# The Isothermal Oxidation of High-Purity Aluminum at High Temperature
## 3 Results and Discussion  
### Thermogravimetric Analysis--Differential Scanning Calorimetry  
TGA and DSC results are shown in Figures 1 and 2, respectively. The TGA data in Figure 1 shows the oxidation behavior versus time during isothermal holds at temperatures between 650 \({}^{\circ}\)C and 1550 \({}^{\circ}\)C. Time zero is defined as the point at which air was first introduced into the TGA, and the data plotted begins 10 min before \(t=0\). The observed behavior (i.e., oxidation mass change versus temperature of exposure to air) is directly comparable to previous literature reports where aluminum samples were heated at various rates under oxidizing conditions, described in the introduction. One significant difference between the present and most of the earlier work is that in the present work, the temperature range for stage I (growth of amorphous oxide layer) was traversed without exposure of the sample to an oxidizing environment; air was introduced in one of the later stages, using the definitions given in the introduction. The TGA data recorded during oxidation at 650 \({}^{\circ}\)C and 670 \({}^{\circ}\)C showed negligible mass increase during a 12 h hold (Figure 1). At an oxidation isotherm of 750 \({}^{\circ}\)C, the onset of slow oxidation was observed, while at 850 \({}^{\circ}\)C a gradual mass increase occurred with  
Figure 1: TGA results for 0.5 mm aluminum heated under argon to the indicated temperature, then exposed to air (at \(t=0\)). Figure 2: DSC results for aluminum heated under argon to the indicated temperature, then exposed to air (at \(t=0\)). a rate that initially accelerated and became significant after about 1 hr of exposure to air. A similar induction period was reported by [PERSON] et al., for commercial grade aluminum (>1500 ppm impurity level) at 850 \({}^{\circ}\)C under pure oxygen [16]. [PERSON] et al. [6] observed the delayed transformation of \(\gamma\)-alumina to \(\alpha\)-alumina by in situ XRD after holding for 160 min in air at 850 \({}^{\circ}\)C. At 950 \({}^{\circ}\)C a similar overall mass increase was seen, but at a more rapid rate and without a significant induction period. The observations of TGA data in this temperature range are consistent with stage III oxidation. The induction period seen at 850 \({}^{\circ}\)C may be a consequence of the necessity to build up a contiguous layer of \(\gamma\)-alumina on the specimen prior to transition to \(\delta\)-, \(\theta\)-, and eventually \(\alpha\)-polymorphs, and that the buildup of \(\gamma\)-alumina is slow at 850 \({}^{\circ}\)C.  
The slow, but gradually increasing (with temperature) oxidation at 1050 \({}^{\circ}\)C, 1150 \({}^{\circ}\)C, and 1250 \({}^{\circ}\)C coincides with the transition from stage III into stage IV oxidation. At 1350 \({}^{\circ}\)C and above, the rate of oxidation becomes increasingly significant. Two other factors are apparent at the highest oxidation temperatures, (1) the sample mass decreases during the isothermal hold before air is admitted, and (2) there is a large, almost instantaneous increase in mass upon initial exposure to air, followed by a more gradual increase. This rapid increase in mass is accompanied by a significant exotherm, as shown in Figure 2. At oxidation temperatures below 1050 \({}^{\circ}\)C the exotherm was barely noticeable. Figure 3 plots the energy released during the initial rapid oxidation event, based upon the measured peak intensity (area) for each exotherm, against oxidation temperature. The decreasing mass prior to oxidation at the highest temperatures is ascribed to vaporization of aluminum. Figure 4 shows an expansion of the region around the introduction of air for experiments conducted at oxidation temperatures of 1350 \({}^{\circ}\)C, 1450 \({}^{\circ}\)C, and 1550 \({}^{\circ}\)C. It can be seen that vaporization becomes noticeable slightly below 1350 \({}^{\circ}\)C, and the rate increases rapidly with temperature.