ELECTROCHEMICAL METHOD FOR FINISHING METALS OR ALLOYS

Abstract

 The present invention refers to a method for finishing a metal or alloy surface, a finished metal or alloy surface obtainable by said method, an electrolytic cell for carrying out the method for finishing a metal or an alloy surface, and the use of a composition of ethylene glycol and sodium chloride as electrolyte.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention may be encompassed within the field of (electro)cleaning of metals or alloys. More particularly, it relates to a method for finishing metals or alloys.

BACKGROUND OF THE INVENTION

[0002] The surface condition of metals and alloys, particularly stainless steel, plays a critical role in their corrosion resistance.

[0003] Electrochemical finishing has emerged as a superior finishing process for metals and metal alloys, offering a non-destructive alternative to conventional mechanical finishing methods. These processes not only produce a smoother and more reflective surface, but also significantly improve corrosion resistance. This makes them particularly valuable for applications that require high durability and performance, such as biomedical and industrial applications. In stainless steel, for example, electropolishing effectively removes the outermost damaged layer created by previous machining operations and forms a denser, more protective surface. This improves the material's ability to resist corrosion and maintain structural integrity. Traditionally, electropolishing and other finishing processes have relied on concentrated acid-based electrolytes, including phosphoric, sulphuric and perchloric acids, or combinations thereof, tailored to the specific metal or alloy. Despite the advantages offered by acid-based electrolytes in electropolishing and other finishing processes, there are significant environmental and safety concerns associated with their use. This highlights the need for the development of novel, environmentally friendly and safe finishing methods that can achieve superior quality finishes on stainless steel and other metals or metallic alloys without compromising environmental or occupational safety standards.

[0004] Han and Fang (Journal of Manufacturing Processes 58 (2020) 1257-1269) carried out an electropolishing process on stainless steel samples using an ethylene glycol solution containing NaCl as the electrolyte. However, they found a brown product formed near the anode surface with the NaCl-based electrolyte, which degrades the electropolishing effect by reducing the diffusion rates of dissolved metal ions and fresh electrolyte towards the anode surface. After further study, Han and Fang concluded that using a composition of ethanol, ethylene glycol and NaCl as the electrolyte improved the electropolishing effect in the stainless steel.

[0005] It is therefore evident that the development of novel environmentally friendly finishing processes and electrolytes for metal and metal alloys is a necessity.

BRIEF DESCRIPTION OF THE INVENTION

[0006] The surface of metals or alloys can be covered with grease ("dirt") and/or scale (oxide layers) which affect their properties and applications. The inventors of the present invention have developed an environmentally friendly method for finishing a metal or alloy surface comprising a degreasing and descaling step that avoids hydrogen embrittlement and that enhances the surface reflectance.

[0007] Thus, a first aspect of the invention is directed to a method for finishing a metal or alloy surface comprising the following steps:

(i) providing:

an electrolytic cell comprising:

• at least two electrodes; wherein one of the at least two electrodes comprises the metal or alloy surface;
• an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and
• means for connecting a power source;

(ii) degreasing and descaling the metal or alloy surface by:

submerging the metal or alloy surface partially or totally in the electrolyte of the cell, and

applying a current density of at least 5 A/dm² at a temperature of at least 20°C for at least 5 minutes;

wherein during the decreasing and descaling step the polarity of the cell changes at least twice;

to obtain a degreased and descaled metal or alloy surface.

[0008] In a second aspect, the invention is directed to a finished metal or alloy surface obtainable by the method for finishing a metal or alloy surface of the invention or any of its particular embodiments.

[0009] In another aspect, the invention is directed to a an electrolytic cell for carrying out the method for finishing a metal or an alloy surface of the invention or any of its particular embodiments; adapted for containing a composition consisting essentially of ethylene glycol and sodium chloride.

[0010] In another aspect, the invention is directed to the use of a composition consisting essentially of ethylene glycol and sodium chloride, as electrolyte for electrochemically processing a metal or an alloy surface.

[0011] These aspects and preferred embodiments thereof are additionally also defined hereinafter in the detailed description and in the claims.

[0012] All the features described in this specification (including the claims, description and drawings) can be combined in any combination, with the exception of combinations of such mutually exclusive features.

BRIEF DESCRIPTION OF THE FIGURES

[0013] To better understand the invention, its objects and advantages, the following figures are attached to the specification in which the following is depicted:

Figure 1 shows the results of anodic polarisation curves showing current density values (A/dm²) at different potentials (V) obtained at different temperatures (70°C, 60°C and 50°C) for a stainless steel surface in a cell using a composition consisting essentially of ethylene glycol and sodium chloride as electrolyte according to example 1.

Figure 2 shows the results of anodic polarisation curves showing current density values (A/dm²) at different potentials (V) obtained at different temperatures (70°C, 60°C and 50°C) for a stainless steel surface in a cell using an acid bath as electrolyte for comparison purposes in example 1.

Figure 3 shows the results (a) before and (b) after the lower part of a stainless steel surface has been degreased and descaled in one step according to example 1.

Figure 4 shows the results obtained after subjecting the upper part of a stainless steel surface to a degreasing and descaling step, followed by an electropolishing step according to example 1.

Figure 5 shows the results obtained after the lower part of a stainless steel surface underwent a degreasing and descaling step followed by an electropolishing step according to example 1.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

[0014] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. As used herein, the singular forms "a" "an" and "the" include plural reference unless the context clearly dictates otherwise. It will be further understood that the terms "comprises" and/or "comprising", when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, and/or components thereof. The term "comprises" encompasses the terms "consisting essentially of" and "consisting of". The term "consisting essentially of" regarding a composition or composition of compounds is interpreted to mean that additional components may be present, provided that they do not significantly alter the fundamental characteristics of the composition. Preferably, the term "consisting essentially of" means unspecified compounds or components may be present up to 5%, 4%, 3%, 2%, 1% or 0.5% by weight based on total weight of the composition.

[0015] As used herein, the term "and/or" is meant to include any combination of elements as well as the alternative inclusion of one or more elements.

[0016] As used herein, the term "approximately" or "about" as applied to one or more values of interest, refers to a value that is similar to a stated reference value. In certain embodiments, the term "approximately" or "about" refers to a value that can vary up to ± 20 %, preferably within ± 10 %, and more preferably within ± 5 % of the stated reference value. When "approximately" or "about" is used before a numerical range, it applies to the upper and lower range end-points.

[0017] Indeed, the skilled person knows that numerical values relating to measurements are subject to measurement errors which place limits on their accuracy. Where terms such as "about" or "approximately" are applied to a particular value (e.g., "about 200 °C" or "approximately 200 °C") or to a range (e.g., "about x to approximately y"), the value or range may be interpreted as being as accurate as the method used to measure it. Unless explicitly stated otherwise, the general convention in the scientific and technical literature may be applied so that the last digit of numerical values preferably indicates the precision of measurement. Thus, unless other error margins are given, the maximum margin is preferably ascertained by applying the rounding-off convention to the last decimal place. For instance, a value of 3.5 preferably has an error margin of 3.45 to 3.54 and a range of 2% to 10% preferably covers a range of 1.5% to 10.4%. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. Further, to provide a more concise description, some of the quantitative expressions given herein are not qualified with the term "about". It is understood that, whether the term "about" is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including equivalents and approximations due to the experimental and/or measurement conditions for such given value.

[0018] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. As an illustration, a numerical range of "about 1 % to about 5 %" should be interpreted to include not only the explicitly recited values of about 1 % to about 5 %, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3 and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting only one numerical value. It should also be understood that ranges formed by combination of any of the end points of different disclosed ranges and/or particular values therein are included in the present disclosure.

Method for finishing a metal or alloy surface

[0019] In the field of the present invention, the term "finishing" refers to the set of processing operations applied to a metal or alloy surface to improve its appearance, handle, properties, and possible applications. Some desired properties and characteristics to be achieved may include improved durability, aesthetics (e.g. a certain roughness or glossiness), solderability, and chemical resistance. The method for finishing a metal or alloy surface proposed by the present invention is characterized by comprising a degreasing and descaling step that avoids hydrogen embrittlement and that enhances the surface reflectance.

[0020] In an embodiment, the method for finishing a metal or alloy surface comprises the following steps:

(i) providing:

an electrolytic cell comprising:

• at least two electrodes; wherein one of the at least two electrodes comprises the metal or alloy surface;
• an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and
• means for connecting a power source;

(ii) degreasing and descaling the metal or alloy surface by: submerging the metal or alloy surface partially or totally in the electrolyte of the cell, and applying a current density of at least 5 A/dm² at a temperature of at least 20°C for at least 5 minutes;

wherein during the decreasing and descaling step (ii) the polarity of the cell changes at least twice;

to obtain a degreased and descaled metal or alloy surface;

(iii) optionally, electropolishing the degreased and descaled metal or alloy surface of step (ii) by applying a current density of at least 10 A/dm² at a temperature of at least 30°C for at least 1 minute, to obtain an electropolished, degreased and descaled metal or alloy surface; preferably, wherein during step (iii) the electrode comprising the metal or alloy surface is an anode;

(iv) optionally, passivating the degreased and descaled metal or alloy surface of step (ii) or the electropolished, degreased and descaled metal or alloy surface of step (iii); and

(v) optionally, recycling the electrolyte after any of the steps (ii) to (iv).

[0021] The method of finishing a metal or alloy surface of the invention may include further steps before, after or in between the steps mentioned.

[0022] In an embodiment, the method for finishing a metal or alloy surface consists of the following steps:

(i) providing:

an electrolytic cell comprising:

• at least two electrodes; wherein one of the at least two electrodes comprises the metal or alloy surface;
• an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and
• means for connecting a power source;

(ii) degreasing and descaling the metal or alloy surface by: submerging the metal or alloy surface partially or totally in the electrolyte of the cell, and applying a current density of at least 5 A/dm² at a temperature of at least 20°C for at least 5 minutes, followed by an optional rinsing step;

wherein during the decreasing and descaling step (ii) the polarity of the cell changes at least twice;

to obtain a degreased and descaled metal or alloy surface;

(iii) optionally, electropolishing the degreased and descaled metal or alloy surface of step (ii) by applying a current density of at least 10 A/dm² at a temperature of at least 30°C for at least 1 minute, to obtain an electropolished, degreased and descaled metal or alloy surface, followed by an optional rinsing step; preferably, wherein during step (iii) the electrode comprising the metal or alloy surface is an anode;

(iv) optionally, passivating the degreased and descaled metal or alloy surface of step (ii) or the electropolished, degreased and descaled metal or alloy surface of step (iii); and

(v) optionally, recycling the electrolyte after any of the steps (ii) to (iv).

Step (i)

[0023] The first step of the method for finishing a metal or alloy surface is providing:
an electrolytic cell comprising:

• at least two electrodes; wherein one of the at least two electrodes comprises the metal or alloy surface;
• an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and
• means for connecting a power source.

[0024] In the context of the present invention the term "electrolytic cell" is understood as known in the art, for example as any device in which electrical energy is converted to chemical energy, or vice versa. Such an electrolytic cell typically comprises at least two electrodes held apart from each other, and in contact with an electrolyte.

[0025] The metal or alloy surface to be finished or treated, which may be also referred to as working surface, is a surface of a piece (working piece). The metal or alloy surface may have any shape; preferably is a substantially flat surface. The metal or alloy surface may have soldered (welded) parts.

[0026] The electrode comprising the metal or alloy surface to be finished or treated is normally referred to as working electrode. That is, the piece which metal or alloy surface is to be finished or treated is or acts as the working electrode of the electrolytic cell.

[0027] The metal or alloy surface may be totally or partially made of any metal or alloy known in the art. In a particular embodiment, the metal or alloy surface is totally or partially made of a metal or alloy selected from: stainless steel, titanium, titanium-based alloy, aluminium, aluminium-based alloy and compositions thereof; preferably is made of stainless steel, more preferably of austenitic stainless steel, much more preferably of austenitic chromium-nickel stainless steel, even much more preferably of austenitic stainless steel of grade 304.

[0028] In an embodiment, the at least two electrodes of the electrolytic cell are two electrodes, namely a working electrode and a counter electrode. Each electrode may become either the anode or the cathode depending on the voltage applied to the cell.

[0029] In an embodiment, the at least two electrodes of the electrolytic cell are three electrodes, preferably a working electrode, a counter electrode, and a reference electrode; more preferably a working electrode, a counter electrode, and a Ag/AgCl reference electrode.

[0030] In an embodiment, the electrolytic cell comprises a plurality of electrodes, e.g. an array of multiple working electrodes in combination with one or more counter electrodes and optionally one or more reference electrodes.

[0031] In an embodiment, the electrolyte is a composition comprising more than 95%, 96%, 97%, 98%, 99% or 99.5% by weight of ethylene glycol and sodium chloride. Preferably, the electrolyte is free of other solvents such as glycerol, methanol, and/or ethanol. In a more particular embodiment, the electrolyte is a composition consisting of ethylene glycol and sodium chloride. In a particular embodiment, the ethylene glycol and sodium chloride are in the composition in a molar ratio of ethylene glycol: sodium chloride of between 15:1 and 21:1; preferably of between 16:1 and 20:1; more preferably of between 17:1 and 19:1; much more preferably between 17.5:1 and 18.5:1; even much more preferably of about 18:1.

[0032] The electrolytic cell comprises means for connecting a power source; preferably connection means configured for electrically connecting the at least two electrodes to a power source; such as electric wires, clips or brackets. In particular, the connection means are configured for electrically connecting independently each of the at least two electrodes to a power source; preferably with a direct current (DC) power source. Preferably, the means for connecting a power source (connection means) are not in contact with the electrolyte. In a particular embodiment, the connection means are configured for switching the polarity of the cell; preferably by changing the electrical connection of the electrodes to a power source.

[0033] In an embodiment, the electrolytic cell comprises a power source; preferably a direct current (DC) power source.

[0034] In an embodiment, the electrolytic cell is adapted for containing a liquid such as an electrolyte solution; preferably a composition consisting essentially of ethylene glycol and sodium chloride. In an embodiment, the electrolytic cell comprises a holding frame; preferably a holding frame adapted for containing a composition consisting essentially of ethylene glycol and sodium chloride. In an embodiment, the holding frame is made of a non-conducting material. In another particular embodiment, the electrolytic cell further comprises an external case.

Step (ii)

[0035] The second step of the method for finishing a metal or alloy surface is degreasing and descaling the metal or alloy surface by: submerging the metal or alloy surface partially or totally in the electrolyte of the cell, and applying a current density of at least 5 A/dm² at a temperature of at least 20°C for at least 5 minutes; wherein during the decreasing and descaling step the polarity of the cell changes at least twice; to obtain a degreased and descaled metal or alloy surface.

[0036] In step (ii), degreasing and descaling are processes that remove grease ("dirt") and scale (oxide layers), respectively, from the metal or alloy surface. As the skilled person will appreciate, descaling only occurs if the surface has actually oxide layers over it.

[0037] In the context of the present invention the expression "current density" is understood as electric current density as known in the art, as the amount of electric current traveling per unit cross-section area. In an embodiment, the current density is applied between the electrodes of the cell.

[0038] In a particular embodiment, the metal or alloy surface is totally submerged in the electrolyte of the cell during step (ii).

[0039] In an embodiment, the current density applied in step (ii) is at least 6 A/dm²; preferably at least 7 A/dm²; more preferably at least 8 A/dm²; much more preferably at least 9 A/dm². In an embodiment, the current density applied in step (ii) is between 5 and 20 A/dm²; preferably 6 and 16 A/dm²; more preferably between 8 and 12 A/dm²; even much more preferably of about 10 A/dm².

[0040] In an embodiment, the current density is applied in step (ii) for at least 5 minutes; preferably at least 10 minutes; more preferably at least 15 minutes. In an embodiment, the current density is applied in step (ii) for between 16 and 26 minutes; preferably between 18 and 24 minutes; more preferably for between 19 and 22 minutes; more preferably for about 20 minutes.

[0041] In an embodiment, step (ii) is performed at a temperature of at least 25°C; preferably at least 30°C; more preferably at least 35°C; even much more preferably at least 40°C. In an embodiment, step (ii) is performed at a temperature of between 40 and 60°C; preferably of between 48 and 58°C; preferably of between 45 and 55°C; more preferably of between 43 and 53°C; even much more preferably of about 50°C.

[0042] The polarity of the cell changes at least two times during step (ii), preferably during the application of the current density. In a particular embodiment, during step (ii) the polarity of the cell changes at least three times; preferably at least five times; more preferably at least 8 times; even more preferably about 10 times.

[0043] In a particular embodiment, during step (ii) the polarity of the cell changes every 1.5 to 2.5 minutes; preferably every 2 minutes.

[0044] In a particular embodiment, the polarity of the cell changes between 1 and 20 times during step (ii); preferably between 5 and 15 times; more preferably about 10 times.

[0045] In an embodiment, step (ii) comprises applying a current density of between 5 and 20 A/dm² at a temperature of between 40 and 60°C for between 16 and 26 minutes; wherein during this step the polarity of the cell changes every 1 to 3 minutes; preferably comprises applying a current density of about 10 A/dm² at a temperature of about 50°C for about 20 minutes; wherein during this step the polarity of the cell changes every 2 minutes.

[0046] The polarity of the electrolytic cell is reversed or inverted at a certain frequency during step (ii) forming a sequence of polarities. In other words, during the decreasing and descaling step the polarity of the cell changes at least twice following a sequence. In a more particular embodiment, the first step of the sequence is a cathodic polarization step.

[0047] In an embodiment, the sequence of polarities ends in a cathodic polarization step or in an anodic polarization step.

[0048] In an embodiment, each step of the sequence of polarities lasts for the same or a different amount of time; preferably for the same amount of time.

[0049] In an embodiment, each step of the sequence of polarities lasts for between 1 to 3 minutes; preferably for between 1.5 to 2.5 minutes; more preferably for about 2 minutes.

[0050] In the context of the present invention, the term 'polarity' as applied to an electrolytic cell is understood as known in the art, particularly it refers to the direction of the electron current, in particular through the circuit of the cell. The polarity of the cell may be a cathodic polarization or an anodic polarization. In the context of the present invention, the term 'anodic polarization' as applied to an electrolytic cell means that the electrode comprising the metal or alloy surface, is connected to the positive terminal of the power source as known in the art. In the context of the present invention, the term 'cathodic polarization' as applied to an electrolytic cell means that the electrode comprising the metal or alloy surface, is connected to the negative terminal of the power source as known in the art.

[0051] The authors have observed that different finishes for the degreased and descaled metal or alloy surface may be obtained by changing the polarity of the last step of the polarity sequence of steps of step (ii) of the method for finishing a metal or alloy surface. For example, when the polarity sequence of steps ends in a cathodic polarization step a blackish finishing of the surface of the metal or alloy surface is obtained. When the polarity sequence of steps ends with an anodic polarization step, a bright finishing of the surface is obtained. A non-limiting example of a polarity sequence of steps is a first step of cathodic polarization, then a step of anodic polarization, then another step of cathodic polarization, etc.

[0052] The authors have observed that during the cathodic polarization step of the polarity changes of step (ii), glycoxide anions and hydrogen gas are generated at the surface of metal or alloy surface due to a reduction reaction: 2HOC₂H₄OH + 2e^- → 2HOC₂H₄O^-+ H₂. Surprisingly it was observed that the glycoxide anions were able to degrease the surface of the metal or alloy while the generated hydrogen gas was able to descale it if any oxide layers are present. However, hydrogen atoms are adsorbed at the metal or alloy surface during this period. After switching the polarity of the cell to an anodic polarisation step, the hydrogen atoms adsorbed on the surface of the surface during the previous cathodic polarisation period are oxidised, thus avoiding the diffusion of these hydrogen atoms into the metal or alloy piece, which could cause hydrogen embrittlement.

Step (iii)

[0053] The third step, optional, of the method for finishing a metal or alloy surface is electropolishing the degreased and descaled metal or alloy surface by applying a current density of at least 10 A/dm² at a temperature of at least 30°C for at least 1 minute; preferably by applying a current density of between 20 and 40 A/dm² to the cell at a temperature of between 60 and 80°C for a period of time of between 1 and 5 minutes; more preferably, wherein during step (iii) the electrode comprising the metal or alloy surface is an anode.

[0054] In step (iii), the term "electropolishing" is understood as an anodic dissolution process that removes material from the metal or alloy surface, reducing the surface roughness and improving the surface finish.

[0055] In an embodiment, during step (iii) the metal or alloy surface is partially or totally submerged in the electrolyte of the cell.

[0056] In an embodiment, the current density applied in step (iii) is at least 12 A/dm²; preferably at least 15 A/dm²; more preferably at least 20 A/dm²; much more preferably at least 22 A/dm²; even much more preferably of about 25 A/dm². In an embodiment, the current density applied in step (iii) is between 15 and 35 A/dm²; preferably 20 and 30 A/dm²; more preferably between 22 and 27 A/dm²; even much more preferably of about 25 A/dm².

[0057] In an embodiment, the current density is applied in step (iii) for at least 1 minute; preferably at least 2 minutes; more preferably about 3 minutes. In an embodiment, the current density is applied in step (iii) for between 2 and 4 minutes; preferably about 3 minutes.

[0058] In an embodiment, step (iii) is performed at a temperature of at least 30°C; preferably at least 40°C; more preferably at least 50°C; much more preferably at least 60; even much more preferably of about 70°C. In an embodiment, step (iii) is performed at a temperature of between 55 and 85°C; preferably of between 60 and 80°C; more preferably of between 65 and 75°C; even much more preferably of about 70°C.

[0059] In an embodiment, during step (iii) the polarity of the electrodes of the electrolytic cell does not change. In an embodiment, during step (iii) the polarity of the electrodes of the electrolytic cell is an anodic polarization. In particular embodiment, during step (iii) the electrode comprising the metal or alloy surface is or acts as an anode.

Passivating step

[0060] The fourth step, optional, of the method for finishing a metal or alloy surface is a passivating step. In an embodiment, during step (iv) the metal or alloy surface is partially or totally submerged in the electrolyte of the cell.

[0061] In step (iii), the term "passivation" is understood as a process that forms a thin protective layer of metal oxide on the metal or alloy surface with the aim of increasing corrosion resistance.

[0062] In an embodiment, the current density applied in the passivating step is at least 5 A/dm²; preferably at least 8 A/dm²; more preferably at least 10 A/dm²; much more preferably at least 12 A/dm²; even much more preferably of about 15 A/dm². In an embodiment, the current density applied in the passivating step is between 5 and 30 A/dm²; preferably 7 and 22 A/dm²; more preferably between 10 and 20 A/dm²; even much more preferably of about 15 A/dm².

[0063] In an embodiment, the current density is applied in the passivating step for at least 1 minute; preferably for at least 2 minutes. In an embodiment, the current density is applied in the passivating step for between 1 and 6 minutes; preferably between 2 and 3 minutes.

[0064] In an embodiment, the passivating step is performed at a temperature of at least 30°C; preferably at least 40°C; more preferably at least 50°C; much more preferably at least 60°C; even much more preferably of about 70°C. In an embodiment, the passivating step is performed at a temperature of between 55 and 85°C; preferably of between 60 and 80°C; more preferably of between 65 and 75°C; even much more preferably of about 70°C.

[0065] In an embodiment, during the passivating step the polarity of the electrodes of the electrolytic cell does not change. In an embodiment, during the passivating step the polarity of the electrodes of the electrolytic cell is an anodic polarization. In a particular embodiment, during the passivating step the electrode comprising the metal or alloy surface is or acts as an anode.

Rinsing step(s)

[0066] The method of the present invention may comprise one or more rinsing steps of the metal or alloy surface after either the degreasing and descaling step (step ii), the optional electropolishing step (step iii) and/or the optional passivating step (step (iv)). During the optional rising step(s) the metal or alloy surface is preferably rinsed with a water or aqueous solution at least once by any method known in the art.

Recycling step

[0067] In an embodiment, the method of the present invention comprises a further step of recycling the electrolyte. In an embodiment, the recycling step comprises the following steps: (i) precipitating the residual particles in the electrolyte that have been generated during any of the previous steps, and (ii) removing said residual particles by for example, decanting or filtering.

A finished metal or alloy surface

[0068] An aspect of the invention is directed to a finished metal or alloy surface obtainable by the method for finishing a metal or alloy surface of the invention or any of its particular embodiments.

[0069] In an embodiment, the finished metal or alloy surface is at least part of the surface of a piece that may or may not be made of a metal or of an alloy.

[0070] The metal or alloy surface may be totally or partially made of any metal or alloy known in the art. In a particular embodiment, the metal or alloy surface is totally or partially made of a metal or alloy selected from: stainless steel, titanium, titanium-based alloy, aluminium, aluminium-based alloy and mixtures thereof; preferably is made of stainless steel, more preferably of austenitic stainless steel, much more preferably of austenitic chromium-nickel stainless steel, even much more preferably of austenitic stainless steel of grade 304.

[0071] In an embodiment, the finished metal or alloy surface has a surface roughness value Ra of between about 5 and about 200 nm; preferably between about 8 and about 120 nm; more preferably of between about 10 and about 100 nm. The Ra roughness value was measured and calculated as known in the art, for example it is calculated as the arithmetic mean of the absolute values of the surface height deviations measured from the mean line of a profile, within a specified evaluation length, using a mechanical profilometer preferably with an about 2 nm probe tip diameter and about a 8 nm profile resolution.

[0072] In an embodiment, the finished metal or alloy surface has a surface glossiness of between about 60 and about 200 Gloss Units (GU); preferably between about 70 and about 150 Gloss Units (GU); more preferably of between about 80 and about 130 Gloss Units (GU). The surface glossiness has been measured as known in the art, for example by directing a constant intensity light beam, at a fixed angle, onto the surface and then measuring the amount of reflected light from the same angle using a glossmeter; preferably wherein the angle was 20° and the glossmeter was previously calibrated against a metal standard.

Cell

[0073] Another aspect of the invention is directed to an electrolytic cell for carrying out the method for finishing a metal or an alloy surface of the invention or any of its particular embodiments; adapted for containing a composition or electrolyte solution consisting essentially of ethylene glycol and sodium chloride. In a particular embodiment, the cell contains a composition consisting essentially of ethylene glycol and sodium chloride.

[0074] In a particular embodiment, the cell is an electrolytic cell comprising:

• at least two electrodes; wherein one of the electrodes comprises a metal or alloy surface;
• an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and
• means for connecting a power source.

[0075] In an embodiment, the at least two electrodes of the electrolytic cell are two electrodes such as an electrode and a counter electrode, or an anode and a cathode. In an embodiment, the at least two electrodes of the electrolytic cell are three electrodes, preferably a working electrode, a counter electrode, and a reference electrode; more preferably a working electrode, a counter electrode, and an Ag/AgCl reference electrode.

[0076] In an embodiment, the electrolytic cell comprises a plurality of electrodes, e.g. an array of multiple working electrodes in combination with one or more counter electrodes and optionally one or more reference electrodes.

[0077] In a particular embodiment, the electrolytic cell comprises means for switching its polarity. In another particular embodiment, the electrolytic cell is adapted for switching its polarity.

[0078] The electrolytic cell comprises means for connecting a power source; preferably connection means configured for electrically connecting the at least two electrodes to a power or load source; such as electric wires, clips or brackets. In particular, the connection means are configured for electrically connecting independently each of the at least two electrodes to a power source; preferably with a direct current (DC) power source. Preferably, the means for connecting a power source (connection means) are not in contact with the electrolyte. In particular, the connection means are adapted for switching the polarity of the electrolytic cell.

[0079] In an embodiment, the electrolytic cell comprises a power source; preferably a direct current (DC) power source.

[0080] In an embodiment, the electrolytic cell is adapted for containing a liquid such as an electrolyte solution; preferably a composition consisting essentially of ethylene glycol and sodium chloride. In an embodiment, the electrolytic cell comprises a holding frame; preferably a holding frame adapted for containing a composition consisting essentially of ethylene glycol and sodium chloride. In an embodiment, the holding frame is made of a non-conducting material. In another particular embodiment, the electrolytic cell further comprises an external case.

[0081] In an embodiment, the cell further comprises means for controlling the temperature (e.g. heating means such as a resistive heater), in particular the temperature of the electrolyte.

Uses

[0082] Another aspect of the invention is directed to the use of a composition consisting essentially of ethylene glycol and sodium chloride, as electrolyte for electrochemically processing a metal or an alloy surface; preferably wherein the ethylene glycol and sodium chloride are in a molar ratio of between 15:1 and 21:1; more preferably of between 16:1 and 20:1 or between 17:1 and 19:1; much more preferably between 17.5:1 and 18.5:1; even much more preferably of about 18:1.

[0083] In an embodiment, the composition comprises more than 95%, 96%, 97%, 98%, 99% or 99.5% by weight of ethylene glycol and sodium chloride. Preferably, the composition does not comprise other solvents such as glycerol, methanol, and/or ethanol. In a more particular embodiment, the electrolyte is a composition that consists of ethylene glycol and sodium chloride; preferably wherein the ethylene glycol and sodium chloride are in a molar ratio of between 15:1 and 21:1; more preferably of between 16:1 and 20:1 or between 17:1 and 19:1; much more preferably between 17.5:1 and 18.5:1; even much more preferably of about 18:1.

[0084] A particular embodiment is directed to the use of a composition consisting essentially of ethylene glycol and sodium chloride, as electrolyte for finishing a metal or an alloy surface; preferably for degreasing and/or descaling a metal or an alloy surface; more preferably for degreasing and descaling a metal or an alloy surface; even much more preferably for degreasing and descaling a stainless steel surface.

[0085] The following examples are merely illustrative of certain embodiments of the invention and cannot be considered as restricting it in any way.

EXAMPLES

Example 1. Method for finishing a stainless steel surface.

[0086] A method for finishing a metal or alloy surface was applied to the surface of a stainless steel piece. Rectangular, flat and optionally welded pieces of an austenitic chromium-nickel stainless steel, AISI 304 SS, were used in the tests of the present example.

[0087] The finishing method was performed on an electrolytic cell wherein the piece with the surface acted as a working electrode. The counter electrode employed was a mesh of Ti/MMOx by Nora^®. A composition consisting essentially of ethylene glycol and sodium chloride in a molar ratio of 18:1 respectively, was used as electrolyte. Said composition had a conductivity of about 5 mS/cm at 22°C. Part of the piece was immersed in the electrolyte. The auxiliary electrode (counter electrode) was as close as possible to the working electrode to minimise the ohmic drop. Electrical contact between electrodes was avoided. In addition, some electrolyte movement was provided to facilitate gas evacuation.

[0088] First, two anodic polarization tests were performed using the cell described above but using a traditional three-electrode cell set up comprising a reference electrode of Ag/AgCl. Each test was done using either (a) a composition of ethylene glycol and sodium chloride or (b) an acid bath as electrolyte (comparative).

[0089] Anodic polarisation curves showing current density values (A/dm²) at different potentials (V), were obtained at different temperatures (70°C, 60°C and 50°C) for an AISI 304 SS surface in the cell described above having a composition of ethylene glycol and sodium chloride in a molar ratio of 18:1 as electrolyte. Results are depicted in Fig. 1.

[0090] A similar experiment was performed but using an acid bath as electrolyte for comparative purposes. The acid bath was an aqueous acid solution having the following amounts in mol/kg: 5.6 of H₃PO₄, 1.4 of H₂SO₄ and 7.2 of H₂O. Fig. 2 shows the results of the polarisation curves at different temperatures (70°C, 60°C and 50°C) of this comparative test.

[0091] The results of the curves in Fig. 1 show no clear passivity domain. The curve with the data collected at 70°C shows a slight current density decrease at potential of about 3V, which can be interpreted as the lower limit of the transpassive domain. Thus, current densities values in the 20-30 A/dm² range are considered suitable for electropolishing stainless steel when using a composition of ethylene glycol and sodium chloride in a molar ratio of 18:1.

[0092] When using an acid bath, the results of Figure 2 showed that the transpassive domain corresponds to potentials higher than 2 V, and the suitable current density values for electropolishing stainless steel under these conditions lie in the 35-40 A/dm² range.

[0093] Thus, advantageously, the use of a composition of ethylene glycol and sodium chloride as electrolyte results in a more environmentally friendly method of finishing stainless steel than the use of acid baths. In addition, the amount of energy required for these methods is similar.

[0094] An example of a surface finishing method was performed on the cell described above. A composition of ethylene glycol and sodium chloride in a molar ratio of 18:1 was used as electrolyte.

[0095] The method was as follows:

• A first step (A) was a degreasing and descaling treatment of the surface of a rectangular, flat piece of AISI 304 SS having a soldered part. This step lasted about 20 minutes in total and was performed at a current density of about 10 A/dm² and at a temperature of about 50°C.

  During said step the polarity of the electrodes was reversed at a frequency of about every two minutes.

  The sequence started with a negative current period (cathodic polarization period). During the cathodic polarization period, glycoxide anions and hydrogen gas are generated at the surface of the stainless steel piece due to the reduction reaction: 2HOC₂H₄OH + 2e^- → 2HOC₂H₄O^- + H₂.

  Surprisingly, it was observed that the glycoxide anions were able to degrease the stainless steel piece while the generated hydrogen gas was able to descaling its surface. After switching the polarity of the cell, an anodic polarization period was produced.

  The cathodic polarization period and the anodic polarization period lasted about 2 minutes each. Thus, during this step the polarity was switched about 10 times. Step (A) might end with a cathodic polarization for a blackish finishing or with an anodic polarization for a bright finishing of the surface of the piece.

  Figure 3 shows the results of the bottom part of a piece of AISI 304 SS (a) before and (b) after having been degreased and descaled. The current ended as anodic, thus, the piece of Figure 3 shows a bright finishing on its treated surface (bottom part).

  Results showed that the use of reversed polarity periods during the degreasing and descaling step has several unexpected advantages. During the negative current periods (cathodic polarization periods), the surface was degreased and descaled. However, during this period hydrogen atoms are also adsorbed at the surface of the stainless steel piece. Then, during the positive current periods (anodic polarization periods), those hydrogen atoms that were adsorbed at the stainless steel piece surface during the previous cathodic polarization period, are oxidized. Thus, avoiding the diffusion of said hydrogen atoms into the stainless steel piece that might produce hydrogen embrittlement.

• Step (B) was an electropolishing treatment of the surface of the piece of AISI 304 SS. This step was performed by switching the polarity of the cell to a positive current (anodic polarization) and increasing the current density compared to the previous step. During this step the metal or metals of the surface of the stainless steel piece are oxidized.

  Fig. 4 shows the results obtained after performing the surface finishing method described above (steps (A) and (B)) on the upper part of the surface of a piece of AISI 304 SS. Said sample was electropolished (step (B)) at a density current of 25 A/dm² for 3 minutes and at a temperature of about 70°C. As can be seen from Fig. 4, the untreated lower part of said piece retained its original appearance and it is showed for comparison purposes.

  The results show that the surface finishing method described above (steps (A) and (B)) reduces the surface roughness parameter Ra of the stainless steel piece by 35% (from 148 nm to 97 nm) and increases its glossiness by about 27 % (from 92 Gloss Units (GU) to 117 GU). The Ra roughness values were measured and calculated as the arithmetic mean of the absolute values of the surface height deviations measured from the mean line of a profile, within a specified evaluation length (4.5 mm) using a mechanical profilometer (2 nm probe tip diameter and 8 nm profile resolution).The glossiness was measured at a 20° incident angle using a gloss meter calibrated against a metal standard.

  Figure 5 shows the results obtained after the bottom part of a piece of AISI 304 SS underwent the surface finishing method described above (steps (A) and (B)). Please note that the upper part of this piece has not been treated for comparison purposes.

  It has been observed that the presence of ethylene glycol in the surface of the pieces treated by the surface finishing method described above is advantageous. In particular it stabilizes the surface by slowing down its passivation process in contact to air, which enhances the surface reflectance.

• An additional optional passivation step (C) may be performed using the cell described above at anodic polarization at 70°C and 15 A/dm² for between 2 and 3 minutes.

[0096] An additional advantage of this process is that the electrolyte formed by a composition of ethylene glycol and sodium chloride, can be recycled and reused after allowing the composition to stand for several hours to precipitate the suspended particles.

Example 2. Method for surface finishing a titanium piece or an aluminium piece.

[0097] A similar method as the one described on example 1 is performed on titanium, and aluminium pieces. Similar results are obtained at room temperature and lower current densities (depending on the metal) but for longer treatment times.

Example 3. Electrolyte for the method for surface finishing.

[0098] Compositions of ethylene glycol (ETG) and sodium chloride with different molar ratios have been prepared as electrolytes for the method of example 1. Their conductivity was measured at about 28°C. Results are shown in the table 1 below. All the compositions were considered suitable for the method of example 1.

Table 1

Composition	1	2	3	4	5
Molar ratio (ETG/NaCl)	16:1	17:1	18:1	19:1	20:1
Mass ratio (ETG/NaCl)	17:1	18:1	19.1:1	20.1:1	21.1:1
Conductivity (mS/cm)	4.93	4.76	4.61	4.52	4.32

Claims

1. A method for finishing a metal or alloy surface comprising the following steps:

(i) providing:
an electrolytic cell comprising:

- at least two electrodes; wherein one of the at least two electrodes comprises the metal or alloy surface;

- an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and

- means for connecting a power source;

(ii) degreasing and descaling the metal or alloy surface by:

submerging the metal or alloy surface partially or totally in the electrolyte of the cell, and

applying a current density of at least 5 A/dm² at a temperature of at least 20°C for at least 5 minutes;

wherein during the decreasing and descaling step the polarity of the cell changes at least twice;

to obtain a degreased and descaled metal or alloy surface.

2. The method according to claim 1, further comprising a step (iii) directed to electropolishing the degreased and descaled metal or alloy surface by applying a current density of at least 10 A/dm² at a temperature of at least 30°C for at least 1 minute; preferably, wherein during the electropolishing step (iii) the electrode comprising the metal or alloy surface is an anode.

3. The method according to claims 1 or 2, wherein the ethylene glycol and sodium chloride are in the composition in a molar ratio of between 15:1 and 21:1.

4. The method according to any one of claims 1 to 3, wherein the metal or alloy surface is totally or partially made of stainless steel, titanium, titanium-based alloy, aluminium or an aluminium-based alloy and compositions thereof.

5. The method according to any one of claims 1 to 4, wherein step (ii) comprises applying a current density of between 5 and 20 A/dm² at a temperature of between 40 and 60°C for between 16 and 26 minutes.

6. The method according to any one of claims 1 to 5, wherein step (ii) comprises changing the polarity of the cell at least three times.

7. The method according to any one of claims 2 to 6, wherein step (iii) comprises applying a current density of between 15 and 40 A/dm² to the cell at a temperature of between 50 and 90°C for a period of time of between 1 and 5 minutes.

8. The method according to any one of claims 1 to 7, further comprising a passivation step.

9. The method according to any one of claims 1 to 8, further comprising a step of recycling the electrolyte.

10. The method according to claim 9, wherein the step of recycling the electrolyte comprises the following sub steps: (i) precipitating the residual particles of the electrolyte that have been generated during any of the previous steps, and (ii) removing said residual particles.

11. The method according to any of claims 1 to 10 consisting of the following steps:

(i) providing
an electrolytic cell comprising:

- at least two electrodes; wherein one of the at least two electrodes comprises the metal or alloy surface;

- an electrolyte; wherein the electrolyte is a composition consisting essentially of ethylene glycol and sodium chloride; and

- means for connecting a power source;

(ii) degreasing and descaling the metal or alloy surface by: submerging the metal or alloy surface partially or totally in the electrolyte of the cell, and applying a current density of at least 5 A/dm² at a temperature of at least 20°C and during at least 5 minutes, followed by an optional rinsing step; wherein during the decreasing and descaling step the polarity of the cell changes at least twice;
to obtain a degreased and descaled metal or alloy surface;

(iii) optionally, electropolishing the degreased and descaled metal or alloy surface of step (ii) by applying a current density of at least 10 A/dm² at a temperature of at least 30°C and during at least 1 minute, to obtain an electropolished, degreased and descaled metal or alloy surface, followed by an optional rinsing step; preferably, wherein during step (iii) the electrode comprising the metal or alloy surface is an anode;

(iv) optionally, passivating the degreased and descaled metal or alloy surface of step (ii) or the electropolished, degreased and descaled metal or alloy surface of step (iii); and

(v) optionally, recycling the electrolyte after any of the steps (ii) to (iv) by: (i) precipitating the residual particles in the electrolyte that have been generated during any of the previous steps, and (ii) removing the residual particles.

12. A finished metal or alloy surface obtainable by the method according to any of claims 1 to 11.

13. A cell for carrying out the method for finishing a metal or an alloy surface described in any one of claims 1 to 11; adapted for containing a composition consisting essentially of ethylene glycol and sodium chloride.

14. Use of a composition consisting essentially of ethylene glycol and sodium chloride, as electrolyte for electrochemically processing a metal or an alloy surface; preferably for electrochemically finishing a metal or an alloy surface.

15. The use according to claim 14 for electrochemically degreasing and descaling a metal or an alloy surface.

Drawing