What Is Pitting Resistance Equivalency Number (PREn)?

The Pitting Resistance Equivalency Number (PREn) is an objective way to do an initial comparison of the relative resistance to corrosion of various metals. It is based on a calculation that considers the weight percentage of alloying elements.

PREn = % Cr + (3.3 x % Mo) + (30 x % N)

A simple examination of the components of the PREn calculation indicates those alloys that contribute to corrosion resistance. Chromium, Molybdenum and Nitrogen are all included because their presence in the alloy grade, in combination, contributes to corrosion resistance.  Sometimes Tungsten is also included in the calculation because of its contribution to pitting resistance.

As there are ranges of alloy percentages that define the alloy grade (MetalTek 304, for example contains 18-21% chromium), the PREn calculation also is a range. PREn ranges of >40 typically describe the Super Duplex or Super Austenitic families, describing a higher concentration of the key elements and, in the right conditions, more corrosion resistance.

A few common alloys and their PREn values: