Self-standing gel electrode device for the selective electrochemical or chemical treatment of a metal surface

Abstract

Self-standing gel electrode device for the selective electrochemical or chemical treatment of a metal surface, comprising or consisting of: a) a gel electrolyte layer comprising: an electrolyte; a physically crosslinked polymer selected from the group consisting of physically crosslinked polyvinyl alcohol, a physically crosslinked cellulose derivative, and a mixture thereof; wherein the gel electrolyte layer comprises perforations; and b) a carbon-based flexible planar structure permeable to gases as a counter electrode in contact with the gel electrolyte layer; wherein the carbon-based flexible planar structure is arranged on a surface of the gel electrolyte layer. Use of the self-standing gel electrode device for the selective electrochemical or chemical treatment of a metal surface. And process for the preparation of the gel electrode device.

Description

[0001] Self-standing gel electrode device for the selective electrochemical or chemical treatment of a metal surface

[0002] This application claims the benefit of European Patent Application 24382756.5 filed on July 12th, 2024.

[0003] Technical Field

[0004] This invention relates to the treatment of a metal surface, particularly to a removable selfstanding gel electrode device and to its use for the selective electrochemical (or chemical) treatment of a metal surface.

[0005] Background Art

[0006] The in-situ application of electrochemical or of chemical surface treatments is essential for a successful selective treatment (e.g., for the reparation) of metallic surfaces, especially big structures such as in aircraft, buildings, or tubing. Several methods are currently used to locally apply electrochemical treatments. However, these methods imply the use of special apparatus and hazard liquids, or they are not suitable to treat complex geometries.

[0007] DALIC process (cf. R.D. Clarke. "DALIC selective brush plating and anodising". International Journal of Adhesion and Adhesives, 1999, Vol. 19 (2-3), pp. 205-207), SIFCO process (a portable method of electroplating localized areas without the use of an immersion tank; cf. https: / / www.sifcoasc.com / wp-content / uploads / Process-in-Brief.pdf) and PAC process (Phosphoric Acid Containment system) need special apparatus and tools for the recirculation of liquid electrolytes and the movement of the tool (electrode with recirculating electrolyte) during the process. These techniques usually need liquids (acidic electrolytes) that could leakage causing safety issues and they need to be carefully managed both during the process and as waste disposal. PANTA process (Phosphoric Acid Non-Tank Anodizing; cf. SY Park et al. "A review of the recent developments in surface treatment techniques for bonded repair of aluminum airframe structures". International Journal of Adhesion and Adhesives, 2018, Vol. 80, pp. 16-29), and the system disclosed in WO2016193736 A1 use gel electrolytes for anodizing and electropolishing processes preventing leakage safety issues. However, the system disclosed in WO2016193736 A1 and the PANTA process imply the use of gel electrolytes that can be placed in the area to be treated together, among other layers, with a metallic mesh that acts as the counter electrode. These gel-based technologies are easier to apply in non-flat surfaces, but the low flexibility of the metallic counter electrode does not allow the treatment of more curved or complex surfaces. Moreover, this metallic mesh is susceptible to corrosion in long term contact with the acid gel electrolyte, so the whole device (gel electrolyte and metallic counter electrode) cannot be storage for a long time. Furthermore, PANTA is only used for anodizing and WO2016193736 A1 only discloses an electropolishing process. In addition, selective anodizing processes with gel electrolytes such as PANTA and the ones used in the process of WO2016193736 A1 leave residues on the surface after the process and it is necessary to clean the treated surface thoroughly to remove these acidic residues.

[0008] WO2012126969 A1 discloses a removable anodizing agent, in particular for local anodic oxidation of metal surfaces. However, selective anodizing processes with gel electrolyte disclosed therein results in anodized surfaces of non uniform thickness, particularly, in the central zone of the area of the surface covered by the anodizing agent due to the formation of bubbles during the anodizing process.

[0009] Thus, there is still a need of providing improved devices and methods for the in situ electrochemical or chemical treatment of metal surfaces having different geometries and solving the problems of the prior art.

[0010] Summary of Invention

[0011] Inventors have found that by providing a flexible self-standing gel electrode device comprising a carbon-based flexible planar structure as a counter electrode and a perforated gel electrolyte, the gel electrolyte comprising an electrolyte and a specific physically crosslinked polymer, it is possible to electrochemically or chemically treat in situ selected areas of a surface of a metal structure. The selective treatment can be performed at room temperature in the absence of corrosive liquid electrolytes, and without the need for additional equipment (except for the power supply).

[0012] The flexibility of the self-standing gel electrode device allows it to be adapted to a wide variety of geometries and surfaces, thus it can be easily positioned even on complex surfaces to selectively treat the areas in need.

[0013] Thus, afirst aspect of the present disclosure relates to a self-standing gel electrode device 10 for the selective electrochemical or chemical treatment of a metal surface 15, the gel electrode device comprising or consisting of: a) a gel electrolyte layer 12 comprising:

[0014] - an electrolyte and

[0015] - a physically crosslinked polymer selected from the group consisting of physically crosslinked polyvinyl alcohol, a physically crosslinked cellulose derivative, and a physically crosslinked mixture of polyvinyl alcohol and a cellulose derivative; wherein the cellulose derivative is selected from the group consisting of carboxymethylcellulose, hydroxypropylcellulose, hydroxypropyl methyl cellulose, and hydroxyethyl cellulose, wherein the gel electrolyte layer comprises perforations 13; and b) a carbon-based flexible planar structure 11 as a counter electrode in contact with the gel electrolyte layer, the carbon-based flexible planar structure being permeable to the gases formed during the electrochemical or the chemical treatment; wherein the carbon-based flexible planar structure 11 is arranged on a surface 121 of the gel electrolyte layer 12 and an opposite surface 122 is configured to be in direct contact with the metal surface 15 to be treated.

[0016] In the context of the present disclosure, it is understood that in order the self-standing gel electrode device of the present disclosure is suitable for the selective electrochemical or chemical treatment of a metal surface 15, the gel electrolyte layer 12 must have a free- surface 122 (the surface opposite to the surface 121 with which the carbon-based flexible planar structure 11 is in contact with), i.e., a surface 122 configured to be in direct contact with the metal surface 15 to be treated so that the selective electrochemical or chemical treatment of a metal surface can take place.

[0017] As shown in Fig. 2B and 2C, the adhesive film 14 is configured to fix the gel electrode device 10 on the area of the metal surface 15 to be treated.

[0018] The self-standing gel electrode device of the present disclosure has no dimensional limitations since its size depends only on the dimensions of the mould used for its fabrication. Thus, the self-standing gel electrode device of the present disclosure can have the required dimension to treat the desired surface area.

[0019] Advantageously, the combination of a gas-permeable carbon-based flexible planar structure 11 as a counter electrode and the perforated gel electrolyte 12 (i.e., the gel electrolyte layer 12 with perforations 13) allows the evacuation of the gasses produced during the chemical and, especially, the electrochemical treatments. In particular, the mentioned perforations allow the removal of the gas generated at the metal surface / gel electrolyte interface (for example O2, in the case of anodic treatments such as anodizing). Besides, an additional advantage of the perforations 13 is that they allow the dissipation of the heat generated during the process, such as in anodic oxidation. As a result of both phenomena, no bubbles stay in the metal surface / gel electrolyte interface and, as a consequence, the whole area covered by the device is uniformly treated. As an instance, when anodizing is performed, the oxide layer formed in the anodized surface has a uniform thickness along the whole anodized surface.

[0020] Besides, as mentioned above, the electrochemical or chemical processes with the gel electrode device of the present disclosure can be carried out at room temperature and the developed surfaces have a performance similar to that of standard liquid-based treatments, which are usually carried out at higher temperatures. In addition, in some embodiments, when the process is carried out by applying a voltage / current, the gel electrode device of the present disclosure maintains its properties during the application of voltage / current, namely it is not degraded due to the local heating produced during the process.

[0021] Thus, another aspect of the present disclosure relates to the use of the self-standing gel electrode device disclosed herein for the selective electrochemical or chemical treatment of a metal surface, particularly, in situ.

[0022] Advantageously, after the electrochemical or the chemical process, the self-standing gel electrode device can be easily removed from the treated surface without hardly any residue remaining on the treated surface, which can be easily cleaned with a wet wipe. Besides, the self-standing gel electrode device of the present disclosure can be reused several times. and, at the end of its service life, can be recovered for its regeneration.

[0023] A further aspect of the present disclosure relates to a process for the preparation of the gel electrode device as defined in claim 1, the process comprising: a) preparing a gel electrolyte precursor comprising i) an electrolyte and ii) a polymer selected from the group consisting of polyvinyl alcohol, a cellulose derivative in the presence of a crosslinking acid, and a mixture of polyvinyl alcohol and a cellulose derivative; wherein the cellulose derivative is selected from the group consisting of carboxymethylcellulose, hydroxypropylcellulose, hydroxypropyl methyl cellulose, and hydroxyethyl cellulose; b) pouring the gel electrolyte precursor in a mould having a base having a surface comprising pins in order to form a layer of gel electrolyte precursor with perforations, wherein the layer of gel electrolyte precursor has a top surface; c) arranging a carbon-based flexible planar structure on the top surface of the gel electrolyte precursor; d) physically crosslinking the polymer by subjecting the gel electrolyte precursor to a freezing-thawing process, to obtain an assembly of a gel electrolyte layer 12 with perforations 13 and the carbon-based flexible planar structure 11 ; and e) demoulding the obtained assembly of gel electrolyte layer 12 and carbon-based flexible planar structure 11 ; f) optionally, disposing a flexible adhesive film 14 on the carbon-based flexible planar structure 11 , wherein flexible adhesive film is permeable to the gases formed during the electrochemical or the chemical treatment; wherein, when the flexible adhesive film 14 is present, the carbon-based flexible planar structure 11 is arranged between the flexible adhesive film and the gel electrolyte layer 12, and the flexible adhesive film 14 surpasses the limits of the carbon-based flexible planar structure 11.

[0024] Brief Description of Drawings

[0025] Fig. 1 shows a scheme of an anodizing process with a gel electrode device of the present disclosure, wherein it is depicted how the O2 produced at the anodized surface / gel electrolyte interface is released through the perforations of the gel electrolyte and the H2 produced at the gel electrolyte / carbon-based flexible planar structure interface is released through the permeable carbon-based planar structure; S: anodizable substrate, C: cathode; G: gel electrolyte.

[0026] Fig. 2 shows: A) a gel electrode device of the present disclosure 10 having a carbonbased flexible planar structure 11 and a gel electrolyte layer 12 comprising perforations 13, the gel electrolyte layer having an surface 121 in contact with the carbon-based flexible planar structure and a surface 122 configured to be in direct contact with the surface to be treated; B) a gel electrode device of the present disclosure 10 having a carbon-based flexible planar structure 11, a gel electrolyte layer 12 comprising perforations 13, and a flexible adhesive film is permeable to gases 14; C) a gel electrode device as in Fig. 2B, wherein the a flexible adhesive film is fixing the gel electrode device on the area of the metal surface 15 to be treated.

[0027] Fig. 3 shows the effect on an anodized metal surface with A) the non-perforated gel electrode of Comparative Example 2, and of B) the perforated gel electrode of Example 1 according to the present disclosure.

[0028] Figs. 4 and 5 show the effect of the number of freezing-thawing cycles on the rheological behaviour of a gel electrolyte of the present disclosure.

[0029] Detailed description of the invention

[0030] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly through-out the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0031] The term “gel”, as used herein, refers to a system comprising a three dimensional network of one or more polymers that is an intermediate between a solid and liquid possessing both an elastic behaviour (gets distorted on applying pressure, but recovers to its actual shape after removal of the stress) and viscous characteristics (as part of the deformation is not recovered after the stress is removed and the gel flows since the bonds are broken and new bonds are formed, as in a liquid when force is applied). Particularly, in the context of the present disclosure, gels have a tan 5 = G” / G’ < 1 , wherein G' and G" are measured in a TA Instruments AR2000 rheometer at 25 °C and in the linear viscoelastic region using a plate-plate geometry in a frequency range from 0.1 to 100 Hz.

[0032] The Linear Viscoelastic Region (LVR) is a safe and stable range for characterizing the rheological properties of materials (such as gels) without altering their internal structure. In this region, the stress-strain relationship is proportional and does not change with the applied strain amplitude. In this context, the plate-plate geometry is commonly used to determine the LVR and to perform measurements in a linear and reproducible manner.

[0033] The term "plate-plate geometry" refers to a rheological measurement configuration in which a sample is confined between two opposing parallel, typically circular plates. In this setup, one plate remains stationary while the other plate is capable of rotational or oscillatory motion relative to the fixed plate.

[0034] The term "planar geometry" referred to a mould means a geometry allowing to obtain a planar structure of a gel electrolyte layer and, as a consequence, of the gel electrode device. Thus, the term "planar" (or flat) referred to a structure, means that the structure has a length and width having the same order of magnitude, and a thickness at least one, preferably two, orders of magnitude smaller than the length and / or the width; or, alternatively, a length greater than the width by at least one or two orders of magnitude, an a thickness at least one, preferably two, orders of magnitude smaller than the width. Thus, the mould has a form allowing to obtain a planar gel electrolyte and, as a consequence, a gel electrode device that is capable of adapting to surfaces having different geometries, such as curved or complex surfaces. A "complex surface" means a surface which is forming part, for instance, of a larger structure having a curved surface, or of a structure requiring application of the gel electrode device on a vertical surface.

[0035] The term "physically crosslinked" means cross-linked by non-covalent bonds such as by the formation of intermolecular hydrogen bonds, or ionic interactions.

[0036] The term “pin” should be understood as a mean to form perforations in the gel electrolyte layer during crosslinking. The perforations in the gel electrolyte are formed by pouring the gel electrolyte precursor in a mould provided with pins (i.e. , having pins embedded in the base of the mould, oriented perpendicular to the mould base surface), such that the height of the layer of poured gel electrolyte precursor is equal or lower than the height of the pins.

[0037] As an instance, pins can be configured as protruding cylindrical elements (such as with the form of nail or rod). The cross section of a pin is the geometric shape formed by a plane cutting across the pin at a right angle to its longitudinal axis. The cross section is understood to be de maximal cross section.

[0038] The term “perforation” means a through-hole, that is a hole made or passing through the gel electrolyte layer, i.e., refers to openings extending fully through the alignment of the gel electrolyte layer and that are clearly distinct to the pores due to the intrinsic porosity of the gel electrolyte layer. Thus, as depicted in Figs. 1 , and 2, the perforations in the gel electrolyte layer have a height equal to the thickness of the gel electrode layer and a cross section having an area equal to or smaller than the area of the cross section of the pins in the mould used for the preparation of the perforated gel electrolyte layer (considering that after demoulding, the cross section of the perforation can be lower due to a certain elasticity of the gel electrolyte). Therefore, in an example, if the area of the cross section of the pins is from 0.2 to 0.8 mm2, such as of 0.5 mm2, after demoulding, the crosslinked gel electrolyte layer will have perforations having a height equal to the thickness of the gel electrode layer and a cross section having an area equal to or smaller than from 0.2 to 0.8 mm2, such as equal to or smaller than 0.5 mm2. Thus, it is understood that perforations are for fluid communicating one surface of the gel electrolyte layer to the opposite surface of the gel electrolyte layer. The perforations are evacuating holes, that are configured to evacuate the gas generated at the metal surface / gel electrolyte interface toward the exterior. Thus, perforations extend perpendicular (or essentially perpendicular) from one surface 121 to the opposite surface 122 of the gel electrolyte layer.

[0039] The apertures are not necessarily uniform in cross section. The cross section is understood to be de maximal cross section.

[0040] Since the gel electrolyte layer is formed by physically crosslinking the polymer comprised in the gel electrolyte precursor in a mould having a base having a surface comprising pins, when demoulded, the physically crosslinked gel electrolyte layer will comprise perforations in the position where the pins where placed. That is, during the polymer crosslinking, perforations going through the gel polymer layer are formed.

[0041] The term "room temperature" refers to a temperature from about 20 °C to about 25 °C.

[0042] As used herein, the indefinite articles “a” and “an” are synonymous with “at least one” or “one or more.” Unless indicated otherwise, definite articles used herein, such as “the” also include the plural of the noun.

[0043] The term "and / or" means that any one of the options to which it relates are possible or the at least two options take place at the same time.

[0044] The term “cellulose derivative” includes carboxymethylcellulose, hydroxypropylcellulose, hydroxypropyl methyl cellulose, and hydroxyethyl cellulose. Carboxymethyl cellulose (CMC; CAS Number 9004-32-4) is a cellulose derivative with a large number of carboxymethyl groups (-CH2-COOH) on a cellulose backbone consisting of a polymeric chain composed of anhydro glucopyranose units linked by 1,4-glycosidic bonds.

[0045] Hydroxypropyl cellulose (HPC; CAS Number 9004-64-2) is an ether of cellulose in which some of the hydroxyl groups in the repeating glucose units have been hydroxypropylated forming -OCH2CH(OH)CH3 groups using propylene oxide.

[0046] Hydroxypropyl methylcellulose (HPMC; CAS Number 9004-65-3) Hydroxypropyl methylcellulose (HPMC) is a cellulose derivative with methoxyl and hydroxypropyl group substituents attached to a cellulose backbone via ether bonds.

[0047] Hydroxyethylcellulose (CAS Number 9004-62-0) is a cellulose derivative having hydroxyethyl (-CH2CH2OH) groups attached to some hydroxyl functional groups of the glucose units in the cellulose chain.

[0048] Polyvinyl alcohol (PVA; CAS Number 9002-89-5) is a water-soluble synthetic polymer. It has the idealized formula [CH2CH(OH)]n.

[0049] The gel formation is a result of the crosslinking which occur by non-covalent bonds such as by the formation of intermolecular hydrogen bonds, or ionic interactions, forming a net structure.

[0050] As mentioned above, one aspect of the present disclosure relates to a self-standing gel electrode device 10 as defined above comprising or consisting of a perforated specific gel electrolyte layer 12 and a carbon-based flexible planar structure 11 as a counter electrode.

[0051] In an embodiment of the first aspect of the present disclosure, the self-standing gel electrode device further comprises a flexible adhesive film 14 which is permeable to the gases formed during the electrochemical or the chemical treatment, wherein the carbonbased flexible planar structure 11 is arranged between the flexible adhesive film 14 and the gel electrolyte layer 12, and the flexible adhesive film surpasses the limits of the carbon-based flexible planar structure. That is, the flexible adhesive film 14 extends out beyond (i.e., is larger than) the carbon-based flexible planar structure, that is, the flexible adhesive film 14 is configured to fix the gel electrode device 10 on the area of the metal surface 15 to be treated.

[0052] The gel electrolyte layer of the self-standing gel electrode device is configured to be in contact with the metal surface to be treated.

[0053] Thus, in the context of the present disclosure, it is understood that in order the self- standing gel electrode device is suitable for the selective electrochemical or chemical treatment of a metal surface, the device must comprise only one carbon-based flexible planar structure as a counter electrode in contact with the gel electrolyte layer. As mentioned above, the carbon-based flexible planar structure is arranged on a surface 121 of the gel electrolyte layer. The opposite surface 122 of the gel electrolyte layer 12 is free of any other material, since it is configured to be in contact with the metal surface 15 to be treated during the chemical or electrochemical process.

[0054] In an embodiment, the self-standing gel electrode device consisting of: a) a gel electrolyte layer comprising:

[0055] - an electrolyte and

[0056] - a physically crosslinked polymer selected from the group consisting of physically crosslinked polyvinyl alcohol, a physically crosslinked cellulose derivative, and a physically crosslinked mixture of polyvinyl alcohol and a cellulose derivative, wherein the gel electrolyte layer comprises perforations; and b) a carbon-based flexible planar structure as a counter electrode in contact with the gel electrolyte layer, the carbon-based flexible planar structure being permeable to the gases formed during the electrochemical or the chemical treatment; and c) optionally, a flexible adhesive film which is permeable to the gases formed during the electrochemical or the chemical treatment, wherein the carbon-based flexible planar structure is arranged on a surface of the gel electrolyte layer, and, when the flexible adhesive film is present, the carbon-based flexible planar structure is arranged between the flexible adhesive film and the gel electrolyte layer, and the flexible adhesive film surpasses the limits of the carbon-based flexible planar structure.

[0057] That is, the self-standing gel electrode device consists of one gel electrolyte layer as defined herein above and below, one carbon-based flexible planar structure as defined herein above and below, and optionally, one flexible adhesive film as defined herein above and below.

[0058] In another embodiment, perforations have a height equal to the thickness of the gel electrode layer and a cross section having an area equal to or smaller than from 0.2 to 0.8 mm2, particularly, equal to or smaller than 0.5 mm2.

[0059] Advantageously, the combination of a gas-permeable carbon-based flexible planar structure as a counter electrode and the perforated gel electrolyte allows the evacuation of the gasses produced during the chemical and, especially, the electrochemical treatments. The self-standing gel electrode device of the present disclosure can take the form, for example, of an essentially flat or a flat structure such as for example a foil or a film. As used herein, the term "essentially flat structure" means a body whose length and width are of the same order of magnitude, whereas the thickness of the body is smaller by at least one, preferably two, orders of magnitude than the length and the width. As used herein, the term "longitudinally extended structure" means a body where the length is greater than the width by at least one, preferably two, orders of magnitude, whereas the thickness of the body is smaller by one, preferably by at least two, orders of magnitude than the width.

[0060] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the perforations are essentially perpendicular to a surface of the carbon-based flexible planar structure and are arranged in a grid pattern, staggered, randomly distributed along gel electrolyte layer, or in any other pattern.

[0061] The term “essentially perpendicular” does not mean that the perforations necessarily have to be exactly aligned at an angle of 90° relative to the surface of the carbon-based flexible planar structure, but includes any angle that allows a crosswise arrangement of the perforations. Consequently, “essentially perpendicular” merely means that the perforations intersect the longitudinal axis of the surface. In some embodiments, the perforations may also be aligned, e.g., at an angle between 70° and 110° relative to the surface of the carbon-based flexible planar structure. In an example, the perforations are perpendicular to a surface of the carbon-based flexible planar structure 11 (or to a surface 121 of the gel electrolyte layer 12), i.e., are aligned at an angle of 90° relative to the surface of the carbon-based flexible planar structure.

[0062] As mentioned above, advantageously, the presence of the perforations in the gel electrolyte layer has the effect that the gases produced in the area of the gel electrolyte layer in contact with the surface to be treated (such as O2 in an anodizing process or H2 in an electrodeposition process) will be able to evacuate, and thus, the selected area of the surface will be uniformly treated.

[0063] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the perforations have a density (i.e. the number of perforations per surface area) from 1 to 16 perforations per cm2, particularly, from 3 to 10 perforations per cm2.

[0064] In another embodiment, the self-standing gel electrode device of the present disclosure, is for the anodizing of a metal surface and the electrolyte comprises an acid selected from the group consisting of phosphoric acid, sulfuric acid, chromic acid and organic carboxylic acids such as tartaric acid, glycolic acid, lactic acid, oxalic acid, citric acid, malic acid and mixtures of said acids.

[0065] In a particular embodiment, the electrolyte consists of an aqueous solution comprising an acid, wherein the acid is in an amount from 10 g / L to 1000 g / L.

[0066] In another embodiment, the acid is a mixture of tartaric acid (TA) and sulfuric acid (SA), particularly, in a weight ratio of TA:SA from 1.5:1 to 2.5:1 , such as of 2:1.

[0067] In another embodiment, the self-standing gel electrode device of the present disclosure is for the passivation of a metal surface and the electrolyte comprises nitric acid. Particularly, the nitric acid is in an amount from 10 g / L to 300 g / L of electrolyte.

[0068] In another embodiment, the self-standing gel electrode device of the present disclosure is for the electropolishing of a metal surface and the electrolyte comprises an acid selected from sulfuric acid, phosphoric acid, and a mixture thereof. Particularly, the acid or mixture of acids is in an amount from 5 g / L to 800 g / L or electrolyte.

[0069] In another embodiment, the self-standing gel electrode device of the present disclosure is for chemical conversion coating of a surface and the electrolyte comprises an inorganic salt of a metal selected from the group consisting of Cr, Ti, Zr and Ce. Inorganic metal salts for chemical conversion coatings and their amounts in the electrolyte are well known by those skilled in the art. Examples of metal salts include, without being limited to, potassium hexafluoro zirconate, potassium hexafluoro zirconate, sodium hexafluoro zirconate, potassium hexafluoro titanate, cerium chloride, trivalent chromium zirconate, trivalent chromium sulfate basic, and trivalent chromium sulfate.

[0070] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the physically crosslinked polymer is physically crosslinked polyvinyl alcohol.

[0071] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the physically crosslinked polymer is carboxymethyl cellulose.

[0072] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the physically crosslinked polymer is a physically crosslinked mixture of polyvinyl alcohol and carboxymethyl cellulose.

[0073] In an embodiment, optionally in combination with one or more features of the embodiments described above, the gel electrolyte layer comprises from 0.5 wt.% to 25 wt.% of a physically crosslinked polymer as defined above by total weight of gel electrolyte. In a particular embodiment, the physically crosslinked polymer is physically crosslinked PVA and is in an amount from 9 wt.% to 25 wt.%, particularly from 10 wt.% to 20 wt.%, by total weight of gel electrolyte. In another embodiment, the physically crosslinked polymer is a physically crosslinked cellulose derivative and is in an amount from 0.5 to 8 wt.%, particularly, from 1 to 5 wt.%, by total weight of gel electrolyte precursor. In a particular embodiment, the physically crosslinked polymer is physically crosslinked CMC and is in an amount from 0.5 to 8 wt.%, particularly, from 1 to 5 wt.%, by total weight of gel electrolyte precursor.

[0074] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the gel electrolyte layer is obtainable by subjecting a gel precursor comprising i) an electrolyte and ii) a polymer selected from the group consisting of polyvinyl alcohol, a cellulose derivative such as carboxymethyl cellulose in the presence of a crosslinking acid, and a mixture of polyvinyl alcohol and cellulose derivative such as carboxymethyl cellulose; to a freezing-thawing process. Particularly, the polymer is polyvinyl alcohol and the freezing-thawing process consists of at least two consecutive cycles of freezingthawing. More particularly, the polymer is polyvinyl alcohol and the freezing-thawing process consists of at least three consecutive cycles of freezing-thawing.

[0075] In an embodiment, optionally in combination with one or more features of the embodiments described above, the gel electrolyte precursor comprises from 0.5 wt.% to 25 wt.% of a polymer as defined above by total weight of gel electrolyte precursor. In a particular embodiment, the polymer is crosslinked PVA and is in an amount 9 wt.% to 25 wt.%, particularly from 10 wt.% to 20 wt.%, by total weight of gel electrolyte precursor. In another embodiment, the polymer is CMC and is in an amount from 0.5 to 8 wt.%, particularly, from 1 to 5 wt.%, by total weight of gel electrolyte precursor.

[0076] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the gel electrolyte layer has a storage modulus G' from 5.000 Pa to 15.000 Pa measured in a rheometer (such as a TA Instruments AR2000 rheometer) at 25 °C and in the linear viscoelastic region using a plate-plate geometry, and wherein tan 5 = G” / G’ < 1 , particularly, from 0.06 to 0.15, more particularly, from 0.08 to 0.1. Particularly, the storage modulus G' is from 8.000 Pa to 12.000 Pa, and the viscous modulus (G") is from 800 Pa to 1000 Pa.

[0077] In a particular embodiment, the physically crosslinked polymer is physically crosslinked PVA, G' is from 8.000 Pa to 12.000 Pa and the viscous modulus (G") is from 800 Pa to 1000 Pa.

[0078] In another embodiment of the self-standing gel electrode device of the present disclosure, optionally in combination with one or more features of the embodiments described above, the carbon-based flexible planar structure is selected from a carbon-based flexible planar fibre, fabric, mesh, or veil.

[0079] Particularly, the carbon-based flexible planar structure has a thickness from 0.05 to 2 mm.

[0080] As mentioned above, the carbon-based flexible planar structure is permeable to the gases formed during the electrochemical or the chemical treatment, such as, for example, H2 and O2 generated during anodic oxidation at the carbon-based flexible planar structure / gel electrolyte interface and at the metal surface / gel electrolyte interface, respectively.

[0081] Besides, the carbon-based flexible planar structure has also a carrier effect for the gel electrolyte layer and, together with the flexible adhesive film, if present, contributes to the mechanical stability of the self-standing gel electrode device of the present disclosure.

[0082] The flexible adhesive film can have a thickness from 20 to 500 pm, particularly from 20 to 200 pm. As mentioned above, the adhesive flexible film is also permeable to the gases formed during the electrochemical or the chemical treatment. In a particular embodiment, the adhesive flexible film is provided with perforations which are coincident with the perforations in the gel electrolyte layer. Suitable flexible adhesive films for being used in the self-standing gel electrode device of the present disclosure are commercially available.

[0083] As derived from the results of the examples, the thickness or viscosity of the gel electrolyte layer has a negligible influence on the obtained anodic layer thickness. Nevertheless, for easy application to the surface to be treated, the thickness of the gel electrolyte layer arranged on the carbon-based flexible planar structure can be from 0.1 mm to 6 cm, or from 0.1 mm to 4 cm, or from 0.1 mm to 2 cm, particularly, from 0.5 mm to 4 cm, more particularly, from 0.5 mm to 1 cm or from 0.5 mm to 4 mm, without counting any portions of the gel electrolyte that can have penetrated into the openings of the carbon-based flexible planar structure.

[0084] As mentioned above, another aspect of the present disclosure is the use of the selfstanding gel electrode device as defined herein for the selective electrochemical or chemical treatment of a metal surface.

[0085] The modulation of the specific formulation of the gel electrolyte allows the development of different kinds of electrochemical and chemical surface treatments. A skilled person in the art will know which electrolyte will be needed for each particular electrochemical or chemical treatment.

[0086] Examples of electrochemical surface treatments include, without being limited to, anodizing, electropolishing, electrocolouring, and electrodepositon.

[0087] Examples of chemical surface treatments include, without being limited to, conversion coatings, passivation, electroless deposition and chemical polishing, etching, and cleaning.

[0088] Thus, in an embodiment of the use of the present disclosure, the electrochemical treatment is an anodic oxidation of the metal surface. Particularly, the anodic oxidation is selected from the group consisting of anodizing and electropolishing.

[0089] In another embodiment of the use of the present disclosure, the electrochemical treatment is a cathodic metallic deposition in the metal surface.

[0090] The metal surface can contain one or a plurality of metals. Examples of metals include, without being limited to, aluminium, titanium, zinc, magnesium copper, and their alloys; steel and brass.

[0091] The present disclosure also relates to a method for treating a metal surface comprising the following steps:

[0092] - bringing a gel electrode device as defined herein into contact with the metal surface to be treated, so that the free-surface of the gel electrolyte layer (i.e., the surface opposite to the one wherein the carbon-based flexible planar structure is in direct contact with) covers and is in direct contact with the metal surface to be treated, without the carbon-based flexible planar structure of the gel electrode device touching the metal surface to be treated; and

[0093] - in the case of electrochemical surface treatments, connecting the metal surface to be treated and the carbon-based flexible planar structure with a direct or pulsed current source or a direct or pulsed voltage source.

[0094] Pulsed current or pulsed voltage refers to an electrical current or voltage that flows in one direction but varies in amplitude over time. Unlike direct current or direct voltage, which provide a constant voltage or current, the pulsed mode has fluctuations or pulsations in its output. These variations can be periodic or sporadic, causing the voltage or current to rise and fall in a pulsating manner.

[0095] The pulsed mode can be unipolar or bipolar. Unipolar pulsed mode generates singlepolarity voltage or current pulses. The output voltage or current is zero between the pulses.

[0096] Bipolar pulsed mode generates dual-polarity voltage or current pulses. The output voltage or current alternately swings between positive and negative. As an instance, in the case of an anodizing treatment, the metal surface to be treated is acting as an anode and the carbon-based flexible planar structure is acting as a cathode. Particularly, the voltage provided by the voltage source can be from 0.5 to 50 V, more particularly from 2 to 40 V, or from 5 to 30 V, or from 10 to 20 V. The anodization can be carried out for a period from one minute and two hours.

[0097] Once anodizing has been performed during the time required to obtain the desired surface quality, i.e. an oxide layer with the desired quality and thickness has been formed, the gel electrode device according to the present disclosure can be removed. Finaly, the treated surface can be rinsed with water or cleaned with a wet wipe.

[0098] In another example, in the case of a chemical treatment, the gel electrode device is left to act for the required period of time.

[0099] As mentioned above, a further aspect of the present disclosure is a process for the preparation of the gel electrode device as defined herein, the process comprising: a) preparing a gel electrolyte precursor as defined above; b) pouring the gel electrolyte precursor in a mould comprising pins in their base; c) arranging the carbon-based flexible planar structure on a surface of the gel electrolyte precursor; d) physically crosslinking the polymer by subjecting the gel electrolyte precursor to a freezing-thawing process; and e) demoulding the obtained assembly of gel electrode and carbon-based flexible planar structure.

[0100] By obtaining the perforations on the gel electrolyte layer during the freeze-thawing process, the perforations remain even after demoulding the matrix. Conversely, if the perforations are performed after the crosslinking (for instance, with a perforation tool), the formed holes will self-repair, that is, they will disappear.

[0101] In an embodiment of the process of the present disclosure, a flexible adhesive film is disposed on the carbon-based flexible planar structure in such a manner that the carbonbased flexible planar structure is arranged between the flexible adhesive film and the gel electrolyte layer, and the flexible adhesive film surpasses the limits of the carbon-based flexible planar structure.

[0102] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the polymer is polyvinyl alcohol and, particularly, the freezing-thawing process consists of at least two consecutive cycles of freezing-thawing, particularly, wherein the PVA is in an amount from 9 wt.% to 25 wt.%, more particularly from 10 wt.% to 20 wt.%, by total weight of gel electrolyte precursor .

[0103] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the polymer is polyvinyl alcohol and, particularly, the freezing-thawing process consists of at least three consecutive cycles of freezing-thawing, particularly, wherein the PVA is in an amount from 9 wt.% to 25 wt.%, more particularly from 10 wt.% to 20 wt.%, by total weight of gel electrolyte precursor.

[0104] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the aqueous solution comprises carboxymethyl cellulose and a crosslinking acid. Particularly, the crosslinking acid is selected from the group consisting of citric acid, malic acid, lactic acid, and hydrogen chloride; more particularly, citric acid.

[0105] In an embodiment, the gel electrolyte precursor comprises from 0.5 wt.% to 5 wt.% of carboxymethyl cellulose by total weight of gel electrolyte precursor and from 0.25 mol / L to 1 mol / L of a crosslinking acid as defined, particularly, of citric acid, malic acid, and lactic acid, more particularly, of citric acid.

[0106] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the polymer is a mixture of polyvinyl alcohol and carboxymethyl cellulose.

[0107] In an embodiment, the gel electrolyte precursor comprises from 3 wt.% to 30 wt.%, particularly from 5 wt.% to 20 wt.%, of an polyvinyl alcohol by total weight of gel electrolyte precursor and from 0.5 wt.% to 10 wt.%, particularly, from 2 wt.% to 5 wt.%, of a carboxymethyl cellulose as defined above by total weight of gel electrolyte precursor.

[0108] In some embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the electrolytes are as defined above in relation to the gel electrode device.

[0109] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above the carbon-based flexible planar structure is selected from a carbon-based flexible planar fibre, fabric, mesh, or veil.

[0110] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, wherein density of pins in the mould (i.e. the number of pins per surface area) from 1 to 16 pins per cm2, particularly, from 3 to 10 pins per cm2.

[0111] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the area of the cross section of the pins is from 0.2 to 0.8 mm2, particularly, of 0.5 mm2.

[0112] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the pins are arranged in a grid pattern, staggered, or randomly distributed along the surface of the base of the mould, or any other arrangement.

[0113] In an embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the mould has a planar geometry.

[0114] Once fabricated, the gel electrode device of the present disclosure can be easily vacuum- packed and conserved during long periods of time before its use.

[0115] In the present disclosure, it is noted that when discussing the process for the preparation of the self-standing gel electrode device of the present disclosure, each one of the embodiments or features defined for the self-standing gel electrode device can be considered applicable to the process for its preparation, when pertinent, whether or not they are explicitly discussed in the context of that other aspect, and vice versa.

[0116] It also forms part of the present disclosure a self-standing gel electrode device obtainable by the process defined herein.

[0117] Throughout the description and claims the word "comprise" and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word “comprise” encompasses the case of “consisting of”.

[0118] The following examples and drawings are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

[0119] Examples

[0120] Materials

[0121] For the gel electrolyte formulation: tartaric acid (TA) (99%; CAS 87-69-4) was purchased from Thermo Scientific; sulphuric acid (SA) (95%; CAS 7664-93-1) was purchased from VWR Chemicals; and poly(vinyl alcohol) (PVA) (CAS 9002-89-5; Mw 89000-98000, +99% hydrolized) was obtained from Sigma Aldrich.

[0122] For the counter electrode preparation: flexible C-based mesh (1071 HCB grade, from AvCarb) was used.

[0123] Characterization techniques Appearance of de anodic coatings was visually evaluated.

[0124] The thickness of the anodic coatings was measured using a Fischer Coating Thickness Gauge based on eddy current testing. (Fischer Dualscope MP20 instrument).

[0125] Top surface morphology and cross section microstructure of the obtained anodic layers was evaluated using a field-emission scanning electron microscopy (FESEM) (Carl Zeiss Ultra Plus). For the cross section observation, the specimens were embedded in cold resin and polished using SiC papers and diamond suspensions to reach a mirror-like surface.

[0126] Corrosion behaviour was determined by potentiodynamic polarization tests that were recorded starting from 0,10 V to 0,50 V vs OCP at 0,167 mV / s. OCP was recorded first for 1 h. The tests were performed in 0.1 M NaCI.

[0127] Contact angle of the anodized surfaces with water was determined by a Theta 200-Basic, goniometer system (Biolin Scientific) at room temperature and ambient humidity.

[0128] Paint adhesion to the produced anodic layer was evaluated using pull-of test. For this purpose, anodized surfaces were painted with 20 micros of Aerodur Barrier Primer 37045 from Azko Nobel.

[0129] G'and G" were measured in a TA Instruments AR2000 rheometer at 25 °C and in the linear viscoelastic region using a plate-plate geometry in a frequency range from 0.1 to 100 Hz.

[0130] Example 1

[0131] 1.1. Gel electrolyte precursor preparation

[0132] First, tartaric acid (TA) and sulfuric acid (SA) were added to distilled water and completely mixed to obtain the tartaric-sulfuric acid (TSA) solution with TA and SA contents of 80 and 40 g / L, respectively. Then, the formed TSA solution was heated in a thermostatic bath to 80 °C, and 10 wt.% of PVA was incorporated under vigorous stirring, which was maintained until a complete transparent solution, indicative of the complete PVA dissolution, was obtained. Finally, the solution was ultrasonically vibrated for 10 minutes to remove bubbles and obtain the TSA / PVA gel electrolyte precursor solution.

[0133] 1.2. Gel electrolyte layer preparation

[0134] About 19.6 cm2of the TSA / PVA precursor solution prepared above was poured into a 70 mm x 70 mm mould x 4 mm comprising 169 pins having a height of 6 mm and subjected to the freezing-thawing cycles method. 3 consecutive cycles were applied with freezing steps of 3 hours at -20 °C and thawing steps at room temperature during the required time to complete thawing. After the final step the obtained gel electrolyte layer was demoulded.

[0135] 1.3. Assembly of the gel electrode (counter electrode incorporation)

[0136] The assembly or fabrication of the gel electrode (an assembly of gel electrolyte layer and counter electrode) was carried out by directly incorporating of a C-based mesh during the jellification of the precursor. In order to do this, firstly, the gel electrolyte precursor was poured into the mould and then, a piece of a C-based mesh was deposited covering the whole surface of the gel electrolyte precursor, which was subjected to the 3 freezingthawing cycles. Then, the obtained gel electrode was demoulded and was ready to be used, in this case, for anodizing. Optionally, after its fabrication, the gel electrode was vacuum-packed for storage.

[0137] Example 2 - Gel electrolyte formulation

[0138] First, the capacity of PVA to jellify the TSA electrolyte was examined. At low PVA contents the obtained gel showed a thixotropic behaviour. Self-supported wet gels were obtained at a PVA content of 10%. Considering that for the subsequent anodizing process a certain degree of humidity between the aluminium substrate and the gel electrolyte is beneficial, the 10% of PVA was selected as the optimum concentration to fabricate the gel electrode.

[0139] The PVA also showed the capacity to jellify sulfuric acid and phosphoric acid with the same trends that those observed for TSA electrolyte (thixotropic gel at low PVA contents, and self-standing gel at 10% PVA).

[0140] Example 3 - Anodization characteristics of the gel electrolyte

[0141] The anodizing performance of the PVA concentrations prepared in Example 2 was studied.

[0142] Gel electrolytes with 2%, 5%, 10% and 20% PVA contents were used to anodize a 2024 aluminium alloy substrate. A carbon cloth was used as a counter electrode. The carbon cloth was connected to a power supply (GVD605, GRLFCO). A ramp of 3 minutes was applied until 25 V and this voltage was maintained during 15 min before switching off. After anodizing, the appearance and the thickness of the produced anodic layers were examined. All the compositions showed the ability to anodize the aluminium surface, and the obtained thickness was not affected by the conditions of the gel formulation.

[0143] Then, the effect of the thickness of the self-supported gel electrolyte was studied. Gels with thicknesses of 2, 3 and 4 mm were obtained and were used for anodizing in the same conditions used before. It was observed that the thickness of the gel electrolyte had a negligible influence on the anodic layer thickness. Nevertheless, the homogeneity, measured as a reduction in the error bars, improved as the thickness of the gel electrolyte increased.

[0144] Finally, the morphology of the anodic layer was examined by FE-SEM. A homogeneous anodic layer was observed along the whole sample. Additionally, a significant disordered pore structure, comparing with that produced in conventional anodizing was manifested. This could be beneficial for a posterior painting process as it could improve the bond between the substrate and paint.

[0145] Comparative Example 1

[0146] Anodizing layers obtained by traditional liquid electrolytes and process (comparative Example 1) and the gel electrode of the present disclosure (10% PVA content) were characterized and compared. It was observed that both coatings showed similar characteristics and properties in terms of coating thickness, corrosion behaviour, contact angle, and adhesion of paint.

[0147] Table 1 shows the applied process parameters and the obtained coating thickness with both systems (traditional anodizing and gel electrode anodizing of the present disclosure).

[0148] Table 1

[0149] Coating morphology in cross section is also similar.

[0150] Regarding corrosion behaviour, polarization curves were recorded for both anodizing layers and no significant differences were observed.

[0151] Contact angle and adhesion to a subsequent paint were also characterized and, again, similar results were achieved in both types of coatings.

[0152] Comparative Example 2

[0153] A non-perforated gel electrolyte layer was prepared as in Example 1 but for the use of a mould without pins (Comparative Example 2). Two test aluminium plane substrates were anodized following the process of Example 3, one with the gel electrolyte layer of Example 1 and another one with the gel electrolyte layer of Comparative Example 2.

[0154] The results shown that in the central zone of the surface of the substrate anodized with the non-perforated gel electrolyte layer of Comparative Example 2, no metal oxide layer was formed (see Fig. 3).

[0155] Thus, it can be seen that the intrinsic porosity of the gel electrolyte is not enough for the evacuation of the gases produced in the area of the gel electrolyte layer in contact with the surface to be treated (such as O2 in an anodizing process or H2 in an electrodeposition process).

[0156] Comparative Example 3

[0157] A gel electrolyte layer was prepared by using the gel electrolyte precursor of Example 1 , section 1.1 and subjecting it to a drying process (48 h at 23 °C and 3 h at 60 °C).

[0158] However, there was not possible to demould the perforated gel electrolyte layer since the loss of water prevented the three-dimensional structure from being maintained.

[0159] Example 4 - Rheological tests

[0160] A gel electrode was prepared as disclosed in Example 1 but for the use of a 13% of PVA.

[0161] The effect of freezing-thawing cycles on the behaviour of the PVA electrolyte was characterized by rheological tests. The storage modulus G' (G prime, in Pa) represents the elastic portion of the viscoelastic behaviour, which quasi describes the solid-state behaviour of the sample. The loss modulus G" (G double prime, in Pa) characterizes the viscous portion of the viscoelastic behaviour, which can be seen as the liquid-state behaviour of the sample.

[0162] It was observed that as the number of freezing-thawing cycles increased a more consistent three-dimensional gel structure was obtained. As can be seen from Figs. 4 and 5, the results with the gel electrolyte precursor show that G"> G, what indicates a predominantly viscous (not of a gel) behaviour. Conversely, after at least one freezingthawing cycle the results show that G'> G", what indicates a predominantly elastic behaviour, i.e., a three-dimensional gel structure. Although after one and two freezingthawing cycles there is already a three-dimensional gel-like structure, it is the third freezing-thawing cycle that produces a notable increase in the level of cross-linking of the gel (the jump in G' and G" is considerably higher), making it possible to achieve greater dimensional stability of the gel. This makes it particularly suitable for the preparation of the gel electrode device of the present disclosure compared to gels subjected to one or two freezing-thawing cycles.

[0163] Citation List

[0164] Patent Literature:

[0165] - WO2016193736 A1

[0166] - WO2012126969 A1 Non Patent Literature:

[0167] - R.D. Clarke. "DALIC selective brush plating and anodising". International Journal of Adhesion and Adhesives, 1999, Vol. 19 (2-3), pp. 205-207

[0168] - The SIFCO Process; Sifco Industries Inc. of Cleveland, Ohio, USA - https: / / www.sifcoasc.com / wp-content / uploads / Process-in-Brief.pdf

[0169] - SY Park et al. "A review of the recent developments in surface treatment techniques for bonded repair of aluminium airframe structures

[0170] For reasons of completeness, various aspects of the invention are set out in the following numbered clauses:

[0171] Clause 1. A self-standing gel electrode device for the selective electrochemical or chemical treatment of a metal surface, the gel electrode device comprising or consisting of: a) a gel electrolyte layer comprising:

[0172] - an electrolyte and

[0173] - a physically crosslinked polymer selected from the group consisting of physically crosslinked polyvinyl alcohol, a physically crosslinked cellulose derivative, and a physically crosslinked mixture of polyvinyl alcohol and a cellulose derivative, wherein the gel electrolyte layer comprises perforations; and b) a carbon-based flexible planar structure as a counter electrode in contact with the gel electrolyte layer, the carbon-based flexible planar structure being permeable to the gases formed during the electrochemical or the chemical treatment; and c) optionally, a flexible adhesive film which is permeable to the gases formed during the electrochemical or the chemical treatment, wherein the carbon-based flexible planar structure is arranged on a surface of the gel electrolyte layer, and, when the flexible adhesive film is present, the carbon-based flexible planar structure is arranged between the flexible adhesive film and the gel electrolyte layer, and the flexible adhesive film surpasses the limits of the carbon-based flexible planar structure. Clause 2. The self-standing gel electrode device of clause 1 , wherein the perforations are essentially perpendicular to the surface of the carbon-based flexible planar structure and are arranged in a grid pattern, staggered, randomly distributed along gel electrolyte, or in any other pattern.

[0174] Clause 3. The self-standing gel electrode device of clauses 1 or 2, wherein the perforations have a density from 1 to 16 perforations per cm2.

[0175] Clause 4. The self-standing gel electrode device of any one of clauses 1 to 3, wherein the electrolyte comprises an acid selected from the group consisting of phosphoric acid, sulfuric acid, chromic acid and organic carboxylic acids such as tartaric acid, glycolic acid, lactic acid, oxalic acid, citric acid, malic acid and mixtures of said acids; or wherein the electrolyte comprises nitric acid; or wherein the electrolyte comprises sulfuric acid, phosphoric acid, or a mixture thereof; or therein the electrolyte comprises an inorganic salt of a metal selected from the group consisting of Cr, Ti, Zr and Ce

[0176] Clause 5. The self-standing gel electrode device of any one of clauses 1 to 4, wherein the electrolyte comprises a mixture of tartaric acid and sulfuric acid.

[0177] Clause 6. The self-standing gel electrode device of any one of clauses 1 to 5, wherein the physically crosslinked polymer is physically crosslinked polyvinyl alcohol.

[0178] Clause 7. The self-standing gel electrode device of any one of clauses 1 to 6, wherein the gel electrolyte has a storage modulus G' from 5.000 Pa to 15.000 Pa when measured with measured in a TA Instruments AR2000 rheometer at 25 °C and in the linear viscoelastic region using a plate-plate geometry, and wherein tan 5 = G” / G’ < 1.

[0179] Clause 8. The self-standing gel electrode device of any one of clauses 1 to 7, wherein the carbon-based flexible planar structure is selected from a carbon-based flexible planar fibre, fabric, mesh, or veil.

[0180] Clause 9. Use of the self-standing gel electrode device of any one of clauses 1 to 8 for the selective electrochemical or chemical treatment of a metal surface.

[0181] Clause 10. The use of clause 9, wherein the treatment is an electrochemical treatment selected from the group consisting of anodizing, electropolishing, electrocolouring and electrodepositon.

[0182] Clause 11. The use of clause 10, wherein the treatment is a chemical treatment selected from the group consisting of conversion coatings, passivation, electroless plating and chemical polishing, etching, and cleaning.

[0183] Clause 12. A process for the preparation of the self-standing gel electrode device as defined in clause 1 , the process comprising: a) preparing a gel electrolyte precursor comprising i) an electrolyte and ii) a polymer selected from the group consisting of polyvinyl alcohol, a cellulose derivative in the presence of a crosslinking acid, and a mixture of polyvinyl alcohol and a cellulose derivative; b) pouring the gel electrolyte precursor in a mould having a base having a surface comprising pins in order to form a layer of gel electrolyte precursor having a top surface; c) arranging a carbon-based flexible planar structure on the top surface of the gel electrolyte precursor; d) subjecting the gel electrolyte precursor to a freezing-thawing process in order to physically crosslink the polymer, to obtain an assembly of gel electrolyte and the carbon-based flexible planar structure; and e) demoulding the obtained assembly of gel electrolyte and carbon-based flexible planar structure; f) optionally, disposing a flexible adhesive film on the carbon-based flexible planar structure, wherein flexible adhesive film is permeable to the gases formed during the electrochemical or the chemical treatment; wherein, when the flexible adhesive film is present, the carbon-based flexible planar structure is arranged between the flexible adhesive film and the gel electrolyte, and the flexible adhesive film surpasses the limits of the carbon-based flexible planar structure.

[0184] Clause 13. The process of clause 12, wherein the pins have a density from 1 to 16 pins per cm2.

[0185] Clause 14. The process of clauses 12 or 13, wherein the polymer is polyvinyl alcohol and the freezing-thawing process consists of at least two consecutive cycles of freezingthawing, particularly, at least three consecutive cycles of freezing-thawing.

[0186] Clause 15. The process of any one of clauses 12 to 14, wherein the polymer is carboxymethyl cellulose in the presence of a crosslinking acid.

Claims

Claims1. A self-standing gel electrode device (10) for the selective electrochemical or chemical treatment of a metal surface (15), the gel electrode device comprising or consisting of: a) a gel electrolyte layer (12) comprising:- an electrolyte and- a physically crosslinked polymer selected from the group consisting of physically crosslinked polyvinyl alcohol, a physically crosslinked cellulose derivative, and a physically crosslinked mixture of polyvinyl alcohol and a cellulose derivative; wherein the cellulose derivative is selected from the group consisting of carboxymethylcellulose, hydroxypropylcellulose, hydroxypropyl methyl cellulose, and hydroxyethyl cellulose, wherein the gel electrolyte layer comprises perforations (13); and b) a carbon-based flexible planar structure (11) as a counter electrode in contact with the gel electrolyte layer, the carbon-based flexible planar structure being permeable to the gases formed during the electrochemical or the chemical treatment; and c) optionally, a flexible adhesive film (14) which is permeable to the gases formed during the electrochemical or the chemical treatment, wherein the carbon-based flexible planar structure (11) is arranged on a surface (121) of the gel electrolyte layer (12), and, when the flexible adhesive film (14) is present, the carbon-based flexible planar structure (11) is arranged between the flexible adhesive film (14) and the gel electrolyte layer (12), and the flexible adhesive film (14) surpasses the limits of the carbon-based flexible planar structure (11).

2. The self-standing gel electrode device of claim 1, wherein the perforations (13) are essentially perpendicular to a surface of the carbon-based flexible planar structure (11) and are arranged in a grid pattern, staggered, randomly distributed along gel electrolyte, or in any other pattern.

3. The self-standing gel electrode device of claims 1 or 2, wherein the perforations (13) have a density from 1 to 16 perforations per cm2.

4. The self-standing gel electrode device of any one of claims 1 to 3, wherein the electrolyte comprises an acid is selected from the group consisting of phosphoric acid, sulfuric acid, chromic acid and organic carboxylic acids such as tartaric acid, glycolic acid, lactic acid, oxalic acid, citric acid, malic acid and mixtures of said acids; or wherein the electrolyte comprises nitric acid; or wherein the electrolyte comprises sulfuric acid, phosphoric acid, or a mixture thereof; or therein the electrolyte comprises an inorganic saltof a metal selected from the group consisting of Cr, Ti, Zr and Ce5. The self-standing gel electrode device of any one of claims 1 to 4, wherein the electrolyte comprises a mixture of tartaric acid and sulfuric acid.

6. The self-standing gel electrode device of any one of claims 1 to 5, wherein the physically crosslinked polymer is physically crosslinked polyvinyl alcohol.

7. The self-standing gel electrode device of any one of claims 1 to 6, wherein the gel electrolyte has a storage modulus G' from 5.000 Pa to 15.000 Pa and a tan 5 = G” / G’ < 1 ; wherein G' and G" are measured in a rheometer at 25 °C and in the linear viscoelastic region using a plate-plate geometry in a frequency range from 0.1 to 100 Hz.

8. The self-standing gel electrode device of any one of claims 1 to 7, wherein the carbonbased flexible planar structure (11) is selected from a carbon-based flexible planar fibre, fabric, mesh, or veil.

9. Use of the self-standing gel electrode device of any one of claims 1 to 8 for the selective electrochemical or chemical treatment of a metal surface.

10. The use of claim 9, wherein the treatment is an electrochemical treatment selected from the group consisting of anodizing, electropolishing, electrocolouring and electrodepositon.

11. The use of claim 9, wherein the treatment is a chemical treatment selected from the group consisting of conversion coatings, passivation, electroless plating and chemical polishing, etching, and cleaning.

12. A process for the preparation of the self-standing gel electrode device (10) as defined in claim 1 , the process comprising: a) preparing a gel electrolyte precursor comprising i) an electrolyte and ii) a polymer selected from the group consisting of polyvinyl alcohol, a cellulose derivative in the presence of a crosslinking acid, and a mixture of polyvinyl alcohol and a cellulose derivative; wherein the cellulose derivative is selected from the group consisting of carboxymethylcellulose, hydroxypropylcellulose, hydroxypropyl methyl cellulose, and hydroxyethyl cellulose; b) pouring the gel electrolyte precursor in a mould having a base having a surface comprising pins in order to form a layer of gel electrolyte precursor with perforations, wherein the layer of gel electrolyte precursor has a top surface; c) arranging a carbon-based flexible planar structure on the top surface of the gel electrolyte precursor;d) physically crosslinking the polymer by subjecting the gel electrolyte precursor to a freezing-thawing process, to obtain an assembly of a gel electrolyte layer (12) with perforations (13) and the carbon-based flexible planar structure (11); and e) demoulding the obtained assembly of the gel electrolyte layer (12) and the carbonbased flexible planar structure (11); f) optionally, disposing a flexible adhesive film (14) on the carbon-based flexible planar structure (11), wherein flexible adhesive film is permeable to the gases formed during the electrochemical or the chemical treatment; wherein, when the flexible adhesive film (14) is present, the carbon-based flexible planar structure (11) is arranged between the flexible adhesive film and the gel electrolyte layer (12), and the flexible adhesive film surpasses the limits of the carbon-based flexible planar structure.

13. The process of claim 12, wherein the pins have a density from 1 to 16 pins per cm2.

14. The process of claims 12 or 13, wherein the polymer is polyvinyl alcohol and the freezing-thawing process consists of at least two consecutive cycles of freezing-thawing, particularly, at least three consecutive cycles of freezing-thawing.

15. The process of any one of claims 12 to 14, wherein the polymer is carboxymethyl cellulose in the presence of a crosslinking acid.

Images