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 the solid-state and dielectric properties of thin anodic films grown on aluminum specimens, anodized in inorganic salt-containing solutions, was investigated in situ by photocurrent and impedance spectroscopy. The presence or the absence of photoresponse under anodic bias of the electrodes was related to the different structure of the surface passive layers on specimens submitted to various surface treatments. The anodic photocurrent behavior was compared with the frequency response of the metal/passive film/electrolyte junction recorded within the blocking potential region. The values derived for the components of the equivalent electrical circuit of the interface were related to the more or less insulating properties of the layers, which affect both the photocurrent were attributed to surface layers with variable hydration degree, having different optical bandgap values. The presence of two absorption edges was related to duplex surface films, with an external more hydrated layer. In this case a more complex electrical equivalent circuit was necessary in order to fit the frequency dispersion of the junction.