DE1521016B2 - Aqueous BATH AND PROCESS FOR THE ANODIC PRODUCTION OF COLORED OXIDE COATINGS ON ALUMINUM AND ALUMINUM ALLOYS - Google Patents

Description

It is known to produce oxide coatings through anodic oxidation of aluminum or its alloys to create. The most common method is a sulfuric acid solution at about Room temperature is used, and therefore the bath must be cooled. The obtained by this procedure Oxide coatings are usually colorless, but in the case of certain alloys they can also be colored.

A process for the production of hard oxide coatings with higher abrasion resistance has recently become known, in which aluminum is anodically oxidized at temperatures around 0 ° C. in an aqueous sulfuric acid solution. The oxide coating obtained in this process not only has excellent abrasion resistance, but is also colored metallic gray, gray-tinged brown or dark brown. Due to this color, the anodized aluminum can be used in construction technology. However, in order to keep the bath temperature at about ⁰ ° C., the costs for the device and the operation must be increased considerably.

From the German Auslegeschrift 1145 888 are also aqueous, sulfosalicylic acid and sulfuric acid containing solutions and a process for producing colored and abrasion-resistant coatings on aluminum and aluminum alloys, in which these solutions are used and that at room temperature is feasible, known. However, the aforementioned treatment solutions have the Disadvantage that sulfosalicylic acid is relatively expensive, which is especially in the case of as outer clothing cost problems arise for aluminum used in large buildings. With other aromatic Hydroxysulfonic acids, such as phenol, cresol or thymol sulfonic acid, cannot be colored at all and at the same time wear-resistant oxide coatings can be produced on aluminum and its alloys. The object of the invention was therefore to provide a new, aqueous, inexpensive bath for anodic production colored oxide coatings on aluminum and aluminum alloys with an aromatic content Sulphonic acid and sulfuric acid and a convenient and economical method of manufacture to provide the aforementioned oxide coatings using such a bath. The invention thus relates to an aqueous bath for the anodic production of colored oxide coatings on aluminum and aluminum alloys with a content of aromatic sulfonic acid and sulfuric acid, which is characterized in that it contains 1.5 to 40 percent by weight of naphthalene disulfonic acid and contains 0.1 to 3.0 percent by weight sulfuric acid.

The naphthalene disulfonic acid contained in the bath according to the invention includes all isomeric naphthalene disulfonic acids or their mixtures. Mixtures containing 1,5-naphthalenedisulfonic acid are preferred contain essential component.

The baths according to the invention have detr in particular important advantage for industrial purposes that naphthalenedisulphonic acid is an easily accessible one and cheap connection is. Nevertheless, the coatings are very hard and evenly colored obtain.

The aqueous bath according to the invention preferably contains 5 to 20 percent by weight of naphthalenedisulfonic acid and 0.3 by weight percent sulfuric acid.

The invention also relates to a method for the anodic production of colored oxide coatings Aluminum and aluminum alloys using the aforementioned bath, which method is characterized in that an initial cell voltage of at least 20VoIt is used will.

The aluminum alloys treated in the process of the invention include any alloy which forms thick oxide coatings by anodic oxidation in a bath containing sulfuric acid.

According to the method according to the invention, it is possible to produce colored oxide coatings with excellent To achieve abrasion and weather resistance at a bath temperature in the range of room temperature.

When using the bath according to the invention, it is possible, even at low temperatures, to produce oxide coatings that are just as hard as those obtained by the aforementioned known process with aqueous sulfuric acid. Furthermore, at a bath temperature of about room temperature, ie 15 to 30 ° C., colored oxide coatings with excellent abrasion and weather resistance can be obtained. The process of the invention is therefore economically more advantageous than some of the known processes in which a bath temperature of 0 ^° C. must be maintained. The main advantage, however, lies in the mentioned cheapness of the naphthalenedisulfonic acid used according to the invention.

The colors of the oxide coatings obtained can

by using an aqueous bath having one within the above range Composition and through the use of various aluminum alloys as well as through

application of different conditions in the processing of aluminum and aluminum alloys and anodizing can be varied.

As the concentration of naphthalene disulfonic acid increases, the cell voltage and the The color of the oxide coating obtained becomes lighter. If, on the other hand, the concentration of naphthalenedisulfonic acid decreases, the cell voltage increases, and the color of the oxide coating gradually becomes darker and changes from brown to dark brown to black. When the concentration of naphthalenedisulfonic acid is less than 1.5 percent by weight, it is difficult to obtain a usable, uniform to obtain colored oxide coating.

If the sulfuric acid concentration is less than 0.1% by weight, it is also difficult to to obtain a uniformly colored oxide coating. With a sulfuric acid concentration above 3.0 percent by weight the color of the oxide coating becomes lighter and lighter, and finally the oxide coating becomes colorless, although the cell voltage drops. When using an aqueous bath that is 10 to 20 percent by weight Contains sulfuric acid and 5 to 25 percent by weight naphthalenedisulfonic acid, the Dissolution of the oxide coating is inhibited by the bath during the electrolysis, so that it is possible to use this bath for anordic oxidation at high current density. The oxide coating obtained however, it is colorless or only slightly colored, the color depending on the type of alloy used depends.

When using the bath according to the invention, the most favorable results can be achieved if aluminum or an aluminum alloy is anodically oxidized ^{at a current density of 1 to 3 A / dm 2.} At a bath temperature around room temperature (about 15 to 3O ⁰ C), the cell voltage is 20 to 80VoIt when the anodic oxidation is carried out at a current density of up to 3 A / dm ^2.

If the initial voltage of the anodic oxidation is to be below 20VoIt, it should be at lower current density or at a higher bath temperature or the sulfuric acid concentration in the bathroom should be increased. In all of these cases, however, there can be no thicker, harder and deeper colored Oxide coating can be obtained.

At constant current density, the cell voltage gradually decreases as the thickness of the oxide coating increases to. There are some cases where depending on the type of aluminum or aluminum alloy used a burn is observed when the cell voltage is applied during anodization same current density has exceeded the value of 60VoIt. In these cases the cell voltage preferably held at a lower value so that combustion does not occur. Accordingly So the anodic oxidation is preferably initially at a constant current density and then at carried out at a constant voltage so that no combustion occurs. The burn can also go through Increase in the bath temperature can be prevented.

The anodic oxidation can be carried out either with alternating current, with direct current or with direct current modulated by means of alternating current. The duration of the treatment depends on the current density and the desired thickness of the oxide coating. Colored oxide coatings with adequate abrasion resistance can be obtained within about 30 to 60 minutes. The thickness of the oxide coating is usually 15 to 20 microns per A.h / dm ² .

The examples illustrate the invention. Percentages relate to weight.

For example

An extruded aluminum alloy (0.93% Si, 0.70% Mg, 0.39% Fe and 0.48% Mn) is freed from dirt in the usual way by degreasing and pickling and then in an aqueous bath, the 9th Contains% naphthalenedisulfonic acid and 0.6% sulfuric acid, anodized at 25 ° C. and a current density of 2 A / dm ² , the bath being passed in by. Keeps air moving. The cell voltage increases gradually from 30 volts to 55 volts towards the end of the anodic oxidation. The waveform of the current used corresponds to a full-wave rectification of a single-phase alternating current.

The oxide coating produced by this anodizing treatment is immersed in boiling, demineralized water re-sealed. The coating is black in color, and its thickness is eiVa 35 microns.

Example 2

An aluminum alloy in the form of extrusions (0.43% Si, 0.55% Mg and 0.17% Fe) is cleaned in a manner similar to that described in Example 1 and in an aqueous bath containing 9% naphthalenedisulphonic acid and 0.6% Contains sulfuric acid, anodically oxidized ^{at 20 0} C and an initial current density of 2.4 A / dm ^2. The cell voltage increases gradually from 30 to 56 volts over a period of 37 minutes. The alloy is then anodized for 24 minutes at a constant voltage of 56VoIt. The current density is 1.3 A / dm ² towards the end of the anodization. The oxide coating produced by this treatment is resealed in a manner similar to that described in Example 1. The coating is brown in color and about 35 microns thick.

Examples 3 to 12

Experiments carried out under various anodizing conditions are explained in Table I. the The color of the oxide coatings obtained is also given.

Tabel example

Components of the bath,%
Rest water

Aluminum or Al alloy Si

Alloy composition.
Mg I Fe j Mn I Cr

Naphthalene disulfonic acid 30

H ₂ SO ₄ '1.8

Naphthalene disulfonic acid 18

H ₂ SO ₄ 1.2

Al

AlMgSi 0.5 ⁺ ) common impurities
0.43 0.55 0.17

continuation

example Components of the bath,%
Rest water Aluminum or
Al alloy Si Legie
Mg 'Incredibly
Fe immense
Mn removal
Cr Cu 5 Naphthalene disulfonic acid 15
H ₂ SO ₄ 1.9 AlMgSi I ⁺⁺ ) 0.63 0.94 0.17 - 0.33 0.23 6th Naphthalene disulfonic acid 9
H ₂ SO ₄ 0.6 AlMgSi I ⁺ ) 0.93 0.70 0.39 0.48 - - 7th Naphthalene disulfonic acid 9
H ₂ SO ₄ 0.6 AlMgSi I ⁺ ) 0.93 0.70 0.39 0.48 ■ - 8th Naphthalene disulfonic acid 9
H ₂ SO ₄ 0.6 AlMgSi I ⁺ ) 0.93 0.70 0.39 0.48 - - 9 Naphthalene disulfonic acid 9
H ₂ SO ₄ 0.6 Al alloy ⁺ ) 0.96 0.67 0.17 - 10 Naphthalene disulfonic acid 9
H ₂ SO ₄ 0.6 Al alloy *) 0.42 0.62 0.16 0.24 - 11th Naphthalene disulfonic acid 4.5
H ₂ SO ₄ 0.3 AlMgSi I ⁺ ) 0.93 0.70 0.39 0.48 - - 12th Naphthalene disulfonic acid 1.5
H ₂ SO ₄ 0.1 Al alloy ⁺ ) 0.96 0.67 0.16 - 0.24 -

(Continuation)

shape sheet Current density Required Bath temperature Time Color of the oxide coatings example Extrusions A / dm ² tension ⁰ C sheet 2.0 V 15 to 20 "■) · ■
60 Yellowish silver 3 Extrusions 2.0 20 to 30 20th 60 Yellowish silver 4th Extrusions 3.0 20 to 30 15 to 20 60 Brown 5 Extrusions 1.0 25 to 60 20th 60 Dark brown 6th Extrusions 1.5 30 to 45 20th 60 black 7th Extrusions 2.0 30 to 55 20th 40 black 8th Extrusions 2.0 30 to 70 20th 60 Brown 9 Extrusions 1.8 28 to 70 20th 60 Brown 10 1.5 * 30 to 55 25th 60 black 11th 1.0 * 30 to 56 25th 60 Brown 12th 35 to 56

* The anodic oxidation is carried out at constant current density until the cell voltage reaches 56 volts, and then at constant 56 volts continued.

⁺ ) Artificial aging after rapid cooling from the processing temperature. ^{+ +} ) Solution annealed, then quenched and aged.

Example 13

The properties, especially the hardness, of the anodic route to aluminum or aluminum alloys applied oxide coatings shown. A bath of the following composition is used:

Naphthalenedisulfonic acid,%

Sulfuric acid,% 0.8

Water rest

The anodic treatment is carried out with a direct current superimposed by alternating current:

Current density of direct current, A / dm ² 2.5

AC voltage (constant), V

Bath temperature, ⁰ C 20 ± t

Electrolysis time, minutes 20 to

The results are summarized in Table II.

Table II

sample Aluminum or
Aluminum alloy Electrolysis time Voltage change
of direct current Properties of the oxide coatings Hardness*) Min. (V) Strength μ 760 1 Al 99 20th 25 to 32 17th 710 2 AlMg ₃ 50 24 to 51 38 650 3 AlMgSi 1 30th 25 to 54 30th 790 4th AlMn 50 28 to 65 25th 760 5 AlMgSi 0.5 50 26 to 52 37 780 6th AlMgSi 1 50 25 to 60 39 720 7th Molding 30th 24 to 50 28 640 8th Molding 50 22 to 55 32

*) The hardness of the coatings is determined using the Micro Vickers Hardness Tester with a load of 25 g.

Claims (5)

Patent claims: 1. Aqueous bath for anodic production of colored oxide coatings on aluminum and aluminum alloys with a content of aromatic sulfonic acid and sulfuric acid, thereby characterized in that it is 1.5 to 40 percent by weight naphthalenedisulfonic acid and 0.1 contains up to 3.0 percent by weight sulfuric acid. 2. Aqueous bath according to claim 1, characterized in that it is 5 to 20 percent by weight Contains naphthalene disulfonic acid and 0.3 to 1.5 percent by weight sulfuric acid. 3. Process for the anodic production of colored oxide coatings on aluminum and aluminum alloys using a bath according to claims 1 and 2, characterized in that at an initial cell voltage of at least 20 volts is used. 4. The method according to claim 3, characterized in that it is carried out at a current density of 1 to 3 A / dm ² and at a bath temperature of about 15 to 30 ^° C. 5. The method according to claim 3 and 4, characterized in that the anodic oxidation first is carried out at constant current density and then at constant voltage.