PREn – Pitting Resistance Equivalent Number
The pitting resistance equivalent number (PREn) is a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels based on the chemical compositions of those alloys. The PREn of a given alloy is determined by a mathematical model that utilises select elements in that alloy’s chemistry. PREn specifically suggests how resistant that alloy is to pitting in a chloride containing environment. PREn is only meaningful within the same stainless steel family (austenitic, ferritic, duplex stainless steels). PREn cannot be interpreted as an absolute value.
There are multiple formulas in use today that incorporate a greater number of alloying elements and/or modestly differing ratios of elements, to fine tune the predictive relationship between the resultant PRE number and actual pitting resistance. Typically, Molybdenum and Nitrogen levels are weighted to take into account their strong influence on pitting corrosion resistance. The most common and widely accepted PREn formula is:
PREn = %Cr + (3.3 x %Mo) + (16 x %N)
The results are very linear as noted in the graph below, for several stainless and duplex alloys. In this case, PREn is plotted against CPT (critical pitting temperature), which is another predictive tool based upon extensive laboratory tests. CPT is a measure of actual alloy performance in a corrosive media, as described by ASTM G48 or ASTM G150, which again suggests relative pitting performance in high chloride environments, like seawater.
Another common PREn formula found acceptable for super duplex stainless steels, takes Tungsten into account. Some super duplex products, like ZERON® 100, contain Tungsten, which appreciably aids in corrosion resistance. For standards ISO 15156/NACE MR0175, that formula is:
PREn = %Cr + 3.3 x (%Mo + 0.5%W) + 16 x %N
The PREn is best suited for comparing groups of alloys or ranking alloys for potential consideration in various applications. It is often used to suggest the suitability of alloys to various service environments. Engineers often consider a PREn of 32-35 as the minimum suitable for seawater service, for austenitic and duplex stainless steels. Duplex stainless steels with a PREn over 40 are considered acceptable for Hydrogen sulfide environs.
PREn is not a suitable tool for specification purposes. One cannot say that an alloy with a PREn of 42.1 will do well in seawater, and a PREn of 41.9 will fail. PREn numbers are not that fine tuned. The British Stainless Steel Association cautions:
PREn numbers are useful for ranking and comparing different grades,
but cannot be used to predict whether a particular grade will be acceptable
for a given application, where pitting corrosion may be a hazard.
In ranking alloys, or selecting likely candidates for service, one should consider the PREn of groups of alloys.
Ultimately, selection of the appropriate specialty steel for a given service depends upon the overall corrosiveness of the environment, including acidity, and other corrosive media beyond just chlorides, as well as service temperatures. Mechanical property requirements and other factors should also be considered including ease of fabrication, availability and cost.
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