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...

The electrochemical dissolution of MnS inclusions and pit initiation processes on type 303 stainless steel were investigated by an electrochemical microcell technique. In 1 M Na 2 SO 4 , the electrochemical dissolution of MnS inclusions started at 0.1-0.3 V vs Ag/AgCl͑3.33 M KCl͒, which was followed by the initiation of a hemispherical and smooth wall pit at MnS/matrix boundary. It was suggested that the dissolution products of MnS inclusions induced the formation of a salt film, which caused localized electropolishing. In 0.1 M NaCl, the onset potential of electrochemical dissolution of MnS inclusions was 0.3-0.4 V and the stable pit growth occurred at a potential of around 0.5 V. Field-emission scanning electron microscopy observations revealed that many metastable pits with a diameter of approximately 1 m were formed at MnS/matrix boundary on the specimen, of which the polarization measurement was stopped immediately after stable pit initiation. From a morphological point of view, the metastable pits were found to be very similar to flat-walled etch pits. The dissolution products of MnS inclusions and chloride ions would attack the metal surface newly exposed by dissolution of the inclusions. It is proposed that the chemistry of dissolved species from MnS inclusions have a substantial influence on pit initiation at MnS inclusions.

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MnS and MnO inclusions with a small amount of chromium were obtained by heat-treatments of Type 304 stainless steel at 1353, 1573, and 1673 K, and the anodic dissolution behavior of the inclusions was investigated using a microelectrochemical technique. In the case of the sulfide inclusions, those with a high dissolution potential were found to provide high pitting potentials in the macroscopic measurements in 0.1 M NaCl. The initiation sites of metastable and stable pits were at the boundaries of the inclusions and the matrix in 3 M NaCl, 3 M MgCl 2 , and 8 M LiCl. The dissolution current densities on the inclusion surface at the moment of the initiation of a stable pit decreased with increasing chloride-ion concentration. The synergistic effect of chloride ions and sulfur-containing species released from the inclusions was thought to play an important role in the pit-initiation process. The oxide inclusions did not dissolve in the passive region of Type 304 stainless steel. In the solutions with chloride-ion concentrations of up to 6 M, pitting did not occur at the oxide inclusions in microscopic polarization measurements. The oxide inclusions exhibited a comparatively inert characteristic as pit-initiation sites.Sulfide inclusions in stainless steels, particularly manganese sulfide, are well known to act as initiation sites for pitting in chloride environments. 1 It is generally agreed that the most significant phenomenon which precedes pitting at the inclusions is sulfide dissolution, and also that the complete dissolution of the inclusions is not required for pit-initiation sites to be produced. 2,3 Sulfur-containing species are released during MnS dissolution, and various aggressive compounds, such as SO 4 2− , HSO 3 − , S 2 O 3 2− , S, and H 2 S, have been proposed as the reaction products. 2-9 Park et al., Paik et al., and Webb et al. demonstrated that the thiosulfate ions released from sulfide inclusions accelerated the dissolution of the inclusions and resulted in stable pitting when their concentration exceeded a critical value. 7-9 Krawiec et al. and Vignal et al. experimentally revealed that the sulfur species produced during MnS dissolution reacted with the passive film on a metal matrix to increase pitting susceptibility and to enhance cathodic reaction kinetics. 10-12 Park and Böhni detected the local acidification on MnS inclusions during the dissolution and pointed out its relationship to pitting at the inclusions. 13 Many researchers have revealed that the MnS/matrix boundaries mainly dissolved, and metastable pits were generated at the boundaries. 3,14-22 In our previous study, the synergistic effect of sulfur-containing species and chloride ions on pit initiation was suggested from the morphological observation of metastable and stable pits in chloride and chloride-free environments. 21 Suter and Böhni examined the anodic polarization behavior of different zones of a single MnS inclusion and revealed that the dissolution potential of the inclusion/matrix boundary was lower than that of the ...