Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relate~ to colouring
anodised aluminium b~ an alternating current electro-
lytic treatment and in particular to a procedure in
which a continuous strip of coloured anodised aluminium
ma~ be produced.
It is well know~ that anodised aluminium ma~ ~e
coloured by the passage of alternating current between
the ~nodised aluminium and a counterelectrode immersed
in an electrolyte, containing certain metallic compound~.
~he electrolyte may contain salts of nickel, cobalt,
copper, tin, chromium, silver, iron, lead or a manganate,
! solenite or tellurite and is maintatned at an acid pH,
dependent on the compound present i~ the bath. In some
instances the bath contains salts of two or more metals,
such as cobalt and nickel salts.
The operability of the process has always been
explained on the a~sumption that, because of the bloc~ing
effe¢t of the anodic oxide film, the total charge passin~
during the intervals when the aluminium is cathodic i8
great~r than when the aluminium i5 anodic, BO that the
material deposited from the electrolyte during the
cathodic intervals is greater than the amount re-
dissolved during the anodic intervals. It has long been
known that the pas~age of alternating current between
anodised aluminium wor~pieces in these electrolytes
j leads at best to very faint colours, because in such
circumstances the waveform Or the current i8 sym-
metrical.
When it i8 desired to colour a moving ~trip of
anodised aluminium by the alternating current process~
it i8 obviousl~ desirable that there should bo no
contQ¢t between the a~odised aluminium and a stationar~
electrode. It i8 well known in the continuous anodi-
sation of aluminium ~trip to employ the bipolar or
"liquid contact" s~tem. However, when that ~ystem i8
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I~OQQ87
employ,ed with an alternating current electrolytic
colouring process, very little colour is produced.
The almost total lack of colour is believed to be due
to the symmetrical nature of the waveform resulting
from passing alternating current from a first elec-
trode through the electrolyte to the anodised
aluminium and from the anodised aluminium through the
electrolyte to a second electrode remote from the first
- electrode.
In order to overcome this difficulty, according
to the present invention all the electrodes out of
direct contact with the strip and facing an anodised
surface in the electrolytic colouring stage are main-
tained at the same polarity. Where the strip is
anodised on both surfaces the alternating current
circuit is completed either by means of a mechanical
contact located at a position in advance of the develop-
ment of the anodic film or by means of a liquid contact
at a position where the anodic film is at most only
partly developed. It follows that, in this procedure,
the colouring of the continuous strip is performed in
line with the anodic oxidation stage.
i, ~.`
~`i.
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Thus, according to one feature of the invention there
is provided a method of producing a coloured anodised alumin-
ium strip which comprises drawing aluminium in strip form
successively through an anodic oxidation treatment cell and an
electrolytic colouring.cell to carry out electrolysis in said
cells to anodize and color the material, characterized in that
just prior to the anodizing~stage, a circuit is formed by
commonly connecting to the aluminum material a power source
for anodizing and a power source for electrolytically
colouring, said power source for anodizing being further con-
nected to the electrolytic colouring cell; said power source
for anodizing being. a current to anodize the aluminum mater-
ial in the anodizing cell and said power source for electro-
lytically colouring being a current having an alternating wave
form to colour the aluminium material in the colouring cell.
According to a further feature of the invention there
is provided a method of producing a coloured anodised
aluminium strip which comprises drawing aluminium in strip
form successively through an anodic oxidation treatment cell
and an electrolytic colouring cell to carry out electrolysis
in said cells to anodize and color the material, characterized
in that just prior to the anodizing stage, a circuit is formed
by commonly connecting~to the aluminum material a power source
for anodizing and a power source for electrolytically
colouring, said power source for anodizing being further
connected to the anodizing cell and said power source for
electrolytically colouring being further connected to the
electrolytic colouring cell; said power source for anodizing
being a current to anodize the aluminum material in the
anodizing cell and said power source for electrolytically
colouring being a current having a~ alternating.wave form to
colour the aluminium material in the colouring cell.
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~here the strip is anodised on one surface
only, it is of course possib e tO co~lplete the alternat- ;
in~ current circuit by means of electrodes in direct
contact with the unanodised surface of the strip or
S facing the unanodised surface, so that the electrolytic
colourin~ may be performed iIl line t~ith the anodic
oxidation stage or, if desired, an already anodised
continuous strip, anodised on one face only, may be
coloured.
With certain colouring electrolytes, parti-
cularly I~i, Sn and Co salts, it has already been found
hi~hly beneficial to superimpose a D.C component on
the alternating current in batch processes for
110~7
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colouring anodised aluminium articles. With the above
electrolytes it has been found that the superimposed
D.C. component leads to more rapid colouring. With
the other electrolytes listed above, superimposed D.C.
has little or no advantage and may indeed be disad-
vantageous.
In one arrangement in accordance with the
presont invention the superimposition of a D.C. compon-
ent is very conveniently achieved in an apparatus in
; 10 which both faces of the strip are anodised in a
preceding D.C. continuous anodising stage.
~ he D.C. component is preferably of such a
~ magnitude that the average current in the portion of
'~ the cycle when the strip is cathodic in the electro-
lytic colouring bath is 2-10 times, preferably 3-6
times, greater than the average current when the strip
i8 anodic. ~he superimposed D.C. component may con-
veniently be derived from the rectifier system employed
for the anodising stage, where the D.C. current is
i
introduced into the strip in the anodiæing stage by
means of longitudinally ~paced cathodes and anodes,
using the "liquid contact" technique.
We have described in Canadian Patent No.
809,640 a method of conti~uously anodising aluminium
strip, in which the strip whilst immersed in a
turbulent ~tream of sulphuric acid electrolyte first
passes one or more anodes to render cathodic that part
of the surface facing the anodes and then pas~es a
series of Gathodes, longitudinally spaced from the
anode or anodes, to render the facing surface anodic
in relation to the cathodes. ~he arrangement described
~n that prior patent is very~ conveniently employed for
the anodic oxidation ætage in the present invention,
when both faces of the strip are to be coloured.
~5 Conveniently the alternating current circuit for the
electrolytic colouring is completed via the anodising
cell anodes, since this results in a small portion of
the direct current flowing from the counterelectrodes
in the electrolytic colouring stage through the strip
to the anodising cell cathodes.
Referring now to the accompanying drawir,gs :-
~igure 1 is a schematic arrangement of an
apparatus for continuous production
of anodised and coloured aluminium
strip,
~igure 2 is a schematic of the D.C. circuit in
Figure 1,
Figure 3 i8 a schematic of the D.C. circuit of
a modification of Figure 1,
~igure 4 is a schematic arrangement for contin-
uous anodising and colouring one
surface of aluminium strip, and
~igure 5 is a schematic of an alternative
arrangement for anodising and colouring
both surfaces of an aluminium strip.
In Figure 1 aluminium strip is drawn from a
supply reel (~ot shown) into an anodising cell of the
type described in Canadian Patent ~o. 809,640, into which
a stream of sulphuric acid electrolyte, for example 15%
sulphuric acid, is supplied through inlets 2 and with-
drawn through outlets 3 for recirculation. The cell 1
is shown as having five pairs of spaced electrodeæ 4.
For perform~nce of the anodising operation the first two
pairs of electrodes 4 are connected to the positive of
a rectifier assembly 5 and the remaining three pairs of
electrodes 4 are connected to the negative. Other
anodising electrolytes, such as oxalic acid or chromic
acid, may be employed in the anodising cell in place of
sulphuric acid.
After leaving the anodising stage the now
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anodised strip is passed through a rinsing stage, shown
diagrammatically at 6, which serves to remove the 8ul-
phuric acid electrolyte. ~he strip then enters the
electrolytic colouring cell 7 in which it passes
between an array of counterelectrodes 8. ~he counter-
;~ electrodes 8 are connected to one terminal of a
variable voltage A.C. supply 9, the other terminal of
which is connected to the anode electrodes 4 in the
cell 1, from which current flows to a substantially
unanodised portion of the surface of the aluminium
strip.
Considering Figure 2, the resistances R1, R2,
R3 are the resistances of the current path~ between the
strip and the counterelectrodes 8, the strip and the
; 15 cathodes 4, the strip and the anodes 4 respectively.
' Because of the growth of the anodic oxide film in the
cell 1 and the lower conductivity of the electrolyte in
, the colouring stage (as compared with the anodising
-I stage), R1~ R2~ R3, and in consequence it will be seen
that the D.C. current i1 in the electrol~tic colouring
stage will be much smaller than the D.C. cuxrent
employed in the anodising stage.
In one example the anodising cell 1 had a length
of 12 feet and the colouring cell 7 had a length of
9 feet. ~he strip (2 inch wide) was drawn through at
a speed of 12 ft./min. to give a dwell time of 1 minute
in the anodising stage and 45 seconds in the colouring
stage. Using a 1~/o sulphuric acid electrolyte at 60C,
a D.C. voltage of 25 volts was found to produce an
~0 anodic film of 5 microns thickne~s at a total current
of 500 amps. It was found possible to provide a
desirable dark bronze colour in this film with an A.C.
current of 30 amps in 45 seconds dwell time in the cell
7, which contained the following nickel electrolyte:-
37
Ni~04 7~2 25 gpl
(NH4)2S04 15
Mg~o4~7H2o 20
~3B03 25
pH 5.7
Alternatively other known baths, such as those contain-
ing salts of tin or cobalt, for electrolytic colouring
of anodic aluminium oxide films by means of alternating
current with superimposed direct current may be
employed.
~ he counterelectrodes 8 were formed of lead
strips and had a dimension in the direction of strip
tra~el of two inches. In the system arranged as des-
cribed above the ratio of the average cathodic current
to the average anodic current in the cell 7 was esti-
mated at about 4:1 by measuring the areas under the
curve of oscillograms.
In the modified arrangement shown in Figure
3, Rl and R2 have the same signific~lce as in ~igure 2.
R4 and R14 are respectively the resist~nce of the path
between the strip and the first pair of electrodes 4
and between the strip and the second pair of elec-
trodes 4. In this arrangement the variable resistance
R5 is connected in the conductor 10 in Figure 1. As
will be seen, this arrangement enables the D.G. current
in the cell 7 to be ~aried by appropriate ad3ustment of
resistor X5. If conductor 10 is disconnected, the
resistance R5 becomes infinite and there is no ~.C.
component superimposed on the alternating current in
the electrolytic colouring stage.
In another example the apparatus of Fi~ure 1 is
operated with no interco~nection between the two pairs
of anodes 4 when e~ploying a copper electrolyte in the
electrocolouring stage. The anodisation was performed
~5 under the same conditions as in the precedin~; example,
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but no superimposed D.C. was employed i~ the electro-
lytic colouring sta~e. It was found possible to obtain
a desirable maroon shade in this film with an A.C.
current of 27 amps at 23 volts in 45 seconds dwell time
with a copper-containing electrolyte having the
following composition :-
~uS04.5~20 35 gpl
MgS04.7H20 20 gplp~ brought to 1.3
by addition of sulphuric acid.
One known system for anodisin~ one surface of an
aluminium strip is described in U.S. Patent No.
3,296,114. ~n anodising cell 11 operating on that
system is illustrated diagrammatically in Figure 4. In
the anodising cell 11 the reverse face of the strip is
in contact with a carbon brush 12 connected to the
positive terminal of a D.C. supply and a cathode 14 is
connected to the negative terminal. ~he electrolyte is
showered onto the strip through a perforated screen, the
streams of electrolyte constituting the current path
between the cathode 14 and the strip, which is brought
into a somewhat surved ~rofile during it~ passage
through the cell so as to avoid much contact between
the electrolyte and the reverse surface of the strip.
~he electrolytic colouring is carried out in a similar
cell 16. The alternating current circuit for the
electrolytic colouring operation is then completed
between the electrode 12 and a counterlectrode 15 in
the electrolytic colouring bath 17~ It is preferable
to superimpose a ~C. component on the current flowing
in the ~.C~ circuit thus constituted, when an appro-
priate electrolyte is employed in the electrocolouring
stage.
In an alternative arran~ement a car~on elec-
trode in contact with the unanodised side o~` the strip
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in the cell 16 may be employed in addition to theelectrode 12. In this case the A.C. circuit is com-
pleted through this second carbon electrode. However
the arrangement illustrated in Figure 4 is preferred for
reasons of economy since these carbon brushes become
worn by contact with the strip and require periodic
replacement.
Figure 5 illustrates an arrangement for
continuous anodisation and electrolytic colouring of
aluminium strip where a contact roll is employed for
introducing current into the strip in the anodisation
stage. In this arrangement the contact roll 21 is
connected to the positive terminal of a D.C. supply and
cathodes 23 in an anodisation cell 22 are connected to
the negative terminal. The roll 21, which is in
contact with unanodised aluminium, is connected to one
terminal of an A.C. supply having a superimposed D.C.
component, whilst the opposite terminal is connected to
the counterelectrodes 8 of the electrolytic colouring
2C cell 7.
One means for generating ~.C. with an 8sym-
metrical waveform (equivalent to A.C. with a superimposed
D.C. component) is shown in Figure 5 and is comprised of
a transformer 24 connected with diodes 25 as shown.
Since there is a voltage drop of the order of 2 volts
across each diode 25, more current can be caused to flow
in the half cycle in which the strip is cathodic in
cell 7 than in the half cycle in which it is anodic.
In some circumstances it may be desirable to
operate the electrolytic colouring stage with pure D.C.
current. In such case the A.C. component may be elimin-
ated and the ratio of D.C. to A.C. becomes infinite.
