Kinetics of Barrier-Film Formation

The compact, nonporous anodic alumina film is the most suitable for fundamental investigations. It is grown by anodization, mostly under constant-current (galvanostatic) conditions, in neutral solutions of borates, tartrates, citrates, and phosphates, all of which possess significant buffering capacity and hence do not allow significant dissolution of the oxide. 

The growth of an anodic alumina film, at a constant current, is characterized by a virtually linear increase of the electrode potential with time, exemplified by Fig. 10, with a more or less notable curvature (or an intercept of the extrapolated straight line) at the beginning of anodization.73 This reflects the constant rate of increase of the film thickness. Indeed, a linear relationship was found experimentally between the potential and the inverse capacitance78 (the latter reflecting the thickness in a model of a parallel-plate capacitor under the assumption of a constant dielectric permittivity). This is foreseen by applying Eq. (38) to Eq. (35). It is a consequence of the need for a constant electric field on the film in order to transport constant ionic current, as required by Eqs. (39)-(43). 

The intercept should reflect the unchanging activation polarization at the two interfaces, as well as some other effects (presence of a film before anodization, time lag in attainment of the steady state, etc.). Nevertheless, the fact that it is small or negligible indicates that charge transfer processes at the interfaces are fast and that the kinetics of the growth are entirely transport controlled. 

The linearity leads to another important conclusion a constant field for a constant current implies a constant overall conductivity throughout the film. Since the conductivity is very structure sensitive, this implies also that either (i) the film grows homogeneously 

The fact is that, on the one hand, a significant field strength, E9 is needed to provide significant current. On the other hand, once in the practical current density range between 0.1 and 10 mA/cm2, a relative insensitivity of the field to the current density is found. In fact, an inverse field of 1.3 to 1.8 nm/V is accepted in the literature as characteristic of the oxide growth, without mention of the current density used. 

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