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

Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers b

Ethyl silicate zinc-rich primers cure by the reaction of the vehicle with moisture, thereby providing a binder. As relative humidity and temperature vary during the day, so does the rate of cure. A certain minimum degree of cure is necessary prior to top-coating. It has been shown that the degree of cure of ethyl silicate zinc-rich primers can be measured by the chemical changes occurring using diffuse reflectance infrared spectroscopy.2 This solvent rub test has been shown to correlate well with the infrared spectroscopic results of some two-component ethyl silicate inorganic zinc systems.

Materials: Methyl Ethyl Ketone (MEK), Cheesecloth, 100 % cotton, Squeeze Bottle

Procedure: 

1- Select area

2- Measure the dry film thickness of the primer

3- Fold the cheesecloth into a pad of double thickness and saturate it to a dipping wet condition with the methyl ethyl ketone.

4- rub the rectangular test area with moderate pressure ( 50 double rubs)

5- Evaluation of result

Most paint has four components: 

The resin is the main part of the paint which forms a film on the surface. It is typically a 

non-hazardous component like linseed or acrylic. 

Solvent keeps the paint a liquid until the solvent evaporates after painting. In oil-based 

paint, the solvent is derived from a petroleum distillate and can include hazardous 

ingredients like mineral spirits, toluene and xylene. The solvent in latex paint is water. 

Pigments provide the color and covering power. The major pigments used presently are 

relatively nontoxic. Some highly colored pigments may contain heavy metals such as 

chromium, cadmium or arsenic. Older paints may contain lead. 

Paint may also have additives. Some types of additives include stabilizers, dryers, 

thickeners and preservatives. Some latex paints contain a mercury-based fungicide 

preservative.