DE1496709B2 - PROCESS FOR ANODIC OXYDATION OF ALUMINUM AND ALUMINI ALLOYS - Google Patents

Description

The present invention relates to a method for anodic oxidation of aluminum and its Alloys in a chromic acid bath with a small cathode area in relation to the anode area.

The surface protection of aluminum and aluminum alloys through anodic oxidation in one Chromic acid bath has been known since 1923. It became a weak chromic acid concentration of 2.5 to 3.5 percent by weight applied. Thin coatings of generally 2 to were obtained with these baths 6 μΐη good corrosion resistance, but succeeded it is not possible to produce thicker and denser layers under these working conditions. Also was consumption chemicals and electricity are high and the operating time of the baths is quite limited. It is also that anodic oxidation in a sulfuric acid bath known. The protective layers obtained were thicker than when using chromic acid (generally 10 to 20 μπι), but they are very brittle and allow only a very low level of protection against corrosion.

Another method of anodic oxidation in a chromic acid bath with a chromic acid concentration between 5 and 10 percent by weight at a temperature of 30 to 40 ° C. and constant voltage has been used on a large scale. It is known that processes using chromic acid at constant voltage have the greatest disadvantage compared to processes using sulfuric acid, the price of chromic acid. Efforts were therefore made to reduce the specific consumption per unit of the anodically oxidized surface. The bath is destroyed, however, as the reduction of chromic acid increases, ie reduction of Cr ⁶⁺ to Cr ³⁺ . This reduction can be suppressed by a cathode surface which is as small as possible compared to the anode surface and thus a high cathode current density.

According to these known processes, the most favorable chromic acid concentration is between 10 and 15%. A higher concentration does not increase the layer thickness per unit of time, but leads to higher chromic acid losses due to discharge with the wet material and extensive reduction of Cr ⁶⁺ to Cr ³⁺ .

At higher operating voltages, more corrosion-resistant layers are obtained with this method, however, the losses are also quite high. At operating temperatures below 40 ° C, only transparent ones are obtained to semi-transparent coatings that are difficult to dye and brittle and also with a duration of treatment cannot be improved for more than 1 hour.

At bath temperatures below 35 ^° C., completely colorless, transparent or only slightly cloudy layers are obtained. For opaque layers, a bath temperature of at least 45 ⁰ C, requires particular from 50 to 55 ° C. The layers produced below 40 ° C have mechanical properties that are not sufficient in practice.

In order to keep the bath consumption within tolerable limits in the known processes, the loss of Cr ⁸¹ 'caused by reduction was generally compensated for by adding chromic acid, but in this way the bath becomes unduly enriched with Cr ³⁺ and dissolved aluminum .

It has also been proposed to keep the bath free of the dissolved aluminum with the aid of cation exchangers, but this measure does not eliminate ^{the main cause of frequent bath renewals, namely the accumulation of Cr 3+.} Attempts have also been made to recover the chromic acid from the used bath. However, this procedure is too complicated and expensive, although the waste water often has to be freed from chromium compounds for reasons of water hygiene. The invention relates to the production of corrosion-resistant, particularly wear-resistant protective layers on aluminum and aluminum alloys in a chromic acid bath with a cathode area that is small in relation to the anode area, whereby the following process conditions must be observed in combination:

a) chromic acid concentration 15 to 19 percent by weight,

b) bath temperature less than 30 ° C,

c) voltage less than 25 V, preferably 21 to 23 V,

d) anode area (1.7 to 2.3) B dm ² , where B is the liter volume of the bath, \

e) cathode area smaller than 0.06 B, preferably (0.03 to 0.05) B dm ² ,

f) ratio Cr ⁶⁺ : Cr ³⁺ in gram-atom greater than 70, preferably greater than 120,

g) no bath movement.

It is particularly useful to use a cation exchange resin for trapping those that have gone into solution Provide aluminum. It has been found that 0.01 to 0.2 percent by weight is used for the first batch Should add hydrogen peroxide (30 percent by volume).

By the method according to the invention

creates opaque oxide layers on objects made of aluminum or aluminum alloys that are characterized by special corrosion resistance and impermeability. Plus, it leads too much Uniform oxide layers on all surfaces of the objects to be coated.

The method according to the invention is particularly distinguished from the known ones in that an unexpectedly high throughput can be achieved with it, ie a very large area can be anodically oxidized with a given bath volume. The loss of chemicals in the bath, in particular the reduction of Cr ⁶⁺ to Cr ^3+, is significantly reduced, the power requirement is much lower than in the known processes, and finally the coatings have a beautiful, decorative appearance.

With the method according to the invention, layer thicknesses of 3 μm after 60 minutes and after 90 minutes 4.5 μπι and received 6 μπι after 120 minutes. In practice, anodizing is seldom longer than 90 minutes.

In general, the process according to the invention takes longer to anodize than is usually the case with chromic acid and constant voltage, since you need for a layer thickness of 3 μπι 60 minutes compared to 40 minutes needed with the well-known bathroom. This disadvantage is, however, made by far by the aforementioned essential Increase in the area oxidizable with one liter of electrolyte compensated for.

Investigations into wear resistance, sealing, bending behavior and dyeability resulted in comparable values with layers in

a conventional chromic acid bath at constant voltage, but are according to the invention The layers obtained are more gentle and show a decorative, enamel-like appearance.

Since according to the invention - in contrast to the known methods - there is practically no bath movement occurs, the rate of dissolution of the aluminum is significantly slower, and so is the enrichment of aluminum ions in the bathroom. The removal of aluminum ions with exchange resins is therefore a very useful and economical measure.

In the known processes, a consumption of 180 to 200 g of chromic acid per m ^{2 of} area is permissible, but according to the invention less than 24 g / m ³ , in particular 16 to 23 g / m ² , is required if the aluminum ions are removed by ion exchangers.

The method according to the invention is characterized by

a particularly low power consumption, and

S although not more than 0.8 kWh / m ² area for a layer thickness of 4.5 μm or 0.55 kWh / m ² for 3 μm.

Also the need for hydrogen peroxide for the first

Batch is comparatively very small, namely only 0.01 to 0.2 percent by weight of the bath. Without the _{addition of H 2} O ₂ at the start of work, the desired ratio of Cr ⁸⁺ : Cr ^{3+ is} only established slowly, so that the batch time would be greatly extended.

The invention is illustrated by the following examples 1 and 2 as well as comparative examples according to the prior art (A, B) explained.

example 1

Layer thickness

Bath composition

Surface of the batch per liter of bath

Batches in 9 hours

Broken surface in 9 hours per liter of bath.

Lifespan of the bath

in days

in batches

Intermediate surface up to bath consumption of 11

Salt requirement g / m ² anodized surface due to bath renewal

Bath loss per m ^{2 of} anodized surface ...
Salt loss per m ^{2 of} anodized surface

Total consumption of the bath, g / m ²

Electricity consumption, kWh / m ²

4.5 μm
200 g / l CrO ₃

2 dm ²
6th
12 dm ²

> 500
> 3000

60 m ²
<3.4

0.091
18th

18 to 21.4
0.8

4.5 μΐη

80 g / l CrO ₃
8 g / l H ₂ C ₂ O ₄
4 g / l H ₃ BO ₃

0.22 dm ²
13th
2.86 dm ²

18th
235

0.5 m ²

160 CrO ₃
16 H ₂ C ₂ O ₄
8 H ₃ BO ₃

0.091

7CrO
0.7 H ₂ C ₂ O ₄
0.4 H ₃ BO ₃

167 CrO ₃
16.7 H ₂ C ₂ O ₄
8.4 H ₃ BO ₃

1.4

15 μm
200 g / l H ₈ SO ₄

0.22 dm ²
13th
2.86 dm ²

175
2300

5 m ²
40

0.075 1
15th

55
1.56

The bath according to the invention contained 200 g / l chromic acid, that is 17.5 percent by weight. The anodic oxidation took place at a bath temperature of 26 ° C. and a voltage of 22 V for 90 minutes. The electrolyte was freed from aluminum ions with the aid of cation exchange resins. The bath container made of polyvinyl chloride had a capacity of 1000 l ("5" = 1000 l). The cathode consisted of aluminum wires with a surface area of 40 dm ² (anode area in dm ² = 0.04 · B = 0.04 · 1000). The aluminum plates to be anodized (99.5%) had a surface area of 20 m ² . The anodic current density varies between 0.15 and 0.27 A / dm ² from the beginning to the end of the batch time. Before the first batch, hydrogen peroxide (30 percent by volume) was added to bath 11.

During an operating time of 100 days (600 batches) the concentration ratio remained 6-valent to trivalent chromium ions between 125 and 130 and to dissolved aluminum at around 5 g / l. Since that If the bath produced oxide layers without any problems up to the 500th batch, the experiment was terminated. Of the Evidence of the superiority of the electrolytes according to the invention was provided.

Comparative experiment A: 50 V at 40 ° C., 40 minutes. The efficiency of the bath of 0.22 dm ² could not be increased without an undue increase in the proportion of trivalent chromium ions at the expense of the hexavalent chromium ions.

Comparative experiment B: 15 V at 20 ° C., 40 minutes.

Example 2

Example 1 was modified so that no hydrogen peroxide was added and none Removal of aluminum ions was done by a cation exchange resin.

In this case the ratio Cr ⁶⁺ : Cr ^{3+ reached} equilibrium values of during the course of the first batch

120 to 130, but this required a batch time of 150 minutes. The other batches were then ready in 90 minutes. The ratio of Cr ⁶⁺ : Cr ³⁺ remained practically constant until the end of the experiment, ie 300 batches corresponding to 6 m ² / l. The bath was already too enriched with aluminum ions for further use. When the bath is renewed, a consumption of CrO ₃ of approximately 34 g / m ² can be expected and is thus far below the consumption of known baths.

Claims (3)

Patent claims: 1. Process for anodic oxidation of aluminum and aluminum alloys in one Chromic acid bath with a small cathode area in relation to the anode area, marked by combining the following features. a) chromic acid concentration 15 to 19 percent by weight, b) bath temperature less than 3O ⁰ C, c) voltage less than 25 V, preferably 21 to 23 V, d) anode area (1.7 to 2.3) · B dm ² , where B is the liter volume of the bath, e) cathode area smaller than 0.06 B, preferably (0.03 to 0.05) · B dm ² , f) ratio Cr ⁶⁺ : Cr ³⁺ in gram-atom greater than 70, preferably greater than 120, g) no bath movement. 2. The method according to claim 1, characterized in that the bath for removing aluminum ions is passed over a cation exchange resin. 3. The method according to claim 1, characterized in that the bath before the first batch 0.01 up to 0.2 percent by weight hydrogen peroxide (30 percent by volume) is added.