Effect of Inclusion Size on Pit Initiation at MnS in Stainless Steel

Sulfide inclusions such as MnS act as initiation sites of pitting on stainless steels in chloride-containing environments. In our previous studies, we succeeded to observe the pit initiation process directly and proposed the following pit initiation mechanism at MnS inclusions: 1) the dissolution of MnS inclusions produces elemental sulfur which deposits on and around the inclusions; 2) the synergistic effect of the elemental sulfur and chloride ions dissolves the steel matrix at the MnS/steel boundary, which results in the formation of trench; 3) in the trench, the hydrolysis reaction and IR-drop produce low pH and low electrode potential, causing the local active dissolution of the steel matrix. The active dissolution initiated in shallow trenches becomes a metastable pit, whereas a stable pit is formed in deep trenches. The pit initiation mechanism was elucidated using re-sulfurized stainless steels with large MnS inclusions elongated in the rolling direction; however, it is well known that when the MnS inclusions are less than 1 μm in diameter, they hardly act as initiation sites of pitting. In addressing the improvement of the pitting corrosion resistance of stainless steels, it is necessary to understand the effect of inclusion size on the pit initiation at MnS.

To clarify the effect of the inclusion size on the pit initiation at MnS in stainless steels, in situ optical microscopic observation of a commercial Type 304 stainless steel (Mn: 0.097, S: 0.004 mass%) surface was carried out in 3 M NaCl during potentiostatic polarization at 0.4 V (electrode area: ca. 200 μm × 220 μm). Figure 1 shows the initiation process of a stable pit at the MnS inclusions with ca. 1 μm in diameter. There were five small MnS inclusions at the center of the image (Fig. 1a). At the places marked by the arrows in Fig. 1b, the dark areas expanded, suggesting that small pits were initiated at the MnS inclusions. The following three steps were noted as the initiation process of the stable pit: 1) the pit was initiated at the MnS/steel boundary (Fig. 1b) and grew with time (Fig. 1c, d); 2) the dissolution of the steel matrix proceeded in the depth direction without changes in appearance; 3) small holes suddenly appeared (marked by the arrows in Fig. 1e) and expanded with time, becoming a large stable pit. This pit initiation process was the same as that in re-sulfurized stainless steels.

Figure 2a indicates the polarization curve of Type 304 stainless steel. Note that the only metastable pit, indicated by the current spike in the polarization curve, was initiated at the MnS inclusion with ca. 1 μm in diameter (Fig. 2b, c). No stable pit was initiated at the MnS inclusions during the potentiodynamic polarization (23 mV/min), while the stable pit was initiated at the inclusions during the potentiostatic polarization described above. The non-initiation of stable pit during the potentiodynamic polarization can be attributed to the uncondensed elemental sulfur prohibiting the formation of a deep trench at the boundary. These results suggest that the critical factor in suppressing pit initiation is to inhibit the production and/or concentration of elemental sulfur at the MnS/steel boundary. Decreasing the size of the MnS inclusion is one way to reduce the amount of elemental sulfur which is produced by the dissolution of inclusion.