The Isothermal Oxidation of High-Purity Aluminum at High Temperature

The isothermal oxidation in air of high purity aluminum sheet was studied as a function of temperature using Thermogravimetric Analysis simultaneously with Differential Scanning Calorimetry (TGA/DSC). The rates and extents of oxidation were found to be non-linear functions of the temperature, in agreement with the literature. Between 650 degrees C and 750 degrees C very little oxidation took place; at 850 degrees C oxidation occurred after an induction period, while at 950 degrees C oxidation occurred without an induction period. At oxidation temperatures between 1050 degrees C and 1150 degrees C rapid passivation of the surface of the aluminum occurred, while at 1250 degrees C and above, an initial rapid mass increase was observed, followed by a more gradual increase in mass. The initial rapid increase in mass was accompanied by a significant exotherm, which was quantified by DSC. At temperatures of 1050 degrees C and above the specimen coalesced into a spheroidal particle, whereas at lower temperatures the original morphology was retained due to the cohesive strength of the native oxide layer. Cross-sections of oxidized specimens were characterized by scanning electron microscopy (SEM); the observed alumina skin thicknesses correlated qualitatively with the observed mass increases. Interrogation of the surface of an oxidized spheroidal particle by SEM showed a fractured alumina shell around a partially hollow core of aluminum which appeared to have grain boundaries.

Automated summary

The isothermal oxidation behavior of high purity aluminum sheet in air was investigated using TGA/DSC. The oxidation rates and extents were found to be temperature-dependent and non-linear. The oxidation process showed different characteristics at various temperatures, including induction period and passivation. The morphology and properties of the oxidized samples were also examined by SEM.