Correlation Between Amount of Bound Water and Stability of Anodic Oxide Film on Aluminum
Authors: Haruna Takumi , Ikeda Takao , Nishimoto Akio 
Published: February 2020

Passive film formed on stainless steel is well known to prevent the steel from corrosion. Okamoto and Shibata tried to obtain bound water in the passive film of stainless steel in 1960’s1,2), pointed out that the bound water played an important role to stability of the passive film3,4), and then proposed a structural model of the passive film3,4). Thereafter, many researchers have tried to understand the correlation between bound water and stability of passive film. We have recently tried to characterize the bound water in the passive film in a different method of a thermal gas-desorption spectroscopy (TDS)5-7).
On the other hand, anodic oxide film formed on Al is also well known to prevent the material from corrosion. However, correlation between the amount of bound water and stability of the anodic oxide film on Al has not been clearly understood. Therefore, this paper aims to determine both the amount of bound water in the film by TDS and pitting potential of the material, and to investigate the correlation between the two factors.
A specimen was Al (99.99 mass%) wire of 1 mm in diameter. Anodic oxide film was formed on the specimen in 1.0 kmol m-3 H2SO4 solution by applying an anodic potential of 20.5 V against Pt counter electrode for 1.8 ks.
Modification of the anodic oxide film on Al was conducted by the following method. The specimen with the film was immersed in 1.0 kmol m-3 NaCl solution, and then a potential was applied to the specimen for 180 s. The specimen of which pitting corrosion does not occur for the period was selected as the specimen with successfully-modified film, and was subjected to the next three tests; the measurement of thickness of barrier layer for the film, the measurement of pitting potential of the specimen, and the measurement of amount of bound water in the film by the thermal gas-desorption spectroscopy (TDS). In the TDS test, the specimen was set in the quartz chamber evacuated to about 10-7 Pa for 54 ks in order to remove the water physically-adsorbed on the film. Thereafter the specimen was gradually heated up at 5.6 x 10-2 K s-1, and then water desorbed out of the film. The desorption rate of water (m/z=18) was measured by a quadrupole mass spectrometer.
First, the anodic oxide film was formed on the specimen at 20.5 V, and then a constant potential from 1.0 to 2.0 VAg/AgCl was applied to the specimen with the film in the NaCl solution to modify the film. The thickness of whole the film and the one of barrier layer showed almost no change even if the modification potential varied. However, the maximum pitting potential was obtained at a modification potential of 1.5 VAg/AgCl. On the other hand, the amount of bound water was detected by the TDS technique, and the value increased with a rise in the potential. Since the modification potential was thought to be applied strongly to the barrier layer, it was considered that most of the bound water existed in the barrier layer. Correlation between the stability of the film evaluated by the pitting potential and the amount of bound water was checked, and it was revealed that the stability of the film improved with an increase in the amount of bound water to 1000 fg mm-3, and the amount beyond the value provided the stability of the film poor. The former relationship was similar to that for passive film of Ti5), and the later one was similar to that for the passive film of type 304 stainless steel6,7).
Acknowledgement
This work was partially supported by The Light Metal Educational Foundation, Inc.
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