Pure polycrystalline Al (99.998%) was oxidized in dry oxygen and water. The surface products formed were analysed by ESCA. The thicknesses of the amorphous Al 2 O 3 formed during oxidation for 1 h in dry oxygen at 25°C and 250°C are 14 and 20 Å, respectively. During exposure of the oxidized sample to water a surface layer of amorphous Al 2 O 3 in hydrated and Al(OH) 3 is formed by the reaction: Al 2 O 3 + 3H 2 O⇄2Al(OH) 3 . The hydroxide is not stable in a vacuum. During analysis in the ESCA instrument, Al(OH) 3 is decomposed to Al 2 O 3 and H 2 O because the equilibrium pressure of H 2 O is higher than the partial pressure of water in the vacuum system. It is found that a 7 Å thick layer of Al 2 O 3 is formed on the surface of Al(OH) 3 . The binding energies of the Al 3+ (2p) and O 2− (1s) signals recorded from Al 2 O 3 formed on Al at 22°C and at 250°C are 75.8 and 532.7 eV, respectively. The binding energies of these peaks are 0.6 eV lower for a sample preoxidized at 250°C and then exposed to a humid atmosphere and immediately analysed. It is suggested that the change of the binding energy is due to set up of a space charge during exposure to a humid atmosphere. After exposure for a long time to a humid atmosphere the charging decays and the binding energies approach their normal values.
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Nylund et al. (1994) studied this question.
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