DE1621073B1 - Process for coloring anodically generated oxide films on aluminum objects - Google Patents

Description

The invention relates to a method for coloring anodically produced oxide films on aluminum objects by electrolytic treatment of the anodized aluminum object with Alternating current in an acidic electrolyte with a counter electrode made of the same metal is used, which is dissolved as a salt in the electrolyte.

By the term "aluminum" as used here, aluminum is intended to be more commonly more commercially available Purity and aluminum-based alloys required for common anodizing treatment are suitable to be understood. The term "aluminum object" as used here Also includes semi-finished products such as rolled aluminum sheet and extruded aluminum profiles. .

It is already known to produce colored aluminum articles by making the aluminum article anodically oxidized and then subjected to an alternating current treatment. A bath is used that contains a soluble compound of a certain metal in an aqueous one contains acidic medium. In this way the anodic oxide film becomes on the aluminum object colored.

In British Patent 1,022,927, the counter-electrodes used are those made of carbon, carborundum and called aluminum. It is also known from German patent specification 741 753, such counter electrodes which consist of the same metal as the salt in the electrolyte is resolved.

It has now been found that coloring anodically generated oxide films on aluminum objects particularly good colorations are obtained if a counter electrode made of nickel and a uses an electrolyte containing nickel salt and operates at a pH of 3.5 to 5.5.

This approach is in contrast to that in British Patent 1,022,927 and the German Patent 741,753 method described. That in British Patent 1022927 for manufacture The materials used for the counter electrode are, without exception, substances that are electrochemical cannot go into solution. In contrast, in the process forming the subject of this invention Electrodes made of nickel are used, which under the prevailing conditions are readily in solution can go. In the process according to German patent 741753, the coloring is a continuation the oxidation that has already been carried out pH values of 0.8 to 1.6 worked. In contrast, the oxide film is colored at pH values from 3.5 to 5.5 instead.

It was therefore in no way obvious that when using a nickel-containing electrolyte, improved results through the use can be achieved by nickel electrodes. When using nickel counter electrodes instead of those made of carbon can be found in electrolytes and anodic oxide films made of nickel Desired dark colors can be obtained more easily within a certain pH value range.

In general, the color developed within a certain period of time depends on the one used Tension off. It has been found that higher voltages are used with nickel electrodes than carbon electrodes without chipping of the anodic oxide film. the The cause of the peeling is not fully known, but it is believed to be a result of development of hydrogen, is on the surface of the metal beneath the anodic oxide film. Even darker and more intense shades result.

Another advantage over carbon electrodes is the lower rate of change the pH value. In order to get even results it is desirable to have an extensive Maintain uniformity of the physical and chemical nature of the bath, and especially with regard to the pH value.

If a nickel counter electrode is used, the pH of the solution remains for a considerable time Period essentially unchanged. In contrast, with a carbon counter electrode frequent readjustments of the pH value by means of suitable chemical additives are necessary.

After cleaning, the aluminum object is anodized to produce the oxide layer, for example using direct current in an acidic bath for a period of e.g. B. 30 to 60 minutes at a conventional voltage and current density. The bath can consist of an aqueous solution of sulfuric acid, sulfonic acid or chromic acid or a mixture of such acids. Treatment and electrolyte in terms of type and concentration should be selected so that a good and permanent anodic oxide layer results. Preferably this is made under conditions which require the use of DC voltages below about 40 volts. Anodic layers made at higher voltages can exhibit poor ink acceptance properties. The article is then subjected to the second stage of the process in which the original anodic oxide coating is converted into a colored coating of essentially the same adhesive strength and protective effect. However, this is characterized by a range of colors and shades that depend on the conditions in the second stage of treatment. In the following specific examples for treating aluminum articles, the article, which consisted of aluminum sheet of commercial grade, was first anodized in a conventional acidic bath. Although, as already mentioned, each anodic treatment may be used, but an appropriate treatment, which was used for the examples was to anodization in an 15% strength ^e n (by weight) sulfuric acid solution by direct current, wherein a current density of 1.62 A / dm ² and a voltage of approximately 17VoIt was applied for 30 minutes at 21 ° C. This created an oxide layer with. a thickness of 15.2 μ obtained. Before anodizing, the surface of the aluminum sheet was cleaned by conventional degreasing and etched in a 5% sodium hydroxide solution for 5 minutes.

During the coloring treatment, the distance between the aluminum object and the counter electrode was approximately 15.2 to 45.7 cm in order to avoid the difficulties of uneven coloring of areas further away from the counter electrode, such as recesses in extruded Γ aluminum profiles . In each case the finished '> - dyed item was subjected to a conventional hand-

treatment to seal the covering, i.e. a sealing treatment in the boiling or almost boiling water.

example

An aqueous electrolyte was used with the following concentration of the constituents: 30 g / l NiSO ₄ -7H ₂ O, _{30 g / 1 H 3} BO ₃ and 15 g / 1 (NHt) ₂ SO ₄ . The pH was adjusted to a value between 3.5 and 5.5 by adding ammonium hydroxide or sulfuric acid as required. Outside this range, no smooth, uniformly colored layers are obtained. An alternating current of 15 volts was applied for 5 minutes. The anodic one

IO flooring assumed a dark bronze color with an unusually good appearance and an unusually good uniformity.

In general, the use of nickel salts and a nickel electrode in the bath results in bronzes Colors of the most varied of hues approaching black.

The following table briefly summarizes the results of a series of experiments that were carried out with Carbon counter electrodes and metal counter electrodes are obtained using the same electrolyte became. In any case, the electrolyte used corresponds to that used in the example Electrolytes.

metal
in the electrolyte Counter electrode Deposition conditions colour Note 1 Ni Ni 10 V-5 minutes Light bronze 2 Ni C. 10 V-5 minutes Light bronze '"■ as with 1 3 Ni Ni 15 V - 10 minutes Dark bronze no peeling 4th Ni C. 15 V - 10 minutes Dark bronze little flaking 5 Ni Fe *) 15 V - 10 minutes Dark bronze no peeling, however
AC corrosion
of steel 6th Ni Ni 13 V — 3 minutes +
17V — 10 minutes Dark bronze easy peeling 7th Ni C. 13 V — 3 minutes +
17 V - 10 minutes Light bronze severe peeling

*) Stainless steel type 304.

The above results show that compared to carbon electrodes with nickel counter electrodes in a nickel-containing electrolyte, considerable improvements can be obtained.

By changing the treatment times and tensions, different shades of color can be achieved will. Treatment times in the range from 3 to 30 minutes and alternating voltages are preferred from 5 to 25 volts applied.

Claims (3)

Claims: 45 1. Process for coloring anodically generated oxide films on aluminum articles by electrolytic treatment of the anodized aluminum object with alternating current in an acidic electrolyte with a counter electrode made of the same metal is used, which is dissolved as a salt in the electrolyte, characterized in that a Counter electrode made of nickel, an electrolyte containing a nickel salt and used in a pH of 3.5 to 5.5 is worked. 2. The method according to claim 1, characterized in that an electrolyte is used which Contains nickel sulfate, ammonium sulfate and boric acid. 3. The method according to claim 1 or 2, characterized in that the electrolytic treatment carried out over a period of 3 to 30 minutes at a voltage of 5 to 25 volts will.