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

 

 

Source: http://www.bssa.org.uk/