Reexamining the Role of Mns Inclusions in Pitting of Stainless Steel

Abstract

Most models of pit initiation in austenitic stainless steels involve the anodic dissolution of manganese sulfide (MnS) inclusions. This work reexamined the role of MnS inclusions and looked at whether they are anodic or cathodic to the matrix metal surrounding them. Specifically, pit formation and the initial stages of growth were investigated using potentiodynamic polarization paired with microscopy.Stainless steel (SS) 304 samples were exposed to 0.5 M NaCl and slowly polarized from 0.2 VSCE to 0.4 VSCE. Polarization scans showed evidence of metastable pits and, in some cases, pit growth. Microscopy of sample surfaces revealed many small open pits with an occasional larger covered pit. The small open pits were associated with inclusions of various types that had fallen to the bottom (Figure 1a). The larger covered pits had MnS inclusions that remained attached to the sample surface (Figure 1b). The attached inclusions acted as a pit cover during initial stages of growth. Growth directions indicated by the perforated pit covers usually showed asymmetric enlargement that is likely controlled by local metallurgical conditions such as crystal orientation and grain boundaries.Examination of MnS inclusions in both open and covered pits showed no evidence of significant corrosion of the inclusion. The electrochemical behavior of MnS samples was analyzed subsequently using potentiodynamic polarization scans. MnS was passive in 0.5 M NaCl up to the maximum potential applied, 0.85 VSCE. The open circuit potential of MnS was approximately -0.26 VSCE, which is slightly more active than that for bulk SS 304, and the passive current density ranged from slightly less than to slightly greater than that of SS 304 up to the pitting potential of SS 304.Based on the results from this study, the following model for pit initiation and initial growth is proposed for SS 304 in NaCl solutions: Pits initiate at cathodic inclusions (MnS, oxides, etc.) with corrosion occurring in the matrix metal at the inclusion / metal interface.When corrosion occurs around the entire interfacial area of the inclusion, the inclusion falls into the pit and repassivation can occur.When corrosion does not occur evenly around the inclusion due to local metallurgical conditions (e.g., composition, crystal orientation, grain boundaries, etc.), the inclusion acts as a cover and pit growth can occur. Figure 1