DE2416027C3 - Process for the electrolytic production of a colored oxide film on a substrate made of aluminum or an aluminum alloy - Google Patents

Description

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The invention relates to a method for electrolytic Producing a colored oxide film on a substrate made of aluminum or an aluminum alloy by direct current anodization and subsequent alternating current electrolysis in an acidic Bath.

Processes of this type are described in JP-PS 310401 and DE-AS 1621073. The coloring is achieved in this process by the fact that in d <yn used in the alternating current electrolysis! Bath a dissolved heavy metal salt is present, these ¹ processes provide coverings with excellent lightfastness and with a wide range of colors, which is why they are widely used. However, the methods have disadvantages in that. it is difficult to monitor the dyebath so that it delivers evenly colored coatings. In addition, the use of two electrolytic baths complicates processes of this type. In addition, the presence of heavy metal ions in the baths creates environmental problems if the spent baths are to be disposed of.

The present invention was now based on the object of the method described at the outset Kind of further training so that it does not have the disadvantages mentioned above:

The object is achieved according to the invention in that, in a method of the type described at the outset, the substrate is _{anodized at a direct voltage E 1} in an electrolyte made of 4 to 1 weight percent sulfuric acid, whereupon the voltage E _{1 is} applied

0 and then the substrate in the same bath is subjected to alternating current electrolysis at a voltage E, from 1 to 5 volts, which is, however, lower than the voltage E ₁ .

The process produces colored deposits, with only a single electrolyte being required for the two stages which does not contain any heavy metal ions.

It is already known per se, colored oxide films on aluminum objects in electrolytes that do not contain heavy metal salts. For example, from JP-PS 296079 a Process known in which a relatively expensive oiganische sulfonic acid is used as the electrolyte will. In addition, high voltages and current densities are required. The costs are therefore disproportionate high

Furthermore, there is a method in JP-AS 9805/69 known, in which anodized aluminum is immersed in a solution of Eiseii (l! I) ammonium oxalate becoming a golden color. This method suffers from the disadvantage that it is very it is difficult to achieve a uniform color and that very narrow pH value limits are adhered to have to. In addition, the range of colors that can be achieved is very limited.

In the method according to the invention, the direct current anodization is carried out in a sulfuric acid-based electrolyte with a concentration of 4 to 15% by weight, for example 10 to 15% by weight. In addition, normal anodizing conditions are used, such as B. a current density of

1 A / dm ^; , for a period of 40 minutes, the applied voltage E ₁ is generally 10 to 20 volts. After a substantially colorless transparent oxide film of the desired thickness has been obtained on the surface of the aluminum or aluminum alloy, the applied spin is brought to zero with interruption of the direct current.

Then, a voltage E ₂ lower than the voltage E _{1 is applied} by an AC power source or a strong pulsating power source to perform AC electrolysis. When the voltage E ₂ , which is lower than the voltage / ·. ',, is applied, an electric current does not immediately flow. This only begins gradually after a few seconds or minutes and after a while it reaches a constant level. This phenomenon is usually referred to as the "increase effect".

The present invention is based on the fixed

ment that said recovery effect plays a role in coloring the anodized oxide film, and therefore a colored film with different shades can be obtained by using the anodizing and / or AC electrolysis conditions, such as. B. the applied voltage, the compositions of the electrolyte and the temperature and time of the electrolysis, selects accordingly.

A colored oxide film can be obtained by allowing the recovery effect to occur only once, in which case the degree of coloring is often low. It is therefore advisable to repeatedly run the recovery effect in order to obtain an oxide film with a stronger color. For this purpose it is essential to increase the voltage E, to a value E ₃ for a short time after the voltage E, has been reapplied for a certain time, but before the constant current has been obtained, the value E ₃ is higher than the value of E ₂ but lower than the value of E ₁ . Then the voltage E ₃ suddenly becomes o »» F A ^ r, a / o ^ t / T ^ Aa. * ; _n A \ *> riantxnri Aac Wirrte T?

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lowered. Two or more repetitions. »It procedure As the voltage rises and falls, the hue of the film can vary.

Each of the voltages E ₂ and E ₃ used in the repetitions can vary somewhat, for example within a limit of up to 1 volt.

It has been found that the hue of the film can be shifted more or less, if you add a small amount of an ammonium salt or a metal salt, such as. B. a salt of Ni, Co, Cu, Nm, Zn, Cr, Sn and V with sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, Chromic acid or oxalic acid.

It has also been found possible to achieve an oxide film with a more opaque hue, if the electrolyte boric acid, an organic d! carboxylic acid, such as. B. oxalic acid, malonic acid, maleic acid odt / tartaric acid, or an organic sulfonic acid, such as B. p-phenolsulfonic acid or sulfosalicylic acid, clogs.

The amount of these salts or acids to be added is generally in the range from O ₁ to K)% by weight, based on the electrolyte used. These salts or acids can also serve as stabilizers for the electrolyte.

The present invention is explained in more detail with reference to the drawing. The drawing includes two graphical representations showing the change in voltage or current versus time during the DC anodization and AC electrolysis show according to an embodiment of the invention.

First, the anodization with a direct current voltage E _{1 is carried out} in the usual way in order to produce a Πχύ! Γihn of the desired thickness (for example K) μΐη) on the surface of the aluminum or the aluminum alloy.

After the current has been interrupted, an alternating current or a highly pulsating current with a voltage E ₁ , which is lower than the voltage E _{1, is applied} . The applied voltage E ₁ (mean value) is generally 1 to 5 volts, preferably 2 to 4 volts. The lower the voltage, the longer the recovery time, which leads to the formation of a more opaque film rrj.i of a grayish black color Consequence. At a higher voltage E ₁ , the film formed looks almost transparent and pale bronze, crimson, or green. When a voltage of more than 4 volts is applied, the recovery time falls below I min, making it difficult to obtain a colored oxide film.

The time during which the voltage E i is applied may vary according to the electrolysis conditions and the desired color tones. However, it is generally between 2 and 20 minutes.

The direct current anodization at the voltage E ₁ and the subsequent alternating current electrolysis at the voltage E ₂ are carried out in the same electrolyte from an aqueous sulfuric acid solution, it being possible to add an above-mentioned metal salt or an above-mentioned acid to the electrolyte of both treatments.

When using an aqueous sulfuric acid as the electrolyte results in a concentration of less than 4 wt .-%, a formation of voids during the first anodization, while a Kanzer ^ ation of more than 1 5 wt -% has a 7 \\ short recovery time for the episode, leading to leads to unsatisfactory staining.

The temperature of the electrolyte is preferably in the range of 20 to 40 ° C. In general, the lower the temperature, the longer the recovery time, whereby the formation of a film having a greyish black color is obtained. At a At temperatures above 40 ° C, there is considerable dissolution by the sulfuric acid, which is why it becomes difficult to obtain the desired colored film.

As a result of the recovery effect upon application of the voltage E ₂ , a colored oxide film can be formed. If it is desired to increase the degree of coloration of the film, then the voltage E _{2 is raised} to a value E ₃ , for example of the order of K) volts, which value is lower than that of E ₁ before the electrical Current reaches a constant value, for example when a current density of 0.3 to 1.3 A / dm ^{2 has} arisen. After applying the voltage E, for 1 to 5 seconds, it is suddenly reduced to the value E ₂ or to a value close to it, so that the recovery effect occurs again. If necessary or desired, the raising and lowering of the tension can be repeated one or more times.

As can be seen from the graph of current versus time, the current falls to zero _{when the voltage suddenly drops from E 1} to E _2.

The method according to the invention can be applied to any anodizable aluminum or any anodizable aluminum Aluminum alloy can be used and has the following advantages:

1. It is sufficient to use an aqueous sulfuric acid alone to be used as an electrolyte, so there are no contamination problems. the This also keeps the costs of the process low in practice. Finally is » the stability of the electrolyte is very good.

2. The desired goal can be achieved using DC and AC power sources which makes the systems cheap.

3. Since the entire procedure is carried out in the same bath, the procedure is short and is his Control easy.

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4. Films of a wide variety of hues can be obtained regardless of the nature of the material being anodized.
The invention is illustrated in more detail by the following examples.

example 1

Using a piece of extruded aluminum alloy (50 X 100 X 1 mm) composed of Al, Mg and Si in a weight ratio of 1: 0.6: 0.45 as an anode and a cathode made of a carbon plate, the anodization was carried out carried out by a direct current source at a current density of 1 A / dm ² in an aqueous 5% (weight) sulfuric acid for 10 min at 25 ° C. In this way, an essentially colorless, transparent oxide film with a thickness of 2.5 μm was formed on the surface of the aluminum alloy.

Then, the anodized aluminum alloy was subjected to AC electrolysis at a voltage of 2.5 volts in the same electrolyte. Initially, no electrical current could be detected but it then began to flow over time. ; It was observed that the color of the film was below Development of gases on the surface of the aluminum alloy moved to gray. 10 minutes later the electricity was switched off, after the current density had reached 0.3 A / dm 'e.

The film thus produced was gray in color, the thickness of 2.5 µm was unchanged.

Example 3

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Example 2

Using the same aluminum alloy molding as in Example 1, direct current anodization was carried out at a current density of 1 A / dm ² in an aqueous 10% (weight) sulfuric acid at 35 ° C. for 10 minutes. Then, a voltage of 3.0 volts was applied by an AC power source. 7 minutes later, the current was switched off after the current density reached 0.7 A / dm ² . The film thus produced was bronze in color.

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DC anodization was carried out in the same manner as in Example 1 except that the Anodizing time was 40 minutes. An oxide film with a thickness of 10 μm was obtained.

Then the anodized aluminum was subjected to such alternating current electrolysis at a voltage of 2.5 volts. Fifteen minutes later, after the current density reached 1.0 A / dm ² , the voltage was increased to 10 volts for 30 seconds. After a few seconds the voltage was suddenly lowered again to 2.5 volts. The current was interrupted by this, but began to flow again after a while. 5 minutes after the voltage had been reduced to 2.5 volts, the current density was 1 A / dm ² , whereupon the electrical current was switched off. eo

The color obtained in this way was a little deeper gray than that of the film obtained in Example 1.

Example 4

The procedure described in Example 3 was followed, except that the lifting and Lowering the voltage was repeated four times (i.e., the recovery effect occurred four times). The film formed was deep grayish to black in color.

Example 5

It was made following the procedure of Example 3 worked, except that the anodized material was replaced by fine aluminum. Of the obtained film was gray in color and was etsvas transparent.

Example 6

By a similar procedure to Example 3, anodization was carried out by a direct current power source carried out in an aqueous 7% strength (by weight) sulfuric acid for 30 minutes at 35.degree.

Then, a voltage of 2.5 volts was applied by an AC power source. 3 minutes later, the

lowered to 2.5 volts. The raising and lowering of the tension was repeated three times. It was obtained a film with a bronze color.

Example 7

The procedure of Example 6 was followed, except that the concentration of the aqueous Sulfuric acid was 10% by weight. The film received had a reddish-purple color.

Example 8

The procedure of Example 6 was followed, but the electrolyte used was one aqueous 10% (weight) sulfuric acid to which 0.5% by weight zinc sulfate had been added. The obtained film was green in color.

Example 9

The direct current anodization was carried out for 40 minutes under similar conditions as in Example 1, However, there was the variant that a 10% (weight) aqueous sulfuric acid was used as the electrolyte was, the 1.0 wt .-% p-phenol sulfonic acid had been added.

The anodized aluminum was subjected to alternating current electrolysis at a voltage of 2.5 volts, 10 minutes thereafter the voltage was increased to 10 volts. The voltage was then suddenly reduced again to 2.5 volts. The electric current was cut off after the current density reached 1 A / dm ² .

The film thus obtained was gray in color.

, Example 10

The procedure of Example 1 was followed except that 1% by weight of ammonium molybdate was used aqueous sulfuric acid was added. The film formed was light reddish gray in color.

Example 11

Using pure aluminum (1100-H24; JIS standard) as the anode and a carbon plate as the cathode, the anodization was carried out with a direct current at a current density of 1 A / dm ² in an aqueous 14% (weight) sulfuric acid containing 1 wt. -% contained copper sulfate, at a temperature of 25 ° C for 10 min.

leads. The oxide film formed had a thickness of 2.5 µm.

The anodized aluminum was then an alternating current electrolysis at a voltage of 2.5 Volts exposed in the same electrolyte, then what

8 minutes later, the voltage was raised to OK volts and then suddenly dropped again to 2.5 volts. The increase and decrease in tension was repeated three times using a film with a reddish bronze color was obtained »

On this J. Blatt drawings

Claims (8)

Patent claims: 1. Method for the electrolytic production of a colored oxide film on a substrate from. Aluminum or an aluminum alloy by direct current anodization and subsequent alternating current electrolysis in an acidic BacL, characterized in that the substrate is anodized at a direct voltage E ₁ in an electrolyte of 4 to 15 weight percent sulfuric acid, then the voltage E _{1 is} lowered to D and then the substrate is at the same time The bath is subjected to alternating current electrolysis at a voltage E, from 1 to 5 volts, but lower than the voltage E ₁ . 2. The method according to claim 1, characterized in that the voltage E, to a, value E ₂ , which is lower than the voltage E ₁ , but higher than the voltage E ₂ , is increased, then the voltage suddenly to a value of E _{2 is} lowered in the vicinity of E ₁ and the raising and lowering of the voltage is repeated one or more times if necessary. 3. The method according to claim 1 or 2, characterized in that a voltage E ₁ of 10 bii. 20 volts is applied. 4. The method according to claim 1 to 3, characterized in that the voltage E _{1 is applied} for a time of 2 to 20 minutes. 5. The method according to claim 1 to 4, characterized in that a voltage E. of 2 to 4 volts is applied. 6. The method according to claim 1 to 5, characterized in that the electrolyte is held at a temperature of 20 to 40 ° C, while the voltage E _{2 is} applied. 7. The method according to claim 2 to 6, characterized in that a voltage E ₃ of 5 to 10 volts is applied for a time of 1 to 5 s. 8. The method according to claim 1 to 7, characterized in that the electrolyte in addition Boric acid, oxalic acid, malonic acid, maleic acid. Tartaric acid, p-phenol isulphonic acid or sulphosacylic acid is added in an amount of 0.1 to 10% by weight. 30th