Initiation of Corrosion Pits at Inclusions on 304 Stainless Steel

Abstract

Onset of pitting corrosion on 304 stainless steel in was investigated at the site of approximately 200 inclusions. A photolithography technique was used to locate individual sites repeatedly in order to characterize them by a combination of scanning electron microscopy, energy dispersive x‐ray, and Auger electron spectroscopy methods. Growth of pits to significant size was found to occur only at and mixed inclusions which had sizes above 0.7 μm. In early stages of initiation, the inclusion dissolved, the Mn and Cl surface concentrations were found to be high within the pit cavity, and the S concentrations were high in the cavity as well as the surrounding area. Smaller sulfide inclusions and all multielement oxide inclusions either did not initiate pits or initiated only small trenches before becoming repassivated. Under conditions of severe corrosion, Cu was detected on inclusions, and was traced to the appearance of flower‐like deposits which formed prior to significant pit growth.