The electrochemical formation of oxide layers on aluminum is a well-known and widely used industrial procedure to produce protective and/or decorative coatings on aluminum and/or aluminum alloys. Electrolytically produced aluminum oxide layers protect the base metal from corrosion and weathering and furthermore may increase the surface hardness and the abrasive resistance of the aluminum part.
Many different processes of anodizing are known. For example, aluminum materials can be anodized in electrolytes such as sulfuric acid, chromic acid, phosphoric acid, and oxalic acid by the application of AC or DC currents at a bath temperature of 10-25° C. Variations in this treatment can change the thicknesses and/or hardness of the anodized aluminum oxide layer.
The porosity of the anodized layer may be favorable for the adhesion of organic coatings, but exhibits a major drawback, namely the lack of protection against corrosive media. Therefore, and to impart maximum corrosion stability, anodized aluminum layers are often sealed in a subsequent process step. During sealing, which might be a hot sealing and/or cold sealing process, the aluminum oxide becomes hydrated and is transformed from its amorphous, essentially water-free constitution to a boehmite structure. This transformation is accompanied by a volume expansion or swelling of the oxide that in turn procures the sealing of the porous structure. Hot sealing of the anodized layer is usually performed in hot water or in steam, whereas the cold sealing process is operated at temperatures close to 30° C. in the presence of nickel fluoride. Sealing improves the corrosion resistance and resistance to weathering of anodized aluminum parts in a pH range from 5-8.
Unfortunately, sealed anodized aluminum surfaces continue to display poor corrosion resistance and stability below pH 4 and/or above pH 9. Additional seals or coatings have been attempted but improved coatings with stability to high and low pH, accelerated corrosion testing, abrasion, and fogging are needed.