Abstract
Colouration in anodic oxide films grown on 99.99% aluminium in sulphuric, oxalic, and phosphoric acid electrolyted has been characterized by curves of optical density (extinction) versus wavelength, by transmission through detached films. At visible wavelengths this extinction includes a small contribution from surface scattering and reflection (0.02–0.04 O.D.), the remainder being due to scattering at the pores within the films. Absorption, in the visible range at least, is negligible. Theoretical calculations of scattering at idealized pores of circular cross-section lying parallel to each and perpendicular to the film surface, using size versus number distributions determined by electron microscopy and a refractive index of the oxide equal to 1.64, gave fair agreement with experiment for sulphuric and oxalic acid films on the basis of water-filled pores, but poor agreement for phosphoric acid films, where interference effects are expected to be important. The O.D. of sulphuric acid films may be increased by pumping in vacuum, which is a further indication that the pores may contain water.
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