Local electrochemical studies after heat treatment of stainless steel: Role of induced metallurgical and surface modific
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. INTRODUCTION
NUMEROUS electrochemical measurements at the macroscale have revealed that under certain conditions MnS inclusions are good precursor sites for pitting corrosion.[1,2,3] Development of high-resolution techniques has recently provided an opportunity to determine in liquids the distribution of surface reactivity with a micron and submicron resolution in order to locate and to investigate single precursor sites. The scanning vibrating electrode technique has been used to image the local current distribution during potentiostatic initiation of pitting close to MnS inclusions,[4] and the scanning electrochemical microscope has been used to study the electroactivity and the release of products above the inclusions.[5,6] The electrochemical microcell technique has been combined with a pH probe in order to determine simultaneously the evolution of the pH in the solution close to the surface and the local electrochemical properties of sites (size between a few microns and several hundreds of microns in diameter) containing a single MnS inclusion.[7] However, a heat treatment may induce irreversible changes in microstructure, which may modify significantly the electrochemical behavior of inclusions-containing stainless steels. It has been shown that MnS inclusions are good precursor sites for the nucleation of microcracks and microvoids during cooling after heat treatment. Transmission electron microscopy has provided a powerful tool for the investigation of the substructure that evolves as metal deforms around elastic inclusions below 1 m in diameter. For large inclusions, other experimental techniques have been used, including etch pit studies to charH. KRAWIEC, Associate Professor, is with the Department of Foundry, AGH University of Science and Technology, 30-059 Kracow, Poland. V. VIGNAL, CNRS Research Scientist, O. HEINTZ, CNRS Research Engineer, and R. OLTRA, CNRS Director of Research, are with the LRSSUniversité de Bourgogne, 21078 Dijon, Cedex, France. Contact e-mail: [email protected] E. FINOT, Associate Professor, is with LPUB-Université de Bourgogne. J.M. OLIVE, CNRS Research Scientist, is with LMP-Université Bourdeaux I, 33405 Talence, Cedex, France. Manuscript submitted March 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
acterize the low density dislocation structure near second phases,[8] conventional or synchrotron X-ray diffraction,[9] and surface strain measurements using either stereoimaging techniques[10] or precoated microgrid displacement measurements.[11] Analytical and numerical methods have also been developed for the stress analysis around inclusions under straining conditions.[12,13] Heat treatment may also induce chemical changes around MnS inclusions. The bulk self-diffusivity of 54Mn in -MnS has been determined within the temperature range 873 to 973 K by a radioactive tracer method.[14] An enrichment in chromium has been detected in the inclusions and the diffusivity of Mn has been found to increase due to an increase in the vacancy concentration in the matrix
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