EP0103234B1 - Process for densifying anodically produced oxide layers on aluminium or aluminium alloys - Google Patents

Abstract

A process for sealing anodically produced oxide layers on aluminum and aluminum alloys by treating them with an aqueous solution at a temperature between about 90 DEG C. and its boiling point, wherein the aqueous solution has a pH of from about 4 to about 8 and and contains from about 0.0005 to about 0.5 g/l of at least one phosphinocarboxylic acid or its water-soluble salt.

Description

The invention relates to a method for compacting anodically produced oxide layers on aluminum or aluminum alloys by treatment with aqueous solutions at elevated temperatures, the occurrence of disruptive aluminum hydroxide coatings (sealing coatings) on the surfaces being prevented.

Anodically produced oxide layers are often applied to aluminum surfaces for the purpose of corrosion protection. These oxide layers protect the aluminum surfaces from the effects of weather and other corrosive media. Furthermore, the anodic oxide layers are also applied in order to obtain a harder surface and thus to achieve increased wear resistance of the aluminum. Due to the intrinsic color of the oxide layers and their partially easy egg coloring, particularly decorative effects can be achieved.

A number of methods are known for applying anodic oxide layers to aluminum. For example, the oxide layers are produced with direct current in solutions of sulfuric acid (direct current-sulfuric acid method).

These layers can be subsequently colored by dipping in solutions of a suitable dye or by alternating current treatment in an electrolyte containing metal salts. However, solutions of organic acids, such as, in particular, sulfophthalic acid or sulfanilic acid, or else these in a mixture with sulfuric acid, are frequently used for the application of the oxide layers. The latter methods are known as color anodization methods.

However, these anodically applied oxide layers do not meet all requirements with regard to corrosion protection, since they have a porous structure. For this reason, it is necessary to densify the oxide layers. This densification is often carried out with hot or boiling water and referred to as «sealing. This closes the pores and thus significantly increases the protection against corrosion.

During the densification of anodically applied oxide layers, not only are the pores closed, but a more or less strong velvety coating, the so-called sealing coating, is also formed on the entire surface. This consists of hydrated aluminum oxide and is not touch-proof, so that the decorative effect of the layer is adversely affected. It also reduces the adhesive strength when gluing such aluminum parts and, due to the increased effective surface area, promotes later contamination and corrosion. For these reasons, it was previously necessary to remove the covering by hand, mechanically or chemically.

It is already known to detach this coating again from compacted surfaces which are covered with sealing coating by means of a mineral acid aftertreatment. With this method, a further treatment step is therefore necessary, and it also requires a very careful aftertreatment with the mineral acid in order to rule out damage to the layer.

It is also part of the prior art to carry out post-compression with solutions which contain nickel acetate and lignin sulfonate in order to prevent sealing deposits. One disadvantage of this procedure is the yellowing of the oxide layers obtained under the influence of light. Finally, processes have already been described in which hot water compression is carried out with the addition of certain polyacrylates (DE-PS 19 38 039) or certain dextrins (DE-PS 1944452) to prevent the sealing coatings. These methods have worked well. In some cases, especially when the molecular weight of the active ingredients used increases, more or less visible polymer films can form on the surface. These are undesirable. It has also already been described to use oxycarboxylic acids such as citric acid (DE-PS 21 62674) and various phosphonic acids (DE-PS 22 11 553) in small quantities as a sealant inhibitor. When using these substances, however, it has been shown that, in particular in large, poorly circulated baths, difficulties can arise with overdosing of the active ingredient. It is not always easy to adhere to the concentration range in which, on the one hand, the sealing coating is prevented with absolute certainty, without, on the other hand, the result of the short-term tests being adversely affected. Although the risk of overdosing could be further reduced by the already known use of certain cycloaliphatic or aromatic polycarboxylic acids (DE-OS 26 50 989), in practice there were still occasional unintentional deviations from the optimal concentration range. Accordingly, the object of the present invention is to further improve the previous methods of operation and to make them even safer in order to arrive at an improved method for compacting anodic oxide layers on aluminum or aluminum alloys.

in der

• R₁ Wasserstoff oder einen der Reste
  
  
  
  
  sowie
• R₂, R₃ und R₄ Wasserstoff oder einen der Reste -CH₃, -C2H5, -COOH, -CH₂-COOH, ―CH₂―CH₂―COOH, bedeuten und wobei mindestens einer der Reste R₁ bis R₄ eine COOH-Gruppe darstellt oder enthält, im Molverhältnis 1 : 1 bis 1 : 8 gebildet werden, oder deren wasserlösliche Salze enthalten.

The invention accordingly relates to a method for compacting anodically produced oxide layers on aluminum or aluminum alloys by treatment with aqueous solutions with a pH of 4 to 8 at temperatures between 90 ° C and the boiling temperature, which is characterized in that the compression with Performs solutions that 0.0005 to 0.5 g / I phosphinocarboxylic acids in the reaction of hypophosphorous acid with unsaturated mono- or polyfunctional carboxylic acids of the general formula I

in the

• R _{1 is} hydrogen or one of the radicals
  
  
  
  
  such as
• R ₂ , R ₃ and R _{4 are} hydrogen or one of the radicals -CH ₃ , -C2H5, -COOH, -CH ₂ -COOH, ―CH ₂ ―CH ₂ ―COOH, and at least one of the radicals R ₁ to R ₄ represents or contains a COOH group, are formed in a molar ratio of 1: 1 to 1: 8, or contain their water-soluble salts.

The phosphinocarboxylic acids used in the process according to the invention can be prepared by known processes. For example, reference is made to the generally known textbook Houben-Weyl, “Methods of Organic Chemistry” 4th Edition, Vol. XII / 1, Stuttgart 1963, pp. 228 to 229.

Accordingly, reaction products of hypophosphorous acid are used, such as those used in the reaction with acrylic acid, methacrylic acid, ethyl acrylic acid, crotonic acid, maleic acid, glutaronic acid, citraconic acid, itaconic acid, 2-butene-2-carboxylic acid, dimethyl maleic acid, 2-methylene glutaric acid, butene polycarboxylic acids and ethylene tetracarboxylic acids, ethylene tetracarboxylic acids, ethylene tetracarboxylic acids and ethylene tetracarboxylic acids, ethylene tetracarboxylic acids, and ethylene tetracarboxylic acids, and ethylene tetracarboxylic acids Cinnamic acid are created. According to the invention, reaction products with hypophosphorous acid derived from higher unsaturated carboxylic acids can also be used. With increasing molecular weight, however, their use in terms of optimal bath management becomes increasingly complex. In addition to the acids, their water-soluble salts, in which all or part of the acidic protons have been replaced, for example, by alkali metal, ammonium, alkaline earth metal, alkylammonium or alkanolammonium ions, can also be used for carrying out the process. The salts are used in an amount which corresponds to 0.0005 to 0.5 g / l of the free acids.

A preferred embodiment of the process is that the compression is carried out in solutions which contain the reaction products formed in the addition of 2-8 molecules of acrylic acid to the two PH functions of the hypophosphorous acid or their water-soluble salts in an amount of 0.0005 to 0.5 g / l included.

A further embodiment of the process consists in that the compaction is carried out in solutions which contain the reaction products obtained in the reaction of maleic acid with hypophosphorous acid or their water-soluble salts.

It has also proven to be advantageous to carry out the compression in solutions which contain the reaction products obtained in the reaction of hypophosphorous acid with itaconic acid or their water-soluble salts.

An equally favorable embodiment of the process is given by the use of addition products of 1-butene-2,3,4-tricarboxylic acid with hypophosphorous acid or its water-soluble salts.

The solutions of the acids or salts according to the invention are adjusted to a pH of 4 to 8, preferably 5 to 6. This setting can be made with ammonia or acetic acid. For the preparation of the solutions, it is advantageous if demineralized or distilled water or condensed water is used.

The compression with the solutions according to the invention is carried out at temperatures between 90 ° C. and the boiling point. In general, a temperature of 95 to 100 ° C is maintained. The compression time is within the usual range and is about 1.5 to 3.5 min / gm layer thickness of the anodic oxide layer. Additives known per se, such as nickel or cobalt acetate, can also be added to the compression solutions for this purpose in small amounts between 0.0001 and 0.5 g / l. The new process makes it possible to prevent the formation of sealing coatings without impairing the anodic oxide layer or reducing the quality of the compression. The special properties of the active ingredients used greatly reduce the risk of unintentional harmful overdosing. Rinsing after compaction or spraying to remove any residue from the surface are not required. The appearance of the surface is not influenced by the method according to the invention; the effects that were achieved by pretreatment and anodization are retained.

In the following examples, the designation of the aluminum alloys was made in accordance with DIN 1725. The quality of the oxide layers was determined by the apparent conductance or y-value according to DIN 50949 and by the loss factor d according to ISO / TC 79 / SC2 (ALL-1) Doc. 65 E. The quality of the compression was also checked using the chromophosphoric acid test (ISO 3210).

example 1

An alkaline degreased and pickled aluminum sheet (Al 99.5) was compressed after the anodic oxidation using the direct current sulfuric acid method (layer thickness 20 µm) in a solution containing 0.01 g / l of a reaction product of 2 mol maleic acid in deionized water Contained 1 mol of hypophosphorous acid and had been adjusted to pH 5.8 with ammonia.

The reaction product of 2 moles of maleic acid with 1 mole of hypophosphorous acid was prepared by adding 45 g of sodium hypophosphite monohydrate to 100 g of maleic acid dissolved in 250 ml of water. The reaction mixture was heated to 60 ° C. and 8 g of ammonium persulfate dissolved in water were added dropwise in small portions over 4 hours. The reaction mixture was kept at 60 ° C. for a further 2 hours. The solution thus obtained was used for the experiments without isolating the reaction product, the active substance content resulting from the raw materials used.

The anodized aluminum was densified for 60 min. performed at 98 ° C. After that, the sheets showed no sealing coating. The layer thickness remained unchanged at 20 µm. The dielectric loss factor was 0.42 and the admittance had dropped from over 400 to 12.5 µS. A weight loss of 12.9 mg / dm ^{2 was} found in the chromophosphoric acid test.

Practically identical results were obtained after appropriate pH adjustment with acetic acid, if one looked at the equivalent amounts of the sodium, potassium, ammonium, magnesium, calcium, tetramethylammonium or alkanolamine salts of the reaction product of 2 moles of maleic acid with 1 mole of hypophosphorous acid ran out of.

Example 2

Alkaline degreased and pickled profiles made from the alloy AIMgSi 0.5 were anodized according to the direct current sulfuric acid-oxalic acid method (layer thickness 19 µm) and colored electrolytically bronze in a tin-containing coloring electrolyte. The profiles were then compacted for a time corresponding to 3 min / μm layer thickness at 98 ° C. in a solution adjusted to pH 6.0, which solution had been obtained in deionized water 0.001 g / l of a reaction product of hypophosphorous acid with 8 mol of acrylic acid.

The reaction product of acrylic acid with hypophosphorous acid was prepared as follows:

The suspension of 100 g of 50% hypophosphorous acid, 40 g of acrylic acid, 4 g of benzoyl peroxide containing 25% of water and 300 ml of water was carefully heated to 95-98 ° C. with stirring. The onset of exothermic reaction was without further heating by adding 340 g of acrylic acid for 45 min. kept going. After the addition and release of the exothermic reaction had ended, the mixture was stirred at 95-100 ° C. for 3 hours. The viscous solution thus obtained, which mathematically contained 430 g of active substance, was used for the experiments.

After the compaction was carried out as described above, the profiles showed no sealing coating or other visible deposits on the surface. The layer thickness was still 19 µm. The apparent conductance had dropped from over 400 to 17 µS. The dielectric loss actuator was 0.45. A weight loss of 9.3 mg / dm ^{2 was} found in the chromophosphoric acid test.

The same results were obtained when the reaction product of acrylic acid with hypophosphorous acid in a molar ratio of 4: 1 or equivalent amounts of its sodium, potassium, ammonium, magnesium, calcium, tetramethylammonium or alkanolamine salt were used after appropriate pH adjustment.

Example 3

Profiles made from the alloy AIMgSi 0.5 degreased and pickled by the usual method were anodized using the direct current sulfuric acid method (layer thickness 18-21 µm). These were at a pH of 5.8 (adjusted with ammonia or acetic acid) in solutions containing the reaction products of hypophosphorous acid with itaconic acid, Citra consic acid or 1-butene-2,3,4-tricarboxylic acid - prepared by the process mentioned in Example 1 - contained in the stated amounts at 97-100 ° C for 60 min. condensed.

The table shows the deposit-preventing effect and the influence of the substances on the layer quality, expressed according to the results of the admittance measurement of the dielectric loss factor and the weight loss in the chromium-phosphoric acid test. When the compounds according to the invention are used in appropriate concentrations, the sealing coating is prevented and the layer quality is not impaired. Comparative experiments with cyclohexane hexacarboxylic acid and phosphonobutane-2,3,4-tricarboxylic acid were included in the table, which show that, although the sealing coating is prevented in the case of the comparison substances, severe layer damage occurs even at the higher concentrations.
(See table on page 6 f.)

Claims (7)

1. A process for sealing anodically produced oxide coatings on aluminium or aluminium alloys by treatment with aqueous solutions having a pH of 4 to 8 at temperatures of from 90 °C to the boiling temperature, characterized in that sealing is performed with solutions containing from 0,005 to 0,5 g/I phosphinocarboxylic acids which are formed in the reaction of hypophosphorous acid with unsaturated mono- or polyfunctional carboxylic acids corresponding to the following general formula

in which

R₁ is hydrogen or one of the radicals

and

R₂, R₃ and R₄ are hydrogen or one of the radicals -CH₃, -C₂H₅, -COOH, -CH₂--COOH, ―CH₂―CH₂―COOH.

at least one of the substituents R₁ to R₄ being or containing a COOH group, in a molar ratio of from 1 : 1 to 1 : 8,
or water-soluble salts thereof.

2. A process as claimed in Claim 1, characterized in that sealing is carried out with solutions containing phosphinocarboxylic acids which are formed in the reaction of 1 mole hypophosphorous acid with from 2 to 8 moles acrylic acid, or water-soluble salts thereof.

3. A process as claimed in Claim 1, characterized in that sealing is carried out with solutions containing phosphinocarboxylic acids which are formed in the reaction of 1 mole hypophosphorous acid with 1 to 2 moles maleic acid, or water-soluble salts thereof.

4. A process as claimed in Claim 1, characterized in that sealing is carried out with solutions containing phosphinocarboxylic acids which are formed in the reaction of 1 mole hypophosphorous acid with 1 to 2 moles itaconic acid, or water-soluble salts thereof.

5. A process as claimed in Claim 1, characterized in that sealing is carried out with solutions containing phosphinocarboxylic acids which are formed in the reaction of 1 mole hypophosphorous acid with 1 to 2 moles citraconic acid, or water-soluble salts thereof.

6. A process as claimed in Claim 1, characterized in that sealing is carried out with solutions containing phosphinocarboxylic acids which are formed in the reaction of 1 mole hypophosphorous acid with 1 to 2 moles 1-butene-2,3,4-tricarboxylic acid, or water-soluble salts thereof.

7. A process as claimed in Claims 1 to 6, characterized in that the pH value is from 5 to 6.