SURFACE-ANALYSIS OF OXIDIZED ALUMINUM .2. OXIDATION OF ALUMINUM IN DRY AND HUMID ATMOSPHERE STUDIED BY ESCA, SEM, SAM AND EDX

Pure Al (99.998%) and technically pure Al (99.5%) were oxidized in oxygen at room temperature, 250-degrees-C and 500-degrees-C. The oxidized samples were exposed to a humid atmosphere (80% RH) at room temperature for times ranging from 1 day to 1 month. The reaction products were analysed by ESCA, scanning Auger microscopy, energy-dispersive x-ray spectroscopy (EDX) and SEM. The thicknesses of the surface oxides formed during oxidation for 5 h at the same temperatures as above are 15, 22 and 65 angstrom, respectively. After exposure of the technically pure Al material to a humid atmosphere the analyses show localized corrosion and formation of hydroxide at noble precipitates like Al3Fe and alpha-AlFeSi. The distribution of hydroxide particles on oxidized Al can be recorded by SEM owing to the fact that the secondary electron yield of Al2O3 is larger than the yield of Al(OH)3. Exposure of pure Al, preoxidized at room temperature, to a humid atmosphere for 1 week causes a growth of the oxide thickness from 15 to 22 angstrom. The corresponding growth in a dry atmosphere for the same time is only 1 angstrom. It is suggested that the growth in a humid atmosphere is due to hydration of Al2O3. Thereby, the oxide becomes thinner and oxidation of Al occurs. The Al(OH)3 formed by hydration is decomposed to Al2O3 in the ultrahigh vacuum (UHV) system and is therefore detected as an oxide growth.