EP0118480B1 - Recompression process - Google Patents

Abstract

Anodized metal surfaces are cold pressed while they are treated at a temperature lower than 30oC in a still wet state with an aqeous solution containing nickel salt and at least one fluoride. The solution may contain further additives for example a preservating agent, optionally dissolved in a solubilization agent such as alcohols.

Description

In the known anodic oxidation of metal surfaces, especially those made of light metals, such as. B. aluminum, by converting the uppermost metal layers, protective layers are formed from the corresponding metal oxide. This produces fine-pore coatings that are suitable for reactions with various reagents and / or dyes, which today make it possible, in particular, to dye aluminum surfaces in a large number of colors. Depending on the intended use of these products, known as «anodized aluminum objects, for indoor or outdoor use, more or less weatherproof surfaces are required. The various colorants suitable for this and their use are well known to the person skilled in the art and do not require any further explanation here.

Common to all anodic oxidation processes is that the surfaces obtained, whether colorless or colored in one or more stages, are sensitive to contact and to corrosive substances. In order to remedy this great disadvantage, various post-compaction processes, so-called seal processes, have been developed, by means of which a contact-resistant and corrosion-resistant surface is obtained. The most common of these post-compaction processes is boiling water or steam treatment, known as hot water and steam sealing. These processes require very high energy consumption. Another disadvantage of hot water treatment is the formation of an undesirable deposit of lime and other salts and impurities dissolved in the bath water, which must then be removed mechanically in an additional operation. To prevent such deposits, so-called sealants have been proposed, which are generally based on high-molecular compounds, but which in turn make the process more expensive because they have to be supplemented.

Metal salts such as nickel salts, e.g. B. nickel acetate, have already been proposed as an additive to the hot water baths in order to intensify the compression. The compacting effect of these salt baths already occurs at a temperature of 70 ° C, so it does not require a boiling temperature, but here too a very thick deposit is formed that cannot be left on the surface.

US-A-3071 494 describes a solution for the recompression of anodized, colored aluminum surfaces in the heat in order to improve the color fastness. This solution contains ammonium complexes of hydrolyzable copper, nickel and cobalt salts.

JP-A-54 116 350 (Metal Finishing Abstracts, Vol. 22, No. 4, July / August 1980, Hampton Hill (GB), page 269 D, left column) proposes a solution for the recompression of the anodized surface, the Ni - and / or co-acetates and / or zirconium fluoride and a complicated polymer sulfonate contains.

According to EP-A-0101820, the densification is already possible at 30 to 35 ° C. with a solution which contains an organic substance containing an OH group, in particular of the R-Si (OH) ₃ type. Suitable auxiliaries can be the solution Ni, F-, SiF _e -, M0 _4, Mo0 ₄ -, Cr0 ₄ ions or contain mixtures thereof.

However, none of these known methods allows the improvement of the anodized and possibly colored aluminum surface to be achieved even at room temperatures.

Another disadvantage of the previously proposed sealing process is that a higher content of silicates, phosphates, chlorides and the like in the water acts as a «seal poison, i. H. Layers of insufficient quality are formed, which are broken down very quickly in the salt test and corrode completely.

Another difficulty of the previously known seal baths is that they usually have a lifespan of about 3 to 6 weeks. After this time they are unusable and must be thrown away.

A new sealing process has now been developed, which allows treatment at temperatures below 30 ° C and in particular also at room temperature, does not cause bath contamination, only requires replacement of the bath components removed by the material to be treated but does not require renewal of the bath and results in excellent compaction of the coatings.

Surprisingly, it was found that these results can be achieved by treating the anodically oxidized and possibly colored metal surfaces, in particular aluminum surfaces, with an aqueous solution which contains a nickel salt and a fluoride.

The present invention therefore relates to a process for the recompression of metals which have been anodically oxidized and optionally colored in an aqueous medium, in particular aluminum, which is characterized in that the still wet surface is brought into contact with an aqueous solution at a temperature below 30 ° C. which contains at least one nickel salt and at least one fluoride.

Suitable nickel salts are inorganic nickel salts, such as nickel chloride, sulfate, carbonate, nitrate, etc., and also organic nickel salts, such as nickel acetate and others. The fluoride is preferably a fluoride of an alkali metal salt such as sodium fluoride, potassium fluoride, etc., or ammonium, e.g. B. (NH ₄ ) HF ₂ , or an organic fluoride.

The salts are preferably used in approximately equimolar proportions of nickel salt to fluoride, the crystal water present also having to be taken into account in the case of the nickel salt. Larger or smaller quantities can also be

nickel or fluoride salt, but the results are generally less good.

The optimal addition of salts per liter of bath liquid depends on the surface to be treated and can be determined by simple tests. In general, total salt addition (nickel salt and fluoride) is about 1 to 20 g / liter, with an amount of about 10 g / liter being preferred for aluminum surfaces.

The pH of the treatment solution is advantageously in the slightly acidic range, e.g. B. at 5.5 to 5.8 and can be mixed with a conventional acid, e.g. B. acetic acid.

The treatment is carried out at a temperature below 30 ° C, preferably at room temperature, which can drastically reduce energy consumption. The contact time of the salt solution with _'the metal surface is not critical. Generally it is at least about 10 minutes. The treatment can be done by immersing in a seal bath or by spraying, brushing or in any way. The treated surface is generally rinsed off with cold water after the treatment, but this rinse can easily be omitted without having an adverse effect on the quality of the surface.

In particular, if baths are used for seal treatment, the addition of a preservative such as z. B. are also common for dye baths and other galvanic baths, for. B. Formalin, «Preventol and others; in general an addition of 0.01 g / liter is sufficient. If the preservative is not water-soluble, it can be in a water-miscible solvent, e.g. B. a lower aliphatic alcohol such as methanol, ethanol, propanol, n- or iso-butanol, etc. dissolved in the aqueous seal solution. Such an additive can also be recommended for solutions for spraying, brushing, etc., if these solutions are kept open for a long time, since they can easily be contaminated by microorganisms.

The seal solutions can contain other conventional additives, such as wetting agents and the like. It is important that the surfaces to be treated are subjected to the seal treatment according to the invention while still wet, i. H. practically immediately after the oxidation or coloring, or after the last rinse. As soon as the oxide layer begins to dry, the process according to the invention no longer results in the desired compaction, even if the surface is wetted again. This is in contrast to the previous hot sealing processes, in which the surface can still be compacted in the dry state even after long storage.

The method according to the invention results in excellent corrosion resistance and a smooth, attractive appearance of the surface. The compression effect has z. B. with aluminum generally reaches its maximum after 24 hours. The treated articles then have a permanent corrosion resistance and are suitable for all common uses. No deposits or hydrate formation are observed on the surfaces.

The removal value for aluminum sheet determined according to ISO 3210, which according to the currently applicable Euras standard may be up to 30 mg / dm ² for hot compression, is approximately 7 to 10 mg / dm ² for cold-compressed sheets according to the invention. The removal values obtained are both with colorless anodized aluminum surfaces and in particular with z. B. with electrolytic metal salt coloring colored aluminum surfaces compared to those of hot-compressed surfaces (with or without added salt) significantly improved.

In addition to the considerable energy savings, the solutions used according to the invention have the advantage of being significantly less polluting than the previous ones, since their service life is practically unlimited, so they do not have to be thrown away.

An agent for carrying out the above cold sealing process is characterized by a content of at least one nickel salt and additionally at least one fluoride of the type described above. The agent can be present as a dry mixture, as a paste or as a concentrate. It preferably contains the salts in an approximately equimolar ratio of nickel ice - H ₂ 0 to fluoride. The agent can already contain other additives such as wetting agents, preservatives, etc., but these can also be added separately when preparing the solution. The agent has an unlimited shelf life and can e.g. B. packaged in such a dosage form that no weighing, measurement or the like when preparing the solution or when supplementing it is necessary.

example 1

A bath is made by dissolving 10 g / liter of a mixture of 262.9 g Ni (S0 ₄ ). 6H ₂ 0 and 42.0 g NaF in water. 0.5 g / liter of the wetting agent «Ekalin F (Sandoz) and 0.01 g / liter« Preventol »(Bayer), dissolved in 10 g / liter butanol, and the pH of the solution to 5.6 with acetic acid are added to this solution set.

Aluminum profiles, which were colorless anodized in the usual way and some of which were additionally colored with «Colinal 3175 (layer thickness 24 µn1), were placed in the above bath, which had a temperature of around 25 ° C, immediately after leaving the last rinsing bath Leave in for 15 minutes. After 24 hours, the removal rate was 6.9 mg / dm ² for the colorless surfaces and 9.4 mg / dm ² for the colored surfaces.

Example 2

For comparison, the same colorless anodized profiles with a layer thickness of 24 ₁ 1m were subjected to the usual hot compression, with removal values of 16.4 mg / dm ^{2 being} obtained. The same colorless anodized, undensified profiles resulted in a removal of 14.4 mg / dm ² after compression as described in Example 1.

Claims (5)

1. A process for after-sealing the surface of metals, in particular aluminium, which have been anodically oxidised in an aqueous medium, and which may be coloured, characterised in that, while it is still wet, the surface is brought into contact at a temperature of less than 30 °C with an aqueous solution which contains at least one nickel salt and at least one fluoride.

2. A process as claimed in Claim 1, characterised in that the cation of the fluoride is an alkali, preferably sodium, potassium, ammonium or a monovalent organic radical.

3. A process as claimed in Claim 1 or 2, characterised in that the solution has a pH which is in the slightly acidic range, for example, between 5.5 and 5.8.

4. A process as claimed in any of Claims 1 to 3, characterised in that the salts are used in a substantially equimolar quantity of nickel salt, including water of crystallisation if present, to fluoride.

5. A process as claimed in any of Claims 1 to 4, characterised in that a preserving agent, and, if desired, other auxiliary agents such as, for example, a wetting agent, are added to the solution.