Investigation Of Nonmetallic Inclusions And Their Corellation To Pitting Corrosion Of Austenitic Stainless Steels PDF Download

Although the effect of non-metallic inclusions on pitting corrosion of stainless steel alloys have been an interesting topic during last decades, the role of the inclusions on pitting initiation and propagation is still under discussion. In this study, different types of inclusions consisting of carbon, nitrogen, silicon, oxygen, manganese, Sulfur etc. on 304L and 303 stainless steels have been investigated using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX) and Focused Ion Beam (FIB) techniques. Preliminary in-situ AFM results in 0.1 M NaCl indicate the deposition of copper on MnS inclusions at anodic potentials but not at the Open Circuit Potential (OCP). No pitting was observed at MnS inclusions or the adjacent area. Pit propagation at the MnS-matrix boundaries was observed in-situ while it was exposed to 0.1 M NaCl solution. In these experiments samples were pretreated by exposure to acidic chloride solution at the OCP. Although it was initially appeared to be pit initiation, it was demonstrated later via SEM-FIB analysis that the observed trenches on the surface were in fact the propagated pits below the surface which reached to the top at some point. Based on SEM-FIB cross-section observation, the MnS inclusions were passivated by a thin layer of copper, presumably as Cu2S. Further examination of other types of inclusions, showed that there was no pitting corrosion at the inclusion-matrix boundaries. This indicates that local chemistry and not the geometry is the more probable reason for the MnS-matrix boundaries to be corroded.