# The Isothermal Oxidation of High-Purity Aluminum at High Temperature
## 1 Introduction  
A considerable body of work exists describing the oxidation or ignition of aluminum particles at elevated temperature. Aluminum particle behavior in an open atmosphere propellant burn has been investigated in detail [1; 2; 3]. For nominal rocket motor pressures, the aluminum particles oxidize to aluminum oxide in the gas phase and liberate significant combustion energy which also minimizes combustor instability. However, for low or atmospheric pressure combustion, which would be expected during rocket abort or launch pad malfunction, the outcome is not clear. Characterization of the thermal-chemical-physical environment in and around a plume formed from a burning fragment of solid rocket fuel at atmospheric pressure would aid in efforts to model accident scenarios. Ejected burning fragments of solid rocket fuel were observed in prior accident scenarios, such as the Titan 34D-9 accident in 1986 which contained 421,569 kg of solid propellant [4]. Burning propellant fragments pose a hazard to first responders and lead to the unintended ignition of nearby fuel sources. The particulate sources, while being representative of commercial products, suffer from broad particle size ranges (hence varying surface areas), varying levels of impurities, and being subjected to dynamic heating; all of which can lead to uncertainties in results that potentially could confound general interpretation. To address this issue, we put forth a single heating experiment that is used to explore and confirm the accepted behavior of aluminum surfaces exposed to high temperature air environments. To gain a clearer scientific understanding of the processes involved in the air-oxidation of aluminum, high purity,low specific surface area sheet was used in the present study. An additional level of control used in the current work (but absent in most previously published work) was to initiate oxidation once the specimen had stabilized at the desired oxidation temperature, i.e., the initial heat-up and stabilization was carried out under high purity inert gas stream. In general, the oxidation of aluminum in air can be described as occurring in four distinct stages. [PERSON] et al. [5] illustrated these oxidation stages clearly, and in situ X-ray diffraction has been used to monitor the phase changes occurring in the Al/Al2O3 system during heat treatment in air [6]. During aluminum combustion, an oxide layer may quickly grow on an aluminum surface which is exposed to an oxidizer. The thickness of the alumina shell depends on several factors including exposure time, temperature and the oxidizer. Oxide layer formation, grain orientation and other physical elements affect aluminum deformation at high temperature [7; 8]. The native amorphous alumina layer covering a particle of aluminum (present even at room temperature) initially grows extremely slowly during the low temperature oxidation stage I, which extends up to about 550 \({}^{\circ}\)C. The rate of this process is believed to be controlled by the outward diffusion of aluminum cations [9]. The amorphous oxide remains stable, due to the energy of the oxide-metal interface, only up to a critical thickness of about 5 nm [10; 11]. The amorphous oxide transforms into \(\gamma\)-alumina once the critical thickness is approached or when the temperature becomes sufficiently high. Since the density of \(\gamma\)-alumina is greater than that of amorphous alumina [12], and the smallest \(\gamma\)-alumina crystallites have a size of about 5 nm [13], if the thickness of the amorphous layer was less than 5 nm, the newly formed \(\gamma\)-alumina crystallites are no longer able to form a continuous layer covering the aluminum surface. Consequently, the beginning of stage II (ca. 550 \({}^{\circ}\)C) is accompanied by an increase in the rate of oxidation, and as the openings in the oxide coating heal, the rate of oxidation decreases. By the end of stage II at around 650 \({}^{\circ}\)C, a polycrystalline layer of \(\gamma\)-alumina has formed that covers the entire surface of the aluminum.