RU2561549C2 - Electrolyte solution and electrochemical methods of surface modification - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to electrochemical processing of work pieces from non-ferrous metals, namely to the water solution of electrolyte used for processing. The electrolyte solution contains lemon acid with the concentration within a range from 1.665 g/l up to 982 g/l, ammonium bifluoride with the concentration from 2 g/l up to 360 g/l and no more than 3.35 g/l of strong acid. The work piece surface treatment includes exposure of the surface to influence of a bathtub with water solution of electrolyte, regulation of temperature of a bathtub is less or equal 85°C, connection of the work piece to the anode of DC power supply and immersion of the cathode of DC power supply into the bathtub and passing through the bathtub of the current less than 255000 amperes per square meter.

EFFECT: invention allows to use the electrolyte water solution for processing of various non-ferrous metals, meanwhile the electrolyte is environmentally safe and doesn't generate hazardous wastes.

29 cl, 12 dwg, 9 tbl

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to a jointly owned application entitled “Electrolyte Solution and Electrolytic Polishing Methods”, which is filed simultaneously with this application.

FIELD OF TECHNOLOGY

[0002] Solutions and methods relate to the general field of electrolytic polishing of non-ferrous metal parts and surfaces, and more particularly to surface treatment by electrolytic polishing, including crack modulation and removal of oxides, on non-ferrous and reactive metals, in particular titanium and titanium alloys.

BACKGROUND OF THE INVENTION

[0003] In the processing of reactive metals by pressure from an ingot to the finished product and after hot processing of the finished part, it is necessary to remove the material of a certain surface layer of metal oxide or, in the case of titanium and titanium alloys, what is commonly called the alpha shell. These oxygen-enriched phases occur when reactive metals are heated in air or in oxygen-containing atmospheres. The oxide layer can affect the strength of the material, fatigue resistance and corrosion resistance of the metal. Titanium and titanium alloys are among the reactive metals, which means that they react with oxygen and form a brittle bonded oxide layer (TiO ₂ for Ti, ZrO ₂ for Zr, etc.) whenever they are heated in air or in The oxidizing atmosphere is above about 480 ° C (900 ° F), depending on the specific alloy and oxidizing atmosphere. The oxide layer is created by heating the metal to temperatures necessary for typical forging in rollers or rolling in a rolling mill, as a result of welding or when heating for forging a finished part or hot forming of a part. Reactive metal oxides and the alpha shell are brittle, and molding is usually accompanied by the formation of a series of surface microcracks that penetrate the metal volume, potentially causing premature failure under tension or fatigue and making the surface more susceptible to chemical attack. Therefore, the oxide layer or the alpha shell layer must be removed before any subsequent hot or cold treatment or before the finished product is put into operation.

[0004] In the pressure treatment of reactive metals, such as titanium and titanium alloys, from an ingot to a finished part, it is also important that the cracks formed during the heat and mechanical treatment are removed. As described above, these cracks can penetrate deeper than the alpha shell and penetrate into the bulk of the metal. Reactive metals are usually heated, subjected to hot processing (for example, forging, rolling, drawing, pressing, extrusion), cooled and heated for additional hot processing from 4 to 8 times, to turn the ingot into a finished product - rolled. Rolled products are often heated again to make the finished part using methods including, but not limited to, hot extrusion, rolling, superplastic molding and stamping in closed dies. Whenever the metal is cooled after hot working, cracks form on the surface that extend into the workpiece. In conventional processing, these cracks are removed by grinding, which involves mechanical removal, or by chemical etching in strong acid, usually HF-HNO ₃ , of a layer of uniform thickness or the amount of material from the workpiece, until the bottom of the deepest crack is opened and removed. Grinding or chemical etching to this depth ensures that all cracks are removed, but takes a considerable amount of time and labor, and also leads to a significant and costly loss of material. This is due to the fact that sometimes cracks extend into the workpiece to a depth of 5% or more of the thickness or diameter of the workpiece or finished part. However, the removal of cracks is necessary because, if the cracks are not removed before the next stage of hot forming or using the finished part in operation, the cracks can propagate and destroy the workpiece or the finished part.

[0005] In chemistry and industry, electrolysis is a method of using direct electric current (DC) to stimulate a non-spontaneous chemical reaction in other circumstances. Electrolytic polishing is a well-known application of electrolysis for deburring metal parts and for obtaining a shiny and smooth surface finish. The workpiece to be electrolytically polished is immersed in a bath with an electrolyte solution and subjected to direct electric current. The workpiece is supported by the anode, and the cathode is connected to one or more metal conductors surrounding the workpiece in the bath. Electrolytic polishing is based on two opposing reactions that control the process. The first reaction is a dissolution reaction, during which the metal from the surface of the workpiece passes into the solution in the form of ions. Thus, metal ion by ion is removed from the surface of the workpiece. Another reaction is an oxidation reaction, during which an oxide layer forms on the surface of the workpiece. The growth of the oxide film limits the development of the ion removal reaction. This film is the thickest on top of the microdrops and the thinnest on top of the microprotrusions, and since the electrical resistance is proportional to the thickness of the oxide film, the highest dissolution rate of the metal occurs at the microprotrusions, and the lowest dissolution rate of the metal occurs at the microdroplets. Therefore, by electrolytic polishing, it is possible to selectively remove microscopic elevations, or "peaks", at a higher rate than the rate of exposure to the corresponding microdepressions or "valleys".

[0006] Another application of electrolysis is in electrochemical dimensional processing (ERO) processes (from the English electrochemical machining processes, ECM). In ERO, a strong current is passed between the electrode and the metal billet (often more than 40,000 amperes and supplied at current densities often more than 1.5 million amperes per square meter), causing the removal of material. An electric current is passed through a conductive fluid (electrolyte) from a negatively charged “tool” electrode (cathode) to a conductive workpiece (anode). The cathode tool is made with a shape corresponding to the desired dimensional processing operation, and is advanced into the anode blank. The electrolyte under pressure is injected at a given temperature into the treated area. The workpiece material is removed, practically translating into a liquid, at a speed determined by the feed rate of the tool into the workpiece. The gap between the tool and the workpiece varies in the range from 80 to 800 microns (80-800 microns) (from 0.003 to 0.030 inches). As the electrons cross the gap, the material on the workpiece dissolves and the tool gives the workpiece the desired shape. The electrolytic fluid carries away the metal hydroxide formed during the reaction between the electrolyte and the workpiece. Washing is necessary, since the process of electrochemical dimensional processing has a low "tolerance" to metal complexes that accumulate in the electrolyte solution. In contrast, methods using the electrolyte solutions disclosed herein remain stable and effective even at high titanium concentrations in the electrolyte solution.

[0007] Electrolyte solutions for electrolytic polishing of metals are usually mixtures containing concentrated strong acids (completely dissociated in water), such as inorganic acids. Strong acids, as described here, are usually qualified as acids that are stronger in aqueous solution than hydroxonium ion (H ₃ O ⁺ ). Examples of strong acids commonly used in electrolytic polishing are sulfuric acid, hydrochloric acid, perchloric acid and nitric acid, while examples of weak acids include acids from the group of carboxylic acids, such as formic acid, acetic acid, butyric acid and citric acid. Organic compounds, such as alcohols, amines or carboxylic acids, are sometimes used in mixtures with strong acids in order to slow the etching reaction with dissolution in order to avoid excessive etching of the workpiece surface. See, for example, US Patent No. 6610194, which describes the use of acetic acid as a reaction inhibitor.

[0008] There is an incentive to reduce the use of these strong acids in metal bathtubs, mainly because of their health risks and the cost of disposing of waste solution used. Citric acid was previously accepted as a passivating agent for stainless steel products according to the standards of both the US Department of Defense and ASTM (American Society for Testing Materials). However, although previous studies had shown and quantified the savings from using an industrial passivation bath with citric acid solution to passivate stainless steel, they were unable to identify a suitable electrolyte solution in which a significant concentration of citric acid could reduce the concentration of strong acids. For example, in a publication entitled Citric Acid & Pollution Prevention in Passivation & Electropolishing, 2002, titled Citric Acid and Pollution Prevention in Passivation and Electrolytic Polishing, several benefits of reducing the amount of strong inorganic acids by replacing them by a certain amount are described. weaker organic acid and, in particular, citric acid due to its low cost, availability and relatively safe disposal, but, in the end, an alternative electrolyte containing a mixture of mainly f Sforno and sulfuric acid with a small amount of an organic acid (no citric acid).

SUMMARY OF THE INVENTION

[0009] Alpha-shell removal and crack modulation are usually not achieved in electrolytic polishing processes. Highly acidic components found in typical electrolytic solutions used in the prior art for electrolytic polishing lead to the migration of hydrogen in the metal surface and aggressive uncontrolled etching, which can deepen cracks. In their development of new chemical foundations for electrolytic polishing baths using solutions of weak acids and ammonium hydrodifluoride (HFA), in the absence of strongly acidic components, the inventors found that both the removal of the alpha shell and modulation of cracks can be effectively achieved by electrolytic polishing. Thus, methods for removing oxides and modulating cracks using electrolytic polishing processes using new chemical bath compositions suitable for these methods are disclosed herein.

[0010] In one embodiment, an aqueous electrolyte solution is disclosed comprising from about 0.1% by weight to about 59% by weight of a carboxylic acid and from about 0.1% by weight to about 25% by weight of a fluoride salt and substantially free of strong acid.

[0011] In yet another embodiment, an aqueous electrolyte solution is disclosed comprising from about 1.665 g / L citric acid to about 982 g / L citric acid and from about 2 g / L ammonium hydrodifluoride to about 360 g / L ammonium hydrofluoride fluoride salt and practically free of strong acid.

[0012] In one embodiment, a method is disclosed for treating a surface of a non-ferrous metal preform, comprising subjecting the surface to a bath with an aqueous electrolyte solution with a citric acid concentration of less than or equal to about 300 g / L and a concentration of ammonium hydrodifluoride of greater than or equal to about 10 g / l, and having no more than approximately 3.35 g / l strong acid, regulating the bath temperature greater than or equal to approximately 54 ° C, connecting the workpiece to the anode of a DC power source and immersing the cathode Source DC power to the bath, and passing a current through a bath.

[0013] In one embodiment, a method for modulating cracks in the surface of a non-ferrous metal preform is disclosed, comprising exposing the surface to a bath with an aqueous electrolyte solution with a citric acid concentration of less than or equal to about 300 g / l and a concentration of ammonium hydrodifluoride greater than or equal to about 60 g / l, and having no more than about 3.35 g / l of strong acid, regulating the bath temperature greater than or equal to about 54 ° C, connecting the workpiece to the anode of a DC power source and immersion the cathode of the DC power into the bath and passing a current through the bath of less than about 53.8 amperes per square meter.

[0014] In one embodiment, a method is disclosed for removing metal oxide from the surface of a non-ferrous metal preform, comprising subjecting the surface to a bath with an aqueous electrolyte solution with a citric acid concentration of less than or equal to about 60 g / L and a concentration of ammonium hydrodifluoride greater or equal to about 60 g / l, and having no more than about 3.35 g / l of strong acid, regulating the bath temperature greater than or equal to about 54 ° C, connecting the workpiece to the anode of a DC power source and angling the cathode of the DC power into the bath and passing a current through the bath of less than about 53.8 amperes per square meter.

[0015] In one embodiment, a method is disclosed for removing an alpha shell from a surface of a titanium or titanium alloy preform, comprising subjecting the surface to a bath with an aqueous electrolyte solution with a citric acid concentration of less than or equal to about 60 g / L and a concentration of hydrodifluoride ammonium greater than or equal to about 60 g / l and having no more than about 3.35 g / l of strong acid, regulating the bath temperature greater than or equal to about 54 ° C, connecting the workpiece to the anode of the power source about current and immersion of the cathode of the DC power source in the bath, and passing through the bath current less than about 53.8 amperes per square meter.

BRIEF DESCRIPTION OF THE FIGURES

[0016] Figures 1A-1B are data graphs showing a material removal rate and a change in surface finish quality as a function of citric acid concentration in an aqueous electrolyte solution having a moderately low concentration of 20 g / L ammonium hydrodifluoride at a high current density of 1076 A / m ² in the temperature range.

[0017] Figures 2A-2B are data graphs showing the material removal rate as a function of the concentration of ammonium hydrodifluoride in an aqueous electrolyte solution containing 120 g / l citric acid, at characteristic low and high temperatures, respectively, in the range of current densities.

[0018] Figures 2C-2D are data graphs showing a change in surface finish quality as a function of ammonium hydrofluoride under conditions corresponding to Figures 2A-2B, respectively.

[0019] Figures 2E-2F are data graphs showing a material removal rate and a change in surface finish quality, respectively, as a function of current density in an aqueous electrolyte solution with virtually no citric acid at a temperature of 85 ° C.

[0020] Figures 3A-3D are data graphs showing a material removal rate as a function of citric acid concentration in an aqueous electrolyte solution for several concentrations of ammonium hydrodifluoride at a current density of 53.8 A / m ² and temperatures of 21 ° C, 54 ° C, 71 ° C and 85 ° C respectively.

[0021] Figures 4A-4D are data graphs showing a material removal rate as a function of citric acid concentration in an aqueous electrolyte solution for several concentrations of ammonium hydrodifluoride at a temperature of 54 ° C and a current density of 10.8 A / m ² , 215 A / m ² , 538 A / m ² and 1076 A / m ^2, respectively.

[0022] Figures 4E-4G are data plots showing a material removal rate as a function of current density at 85 ° C in an aqueous solution containing 120 g / l, 600 g / l and 780 g / l of citric acid, respectively, for several ammonium hydrodifluoride concentrations.

[0023] Figures 4H-4J are data graphs showing a change in surface finish quality as a function of current density under conditions corresponding to Figures 4E-4G, respectively.

[0024] Figures 5A-5B are data graphs showing the amount of material removed and surface finish quality changes, respectively, for various combinations of citric acid and ammonium hydro difluoride concentrations at low temperature (21 ° C) and high current density (538 A / m ² ) .

[0025] Figures 6A-6B are data graphs showing the amount of material removed and the quality of the surface finish, respectively, for various combinations of citric acid and ammonium hydro difluoride concentrations at low temperature (21 ° C) and high current density (1076 A / m ² ) .

[0026] Figures 7A-7B are data graphs showing the amount of material removed and surface finish quality changes, respectively, for various combinations of citric acid and ammonium hydro difluoride concentrations at high temperature (85 ° C) and high current density (1076 A / m ² ) .

[0027] Figures 8A-8B are graphs of data showing the amount of removed material and the change in surface finish quality, respectively, for various combinations of citric acid and ammonium hydrofluoride concentrations at characteristic high temperature (85 ° C) and low current density (10.8 A / m ² ).

[0028] Figures 9A-9B are data graphs showing the amount of material removed and surface finish quality changes, respectively, for various combinations of citric acid and ammonium hydro difluoride concentrations at characteristic high temperature (85 ° C) and high current density (538 A / m ² )

[0029] Figures 10A-10B are data graphs showing the amount of material removed and surface finish quality changes, respectively, for various combinations of citric acid and ammonium hydro difluoride concentrations at a characteristic moderately high temperature (71 ° C) and moderate current density (215 A / m ² ).

[0030] Figure 11 is a schematic representation of a sequence that occurs in a prior art crack removal process extending into a preform from a material surface.

[0031] Figure 12 is a schematic representation of a sequence that takes place in a process using the electrolyte disclosed herein to modulate a crack extending into a preform from a surface of a material.

DETAILED DESCRIPTION

[0032] Water electrolyte solutions are disclosed herein that are particularly useful for surface treatment of reactive metals, including, but not limited to, titanium and titanium alloys. Relatively small amounts of fluoride salt and carboxylic acid are dissolved in water, practically in the absence of a strong acid, such as an inorganic acid, so that the solution contains practically no strong acid. This electrolyte solution is a noticeable departure from earlier attempts to surface reactive metals in electrolyte baths, including, but not limited to, titanium and titanium alloys, which typically use strong acids and require that the amount of water in the electrolyte solution be kept at an absolute minimum.

[0033] The fluoride salt provides a source of fluoride ions for the solution and can be, but is not limited to, ammonium hydrodifluoride, NH ₄ HF ₂ (sometimes abbreviated here as “HFA”). Without being attached to theory, it seems that carboxylic acid softens the effect of fluoride ions on the treated surface of a reactive metal and can be, but is not limited to, citric acid. No amount of strong acid or inorganic acid is intentionally added to the solution, although a trace amount of strong acid may be present. As used herein, the terms “substantially absent” and “substantially free” are used to mean concentrations of strong acid less than or equal to about 3.35 g / L, preferably less than or equal to about 1 g / L, and more preferably less than about 0 , 35 g / l.

[0034] Samples of technically pure (PM) titanium were immersed in a bath with an aqueous solution containing 60 g / L citric acid and 10 g / L HFA at 54 ° C, and a current was applied at 583 A / m ² . A sample cut off from a strip of titanium with a surface obtained after rolling (with a surface roughness of 0.52 μm), exposed to this solution for 15 minutes, became uniformly smooth (with a surface roughness of 0.45 μm) and cosmetically reflective. Then, small amounts of HNO ₃ (nitric acid) with 42 ° Be were added step by step, and the prepared test sample was processed several times until surface changes were detected. The samples did not experience treatment after each addition of nitric acid until a nitric acid concentration of 3.35 g / l was reached, in which the control panel showed a non-uniform cosmetic appearance, including pitting corrosion and peeling, with irregular effects around the perimeter of the sample, with a surface roughness in the range from 0.65 to 2.9 μm and above. Nitric acid is considered a border strong acid, with a dissociation constant not much greater than the dissociation constant of the hydroxonium ion. Therefore, it is expected that for other stronger acids having the same or greater dissociation constants than nitric acid, such an electrolyte solution would be similarly effective in controlled material removal and micropolishing at strong acid concentrations of less than about 3.35 g / L. However, it is expected that the other electrolyte solutions disclosed herein having different concentrations of citric acid and HFA and different ratios of concentrations of citric acid and HFA may have a lower "tolerance" to the presence of a strong acid, depending on the particular strong acid, as well as the operating parameters, such as temperature and current density. Therefore, to ensure the effective use of aqueous solutions of electrolytes to remove material and fine surface finish in a wide range of concentrations of citric acid and HFA and a wide range of temperatures and current densities should be present no more than about 1 g / l of strong acid, and preferably not more than about 0 , 35 g / l of strong acid.

[0035] Extensive electrolytic polishing tests have been performed on samples of titanium and titanium alloys using a series of chemical concentrations, current densities, and temperatures. In particular, the test was conducted on “clean” steel (representing a typical state of “delivery” of metal by rolled metal manufacturers meeting ASTM or Aerospace Material Specifications (AMS) standards to measure the ability of various solutions and methods to remove volume metal, improve or refine the surface finish on sheet metal products with low material removal rates and / or micro-polish the metal surface for a very fine surface finish those with very low material removal rates. In addition, while most tests focused on titanium and titanium alloys, the test also showed that the same solutions and methods are more generally applicable to the processing of many non-ferrous metals. For example, good results have been obtained, in addition to titanium and titanium alloys, on metals, including, but not limited to, gold, silver, chromium, zirconium, aluminum, vanadium, niobium, copper, molybdenum, zinc and nickel. Additionally, alloys such as titanium-molybdenum, titanium-aluminum-vanadium, titanium-aluminum-niobium, titanium-nickel (Nitinol®), titanium-chromium (Ti 17®), Waspaloy and Inconel® (based on nickel).

[0036] An electrolyte solution containing citric acid and ammonium hydrodifluoride proved to be effective in etching non-ferrous metals and metal alloys at an unexpectedly diluted concentration of both components. In this context, etching should be understood as encompassing substantially uniform surface removal. In addition, improvements in surface finish have been demonstrated over a wide range of concentrations of both citric acid and ammonium hydrodifluoride. Although any concentration of citric acid can be used up to the point of saturation with water (59% by weight or approximately 982 g / l of an aqueous solution at standard temperature and pressure), there seems to be a correlation between the concentration of citric acid and the concentration of ammonium hydrodifluoride, at which citric acid sufficiently weakens the effects of etching with fluoride ions formed during dissociation of ammonium hydrodifluoride, which reduces the rate of material removal, while the micropolishing of the surface of the material increases tsya. For both etching and micropolishing, some mixtures having citric acid concentrations as low as 3.6 wt% or about 60 g / l of solution showed etching rates and micro-polishing of the titanium surface, comparable to mixtures having significantly higher concentrations of citric acid, including up to about 36 wt.%, or about 600 g / l of solution. Thus, in these solutions, the etching rate is clearly more directly dependent on the concentration of HFA than on the concentration of citric acid. Effective etching and micropolishing have been demonstrated even at extremely low concentrations of citric acid, less than about 1 wt.%, Or about 15 g / l of solution. However, the presence of even a minimal amount of fluoride ions is sufficient to allow some removal of the metal.

[0037] The etching rate drops significantly at citric acid concentrations in excess of about 600 g / L. However, at this high concentration of citric acid, at least in cases of moderate to high current density, the surface finish results are improved, while the etching rate decreases. Thus, when direct current is applied, more dilute citric acid mixtures provide greater rates of material removal from the surface, while more concentrated citric acid mixtures, up to a mixture concentration as high as about 42% by weight, or about 780 g / l of solution, provide a smoother and more mirrored surface, with a uniform fine-grained structure and no corona effect, compared to parts treated with less concentrated mixtures of citric acid.

[0038] Highly controlled metal removal can be achieved using the bath solutions and methods described herein. In particular, the level of control is so thin that the bulk metal can be removed with thicknesses as small as 0.0001 inches and as large and precise as 0.5000 inches. Such a fine control can be achieved by regulating the combination of citric acid and HFA concentrations, temperature and current density, as well as by varying the duration and cyclicity of the direct current supply. Removal can be performed substantially uniformly on all surfaces of the workpiece, or it can be selectively applied only on certain selected surfaces of a rolled product or a manufactured structural element. Removal control is achieved by fine-tuning several parameters, including but not limited to temperature, power density, power cycle, HFA concentration and citric acid concentration.

[0039] The removal rates directly vary with temperature, which means that while all other parameters are kept constant, removal becomes slower at lower temperatures and faster at higher temperatures. However, by maintaining concentrations of citric acid and HFA within certain preferred ranges, high levels of micropolishing at high temperatures can also be achieved, which is the opposite of what might be expected.

[0040] The removal rate depends on the DC power supply mode. Unlike what you would expect, the removal rate appears to be inversely dependent on the continuously supplied DC power, and with continuous supply, increasing the DC power density reduces the removal rate. However, by cyclic switching the DC power, removal rates can be increased. Therefore, when significant rates of material removal are desired, during the entire processing operation, the DC power is cyclically switched between “ON” and “OFF”. On the contrary, when fine control of removal rates is desired, DC power is supplied continuously.

[0041] Without adhering to theory, it seems that the removal is slowed down in proportion to the thickness of the oxide layer that forms on the metal surface, and a higher DC power supply leads to greater oxidation on the metal surface, which can act as a barrier to the influence of fluoride ions on metal. Accordingly, cyclic switching of DC power between “on” and “off” at a given speed can overcome this oxide barrier or create a mechanism that causes thick oxide to periodically flake from the surface. As described here, by varying the operating parameters of the bath temperature, applied voltage, concentration of citric acid and concentration of ammonium hydrodifluoride, the electrolyte provides the opportunity to select the desired results, namely highly controlled bulk metal removal and micropolishing, for a specific application. In addition, the variation of operating modes within a given technological set of operating parameters can modify and increase the possibility of fine-tuned control of metal removal and surface finish quality.

[0042] For example, Figures 8A and 9A demonstrate that at 85 ° C, 300 g / L of citric acid and 10 g / L of ammonium hydrogen difluoride, material removal rates increase with increasing current density from 10.8 A / m ² to 538 A / m ² . At the same time, figures 8B and 9B demonstrate that under the same conditions, with an increase in current density from 10.8 A / m ² to 538 A / m ^{2, the} quality of the surface finish deteriorates. By cyclic switching the DC power source between these two current densities, you can achieve the final result, which is better than when working exclusively with any one of these current densities throughout the process. In particular, the duration of the process of removing a specific amount of material can be reduced compared to operating exclusively at 10.8 A / m ² . Additionally, due to the smoothing effect at a lower current density, the finish quality of the entire surface of the final product is superior to that obtained by processing exclusively at 538 A / m ² . Therefore, cyclic switching between two or more preset power modes (which is manifested in current density) provides favorable results in terms of both improving surface quality and precision removal of bulk metal, using a process that requires less total time than separate processes to improve quality surface or removal of bulk metal separately.

[0043] In addition to varying the duty cycle, electric current can be supplied through an electrolyte solution and through a blank with various waveforms that are available from DC power supplies, including, but not limited to, half wave, half wave, square wave, and other intermediate rectifications , to obtain additional useful results and / or increase operating speeds, without compromising the final quality of the surface finish. Fast DC switching frequencies such as 50 kHz - 1 MHz or slow cycles such as 15-90 minutes may be beneficial, depending on the surface area to be treated, the mass of the workpiece, and the particular surface condition of the workpiece. Additionally, the DC switching cycle itself may optimally require its own cycle. For example, a large mass blank with a very rough initial surface finish may be best suited at first by a slow switching cycle, followed by an increased frequency switching cycle as the material is removed and the quality of the surface finish improves.

[0044] Testing the electrolytic baths of the type described herein also revealed that electrolytic polishing in certain embodiments takes place without increasing the concentration of hydrogen on the metal surface and in some cases lowers the concentration of hydrogen. For the absence of hydrogen migration into the metal matrix, an oxygen barrier on the surface of the material may be responsible. The data indicate that this oxygen barrier can also remove hydrogen from the metal surface. Higher concentrations of fluoride ions lead to faster removal rates, but have an unknown effect on hydrogen adsorption in the metal matrix. Higher concentrations of citric acid tend to reduce the removal rate and require higher power densities during electrolytic polishing, but also give the surface a “smoothing” or “gloss” effect.

[0045] Some of the advantages over prior art solutions are the result of using an aqueous solution of HFA electrolyte and citric acid to finish and / or etch the surface of metal products. The disclosed electrolyte solutions allow precise control of the degree of surface finish achieved. The surface finish of conventional flat products from alloy manufacturers (sheets and plates) involves multi-stage grinding to the final degree of surface finish using increasingly thin abrasive materials, usually followed by “flushing etching” in an acid bath containing hydrofluoric acid (HF) and nitric acid (HNO ₃ ) to remove residual abrasive materials, ground smeared metal and surface anomalies. Acid etching in HF-HNO ₃ is exothermic and therefore difficult to control and often leads to the metal going beyond a given caliber, which leads to an increase in the percentage of rejects or a change in the purpose of the metal to a lower quality. When using the disclosed electrolyte solutions, typical secondary and tertiary resurfacing, which may be necessary in the case of washing etching, can be avoided. A precisely specified degree of surface finish can be achieved, which cannot be achieved with the current state of grinding and pickling technology. In addition, the disclosed electrolyte solutions do not introduce mechanical stresses into the workpiece. For comparison, any technology of mechanical grinding gives the surface significant mechanical stresses, which causes warpage of the material and leads to the fact that a certain percentage of the material becomes incapable of meeting the planarity requirements typical or specified by the customer.

[0046] A typical acid etching technique in HF-HNO ₃ will incorporate hydrogen into the target material, which often needs to be removed by expensive vacuum degassing to prevent embrittlement of the material. The tests carried out using baths with an aqueous electrolyte containing citric acid and HFA on typical Ti-6Al-4V full-size sheets typical for rolling production and on test samples of PM titanium, 6Al-4V titanium, and nickel-based alloy 718 showed results with reduced impregnation ( saturation) with hydrogen compared to samples exposed to conventional strong acid etching solutions. In particular, when treating Ti-6Al-4V and titanium PM to achieve the same final result with an alpha-free, clean surface, which is usually achieved by etching in strong acid, using aqueous electrolyte compositions containing ammonium hydrodifluoride and citric acid, a range of regimes was revealed in terms of temperature and current densities, at which there was a complete absence of hydrogen penetration into the workpiece material, and in many of these operating modes, hydrogen was actually displaced from the mother la. For all metals and alloys, although tests to refine the preferred operating ranges are still ongoing, the results still invariably indicate that even under conditions that may be suboptimal, less hydrogen enters the material than would be possible with the same working conditions using a strong acid pickling bath. Typically, lower concentrations of ammonium hydrodifluoride lead to a greater removal of hydrogen from the material exposed to the electrolyte solution, or to a smaller penetration of hydrogen into the material exposed to the electrolyte solution.

[0047] Highly controlled metal removal, surface finish and micropolishing

[0048] Micro-polishing or micro-smoothing of structural elements and, in particular, micro-smoothing of already relatively smooth surfaces can be achieved using the solutions and methods described herein with greater accuracy than manual or machine polishing. Micropolishing takes place without the formation of harmful residual mechanical stresses in the target workpiece or material, as well as without smearing the metal in the workpiece, both of which are problems inherent in modern mechanical methods. Additionally, by eliminating the human factor, the polishing levels obtained are accurate and reproducible. When using the presented electrolyte solution, cost savings are also achieved compared to existing methods.

[0049] In the tests, good results were obtained for micropolishing at high concentrations of citric acid, low to moderate concentrations of HFA, high temperature and high direct current density, which can be supplied continuously or cyclically. However, the DC power density should be adjusted based on the alloy being processed. Aluminum-containing titanium alloys (typically alloys with an alpha-beta metallographic structure, including the well-known Ti-6Al-4V alloy) tend to lose their gloss when applied DC voltages in excess of 40 volts. However, for these metals, limiting the voltage to about 40 volts and applying a higher current (i.e., to achieve a higher power density) allows again to realize the gloss of the material. Without being tied to theory, this may be the result of the action of an alpha stabilizing element, which in the case of most alpha-beta alloys (including Ti-6Al-4V) is aluminum, anodized to Al ₂ O ₃ and not polished. In addition, titanium-molybdenum (fully beta-phase metallographic structure) and technically pure (PM) titanium (completely alpha-phase), however, become more brilliant with increasing power densities of the direct current, without explicit limitation to the same upper voltage limit. In particular, for other metals, it was found that higher voltages up to at least 150 volts can be used, for example, for nickel-based alloy 718, to obtain favorable electrolytic polishing, micropolishing and surface treatment results using the electrolyte solutions disclosed herein .

[0050] The solutions and methods disclosed herein can be used to remove burrs from machined parts by preferentially deburring machined metal components, especially when the parts are made of hard to cut metals such as titanium and nickel based alloys. In the current state of technology, deburring from machined structural elements is usually performed as a manual operation, and thus it is associated with many problems caused by human errors and human impermanence. Testing with open solutions showed that deburring is most effective at low citric acid concentrations, due to the resistive nature of citric acid in the electrochemical cell, and best at high levels of fluoride ions from HFA. Such solutions can also be used to remove impurities from the surface or to clean the workpiece after machining, as could otherwise be done using strong acid etching in an etching bath with HF-HNO ₃ .

[0051] Non-ferrous and especially reactive metals exhibit an effective chemical etching rate over a wide range of dilute citric acid mixtures, as described above. This makes it possible to tailor custom surface technology to a particular non-ferrous metal blank, which may include a selected residence time in the bath before electric current is applied to remove and react some part of the surface metal before electrolytic polishing to selectively reduce peak areas.

[0052] A citric acid-based electrolyte has a much lower viscosity than traditional electropolishing mixtures, in part because of the much lower dissociation constant of citric acid compared to the strong acids commonly used in electro-polishing electrolytes. A lower viscosity promotes material transfer and reduces electrical resistance, so that lower voltages can be used than with conventional electrolytic polishing. The resulting surface finish quality by electrolytic polishing largely depends on the viscosity and electrical resistivity of the electrolyte used. It has been found that the finest surface finish (carefully micropolished) can be achieved using a high-resistivity electrolyte solution in combination with a high electrolytic polishing voltage (and therefore with a moderate to high current density). In addition, when using a slightly more electrically conductive (less highly resistive) electrolyte solution, fine micropolishing can still be achieved at high voltages and high current densities.

[0053] From this it follows that the corresponding advantages will be applicable to electrochemical dimensional processing. In particular, it is expected that electrolyte baths with the compositions described herein can be effectively used instead of conventional solutions for electrochemical sizing and / or etching, with significant environmental benefits and cost savings. Since the electrolyte solutions disclosed here are virtually free of strong acid, the problems of hazardous waste disposal and handling are minimized. Moreover, the required current densities will be much lower than those required for conventional electrochemical dimensional processing.

[0054] Generally, increasing the concentration of ammonium hydrodifluoride tends to lower the electrical resistance of the electrolyte solution (ie, ammonium hydrodifluoride increases the electrical conductivity of the electrolyte solution), while the presence of citric acid or increasing the concentration of citric acid relative to the concentration of ammonium hydrogen difluoride tends to weaken the effects of ammonium hydrodifluoride on the electrical resistance. In other words, in order to maintain the electrical resistance of the electrolyte solution at a high level to facilitate micropolishing, it is desirable to maintain low concentrations of ammonium hydro difluoride or to use a higher concentration of ammonium hydro difluoride in combination with a higher concentration of citric acid. Thus, by varying the concentration of ammonium hydrodifluoride and the relative concentrations of ammonium hydrodifluoride and citric acid, it is possible to successfully control the electrical resistance of the electrolyte solution to achieve the desired levels of micropolishing of the surface of the workpiece.

[0055] In the methods disclosed herein, it is not necessary that the proximity of the preform (anode) to the cathode be accurate, in contrast to conventional electrolytic polishing or electrochemical dimensional processing. Successful processing took place with the cathode at a distance in the range of about 0.1 cm to about 15 cm from the workpiece. Practical limitations on the maximum distance between the cathode and the anode preform are mainly industrially determined, including the size of the bath, the size of the preform, and the electrical resistance of the electrolyte solution. Since the total current densities are lower and often much lower than those required for electrochemical dimensional processing, it is possible to use longer distances from the workpiece to the cathode, and then simply increase the power of the power source accordingly. Moreover, since the low viscosity electrolyte solutions disclosed herein provide highly controlled bulk metal removal, surface finish and micropolishing, it is expected that the same solutions will also be effective in electrochemical dimensional processing.

[0056] Electrolytic polishing of the metal billet is performed by exposing the billet and at least one cathode electrode to a bath with an electrolyte solution and connecting the billet to the anode electrode. The electrolyte solution contains an amount of carboxylic acid in the range of from about 0.1% by weight to about 59% by weight. The electrolyte solution may also contain from about 0.1% by weight to about 25% by weight of a fluoride salt selected from alkali metal fluorides, alkaline earth metal fluorides, etching silicates of compounds and / or combinations thereof. Current is supplied from a power source between the at least one anode electrode connected to the workpiece and the cathode electrode immersed in the bath to remove metal from the surface of the workpiece. The current is supplied at a voltage in the range of about 0.6 millivolts DC (mVDC) to about 100 volts DC (VDC). Citric acid is the preferred carboxylic acid, although other carboxylic acids may be used, including, but not limited to, formic acid, acetic acid, propionic acid, butyric acid, valerianic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, palmitic acid and stearic acid. The preferred fluoride salt is HFA.

[0057] In another aspect of the electrolytic polishing method, current is supplied at a voltage of from about 0.6 VDC to about 150 VDC. The current can be supplied at a current density of less than or equal to about 255,000 amperes per square meter ((A / m ² ) (about 24,000 amperes per square foot), where the denominator displays the total effective surface area of the workpiece. For some non-ferrous metals, such as based on nickel, current densities up to about 5000 A / m ² (about 450 A / ft ² ) inclusive can be used, and for titanium and titanium alloys, current densities from about 1 to about 1100 A / m ² (about 0.1 -100 A / ft ² ). Electrical processes of polishing using an electrolyte solution can operate between the freezing and boiling points of the solution, for example, at a temperature of from about 2 ° C to about 98 ° C, and preferably in the range of from about 21 ° C to about 85 ° C.

[0058] In practice, the material can be removed from the metal substrate at a rate of from about 0.0001 inch (0.00254 mm) to about 0.01 inch (0.254 mm) per minute. The following examples show the effectiveness of the electrolyte with varying concentrations and operating conditions.

[0059] Example 1: etching of technically pure titanium

[0060] In an electrolyte consisting essentially of approximately 56% water by weight, 43% citric acid (716 g / L) and 1% ammonium hydrodifluoride (15.1 g / L), operating at 185 ° F (85 ° C) ), a sample of a plate of technically pure titanium was processed to improve the surface finish of the material (i.e., to make the surface finish standard for a rolling mill smoother). Material processing began with a surface roughness of approximately 160 microinches, and after processing the surface roughness was reduced by 90 microinches to a final reading of 50 microinches, or, in other words, the improvement was approximately 69%. The process was carried out over a period of 30 minutes, which led to a decrease in the thickness of the material by 0.0178 inches.

[0061] Cold formability, a key characteristic of a titanium plate as a product for many end-use applications, is highly dependent on the surface finish of the product. By using the embodiments of the electrochemical process disclosed herein, an improvement in the quality of the surface finish of the material can be achieved at a lower cost than with conventional grinding and etching methods. The degree of finish obtained using the embodiments of the disclosed solutions and methods has demonstrated an improvement in the cold forming characteristics of the plate product to a higher level than in conventional methods.

[0062] Example 2: etching of a test sample 6Al-4V

[0063] In the following examples, test samples of 6Al-4V titanium alloy sheet blanks with dimensions of 52 mm × 76 mm were processed. The electrolyte consisted of water (H ₂ O), citric acid (LA) and ammonium hydrodifluoride (HFA) at varying concentrations and temperatures. The observations and indications are shown below in table 1.

Table 1 H ₂ O (% wt.) LC (% wt.) HFA (% wt.) Temperature (° F) Time (min) Loss of material (inch) 77.90 21.45 0.65 178 1,0 0,00065 77.25 21.45 1.30 185 1,0 0,00085 75.95 21.45 2.60 189 1,0 0.00120 74.65 21.45 3.90 188 1,0 0.00120 56.45 42.90 0.65 184 1,0 0.00005 55.80 42.90 1.30 195 1,0 0,00030 54.50 42.90 2.60 193 1,0 0.00005 53.20 42.90 3.90 188 1,0 0,00035 53.20 42.90 3.90 191 5,0 0.00140 75.95 21.45 2.60 190 3.0 0,00205 88.95 10,725 0.325 180 1,0 0,00020 88,625 10,725 0.650 180 1,0 0,00020 87,975 10,725 1.30 182 1,0 0,00060 99.25 0,100 0.65 188 1,0 0.00010 98.60 0,100 1.30 182 1,0 0,00065 97.30 0,100 2.60 195 1,0 0,00095

[0064] Example 3: electrolytic polishing of a test sample 6Al-4V

[0065] In the following examples, sample samples of 6Al-4V titanium alloy sheet blanks with dimensions of 52 mm × 76 mm were processed. The electrolyte consisted of water (H ₂ O), citric acid (LA) and ammonium hydrodifluoride (HFA) at varying concentrations and temperatures. The observations and data are shown below in table 2.

table 2 H ₂ O (% wt.) LC (% wt.) HFA (% wt.) Temperature (° F) Time (min) Power (V / A) Loss of material (inch) 77.90 21.45 0.65 190 1,0 (50/7) 0,00025 77.25 21.45 1.30 195 1,0 (50/8) 0,00070 75.95 21.45 2.60 191 1,0 (50/10) 0.00130 74.65 21.45 3.90 190 1,0 (50/12) 0.00130 74.65 21.45 3.90 188 1,0 (20/6) Not recorded 74.65 21.45 3.90 184 1,0 (6/2) Not recorded 74.65 21.45 3.90 180 1,0 (12/3) Not recorded 56.45 42.90 0.65 182 1,0 (50/3) 0.00010 55.80 42.90 1.30 200 1,0 (50/5) 0,00045 54.50 42.90 2.60 189 1,0 (50/8) 0,00055 53.20 42.90 3.90 190 1,0 (50/12) 0,00045 53.20 42.90 3.90 203 5,0 (50/5) 0.00115 75.95 21.45 2.60 172 3.0 (12/3) 0.00015 88.95 10,725 0.325 180 1,0 50V 0.00000 88,625 10,725 0.650 180 1,0 50V 0.00010 87,975 10,725 1.30 184 1,0 50V 0,00060 99.25 0,100 0.65 190 1,0 50V 0,00060 98.60 0,100 1.30 184 1,0 (50/19) 0.00145 97.30 0,100 2.60 190 1,0 (50/38) 0.00360

[0066] An additional extensive test was conducted using aqueous electrolyte solutions containing citric acid in a concentration range from about 0 g / l to about 780 g / l (from about 0% to about 47% by weight) and ammonium hydrodifluoride in a concentration range from about 0 g / l to about 120 g / l (from about 0% to about 8% by weight) and virtually no strong acid (i.e., less than about 1 g / l or less than 0.1% by weight mass), at bath temperatures in the range of about 21 ° C to about 85 ° C, and with densities applied and a current in the range of about 0 A / m ² to about 1076 A / m ² surface area of the preform. (It should be noted that 780 g / l of citric acid in water is a saturation concentration at 21 ° C.) High current densities, such as at least 225,000 A / m ² , can be used at applied voltages of 150 volts or more. The metals tested included technically pure titanium, as well as some spot testing on 6Al-4V titanium alloy and nickel-based alloy 718. Based on these results, it is expected that similar results of electrolytic polishing, micropolishing and surface treatment can be obtained for various classes of non-ferrous metals and alloys. The results are summarized in the following tables and description and with reference to the figures. Unless otherwise specified, tests were performed at temperatures of approximately 21 ° C, approximately 54 ° C, approximately 71 ° C, and approximately 85 ° C and at current densities of approximately 0 A / m ² , approximately 10.8 A / m ² , approximately 52.8 A / m ² , about 215 A / m ² , about 538 A / m ^2, and about 1076 A / m ² . No amount of strong acid has been added to any of the tested solutions, although trace amounts are unlikely to significantly affect the results.

[0067] Figures 1A-1B show a material removal rate and a change in surface finish quality at four different temperatures, respectively, using an aqueous electrolyte solution having a moderately low concentration of ammonium hydrodifluoride of 20 g / L and a concentration of citric acid from about 0 g / L to about 780 g / l and at a current density of 1076 A / m ² . Figure 1A shows that the material removal rate varies proportionally with temperature, especially at lower concentrations of citric acid. As the bath temperature rises, the rate of removal increases. At lower temperatures of 21 ° C, 54 ° C and 71 ° C, a concentration of citric acid of 180 g / l is sufficient to begin to restrain the removal efficiency of the material with ammonium hydrodifluoride, while at a higher temperature of 85 ° C, relatively rapid removal of the material continues up to concentration of approximately 300 g / l of citric acid. At higher concentrations of citric acid of 300 g / l or more, removal rates are reduced at all temperatures. On the contrary, figure 1B shows that at lower concentrations of citric acid, in particular, at a concentration of from 120 g / l to 180 g / l or lower, the surface finish generally deteriorates at the lowest temperature. In other words, fluoride ion, which is responsible for significant material removal at low concentrations of citric acid, also causes surface damage, but the presence of citric acid in significant concentrations appears to act as a beneficial barrier against fluoride ions. However, with an increase in the concentration of citric acid to 180 g / l and higher, the quality of the surface finish actually increases, in particular, at citric acid levels of 600 g / l or more, and the material removal rate is significantly reduced. Moreover, even with citric acid levels between about 120 g / l and about 600 g / l, where material is still being removed, improvements in surface finish quality can be achieved at the same time.

[0068] The test found that to achieve the desired improvements in material removal and surface finish, a source of fluoride ions such as ammonium hydrodifluoride is needed. In electrolyte solutions, consisting essentially of citric acid in water alone, in the absence of ammonium hydrodifluoride, there is practically no removal of material, regardless of bath temperature or current density, and changes in surface finish are also minimal. It is believed that when titanium or another reactive metal is treated in an aqueous electrolyte containing only citric acid, the surface of the material is mainly anodized to form an oxide layer that is very thin (i.e., from about 200 nm to about 600 nm thick) and forms quickly . After the formation of the anode oxide layer, since the supplied direct current power can no longer affect the surface of the material, it hydrolyzes the water. The resulting oxygen, which is formed rapidly, finds another monatomic oxygen and is released on the anode in the form of gaseous O ₂ .

[0069] Figures 2A-2B and 2C-2D show the material removal rate and surface finish quality changes, respectively, using an aqueous electrolyte solution having a citric acid concentration of 120 g / L and an ammonium hydrodifluoride concentration of from about 0 g / L to about 120 g / l Figures 2A and 2C show data at a characteristic low temperature of 21 ° C, and Figures 2B and 2C show data at a characteristic high temperature of 71 ° C. Figures 2A-2B show that material removal is strongly correlated with ammonium hydrodifluoride concentration and temperature, but is minimally affected by current density. Higher rates of material removal are usually achieved by increasing either the concentration of ammonium hydrodifluoride, or the temperature, or both. Figures 2C-2D show that material removal is accompanied by some deterioration in surface quality. However, it was unexpectedly found that with increasing temperature and material removal rate, the degree of deterioration of surface finish decreases. At a low temperature of 21 ° CC, as in Figure 2C, an increase in current density mitigates the effects of deterioration in surface quality, and at maximum current density, some improvement in surface finish becomes apparent. At a higher temperature of 71 ° C, as in figure 2D, with changes in current density, the quality of the surface finish does not change significantly.

[0070] Figures 2E-2F show that the removal rate of the material and the change in surface finish quality, respectively, when using an aqueous electrolyte solution consisting essentially of ammonium hydrodifluoride in water, without intentionally adding citric acid, depends on the current density when operating at high temperature 85 ° C. High rates of material removal can be achieved using an electrolyte only with HFA, but this removal of material occurs due to the quality of the surface finish, which often becomes significantly worse under the action of an electrolyte solution. However, under certain operating conditions (not shown in the figures), minimal deterioration or the most moderate improvement in surface finish was achieved. For example, improvements in the quality of surface finish by means of electrolyte solutions with HFA only were achieved with a solution of 10 g / l HFA at 21 ° C and 215-538 A / m ² and at 54-71 ° C and 1076 A / m ² , with a solution 20 g / l HFA - at 21 ° C and 215-1076 A / m ² , and with a solution of 60 g / l HFA - at 21 ° C and 538-1076 A / m ² .

[0071] Without being bound by theory, a possible explanation for the ability of an increased current density to improve the quality of surface finish, with minimal effect on the material removal rate, is that one of the functions of the electric current is to grow a natural oxide layer on the surface of the material. This excess oxygen in combination with citric acid appears to act as a beneficial barrier to the surface of the material. Accordingly, it seems that as the current density increases, oxygen with a higher concentration is formed on the anode, which, in turn, can act as a barrier to mass transfer. Alternatively, if we simplify the surface morphology of the material as a sequence of “peaks” and “valleys”, we can assume that citric acid and oxygen settle in the valleys, leaving only the peaks of surface morphology for exposure to fluoride ions. As citric acid and oxygen barriers become stronger (i.e., citric acid concentrations increase and current densities increase), only the highest surface peaks become available for chemical exposure. According to this theory, it can be expected that low current densities and low concentrations of citric acid will provide a minimally capable surface smoothing process, while high current densities and high citric acid concentrations can be expected to provide the most capable surface smoothing process. Regardless of whether these theories are accurate, there is every reason to argue that the results are consistent with the above analysis.

[0072] Understanding that oxygen (produced by electric current) and citric acid appear to act as microbarriers to the removal process, it helps to clarify that the HFA concentration and temperature are the variables that are probably the most readily available for controlling the removal of material and micropolishing. Therefore, in the processes described here, the current density seems to act mainly on the formation of oxygen, and for the most part is not a significant factor enhancing the total removal of the material. Rather, material removal seems to occur almost exclusively under the influence of fluoride ions, the activity of which is to some extent controlled by the thermodynamic influence of temperature. In short, it unexpectedly turned out that the current density as a control variable has a relatively low value, that the presence of fluoride ions prevails over the influence of current density.

[0073] Figures 3A-3D show that at a typical current density of 53.8 A / m ^2, the material removal rate can vary in direct proportion to temperature, as a result of which more material is removed for the same mixture of citric acid, ammonium hydrodifluoride and water at higher temperatures. Similar trends were observed at all current densities from 0 A / m ² to 1076 A / m ² .

[0074] Figures 4A-4D show that at a typical temperature of 54 ° C, the material removal rate is relatively constant with current density, so that for the same mixture of citric acid and ammonium hydrodifluoride at any given bath temperature, the material removal rate is relatively immune to changes in current density. Similar trends were observed at all temperatures from 21 ° C to 85 ° C, and it seems that these trends persist at temperatures below 21 ° C (but above the freezing temperature of the solution) and above 81 ° C (but below the boiling point of the solution). As occurs under almost all conditions of temperature and current, regardless of the concentration of HFA, when the concentration of citric acid increases above a certain level, usually between 600 g / l and 780 g / l, the material removal rate is significantly reduced. Therefore, in order to maintain the ability to achieve a certain level of material removal, when it is desirable to give the workpiece a certain shape, the concentration of citric acid should usually be kept at less than 600 g / l.

[0075] Figures 4E-4G show, at a characteristic high temperature of 85 ° C and three different citric acid concentrations, the effect of current density on the material removal rate, and Figures 4H-4J show the effect of current density on surface finish under the same set of conditions. Figure 4E shows, like figures 4F and 4G, but to a lesser extent, that the ability to remove material from the electrolyte solution is maximum at maximum concentrations of ammonium hydrodifluoride and quite significant at high temperature. It should be noted that although Figure 4E only shows data at 120 g / L citric acid, substantially the same material removal rates are observed at citric acid concentrations of 60 g / L, 120 g / L and 300 g / L. But, as shown in FIG. 4F, at 600 g / L citric acid, the concentration of citric acid appears to provide some degree of surface protection against large-scale exposure, and material removal rates drop compared to lower citric acid concentrations. At 780 g / l, as shown in Figure 4G, removal rates are reduced even more. Regardless of the concentration of ammonium hydrodifluoride and citric acid, the removal of the material seems to be independent of the current density.

[0076] Figure 4H shows that at high temperature and the most moderate concentration of citric acid, a moderate degree of deterioration in surface finish is observed at almost all concentrations of ammonium hydrodifluoride and current densities. However, when considering together figures 4E and 4H it is seen that stands out one processing mode. At a concentration of citric acid of 120 g / l, a low level of ammonium hydro difluoride of 10 g / l and a high current density of 1076 A / m ² , material removal is suppressed, and as a result, a significant improvement in the quality of surface finish is achieved. This may provide additional confirmation of the theory discussed above and that the increased current density can create an excess of oxygen on the surface of the material, sufficient to fill the "valleys" in the surface morphology, so that the "peaks" are preferably exposed to fluoride ions formed by dissociation ammonium hydrodifluoride. This effect, combined with the possible microbarrier effect of citric acid, can even be more strongly observed in figure 4I (at 600 g / l of citric acid) and in figure 4J (at 780 g / l of citric acid), which show a reduced quality deterioration surface finishes, and in some cases, improved surface finishes, at higher concentrations of citric acid and higher current densities separately, and even more so, when a combination of higher concentrations of citric acid and higher densities current. For example, a significant improvement in the quality of surface finish occurs at 10 g / l and 20 g / l of ammonium hydrodifluoride when switching from 600 g / l to 780 g / l of citric acid.

[0077] However, this effect seems to have a limit, since it can be seen that the quality of the surface finish deteriorates sharply at the highest concentration of ammonium hydrodifluoride 120 g / l and higher current densities when switching from 120 g / l to 600 g / l and further to 780 g / l of citric acid. A similar result was obtained with 60 g / L ammonium hydrodifluoride, at least with an increase in the concentration of citric acid from 600 g / L to 780 g / L.

[0078] As shown in Tables 3A-3C and 4A-4C below, operating conditions for surface finishing of sheet products that require minimal removal of material and it is desirable to improve the quality of surface finish from the most moderate to high, and for micropolishing, in which no material removal is required, and a strong improvement in the quality of the surface finish is desirable, can be achieved for a wide range of mixtures of electrolytes, temperatures and current densities. Tables 3A-3C and 4A-4C do not include electrolytes consisting essentially of water and citric acid and practically free of ammonium hydrodifluoride, although with this solution it is possible to obtain essentially zero removal of material, as well as improving the quality of the surface from very moderate to high in a wide range of temperatures and current densities, since these conditions are discussed separately with reference to figures 1A-1C. Similarly, Tables 3A-3C and 4A-4C do not include electrolytes consisting essentially of water and ammonium hydro difluoride and substantially free of citric acid, since these conditions were discussed separately with reference to FIGS. 2A-2D. Tables 3A-3C are separated by fineness levels and then arranged in increasing order of HFA concentration. Tables 4A-4C are separated by levels of citric acid concentration and then organized in order of increasing concentration of HFA.

[0079] From the data presented in tables 3A-3C, some trends follow. Firstly, over the entire range of citric acid concentrations (from 60 g / l to 780 g / l), ammonium hydrodifluoride concentrations (from 10 g / l to 120 g / l), temperatures (from 21 ° C to 85 ° C) and current densities (from 10.8 A / m ² to 1076 A / m ² ), low or almost zero results were obtained on material removal and improving the quality of surface finish. Therefore, aqueous solutions of citric acid and HFA, with the practical absence of a strong acid, can provide a fine surface finish with minimal material loss at such low concentrations as 60 g / l citric acid and 10 g / l HFA, and such high concentrations as 780 g / l of citric acid and 120 g / l of HFA, and with some combinations between them.

Table 3A Superfine finishes Citric Acid (g / l) HFA (g / l) Temperature (° C) Current density (A / m ² ) Material removal (mm / h) Change in surface finish (%) 780 10 85 1076 0.168 39.2 180 10 85 1076 0.208 36,4 120 10 85 1076 0.232 33.3 300 10 71 1076 0.156 30,4 780 10 71 53.8 0,100 30,4 780 10 71 10.8 0.108 30,2 300 10 54 1076 0.640 38.9 780 twenty 71 538 0,100 44.8 600 twenty 71 1076 0.188 40,0 180 twenty 54 1076 0.168 31.9 780 twenty 21 1076 0,044 30.9 780 60 54 1076 0.160 36.1 600 60 21 1076 0,200 46.9 780 60 21 538 0,088 42.0 600 60 21 538 0,080 37.9 780 60 21 1076 0.204 34.6 780 120 21 538 0.116 49.1 600 120 21 1076 0.168 44.7

[0080] In general, as shown in Table 3A, maximum levels of improvement in surface finish (ie, more than a 30% reduction in surface roughness) were obtained at higher current densities of 538-1076 A / m ² , at moderate and higher concentrations of citric acid 120-780 g / l and, as a rule, at lower concentrations of HFA 10-20 g / l. At lower HFA concentrations, in the range of 10-20 g / l, higher temperatures 71-85 ° C tend to give better surface finishes at higher concentrations of citric acid 600-780 g / l, while a more moderate temperature is 54 ° C provided a fine surface finish at moderate concentrations of citric acid 120-300 g / l. However, significant improvements in surface finish were also obtained under conditions of low HFA concentration, moderate concentration of citric acid and high temperature (10 g / l HFA, 120 g / l citric acid, 85 ° C) and low concentration of HFA. moderate concentration of citric acid and lower temperature (20 g / l HFA, 180 g / l citric acid, 54 ° C). With an increased concentration of HFA, in the range of 60-120 g / l, lower temperatures of 21-54 ° C tend to give better surface finishes at higher citric acid concentrations of 600-780 g / l and higher current densities. In addition, a significant fineness of the surface finish was achieved at lower current densities of 10.8-53.8 A / m ² at high concentrations of citric acid 780 g / l and high temperatures 71-85 ° C as at a low HFA concentration of 10 g / l , and at a high concentration of HFA 120 g / l, as shown in figure 4H.

Table 3B High fineness Citric Acid (g / l) HFA (g / l) Temperature (° C) Current density (A / m ² ) Material removal (mm / h) Change in surface finish (%) 780 10 85 538 0.132 28.8 60 10 85 1076 0.276 28.0 300 10 85 1076 0.216 25.6 600 10 85 538 0,084 25.0 600 10 85 1076 0.220 24.5 780 10 85 10.8 0.136 17.9 600 10 71 538 0,076 19.6 180 10 71 1076 0.192 18.8 180 10 54 1076 0,200 25.0 780 10 54 538 0.024 21,2 780 10 54 53.8 0,088 15.3 300 twenty 85 1076 0.212 30,0 780 twenty 85 10.8 0.244 15.7 780 twenty 71 1076 0.196 27.1 780 twenty 71 0 0.176 22.1 180 twenty 71 1076 0.188 15.1 780 twenty 54 1076 0.228 28.6 300 twenty 54 1076 0.144 25.0 600 twenty 54 1076 0.164 18.0 780 twenty 54 538 0,100 16.7 780 twenty 54 215 0.108 15.6 780 twenty 21 538 0.016 20.3 300 60 21 1076 0.192 21.3 780 120 85 10.8 0.004 30,0 780 120 71 10.8 0,000 25.0 780 120 71 53.8 0.002 23.7 780 120 54 10.8 0,032 16,4 780 120 21 1076 0.196 16.3

[0081] In general, as shown in Table 3B, high, but not the highest levels of improvement in surface finish (ie between about 15% and about 30% reduction in surface roughness) were obtained at lower HFA concentrations 10-20 g / l and moderate - higher temperatures of 54-85 ° C, and mainly, but not exclusively, at higher current densities of 538-1076 A / m ² . As a rule, these results were achieved with high concentrations of citric acid 600-780 g / l. For example, while HFA concentrations of 10–20 g / L usually produced excellent results at higher current densities and higher citric acid concentrations, excellent results were also obtained using low citric acid concentrations of 60–300 g / L at low density. a current of 10.8 A / m ² and a high temperature of 85 ° C, as well as at a low current density of 53.8 A / m ² and the most moderate temperature of 54 ° C. High levels of improvement in surface finish were also achieved at high HFA levels of 120 g / l, both at high temperature and low current density (71-85 ° C and 10.8-53.8 A / m ² ), and at low temperature and high current density (21 ° C and 1076 A / m ² ), in all cases - at high concentrations of citric acid 780 g / l. In this regard, it seems that there is some complementary activity between temperature and current density in that similar surface finishes can be achieved for a solution with a high concentration of citric acid by using a higher current density with a lower temperature or by using a lower current density with a higher temperature. See also Figures 4H-4J, which show that high temperature conditions combined with high current density tend to give maximum surface finish improvements.

Table 3C Moderate fineness Citric Acid (g / l) HFA (g / l) Temperature (° C) Current density (A / m ² ) Material removal (mm / h) Change in surface finish (%) 600 10 85 10.8 0.216 4.0 600 10 85 215 0.232 1.9 780 10 71 0 0,100 14.3 780 10 71 215 0,048 9.8 600 10 71 0 0.164 6.0 780 10 71 538 0,064 5,4 780 10 21 53.8 0,040 14.5 60 10 21 1076 0.148 13.5 780 twenty 85 215 0.260 7.7 780 twenty 85 53.8 0.216 7.7 780 twenty 85 0 0.232 5.7 600 twenty 85 1076 0.184 6.2 300 twenty 71 1076 0,200 7.1 780 twenty 71 53.8 0.172 2.0 600 twenty 54 538 0,064 8.2 600 twenty 21 538 0,032 13,2 120 twenty 21 1076 0.164 10.6 300 twenty 21 1076 0.148 10,4 600 twenty 21 1076 0,032 6.7 60 twenty 21 1076 0.124 6.8 180 twenty 21 1076 0.132 4.2 780 twenty 21 53.8 0,032 1.7 120 60 21 1076 0.196 11.3 60 60 21 1076 0.224 4.2 780 120 85 0 0.016 11.1 780 120 85 53.8 0.016 2.2 780 120 54 0 0.008 13.5 780 120 54 53.8 0,020 5.9 780 120 21 10.8 0.004 7.8 300 120 21 1076 1,400 2,3

[0082] In general, as shown in Table 3C, the most moderate levels of improvement in surface finish (ie, less than about 15% reduction in surface roughness) were obtained at lower HFA concentrations of 10-20 g / L or more high temperatures 71-85 ° C and mainly over the entire range of current densities of 10.8-1076 A / m ² . As a rule, these results were achieved with high concentrations of citric acid 600-780 g / l. One notable exception to this trend is that moderate to high surface finishes were also obtained at all HFA concentrations of 10-120 g / L and low to moderate citric acid concentrations of 60-300 g / L at low temperature. 21 ° C and high current density 1076 A / m ² .

Table 4A The lowest concentration of citric acid Citric Acid (g / l) HFA (g / l) Temperature (° C) Current density (A / m ² ) Material removal (mm / h) Change in surface finish (%) 180 10 85 1076 0.208 36,4 120 10 85 1076 0.232 33.3 60 10 85 1076 0.276 28.0 180 10 54 1076 0,200 25.0 180 10 71 1076 0.192 18.8 60 10 21 1076 0.148 13.5 180 twenty 54 1076 0.168 31.9 180 twenty 71 1076 0.188 15.1 120 twenty 21 1076 0.164 10.6 60 twenty 21 1076 0.124 6.8 180 twenty 21 1076 0.132 4.2 120 60 21 1076 0.196 11.3 60 60 21 1076 0.224 4.2

[0083] As shown in table 4A, at low concentrations of citric acid 60-180 g / l, a uniform improvement in surface finish seems to require a high current density. In general, the maximum improvement in the quality of surface finish was obtained at low HFA concentrations of 10–20 g / L and at moderate to high temperatures of 54–85 ° C. A low to moderate improvement in surface finish was achieved with HFA concentrations of 10-60 g / L and a low temperature of 21 ° C.

Table 4B Moderate citric acid concentrations Citric Acid (g / l) HFA (g / l) Temperature (° C) Current density (A / m ² ) Material removal (mm / h) Change in surface finish (%) 300 10 54 1076 0.188 38.9 300 10 71 1076 0.156 30,4 300 10 85 1076 0.216 25.6 600 10 85 538 0,084 25.0 600 10 85 1076 0.220 24.5 600 10 71 538 0,076 19.6 600 10 71 0 0.164 6.0 600 10 85 10.8 0.216 4.0 600 10 85 215 0.232 1.9 600 twenty 71 1076 0.188 40,0 300 twenty 85 1076 0.212 30,0 300 twenty 54 1076 0.144 25.6 600 twenty 54 1076 0.164 18.0 600 twenty 21 538 0,032 13,2 300 twenty 21 1076 0.148 10,4 600 twenty 54 538 0,064 8.2 600 twenty 21 1076 0,032 6.7 300 twenty 71 1076 0,200 7.1 600 twenty 85 1076 0.184 6.2 600 60 21 1076 0,200 46.9 600 60 21 538 0,080 37.9 300 60 21 1076 0.192 21.3 600 120 21 1076 0.168 44.7 300 120 21 1076 1,400 2,3

[0084] As shown in table 4B, at moderate concentrations of citric acid 300-600 g / l, a significant improvement in the quality of surface finish usually requires higher current densities of 538-1076 A / m ² and occurs mainly at low HFA concentrations of 10-20 g / l At the lowest HFA concentration of 10 g / l, higher temperatures of 54-85 ° C lead to the achievement of the best results, while at an HFA concentration of 20 g / l, good results are also achieved in the temperature range of 21-85 ° C. At high HFA concentrations of 60-120 g / l, an improvement in the quality of the surface finish most typically occurs at a lower temperature of 21 ° C.

Table 4C The highest concentration of citric acid Citric Acid (g / l) HFA (g / l) Temperature (° C) Current density (A / m ² ) Material removal (mm / h) Change in surface finish (%) 780 10 85 1076 0.168 39.2 780 10 71 53.8 0,100 30,4 780 10 71 10.8 0.108 30,2 780 10 85 538 0.132 28.8 780 10 54 538 0.024 21,2 780 10 85 10.8 0.136 17.9 780 10 54 53.8 0,088 15.3 780 10 21 53.8 0,040 14.5 780 10 71 0 0,200 14.3 780 10 71 215 0,048 9.8 780 10 71 538 0,064 5,4 780 twenty 71 538 0,100 44.8 780 twenty 21 1076 0,044 30.9 780 twenty 54 1076 0.228 28.6 780 twenty 71 1076 0.196 27.1 780 twenty 71 0 0.176 22.1 780 twenty 21 538 0.016 20.3 780 twenty 54 538 0,100 16.7 780 twenty 85 10.8 0.244 15.7 780 twenty 54 215 0.108 15.6 780 twenty 85 53.8 0.216 7.7 780 twenty 85 215 0.260 7.7 780 twenty 85 0 0.232 5.7 780 twenty 71 53.8 0.172 2.0 780 twenty 21 53.8 0,032 1.7 780 60 21 538 0,088 42.0 780 60 54 1076 0.160 36.1 780 60 21 1076 0.204 34.6 780 120 21 538 0.116 49.1 780 120 85 10.8 0.004 30,0 780 120 71 10.8 0,000 25.0 780 120 71 53.8 0.008 23.7 780 120 54 10.8 0,032 16,4 780 120 21 1076 0.196 16.3 780 120 54 0 0.008 13.5 780 120 85 0 0.016 11.1 780 120 21 10.8 0.004 7.8 780 120 54 53.8 0,020 5.9 780 120 85 53.8 0.016 2.2

[0085] From a comparison of Table 4C with Tables 4A and 4B, it can be seen that many operating conditions to achieve improved surface finish, with virtually no or minimal material loss, occur at high concentrations of citric acid 780 g / L. As shown in table 4C, at high concentrations of citric acid 780 g / l, a significant improvement in the quality of surface finish can be obtained at almost all current densities of 10.8-1076 A / m ² and from low to high temperatures of 21-85 ° C and how at low concentrations of HFA 10-20 g / l, and high concentrations of HFA 120 g / l.

[0086] Figures 5A and 5B show material removal rates and changes in surface finish quality at a characteristic low temperature of 21 ° C and a characteristic high current density of 538 A / m ² . As can be seen in Figure 5B, the deterioration of the surface finish is most moderate at all citric acid concentrations below 600 g / L, for HFA concentrations below 60 g / L, and that the surface finish is actually improved for all HFA concentrations from 10 to 120 g / l at high concentrations of citric acid above 600 g / l and especially at 780 g / l. In addition, FIG. 5A shows that the material removal rate under these operating conditions is relatively low. Therefore, working in this range of composition, temperature and current density would be desirable to achieve the most moderate controlled removal of material with minimal surface deterioration, or perhaps with the most moderate improvement in surface finish, but would not be particularly effective for large-scale material removal.

[0087] Similarly, Figures 6A and 6B show material removal rates and changes in surface finish quality at a characteristic low temperature of 21 ° C and high current density of 1076 A / m ² . From Figure 6B, it can be seen that a slight to moderate improvement in surface finish is achieved at all citric acid concentrations below 600 g / L for HFA concentrations of more than 10 g / L and less than 120 g / L, and that surface finish is most significantly improved at citric acid concentrations of 600 g / l and higher. In addition, Figure 6A shows that the material removal rate under these operating conditions is relatively low, with the exception of compositions of about 300 g / L citric acid and 120 g / L HFA, where the material removal rate is higher without causing any significant surface deterioration. Therefore, working in these ranges of composition, temperature and current density would be desirable to achieve the most moderate controlled removal of material with minimal surface deterioration, or perhaps the most moderate improvement in surface finish, but would not be particularly effective for large-scale removal of material.

[0088] Figures 7A and 7B show that under certain conditions, controlled removal of material and improved surface finish can be achieved simultaneously. In particular, at a HFA concentration of about 10 g / l, Figure 7A shows the most stable steady material removal rates at all citric acid concentrations when the workpiece is exposed to an electrolyte solution at a high temperature of 85 ° C and at a high current density of 1076 A / m ² . Under the same conditions, Figure 7B shows an actual improvement in surface finish at all citric acid concentrations greater than or equal to 60 g / L. Even at higher concentrations of HFA from 20 g / l to 120 g / l, the resulting removal of material can directly depend on the concentration of HFA, without actually affecting the quality of the surface finish. However, at maximum citric acid concentrations of 600 g / l or more, material removal rates are significantly reduced.

[0089] Some ranges of operating conditions have been identified in which controlled removal of material can be achieved, while the deterioration in surface finish is only the most moderate, usually with an increase in roughness of less than about 50%. Figures 8A-8B, 9A-9B, and 10A-10B illustrate exemplary modes of operation in this category.

[0090] Figure 8A shows that under conditions of high temperature (85 ° C) and low current density (10.8 A / m ² ), an almost constant rate of material removal can be achieved at all HFA concentrations for citric acid concentrations in the range of about 60 g / l to about 300 g / l, with higher material removal rates that are directly dependent on the concentration of HFA. Figure 8B shows that for these ranges of concentrations of citric acid and HFA, the deterioration in surface finish is consistently the most moderate, with little or no reference to specific concentrations of citric acid and HFA. Concentrations of citric acid of 600 g / l and higher greatly reduce or even eliminate the ability of the electrolyte solution to remove material, as well as, with the exception of the HFA concentration of 60 g / l, moderate deterioration in surface finish, and may even show a slight increase in finish quality surface. Figures 9A and 9B show very similar results under conditions of high temperature (85 ° C) and high current density (538 A / m ² ), and figures 10A and 10B show that similar results can be approached even at a slightly lower temperature of 71 ° C and at the most moderate current density of 215 A / m ² .

[0091] Based on the test results disclosed herein, it becomes clear that by controlling the temperature and current density, the same bath with an aqueous electrolyte solution can be used in a multi-stage process, which involves first removing the most moderate and controlled amount of material at a relatively low density current, and then repair the damaged surface by increasing the current density to a high level while maintaining or slightly lowering the temperature. For example, using a solution containing 300 g / l of citric acid and 120 g / l of HFA, you can get the most moderate rates of material removal at a temperature of 85 ° C and a current density of 53.8 A / m ² (see figure 3D), with deterioration in quality surface finishes by less than 30%, and then improved surface quality can be obtained at the same temperature and current density of 1076 A / m ² (see figures 7A and 7B), while removing less material.

[0092] Numerous different combinations of multi-stage processing conditions can be selected by varying the concentration of citric acid in addition to temperature and current density, due to the effect of greatly softening the removal of material that results when the concentration of citric acid increases to 600 g / l or higher. For example, Figures 8A and 8B show that using an electrolyte solution with 120 g / l HFA at a temperature of 85 ° C and a current density of 10.8 A / m ² , aggressive material removal can be achieved with the most moderate deterioration in surface quality at a concentration of citric acid 300 g / l in the first stage of processing, and then simply by increasing the concentration of citric acid to 780 g / l in the second stage of processing, the removal of material can actually be stopped, while the quality of surface finish is significantly improved. Similar results can be obtained using conditions of high temperature, higher current density according to figures 9A and 9B, or conditions of moderate high temperature, moderate current density according to figures 10A and 10B.

[0093] It has been found that very low concentrations of ammonium hydrodifluoride are effective both in material removal and in micropolishing. As shown in FIG. 1A, material removal rates are greatest at elevated temperatures, therefore, lower concentrations of ammonium hydrodifluoride are expected to be more effective at higher temperatures such as 85 ° C or more. In one exemplary electrolyte solution having both citric acid and ammonium hydrodifluoride concentrations of 2 g / L, material removal and surface finish quality changes were observed. At a current density of 285 A / m ² , material removal rates of 0.008 mm / h were recorded with a corresponding change (deterioration) in surface finish by -156%. At 0 A / m ² , material removal rates of 0.0035 mm / h were recorded with a corresponding change in surface finish quality of -187%.

[0094] Similarly, when 2 g / L HFA was treated in an aqueous solution and without citric acid with a supplied current of 271 A / m ² , material removal rates of 0.004 mm / h were recorded with a corresponding change (deterioration) in surface finish by -162%. At 0 A / m ² , material removal rates of 0.0028 mm / h were recorded with a corresponding change in surface finish quality of -168%.

[0095] Although it would be preferable to use the smallest amount of HFA necessary to be effective, concentrations well above 120 g / L can be used, including concentrations of ammonium hydrodifluoride at levels as high as 240 g / L to 360 g / l, and even concentrations exceeding the saturation concentration in water. The efficiency of electrolyte solutions at high concentrations of HFA was tested by adding HFA to a solution of 179.9 g / l citric acid with a temperature fixed at 67 ° C and current densities in the range from 10.8 A / m ² to 255000 A / m ² . Since this solution has a relatively low electrical resistance, it was expected that higher HFA concentrations could provide higher solution conductivity, especially at higher current density levels. The temperature was also raised above room temperature to reduce electrolyte resistance. Samples of both PM titanium and nickel-based alloy 718 were subjected to electrolyte and, as HFA was added, the bulk material was removed and micropolished. HFA was added up to its saturation point in and for the electrolyte. The HFA saturation point (which varies with temperature and pressure) at these parameters was between about 240 g / L and about 360 g / L. The data shown in table 5 indicate that the electrolyte solution was effective both for the removal of bulk metal and for micropolishing at HFA concentrations up to and above saturation concentrations in water.

[0096] A test was conducted to determine the effectiveness of electrolyte solutions for micropolishing and bulk metal removal at relatively high current densities, including those approaching 255,000 A / m ² . From the literature it is clear that electrolytes with low resistance values can withstand high current densities. Certain combinations of citric acid concentration and HFA concentration show a particularly low resistance. For example, an electrolyte solution containing about 180 g / l citric acid in the temperature range of about 71 ° C-85 ° C was investigated at high current densities. Samples of technically pure (PM) titanium and nickel-based alloy 718 were exposed to this electrolyte solution with a gradual increase in current density in the range from 10.8 A / m ² to 255000 A / m ² . The data shown in table 5 indicate that the removal of bulk material and micropolishing were achieved at all investigated current densities in this range, including 255,000 A / m ² . Compared to the treatment of titanium and titanium alloys, higher current densities, in particular at about 5000 A / m ^2, can be applicable to nickel-based alloys.

[0097] While titanium PM is efficiently processed using relatively low voltages less than or equal to about 40 volts, higher voltages can also be used. In one exemplary test, titanium PM was treated in a bath with an aqueous electrolyte solution containing about 180 g / L citric acid and about 120 g / L HFA at 85.6 ° C, applying a potential of 64.7 VDC and a current density of 53160 A / m ² . Under these conditions, a volumetric metal removal rate of 5 mm / h was achieved, along with a 37.8% improvement in surface roughness according to the profilometer, resulting in a uniform, visually shiny reflective appearance. An electrolyte of the same chemical composition remained effective on PM titanium samples to remove bulk metal after increasing the voltage to 150 VDC and lowering the current density to 5067 A / m ² , but under these conditions the metal removal rate slowed down to 0.3 mm / h, and the quality surface finishes slightly deteriorated to a matte appearance.

[0098] For some metals and alloys, higher stresses can be equally or even more effective in achieving improved bulk removal of the material, surface finish, or both. In particular, certain metals, including, but not limited to, nickel-based alloys (such as Waspaloy and 718 nickel alloys), 18 carat gold, pure chromium and Nitinol alloys seem to benefit from higher voltage processing in terms of faster removal of bulk metal and / or better surface finish. In one exemplary experiment at a relatively high voltage for nickel alloy 718, samples treated in an aqueous electrolyte solution containing about 180 g / L citric acid and about 120 g / L HFA, at 86.7 ° C, using a potential of 150 VDC and density current 4934 A / m ² , led to a rate of volumetric metal removal of only 0.09 mm / h, but, based on surface measurements by filometer, a uniform improvement in surface finish by 33.8%.

[0099] To evaluate the effect of the accumulated dissolved metal in the electrolyte solution, a batch of rectangular bars Ti-6Al-4V with dimensions of 6.6 cm by 13.2 cm by approximately 3.3 meters in an amount of 21 pieces was sequentially processed in a bath of approximately 1135 liters. The treatment was intended to demonstrate highly controlled metal removal in typical types of rolled products. From a total of 21 rectangular bars, the material was removed in a total amount of 70.9 kg and suspended in an electrolyte solution. The treatment of the first bar was started at 0 g / L of dissolved metal in solution, and the final bar was processed at a dissolved metal content of more than 60 g / L. From the beginning to the end of the treatment, no harmful effects were found on the state of the metal surface or the rate of metal removal, and as a result of the increase in the dissolved metal content in the electrolyte solution, no significant changes in any operating parameters were required. This differs from the results of acid etching of titanium in HF / HNO ₃ , in which the solution becomes significantly less effective even at titanium concentrations in the solution of 12 g / L. Similarly, electrochemical sizing is prevented by high levels of dissolved metal in the electrolyte solution, since metal particles can clog the gap between the cathode and the anode preform and, if the solid is electrically conductive, can even cause a short circuit.

[0100] Modulation of cracks and removal of the oxide layer

[0101] The aqueous electrolyte solution disclosed herein can be used to modulate or round the bottom of surface cracks, thereby eliminating the pointed ends of cracks that form at varying depths on the surface of the metals and are most dangerous in the family of reactive metals, including, but not limited to, titanium and titanium alloys, nickel-based alloys, zirconium and the like, when these metals are cooled from elevated temperatures in an oxygen-containing atmosphere. For example, such surface cracks can occur during cooling from processes including, but not limited to, hot working (for example, forging, rolling, superplastic molding, and the like), welding, and heat treatment. Moreover, an aqueous electrolyte solution can modulate these cracks with relatively small loss in yield. The rounded or modulated end of the crack is suitable for subsequent hot processing of metal, since such a crack is able to “heal” during subsequent hot processing, whereas a typical crack that occurs when the metal is cooled, if it is not completely removed, “creeps” and the metal breaks or crack when post-processing.

[0102] Figure 11 illustrates a conventional crack removal process that involves mechanical removal, usually grinding or grinding a completely uniform layer of material on a machine to expose the bottom of the deepest surface crack. As schematically shown, this leads to a significant loss of material. In contrast, Figure 12 illustrates a crack modulation process using an electrolyte solution in combination with an electric current supply to expand the crack and round the end of the crack at its bottom so that the crack will not propagate when the workpiece is subjected to further hot working operations or put into operation.

[0103] Accordingly, the preferred conditions of the modulation process of the end of the crack or removal are those that give very high removal rates, while minimizing or eliminating hydrogenation. As discussed below, these conditions are generally obtained when the concentration of carboxylic acid (e.g., citric acid) is low, the concentration of fluoride ions (e.g., in the form of ammonium hydrodifluoride) is high and the temperature is high. Although effective large-scale removal can be carried out at all power densities, good results were achieved when the power density is low and the power is cyclically changed to have the “DISABLED” period as long as possible to minimize the amount of hydrogen introduced into material.

[0104] When processing within these process parameters, the ability of an aqueous electrolyte solution to modulate cracks provides a significant overall improvement in metal yield compared to modern processing methods in which the metal surface is uniformly or locally removed mechanically (by grinding or grinding) until the bottom of the deepest crack will not be sanded flush with the surrounding metal. In addition, the processing costs and consumables are significantly lower when using the electrolyte solution and the method disclosed herein compared to modern mechanical removal methods.

[0105] Another significant advantage of the method achievable with an aqueous electrolyte solution is the removal of a layer of reactive metal oxide or, in the case of titanium and titanium alloys, an alpha shell. Like the oxide layers of other reactive metals, the alpha shell is an oxygen-enriched phase that occurs when titanium and its alloys are exposed to heated air or oxygen. The alpha shell is fragile and tends to create a series of surface microcracks that will degrade performance, including strength, fatigue properties and corrosion resistance of the metal part. Titanium and titanium alloys are among the reactive metals, which means that they react with oxygen and form a brittle bonded oxide layer (TiO ₂ for Ti, ZrO ₂ for Zr, etc.) whenever they are heated in air or in oxidizing atmosphere at one or more high temperatures at which a natural oxide layer forms, which depends on the particular alloy and oxidizing atmosphere. As noted above, the oxide layer or the oxide layer of the alpha shell can be created by any heating of the metal to temperatures necessary for forging in rollers or rolling in a rolling mill, as a result of welding or by heating to forge a finished part or hot forming a part. The alpha shell is fragile and full of microcracks that penetrate the metal volume, potentially causing premature failure upon stretching or fatigue and making the surface more susceptible to chemical attack.

[0106] Therefore, the alpha-shell layer must be removed before any subsequent hot or cold pressure treatment or operation of the finished product. The aqueous electrolyte solutions described herein and methods using these solutions can remove the alpha shell until an unaffected base metal appears. Removing the alpha shell is a difficult problem when machining titanium and titanium alloys, since the alpha shell is exceptionally resistant to impact, and conventional wisdom suggests that some mechanical intervention is required before electrochemical treatment.

[0107] This difficulty was clearly manifested in an early test in which the titanium alpha shell was first blown with a shot and / or lightly ground as a pretreatment before electrochemical treatment. Having been subjected to such abrasive treatment, the rest of the oxide layer is most easily removed during the “ON” DC power cycle, followed by a simple brushing off with a hard brush, followed by the “OFF” DC power source cycle, in a cyclic process, which is then repeated several times. It should be noted that an alternative to brushing with a stiff brush would be a high-pressure injection system. As soon as the alpha-shell layer is removed, the rates of material removal during electrochemical etching increase. However, the implementation of such a multi-stage technological cycle is very expensive and gives a low income.

[0108] To overcome these difficulties, a preform with a layer of oxide or alpha shell is treated in a bath with an aqueous electrolyte solution having low concentrations of carboxylic acid (eg, citric acid), high concentrations of fluoride ions (eg, ammonium hydrodifluoride), high temperatures and preferably at low power densities. Low concentrations of citric acid, high concentrations of ammonium hydrodifluoride and high temperatures maximize material removal rates, and since removal rates are relatively insensitive to current density when power is applied cyclically, a lower power density is used to provide less hydrogen penetration into the material surface.

[0109] In general, the aqueous electrolyte solution disclosed herein and the method using such a solution make it possible to remove oxide layers and a titanium alpha shell from the surface of the reactive metal in a controlled reproducible manner. In this way, they differ from modern acid etching of HF-HNO ₃ , in which a strongly exothermic reaction takes place, which in the case of titanium uses the liberated titanium as a catalyst in the reaction, thereby causing continuous changes in acid concentration and reaction rates, making reproducibility difficult and making it almost impossible to accurately surface metal removal. Moreover, the disclosed aqueous electrolyte solution and method do not introduce harmful hydrogen into the metal volume. Indeed, the disclosed method can be implemented in such a way as to remove hydrogen from the bulk of the metal. On the contrary, the modern method, by its very nature, introduces harmful hydrogen into the bulk of the metal, making it necessary for additional expensive degassing stages to remove hydrogen.

[0110] The aqueous electrolyte solution is environmentally friendly and does not create hazardous waste, while the modern method uses environmentally hazardous and dangerous hydrofluoric acid (HF) and nitric acid (HNO ₃ ), which are extremely difficult to handle and which can only be used in strictly regulated programs. As a result of this, processes using such an aqueous electrolyte solution can be carried out without involving significant air-handling equipment and do not require the operator to wear protective clothing against hazardous chemicals, as is necessary when using a modern process based on HF-HNO ₃ .

[0111] Although the surface finish is not particularly important for modulating cracks and removing the alpha shell, and improving the surface finish is not necessary due to the further processing steps that will be carried out subsequently, it is still advisable not to cause serious damage to the surface of the material or cause pitting or other deep defects in the material.

[0112] In figures 3D, 4E-4J, 7A, 8A, 9A and 10A, it can be seen that the highest rates of material removal were obtained by maximizing the concentration of HFA and temperature (ie, at a HFA concentration of 120 g / l and a temperature of 85 ° C, as shown in the graphs), while maintaining the concentration of citric acid at or below 300 g / l. In addition, as emphasized in some of the figures, including in figures 4E-4G, since citric acid tends to mitigate the effect of fluoride ions on the surface of the material, it can be seen that the removal rate of the material tends to increase sharply as the concentration of citric acid approaches to 0 g / l. However, under conditions of an almost zero concentration of citric acid, figures 4H, 7B, 8B, 9B, and 10B show that surface pitting and serious deterioration of surface finish can occur.

[0113] Therefore, when using an aqueous electrolyte solution to modulate cracks, where it is preferable to avoid severe surface damage, at least a small amount of citric acid, for example, 1 g / l or 10 g / l, should be used to mitigate the most serious effects of fluoride- ions. However, when using an aqueous electrolyte solution to remove the alpha shell, where severe surface damage would not be particularly harmful, the most aggressive exposure to fluoride ions may be acceptable without severe pitting, and therefore, the concentration of citric acid can be reduced to almost zero .

[0114] In both a crack modulation situation and an alpha shell removal situation, it is desirable to avoid introducing hydrogen into the material so as to prevent embrittlement. Since, in particular, in Figures 1A-1C, 2A-2B and 4E-4G, the material removal rates are relatively insensitive to current density, and since higher current densities tend to cause hydrogen to enter the material, it is preferable to operate at the lowest effective current densities.

[0115] Although the description is given in connection with exemplary embodiments, those skilled in the art will understand that specific, undescribed additions, deletions, modifications, and substitutions can be made without departing from the spirit and scope of the invention as defined in the appended claims, and that the invention is not limited to the disclosed specific embodiments.

Claims (29)

1. The use of an aqueous electrolyte solution for electrochemical surface treatment of a metal billet, which is the anode, containing:
from 0.1% by weight to 59% by weight of carboxylic acid; and
from 0.1% by weight to 25% by weight of fluoride salt; and
containing not more than 3.35 g / l strong acid. 2. The use according to claim 1, wherein the fluoride salt is selected from the group consisting of alkali metal fluorides, alkaline earth metal fluorides, etching silicates of compounds and their combinations. 3. The use according to claim 2, wherein the fluoride salt is ammonium hydrodifluoride. 4. The use according to any one of paragraphs. 1-3, wherein the carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, valerianic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, palmitic acid, stearic acid acid and citric acid. 5. The use according to claim 4, wherein the carboxylic acid is citric acid. 6. The use of an aqueous electrolyte solution for electrochemical surface treatment of a metal billet, which is the anode, containing:
from 1.665 g / l to 982 g / l of citric acid; and
from 2 g / l to 360 g / l ammonium hydrodifluoride; and
containing not more than 3.35 g / l strong acid. 7. A method for electrochemical surface treatment of a non-ferrous metal billet, comprising the steps of:
subjecting the surface of the workpiece to an aqueous electrolyte solution in a bath, including citric acid with a concentration of 1 g / l to 982 g / l and ammonium hydrodifluoride with a concentration greater than or equal to 2 g / l, and having no more than 3.35 g / l strong acid;
connect the workpiece to the anode of the DC power source and immerse the cathode of the DC power source in the bath with said electrolyte solution; and
pass current through the bath. 8. The method according to claim 7, in which the temperature of the bath is regulated greater than or equal to 71 ° C. 9. The method according to claim 7, in which the current passed through the bath is less than or equal to 255000 amperes per square meter. 10. The method according to claim 9, in which the current passed through the bath is less than or equal to 538 amperes per square meter. 11. The method according to claim 10, in which the current passed through the bath is less than or equal to 53.8 amperes per square meter. 12. The method according to any one of paragraphs. 7-11, in which the concentration of citric acid is less than 60 g / l. 13. The method according to any one of paragraphs. 7-11, in which the concentration of ammonium hydrodifluoride is greater than or equal to 60 g / l. 14. A method for modulating cracks in the surface of a non-ferrous metal billet, comprising the steps of:
subjecting the surface of the workpiece to an aqueous electrolyte solution in a bath, including carboxylic acid with a concentration of from 1 g / l to 982 g / l and a fluoride salt with a concentration of from 1 g / l to 360 g / l and having no more than 3.35 g / l strong acid;
connect the workpiece to the anode of the DC power source and immerse the cathode of the DC power source in the bath with said electrolyte solution; and
less than 255,000 amperes per square meter are passed through the bath. 15. The method according to 14, further comprising stages, in which:
maintain current for a time sufficient to round the bottom of the cracks; and
interrupt the current and remove the workpiece from the bath before the workpiece surface is removed to the level of the bottom of the cracks. 16. The method according to 14, in which the carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, valerianic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid , palmitic acid, stearic acid and citric acid. 17. The method according to clause 16, in which the carboxylic acid is citric acid. 18. The method according to any one of paragraphs. 14-17, in which the fluoride salt is selected from the group consisting of alkali metal fluorides, alkaline earth metal fluorides, ammonium fluorides, etching silicates of compounds and their combinations. 19. The method of claim 18, wherein the fluoride salt is ammonium hydrodifluoride. 20. A method of removing metal oxide from the surface of a substrate of a non-ferrous metal workpiece, comprising the steps of:
expose the surface of the workpiece to an aqueous solution of an electrolyte in a bath, including carboxylic acid with a concentration of from 1 g / l to 180 g / l and a fluoride salt with a concentration of from 1 g / l to 120 g / l and having no more than 3.35 g / l strong acid;
regulate the temperature of the bath with said electrolyte solution less than or equal to 85 ° C;
connect the workpiece to the anode of the DC power source and immerse the cathode of the DC power source in the bath; and
less than 255,000 amperes per square meter are passed through the bath. 21. The method according to claim 20, in which the carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, valerianic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid , palmitic acid, stearic acid and citric acid. 22. The method according to item 21, in which the carboxylic acid is citric acid. 23. The method according to any one of paragraphs. 20-22, in which the fluoride salt is selected from the group consisting of alkali metal fluorides, alkaline earth metal fluorides, ammonium fluorides, etching silicates of compounds and their combinations. 24. The method according to item 23, in which the fluoride salt is an ammonium hydrodifluoride. 25. A method of removing an alpha shell from a surface of a substrate of a preform of titanium or a titanium alloy, comprising the steps of:
expose the surface of the workpiece to an aqueous solution of an electrolyte in a bath, including carboxylic acid with a concentration of from 1 g / l to 180 g / l and a fluoride salt with a concentration of from 1 g / l to 120 g / l and having no more than 3.35 g / l strong acid;
regulate the temperature of the bath with said electrolyte solution less than or equal to 85 ° C;
connect the workpiece to the anode of the DC power source and immerse the cathode of the DC power source in the bath; and
less than 5000 amperes per square meter is passed through the bath. 26. The method according A.25, in which the carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, valerianic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid , palmitic acid, stearic acid and citric acid. 27. The method according to p, in which the carboxylic acid is citric acid. 28. The method according to any one of paragraphs. 25-27, in which the fluoride salt is selected from the group consisting of alkali metal fluorides, alkaline earth metal fluorides, ammonium fluorides, etching silicates of compounds and their combinations. 29. The method of claim 28, wherein the fluoride salt is ammonium hydrodifluoride.

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