DE1248427B - Method and solution for applying coatings to aluminum and its alloys - Google Patents

Description

GERMAN PATENT OFFICE

German class: 48 dl -7/26

EDITORIAL

Number: 1248 427

File number: M 63930 VI b / 48 dl

1 248 427 filing date: January 26, 1965

Opened on: August 24, 1967

It is known to have firmly adhering and anti-corrosive coatings on aluminum and its alloys to be applied with the help of aqueous acidic solutions containing hexavalent chromium and fluoride ions and can be modified by certain additives. Such methods are for example U.S. Patents 2,276,353, 2,471,909, 2,472,864, 2,796,370, 2,796,371, 2,507,956, 2,843,513, 2 859144, 2 868 679, 2 839 439 or 3 009 842. The known proposals leave the endeavor realize an effective process to be obtained simultaneously for a wide variety of surfaces made of aluminum and aluminum alloys usable and yet easy to manufacture Solutions and their preservation of effectiveness during use. In most cases, additional anions or cations are added to the treatment solutions, to achieve a certain effect, but with such modifications the monitoring the solutions cumbersome and difficult during their continuous use. In the one in that US Pat. No. 2,796,370 described method is only a single modifying anion, namely Ferricyanide, used; but also these ferricyanide-containing chromate-fluoride solutions show in their technical use still presents a number of procedural difficulties. So one technically Layer formation speed that is justifiable in terms of both quality and economy and If the stratification strength of the solutions is retained, they must always have a certain minimum content Have ferricyanide. However, it has been found that the ferricyanide is above about 49 to 52 ° C is temperature sensitive. If, therefore, care is not always taken, it will be exceeded To avoid temperature even at individual points of the treatment bath, the ferricyanide is decomposed and is then no longer able to keep the solution in sufficient film-forming ability. It has also been found that these ferricyanide-containing solutions are relative in terms of acidity are sensitive so that their acidity must be maintained within fairly narrow limits. Further Difficulties consist in the fact that the solutions even when simply standing at room temperature, without that they are used, decrease in their ability to form layers and that it is difficult to replenish them is because in order to avoid a reaction between chromate and ferricyanide a separate addition of Ferricyanide on the one hand and chromate fluoride on the other hand, as well as separate storage of the components is required.

Method and solution for applying
Coatings on aluminum and its
Alloys

Applicant:

Metallgesellschaft Aktiengesellschaft,
Frankfurt / M., Reuterweg 14

Named as inventor:

Herman John Lodeesen, Gaylord, Mich .;

William Sykes Russell, Warren, Me. (V. St. A.)

Claimed priority:
V. St. v. America 3 February 1964
(342275)

It has now been found that these disadvantages have been avoided and results in improved application results of coatings on aluminum and its alloys with aqueous acidic solutions, the hexavalent ones Chromium and fluoride ions can be achieved in that the surfaces with Solutions are brought into contact which contain tungsten anions.

It has been found that even small amounts of tungsten anions lead to fast-working, highly effective treatment solutions which are easy to control and can be kept in optimal working conditions. In the case of solutions which contain 0.5 to 10 g / 1 Cr ^vi , calculated as CrO ₃ , and an amount of fluoride ions sufficient for the pickling attack and layer formation, at least 0.1 g / 1 tungsten anion are required to achieve a noticeable improvement in speed and film formation efficiency.

This small amount of tungsten is particularly effective when complex aluminum fluoride is also present in the solution in amounts equivalent to about 2.2 to 32 g / l Al (F) _x. If such amounts are not present, the highest layer formation speed and layer formation strength of the solution will not be achieved, even if the tungsten anion alone already causes a noticeable increase in the layer formation ability of the solutions. The complex aluminum fluoride is designated as Al (F) _x because it is present in the treatment solution as an equilibrium of different aluminum fluorides, which can contain 1 to 6 fluorine atoms per aluminum atom.

709 638/540

In the solutions used according to the invention, this equilibrium is approximately equivalent to _{the AlF 3 ion.} The term Al (F) _x is therefore intended to encompass all aluminum fluoride ions, and the amounts given here relate to the equilibrium which is equivalent _{to AlF 3.}

The method according to the invention has the advantage that the coatings move at high speed form and with regard to corrosion protection and adhesion promotion for subsequently applied organic Coatings are of high effectiveness. The solutions used in the process can be used in a wide pH range and at temperatures even up to the boiling point. The solutions are stable on standing at room temperature without being used. A special The advantage is that they are easily kept in optimal film-forming conditions can be done using just one supplement.

The solutions have a high layer formation strength. The expression “layer formation strength” is understood here to mean the ratio of the metal detached from the metal surface in mg / m ^{2 of} treated surface to the coating formed on this surface in mg / m ^2. Lower numbers thus indicate a higher layer formation strength, that is to say a higher effectiveness. For example, a chromic acid solution containing fluoride ions as the only activator ions dissolves between 0.44 and 2.2 g / m ^{2 of} aluminum in order to form a coating with a weight of 1.1 g / m ² on this surface. Such coating formation takes place relatively slowly, and the layer formation thickness is between 0.4 and 2.0, that is to say it is relatively low. In comparison, when using the solutions according to the invention, a layer weight of 1.1 g / m ^{2 is obtained} with a metal removal of only 0.165 to 0.363 g / m ² . The coatings are thereby formed at a high speed and the film thickness is between 0.15 and 0.33. So it is in a relatively high range.

The solutions used in the method according to the invention expediently contain hexavalent Chromium, fluoride ions, tungsten anion and complex aluminum fluoride in the following quantities:

Solution component Concentration
tion
g / 1 CrO ₃
Fluoride ion
Wolframamanion (calculated as tungsten)
AI (F) ^ complex (calculated as AlF ₃ ) ..
A preferred composition
solution results from the following T 0.5 to 10
1.6 to 27
0.1 to 4
2.2 to 32
the treatment
abelle: Solution component Concentration
tion
g / 1 CrO ₃
Fluoride ion
Wolframamanion (calculated as tungsten)
Al (F) _x complex (calculated as AlF ₃ ) .. 2 to 5
1.6 to 16
0.3 to 1.0
2.2 to 19

65

The hexavalent chromium can be in the form of chromic acid or one or more of its water-soluble

Salts, for example in the form of sodium, potassium or ammonium chromate or dichromate, introduced will. Mixtures of chromic acid and its salts can also be used.

The fluoride ions can use any fluorine-containing compounds that are present in the aqueous acidic solutions dissociate, are introduced, with the formation of fluoride ions. Suitable compounds are hydrofluoric acid, fluosilicic acid, fluoboric acid and the sodium, Potassium and ammonium salts of the acids mentioned. The aluminum fluoride complex compounds can be added as such or formed in the bath from free aluminum and fluoride ions.

The tungsten anion can be in the solutions in the form of any dissociating tungsten compounds, the tungsten anions may be formed by oxidation by hexavalent chromium. Tungstic acid or its sodium, potassium or ammonium salts are particularly suitable.

The solutions can be made by dipping, brushing or spraying with the surfaces of aluminum and whose alloys are brought into contact after they have been removed from oil, grease, Oxide or the like. Were cleaned. The solutions can also be obtained by spraying on previously heated surfaces be applied. Application by spraying is carried out after the surface of aluminum or Aluminum alloys has been brought to a temperature above 66 ° C. The amount of Measure the treatment solution to be sprayed on so that no drops are caused by the spray particles flowing together arise on the surface. The coating then arises from the almost instantaneous Evaporation of the solvent, so that the layer formed from the individual spray particles in the essentially occurs at the point where the spray particle comes into contact with the metal surface has come.

The coatings produced according to the invention are slightly colored, with increasing layer weight from iridescent to light gold and yellow to brown.

It has been found that the rate and strength of film formation can be improved by simultaneously selecting and controlling the acidity of the solution and its application temperature. When the temperature of the treatment solution is increased from room temperature to 49 ° C, the film formation speed also increases. Depending on the acidity of the solution and the concentration of tungsten anion and aluminum fluoride complex compound in the solution, with constant amounts of chromic acid and fluoride ions, an increase in the rate of formation of two to five times the value compared to room temperature can be obtained. The rate of layer formation increases significantly more slowly between 49 and 71 ^° C., so that it is essentially the same for practical purposes in this range. The temperature range from 49 to 71 ^° C. is therefore preferably used. Although temperatures higher than 71 ° C can also be used, e.g. B. 82 ° C or temperatures close to the boiling point of the solution, but no particular advantages are obtained.

Although the pH of the treatment solution, the film forming speed and the film forming strength the solution clearly influenced, but good results can be broadly

Maintain pH area. The solutions work satisfactorily in the pH range 1.1 to 2.3. Preferably the pH range is from 1.6 to 2.1.

The pH values refer to measurements made with the aid of an electrical pH meter using a glass and a calomel electrode by immersing the electrodes in fresh portions the solution can be made. The glass electrode is kept in optimal condition if you care for it Care must be taken that immediately after removal from the measurement solution, it is immersed in a 5N hydrochloric acid solution for 2 or 3 minutes is immersed and then rinsed with distilled water. The electrode is between Leave the measurements in distilled water and with a buffer solution before each use calibrated.

The chromate fluoride solutions containing tungsten anion used according to the invention have a number of advantages which are not obtained in the known processes. The reason for this fact is not known with certainty. From the observed differences compared to the behavior of the known solutions, however, it clearly emerges that the presence of the tungsten anion is responsible for this. as

When treating aluminum and aluminum alloys with chromate fluoride solutions As is well known, different chemical reactions take place at the same time, whereby the respective proportions of the Solutions of active ingredients change and the film-forming ability of the solution decreases. However, when using tungsten anions in the solutions, these changes are not as strong. So are for example the decrease in fluoride in a simple chromic acid-fluoride solution and thus the Follow-up reactions are much stronger compared to a solution that is composed accordingly, however additionally contains tungsten anions. This is apparently due to the fact that with the tungsten anions additionally stabilizing complexes are formed. The used according to the invention Solutions therefore have an improved film-forming ability.

When using the known chromating solutions, it is customary to add acids, e.g. B. Sulfuric acid, nitric acid or mineral acids, to add to the layer-forming ability of the solution restore and increase the film formation speed. This effect can be achieved through it explain that the balance of the various aluminum fluoride complexes to compounds with relatively smaller fluorine content is shifted. Such a mineral acid addition to tungsten anions In contrast, the solutions containing them do not produce a comparable change in the rate of layer formation emerged. Rather, as already mentioned, this is over a wide range in the case of the solutions according to the invention pH range is the same and therefore more advantageous.

In the known chromating solutions, the layer weights obtained increase with the treatment temperature. On the other hand, when using the solutions according to the invention containing tungsten anions, approximately the same layer weights ^{result in the temperature range from 49 to 71 ° C.} This has the advantage that, when working in this temperature range, certain temperature fluctuations can be accepted without the layer weight changing significantly.

The method according to the invention can be obtained by combination in a particularly advantageous embodiment with the method of the USA patent 2 967 791, in which the use of cation exchangers to remove interfering foreign ions, especially aluminum ions.

The inventive method allows the formation of corrosion-resistant, firmly adhering coatings within a few seconds, for. B. in about 1 to 20 seconds. Depending on the content of tungsten anion, fluoride, aluminum fluoride complex in the solution and depending on the acidity of the solution, a coating with a layer weight of 0.16 to 0.43 g / m ² can be applied to strip material made of aluminum or aluminum alloy in 1 to 2 seconds of treatment time will.

In the following examples, the aluminum fluoride complex content is _{given as AlF 3} as the average value for the Al (F) _x complexes in equilibrium.

The content of tungsten anion is given as tungsten. It was introduced as Na ₂ WO ₄ · 2 H ₂ O in each case.

example 1

A solution containing 5 g / l chromic acid, 3 g / l fluoride, introduced as HBF ₄ , 1 g / l tungsten and 3.7 g / l AlF ₃ has a pH value of 1.75 and a total fluoride content of 5.5 g / 1.

Aluminum sheets of the quality AlMn (0.9% Mn, 0.5% Mg) were treated by ^{spraying for 7 to 15 seconds with the above-described solution heated to 49 ° C.} The coatings obtained are firmly adherent, colored light gold and, depending on the duration of the treatment, have layer weights between 0.22 and 0.54 g / m ² . The thickness of the layer formation (metal loss [mg / m ² ] divided by layer weight [mg / m ² ]) averaged 0.2. The solution was kept free of trivalent chromium and the content of aluminum fluoride complex compound was kept constant by passing part of the treatment solution over a strongly acidic cation exchange resin. After a large number of aluminum sheets had passed through, the coating weight and the layer formation thickness had remained the same.

Example 2

A treatment solution containing 2.5 g / l chromic acid, 0.2 g / l fluoride, introduced as HF, 0.2 g / l tungsten and 2.2 g / l AlF ₃ , has a pH value of 1, 68 and a total fluoride content of 1.7 g / l. When aluminum sheets of the quality AlMn (0.9% Mn, 0.5% Mg) were sprayed with the solution heated to 49 ° C. for 7 to 15 seconds, coatings were obtained which had almost the same layer weight as the cover sheets according to Example 1 are obtained. The average film thickness was 0.25.

Example 3

A solution containing 5 g / 1 chromic acid, 2.5 g / 1 fluoride, introduced as H ₂ SiF ₆ , 0.4 g / 1 tungsten, 2.3 g / 1 chromium (III) nitrate and 2.2 g / 1 _{contains AlF 3} , has a pH value of 1.55 and a total fluoride concentration of 4 g / 1. At a bath temperature of 49 ⁰ C of the aluminum plates quality AlMn (1.25% Mn) were treated by being dipped into the solution. After an immersion time of 30 to 60 seconds, firmly adhering, gold-colored coatings with a transparent

average layer weight of 0.43 to 0.65 g / m ^{2 was} obtained. The film thickness was 0.25.

Was the solution described at different temperatures in the spray with a spray duration of Applied for 15 seconds, the following were obtained, depending on the respective treatment temperature Layer weights:

Treatment temperature
(° Q Layer weights
(g / m ² ) 37.8 0.594 43 0.669 49 0.907 57 0.874 66 0.723 71 0.540

Example 4

A solution containing 5 g / 1 chromic acid, 0.8 g / 1 tungsten, 13.7 g / 1 chromium (III) nitrate, 0.2 g / 1 fluoride, introduced as HF, and 18.7 g / 1 Contains AlF ₃ , has a pH of 1.71 and a total fluoride concentration of 12.3 g / 1. The solution was used at 49 ⁰ C for the spray treatment of aluminum sheet of the quality AlMn (1.25% Mn) with a treatment time of 7 to 15 seconds. Firmly adhering light to dark gold coatings with an average layer weight between 0.22 and 0.54 g / m ^{2 were formed} . The film thickness was 0.3.

Example 5

A solution containing 10 g / 1 chromic acid, 4.1 g / 1 fluoride, introduced as HBF ₄ , 22.8 g / 1 chromium (III) nitrate, 3.1 g / 1 tungsten and 31.1 g / 1 Contains AlF ₃ , has a pH of 1.86 and a total fluoride concentration of 25.1 g / l. When aluminum sheets of the type described in Example 1 were treated under the treatment conditions described in Example 1, coatings were obtained which are comparable to those obtained in Example 1. The average film thickness was 0.33.

Example 6

A solution containing 4.9 g / l chromic acid, 0.4 g / l fluoride introduced as HF, 0.9 g / l tungsten and 2.36 g / l AlF ₃ has a total fluoride concentration of 2 g / l and a pH of 1.68. The solution, heated to 49 ° C., was applied to aluminum sheets of the quality AlMn (1.25 Mn) with a treatment time of 15 seconds by spraying. Uniform, firmly adhering coatings with an average layer weight of 0.842 g / m ^{2 were produced} . The film thickness was 0.19.

At different treatment temperatures, but otherwise identical conditions, were obtain the following layer weights with the bath described above:

Treatment temperature
( ⁰ Q Layer weights
(g / m ² ) 32.2 0.227 37.8 0.583 54 1.058 60 1.058 66 0.928 71 0.821

Different amounts of sodium hydroxide were added to individual parts of the treatment solution described above, thereby setting different pH values of the solution. Using the 5 treatment conditions given above, the following layer weights and thicknesses were obtained:

pH the
Treatment solution Layer weights
(g / m ² ) Stratification
strength 1.81 0.831 0.20 1.91 0.767 0.24 2.12 0.486 0.27 2.28 0.238 0.28

Another treatment solution contains 4.9 g / l

so chromic acid, 0.9 g / l tungsten, 0.2 g / l fluoride, introduced as HF, and 2.66 g / l AlF ₃ . The solution has a pH of 1.70 and a total fluoride concentration of 2 g / l. It was 15 seconds for spray treatment of aluminum sheets of the quality AlMn applied at 49 ⁰ C (1.25% Mn) and generated layers with an average weight of 0.400 g / m ^2. The film thickness was 0.28. When the pH of the solution was lowered to 1.21 by adding 4.8 g / 1 nitric acid, but otherwise the same conditions were maintained, firmly adhering coatings with an average layer weight of 0.713 g / m ^{2 were} obtained, and the The film thickness was 0.27.
The method according to the invention has the particular advantage that a supplementary material consisting of a mixture can be used to maintain the effectiveness of the baths.

A suitable supplement has the following composition:

Components parts by weight

CrO ₃ 15 to 20

HF 7 to 12

ENT ₃ 1.4 to 7

Na ₂ WO ₄ -2 H ₂ O 3 to 6.5

AlF ₃ 0.6 to 1.6

The supplement can optionally contain 1.2 to 2 parts of H _{3 BO 3.}

A preferred supplement material that is particularly suitable for supplementing treatment fluids, which are used in conveyor systems and which are partly passed through ion exchangers, suitable, has the following composition:

Components parts by weight

CrO ₃ 17 to 19

HF 7 to 8

HNO ₃ 1.4 to 1.6

Na ₂ WO ₄ -2 H ₂ O 3 to 5

AlF ₃ '. 0.6 to 1.0

H ₃ BO ₃ 1.2 to 2.0

A supplement that is particularly suitable for supplementing treatment fluids used in conveyor systems come into use, but not over

Claims (5)

Ion exchangers, is composed as follows: Components Parts by weight CrO3 18 to 20 HF 10 to 12 HNO3 5 to 7 Na2WO2 -2 H2O 3 to 6 AlF3 0.8 to 1.5 A supplementary material that is particularly suitable for such systems , in which the log parts to be treated are guided on a conveyor system through a spray device, has the following composition: ao components parts by weight CrO3 15 to 17 HF 8 to 11 HNO3 4.5 to 6.5 Na2WO4-2 H2O 3.5 to 6 , 5 as AlF3 1 to 1.6 claims: 1. Process for applying coatings to aluminum and its alloys by means of aqueous acidic solutions containing hexavalent chromium and fluoride ions, characterized in that, that the surfaces with a solution containing tungsten anion, preferably at temperatures of 49 to 71 ° C in Be brought into contact. 2. The method according to claim 1, characterized in that part of the treatment solution over a cation exchanger is passed. 3. Aqueous acidic solution for carrying out the method according to claim 1 and 2, characterized by a content of 0.5 to 10 g ^{/ l Cr vl} , calculated as CrO ₃ , 1.6 to 27 g / l fluoride, 2.2 to 32 g / l Al (F) ₂ , 0.1 to 4g / l tungsten amine, calculated as tungsten. 4. Solution according to claim 3, characterized by a content of 2 to 5 g / 1 Cr ^vi , calculated as CrO ₃ , 1.6 to 16 g / 1 fluoride, 2.2 to 19 g / 1 Al (F) ₂ ; , 0.3 to 1.0 g / l tungsten anion calculated as tungsten. 5. Solution according to claim 3 and 4, characterized by a pH of 1.1 to 2.3, preferably 1.6 to 2.1. 709 638/540 8.67 © Bundesdruckeiei Berlin