Effects of oxygen evolution on the voltage and film morphology during galvanostatic anodizing of AA 2024-T3 aluminium alloy in sulphuric acid at −2 and 24 °C

The effects of oxygen evolution on the voltage-time response and film morphology during galvanostatic anodizing of AA 2024-T3 alloy at 50 mA cm−2 in sulphuric acid have been investigated at −2 and 24 °C. The study employed interrupted anodizing experiments and real-time gravimetric measurements of the oxygen generated. The results showed that similar amounts of oxygen were evolved at the two temperatures, but with significantly different film morphologies and voltage responses. At −2 °C, a relatively large voltage increment accompanied the formation of linear cells in a relatively compact arrangement. The increment was mainly due to increase in the barrier layer thickness. In contrast, at 24 °C, the voltage increase was comparatively negligible and a sponge-like film morphology was generated that contained significant inter-cell porosity. It is proposed that the anodizing voltage and film morphology are dependent on the transport paths for oxygen gas escaping the film, in particular the relative proportions of gas escaping from the film via intra-cell and inter-cell porosity.