GB2112814A - Improved plant and process of electrocolouring aluminium - Google Patents

Abstract

A plant and process of electrocolouring aluminium are disclosed. The plant includes an electrocolouring tank, wherein at least one group is arranged which comprises an aluminium electrode located centrally between two counter-electrodes whose mutual distance apart is 80 cm at the most. Each electrode comprises a plurality of elements such as sheet metal strips, plates, pipes, or rods spaced apart from one another and formed from graphite, special cast iron, or stainless steel. The process involves raising the supply voltage to the electrodes up to 16 to 20 Volts, with a programmed intermediate dwell at 8 to 10 Volts, and the use of an electrolytic bath containing stannous salts, silver salts, or copper salts.

Description

SPECIFICATION Improved plant and process of electrocolouring aluminium The present invention relates to electrochemical colouring, or electrocoiouring of aluminium, in particular of extruded aluminium workpieces, such as sectional members.

As is known, aluminium electrocolouring involves the electrodeposition of a colouring material from an electrolytic bath onto aluminium functioning as an electrode in an electrolytic cell, by flowing an AC current therethrough.

The electrocolouring of aluminium, albeit representing a highly promising alternative method to prior colouring methods by immersion and impregnation, has posed some significant problems for its application on an industrial scale.

The electrocolouring process has in the past represented a "bottleneck" in the industrial aluminium anodizing plant, in that it could only be carried out on limited aluminium surface areas and with excess dissolved reactants, for the purpose of ensuring uniformity of the electrocoloured products and the achievement of the deepest colour hues. This is reflected in a lower output of the industrial production process and consequently higher production costs brought about among others by higher consumption rates for the entrainment of more concentrated solutions.

It is a primary object of this invention to obviate the problems associated with prior electrocolouring methods, by providing a plant and a method for electrocolouring aluminium, which enable aluminium colouring on an industrial scale at a high output rate comparable to that of the upstream process of anodic oxidation of aluminium, thus avoiding "bottlenecks" in the conventional production line.

A further object of this invention is to provide a plant and a method of electrocolouring aluminium, which can afford high quality products which are desirable above all on account of the uniformity and reproducibility of the colouring.

Another object of the invention is to provide a plant and a method of electrocolouring aluminium, which allow the electrocolouring process to be performed with a minimum of maintenance, mainly required to minimise corrosion phenomena and possible contamination of the respective colouring baths.

Yet another object of this invention is to provide a plant and a method for electrocolouring aluminium, which afford the possibility of lowering production costs also by virtue of material savings accomplished thereby.

According to one aspect of the present invention there is provided a plant for electrocolouring aluminium following the anodic oxidation thereof, which comprises an electrolytic tank arranged to contain an electrocolouring electrolytic bath, at least one respective electrode which in use includes at least one aluminium piece to be coloured and at least two counter-electrodes placed on either sides of said electrode and arranged to contact said electrolytic bath in said tank, wherein said two counter-electrodes are located at a mutual distance apart of up to 80 cm and comprise a plurality of sheet metal strips, plates, pipes, and/or rods arranged at intervals from one another.

According to another aspect of this invention, there is provided a process of electrocolouring aluminium following the anodic oxidation thereof, which comprises the steps of pre-arranging in an electrocolouring electrolytic bath at least one electrode including aluminium to be coloured and at least two counter-electrodes located on either sides of said electrode and being included in a respective external electric circuit to pass an AC current through said electrolytic bath, wherein said bath comprises an acidic electrolyte containing metal salts which are stannous salts stabilised against oxidation, silver salts, and/or copper salts.

Further features and advantages will be more apparent from the following detailed description of preferred embodiments of a plant and process for electrocolouring in accordance with this invention.

The plant of this invention comprises, as mentioned hereinabove, a tank arranged to contain an electrolytic bath, in which are immersed at least one respective electrode including at least one workpiece of aluminium to be coloured and at least two respective counterelectrodes, said counter-electrodes being suitably incorporated in an electric circuit also including a generator and associated AC current transformer.

According to a specially advantageous aspect of this invention a "dual" such group comprising two aluminium electrodes each being positioned between two counter-electrodes is used. In practice, three counter-electrodes will be prearranged in the tank, one being located centrally between two aluminium electrodes, and the remaining two being arranged to lie alongside each of the two aluminium electrodes externally thereof.

The provision of this arrangement, with a counter-electrode positioned at a central location, affords the possibility of achieving effective and uniform colouring results, in particular over the inside surfaces of sectional workpieces, e.g.

channel sections.

Of course, in actual practice, it will be possible to use a plurality of such groups each comprising one electrode with two counter-electrodes alongside it, if the size of the available tanks and with the critical spacing between two consecutive counter-electrodes (to be explained hereinafter) allow it.

It has indeed been found that, by arranging two consecutive ones of the counter-electrodes, e.g.

the two counter-electrodes on either side of the one aluminium electrode, at a distance apart of, no more than 80 cm, and preferably from 40 to 50 cm, maximum electrical efficiency can be achieved, which allows for the electrocolouring tank to be loaded with a maximum amount of colouring material and maximum surface area of the aluminium to be coloured.

The counter-electrodes used in the plant of this invention comprise a plurality of elements in the form of sheet metal strips or plates or pipes and/or rods, which are arranged spaced from one another to define rows of such elements along the length of the tank. It has been found, in fact, that this form of the counter-electrode, contrary to the relatively large size sheets employed conventionally, favours a uniform current distribution, thus affording improved results as regards colour uniformity and production output.

When the counter-electrode comprises a plurality of strips or plates, these will have a width in the range of from 10 to 40 cm, preferably from 10 to 20 cm, and their spacing may be up to twice their width. Where instead the counter-electrode comprises a plurality of pipes or rods, these will have preferably diameters in the range of from 2 to 6 cm, and may be spaced apart at intervals apart of up to 4 times their diameter, preferably from 3 to 4 times their diameter.

The counter-electrodes of the plant according to this invention are preferably formed from graphite, especially when intended for use with certain electrolytic baths containing silver salts, which will be explained hereinafter, or from special cast iron, preferably silicon cast iron, or from stainless steel, preferably a high alloy stainless steel, e.g. AISI 316 or higher ratings.

Where a counter-electrode of stainless steel is used, it is preferable to select a ratio of the surface area of the steel counter-electrode (being the surface area facing the aluminium workpieces to be coloured and acting as electrodes) to that of the aluminium electrode equal at least to 1:1, and preferably higher than this minimum critical ratio.

In fact it has been found that, by using such a ratio, the corrosion phenomena on the counterelectrode are minimised, as is, accordingly, the contamination of the electrolyte by metal ions released by the corrosion process.

The aluminium electrocolouring process involves the use of an acidic electrolyte having a pH of about 0.5 to 1.5, and comprising as the colouring salt a stannous salt, silver salt, or copper salt.

As the stannous salt, use is preferably made of stannous sulphate in a concentration which lies closest to the minimum required to ensure the highest electrocolouring output and uniformity, which equals 1 2 g/litre. It has been found that an excellent stabilisation of stannous sulphate against oxidation, (to which stannous salt is readily prone) can be achieved by using as antioxidizer a mixture of creosol-sulphonic acid and naphthol .sulphonic acid, in a 1:1.5 to 1:0.005 ratio, preferably 1:0.5, ratio.

Said stabilising mixture is used in the electrolyte in a ratio of stannous sulphate to stabilising mixture of 1:2 to 1:0.1, preferably 1:0.7.

It has been ascertained, in fact, that the above mentioned mixture of creosol-sulphonic acid and naphthol-sulphonic acid in the specified ratios, has a synergistic action leading to the obtainment of superior stabilisation results, as is proved by the comparative tests related herein below. Such tests show in fact that, by using the subject mixture, the resulting stabilisation is much higher than through the use of individual components of this mixture or the use of other chemicals known for being antioxidizers of stannous salts.

Additionally to stannous sulphate and the stabilising mixture, the stannous salt-based electrolytic bath also comprises other compounds, known per se, such as sulphuric acid, hydroxy acids (e.g. citric acid, tartaric acid, glycolic acid, etc.), sulphamic acid, and preferably anionic, or non-ionic, surfactants, such as alkylarylsuiphonates or ethoxylated alcohols, to achieve the highest degree of wettability.

The colours which may be obtained with this type of electrolytic bath range from bronze to black, the hue being the darker the longer is the treatment time in the electrolytic bath. Such treatment time will usually vary from 30 seconds to substantially 1 5 minutes.

Alternatively, the electrolytic bath may comprise silver salts, such as silver nitrate or silver sulphate. In that case, the provision of a graphite counter-electrode is preferred on account of the highly electropositive character of silver, and hence of its higher corrosive power on steel.

Preferably, because of the well-known high cost of silver, a concentration of silver salts in the electrolytic bath is used which lies closest to the minimum effective concentration, which is 1 g/litre. The electrolytic bath comprises, in this case as well as the silver salts, further additions of compounds normally employed in connection with electrocolouring processes, such as sulphuric acid, boric acid, and sulphamic acid.

Yet a further embodiment uses an electrolyte of copper salts, preferably copper sulphate pentahydrate, again in concentrations which iie closest to the effective minimum for securing the highest process output, which is equal to 10 g/litre. Additional components of the electrolyte are: mineral acids, such as sulphuric acid: organic acids, such as creosol- or phenol-sulphonic or hydroxy acids, to improve the uniformity of the electrocoloured products; or organic substances such as dextrine and/or thiourea and/or gelatine.

It has been found particularly advantageous to carry out the process so as to provide an initial step of programmed current rise and thus to obtain improved colouring results. More specifically, this process provides, after the electric circuit has been closed, a progressive voltage increase up to 8 to 10 Volts within a time period of from 10 to 20 seconds, then an intermediate dwell time at this voltage level for 20 to 60 seconds more, and then after this dwell an increase of the voltage to a value in the range of from 16 to 20 Volts, which is the effective rating of electrodeposition during the electrocolouring process.With this procedure, or by including the intermediate dwell step at 8 to 10 Volts, the porous layer of aluminium oxide is better predisposed, as created by anodization, for the subsequent electrocolouring, thus achieving improved uniformity in the ultimate electrocolouring and improved capacity to develop an anodised layer.

Preferably, electrocolouring is conducted at an initial peak current density upon reaching the working voltage level of from 1 6 to 20 Volts equal to 1.2 to 1.3 Amp/dm2, and then at a current density in the range 0.2 to 1.0 Amp/dm2 until electrocolouring has been completed. This is accomplished by using a transformer which can deliver an adequate power, and it has been found that by so doing the utmost exploitation of the electrolytic process can be secured along with maximum production output.

It will be appreciated from the foregoing that the invention achieves its objects. In fact, it enables the maximum production output and electric efficiency to be achieved through the use of a plant which comprises a particular arrangement of the counter-electrodes and electrodes, with a critical spacing maintained between any two consecutive counter-electrodes, and the use of a top critical power of the transformer whereby a starting current density equal to 1.2 to 1.3 Amp/dm2 can be delivered.

The invention also affords the obtainment of optimum colouring results, especially from the standpoint of colour uniformity and reproducibility, thanks to such expedients as the use of counterelectrodes formed from a plurality of spaced apart elements in the form of strips or plates, or pipes, or rods, one of the counter-electrodes located centrally between two respective aluminium electrodes, and two others located on either sides of the two aluminium electrodes externally of the central counter-electrodes, and by virtue of the process being conducted with an initial step of programmed increase of the supply voltage. The invention further enables the achievement of substantial energy and material savings, owing to the higher production output secured as mentioned hereinabove, and thanks to the use of particular electrolytic baths, mainly of stannous salts, which are fully stabilised and involve only a very small waste of such raw material.

Furthermore, the utilisation in said electrocolouring baths of counter-electrodes of stainless steel in the critical "facing surface" area ratio of at least 1:1 relative to the aluminium, as mentioned above, or made of special cast iron or graphite, minimises or even completely eliminates bath corrosion and contamination phenomena, thus lowering also the costs involved by the need for replacement of the counter-electrodes.

Claims (9)

1. A plant for electrocolouring aluminium after the anodic oxidation thereof, comprising: an electrolytic tank arranged to contain an electrocolouring electrolytic bath; at least one respective electrode to include, in use, at least one aluminium piece to be coloured: and at least two respective counter-electrodes located on either sidle of the first mentioned said electrode and arranged to contact said electrolytic bath in said tank, wherein said at least two counter-electrodes are mutually spaced apart by a distance of up to 80 cm and comprise a plurality of mutually spaced apart sheet metal strips, plates, pipes, and/or rods.

2. A plant according to claim 1, wherein the spacing of said at least two counter-electrodes is from 40 to 50 cm.

3. A plant according to claim 1, wherein it comprises two such first mentioned electrodes one said counter-electrode located centrally therebetween, and two further said counter electrodes laid side-by-side with each said aluminium electrodes externally thereof, the spacing of any two consecutive said counter electrodes being no greater than 80 cm.

4. A plant according to any of the preceding claims, wherein each said counter-electrode comprises a plurality of strips and/or plates having a width dimension in the range of from 10 to 40 cm and spaced apart by a maximum distance approximately equal to the width thereof.

5. A plant according to any of claims 1 to 3, wherein each said counter-electrode comprises a plurality of pipes and/or rods with a diameter ranging from 2 to 6 cm and spaced apart by a distance of up to 4 times their diameter.

6. A plant according to claim 5, wherein said distance is from 3 to 4 times the diameter of the pipes and/or rods.

7. A plant according to claim 1, wherein each said counter-electrode is formed from graphite, cast iron, and/or stainless steel.

8. A plant according to claim 7, wherein each said counter-electrode is of stainless steel, and the ratio of the counter-electrode surface area facing said first mentioned electrode to the surface area of the aluminium workpiece acting as an electrode is at least 1:1.

9. A process of electrocolouring aluminium subsequently to the anodic oxidation thereof, comprising:-- providing an electrocolouring electrolytic bath of an acidic electrolyte including stannous salts stabilised against oxidation, silver salts, or copper salts; arranging in said bath at least one electrode made of aluminium to be coloured and at least two counter-electrodes located on either side of the first mentioned said electrode as an integral part of an external electric circuit; and passing an AC electric current through said electrolytic bath.

9. A process of electrocolouring aluminium subsequently to the anodic oxidation thereof, comprising:-- providing an electrocolouring electrolytic bath of an acidic electrolyte including stannous salts stabilised against oxidation, silver salts, or copper salts: arranging in said bath at least one electrode made of aluminium to be coloured and at least two counter-electrodes located side-by-side with the first mentioned said electrode as an integral part of an external electric circuit; and passing an AC electric current through said electrolytic bath.

10. A process according to claim 9, wherein said electrolyte comprises stannous sulphate with an anti-oxidiser of a mixture of creosol-sulphonic and naphthol-sulphonic acids, the ratio of said anti-oxidisers in the electrolyte being in the range of from 1:1.5 to 1:0.005.

ii. A process according to claim 9, wherein said electrolyte comprises a salt selected from silver nitrate and silver sulphate, in concentrations of at least 1 g/litre of said salt based on the overall volume of said bath.

12. A process according to claim 9, wherein said electrolyte comprises copper sulphate pentahydrate in a concentration of at least 10 g/litre.

13. A process according to any of claims 9 to 12, comprising initially completing said circuit then increasing the supply voltage up to a level of from 8 to 10 Volts within a time period of from 10 to 20 seconds, then holding the 8 to 10 Volts level for an intermediate dwell step lasting 20 to 60 seconds, and then increasing the voltage to 8 to 20 Volts, and finally maintaining the voltage at 1 6 to 20 Volts until the electrocolouring has been completed.

14. A process according to claim 13, wherein after the 1 6 to 20 Volt voltage level has been reached, current is passed at an initial peak density of 1.2 to 1.3 Amp/dm2 and then at a current density of 0.2 to 1.0 Amp/dm2 until the electrocolouring has been completed.

1 5. A process of electrocolouring aluminium according to any of claims 9 to 14 when carried out in an electrocolouring plant according to any of claims 1 to 8.

16. A process according to any of claims 9 to 14, wherein there are arranged in said electrolytic bath one of said counter-electrode located centrally between two said aluminium electrodes, and two additional said counter-electrodes located side-by-side with each said aluminium electrodes externally thereof, the spacing between any two consecutive said counter-electrodes being no greater than 80 cm.

17. A process accordirig to any of claims 9 to 14, and 16, wherein each said counter-electrode comprises a plurality of mutually spaced apart elements which are strips, plates, pipes, and/or rods, and are formed from graphite, cast iron, and/or stainless steel.

1 8. A process according to claim 17, wherein each said counter-electrode comprises a plurality of strips having a width dimension in the range 10 to 40 cm, said strips being spaced apart by a maximum distance which is twice their width.

1 9. A process according to claim 17, wherein each said counter-electrode comprises a plurality of pipes and/or rods having a diameter of from 2 to 6 cm and being arranged at a maximum mutual spacing of substantially 4 times their diameter.

20. A plant for electrocolouring aluminium according to claim 1 and substantially as herein described.

21. A process of electrocolouring aluminium according to claim 9 and substantially as herein described.

New claims or amendments to claims filed on 14 January 1983 Superseded claim 9 New or amended claims: