Fe, Ni and Co are known as catalysts for carbon formation. They are also main constituent elements of industrial alloys due to e.g. temperature properties. Alloys based on these metals are therefore susceptible to metal dusting corrosion; a catastrophic degradation phenomenon that proceeds by a gradual breakdown of the material into a powdery mixture of graphite, carbide and metal particles. It constitutes a major issue in the synthesis gas production, where (heat exchange) surfaces are extensively exposed to carbon-saturated gaseous environments with low partial pressures of oxygen and/or steam in a critical temperature range of 400–900 ˚C [1].
Cr2O3 is known as impermeable to carbon and remains stable in carbonaceous atmospheres down to very low oxygen partial pressure [2]. One measure to prevent metal dusting is the incorporation of chromium (Cr) in the alloy to facilitate formation of a protective oxide layer at the surface. The inhibition of carbon formation that a dense Cr2O3 layer that prohibits access to the bulk matrix can be formed and maintained during industrial operation.
The overall objective of our investigations is to provide a better general understanding and prediction of carbon formation phenomena leading to metal dusting, as well as enabling alloy selection and alloy pre-treatment procedures that minimize metal dusting corrosion. We have previously demonstrated in a qualitative way that the tendency to form solid carbon on the Inconel 601 alloy sample surface is strongly linked to the pre-oxidation conditions, through the composition and structure of the Cr-rich oxide layer formed [3]. The carbon formation was catalyzed by Fe and/or Ni (alloy) particles, seemingly originating from the reducible phases presence within this layer. Moreover, we used exposures to a model syngas at varying temperature to show that features resembling the so-called pitting observed in alloys applied in industrial process equipment could be formed within relatively short exposures [4]. In this study, by varying the exposure time we assess the point at which the reducible phases are initiating the first carbon formation as well as how the accelerated pitting may occur.
Inconel 601 alloy samples were first subjected to a polishing and oxidation procedure intended to yield a consistent starting point. Two types of gas mixtures were used for the carburizing experiments at 750 ˚C; one representing infinite carbon activity and consisting of 10% CO in Ar at 1bar (I-aC), the other simulating syngas at industrially relevant conditions, i.e. a finite low carbon activity mixture of H2 (25%), CO (20%), CO2 (15%), H2O (10%) and Ar (30%) at 20 bar total pressure (FL-aC).
The resulting samples were characterized by a range of techniques (in an attempt to relate the carbon formation propensity and potential progression of the metal dusting to the initial structure and composition, as well as the development of the metal/oxide matrix during the carburizing exposures; i.e. field emission gun scanning electron microscopy (FE-SEM), Auger and Raman spectroscopy, x-ray diffraction (XRD) and, finally, transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDS) and electron energy loss spectrometry (EELS) analysis of near-surface cross-section lamellas prepared by focused ion beam (FIB) milling.
The nature of the surface oxide layer, i.e. microstructure and composition as resulting from the initial polishing to uncover the bulk and subsequent oxidation in steam at elevated temperature, represents the starting point of this investigation. Combined, the information from XRD, Raman spectroscopy and Auger depth profiling shows that very thin oxide scale has formed after oxidization in 10% steam in Ar at540 ˚C that contains Cr2O3 as well as (Ni, Fe, Cr)3O4 spinel.
Carbon filaments are found on the surface for nearly all I-aC and FL-aC exposure times (1 to 20h), with density, length diameter increasing with time. Metal particles are found at the tip of the filament as well as along the center axis as the length increases, typical of a Ni/Fe catalyzed growth mode. The induction period appeared, however, longer under FL-aC at 20 bar than under I-aC at 1 bar, but eventually the carbon filaments were usually larger in diameter in the former case.
In Raman spectroscopy comparison of the so-called G (1577-1608 cm-1), D (1326-1328 cm-1) and D’ (1600-1610 cm-1) bands can be used to investigate the structural properties of the carbon on the surface [5-8]. The results imply that, while some disordered and amorphous carbon may synthesize first, the overall crystallinity of the carbon formed is improved by prolonged exposure to CO only (I-aC). When it comes to the FL-aC condition increase in the ID/IG value implies that there is no improved crystallinity of the growing carbonaceous deposits. Raman and XRD jointly indicate that the pre-oxidized alloy samples continue to develop near-surface oxides phases during exposure to the – essentially reducing - carburizing gas mixture at 750 ˚C. But while Cr2O3 becomes predominant for I-aC, the major additional phase observed after FL-aC exposure is (Ni, Fe, Cr)3O4 spinel. Such spinels are thermodynamically less stable and can also be spalled more easily as a result of stress [9].
The TEM/EDS/EELS cross-section analysis of the near-surface region reveals that the changes in structure and composition have penetrated significantly deeper into the alloy matrix for the simulated synthesis gas exposure than for the CO only case ( I-aC), with clear evidence of pitting. The synthesis gas thus seem to facilitate continuous formation of (Ni, Fe, Cr)3O4 species that can be subsequently reduced to metallic (Ni, Fe) catalyst particles, while under CO a thin Cr2O3 layer can be maintained even if carbon forms on the surface. The results also reveal how the underlying alloy grain structure affects the propensity to form carbon on the surface. A fine-grain structure is believed able to provide easy-diffusion paths such as grain boundaries, sub-boundaries and dislocations resulting in faster formation of the protective Cr-rich oxide layer [10, 11].
[1] M. Holland and H. De Bruyn, International journal of pressure vessels and piping. 66 (1996) 125-133.
[2] Z. Zeng et al., Corrosion 60 (2004) 632-642.
[3] P. D. S. Gunawardana, H. Venvik, and J. Walmsley, "Investigation of the initial stage of metal dusting corrosion in the conversion of natural gas to synthesis gas," in CORROSION 2013, 2013: NACE International.
[4] P. D. S. Gunawardana, T. T. M. Nguyen, J. C. Walmsley, and H. J. Venvik, "Initiation of metal dusting corrosion in conversion of natural gas to syngas studied under industrially relevant conditions," Industrial & Engineering Chemistry Research, vol. 53, no. 5, pp. 1794-1803, 2013.
[5] M. M. Lucchese et al., "Quantifying ion-induced defects and Raman relaxation length in graphene," Carbon, vol. 48, no. 5, pp. 1592-1597, 2010.
[6] L. G. Cançado et al., "Quantifying defects in graphene via Raman spectroscopy at different excitation energies," Nano letters, vol. 11, no. 8, pp. 3190-3196, 2011.
[7] I. Childres, L. A. Jauregui, W. Park, H. Cao, and Y. P. Chen, "Raman spectroscopy of graphene and related materials," New developments in photon and materials research, vol. 1, 2013.
[8] R. Saito, M. Hofmann, G. Dresselhaus, A. Jorio, and M. Dresselhaus, "Raman spectroscopy of graphene and carbon nanotubes," Advances in Physics, vol. 60, no. 3, pp. 413-550, 2011.
[9] M. W. Edwards and N. S. McIntyre, "Gas Phase Initial Oxidation of Incoloy 800 Surfaces," Oxidation of metals, vol. 79, no. 1-2, pp. 179-200, 2013.
[10] H. Grabke, E. Muller-Lorenz, S. Strauss, E. Pippel, and J. Woltersdorf, "Effects of grain size, cold working, and surface finish on the metal-dusting resistance of steels," Oxidation of Metals, vol. 50, no. 3, pp. 241-254, 1998.
[11] N. Parimin, E. Hamzah, and A. Amrin, "Effect of grain size on the isothermal oxidation of superalloys," Advances in Environmental Biology, vol. 7, no. 12, pp. 3720-3726, 2013.
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