Short duration hot seal for anodized metal surfaces

Details Short duration hot seal for anodized metal surfaces Short duration hot seal for anodized metal surfaces

1998-04-20

1999-08-10

Cameron, Erma

Coating processes

With post-treatment of coating or coating material

Heating or drying

148250, 148252, 148272, 148274, 148275, 106 1412, 106 1413, 106 1415, 106 1441, 106 1442, 106 1444, B05D 714

Patent

active

059356561

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to the production of anti-corrosive and/or decorative coatings on metals by means of anodic oxidation. It relates to an improved process for sealing the electrochemically-produced porous anodic coatings in order further to improve the properties thereof.


TECHNICAL BACKGROUND AND RELATED ART

The electrochemical anodic oxidation of metals in suitable electrolytes is a widespread practice for forming anti-corrosive and/or decorative coatings on suitable metals. Such processes are briefly described, for example, on pp. 174-176 of Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. 9 (1987). According to this reference, titanium, magnesium and aluminum, as well as alloys thereof, may be anodized, the anodizing of aluminum and alloys thereof being the most important from a technical point of view. The electrolytically-produced anodic coatings protect the aluminum surfaces from the effects of weathering and other corrosive media. Anodic coatings are also applied in order to obtain a harder surface and therefore to make the aluminum more wear-resistant. Specialized decorative effects may be achieved by the natural color of the anodic coatings and/or by means of absorptive and/or electrolytic coloring. The aluminum is anodized in an acid electrolyte, sulphuric acid being the most common. Further suitable electrolytes include phosphoric acid, oxalic acid and chromic acid. The properties of the anodic coatings may be varied within wide limits by means of the selection of the electrolyte and the temperature thereof, as well as the current density and the anodizing time. Anodizing usually takes place using direct current or using direct current on which alternating current is superimposed.
The fresh anodic coatings may be subsequently colored by dipping in solutions of a suitable dye or by means of treatment with an alternating current in an electrolyte which contains a metal salt, preferably one which contains tin. As an alternative to subsequent coloring, colored anodic coatings may be obtained by means of so-called "color anodizing processes", which involve anodizing in solutions of organic acids, such as sulfophthalic acid or sulfanilic acid in particular, optionally mixed with sulfuric acid in each case, for example.
These anodically-produced protective coatings, the structure of which has been discussed in scientific papers (R. Kniep, P. Lamparter and S. Steeb: "Structure of Anodic Oxide Coatings on Aluminum", Angew. Chem. Adv. Mater. 101 (7), pp. 975-977 (1989)), are often called "oxide coatings". The above-mentioned paper has, however, shown that these coatings are glass-like and contain tetrahedrally-coordinated aluminum. Octahedrally-coordinated aluminum, as in the aluminum oxides, was not found. For the present purposes, therefore, the more general term "anodic coatings" will be used instead of the misleading term "oxide coatings".
However, these coatings do not yet meet all requirements with respect to corrosion protection because they still have a porous structure. This is why it is necessary to densify the anodic coatings. This densification is often undertaken using hot and/or boiling water or steam and is known as "sealing". It closes the pores and thus considerably increases the corrosion protection. There is extensive literature about this sealing process, examples of which are: S. Wernick, R. Pinner and P. G. Sheasby: The Surface Treatment and Finishing of Aluminum and its Alloys", Vol. 2, 5th edition, Chapter 11: "Sealing Anodic Oxide Coatings", (ASM International, Metals Park, Ohio, U.S.A., and Finishing Publications Ltd., Teddington, Middlesex, England, 1987).
When the anodic coatings are sealed, however, not only are the pores sealed, but a more or less thick velvet-like coat is formed on the entire surface, the so-called sealing "smut". This smut, which comprises hydrated aluminum oxide, is visually unattractive, reduces the adhesive strength when such aluminum components are bonded and promotes subsequent contamination and corrosio

REFERENCES:
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Huang, Diandu Yu Huanbao (1991), 11(1), pp. 15-20. Sealing of anodized aluminum.
Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., vol. 9., 174-176 (1987).
Angew. Chem. Adv. Mater 101(7): 975-977 (1989).
The Surface Treatment and Finishing of Aluminum and its Alloys, 5th ed., ASM International and Finishing Publications Ltd, vol. 2, Chap. 11, 773-856 (1987).

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