Experiments on Oxidation and Combustion Behaviors of Cerium Metal Slice with Slow Heating under O2/Ar Atmospheric Conditions

Abstract

Cerium (Ce) metal is commonly involved in fires due to its high activity in terms of chemical properties, posing a critical threat to equipment and human health. The oxidization, combustion and oxidization-to-combustion transition of cerium are complicated processes, and a full understanding of detailed evolution behaviors is lacking. A series of experiments are executed to study the oxidation-to-combustion process of cerium metal slices (CMSs) in an O2/Ar atmosphere of 0.3 mg/mL O2. Macroscopic features and micro-transformation behaviors of the physicochemical process are characterized using high-speed imaging, spectroscopy, XRD, AFM, SEM-EDX, and TGA. Results show that the evolution behaviors of CMS present three critical transitions, namely, the oxidation stage (OS), ignition and combustion stage (ICS) of heterogeneous reaction, and extinction stage (ES). The evolutions of CMS structure, oxide layer thickness, surface morphology and micro-zone composition at several key moments during the OS elucidate the transformation mechanism. The surface of CMS is firstly oxidized to Ce2O3 and then to CeO2, and these oxides experience their formation, grow, and gradually aggregate to form dense oxide layers. Fissures have been observed in the micro-morphology of the dense oxide layers at the initial ICS, implying that oxygen could diffuse through the fissures of the oxide layers and fiercely react with molten Ce inside during the ICS. The reactivities of Ce in OS and ICS are quantitatively evaluated with thermodynamic data. The qualitative and quantitative mechanism of the oxidization-to-combustion transition of Ce greatly contributes to the optimal design and safe operation of active metal equipment.