Method of compacting anodized metals with lithium and fluoride-c

Details Method of compacting anodized metals with lithium and fluoride-c Method of compacting anodized metals with lithium and fluoride-c

1998-01-07

1999-04-06

Gorgos, Kathryn

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

148276, 205199, 205200, 205203, 205204, C25D 1118

Patent

active

058912690

DESCRIPTION:

BRIEF SUMMARY
This is a national stage application of PCT/EP96/02848, filed Jun. 29, 1996.


FIELD OF THE INVENTION

This invention relates generally to the production of corrosion-controlling and/or decorative coatings on metals by anodic oxidation. More particularly, the invention relates to a new process for sealing the electrochemically produced porous anodizing layers for further improving their properties.


TECHNICAL BACKGROUND AND RELATED ART

The electrochemical anodic oxidation of metals in suitable electrolytes is a widely used process for forming corrosion-controlling and/or decorative coatings on suitable metals. These processes are briefly characterized, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. 9 (1987), pages 174 to 176. According to this literature reference, titanium, magnesium and aluminium and their alloys can be anodized, the anodization of aluminium and its alloys having the greatest industrial significance. The electrolytically produced anodizing layers protect the aluminium surfaces against the effects of weathering and other corrosive media. Anodizing layers are also applied to obtain a harder surface and hence to increase the resistance of the aluminium to wear. Particular decorative effects can be obtained through the color of the anodizing layers and through absorptive or electrolytic coloring. The anodization of aluminium takes place in an acidic electrolyte, sulfuric acid being the most widely used. Other suitable electrolytes are phosphoric acid, oxalic acid and chromic acid. The properties of the anodizing layers can be varied within wide limits through the choice of the electrolyte and its temperature and through the current density and anodizing time. The anodizing process is normally carried out with direct current or with direct current superimposed on alternating current.
The fresh anodizing layers may subsequently be colored by immersion in solutions of a suitable dye or by an alternating-current treatment in an electrolyte containing a metal salt and preferably in a tin-containing electrolyte. As an alternative to subsequent coloring, colored anodizing layers can be obtained by so-called color anodizing processes which are carried out, for example, in solutions of organic acids, more particularly sulfophthalic acid or sulfanilic acid, optionally in admixture with sulfuric acid.
These anodically produced protective layers, of which the structure has been scientifically investigated (R. Kniep, P. Lamparter and S. Streeb: "Structure of Anodic Oxide Coatings on Aluminium", Angew. Chem, Adv. Mater 101 (7), pages 975 to 977 (1989)), are frequently referred to as "oxide coatings". However, the study mentioned above revealed that these coatings are glass-like and contain tetrahedrally coordinated aluminium. No octahedrally coordinated aluminium, as present in the aluminium oxides, was found. Accordingly, the more general term "anodizing layers" is used in this patent application instead of the misleading term "oxide coatings".
However, these layers are still not entirely satisfactory in regard to corrosion control because they still have a porous structure. For this reason, the anodizing layers have to be sealed. The sealing process is often carried out with hot or boiling water or, alternatively, with steam. Sealing closes the pores and hence considerably increases protection against corrosion. Extensive literature is available on the sealing process, cf. for example S. Wemick, R. Pinner and P. G. Sheasby: The Surface Treatment and Finishing of Aluminium and its Alloys (Vol. 2, 5th Edition, Chapter 11: "Sealing Anodic Oxide Coatings", (ASM International, Metals Park, Ohio, USA and Finishing Publications LTD, Teddington, Middlesex, England, 1987).
In the sealing of anodizing layers, however, not only are the pores closed, a more or less thick velvet-like coating, the so-called sealing film, is formed over the entire surface. This film, which consists of hydrated aluminium oxide, is visually unattractive, reduces bond strength in the bonding of corres

REFERENCES:
patent: 4121980 (1978-10-01), Gohausen et al.
patent: 4225398 (1980-09-01), Hasegawa et al.
patent: 4939001 (1990-07-01), Brodalla et al.
patent: 5411607 (1995-05-01), Basaly et al.
Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., vol. A9 pp. 174-176 (1987).
Angew. Chem. Adv. Mater. 101(7):975-77 (1989).
The Surface Treatment and Finishing of Aluminum and its Alloys, vol. 2, 5th Ed., Chap.11, ASM International and Finishing Publications LTD pp. 773-856 (1987).

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