2015 Volume 56 Issue 12 Pages 2000-2005
It has been investigated that the tiny amount of bound water in anodic oxide film on Al was quantitatively measured by thermal gas desorption spectroscopy (TDS) and correlation between the amount of bound water in the film and stability of the film was discussed. Al wire specimens were anodically polarized at 20.5 V in sulfuric acid solution to prepare the anodic oxide film on the specimen. The specimen with the film was then subjected to the film-modification process in which an anodic potential from 1.0 to 2.0 VAg/AgCl was applied to the specimen for 180 s in chloride solution. The specimens for which pitting corrosion did not occur were subjected to the four tests; measurements of thickness of barrier layer in the film, pitting potential, the amount of bound water in the film by the TDS and the amount of water in the porous layer by grow discharge optical emission spectroscopy (GD-OES). As a result, the thickness of the anodic oxide film formed on Al at 20.5 V was about 25 µm and independent of the modification potential. The amount of bound water was quantitatively detected by the TDS, and increased with a rise in the modification potential. However, depth profile of hydrogen, which is considered to correspond to water, in the porous layer of the film detected by the GD-OES was independent of the modification potential. The findings suggested that the bound water is included in the barrier layer. The pitting potential exhibited the maximum value when a modification potential of about 1.5 VAg/AgCl was applied to the specimen. According to the correlation between the amount of bound water in the film and the pitting potential, the pitting potential rose with an increase in the amount of bound water in smaller amount range, and the amount beyond the small range induces the pitting potential higher. The evidence suggests that control of the amount of bound water is key factor for improving stability of the anodic oxide film on Al.