(Mn,Ti)S inclusions as pit-initiation sites in Ti-containing Fe–16Cr stainless steel: insights from microelectrochemical analysis using artificial inclusions

Type 430 stainless-steel specimens with large sulfide inclusions were fabricated by mixing MnS or TiS 2 powder with Type 430 stainless-steel powder and consolidating the mixtures by spark plasma sintering. MnS addition produced MnS inclusions, whereas TiS 2 addition produced complex inclusions comprising TiS 2 , TiN, and (Mn,Ti)S. Microscale polarization of individual inclusions in 0.1 M Na 2 SO 4 yielded dissolution-potential ranges of MnS (0.2–0.7 V) < (Mn,Ti)S (0.5–1.0 V) < TiS 2 (1.0–1.8 V). TiN did not dissolve up to 1.8 V. In 0.1 M NaCl, pitting corrosion initiated at approximately 0.37 V and 0.61 V in the MnS- and TiS 2 -added specimens, respectively, indicating that dissolution of MnS and (Mn,Ti)S triggered pit initiation in the respective specimens. In commercial Fe–16Cr stainless steel, the inclusions were identified as MnS. In arc-melted Fe–16Cr–0.17Ti, they included Ti-containing oxides, TiN, and TiS. The average pitting potentials were 0.38 V for Fe–16Cr and 0.51 V for Fe–16Cr–0.17Ti. Because inclusion dissolution induces pit initiation, MnS and (Mn,Ti)S were expected to be the respective pit-initiation sites in Fe–16Cr and Fe–16Cr–0.17Ti, although electron microscopy and energy-dispersive X-ray spectroscopy did not reveal (Mn,Ti)S. Galvanostatic polarization in 0.1 M NaCl produced pits several micrometers in diameter. MnS remained at the pit-initiation sites in Fe–16Cr, whereas Mn and S were detected at the pit-initiation sites in Fe–16Cr–0.17Ti. These findings verify the predicted pit-initiation sites.