Note: Descriptions are shown in the official language in which they were submitted.
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The pxesent invention relates to a process for the
electrolytic colouring of anodized aluminium.
Although the baths and procedures used in the pro-
cesses for the el~ctrolytic colouring of anodized aluminium
with inorganic pigments have increased the number of patents
existing on thls subject, it is a fact that the colours pro-
duced with the mentioned technique are, at an industrial scale,
rather poor.
The search for new baths and processes, with the
idea of obtaining new colours, has been the general trend in
this field and thus in 1968 French patent 1,605,100 developed
a novel method for colouring, whereby yellow and brick-red
colours were obtained by the partial anodic dissolution of the
particles deposited in the bottom of the blister in a sodium
thiosulphate solution.
Subsequently German patents 2,106,388 and 2,106,339
cited a new process for obtaining bluish colours which mainly
consisted in electrodepositing metals such as Cu, Co and Ni on
a chromic acid formed anodic layer and simultaneously stamping
the samples under special conditions. This process has the
disadvantage that the colouring can only take place on chromic
acid anodized samples to produce the new colours and besides
stamping should take place in specific solutions.
From 1974 onwards there appeared various patents,
inter alia, Spanish patent 437,604 and French patent 2,236,029,
whereby colours other than those normally obtained in the
electrolytic colouring baths were achieved by using high con-
centrations of sulphuric acid.
These processes have the typical disadvantage of
working with high proton concentrations, i.e. the possibility of
producing spalling at not very high voltages. There is another
important problem which should be emphasized, i.e. the com-
petition between the discharge of the ions, less noble thanhydrogen, and the proton itself. Due to this disadvantage dark
tones cannot be achieved in the majority of the baths. However,
the high proton concentration can cause losses in the intensity
of the colour and in the washing and stamping processes.
Subsequently, French Patent 2,318,245, using a double
anodizing process, obtained a wlde range of colours and tones
~y the optical interference between the light reflected by the
colouring pigment and that reflected by the surface of the
aluminium.
The process for obtaining this type of colouring by
optical interference has the disadvantage that, in order to ob-
tain the new colours, the elements should be subjected to a
chemical or an electrolytic treatment between the anodizing and
the colouring process, so that the complete blister, or at least
the bottom thereof, is widened.
Another feature of this patent resides in using in the
anodizing process voltages greater than 35 volts. This produces
serious disadvantages in the baths normally used in these pro-
cesses.
Subsequently, French Patent 2,380,257, produced dif-
ferent coloured electro-deposits on the aluminium, which electro-
deposits had the disadvantage of not being adherent nor resis-
tant to corrosion. Therefore, it was necessary to apply a
layer of lacquer or varnish to the samples treated with said
process.
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The process of the present invention has the fo'
lowing advantages when compared with the known techniqu2 set
forth in the aforementioned patents:
1. It permits pH values greater than 0.8 ~o be
operated with.
2. It does not require a re-anodizing bath, i.e.
this process fo~ows the known two-phase colouring process.
It produces finishes having a wide range of
colours and tones resistant to light and to corrosion.
The process for the electrolytic colouring of ano-
dized aluminium of this invention~ ~ommences by producing
an aluminium oxide layer on the metal using the conventional
method in a sulphuric acid bath, but with a concentration
of ~rom lSO to 200 g/l, a voltage between ele~trodes of from
12 to 20 volts, and a current density of from 1 to 2 A/dm2.
The duration of this treatment ~ be of from 15 minutes to
1 hour.
Once the anodic layer of the pleces is obtainad~
these are introduced in the colouring bath.
In the mentioned colouring bath the piece is sub-
~ected to two different elec~rolytic steps. Thus, the irst
step consists in subjectLng the anodized aluminium piece to
a continuous puIsating voltage, fixed or programmed, the
duration o~ which electrolytic treatment Yarying ~rom 3 to
10 minutes and the average voltages used also varyi~g from
7 ts 35 volts. The type of waYe used ~n this first step of
the colouring treatment can be wavy or of any o~her type.
The second step of the treatment consists of
various possibilitiesO ~irstly~ a nesative ~:ontinuQus pul
sating ~ol tage can be programmed on the wava ~orm of the
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first step for a period of time of from 2 to 30 minutes, depend-
ing on the colour and tone to be obtained. The peak voltages of
the negative semi-wave, to obtain different colours and tones,
depend on the required treatment times and on the peak voltage
of the positive semi-wave, but they can .range of from 7 to 25
volts.
The aforegoing description will more clearly be under-
stood when taken in conjunction with the set of accompanying
graphs, wherein the following is represented:
Figure 1 illustrates the type of voltage to which the
aluminium piece is subjected in the first of the two electroly-
tic steps of which the second phase of the process is comprised;
Figure 2 illustrates the voltage corresponding to the
second electrolytic step to which the piece is subjected;
Figure 3 illustrates a variant of the voltage that can
be applied to the aluminium piece in the mentioned second elec-
trolytic step;
Figure 4 illustrates another variant of this same vol-
tage applied to the second step;
Figure 5 illustrates the anodic and cathodic peak
voltages used in an example of practical application;
Figure 6 illustrates the cathodic current density cor-
responding to the same example of practical application;
Figure 7 illustrates the anodic current density in the
mentioned example; and
Figure 8, which is on the same sheet as Figure 4,
is another variant of the voltage applied in the second step.
Thus, as can be seen, in the first electrolytic step
corresponding to the second phase of the process the aluminium
piece is subjected to a fixed or programmed con-
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tinuous ~ulsating voltage, as lll~strated in figure l~ ~Jhile
in the second step of the treatment a negative continous
pulsating voltage can be programmed~ as illustrated in
f~gure 2, on the wave form of the irst step7 this second
step urthermore permitting the possibilities re1ected in
figures 3 and ~ according to which the aluminium piece, a-
h~4v
ter the first step has~ ompleted, is subjected to a treat-
ment under potentiostatic conditions with polarized or non-
polarized currents.
The baths used in ~hese electric treatments are
characteris~d in ~hat they conta~-., a3 the colou~i.ns sub-
stances, metal salts of the group o Cu~ Sn~ Ni and Co~ or
mixtures of said 5alts. Any type o anion can be used with
said catlons9 although sulphates will preferably be used.
The pH of the work should b~ greater than 0~8
and the working temperature can be of from 15 to 25
without noticising remarkable changes in colour.
The concentrations of the colour producing salts
can be of ~rom 5 to S0 g/lo
The following examples illustrate practical appli-
cations of the process for the electrolytic colouring of
anodized aluminium~
EXA~PI,E 1
. A test sample? anodized in a mixture of sul-
phuric and ~xalic acids, the concentration o the former
bei.ng of.l65 9~1 and that of the latter being of 30 g/l,
a~ introduced in a bath cQntainlng:
- S04Cu XH20 ~Ø.............. ~0 q/l
Tal^tarlc acid 0~ O~-~O 30 yfl
Boric acid 9 ~ 0 ~ 0 ~ 30 g/l
pH ~ l.S
.. , . , ... , ,, , ., , . ~ ... , .. , .. . . . . ,, .. ., . , . ~ .. .. . . .. . .. . . . . .
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The pH was adjusted by using 1 N sulphuric acid
or MgO where the resulting pH was lower than 1.5.
The anodized test sample ~as subjected in the
first step to an alternating voltage as that illustrated
in figure 19 i.e. a positive peak vol~age of 25 volts for
a period of time of 7 minutes. This period of time ha~ing
lapsed, the negative voltage was increased to 12 peak volts,
whereby the colouring processes commenced. The following
chromatic scale was obtained.
Time (minutes) ColoOur
1 .~..~OO~.OO....O Violet grey
2 ~ ............... O.O ~ronze
3 ~ 0~5~0~ 0~ Blue-grey
4 .~....... OO...... ...Violet
5 D ~ O ~ Brown
EXAMPLE 2
` A sample, anodiæed in a so1utisn of sulphuric acid
having a concentration of 165 gJl, was introduced in a bath
containing:
BO3H3 ~ 400~00~0~ 30 g/l
n o-~c---.-o-.~.A~ 0
5~4H 2 ' ~ D <~ O ~ lQ
Tartaric acld ~ .c.~.~.. 15 t~
pH = OO9
The anodized sample was sub~ected~ in the first
step9 to a continuous voltage as that illustrated in figure
1. That is~ a positlve peak voltage of 30 volts for a
period of 12 minutes. Once this period of time had lapsed~
the negativ2 voltage was increased to 14 volts~ whereby
the colouring process commenced ? obtaining the following
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chromatlc scale:
Ti~e (minutes) Colour
~ 0O.OO~ ...O Llght bronze
5 ~ ~ o ~ o r ~ llow
10 .~..Ø~...O Blue-grey
14 ~Q-~ O~ Green
18 0~.O.. ~....... ..Bronze
25 ~.. 0.......... ~ Black
eXAMPLE 3
A test sample, anodized follQwing the procedure
o4 Ex~mple 17 was ~ntroduced in the ~olourln~ solution of
the same example.
The anodized test sample was subjected, ln the
first step, to a half-~ave continuous pulsating voltage
- having a maximum voltage of 30 ~olts and it was maintain-
ed under these conditions for 5 minutesc This period of
time having lapsed, the increase of the negative peak Yol
~ o
tage was programmed at a speed o 4Vtminute, un-til a maxi-
mum voltage o4 12 volts. At the end of ~hese 7 minutes,
from the initiation of the programming of the negative
voltage, the sample adopted a uni~orm olive-green colour.
. Subsequent research in this same field ~ to
thè conclusion that the continuous pulsating voltage used
~n the first step of the second phase of the process, with
values of ~rom 7 to 35 vol ts, should be usf~d for a maxi-
mum period of ti~e of 30 minutes, maintaining the minimum
duration thereo at 3 minutes.
It was likewise concluded tha~ the metal should
be sub~ect~d to potentiostatlc conditions with polari7.ed
or non-polarized currents during this first st~p of the
pr o ~
. s~cond phase o~ the process. In this ~et~en~ the vol~-
2~
tage applied during the first step can colncide with that
previously applied in the second step as illustrated in
figure 2, or a voltage of the type illustrated in figure 8
A programmed or fixed negati.ve continuous pul
sating ~oltage can be applied to the previously mentioned
wave form, corresponding to this first step of the second
phase of the process, also for a period of time of from 3
to 30 minutes7 depending on the colour and tone to be ob-
tained in the following stepO
The peak voltages of the negative semi-wave to
o~ain d~fr~rerlt colours and tones depend or, th~ r2q~ir
treatment times and on the peak voltage of the positive
semi-wave, but can range from 2 to 15 volts, provided that
this voltage does not colour.
During the second step of this second phase of
the process, the piece already prepared is subjected to
colouring. The baths used for thesè electric treatments
are those already rnentioned and are characterised in that
they contain colourin~ substances, such as metal salts of
the group Cu, Sn, N1 and Co1 or mixtures of said metals~
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