Microscale investigations of the metal-dusting corrosion mechanism on mild steel
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. INTRODUCTION
METAL-DUSTING corrosion involves the progressive disintegration of steel surfaces into a carbonaceous deposit containing amorphous and crystalline carbon, finely divided metal particles, metal oxides, and carbides, when they are exposed to highly carburizing and weakly oxidizing environments at high temperatures. Although this corrosion type does not occur very frequently, damage to structural parts in industrial plants can be hazardous and expensive. By way of example, case-failure histories are reported from a plant for direct reduction of iron ore,[1] a synthetic fuel production plant,[2] and an ammonia production plant.[3] The explanation of the reaction processes in metal-dusting corrosion is based on the suggestions of Hochman,[4] which were further refined by Grabke.[5] Briefly, iron-based alloys disintegrate through a reaction sequence which involves the supersaturation of the metal matrix, formation of a surface cementite phase, and the deposition of a graphite layer due to decreased carbon transport into the metal bulk. The graphite layer is thought to reduce the local thermodynamic activity of carbon, causing a destabilization of the cementite phase. The cementite decomposes to graphite and iron, where the metal diffuses to the outermost surface, enabling additional carbon deposition by filamentous carbon formation. Microscale investigations employing transmission electron microscopy (TEM) have uncovered processes at the reacting interface between the metal substrate and the carbon overlay. Cox[6] exposed vapor-deposited iron films of 100 nm in thickness to pure carbon monoxide at 922 K and proved the transformation of iron into different carbides on ˚ RD, Director BERND SCHMID, Senior Engineer, and ROLF ØDEGA of Oil and Gas Refining, are with the Research Centre, STATOIL, N-7005 Trondheim, Norway. Contact e-mail: [email protected] JOHN CHARLES WALMSLEY, Senior Scientist, is with SINTEF Materials Technology, N-7465 Trondheim, Norway. ØYSTEIN GRONG, Professor, is with the Department of Materials Technology and Electrochemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Manuscript submitted June 18, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
simultaneous carbon deposition. Pippel et al.[7] prepared cross-section-oriented samples from 10-m-thick iron foils corroded in a mixed gas environment at 923 K. The formation of surface cementite and its oriented decomposition to graphite was observed. It was proposed that dissolved iron in the graphite diffuses to the outermost surface, where it nucleates to particles which are catalytically active for carbon deposition. Chun et al.[8] proposed the direct intercalation of iron atoms into a growing graphite which had a perpendicular orientation to the cementite surface. Crosssectional samples from 100-m-thick iron foils were investigated in that study. The present study considers the reaction products from metal-dusting corrosion at the substrate-deposit interface on a polycrystalline mild steel. Following initial
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