EP0293774B1 - Process for electrolytic coloring of anodised aluminium - Google Patents

Abstract

A process for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys using alternating current or direct current superimposed on alternating current, the electrolytic coloring being carried out with an electrolyte which contains cationic organic dyes and, optionally, conducting salts.

Description

The invention relates to a method for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys using alternating current or alternating current superimposed direct current, the electrolytic coloring being carried out with an electrolyte containing cationic organic dyes.

To increase the corrosion resistance and to achieve decorative effects, the surface of the aluminum and its alloys can be changed mechanically or provided with metallic or non-metallic coatings. The enhancement of the natural protective oxide film by chemical or electrical processes is of great importance.

In the processes for coloring surfaces of aluminum or aluminum alloys, a distinction is made according to the prior art, his Wernick, Pinner, Zurbrügg, Weiner "The surface treatment of aluminum", Leuze Verlag, Saulgau / Württ. (1977), pages 354 to 374 and 309 to 312, the adsorptive coloring, the color anodizing and the electrolytic coloring.

In the case of adsorptive coloring, for example, an organic dye is introduced into the openings in the pores of the oxide layer, and this remains adsorbed in the surface region of the surface. The entire color spectrum can be obtained with great uniformity and reproducibility by the process mentioned. Various dyes that can be used for this are commercially available.

Furthermore, the so-called color anodization (integral process) has been in use for years. In an integrally colored film, the finely divided inorganic color particles are not in the pores of the oxide layer, but remain as an alloy component in the aluminum oxide layer. Here, special aluminum alloys are mostly anodized and colored using DC voltages up to 150 V in only one process step, suitable organic acids, e.g. Maleic acid, oxalic acid, sulfosalicylic acid or sulfophthalic acid can be used. However, for cost reasons (high power consumption, complex cooling devices), the integral method is used less and less in practice.

In the electrolytic coloring using metal salt solutions, however, anodic oxidation using direct current in aqueous sulfuric acid and / or other electrolytic solutions first produces a colorless, transparent oxide layer, the coloring of which is then carried out in a second process step - in contrast to adsorptive coloring - by deposition of metal particles from metal salt solutions on The oxide layer pores are made by means of alternating current. The color shades range from light bronze to dark bronze to black. The storage on the pore base gives completely lightfast stains (W. Sautter, Metallfläche, 32 , 1978, pages 450 to 454).

The electrolytic coloring processes are mainly used for coloring aluminum - which is to be used in architecture - due to its advantages such as higher light resistance and weather resistance. In the electrolytic coloring process, because of lower costs and therefore greater economy, the electrolytic metal salt coloring clearly outweighs the integral coloring, with Sn (II), Co, Ni and Cu-containing electrolyte solutions preferably being used for this purpose.

DE-OS 28 50 136 describes a process for the electrolytic metal salt coloring of aluminum, in which a defined oxide layer is first generated by means of direct current in acidic solution and this is then colored by means of alternating current using an acidic electrolyte containing tin (II) salts, the Electrolyte also contains stabilizers for the tin (II) salts. Such coloring electrolytes containing metal salts are, however, unsuitable for producing any color and brightness on the aluminum and aluminum alloy surfaces.

DE-PS 32 48 472 describes a method for coloring anodically produced oxide layers on aluminum and aluminum alloys, in which a coloring electrolyte is used, with different colors Colorfulness and brightness, in particular for use in profiles for windows, doors, facade elements and the like, can be produced on anodized aluminum surfaces. In order to be able to produce such colorations economically and reproducibly at any time, even with different color shades, the coloring electrolyte contains an organic dye component in addition to a metal salt. A metal complex-containing azo dye is proposed as the organic dye component. DE-OS 32 48 472 thus describes a process for coloring anodically produced oxide layers in an electrolyte containing metal salts with simultaneous adsorptive coloring with a metal complex-containing azo dye.

However, the dyeing processes described above are not completely satisfactory in terms of application technology: the electrolytic dyeing processes - both the integral process and the metal salt dyeing - do not produce bright colors, but rather only gray or bronze to black shades. A wide range of bright colors can be achieved using adsorptive processes; however, the dyes are only adsorbed in the upper pore area. Such stains are therefore not resistant to abrasion: the surface is attacked by mechanical loads, ie the dyes are removed and the stain is thus removed. Since such loads usually occur in a locally irregular manner, the scratches, stains, discolorations and the like produced in this way are particularly noticeable. The usability of such colored aluminum parts is therefore greatly impaired. Also for aluminum facades Such surface coloring is unsuitable because cleaning them with agents that usually contain abrasives leads to fading.

The object of the present invention is therefore to provide an improved method for the electrolytic coloring of anodic surfaces of aluminum or aluminum alloys using alternating current or alternating current superimposed direct current, which does not have the above disadvantages.

The object of the present invention is achieved in that the electrolytic coloring is carried out using an electrolyte containing cationic organic dyes.

The present invention accordingly relates to a process for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys using alternating current or alternating current superimposed on direct current, the electrolytic coloring being carried out with an aqueous electrolyte containing cationic organic dyes, which optionally also contains conductive salts.

The advantage of the method for electrolytic coloring according to the invention compared to adsorptive coloring is that the cationic organic dyes penetrate to the bottom of the pores of the oxide layer during electrolytic coloring, whereby a better protection of the dyes against abrasion and corrosion is proven. As a result of this deep deposit in the bottom of the pores, it succeeds in being more economical How to create extremely abrasion-resistant, colorful shades on anodized aluminum.

The electrosorptive coloring of organic dyes known in the prior art has so far only made it possible to obtain so-called achromatic colors, such as gray tones, on anodized aluminum. In contrast, the method according to the invention enables the generation of a large variety of colors with a simultaneously high penetration depth.

In principle, all cationic organic dyes can be used in the process according to the invention. Examples of these are dyes from the groups of triphenylmethane dyes, cyanine dyes, xanthene dyes (xanthene dyes of the rhodamine group), acridine dyes, azine dyes, thiazine dyes or pyrylium dyes. Of these, dyes from the groups of triphenylmethane dyes, xanthene dyes and azine dyes are particularly preferred for the purposes of the process according to the invention. Examples of representatives from these preferred groups of the cationic dyes are: crystal violet, malachite green, methyl violet, rhodamine 6G, methylene blue. Such dyes can be used both individually and in the form of mixtures in the process according to the invention.

Due to the positive charge of the cationic organic dyes, these are deposited on the pore base during the negative half-wave of the alternating current.

In general, the cationic organic dyes can have all possible anions, provided that these have no disruptive influence on the electrolytic deposition of the cationic organic dyes. In this sense, when choosing the anion, it is of course important to note that the dye salt is soluble in water. In principle, the anions for the dye cations are the anions of the mineral and carboxylic acids, for example chloride, sulfate, perchlorate, acetate, tetrafluoroborate or oxalate. Preferred anions for the cationic organic dyes for the purposes of the invention are: chlorides, perchlorates and / or oxalates. Some of these dye salts are commercially available or their preparation is familiar to the person skilled in the art.

The process according to the invention is carried out in the voltage and current density ranges customary in the prior art, which are usually used for electrolytic metal salt coloring. As a rule, the method according to the invention is carried out at a voltage in the range from 8 to 30 V, which is dependent on the electrode spacing, and at the current densities which arise under these conditions. The frequency of the alternating current is usually 50 to 60 Hz. Stainless steel is usually used as the material for the counterelectrode, but other materials, for example graphite, can also be used for this purpose.
If there is talk of direct current superimposed on alternating current in connection with the method according to the invention, this is understood to mean an asymmetrical alternating current whose amplitude levels of the positive or negative half-waves have different values. Corresponding circuits for generating such alternating current superimposed direct currents are known to the person skilled in the art from the relevant prior art. In this connection, however, reference is made to DE-A-3 718 741.

According to a preferred embodiment of the present invention, the method is carried out at a voltage of 10 to 22 V and the resulting current density.

Electrolytic coloring according to the invention is carried out in aqueous solutions. Accordingly, the upper limit of the concentration of the cationic dye in the aqueous electrolyte solution is determined by the upper solubility limit of the respective dye in water. With regard to the lower concentration limit of the dye, it should be noted that an excessively low concentration of the dye in the electrolyte does not permit economical operation in the sense of the process according to the invention. According to the invention, the concentration of the cationic dyes in the electrolyte solution is therefore in the range from 0.01 g / l to the upper solubility limit of the respective dye. In general, the aqueous electrolyte solutions in the process according to the invention contain cationic dyes in concentrations of 0.01 to 10 g / l; the concentrations are preferably in the range from 0.05 to 5 g / l.

In addition to the cationic organic dyes, the electrolyte solution used in the process according to the invention can contain conductive salts in order to increase the conductivity of the solutions. Corresponding conductive salts are known to those skilled in the relevant State of the art known; for example, they can be selected from the group of the water-soluble alkali metal, ammonium and / or alkaline earth metal salts of those acids which also form the anion of the cationic dyes. For the purposes of the present invention, sulfates, preferably sodium sulfate or magnesium sulfate, are generally used as conductive salts. According to the invention, the concentration of the conductive salts in the aqueous electrolyte solutions is generally in the range from 1 to 50 g / l; a concentration range of 5 to 20 g / l is preferred.
The addition of such conductive salts can, in individual cases, lead to a more intense color tint of the color obtained. The person skilled in the art will therefore decide in individual cases - ie depending on the dye used and on the type and intensity of the desired coloring - whether such an addition is desired.

Further - also non-critical - influencing variables of the process according to the invention are the pH and the temperature of the electrolyte solution and the residence time of the material to be colored in it.
With regard to the pH of the electrolyte solution, it is generally the case that the pH value that is optimal for the respective dye is that which occurs in the aqueous electrolyte solution when this dye is dissolved, in the concentration range indicated. In addition, however, it is possible in principle to work at other pH values of the electrolyte solution. For example, the pH of the electrolyte solutions in the process according to the invention is generally in the range from 1 to 9; with regard to what was said first, the pH range from 2 to 5 is preferred, provided, however, that a pH adjustment of the aqueous If an electrolyte solution is desired, acids or alkalis are used for this purpose, which do not have a disruptive effect on the electrolytic deposition of the cationic dyes, for example dilute aqueous sulfuric acid or sodium hydroxide solution.

Regarding the temperature of the electrolytic solution, it is preferable to - at least in view of the associated energy saving - at room temperature, i.e. in a temperature range of approx. 15 to 25 ° C. In individual cases, i.e. again depending on the chosen dye, it may be advisable to choose higher temperatures - for example up to approx. 60 ° C - in order to support the diffusion of the dye molecules and thus to achieve a more uniform coloring.

The residence time of the material to be colored in the electrolyte solution depends primarily on the desired depth of color of the coloring. No generally applicable, binding guideline values can be given for this, rather the optimal dwell time must be tried out on a case-by-case basis. However, dwell times of approximately 15 to 30 minutes may be mentioned here as examples.
The parameters temperature and residence time discussed last serve in particular to optimize the desired coloring and, in individual cases, require a few preliminary tests to be carried out.

According to a preferred embodiment of the method according to the invention, the material to be colored; ie the anodized workpieces made of aluminum or aluminum alloys, before the actual coloring treatment with the use of alternating current or alternating current superimposed direct current - in the same electrolyte - first subjected to treatment with direct current. Here, the workpiece or the workpieces is switched as an anode. The voltage of the direct current during this treatment is in the range mentioned above; the statements made above also apply to the other parameters. The actual dyeing process does not yet take place during this pretreatment; rather, this pretreatment requires an increased uniformity of the subsequent coloring and a better depth dispersion of the same. Further details on such a pretreatment by means of direct current are described in DE-OS 26 09 146.

By using several successive treatments according to the method according to the invention, the most varied color shades of the aluminum oxide layers can be achieved by specifically coordinating the influencing variables of the individual treatments.

Before the electrolytic coloring of the anodized surfaces according to the invention, the objects made from aluminum or its alloys are subjected to a customary pretreatment for producing the oxidic surface layer. In this first treatment stage, the condition of the semi-finished products to be anodized, ie the degree of gloss or mattness of the surfaces, as well as the electrolyte composition and the working conditions during the anodizing process are important influencing factors. The conditions known to the person skilled in the art from the relevant prior art, for example the monograph cited at the beginning by Wernick, Pinner, Zurbrügg and Weiner, apply here.

The exemplary embodiments mentioned below serve to explain the invention, but without restricting the present invention to the details mentioned in the exemplary embodiments.

Examples Pretreatment:

For the following examples, test sheets (dimension 50 mm x 40 mm x 1 mm) made of Al 99.5 (DIN material No. 3.0255) were used.

Before the anodizing, the sheets were degreased, pickled and pickled using conventional methods. Degreasing was carried out using an alkaline cleaner containing borates, carbonates, phosphates and nonionic surfactants (P3-almeco® 18, from Henkel KGaA, Düsseldorf); Bath concentration: 5% by weight, temperature: 70 ° C, immersion time: 15 minutes. For the pickling, a mixture (3: 1) of NaOH and a pickling agent containing alkali, alcohols and salts of inorganic acids (P3-almeco® 46, from Henkel KGaA, Düsseldorf) was used; Bath concentration: 8% by weight, temperature: 55 ° C, immersion time: 10 minutes. The pickling was carried out using an acidic pickling agent containing salts of inorganic acids and inorganic acids (P3-almeco® 90, Henkel KGaA, Düsseldorf), bath concentration: 15% by weight, temperature 20 ° C., immersion time: 10 minutes. After each process step, the sheets were thoroughly rinsed with deionized water.

The subsequent anodization was carried out using the direct current sulfuric acid method; Bath composition: 200 g / l H₂SO₄, 10 g / l Al; Air injection: 8 m³ / m².h; Temperature: 18 ° C; DC voltage: 15 V. The anodizing times were about 3 minutes per µm layer build-up; ie the total anodizing times for the oxide layer thicknesses of 15 to 25 μm given in the examples below were between 45 and 75 minutes.

After thorough rinsing again with deionized water, the electrolytic dyeing treatment according to the invention was carried out (details below). The sheets were then rinsed again and then compacted in hot water with the addition of a sealing deposit inhibitor based on salts of organic acids and nonionic surfactants (P3-almecoseal® SL, from Henkel KGaA, Düsseldorf); Bath temperature: 98 to 100 ° C, immersion time: 60 minutes, concentration of the sealing deposit inhibitor: 0.2% by weight.

Examples 1a to 1f

• 1a: Rhodamin 6G, als Perchlorat (Xanthenfarbstoff)
• 1b: Kristallviolett, als Chlorid (Triphenylmethanfarbstoff)
• 1c: Malachitgrün, als Oxalat (Triphenylmethanfarbstoff)
• 1d+e: Methylviolett, als Chlorid (Triphenylmethanfarbstoff)
• 1f: Methylenblau, als Chlorid (Azinfarbstoff).

In the examples below, the cationic dyes used and the thickness of the oxide layers were varied.
The following cationic dyes were used:

• 1a: Rhodamine 6G, as perchlorate (xanthene dye)
• 1b: crystal violet, as chloride (triphenylmethane dye)
• 1c: malachite green, as oxalate (triphenylmethane dye)
• 1d + e: methyl violet, as chloride (triphenylmethane dye)
• 1f: methylene blue, as chloride (azine dye).

The dye concentration in the aqueous electrolyte was 5 g / l, the temperature of the electrolyte was 20 ° C. and the treatment time (dyeing time) was 15 minutes. The pH values of the electrolyte were obtained in each case by dissolving the dye mentioned in the stated concentration. Only in the case of Example 1e was a lower pH set using H₂SO₄.
In each case an AC voltage of 15 V (50 Hz) - counter electrode made of stainless steel - was used.

The thickness of the oxide layer was measured according to the eddy current principle in accordance with DIN 50984. Following the electrolytic coloring, the depth of penetration of the color was determined by rubbing the oxide layer until it began to lighten with an abrasion tester according to ISO / TC 79 / SC 2 N420E and then measuring the remaining layer thickness determined as indicated above.
The values determined are summarized in Table 1 below: Table 1 No. Layer thickness / µm dye pH Depth of penetration / µm colour 1a 20th Rhodamine 6G 3.3 19th pink 1b 24th Crystal violet 4.8 22 blue violet 1c 18th Malachite green 2.3 16 green 1d 22 Methyl violet 2.6 20th light violet 1e 22 Methyl violet 0.7 7 light violet 1f 25th Methylene blue 3.3 17th blue

The above values show that the method according to the invention makes it possible to produce different colorations of the oxide layer with high penetration depths at the same time. Example 1e illustrates that the depth of penetration of the color can be influenced or controlled by varying the pH.

Examples 2a to 2i

These examples were carried out exclusively with the cationic dye malachite green (as oxalate) - with variation of the dye concentration, the tension and the dyeing time. The following parameters were kept constant in all examples: oxide layer thickness: 22 μm in each case; pH of the aqueous electrolyte: 2.3; Electrolyte temperature: 20 ° C. In Example 2i, a conductive salt - 10 g / l MgSO₄ - was added to the electrolyte.

The results determined are summarized in Table 2: Table 2 No. Conc. (G / l) Voltage (V) Dyeing time (minutes) colour Depth of penetration (µm) 2a 0.2 15 30th light green 19th 2 B 0.5 15 25th light green 16 2c 1 15 15 green 18th 2d 5 15 15 green 20th 2e 8th 15 15 dark green 20th 2f 3rd 12 20th light green 16 2g 3rd 22 15 green 19th 2h 3rd 25th 15 dark green 20th 2i 3rd 10th 20th green 18th

The values determined in Examples 2a to 2e show that with increasing concentration of the dye, more intense shades of color result - with approximately the same depth of penetration. An increase in voltage - examples 2f to 2h - also leads to this result. In contrast, the influence of different dyeing times is less pronounced.

The addition of the conductive salt in Example 2i likewise leads to a more intensive color tinting in comparison to Example 2f, but with the same dyeing time but less tension; however, the penetration depth is not significantly influenced by this.

Comparative experiments 3a to 3d

For the comparative tests, test sheets were used which were pretreated in the same way as in the examples according to the invention. Commercial anionic aluminum dyes were used to color the oxide layer. Work was carried out on the one hand in the conventional immersion process and on the other hand using AC - 15 V, 50 Hz. The temperatures of the aqueous bath and the electrolyte were 60 ° C; the dyeing times are 15 minutes. The pH of the baths corresponded to the values that resulted when the respective dye was dissolved in water.
Dye type, concentration and thickness of the oxide layer as well as the coloration achieved and in particular the depth of penetration into the oxide layer - without and with the use of alternating current - are shown in Table 3.

It turns out that in all cases there is only an insufficient penetration depth of the coloring into the oxide layer, and the use of alternating current does not cause any significant changes - in the sense of higher penetration depths.

Claims (9)

1. A process for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys comprising utilizing alternating current or alternating current-superimposed direct current,
   characterized in that
   the electrolytic coloring is carried out by means of an aqueous electrolyte containing cationic organic dyes and optionally containing additional conductive salts.
2. The process of claim 1, characterized in that said cationic organic dyes are selected from the groups of the triphenylmethane, xanthene, and/or azine dyes.
3. The process of claim 2, characterized by utilizing said cationic organic dyes in the form of their chlorides, oxalates and/or perchlorates.
4. The process of claims 1 to 3, characterized by utilizing for the electrolytic coloring an alternating current or an alternating current-superimposed direct current in the range of 8 to 30 V.
5. The process of claim 4, characterized by utilizing a voltage in the range of 10 to 22 V.
6. The process of claims 1 to 5, characterized in that said cationic organic dyes are present in said electrolyte in a concentration of 0,01 g/l up to the upper solubility limit of said dyes.
7. The process of claim 6, characterized in that said cationic organic dyes are present in said electrolyte in a concentration in the range of from 0,01 to 10 g/l, preferably from 0,05 to 5 g/l.
8. The process of claims 1 to 7, characterized in that said electrolyte contains sodium sulfate and/or magnesium sulfate in a concentration of from 1 to 50 g/l, preferably from 5 to 20 g/l, as said conductive salt.
9. The process of claims 1 to 8, characterized in that said anodized surfaces of aluminum are aluminum alloys are subjected to a treatment with direct current in the same electrolyte prior to said electrolytic coloring by means of alternating current or alternating current-superimposed direct current.