Since the oxidation reactions in the process of steel production occur in harsh conditions (i.e., high temperatures and gas atmospheres), it is practically impossible to observe in situ the compositional changes in the steel and the formed oxide scale. Hence, a coupled thermodynamic-kinetic numerical model is developed that predicts the formation of oxide phases and the composition profile of the steel alloy’s constituents in a short time due to external oxidation. The model is applied to high-temperature oxidation of Fe–Mn alloys under different conditions. Oxidizing experiments executed with a thermogravimetric analyzer (TGA) on Fe–Mn alloys with different Mn contents (below 10 wt %) are used to determine kinetic parameters that serve as an input for the model. The mass gain data as a function of time show both linear and parabolic regimes. The results of the numerical simulations are presented. The effect of different parameters, such as temperature, Mn content of the alloy, oxygen partial pressure, and oxidizing gas flow rate on the alloy composition and oxide phases formed, is determined. It is shown that increasing the temperature and decreasing the oxygen partial pressure both lead to a thicker depleted area.

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High-temperature oxidation of steels can be relatively fast when exposed to air. Consequently, elucidating the effect of different parameters on the oxidation mechanism and kinetics is challenging. In this study, short-time oxidation was investigated to determine the oxidation mechanism, the affecting parameters, and the linear-to-parabolic growth transition of different Fe–Mn alloys in various oxygen partial pressures (10–30 kPa) and gas flow rates (26.6 and 53.3 sccm) in a temperature range of 950–1150 °C. Oxidation kinetics was investigated using a thermogravimetric analyzer (TGA) under controlled atmosphere. Linear oxide growth was observed within the first 20 minutes of oxidation. The linear rate constant was significantly increased by increasing the oxygen partial pressure or the flow rate of the oxidizing gas. The morphology of the oxide layer was determined by scanning electron microscopy (SEM). The crystal structure of the oxides formed was followed by in-situ X-ray diffraction (XRD), confirming that the growing layer consists of wustite mainly, which upon slow cooling to room temperature, transformed into magnetite. Energy-dispersive X-ray spectroscopy (EDS) showed that the atomic ratio of Fe+Mn to O was ~ 1.03:1 in the oxide scale, corresponding to Fe(Mn)O formation. Based on the characterization and a model for linear growth kinetics, it is concluded that the oxidation rate is controlled by the diffusion of oxidizing molecules through the gas layer to the sample’s surface. The findings led to a better understanding of initial oxidation behavior and provided a pathway for improved insight into the high-temperature oxidation behavior for more complex alloys.

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High-temperature corrosion is playing a critical role now in many modern industries such as aerospace, gas turbines manufacturing, and steelmaking. For instance, during hot rolling of the steel sheets -as one of the later steps in steel production- before coiling, high-temperature corrosion occurs in the form of oxidation. Such reaction in air is fast and causes the formation of an oxide scale at the surface. This can change the substrate metal in terms of composition and mechanical properties. Therefore, it is essential to study the initial oxidation behavior of steels at high temperatures when exposed to air. There are different mechanisms that can control the oxidation growth rate which lead to either linear or parabolic growth regimes. The goal of this work is to study the initial high-temperature oxidation behavior of binaries with different alloying element contents (<5 wt%). The kinetics, affecting parameters, and oxidation mechanism at different stages of the process were studied. The experiments were conducted in a Thermogravimetric Analyzer (TGA) to obtain continuous mass-gain data while controlling the atmosphere and the temperature. Fe-Mn and Fe-Si samples with different compositions were chosen for isothermal experiments at a range of temperatures (between 950 °C and 1150 °C). The chamber contained gas mixtures, including oxygen, nitrogen, and water vapor, applied with different flow rates. Depending on the alloying element content of the samples or the oxidizing condition such as gas mixture, it is expected to see different oxide phases, oxidation behavior, and growth regimes. Therefore, to find the phases present in the oxide layer and the elemental composition of the oxide layer as well as the metal substrate, X-Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) measurements were conducted. Since the oxide phases present at room temperature after cooling the oxidized sample can be different with the ones forming at the oxidation temperature, in-situ XRD experiments were conducted. Moreover, both the surface and cross-section of the oxidized samples were characterized via SEM to observe the roughness and thickness of the oxide layer. Linear growth of the oxide layer was observed in the beginning of oxidation for all the samples at different oxidizing conditions, and parabolic growth of the oxide layer was seen only at higher oxygen partial pressures (> 25 kPa). Furthermore, the oxidation mechanism was observed to be gas-phase diffusion within the linear regime, where the linear rate constants agreed with the values calculated via an existing analytical model. The temperature and alloying element content had a negligible effect on the growth rate of the oxide layer. On the other hand, within the parabolic regime, those parameters were much more influential on the kinetics, where the diffusion of species through the oxide layer was the rate-limiting step in oxidation. The result of this work helps in better understanding the initial oxidation behavior of more complex steels at high oxygen partial pressures and determining the surface composition of the steels.