EP0241415B1 - 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 a method for dyeing anodically produced oxide layers on aluminum or aluminum alloys with an acidic Sn (II) -containing electrolyte, which is stabilized by certain additives, the Sn (II) electrolyte composition used therefor and its use for the dyeing of anodized aluminum or aluminum alloys.

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), the quality of the surface treatment deteriorates in considerable amounts due to insufficient stabilization of the Sn (II). It is therefore desirable to obtain the tin in acidic solutions as Sn (II).

From FR-A 2095375 (1) and GB-A 1089479 (2) methods for electroplating tin are known, the improvement of which consists in that a desired extension of the range of the applicable current densities is achieved by certain additives to the electroplating bath.
In (1) a two-component addition fulfills this purpose, an aromatic compound with a basic nitrogen atom such as diphenylamine, N, N-diphenylmethylamine and 2,4-diaminodiphenylamine being mentioned as the first component.
In (2) this addition belongs, for example, to the class of aromatic amines such as aminodiphenylmethane and derivatives thereof.

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 coloration 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 providing a process for coloring anodized aluminum or aluminum alloys using an acidic electrolyte containing Sn (II), which contains substances with a well-stabilizing effect of Sn (II) ions and a better scattering and thus coupled has a better current distribution.

worin R₁ bis R₁₀ unabhängig voneinander Wasserstoff, Halogen (höchstens einmal in jedem Ring), Nitro (höchstens einmal in jedem Ring), -COOM (höchstens zwei in jedem Ring), -SO₃M (höchstens zwei in jedem Ring), C₁₋₄Alkyl, -NH₂ (höchstens zwei in jedem Ring) oder Phenylamino (höchstens einmal in jedem Ring) bedeuten, R₁₁ für Wasserstoff, C₁₋₄Alkyl, Hydroxy-C₁₋₄alkyl, Phenyl oder eine Gruppe -(B-O)_n-R₁₂ (a) steht, wobei jedes B, unabhängig voneinander, für -C₂H₄-, -C₃H₆- oder -C₄H₈-, n für eine Zahl von 1 bis 20, und R₁₂ für Wasserstoff, -SO₃M oder -CH₂COOM stehen, und M Wasserstoff oder ein Kation bedeutet, oder einer Mischung solcher Verbindungen der Formel I stabilisiert ist.According to the invention, the object is achieved by an electrolyte which, by adding a compound of the formula I which is soluble in an acid medium,

wherein R₁ to R₁₀ 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), -SO₃M (at most two in each ring), C₁₋₄alkyl, -NH₂ (at most two in each ring) or phenylamino (at most once in each ring), R₁₁ represents hydrogen, C₁₋₄alkyl, hydroxy-C₁₋₄alkyl, phenyl or a group - (BO) _n -R₁₂ (a), where each B, independently of one another, is -C₂H₄-, -C₃H₆- or -C₄H₈-, n is a number from 1 to 20, and R₁₂ is hydrogen, -SO₃M or -CH₂COOM, and M is hydrogen or a cation, or a mixture of such compounds of formula I is stabilized.

When compounds of the formula I are used in electrolysis baths, surprisingly, errors relating to over-coloration no longer occur during the dyeing process of anodically oxidized aluminum layers with advanced dyeing time. The lack that dark shades were often inadequately obtained and the unevenness of coloration that repeatedly occurred when the layers were colored in the presence of the stabilizers customary hitherto have been substantially improved or completely prevented by the additives according to the invention. The excellent coloring properties apply not only to the light shades, but entirely especially the dark ones, which until now were very difficult to achieve both in terms of depth of color 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 disadvantages mentioned at the outset of known stabilizers for acidic Sn (II) -containing electrolytes for coloring anodized aluminum layers did not occur in the electrolytes used according to the invention, or only to a non-disturbing and therefore negligible extent.

In compounds of formula I, the groups R₁ to R₁₀ are preferably H, -NH₂, -COOM or -SO₃M and R₁₁ is preferably hydrogen or a group (a), in which n is a number from 1-5.

The following substances and their mixtures from the described group of compounds of formula I 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.

Diphenylamine-4-sulfonic acid or a mixture of this compound with diphenylamine-4,4'-disulfonic acid is preferably used.

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

In order to show 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 used in each case using a rapid test with gassing with pure oxygen.

Test series 1

Bad 1:

ohne Zusatz

Die anderen Bäder enthielten folgende Zusätze:

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/l

Bad 6:

2-Amino-diphenylamin-4-sulfonsäure, 100 mg/l

Bad 7:

4-Amino-diphenylamin-2-carbonsäure, 100 mg/l

In allen Bädern war der pH-Wert gleich 1; die Bäder befanden sich auf Raumtemperatur und wurden mit einem Magnetrührer ständig bewegt. Jedes Bad wurde durch eine Glasfritte mit 200 cm³ reinem Sauerstoff pro Minute begast und alle halbe Stunde wurde der Gehalt an Sn(II) analysiert. Die Ergebnisse sind in Figur 1 wiedergegeben (Ordinate: Verlust an Sn(II) in g/l; Abszisse: Zeit in Stunden).An aqueous electrolyte was prepared, consisting of H₂SO₄ (10 g / l) and SnSO₄ (20 g / l). Under these conditions, tin is initially available as Sn (II). The electrolyte was distributed over 7 vessels of the same geometric shape. There was 1 liter of electrolyte in each vessel.

Bathroom 1:

without addition

The other baths contained the following additives:

Bathroom 2:

Paraphenolsulfonic acid, 20 g / l, additive commonly used so far

Bathroom 3:

N, N-dimethylaniline, 100 mg / l, known additive

Bathroom 4:

Diphenylamine, 100 mg / l

Bathroom 5:

Diphenylamine-4-sulfonic acid, 100 mg / l

Bathroom 6:

2-amino-diphenylamine-4-sulfonic acid, 100 mg / l

Bathroom 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 cc 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 was prepared as in test series 1, which was placed in the same vessels of 1 liter of electrolyte as in test series 1. 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 .

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

Bathroom 9:

20 mg / l

Bathroom 10:

100 mg / l

Bathroom 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 (ordinate: 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. Although much larger amounts of the known additives paraphenolsulfonic acid and N, N-dimethylaniline have been added, their action as a stabilizer for Sn (II) is much worse than that of the additives used according to the invention.

Staining attempts

With the electrolytes specified in test series 1 and 2, which contain the additives to be used according to the invention, dyeing tests with anodically oxidized aluminum layers were carried out in 60 liter baths. The voltage was 15 volts in all cases, the treatment time varied between 1 and 12 minutes. The sample plates consisted of PERALUMAN-100 (semi-hard), an aluminum alloy with 1% magnesium, and had the dimensions 200 x 300 x 1.5 mm. The plates were anodized according to normal direct current / H₂SO₄ method. The layer thickness was 20 »m.

In all cases, a perfect regular bronze coloring 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. This means that with the Sn (II) -containing electrolyte used according to the invention, the scattering 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 used according to the invention per liter of electrolyte were found as a sensible upper limit, without wishing to be determined, and accordingly about 20 mg as a sensible lower limit. A mixture or combination of different additives does not interfere. Additional amounts beyond this have practically no additional effect. Quantities of 20 to 500 mg / l, preferably 10 to 20 mg / l, of electrolyte have proven to be advantageous.

Claims (9)

1. Process for dyeing anodically produced oxide layers on aluminium or aluminum alloys, characterized in that an acidic Sn(II)-containing electrolyte is used, which is stabilized by adding a compound of formula I,

   

   which is soluble in an acidic medium, and
   wherein R₁ to R₁₀, independently of one another, signify hydrogen, halogen (at most one in each ring), nitro (at most one in each ring), -COOM (at most two in each ring), -SO₃M (at most two in each ring), C₁₋₄alkyl, -NH₂ (at most two in each ring) or phenylamino (at most one in each ring), R₁₁ is hydrogen, C₁₋₄alkyl, hydroxy-C₁₋₄alkyl, phenyl or a group -(B-O)_n-R₁₂ (a), whereby each B, independently of one another, is -C₂H₄-, -C₃H₆-, -C₄H₈-, n is a number from 1 to 20, and R₁₂ is hydrogen, -SO₃M or -CH₂COOM, and M signifies hydrogen or a cation,
   or a mixture of such compounds of formula I.
2. Process according to claim 1, characterized in that the electrolyte contains an admixture of diphenylamine-4-sulphonic acid or a mixture of this compound with diphenylamine-4,4'-disulphonic acid.
3. Process according to claim 1 or 2, characterized in that the electrolyte contains the single or combined admixture in a quantity of 20 mg to 500 mg per litre of electrolyte.
4. Use of a compound of formula I according to claim 1, or of a mixture thereof, for dyeing anodically produced oxide layers on aluminium or aluminium alloys, in an acidic Sn(II)-containing electrolyte.
5. Use according to claim 4, whereby a compound from the series
      2-amino-diphenylamine
      4-amino-diphenylamine
      4-amino-diphenylamine-2-carboxylic acid
      diphenylamine-4-sulphonic acid
      2-amino-diphenylamine-4-sulphonic acid
      4-amino-diphenylamine-2-sulphonic acid
      4,4'-diamino-diphenylamine-2-sulphonic acid
      4'-amino-4-nitrodiphenylamine-2-sulphonic acid
      1-amino-2,4-di(phenylamino)benzene-5-sulphonic acid
      and diphenylamine-4,4'-disulphonic acid
   or a mixture thereof, is employed.
6. Use according to claim 5, whereby diphenylamine-4-sulphonic acid is employed, or a mixture of this compound with diphenylamine-4,4'-disulphonic acid.
7. Acidic Sn(II)-containing electrolyte for dyeing anodically produced oxide layers on aluminium or aluminium alloys, characterized in that the electrolyte contains a compound from the series
      4-amino-diphenylamine-2-carboxylic acid
      diphenylamine-4-sulphonic acid
      2-amino-diphenylamine-4-sulphonic acid
      4-amino-diphenylamine-2-sulphonic acid
      4,4'-diamino-diphenylamine-2-sulphonic acid
      4'-amino-4-nitrodiphenylamine-2-sulphonic acid
      1-amino-2,4-di(phenylamino)benzene-5-sulphonic acid
      and diphenylamine-4,4'-disulphonic acid
   or a mixture thereof.
8. Acidic Sn(II)-containing electrolyte according to claim 7, characterized in that the electrolyte contains an admixture of diphenylamine-4-sulphonic acid, or a mixture of this compound with diphenylamine-4,4'-disulphonic acid.
9. Acidic Sn(II)-containing electrolyte according to claim 7 or 8, characterized in that the electrolyte contains the single or combined admixture in a quantity of 20 mg to 500 mg per litre of electrolyte.

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