Dissolution of chromium-enriched inclusions and pitting corrosion of resulfurized stainless steels
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NUMEROUS electrochemical studies at the macroscale have revealed that under certain conditions MnS inclusions are good precursor sites for pitting corrosion of stainless steels.[1,2,3] The first steps leading to pit initiation are the dissolution of inclusions and the adsorption of sulfur species released into the solution on the surrounding passive film. In aqueous media, both chemical and electrochemical dissolution of MnS may occur, and the presence of a small amount of chromium (;5 at. pct) and iron (around 3 at. pct) in the inclusions affects these processes. A survey of the literature indicates that much attention has been paid to the mechanisms involved in the electrochemical dissolution of inclusions.[2–8] Regarding the chemical dissolution path, several mechanisms involving the formation of H2S, elemental sulfur, or HS" have been proposed.[3–6] An insulating Ag wire has been used as a probe inside a microelectrochemical cell to detect HS" produced during the dissolution of MnS inclusions in 304 stainless steel at the OCP value.[9] Ag has been known to react in the presence of sulfide and hydrosulfide species[10] by one-electron oxidative adsorption of HS" (Ag 1 HS" 5 Ag-SH 1 e") followed by a one-electron oxidative phase transition to yield a complete Ag2S monolayer (Ag-SH 1 Ag 1 OH" 5 Ag2S 1 H2O 1 e"). In addition, Ag2S/Ag ion-selective electrode has been reported to detect sulfide ions present as low as 10"7 M[11] and does not react with SO42", SO32", and S2O32".[9] The voltammetric response by Ag showed the presence of hydrosulfides at the open circuit potential (located in the passive range) in 0.1 M Na2SO4 (pH 5 2) H. KRAWIEC, Postdoctoral Fellow, V. VIGNAL, CNRS Research Scientist, and O. HEINTZ, CNRS Research Engineer, and R. OLTRA, CNRS Director of Research, are with the Laboratoire de Recherches sur la Re´activite´ des Solides, Universite´ de Bourgogne, 21078 Dijon Cedex, France. Contact e-mail: [email protected] H. KRAWIEC is also Associate Professor in the Department of Foundry, AGH University of Science and Technology, 30-059 Cracow, Poland. E. CHAUVEAU, Research Manager, is in the Corrosion Oxidation Department with OUGITECH, Arcelor Group, Av. P. Girod, 73403 Ugine, France. Manuscript submitted September 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
solution, indicating that inclusions dissolved chemically to form HS" according to:[9] MnS 1 H 1 ¼ Mn2 1 1 HS"
[1]
Recently, the influence of the chemical and electrochemical dissolution of MnS inclusions on the composition of passive films and the corrosion resistance of stainless steels has been studied using the electrochemical microcell technique, in situ atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectroscopy (SIMS).[12,13,14] If stainless steels contain a low amount of manganese or other sulfur-gettering elements (calcium, titanium, or rare earth elements), sulfur can be gettered as inclusions rich in chromium.[15] It has been reported that chromium can substitute for up to 65 pc
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