DK150187B - PROCEDURES FOR ELECTROLYTIC COLORING OF ALUMINUM - Google Patents

Description

150187

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a process for dyeing aluminum or aluminum alloys already anodically oxidized by electrolytically treating the alumina layer with alternating current in an acidic aqueous electrolyte containing colorant metal salts, preferably ions of at least sections of the metals Ni, Co, Cu. Sn, Ag and Fe ·

It is known that oxide layers on aluminum and aluminum alloys produced, for example · by the so-called GS method, can be dyed by subsequent alternating current treatment in acidic aqueous metal salt solutions · Due to the valve effect of the oxide layer, the alternating current is extensively rectified and in the phase where the workpiece is cathodic, the metal cations in the oxide pores are reduced by reducing the cations ·

For this metal separation, the pH range of the metal salt solution is crucial · The protons thus released have little effect in a highly acidic environment where the proton concentration is already high. On the other hand, in a weakly acidic - 2 - 150187 milieu where the proton concentration is some tier potencies lower, the additional protons have a significant effect. Therefore, a significant pH shift can be expected in electrolytes with metal salts whose metals are separated from weakly acidic solutions, e.g. in the pH range between 4 and 6. The buffering effect of the boric acid is not sufficient to prevent a strong decrease in the pH value.

The increase in the amount of free acid during dyeing occurs especially if working with inert model electrodes, e.g. of graphite.

The imbalance of the electrode reaction has so far been countered by the use of ammonium hydroxide or optionally alkali hydroxides for the pH adjustment.

However, this rather has a disadvantageous effect because these additions in certain electrolytes can lead to so-called pitting, ie. needle stick or dot-shaped defect sites, and local peeling of the oxide layers.

SUMMARY OF THE INVENTION The object of the invention is to provide a process for dyeing aluminum or aluminum alloys already anodically oxidized by electrolytic treatment of alternating current alumina layers in an acidic aqueous electrolyte containing colorant metal salts, by which a process can be obtained quickly and reproducibly. -egalitet. Thus, the aforementioned shortcomings must also be remedied.

The task is solved according to the invention by adding at least 1 g / l amino alcohol to the electrolyte. This addition can be made by an electrolyte already in operation to correct the pH decreasing during the dyeing process. flmino alcohols are added continuously or periodically until the desired pH is reached again.

It is advantageous to make an amino alcohol addition already when preparing a new electrolyte. If the pH increases too strongly with this addition, it can be corrected by the addition of sulfuric acid.

The color electrolyte may contain all known metal salts. The amino alcohol additions, in particular mono-, di- and triethanolamine, have the advantage, partly that the pH can be increased and partly that more favorable excretion conditions for the metal ions occur. Thus, e.g. the color equality of the colored oxide layers due to the increased conductivity. A further advantage of the addition of amino alcohols is that an intensified, darker tint is obtained at the staining times commonly used. Thus, for a particular hue, a shorter staining time and / or AC voltage can be reduced. Furthermore, the sensitivity of the electrolytes to random contaminants in the form of alkali, alkaline earth and ammonium ions is diminished, which is indicated by the fact that under normal conditions, even in dark colors, no needle-stick and dot-shaped fault points occur.

The colored aluminum objects are preferably used, but not exclusively for decorative purposes and in exterior architecture.

Example 1:

Plate specimens of an aluminum alloy consisting of 1.4 - 1.8%

Mg, 0.4% Fe, 0.4% Si, 0.1 - 0.3% Mn, 0.1% Zn, 0.05% Cu, and the residual aluminum with ordinary contaminants are anodized in sulfuric acid and then treated in sulfuric acid. an aqueous color electrolyte containing: 80 g / l CoSO 4 * 7 H 2 O 40 g / 1 NiSO 4 · 7 H 2 O 40 g / 1 H 3 BO 3 1 g / l CuSO 4 at a temperature of 20-25 ° C with alternating current. Across the center of a test surface, a 6 mm diameter graphite rod, at a distance of 8 cm, is used as a model electrode.

By applying a voltage of 16 V alternating current for 120 sec. you get a staining with a medium source zone. on the surface facing away from the graphite model electrode, the samples show color irregularities in the form of dark edge surfaces.

If the samples are subjected to the same conditions for 15 min. for an alternating current of 16 V, they show a black color. Due to the deep black coloring, irregularities can no longer be detected.

Example 2: 2,100 pre-anodized samples with a total surface of 1 m are colored successively in an electrolyte, where the alloy of the samples, the composition of the electrolyte, the geometric arrangement and the alternating current voltage correspond to Example 1. During the dyeing, the pH drops from 4 , 7 to 4.2. All samples exhibit the color irregularity mentioned in Example 1.

By the addition of 4 g / l triethanolamine to the electrolyte, the pH is again increased from 4.2 to 4.7. Then 100 samples are again stained, with alternating current at 16 V. After only 90 seconds of processing time, instead of 120 sec ·, the same intermediate source zone as in Example 1. is obtained. All samples exhibit regular coloring on both surfaces. During the staining of the 100 samples - 4 - 150187 the pH drops again to 4 »2. It can be increased again by a renewed triethanolamine screening.

For a black stain with the same intensity as in Example 1, a processing time of only 10 min ·, instead of 15 min, is needed.

Thus, with the addition of triethanolamine, not only can the original pH value of the color sheet be restored, but one can also shorten the staining time as well as improve the color quality.

This example also shows that the colored samples show the desired color quality only after triethanolamine has been added. Therefore, in an industrial application of the process, at least 1 g / l of amino alcohol must be added before dyeing.

Example 3

Pre-anodized samples with the same alloy composition as in Example 1 s are treated in an aqueous color electrolyte containing 20g / l SnSO4 and 7g / l H2SO4, giving a pH of 1.4, at 20-25 ° C with alternating current.

When the pH is lowered to 1.3, 3 g / l of monoethanolamine is added. This not only increases the pH to 1.4, but also improves color quality and shortens the staining time. Thus, except for the monoethanolamine addition, working under the same conditions as in known methods can produce a certain hue from the obtainable color scale faster and in better quality.