Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
16~50470
The present invention relates to a process ~r ~olo~i~g,
by electroplating, aluminium ~r aluminium a]loy article~ whose
surface has previously been subjected to anodic oxidation.
- The term "aluminium" as used herein means the metal
and its alloys and "anodized aluminium" means any aluminium or
aluminium alloy articles whose surface has been subjected to
anodic oxidation.
Various processes are known ~or coloring, by electro-
plating, aluminium, amongst which may be mentioned those involving
the so-called "two phase" technique comprising formation of an
aluminium oxide layer and then electrodeposition of coloured @
metallic compounds.
Norwegian patent No. 69,930, which discloses the
"two-phase" technique, discloses in particular the use of an acid
electroplating bath through which an alternating current is
passed and which contains one or more salts selected from the ~
group Fe, Co, Ni, Mn and Cr salts and, in an amount less than C
10 g/l one or more soluble compounds selected fromthe group of t
salts As, Sb, Bi, Se, Te and Sn soluble salts. Using a bath of
this type, various bronze tones have been obtained on the surface
of the aluminium articles when coloured compounds of metal such
as Fe, Co, Ni or Mn were deposited in the pores of the oxide ~
layer. To carry out the co-deposition of the metals of the two .
groups, it was necessary to maintain the As, Sb, Bi, Se and Sn
content below a certian limit, in order to maintain a current F
` density which can be used industrially. This process has various
drawbacks. The current density, when the electroplating bath
contains primarily nickel, must be as low as possible to avoid
perforated discharges or a concentric spalling of the oxide layer,
- 30 and the duration of the treatment must be increased, to obtain .
adequate coloring, thus decreasing the economical desirability of
the process, and increasing the risk of disruptive discharges.
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British Patent No. 1,022,927 discloses a process for
coloring, by electroplating, aluminium in which an alternating
current is passed between an aluminium article and a counter-
electrode, particularly one made of retort carbon or gas carbon,
carborundum or aluminium The electroplating bath contains
a small amount of Ni, Co, Cr, Cd, V, Au, Ag, Fe or Pb salt.
As this process has the same drawbacks as those indicated
previously, consideration was given to French patent 2,011,376
which discloses the application of an assymmetric alternating
voltage between an anodized aluminium article and a counterelectrode
immersed in a bath containing salts capable of coloring the
aluminium oxide layer.
Further, to overcome the above drawbacks and to deposit
tin on anodized aluminium, to obtain coloration, by the electro-
plating of tin, French patent ~o. 2,047,917 discloses a process
in which an acid bath containing tin salts and a tin complexing
agent is used to avoid the oxidation of tin (II) to tin (IV).
An alternating current, which is applied across the aluminium
article to be treated and a tin or titanium counterelectrode, is
passed through said bath. The pH of the bath is preferably in
the range of from 0.8 to 1.5 and the current density may vary
from 0.2 to 1.0 A/dm , and it can even be above 1.0 ~/dm2. It
is evident that only specific coloringsof the tin can be obtained
by this process, i.e. bronze colours with greenish tints, which
can extend to black. However, at present, the colourings required
by the construction and architectural industries have warmer
tones which cannot be obtained with the tin salt baths contemplated
in French patent 2,047,917.
The present invention provides a process for producing
~0 on the surface of an anodized aluminium article, a bronze tone
by the deposition of one or more metals selected from Ni, Co and
Cd, so that:
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a) said process does not have the aforesaid drawbacks;
b) the shade obtained is uniform and sufficiently
dark; and
c) the process is industrially economically desirable.
In accordance with the present invention, a solution
which differs completely from those contemplated in French
patents 2,011,376 and 2,047,917 has been found which overcomes the
insufficiency of distributing the current in conventional
electroplating baths, which insufficiency is mainly due to the
reduced conductivity of said baths. To increase the heat
conductivity of the electroplating bath, it is necessary to
operate at a very low p~l~ However, this pH does not allow
deposition of the metals of the group consisting of nickel, cobalt
and cadmium. This group will hereinafter be referred to as the
q:
"fundamental group". ~
~ It has surprisingly been found, to obtain the Ni, Co ~ ~;
- or Cd electropl~ating tones, one or more metals of the fundamentalgroup may be deposited at a pH below or equal to 1.8 in the
presence of one or more auxiliary metals provided that a special
type of alternating voltage is applied.
Thus, since when operating at a pH below or eaual to
1.8, it is not possible to deposit one or more metals of the
fundamental group on the anodized aluminium, a co-deposition of
I one or more metals of the fundamental group with one or more ~
nobler auxiliary metals should be effected and a portion of r
the auxiliary metal or metals deposited on the aluminium oxide
layer should be re-dissolved.
The main difficulty of this concept according to which
penetration of the ion or ions of the metals of the fundamental
group facilitated by the use of one or more auxiliary cations,
is that the auxiliary cation is deposited with more ease than
the fundamental cation andis preferentially deposited. Therefore,
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use is made of the favourable action of the auxiliary cation to
its utmost. Consequently, it is necessary to use conditions
which tend to reduce the effect of the presence of said auxiliary
cation in the ~inal deposition.
In practice, to overcome the difficulties posed by the
use of an auxiliary metal salt under the relatively unfavourable
deposition conditions of said auxiliary metal, it has been
necessary to determine the precise operative conditions which
permit the exact reproduction of the desired shades, so that:
a) the influence of the color corresponding to the
auxiliary metal is as weak and uniform as possible, in the
colour obtained; and
b) a dissolving effect in ~he bath, due to the
auxiliary metal which is deposited, is produced during deposition.
According to the present: invention therefore there
is prcvided a process for coloring, by electroplating, an aluminium
or aluminium alloy article which has previously been subjected
to anodic oxidation, wherein co-deposition of at least one
~ metal selected from nickel, cobalt and cadmium with at least one
-I 2~ auxiliary metal is carried out by stirring, applying an alternat-
; ing voltage between said article and a counterelectrode
immersed in a bath containing the salts of said metals, the
, alternating voltage, for each period, having a zero value, at
least for a certain period, the pH of the bath being below or
equal to 1.8, the bath containing at least one auxiliary metal
salt selected from copper or tin salts, and the concentration
in the bath of the salt selected from Ni, Co and Cd being less
than 200 g/l. The concentration of the auxiliary metal salt
is always less then 2.5 g/l, so that its influence on the
. 30 coloring obtained is minimum. The practical coloring effectobtained for auxiliary metal salt concentrations of about 10
g/l is that which corresponds to the auxiliary metal,
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primarily that o~ tinr when this is used. It has been obser~ed
that with this concentration, when a normal alternatin~ current
and only the auxiliary metal cation are used and when the
current ceases, the dissolving action of the bath on the
deposit obtained is very important and the color obtained can
disappear completely. This action is reduced when the concen-
tration of the auxiliary metal used alone is higher. Therefore,
this action is practically negligible when concentrations of
about 10 g/l are used,
Therefore, under the operating conditions, in the
process of the present invention, the auxiliary metal salt
concentrations used are as low as possible, in order to reduce
as much as possible the ratio between the elements which are
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jointly deposited.
, Thus under these working conditions and to enhance
the dissolving action of the bath on the auxiliary cation, it is
necessary, once the deposition has started, to have periods of
time durlng which current does not flow. Consequently, a normal
alternating current is not suitable for this process. A
symmetric alternating voltage is preferably used for each
period which has a zero value for a total duration corresponding
'~ to hal,f a period. ,
1, Suitable pH conditions are obtained by adding acids ,
which can act on the auxiliary cation, tending to remove it
i~ :.
` ,after its deposition. Naturally, the removal or dissolving
action of the acid on the auxiliary cation can be promoted by
varying the temperature. Likewise, organic acids which favour
the stability of the auxiliary cation, when this is necessary,
~ can be added to the bath. Should the auxiliary cation be tin,,~ 30~ the aforesaid stability can be obtained by conventional means in
tinplating deposition, that is by adding organic acids selected
from phenols and sulfones mainly sulfophthalic, phenolsulfonic
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and cresolsulfonic acids.
In a particular embodiment of the present invention
the temperature of the bath is in the ranye from about 15 to
about 25C, i.e. close to ambient temperature.
The alternating voltages are used with time int~rvals
during which current does not flow. The accompanying drawings
illustrate examples of usable voltages.
In the drawings;
Figure 1 illustrates a voltage which voltage is
aiternating for a 0.5 N period and zero for a 0.5 N period,
Figure 2 illustrates a voltage which is alternating for
an N-period and zero for an N-period and
Figure 3 illustrates a square wave voltage which voltage
is uniform and positive for a O.S N period, zero for a 0.5 N
period uniform and ~egative for a 0.5 N period and zero for a 0.5
N period.
The voltages of Figures 1 and 2 are obtained according
to the technique of controlled diodes, mainly for example by
means of thyristors and triads. The square wave represented
in Figure 3 is obtained by starting with a tri=phase alternating
current, contrary to the conventional practice in the technique
~ of coloring/ by electroplating, which starts with monophasic
-~' alternating currents. The use of this type of wave allows a
3 more effective action of the current to be obtained since the
wave starts with a constant current value for each polarity.
Further, in practice, the use of triphasic transformers avoids,
when their voltage is high, the unbalances between phases produced
'~ when tri-monophasic transformers are used.
$: Copper and/or tin salts are the auxiliary metal salts.
, .
30 Under the aforesaid operative conditions neither of these cations ~
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give rise, independently, to a useful effect. Thus, when the
auxiliary cation e.g. the tin cation, is used alone, a colour
is produced, but the chemical action of the bath ten~s to
remove it. If the nickel cation is used alone, no colour is
produced.
` The requirement to operate at low pH values is
determined by the need to obtain the highest possible conduct-
ivity of the electrolyte, so that the predominant resistance,
in the coloring process, is that of the aluminium oxide film.
Consequently, the process is such that deposition is auto-
controlled by the resistance of said aluminium oxide film
and deposition results in a perfect uniformity even in poorly
accessible zones, thus eliminating the shade effect which is
an undesirable feature in all galvanizing processes.
According to the process of the present invention,
a darker and uniform coloration is obtained even on articles
whose anodic film has discontinuities affecting the electrical
'1 resistances, as in the case of casting articles, aluminium
alloys containing copper, and aluminium alloys having a high
silicon content. The importance of operating under pH
conditions below 1.8 is important because if a pH above 1.8 is
, used the silicon content of the alloys used in architecture
would have such an influence on the coloration that when the
~, silicon content is high, within permissible limits, various
~¦ difficulties appear if one is to obtain dark shades, since
the aluminium alloys for use in architecture have, according
! to the diverse international norms, a silicon content of
~ approximately 0.2 to 0.6% in weight.
`~ Furthermore, better results are obtained with the
¦~ 30 process of the present invention than with classic methods of
.,
coloration by immersion, even with pieces having residues of
~ acids from anodic oxidation baths, as in the case of wedged or
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nailed pieces or faulty weldings.
It is desirable to stir the bath during the process
of the present invention. ~,enerally, an inert counterelectrode,
mainly graphite electrodes, is used. The duration of the
treatment, which affects the ~inal result of the colorations
and the properties cf the colora~ions, is at least 3 minutes,
and is preferably from about 3 to about 30 minutes.
In a preferred embodiment of the present invention the
process comprises depositing on the surface of a previously
- 10 anodized aluminium article a fundamental metal with a nobler
; auxiliary metal, applying to a bath containing the salts of
these metals, at a p~ of no more than 1.5 and a temperature of e
about 15 to 25C a symmetric alternating voltage which, during
each period, has a zero value for a total time corresponding
to half a period, so that:
1) operation is carried out with a square wave voltage
or 15 V for 2 minutes and then
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2) operation is carried out with a square wave voltage
of 18 V for 5 to 25 minutes.
The process of the present invention also allows
the simultaneous treatment of articles having different shapes ~-
and sizes, precisely due to the ease with which penetration is
obtained. This applies to soldered frames, to profiles having
different shapes and lengths, open and closed.
The-present invention will be further illustrated by
way of the following Examples.
EXAMPLE 1
Coloration, by electroplating, of an aluminium plate
' previously subjected to anodic oxidation i5 carried out by
i 30 àpplying to the terminals of the bath an alternating voltage, ~.
according to Figure l! wherein each half-period of a pure
alternating current is followed by a half~period of zero current.
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The electroplating bath has the following composition and criteria:
NiSO4 150 g/l
SnSO4 2 g/l
o-Phenosulfonic acid 1 g/l
H2SO4 in a sufficient amount
for a pH of 1.5
Ambient temperature 15-25C
A root mean square voltage of 15 V, according to Figure 1,¦
is applied for 2 minutes followed by a root mean square voltage
of 18 V, always according to Figure 1, for 5 minutes. A uniform
bluish-bronze tone differing from the greenish-bronze solely
due to the tin is obtained.
EXAMPLE 2
:, . .
Coloration, by electroplating, of an aluminium plate
previously subjected to anodic oxidation is carried out by
applying to the terminals of the bath a voltage, according to
Figure 2, which is an alternating wave voltage, wherein each
one of the periods is followed by a break having a duration
corresponding to said period of the wave voltage. The bath has
the following composition and criteria:
,~ CoSO4 160 g/l
SnSO4 2 g/l
4-(o-cresol) sulfonic acid 2 g/l
H2SO4 in a sufficient
amount for a pH of 1.4
Ambient temperature 15-25C
Following the procedure of Example 1, a root mean
square voltage of 15 V is applied for 2 minutes followed by a root
mean square voltage of 18 V for 5 minutes A uniform bronze tone
differing from the greenish bronze due solely to tin, is obtained.
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- EX ~ PLE 3
Coloration, by electroplating, of an aluminium plate
is carried out using a bath identical to that represented in
; Example 1. The alternating voltage used is that of Figure 3
(direct current which has, with respect to an alternating current
which starts with an N period of time, a positive value for
a 0.5 N period of time, a break which lasts for 0.5 N of time,
a negative value for a 0.5 N period of time and a break which
lasts for 0. 5 N of time).
A root mean square voltage of 15 V is applied for 2 ,~
minutes which is converted within 2 minutes to a root mean ,;
square voltage of 18 V, this latter being maintained for 20 to
25 minutes. A blackish-bronze tone is obtained, I
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