Note: Descriptions are shown in the official language in which they were submitted.
1066650
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This invention relates to electroplating an elon-
gate aluminium stock, such as strip, rod or wire.
One purpose of electroplating aluminium stock is to
reduce its electrical contact resistance. Electroplating
with tin, for example, avoids the formation of a high resi~-
tance, surface film of aluminium oxide.
One known technique for plating aluminium stock with
l tin involves the successive steps of degreasing, etching, de- -
; smutting, immersion tinning, bronze strike? acid conditioning; 10 and finally tin plating. The bronze strike and the final tin
plating are the only electrolytic steps. Similar treatments
are used for plating aluminium stock with other metals.
We have now found that a 6implified process of metal
plating aluminium ic 6atisfactory, this process involving an
electrolytic cleaning treatment of the aluminium stock in
acid or alkali under anodic conditions followed by metal
plating with, if necessary, intermediate conditioning steps.
~he electrolytic treatment which is preferably in hot acid
can, for instance, replace the non-electrolytic degreasing
and etching steps of the tin plating process described above,
~; and can replace similar steps in other metal plating
processes.
It is well known to carry out electrolytic cleaning
of aluminium under cathodic conditions in continuous anodis-
` 25 ing treatment of al1m;nium, but in the present treatment
anodic conditions are employed.
, In the electrolytic clePn;ng treatment step, a high
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~066650
concentration of strong mineral acid i~ preferred, such as
20-5~/o~ for example, 37% H3P04 in admixture with 10-2~/o~ for
example, 1~/o H2S04, or other mixture~ of mineral acid~ havi~g
equivalent dissolving power for aluminium oxide, for example,
75% H3P04 and 5% H~03, or H2S04 (8~o by volume) and CrO3
(25 gms/litre). Other acid~, and even alkaline solution~ may
also be employed provided that they possess suf~icient dis-
solving power for anodic aluminium oxide (~imilar dissolving
power to the above-mentioned mixed phosphoric acid and
sulphuric acid solution), as ideally anodic oxide should be
removed from the aluminium as rapidly as it rorms in the
course of this treatment. In general, bath compositions
suitable for electropolishing of aluminium will be satis-
factory. With the first particular composition given above,
a bath temperature of 80-95C is satisfactory, and adequate
cleaning of aluminium stock occurs with a current density of
100 A/dm2 in about 5 or 6 seconds. Higher bath temperatures
may also be used, e.~. up to the boiling point of the 801u-
tion, as may also lower temperatures, so long as the rate of
redissolution Or the anodic oxide rilm does not become un-
desirably low. A non-electrolytic treatment in the same or a
similar bath before and/or after the electrolytic treatment
may be of further benefit. These non-electrolytic treatments
may be of a duration of 2 seconds each, although a post-
treatment of 1 second and no pretreatment can be adequate.
We have found that this method can be conveniently
performed during continuous processing, in which the bath
~,~ liquids themselves are employed as electrical contacts.
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~066651)
Thus anodes in the metal plating bath and cathodes in the hot acid or alkali
electrolytic cleaning treatment stage may be connected to opposite poles of
a D.C. supply. This removes the need to use sliding or rolling contacts
between the stock and an electrical supply. Such contacts have for long
been a source of difficulty in continuous plating operations and can lead to
severe maintenance problems due to build up of corrosion products and oxides
on the contacts which result in arcing and pitting of both contact and stock.
According to the present invention there is provided a process for
the production of metal-plated, elongated aluminium stock which comprises
passing the aluminium stock continously ~1) during a minor part of a minute
through a hot acid electrolytic cleaning bath containing an electrolyte having
a high dissolving power for aluminium oxide, said bath consisting essentially
of an aqueous, acid solution selected from the group consisting of: 20 to
50% H3P04 and 10 to 25% H2S04, 75% H3P04 and 5% HN03, and 80% by volume
H2S04 plus 25 grams per litre CrO3; and subsequently (2) through an electro-
plating bath containing a metal plating electrolyte, the first-mentioned bath
having a cathode electrode immersed therein and the second-mentioned bath
having an anode electrode immersed therein, whereby to render the stock anodic
in the first bath; the composition and temperature of the solution of the
first bath being sufficient for removal of anodic oxide from the aluminium
essentially as rapidly as it forms in this first electrolytic treatment, and
said first treatment in the hot acid bath being effective to clean the alumi-
nium during its passage therethrough and to deliver the stock with an essen-
tially bare aluminium surface; and electric current, from a supply, for
electrolytic cleaning in the first bath and for plating the stock with metal
in the second bath being passed from said anode successively through the
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second bath, the stock and the first bath, to the cathode without sliding or
` rolling contacts between the stock and the electrical supply.
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~066650
:The process of the invention so far described is applicable to the
plating of aluminium stock with a variety of metals, including tin, and there
4,
- is particularly provided by the present invention a method of metal plating
aluminium stock including the steps of electrolytically cleaning the stock
under anodic conditions in hot acid, immersion tinning or zincating the stock,
an electrolytic bronze strike (i.e. electrolytic deposition of very thin
coating of copper-tin
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alloy) and electrolytic metal plating. ~ihese steps are
particularly suitable when tin iB the plating metal, but can
also be used when, for instance, ~luminium is to be plated
with brass, zinc, lead, nickel or copper. Preferably the
above-described liquid contact principle is uæed in the
cleaning, bronze strike and metal plating steps, in which case
the electrodes in the bronz-e strike and metal plating baths
can be connected to the positive terminal of a current source
and an electrode in the bath of hot acid to the negative
terminal of the source.
Either a non-electrolytic immersion tinning or a
zincating step can be used prior to the bronze strike with
good results. ~ihese step~ are needed to prepare the aluminium
surface to receive the metal plate from those plating baths
with which it is not in it~elf compatible.
A preferred acid cleaning solution has been given
above. If zincating i8 performed as the second step, an
aqueous bath composition as follows may be used:
40 ~pl Zn~04.7H20
106 gpl NaOH
40 gpl KHC4H406
to which may be added 10 gpl KC~.
A residence time of 2 seconds with the bath at 40C
is found to be satisfactory.
If, on the other hand, tinning is selected as the
second step, then an aqueous bath composition as follows may
be used:
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:~066650
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50 gpl ~2SnO3.3H20
15 gpl H ~03
A residence time of 2 seconds is suitable with the
bath at 45C.
~or the third step, the bronze strike, a preferred
aqueou~ bath solution is as $ollows:
: 140 gpl K ~nO3.3H20
36.5 gpl CuCN
75.5 gpl ECN
7.5 gpl KOH
A temperature of 40C, a residence time of 2 to 3
; seconds and a current density of 20 to 35 A/dm2 provide satisfactory conditions for aluminium stock.
An alternative bath solution for the bronze strike
` 15 i8 obtainable from M & ~ Chemicals Inc. and comprises
., ~lstan 71 ~a powder of which 180 gpl may be used) plu8
Alstan 72 (a concentrate o~ which 50 ml/l may bs u~ed).
; ~his may be employed satisfactorily at 40C with a residence
time of 2 seconds and a current density of 30 A/dm .
The metal plati~g bath,.where the metal is ti~, may
be a~ follows:
300 gpl Sn(EF4)2
200 gpl HBF4
:l 25 gpl ~3B03
,~ 25 30 gpl gelatin
i~ 1 gpl ~-naphthol
.~ Alternatively, ~3B03 and gelatin may be.omitted and
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H~F4 content reduced to 50 gpl. In both cases a temperature
of 35C, residence time of 5 seconds and current density of
100 to 120 A/dm2 are preferably employed where a tin coating
of 5 ~m is plated.
~he above particularly described conditions are
suitable for tin plating aluminium wire of 3.2 mm diameter
to a thickness of 5 ~m. A throughput speed of 36 metres/min.
i8 obtainable with these conditions in conjunction with bath
lengths of 3.6, 0.9, 0.9 ~nd ~ metres respectively.
It will be seen that this preferred method of the
invention allows the omission of the acid conditioning step
when compared with the prior known treatment method discussed
above. Thus the steps required in conditioning the bare
aluminium surface prior to metal plating~ particularly tin
plating, have been reduced with the advantage that where the
liquid contact system is used a minimum length of the
aluminium will be required to carry current. ~his reduces
heating and possible wire breaking dirficulties.
It has also been discovered that when high current
~ 20 dencities of the order of 70-80 A/dm2 are employed in the
i plating bath the invention can be performed with a much
greater efficiency if the stock or electrolytic solution i~
agitated in the metal plating bath, particularly where tin
plating is concerned. ~his may be conYeniently achieved by
l 2~ agitating the stock, for instance by passing the stock
¦ through a ring located centrally of the bath and oscillating
~ the ring. ~he ring may suitably be of polytetrafluoro-
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~)66650 -
ethylene and located on an arm connected ecentrically to
the drive shaft of an electric motor. The improvement
achieved by agitation decrea~es progressively when lower
current densities are employed. Agitation may conveniently
be carried out by oscillating the ring at 2-20 cycles/sec.,
more prefera~ly 5-15 cycles/sec. ~he amplitude of the
oscillation may conveniently be in the range 1.5-75 mm, but
most usually in the range 5-25 mm. The effect Or the
oscillation of the Qtock or agitation of the electrolyte is
believed to result in bringing the metal surface into contact
with fresh electrolyte, thus continuously replenishing the
metal ions in the electrolyte in the immediate vic;nity of
,;
' the metal surface.
~he invention will be more clearly understood from
the following description, which is given merely by way of
~ example, with reference to the accompanying drawings, in
;~l which :
~` Figures 1 and 2 show schematically apparatus for
` performi~g the present invention; and
'; 20 ~igure 3 shows one form of apparatus for oscillating
,~! the aluminium stock in the metal plating bath.
~1, , .
~igure 1 shows the liquid contact principle as may
be simply applied according to the pre~ent invention. There
are three baths, each containing an appropriate solution,
and the aluminium stock S moves through them in the direction
~; of the arrow. In the first (leftmost) bath 10 electrolytic
cleansing of the stock in hot acid or alkali takes place,
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~06665l~
in the ~econd bath 11 the stock is treated non-electro-
lytically with a conditioner while metal plating is carried
out in the third bath 12. ~he conditioning bath 11 may be
omitted in plating certain metals from baths which are
compatible with bare aluminium, e.g. direct plating. Zinc
can be plated on aluminium in this way.
In the first and third baths are respective elec-
trodes 13 and 14, respectively connected to the negative Pnd
positive terminals of a current source. In use, current from
t~e source pasæes from electrode 14, the anode, through the
solution to the aluminium stock in bath 12 which is therefore
the cathode. ~he current then passes through the stock to
bath 10, leaves the stock and travels to electrode 13 and
thence to the source. In bath 10 the stock iæ anodic and the
electrode cathodic. ~he electrode 13 may be of lead,
, graphite or stainlesQ Qteel.
-~ Hot acid which may be such as has been described
may be contained in bath 10 and plating solution is in bath
12. ~he conditioner will be selected according to the
plating taking place. Obviously more than one conditioning
step can be used, although this may increase the stock
lèngth carrying the current.
~igure 2 shows the application of the liquid contact
principle with a further plating step. ~ike parts are given
~ 25 like reference numeral_, when compared with Figure 1, and it
i3 will be seen that the only difference from Figure 1 is the
provision of a second plating bath 20, and a corre~ponding
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~o6665
further electrode 21 connected to the positive terminal of
the current source.
An arrangement of this type would be used when
performing the preferred method of the invention involving
hot acid electrolytic cleaning (in bath 10), zincating or
tinning (in bath 11), bronze strike (in bath 12) and metal,
particularly tin, plating (in bath 20).
Finally~ ~igure 3 shows schematically one method Or
agitation of the aluminium stock in the metal plating bath.
~he stock ~ passe~ through a ring 30, suitably of polytetra-
fluoroethylene on one end of an arm 31 pivoted in the bath
;, wall at 32. 'he other end of the arm is eccentrically
~ mounted on a disc 33 on the shaft of a drive means such as
`-~, an electric motor (not shown). ~he ring 30 is suitably half-way along the plating bath, and it has been found that --
~ibrations of amplitude about 10-15 mm at 10 C/8 frequency
increases greatly the current density which can be used in
~` the platiug bath. If the wire i8 not vibrated in this way, ori, if the solution in the bath i8 not agitated, then a longer
plating time provided by a longer bath or lower stock speed
would be required.
~he current which can be used i8 limited by temper-
ature factors, and the current path in the stock i8 therefore
kept as short as possible.
The vibration of the wire or agitation of the bath
i8 also effective in reducing "treeing" of deposited tin.
~lumi~ium wire or rod stock can be plated with tin
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: ~066650
or other metals with the method of the invention. Rod
i stock can be drawn to smaller diameters, such as normal
' wire diameters, after tin plating.
In performing the preferred method of the invention
with the apparatus of Figure 2, it will generally be the case
that up to 1~/o of the total current entering the wire will
do æo in the bronze strike bath, the remainder in the metal
plating bath. As the voltages suitable for these operations
are similar, in the case of the present example, a single
current source can be used although obviou~ly two could be
used if desired.
~he thickness of the metal deposits can be varied
-,l by varying the speed or current.
It will be understood that in the systems of Figures
1 and 2 the strip will be washed with water, either by
~ immersion or ~praying, in the course of transfer ~rom one
;~l treatment bath to the next.
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