Investigation of the Oxidation Kinetics of Fe-Cr and Fe-Cr-C Melts under Controlled Oxygen Partial Pressures

Abstract

In the current work, oxidation kinetics of Fe-Cr and Fe-Cr-C melts by gas mixtures containing CO2 was investigated by Thermogravimetric Analysis (TGA). The experiments were conducted keeping the melt in alumina crucibles, allowing the alloy melt to get oxidized by an oxidant gas. The oxidation rate was followed by the weight changes as a function of time. The oxidation experiments were conducted using various mixtures of O2 and CO2 with $$ P_{{{\text{O}}_{2} }} $$ = 10−2 to 104 Pa. In order to understand the mechanism of oxidation, the wetting properties between the alumina container and the alloys used in the thermogravimetric analysis (TGA) experiments and the change of the alloy drop shape during the course of the oxidation were investigated by X-ray radiography.The experiments demonstrated that the oxidation rate of Fe-Cr melt increased slightly with temperature under the current experimental conditions, but it is strongly related to the Cr-content of the alloy as well as the oxygen partial pressure in the oxidant gas mixture, both of which caused an increase in the rate. For the Fe-Cr-C system, the oxidation rate has a negative relationship with carbon content, viz. with increasing carbon, the oxidation rate of the alloy melt slightly decreased. The chemical reaction was found to be the rate determining step during the initial stages, whereas as the reaction progressed, the diffusion of oxygen ions through slag phase to the slag–melt interface was found to have a strong impact on the oxidation rate. The overall impact of different factors on the chemical reaction rate for the oxidation process derived from the current experimental results can be expressed by the relationship: $$ k_{1} = \frac{{dm}}{{dt}} = \Uplambda {\text C}_{\text{Cr}}^{0. 2 3} {\text{C}}_{{\text{CO}}_{ 2} } ^{ 0. 4 1}{\text{exp}}(\frac{{{{ - E}}_{\text{a}} }}{{{\text{R}}T}} ). $$ A model for describing the kinetics of oxidation of Fe-Cr and Fe-Cr-C alloys under pure CO2 was developed. Simulation of the oxidation kinetics using this model showed good agreement with the experimental results.