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Aluminum is able to form a self-healing, oxide layer on its surface which reliably insulates and protects it from the ambient environment. This ability is the basis of its widespread use in many applications. However, the mechanism of the oxide layer formation is largely unknown. Although the early stages of oxygen molecule absorption have been effectively elucidated with surface science techniques (scanning tunneling microscopy /l/, work function measurements /l/, and X-ray photoemission spectroscopy /l/), the overall kinetics of oxide layer growth remains poorly understood.
In nature, a passive oxide layer of about a monolayer covers the aluminium surface and stops oxidation by the action of an electric field generated by charge separation between aluminum ions at the oxide-gas interface and oxygen ions at the metal-oxide interface. The net reaction rate is zero, indicating that the formation of the passive oxide layer is an equilibrium process.
When anodic polarization is applied to a single-crystal aluminium substrate with (100), (110) and (111) faces, the oxide layer begins to grow quickly in dry oxygen at 25 degrees C and 760 mm Hg pressure. After a few days of exposure the aluminium oxide layer grows to an 'effective limit' of approximately 30 angstroms, measured by three different methods (anodic polarization, electron diffraction and capacity).
As the oxide layer thickness increases the atomic structure of the metal becomes more ordered with a reduction in lattice spacing. The increased crystalline order also leads to an increase in the electrostriction stress that develops between adjacent atoms of the forming oxide. This additional tensile component of the electrostriction stress causes each oxide flake to exert lateral forces against its nearest neighbor, with each flake constraining its neighbor in a hexagonal arrangement.
