Oxidation Behavior of Steel and Iron pellets at High Temperature

Details

Supervisors

Jacques, Pascal ; Kuypers, Bénédicte ; Choisez, Laurine ; Pardoen, Thomas

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

Iron has recently attracted increasing interest as a potential energy carrier for largescale energy storage. Through reversible oxidation–reduction cycles, iron can store and release energy while offering advantages such as high volumetric energy density, safe handling, and abundant availability. To support and optimize the development of such systems, a better understanding of the oxidation behavior of iron is required. This Master’s thesis investigates the high-temperature oxidation behavior of pellets made from compressed steel wool and pure iron wires. Three steel wire dimensions (35, 75, and 100 μm) and one iron wire dimension (75 μm) were studied. Thermogravimetric analyses (TGA) were performed between 750°C and 900°C to evaluate oxidation kinetics. Additional characterization techniques, including scanning electron microscopy (SEM), X-ray tomography, XPS, and EBSD analyses, were used to investigate the microstructural evolution of the materials during oxidation. The results showed that smaller wire dimensions promote faster oxidation. Furthermore, oxidation proceeds through two distinct regimes. The first regime is characterized by a rapid oxidation rate and appears to be mainly limited by oxygen transport within the porous pellet structure. The second regime is significantly slower and is controlled by diffusion through the growing oxide layers. Eccentric cavities were observed in all oxidized wires, even after very short oxidation times. Their formation was attributed to the Kirkendall effect and suggests that oxidation is mainly driven by the outward diffusion of iron cations. The early appearance of these cavities indicates that diffusion processes are already active during the first stage of oxidation. The influence of several parameters, including temperature, wire dimensions, air availability, surface condition, and material composition, was investigated. Acid pickling was found to significantly accelerate oxidation. Pure iron pellets achieved higher conversion degrees than steel pellets under similar conditions. Significant morphological differences were observed between steel and iron pellets, including vein-like structures in steel wires and dense whisker formations in pure iron wires. Kinetic fitting performed on the Steel 35 pellet gives an apparent activation energy of approximately 30.44 kJ/mol, which is significantly lower than values commonly reported in the literature.