High frequency pulse anodising of aluminium: Anodising kinetics and optical appearance

High frequency pulsed anodising of pure aluminium was investigated with an aim to understand the effect of the anodising parameters on the growth kinetics of the anodic layer and optical appearance of the anodised surface. Anodising was performed in sulphuric acid, and the effect of the pulse duty cycle, applied potential offset, and pulse frequency was investigated.

Optical properties of the anodised surfaces are improved upon lowering the anodising potential and by increasing the frequency of the applied potential pulses. Temperature evolution of the samples during anodising was investigated by employing a special flat cell setup equipped with a thermocouple close to the sample.

The effect of high frequency pulsing of the anodising potential on the anodising kinetics is presented, which is related to the temperature evolution and dielectric losses, and the effect is compared to the traditional DC anodising process. From the observations, it is postulated that the dominant factor responsible for the improved growth kinetics during high frequency pulsed anodising might not be dielectric losses instead a thickness reduction in the Gouy-Chapman/Helmholtz layers.