Thermodynamic and kinetic consideration on the corrosion of Fe, Ni and Cr beneath a molten KCl–ZnCl2 mixture

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.