The oxidation of aluminium in dry and humid oxygen atmospheres

Abstract

The oxidation of electrolytically polished aluminium single-crystal surfaces, presenting (100), (110) and (111) faces, has been studied in dry oxygen at 20° C and 760 mm Hg and in 80% saturated oxygen at 25° C and 760 mm Hg; three methods (anodic polarization, electron diffraction and capacity) have been employed to identify and estimate the surface films. Film growth in dry oxygen is at first rapid and then slow, the growth rate being inverse logarithmic (1/8 = — K log₁₀ (t + t₀) + K'). A ‘practical limit’ of approximately 30 A is reached after several days’ exposure. The presence of water vapour increases film growth; after 7 days’ exposure aluminium carries films which are about 10 A thicker than those formed by exposure for the same time to dry oxygen. In humid oxygen, film growth does not cease at a ‘practical limit’ but continues at a slow rate after the initial period of rapid thickening. The growth law is direct logarithmic (8 = K log₁₀ (t + t₀) + K') during the first 10h of exposure but thereafter inverse logarithmic (1/8 = — K log₁₀ (t + t₀) + K '). The growth of thicker films in humid oxygen is caused by small, though continual, changes in the electrical properties of these films during thickening. The films formed in both dry and humid oxygen were found by electron diffraction to be ‘amorphous’. Anisotropy in the growth rate of films on differently oriented metal faces has not been experimentally verified with ‘amorphous’ films which thicken according to either the direct or inverse logarithmic laws.