Abstract
Metal dusting is a catastrophic carburisation phenomenon that occurs at temperatures of 450-850°C in atmospheres of high carbon activity. The resistance of alloys to corrosion, including metal dusting, relies on the formation of a dense, adherent oxide layer that separates the alloy from the corrosive environment. For such an oxide layer to be protective, it must achieve full surface coverage, be crack-free and be established before significant material degradation has occurred. Formation of a protective oxide scale can be enhanced by increasing the population of rapid diffusion paths for the protective elements (e.g. Cr and Al) to reach the alloy surface. In this work, laser surface melting has been used to improve the metal dusting resistance of Alloy 800H by creating a rapid solidification microstructure and, thereby, increasing the density of rapid diffusion paths. Oxidation during laser processing has been found to be detrimental to metal dusting resistance. However, it has been demonstrated that the resulting oxide can be removed without compromising metal dusting resistance. Results of exposure to a metal dusting atmosphere (20% H2 80% CO at 650°C) are presented. Samples have been examined in plan and cross-section using optical and scanning electron microscopy. Selected samples were also examined by electron probe microanalysis and X-ray diffraction.
Highlights
Metal dusting is a catastrophic carburisation phenomenon that occurs at 450-850°C in atmospheres of high carbon activity, conditions typical of methanol, steam reformers and other chemical plants that use synthesis gas (H2 + CO)
Inward growth of graphite disintegrates the alloy. Both these mechanisms lead to the formation of metal particles that catalyse further carbon deposition, promoting further metal dusting attack
The metal dusting resistance of Alloy 800H has been successfully improved by laser surface melting
Summary
Metal dusting is a catastrophic carburisation phenomenon that occurs at 450-850°C in atmospheres of high carbon activity, conditions typical of methanol, steam reformers and other chemical plants that use synthesis gas (H2 + CO). All Fe-, Ni- and Co-based alloys are susceptible to metal dusting. The alloys disintegrate into a dust of coke and metal particles, leading to two undesirable effects: metal wastage and coke accumulation. In Febased alloys, carbon deposits on the surface and diffuses into the alloy. The alloy becomes saturated with carbon and cementite, Fe3C, forms. The presence of cementite restricts carbon ingress. The metastable cementite decomposes into graphite and metal particles. In Ni-base alloys, there is no carbide intermediary; graphite forms directly from the carbon-saturated alloy. Inward growth of graphite disintegrates the alloy. Both these mechanisms lead to the formation of metal particles that catalyse further carbon deposition, promoting further metal dusting attack
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