Electrolyte additive for a colorant bath for coloring aluminum and process for coloring aluminum

The invention describes a novel electrolyte additive for a sulfuric acid tin(II) containing colorant bath for the alternating current coloring of anodized aluminum surfaces, consisting of a synergistic mixture of at least one antioxidant of one of the general formulas (I to IV) and at least one throwing power improver of general formula (V), and to a process for the alternating current coloration of anodized aluminum surfaces using the electrolyte additive of the invention.

FIELD OF THE INVENTION 
This invention relates to a new electrolyte additive for a sulfuric acid 
coloring bath containing tin(II) for the alternating current coloring of 
anodized aluminum surfaces, which consists of a synergistic mixture of at 
least one antioxidant corresponding to one of general formulae I to IV and 
at least one throwing power improver corresponding to general formula V, 
and to a process for the alternating-current coloring of anodized aluminum 
surfaces using the electrolyte additive according to the invention. 
STATEMENT OF RELATED ART 
It is known that, on account of its base character, aluminum becomes 
covered with a natural oxide coating generally below 0.1 .mu.m in 
thickness (Wernick, Pinner, Zurbrugg, Weiner, Die Oberflachenbehandlung 
von Aluminum (The Surface Treatment of Aluminum), 2nd Edition, (Eugen 
Leuze Verlag, Saulgau/Wurtt., 1977). 
Considerably thicker oxide coatings can be obtained by electrolytic 
oxidation of aluminum. This process is known as anodizing. Sulfuric acid, 
chromic acid or phosphoric acid is preferably used as the electrolyte. 
Organic acids, such as for example oxalic acid, maleic acid, phthalic 
acid, salicylic acid, sulfosalicylic acid, sulfophthalic acid, tartaric 
acid or citric acid, are also used in some processes. 
However, sulfuric acid is the most commonly used electrolyte. Depending on 
the anodizing conditions, layer thicknesses of up to 150 .mu.m can be 
obtained in this process. However, layer thicknesses of 20 to 25 .mu.m are 
sufficient for external applications, such as for example facade facings 
or window frames. 
The anodizing process is generally carried out in 10 to 20% sulfuric acid 
with a current density of 1.5 A/dm.sup.2, at a temperature of 18.degree. 
to 22.degree. C. and over a period of 15 to 60 minutes, depending on the 
required layer thickness and the particular application. 
The oxide coatings thus produced have a high absorption capacity for a 
number of organic and inorganic substances or dyes. 
Electrolytic coloring processes, in which anodized aluminum is colored by 
treatment with alternating current in heavy metal salt solutions, have 
been known since the middle of the thirties. The heavy metals used are, 
above all, elements of the first transition series, such as Cr, Mn, Fe, 
Co, Ni, Cu and, in particular, Sn. The heavy metal salts are generally 
sulfates, pH being adjusted to a value of 0.1 to 2.0 with sulfuric acid. 
The coloring process is carried out at a voltage of around 10 to 25 V and 
the resulting current density. The counter-electrode may either consist of 
graphite or stainless steel or of the same material which is dissolved in 
the electrolyte. 
In this process, the heavy metal pigment is deposited in the pores of the 
anodic oxide coating in the half cycle of the alternating current in which 
aluminum is the cathode, the aluminum oxide coating being further 
thickened by anodic oxidation in the second half cycle. The heavy metal is 
deposited at the bottom of the pores and thus colors the oxide coating. 
However, one of the problems encountered where coloring is carried out in 
tin electrolytes is that the tin readily oxidizes, so that basic tin(IV) 
oxide hydrates (stannic acid) are rapidly precipitated during the 
application and, in some cases, even during the storage of the Sn 
solutions. It is known that aqueous tin(II) sulfate solutions are oxidized 
to tin(IV) compounds simply by exposure to atmospheric oxygen or by 
reaction at the electrodes when current is applied. This is highly 
undesirable in the coloring of anodized aluminum in tin electrolytes 
because, on the one hand, it disrupts the process sequence (frequent 
renewal or topping up of the solutions rendered unusable by the formation 
of precipitates) and, on the other hand, leads to considerable extra costs 
because of the tin(IV) compounds which cannot be used for coloring. 
Accordingly, various processes have been developed, differing in 
particular in the means used to stabilize the generally sulfuric acid 
tin(II) sulfate solutions for the electrolytic coloring of aluminum. 
Phenol-like compounds, such as phenol sulfonic acid, cresol sulfonic acid 
or sulfosalicylic acid, are by far the most commonly used (see, for 
example, in S. A. Pozzoli, F. Tegiacchi; Korros. Korrosionsschutz Alum., 
Veranst. Eur. Foed. Korros. Vortr. 88th 1976, 139-45 or in published 
Japanese patent applications JP-A-78 13583, 78 18483, 77 135841, 76 
147436, 74 31614, 73 101331, 71 20568, 75 26066, 76 122637, 54 097545, 56 
081598 and in GB-C-1,482,390). 
Polyhydric phenols such as, for example, the diphenols hydroquinone, 
pyrocatechol and resorcinol (see published Japanese applications JP-A-58 
113391, 57 200221 and in FR-C-23 84 037) and the triphenols phloroglycinol 
(JP-A-58 113391 ), pyrogallol (S. A. Pozzoli, F. Tegiacchi; Korros. 
Korrosionsschutz Alum., Veranst. Eur. Foed. Korros., Vortr. 88th 1976, 
139-45 or in published Japanese patent applications JP-A-58 113391 and 57 
200221) and gallic acid (JP-A-53 13583) have also been described in this 
connection. 
Another significant problem in electrolytic coloring is so-called throwing 
power (depth throwing) which is understood to be the ability of a product 
to color anodized aluminum parts situated at different distances from the 
counter-electrode with a uniform color. Good throwing power is 
particularly important when the aluminum parts used are complicated in 
shape (coloring of depressions), when the aluminum parts are very large 
and when, for economic reasons, several aluminum parts have to be 
simultaneously colored in a single coloring process and medium color tones 
are to be obtained. In practice, therefore, high throwing power is highly 
desirable because faulty production is avoided and the optical quality of 
the colored aluminum parts is generally better. The process is made more 
economical by good throwing power because several parts can be colored in 
a single operation. 
Throwing power is not the same as uniformity and a clear distinction has to 
be drawn between the two. Uniformity is coloring with the fewest possible 
local variations in color tone (patchy coloring). Poor uniformity is 
generally caused by impurities, such as nitrate, or by errors in the 
anodizing process. A good coloring electrolyte should never adversely 
affect the uniformity of coloring. 
Although a coloring process may achieve high uniformity, it may still have 
poor throwing power; the reverse is also possible. In general, uniformity 
is only influenced by the chemical composition of the electrolyte while 
throwing power is also dependent upon electrical and geometric parameters, 
such as for example the shape of the workpiece or its positioning and 
size. 
DE-A-26 09 146 describes a process for coloring in tin electrolytes, in 
which throwing power is established through the particular circuit and 
voltage arrangement. 
DE-A-24 28 635 describes the use of a combination of tin(II) salts and zinc 
salts with addition of sulfuric acid and boric acid and also aromatic 
carboxylic and sulfonic acids (sulfophthalic acid or sulfosalicylic acid) 
in the electrolytic coloring of anodically oxidized aluminum articles in 
grey tones. Excellent throwing of the coloring effect is said to be 
obtained in particular when the pH value is between 1 and 1.5. pH 
adjustment to 1-1.5 is a basic prerequisite for good electrolytic 
coloring. There is no mention of whether the organic acids added have an 
effect on throwing power, nor is the throwing power achieved 
quantitatively described. 
DE-C-32 46 704 describes a process for electrolytic coloring in which good 
throwing power is guaranteed by the use of special geometry in the 
coloring bath. In addition, cresol and phenol sulfonic acid, organic 
substances, such as dextrin and/or thiourea and/or gelatine, are said to 
guarantee uniform coloring. The disadvantage of this process lies in the 
high capital outlay involved in installation of the necessary equipment. 
The addition of deposition inhibitors, such as dextrin, thiourea and 
gelatin, has only a slight influence on throwing power because the 
deposition process in electrolytic coloring differs significantly from 
that in electro-tin-plating. There is also no reference in the document in 
question to possible methods of measuring the improvements in throwing 
power. 
In addition, European patent application EP-A-354 365 describes a process 
for the electrolytic coloring of anodized aluminum surfaces using metal 
salts, in which the antioxidants corresponding to general formulae I and 
IV (cf. the claims) are used together with the throwing power improvers 
p-toluene sulfonic acid and/or naphthalene sulfonic acid. However, the 
throwing power improvers mentioned in this document lead during 
electrolysis to foul-smelling decomposition products so that these 
throwing power improvers are no longer being used. 
DESCRIPTION OF THE INVENTION 
Object of the Invention 
Now, the problem addressed by the present invention was to provide a new 
electrolyte additive for a sulfuric acid coloring bath containing tin(II) 
for the alternating-current coloring of anodized aluminum surfaces which 
would overcome the problems known from the prior art discussed in the 
foregoing, such as guaranteeing lasting stability of the coloring bath, 
avoiding the oxidation of Sn(II) and, at the same time, guaranteeing good 
throwing power. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention relates to an electrolyte additive for a 
sulfuric acid coloring bath containing tin(II) for the alternating-current 
coloring of anodized aluminum surfaces containing at least one antioxidant 
and at least one throwing power improver, characterized in that the 
electrolyte additive contains 
a) as antioxidant at least one compound corresponding to general formulae I 
to IV: 
##STR1## 
in which R.sup.1 and R.sup.2 represent hydrogen, alkyl, aryl, alkylaryl, 
alkylaryl sulfonic acid, alkyl sulfonic acid containing 1 to 22 carbon 
atoms and alkali metal salts thereof and 
R.sup.3 represents one or more hydrogen and/or alkyl, aryl, alkylaryl 
moieties containing 1 to 22 carbon atoms, at least one of the substituents 
R.sup.1, R.sup.2 and R.sup.3 not being hydrogen, and 
b) as throwing power improver at least one aromatic carboxylic acid 
corresponding to general formula V: 
##STR2## 
in which R.sup.1 to R.sup.5 represent hydrogen, hydroxyl, carboxyl and/or 
sulfonic acid groups. 
The present invention also relates to a process for the alternating-current 
coloring of anodized aluminum surfaces in a sulfuric acid coloring bath 
containing tin(II), characterized in that an electrolyte additive as 
defined above is used for electrolytic coloring in the sulfuric acid 
coloring bath containing tin(II) at a pH value of 0.1 to 2.0, at a 
temperature of 10.degree. to 30 .degree. C. and with an alternating 
current voltage with a frequency of 50 to 60 hertz and a terminal voltage 
of 10 to 25 V. 
A major advantage of the electrolyte additive according to the invention 
lies in the use of oxidation-stable, water-soluble throwing power 
improvers. It is particularly after relatively long periods of operation 
that the p-toluene sulfonic acid known from the teaching of EP-A-354 365 
emits foul-smelling vapors through oxidation of the methyl group so that 
the coloring bath cannot be used for prolonged periods. According to the 
invention, therefore, it is particularly important to provide the throwing 
power improver with oxidation-stable functional groups, such as carboxyl, 
hydroxyl and/or sulfonic acid groups. In addition, the functional groups 
mentioned guarantee the necessary solubility in water. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
In one preferred embodiment of the invention, the electrolyte additive 
contains at least one of the compounds corresponding to one of general 
formulae I to IV in a quantity of 0.01 to 2 g/l as antioxidant and at 
least one of the compounds corresponding to general formula V in a 
quantity of 0.1 to 30 g/l, based on the coloring bath, as throwing power 
improver. 
According to the invention, 2-tert-butyl-1,4-dihydroxybenzene (tert-butyl 
hydroquinone),methylhydroquinone, trimethyl hydroquinone, 
4-hydroxy-2,7-naphthalene disulfonic acid and/or p-hydroxyanisole in 
particular are used in the concentrations mentioned above as the 
antioxidants corresponding to general formulae I to IV. 
According to the invention, 5-sulfosalicylic acid, 4-sulfophthalic acid, 
2-sulfobenzoic acid, benzoic acid and/or benzene hexacarboxylic acid are 
used as throwing power improvers corresponding to general formula V. The 
use of 5-sulfosalicylic and 4-sulfophthalic acid together has proved to be 
particularly effective by virtue of the synergistic effect produced. 
In one preferred embodiment of the invention, therefore, the electrolyte 
additive according to the invention contains (based on the total volume of 
the coloring bath): 
a) t-butyl hydroquinone in a quantity of 0.01 to 2 g/l as antioxidant and 
b) 5-sulfosalicylic acid in a quantity of 0.5 to 6 g/l and 4-sulfophthalic 
acid in a quantity of 5 to 20 g/l as throwing power improvers. 
In one particularly preferred embodiment, the electrolyte additive 
according to the invention contains in particular (based on the total 
volume of the coloring bath): 
a) t-butyl hydroquinone in a quantity of 0.1 to 0.5 g/l and preferably in a 
quantity of 0.2 to 0.3 g/l as antioxidant and 
b) 5-sulfosalicylic acid in a quantity of 1 to 3 g/l and preferably 1.5 to 
2.5 g/l and 4-sulfophthalic acid in a quantity of 8 to 12 g/l and 
preferably 10 g/l as throwing power improvers. 
The coloring process is normally carried out using a tin(II) sulfate 
solution containing approximately 3 to 20 g and preferably 7 to 16 g 
tin(II) per liter. In a preferred embodiment, the coloring process is 
carried out at a pH value of 0.1 to 2.0, corresponding to 16 to 22 g 
sulfuric acid per liter, and at a temperature of approximately 14.degree. 
to 30.degree. C. The alternating current voltage or the alternating 
current voltage superimposed on direct current (50 to 60 hertz) is 
preferably adjusted to a value of 10 to 25 V and, more preferably, to a 
value of 15 to 18 V with an optimum at approximately 17 V.+-.3 V. 
In the context of the invention, alternating current coloring is understood 
to be either coloring with pure alternating current or coloring with 
"alternating current superimposed on direct current" or "direct current 
superimposed on alternating current". The figure shown is always the 
terminal voltage. Coloring begins at a resulting current density of 
generally about 1 A/dm.sup.2 which then falls to a constant value of 0.2 
to 0.5 A/dm.sup.2. The color tones obtained, which can vary from champagne 
through various bronze tones to black, differ according to the voltage, 
the metal concentration in the coloring bath and the immersion times. 
In another embodiment, the process according to the invention is 
characterized in that the electrolyte contains other heavy metal salts 
besides tin, for example nickel, cobalt, copper and/or zinc (see Wernick, 
et al., loc. cit.). 
The electrolyte additive according to the invention is illustrated by the 
following Examples:

EXAMPLES 
Test Methods 
a) Accelerated Test for Evaluating the Stability of the Baths in Storage 
(Test 1 ) 
An aqueous electrolyte was prepared, containing 20 g/l of sulfuric acid and 
10 g/l of Sn(II) ions and corresponding quantities of an electrolyte 
additive. 1 liter solutions were vigorously stirred with a magnetic 
stirrer at room temperature and aerated through a glass frit with 12 l/h 
of pure oxygen. The content of Sn(II) ions was iodometrically determined 
after 4 hours. The percentage reduction in the concentration of Sn(II) was 
recorded. 
b) Evaluation of the Antioxidant Effect Under Current (Test 2) 
An aqueous electrolyte was prepared, containing 20 g/l of sulfuric acid, 10 
g/l of Sn(II) ions and corresponding quantities of an electrolyte 
additive. The continuous electrolysis (alternating current 50 hertz, 
voltage 12 V) was carried out with stainless steel electrodes. The 
quantity of current flowing was recorded with an ampere-hour counter. The 
characteristic behavior of the oxide coating to be colored was simulated 
by corresponding sine distortion of the alternating current under a high 
capacitive load. The quantity of Sn(II) ions oxidized by electrode 
reactions was determined by continuous iodometric titration of the 
electrolyte and by gravimetric determination of the reductively deposited 
Sn and the difference between the sum of these two values and the starting 
quantity of dissolved Sn(II). The ampere-hour value at which a reduction 
in the Sn(II) concentration of 5 g/l can no longer be prevented was 
selected as a measure of the antioxidant effect. 
c) Evaluation of Throwing Power (Test 3) 
50 mm.times.460 mm.times.1 mm sample plates of the DIN material Al 99.5 
were conventionally pretreated and then electrolytically colored in a 
coloring bath of suitable geometry (electrode at a distance at 1 to 5 cm 
from the counter electrodes). In addition to 10 g/l of Sn(II) and 20 g/l 
of sulfuric acid, the coloring bath also contained various quantities of 
the test substances (see Examples and Comparison Examples). Coloring was 
routinely carried out for 5 minutes at 16 V (alternating current 50 
hertz). The coloring result was numerically determined as follows: first 
the distribution of tin over the test plate was determined at ten 
different places in the longitudinal direction (i.e. at 5 cm intervals) by 
measurement with a scattered light reflectometer against the white 
standard titanium dioxide (=99% ). The "average coloring" is obtained from 
the individual measurements. Throwing power is determined therefrom as a 
measure of the accordance of each measuring point with the average value 
and is expressed as a percentage. A throwing power of 100% means that the 
test plate is evenly colored over its entire length. The closer the values 
come to the value 0%, the more differently the ends of the plate are 
colored. 
Electrolytic Coloring 
Test plates of the DIN material Al 99.5 (No. 3.0255) were conventionally 
pretreated (degreased, pickled, descaled) and anodized for 60 minutes by 
the GS process (200 g/l of sulfuric acid, 10 g/l of Al(III), air 
throughput, 1.5 A/dm.sup.2, 18.degree. C.). A layer thickness of about 20 
.mu.m was obtained. The plates thus pretreated were electrolytically 
colored with alternating current (50 hertz) as described in the following 
Examples. The results are set out in Table 1. 
TABLE 1 
______________________________________ 
Test 1 Test 2 Test 3 
[stability 
[Antioxidant 
[Throw- 
in storage 
effect in ing pow- 
in %] Amp-hours] er %] 
______________________________________ 
Example 1 0 810 99 
Example 2 0 810 83 
Example 3 0 809 90 
Example 4 0 800 99 
Example 5 0 880 99 
Example 6 0 810 99 
Example 7 0 875 97 
Comparison Example 1 
72 560 54 
Comparison Example 2 
0 810 55 
Comparison Example 3 
69 570 86 
Comparison Example 4 
70 565 90 
Comparison Example 5 
0 800 96 
______________________________________ 
______________________________________ 
Example 1 
Electrolyte: 10.0 g/l of Sn(II) 
10.0 g/l of sulfuric acid 
0.2 g/l of t-butyl hydroquinone 
2.0 g/l of 5-sulfosalicylic acid 
10.0 g/l of 4-sulfophthalic acid 
Coloring parameters: 
16 V, 5 minutes 
Example 2 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of t-butyl hydroquinone 
2.0 g/l of 5-sulfosalicylic acid 
Coloring parameters: 
16 V, 5 minutes 
Example 3 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of t-butyl hydroquinone 
10.0 g/l of 4-sulfophthalic acid 
Coloring parameters: 
16 V, 5 minutes 
Example 4 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of methyl hydroquinone 
2.0 g/l of 5-sulfosalicylic acid 
10.0 g/l of 4-sulfophthalic acid 
Coloring parameters: 
16 V, 5 minutes 
Example 5 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of trimethyl hydroquinone 
2.0 g/l of 5-sulfosalicylic acid 
10.0 g/l of 4-sulfophthalic acid 
Coloring parameters: 
16 V, 5 minutes 
Example 6 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 911 of t-butyl hydroquinone 
10.0 g/l of benzene hexacarboxylic 
acid 
Coloring parameters: 
16 V, 5 minutes 
Example 7 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of trimethyl hydroquinone 
20.0 g/l of sulfobenzoic acid 
Coloring parameters: 
16 V, 5 minutes 
Comparison Example 1 
Electrolyte: 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
Coloring parameters: 
16 V, 5 minutes 
Comparison Example 2 
Electrolyte: 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of t-butyl hydroquinone 
Coloring parameters: 
16 V, 5 minutes 
Comparison Example 3 
Electrolyte: 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
2.0 g/l of 5-sulfosalicylic acid 
Coloring parameters: 
16 V, 5 minutes 
Comparison Example 4 
Electrolyte: 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
10.0 g/l of 4-sulfophthalic acid 
Coloring parameters: 
16 V, 5 minutes 
Comparison Example 5 
Electrolyte: 10.0 g/l of Sn(II) 
20.0 g/l of sulfuric acid 
0.2 g/l of t-butyl hydroquinone 
20.0 g/l of p-toluene sulfonic acid 
Coloring parameters: 
16 V, 5 minutes 
______________________________________ 
The results in Table 1 clearly show that an electrolyte additive containing 
a mixture of an antioxidant corresponding to one of general formulae I to 
IV and a throwing power improver corresponding to general formula V 
(Examples 1 to 7) clearly improves the coloring properties of the tin(II) 
salt electrolyte, such as stability in storage, antioxidant effect and 
throwing power, in relation to Comparison Examples 1 to 4. In the case of 
Comparison Example 5, an intensifying foul-smelling odor occurs after only 
15 minutes.