D E S C R I P T I O N

"Electrolytic solution for the electrochemical polishing of metal articles."

Technical field of the invention

The present invention relates to an electrolytic solution for the electrochemical polishing of metal articles which constitute the anode in a polishing procedure by application of electrical current. It also relates to a new procedure for electrochemical polishing and to a device for performing said procedure.

Background of the invention

Metal electropolishing processes are based on the extraction of ions from the surface of said metals in a geometrically selective form. Indeed, this geometric selectivity, or of form, depends on what are the restrictions to ion transport determined by the electrolyte layer which comes into immediate contact with the surface of the metal piece to polish, i.e. the electrolyte layer known as anodic layer. The restrictions to ion transport referred to are due to polarization. In conventional electropolishing processes, polarization produces electrical conductivity gradients in the anodic layer, basically due to differences in ion concentrations of the metal to polish and to the increase in viscosity caused by the increase in ion concentration.

Thus, in the protuberant areas of the metal piece to polish, passivity minimums take place because they hinder the aforementioned ion concentration differences, whilst the viscosity is less than the cavities of the relief where passivity minimums usually occur.

As a result, the metal is extracted in greater proportion in the protuberant areas with respect to the cavities of the relief achieving the polishing of the surface of the metal piece.

Nevertheless, the field of application of these conventional electropolishing processes is strongly conditioned because only thicknesses of a few microns of the anodic layers or film can be achieved, for which reason, in short, only reliefs of a few microns can be smoothed. This fact means that in order to polish the metal pieces the relief should first be polished by strictly mechanical means and then profiled or brightened by application of conventional electropolishing, which makes the process complex and lengthens it.

Furthermore, it should be highlighted that in order to suitably respect the

large-scale geometry of the pieces to polish, in a mechanical polishing process it is necessary to use, necessarily, tools provided with a certain rigidity (bands, rotating discs, rotating cylinders...), which confine the erosive action to predetermined areas.

This enormously hinders the automation of the processes, especially when trying to polish pieces of complex geometry and which are different from one another.

Therefore, the inventors have developed a new electrolytic solution which permits polishing metal pieces or articles without the use of strictly mechanical procedures and which, being used in devices also developed by the inventors, enable the polishing of metal pieces of any geometric form and in a totally automatically manner.

Explanation of the invention

In essence, the electrolytic solution object of the invention is characterized in that it comprises at least one compound of the alkylbenzene sulfonic family of formula I,

its salts or derivatives, where R ₁ corresponds to an alkyl radical of 10 to 14 carbons; and R ₂ is selected from a hydrogen atom and an alkyl radical of 10 to 14 carbons.

The electrolytic solution according to the invention is also characterized in that it comprises an alkylbenzene sulfonic compound of formula I where R1 is an alkyl radical of 10 to 14 carbon atoms and R2 is hydrogen; and an alkylbenzene sulfonic compound of formula I where R1 and R2 correspond to alkyl radicals of 10 to 14 carbon atoms.

Preferably, the percentage by weight of alkylbenzene sulfonic compound of formula I, where R1 is an alkyl radical of 10 to 14 carbon atoms and R2 is hydrogen, is between 2.0% and 4.0%. Similarly, the percentage by weight of alkylbenzene sulfonic compound of formula I where R1 and R2 correspond to alkyl radicals of 10 to 14 carbon atoms is between 10.0% and 17.0%.

According to another characteristic of the invention, the electrolytic solution comprises a hydrocarbon in a proportion by weight with respect to the alkylbenzene sulfonic compound or compounds between 15% and 75%.

Furthermore, the electrolytic solution according to the invention is characterized in that it comprises chemically inert particles.

Another object of the present invention is the use in a metal polishing procedure by application of electrical current of an electrolytic solution which comprises at least one compound of the alkylbenzene sulfonic family of formula I, its salts or derivatives, where R ₁ corresponds to an alkyl radical of 10 to 14 carbons; and R ₂ is selected from a hydrogen atom and an alkyl radical of 10 to 14 carbons.

Preferably, solutions according to the invention are used for the polishing procedure of metals by application of electrical current electrolytic, which comprise an alkylbenzene sulfonic compound of formula I, where R1 is an alkyl radical of 10 to 14 carbon atoms and R2 is hydrogen; and an alkylbenzene sulfonic compound of formula I where R1 and R2 correspond to alkyl radicals of 10 to 14 carbon atoms.

More preferably, the electrolytic solution used comprises chemically inert particles in suspension and between 15% and 75% by weight of a hydrocarbon with respect to the percentage by weight of the alkylbenzene sulfonic compound or compounds of formula I.

A further object of the present invention is a polishing procedure of metal articles by application of electrical current between an anode and a cathode, the anode being the metal article to polish, which comprises at least one stage of interaction of said anode with inert dielectric particles of size over 100 microns.

The procedure according to the invention is also characterized in that it comprises cycles wherein a half-period occurs wherein the anode is in contact with the electrolytic solution free from chemically inert particles and a half-period wherein the anode is in contact with an electrolytic solution which contains chemically inert particles in suspension.

According to another characteristic of the procedure object of the invention, the half-period, wherein the anode is in contact with an electrolytic solution which contains chemically inert particles in suspension comprises, in turn, a stage of fluidization of said particles and a compacting stage of said particles. Another object of the present invention is a device for the polishing of metal articles by application of electrical current between an anode and a cathode, the anode being the metal article to polish, designed to perform the procedure as has been described in the previous paragraphs.

The device according to the invention is characterized in that it comprises at least one receptacle divided by a vertical partition wall which defines two vessels connected by their lower part, the first of them designed to contain the anode of the

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electrochemical reaction and provided with means of separation permeable to liquid solutions located in its lower part; and the second vessel provided with means of oscillation of the liquid contained in its interior.

According to another characteristic of the device of the invention, the means of separation permeable to liquid solutions consists of a strip provided with multiple orifices.

According to another characteristic of the device of the invention, the means of separation permeable to liquid solutions consists of a mesh.

Brief description of the drawings

The attached drawings illustrate, by way of non-limitative example, an embodiment of a device according to the invention, where the steps of the procedure used thereby can also be appreciated. It also presents diagrams which try to clarify the behaviour of the electrolytic solution around the metal piece to polish throughout the polishing procedure. In said drawings:

Fig. 1 corresponds to an elevational and schematic view of the device object of the invention showing two vessels interconnected by permeable means of separation;

Fig. 2 corresponds to the same view as Fig. 1 but in a more advanced phase of the polishing procedure, specifically when the metal piece to polish comes into contact with the chemically inert particles;

Fig. 3 shows the same device of Figs. 1 and 2 but in another stage of the half- period wherein the metal piece is in contact with the chemically inert particles, specifically when they are compacted; Fig. 4 is a diagram of a rough surface of metal to polish, where a chemically inert particle positioned in contact with said piece is displayed; and Fig. 5 corresponds to another diagram which illustrates the behaviour of the electrolytic solution according to the invention in the final phases of the electrolytic polishing procedure.

Detailed description of the drawings

Below, and by way of non-limitative example, the components and the percentages by weight of an electrolytic solution 1 according to the invention are detailed, useful for the electrochemical polishing of metal articles 2 where said article 2 constitutes the anode in the polishing procedure.

Example 1 : Electrolytic solution for the electropolishing of articles of copper

and its alloys.

The final solution should have a pH between 7.5 and 9, preferably 8. The working conditions in this case and for this solution are:

Voltage = 14V Temp = 2O ⁰ C - 3O ⁰ C tcycle ⁼ 1 .2 S. t TOTAL = 70 min

The t _TO T _AL relates to the time used to smooth roughness 11 at maximum in the order of tens of microns. Glass spheres of 2 mm diameter are used as chemically inert particles 3.

It also provides the presence of chemically inert particles 3 of other materials, such as particles of different types of polyolefins or wood, provided that they have a size over 100 microns.

In this solution 1 described in Example 1 , the sodium salt of decylbenzenesulfonic acid is used as compounds of the alkylbenzene sulfonic family of formula I, which corresponds to the case of Ri equal to an alkyl radical of 10 carbon atoms and to an R ₂ equivalent to a hydrogen atom. It also uses the sodium salt of any dialkylbenzene sulfonic acid with alkyl radicals of 10 to 14 carbon atoms.

In a preferred formulation of the electrochemical solution 1 in accordance with the invention, compounds are included which act as coadjuvants, i.e. which optimize the rheology of the anodic layer, such as fatty acid molecules.

In order to detail the procedure of electrochemical polishing of metal articles 2 object of the invention, reference is made to attached Figs 1 to 3. Based on these figures, it also illustrates an embodiment of the device 4 for the polishing of metal

articles 2 according to the invention.

Thus, Fig. 1 shows a receptacle 5 which comprises the electrolytic solution 1 according to the invention, which receptacle 5 is divided in two tanks or vessels 7 and 8 interconnected by their lower part and which are defined by a vertical partition wall 6. The first vessel 7 is designed to contain the anode of the electrochemical reaction, which, as has been stated above, consists of the metal piece 2 to polish. Said metal piece or article 2 is fixed by fastening means or metal frames (such as hooks) which ensures the electrical contact therewith as well as its positioning in the first vessel 7 and which, in turn, are in contact (although they do not appear represented) with the anode terminal (positive pole) of a source of electrical current.

Furthermore, in its lower part the first vessel 7 of the receptacle 5 comprises means of separation permeable to liquid solutions 9, the second vessel 8 with electrolytic solution 1 which is connected with the first vessel 7 via said permeable means of separation 9, being thus defined and in collaboration with the vertical partition wall 6,. These means of separation 9 consists of a strip provided with multiple orifices. Preferably the means of separation consists of a mesh or net.

The permeable means of separation 9 also acts as support of chemically inert particles 3 which act by eroding the anodic layer 12 existing on the surface 12 of the pieces to polish 2. The second vessel or tank 8 of the receptacle 5 comprises means of oscillation 10 of the liquid or electrolytic solution 1. In the case, represented in Figs. 1 to 3, said means of oscillation 10 is constituted by a plunger which vertically slides inside this second vessel 8, causing the electrolytic solution 1 of the receptacle 5 to increase in level in the first vessel 7 when said solution 1 passes through the permeable means of separation 9.

With the passage of the electrolytic solution 1 from one vessel to another, it enables the formation of a fluidized bed of chemically inert particles 3 in the first vessel 7, which are in suspension as can be seen in Figs. 2 and 3.

Fig. 2 represents with vertical arrows the direction of advance of the plunger 10 and of the metal piece to polish 2, whilst the horizontal arrow aims to detail the direction that the electrolytic solution 1 follows when the plunger 10 descends through the second vessel 8.

The same scheme of arrows is used in Fig. 3, where the direction of advance of the plunger 10 has been reversed, which is now moving backwards, and of the anode or metal piece to polish 2 which is also rising. This change in direction of the plunger 10 movement achieves that a fraction of the electrolytic solution 1 returns to

the second vessel 8. This causes the compacting of the chemically inert particles 3. In this way, maximum particle 3 density is reached around the metal pieces 2 and with this, maximums of their frictions with respect to the metal surface 13 to polish.

With a device 4 for the polishing of metal articles 2 such as the one described, which admits multiple variations without, for this reason, moving away from the object of the invention, a polishing procedure can be applied by application of electrical current between a cathode and an anode, the anode being the metal article 2 to polish, which comprises at least one stage of interaction of said anode with chemically inert particles 3 with size over 100 microns. The procedure which can also be gathered from Figs. 1 to 3 comprises cycles wherein a half-period of interaction or immersion of the anode with electrolytic solution 1 free from inert particles 3 (See Fig. 1) occurs; and a half-period wherein the metal article 2 to polish or anode is in contact with an electrolytic solution 1 which contains inert particles 3 in suspension (see Figs. 2 and 3). Within the half-period wherein the anode is in contact with the electrolytic solution 1 with chemically inert particles 3 in suspension, two stages or phases can be distinguished: a first stage where the particles are fluidized and the anode is immersed in this fluidized bed 14; and a second stage where the chemically inert particles 3 are compacted around the metal article to polish coming into contact therewith in the most intimate form as can be gathered from Fig. 3.

Finally, although it does not appear represented, after the compacting stage of the chemically inert particles 3 around the metal piece or article 2 to polish, a half- period occurs wherein said article 2 is in contact with the electrolytic solution 1 free from chemically inert particles 3, a situation similar to that of Fig. 1. The conjunction of an electrolytic solution 1 such as that described and the application of this polishing procedure permits the polishing of articles 2 whose relief or geometry is relatively complex and have problems when being treated with mechanical methods or procedures; from these problems we can highlight the excessive rounding of protruding corners, the deficient smoothing in the cavities 15, excessive treatment or polishing time, deformations, pitting, etc.

Nevertheless, with the electrolytic solution 1 and the procedure according to the invention, perfect polishes are achieved without ruggedness or roughness 11 in the order of up to tens of micrometers, because the presence of at least one compound of the alkylbenzene sulfonic family of formula I, and more preferably the presence of at least one dialkylbenzene sulfonic compound with alkyl radicals of 10 to 14 carbon atoms and of at least one alkylbenzene sulfonic compound with an

alkyl radical of 10 to 14 carbon atoms, enables the formation of an anodic layer 12 around the metal article 2 to polish which has a supramolecular structure.

Fig. 4 illustrates a diagram where we can see a detail of a metal article 2 with protuberances or roughness 11 which configures the anode of the electrochemical reaction, surrounded by a film or anodic layer 12, represented by broken lines which follow the profile of the metal 2 and which represent said supramolecular order of the electrolytic solution 1. Chemically inert particles 3 interacting with the metal 2 have also been represented.

This anodic layer 12 with supramolecular structuring may come to have a stationary thickness in the order of tens of millimetres. Furthermore, it develops greater electrical resistance than the rest of the electrolyte 1 and great mechanical susceptibility, so that in the half-period wherein the article 2 to polish is in contact with electrolytic solution 1 with chemically inert particles 3 in suspension, the latter tend to eliminate the anodic layer 12 by mechanical erosion and preferably, of the protruding parts 11 of the relief to polish, easily producing an effective displacement of the anodic layer 12 existing on said protuberances or roughness 11 towards the adjacent spaces or cavities 15 below them.

The formulation of the electrolytic solution 1 according to the invention facilitates the formation of anodic layer 12 which develop gradients, perpendicular to its thickness, of physico-electrical properties with no limit of continuity. On producing this type of anodic layers 12, of thicknesses of the same order as the ruggedness 11 to level, high efficiency of levelling of ruggedness or roughness 11 is achieved of very different order of magnitude.

Thus, as can be seen in Fig. 4 indicated with the arrows, when a chemically inert particle 3 interacts with the anodic layer 12 of supramolecular structuring, the material or the solution 1 close to the protuberant points 11 of the metal article 2 is displaced towards the adjacent recessed or concave areas 15. As the polishing procedure advances, increasingly phenomena predominate which hinder direct contact of the inert particles 3 with the metal surface 13. In other words, because the most viscous layers next to the anode statistically behave as a limit layer without turbulence, and which, therefore, has little renewal.

In this way, as seen in Fig. 5, the final smoothing of the metal surface 13 is intermediated by said limit layer, thus achieving the smoothing in the order of tens of micrometers. With the electrolytic solution 1 object of the invention, polishes are achieved which are surprisingly fine in metal pieces of copper or its alloys.

This new electrolytic solution 1 permits the polishing of metal pieces 2 without the use of strictly mechanical procedures, it further being possible to smooth, in a single stage, reliefs in the order of tens of millimetres thickness, achieving final roughness 11 in the order of only tens of micrometers. Although Figs 1 to 3 represent a device 4 for the polishing of metal articles, which comprises a receptacle 5 wherein at least two vessels 7 and 8 are distinguished, 2, devices 4 which have more than two interconnected vessels 7 and 8 where at least one of them contains the chemically inert particles 3 and in a form so that they can be fluidized, are equally applicable for the polishing of metal pieces. Devices 4 are also provided for the polishing of metal articles 2 with the electrolytic solution 1 of the invention where the means of oscillation 1 is constituted by other alternative devices such as blade stirrers or gas insufflation systems.

Evidently, any device 4 wherein either an internal tank or vessel 7 or 8 with a permeable base which contains the chemically inert particles 3, which is subject to a relative movement with respect to the walls of the outer receptacle 5 in order to fluidize the bed of particles 3 during the half-period of the stage of interaction of the anode with the latter, is also applicable to perform the polishing procedure according to the invention.