CA1066650A - Electroplating aluminium stock - Google Patents

Abstract

A B S T R A C T

A process for the production of electroplated aluminium stock, such as strip or wire, comprises passing the stock continuously through a bath having a high dissolving power for aluminium oxide, such as strong aqueous sulphuric acid and phosphoric acid and subsequently through an electroplating bath, the first bath having a cathode electrode and the electroplating bath having an anode electrode so that the stock is anodic in the first bath. The stock may pass through one or more intermediate non-electrolytic treatment stages, such as immersion tinning or zinoating. It may also pass through one or more electrolytic pretreatment stages during its passage between the first bath and the electroplating bath. In such pretreatment stages, such as the appli-cation of a bronze strike, there may be an anode electrode at the same potential as the anode in the electroplating bath.

Description

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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 ~JH/1496 -2-. , ' " :'.. " ' ~' . ... .: ';
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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 `;
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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' " , ' ~ ' ', ~ ' ' ': . ' ', ' "' ' ' ' . : ' r~ -~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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. 1~66650 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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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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~ ~066650 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.
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~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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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 _9_ ., .

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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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Claims (7)

THE EMBODIMENT OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the production of metal-plated, elongated aluminium stock which comprises passing the aluminium stock continuously (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 electroplating 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 aluminium during its passage therethrough and to deliver the stock with an essentially 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 second bath, the stock and the first bath, to the cathode without sliding or rolling contacts between the stock and the electrical supply.

2. A process according to claim 1 in which said stock passes through a bronze strike bath intermediate the first bath and said electroplating bath, said bronze strike bath having an anode therein at the same potential as the anode of the plating bath, electric current being passed from the anode of said bronze strike bath through said strike bath to the stock in parallel with the current through the plating bath, and the total current from said plating and strike baths being passed from the stock through the electrolytic cleaning bath to the cathode.

3. A process according to claim 1 in which the first mentioned bath is an aqueous solution consisting essentially of 20 to 50% H3P04 and 10 to 25% H2504, and is maintained at a temperature in the range of 80°C. and higher.

4. A process according to claim 1, 2 or 3 which is for production of tin-plated aluminium stock and in which the electroplating bath contains a tin-plating electrolyte.

5. A process according to claim 1 or 2 in which after passage through said first-mentioned bath the stock is passed through an aqueous solution of a strong mineral acid under non-electrolytic conditions.

6. A process according to claim 1 in which the stock passes through at least one non-electrolytic intermediate immersion treatment bath between said first bath and said electroplating bath.

7. A process according to claim 6 in which the non-electrolytic treatment bath is an immersion tinning or zincating bath.