Surface analysis of oxidized aluminium. 2. Oxidation of aluminium 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°C and 500°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 Å, 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 Al 3 Fe and α‐AlFeSi. The distribution of hydroxide particles on oxidized Al can be recorded by SEM owing to the fact that the secondary electron yield of Al 2 O 3 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 Å. The corresponding growth in a dry atmosphere for the same time is only 1 Å. It is suggested that the growth in a humid atmosphere is due to hydration of Al 2 O 3 . Thereby, the oxide becomes thinner and oxidation of Al occurs. The Al(OH) 3 formed by hydration is decomposed to Al 2 O 3 in the ultrahigh vacuum (UHV) system and is therefore detected as an oxide growth.