Pitting resistance equivalency number (PREN) – The corrosion resistance of pit-resistant stainless steels (e.g. AISI 316 and 317, and alloy 25-6MO) is largely determined by their composition. The PREN typically used to compare stainless steels is determined by a calculation based on their contents of chromium, molybdenum and nitrogen: PREN = %Cr + 3.3 (%Mo) + 30 (%N).

As a general rule, the higher the PREN, the better the resistance to pitting. However, alloys having similar values may differ considerably in actual service. Those with values greater than 38 on the PREN scale offer more resistance than the 304 and 316 stainless steels. Alloy 25-6MO, with a PREN of 47, offers a cost-effective means of avoiding aggressive chloride attack. PRENs for nickel-based corrosion-resistant alloys (e.g. alloy 825 and alloys G-3, 622, 625, 686 and C-276) are calculated by a somewhat different equation. For such alloys, molybdenum, chromium, tungsten and niobium are the most influential alloying elements. PREN = %Cr + 1.5 (%Mo + %W + %Nb).

Addition of the nitrogen factor to this equation as in below table will allow calculation of PRENS for both iron-based and nickel-based alloys. As with stainless steels, a higher PREN for a corrosion-resistant alloy is generally indicative of superior resistance to corrosion. A ranking of stainless steels and alloys by PREN is presented in the below table. PREN = %Cr + 1.5(%Mo +%W + %Nb) + 30(%N).