HU208344B - Direct dyeing anodic oxidation process for producing bluish grey or blue oxide layer on aluminium and aluminium alloys - Google Patents

Abstract

The electrolyte comprises (g/l): 20-250 sulphuric acid, 1-20 alkaline hexacyano-ferrate (II), 1-50 ascorbic acid as reductant, 10-80 sulphonic acid and 5-50 additives. Voltage is 10-50 V, and current density 0.1-10 A/dm2 single current. The obtd. oxide layer powder is treated in the usual way.

Description

The present invention relates to a process for the production of blue-gray or blue oxide layers by direct anodic oxidation of aluminum and its alloys.

It is known to produce anodic oxidized colored surface aluminum or aluminum alloy by electrolytic formation, direct dyeing, or subsequent electrolytic dyeing of the colorless oxide layer, by indirect dyeing, or by adsorption dyeing of a colorless oxide layer with dyes. Known dyeing capabilities are the recently developed interference dyeing (European Patent No. 3,175) which is not yet widely used in the industry.

These methods do not allow the production of self-gray bluish or ultra-blue oxide layers that are in harmony with conventional architectural materials.

The GRINATAL process (Zurbrügg, É .: Schweizer Aluminum Rundschau 08-08-1964) is known for the production of gray oxide layers by spot dyeing. By this method, the production of gray oxide layers of different shades is solved by:

They contain 3.5 to 5% of silicon in the metal. If this AlSi ₅ alloy is anodized in the classical manner in 20% sulfuric acid, the resulting oxide layer will be gray. The color of the layer is caused by the silicon crystals embedded in the structure of the oxide, which are already present in the alloy with a fine, relatively uniform distribution. The finely dispersed embedded silicon particles reduce the transparency of the oxide layer, imparting a gray color to the material. However, this process is not widespread and is not used today because the production of high silicon alloys requires special metallurgical technology and is therefore quite expensive.

The GRINATAL process is available in a gray color with a mixture of white and black in varying proportions to form the true color, depending on the degree and degree of dispersion of the uniformly distributed silicon crystals embedded in the oxide layer.

The production of a gray oxide layer by spot dyeing technique is also known in U.S. Patent No. 160,088. Hungarian patent specification. According to this, in electrolytes containing organic substances, the metal is anodized at a voltage of 50-100 V dc with a current density of 1-10 A / dm ² . However, this process also produces a gray color that lacks the blue components that produce the blue tone, and the color of the layers corresponds to different shades of pure gray (from light gray to black) or to brownish or yellowish tones.

The use of potassium hexacyanoferrate (H) as an additive in an anodizing bath is mentioned by Y. Shimajiri (Aruminyumu Kenkyu Kaishi 1976, 109, 25-33). In its direct dyeing process, it is subjected to alternating treatment of aluminum in a solution containing sulfuric acid, potassium hexacyanoferrate (H) and boric acid, whereby a distinctly blue oxide layer is formed. A process for the preparation of higher hardness anodic alumina is known in U.S. Patent No. 235,081. NDK patent, according to which sulfuric acid containing potassium hexacyanoferrate (II) is also used as an electrolyte. By using direct current anodization, a blue oxide layer is formed as an additional result.

The aforementioned processes produce oxide layers whose blue color is formed by blue iron cyanide compounds deposited on the upper part of the layer or on the surface. It follows that these processes do not allow the formation of a reproducible blue oxide layer.

A further disadvantage of the processes is that baths containing potassium hexacyanoferrate (II) complex decompose very rapidly and the solutions turn blue within 10-12 hours due to the rapid precipitation of blue precipitate. As a result, electrolytes will soon become unsuitable for further anodization. Thus, the industrial application of baths containing potassium hexacyanoferrate (II) complex for anodizing is only possible if the self-decomposition of the iron cyanide complex is prevented and the bath is stabilized.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process using an alkaline hexacyanoferrate (II) dyeing agent which allows direct dyeing of a single, uniformly blue-gray, greyish-blue, or blue-colored oxide layer on aluminum or aluminum alloys, the self-decomposition of anion with formation of blue precipitate is suppressed to such an extent that the stationary conditions for electrolytic dyeing are maintained for a long period of time.

It has been found that this object can be achieved by anodizing the aluminum or aluminum alloy in the electrolyte containing sulfuric acid, alkaline hexacyanoferrate (II) and reducing agent in the spot dyeing process so that the anodic treatment is carried out with 20,250 g / dm ³ g / dm ³ alkaline hexacyanoferrate (II) electrolyte with a DC voltage of 10-50 V and a current density of 0.1-10 A / dm ² , which is 150 g / dm ^{3 of} alkaline iodide or bromide and 1- It contains 50 g / dm ^{3 of} ascorbic acid and optionally 10 to 80 g / dm ^{3 of} organic sulfonic acid and / or 5 to 50 g / dm ^{3 of} known anodizing excipient and seals the pores of the resulting oxide layer in a known manner.

The present invention is based on the discovery that high sulfuric acid solutions containing hexacyanoferrate (II) anions exhibit an outstanding and special stabilizing effect over conventional organic and inorganic reducing agents, as well as alkaline iodides or bromides, and especially the combination thereof. that, in their presence, the solution does not undergo blue-blue decomposition, even after months of anodization, and the solution remains clear. As potent reducing agents, these additives inhibit the formation of the Fe ³⁺ ions required for the formation of blue compounds, and also strongly inhibit the catalytic self-decomposition process.

The two reducing agents mentioned are constant re2

HU 208 344 Β. For example, potassium iodide reduces the oxidized dehydro-ascorbic acid to ascorbic acid,

The iodine formed in the redox reaction of 2Fe ³⁺ + KJ o 2Fe ²⁺ + J ₂ is converted by ascorbic acid to iodide. Nascent hydrogen evolving on the cadaver also exerts a reducing effect on oxidized reducing compounds. As a result of these interactions, the reducing additive concentrations remain constant over a long period.

It was observed that the concentrations of hexacyanoferrate additives remained constant for a long time.

It has been observed that in the presence of ascorbic acid and alkali iodide or bromide in the presence of ascorbic acid and sulfuric acid containing hexacyanoferrate (II), no harmful by-products resulting from self-decomposition occur. As a result, during the anodization, the layer coloration is solely based on the reaction of the iron and other accompanying metal ions liberated from the aluminum with the hexacyanoferrate (H) ions in the solution, which ensures uniform coloring at all times.

It has also been found that the use of organic sulfonic acids, especially sulfosalicylic acid, is very advantageous in avoiding the possible formation of colored deposits on the surface by more intensive anodization and high current densities. In the presence of this, the formation of surface precipitation is not achieved even at very high current density anodisation, thus ensuring uniform coloring under such current conditions. Organic sulfonic acid additives also attenuate the extremely potent effect of sulfuric acid. Sulfuric acid, when used alone, can initiate oxidation-reduction side-processes that can lead to surface deposits not incorporated in the oxide layer on the surface of the anodized body. With moderate sulfuric acid content and sufficient sulfonic acid content, undesirable side reactions are completely eliminated.

The process according to the invention is preferably carried out as follows. The counter electrode used is aluminum metal. The anodic oxidation itself is carried out in the usual manner. The bath is vigorously stirred and the temperature is preferably adjusted to between 15 and 25 ° C and maintained to within ± 2 ° C. Treatment

It is continued for 10-90 minutes, preferably 40-60 minutes. If desired, anodizing excipients are added to the electrolyte at a concentration of 5 to 50 g / dm ³ . Anodizing aids include, for example, boric acid, tartaric acid, citric acid or other known acidic additives which dampen the aggressive action of sulfuric acid and thus advantageously influence the structural properties of the oxide layer formed. Continuing the anodic treatment in this manner produces a uniform color oxide layer of 20-45 µm on the metal surface in about 40-60 minutes.

The colored oxide layer may be subjected to any known hot water or cold pore sealing process. This after-treatment does not change the nature, intensity and UV light resistance of the resulting color.

The oxide layer produced by the process according to the invention is not always yellowish, brownish, bronze-gray or pure gray, but in all cases the base gray, in both lighter and darker versions, is blue. Such blue tones of gray can not be produced by any known method, even by adsorption dyeing, although in the latter case the range of colors is almost unlimited.

The particular bluish-gray color of the architectural aluminum profiles produced by the process of the present invention, or the lighter or darker versions and shades of the aluminum profiles as desired, fit better and more harmoniously with the materials of the building industry and create a very good color harmony.

A further advantage of the process according to the invention is that the power consumption of the anode treatment is low. In fact, known methods of direct dyeing with organic electrolytes require a voltage of 30-100 volts DC or AC to adjust the current density, usually 6-8 A / dm ² . Since our process is based on anodizing in sulfuric acid electrolytes, a voltage of 18-26 V is sufficient to achieve a current density of 2-4 A / dm ² .

The process of the present invention produces oxide layers that vary in color from light blue to dark blue to blue, depending on the various types of aluminum alloys and current parameters, while retaining the metallic nature of the workpiece. There is an inverse proportion between the film thickness and the degree of lightness of the color, which means that the thicker the oxide layer, the darker the color. A special advantage of this method is that while with most spot coloring methods the color coordinates change with increasing layer thickness and darkening of the color, ie the spectral composition of the blue-gray color is practically unchanged, only the Y luminance degrees change. This means, for example, that for metallic aluminum, the color coordinates are unchanged (x 0.30 + 0.01, y = 0.32 + 0.01), measured with a MOMCOLOR-D tristimulus colorimetric meter, only at the Y brightness its value decreases with the thickness of the layer, strictly linear function

The hardness of the colored oxide layer is higher than that of the classical sulfuric acid anodization, so that the Vickers hardness of the oxide layer is HV = 450600 kp / cm ² . Its abrasion resistance is also significantly improved and its mechanical properties meet practical requirements.

By virtue of the special advantages listed above, the invention permits the formation of a uniform, durable and weather-resistant bluish-gray color on the aluminum surface. The process is easily industrially feasible. Due to the stability of the electrolyte, only the inevitable evaporation and delivery volume losses as well as the inevitable consumption of ascorbic acid through oxidation need to be replaced.

HU 208 344 B

The invention is illustrated by the following examples.

Example 1

180 g / dm ³ H2SO4, 1.5 g / dm ³ ^ [FefCNU 5.0 g / dm ³ potassium iodide, 3.0 g / dm ³ ascorbic acid, 5.0 g / dm ³ H3BO ₃ and 20 g / In a solution of dm ³ citric acid at 20 [deg.] C., 5 x 5 cm aluminum sheets are placed as anode or cathode.

A DC voltage is applied to the electrodes and oxidation is initiated with constant stirring of the solution by setting a power consumption of 2.0 A / dm = 2. After 45 minutes, the anodized aluminum plate forms a 24 µm thick uniform blue-gray oxide layer. The pores of the oxide layer were sealed in distilled water at 100 ° C for 50 minutes.

Example 2 g / dm ³ H2SO4, 60 g / l sulfosalicylic acid, 1.0 g / dm ³ K4 [Fe (CN) 6], 10 g / dm ³ potassium iodide and g / dm ³ ascorbic acid at 18 ° C. 3M x 10 cm AlMgSiO ₅ alloy aluminum plates are placed in an continuously tempered solution as an anode and a cathode aluminum plate as the cathode, then anodized for 50 minutes at a voltage of 26 V and an initial current of 2.6 A / dm ² . An anodic blue oxide layer of 24 µm thickness is formed on the anode plate. The pores of the oxide layer were sealed as in Example 1.

Example 3 g / dm ³ H2SO4, 10 g / dm ³ of K4 [Fe (CN) 6] continuously annealed to a temperature of 20 g / dm ³ potassium iodide and 25 g / dm ³ concentration of ascorbic acid solution at 22 ° AlMg ₃ alloy 3x10 cm aluminum plates are anodized for 26 minutes at 26 V DC and 3 A / dm ^{2 of} initial power consumption. The anodized aluminum alloy forms a highly bluish-gray oxide layer with a thickness of 30 µm. The pores were sealed as in Example 1.

Example 4

As a first step, a 5 x 5 cm forged aluminum plate is anodized for 10 minutes in a sulfuric acid bath of 186 g / dm ³ at a voltage of 20 V to form a pre-layer having a thickness of about 10 µm. As a second step, the metal plate is further anodized for 50 minutes at a direct current density of 4 A / dm ^{2 in} a spot color bath of Example 1 at 22 V DC. A total of 45 µm of a very attractive dark blue-gray oxide layer is obtained. The pores were sealed as in Example 1.

Example 5

200 g / dm ³ H2SO4, 5 g / dm ³ sulfosalicylic acid, 2 g / dm ³ Na4 [Fe (CN) ₆ ], 5 g / dm ³ potassium bromide,

In a solution of 2.5 g / dm ³ ascorbic acid and 10 g / dm ³ citric acid at 20 ° C, AlMg _s alloy 3x10 cm aluminum plates were anodized for 24 minutes at 24 V DC and 2.0 A / dm ² . This produces a very nice blue-gray oxide layer of 25 µm thickness. The pores were sealed as in Example 1.

Example 6

This example illustrates the effect of the type of metal alloy on the color tint when using the anodizing parameters of Example 1. Table 1 illustrates the color properties of the oxide layers by numerical values of the x and y color coordinates obtained with the MOMCOLOR-D measuring apparatus and Y luminance degree.

Table 1

Alloy type Layer thickness pm color Coordinate Degree of brightness Y Visual characterization X y Al 99.5 24.0 0.308 0.322 27.92 blue gray AlZn ₅ Mgl 11.5 .309 0.320 32.46 blue gray AlMgSil 26.3 0.305 .319 30.64 blue gray AlMg ₄ , 5Mn 27.6 0.303 0.313 10.40 navy blue AlMgSiCu 5.2 0.304 0.316 12.77 gray-blue AlMg ₃ 22.0 0.310 0.324 29.86 blue gray AlMg ₄ 26.8 0.322 .332 33.65 blue gray AlMgj 24.3 0.310 0.325 26.74 blue gray

PATENT CLAIMS

Claims (2)

A direct dye anodic oxidation process for the production of a blue-gray or blue-colored oxide layer on aluminum and aluminum alloys by treating the aluminum or aluminum alloy with an electrolyte containing sulfuric acid, an alkali hexacyanoferrate (II) and a reducing agent. in electrolyte containing dm ³ sulfuric acid and 120 g / dm ³ alkaline hexacyanoferrate (II) with a DC voltage of 10-50 V and a current density of 0.50 A / dm ² , which as the electrolyte reducing agent is 1-50 g / dm ³ alkaline iodide or bromide and 1-50 g / dm ³ ascorbic acid, and given In 55 cases, it contains 10-80 g / dm ^{3 of} organic sulfonic acid and / or 550 g / dm ^{3 of} known anodizing excipient and the pores of the resulting oxide layer are sealed in a known manner. 2. The method of claim 1, wherein the anode treatment is at a temperature of 15-25 ° C 60 10-90 minutes.