Thermodynamic and kinetic consideration on the corrosion of Fe, Ni and Cr beneath a molten KCl-ZnCl2 mixture |
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Authors: | Andreas Ruh |
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Affiliation: | Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany |
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Abstract: | Thermogravimetric (TG) experiments have been carried out to study the kinetics of hot corrosion of Fe, Cr and Ni, covered by a molten KCl-ZnCl2 mixture of a composition close to the eutectic (50 mol% KCl-50 mol% ZnCl2). Furthermore binary and ternary phase diagrams were calculated in order to describe the corrosion process. The tests were conducted at a temperature of 320 °C in an atmosphere consisting of argon and oxygen. For iron different stages are observed in a TG curve. They can be attributed to the different reaction steps of iron chloride formation (incubation phase), oxide precipitation (linear stage) and scale formation (parabolic or logarithmic stage). Based on these observations a model, described by Spiegel [A. Spiegel, Molten Salt Forum 7 (2003) 253], is confirmed. For Cr and Ni these stages are not observed. At 8 vol% O2 only slight oxidation of Cr and Ni was observed accompanied by evaporation of the salt deposit. At 16 vol% O2 the rate of oxidation increases and the experiments yield a curve that is either parabolic or logarithmic for both Ni and Cr. As a result it is shown that the solubility of iron chloride in the KCl-ZnCl2 melt is higher than the solubility of nickel chloride and chromium (III) chloride in the KCl-ZnCl2 melt. This enables a higher diffusibility of iron chloride to the upper region of the melt where a higher oxygen partial pressure (p(O2)) is present leading to a higher oxidation rate of iron. |
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Keywords: | C. Hot corrosion C. Thermodynamic diagrams A. Molten salts A. Iron A. Nickel |
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