EP0186897B1 - Agent and process for the production of colourless sealing layers on anodized aluminium surfaces - Google Patents

Abstract

A process and composition for producing colorless, cold-sealed, anodized aluminum or aluminum alloy surfaces. The composition comprises an aqueous solution containing nickel ions and a solution of at least one azo or azo metal dye, the colors imparted by the nickel ions and dye offsetting each other.

Description

The invention relates to a solution and a method for producing colorless, compacted layers on anodized aluminum surfaces in the course of the so-called “cold compression”.

In contrast to the so-called "hot compression", in which the pores of anodized aluminum surfaces are closed by treatment with water, water vapor or metal salt solutions above 90 ° C and aluminum surfaces are made corrosion-resistant and resistant to other external influences, the technology generally means the so-called «Cold compression or« cold impregnation or «low-temperature compression processes with which the porosity of anodized aluminum surfaces is reduced at temperatures between 15 and 70 ° C and the surface properties are significantly improved. The main goal is improved corrosion protection compared to an undensified surface.

Even if the exact mechanisms of the actual compression process ("sealing") have not yet been elucidated in every detail, it can be said that in the near-surface area of the aluminum oxide layer, which forms on aluminum metal in contact with oxygen, through the pores Storage of alumina hydrates, e.g. B. boehmite can be closed. However, it is undesirable that such a mineral coating also forms on the surface of the oxide layer, since this coating is not grippy and the anodized aluminum parts have a spotty and faulty appearance. Methods for sealing metallic surfaces with the formation of additional inorganic protective layers, such as have been described, for example, in US Pat. No. 3,012,917, have therefore not been able to establish themselves in application technology.

Methods for the cold compression of anodized workpieces made of aluminum and their alloys are known from the prior art. So in Chem. Abstr. 87, 75493 t (1977) describes using solutions of various metal fluorides, for example CrF ₃ , MnF ₂ , CoF ₂ or NiF ₂ , for the treatment of anodized aluminum surfaces at room temperature to 50 ° C. Accordingly, JP-A-75117648 describes the sealing of anodized aluminum surfaces by immersion in a solution containing nickel fluoride and isoamyl alcohol at 30 ° C.

DE-A-3 301 507 also describes a method for cold-compacting workpieces made of aluminum and its alloys, in which solutions containing fluorine or fluoride, for. a. solutions containing nickel fluoride and / or cobalt fluoride can also be used. Likewise, DE-A-3411 678 discloses a process for the post-compression of aluminum and aluminum alloys after the anodization, in which aqueous solutions of at least one nickel salt are added nonionic surfactants which are capable of lowering the surface tension of the compression bath. Organosilicon compounds are preferably used for this purpose.

All of the described processes have in common that aqueous solutions of certain nickel salts are used. The nickel ions are incorporated into the surface layer when they come into contact with the freshly anodized aluminum surfaces, whereby - depending on the anions that are also incorporated - there is a more or less strong greenish discolouration of the aluminum surfaces, particularly when viewed from an angle or at an acute angle becomes visible. For workpieces that are used for decorative purposes, the greenish surface discoloration is very annoying because it changes the actual natural tone of the aluminum metal.

Methods for eliminating undesirable discoloration of anodized surfaces of certain aluminum alloys are described in DE-A-2 510 246. In the disclosed method, changes in the color of aluminum surfaces made from foreign metals of the aluminum alloys, e.g. B. copper result, eliminated by adding a monoazo dye to the baths at temperatures above 82 ° C. With this additive, however, only those discolorations can be eliminated which have arisen from alloy components of the aluminum during the anodization preceding the sealing. Metal salts, for example cobalt or nickel salts, can be added as treatment accelerators. However, adding accelerators makes color control difficult.

Monoazo dyes are mainly used in the aluminum industry to consciously color anodized aluminum surfaces. The dyes penetrate into the porous surfaces of anodized aluminum parts under adsorption, after which the colored layers are usually compacted by treatment in hot aqueous solutions. In addition, substances are added to the compression solutions that prevent the formation of sealing coatings (see: Aluminum 47, 245 (1971)). In such cases, it is often necessary to add small amounts of nickel salts, such as nickel acetate, or to pretreat the surfaces with solutions containing nickel salt in order to avoid "bleeding" of the dyes from the pores and thus an undesirable change in the hues achieved by organic dyes.

In cold compaction with solutions containing nickel salt, components of the solutions required for the compaction process cause a greenish discoloration of the surface, which is undesirable. The invention is therefore based on the object of providing a method for cold compression of anodized aluminum surfaces in which, despite the use of nickel salt aqueous solutions, colorless layers can be generated and the greenish discoloration caused by nickel ions in the surface layers can be avoided. Aqueous solutions of nickel fluoride absorb light in the wavelength ranges 350 to 450 nm and 650 to 850 nm. It has surprisingly been found that selected dyes with absorption maxima in the range from 450 to 600 nm, preferably in the range from 490 to 560 nm, together with aqueous solutions of Nickel salts enable the cold compression of anodized aluminum surfaces without the greenish discoloration of the surfaces being observed. The cold compression of aluminum surfaces with aqueous nickel salt solutions containing such dyes results in surfaces with the pure natural tone of aluminum.

The invention relates to an aqueous solution for producing colorless compression layers on anodized aluminum surfaces at temperatures in the range from 15 to 70 ° C. and pH values in the range from 5 to 7.5, the 1 to 5 g of nickel per liter of solution in the form of a water-soluble nickel salt , and optionally further organic and inorganic auxiliaries customary in the cold compaction of anodized aluminum surfaces, one or more organic dyes which have an absorption maximum in the range from 450 to 600 nm, an extinction coefficient of at least 1 - 10 ³ l · mol- '- cm-' have in a molecularly dispersed solution in the solution and do not undergo any precipitation reactions with nickel ions and any auxiliaries present in the solution.

The invention also relates to a process for producing colorless compression layers by treating anodized aluminum surfaces with nickel ions and optionally further aqueous solutions containing organic and / or inorganic auxiliaries customary in cold compression anodized aluminum surfaces at temperatures in the range from 15 to 70 ° C. and pH values in Range from 5 to 7.5, which is characterized in that one or more organic dyes are added to the compression solutions, which have an absorption maximum in the range from 450 to 600 nm, an extinction coefficient of at least 1 103 1 - mol ^-1 · cm ^{- 1,} have a molecular dispersion in the solution and do not undergo precipitation reactions with nickel ions and / or the other constituents of the solution.

• (a) Die verwendbaren Farbstoffe müssen ein Maximum der Lichtabsorption im sichtbaren Bereich bei Wellenlängen zwischen 450 und 600 nm aufweisen. Ein bevorzugter Absorptionsbereich der erfindungsgemäß verwendbaren Farbstoffe liegt bei 490 bis 560 nm. Zugabe von wässrigen Farbstofflösungen, die in Abwesenheit anderer farbgebender Substanzen eine rote Farbe aufweisen, zu wässrigen, grün gefärbten Lösungen wasserlöslicher Nickelsalze in geeigneter Konzentration führt dazu, daß eine Entfärbung beider Lösungen eintritt bzw. daß die Mischungen farblos erscheinen.
• (b) Die erfindungsgemäß verwendbaren organischen Farbstoffe müssen einen Extinktionskoeffizienten von mindestens 1 10³ 1 · ml^-1· cm-' haben. Bei gegebenem Nickelgehalt der Lösung liegt die Konzentration der erfindungsgemäß verwendbaren Farbstoffe in der Lösung im Bereich 0,5 bis 80 mg/l, ist jedoch direkt von der spezifischen Extinktion dieser Farbstoffe abhängig. Eine hohe Farbintensität - entsprechend einem hohen Extinktionskoeffizienten - im Bereich von vorzugsweise 5·10³ bis 5 . 10⁵ i · mol^-1 · cm-' führt zu den erfindungsgemäß bevorzugten niedrigen Einsatzkonzentrationen der Farbstoffe, die im Bereich 1,0 bis 10,0 mg, besonders bevorzugt bei 1 bis 2,9 mg/I Lösung liegen. Da Farbstoffe niedriger Farbintensität in entsprechend größeren Konzentrationen eingesetzt werden müssen und zu hohe Einsatzkonzentrationen die Qualität des Verdichtungsvorgangs negativ beeinflussen können, sind Farbstoffe mit niedrigen Extinktionskoeffizienten im Sinne der Erfindung nicht geeignet.
• (c) Um eine dauerhafte Entfärbung anodisierter Aluminiumoberflächen bei Anwendung des erfindungsgemäßen Verfahrens zu erreichen, müssen die Farbstoffmoleküle wie auch die Nickelionen in die Poren der anodisierten Aluminiumoberflächen eindringen. Bei gegebenem Porendurchmesser der Aluminiumoxidhydratschicht dürfen die Farbmoleküle eine bestimmte Molekülgröße nicht überschreiten. - Sie müssen dazu molekulardispers gelöst sein, mit anderen Worten : in Form einer echten Lösung vorliegen. Es muß somit gewährleistet sein, daß bei einer gegebenen Einsatzkonzentration an Nickelionen einerseits und an Farbstoffmolekülen die Nickelionen und Farbstoffmoleküle aus der Lösung in einem solchen Verhältnis in die Aluminiumoxidhydratschicht eingelagert werden, daß eine Absorption der Lichtenergie des gesamten sichtbaren Spektrums die Folge ist.
• (d) Wenn die Nickelkonzentration in der Lösung im Bereich von 1 bis 5 g/l Lösung, bevorzugt bei 1,4 bis 2,8 g/I Lösung und idealer Weise bei 2 g Nickel pro Liter Lösung liegt, darf der erfindungsgemäß zugesetzte Farbstoff keine Ausfällungen beispielsweise Umkomplexierungsreaktionen mit den Nickelsalzen eingehen. Der Farbstoff muß zwingend auch mit den anderen Bestandteilen der Lösung verträglich sein, da Folgereaktionen der Lösung eine unter Umständen notwendige Menge Farbstoff entziehen würden.

Dyes which are suitable for the agents according to the invention for producing colorless compression layers and can therefore be used in the proposed method can only be dyes which meet all of the following criteria (a) to (d).

• (a) The dyes that can be used must have a maximum of light absorption in the visible range at wavelengths between 450 and 600 nm. A preferred absorption range of the dyes which can be used according to the invention is 490 to 560 nm. Addition of aqueous dye solutions, which are red in the absence of other coloring substances, to aqueous, green-colored solutions of water-soluble nickel salts in a suitable concentration leads to decolorization of both solutions or that the mixtures appear colorless.
• (b) The organic dyes which can be used according to the invention must have an extinction coefficient of at least 1 10 ³ 1 · ml ^-1 · cm- '. For a given nickel content of the solution, the concentration of the dyes which can be used according to the invention in the solution is in the range from 0.5 to 80 mg / l, but is directly dependent on the specific extinction of these dyes. A high color intensity - corresponding to a high extinction coefficient - in the range of preferably 5 · 10 ³ to 5. 10 ⁵ i · mol ^-1 · cm- 'leads to the low use concentrations of the dyes preferred according to the invention, which are in the range from 1.0 to 10.0 mg, particularly preferably from 1 to 2.9 mg / l of solution. Since dyes of low color intensity have to be used in correspondingly larger concentrations and too high use concentrations can negatively influence the quality of the compression process, dyes with low extinction coefficients are not suitable for the purposes of the invention.
• (c) In order to achieve permanent decolorization of anodized aluminum surfaces when using the method according to the invention, the dye molecules as well as the nickel ions must penetrate into the pores of the anodized aluminum surfaces. For a given pore diameter of the aluminum oxide hydrate layer, the color molecules must not exceed a certain molecular size. - To do this, they must be molecularly dispersed, in other words, they must be in the form of a real solution. It must therefore be ensured that, for a given concentration of nickel ions on the one hand and on dye molecules, the nickel ions and dye molecules from the solution are incorporated into the aluminum oxide hydrate layer in such a ratio that the result is absorption of the light energy of the entire visible spectrum.
• (d) If the nickel concentration in the solution is in the range from 1 to 5 g / l solution, preferably 1.4 to 2.8 g / l solution and ideally 2 g nickel per liter solution, the dye added according to the invention may be used no precipitates, for example, undergo complexing reactions with the nickel salts. The dye must also be compatible with the other components of the solution, since subsequent reactions of the solution would remove a necessary amount of dye.

Of numerous dyes, surprisingly, in particular, selected azo or azo metal dyes proved to be suitable for fulfilling all of the criteria (a) to (d) mentioned. From the large group of azo or azo metal dyes, however, those are not suitable which either cannot diffuse into the pores of the aluminum oxide hydrate surface due to their molecular size or which result in the precipitation with the nickel ions of the solutions.

The use of the azo and azo metal dyes suitable according to the invention means that, for a given nickel concentration of, for example, 1 to 5 g of nickel / I on the one hand and concentrations of dye molecules of, for example, 1 to 10 mg of dye / I on the other hand, within a treatment time of 0.1 to 1 , 5 min per micron layer thickness, the nickel ions and dye molecules are embedded in the aluminum oxide hydrate layer in such a ratio that the result is absorption of the light energy of the entire visible spectrum.

Azo dyes which are sold by the company SANDOZ under the names Aluminum Red GLW or Aluminum Violet CLW were preferably used in the context of the present invention. Aluminum red GLW is an azo metal complex with copper and aluminum violet CLW is a purely organic azo dye. At high color intensity (extinction coefficient about 1. 10 ⁴ I - moH - cm- ¹ ), the dyes mentioned have an absorption maximum at 500 or 555 nm and, due to their molecular size (molecular weight approx. 800 to 1,000), diffuse easily into the pores of the anodized Aluminum surfaces. Using these dyes have proven to be expedient in the compression process at nickel concentrations in the range of approximately 2 g nickel / l dye concentrations of approximately 2.5 mg dye / l, the treatment solution containing all components appearing colorless.

In contrast to several other dyes from the class of azo and azo metal dyes (aluminum red RLW, aluminum copper, aluminum bordeaux RL, aluminum fire red ML), the dyes mentioned aluminum red GLW and aluminum violet CLW do not precipitate through metal complexes, but remain molecularly dispersed in the water even after a long time Solutions solved.

Aqueous solutions of nickel salts, which are prepared by dissolving in particular NiF ₂ - 4 H ₂ O or other nickel salts such as nickel sulfate or nickel acetate, and adding appropriate amounts of alkali metal fluorides, are used for the compression. The solutions according to the invention can optionally contain further organic and / or inorganic auxiliaries customary in cold compression anodized aluminum surfaces. Surfactants and / or organic compounds such as alcohols, amines, ketones and / or ethers and / or organosilicon compounds and / or fluorides of different metals and / or salts with complex anions are suitable as such. However, these solutions are not essential and the anodized aluminum surfaces are colorlessly compacted even in the absence of such customary auxiliaries.

Within the scope of the method according to the invention, it is fundamentally possible to treat anodized surfaces made of aluminum or its alloys primarily with an aqueous solution of the dyes which can be used according to the invention and then to compact them with an aqueous nickel salt solution using the cold method. The reddish coloration of the aluminum oxide surface obtained in the first process step compensates for the subsequent greenish coloration resulting from the sealing process, so that aluminum oxide surfaces are formed with the natural color of the aluminum. However, such a dye rinsing bath must always be operated with a water overflow, which makes it difficult to maintain a certain dye concentration and results in large dye losses. In addition, anodizing lines that are commonly used for the fully automatic treatment of surfaces made of aluminum and its alloys have no space for the installation of a separate pre-dye bath.

Conversely, it is also possible in principle to compact anodized aluminum surfaces in a first process step with aqueous solutions containing nickel salt and only in the subsequent process step to compensate for the greenish surface color by incorporating the dye molecules into the pores by incorporating the dye molecules. Apart from the fact that in this case too, the conventional anodizing lines do not offer any space for setting up a separate dye bath, this procedure has the disadvantage that the dye molecules can penetrate the pores, which have already been partially closed by the sealing process, much more poorly, and thus completely Compensation of the greenish discoloration is no longer guaranteed by the inclusion of nickel ions. Above all, a permanent compensation of the greenish discoloration is difficult because the only superficially applied dyes can be split off or faded by other environmental influences.

An integrated process in which coloring and sealing take place simultaneously is therefore preferably proposed according to the invention. For this purpose, aqueous solutions are prepared which contain 1 to 5 g of nickel per liter of solution in the form of a water-soluble nickel salt, 0.5 to 80 mg per liter of solution of one or more organic dyes which meet the above-mentioned criteria (a) to (d) must and, if necessary, contain further organic and / or inorganic auxiliaries customary in the cold compaction of anodized aluminum surfaces. The solutions preferably have a dye content of 1 to 10 mg, particularly preferably 1 to 2.9 mg of dye per liter of solution, this amount of dye being dependent on the one hand on the nickel concentration and on the other hand on the color intensity of the dye used.

In a further preferred embodiment of the method according to the invention Treatment solutions are used in which the coloring of the nickel ions is compensated for by the red coloring caused by the dye molecules and which thus appear colorless. Dyes with an extinction coefficient in the range from 5. 103 to 5 · 10 ⁵ l · ml ^-1 · cm- 'with an absorption maximum in the range from 490 to 560 nm decolorize in a concentration of 1 to 10 mg / l solutions containing 1 to Contain 5 g / l nickel.

At treatment temperatures in the range from 15 to 70 ° C., preferably 20 to 40 ° C., particularly preferably at 25 to 32 ° C. and pH values in the range from 5.0 to 7.5, preferably in the range from 5.5 to 7 , 0 and ideally at a pH of 6.5, anodized surfaces made of aluminum or its alloys are treated with such dye solutions. This takes place in that the corresponding aluminum parts are immersed in the solutions containing the components according to the invention for a time of 0.1 to 1.5 min / μm layer thickness, preferably for a time of 0.4 to 1.2 min / μm layer thickness . It is then conveniently rinsed with demineralized water.

In the course of the treatment, nickel ions and dye molecules are embedded in the pores of the aluminum oxide surface; consequently, the solution becomes poor in these components, so that the concentration of the solution components must be checked continuously. This can be monitored on the one hand by complexometric titration of the nickel content of the solutions, and on the other hand by spectrophotometric monitoring of the extinctions of the solutions at the characteristic absorption wavelengths for nickel (395 and 720 nm) and the dyes used (500 and 555 nm). A steady decrease in the concentration of both coloring components indicates that both components are embedded in the pores of the aluminum oxide surfaces and that there is mutual color compensation. A constant value for the dye content in the solutions indicates that the dye molecules are not stored. As a result, colorless aluminum surfaces with a natural metal color result in the former case, while in the second case the surfaces are colored greenish.

In a preferred embodiment of the process according to the invention, the solutions are re-sharpened in accordance with the consumption of their components, so that continuous process control is possible. By adding the bath components in solid form or in the form of suitable supplementary solutions, both the nickel content and the dye content are set to a constant value and the exact observance of these and the other bath parameters is continuously monitored.

When using the agents according to the invention, colorless compaction layers of anodized surfaces of aluminum and its alloys are produced which have no discoloration. Due to the natural tone of the aluminum metal, surfaces treated in this way are ideally suited for decorative purposes. In addition, the improved process does not affect corrosion protection in any way. The method according to the invention therefore also makes it possible to produce naturally colored aluminum surfaces for decorative purposes by means of cold compression.

The invention is illustrated by the following examples.

• Stromdichte : 1,5 A· dm-² ;
• Temperatur: 20 °C ;
• Schichtstärke : 20 µm und
• Schwefelsäuregehalt : 180 g · 1-¹.

For the following examples, sheets of alloy AIMg 3 (DIN material No. 3.3535) were degreased, rinsed, pickled, rinsed and anodized in the GS process in compliance with the following process parameters:

• Current density: 1.5 A · dm- ² ;
• Temperature: 20 ° C;
• Layer thickness: 20 µm and
• Sulfuric acid content: 180 g · 1- ^first

Then it was rinsed with deionized water.

In the process of actually compacting the aluminum oxide surfaces, aqueous solutions were used which had the composition mentioned in the individual examples. The pH was between 5.5 and 6.5 and was corrected with acetic acid if necessary. The treatment temperature was 28 to 32 ° C, the treatment time was 0.5 min / µm layer thickness.

The nickel content of the solutions was monitored by complexometric titration. In addition, the solution was photometrized in cuvettes with a layer thickness of 1 cm in a spectrophotometer. The extinctions (cm- ') at the characteristic absorption wavelength (Ni: 395 and 720 nm; dyes: 500 and 555 nm) are directly dependent on the respective concentration and can therefore be correlated.

example 1

• 7,0 g NiF₂ · 4 H₂0 pro Liter Lösung und
• 3,0 mg Aluminiumviolett CLW pro Liter Lösung.

The starting solution contained

• 7.0 g NiF ₂ · 4 H ₂ 0 per liter solution and
• 3.0 mg aluminum violet CLW per liter of solution.

The pH was 5.8.

The change in the values for the concentrations of the coloring bath components were tracked complexometrically and photometrically; the result is shown in Table 1 below.

Result

With increasing throughput of anodized aluminum surface, a decrease in the absorptions characteristic of nickel or the dye as well as a decrease in the nickel content determined by complexometry was observed. Nickel ions and dye were thus simultaneously embedded in the pores of the aluminum oxide hydrate layer.

Sheets were obtained which showed no discoloration but only had the natural metallic sheen.

Comparative Example 1

Under the same conditions as in Example 1, a solution was used for densification which contained only 7.0 g of NiF ₂ .4H ₂ 0 per liter of solution, ie no dye. A comparable drop in the absorption typical of nickel was observed, but surfaces with greenish discoloration resulted.

Comparative Example 2

• 7,0 g NiF₂ - 4 H₂0 pro Liter Lösung und
• 5,0 mg Aluminiumkupfer pro Liter Lösung.

The starting solution contained

• 7.0 g NiF ₂ - 4 H ₂ 0 per liter solution and
• 5.0 mg aluminum copper per liter of solution.

The pH was 5.8.

ErgebnisAfter allowing the solution to stand for a long time, it was observed that a colloidally disperse solution was formed and that part of the dye precipitated out of the solution. The results for the spectrophotometric and complexometric determination of the nickel or dye content can be found in Table 1 (a) below.

Result

While a gradual decrease in the absorption value for nickel was found, the absorption value for the dye remained almost constant at 505 nm. A joint incorporation of dye and nickel ions into the pores of the aluminum oxide hydrate surface therefore did not take place. The surfaces obtained consequently also showed the usual greenish discoloration.

Example 2

• 7,0 g NiF₂ · 4 H₂O und
• 5,0 mg Aluminiumrot GLW pro Liter Lösung.

The starting solution contained

• 7.0 g NiF ₂ .4 H ₂ O and
• 5.0 mg aluminum red GLW per liter of solution.

The pH was 5.8.

The consumption of both components of the solution was determined spectrophotometrically. The results are shown in Table 2 below.

Result

Both components diffused into the pores of the alumina hydrate surface. As a result, colorless surfaces with natural aluminum luster were obtained.

Example 3

• 5,5 g NiF₂ · 4 H₂0 pro Liter Lösung,
• 1,0 mg Aluminiumrot GLW pro Liter Lösung und
• 2 mg Aluminiumviolett CLW pro Liter Lösung.

The starting solution contained

• 5.5 g NiF ₂ · 4 H ₂ 0 per liter solution,
• 1.0 mg aluminum red GLW per liter of solution and
• 2 mg aluminum violet CLW per liter of solution.

The results of the spectrophotometric absorbance measurements are shown in Table 3.

Result

The decrease in all four extinction values indicates a simultaneous incorporation of nickel ions and dye molecules into the pores of the surface layers. As a result, colorless alumina hydrate surfaces with a natural metallic luster are obtained.

Example 4

• 5,5 g NiF₂ · 4 H₂0 pro Liter Lösung,
• 1.25 mg Aluminiumrot GLW und
• 1,25 mg Aluminiumviolett CLW pro Liter Lösung.

The starting solution contained

• 5.5 g NiF ₂ · 4 H ₂ 0 per liter solution,
• 1.25 mg aluminum red GLW and
• 1.25 mg aluminum violet CLW per liter of solution.

• 32.7 g NiF₂ - 4 H₂0,
• 7,5 mg Aluminiumrot GLW und
• 7,5 mg Aluminiumviolett CLW pro Liter Zusatzlösung enthielt.

Depending on the nickel content, a solution was added to supplement it

• 32.7 g NiF ₂ - 4 H ₂ 0,
• 7.5 mg aluminum red GLW and
• Contained 7.5 mg of aluminum violet CLW per liter of additional solution.

ErgebnisThe spectrophotometric and complexometric values for the concentration of nickel and dyes can be found in Table 4 below.

Result

By adding the supplementary solution, the content of nickel ions was kept almost constant, while the dye concentration was still subject to wide fluctuations. However, nickel and dyes were incorporated into the pores of the aluminum oxide hydrate surfaces and consequently colorless surfaces with a natural metallic luster were obtained.

Example 5

• 5.7 g NiF₂ · 4 H₂0,
• 1,25 mg Aluminiumrot GLW und
• 1,25 mg Aluminiumviolett CLW pro Liter lösung

The starting solution contained

• 5.7 g NiF ₂ · 4 H ₂ 0,
• 1.25 mg aluminum red GLW and
• 1.25 mg aluminum violet CLW per liter of solution

• 40,2 g Nickel pro Liter Lösung und je
• 26,8 mg der in Beispiel 4 genannten Farbstoffe pro Liter Lösung enthielt.

Depending on the nickel content, a supplementary solution was metered in

• 40.2 g nickel per liter solution and each
• Contained 26.8 mg of the dyes mentioned in Example 4 per liter of solution.

The results of the spectrophotometric and complexometric determination of the concentrations on nickel and dye are shown in Table 5.

Result .

By adding the supplementary solution in suitable quantities, the nickel ion and dye content could be adjusted to almost constant values. The present supplementary solution proved to be the most suitable for the given experimental setup.

Colorless surfaces with a natural metallic luster were obtained.

Examples 6 and 7

• 5,7 g NiF₂ · 4H₂0,
• 1,25 mg Aluminiumrot GLW und
• 1,25 mg Aluminiumviolett CLW pro Liter Lösung.

The starting solution contained

• 5.7 g NiF ₂ · 4H ₂ 0,
• 1.25 mg aluminum red GLW and
• 1.25 mg aluminum violet CLW per liter of solution.

• 30 g Nickel
• 18,75 mg der beiden genannten Farbstoffe pro Liter Lösung.

The supplementary solution contained

• 30 g nickel
• 18.75 mg of the two dyes mentioned per liter of solution.

Sheets with layer thicknesses of 20 µm (Example 6) and 5 µm (Example 7) were compressed.

The results of the spectrometric and complexometric concentration determinations can be found in Tables 6 and 7 below.

Result

Regardless of the layer thickness, the content of nickel ions and dye molecules could be kept almost constant by re-sharpening the solution with the supplementary solutions mentioned. To the extent necessary, both coloring components were built into the pores of the aluminum oxide hydrate surface. This resulted in undyed layers with a natural metallic sheen.

Example 8

A cold impregnation solution containing 2 g / l of nickel and 1.4 g / l of fluoride was prepared in an 18 m ³ bath container. 1.25 mg / l aluminum red GLW and 1.25 mg / l aluminum violet CLW were added so that the solution appeared colorless when visually assessed. Over a first test period of 8 weeks, a total of 11,500 m ^{2 of} anodized aluminum parts with layer thicknesses of the oxide layer between 2 and 25 µm and different anodization conditions were treated in this bath.

The nickel content was determined by complexometric titration. The dye content was checked photometrically. If necessary to maintain the nickel concentration at 2 g / l, a nickel salt solution, which also contained the above-mentioned dyes, was added. This solution contained nickel and dye (50% aluminum red GLW and 50% aluminum violet) in a weight ratio of 1: 0.0015. The total consumption was 12.3 kg Ni and 18 g dye mixture. All parts treated in this way could be colorless in this bath, i. H. without greenish discoloration. The solution also remained colorless when viewed visually. (In one case, a slight drop in the dye concentration could be measured photometrically because the introduction of hard water led to precipitation of calcium fluoride and the precipitated calcium fluoride adsorbed parts of the dye. Due to the absorbance measurement, the missing amount of dye of 7 g could be easily supplemented ).

Claims (20)

1. A process for forming colourless sealing layers on anodized aluminium surfaces by treatment of the surfaces with aqueous solutions containing nickel ions and, optionally, other organic and/or inorganic auxiliaries typically used in the cold sealing of anodized aluminium surfaces at temperatures in the range from 15 to 70 °C and at pH values in the range from 5 to 7.5, characterized in that the aluminium surfaces are treated with an aqueous solution of one or more organic dyes which

a) show an absorption maximum in the range from 450 to 600 nm,

b) have an extinction coefficient of at least 1 x 10³ I × mol-¹ x cm-¹,

c) are present dissolved in molecular dispersion in the solution and

d) do not enter into any precipitation reactions with nickel ions and/oder the other constituents of the solution.

2. A process as claimed in claim 1, characterized in that one or more organic dyes having an absorption maximum in the range from 490 to 560 nm is/are added to the solutions.

3. A process as claimed in claims 1 and 2, characterized in that one or more azo or azo metal dyes is/are added to the solutions.

4. A process as claimed in claims 1 to 3, characterized in that aluminium red GLW and/or aluminium violet CLW is/are added to the solutions.

5. A process as claimed in claims 1 to 4, characterized in that the dye concentration in the solutions is adjusted to a value of from 0.5 to 80 mg of one or more dyes having an extinction coefficient of at least 1 x 10³ 1 x mol^-1 x cm^-1 per litre solution.

6. A process as claimed in claims 1 to 5, characterized in that the dye concentration in the solutions is adjusted to a value of from 1 to 10 mg of one or more dyes having an extinction coefficient of from 5 x 10³ to 5 x 10⁵ 1 x mol-' x cm^-1 per litre solution.

7. A process as claimed in claims 1 to 6, characterized in that the dye concentration in the solutions is adjusted to a value of from 1 to 2.9 mg of one or more dyes having an extinction coefficient of 1 x 10" I x mol-' x cm-' per litre solution.

8. A process as claimed in claims 1 to 7, characterized in that the anodized aluminium surfaces are integrally cold-sealed with aqueous solutions containing nickel, ions and one or more organic dyes and, optionally, other typical organic and/or inorganic auxiliaries.

9. A process as claimed in claims 1 to 8, characterized in that the anodized aluminium surfaces are integrally cold-sealed with aqueous solutions containing nickel ions and one or more organic dyes and. optionally, other typical organic and/or inorganic auxiliaries, the greenish colouration produced by the nickel ions in the solutions used being compensated by the reddish colouration produced by the dye molecules and the solutions appearing substantially colourless.

10. A process as claimed in claims 1 to 9, characterized in that it is carried out at temperatures of from 20 to 40 °C and preferably at temperatures of from 25 to 32 °C.

11. A process as claimed in claims 1 to 10, characterized in that the pH value is adjusted to between 5.5 and 7.0 and preferably to 6.5.

12. An aqueous solution for forming colourless sealing layers on anodized aluminium surfaces at temperatures of from 15 to 700C and at pH values of from 5 to 7.5 containing

a) 1 to 5 g nickel per litre solution of a water-soluble nickel salt and, optionally, other organic and inorganic auxiliaries typically used in the cold sealing of anodized aluminium surfaces,

b) one or more organic dyes having an absorption maximum in the range from 450 to 600 nm and an extinction coefficient of at least 1 x 10³ I x mol-¹ x cm-¹, being present dissolved in molecular dispersion in the solution and not entering into any precipitation reactions with nickel ions and the auxiliaries optionally present.

13. A solution as claimed in claim 12, characterized in that it contains 1 to 5 g nickel per litre solution in the form of NiF₂ x 4H₂0.

14. A solution as claimed in claims 12 and 13, characterized in that it contains one or more dyes having an absorption maximum in the range from 490 to 560 nm.

15. A solution as claimed in claims 12 to 14, characterized in that it contains one or more azo or azo metal dyes.

16. A process as claimed in claims 12 to 15, characterized in that it contains aluminium red CLW and/or aluminium violet CLW as dyes.

17. A solution as claimed in claims 12 to 16, characterized in that contains 0.5 to 80 mg of one or more dyes having an extinction coefficient in the range from 5 x 10³ to 5 x 10⁵ I x mol^-1 x cm^-1 per litre solution.

18. A solution as claimed in claims 12 to 17, characterized in that it contains 1 to 2.9 mg of one or more dyes having an extinction coefficient of 1 x 10⁴ I x mol-¹ x cm-¹ per litre solution.

19. A solution as claimed in claims 12 to 18, characterized in that it contains nickel ions and one or more organic dyes and, optionally, other typical organic and/or inorganic auxiliaries in such concentrations that the greenish colouration produced by the nickel ions is compensated by the reddish colouration produced by dye molecules and the solutions appear colourless.

20. A solution as claimed in claims 12 to 19, characterized in that it has a pH value of from 5.5 to 7.0 and preferably of 6.5.