Application of Different Anodizing Methods in Aerospace

The most commonly used anodizing process is sulfuric acid anodizing, but it differs in aerospace applications. Chromic acid anodizing is mainly used to protect key structures at various junctions. The excellent corrosion resistance is related to the thickness of the coating, usually ranging from 0.08 to 0.2 mils. Compared with other processes, the oxide film formed by this process is softer, with fewer pores, and no significant material fatigue loss. The film is easily damaged and has an opaque light gray color. When the film is sealed with a double chromate sealant, it will appear green.
The main uses of sulfuric acid anodizing can be divided into two types, type II coating and type III coating. Type II coating is mainly used for decorative or protective applications, while hard coating oxide film (Type III) is suitable for engineering applications, such as aerospace industry. MIL-A-8625F specifies the uniform production of anodized coatings from type III coatings treated with electrolytic aluminum and aluminum alloys. As long as thick and dense coatings can be produced, this type of coating can provide diversity for the operational process.
Phosphoric acid anodizing is basically used for structural bonding in high humidity environments. This process is called Boeing machining and is carried out at 10-15V. When under unfavorable conditions, the oxide film formed by this process has better durability than the film formed in chromic acid and sulfuric acid. One of the reasons for the formation of this important characteristic of adhesive is said to be the form of oxide film, which should be a film containing whisker pores or a film with protrusions on the top of the surface.
The last mentioned anodic oxidation process is the new boric acid sulfuric acid anodic oxidation, which is a substitute for the chromic acid electrolyte containing hexavalent chromium (hexavalent chromium is a carcinogen and has been eliminated as a metal treatment process, therefore, the electrolyte without hexavalent chromium is particularly necessary). The coating adhesion of the oxide film formed by boric acid sulfuric acid electrolyte is equivalent to or even better than that of the coating formed by chromic acid. This process is controlled by the voltage, and the sealant in the diluted chromate solution heated to 15V must achieve satisfactory corrosion resistance.
The above anodizing process is the foundation of the current aerospace industry. It should be remembered that the operating conditions vary within different ranges, and most specifications are general guidelines. Therefore, the most important thing to remember is to determine the performance criteria before selecting the correct anodizing process.