(Invited) Effect of Cr Concentration on Dissolution of (Mn,Cr)S Inclusions in Stainless Steel

Abstract

The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. The chemical composition of the inclusions affects the pitting corrosion resistance of stainless steels. In general, MnS inclusions in stainless steels contain a small amount of Cr, and thus they are accurately written as (Mn,Cr)S inclusions. It has been reported that Cr in (Mn,Cr)S inclusions have an inhibitory effect on the dissolution of the inclusions. However, the extent to which the Cr concentration in the inclusions prevents the dissolution of the inclusions remains unclear. In this study, spark plasma sintering was used in the facile fabrication of stainless steel specimens containing artificial (Mn,Cr)S inclusions with different Cr concentrations.1 A microelectrochemical technique was applied to elucidate the effect of Cr concentration on the dissolution of the inclusions in stainless steel.Type 304 stainless steel powders were mixed with either MnS or Cr2S3 powders to prepare sintered specimens containing 0.06 mass% S. The mixed powders were loaded into a cylindrical graphite die and subsequently compressed into a pellet. The pressed compact was sintered under vacuum conditions and uniaxial pressure in a spark plasma sintering system. The sintered specimens were ground with SiC papers and then polished with diamond pastes. Potentiodynamic polarization was conducted in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The specimen surface was masked to make an electrode area of approximately 100 µm × 100 µm containing a single artificial inclusion. All the potentials reported in this study refer to the Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogen electrode at 298 K). During polarization, the electrode area was observed using an in situ real-time optical microscopy system.2 The anodic polarization curves were measured to analyze the dissolution and pit initiation behavior of the artificial (Mn,Cr)S inclusions. The onset potential of inclusion dissolution and the pitting potential increased with increasing Cr concentration in the inclusions. The onset potential of inclusion dissolution reached approximately 0.7 V when the Cr/(Mn+Cr) atomic ratio in the inclusion was more than 0.5. This potential is almost the same as the transpassive region of Type 304 stainless steel. There was not much difference even if the Cr/(Mn+Cr) ratio of the inclusions was higher than 0.5. We concluded that the inhibitory effect of Cr on the dissolution of (Mn,Cr)S inclusions is most effective when the Cr/(Mn+Cr) ratio is 0.5.