Effect of the initial treatment on the structure of thin anodic films on aluminum

The influence of different initial treatments of the metallic surface on th e solid-state and dielectric properties of thin anodic films grown on alumi num specimens, anodized in inorganic salt-containing solutions, was investi gated in situ by photocurrent and impedance spectroscopy. The presence or t he absence of photoresponse under anodic bias of the electrodes was related to the different structure of the surface passive layers on specimens subm itted to various surface treatments. The anodic photocurrent behavior was c ompared with the frequency response of the metal/passive film/electrolyte j unction recorded within the blocking potential region. The values derived f or the components of the equivalent electrical circuit of the interface wer e related to the more or less insulating properties of the layers, which af fect both the photocurrent behavior and the kinetics of growth of the anodi c films. Different absorption edges for the anodic photocurrent were attrib uted to surface layers with variable hydration degree, having different opt ical bandgap values. The presence of two absorption edges was related to du plex surface films, with an external more hydrated layer. In this case a mo re complex electrical equivalent circuit was necessary in order to fit the frequency dispersion of the junction. (C) 1999 The Electrochemical Society. S0013-4651(97)10-041-6. All rights reserved.