Calculation of pitting resistance equivalent numbers (PREN)

Introduction

Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels, based on their chemical compositions.
The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.
Actual or specified range compositions can be used and usually involve chromium, molybdenum and nitrogen in the calculations. Tungsten also appears in some versions of the calculation.

In some industries, notably the oil and gas sector, specifications may place tighter restrictions on the PREN for specific grades than that implied by the minimum composition of the grade defined in EN or ASTM Standards.

Affect of alloying elements on pitting resistance

These are 'linear' formulas, where the molybdenum and nitrogen levels are 'weighted' to take account of their strong influence on pitting corrosion resistance.

They typically take the form

PREN = Cr + m Mo + n N

where 'm' and 'n' are the factors for molybdenum and nitrogen.

The most commonly used version of the formula is

PREN = Cr + 3.3Mo + 16N

Some formulas weight nitrogen more, with factors of 27 or 30, but as the actual nitrogen levels are quite modest in most stainless steels, this does not have a dramatic effect on ranking. Tungsten is also included in the molybdenum-rating factor to acknowledge its affect on pitting resistance in the tungsten bearing super-duplex types, for example 1.4501. A modified formula is then used:

PREN = Cr + 3.3(Mo +0.5W) + 16N

Calculated pitting resistance numbers

Nitrogen ranges are not specified in standards such as BS EN 10088-1 for all but specific grades, such as 1.4311 (304LN), 1.4406 (316LN) austenitics. In contrast all the duplex grades have specified nitrogen ranges. It can then be misleading to use just specified ranges as the residual nitrogen in commercially produced austenitics will benefit the pitting resistance.

The table below shows a range of calculated PREN values for comparison. A full theoretical range is shown, using a combination of the lowest and highest specified values for a selection of ferritic, austenitic and duplex grades.
The values are rounded for convenience of display.
The PREN values for commercially available grades will of course lie somewhere between these minimum and maximum values and so commercially available steels in grades 1.4410, 1.4501 and 1.4507 are often stipulated to have actual PREN values over 40.

Grades with a PREN of 40 or more are known as 'super' austenitics or 'super' duplex types, depending to which basic family they belong.

PREN = Cr + 3.3Mo + 16N

Tungsten (W) is known to have an effect on the pitting resistance and for some grades a modified formula is used:

PREN = Cr + 3.3(Mo +0.5W) + 16N

Grade Type Cr Mo N PREN
Ferritics
1.4003 - 10.5-12.5 NS 0.030 max 10.5-12.5
1.4016 430 16.0-18.0 NS NS 16.0-18.0
1.4113 434 16.0-18.0 0.9-1.4 NS 19.0-22.6
1.4509 441 17.5-18.5 NS NS 17.5-18.5
1.4521 444 17.0-20.0 1.8-2.5 0.030max 23.0-28.7
Austenitics
1.4301 304 17.5-19.5 NS 0.11max 17.5-20.8
1.4311 304LN 17.5-19.5 NS 0.12-0.22 19.4-23.0
1.4401/4 316/316L 16.5-18.5 2.0-2.5 0.11max 23.1-28.5
1.4435 316L (2.5% min Mo) 17.0-19.0 2.5-3.0 0.11max 25.3-30.7
1.4406 316LN 16.5-18.5 2.0-2.5 0.12-0.22 25.0-30.3
1.4539 904L 19.0-21.0 4.0-5.0 0.15max 32.2-39.9
1.4563 Sanicro 28 24.0-26.0 3.0-4.0 0.11max 35.9-43.0
1.4547 254SMO 19.5-20.5 6.0-7.0 0.18-0.25 42.2-47.6
1.4529 1925hMo 19.0-21.0 6.0-7.0 0.15-0.25 41.2-48.1
1.4565 4565S 24.0-26.0 4.0-5.0 0.30-0.60 42.0-52.1
Duplex
1.40621 2202 22.0 0.4 0.20 26.5
1.4162 2101LDX 21.0-22.0 0.1-0.8 0.20-0.25 24.5-28.6
1.4362 SAF 2304 22.0-24.0 0.1-0.6 0.05-0.20 23.1-29.2
1.4462 SAF 2205 21.0-23.0 2.5-3.5 0.10-0.22 30.8-38.1
1.44102 SAF 25072 24.0-26.0 3.0-4.0 0.24-0.35 > 402
1.45012 Zeron 1002 24.0-26.0 3.0-4.0 0.20-0.30 > 402
1.45072 Ferrinox 2552 24.0-26.0 3.0-4.0 0.20-0.30 > 402

The nearest AISI grade is shown, where appropriate, otherwise a typical trade name used by some of manufacturers is shown.

NS - Not specified

1 Typical composition only available

2 The “superduplex” steels in particular are subject to tighter restrictions on PREN than that implied by the minimum composition of the EN grade. For example, the equivalent ASTM grade or the specifications of the oil and gas industry e.g. NORSOK or NACE typically require a minimum of PREN =40.

K03 ArcelorMittal Stainless UK Ltd
2101LDX Outokumpu Ltd
904L Outokumpu Ltd
254SMO 4565S Outokumpu Ltd
SAF 2304, SAF 2205, SAF 2507, Sanicro 28 Sandvik Steel UK
Zeron 100 Rolled Alloys

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