A microelectrochemical system for in situ high-resolution optical microscopy was fabricated and applied to the real-time observation of pit initiation at MnS inclusion in type 304 stainless steel in NaCl solutions. It was directly observed that the metastable and stable pits were initiated at the MnS/steel boundaries, and that deep trenches were generated at these boundaries during anodic polarization. The initial rounded form of metastable and stable pits became polygonal in shape within 1 s. After that, the dissolution proceeded in the depth direction with no change in the appearance of the pit as observed externally. In the case of the metastable pitting, the duration of this stage was ca. 1.5 s, and then the pit repassivated, and the polygonal metastable pit remained on the electrode surface. The in-depth growth stage for stable pitting was relatively longer (ca. 3.5 s), and the pit grew deeply into the steel matrix and wrapped beneath the inclusion, leading to the formation of a large occluded cavity, in which the corrosivity considerably exceeded the critical conditions for autocatalytic pit growth. Chloride ions were shown to increase the probability of metastable pit initiation and affected the surface and cross-sectional morphology of stable pits.

Microscopic polarization, scanning transmission electron microscopy, and Raman spectroscopy were performed to ascertain the pit initiation mechanism at MnS inclusions in stainless steels. While the inclusion surfaces dissolved under anodic polarization in 0.1 M Na 2 SO 4 as well as 0.1 and 3 M NaCl solutions, the boundaries between the inclusions and the steel matrix dissolved selectively only in the NaCl solutions. This selective dissolution resulted in the formation of trenches, in which metastable and stable pits were initiated. The trenches were shown to be formed by the active dissolution of the steel sides of the boundaries, where no anomalous phase and no compositionally altered zone was observed. It was found that elemental sulfur was deposited on the inclusions and at the boundaries after anodic polarization in 3 M NaCl. The active dissolution of the steel matrix occurred in solutions in which chloride ions and elemental sulfur coexist. The synergistic effect of the elemental sulfur produced by MnS inclusion and chloride ions is likely to cause the trenches, and the decrease in both pH and potential inside the trenches results in pit initiation.Stainless steels suffer from pitting corrosion in chloride-containing environments. 1 Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting. 2-7 It is known that chromium in sulfide inclusions provides higher pitting potentials of the steels. 8 The sensitivity to pit initiation seems to be directly related to the solubility of sulfide inclusions; 9,10 however, how the dissolution of the inclusions affect the pit initiation is not well understood. In order for stainless steels to be used safely in severe environments, it is necessary that the mechanism of the pitting at MnS inclusions in stainless steels be clarified.Muto et al. demonstrated that the pitting was initiated at the boundaries between MnS inclusions and steel matrix. 11 It has been proposed that the synergistic effect of MnS dissolution products and chloride ions plays an important role in the initiation of pitting. 12,13 Under anodic polarization, the passivation of the steel matrix proceeds in near-neutral environments; however, MnS inclusions dissolve electrochemically in the passive region of the steels, which causes pitting corrosion in chloride-containing solutions. 12,14,15 Among the many types of sulfur-containing species that the electrochemical dissolution of MnS inclusions can produce are thiosulfate ion (S 2 O 3 2− ), solidstate elemental sulfur (S), hydrogen sulfide (H 2 S), and hydrogen sulfide ion (HS − ). This diversity is possible because sulfur can present itself in a range of states from the negative bivalent state to positive sexivalent state. Webb et al. and Lott et al. detected thiosulfate ions released from MnS inclusions under anodic polarization. 16,17 Krawiec et al., Ke et al., and Castle et al. demonstrated that the dissolution of MnS inclusions produces sulfur deposition on stainless steel matrix near the MnS inclusions. 18-20 Brossia et al. re...

In situ and real-time optical microscopic observations of pit initiation process on a commercial Type 304 stainless steel with low-sulfur content (0.004 mass%) were performed in 3 M NaCl solution at 298 K. MnS inclusions with diameters of ca. 1 µm were found to act as the initiation sites of pitting, as was the case in a re-sulfurized Type 304 stainless steel (0.027 mass%). The pit was initiated at the boundary between the MnS inclusion and the steel matrix, and grew with time. After a few seconds, no visible change was observed for 1 s, even though the anodic current was measured, suggesting that the steel dissolution proceeded in the depth direction. After that, a small hole suddenly appeared on the steel surface. The hole widened with time, steadily becoming a large stable pit. This pit initiation process in the low-sulfur stainless steel is much like that found in the re-sulfurized stainless steel.

Type 304 stainless steel was subjected to low-temperature carburizing treatment to add excess interstitial carbon to its surface. The average carbon concentration in the carburized layer was ca. 2.6 mass%, and the lattice parameter was expanded by ca. 2.5% without any carbide precipitation. No pitting was initiated on the carburized stainless steel in 0.1 M NaCl. Microscopic polarization measurements of a small area with a MnS inclusion were performed in NaCl and Na 2 SO 4 solutions to clarify the mechanism of the improved resistance against pitting corrosion. The anodic polarization in 0.1 M NaCl and 0.1 M Na 2 SO 4 demonstrated that the carburizing treatment has little or no effect on the electrochemical dissolution behavior of the MnS inclusions. However, from the anodic polarization of the steel in the solution that simulates the vicinity of the dissolved MnS inclusions in chloride-containing environments, it was clarified that the carburizing treatment inhibits the active dissolution rate of the steel matrix to about one hundredth. It would appear that interstitial carbon inhibits the dissolution rate of the steel, resulting in a reduction in the dissolution depth of the trenches at the MnS/steel boundaries. It is likely that the carburization resulted in a suppression of both the acidification due to the hydrolysis reaction of Cr 3+ and the potential decrease due to IR-drop in the trenches to the extent that the corrosivity inside the trenches is insufficient for the localized transition from the passive to active state. Therefore, pit initiation does not occur at the MnS inclusions on the carburized stainless steel in chloride-containing environments. Stainless steels suffer from pitting corrosion in chloride-containing environments.1 Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting on stainless steels.2-7 MnS inclusions dissolve electrochemically in the passive region of stainless steels, causing the pit initiation on the steels in near-neutral chloride-containing environments. [8][9][10][11][12][13] It has been proposed that the dissolution products of the MnS inclusions play an important role in pit initiation.12,14 The dissolution of MnS inclusions produces many types of sulfur-containing species; Lott et al. 15 proposed that thiosulfate ion (S 2 O 3 2− ) is produced, and Webb et al. 16 In previous work, we studied the pit initiation behavior at the MnS inclusions in stainless steels. The pit initiation mechanism was found to be as follows [21][22][23] : 1) the dissolution of MnS inclusions in chloridecontaining 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 stainless steel matrix at the MnS/steel boundaries, resulting in the formation of trenches; 3) the hydrolysis reaction of Cr 3+ 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 trenche...

Microelectrochemical measurements were performed to ascertain the effects of atmospheric exposure on pitting at MnS inclusion on Type 304 stainless steel. The exposure to air at 298 K and 50% RH for 30 and 90 days improved the resistance of the investigated steel to pitting. The thickness of the oxide film on the inclusions increased with the exposure time, and the dissolution of the inclusion was suppressed during anodic polarization in NaCl and Na 2 SO 4 solutions. The 90 days exposure effectively inhibited the trench formation at the MnS/steel boundaries, making the inclusions less active as the initiation sites for pitting.