The oxide films obtained after anodizing have considerable porosity and high adsorptive capacity. Sealing of the film increases their resistances to corrosion and improves their electrical insulating properties. Sealing is essential for films that have been produced by anodizing in sulfuric acid, because only after sealing do the film acquire a sufficiently high resistance to corrosion. The following methods can be used for sealing anodic oxide films:
- Filling of the pores with organic materials such as fats and greases (paraffin wax, lanolin, ceresin wax), solution of film forming substances (boiled linseed oil, varnishes) and resins. If necessary impregnation of the film is followed by drying or polymerization of the impregnating material.
- Precipitation of insoluble substances in the pores by exchange or decomposition reactions. For this purpose the film is impregnated successively with solutions which react to form insoluble precipitates (barium chloride followed by sodium sulfate, etc.); with solutions of salt which, on heating form insoluble hydrated oxides or successively, with a solution of sodium silicate and acetic acid to precipitate silicic acid.
- Filling of the pores with hydrated aluminum oxide by hydrating the oxide forming the film itself by treatment with hot water or steam.
- Passivating the metal at the base of the pores by impregnation of the film with passivating agents (solutions of chromates).
The last is the most widely used method of increasing the resistance of anodized parts of corrosion. The treatment is carried out in a 5 – 15 percent solution of potassium dichromate at a temperature of 90 – 95 oC for 20 minutes. The optimum composition of the solution is 8.5 percent potassium dichromate, 1.5 percent anhydrous sodium carbonate and balance water. The pH of the solution is 6-7. since carbonate interferes with the normal operation of the carbonate sealing process, each addition of soda to the solution should be followed by boiling to remove the carbon dioxide.