# The Isothermal Oxidation of High-Purity Aluminum at High Temperature
## 4 Conclusions  
From the isothermal oxidation experiments described in this paper, several observations can be made:  
* The rate or extent of oxidation in air was found to be a non-linear function of the temperature. Different temperature ranges resulted in different behavior, in agreement with the literature.
* At temperatures between 650 \({}^{\circ}\)C and 750 \({}^{\circ}\)C, very little oxidation took place, even after 12 h exposure to air. At 850 \({}^{\circ}\)C oxidation occurred after an induction period, while at 950 \({}^{\circ}\)C a similar amount of oxidation occurred as at 850 \({}^{\circ}\)C, but more promptly.
* Raising the temperature further resulted in rapid passivation of the surface of the aluminum sheet, as only small and gradual mass increases were measured at oxidation temperatures of 1050 \({}^{\circ}\)C, 1150 \({}^{\circ}\)C, and 1250 \({}^{\circ}\)C. Although the amounts of oxidation occurring at these three temperatures were very small, it was observed that the rates of mass increase was proportionally to temperature.
* At 1250 \({}^{\circ}\)C and above, an initial extremely rapid mass increase was observed followed by a more gradual increase in mass. The initial rapid increase was accompanied by a significant exotherm as measured by DSC.
* At temperatures of 1350 \({}^{\circ}\)C and above, mass loss was recorded during the temperature ramp under inert atmosphere; the mass loss began at about 1350 \({}^{\circ}\)C and became significant above 1400 \({}^{\circ}\)C. This is attributed to vaporization of aluminum.
* By scanning electron microscopy (SEM); the observed alumina skin thicknesses correlated qualitatively with the observed mass increases for each oxidation temperature studied. The outer surface of a spheroidal particle produced by oxidation of an aluminum specimen at 1050 \({}^{\circ}\)C revealed a partially hollow sphere of aluminum, with a fractured outer alumina shell.
* Specimens oxidized at temperatures below 1050 \({}^{\circ}\)C did not transform into spheroidal particles, rather they retained the overall morphology of the aluminum material, presumably due to the stabilizing effect of the native alumina shell at these temperatures.  
Since the oxide shell inhibits the hot internal aluminum vapor from coming with the external oxidizing atmosphere, the shell thickness and quality will influence aluminum particle ignition conditions; this conjecture may address the difficulty which has been experienced in modeling ignition.  
Conceptualization, E.N.C., B.D., W.G., N.Y. and F.M.V.; Data curation, N.Y. and F.M.V.; Funding acquisition, E.N.C. and W.G.; Writing--review and editing, E.N.C., B.D., W.G., N.Y. and F.M.V. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc. (Charlotte, NC, USA), for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract No. 89233218 CNA000001).  
The authors declare no conflict of interest.