(Digital Presentation) Initial High-Temperature Oxidation Behavior of Iron Binary Alloys in Air

Abstract

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.