DE2106388B2 - Process for the electrolytic production of blue oxide layers on aluminum alloys - Google Patents

Description

15th

The invention relates to a method for producing colored oxide layers with a blue base tone Aluminum alloys by anodic oxidation in a chromic acid bath using direct current and Subsequent electrolytic treatment in an aqueous-acidic bath containing nickel, cobalt or copper salts by means of alternating current and recompaction.

According to FR-PS 880095 pigments are on the Based on metal compounds in the pores of the aluminum oxide layer on the aluminum backing stored by electrolysis with alternating current in an acidic solution of a copper, nickel or silver salt. With this one achieves pink, bronze, red or black shades and, as a modification, yellow, brick red, chestnut brown and chocolate brown. From FR-PS 14 77 823 is a process for the production of a blue color on pure aluminum (99- to 99.9%) by means of electrolysis Direct current in a bath containing 80 g / l chromic acid and then by means of alternating current in one Bath containing 25 g / l boric acid, 20 g / l nickel sulphate and 15 g / l ammonium sulphate are known. For the Anodic oxidation can also be done in a bath with a content of 80 g / l sulfuric acid and 15 g / l Oxalic acid can be used. In this case the coloring electrolyte contains 25 g / l nickel sulfate and 15 g / l Ammonium sulfate The layers obtained are pale blue. It turned out, however, that this procedure was not is reproducible with certainty, and which one obviously depends on unknown factors Shades and depths of color are obtained It is worth noting that the US Pat. No. 3,382,160, which corresponds approximately to the French patent, is used the same aluminum material and the same electrolyte compositions with the same process conditions to brownish or bronze shades comes, which confirms the general practice contrary to the statement of the French patent specification

It is known that anodic oxidation occurs under direct current in chromic acid baths a certain reduction in chromium (VT> ions in Chromium (iII) ions come but this is after previous method very undesirable, so that in the sense of FR-PS 13 99 797 at the beginning of the electrolyte an oxidizing agent, such as hydrogen peroxide, added to suppress the formation of chromium (III) ions.

It has now been shown that, surprisingly, by adhering to a special composition of the aluminum material and d *. ~ Composition of the electrolytes for the anodii »oxidation under direct current and for the coloring of the oxide layer under alternating current, well reproducible blue oxide layers of different shades and depths of color can come. The invention thus relates to a method for producing colored oxide layers with a blue base tone on aluminum alloys by anodic oxidation in a chromic acid bath using direct current and subsequent electrolytic treatment in an aqueous acid bath containing nickel, cobalt or copper salts using alternating current and recompaction and is characterized in that an aluminum alloy containing 0.45 to 6% magnesium and / or 0.5 to 2% cobalt and / or 1 to 6% nickel and optionally up to Wo zinc, up to 0.3% copper and up to 1.18% Silicon and up to 1% manganese are used and the oxidation is carried out in a chromic acid bath containing chromium (II) ions. The oxide layers obtained according to the invention can range from a simple violet shimmer through a vivid blue to a dark blue.

As mentioned, the electrolyte for the anodic oxidizer must contain not only hexavalent chromium but also trivalent Ch mm ions, which according to the prior art should be avoided. Such an electrolyte is obtained by adding 10 to 200 g / l of chromic acid in an aqueous solution Reacts oxalic acid, which reduces a certain amount of chromic acid. According to the invention, preference is given to using an electrolyte obtained from 60 g / l chromic acid and 20 g / l oxalic acid. The anodic oxidation should take about 50 minutes at 50 V and 50 ° C., so that a layer thickness of 5 to 6 μm is obtained. In principle, layer thicknesses of 1 to 20 μm, which can be obtained at a current density of 0.1 to 5 A / dm or 10 to 100 V, can also be used. It was found that the blue hue occurs in the coloring electrolyte in the presence of a wide variety of metal compounds, the type of metal compound used being of no essential importance. The pH of the coloring electrolyte depends on the metal salt used and its position in the electrochemical series. The concentration of metal salt is generally 5 to 100 g / l. The counter electrode can be inert, e.g. B. made of corrosion-resistant steel or self-consuming and then consists of the metal that corresponds to the metal salt used. The electrolysis should take 30 seconds to 15 minutes and take place at a bath temperature between 18 and 22 ° C. The alternating current is set with a constant current density of 0.1 to 5 A / dm ² (eff.) Or a constant voltage of 10 to 50 V (eff.)

The preferred electrolytes are: Electrolyte A

20 g / l cobalt sulfate,

25 g / l boric acid,

15 g / l ammonium sulfate,

Electrolyte B 100 g / l nickel sulfate, 25 g / l boric acid, 30 g / l ammonium sulfate,

Electrolyte C 15 g / l copper sulfate,

5 g / l sulfuric acid. j

The blue hue, which occurs after a few 10 coulombs / dm ² under the process conditions described below, can be preceded by a purple sheen. The darker shades are generally achieved after a current of the order of 100 coulombs / dm ²

The colored oxide layer obtained in this way is porous and therefore not corrosion-resistant and must therefore be sealed or re-compacted. It has been shown that the usual recompression in boiling Water with the optional addition of a nickel salt frequently changes the color shade achieved according to the invention. On the other hand, good results are achieved when the workpiece is 1 to 45 minutes in a simmering aqueous sodium silicate solution containing 0.1 to 1 g / L sodium silicate is immersed.

Examples

In all examples, the workpieces were subjected to the following operations:

1. Degreasing, pickling or pickling and mechanical polishing or chemical or electrolytic polishing.

2. Anodic oxidation.

3. Coloring electrolysis.

In a first series of tests, an aluminum alloy with Mg 0.47%, Si 0.45%, Fe 0.24%, Cu 0.01% and Mn 0.05% of the anodic oxidation in chromic acid baths with increasing oxalic acid content and the coloring electrolysis in electrolyte A subjected. The results are summarized in Table 1.

Tabel

Anodic oxidation Oxalic acid A / dm ² Coloring electrolysis V (end) hue Brown example Chromic acid fe / 1) 0.6 25th brown with blue (g / l) 0 0.6 30th shine 1 (comparison) 60 5 bluish 2 60 0.6 30th bright blue 10 0.6 32 blue 3 60 20th 0.6 31 4th 60 50 5 100 Min. 2 2 2 2 2

The comparison with the comparison example 1 shows that the addition of oxalic acid turns the hexavalent chromium ions into trivalent ones Chromium ions are reduced, is essential to achieve a blue hue.

In a further series of tests, the magnesium content of the aluminum material was varied Working conditions were kept constant.

The electrolyte for the anodic oxidation contained 60 g / l chromic acid and 20 g oxalic acid, in the coloring electrolysis electrolyte A was used and a current density of 0.4 A / dm ^{2 for} 2 and 4 minutes at a final voltage of about 30 V (eff .) maintained. The results are summarized in Table 2.

Fe Si Al with Tabel : 2 Mn C / dm ² hue (%) (V.) Cu (V.) example 0.39 0.13 (V.) Mg <0.04 36 gray (comparison) 0.46 0.11 0.040 C /.) <0.04 48 playing into the violet 6th 0.46 0.11 0.025 0.03 <0.04 96 dark purple 7th 0.52 0.13 0.025 . 0.56 <0.04 48 playing into the violet 8th 0.52 0.13 0.030 0.56 <0.04 96 violet 9 0.43 0.12 0.030 1.40 <0.04 48 bright blue 10 0.43 0.12 0.025 1.40 <0.04 96 bright blue 11 0.39 0.14 0.025 3.34 <0.04 96 bright blue 12th 0.030 3.34 13th 5.21

Comparing Example 6 with Examples 7 A further series of tests shows the influence of

up to 13 according to the invention, it turns out that another alloy constituents and / or accompanying substances,

Magnesium content within the specified limits such as silicon, manganese, nickel, cobalt, zinc and

is necessary for the blue base tone to appear. Copper. The limit value is <0.6% magnesium.

Tabel

<t

example Legicration
additive Fe Si Cu Mg Mn Other
elements _ C / dm ² hue 14th Mn 0.58 0.20 0.13 0.005 1.20 108 Gray - (Comparison) 15th Si 0.42 6.7 0.08 0.05 0.3 - 240 Gray - (Comparison) 16 Cu, Mg 03 0.5 4th 0.6 - 48 Gray 0.9 (Comparison) 17th Co 0.12 0.06 0.03 0.01 0.03 Co 4.5 72 dark blue 18th Ni 0.11 0.07 0.02 G, 01 0.02 Ni - 144 Light Blue 19th Mg, Mn 0.31 0.06 0.01 0.99 0.74 - - 108 dark blue 20th Si, Mg, Mn 0.44 1.18 0.01 0.92 0.46 - - 108 dark blue 21 Si, Mg, Cu 0.19 039 0.23 0.47 <0.01 - 4.61 48 blue-violet 22nd Zn 0.08 0.06 0.1 1.31 <0.005 Zn 48 blue-violet

It can be seen from Table 3 that a blue color tone cannot be obtained by silicon alone. On the other hand, silicon in an alloy containing magnesium does not change the hue (example 5) and even lowers the lower limit for the magnesium content: an addition of 0.56% magnesium alone (example 7) results in a hue that is violet; on the other hand, when 0.45% silicon and 0.47% magnesium (Example 5) are added at the same time, a bright blue color is achieved. Manganese also has a reinforcing effect, as a comparison of Examples 19 and 9 shows, without affecting the blue hue .

A high content of copper (Example 16) prevents the appearance of a blue hue; However, copper can be contained in the alloy in an amount up to 0.3% (Example 21). Zinc in an amount up to 4.6% does not prevent the blue coloration (Example 22); In other tests it has been shown that the zinc content of the alloy can be up to 6%; however, in the presence of zinc , like copper, there is a tendency towards a shade of violet.

A fourth series of tests shows that the particles of a metal compound can be deposited in the pores of the aluminum oxide layer in very different ways and that the metal of the metal compound in particular has little or no effect on the color shade obtained. Table 4 summarizes the conditions and results of this series of experiments (anodic oxidation in a bath with a content of 60 g / l chromic acid and 20 g / l oxalic acid, layer thickness 8 μm). Examples 24 to 28 show that on the same aluminum materials, the more intense blue tones are achieved, the greater the passage of current. As can be seen in Example 23, a yellowish color is initially obtained with the same aluminum. It can also be seen from Examples 24 to 30 that the blue hue is obtained regardless of the counter-electrode used.

Tabel

example Mg Si Mn electrolyte Counter electrode C / dm ² hue (V.) 23 0.92 1.18 0.46 A. Co 36 yellowish (comparison) 24 0.92 1.18 0.46 A. Co 72 Light Blue 25th 0.92 1.18 0.46 A. Co 90 blue 26th 0.92 1.18 0.46 A. Co 108 dark blue 27 0.92 1.18 0.46 A. Co 180 dark blue 28 0.92 1.18 0.46 B. Ni 90 dark blue 29 0.56 0.45 0.05 C. Cu 480 blue 30th 0.56 0.45 0.05 D. stainless steel 96 bright blue

D = cobalt sulfate 50 g / l, boric acid 35 g / l, ammonium sulfate 20 g / l.

Claims (1)

Claim: Process for the production of colored oxide layers with a blue base tone on aluminum alloy by anodic oxidation in a chromic acid bath using direct current and subsequent electrolytic treatment in one aqueous acidic bath containing Ni, Co or Cu salts by means of alternating current and compress, d a d u r c h e k e π η ζ e i c h π e t that an aluminum alloy containing 0.45 to 6% magnesium and / or 0.5 to 2% cobalt and / or 1 to 6% nickel and optionally also up to 6% zinc, up to 0.3% copper and up to 1.18% silicon and up to 1% manganese and anodic oxidation in one Chromic acid bath containing chromium (III) ions is carried out.