Abstract
Aluminium specimens anodized to form porous oxide films in an oxalic acid solution were reanodized in a neutral boric acid-borate solution (pH=7.4, 20°C) at a constant c.d. of 0.5mA/cm2 and changes in the anodizing voltage and geometrical structure of the film were examined. An automatic apparatus was used for the current-controlled electrolysis and an ultra-thin sectioning technique was utilized in electron-microscopy. It was found that the oxide formation occurs both at the barrier oxide/solution and oxide/metal interphases. The former which leads to pore-filling is due to the transport of Al3+ ions through the barrier oxide whereas the latter is caused by the O2- ion transportation. The voltagetime relationship is represented by two successive straight lines of different slopes and the pore-filling was found to be completed at the time when the slope changes. Apparent transport numbers for Al3+ and O2- ions were determined respectively to be 0.40 and 0.60 and they were almost independent of the applied c.d. It is shown that the porosity and thickness of the film could be estimated by an analysis of the voltage-time relationship using the above transport numbers.
