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corrosion in stainless

The PREN: how to measure the pitting resistance of stainless steel
Posted: 10 Jun 02 (Edited 3 Oct 03)

The pitting resistance of stainless steel is primarily determined by its composition. The three elements which have a significant beneficial effect are chromium, molybdenum, and nitrogen. The formula which gives their relative contribution is:

PREN=%chromium+3.3x%molybdenum+(16 to 30)x%nitrogen,
where PREN is pitting resistance equivalent number and percents are in weight percent of elements IN SOLID SOLUTION. Ferritic stainless steels, which can hold no nitrogen in solid solution, must obtain all their corrosion resistance from chromium and molybdenum. The nickel in austenitic stainless steel confers no benefit; it only stabilizes the austenitic structure, which can hold appreciable nitrogen.

Recent research has shown that pitting susceptibility arises from the bulk material around the manganese sulfide inclusions losing chromium as sulfur precipitates as chrome sulfide around the pre-existing primary manganese sulfide inclusion. This occurs during cooling from melting temperatures. This region can have less than 10% chrome. Thus, it corrodes away as would carbon steel, initiating the pitting process. The inclusion itself is not the initiator of pitting.
Welds (GTAW or TIG)have significantly lower pitting resistance. Their PREN is reduced by 335x(%S) and 700 to 1000 x (%O). These elements form chromium bearing inclusions during weld cooling and deplete the surrounding matrix of chromium. Removing oxides from welds is important, but so is removing the chromium depleted regions by pickling and passivating.

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