What is a pitting resistance equivalent number?

As a first review, the Pitting Resistance Equivalent Number (PREN) is a helpful theoretical way of comparing the pitting corrosion resistance of various types of metals based on their chemical compositions. The PREN numbers can be used to compare different grades, but cannot be used to predict absolute performance of a grade in a specific application.

The most widely accepted version of the PREN formula utilises the content of chromium, molybdenum and nitrogen as: PREN = %Cr + 3.3x %Mo + 16x %N. Due to the fact that most product specifications allow a range of compositions, certain end users will also specify a minimum PREN value to ensure adequate levels of corrosion resistance. More highly-alloyed grades will generally have a higher PREN value, and therefore will generally have a better resistance to pitting corrosion i.e. the PREN values of 316L, 2205 duplex and super duplex stainless steels are 25, 32 and >40 respectively, primarily increased by increasing chromium content.

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