NO137241B - PROCEDURES FOR ELECTROLYTICAL COLORING OF ALUMINUM - Google Patents

Description

The present invention relates to a method for colouring

of previously anodically oxidized objects of aluminum or aluminum alloys by electrolytic treatment of the aluminum oxide layer with alternating current in an acidic, aqueous electrolyte, which contains dissolved color-giving metal salts, preferably ions of at least one of the metals Ni, Co, Cu, Sn, Ag, Se, and the distinctive feature of the method according to the invention is that amino alcohol is added to the electrolyte, so that the amino alcohol content amounts to at least 1 g/l, preferably at least 2 g/l.

It is known that oxide layers on aluminum and aluminum alloys,

like for example. is produced using the direct current method, can be colored by subsequent alternating current treatment in acidic, aqueous metal salt solutions. By the valve action of the oxide layer, the alternating current is rectified to a great extent and in the phase in which the workpiece is cathodic, a metal separation takes place in the oxide pores when the cations are reduced.

For this metal precipitation, the pH range of the metal salt solution is of decisive importance. The protons thereby released are not particularly effective in a strongly acidic area in which the proton concentration is already high. In a weakly acidic region, in which the proton concentration is a few orders of magnitude lower, the additional protons will, however, have a significant effect. Therefore, a significant pH shift is expected for electrolytes with metal salts, whose metals are excreted from weakly acidic solutions, e.g. in the pH range between 4 and 6. The buffering effect of boric acid is not sufficient to avoid

a sharp drop in the pH value.

The increase in free acid content during dyeing occurs above all

when working with inert counter electrodes, e.g. of graphite*. The lack of equilibrium in the electrode reaction has previously been sought

remedied by using ammonium hydroxide and/or alkali hydroxides for the pH setting. However, this seems unfortunate, as these additions in some electrolytes can lead to localized deep corrosion, i.e. to needle-shaped or point-shaped defects and local peeling of the oxide layers. The task underlying the present invention is to develop a method for coloring pre-anodically oxidized objects made of aluminum or aluminum alloys by electrolytic treatment of the aluminum oxide layer with alternating current in an acidic, aqueous electrolyte, which contains coloring metal salts, and with which an increased color uniformity can be achieved quickly and reproducibly. In this way, the aforementioned shortcomings will also be overcome.

The amino alcohol addition can be made to an electrolyte that has already been used in order to correct the decreasing pH during the course of the dyeing process. The amino alcohols are added continuously or periodically until the desired pH value is again achieved.

When a new electrolyte is used, the desired effect can be achieved

when at least 1 g/l, preferably at least 2 g/l of amino alcohol is added. If the pH value rises too strongly with this addition, it can be corrected by adding sulfuric acid.

The color electrolyte can contain all known metal salts, preferably the ions of at least one of the metals Ni, Co, Cu, Sn, Ag, Fe.

Addition of amino alcohol, especially mono-/di- and tri-ethanolamine, has the advantage that, on the one hand, the pH value can be increased, and on the other hand, more favorable separation conditions can be created for. the metal ions. Thus, e.g. the color uniformity of the colored oxide layers due to the increased conductivity. A further advantage of the addition of amino alcohols is that,

that a more intense, darker tinting is obtained at commonly used dyeing times. For a specific color tone, you only need a shorter color time and/or the alternating current voltage can be reduced. Furthermore, the sensitivity of the electrolytes to accidental contamination by alkali, alkaline earth and ammonium ions is reduced, and this manifests itself in the fact that under normal conditions, even at darker

colors, no pinprick and point-shaped defects appear.

As amino alcohols come next to the aforementioned ethanolamines

also further amines in question, where one to three of the H atoms connected to the N atom are substituted with at least one alcohol group with an 0.2- or C3 chain and a further H atom is substituted with a methyl, ethyl, propyl, butyl or phenyl group, such as e.g. methyldiethanolamine, diethylethanolamine, butyl-ethanolamine, phenylethanolamine, the quantity that is added being determined simply on the basis of the correction of the pH value in the bath to be achieved, resp. when preparing a fresh bath, and which amounts to at least one gram/litre and preferably at least 2 g/l.

The colored aluminum objects are used preferably, but not exclusively, for decorative purposes and in exterior architecture0

Eat empel 1.

Tin-shaped samples 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 rest aluminum with common impurities, was anodized in sulfuric acid and then treated in an aqueous dye electrolyte containing

at a temperature of 20-25°C with alternating current. A graphite rod with a diameter of 6 mm, at a distance of 8 cm, was used as a counter electrode on one side of the center of a sample surface.

By applying a voltage of 16 V*~* for 120 sec. a coloring with a medium bronze tone is obtained" On the surface facing away from the graphite counter electrode, the sample shows color unevenness in the form of dark edge surfaces"

If, under otherwise identical conditions, the sample is subjected to an alternating current of 16 Vi 15 min., they show a black colour. Due to the deep black coloring, irregularities can no longer be detected.

Example 2.

* o 2 Approximately 100 pre-anodized samples with a total surface of 1 m were

colored successively in an electrolyte, the alloy in the sample, the composition of the electrolyte, the geometric arrangement and the alternating current voltage being consistent with example 1.

During the coloring, the pH drops from 4.7-4.2 and all samples show

the color unevenness mentioned in example 1.

By adding 4 g/l triethanolamine to the electrolyte

the pH is raised again from 4.2 to 4.7. Approximately 100 samples are then dyed again using alternating current of approximately 16 V. After only 90 instead of 120 sec.'s processing time, the same medium bronze tone is obtained as in ex. 1 and all samples show regular staining on both surfaces. During the coloring of the 100 samples, the pH drops again to 4.2 and it can be raised again by renewed addition of triethanolamine.

For a black coloring with the same intensity as in ex. 1, a processing time of only 10 instead of 15 min0 is required

With the addition of triethanolamine, not only can the original pH value in the dye bath be re-established, but a shortened dyeing time can also be achieved as well as an improvement in color evenness.

This example also shows that the dyed samples first exhibit the desired color uniformity when triethanolamine is added. In an industrial application of the method, at least 1 g/l of an aminoalcohol must therefore already be added for the colouring.

Example 3.

Pre-anodized samples with the same alloy composition as in

e.g. 1 is treated in an aqueous color electrolyte, which contained 20 g/l SnS04 and 7 g/l H2S04, which gave a pH of 1.4 at 20 - 25°C with alternating current.

After the pH has dropped to 1.3, 3 g/l monoethanolamine is added. This not only raises the pH again to 1.4, but also improves color uniformity and shortens the dyeing time. Even if apart from the addition of monoethanolamine, the same conditions are used as in conventional methods, a specific hue from the achievable color scale can thus be achieved more quickly, and with better quality.

Claims (4)

1. Method for coloring pre-anodically oxidized objects made of aluminum or aluminum alloys by electrolytic treatment of the aluminum oxide layer with alternating current in an acidic, aqueous electrolyte, which contains dissolved coloring metal salts, preferably ions of at least one of the metals Ni, Co, Cu, Sn Ag ,See, characterized in that amino alcohol is added to the electrolyte so that the amino alcohol content in the bath amounts to at least 1 g/l, preferably at least 2 g/l. 2. Method as stated in claim 1, characterized in that the pH value, which decreases during the coloring, is corrected by adding amino alcohol. 3. Method as stated in claim 1 or 2, characterized in that a strongly elevated pH value is corrected with sulfuric acid when amino alcohol is added for the coloring. 4. Method as stated in claims 1-3, characterized in that mono-di- or triethanolamine or mixtures thereof are used as amino alcohols.