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 corrosion. As the subsequent removal of this sealing coat by hand using mechanical or chemical means is complex, attempts are made to prevent the formation of this sealing coat by the addition of chemicals to the sealing bath. According to DE-C-26 50 989, additions of cyclic polycarboxylic acids having from 4 to 6 carboxyl groups in the molecule, particularly cyclohexane hexacarboxylic acid, are suitable. According to DE-A-38 20 650 certain phosphonic acids, such as 1-phosphonopropane-1,2,3-tricarboxylic acid, may also be used. The use of other phosphonic acids is known from EP-A-122 129. DE-C-22 11 553 describes a process for sealing anodic oxide coatings on aluminum and aluminum alloys in aqueous solutions containing phosphonic acids or salts thereof and calcium ions, the molar ratio of calcium ions to phosphonic acid being at least 2:1. A higher ratio of calcium ions to phosphonic acid of about 5:1 to about 500:1 is preferably used. Examples of suitable phosphonic acids are: 1-hydroxypropane-, 1-hydroxybutane-, 1-hydroxypentane-, 1-hydroxyhexane-1,1-diphosphonic acid, as well as 1-hydroxy-1-phenylmethane-1,1-diphosphonic acid and preferably 1-hydroxyethane-1,1-diphosphonic acid, 1-aminoethane-, 1-amino-1-phenylmethane-, dimethylaminoethane-, dimethylaminobutane-, diethyl-aminomethane-, propyl- and butyl-aminomethane-1,1-diphosphonic acid, aminotrimethylene phosphonic acid, ethylene diamine tetramethylene phosphonic acid, diethylene triamine pentamethylene phosphonic acid, aminotri-(2-propylene-2-phosphonic acid), phosphonosuccinic acid, 1-phosphono-1-methylsuccinic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid. It is apparent that this disclosure concerns a conventional heat sealing process using sealing times between 60 and 70 minutes for anodic coating thicknesses between about 18 and about 22 .mu.m. The sealing time is therefore about three minutes per .mu.m of coating thickness.
When using water, containing no further additives other than the sealing smut inhibitors mentioned, high temperatures (at least 90.degree. C.) and relatively long treatment times of the order of about one hour have previously been required for effective sealing in the case of an anodic coating of about 20 .mu.m. This corresponds to a sealing time of about three minutes per micrometer of anodic coating thickness. The sealing process therefore consumes a great deal of energy and may represent a production bottleneck because of its duration. This is why attempts have already been made to find additives for the sealing bath which support the sealing process, so that it may proceed at lower temperatures (so-called "cold sealing") and/or using shorter treatment times. The following have been proposed as additives which permit sealing at temperatures below 90.degree. C., for example: nickel salts, particularly fluorides, which are used in practice to some extent (EP 171 799), nitrosylpentacyanoferrate, complex fluorides of titanium and zirconium, as well as chromates and/or chromic acid, optionally combined with further additives. As an alternative to true sealing, the hydrophobizing of the oxide coating by means of long-chain carboxylic acids or waxes has been recommended, as well as treatment with acrylamides, which should be polymerized in the pores. Further information may be found in the above-mentioned literature reference by S. Wernick et al. Apart from sealing using nickel compounds, these proposals have not been able to gain acceptance in practice.
Processes for cold sealing using nickel fluoride have been introduced on an industrial scale. Complex measures for waste water treatment are, however, required in this case because of the toxic properties of nickel salts.
There is, therefore, still a need for alternative sealing processes for anodized surfaces which enable the production rate to be increased by means of reduced sealing times and/or the energy required for the sealing to be reduced, without using heavy metals, such as nickel, which are questionable from the ecological and health standpoints. An object of the present invention is to provide such a process.