Study of initiation and development of local burning phenomena during anodizing of aluminium under controlled convection

Abstract

A study of burning during galvanostatic anodizing of aluminium in sulphuric acid is presented. Experiments are performed under controlled convection in wall-jet configuration, while in situ measuring local anode temperatures. Burning is a breakdown phenomenon manifesting itself in the local temperature evolutions and in the electrochemical behaviour of the electrode. Undulating oxide surfaces with protruding oxide hillocks, displaying a twofold microstructure, are formed. Focusing on short anodizing times the initiation of burning is determined in the period of declining potential following the maximum potential, linking burning to the development of the final porous structure. The influence of enhanced Joule heat generation during the onset of anodizing is investigated by anodizing electrodes covered by barrier-type oxide films, formed by an a priori performed barrier anodisation. Increased initial oxide thicknesses intensify burning, in the extreme case resulting in non-quenched phenomena involving quasi complete concentration of current in the burning area. However, the occurrence of burning during experiments on a priori barrier anodized electrodes at reduced current density exclude the conditions of heat transfer as key parameter for triggering burning. Results indicate the existence of a critical barrier layer thickness above which burning is encountered during the period of development of the regular porous structure.