Sammendrag
The initial stages of metal dusting corrosion and carbon formation were studied under syngas at industrially relevant conditions. A set of Inconel 601 alloy coupons were preoxidized in 10% steam in N2 at 540 °C for 6 h, followed by exposure to carburizing (ac >1) syngas mixtures. The exposure was performed at 550, 650, or 750 °C at 20 bar for 20 h under a CO/H2/CO2/H2O/Ar mixture of composition 20/25/15/10/30 or 50/25/15/10/0 (%). The resulting surfaces and carbonaceous products were then characterized by means of optical imaging and light optical microscopy, scanning electron microscopy (SEM), depth profile analysis by Auger electron spectroscopy under ion-sputtering and transmission electron microscopy/energy dispersive spectroscopy (TEM/EDS). Little or no carbon formed at 550 °C, and the alloy matrix remained reasonably intact, with some smaller changes occurring within the Cr rich oxide layer covering the underlying metallic bulk. The higher carbon formation occurring under the exposure at 650 and 750 °C demonstrate that the reactions leading to carbon are kinetically limited. Cocurrent oxidation and Cr-enrichment of the near-surface layer was also observed under the mixture containing 20% CO at 650 and 750 °C, and pitting of the samples was revealed after removal of most of the carbon. The carbon formation is higher under the exposure to the 50% CO mixture compared to the one containing 20% CO. Severe metal spalling of the alloy was observed at 650 °C, with complete disintegration of the Cr-rich surface oxide layer and carburization of the alloy matrix extending far into the bulk. Presence of reducible Ni and Fe phases within the Cr-rich oxide formed during the preoxidation were associated with the carbon formation and the TEM/EDS evidence confirms that Ni–Fe alloy particles have acted as catalysts for the formation of filamentous carbon. Carbon filament size effects were attributed to the temperature dependent sintering of the Ni–Fe particles upon their reduction.