EP0241415A1 - Acid tin-(II)-containing electrolyte - Google Patents

Abstract

In acid Sn(II)-containing electrolytes for staining anodically produced oxide layers on aluminium or aluminium alloys or for electroplating, the Sn(II) becomes oxidised. Adding substances to the electrolyte, so called stabilisers, may prevent the oxidation to a greater or lesser degree, i.e. Sn(II) can be stabilised to a greater or lesser degree. The effectiveness of the stabiliser furthermore governs the quality of the deposits or the coloration of the anodised layer. The quality of the surface treatment is known to deteriorate in the presence of Sn(IV) in substantial amounts alongside Sn(II) as a consequence of inadequate stabilisation of the Sn(II). An acid Sn(II)-containing electrolyte with a soluble diphenylamine or substituted diphenylamine derivative added stabilises the Sn(II) and yields faultless colorations.

Description

The invention relates to the stabilization of acidic Sn (II) -containing electrolytes which are used for coloring anodically produced oxide layers on aluminum or aluminum alloys or for electroplating.

Both in electroplating and in the dyeing of anodically applied aluminum oxide layers on aluminum or aluminum alloys, Sn (II) salts are used in acidic solutions, e.g. used in the form of sulfate, fluoroborate or chloride solutions.

It is known that practically only the acidic sulfate bath contains simple Sn (II) ions. With all other electrolytes, the tin is at least partially complexly bound.

A difficulty in electroplating or coloring anodically produced oxide layers on aluminum or aluminum alloys in acidic solutions is that the Sn (II) is oxidized to Sn (IV) during the working process. By adding substances to the electrolyte, so-called stabilizers, the oxidation can be more or less prevented, i.e. Sn (II) can be more or less stabilized.

The effectiveness of the stabilizer is decisive for the quality of the precipitation or the coloring of the anodized layer. It is generally known that in the presence of both types of ions Sn (II) and Sn (IV) inadequate stabilization of Sn (II) results in significant quantities the surface treatment deteriorated. It is therefore desirable to obtain the tin in acidic solutions as Sn (II).

In the case of acidic electrolytes, organic aromatic compounds are also known as stabilizers which contain amino groups, e.g. Aminophenol, dimethylaniline.

These compounds have a stabilizing effect for Sn (II). However, you cannot prevent part of the Sn (II) from being oxidized to Sn (IV). This particularly affects the quality of the coloring of anodic oxide layers on aluminum or aluminum alloys. If Sn (II) is not sufficiently stabilized, the depth of color of dark shades and the uniformity of the coloring are often poor. This is a major disadvantage of the stabilizers previously used. An ideal stabilizer would be one that effects full stabilization of the Sn (II) ion. In fact, when coloring anodized oxide layers on aluminum or aluminum alloys, it is difficult to produce colors such as dark bronze or black using the additives mentioned above. Usually the scattering of the electrolyte also leaves something to be desired. This is noticeable by the lighter and darker coloring of the edge zones. With an advanced coloring time of the anodized layer - from about 10 minutes - over-coloring occurs and a metallic coating is obtained on the surface, which results in cleaning problems of the colored surface, contamination of the compression baths and corrosion problems.

The invention is therefore based on the object of finding an acidic Sn (II) -containing electrolyte for coloring anodically oxidized aluminum oxide layers or for electroplating, which contains substances with a better stabilizing effect of Sn (II) than those known hitherto and better scattering and coupled with it has a better power distribution.

According to the invention, the object is achieved by an electrolyte which is distinguished by the wording of claim 1. Further advantageous embodiments of the invention are characterized by the features of claims 2 to 5. A further object of the invention is the use ^g emäss Claim 6, _1, the method according to the dyeing claims 7 to 9 and the colored aluminum articles according to claim 10 degrees.

When compounds from the group according to the invention are used in electrolysis baths, surprisingly, errors in the over-coloring no longer occur during the dyeing process of anodically oxidized aluminum layers with an advanced dyeing time. The lack of the often disadvantageous preservation of the dark hue and the non-uniformity of the coloration, which repeatedly occurred when the layers were colored in the presence of the previously customary and initially mentioned stabilizers, have been substantially improved or completely prevented by the additives according to the invention. The excellent coloring properties not only hit the light colors, but especially the dark ones, which until now have been very difficult to achieve both in terms of color depth and uniformity of color.

It is particularly surprising that these compounds are highly effective even in very small concentrations. Even an addition of 20 ppm has a significant stabilizing effect.

Overall, it was found that the aforementioned disadvantages of the known stabilizers for acidic Sn (II) -containing electrolytes for coloring anodically oxidized aluminum layers or for galvanizing did not occur in the electrolytes according to the invention, or only to a non-disturbing and therefore negligible extent.

In the formula I of claim 2, the groups R ₁ to R _{10 are} preferably H, -NH ₂ , -COOM or -S0 ₃ M and R _{11 is} preferably hydrogen or a group (a), in which n is a number from 1-5 means.

• 2-Amino-diphenylamin
• 4-Amino-diphenylamin
• 4-Amino-diphenylamin-2-carbonsäure
• Diphenylamin-4-sulfonsäure
• 2-Amino-diphenylamin-4-sulfonsäure
• 4-Amino-diphenylamin-2-sulfonsäure
• 4,4'-Diamino-diphenylamin-2-sulfonsäure
• 4'-Amino-4-nitrodiphenylamin-2-sulfonsäure
• 1-Amino-2,4-di(phenylamino)benzol-5-sulfonsäure und Diphenylamin-4,4'-disulfonsäure, vorzugsweise deren Mischungen mit der Diphenylamin-4-sulfonsäure.

The following substances and their mixtures from the compound group described according to the invention are particularly advantageous as additives:

• 2-amino-diphenylamine
• 4-amino-diphenylamine
• 4-amino-diphenylamine-2-carboxylic acid
• Diphenylamine-4-sulfonic acid
• 2-amino-diphenylamine-4-sulfonic acid
• 4-amino-diphenylamine-2-sulfonic acid
• 4,4'-diamino-diphenylamine-2-sulfonic acid
• 4'-amino-4-nitrodiphenylamine-2-sulfonic acid
• 1-amino-2,4-di (phenylamino) benzene-5-sulfonic acid and diphenylamine-4,4'-disulfonic acid, preferably their mixtures with the diphenylamine-4-sulfonic acid.

In order for the advantages of the electrolyte according to the invention to be fully recognized compared to those previously used, all factors - stabilizing effect of Sn (II), scattering / current distribution, coloring effect - must of course be assessed together.

In order to demonstrate the excellent effect of stabilization and the associated better coloring properties of anodically produced oxide layers on aluminum or aluminum alloys in acidic solutions, the following two test series and coloring tests are given. The purpose of these comparative experiments is to show the stabilizing effect of the additives according to the invention using a rapid test with gassing with pure oxygen.

Test series 1

An aqueous electrolyte was produced, consisting of H ₂ S0 ₄ , 10 g / l, and SnS0 ₄ , 20 g / l. Under these conditions, tin is initially available as Sn (II). The electrolyte was divided into 7 vessels of the same geometric shape. There was 1 liter of electrolyte in each vessel.

Bathroom 1: without additives

• Bad 2: Paraphenolsulfonsäure, 20 g/l, bisher üblicherweise verwendeter Zusatz
• Bad 3: N,N-Dimethylanilin, 100 mg/l, bekannter Zusatz
• Bad 4: Diphenylamin, 100 mg/l
• Bad 5: Diphenylamin-4-sulfonsäure 100 mg/1
• Bad 6: 2-Amino-diphenylamin-4-sulfonsäure 100 mg/l
• Bad 7: 4-Amino-diphenylamin-2-carbonsäure 100 mg/l

The other baths contained the following additives:

• Bath 2: Paraphenolsulfonic acid, 20 g / l, additive previously used
• Bath 3: N, N-dimethylaniline, 100 mg / l, known additive
• Bath 4: diphenylamine, 100 mg / l
• Bath 5: Diphenylamine-4-sulfonic acid 100 mg / 1
• Bath 6: 2-amino-diphenylamine-4-sulfonic acid 100 mg / l
• Bath 7: 4-amino-diphenylamine-2-carboxylic acid 100 mg / l

In all baths the pH was 1; the baths were at room temperature and were constantly moved with a magnetic stirrer. Each bath was gassed through a glass frit with 200 cm ^{3 of} pure oxygen per minute and the Sn (II) content was analyzed every half hour. The results are shown in FIG. 1 (ordinate: loss of Sn ^II in g / L; abscissa: time in hours).

Test series 2

The same stock solution as in test series 1 was prepared and likewise placed in the same vessels as in test series 1, 1 liter of electrolyte. In all baths the pH was 1; the baths were at room temperature and were constantly moved with a magnetic stirrer.

• Bad 9: 20 mg/l
• Bad 10: 100 mg/l
• Bad 11: 200 mg/l

The bath 8 corresponded to the bath 1 of the test series 1 and contained no additive. The following three baths all contained diphenylamine-4-sulfonic acid

• Bath 9: 20 mg / l
• Bath 10: 100 mg / l
• Bath 11: 200 mg / l

Bath 10 thus corresponds to bath 5 of test series 1.

As in test series 1, each bath was gassed with 200 cm ^{3 of} pure oxygen per minute through a glass frit and the Sn (II) content was determined every half hour. The results are shown in FIG. 2 (0dinate: loss of Sn ^II in g / L; abscissa: time in hours).

It can be seen from the figures that without addition, a large proportion of the Sn (II) has changed into Sn (IV) after a relatively short time. It can also be seen that the stabilizing effect is dependent both on the additional substance and on the additional amount. Despite the much larger amount added, the known additive paraphenolsulfonic acid works just like the known additive N, N-dimethylaniline much worse than the stabilizer of (SnII) than the additives used according to the invention.

Staining attempts

With the electrolytes, as stated in test series 1 and 2, which contain the new additives according to the invention, coloring tests of anodically oxidized aluminum layers were carried out in 60 liter baths. The voltage was 15 V in all cases, the treatment time varied between 1 and 12 minutes. The sample panels consisted of PERALUMAN-100 (semi-hard) and had the dimensions 200 x 300 x 1.5 mm. They were anodized using the normal direct current / H ₂ S0 ₄ method. The layer thickness was 20 µm.

In all cases where a stabilizing compound according to the invention was used, a perfect regular bronze coloration was achieved which was free from edge discoloration. With the additives that have been used up to now, edge effects often occur, particularly in the case of bronze color, due to poor scattering of the electrolyte. That is, with the Sn (II) -containing electrolyte according to the invention, which contains the new additives, the scatter is better than with the electrolytes previously used for this purpose. The result is a better current distribution and associated with this a more regular coloring of the anodized layer.

Amounts of 1 ^{g of} the additives according to the invention per liter of electrolyte were found as a sensible upper limit, without wishing to be determined in the process; accordingly as a reasonable lower limit about 20 mg. A mixture is disturbing

or combination of the various additives. Additional amounts beyond this have practically no special effect. Quantities of 20 to 500 mg / l, preferably 100 to 200 mg / l, of electrolyte have proven to be advantageous.

Claims (17)

1. Acidic Sn (II) -containing electrolyte for coloring anodically produced oxide layers on aluminum or aluminum alloys or for galvanizing metal surfaces in general, characterized in that the electrolyte contains an addition of at least one diphenylamine and / or substituted diphenylamine derivative which is soluble in an acid medium. 2. Acidic Sn (II) -containing electrolyte according to claim 1, characterized in that the electrolyte is an addition of at least one diphenylamine and / or substituted diphenylamine derivative of the formula I.

or contains a mixture of such compounds, where R ₁ to R ₁₀ independently of one another are hydrogen, halogen (at most once in each ring), nitro (at most once in each ring), -COOM (at most two in each ring), -S0 ₃ M ( at most two in each ring), C _1-4 alkyl, -NH ₂ (at most two in each ring) or phenylamino (at most once in each ring), R ₁₁ for Is hydrogen, C _1-4 alkyl, hydroxy-C _l-4 alkyl, phenyl or a group - (BO) n-R12 (a), where each _B , independently of one another, is -C ₂ H ₄ , -C ₃ H ₆ - or -C ₄ H ₈ -, n is a number from 1 to 20, preferably 1 to 5, and R _{12 is} hydrogen, -S0 ₃ M or -CH ₂ COOM, and M is hydrogen or the equivalent of a cation . 3. Acidic Sn (II) -containing electrolyte according to claim 2, characterized in that the electrolyte contains an addition of diphenylamine-4-sulfonic acid or a mixture of this compound with diphenylamine-4,4'-disulfonic acid. 4. Acidic Sn (II) -containing electrolyte according to one of claims 1 to 8, characterized in that the electrolyte contains the additive individually or in combination in an amount of 20 mg to 1 g per liter of electrolyte. 5. Acidic Sn (II) -containing electrolyte according to claim 9, characterized in that the electrolyte contains an additive in an amount of 20 to 500 mg per liter of electrolyte. 6. Use of a compound of formula I according to claim 2 as a stabilizer for an acidic Sn (II) -containing electrolyte for coloring anodic oxide layers on aluminum or aluminum alloys or for galvanizing metal surfaces in general. 7. A method for dyeing anodically produced oxide layers on aluminum or aluminum alloys, characterized in that an acidic Sn (II) -containing electrolyte according to claim 1 is used. 8. The method according to claim 7, characterized in that the electrolyte contains an addition of diphenylamine-4-sulfonic acid or a mixture of this compound with diphenylamine-4,4'-disulfonic acid. 9. The method according to claim 7 or 8, characterized in that the electrolyte contains an additive in an amount of 20 mg to 1 g, preferably 20 to 500 mg per liter of electrolyte. 10. Anodically produced oxide layers on aluminum or aluminum alloys which have been colored by a process according to claims 7, 8 or 9. P _A TENT REQUIREMENTS for Austria and Spain 1. A method for coloring anodically produced oxide layers on aluminum or aluminum alloys or for electroplating metal surfaces in general, characterized in that an acidic Sn (II) -containing electrolyte is used which comprises the addition of at least one diphenylamine and / or soluble in acidic medium contains substituted diphenylamine derivative. 2. The method according to claim 1, characterized in that the electrolyte is an addition of at least one diphenylamine and / or substituted diphenylamine derivative of the formula I.

or contains a mixture of such compounds, where R ₁ to R ₁₀ independently of one another are hydrogen, halogen (at most once in each ring), nitro (at most once in each ring), -COOM (at most two in each ring), -SO ₃ M ( at most two in each ring), C _1-4 alkyl, -NH ₂ (at most two in each ring) or phenylamino (at most once in each ring), R ₁₁ for Is hydrogen, C _1-4 alkyl, hydroxyC _1-4 alkyl, phenyl or a group - (BO) _n -R ₁₂ (a), where each B, independently of one another, is -C ₂ H ₄ -, -C ₃ ^H ₆ - or -C ₄ H ₈ -, _{n is} a number from 1 to 20, preferably 1 to 5, and R _{12 is} hydrogen, -S0 ₃ M or -CH ₂ COOM, and M is hydrogen or the equivalent of one Cations means. 3. The method according to claim 2, characterized in that the electrolyte contains an addition of diphenylamine-4-sulfonic acid or a mixture of this compound with diphenylamine-4,4'-disulfonic acid. 4. The method according to any one of claims 1 to 3, characterized in that the electrolyte contains the additive individually or in combination in an amount of 20 mg to 1 g per liter of electrolyte. 5. The method according to claim 4, characterized in that the electrolyte contains an additive in an amount of 20 to 500 mg per liter of electrolyte. 6. Anodically produced oxide layers on aluminum or aluminum alloys which have been colored by a process according to claims 1 to 5.

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