DE1496709A1 - Process for surface protection of aluminum and aluminum alloys - Google Patents

Description

Process for surface protection of aluminum and aluminum alloys.

The present invention relates to a method for surface protection of aluminum and its alloys by anodic oxidation in a chromic acid bath.

The surface protection of aluminum and aluminum alloys through anodic oxidation in a chromic acid bath was developed in 1923 by "BEIGQUGH and STUART". These authors used weak chromic acid concentrations of 2.5 to 3.5 wt% and obtained thin coatings generally 2-6 / U of good corrosion resistance

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persistence, this procedure had a great one. Success. However, the voltage shows a rather complicated course as a function of time ... ■ during the course of the procedure.

Then came the anodic protection of aluminum and aluminum alloys im. Sulfuric acid bath for. Application whose industrial realization goes back to "Gaver and O'Brien".

The resulting protective layer of aluminum oxide is thicker than in all of the process the use of chromic acid (generally 10 to 20 / u). On the other hand, it is' less pliable and it is assumed that it is of the same thickness is significantly less corrosion-resistant.

Another method of anodic oxy-. dation in the chromic acid bath was worked out around 1937 by RW, BUZZAHD of the "Bureau of Standards" in the United States. In this process, a chromic acid concentration between 5 and 10 wt .- ^ at a temperature of 30-4Q ⁰ C is practically used. A constant -... voltage can be used. This simplification of work makes it possible to understand that this procedure ^{has "superseded the procedure: according to 11} BEHGAUGH - STUART" » - - -.>. :; ■■ ._

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Since then, a large number of publications have appeared on processes for anodic oxidation in a chromic acid bath under constant voltage.

An overview of this work is in the German magazine "Aluminum", 39th year, March 1963, pages 169-180, published by "F. MODIC" been.

It is known that the processes using chromic acid at constant voltage have the greatest disadvantage compared to the process using sulfuric acid, the price of chromic acid, which is much higher than that of sulfuric acid. Research has therefore tried hard to reduce the specific consumption of chromic acid, ie per unit of the anodically oxidized surface, for which all factors exerting an influence were varied to the extreme limit. It is known that the most important cause of the destruction of the bath is the gradual reduction of the chromic acid contained in it, the chromium ions changing from the hexavalent to the trivalent state. It is known that such a reduction is made more difficult if the cathode surface is as small as possible (above journal, page 179t ¹ · column), and as a result works with a high density of the cathodic current.

BATH ORIGINAL

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It is stated that the most favorable chromic acid concentration is between 10 and 15 ° . A higher concentration does not allow an increase in the thickness of the coating obtained per unit of time, but brings the disadvantage of the occurrence of greater losses through entrainment, a greater heduction of sea worth

2
trivalent chromium per dm of the anodically oxidized surface (aabove-mentioned magazine, page 171, 2 «column, paragraph 3.1) ·

It is also stated that if the voltage is increased during the electrolysis, the corrosion resistance of the coating obtained also increases and can assume a very high amount _e (see above journal, page 174, 1st column, paragraph 4).

It is further asserted that at a treatment temperature of less than 40 0 thin, transparent to semi-transparent coatings can be obtained which are difficult to dye and are mechanically too brittle (see above-mentioned journal, page 171, 2nd column, paragraph 3.2), and that this condition is due to an extension of the

1 hour duration of the operation can be improved by itself more than / not significantly (see above ^ eit- sohr.ift, page 172, 1st column, paragraph 3.2).

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Completely colorless, transparent or only slightly cloudy layers are obtained up to 35 ⁰ O or below (see above journal, page 172, 2nd column). To preserve entirely opaque oxidized layers, a bath temperature of at least 45 ⁰ C, and more preferably such of 50 to 55 ⁰ O requires · (s. Above-mentioned magazine, page 173, column 1, paragraph 3.21). At a temperature of less than 40 G, the layers obtained have mechanical properties without any practical interest (see above-mentioned journal, page 173, 1st column, paragraph 3i22).

In the absence of a means of suppressing the consumption of the bath (see above magazine, Page 178, 2 “Column, Paragraph 7) was destruction of the hexavalent ear is generally balanced by adding chromic acid in a calculated amount. However, this bath must be partially or completely replaced after a certain time, as it is too concentrated in trivalent chromium and aluminum salts.

It has been suggested to clean the bath with the help of cation exchange hairs, which the Hold on to dissolved aluminum salts ·. However, this will the main factor in bathroom consumption,

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which consists in the enrichment of salts of trivalent chromium, is not "eliminated".

It has also been suggested that the chromic acid be removed from the spent bath before discarding to regain it. However, this way of working is complicated and expensive in practice, so that the used bath is mostly simply discarded as waste eye. However, the chrome must be be retained from these waste eyes before they are discharged into streams and rivers, since the public "administration regarding the maximum Chromic acid content in waste waters is often very severe. (see "la Metallurgie et la Construction Meeanique ", Volume 95» December 1963, Pp. 1071 and '1075-1076).

It is known that the object to be anodized is carefully connected to the power source must be, thereby causing high labor costs.

One attempt to reduce this cost is anodic oxidation in a "basket", which is Holding on to the objects is not so important · This way of working, however, can only be applied to pieces use smaller dimensions and delivers unequal

ORJGlNAL INSPECTED

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more moderate results than the more carefully performed currents.

According to the invention, in particular, the destruction of the chromic acid bath as a result of the reduction of the hexavalent chromium to be completely "eliminated" in trivalent chromium.

According to the invention, the specific Consumption of chromic acid as well as electrical energy is noticeably reduced and that per day. in a certain bath volume increased anodically oxidized surface and at the same time a coating with at least e. have properties that are as good as those of the known processes for anodic oxidation Can be obtained with the help of chromic acid.

The inventive method is concerned with the surface protection of aluminum and aluminum alloys duroh oxidation in the chromic acid bath under constant voltage with the aid of direct current, the cathodic surface being as small as possible and is characterized by the following measures in combination: the concentration of chromic acid is essentially between 15 and 19% by weight _for the bath temperature is less than 30 ^° O,

BAD 0RK3lfNAL

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the electrolysis voltage is less than 25 volts and preferably between 21 and 23 yoIt, the anodic surface is essentially between "1.7.B" dm ² and "2.3oB" dm ² , with "B" denoting the bath volume in liters is expressed the cathodic

2 surface is less than "0.06.B" dm and preferably

2 2

essentially "0.03.B" dm to "0.05.B" dm, the electrolysis is preferably carried out without any stirring,

so that the ratio of the number of atomic grams of the im Hexavalent chromium dissolved in the bath to the number of atomic grams of the trivalent chromium dissolved in the bath constantly maintains a value of more than 70 and even more than 120.

The characteristic feature of the subject matter of the invention is in stark contrast to the prior art indicated above. According to the prior art is not recommended according to the invention applied strong chromic acid concentration of 15 to 19 wt .- ^ "In particular, according to the prior art to be avoided at a temperature of less than 3 ¥ e JO ⁰ C.

The prior art has therefore actually diverted from the present subject matter of the invention “Nonetheless are examinations in a concentration area of more than $ 15 and at a temperature

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en, ν In contrast to the state of the art, there is _β

has been performed by less than 40 0, which is im

The measures specified above, which represent the subject of the invention, can only be implemented in Combination can be used with each other. If one of these measures is omitted from this combination, then the results according to the invention described below no longer occur.

To show more clearly that the characteristic features are closely linked, it should be noted that the density of the anodic current as a consequence of the bath temperature ("aluminum" , 's. top page 173, 2nd column and page 174, 1st column Column, paragraph 3 3) as well as the concentration must be considered. The density of the anodic current still depends on the tension (according to the JOUIiE Law), on the global strength of the current passing through the electrolytic cell, such as also depends on the anodic surface. This global current strength in turn has an effect on the Density of the cathodic current. This latter density must be taken into account when considering the cathodic Surface is determined.

It has been shown on the basis of experiments (see below) that among the inventive

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Working conditions Peroxide reactions occur at the anode, it is clear that such reactions under the influence of temperature and concentration stand"

The combination character of the features according to the invention is based in particular on technical aspects Results obtained thanks to this new combination, which is clear from the state of the art differs from the technique, as shown below.

These attempts are as follows: ^ "ird the aluminum plate to be anodically oxidized taken out of the bath at the moment the coating begins to become opaque M / su, i.e. After a treatment time of 15-30 minutes, the formed layer shows aluminum oxide a blue color when rinsed off. This coloring does not occur towards the end of treatment after the coating has become completely opaque. This phenomenon cannot be attributed to the addition of hydrogen peroxide, as the affected Baths have never been treated in such a way. These experiments indicate that while the anodic oxidation of aluminum in a chromic acid bath Peroxide compounds occur. (In "Gmelln's Handbook of Inorganic Chemistry "Syst. Fr. 52,

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8th edition, 1962, part B, pages 152, 153, 156 and Pascal "Nouveau Traite de ^u himie Minerale", Volume H, edition 1959, pages 77 and 306) are given blue peroxide compounds of the hexavalent odor.

Such phenomena have not yet been published,

According to the invention, after a treatment time of 60 minutes, coatings with a thickness of 3 / U, after 90 minutes with a peck of 4.5 αχ and after 120 minutes with a thickness of 6 αχ . In practice, a treatment duration of more than 90 minutes is rarely exceeded.

The treatment time according to the invention is longer than that according to the method of anodic oxidation with chromic acid under constant voltage, _β in which a treatment time of 40 minutes instead of 60 minutes according to the invention is generally required to maintain a thickness of 3 / u.

This part of a long treatment

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However, it will last, by far, through an increase compensated for the anodically oxidized surface per liter of bath during each work step. More details about this have already been given above. This anodically oxidized surface is significantly larger than that obtained in the case of the known method using chromic acid constant voltage can be used technically.

The coatings obtained according to the invention are completely opaque. This is a surprising result that looks- " lent a temperature less than 40 completely is in contrast to the prior art.

The corrosion resistance of the coatings obtained according to the invention is greater than that the coatings obtained in the chromic acid bath according to the prior art.

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According to the state of the art was lower because of the relatively low voltage during the process of the invention Expect corrosion resistance. (The corrosion tests are comparatively according to the test with normalized salt spray, according to the test with acetic acid salt spray, also "Cass experiment" called, using copper chloride, as well as in the industrial atmosphere, the marine atmosphere and harbor atmosphere).

The investigation of wear resistance,

de of density stability (resistance / colmatage), des Flexibility, the coloring ability gives similar results that are very comparable to those which are obtained by anodic oxidation in a chromic acid bath under constant voltage. According to the invention The coating that has been preserved has a beautiful, enamel-like appearance Appearance that is easily comparable to that in the chromic acid bath.

The surfaces treated according to the invention be with a very even and regular

Sow the coating, even in places on the anode that are very to

^ are far from the cathodic surface. co

** «. The alloys which according to the invention

* °. That can be traded are practically the same ° *. According to the prior art in the chromic acid bath under constant Voltage can be anodically oxidized,

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The fact that electrolysis without

Stirring takes place, "means a decrease in the rate of dissolution of the aluminum, which is significantly slower is than in the lall of the known methods. Despite its insignificance, this solution process represents because of the Enrichment of aluminum salts in the bathroom in the long run. As a result, the chromic acid bath is advantageously made by cation exchange on exchange resins cleaned, whereby the dissolved aluminum salts are retained. Albeit such a cleaning was already known, but their combination with the method according to the invention is new, since they have a causes a new technical effect, as can be clearly seen from a comparison of Examples 1 and 4 below. This cleaning can be carried out continuously or discontinuously.

According to the previous anodic oxidation process under constant tension a consumption of .180

ρ
up to 200 g chromic acid per m of the surface to be anodically oxidized is still acceptable (see "Aluminum", above-mentioned magazine, page 180, last column, ab-

sentence 9) · According to the invention, the specific consumption is an

ρ J ₀ chromic acid, on the other hand, is generally less than 24 g per m

eo of the anodically oxidized surface, and is in particular oo _p

^> 16 to 23 g / m.

_k Such a result represents a new and

'. surprising technical effect, which can be achieved thanks to the combination of the inventive features. BATH ORIGINAL

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These numbers "refer to the improved Process according to the invention, the bath being cleaned as above with the aid of ion exchange resins will. According to this process, the bath does not become enriched in salts of trivalent chromium Aluminum salts, as can be seen from the analyzes.

According to the invention, a very low specific Consumption of electrical energy (direct current)

of not more than 0.8 KWh per m of the anodically oxidized surface can be achieved, whereby the thickness of the layer 4.5 yu. In the case of a thickness of the protective layer of 3 / u as they come in handy for comparison with previous ones The process of anodic oxidation in a chromic acid bath under constant voltage must be taken into account, if the consumption does not exceed 0.55 KWh / m.

Such a consumption of electrical energy is significantly lower than in the case of the known method anodic oxidation in a chromic acid bath.

Accordingly, a modified invention As a procedure, a calculated amount of 30 vol. Hydrogen peroxide can be added to the bath when it is used for the first time which corresponds to 0.02 to 0.2 wt .- ^ of the bath.

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Thanks to such addition, the equilibrium ratio becomes between hexavalent and trivalent chromium from the beginning of the first work step realized.- If such an addition were not made, the advantageous inventive Results can be achieved nonetheless. The necessary duration of the first work stage would then, however can be extended so that the treated pieces have a similarly satisfactory opacity as in The case of the following operations.

According to a preferred procedure according to the invention, very uniform and homogeneous Coatings are obtained when the electrical contact between the object to be anodized and the anodic power source is made simply by gravity. For this purpose, the object to be anodically oxidized is placed on a metallic carrier, the one with the positive pole. the power source is connected, the weight of the object is sufficient to ensure a Konak ^. Included it is by no means necessary to press the object in question against the anode of the container or to attach it to it. Here, too, the method according to the invention has a new feature compared to the prior art. This Feature is of great economic importance because it represents a significant reduction in the cost price connected is.

ORIGINAL BATHROOM

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It can be pieces of large dimensions, that is, those whose maximum dimensions with the Containers are compatible. This modified procedure is explained in more detail in FIGS.

In Figure 1, the container of the anodic oxidation bath and the support are in vertical section while in Figure 2 a longitudinal section thereof is shown.

In FIG. 1, 1 denotes a container made of polyvinyl chloride, 2 the bath 3 denotes the one with the anodic power source related beam, 4- one of the horizontal arms of the beam, 5 cross members through which the Components of the carrier are held together, and 6 the object to be anodically oxidized. (Profiles in the form of a TJ) It is clear that other modes of implementation can also be derived from the subject matter of the invention, the anodic oxidized object is kept in electrical contact simply by the action of its weight, in which the characteristic Feature of this modified procedure is.

The method according to the invention is particularly recommended for applications in architecture, where the beautiful Appearance of the treated surface and its special corrosion resistance are of particular importance.

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The invention is explained in more detail by the comparative examples given below.

The 3 first examples are shown schematically in the following table.

Tabel

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example 1
Today's procedure Example 2
State of
technology Example 3
State of
technology Thickness of the coating 4.5 / u 4.5 / u 15 / U Composition of the bath 2OOgA · OrO, 80g / l.OrO,
8g / 10O ₂ O ^ H ₂ 200g / lH ₂ S Per operation and liter
Bath treated surface ■ 2 dm ² 0.22 dm ² 0.22 d Number of operations each
9 hour day: 6th 13th Per day and liter of bath
treated surface 12 dm ² 2.86 dm ² 2.86 Expressed in working days
te period of use of the
Bath > 500 T. 18 T 175 T Expressed in operations
te period of use of the
Bath 235 2.30C After. · Consumption of 1 1 bath
treated surface > 60 m ² / l 0.5 m ² / l 5 dl Consumption in g because of
Renewal of the bathroom
Bath consumption based on ver
lust by being carried away
decreases in l / m 0.09 l / m ² 160g 0r0 ₅ / m ²
/ m ₂
0.09 l / m ² 4Og ^
0.07 -id-ing / m 18g CrO ₃ / m ² 7g Cr0, / m ² 1SgH ₂ SO ₁ Global consumption of
Bath in g / m between 18
and 21 ₂ 4 g 0.4 gB0, H ^ / m
167 grams of CrOyif; _55S H ₂ SO. DC power consumption
per m of treatment
th surface 0.8 kWh / m ² 1.4 kWh / m ² 1.56 I.

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INSPECTED

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In connection with this table, the following additional explanations must be provided are given:

Example 1 deals with the method according to the invention. The bath contained chromic acid per 1,200 g, which corresponds to 17.5 Grew .- $. The anodic oxidation took place at 26 ^° C. under a voltage of 22 volts. The treatment time was 90 minutes in each case. A device for cleaning the aluminum salts while passing the bath over cation exchange resins was provided. The container was made of polyvinyl chloride and had a capacity of 1,000 1 x ("B" = 1,000 1). The method consisted of aluminum wires

'2

a surface of 40 dm (anodic surface in dm = 0.04 XB = 0.04 x 1 000), The by anodic oxidation The parts to be treated were in each case plates made of $ 99 »5

Aluminum with a surface of 20 m. The anodic current density varied from the beginning to the end of a work

* 2

Ganges between 0.15 and 0.27 A / dm. Before the first step, 1 1 30 vol. Hydrogen peroxide was added to the bath added.

For 100 days corresponding to a working period of 4 1/2 months or 600 work cycles *! kept that Ratio of the concentration of 6-valent to 3-valent Ghrom, expressed in dissolved atomograms, from the first work step constant between 125 and 13Q, the concentration of

* ■■. *

dissolved aluminum salts "kept a constant value of un-

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drive j? g / l. The obtained, anodically oxidized Surfaces consistently showed the same satisfactory result from the first to the 500th pass Appearance and the same advantageous characteristic properties already described above. After the bathroom had no consumption during this working period of 4 1/2 months showed what by. Analysis could be proven, the experiment should be sufficiently convincing that it need not continue any longer. The useful life of the bath is therefore with certainty more than 500 working days. This corresponds to a treated surface of more than 60 m / 1 bath and accordingly less consumption until the bathroom is renewed

than 5> 4 g chromic acid / m of the treated surface.

Example 2 deals with a known process for anodic oxidation in an odorous acid bath under a constant voltage of 40 volts at 40 ^G 0 for 40 minutes each time. The cathode consisted of rods and plates made of pure iron. The surface treated per 1 bath

of 0.22 dm cannot be increased without a simultaneous increase in the amount of per unit time in the bath formed 3-valent öhroms takes place.

Beis £) iel 3 deals with a known process of anodic oxidation in a sulfuric acid ^{bath under a constant voltage of 15 volts at 20 0} O during each. .y / eils 40 min.

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Example 4 deals with a treatment according to the invention. The same won't happen in the above Table described. It represents a variant of example 1 and is different from this Example that no addition of hydrogen peroxide and no cleaning (fixation) of the aluminum salt of the Bathing while passing it over a cation exchange resin took place.

Cr 6+

In this case, the ratio reached Cr 3+

of the bath its equilibrium value between 120 and 130 from first step. However, as stated above, it was necessary to lengthen the duration more (Duration of the 1st step: 150 minutes). The remaining operations had the same duration as in Example 1, i.e. 90 minutes. The ratio remained practically to the end of the experiment, i.e. during 300 work steps 6 m treated. Surface / l bath, unchanged from the value given above. The bathroom was above this limit do not use it anymore because it has become too rich in aluminum saltsii. However, this bath can be re-opened just like a Use a new bath provided that the aluminum salts can be removed therefrom via a cation exchange resin. However, if such a cleaning device is not available, then the bathroom has to be renewed. In this case, the consumption due to the renewal of the bathroom increases about 34 g chromic acid / m of the treated surface. Even in this unfavorable case, this consumption remains well below from the one corresponding to the earlier state of

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Technology. This consumption must be compared with that corresponding to a device which has a possibility of cleaning from the aluminum salts, (Example 1) which is less than 3.4 g / m is

The table reproduced above, which corresponds to Examples 1 to 3, leaves in particular the exceptionally long period of use of the bath

know there that can still be used after / with pro

Bath (Example 1) anodically oxidized a surface of 60 m became. A comparison of this number with the corresponding number from Examples 2 and 3 shows that it is is a completely different order of magnitude of the results and leaves a surprising technical effect and, a recognize significant technical progress. This progress comes from the penultimate horizontal column of the Table, and expresses such a small one Consumption of raw materials assumes that the order of magnitude really differs from the state of the art.

Claims

BATH ORIGINAL

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Claims (4)

- 24 - ■ 1A-27 893 Claims MJ Process for the surface protection of aluminum and aluminum alloys by anodic oxidation in a chromic acid bath under constant direct current voltage, the cathodic surface being as small as possible, characterized in that the concentration of the chromic acid is essentially between 15 and 19 percent by weight, the temperature the bath is less than 3O ⁰ C, the ^ l.ektrolysespannung is below 25 volts, and preferably substantially 21 to 23 volts, the anodic 2 The surface area is essentially between "1.7.B" dm and "2.3" B "dm, with" B "representing the volume of the bath in FIG. 1 that the cathodic surface area is less than" 0.06.B "dm and is preferably between "0.03.B" and "0.05.B" dm ² , and that the electrolysis is preferably carried out without any stirring, so that the atomic gram ratio of the 6-valent chromium dissolved in the bath to the 3-valent chromium dissolved therein valuable chromium remains constant at a value of more than 70 and even more than 120, the above measures being applied in combination. 2. The method according to claim 1, characterized in that the bath has a cation exchange resin is cleaned to fix the aluminum salts. 3. The method according to claim 1, characterized in that the bath before the first operation Hydrogen peroxide in an amount of 0.01 to 0.2 wt .- ^ calculated on 30 vol. hydrogen peroxide, added ^becomes · 909823/0916 U-27 893 • H96709 4. The method according to claim 1, characterized in that the electrical contact between the object to be anodically oxidized and the anode of the container is achieved in that the object is placed on a support which is electrically connected to the anode, the weight of the object alone is sufficient to ensure electrical contact. "- BATH 90 9823/09 16 -U- Blank page