The Role of Oxide Films on TiS and Ti₄C₂S₂Inclusions in the Pitting Corrosion Resistance of Stainless Steels

Abstract

Microelectrochemical polarization, thermodynamic calculations, and Auger electron spectroscopy were performed to determine the predominant factors affecting the pit initiation resistance at sulfide and carbosulfide inclusions in stainless steels. TiS and Ti4C2S2 inclusions did not dissolve in the passive or the trans-passive regions of stainless steels in NaCl solutions, and no pits were initiated at these inclusions in NaCl solutions. The calculations made from the potential-pH diagrams for Ti-S-TiS-H2O and Ti-S-Ti4C2S2-H2O systems indicated a high likelihood that Ti-oxide enriched surface films would form, and this was confirmed by surface analysis. The Ti-oxide enriched layers on the inclusions inhibited the dissolution of the TiS and Ti4C2S2 inclusions in the passive and trans-passive regions of stainless steels. In contrast, MnS inclusions dissolved in NaCl solutions, and pits were initiated at the inclusions. While MnS inclusion surfaces were covered by Mn-oxides, the corrosion resistance of Mn-oxides was not sufficient to prevent inclusion dissolution and subsequent pit initiation. The oxide enriched surface layers on the sulfide and carbosulfide inclusions were shown to significant enhance pit initiation resistance at the inclusions.

Artificial MnS Inclusions in Stainless Steel: Fabrication by Spark Plasma Sintering and Corrosion Evaluation by Microelectrochemical Measurements

“…1) The relationship between the chemical composition and the electrochemical properties of the inclusions is a topic of interest in corrosion-resistant alloy design. [2][3][4][5][6] A systematic investigation of inclusion modification by alloying necessitates a facile fabrication method of steel specimens containing artificial sulfide inclusions. However, research in this area has been very limited.…”

Corrosion Resistance of a Free-Cutting Soft-Magnetic Stainless Steel in Pure Water

“…The interference colors observed on the Ti 4 C 2 S 2 inclusions in K-M38CS suggest the growth of oxide film. 14) The oxide film should have inhibited the dissolution of the inclusions under Table 2 Relative composition of the points marked in Fig. 2 (at% anodic polarization.…”

“…Figure 8 shows the potential-pH diagrams for the MnS-H 2 O system at 298 K. MnS is not stable in neutral solutions. Although Mn-oxide layers are formed on the MnS inclusion surface in the as-polished condition, 14) both Mn oxide and hydroxide (MnO 2 , Mn 2 O 3 , Mn 3 O 4 , and Mn(OH) 2 ) are unstable at the corrosion potential of ¹0.05 V in the deionized water. Thereby, MnS and native Mn-oxide films are expected to dissolve as Mn 2+ and S 2 O 3 2¹ in the deionized water, as shown in Fig.…”

“…12,13) It has been reported that the absence of anodic dissolution of Ti 4 C 2 S 2 inclusions in a conventional 18Cr-8Ni austenitic stainless steel can be attributed to the formation of a Ti-enriched oxide layer, and that no pitting initiates at the inclusions as a result. 14) The corrosion resistance of the new soft-magnetic stainless steel with Ti 4 C 2 S 2 is expected to be high. In order to investigate the applicability of the new soft-magnetic stainless steel with Ti 4 C 2 S 2 to solenoid valves for water systems of PEFCs, the influence of both Ti 4 C 2 S 2 and MnS inclusions on the corrosion resistance in pure water environments was determined in this study.…”

Effect of Applied Stress on Dissolution Morphology and Pit Initiation Behavior of MnS Inclusion in Stainless Steel

“…2a, since the surface oxide acts as a barrier against the inclusion dissolution. 7,12,13,18 From a thermodynamic point of view, it is expected that the anodic polarization causes the composition change from (Mn,Cr)S to CrS.…”