Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a process for producing
coloured, protective coatings on workpieces made of aluminum or alloys
thereof, to which a protective layer of oxide has already been applied.
The increasing use of aluminum or alumlnum alloys for
internal and external applications has resulted in the development of
numerous processes for the surface treatment of anodically produced
layers of oxide on aluminum and alloys thereof, in order to increase
the decorative effect and/or stability of such products.
A process for dyeing anodically oxidized aluminum
objects with aniline dyestuffs was developed at an early stage.
However, ob~ects thus dyed had poor resistance to atmospheric action.
Italian Patent 339,232 discloses a dyeing process
whereby anodized aluminum is immersed in the solution of a metal salt
which is more highly electro-positive than aluminum. The metal is
reduced by the aluminum and is precipitated on the surface thereof.
In practice, this method produced no useful results.
According to another known process, the aluminum
workpiece is anodized and is immersed in chemicals which penetrate
; into the pores of the oxide layer. The workpiece thus treated is
then immersed in an aqueous solution of metal salts which also enter
the pores and unite with the chemical substance first used. It has
been found that, in practice, this process easily leads to different
shades,
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when applied -to large areas: it is also difficult to
reproduce.
Also known are processes for simultaneously anodiz-
ing and dyeing alumnum workpieces. However, the choice
of colours afforded by these processes is limited. Fur-
thermore they are expensive and difficult to carry out,
since the metallic structure and composition of the
aluminum is extremely important in achieving the desired
results. For instance, it it is desired to produce a
grey, aluminum alloys containing a relatively large
amount of silicon must be used. Alloys of this kind are
dfficult to form by pressure and the producer is faced
with storage problems.
Aslo known are numerous processes for dyeing anod-
ized aluminum electrolytically. In these processes, the
anodized workpiece is immersed in an acid bath contain-
ing the dissolved salt of a metal adapted to precipitate
electrolytically in the pores of the layer of aluminum
oxide, thus forming coloured compounds. At the present
time, this is the process most commonly used for dye-
ing anodized workpieces, especially if alternating cur-
rent is to be used. The electrolyte proposed for-such
processes is an aqueous solution of the metal vanadium,
chromium, manganese, iron, cobalt, nickel, copper, sel-
enium, silver, cadmiun, tin, tellurium, gold and lead.
Some of these salts procues colours varying between a
black bronze and black, depending upon the current in-
tensity and the dyeing time, whereas others produce
yellow and red colours.
Norwegian Patent 125,236 discloses a process for
producing blue colours, in which the anodized aluminum
or aluminum-alloy workpiece is dyed electrolytically w
with the aid of an acid aqueous solution of a hetero-
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polybasic acid of` silicon or phosphorus with molyb-
denum or tungsten, of the formula H85i(mo207)6 or H8
Si(W207)6~ It was
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also proposed in this patent to use a dyeing electroly-
sis bath of this kind for over-dyeing anodized aluminum
which has been dyed, after anodizing, with aqueous sol-
ution of salts of the above-mentioned metals, using
the electrolytic method. Depending upon the metal salts
used for the initial electrolytic deying, this produced
mixed colours, e.g. grey-green, yellow-green and violet
in a number of attractive shades. After the conventional
treatment in boiling water, these colours proved to be
extremely fast and durable.
In the case of large workpieces, however, this pro- ~-
cess had its disadvantages, since the electrolysis bath
of hetero-polybasic acids of silicon or phosphorus with
molybdenum or tungsten gave off gas bubbles which, in
ascending, left vertical strips on the workpiece. Further-
more the known difficulties associated with two-stage
dyeing processes were increased still further by the two
fold electtrolytic treatment, as a result of occasional
secondary precipitations of silicon, which bring
about differences in colour.
It is therefore the purpose of this present inven-
tion to provide a process for the production of combin
ation dyeing on workpieces of oxidized aluminum or
aluminum alloys, which will el2minate the disadvantages
outline above. ;
This purpose is achieved in that, after being anod-
ized, workpieces of aluminum or aluminum alloys are dyed
electrolytically, with an alternating current, using a
aqueous bath consisting of solutions of metal salts,
after which they are imrnersed in an aqueous bath contain-
ing hetero-polybasic acids of silicon or phosphorus with
molybdenum or tungsten. Examples of such metals salts
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are the salts of the me-tals nickel, manganese,
cobalt, cadmium, iron and lead. In the case of sa:L-ts
of copper, and o:f metals nobler than copper
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it is desirable to add a small amoun-t of an appropriate
metal salt -to the elec-trolysis bath, appropr:iate metals
salts being, for example, stannous salts, or nickeL,
cobalt and zinc salts, in amount of about 0.5% by weight .
The admixture of small amount of a stannous salt,
for instance, apparently causes very small particles of
tin to be incorporated into the oxide layer during the
electrolytic treatment. Although this does effect the
dyeing action of -the noble-metal salts, for example
silver sulphate, it brings about a change in colour upon
immersion in the hetero-polybasic acid solution, which
is not brought about by the noble-metal salt i-tself.
The invention therefore relates to a proce.ss for
the production of combination dyeing on workpieces of
aluminum or aluminum alloys, whereby the workpiece, I
which has been anodized and possibley dyed electrolyti-
cally with the addition of a small quantity of an
appropriate metal salt, is immersed in an aqueous solu-
tion of a hetero-polybasic acid of silicon or phosphorus
with molybdenum or phosphorus 9 as a result of wh~ch an
electro-chemical reaction takes place, evidently withcut
a supply of current, with the metal ions previously
embedded electrolytically, the said reaction leading to
the desired combination dyeing.
In one particular aspect the present invention pro-
vides a process for the production of combination dyeing
on a workpiece of aluminum or aluminum alloys previously
anodically oxidized and then dyed electrolytically in
an acid solution of a metal salt, for between 0.1 and
15 minutes, with alternating current of 1 - 50 V, charac-
-te-rized in that the anodically oxidized and electrolyti-
cally dyed workpiece is immersed in an aqueous solution of
a heteropolybasic acid of silicon or phosphorus with
molybdenum, after which
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it is re-condensed in boiling wa-ter.
reStS have shown that if aluminum which has been
merely anodically oxidized is immersed in a dyeing bath
of this kind does not produce colour in the layer of
aluminum oxide, and it was therefore surprising that it
was possible to achieve chemcial dyeing after the anod-
ically oxidized aluminum had been dyed electrolytically,
using a bath consisting of dissolved salts of the above-
mentioned metals.
It has also been found, surprisingly enough, that,
with the process according to the invention - in cont-
rast to the process according to N~rwegian Patent 125,236 - it is unnecessary to use sufficiently pure hetero-
polybasic acids, which are very expensive to manufact~e
instead, in the process according to the invention,
these hetero-polybasic acids may be produced in situ
in the dyebath, in that sodium molybdate or sodium
tungstate may be caused to react, in an aqueous solution,
with soldium silicate or sodium phosphate and hydrochlor-
ic acid in an approximately stoichiometric ratio, for
example in accordance with the following equations:
12Na2ZO + Na2SiO3 + 26HCL + H20 r H4SiZ12040 X H20 +
26NaCl + llH20
or
12Na2Z04 + NaHP04 + 26HCL + H20~ H7P(Z207)6 x H20 +
26NaCl +llH20 wherein Z signifies Mo ore W.
The above reaction equations indicate that consid-
erable quantities of sodium chloride are formed duringthe production of hetero-polybasic acids. It was found,
surprisingly enough, this sodium-chloride content leads
to improved dyeing results, as is the case in a bath to
which pure manufactured hetero-polybasic acids have been
added. Furthermore, tests have shown that these improved
dyeing results are also obtained if sodium chloride is
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added t~ a dye-bath in which pure manufactured
hetero-polybasic acids have been dissolved.
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These improved results may be recognized by the more
pronounced grey shade of -the electrolytically applied
base colour. However, the sodium-chloride addition must
not be too large, since it has been found in practice
hat a sodium-chloride content more than double that
formed during the stoichiometric reaction, in relation
to the heteropholybasic acid, leads to a more brownish
grey~colour
The concentration of het~ro-polybasic acid in the
dyebath used may vary withih wide limits, concentrations
between 0.1 and 20% by weight having been found to give
good results. A concnetration of between 0.5 and 2% by
weight has been found preferable.
Particularly good dyeing results were obtained after
electrolytic dyeing in an acid, aqueous bath of a dis-
solved tin salt. Depending upon current density, tin-
salt concentration and dyeing time, the colours obtained
varied between from a weak bronze to a black. After a
workpiece so dyed electrolytically had been immersed in
a bath containing one of the above mentioned hetero
polybasic acids with tungsten or molybdenum, grey shades
were obtained, varying between dark grey - when the tin
base-colour was very dark, to a light grey - when the
base colour was a weak bronze. The period of immersion
also affected the colour obtained, a very long period
resulting in an unwanted colour change to blue. Good
results were obtained with immersion periods of between
1 and 10 minutes.
The grey colours obtained by the process according
tothe invention are extremely resistant to wear, light-
fast, and weatherproof and are therefore outstandingly
suitable for producing, on aluminum or aluminum-alloy
workpieces, the durable greys demanded for numerous
applications.
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Since the condition~ to be maintained during the
anodic oxidation of aluminum and the electrolytic
dyeing of anodically
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oxidized alumin~ are already described in detail in t
the relevant literature, they require no further ex- .
planation herein. A summary of the prior art may be
found in an article entitle "Electrolytic dyeing of an-
odized aluminum", Galvanotechnik 7968 Saulgau, 63 (1972)
No. 2 pages 110 to 121.~
The invention is explained hereinafter with the
aid of some examples.
EXAMPLE 1
As aluminum workpiece, previously anodically oxid-
ized in a 15% aqueous solution of sulphuric acid, was
connected to a tin counter-electrode in an aqueous
electrolyte containing 2% of stannic chloride and 2%
of concentrated sulphuric acid per litre. An alter-
nating voltage of between 5 and 8 V was applied to the
electrodes at room temperature for a period of time
varying between 5 and 15 minutes, while the current
density used varied between 0.2 and 0.8 amp/dm .
Depending on the length of time during which the alter-
nating current was applied, highly attractive bronze
tones were obtained. The bronze tone became deeper as
the period during which the alternating current was
applied was increased.
Workpieces thus electrolytically dyed were washed
with water and were immersed in a bath containing 2%
by weight of dissolved hetero-polybasic acid made of
silicon with molybdenum. The immersion period varied
between 1 and 10 minutes. Depending upon the electro-
lytically applied base colour and the length of the
immersion period, grey colours were obtained, varying
between light and dark grey, buth with a tine go green
in the lighter greys and a brownish-green tinge in the
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darker greys. After the conventional treatment in
boiling water, the greys obtained were highly resistant
to weathering.
EXAMPL.E 2
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Anodically oxidized workpieces electrolytically
dyed as in Example 1, were immersed in a bath -to which
was added about 80C~o by weight ol` NaCl of the stoichio-
metric amount in relation to the hetero-polybasic acid ,.
The grey colours obtained were purer than in Example 1.
EXAMPLE 3
An aluminum workpiece, previously anodically oxi-
dized as in Example 1, was connected to a graphite
counter-electrode in an aqueous electrolyte containing
30 g/l of boric acid and 50 g/l of cobalt acetate.
Alternating current at 14V and about 0.5 amps/dm was
passed through the electrolyte between the workpiece to
be dyed and the counter-electrode Dyeing was carried
on for 10 minutes, and the colour obtained was brown.
The workpiece was rinsed and was immersed in a dyebath
of the above-mentioned composition, with and with,out
the addition of sodium chloride. The colour obtaned
was an attractive grey with a green-brown shimmer, which
became purer after the workpiece was immersed in a bath
containing sodium chloride. After the ususal treatment
in boiling water, the colors thus obtained were highly
weatherproof.
The same results were obtained after basic electro-
lytic dyeing with dissolved nickel salts and subsequent
immersion in a bath containing the hetero-polybasic acid
of silicon with molybdenum or tungsten in the given
conentration, both with and t~ithout the addition of
sodium chloride.
EXAMPLE 4
Sodium molybdate in approximately stoichiometric
ratio to sodim silicate and hydrochloric acid was
reacted in an aqueous solution. The concentration of
the hetero-polybasic acid produced from silicon with
molybdenum in the aqueous bath amount to about
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0.5%, and when aluminum objects, previously dyed electro~
lytically as described in the foregoing examples - were
immersed in this bath, very pure and durable colours
were obtained after the usual treatment in boiling water.
This example shows that better dyeing was obtained in
this bath than in a bath in which pure manufactured
hetero-polybasic acids from silicon and molybdenum or
tungsten were dissolved.
EXAMPLE 5
These tests according to Examples l to 4 were re-
peated, but with hetero-polybasic acids from phosphorus
with molybdenum or tungsten. Substantially the same
results were obtained, but a somewhate longer period
of immersion was required to obtain the same chemical
dyeing.
EXAMPLE 6
A strip measuring approximately 5 x 12 cm of the
standard AlMg3 alloy was pickled in caustic soda, rinsedm
and anodized in a conventional sulphuric-acid electrolyre
according to German Industrial Standard 17 612, with
a minimal coating thickness of 20/um.
The test piece was then well rinsed and dyed in a
0.5% sulphuric silver-sulphate solution with no further
additions.
Voltage: 8V
Time: 0.25 minute
Colour: intense lemon yellow
The test piece was then well rinsed and immersed
for about 3.5 minutes, to a depth of a few centimeters
in a 2% molybdate-silicic-acid solution.
The immersed portion of the test piece was not of
the mixed colour desired, but~lwas merely lighter than
before, as would occur had it been immersed in another
acid solution.
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EXAMPI.E 7
A test piece was anodized as in Example 6, rinsed
and dyed in a 0.5% sulphuric silver-sulphate solution
to which 2.5 g/1 of` stannic sulphate has been added.
In order to prevent the silver from being completely
reduced to metal and precipitating, l~ of cresol sulph-
uric acid and 0.5% of gelatine were added.
Voltage: 8V
Time: l minute
Colour: dark brass shade
The test peice was then rinsed and immersed for
12 minutes in a molybdato-silicic-acid solution.
The immersed portion of the test piece showed a
change of colour towards green, but not the desired com-
bination colour.
EXAMPLE 8
Example 7 was repeated except that 5 g/l of stannic
sulphate were added to the electrolytic dyebath. However
this amount is so small that the carrier still consisted
substnatially of silver.
The test piece was immersed for 3 minutes in a
molybdatosilicic-acid solution, after which the brass
shade changed to a distinctly greenish shade.
E~AMPLE 9
Example 8 was repeated, except that the electrolytic
dyeing was carried out in a 2% sulphuric copper-sulphate
solution also containing 5 g/l of stannic sulphate.
The test piece was reddish brown. After rinsing and im-
mersion in a 2% molybdator-silicic acid solution, an
intense blue-grey colour was obtained after an immersion
time of 0.5 minutes.
Further tests have shown that combination dyeing
with
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copper and noble-metal salts is also possible by adding
small quantities, prererably between 0.5 and 1.~% of
metal salts other than the stannic salts mentioned above
for example nickel, cobalt and zinc salts.
In the case of thin, fine-pored layers of oxide,
the subsequent second dyeing in hetero-polybasic acid
may possibly turn out too weak. In order to counter-
act this, it has been found advantageous to immerse the
dyed aluminum objects in a solution of a reducing agent.
A highly dilute solution of a bivalent tin salt has
been found particularly suitable for this purpose.
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