The Growth of Anodic Aluminum Oxide Layers after a Heat‐Treatment

Abstract

Aluminum was anodized in a solution of ammonium pentaborate in ethylene glycol with a current density of 0.25 mA/cm2 to voltages between 9.8 and 183V. After annealing to 400°C in oxygen the specimens were reanodized in the same solution, either with a constant rate of voltage increase or at constant voltage. In both cases the current displays a peak as a function of time. With increasing voltage the peak occurs slightly above the original forming voltage, at constant voltage an induction period occurs before the current starts to increase. After passage of the peak annealed and unannealed specimens behave in the same way. The efficiency of the reanodization after annealing has remained unity: the layer grows. Transmission electron micrographs of layers stripped after interruption of the reanodization show that in the peak growth occurs inhomogeneously: black spots occur which are thickenings of the layer, the surface of which increases as the reanodization is interrupted in a later stage. The effect can formally be described as a two dimensional phase transformation. A model in which the theory of the kinetics of phase transformations is combined with the experimental exponential relation between ionic current and field during anodization leads to an equation which in most cases accurately describes the reanodization curves.