Corrosion performance and microstructure of anodic film formed on aluminum after sealing in lithium hydroxide solution

摘要

Anodization has been widely used as an excellent surface finishing process of aluminium to confer high corrosion resistance and fair appearance to the substrate surface. To further improve the corrosion resistance, pores of the anodic film are sealed by treating the film in boiling water or hot steam to hydrate the oxide. The mechanism of such conventional hydration sealing was explained as a volume expansion of hydrated alumina that fills the pores. These explanations were based on the uniform pore shrinkage during sealing treatment. Previously, we reported hydration behaviour through the dissolution of pore walls and the precipitation of platelet-like hydroxide in pores as well as on the top surface as determined by direct TEM observation ofthe cross sections of the films. Recently, a novel sealing process has been developed that entails dipping a specimen in a lithium hydroxide solution at room temperature. In this study, we investigated in detail the change in microstructure of porous anodic oxide films with sealing time in lithium hydroxide solution using SEM and TEM to onfirm the sealing mechanism. In addition, the corrosion performance of the sealed anodic films using lithium hydroxide solution was investigated with comparing those of the other sealing methods. After sealing of anodic films in a lithium hydroxide solution at 25 °C for 1-2 minutes, platelet-like precipitates composed of LiH(AlO_2)5H_2O and hydrated alumina were formed on the surface as well as in the pores. The sealing in lithium hydroxide solution is explained by dissolution/precipitation process at the pore wall of anodic oxide. The sealing behavior was significantly dependent on the type of anodizing electrolyte used since the sealing proceeded through dissolution/precipitation stages of a pore wall in which electrolyte anion species was incorporated. Although the sealing temperature was lower and the treatment time was fairly shorter than those of conventional sealing processes such as boiling hydration sealing and nickel salt sealing, a comparable or rather superior corrosion resistance of the anodic film treated in lithium salt was confirmed from the result of the CASS test. 1) Japan patent 2010-77532