Papers by Keyword: Metal Dusting

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Authors: P. Szakàlos
Abstract: This paper gives an overview of the different processes of metal dusting (MD) that operate on low and high alloyed iron and nickel base alloys exposed in CO+H2–containing environments with or without water vapour. MD of pure metals like iron and nickel occur with a solely carbon-induced corrosion mechanism. However, in high alloyed materials with strong oxide formers such as Cr and Al, a more complex MD-process takes place which involves both carbon and oxygen in close collaboration. The “alloyed” carbides, i.e normally Cr-containing carbides, formed in such materials are thermodynamically stable with respect to the carbon activity. However, in the reaction front of a MD-pit with non-protective spinel oxide, they destabilise and dissolve due to the influence of the low oxygen activity determined by CO-dissociation. Based on recent results in the field of MD a chart with tentative MD mechanisms is presented as a function of alloy composition and temperature.
571
Authors: Hisao Fujikawa
Abstract: Among the high temperature damages, this paper was discussed on the cases in which the materials were damaged mainly by the effect of environmental factors. That is, high temperature oxidation, steam oxidation, molten salt corrosion, high temperature particle erosion and corrosion, high temperature sulfurization, carburizing, metal dusting, nitriding, high temperature chloride corrosion and so on were introduced using my research data. Finally, anticipation to future research of high temperature corrosion was maintained.
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Authors: Yoshitaka Nishiyama, Koji Moriguchi, Nobuo Otsuka
Abstract: Laboratory metal dusting test of several Ni binary alloys containing the representative element was conducted in a simulated syngas atmosphere at 650°C for 100h. The Ni alloys containing element belonging to Group 14 and 15 in the periodic series exhibited excellent metal dusting resistance, while those containing Group 13 did not. This behavior was able to be reasonably interpreted from the Blyholder mechanism and the concept of Pauling’s electronegativity.
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Authors: F.Howard Stott, F. Di Gabriele, Zhu Liu
Abstract: Selection of alloys for components in industrial environments at high temperature is often based on long-term tests in environments that simulate as closely as possible the actual service conditions. However, particularly for alloys that rely on formation and retention of oxide scales for protection, such tests may not be sufficient for this purpose. Several nickel-base alloys have been exposed in an 80% CO-20% H2 gas mixture at 650oC for up to 5,000h, in order to give conditions similar to those that can occur in petrochemical plant where high carbon and low oxygen activities can lead to scale breakdown and the onset of metal dusting damage. The study has shown that small changes in test conditions, such as the introduction of contaminants, the addition of CO2, the proximity to alloy surfaces on which carbon deposition and metal dusting have already been initiated and the proximity to other surfaces that can initiate such deposition, can lead to very significant decreases in the induction period, prior to breakdown of the scale and the onset of dusting.
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Authors: Jan Dampc, Marek Szkodo
Abstract: The work shows the results of the tests of 9Cr-1Mo steel, which was for 10 years operated in the CCR platforming unit in Group Lotos SA in Gdańsk, and then in the laboratory was sulphidised during 166 h at a temperature of 600 °C. Sulphidation was performed in a mixture of H2-H2S gases at the vapour pressure of sulphur 4.1·10-14 atm, so the order of magnitude of vapour pressure was less than that of the dissociation pressure of FeS. Although sulphidising took place in conditions which preclude any iron sulphide formation, research results have demonstrated that after 166 hour exposure in reaction mixture in 9Cr-1Mo steel produces iron and chromium sulfides. Sulphide precipitates form under a layer of fine carbides, located directly on the surface of steel. Formation of scale sulphide here is most likely caused by the penetration of hydrogen sulphide through the porous layer of carbides that increases the pressure of hydrogen sulphide. Thus the vapour of sulphur in apertures and narrow passages occurs between the layer of carbides and the rest of oversaturated carbon layer top, until its pressure value is reached allowing the formation of sulphide scaling.
139
Authors: Carsten Strübbe, Gabriela Marginean, Viorel Aurel Serban
Abstract: The high temperature oxidation behaviour of Ni-Cr-B-Si coatings with a higher Si-content was investigated in order to evaluate the suitability of such materials especially for novel applications concerning highly aggressive environments like metal dusting. Metal dusting is a corrosion phenomenon that occurs in reducing-, carbon-supersaturated (ac>1) gaseous atmosphere, containing CO, H2, CO2 and H2O, at elevated temperatures between 400 and 800°C. Metal dusting reactions can be classified into two types. The first one concerns Fe-alloys, where Fe3C is growing on the surface. The second one is related to the reaction of Ni, Co and their alloys, where the destruction takes place through inward growth or direct ingrowth of graphite, without forming the metastable Fe3C. Regarding to the literature, metal dusting is typically encountered in industrial furnaces, but mainly in the chemical or petrochemical industry. The way to suppress metal dusting is to stop the dissociation of the carbon source or to stop the carbon ingress in the material. One possibility in order to avoid the carburization of Fe, Ni, Co and their alloys is to preoxidize the samples. Based on the reducing atmosphere, where metal dusting occurs, the isothermal outsourcing for the formation of a protective Al-, Cr- or Si-oxide layer on the samples in air is mostly necessary. The role of a stable Al2O3 and Cr2O3-layer on the sample as a diffusion barrier against the carbon ingress, based on their low solubility for carbon, has already been investigated and proved by many scientists. The formation of a protective and thermodynamically very stable SiO2 scale was also investigated. Within the scope of this work, the influence of a higher Si-content (4,5 wt%) in NiCrBSi-alloys, depending on the temperature, was analyzed. For this purpose the samples were oxidized in air at 600, 700 and 800°C respectively. The surface morphology and the phase composition of the grown oxide scales were characterized by means of scanning electron microscopy combined with energy dispersive X-ray (SEM/EDX) and by X-ray diffraction (XRD) technique. The experimental results demonstrate the importance of silicon content on the coatings properties, respectively on the stability of the formed oxide scale (free of micro cracks, no spallation). This element is able to form beside chromium, a dense oxide layer on the sample surface, protecting it against further degradation induced by the atmosphere in different high temperature applications. Moreover, the increased chromium content of the feedstock powder (from 10 wt% in previous work to 12,5 wt%) demonstrated that the Ni-Cr-B-Si coatings exposed at 600°C, 700°C as well as at 800°C were not susceptible to internal oxidation.
79
Authors: S.K. Varma, Aditya Putrevu, Maduri Pasala, Z. Zeng, Ken Natesan
Abstract: Experiments involving metal dusting in carbonaceous atmosphere and oxidation in air have been carried out at 593 and 704oC using T22, T91 and 800 alloy steels. The effect of Cr on these two processes can be understood because of their compositions. In general, the resistance to both metal dusting up to 1000 hours and oxidation up to 456 hours has been found to improve with increase in Cr content from 2.36 to 21.6%. However, mechanisms change from spalling to oxide scale in oxidation and from uniform metal dusting to pitting in metal dusting as the amount of Cr increases in these steels. Pitting corrosion shows higher C carbon concentration in the pits than at locations away from the pits. A detailed analysis has been performed to evaluate the scale and spalled materials in terms of oxide characterization using XRD, AES, SEM, EDS on SEM, optical microscopy and TEM.
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Authors: Yoshitaka Nishiyama, Kazuyuki Kitamura, Nobuo Otsuka
Abstract: Metal dusting behaviour of alloy800H was investigated in a laboratory-pressurized reactor where CO-H2-CO2-H2O gas mixtures flowed continuously, simulating the gas environments in actual syngas production plant. Four test conditions, having a variety of gas pressures and H2O contents, were conducted for a long rod specimen that was imposed thermal gradient of 777°C to 454°C in a longitudinal direction. After an exposure of 100h at high pressure, metal dusting has initiated with a bell-shape distribution on the test specimens at temperatures between 600°C and 700°C. Gas pressure has accelerated metal dusting even if the gas atmospheres had the same gas composition. On the contrary, metal dusting has been alleviated with increasing the H2O content in the test gas. Oxide scale characterisations were analyzed by using Raman spectroscopy and XPS at the test specimen surfaces exposed at different temperature. The ratio of a protective Cr2O3 scale to spinel oxides such as FeCr2O4 and MnCr2O4 has increased with increasing the exposure temperature, with reducing the gas pressure, and with lowering the H2O content. These behaviours have been interpreted from thermodynamic indices: carbon activity ac and the oxide scale integrity related to oxygen potential Po2 as a function of temperature.
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