Sealing the Oxide Film
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 ats and (paraffin 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).