the growth of new oxide either proceeds at the interface between metal and oxide or at the interface between oxide and electrolyte (compact anodic TiO2 layers may in principle grow at either interfaces, but under most experimental conditions oxide grows at the metal–oxide interface). Under a constant voltage U, the field F = U/d drops constantly, thus lowering the driving force (for solid-state ion migration) with increasing film thickness d. The result is an (exponential) drop in the anodic current with time (Figure 2a) until the field effect is lost (that is, is on the order of kT). At this point, a (practically) finite thickness is reached that mainly depends on the anodization
voltage. For many transition metals (so-called valve metals), this final thickness is given by d = fU, where f is the so-called growth factor of the oxide (typically in the range of 2– 4 nm V1). Typically, the layer which is grown at the oxide/ electrolyte interface consists of less-dense oxide containing oxyhydroxides,[117, 118] while the layer at the metal–oxide interface consists of dense and stable TiO2-