Effect of Low-Temperature Carburizing Treatment on Improving Pitting Corrosion Resistance at Manganese Sulfide Inclusion in Type 304 Stainless Steel

Stainless steels suffer from pitting corrosion in chloride-containing environments. Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting on stainless steels. In our previous works, we studied the pit initiation behavior at the MnS inclusions in stainless steels. The pit initiation mechanism was found to be as follows: 1) the dissolution of the MnS inclusions in chloride-containing solutions leads to the deposition of elemental sulfur on and around the inclusions; 2) the synergistic effect of the elemental sulfur and chloride ions causes the dissolution of the steel matrix at the MnS/steel boundaries, resulting in the formation of trenches; 3) the hydrolysis reaction of Cr³⁺ released from the steel matrix dissolution decreases the pH in the trenches, and at the same time, the electrode potential at the bottom of the trenches decreases due to the IR-drop. Finally, the pit initiation is defined as the local transition from the passive to active state at the bottom of the trenches. It would appear that inhibiting the dissolution of the steel side at the MnS/steel boundaries improves the pitting corrosion resistance of stainless steels in chloride-containing environments. In recent years, it has been reported that low-temperature carburizing treatments which result in interstitial carbon content of more than 10 at% with no precipitation of carbides improve the corrosion resistance of stainless steels; however, the mechanism is not well understood. In addressing the improvement of the pitting corrosion resistance of stainless steels, it is necessary to understand the mechanism of the effect of low-temperature carburizing treatments on the pit initiation at the MnS inclusions in stainless steels.

The anodic polarization curves of a micro-scale electrode area with a MnS inclusion of the untreated and the carburized re-sulfurized Type 304 stainless steels (Mn: 1.32, S: 0.029 mass%) were measured in 0.1 M NaCl. Figure 1 shows the microscopic anodic polarization curves and the surface appearances of the inclusions after the polarization. The surface dissolution of the MnS inclusions resulted in broad peaks around 0.4 V in the anodic currents both of the untreated and the carburized specimens (Fig. 1a). In the polarization curve of the untreated specimen, sharp increases in current density due to the pit initiation were measured in the potential range of the dissolution of the MnS inclusion. These pits were initiated at the MnS inclusion marked by the arrows in Fig. 1b. The current peaks in the polarization curve of the carburized specimen were much smaller than those indicating the pit initiation on the untreated specimen. Neither metastable nor stable pits were initiated at the MnS inclusions in the carburized specimen (Fig. 1c). Figures 1d-e display enlarged views of the areas enclosed by the broken lines in Figs. 1b-c, respectively. A wide and deep trench was formed at the MnS/steel boundary on the untreated specimen (Fig. 1d). In contrast, a narrow and shallow trench was formed at the boundary on the carburized specimen (Fig. 1e). These results suggest that the improved resistance to pitting corrosion at MnS inclusions after the carburizing treatment can be attributed to the reduced scale of the trenches formed at the MnS/steel boundaries in chloride-containing solutions.

It is assumed that the dissolution of the steel matrix forming the trenches at the MnS/steel boundaries is caused by the synergistic effect of elemental sulfur and chloride ions. To compare the active dissolution rate of the untreated and the carburized stainless steel matrix in a sulfur suspension with chloride ions, macroscopic anodic polarization curves were measured. The low-sulfur Type 304 stainless steel (Mn: 0.82, S: 0.0002 mass%) was used. The sulfur suspension was produced by the acidification of a thiosulfate-containing solution. It has been reported that the dissolution of the MnS inclusions produces thiosulfate ions (S₂O₃^2-) and hydrogen ions which decrease the solution pH around the inclusions to approximately 3. The polarization curves were therefore measured in 3 M NaCl – 1 mM Na₂S₂O₃ solution at pH 3.0 (Fig. 2). The active dissolution rate at ‑0.3 V of the untreated stainless steel was about 500 A/m², whereas that of the carburized stainless steel was reduced to about 5 A/m², approximately one hundredth that of the untreated stainless steel. From these results, we can conclude that the carburizing treatment inhibits the active dissolution rate of stainless steel matrix in solutions with elemental sulfur and chloride ions, resulting in a reduction in the dissolution depth of the trenches at the MnS/steel boundaries. Therefore, no pit initiation occurs at the MnS inclusions on the carburized stainless steel in chloride-containing solutions.