HU208346B - Direct dyeing oxidation process for producing hard oxide layer of chocolate brown basic colour on aluminium and its alloys - Google Patents

Abstract

Anodic treatment of Al (alloy) is carried out in an electrolyte comprising (g/l) 3-100 sulphosalycilic acid, 20-80 maleic acid and 2-20 boric acid at 30-70 V and 0.5-5 A/dm2 current. The reducing agent is 5-50 g/l ascorbic acid and 0.5-2 g/l sulphuric acid. The produced oxide powder layer is treated by usual method

Description

FIELD OF THE INVENTION The present invention relates to a process for the preparation of specific chocolate-colored hard oxide coatings on aluminum and its alloys by direct color anodic oxidation.

The two most important general methods of coloring anodic alumina are direct and indirect coloring. The essence of spot coloring is mainly based on the electrochemical oxidation of the metal in solutions containing organic compounds, which results in a gradient of brown to basic or coloration on the aluminum surface, possibly ranging from greyish blue to black.

In the case of subsequent or indirect tinting, the colorless anodic oxide layer, usually formed in a sulfuric acid medium, is subsequently tinted by electrodeposition of very fine particles of colloidal metal or metal compound into the pores of the oxide layer.

For less demanding purposes, adsorption dyeing by dipping the oxide layer produced by anode treatment in sulfuric acid electrolyte into inorganic or organic dyes is also a common method. In this way, color variations in almost the entire spectrum of the visible spectrum can be produced on the oxide layer. However, organic dyes or inorganic pigments are generally poor in weathering and light fastness, so that adsorption dyeing on aluminum structural materials used in architectural and other outdoor applications is limited.

Current and voltage requirements for spot dyeing are generally higher than two-stage indirect dyeing, but are still preferred for oxide coatings, since the weathering and color stability of the oxide layers thus produced are often superior to those achieved with post-dye oxide layers.

The absolute advantage of spot dyeing methods is that the dyeing and anodising and shorter anodising take place in one step, which, together with the increased production capacity, largely compensates for the increased electricity costs.

A number of processes are known for producing spot-tinted oxide coatings. They are based, inter alia, on the application of Resolution 962 048. and U.S. Patent No. 973,391. British Patent No. 3,031,387; U.S. Pat. Hungarian patent specification. A common feature of the processes is that the main coloring component in the anodizing electrolyte is an aromatic sulfonic acid, most often sulfophthalic acid or sulfosalicylic acid. Given that oxidative decomposition of organic sulfonic acids also occurs during anodization, baths always contain decomposition additives, usually organic or inorganic reducing agents. In addition, although the bath undergoes a slower oxidative decomposition, the bath itself becomes discolored, while the concentration of the original non-degraded aromatic sulfonic acid decreases relatively rapidly. Taken together, these effects quickly change the initial optimum conditions for anodizing.

The disadvantage of sulfosalicylic acid-based spot color baths is that the spectral composition of the color is strongly dependent on the voltage and current, so that the most varied shades of brown, golden, tan, greyish brown may develop depending on the change of current parameters. It also has the disadvantage that the spectral composition of the resulting color is strongly influenced by the type of aluminum alloy to be anodized. Taken together, this results in the fact that only metal parts of the same alloy type can be counted for spectrally uniform colors, with strictly specified current parameters.

A further disadvantage of spot dye baths containing sulfosalicylic acid is that sulfosalicylic acid forms a red colored stable complex with the metal ions liberated from the metal during anodization, and thus the concentration of free acid involved in the anodization is reduced relatively rapidly. Exceeding the solubility value, the solid metal complex appears over time in the bath.

Experience has shown that known sulfosalicylic acid baths do not provide sufficient stability of the electrolyte in the presence of stabilizing additives, such as the sulfosalicylic acid-formic-hydroquinone three-component system disclosed in U.S. Patent No. 155,034. In the Hungarian patent, it is known that even in the presence of the stabilizing hydroquinone, it rapidly becomes reddish during anodization and irreversibly decomposes.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome the above-mentioned disadvantages, with the further object of providing a spot-dye anodic oxidation process capable of producing a reproducible spectral composition of a chocolate-colored base oxide on aluminum and aluminum alloys.

In the course of our research, we have investigated a process with sulfosalicylic acid baths already used for similar purposes and considered it an improvement of the invention.

It has been found that this object can be achieved by anodizing the aluminum or aluminum alloy in an electrolyte containing sulfosalicylic acid, maleic acid, boric acid and reducing agent by treating the sulfosalicylic acid with ^{3 to} 100 g / dm ³ , in an electrolyte containing dm ³ maleic acid and 2-20 g / dm ³ boric acid with a DC voltage of 30-70 V and a current density of 0.5-5 A / dm ₂ , which is 550 g / dm ^{3 of} ascorbic acid as an electrolyte; It contains 0 g / dm ³ sulfuric acid and the pores of the resulting oxide layer are sealed in a known manner.

It has been observed that when ascorbic acid is used as a special reducing agent in the sulfosalicylic acid electrolyte of the composition defined above, the electrolyte is discolored and the ascorbic acid ensures bath stability. This unexpectedly good result can be attributed to the fact that ascorbic acid uniquely reduces the iron dissolved in the treated metal and holds it in the form of ferro-ion, so that sulfosalicylic acid cannot form a complex with iron.

HU 208 346 Β keeps the bath stable. Sulfuric acid 0.5-2.0 g / dm ³ , preferably 1.5-2.5 g / dm ^{3, is} added to increase the conductivity, thus accelerating the oxidation, to obtain an electrolyte in which the aluminum ion or aluminum alloy is treated with anode Using a DC current of 40-70 V, a warm-toned chocolate bama colored oxide coatings are formed. In this electrolyte system, specifically, the reducing ascorbic acid provides bath stability. Providing a steady supply of ascorbic acid, which is slowly oxidized by self-decomposition, will not cause any noticeable decomposition of the bath, even after regular prolonged use, and consequently will remain clear and clear even after months. The use of this special electrolyte combination also has the advantage that changing the voltage and current density within a relatively wide range does not significantly affect the spectral nature of the color, such as the development of a warm-toned chocolate bama color. At most, the depth of the color can be varied by accelerating the oxidation by increasing the voltage, i.e., increasing the thickness of the oxide layer formed over time. In contrast to other sulfosalicylic acid-based spot dyeing processes, the present invention utilizes the oxide layers of practically constant spectral composition in the alloy types used, which means that the diversity and types of alloying elements in the final color are hardly relevant.

The advantages of using the electrolyte combination defined above are also reflected in the favorable evolution of the current parameters. While most organic sulfonic acid-based spot dyeing processes require a voltage of 70 to 100 V, the process of the present invention already provides satisfactory results at 40 to 55 V.

There is also a positive difference in the current density. For example, U.S. Patent No. 155,034. In the case of the sulfosalicylic acid - formic acid - hydroquinone system according to the Hungarian patent, the starting current may be 1015A / dm ² , but in the process according to the invention, the current consumption in the direct dye bath described above does not exceed 3.03,5 A / dm ² . .

Also, the time required for anodizing is particularly advantageous due to the high value of the anodizing rate. When using 55 V DC, eg. achieving a thickness of 20 μπι requires only 15 to 20 minutes of anodizing time, which means that the capacity of the bath allows for very significant bath utilization.

The process according to the invention is preferably carried out in such a way that the anodic treatment is carried out at a temperature of 15-25 ° C for 530 minutes at said current parameters. Upon completion of the treatment, the pores of the oxide layer are sealed by the known hot water treatment.

In this way, a uniformly colored oxide layer of 5-25 μπι is formed on the surface of the aluminum and its alloy grades, depending on the selected current parameters.

Together, these advantages play a special role in the application of the method, which is preferably supported by more than one measurement.

Between the thickness of the layer and the degree of luminance of the color measured by the optical measuring instrument M0MC0L0R-D up to about 20 μιη in practical use, there is a linear curtain proportional to the proportionally darker color of the thicker layer. Above 20 μπι, the function points in the direction of the saturation curve.

In addition to the above mentioned advantages, the use of sulfosalicylic acid-maleic-ascorbic acid boric acid-sulfuric acid electrolyte combination has the outstanding advantage of the hardness of the oxide layer formed in the bath to reach or exceed the hard oxide formed in the electrolyte specially formulated for the production of hard oxide. Extreme hard aluminum oxide layers are those with a Vickers hardness greater than HV = 400. Accordingly, the hardness of the oxide layers produced by the spot dyeing process of the present invention far exceeds the requirements for the extra hard oxide layer. Depending on the type of alloy, the following hardness values characterize the oxide layers:

Aluminum: HV = 530-540

AlMgSiO ₅ alloy: HV = 550-560

AlMg ₃ on metal: HV = 560-570

On AlSi ₃ metal: HV = 630-640

The oxide layers also have excellent light fastness. Irradiated with a 400 W medical quartz lamp at 60 cm, the anodized specimens were not subject to the slightest discoloration or fading in 2000 hours, either visually or when measuring the Y brightness with a M0MC0LOR-D tristimulus optical meter.

The invention is illustrated by the following examples.

Example g / dm ³ sulfosalicylic acid, 50 g / dm ³ of maleic acid, 10 g / dm ³ of ascorbic acid, 10 g / dm ³ and 1.7 g of boric acid / dm ³ sulfuric acid concentration of 18 to 20 ° C to solution annealing in the usual 1 After pickling pre-treatment, a slab of aluminum was placed, also against a slab of aluminum. A 55 V DC is applied to the electrodes and anodized oxidation is initiated at a current of 2.3 A / dm ² . After 15 minutes, the metal plate connected to the anode forms a medium-strength chocolate brown oxide 17 μπι thick. The pores of the oxide layer were sealed by boiling in 100 ° C water for 20 minutes.

EXAMPLE solution g / dm ³ sulfosalicylic acid, 60 g / dm ³ of maleic acid, 40 g / dm ³ of ascorbic acid, 5 g / dm ³ of boric acid and 1.0 g / dm ³ sulfuric acid containing from 18 to 20 ° C on an annealed second In contrast to the aluminum aluminum cathode plate, an anodic plate made of AlMg ₃ alloy pretreated with conventional alkaline pickling is placed. A 70 V DC is applied to the electrodes and anodized for 15 minutes at 3.5 A / dm ²

HU 208 346 B at a density. Upon completion, a dark, slightly greyish-brown chocolate brown solid oxide layer is formed on the metal surface. The pores of the oxide layer were sealed as in Example 1. The resulting oxide layer had a Vickers hardness of HV = 570.

Example 3 g / dm ³ sulfosalicylic acid, 30 g / dm ³ of maleic acid, 20 g / dm ³ of ascorbic acid, 15 g / dm ³ and boric acid

In a solution containing 1.4 g / dm ³ sulfuric acid, a profile piece made of AlMgSiO ₅ is placed as an anode against the aluminum cathode plate. The temperature of the solution was brought to 22 ° C. A 60 V dc voltage was applied to the electrodes and the anode treatment was continued for 20 minutes at a starting current density of 2.8 A / dm ² . The profile pieces produce a very attractive dark chocolate bama colored oxide coating having a micro hardness of HV-560.

Example 4 An aluminum alloy containing 2.5% SiC in an electrolyte containing g / dm ³ sulfosalicylic acid, 70 g / dm ³ maleic acid, 30 g / dm ³ ascorbic acid, 20 g / dm ³ boric acid and 2 g / dm ³ sulfuric acid was anodized with 50 V with a DC current of 2.5 A / dm ² for 40 minutes. The oxide layer was sealed in hot water for 60 minutes. This results in an HV = 630 extra hardness oxide coating on the surface of the aluminum moldings.

Example 5

In the example we show how the type of metal alloy affects the nature and hue of the color,

With the electrolyte given in Example 1, current parameters and anodizing time of 15 minutes. In addition to the visual characterization in Table 1, the color properties of the oxide layers are also characterized by the numerical values of the x, y color coordinates obtained with the M0MC0LOR-D tristimulus optical measuring instrument and the degree of luminance Y.

Table 1

alloy Type Layer- thick- pany Visual color characterization Színkoordiná- Tak Degree of brightness pm X y Y A199.95 20.0 chocolate bronze brown 0.336 0.344 14.14 AlZn ₅ Mgl 18.2 dark chocolate bama 0.336 0.342 7.24 AlMgSil 15.0 chocolate- brown .340 0.345 13.84 AlMg ₄ 4Mn 15.0 brownish black 0.320 0.321 6.55 AlMgSiCu 5.0 bluish black 0.312 0.329 8.14 AlMg ₃ 16.0 with a greyish hue 0.342 0.349 11.26 AlMg ₄ 17.0 chocolate- brown 0.342 0.350 13.62 AlMg ₅ 19.0 chocolate- brown 0.313 0.322 13.16

Example 6

Within a given type of metal alloy, there is an approximate linearity between the thickness of the layer and the intensity of its color up to different layer thicknesses. Electrolyte and under the current conditions of Example 1 primary aluminum and anodised AlMgSiO ₅ discs of the color intensity, expressed as the lightness Y degrees (DIN 17050 and 17054) in Table 2 shows the results.

Table 2

primary aluminum AlMgSiO ₅ Rétegvastag- pany pm light degree Y Rétegvastag- pany pm light degree Y 12.5 14.50 6.1 25.55 16.0 13.70 11.0 20.44 23.0 12.01 15.0 16.80 32.0 9.53 18.7 14.00 43.0 7.95 22.0 13.10 25.0 12.58

Example 7

Similarly, anodized aluminum and AlMgSiO ₅ plates according to Example 1 also have the characteristic within the exemplary film thickness range that the optical composition and color tone of the color are almost independent of the film thickness, since they remain constant throughout. To confirm this experience, Table 3 summarizes the color coordinate values measured with the MOMCOLOR-D optical instrument.

Table 3

primary aluminum AlMgSiO ₅ Layer- thickness color Coordinate Layer- thickness color Coordinate pm X y pm X y 12.5 0.342 .340 6.1 0,356 0.358 16.0 0.338 .340 11.0 0.353 0.360 23.0 0.341 0.342 15.0 0.355 0.362 32.0 0.344 0.343 18.7 0.360 0,357 43.0 0.350 0.342 22.0 0.359 0.362 25.0 0.354 0.359

The present invention has been compared to the known sulfosalicylic acid anodic oxidation process mentioned in the introduction for electrolyte durability. In the comparative experiment, the process of the invention was continued as in Example 1, in the electrolyte and current parameters described therein for 100 hours. During this time, the electrolyte remained stable, no precipitate formed, and no turbidity occurred. In parallel, an experiment was carried out using an electrolyte containing the same amount of formic acid instead of ascorbic acid and an equal amount of hydroquinone instead of ascorbic acid.

EN 208 346 B, which was also performed with the parameters mentioned in Example 1. It was found that the electrolyte was discolored and clouded after 1-2 hours.

As can be seen from the comparison, the process according to the invention can be used for anodic oxidation treatment of aluminum or aluminum alloy over a long period of time under stable technological conditions.

Claims (3)

PATIENT INDIVIDUAL POINTS 1. A spot coloring oxidation process for the production of a chocolate base-based hard oxide layer on aluminum and its alloys, wherein the aluminum or aluminum alloy is anodized in an electrolyte containing sulfosalicylic acid, maleic acid, boric acid and reducing agent, characterized in that the treatment comprises 3 to 100 g / dm ^{3 of} sulfosalicylic acid, -80 g / dm ^{3 of} maleic acid and 2 to 20 g / dm ^{3 of} boric acid with a DC current of 30-70 V and 0.5-5 A / dm ² , which is an electrolytic reducing agent of 5-50 g / dm ³ ascorbic acid, in addition 0.52.0 g / dm ^{3 of} sulfuric acid and the pores of the oxide layer obtained are sealed in a known manner. The method according to claim 1, wherein the anode treatment is at a temperature of 15-25 ° C 5 to 30 minutes.