Influence of Alloy Sulfur Content and Bulk Electrolyte Composition on Crevice Corrosion Initiation of Austenitic Stainless Steel

The influence of alloy sulfur content and bulk solution composition on crevice corrosion initiation of type 304 (UNS S30400) stainless steel (SS) in neutral chloride solutions was determined experimentally, with particular emphasis on the solution chemistry that develops in the occluded region. Sulfide was the only dissolved sulfur species detected during initiation, with sulfite appearing during the propagation stage after longer times as a result of homogeneous oxidation of sulfide within the crevice. Although the time to develop the critical chemistry, manifested through the initiation time (τ), depended strongly upon alloy sulfur levels (–25 min/decade-sulfur), the crevice solution composition (in terms of the chloride [Cl^–] to sulfide [HS^–] concentration ratio) at the initiation of crevice corrosion was found to be independent of alloy sulfur content. The beneficial influence of alloyed molybdenum was manifested as a threefold increase in τ for type 316L SS (UNS S31603) compared to τ for type 304 SS at comparable sulfur levels, although the crevice solution composition was the same within experimental error. The inhibiting action of sulfate in the bulk solution on crevice corrosion initiation was due solely to a supporting electrolyte effect. Nitrate was a far stronger inhibitor for crevice corrosion than sulfate and operated via a mechanism other than a supporting electrolyte effect.