Stainless steel is known for its corrosion resistance, which depends on its chemical composition. To compare alloys’ anti-corrosion capabilities, a predictive measurement called the Pitting Resistance Equivalent Number (PREN) is used. This method provides a reliable way to assess and rank their durability in various environments.
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ToggleHow to Measure Corrosion Resistance
Pitting Resistance Equivalent Number (PREN) is officially considered as a scientific calculation to compare the pitting corrosion resistance among stainless alloys with calculating chemical compositions. The formula of PREN is widely accepted as:
PREN = %Cr + (3.3 x %Mo) + (16 x %N)
Elements Influencing PREN
Regarding elements of formula, it is apparent that Chromium, Molybdenum and Nitrogen play vital roles in influencing pitting corrosion resistance. Sometimes Tungsten is included in some versions of the PREN formula as well. Specifications may be restricted tighter on the PREN for grades in accordance with specific petrochemical and marine industry demands.
PREN Value Application
Pitting Resistance Equivalent Number cannot be seen as an absolute value but be used as a useful method of ranking among the anti-corrosion abilities. Mostly, a PREN value of 32-35 is least acceptable in seawater devices. Waterson 304 stainless steel as an example contains chromium in the specific range of 18-20%. Super Duplex and Super Austenitic families of PREN over 40 can be illustrated as greater resistance to crevice corrosion under tougher conditions, such as high chloride seawater or hydrogen sulfide environments.
Reference: International Molybdenum Association, PREN Comparison of Stainless Steel and Duplex Steel Alloys
Waterson Stainless Steel Investment Casting
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