Corrosion Resistance Improvement Technology of Anodic Oxide Films on Aluminum Alloy that uses a Lithium Hydroxide Solution

The anodic oxide films are formed to improve the corrosion resistance on aluminum alloy that used as the parts of engines and car bodies. Because these films are porous structure, it is necessary to seal the pores to further improve the corrosion resistance. The pores are sealed with hydrated alumina by treating the films in boiling water or solution that added sealing additives. These hydration sealing has a problem that energy consumption is large because of long sealing time and high temperature of solution. In this study, the authors have developed a new sealing treatment (Lithium sealing) using a lithium hydroxide solution to solve above problem. Lithium sealing mainly sealed the pores with lithium aluminate double salt (LiH(AlO₂)₂·5H₂O). This salt was rapidly formed in strong alkaline solution at room temperature, so that the sealing time was reduced to about 1/10 compared with the conventional sealing. This salt was formed from the surface of the film to the depth part, especially, much more salt was formed in the upper part of the film. It contributed to the corrosion resistance improvement. We have developed to apply Lithium sealing to the outboard machine parts used in a severe erosive environment.