Pitting corrosion resistance indices – PREN

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What is the PREN index?

PREN (Pitting Resistance Equivalent Number) is an index used to compare the resistance of stainless steels and other corrosion-resistant alloys to pitting corrosion, especially in environments containing chlorides.
Pitting corrosion is one of the most dangerous forms of local corrosion. Unlike uniform corrosion does not lead to uniform material loss across the entire surface. The attack is concentrated in small areas, where local damage to the passive layer occurs. The resulting pitting can progress deeper into the material and lead to perforation of the component, with a relatively small overall mass loss.
A particularly important factor contributing to the formation of pitting is chloride ions Cl⁻, therefore this issue is of great importance, among others, in the chemical, petrochemical, marine, food and energy industries as well as in installations working with seawater, brines and solutions containing chlorides.
PREN allows you to easily compare the potential resistance of different steel grades to this type of corrosion based on their chemical composition.

How is PREN calculated?

One of the most commonly used formulas is:
PREN = %Cr + 3,3 × %Mo + 16 × %N
where:

• Cr – chromium content [% wt.],
• Mo – molybdenum content [% wt.],
• N – nitrogen content [% wt.].

For alloys containing tungsten, the extended form is also used:
PREN = %Cr + 3,3 × (%Mo + 0,5 × %W) + 16 × %N
where W is the tungsten content in mass percentage.
This method of taking into account Cr, Mo, W and N is used, among others, in standards used in the selection of materials for the oil and gas industry.
However, it's important to remember that several variations of the PREN equation can be found in the technical literature. For example, the coefficient assigned to nitrogen may have different values. Therefore, when comparing PREN values ​​from different sources, always check the equation used to calculate them.

Why do chromium, molybdenum and nitrogen affect PREN?

In the PREN equation, individual elements are assigned different weights. This is not accidental – it results from their influence on the behavior of steel in environments conducive to localized corrosion.

Chromium – Cr

Chromium is the basic element responsible for the corrosion resistance of stainless steels.
A sufficiently high chromium content allows the formation of a very thin, dense passive layer on the material's surface. This layer separates the metal from the environment and can self-regenerate after minor damage if environmental conditions permit.
In the PREN equation, the influence of chromium was assumed with a coefficient 1.

Molybdenum – Mo

Molybdenum plays a particularly important role in increasing resistance to localized corrosion in chloride-containing environments.
In the classical PREN equation its contribution is multiplied by 3,3, which shows how important its influence on resistance to the formation and development of pitting is.
Therefore, AISI 316L steel containing molybdenum generally has a higher resistance to pitting corrosion than AISI 304L, even though both grades are common austenitic stainless steels.

Nitrogen – N

Nitrogen is very heavily factored into the PREN equation:
16 × %N
This means that even a relatively small increase in nitrogen content can noticeably increase the calculated PREN value.
The importance of nitrogen is particularly evident in modern duplex and superduplex steels and high-alloy austenitic steels.

Tungsten – W

Tungsten is also present in some high-alloy materials. In the extended PREN equation, its contribution is considered along with molybdenum:
Mo + 0,5W
This means that in this model, 1% W has about half the effect on PREN of 1% Mo.

PREN calculation example

Let us assume steel containing:

• Cr = 22,0%
• Mo = 3,2%
• N = 0,18%

PREN is calculated:
PREN = 22,0 + (3,3 × 3,2) + (16 × 0,18)
that is:
PREN = 22,0 + 10,56 + 2,88
PREN = 35,44
The material therefore has a calculated PREN value of approximately 35,4.
If the second alloy has a PREN of, for example, 25, it can be expected that – with comparable material and surface conditions – the alloy with a PREN of 35,4 will have a greater potential for resistance to pitting corrosion in a chloride environment.
This does not mean, however, that it will be resistant to corrosion in all operating conditions.

How to interpret the PREN value?

The basic principle is simple:
the higher the PREN value, the greater the expected resistance of the material to pitting corrosion in chloride environments.
However, PREN should be treated as a comparative indicator and not an absolute scale of resistance.

These limits are intended as a guide only. PREN is not a universal scale that can automatically classify steel for a specific application.

PREN popular stainless steel groups

Differences in chemical composition cause significant differences in PREN values ​​between individual material groups.

AISI 304 / 304L

Steels of the 18Cr-8Ni type contain practically no molybdenum, therefore their PREN is relatively low and is usually around:
PREN ≈ 18–20
These steels are very widely used, but in environments with higher chloride content their resistance to pitting corrosion may be insufficient.

AISI 316 / 316L – 1.4401 / 1.4404

The addition of approximately 2–2,5% molybdenum significantly increases resistance to local corrosion.
Typical values ​​are approximately in the range:
PREN ≈ 23–27
The presence of molybdenum is one of the main reasons for the advantage of 316L over 304L in many chloride environments.

Duplex 1.4462 / UNS S31803 / S32205

Duplex steels typically contain about 22% Cr, about 3% Mo and an increased nitrogen content.
As a result, they achieve:
PREN ≈ 32–36
This provides significantly higher resistance to pitting corrosion than classic 304L and 316L steels.

Superduplex 1.4410 / UNS S32750

Due to the content of approximately 25% Cr, increased content of Mo and N, the PREN value usually exceeds:
PREN ≥ 40
Materials from this group are used in particularly aggressive chloride environments, including offshore installations, systems using seawater and the chemical industry.

Austenitic steels of the 6Mo type

This group includes high-alloy austenitic steels containing about 6% molybdenum, e.g. grades 1.4547, 1.4529 or UNS N08367.
Due to the high content of Cr, Mo and N their PREN can exceed 40, ensuring very high resistance to local corrosion.

PREN ≥ 40 – why is this value important?

Value PREN 40 is a common technical limit when classifying materials intended for very demanding environments.
For example, in the classifications used in the oil & gas sector, 6Mo materials are described as steels with a PREN of at least 40 and a nominal content of approximately 6% Mo. For duplex 25Cr steels, a PREN range of approximately 40–45 is encountered.
This does not mean, however, that:
PREN = 39 – the material is not resistant,
PREN = 40 – the material is resistant.
There is no such sudden physical boundary.
The value of 40 is primarily a useful classification and design criterion.

PREN and pitting corrosion

A pit is formed at the site of a local breakdown in the passivity of the steel surface.
In the presence of chlorides, the process can become self-sustaining. Within the developing pit, the local chemical composition of the environment changes, the pH decreases, and the solution becomes more aggressive. As a result, an initially small defect can develop deep within the material.
This is particularly dangerous because the surface of the component may appear almost undamaged, while individual pits reach considerable depth.
Therefore, in pressure systems, tanks, heat exchangers and pipelines, pitting corrosion can lead to loss of tightness without prior occurrence of large, easily visible material loss.

PREN and crevice corrosion

High PREN is also typically associated with better crevice corrosion resistance, but the two phenomena should not be equated.
Crevice corrosion can develop, among others:

• under the seals,
• in flange connections,
• under the settlements,
• in threads,
• in tabs,
• in places of medium stagnation.

The chemical conditions created inside the fracture may be much more aggressive than in the main volume of the medium.
Therefore, a material that shows good resistance on a freely exposed surface may corrode in a tight crevice.

PREN and temperature

Temperature is one of the factors whose simple formula PREN does not take into account.
This has enormous practical importance.
The same material may behave correctly in chloride solution at low temperature, but become susceptible to localized corrosion at elevated temperatures.
Therefore, when assessing materials more precisely, the following concepts are used:
CPT – Critical Pitting Temperature, i.e. the critical pitting corrosion temperature.
This is the temperature above which, under certain test conditions, stable pitting development can begin.

PREN and ASTM G48 testing

PREN is a calculated value derived from the chemical composition. If a true assessment of material behavior is required, laboratory testing is used.
One of the most important methods is ASTM G48 – Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution.
The standard covers test methods for the pitting and crevice corrosion resistance of stainless steels and related alloys in aggressive chloride environments.
This type of research allows, among other things, the comparison of materials and the determination of critical initiation temperatures for pitting or crevice corrosion.

Why is PREN not enough for steel selection?

This is the most important limitation of the entire method.
PREN is not a guarantee of the corrosion resistance of the material.
The formula primarily takes into account the chemical composition, but the actual resistance also depends on many other factors.

Temperature

An increase in temperature can significantly increase the risk of local corrosion.

Chloride concentration

Higher Cl⁻ content usually increases the aggressiveness of the environment.

pH

An acidic environment can destabilize the passive layer.

Electrochemical potential and the presence of oxidants

Oxidizing conditions can significantly change the susceptibility of a material to pitting initiation.

Surface condition

Roughness, scale, iron contamination, welding discoloration and improper surface treatment may reduce the actual resistance.

Welding

Stainless steel welding process, can locally change the microstructure and distribution of alloying elements.
This is particularly important in duplex and superduplex steels, where the correct proportion of ferrite and austenite and the absence of unfavorable precipitations are of great importance for corrosion resistance.

Microstructure

PREN calculated from the average chemical composition does not accurately describe the local composition of the individual phases.
This is particularly important for duplex steels. Chromium, molybdenum, and nitrogen are not distributed identically between ferrite and austenite. Therefore, the local resistance of one of the phases may be lower than the PREN calculated for the entire material.

Element construction

Even a very resistant material can corrode if the design causes:

• stagnation of the medium,
• formation of deposits,
• the occurrence of tight gaps,
• insufficient rinsing of surfaces,
• chloride concentration.

Therefore, the correct selection of material must take into account both the steel grade and the device design.

Nominal PREN vs. Actual Melt PREN

Another important issue is how PREN is calculated.
The chemical composition of the steel grade specified in the standard is usually as range of permissible element contents, e.g.

• Cr = 21,0–23,0%
• Mo = 2,5–3,5%
• N = 0,10–0,22%

There is therefore no single absolute PREN value for each material belonging to a given species.
You can calculate PREN based on:

• minimum content of elements according to the standard,
• nominal values,
• average composition values,
• actual chemical analysis of a specific melt.

In applications requiring confirmation of the minimum PREN value, the most reliable method is to calculate it based on the actual chemical composition of a specific batch of material , e.g. data contained in the 3.1 acceptance certificate.

Example of the influence of actual composition on PREN

Consider two melts of the same grade of steel.
Melt A:

• Cr = 21,2%
• Mo = 2,6%
• N = 0,12%

PREN:
21,2 + 3,3 × 2,6 + 16 × 0,12 = 31,70
Melt B:

• Cr = 22,8%
• Mo = 3,4%
• N = 0,20%

PREN:
22,8 + 3,3 × 3,4 + 16 × 0,20 = 37,22
Both materials can formally belong to the same species, yet their calculated PREN differs by more than 5 points.
This shows why, in demanding technical applications, species identification alone may not be sufficient.

Does nickel increase PREN?

In the classical PREN equation nickel is not present.
This does not mean, however, that nickel has no importance for the corrosion properties of steel.
Nickel strongly influences, among others, the stability of austenite, the microstructure of the material and resistance in many chemical environments. However, PREN was constructed as an indicator relating primarily to resistance to pitting corrosion in chloride environments.
Therefore, PREN must not be interpreted as a general indicator of the "chemical resistance" of steel.
A material with a higher PREN is not necessarily more resistant than a material with a lower PREN to every acid, base or process environment.

PREN does not indicate resistance to all types of corrosion

A high PREN value does not automatically mean high resistance to:

• uniform corrosion,
• intergranular corrosion,
• SCC stress corrosion cracking,
• galvanic corrosion,
• high-temperature corrosion,
• oxidation,
• carburizing,
• sulfurization.

PREN should therefore be used exactly for its intended purpose – as a comparative indicator of pitting corrosion resistance , primarily in environments containing chlorides.

PREN, CPT and actual material selection

For initial species comparison, PREN is an extremely convenient parameter.
However, when selecting the material responsibly, it is worth analyzing at the same time:

• PREN – pitting resistance potential resulting from the chemical composition,
• CPT – critical pitting corrosion temperature,
• CCT – critical crevice corrosion temperature,
• corrosion test results, e.g. according to ASTM G48,
• working temperature,
• chloride concentration,
• pH,
• presence of oxidants,
• flow speed,
• possibility of stagnation,
• presence of crevices and sediments,
• method of construction and welding of the installation,
• surface condition.

Only the compilation of this information provides the basis for the proper selection of material.

Summary

PREN (Pitting Resistance Equivalent Number) is one of the most useful indicators for quickly comparing the potential resistance of stainless steels to pitting corrosion.
The most commonly used formula is:
PREN = %Cr + 3,3 × %Mo + 16 × %N
and for materials containing tungsten:
PREN = %Cr + 3,3 × (%Mo + 0,5 × %W) + 16 × %N
A higher PREN value generally indicates greater resistance to pitting corrosion initiation in chloride environments.
However, PREN should not be considered a standalone criterion for steel selection. The index does not take into account temperature, pH, chloride concentration, microstructure, surface quality, welding conditions, or device design.
Therefore, PREN is best treated as the first stage of material selectionIn demanding applications, it should be supplemented with an analysis of the operating environment and, if necessary, pitting and crevice corrosion resistance test results.

Comparison of the PREN values ​​of stainless steels, duplex steels, and nickel alloys

The table below shows approximate values PREN (Pitting Resistance Equivalent Number) for popular stainless steels, duplex and superduplex steels, high-alloy austenitic steels and selected nickel alloys.
For most stainless steels, the PREN value is calculated using the formula:
PREN = %Cr + 3,3 × %Mo + 16 × %N
The PREN value is a comparative parameter. The higher the value, the greater the material's resistance to pitting corrosion initiation in chloride-containing environments. However, it is not a universal indicator of a material's overall corrosion resistance.

* PREN values ​​marked with an asterisk for selected Ni-Cr-Mo nickel alloys are derived from calculations according to the modified equation used by VDM Metals:
PREN = %Cr + 3,3 × (%Mo + 0,5 × %W) + 30 × %N
This equation takes into account the effect of tungsten and uses a higher factor for nitrogen than the classical equation commonly used for stainless steels. Therefore, PREN values ​​marked with an asterisk should not be directly compared numerically with PREN values ​​calculated using the formula Cr + 3,3Mo + 16N.

How to interpret differences in PREN values?

The table shows the effect of chemical composition on pitting corrosion resistance. Typical austenitic steels without molybdenum addition, such as 1.4301 / AISI 304 i 1.4307 / AISI 304L, reach a PREN of about 18–20. Addition of molybdenum in steels of the type 316L/1.4404 raises the PREN value to approximately 24–25.
The next level of resistance is represented by steels with a higher molybdenum content, such as 317L i 904L/1.4539For 904L steel the PREN value is approximately 34.
A very significant increase in resistance occurs in duplex steels. duplex 2205 / 1.4462 the typical PREN value is around 35, while for steel superduplex 2507 / 1.4410 exceeds 40. Manufacturers' data indicate for 2507 a minimum value of approximately 42,5For comparison, Outokumpu's table shows PRE values ​​of approximately 25 for 2304, 26 for LDX 2101, 35 for 2205, and 41 for 2507.

6Mo and PREN steels above 40

The group of materials intended for very aggressive chloride environments includes high-alloy austenitic steels of the type 6MBOne of the most famous examples is 254 SMO / 1.4547 / UNS S31254The manufacturer provides a minimum value for this material. PREN ≥42,5.
A similar class of resistance is represented by species 1.4529 / Alloy 926 and AL-6XN / UNS N08367Thanks to the combination of high chromium content, approximately 6–7% molybdenum and added nitrogen, they achieve PREN values ​​in excess of 40.

Sanicro 35 and Alloy 625 – PREN approximately 52

Even higher is Sanicro 35 / 1.4562 / UNS N08935Alleima lists a nominal PREN value of approximately 52, calculated according to the classical formula Cr + 3,3Mo + 16NThe manufacturer also indicates that this level is comparable to Alloy 625.
Alloy 625 / 2.4856 / UNS N06625 contains about 20–23% Cr and as much as about 8–10% Mo. The very high molybdenum content causes its approximate PREN value to be about 50–53.

Why is there an asterisk next to C-276, Alloy 22, Alloy 59, and Alloy 2120?

For highly alloyed Ni-Cr-Mo materials belonging to the so-called C-family, VDM Metals uses a modified equation:
PREN = %Cr + 3,3 × (%Mo + 0,5 × %W) + 30 × %N
According to this method, VDM provides approximate values:

• Alloy C-276 / 2.4819 – PREN approx. 75*,
• Alloy 22 / 2.4602 – PREN approximately 70*,
• Alloy 59 / 2.4605 – PREN approximately 76*,
• Alloy 2120 / 2.4700 – PREN approximately 86*.

These values ​​are taken directly from the VDM Metals specification, which typically assumes approximately 16% Cr, 16% Mo and 3,5% W for C-276, approximately 22% Cr, 13% Mo and 3% W for Alloy 22, approximately 23% Cr and 16% Mo for Alloy 59, and approximately 21% Cr, 19% Mo and 0,075% N for Alloy 2120.
So the asterisk is important, because the number 75 or 86 may suggest that it is directly comparable to, for example, PREN 42,5 of 254 SMO steel. However, these values ​​were calculated using a different equation.

Is PREN 50 material always better than PREN 40 material?

No. PREN primarily describes potential resistance to pitting corrosion in chloride environments. It is not a universal ranking of chemical resistance.
The actual resistance of a material is also influenced by temperature, pH, chloride concentration, the presence of oxidizing and reducing agents, stresses, surface condition, microstructure, welding process and the possibility of crevices and deposits.
A good example is Alloy 825Its PREN may be lower than that of duplex 2205, even though it is a high-alloy nickel material that performs very well in certain acidic environments. The PREN alone does not account for the full impact of nickel, copper, or other alloying elements on chemical resistance.

PREN and CPT – two different parameters

When comparing materials, it is worth analyzing not only PREN but also CPT (Critical Pitting Temperature), or critical pitting corrosion temperature. PREN results from the chemical composition, while CPT describes the behavior of the material under specific test conditions.
Therefore, two materials with a similar PREN value may not necessarily have identical resistance in a real installation. For very high-alloy materials, differences are best assessed by testing according to ASTM G48 or ASTM G150. For example, Alleima lists a very high critical pitting temperature for Sanicro 35, while also comparing this material with 254 SMO, Alloy 625, and C-276.
Proposal: PREN is an excellent tool for preliminary material comparisons, especially between stainless steels with similar passivation mechanisms. However, as we move further from classic stainless steels to highly alloyed nickel alloys (Ni-Cr-Mo-W), the simple numerical interpretation of PREN should be treated with greater caution.

Types of corrosion of steel and metals

Uniform corrosion of steel and metals
Pitting corrosion of stainless steels
Crevice corrosion of stainless steels
Intergranular corrosion of stainless steel
Stress corrosion cracking SCC
Galvanic corrosion
Erosive corrosion
Cavitation corrosion
Corrosion fatigue
Fretting corrosion
Microbiological corrosion MIC
High-temperature corrosion

Division of corrosion according to the environment

Marine corrosion – corrosion of steel and metals in seawater
Atmospheric corrosion
Ground corrosion
Corrosion in fresh and industrial water
Corrosion in chloride environments
Corrosion in acidic environments
Corrosion in alkaline environments
Corrosion in gases and exhaust gases
Corrosion in sewage and sewer systems
Corrosion in chemical and petrochemical environments

Related topics

CPT – critical pitting corrosion temperature of stainless steels and nickel alloys
PREN – pitting corrosion resistance index for stainless steel
CCT – Critical crevice corrosion temperature of stainless steels and nickel alloys