Calculation of Pitting Resistance Equivalent Numbers PREN By yaang.com

Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pitting corrosion resistance of various types of stainless steel, based on their chemical composition.
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 composition 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 Standard.

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 steel, 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 stainless steel 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 EN 10088-1 for all but specific grades, such as 1.4311 (304LN), 1.4406 (316LN) austenitic stainless steel. In contrast all the duplex stainless steel grades have specified nitrogen ranges. It can then be misleading to use just specified ranges as the residual nitrogen in commercially produced austenitic 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 stainless steel 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 S32750 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' austenitic or 'super' duplex stainless steel 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

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 “super duplex” stainless 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.

Source: Yaang Pipe Industry Co., Limited (www.yaang.com)

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