SURFACE-ANALYSIS OF OXIDIZED ALUMINUM .1. HYDRATION OF AL2O3 AND DECOMPOSITION OF AL(OH)3 IN A VACUUM AS STUDIED BY ESCA

Pure polycrystalline Al (99.998%) was oxidized in dry oxygen and water . The surface products formed were analysed by ESCA. The thicknesses o f the amorphous Al2O3 formed during oxidation for 1 h in dry oxygen at 25-degrees-C and 250-degrees-C are 14 and 20 angstrom, respectively. During exposure of the oxidized sample to water a surface layer of amo rphous Al2O3 is hydrated and Al(OH)3 is formed by the reaction: Al2O3 + 3H2O reversible 2Al(OH)3. The hydroxide is not stable in a vacuum. D uring analysis in the ESCA instrument. Al(OH)3 is decomposed to Al2O3 and H2O because the equilibrium pressure of H2O is higher than the par tial pressure of water in the vacuum system. It is found that a 7 angs trom thick layer of Al2O3 is formed on the surface of Al(OH)3. The bin ding energies of the Al3+(2p) and O2-(1s) signals recorded from Al2O3 formed on Al at 22-degrees-C and at 250-degrees-C are 75.8 and 532.7 e V, respectively. The binding energies of these peaks are 0.6 eV lower for a sample preoxidized at 250-degrees-C and then exposed to a humid atmosphere and immediately analysed. It is suggested that the change o f the binding energy is due to set up of a space charge during exposur e to a humid atmosphere. After exposure for a long time to a humid atm osphere the charging decays and the binding energies approach their no rmal values.