convert tungsten into molybdenum in the alloy itself. A steel with 3% Mo and 1% W is not chemically equivalent in every respect to a steel with 3.5% Mo. PRENW assigns the two compositions the same incremental index contribution for those terms—3.3(3.0 + 0.5) equals 11.55—but their phase chemistry, fabrication history and localized-corrosion response may still differ. For that reason, the arithmetic should be shown when PRENW is reported. Writing only “the grade has a PREN of 40” conceals whether the calculation used 1.65W, no tungsten term, or another published convention. A transparent report should identify the formula, list the composition basis, retain sufficient decimal places during calculation and state whether the result is PREN or PRENW. ### When the tungsten-bearing convention should be identified The tungsten-bearing convention should be identified whenever tungsten is included in the calculation, whenever a source labels a value PRENW, and whenever grades with meaningful tungsten additions are being compared with grades that contain little or none. This is particularly important for specifications and datasheets covering tungsten-bearing duplex stainless steels, including designations such as UNS S32760 where the specified chemistry includes tungsten. The exact product specification still governs the permitted composition; a nominal grade description should not replace the heat-analysis values used in the calculation. The label should also appear in technical reports, material-selection documents and corrosion-test comparisons. A clear statement might read: “PRENW calculated as %Cr + 3.3(%Mo + 0.5%W) + 16(%N), using weight-percent heat-analysis values.” If tungsten was not included, state that the common expression was used instead: “PREN calculated as %Cr + 3.3(%Mo) + 16(%N).” Do not silently add a tungsten correction merely because a grade contains a detectable amount of W. That last point is essential. The common PREN expression is still the correct convention for many stainless-steel comparisons, including comparisons in which tungsten is absent or not treated as a contributing term. ISSF also reports a manganese-adjusted expression, PRENMn: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn The existence of PRENW and PRENMn demonstrates that “PREN” is not one immutable equation across all publications. The suffix tells the reader which composition convention was selected. If a source gives an unsuffixed value for a tungsten-bearing grade, the calculation basis should be checked rather than guessed. Finally, PRENW remains an index for ranking localized-corrosion resistance under chloride-containing conditions, not validation of a component in service. ASTM G48 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Those tests can support a calculation-based comparison, but they do not reproduce every combination of temperature, chloride activity, deposits, welding, crevices, oxygen availability and mechanical damage found in service. A higher PRENW can support the expectation of greater resistance within a relevant comparison set. It cannot guarantee that a tungsten-bearing grade will resist attack in every chloride environment, nor can it erase the need for testing and design review. State the variant, show the arithmetic and treat the result as empirical evidence—not as a universal corrosion rating. ## PRENMn and the Treatment of Manganese PRENMn is a manganese-adjusted form of the pitting resistance equivalent number. Its purpose is not to redefine PREN for every stainless steel, but to provide an alternative composition-based comparison when manganese is treated as a factor that reduces localized-corrosion resistance. The distinction matters because manganese appears in many stainless-steel compositions, yet it is absent from the common PREN equation. PREN remains an empirical index. A higher value generally corresponds to greater resistance to localized attack in chloride-containing solutions, as reported in the 2021 *Metals* paper, but the number is not a service-life prediction, a guaranteed pitting potential or a substitute for corrosion testing. The manganese term changes the calculated index; it does not turn the calculation into a direct corrosion model. ### The ISSF PRENMn expression The International Stainless Steel Forum (ISSF) gives the manganese-adjusted expression as: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn This is the exact form supplied by ISSF in its 2025 *Duplex Stainless Steels* publication. Chromium, molybdenum, nitrogen and manganese are expressed as weight percentages. The manganese content is subtracted directly, with a coefficient of one. That construction produces a lower value than the common expression when the same chemical analysis is used: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) The British Stainless Steel Association published this common equation in 2025. It assigns no direct term to manganese. The reason is not that manganese has no metallurgical relevance. Manganese can affect austenite stability, nitrogen solubility, phase balance, inclusions and the composition of the passive film. Its effect on localized corrosion also depends on the wider alloy system and processing history. The common PREN expression is simply an established empirical convention whose principal terms are chromium, molybdenum and nitrogen. Consider a steel analysis containing 22.0% Cr, 3.0% Mo, 0.18% N and 2.0% Mn. The common calculation is: 22.0+3.3⁢(3.0)+16⁢(0.18)=34.38 Applying the ISSF PRENMn expression gives: 22.0+3.3⁢(3.0)+16⁢(0.18)−2.0=32.38 Manganese adjustment changes the reported index without changing the alloy chemistry. | Calculation | Expression applied | Result | |---|---|---| | Common PREN | 22.0 + 3.3(3.0) + 16(0.18) | 34.38 | | PRENMn | 22.0 + 3.3(3.0) + 16(0.18) − 2.0 | 32.38 | The two figures describe the same heat, but they are not the same index. Calling both simply “PREN 34.4” and “PREN 32.4” without naming the equations invites a false comparison. This treatment is especially relevant when comparing duplex, ferritic and other stainless-steel families with different manganese levels. For example, UNS S32205, commonly identified as 2205 duplex stainless steel, is often discussed using the conventional PREN relationship. A manganese-adjusted value may also be calculated if that convention is explicitly selected. Neither result should be presented as the sole intrinsic corrosion rating of the grade. The actual response depends on phase balance, segregation, heat treatment, surface condition, weld-zone microstructure and the chloride environment. Nickel does not appear in either the common PREN or the ISSF PRENMn equation. The Society of Petroleum Engineers states that “nickel is not included in the equation.” That omission does not mean nickel is metallurgically unimportant. Nickel influences phase stability and the ferritic-austenitic balance, among other effects, but the PREN family of equations does not assign it a numerical coefficient. ### Why formula variants must be labeled PRENMn, ordinary PREN and tungsten-bearing PRENW are related conventions, not interchangeable equations. ISSF gives PRENW as: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) The tungsten term is therefore multiplied by 0.5 inside the molybdenum-equivalent bracket, while manganese in PRENMn is subtracted outside the bracket. A report that states only “PREN = 40” leaves unanswered which relationship produced the number. The composition basis must also accompany the equation. A calculation should state whether the inputs are nominal grade limits, specified product chemistry, ladle analysis, check analysis or measured local composition. Standards and datasheets commonly list ranges, not a single chemical result. Substituting a midpoint for every range can produce a convenient estimate, but it is not equivalent to calculating from the certified heat analysis. Nitrogen deserves particular care because small changes are multiplied by 16, while molybdenum is multiplied by 3.3. Units need the same discipline. These expressions use weight percentages, not mass fractions written as decimals. Entering 0.22 for 22% Cr instead of 22.0 lowers the chromium contribution by almost two orders of magnitude. The notation “%Cr” means the chromium content in percent by mass within this calculation convention. Rounding must be declared as well. If each composition value is rounded before calculation, the result can differ from a calculation that retains analytical digits until the final step. A useful report therefore identifies the equation, the source and basis of the composition, the calculation date or document revision where relevant, and the final rounding rule. “PRENMn = 32.4, ISSF equation, wt% heat analysis, rounded to one decimal place” is reproducible. “PREN approximately 32” is not. The subtraction of manganese should not be interpreted as a universal correction that must be applied to every stainless steel. ISSF’s expression is an empirical convention for a particular comparison framework. Applying it to a nickel-rich austenitic grade, a ferritic grade or a duplex grade does not erase differences in microstructure and corrosion history. Nor does it establish that manganese reduces pitting resistance by exactly one PREN point for every additional weight percent in every environment. ### Comparing results calculated by different conventions A comparison is meaningful only when the same chemical analysis is run through clearly identified equations. Suppose an alloy contains 25.0% Cr, 4.0% Mo, 0.30% N, 2.0% Mn and 0.5% W. The common PREN is: 25.0+3.3⁢(4.0)+16⁢(0.30)=40.60 The manganese-adjusted result is: 40.60−2.0=38.60 The tungsten-bearing result is: 25.0+3.3⁢(4.0+0.5×0.5)+16⁢(0.30)=41.425 Rounded to one decimal place, these become 40.6, 38.6 and 41.4. The spread is generated by convention, not by three different steels. Reporting the figures without their labels could make PRENW appear to show higher resistance than PRENMn, although the formulas are answering different empirical comparison questions. For the same reason, a table comparing UNS S32750, UNS S32205 and UNS S31603 should place the equation in the heading or in a nearby note. A table of “PREN values” that mixes conventional PREN, PRENW and PRENMn is methodologically unsound unless every entry is labeled. The ranking itself can change, particularly where grades have similar chromium, molybdenum and nitrogen contents but different tungsten or manganese levels. Calculation still requires validation. ASTM G48 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Those tests can compare specimens under controlled procedures; they do not reproduce every service environment. Chloride concentration, temperature, oxygen access, deposits, crevices, welding, surface finish and exposure time can alter the result. PRENMn should therefore be reported as an identified index, not as a universal corrosion rating. Use the ISSF equation when that convention is intended, retain the common PREN equation where it is the established basis of comparison, and never place results from different formulas in the same ranking without showing how each number was calculated. ## PREN Across Ferritic, Duplex and Other Stainless Steels Pitting Resistance Equivalent Number (PREN) is used across several stainless-steel families, including ferritic, duplex, austenitic and martensitic grades. That common use does not make PREN a universal corrosion rating. It remains an empirical, composition-based index for comparing likely resistance to localized corrosion in chloride-containing conditions. The commonly reported calculation is: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) Chromium, molybdenum and nitrogen are entered as weight percentages. The British Stainless Steel Association gave this expression in its 2025 explanation of PREN calculations. A higher result generally indicates greater resistance to chloride-induced pitting, because molybdenum and nitrogen contribute strongly to the calculated value in addition to chromium. The 2021 *Metals* paper published by MDPI likewise describes the relationship as higher PREN corresponding to greater resistance to localized attack in chloride-containing solutions. The number is not a pitting potential, a service-life prediction or a pass/fail limit. Outokumpu describes “PRE or PREN” as “a composition-based index used to rank and compare stainless steels for resistance to pitting corrosion.” That wording matters. PREN compares compositions under a defined interpretation; it does not reproduce the effects of welding, surface contamination, heat treatment, crevices, deposits, temperature, acidity, oxygen concentration or flow. Nickel is omitted from the common PREN expression even though it affects phase balance and other metallurgical and corrosion properties. Limited evidence Nickel is not included in the common equation. The Society of Petroleum Engineers made that point explicitly in 2025. Nickel still affects phase stability, austenite formation, toughness and general corrosion behaviour, but its absence from the formula prevents PREN from being treated as a complete description of an alloy. ### Ferritic stainless steels Ferritic stainless steels have a body-centred cubic ferritic matrix and are generally designed with chromium as the principal alloying addition. Their compositions may also contain molybdenum, nitrogen, titanium, niobium or other additions, depending on the grade and intended service. Since the common PREN equation uses chromium, molybdenum and nitrogen, it can be calculated for ferritic grades in the same way that it can be calculated for other stainless steels. That calculation is useful for placing ferritic compositions on a comparative scale. It does not erase the metallurgical features that distinguish ferritic stainless steels from austenitic or duplex steels. Ferritic grades can respond differently to welding, thermal exposure and surface damage, even where their calculated PREN values are similar to those of grades in another family. Grain structure, precipitates, weld-zone chemistry and the condition of the passive film can all affect the observed onset of pitting. The ferritic family includes designations such as EN [1.4003](/materials/material-no/1.4003 " — composition, equivalents and standards"), EN 1.4016 and EN 1.4521, while higher-alloy ferritic compositions are also covered by other national, regional or product standards. These designations should not be treated as interchangeable names for a single corrosion class. Each refers to a specified chemical range and product designation, and the actual PREN calculation should use the composition applicable to the material being assessed. A nominal grade designation may provide a range rather than one exact analysis. Consequently, two heats meeting the same specification can produce slightly different calculated PREN values. If a supplier certificate gives measured chromium, molybdenum and nitrogen contents, those values can be inserted into the equation. If only nominal or maximum values are available, the result is an estimate tied to those assumptions. Ferritic stainless steels also illustrate why surface condition matters. Grinding, pickling, polishing and exposure to iron contamination can change the surface presented to a chloride solution without changing the bulk PREN. A mechanically damaged or contaminated surface may initiate attack sooner than a clean, properly treated surface made from the same heat of steel. PREN cannot account for that difference. The ferritic guidance published by the International Stainless Steel Forum (ISSF) presents PREN as one composition-related way to discuss localized-corrosion resistance across stainless-steel products. It should therefore be read as a comparison aid, not as evidence that every ferritic grade with a particular calculated number will behave identically in service. ### Duplex stainless steels Duplex stainless steels contain both ferritic and austenitic phases. Their performance depends not only on bulk chemistry but also on the balance, distribution and condition of those phases. The duplex family includes designations such as EN 1.4462 and the corresponding UNS designations UNS S31803 and UNS S32205, provided the particular product standard identifies them as applicable. Leaner and higher-alloy duplex grades use other exact designations. The designation alone does not remove the need to check the applicable chemical and product requirements. PREN is especially common in discussions of duplex stainless steels because nitrogen, chromium and molybdenum are important to their localized-corrosion resistance. Nitrogen supports austenite formation and contributes a large coefficient in the equation. Molybdenum and chromium contribute through the other terms. For tungsten-bearing duplex compositions, the ISSF’s 2025 duplex document reports the alternative expression: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) Here, tungsten is included at half its weight percentage within the molybdenum-related term. This expression must not be mixed casually with the ordinary PREN calculation. A reported value should state whether PREN or PRENW was used. The same ISSF document also reports a manganese-adjusted expression: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn That form reflects an attempt to account for manganese in comparisons involving duplex compositions. It is not the same index as the conventional expression. A numerical result calculated with PRENMn should be labelled as such, because comparing it directly with an unqualified conventional PREN can create a false impression of precision. Duplex metallurgy makes the limits of a single number particularly clear. Welding can alter the ferrite–austenite balance in the weld metal and heat-affected zone. Excessive ferrite, unsuitable cooling, or precipitation during thermal exposure may reduce localized-corrosion resistance even when the parent-metal chemistry produces a high PREN. Conversely, a controlled fabrication procedure can preserve a more favourable phase balance and surface condition. Heat treatment also matters. Duplex products are manufactured and solution-treated under conditions intended to control phase balance and avoid harmful intermetallic phases. Sigma phase and related precipitates can consume chromium and molybdenum locally, creating regions with lower resistance than the bulk composition suggests. PREN calculated from the certificate cannot detect such microstructural depletion. The ISSF duplex document also describes a higher value as indicating semi-quantitatively greater resistance to localized corrosion in chloride-bearing soils. “Semi-quantitatively” is the important qualification. The index supports ranking; it does not establish a universal threshold for every soil, seawater, process stream or crevice geometry. ### Why grade family changes the interpretation A PREN value has meaning only alongside the stainless-steel family, specification, condition and intended environment. The same numerical index calculated for a ferritic steel and a duplex steel does not guarantee equal pitting performance. Their phase structures differ, their welding responses differ, and their susceptibility to precipitation or phase imbalance differs. The same caution applies when comparing a duplex grade with an austenitic or martensitic grade. The common formula treats selected elements as weighted contributions, but it does not include nickel, copper, silicon, carbon, surface finish, grain structure or fabrication history. Nor does it specify whether the reported composition is a heat analysis, a product analysis or a nominal value. Validation requires testing and engineering judgement. ASTM G48:2011 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Those methods provide controlled comparisons, but ferric chloride is not a complete model of every operating environment. A chloride-bearing process solution may differ in temperature, pH, oxidizing power and wetting behaviour; a buried component may experience deposits and differential aeration; a welded assembly may contain several metallurgically distinct zones. For that reason, PREN is most defensible when reported with its formula, composition basis and material condition. “PREN 40,” without those details, is incomplete. A useful comparison identifies whether the value is conventional PREN, PRENW or PRENMn, names the grade designation, states whether the chemistry is nominal or measured, and separates parent-metal calculations from weld or heat-affected-zone assessment. Used in that disciplined way, PREN allows ferritic, duplex and other stainless steels to be discussed on a common compositional scale. It does not make those families metallurgically equivalent, and it cannot replace testing where localized corrosion has serious consequences. ## Nickel, Alloy Design and a Common Misreading ### Why nickel is absent from the common equation The common calculation is: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) The alloying contents are weight percentages. Chromium, molybdenum and nitrogen receive terms; nickel does not. The Society of Petroleum Engineers states this directly in its 2025 discussion of material selection: “nickel is not included in the equation.” That is not an accidental omission or a typographical gap in the formula. The reason is that PREN is a limited empirical index, not a complete numerical description of stainless-steel metallurgy. The index was developed to compare the tendency of stainless steels toward localized corrosion, particularly pitting, in chloride-containing conditions. In the common expression, chromium represents the principal passive-film-forming contribution, while molybdenum and nitrogen generally increase resistance to local breakdown. The coefficients—3.3 for molybdenum and 16 for nitrogen—are empirical weighting factors, not universal physical constants. The British Stainless Steel Association gives the equation in precisely this form in its 2025 explanation of PREN calculations. A higher calculated value normally indicates greater resistance to localized attack in chloride-bearing environments, as reported by the 2021 *Metals* paper, but “normally” matters. PREN ranks compositions under a defined comparison scheme; it does not calculate a service lifetime, a guaranteed pitting potential or a guaranteed minimum temperature for attack. Alternative expressions reinforce the point that PREN is a family of empirical comparison formulas rather than a complete corrosion model. The International Stainless Steel Forum reports: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) for a tungsten-bearing expression, and: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn for a manganese-adjusted expression. Nickel remains absent from both of those reported forms. Changing the formula to account for tungsten or manganese does not convert PREN into an all-purpose alloy-performance score. It only changes the empirical comparison used for a particular group of stainless steels or a particular interpretation of composition. A simple calculation shows why nickel-free arithmetic should not be read as nickel-free metallurgy. Using approximate nominal compositions, Type 304 stainless steel with 18 wt% Cr, no intentional Mo term and 0.08 wt% N gives a common PREN of about 19.3. Type 316L stainless steel with 17 wt% Cr, 2.5 wt% Mo and 0.05 wt% N gives about 26.1. Nickel is present in both grades, but its percentage is not entered. The difference in these illustrative PREN values comes from the terms that the equation was constructed to weight, not from a claim that nickel has no effect on either alloy. Composition also is not the same as the condition of the material. Segregation, inclusions, heat treatment, weld thermal cycles, surface contamination, phase balance and surface finishing can alter localized-corrosion performance. Two heats with similar nominal PREN values may not behave identically in a test or in service. ASTM G48:2011 specifies laboratory methods for comparing stainless steels and related alloys by measuring resistance to initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Such testing can validate a comparison for the selected procedure. It does not make the PREN equation a substitute for testing. ### Nickel's metallurgical role versus PREN's scope Nickel has a major metallurgical role even though it has no term in the common PREN equation. In iron-chromium-nickel stainless steels, nickel stabilizes the austenitic phase. Type 304, designated UNS S30400, is an austenitic stainless steel whose chromium-nickel balance supports an austenitic structure over a broad range of processing conditions. Type 316L, designated UNS S31603, uses nickel as part of that phase balance while also adding molybdenum for improved resistance to localized corrosion relative to the standard 18Cr-8Ni family. That phase effect influences more than crystal structure. Nickel affects toughness, especially at low temperature; ductility; work hardening; magnetic response; hot and cold forming behavior; and the balance between ferrite and austenite during welding. In austenitic grades, sufficient nickel helps suppress unwanted transformation to ferrite or martensite during processing, although the final phase balance depends on the whole composition and thermal history. Nickel also interacts with carbon, nitrogen, manganese, chromium and molybdenum. It is therefore part of alloy design even when a particular corrosion index assigns it no numerical coefficient. The contrast is clear in duplex stainless steels. A grade such as 2205, commonly designated UNS S32205 or UNS S31803 depending on the specified product and designation, is designed to contain both ferrite and austenite. Nickel helps establish the austenite fraction, while chromium, molybdenum and nitrogen contribute strongly to localized-corrosion resistance. Nitrogen is especially important because it supports austenite formation and strengthens the alloy while also appearing with a large coefficient in the PREN expression. A PREN calculation captures only selected aspects of that design. Ferritic grades make the same limitation visible from another direction. Type 430, designated UNS S43000, contains little or no intentional nickel compared with austenitic grades, yet its corrosion behavior cannot be inferred from nickel content alone. Its ferritic structure, chromium level, surface condition, inclusions and environment all matter. A nickel-free or low-nickel composition does not automatically have low localized-corrosion resistance, just as a nickel-bearing composition does not automatically have a high PREN. Nickel can also influence corrosion behavior outside the narrow pitting comparison. It affects general corrosion rates in some acids and alkaline media, resistance to stress-corrosion cracking in particular environments, hydrogen-related behavior, corrosion fatigue and the stability of passive or active surface states. Those effects are environment-specific. A single coefficient for nickel could not faithfully represent all of them, and inserting one into the common formula without a validated basis would create false precision. This is why the Outokumpu 2025 explanation describes PRE or PREN as a composition-based index used to rank and compare stainless steels for resistance to pitting corrosion. “Rank and compare” is the operative scope. The expression is useful precisely because it is narrow enough to support a rough comparison, not because it predicts every property of an alloy. ### Avoiding the claim that omitted elements are unimportant An omitted element is not an irrelevant element. It is an element that the selected index does not score. That distinction should be stated plainly whenever PREN is presented. The absence of nickel means only that the common empirical pitting-resistance index assigns nickel no direct term. It does not mean nickel has no role in phase stability, mechanical properties, weldability or broader corrosion behavior. It also does not mean that PREN can compare every stainless-steel family with equal confidence. A comparison between austenitic, ferritic and duplex grades must account for their different microstructures and processing histories, even when the same arithmetic is applied to each composition. Nor should a calculated value be detached from the environment. Chloride concentration, temperature, acidity, oxygen availability, deposits, crevices, flow, microbiological activity and galvanic coupling can change the likelihood and severity of localized attack. The International Stainless Steel Forum describes a higher value as indicating, semi-quantitatively, greater resistance to localized corrosion in chloride-bearing soils. “Semi-quantitatively” limits the claim. Soil service is not identical to seawater, a hot chloride process stream or a ferric-chloride laboratory test. PREN can therefore support an initial materials comparison, especially when grades belong to related families and their compositions are known. It cannot establish that one component will outlast another in every chloride environment. A material with a higher PREN may still pit because of a weld defect, a contaminated surface, an inclusion, an unexpected crevice or a temperature outside the comparison basis. Conversely, a lower-PREN grade may perform adequately where chlorides, temperature and exposure time remain modest. The sound reading is narrow but useful: nickel is absent from the equation because this particular index was built around selected composition terms associated with localized-corrosion resistance. Alloy designers still need nickel to control phase constitution and property balance. Engineers still need standards, fabrication controls and corrosion testing—such as the defined procedures in ASTM G48—when a service decision depends on actual performance rather than a composition-based ranking. ## PREN Versus ASTM G48 Testing PREN and ASTM G48 answer related but different questions. PREN is calculated from chemical composition. ASTM G48 produces an experimental response from a particular specimen exposed under a specified laboratory procedure. Treating the two as interchangeable is a common source of bad corrosion comparisons. The commonly used expression is: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) Chromium, molybdenum and nitrogen are entered as weight percentages. The result is dimensionless. It is an empirical index for ranking stainless steels by expected resistance to localized corrosion in chloride-containing environments, not a direct measurement of corrosion rate, service life or pitting potential. The British Stainless Steel Association gave this formula in its 2025 explanation of PREN calculations. \[4\] \[4\] [**How to Optimize Tubing Material Selections Using PREN Value**](https://jpt.spe.org/how-optimize-tubing-material-selections-using-pren-value). Society of Petroleum Engineers. Journal of Petroleum Technology, 2025. The equation also leaves out nickel. The Society of Petroleum Engineers stated in 2025 that “nickel is not included in the equation.” That omission does not mean nickel has no metallurgical importance. Nickel affects phase balance, austenite stability, toughness and general corrosion behaviour, particularly in duplex and austenitic grades. It means only that nickel is not assigned a coefficient in this particular pitting-resistance index. The International Stainless Steel Forum also reports two related expressions. For tungsten-bearing alloys: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) For the manganese-adjusted form: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn These variants can change the numerical ranking, especially for duplex stainless steels or compositions with significant tungsten or manganese. A reported number therefore needs its formula, not just the label “PREN.” ![Technician examining stainless-steel coupons after ferric-chloride pitting and crevice testing.](/images/uploads/8d001fa1-084a-48d8-9dd4-925c4dd00273/wiki-inline-a-laboratory-technician-inspecting-stainless-steel-specimens-after-ferric-chlori-1920x1288.jpg)[](/images/uploads/8d001fa1-084a-48d8-9dd4-925c4dd00273/wiki-inline-a-laboratory-technician-inspecting-stainless-steel-specimens-after-ferric-chlori-1920x1288.avif "Enlarge image — Technician examining stainless-steel coupons after ferric-chloride pitting and crevice testing.")ASTM G48 measures specimen response under defined ferric-chloride laboratory conditions. ### What ASTM G48 is designed to compare ASTM G48 is a laboratory testing standard for comparing stainless steels and related alloys by their resistance to the initiation of pitting and crevice corrosion under defined ferric-chloride conditions. ASTM International described this purpose in the 2011 edition of ASTM G48. The standard does not calculate a composition index. It exposes specimens to an aggressive chloride environment and records whether localized attack initiates, and under what conditions. That distinction matters. A mill certificate can provide chromium, molybdenum, nitrogen and other chemical contents from which PREN is calculated. It cannot provide an ASTM G48 critical pitting temperature or mass loss. Conversely, a G48 result belongs to the tested material condition and procedure; it cannot automatically be converted into a universal PREN value. ASTM G48 includes methods addressing pitting and crevice corrosion, including Method A for ferric-chloride pitting tests and Method B for ferric-chloride crevice tests. Other methods in the standard address temperature-based determinations such as critical pitting temperature or critical crevice temperature. The applicable edition and method must be stated because “tested to ASTM G48” is incomplete by itself. ASTM G48 results are measured responses from defined laboratory procedures, not calculated PREN values. | ASTM G48 output | What it represents | What it does not establish | |---|---|---| | Critical pitting temperature | Temperature associated with pitting initiation under the selected method | A universal service temperature limit | | Critical crevice temperature | Temperature associated with crevice attack under the selected method | Performance in every crevice geometry | | Mass loss after exposure | Measured material loss during the defined exposure | A direct conversion to service life | | Pass or fail at selected conditions | Response under specified ferric-chloride conditions | A universal corrosion rating | A typical result might be a pass or fail at a selected temperature and exposure time, a critical pitting temperature, a critical crevice temperature, or an observation of attack after exposure. The reporting format depends on the method. A specimen that remains free from visible pits at one temperature has not demonstrated that it will resist every chloride solution in service, and a specimen that pits in ferric chloride has not necessarily predicted the exact temperature or potential at which it will fail in a process stream. ASTM G48 is particularly useful for comparing grades such as UNS S30400, commonly designated Type 304; UNS S31603, commonly designated Type 316L; UNS S32205, commonly designated 2205 duplex stainless steel; and higher-alloyed grades such as UNS S32750, commonly designated 2507 super-duplex stainless steel. Those comparisons are meaningful only when the samples, preparation and test conditions are controlled. A comparison between solution-annealed plate and welded pipe, for example, may reflect heat-affected-zone condition, surface scale or fabrication history as much as nominal grade chemistry. The test is a screening and comparison tool, not a miniature service installation. Ferric chloride is deliberately aggressive and can produce localized attack rapidly. Its severity helps separate alloys in the laboratory, but it does not reproduce all combinations of oxygen content, pH, chloride activity, flow, deposits, biofilms, pressure, temperature and electrochemical potential found in equipment. ### Ferric-chloride laboratory conditions Ferric-chloride testing uses an oxidizing chloride solution, commonly prepared from ferric chloride hexahydrate, FeCl₃·6H₂O, at the concentration and volume specified by the applicable ASTM G48 procedure. The solution is not simply “salt water.” Ferric ions increase the oxidizing character of the exposure, while chloride supports passive-film breakdown and stabilizes the small anodic regions associated with pit growth. Temperature is a major control variable. Some G48 procedures expose specimens at a fixed temperature for a specified period; others increase or otherwise evaluate temperature to determine a critical value. Exposure time, solution preparation, vessel arrangement and specimen orientation can affect the observed result. A higher critical pitting temperature generally indicates better resistance within that test system, but only when the same method and reporting criteria are used. Surface condition is equally important. Grinding, polishing, pickling, passivation, heat tint, embedded particles, scratches and surface contamination alter the sites at which attack can begin. Stainless steel with weld discoloration or oxide scale may perform differently from the same nominal alloy after appropriate cleaning. The result should therefore identify the surface finish and cleaning procedure rather than presenting a grade designation alone. Crevice testing adds another variable: the crevice former. A loaded washer or other specified assembly creates a restricted oxygen-access region where passive-film repair is more difficult. Contact pressure, material pairing, geometry and assembly method influence the severity and location of attack. Pitting and crevice results should not be merged. An alloy may show little free-surface pitting while developing corrosion beneath a crevice former. After exposure, the specimen is examined according to the method’s criteria. That examination may include visual inspection, cleaning before evaluation, measurement of pit depth or assessment of crevice corrosion. The interpretation must state what counted as failure. “No corrosion” can mean no visible attack under a particular inspection procedure; it does not mean that electrochemical changes were absent. ### Why calculated ranking and test response can diverge PREN is a useful first comparison because alloy chemistry influences passive-film stability. The 2021 paper in *Metals* reported that higher PREN generally corresponds to greater resistance to localized attack in chloride-containing solutions. The International Stainless Steel Forum similarly describes a higher value as semi-quantitatively indicating greater resistance to localized corrosion in chloride-bearing soils. “Semi-quantitative” is the important limitation. The calculation treats composition as the controlling input, while ASTM G48 tests a real material with a particular microstructure and surface. Solidification segregation, intermetallic phases, precipitated nitrides, chromium-depleted regions, ferrite-austenite balance and heat-treatment history can change localized-corrosion behaviour without changing the nominal grade designation. This is especially relevant to duplex stainless steels, where phase balance and partitioning of chromium, molybdenum and nitrogen can differ between ferrite and austenite. Manufacturing condition can also reverse an expected ranking. A high-PREN alloy with weld heat tint, poor pickling or an unfavourable heat treatment may initiate attack before a lower-PREN alloy with a clean, well-passivated surface. Plate, bar, tubing and weld metal may have different thermal histories. The chemical analysis used for PREN may be a nominal specification range, whereas the G48 specimen contains the actual local composition and defects of that product. The formula itself introduces another source of divergence. Nitrogen has a coefficient of 16, so a small analytical or specification difference can affect the calculated value substantially. Tungsten-bearing grades may be compared with PRENW rather than the basic PREN expression. Applying the manganese-adjusted PRENMn equation to one alloy and the basic equation to another makes the ranking inconsistent unless the reason is clearly stated. Neither PREN nor G48 fully represents service conditions. Natural seawater, for example, may involve biofouling, deposits and changing oxygen concentration; a hot chloride process may involve acidic chemistry, evaporation and tensile stress; and sour-service equipment may require separate assessment of sulfide stress cracking and hydrogen effects. G48 does not replace those evaluations. A technically useful report should therefore give both the index and the evidence behind it: the exact PREN expression, each elemental value and its analytical basis; the alloy designation and product form; the ASTM G48 edition and method; ferric-chloride concentration, temperature and exposure duration; specimen orientation and surface finish; crevice assembly where applicable; and the criterion used to identify attack. Without those details, a PREN number and a G48 result can create false precision. The sound interpretation is limited but useful: PREN helps screen and rank compositions, while ASTM G48 shows how a prepared specimen responds under a defined accelerated laboratory exposure. Neither number is a universal corrosion rating. ## Limits of PREN in Real Service Environments PREN is useful only when its meaning is kept narrow. The common expression, PREN = %Cr + 3.3(%Mo) + 16(%N), converts selected alloy contents, expressed as weight percentages, into an empirical comparison index. A higher value generally indicates greater resistance to localized attack in chloride-containing solutions, as described in the 2021 *Metals* paper. It does not specify a service lifetime, guarantee a pitting potential, or establish that one component will outlast another in every plant or marine exposure. That distinction matters because PREN ranks nominal chemistry while corrosion occurs at an actual surface in an actual environment. The surface may contain oxide, scale, weld discoloration, embedded iron, inclusions, or deposits. The environment may be stagnant rather than aerated, hot rather than ambient, and concentrated inside a crevice rather than equal to the bulk solution. PREN cannot encode all those conditions. The International Stainless Steel Forum (ISSF) also reports modified expressions, including PRENW = %Cr + 3.3(%Mo + 0.5%W) + 16(%N) for tungsten-bearing alloys and PRENMn = %Cr + 3.3(%Mo) + 16(%N) − %Mn. These alternatives reinforce the same limitation: the result depends on the selected empirical formula and the composition used. Nickel is not included in the conventional equation, as the Society of Petroleum Engineers stated in 2025, although nickel still affects phase stability, structure and corrosion behavior. A single number cannot stand in for metallurgical examination and service-relevant testing. ### Chloride concentration, temperature and oxygen conditions Chloride is not a single, fixed exposure condition. Its concentration at the metal surface can differ greatly from the concentration measured in the surrounding water. Evaporation can concentrate chloride in a thin wet film. Salt deposits can retain moisture against a pipe or vessel wall. A crevice beneath a gasket, flange, support or deposit can develop a chemistry unlike the bulk liquid, with increased acidity and altered chloride activity. PREN does not calculate those changes. Temperature adds another major variable. Raising temperature commonly makes passive-film breakdown easier and can accelerate the propagation of an existing pit. A grade that remains passive in a cool, flowing chloride solution may suffer attack when the same chloride is present beneath insulation or inside a heated process line. Yet no single temperature correction can be applied to PREN across ferritic, austenitic and duplex stainless steels, because alloy structure, surface condition and solution chemistry also affect the result. Flow can either reduce or increase risk. Moving liquid may remove deposits and prevent stagnant concentration cells, but high velocity, suspended solids or turbulence can damage the passive film. Erosion-corrosion can repeatedly expose fresh metal, while a dead leg may remain chemically stagnant. A PREN comparison does not reveal which of these hydraulic conditions exists. Oxygen conditions are equally important. Stainless steel depends on a thin chromium-rich passive film, and oxygen availability helps that film form and repair. A well-aerated surface and an oxygen-starved crevice can therefore behave differently even when they contact the same liquid. Differential aeration may make the less-oxygenated region anodic relative to the exposed surface. Deposits can create that separation, as can biofilms, insulation, sludge and sediment. At the same time, oxygen-rich water can support a strong cathodic reaction outside a pit, increasing the driving force for localized dissolution once a small site has become active. \[5\] \[5\] [**Stainless Steel Pipe**](https://worldstainless.org/Files/issf/non-image-files/PDF/Stainless_Steel_Pipe.pdf). International Stainless Steel Forum. International Stainless Steel Forum publication, 2025. This is why PREN should not be treated as a universal chloride rating. ASTM G48, first issued in the cited 2011 version, specifies laboratory methods for comparing stainless steels and related alloys for initiation of pitting and crevice corrosion under defined ferric-chloride conditions. Those tests are controlled comparisons, not replicas of every seawater, process-water, soil or atmospheric exposure. The ISSF describes a higher PREN as indicating semi-quantitatively greater resistance to localized corrosion in chloride-bearing soils; “semi-quantitatively” is the important qualification. The index supports ranking, but it does not supply a universal threshold for acceptable service. ### Surface condition, inclusions and passive-film damage The calculated value normally uses bulk or specified composition. Corrosion begins at the surface, where the chemistry may differ from the heat analysis. A stainless component can have the correct nominal grade and still perform poorly after fabrication if its surface carries free iron, carbon-steel grinding debris, welding oxides, heat tint, scale or other contamination. These materials can disrupt passivation or create local electrochemical cells. Iron contamination is a familiar example. Carbon-steel tools, lifting devices and wire brushes can transfer iron particles to stainless steel. The particles may rust and stain the surface, but staining alone is not identical to a pit in the stainless substrate. More importantly, contamination can interfere with a uniform passive film and make inspection difficult. Pickling, appropriate mechanical finishing and chemical passivation may restore a cleaner surface, but their suitability depends on the grade, geometry and process controls. PREN cannot tell whether those controls were applied. Roughness also changes exposure. Valleys can retain chloride solution and deposits, while scratches can act as initiation sites if they contain smeared metal or embedded particles. A polished surface and a heavily ground surface made from the same 316L stainless steel have the same nominal PREN, but they do not present the same population of defects to the environment. A surface that has been abraded during installation may need time to repassivate before it encounters a severe chloride condition. Nonmetallic inclusions are another reason a bulk composition number has limits. Sulfide inclusions, oxide inclusions and complex inclusion clusters can become preferential initiation sites. Their chemistry, size, distribution and proximity to the surface matter. Molybdenum and nitrogen in the average alloy composition do not guarantee that every microscopic region has the same resistance as the matrix. A clean, fine inclusion population may behave differently from a coarser or more reactive population at the same nominal grade. Passive-film damage can be mechanical, chemical or thermal. Sliding contact, impact, cavitation and erosion expose fresh metal. Acidic deposits or cleaning residues can prevent rapid repassivation. Chloride trapped under a damaged coating or gasket can then attack a small area while the surrounding stainless surface remains passive. Once a pit forms, its interior becomes a confined electrochemical environment; metal ions hydrolyze, acidity increases and chloride migrates into the cavity. The original alloy ranking still has relevance, but initiation and propagation now depend on local conditions that PREN does not describe. ![Macro view of a duplex stainless-steel weld showing the heat-affected zone and ferrite-austenite phases.](/images/uploads/25dd9254-0ebd-407d-90f7-740715503463/wiki-inline-a-welded-duplex-stainless-steel-joint-showing-the-weld-metal-heat-affected-zone-1920x1434.jpg)[](/images/uploads/25dd9254-0ebd-407d-90f7-740715503463/wiki-inline-a-welded-duplex-stainless-steel-joint-showing-the-weld-metal-heat-affected-zone-1920x1434.avif "Enlarge image — Macro view of a duplex stainless-steel weld showing the heat-affected zone and ferrite-austenite phases.")Bulk PREN cannot establish the corrosion behavior of a welded joint or its heat-affected zone. ### Welding, heat-affected zones and local composition #### Terms used in weld assessment Base metal The original product material away from the weld thermal cycle. Weld metal Material solidified from filler metal, parent-metal dilution or both. Heat-affected zone (HAZ) Parent material whose microstructure or properties changed during welding without melting. Phase balance The relative ferrite and austenite proportions, particularly important in duplex stainless steels. Welding makes the gap between nominal composition and service behavior especially clear. A welded assembly is not one uniform alloy. It may contain base metal, weld metal, a fusion boundary and one or more heat-affected zones (HAZs), each with different thermal histories and sometimes different compositions. The weld filler can have a PREN different from the parent grade, while dilution changes the final weld-metal chemistry. For duplex stainless steels such as UNS S32205, commonly designated 2205, the weld thermal cycle must preserve a suitable balance of ferrite and austenite. Excessive ferrite, excessive austenite or harmful secondary phases can alter localized-corrosion behavior even when the certified bulk chemistry gives a high PREN. Rapid cooling, excessive heat input, unsuitable interpass temperature or poor shielding can all affect that balance. In some conditions, chromium nitrides or intermetallic phases such as sigma can form, depleting nearby regions of chromium or molybdenum. The depleted zone may be much more vulnerable than the average composition suggests. Austenitic grades also have welding concerns. For 304L and 316L, low-carbon designations reduce the risk of chromium-carbide sensitization during welding, but low carbon does not remove every fabrication risk. Incorrect heat treatment, contamination, excessive heat tint or a poorly controlled weld procedure can leave a surface that requires cleaning and restoration. A welded 316L component should not be judged solely by the PREN calculated from its mill certificate. Ferritic stainless steels can develop grain coarsening or other HAZ changes that affect toughness and corrosion response. Weld-metal solidification may produce segregation, so interdendritic regions do not necessarily match the nominal analysis. These local variations are small in volume but significant if they form a continuous path or sit at a highly stressed, exposed location. Post-weld cleaning is therefore part of corrosion control, not merely appearance work. Removing heat tint and contamination can restore access to a chromium-rich passive surface, but treatment must match the stainless grade and fabrication procedure. Inspection should consider weld toes, crevices, undercut, lack of fusion, slag residues and areas where deposits can collect. PREN remains valuable for comparing grades such as 316L, UNS S32205 and higher-alloy duplex steels under similar assumptions. It becomes misleading when used as a stand-alone acceptance rule for a welded, contaminated or geometrically complex component. Composition establishes a starting rank. Service performance depends on the environment, the surface and the microstructure that actually reach the chloride. ## How to Report a PREN Calculation Responsibly A PREN value is meaningful only when its calculation can be reconstructed. Reporting “PREN 40” without identifying the chemistry, equation, units, or source is not a technical result; it is an unsupported comparison. PREN is an empirical, composition-based index for ranking stainless steels under chloride-containing conditions. It is not a universal corrosion rating, a guaranteed pitting potential, or a prediction of service life. The British Stainless Steel Association states the commonly used expression as: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) Chromium, molybdenum and nitrogen are entered as weight percentages. The result is dimensionless, although it is often described informally as a “PREN number.” The equation does not include nickel. The Society of Petroleum Engineers specifically identifies this omission, which matters because nickel content is often prominent in a stainless-steel grade designation or datasheet even though it makes no direct contribution to this particular calculation. ### Formula and variant identification The first reporting requirement is to state the exact equation used. “PREN” is not one perfectly uniform formula across every stainless-steel family and technical publication. For the common chromium-molybdenum-nitrogen expression, report: PREN=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N) For tungsten-bearing alloys, the International Stainless Steel Forum (ISSF, 2025) reports: PRENW=%⁢Cr+3.3⁢(%⁢Mo+0.5⁢%⁢W)+16⁢(%⁢N) For the manganese-adjusted expression, ISSF reports: PRENMn=%⁢Cr+3.3⁢(%⁢Mo)+16⁢(%⁢N)−%⁢Mn These are not interchangeable labels. A report that gives “PREN 39” must say whether the result is PREN, PRENW or PRENMn. Otherwise, another engineer cannot reproduce the number or determine whether two values were calculated on the same basis. The selected expression should reflect the purpose of the comparison and the convention used by the governing specification, organization or project document. The ordinary formula may be appropriate for comparing common austenitic, ferritic and duplex stainless steels when the source uses that convention. A tungsten-bearing duplex grade may require explicit consideration of the PRENW expression. The manganese-adjusted form should not be inserted merely because manganese appears in the chemical analysis; it must be identified as a deliberate formula choice. A worked calculation should show every term. For an illustrative nominal composition of 22.0% Cr, 3.0% Mo and 0.18% N: PREN=22.0+3.3⁢(3.0)+16⁢(0.18)=34.68 If the result is reported as 34.7, the report should state that it was rounded to one decimal place. Rounding can affect close comparisons. A difference of only 0.01 percentage points in nitrogen changes the common PREN result by 0.16, while a 0.10 percentage-point difference in molybdenum changes it by 0.33. The formula also needs to be separated from the interpretation. The 2021 *Metals* paper associated higher PREN with greater resistance to localized attack in chloride-containing solutions, but that relationship is semi-quantitative. It does not establish that a steel with PREN 40 will resist a particular environment twice as long as one with PREN 20, or that every steel above a stated threshold will pass a service requirement. ### Composition source and analytical basis The calculation is only as reliable as the composition entered into it. A report must identify whether it uses a heat analysis, a product analysis, a certified material test report, an independent laboratory result, or nominal values taken from a grade table. The grade designation comes first. For example, write **UNS S31603**, not simply “316L,” when the UNS designation is the controlling identification. If the material is specified to **ASTM A240**, state that standard and the product form where relevant. For duplex material, distinguish designations such as **UNS S31803** and **UNS S32205** rather than treating “2205” as a complete material identity. The same commercial shorthand can cover different specification requirements, and the chemistry limits may not be identical. Next, state whether the input is nominal or measured. A nominal calculation based on rounded grade chemistry is useful for preliminary comparison, but it is not the PREN of a particular heat. A measured heat analysis can produce a different value while remaining fully within the specified composition limits. This is especially significant for nitrogen and molybdenum because their coefficients are relatively high. The report should state that all alloying values are in weight percent, not mass fractions expressed as decimals. Entering nitrogen as 0.18 wt% is correct for the equation above; entering 18% would produce an absurd result. If a laboratory reports 1,800 ppm nitrogen, convert it to 0.18 wt% before calculation. Record the precision supplied by the source rather than inventing extra digits. A result calculated from chemistry reported only to the nearest 0.1% should not be presented as 36.274. A reproducible record can follow this format: **Grade and designation:** UNS S32205, product specification and product form **Composition basis:** heat analysis from material test report, or nominal specification limits **Inputs, wt%:** Cr = \_\_; Mo = \_\_; N = \_\_; W = \_\_ if applicable; Mn = \_\_ if applicable **Formula:** PREN, PRENW or PRENMn, written in full **Calculation:** substituted values and arithmetic **Result:** \_\_, rounded to \_\_ decimal places **Source:** document number, revision, heat number, laboratory method, or specification table **Validation evidence:** ASTM G48 method, test temperature, ferric-chloride concentration, exposure time, specimen condition and observed result When specification limits are used, calculate carefully. A “minimum PREN” assembled from minimum chromium, minimum molybdenum and minimum nitrogen may be mathematically conservative for those terms, but it may not correspond to an actual heat. If nitrogen has only a minimum limit and the report uses that minimum, say so. If maximum manganese is used in PRENMn, identify that treatment as well. Do not label a limit-based estimate as measured performance. ### Pairing PREN with testing and design evidence PREN should support a materials comparison, not replace qualification or design analysis. The International Stainless Steel Forum describes higher values as indicating semi-quantitatively greater resistance to localized corrosion in chloride-bearing soils. That wording is important: the index gives a direction for comparison, not a universal pass/fail boundary. ASTM G48 provides laboratory methods for comparing stainless steels and related alloys by exposing specimens to ferric-chloride solution under defined conditions. The standard addresses initiation of pitting and crevice corrosion, with the selected method, temperature, exposure period, specimen preparation and acceptance criteria affecting the result. A report citing “passed G48” is therefore incomplete unless it identifies the method and test conditions. **ASTM G48 Method A** is commonly associated with pitting-corrosion testing, while other ASTM G48 methods address crevice-corrosion evaluation. Test evidence must also be tied to the material condition. Welded and solution-annealed specimens may behave differently from parent plate. Surface finish, heat tint, fabrication contamination, inclusions, phase balance in duplex stainless steel and post-weld treatment can alter localized-corrosion behavior without changing the nominal grade designation. A PREN calculated from bulk chemistry cannot capture every one of those factors. Service design evidence belongs alongside the calculation. State the chloride concentration, temperature, oxygen exposure, pH, crevice conditions, flow regime, deposits, cleaning chemicals and expected weld condition when those factors are relevant. A steel can rank higher by PREN and still suffer attack in a hot, stagnant, acidic crevice. Conversely, a lower calculated value may perform acceptably in a less severe environment. A responsible conclusion therefore reads like this: the measured or nominal composition gives a calculated PREN of a specified type; the value supports a relative comparison with named grades on the same formula basis; and ASTM G48 or service-specific evidence provides separate validation. Keeping those three statements separate prevents a composition index from being mistaken for a corrosion guarantee. ## References 1. \[1\] International Stainless Steel Forum. [Duplex Stainless Steels](https://worldstainless.org/wp-content/uploads/2025/02/ISSF_Duplex_Stainless_Steels.pdf). International Stainless Steel Forum publication, 2025. [](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#wiki-cite-ref-1) https://worldstainless.org/wp-content/uploads/2025/02/ISSF\_Duplex\_Stainless\_Steels.pdf 2. \[2\] Metals authors. [Localized corrosion resistance and PREN relationship](https://www.mdpi.com/2075-4701/11/5/836). Metals, 2021. [](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#wiki-cite-ref-2) https://www.mdpi.com/2075-4701/11/5/836 3. \[3\] ASTM International. [Standard test methods for pitting and crevice corrosion resistance of stainless steels](https://store.astm.org/g0048-11.html). ASTM G48, 2011. [](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#wiki-cite-ref-3) https://store.astm.org/g0048-11.html 4. \[4\] Society of Petroleum Engineers. [How to Optimize Tubing Material Selections Using PREN Value](https://jpt.spe.org/how-optimize-tubing-material-selections-using-pren-value). Journal of Petroleum Technology, 2025. [](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#wiki-cite-ref-4) https://jpt.spe.org/how-optimize-tubing-material-selections-using-pren-value 5. \[5\] International Stainless Steel Forum. [Stainless Steel Pipe](https://worldstainless.org/Files/issf/non-image-files/PDF/Stainless_Steel_Pipe.pdf). International Stainless Steel Forum publication, 2025. [](/wiki/calculated-values/pitting-resistance-equivalent-number-calculations#wiki-cite-ref-5) https://worldstainless.org/Files/issf/non-image-files/PDF/Stainless\_Steel\_Pipe.pdf **PREN at a glance** Common equation PREN = %Cr + 3.3(%Mo) + 16(%N) Inputs Chromium, molybdenum and nitrogen in wt% Result Dimensionless empirical index Primary use Comparing resistance to chloride-induced pitting Does not establish Service life, pitting potential or a universal corrosion limit [Back to Calculated Values](/wiki/calculated-values "Calculated Values — Steel Wiki")