The onset of pitting corrosion at MnS inclusions on 304 stainless steel in 1 M NaCl was studied with and without applied mechanical stress with use of microelectrochemical cells. Polarization curves of areas ͑100 m diam͒ without inclusion showed no pitting at potentials below that of oxidation evolution; stress had no effect on the corrosion behavior. Areas containing five round inclusions of about 4 m in size showed stable pitting at about 400 mV; the effect of stress shifted the pitting potential to values that were 150 mV more negative. Polarization curves measured on large deep MnS inclusions showed active pitting. Curves of areas with single large, shallow MnS inclusions showed multiple current transients during dissolution of the inclusion without stress, but the metastable events did not initiate stable pitting. The dissolution of shallow MnS inclusion did not form a deep microcrevice between the MnS and stainless steel matrix. However, under applied stress, cracks were formed within the shallow MnS inclusion and active pitting occurred. To explore whether such cracks might serve to generate locally high concentrations of aggressive species, the pH and chloride concentration inside a crack were simulated using a finite difference model. For experimental conditions where stable pitting was observed, the simulations predicted that the solution composition at the base of a typical 13 m deep crack correspond to a pH of around 2, and a chloride concentration of about 6 M led to stable pitting.

Electrochemical measurements were performed on single sulfide inclusions within a microcell to study their dissolution behavior and role in the initiation of pitting corrosion of 304 stainless steel in NaCl solutions. It was found that chloride ions catalyzed inclusion dissolution and caused metastable pits, and that no metastable pits formed in thiosulfate solutions without chloride. The thiosulfate ion accelerated inclusion dissolution, and when the thiosulfate ion was present above a critical concentration it caused stable pitting, at low concentrations the chloride ion inhibited the aggressive nature of the thiosulfate ion and stifled stable pitting, while at high concentrations the thiosulfate ion inhibited the ability of the chloride ion to cause metastable pitting events. The addition of xanthate ion in combination with electrode activation in copper sulfate was found to inhibit the rate of inclusion dissolution and increase the pitting potential. The initiation of stable pitting was found to depend on the inclusion geometry.The addition of sulfur to stainless steel ͑SS͒ improves machinability but decreases the corrosion resistance due to the presence of sulfur-rich inclusions on the surface. 1-3 Such inclusions, mainly sulfide inclusions and mixed sulfide/oxide inclusions, have been the emphasis of extensive studies to understand their important role in the initiation mechanism of localized corrosion of SS. 4-9 It is generally agreed that anodic dissolution of sulfide inclusions leads to locally aggressive solution compositions near the inclusion, under which the passive film on SS cannot be sustained. However, direct experimental measurement of the local environment during initiation is usually difficult owing to the small distance-and time-scales over which critical events occur. In the present series of papers, a suite of techniques was applied in order to obtain electrochemical data at single inclusions by a microcell technique ͑Part I͒, local chemical compositions in the vicinity of a single inclusion ͑Part II͒, and a mathematical model ͑Part III͒.Mechanisms for the dissolution of sulfide inclusions and initiation of localized corrosion have been proposed by many researchers. Eklund 4,5 performed thermodynamic calculations and concluded that MnS was thermodynamically unstable above a potential of approximately Ϫ100 mV ͑SHE͒ and can only exist in the pH range 4.8-13.8, and suggested the following dissolution mechanismEklund proposed that the sulfide inclusions were unstable at the passive potential and dissolved, with dissolution occurring primarily at the edge of the inclusions. Thus a small microcrevice formed with one wall consisting of the MnS inclusion surface and the other wall consisting of the metal matrix. He proposed initiation due to the local acidification within the microcrevice region, and that the local acidification was attributed to the presence of a significant proportion of Cr within the sulfide inclusion. Similarly, Wranglen 6 suggested the following dissolution mechanism of sulfid...

A combination of mathematical modeling and experiments on single MnS inclusions was used to investigate the role of MnS inclusions on the initiation of pitting corrosion of 304 stainless steel. Isolation of single MnS inclusions in chloride-containing solutions with use of microcapillaries as electrochemical cells showed that the orientation of the MnS inclusion played a significant role in pit initiation. Large, shallow MnS inclusions failed to initiate pitting corrosion, while the same inclusions, oriented narrow and deep, consistently exhibited the onset of localized corrosion. The formation of a microcrevice was observed between the dissolving MnS inclusion and stainless steel. The microcrevice resulted in a locally occluded region where aggressive ions can accumulate. A mathematical model was developed to explore the local chemistry within a one-dimensional microcrevice of which one wall was MnS and the other was stainless steel. The results of the simulations supported the view of a critical solution chemistry of sulfur species, in a chloride environment, as a possible trigger mechanism for localized corrosion. A critical microcrevice geometry of approximately 1 m was predicted to be sufficiently large to generate stable pitting in the system under study.MnS inclusions play an important role in the initiation of localized corrosion of stainless steel. The importance of MnS inclusions has been extensively reported 1-18 and it is generally agreed that anodic dissolution of MnS inclusions results in a change of the local solution composition near the inclusion, resulting in a condition where the passive film on stainless steel can no longer be sustained. While the mechanism by which initiation occurs is not fully understood, there is evidence that the process of pit initiation involves transport of chemical species to and from the inclusion, which leads to the development of a complex chemical environment that includes sulfur species from the inclusion as well as metal species from the stainless steel. Because these events are difficult to observe directly, in the present study a combination of experimental and mathematical tools were used to explore the role of MnS inclusions on pit initiation. The integration of experimental data with numerical simulations was carried out to test one hypothesis of the mechanism of pit initiation at MnS inclusions.It is generally agreed that the initial step in the initiation of pitting corrosion at MnS inclusions is the dissolution of the MnS inclusion. Previous investigators have reported the formation of a crevice between a dissolving MnS inclusion and the stainless steel. Eklund 2 observed the formation of a small crevice between the sulfide and the metal matrix to be the initial stage of pit initiation. He attributed the formation of the small crevice to the lower electronic conductivity of the sulfide inclusion relative to the steel. Wranglen 3 reported that the first stage of initiation is due to the dissolution of the inclusion, where the current density is the highes...

Small wires of W, Ag, and Pt were placed within an electrochemical microcell above single sulfide inclusions to detect the pH, sulfide, and thiosulfate, respectively. In chloride-free electrolytes the onset of sulfide dissolution was accompanied by a decrease in pH of 0.5 units to 4.2, a value that is not low enough to initiate active metal dissolution. In chloride-containing electrolytes, metastable pitting events were found to cause rapid temporary periods during which the pH decreased to values as low as zero. However, the results suggest that sustained pH decreases occur only with onset of stable pitting. Sulfide species were only detected during chemical dissolution in a pH 2 electrolyte at the rest potential and not as a result of anodic dissolution at neutral pH. A Pt wire along with an iodide/triiodide couple was used to detect thiosulfate as a result of anodic dissolution. The results support the mechanism of electrochemical dissolution of sulfide inclusions to produce thiosulfate at neutral pH.

Mathematical modeling was used to simulate the dissolution of a single MnS inclusion within an electrochemical microcell. The model allowed for the evaluation of the hypothesis of mechanism of pit initiation based on a critical concentration of thiosulfate ions in the presence of chloride ions, which results in depassivation of the stainless steel. As a result of inclusion dissolution, the concentration of thiosulfate increased to values consistent with critical concentrations observed experimentally for pit initiation. Dissolution of inclusions led to modest local acidification, but not to the low pH values associated with passivity breakdown. The model defined the dimensionless parameter space under which thiosulfate and sulfide species would be expected to occur as a consequence of inclusion dissolution. Based on the hypothesis of critical thiosulfate initiation mechanism, the predictions of pitting potential for 304 stainless steel in chloride solutions were within experimental variation; the chloride concentration dependence was smaller than observed experimentally. © 2002 The Electrochemical Society. All rights reserved.

Copper electrodeposition in submicron trenches

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