EA036890B1 - Electrolyte solution and electrochemical methods for surface modification - Google Patents

Abstract

An aqueous electrolyte solution including a concentration of citric acid in the range of about 1.6 to about 982 g/L and an effective concentration of ammonium bifluoride (ABF), and being substantially free of a strong acid. Methods of treating the surface of a non-ferrous metal workpiece include exposing the surface to a bath of an aqueous electrolyte solution including a concentration of citric acid less than or equal to about 300 g/L and a concentration of ammonium bifluoride greater than or equal to about 10 g/L, and having no more than about 3.35 g/L of a strong acid, controlling the temperature of the bath to be greater than or equal to about 54°C, connecting the workpiece to the anode of a DC power supply and immersing a cathode of the DC power supply in the bath, and applying a current across the bath.

Description

Cross-reference to related claims

The present application is related to a jointly owned application entitled Electrolyte Solution and Electrolytic Polishing Methods, which is filed concurrently with this application.

Technology area

The 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 oxide removal, on non-ferrous and reactive metals, in particular titanium and titanium alloys.

Background of the invention

In the pressure treatment of reactive metals from the ingot into the finished product and after hot working the finished part, it is necessary to remove material from a specific surface layer of the metal oxide or, in the case of titanium and titanium alloys, what is commonly referred to as an α-shell. These oxygen-rich phases arise when reactive metals are heated in air or oxygen-containing atmospheres. The oxide layer can affect the strength of the material, the fatigue resistance, and the corrosion resistance of the metal. Titanium and titanium alloys are reactive metals, which means they react with oxygen and form a brittle cohesive oxide layer (TiO2 for Ti, ZrO2 for Zr, etc.) whenever they are heated in air or in an oxidizing atmosphere above approximately 480 ° C (900 ° F) depending on specific alloy and oxidizing atmosphere. An oxide layer is created when the metal is heated to temperatures required for typical roll forging or rolling in a rolling mill, by welding, or by heating to forge a finished part or thermoform a part. Reactive metal oxides and the α shell are brittle, and molding is usually accompanied by the formation of a series of surface microcracks that penetrate the bulk of the metal, potentially causing premature fracture due to tension or fatigue and making the surface more susceptible to chemical attack. Therefore, the oxide layer or the α-shell layer must be removed before any subsequent hot or cold working or before putting the finished product into service.

When forming reactive metals such as titanium and titanium alloys from ingot to finished part, it is also important that cracks formed during heat treatment and machining are removed. As described above, these cracks can go deeper than the α-shell and penetrate into the bulk of the metal. Reactive metals are typically heated, hot worked (eg, forged, rolled, drawn, pressed, extruded), cooled and reheated for additional hot working 4 to 8 times to convert the ingot into the final rolled product. The rolled stock is often reheated to form a finished part using techniques including, but not limited to, hot extrusion, rolling, superplastic forming, and closed die forging. Whenever the metal is cooled after hot working, cracks form on the surface that extend into the workpiece. Conventionally, these cracks are removed by grinding, which involves mechanical removal, or by chemical etching in a strong acid, usually HF-HNO3, of a uniform thickness or amount of material from the workpiece until the bottom of the deepest crack is exposed and removed. Grinding or chemical etching to this depth will ensure that all cracks are removed, but it takes a significant amount of time and labor and also results in significant and costly material loss. This is because cracks sometimes extend into the workpiece to a depth of 5% or more of the thickness or diameter of the workpiece or finished part. Crack removal is necessary, however, because if the cracks are not removed before the next hot forming step or using the finished part in service, cracks can propagate and destroy the workpiece or finished part.

In chemistry and industry, electrolysis is a method of using direct electric current (DC) to stimulate an otherwise non-spontaneous chemical reaction. Electrolytic polishing is a well-known electrolysis application to remove burrs from metal parts and to obtain a shiny and smooth surface finish. The workpiece to be electrolytically polished is immersed in a bath with an electrolyte solution and exposed to a direct electric current. The workpiece is supported by the anode, and the cathode is connected to one or more metallic conductors surrounding the workpiece in the bath. Electrolytic polishing is based on two opposite reactions that control the process. The first reaction is a dissolution reaction, during which metal from the surface of the workpiece goes into 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 build-up of the oxide film limits the development of the ion removal reaction. This film is the thickest over the microdepressions and the thinnest over the microprotrusions, and since the electrical resistance is proportional to the thickness of the oxide

- 1 036890 films, the highest rate of metal dissolution occurs at micro-protrusions, and the lowest rate of metal dissolution occurs at micro-cavities. Consequently, microscopic bumps, or peaks, can be selectively removed by electrolytic polishing at a higher rate than the rate of exposure to the corresponding microcavities or valleys.

Another application of electrolysis is in electrochemical machining processes (ECM). With EDT, a strong current is passed between the electrode and the metal workpiece (often over 40,000 A and applied at current densities often over 1.5 million A per square meter), causing material to be removed. An electric current is passed through a conductive fluid (electrolyte) from a negatively charged electrode - tool (cathode) to a conductive workpiece (anode). The cathode tool is shaped to match the desired sizing operation and is advanced into the anode blank. Pressurized electrolyte is injected at a predetermined temperature into the area to be treated. The workpiece material is removed, practically liquidated, at a rate determined by the feed rate of the tool into the workpiece. The gap between tool and workpiece ranges from 80 to 800 microns (0.003 to 0.030 inches). As the electrons cross the gap, the material on the workpiece dissolves and the tool shapes the workpiece into the desired shape. The electrolytic fluid carries away the metal hydroxide formed during the reaction between the electrolyte and the workpiece. Flushing is necessary because the electrochemical sizing process has a low tolerance for metal complexes accumulating 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.

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 herein, are generally classified as acids that are stronger in aqueous solution than hydronium 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 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 down the dissolution etching reaction in order to avoid excessive etching of the workpiece surface. See, for example, US Pat. No. 6,610,194 describing the use of acetic acid as a reaction retardant.

There is an incentive to reduce the use of these strong acids in metal finishing baths, mainly due to their health hazards and waste disposal costs. Citric acid has previously been accepted as a passivating agent for stainless steel products by both the US Department of Defense and ASTM (American Society for Testing Materials) standards. However, while previous studies have shown and quantified the savings from using an industrial citric acid passivation bath to passivate stainless steel, they have been unable to identify a suitable electrolyte solution in which a significant concentration of citric acid could reduce the concentration of strong acids. For example, a 2002 publication entitled Citric Acid & Pollution Prevention in Passivation & Electropolishing describes several advantages of reducing the amount of strong inorganic acids by replacing them with some weaker organic acid. and, in particular, citric acid because of its low cost, availability and relatively safe disposal, but ultimately an alternative electrolyte containing a mixture of mainly phosphoric and sulfuric acids with a small amount of organic acid (not citric acid) was evaluated.

The essence of the invention

Α-shell removal and crack modulation are usually not achieved in electrolytic polishing processes.

The highly acidic components found in typical electrolytic solutions used in the prior art for electrolytic polishing result in hydrogen migration at the metal surface and aggressive, uncontrolled etching that can deepen cracks. In their development of new chemical bases for electrolytic polishing baths using solutions of weak acids and ammonium hydroxide fluoride (HFA), in the absence of strongly acidic components, the inventors have found that both α-shell removal and crack modulation can be effectively achieved by electrolytic polishing. Thus, there are disclosed methods for removing oxides and modulating cracks using electrolytic polishing processes using new bath chemistries suitable for these methods.

In one embodiment, an aqueous electrolyte solution is disclosed comprising from about 0.1

- 2,036890 to about 59 wt.% Carboxylic acid and from about 0.1 to about 25 wt.% Fluoride salt and is substantially free of strong acid.

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 bifluoride to about 360 g / L ammonium bifluoride fluoride salt and is substantially free of strong acid.

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

In one embodiment, a method is disclosed for modulating surface cracks in a nonferrous metal workpiece comprising exposing the surface to an aqueous electrolyte bath having a citric acid concentration less than or equal to about 300 g / L and an ammonium bifluoride concentration greater than or equal to about 60 g / L, and having no more than about 3.35 g / L strong acid, adjusting the bath temperature greater than or equal to about 54 ° C, connecting the workpiece to the anode of the DC power supply and immersing the cathode of the DC power supply into the bath, and current flow through the bath is less than about 53.8 A per square meter.

In one embodiment, a method of removing a metal oxide from a surface of a non-ferrous metal preform is disclosed comprising exposing the surface to an aqueous electrolyte bath having a citric acid concentration of about 1 to about 180 g / L, such as less than or equal to about 60 g / L and an ammonium bifluoride concentration of from about 1 to about 120 g / L, such as greater than or equal to about 60 g / L, and having no more than about 3.35 g / L of strong acid, adjusting the bath temperature to be greater than the temperature freezing the bath and less than or equal to about 85 ° C, for example, greater than or equal to about 54 ° C, connecting the workpiece to the anode of the DC power supply and immersing the cathode of the DC power supply in the bath and passing a current through the bath with a density less than or equal to about 255,000 And per square meter, for example, less than or equal to about 5000 A per square meter, in particular less than about 53.8 A per square meter.

In one embodiment, a method for removing an α-shell from a surface of a titanium or titanium alloy preform is disclosed comprising exposing the surface to a bath of an aqueous electrolyte solution having a citric acid concentration less than or equal to about 60 g / L and an ammonium bifluoride concentration greater than or equal to about 60 g / L, and having no more than about 3.35 g / L strong acid, adjusting the bath temperature greater than or equal to about 54 ° C, connecting the workpiece to the anode of the DC power supply and immersion of the cathode of the DC power supply into the bath and passing a current through the bath of less than about 53.8 A per square meter.

Brief description of figures

FIG. 1A-1B are data plots showing material removal rate and surface finish variation as a function of citric acid concentration in an aqueous electrolyte solution having a moderately low concentration of 20 g / L ammonium bifluoride at a high current density of 1076 A / m ² over a temperature range ;

fig. 2A-2B are data plots showing material removal rate as a function of ammonium bifluoride concentration in an aqueous electrolyte solution containing 120 g / L citric acid at characteristic low and high temperatures, respectively, over a range of current densities;

fig. 2C-2D are data graphs showing variation in surface finish as a function of ammonium bifluoride under conditions corresponding to FIG. 2A-2B, respectively;

fig. 2E-2F are data plots showing material removal rate and surface finish change, respectively, as a function of current density in an aqueous electrolyte solution with substantially no citric acid at 85 ° C;

fig. 3A-3D - data graphs showing the rate of removal of material as a function of citric acid concentration in the aqueous electrolyte solution for several concentrations of ammonium bifluoride at a current density of 53.8 A / m ² and a temperature of 21 ° C, 54 ° C, 71 ° C and 85 ° C respectively;

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

fig. 4E-4G - Data plots showing 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 citric acid

- 3 036890, respectively, for several concentrations of ammonium bifluoride;

fig. 4H-4J are data graphs showing variation in surface finish as a function of current density under conditions corresponding to FIG. 4E-4G respectively;

fig. 5A-5B are data plots showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at low temperature (21 ° C) and high current density (538 A / m ² );

fig. 6A-6B are data plots showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at low temperature (21 ° C) and high current density (1076 A / m ² );

fig. 7A-7B are data plots showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at high temperature (85 ° C) and high current density (1076 A / m ² );

fig. 8A-8B are data plots showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at characteristic high temperature (85 ° C) and low current density (10.8 A / m ² );

fig. 9A-9B are data plots showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at characteristic high temperature (85 ° C) and high current density (538 A / m ² );

fig. 10A-10B are data plots showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a characteristic moderately high temperature (71 ° C) and moderate current density (215 A / m ² );

fig. 11 is a schematic diagram of the sequence that occurs in the prior art process for removing a crack extending into a workpiece from a material surface;

fig. 12 is a schematic diagram of a sequence that occurs in a process using an electrolyte disclosed herein to modulate a crack extending into a workpiece from a material surface.

Detailed description

Disclosed herein are aqueous electrolyte solutions 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 in the substantially absence of a strong acid, such as an inorganic acid, so that the solution contains practically no strong acid. This electrolyte solution is a marked departure from earlier attempts at electrolyte bath treatment of the surface of reactive metals, including but not limited to titanium and titanium alloys, which typically use strong acids and require the amount of water in the electrolyte solution to be kept to an absolute minimum.

The fluoride salt provides a source of fluoride ions for the solution and may be, but is not limited to, ammonium bifluoride, NH ₄ HF ₂ (sometimes abbreviated herein as HFA). While not wishing to be bound by theory, the carboxylic acid appears to mitigate the effect of fluoride ions on the treated surface of the reactive metal and may be, but is not limited to, citric acid. No 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 refer to strong acid concentrations of 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 ...

Trial samples of commercially pure (PM) titanium were immersed in a bath of an aqueous solution containing 60 g / l of citric acid and 10 g / l of HFA at 54 ° C, and the current was supplied at 583 A / m ^2. A sample cut from a titanium strip with the surface obtained after rolling (with a surface roughness of 0.52 μm), exposed to this solution for 15 min, became uniformly smooth (with a surface roughness of 0.45 μm) and cosmetically reflective. Small amounts of HNO ₃ (nitric acid) with 42 ° Be were then added step by step and the prepared test sample was processed several times until surface changes were detected. The samples were not exposed to treatment after each addition of nitric acid until a nitric acid concentration of 3.35 g / L was reached, at which the control panel showed an uneven cosmetic appearance, including pitting corrosion and flaking, with irregular exposure around the sample perimeter, with surface roughness in the range from 0.65 to 2.9 microns and above. Nitric acid is considered to be a borderline strong acid with a dissociation constant not much higher than the dissociation constant of the hydronium ion. Therefore, it is expected that for other stronger acids having the same or greater

- 4036890 dissociation constants than nitric acid, such an electrolyte solution would be similarly effective in controlled material removal and micropolishing at strong acid concentrations less than about 3.35 g / L. However, it is expected that other electrolyte solutions disclosed herein having different citric acid and HFA concentrations and different citric acid and HFA concentration ratios may have a lower tolerance for the presence of a strong acid depending on the specific strong acid as well as operating parameters such as temperature. and current density. Therefore, to enable the efficient use of aqueous electrolyte solutions for material removal and fine surface finishing over a wide range of citric acid and HFA concentrations and over a wide range of temperatures and current densities, no more than about 1 g / L of strong acid should be present, and preferably no more than about 0 , 35 g / l strong acid.

Extensive electrolytic polishing tests have been performed on titanium and titanium alloy specimens using a range of chemical concentrations, current densities and temperatures. Specifically, the test was performed on clean rolled stock (representing the typical metal supply condition of the roll manufacturer, meeting the American Society for Testing Materials (ASTM) or Aerospace Material Specification (AMS) standards) to measure the ability of various solutions and methods to remove metal volume, improve or thinning surface finishes on sheet metal products with low material removal rates; and / or micropolishing metal surfaces for very fine surface finishes with very low material removal rates. In addition, while most of the tests focused on titanium and titanium alloys, the test also showed that the same solutions and methods are more generally applicable to 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® (nickel-based alloy) have also been successfully processed ...

An electrolyte solution containing citric acid and ammonium bifluoride has proven effective in pickling non-ferrous metals and metal alloys at unexpectedly diluted concentrations of both components. In this context, etching is to be understood as encompassing a substantially uniform surface removal. In addition, improvements in surface finish quality have been demonstrated across a wide range of concentrations of both citric acid and ammonium bifluoride. Although any citric acid concentration can be used up to the point of saturation with water (59 wt% or about 982 g / L aqueous solution at standard temperature and pressure), there appears to be a correlation between citric acid concentration and ammonium bifluoride concentration at which citric acid acid sufficiently weakens the effects of etching by fluoride ions formed during the dissociation of ammonium bifluoride that the rate of material removal is sharply reduced, while the micropolishing of the material surface is enhanced. For both etching and micropolishing, some blends having citric acid concentrations as low as 3.6 wt%, or about 60 g / L solution, exhibited etch rates and titanium surface micropolishing results comparable to blends having significantly higher concentrations of citric acid, including up to about 36 wt.%, or about 600 g / L solution. Thus, in these solutions, the etching rate clearly depends more directly on the HFA concentration than on the citric acid concentration. Effective etching and micropolishing have been demonstrated even at extremely low citric acid concentrations, less than about 1 wt%, or about 15 g / L solution. However, the presence of even a minimal amount of fluoride ions is sufficient for some metal removal to occur.

The pickling rate drops significantly at citric acid concentrations above about 600 g / L. However, at this high citric acid concentration, at least in moderate to high current density cases, the surface finish results are improved while the etching rate decreases. Thus, when DC is applied, more dilute citric acid mixtures provide higher rates of material removal from the surface, while more concentrated citric acid mixtures, up to such a high mixture concentration as about 42 wt.%, Or about 780 g / L solution, Provides a smoother and smoother surface with a uniform fine grain structure and no crown effect compared to parts treated with less concentrated citric acid blends.

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 bulky metal can be removed as thin as 0.0001 "and as large and accurate as 0.5000". Such fine control can be achieved by adjusting the combination of citric acid and HFA concentrations, temperature and current density, as well as by varying

- 5 036890 sti and cyclic DC supply. Removal can be performed substantially uniformly on all surfaces of the workpiece, or it can be selectively applied only to certain selected surfaces of the rolled or manufactured component. Removal control is achieved by fine-tuning several parameters, including but not limited to temperature, power density, power injection cycle, HFA concentration, and citric acid concentration.

Removal rates vary directly with temperature, which means that by keeping all other parameters constant, removal becomes slower at lower temperatures and faster at higher temperatures. However, by keeping citric acid and HFA concentrations within certain preferred ranges, high levels of micropolish at high temperatures can also be achieved, which is the opposite of what one would expect.

The removal rate depends on the DC power supply mode. Contrary to what might be expected, the removal rate appears to be inversely related to the continuously supplied DC power, and with continuous supply, increasing the DC power density decreases the removal rate. However, when cycling the DC power supply, the removal rates can be increased.

Therefore, when significant material removal rates are desired, the DC power is cycled between ON and OFF throughout the processing operation. In contrast, when fine control of the removal rates is desired, DC power is supplied continuously.

Without being bound by theory, it appears that removal slows in proportion to the thickness of the oxide layer that forms on the metal surface, and higher DC power applied results in more oxidation at the metal surface, which can act as a barrier to fluoride ion exposure to the metal. Accordingly, cycling the DC power supply between on and off at a given rate can overcome this oxide barrier or create a mechanism that causes the thick oxide to periodically flake off the surface. As described herein, by varying the operating parameters of bath temperature, applied voltage, citric acid concentration, and ammonium bifluoride concentration, the electrolyte provides the ability to tailor the desired results, namely highly controlled bulk metal removal and micropolishing, to a particular application. In addition, varying operating modes within a given technological set of operating parameters can modify and enhance the ability to fine-tune metal removal and surface finish control.

For example, FIG. 8A and 9A demonstrate that at 85 ° C, 300 g / L citric acid and 10 g / L ammonium bifluoride, material removal rates increase with increasing current density from 10.8 to 538 A / m ² . At the same time, FIG. 8B and 9B show that under the same conditions, as the current density is increased from 10.8 to 538 A / m ² , the surface finish deteriorates. By cyclically switching the DC power supply between these two current densities, an end result can be achieved that is better than operating solely with either one of these current densities throughout the entire process. In particular, the duration of the process for removing a specific amount of material can be shortened compared to operating at only 10.8 A / m ² . Additionally, due to the smoothing effect at a lower current density across the finish quality of the final product obtained by treating the surface exceeds exclusively at 538 A / m ^2. Therefore, cycling between two or more preset power modes (as manifested in current density) provides favorable results in terms of both surface quality improvement and precision metal removal, with a process that requires less overall time than individual processes to improve quality. surface or bulk metal removal separately.

In addition to varying the duty cycle, electric current can be applied through the electrolyte solution and through the workpiece with various waveforms that are available from DC power supplies, including but not limited to half-wave, full-wave, square-wave, and other intermediate rectifications, to produce additional benefits and / or increased operating speeds, without compromising the final surface finish. Fast DC switching frequencies such as 50 kHz to 1 MHz or slow cycles such as 15 to 90 minutes may be beneficial, depending on the surface area to be machined, the weight 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 workpiece with a very rough initial surface finish may be best suited to a slow switching cycle first, followed by a higher frequency switching cycle as material is removed and the surface finish improves.

Testing of electrolytic baths of the type described herein also revealed that electrolytic

- 6,036890 polishing in certain embodiments takes place without increasing the hydrogen concentration on the metal surface and in some cases lowering the hydrogen concentration. The oxygen barrier on the material surface may be responsible for the absence of hydrogen migration into the metal matrix. The data indicate that this oxygen barrier can also remove hydrogen from the metal surface. Higher concentrations of fluoride ions result in faster removal rates, but have an unknown effect on the adsorption of hydrogen in the metal matrix. Higher citric acid concentrations tend to slow the removal rate and require higher power densities during electrolytic polishing, but also give the surface a smooth or glossy effect.

Several advantages over prior art solutions result from the use of an aqueous HFA electrolyte solution and citric acid for finishing and / or etching the surface of metal products. The disclosed electrolyte solutions allow precise control of the achieved surface finish. The surface finish of conventional flat products from alloy manufacturers (sheets and plates) involves multi-stage grinding to a final surface finish using finer and finer abrasive materials, usually followed by rinsing etching in an acid bath containing hydrofluoric acid (HF) and nitric acid (HNO3) to remove residual abrasive materials, ground oiled metal and surface anomalies. Acid pickling in HF-HNO3 is exothermic and therefore difficult to control and often results in metal overshoot, resulting in an increased scrap rate or a change in the metal's purpose to a lower grade. By using disclosed electrolyte solutions, the typical secondary and tertiary resurfacing that may be required in case of wash etching can be avoided. A precisely defined surface finish can be achieved which cannot be achieved with the current state of the grinding and etching technology. In addition, the disclosed electrolyte solutions do not introduce mechanical stress into the workpiece. By comparison, any mechanical grinding technique imposes significant mechanical stress on the surface, which causes warpage of the material and causes a percentage of the material to fail to meet typical or customer-specified flatness requirements.

Typical HF-HNO3 acid etching technology will incorporate hydrogen into the target material, which often needs to be removed by expensive vacuum degassing to prevent material embrittlement. Tests carried out using baths with an aqueous electrolyte containing citric acid and HFA on full-size Ti-6Al-4V sheets typical for rolling production and on test specimens of PM titanium, 6Al-4V titanium and 718 nickel-based alloy showed results with reduced impregnation ( saturation) with hydrogen compared to samples exposed to conventional strong acid pickling solutions. In particular, when treating Ti-6Al-4V and titanium PM to achieve the same end result with an α-shell-free clean surface, which is usually achieved by etching in a strong acid, using aqueous electrolyte formulations containing ammonium bifluoride and citric acid, a range of modes in temperature and current densities was identified, 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 material. For all metals and alloys, while testing is still ongoing to refine the preferred operating ranges, results still consistently indicate that even under conditions that may be suboptimal, less hydrogen is introduced into the material than would otherwise have been possible with the same operating conditions. conditions using a pickling bath with strong acid. Typically, lower concentrations of ammonium bifluoride will result in more hydrogen removal from the material exposed to the electrolyte solution, or less hydrogen penetration into the material exposed to the electrolyte solution.

Highly controlled metal removal, surface finishing and micro-polishing.

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 precision than manual or machine polishing. Micropolishing proceeds 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 human error, the resulting polish levels are accurate and reproducible. Using the presented electrolyte solution also achieves cost savings over existing methods.

Tests have shown good results for micropolishing at high citric acid concentrations, low to moderate HFA concentrations, high temperatures and high DC densities that can be applied continuously or cyclically. However, the DC power density should be adjusted based on the alloy being processed. Aluminum

- 7,036890 containing titanium alloys (typically α-β-metallographic alloys, including the well-known Ti-6Al-4V alloy) tend to lose their gloss when applied DC voltages above 40 V. However, for these metals, the voltage is limited to about 40 V and supplying a higher current (i.e. to achieve a higher power density) allows the gloss of the material to be realized again. Without being bound by theory, this may be the result of the action of an α-stabilizing element, which in the case of most α-β alloys (including Ti-6Al-4V) is aluminum, anodized to Al2O ₃ , and not subjected to polishing. In addition, titanium-molybdenum (all-β-phase metallographic structure) and commercially pure (PM) titanium (all-α-phase), however, become more shiny with increasing DC power densities, without being explicitly limited by a similar upper voltage limit. In particular, for other metals, it has been found that higher voltages up to at least 150 V can be used, for example, for nickel-based alloy 718, with favorable results of electrolytic polishing, micro-polishing and surface treatment using the electrolyte solutions disclosed herein. ...

The solutions and methods disclosed herein can be used to debur cut machined parts by preferentially deburring machined metal structures, especially when the parts are made from difficult to cut metals such as titanium and nickel alloys. In the current state of the art, deburring of machined structural elements is usually performed as a manual operation and thus poses many problems due to human error and human inconsistency. Testing with disclosed solutions has shown 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 HFA fluoride levels. Similar solutions can also be used to remove impurities from the surface or to clean the workpiece after cutting, as might otherwise be done using strong acid pickling in an HF-HNO3 pickling bath.

Colored and especially reactive metals exhibit an effective chemical etch rate over a wide range of dilute citric acid mixtures as described above. This allows customization of the surface finish technology for a specific non-ferrous workpiece, which may include a selected residence time in the bath before the electric current is applied to remove and react some of the surface metal prior to electrolytic polishing to selectively reduce peak areas.

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

It follows that the corresponding advantages will apply to electrochemical sizing. In particular, it is expected that electrolyte baths of the compositions described herein can be efficiently used in place of conventional electrochemical sizing and / or etching solutions, with significant environmental benefits and cost savings. Since the electrolyte solutions disclosed herein are virtually free of strong acid, hazardous waste disposal and handling problems are minimized. Moreover, the required current densities will be much lower than those required for conventional electrochemical sizing.

As a rule, an increase in the concentration of ammonium bifluoride tends to reduce the electrical resistance of the electrolyte solution (i.e., ammonium bifluoride increases the conductivity of the electrolyte solution), while the presence of citric acid or an increase in the concentration of citric acid relative to the concentration of ammonium bifluoride tends to weaken the effect of ammonium bifluoride on electrical resistance. In other words, in order to maintain the electrical resistance of the electrolyte solution at a high level to aid micropolishing, it is desirable to maintain low concentrations of ammonium bifluoride or use a higher concentration of ammonium bifluoride in combination with a higher concentration of citric acid. Thus, by varying the concentration of ammonium bifluoride and the relative concentrations of ammonium bifluoride and citric acid, one can successfully regulate the electrical resistance

8 036890 pouring electrolyte solution to achieve desired levels of micro-polish on the workpiece surface.

In the methods disclosed herein, it is not necessary for the proximity of the workpiece (anode) to the cathode to be accurate, unlike conventional electrolytic polishing or electrochemical sizing. Successful processing took place with the cathode at a distance ranging from about 0.1 to about 15 cm from the workpiece. Practical limitations on the maximum distance between the cathode and the anode preform are mainly due to industry, including the size of the bath, the size of the preform, and the electrical resistance of the electrolyte solution. Since the overall current densities are lower, and often much lower than those required in electrochemical sizing, large workpiece-to-cathode distances can be used, and then simply increase the power supply accordingly. Moreover, since the reduced viscosity electrolyte solutions disclosed herein provide highly controlled bulk metal removal, surface finish, and micropolishing, the same solutions are expected to be effective in electrochemical sizing as well.

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

In yet another aspect of the electrolytic polishing method, the current is applied at a voltage of about 0.6 to about 150 VDC. The current can be applied at a current density less than or equal to about 255,000 A / m ² (about 24,000 A / ft ² ), where the denominator represents the total effective surface area of the workpiece. For some non-ferrous metals, such as nickel-based alloys, 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 are preferred. A / m ² (approximately 0.1-100 A / ft ² ). Electrolytic polishing processes using an electrolyte solution can operate between the freezing and boiling points of the solution, for example, at temperatures from about 2 to about 98 ° C, and preferably in the range from about 21 to about 85 ° C.

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

Example 1. Etching of commercially pure titanium.

In an electrolyte consisting essentially of approximately 56% water, 43% citric acid (716 g / L), and 1% ammonium bifluoride (15.1 g / L) by weight, operating at 185 ° F (85 ° C) , a sample of the commercially pure titanium plate was treated to improve the surface finish of the material (i.e., to smoothen the mill standard surface finish). The material began with a surface roughness of approximately 160 micro-inches, and after processing, the surface roughness was reduced 90 micro-inches to a final reading of 50 micro-inches, or, in other words, the improvement was approximately 69%. The process was run over a period of 30 minutes, resulting in a 0.0178 inch reduction in material thickness.

Cold formability, a key product characteristic of titanium plate for many end uses, is highly dependent on the surface finish of the product. By using the electrochemical process embodiments disclosed herein, an improved surface finish of a material can be achieved at a lower cost than conventional grinding and etching methods. Finishings obtained using embodiments of the disclosed solutions and methods have shown improved cold forming characteristics of the board product to a higher level than conventional methods.

Example 2. Etching of a test sample 6Al-4V.

In the following examples, test specimens of sheet stocks of titanium alloy 6Al4V with dimensions of 52 mm x 76 mm were processed. The electrolyte consisted of water (H2O), citric acid (LA), and ammonium bifluoride (HFA) at varying concentrations and temperatures. Observations and readings obtained are given in table below. one.

- 9 036890

H ₂ O (% wt.) LC (% mass.) HFA (% wt.) Temperature (° F) Time (min) Material loss (inch) 77.90 21.45 0.65 178 ten 0.00065 77.25 21.45 1.30 185 ten 0.00085 75, 95 21.45 2, 60 189 ten 0.00120 74, 65 21.45 3, 90 188 ten 0.00120 56, 45 42.90 0.65 184 ten 0.00005 55, 80 42.90 1.30 195 ten 0.00030 54.50 42.90 2, 60 193 ten 0.00005 53.20 42.90 3, 90 188 ten 0.00035 53.20 42.90 3, 90 191 fifty 0.00140 75, 95 21.45 2, 60 190 thirty 0.00205 88.95 10,725 0.325 180 ten 0.00020 88.625 10,725 0, 650 180 ten 0.00020 87.975 10,725 1.30 182 ten 0,00060 99, 25 0, 100 0.65 188 ten 0.00010 98, 60 0, 100 1.30 182 ten 0.00065 97.30 0, 100 2, 60 195 ten 0.00095

Table 1

Example 3. Electrolytic polishing of test sample 6A1-4V.

In the following examples, test specimens of sheet stocks of titanium alloy 6Al4V with dimensions of 52 mm x 76 mm were processed. The electrolyte consisted of water (H ₂ O), citric acid (LA), and ammonium bifluoride (HFA) at varying concentrations and temperatures. Observations and data obtained are shown below in table. 2.

- 10 036890

table 2

H ₂ O (% wt.) LC (% mass.) HFA (% wt.) Temperature (° F) Time (min) Power (W / A) Material loss (inch) 77.90 21.45 0.65 190 ten (50/7) 0.00025 77.25 21.45 1.30 195 ten (50/8) 0.00070 75.95 21.45 2.60 191 ten (50/10) 0.00130 74.65 21.45 3.90 190 ten (50/12) 0.00130 74.65 21.45 3.90 188 ten (20/6) Not recorded 74.65 21.45 3.90 184 ten (6/2) Not recorded 74.65 21.45 3.90 180 ten (12/3) Not recorded 56.45 42.90 0.65 182 ten (50/3) 0.00010 55,80 42.90 1.30 200 ten (50/5) 0.00045 54.50 42.90 2, 60 189 ten (50/8) 0.00055 53.20 42.90 3.90 190 ten (50/12) 0.00045 53.20 42.90 3.90 203 5.0 (50/5) 0.00115 75.95 21.45 2.60 172 thirty (12/3) 0.00015 88.95 10,725 0.325 180 ten 50V 0.00000 88.625 10,725 0.650 180 ten 50V 0.00010 87.975 10,725 1.30 184 ten 50V 0.00060 99.25 0, 100 0.65 190 ten 50V 0.00060 98.60 0, 100 1.30 184 ten (50/19) 0.00145 97.30 0, 100 2, 60 190 ten (50/38) 0.00360

An additional extensive test was performed using aqueous electrolyte solutions containing citric acid in a concentration range of about 0 to about 780 g / L (about 0 to about 47 wt%) and ammonium bifluoride in a concentration range of about 0 to about 120 g / L (from about 0 to about 8 wt%) and substantially free of strong acid (i.e., less than about 1 g / L or less than 0.1 wt%), at bath temperatures ranging from about 21 to about 85 ° C and with applied current densities in the range from about 0 to about 1076 A / m ^{2 of} the surface area of the preform (note that 780 g / L citric acid in water is the saturation concentration at 21 ° C). Current densities as high as at least 225,000 A / m ² can be used at applied voltages of 150 V or more. Metals tested included commercially 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 electrolytic polishing, micro-polishing and surface treatment results can be obtained for different grades of non-ferrous metals and alloys. The results are summarized in the following tables and descriptions and with reference to figures. Unless otherwise stated, the tests were conducted at temperatures of about 21 ° C, about 54 ° C, about 71 ° C and about 85 ° C and at a current density of 0 A / m ^2, approximately 10.8 A / m ^2, about 52.8 A / m ² , about 215 A / m ² , about 538 A / m ² and about 1076 A / m ² . No strong acid was added to any of the solutions tested, although trace amounts are unlikely to significantly affect the results.

FIG. 1A-1B show material removal rates and surface finish changes, respectively, at four different temperatures using an aqueous electrolyte solution having a moderately low ammonium bifluoride concentration of 20 g / L and a citric acid concentration of about 0 to about 780 g / L and at a current density 1076 A / m ² . FIG. 1A shows that material removal rate varies proportionally with temperature, especially at lower citric acid concentrations. As the bath temperature rises, the removal rate increases. At the lower temperatures of 21 ° C, 54 ° C and 71 ° C, a citric acid concentration of 180 g / L is sufficient to begin to inhibit the removal efficiency of the ammonium bifluoride material, while at the higher temperature of 85 ° C, the relatively rapid removal of material continues up to concentration of about 300 g / l of citric acid. At higher citric acid concentrations of 300 g / l or more, the removal rates at all temperatures decrease. On the contrary, FIG. 1B shows that at lower citric acid concentrations, in particular at end

- 11 036890 centering from 120 to 180 g / l or less, the surface finish generally deteriorates at the lowest temperature. In other words, the fluoride ion, which is responsible for significant material removal at low citric acid concentrations, also causes surface damage, but the presence of citric acid in significant concentrations appears to act as a beneficial barrier against exposure to fluoride ions. However, as the citric acid concentration rises to 180 g / L and above, the surface finish actually improves, particularly at citric acid levels of 600 g / L or more, with the material removal rate being significantly reduced. Moreover, even at citric acid levels between about 120 and about 600 g / L, where material removal is still occurring, improvements in surface finish can be achieved concurrently.

The test found that a fluoride ion source such as ammonium bifluoride was needed to achieve the desired improvements in material removal and surface finish. In electrolyte solutions consisting essentially of only one citric acid in water, practically in the absence of ammonium bifluoride, material removal practically does not occur regardless of the bath temperature or current density, and changes in the quality of surface finish are also minimal. It is believed that when titanium or other reactive metal is treated in an aqueous electrolyte containing only citric acid, the surface of the material is generally anodized to form an oxide layer that is very thin (i.e., about 200 to about 600 nm thick) and forms rapidly. After the formation of the anode oxide layer, since the supplied DC power can no longer act on the surface of the material, it hydrolyzes the water. The resulting oxygen, which is generated quickly, finds another monatomic oxygen and is released at the anode as O ₂ gas.

FIG. 2A-2B and 2C-2D show material removal rates and surface finish changes, respectively, using an aqueous electrolyte solution having a citric acid concentration of 120 g / L and an ammonium bifluoride concentration of about 0 g / L to about 120 g / L.

FIG. 2A and 2C show data at a typical low temperature of 21 ° C, and FIG. 2B and 2C show data at a characteristic high temperature of 71 ° C. FIG. 2A-2B show that material removal is highly correlated with ammonium bifluoride concentration and temperature, but minimally influenced by current density. Higher material removal rates are usually achieved by increasing either the concentration of ammonium bifluoride, or the temperature, or both.

FIG. 2C-2D show that material removal is accompanied by some degradation in surface quality. However, it has surprisingly been found that with increasing temperature and material removal rate, the degree of deterioration in surface finish decreases. At a low temperature of 21 ° C, as in FIG. 2C, increasing the current density mitigates the effects of surface degradation, and at maximum current density some improvement in surface finish becomes apparent. At a higher temperature of 71 ° C, as in Figure 2D, the surface finish does not change significantly with changes in current density.

FIG. 2E-2F show that the rate of material removal and the change in surface finish, respectively, when using an aqueous electrolyte solution consisting essentially of ammonium bifluoride in water, without intentionally adding citric acid, is dependent on the current density when operating at a high temperature of 85 ° C. High material removal rates can be achieved with an HFA-only electrolyte, but this material removal comes at the expense of the surface finish, which is often significantly impaired by the electrolyte solution. However, under certain operating conditions (not shown in the figures), the least deterioration or the most moderate improvement in surface finish was achieved. For example, improvements in the quality of surface finish by electrolyte solutions only with HFA have been achieved with a solution of 10 g / l of 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 .

Without being bound by theory, a possible explanation for the ability of increased current density to improve surface finish with minimal impact on material removal rate is that one of the functions of electric current is the growth of 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 against the surface of the material. Accordingly, it appears that as the current density increases at the anode, oxygen with a higher concentration is formed, which, in turn, can act as a barrier to mass transfer. Alternatively, if we simplify the morphology of the material surface as a sequence of peaks and valleys, it can be assumed that citric acid and oxygen settle in the valleys, leaving only the peaks of the surface morphology for exposure to fluoride ions. As the citric acid and oxygen barriers are strengthened (i.e., citric acid concentrations increase and current densities increase), only the highest peaks of the surface become available for chemical attack. According to this theory, low current densities and low concentrations of lemon

- 12,036890 acidic acid will provide the least capable surface smoothing process, while high current densities and high citric acid concentrations would be expected to provide the most efficient surface smoothing process. Whether or not these theories are accurate, there is every reason to believe that the results are consistent with the analysis above.

Understanding that oxygen (generated by electric current) and citric acid appear to act as micro-barriers to the removal process helps to clarify that HFA concentration and temperature are variable, probably the most readily available to control material removal and micropolishing results. Therefore, in the processes described here, the current density appears to act mainly on the formation of oxygen, and for the most part is not a significant factor that enhances the overall removal of material. Rather, material removal appears to be almost exclusively driven by fluoride ions, the activity of which is controlled to some extent by the thermodynamic effects of temperature. In short, it was surprisingly found that the current density as a control variable has a relatively low value, that the presence of fluoride ions prevails over the influence of the current density.

FIG. 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, so that for the same mixture of citric acid, ammonium bifluoride and water, more material removal occurs at higher temperatures. Similar trends were observed at all current densities from 0 to 1076 A / m ² .

FIG. 4A-4D show that at a typical temperature of 54 ° C, the material removal rate is relatively constant with current density, such that for the same citric acid / ammonium bifluoride mixture at any given bath temperature, the material removal rate is relatively insensitive to changes in current density. Similar trends were observed at all temperatures between 21 and 85 ° C, and these trends appear to persist below 21 ° C (but above the freezing point of the solution) and above 81 ° C (but below the boiling point of the solution). As occurs under almost all temperature and current conditions, regardless of the HFA concentration, when the citric acid concentration rises above a certain level, usually between 600 and 780 g / L, the material removal rate is greatly reduced. Therefore, to maintain the ability to achieve some level of material removal when shaping is desired, the citric acid concentration should generally be kept below 600 g / L.

FIG. 4E-4G show, at a characteristic high temperature of 85 ° C and three different citric acid concentrations, the effect of current density on material removal rates, and FIG. 4H-4J show the effect of current density on surface finish quality under the same set of conditions. FIG. 4E shows, like FIG. 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 bifluoride and quite significant at high temperatures. It should be noted that although FIG. 4E shows data at only 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 citric acid concentration appears to provide some degree of surface protection against large scale exposure and material removal rates are reduced compared to lower citric acid concentrations. At 780 g / L, as shown in FIG. 4G, removal rates are reduced even more. Regardless of the concentration of ammonium bifluoride and citric acid, the removal of the material appears to be slightly dependent on the current density.

FIG. 4H shows that at high temperature and at the most moderate citric acid concentration, a moderate degree of deterioration in surface finish is observed at almost all ammonium bifluoride concentrations and current densities. However, when viewed together with FIG. 4E and 4H show that one processing mode stands out. With a citric acid concentration of 120 g / L, a low ammonium bifluoride level of 10 g / L, and a high current density of 1076 A / m ² , material removal is suppressed, resulting in a significant improvement in surface finish. This may further support the theory discussed above that the increased current density can create an excess of oxygen on the surface of the material sufficient to fill valleys in the surface morphology so that the peaks are preferentially exposed to fluoride ions formed by dissociation of ammonium bifluoride. This effect, combined with the possible micro-barrier effect of citric acid, can be observed even more strongly in FIG. 4I (at 600 g / L citric acid) and FIG. 4J (at 780 g / L citric acid), which show a reduced deterioration in surface finish and, in some cases, an improvement in surface finish at higher citric acid concentrations and higher current densities individually, and even more so when more high concentrations of citric acid and higher current densities. For example, a significant improvement in the quality of the surface finish occurs at 10 and 20 g / l ammonium bifluoride going from 600 to 780 g / l citric acid.

However, this effect seems to have a limit, as it can be seen that the quality of the surface finish deteriorates sharply at the highest concentration of ammonium bifluoride 120

- 13,036890 g / l and higher current densities when going from 120 to 600 g / l and further to 780 g / l of citric acid. A similar result was obtained at 60 g / L of ammonium bifluoride, at least with an increase in the concentration of citric acid from 600 to 780 g / L.

As shown below in table. 3A-3C and 4A-4C, modes of operation for surface finishes of sheet products that require minimal material removal and desirably improve surface finishes from the most moderate to high, and for micro-polishing, where essentially no material removal is required, but desirable strong improvement in surface finish can be achieved for a wide range of electrolyte mixtures, temperatures and current densities. Tab. 3A-3C and 4A-4C do not include electrolytes consisting essentially of water and citric acid and practically free of ammonium bifluoride, although with such a solution, essentially zero material removal can be obtained, as well as an improvement in surface quality from the very moderate to high over a wide range of temperatures and current densities, as these conditions were discussed separately with reference to FIG. 1A-1C. Similarly, tab. 3A-3C and 4A-4C do not include electrolytes consisting essentially of water and ammonium bifluoride and practically free of citric acid, as these conditions were discussed separately with reference to FIG. 2A-2D. Tab. 3A3Cs are separated by fineness levels of surface finish and then organized in ascending order of HFA concentration. Tab. 4A-4C are separated by citric acid concentration levels and then organized in ascending order of HFA concentration.

From the data presented in table. 3A-3C, some trends are following. First, over the entire range of citric acid concentrations (from 60 to 780 g / l), concentrations of ammonium bifluoride (from 10 to 120 g / l), temperatures (from 21 to 85 ° C) and current densities (from 10.8 up to 1076 A / m ² ) low or almost zero results were obtained in material removal and improved surface finish. Therefore, aqueous solutions of citric acid and HFA, with virtually no strong acid, can provide a fine surface finish with minimal material loss at concentrations as low as 60 g / L citric acid and 10 g / L HFA, and at concentrations as high as 780 g / L citric acid and 120 g / l HFA, and with some combinations between them.

Table 3A Highest surface fineness

Citric acid (g / l) HFA (g / L) Temperature (° C) Current density (A / m ² ) Deleting material (mm / h) Change in surface finish quality (%) 780 ten 85 1076 0, 168 39, 2 180 ten 85 1076 0.208 36, 4 120 ten 85 1076 0.232 33.3 300 ten 71 1076 0.156 30, 4 780 ten 71 53.8 0, 100 30, 4 780 ten 71 10.8 0, 108 30.2 300 ten 54 1076 0, 640 38.9 780 20 71 538 0, 100 44, 8 600 20 71 1076 0.188 40, 0 180 20 54 1076 0, 168 31, 9 780 20 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

In general, as shown in table. 3A, maximum levels of improvement in surface finish (i.e. more than 30% reduction in surface roughness) were obtained at higher current densities of 538-1076 A / m ² , with moderate to higher citric acid concentrations of 120-780 g / L and, as a rule, at lower HFA concentrations of 10-20 g / L. At lower HFA concentrations in the 10-20 g / L range, higher temperatures of 71-85 ° C tend to give better

- 14 036890 surface finish quality at higher citric acid concentrations of 600-780 g / l, while a milder temperature of 54 ° C provided a fine surface finish at moderate citric acid concentrations of 120-300 g / l. However, significant improvements in surface finish have also been obtained under conditions of low HFA concentration, moderate citric acid concentration and high temperature (10 g / L HFA, 120 g / L citric acid, 85 ° C) and low HFA concentrations. moderate concentration of citric acid and lower temperature (20 g / L HFA, 180 g / L citric acid, 54 ° C). At elevated HFA concentrations, in the 60-120 g / L range, lower temperatures of 21-54 ° C tend to give better surface finish at higher citric acid concentrations of 600-780 g / L and higher current densities. In addition, substantial fineness of the surface finish was achieved at lower current densities of 10.8-53.8 A / m ² at high citric acid concentrations of 780 g / L and high temperatures of 71-85 ° C as at a low HFA concentration of 10 g / L and at a high HFA concentration of 120 g / L, as shown in FIG. 4H.

Table 3B ________________ High surface fineness _______________

Citric acid (g / l) HFA (g / L) Temperature (° C) Current density (A / m ² ) Deleting material (mm / h) Change in surface finish quality (%) 780 ten 85 538 0, 132 28.8 60 ten 85 1076 0.276 28.0 300 ten 85 1076 0.216 25.6 600 ten 85 538 0, 084 25.0 600 ten 85 1076 0.220 24.5 780 ten 85 10.8 0.16 17.9 600 ten 71 538 0.076 19, 6 180 ten 71 1076 0, 192 18.8 180 ten 54 1076 0.200 25.0 780 ten 54 538 0.024 21.2 780 ten 54 53.8 0.088 15.3 300 20 85 1076 0.212 30.0 780 20 85 10.8 0.244 15.7 780 20 71 1076 0.196 27.1 780 20 71 0 0, 176 22.1 180 20 71 1076 0.188 15.1 780 20 54 1076 0.228 28.6 300 20 54 1076 0, 144 25.0 600 20 54 1076 0.164 18.0 780 20 54 538 0, 100 16.7 780 20 54 215 0, 108 15, 6 780 20 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

In general, as shown in table. 3B, high but not the highest levels of improvement in surface finish (i.e. between about 15% and about 30% reduction in surface roughness) were obtained at lower HFA concentrations of 10-20 g / L and moderate higher temperatures. 54-85 ° C, and mostly, but not exclusively, at higher current densities of 538-1076 A / m ² . Typically, these results were achieved at high citric acid concentrations of 600-780 g / l. For example, while HFA concentrations of 10-20 g / L generally yielded excellent results

- 15 036890 at higher current densities and higher concentrations of citric acid, excellent results were also obtained with lower concentrations of citric acid 60-300 g / l at a low current density of 10.8 A / m ² and a high temperature of 85 ° C, and also 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 at both high temperature and low current density (71-85 ° C and 10.8-53.8 A / m ² ) and low temperature and high current density (21 ° C and 1076 A / m ² ), in all cases at high citric acid concentrations of 780 g / l. In this regard, there appears to be some complementary activity between temperature and current density in that similar surface finish results can be achieved with a high citric acid solution using a higher current density with a lower temperature or using a lower current density. with a higher temperature. See also FIG. 4H-4J, which show that high temperature conditions, combined with high current density, tend to produce maximum improvements in surface finish.

Table 3C

Moderate fineness of surface finish

Citric acid (g / l) HFA (g / L) Temperature (° C) Current density (A / m ² ) Deleting material (mm / h) Change in surface finish quality (%) 600 ten 85 10.8 0.216 4.0 600 ten 85 215 0.232 1.9 780 ten 71 0 0, 100 14.3 780 ten 71 215 0.048 9.8 600 ten 71 0 0.164 6.0 780 ten 71 538 0.064 5.4 780 ten 21 53.8 0.040 14.5 60 ten 21 1076 0, 148 13.5 780 20 85 215 0.260 7,7 780 20 85 53.8 0.216 7,7 780 20 85 0 0.232 5.7 600 20 85 1076 0.184 6.2 300 20 71 1076 0.200 7.1 780 20 71 53.8 0.172 2.0 600 20 54 538 0.064 8.2 600 20 21 538 0.032 13.2 120 20 21 1076 0.164 10, 6 300 20 21 1076 0, 148 10, 4 600 20 21 1076 0.032 6,7 60 20 21 1076 0, 124 6.8 180 20 21 1076 0, 132 4.2 780 20 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

In general, as shown in table. 3C, the most moderate levels of improvement in surface finish (i.e. less than about 15% reduction in surface roughness) were obtained at lower HFA concentrations of 10-20 g / L and higher temperatures of 71-85 ° C and generally all over the dia

- 16 036890 in the range of current densities 10.8-1076 A / m ² . Typically, these results were achieved at high citric acid concentrations of 600-780 g / l. One notable exception to this trend is that moderate to high surface finish improvements were also obtained at all HFA concentrations of 10-120 g / L and low to moderate citric acid concentrations of 60300 g / L at a low temperature of 21 ° C. and a high current density of 1076 A / m ² .

Table 4A ______________ Lowest citric acid concentrations ______________

Citric acid (g / l) HFA (g / L) Temperature (° C) Current density (A / m ² ) Deleting material (mm / h) Change in surface finish quality (%) 180 ten 85 1076 0.208 36.4 120 ten 85 1076 0.232 33.3 60 ten 85 1076 0.276 28.0 180 ten 54 1076 0.200 25.0 180 ten 71 1076 0, 192 18.8 60 ten 21 1076 0, 148 13.5 180 20 54 1076 0, 168 31.9 180 20 71 1076 0.188 15.1 120 20 21 1076 0.164 10.6 60 20 21 1076 0, 124 6.8 180 20 21 1076 0, 132 4.2 120 60 21 1076 0.196 11.3 60 60 21 1076 0.224 4.2

As shown in table. 4A, at low citric acid concentrations of 60-180 g / L, a uniform improvement in surface finish appears to require a high current density. In general, maximum surface finish improvements were obtained at low HFA concentrations of 10-20 g / L and at moderate high temperatures of 54-85 ° C. Low to moderate improvement in surface finish was achieved at 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 ) Deleting material (mm / h) Change in surface finish quality (%) 300 ten 54 1076 0.188 38.9 300 ten 71 1076 0.156 30.4 300 ten 85 1076 0.216 25.6 600 ten 85 538 0.084 25.0 600 ten 85 1076 0.220 24.5 600 ten 71 538 0.076 19, 6 600 ten 71 0 0.164 6, 0 600 ten 85 10, 8 0.216 4.0

- 17 036890

600 ten 85 215 0.232 19 600 20 71 1076 0.188 40.0 300 20 85 1076 0.212 30.0 300 20 54 1076 0.144 25.6 600 20 54 1076 0.164 18.0 600 20 21 538 0.032 13.2 300 20 21 1076 0.148 10.4 600 20 54 538 0.064 8.2 600 20 21 1076 0.032 6, 7 300 20 71 1076 0.200 7.1 600 20 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

As shown in table. 4B, at moderate citric acid concentrations of 300-600 g / L, significant improvement in surface finish typically 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 best results, while at 20 g / L HFA concentration good results are also achieved in the temperature range 21-85 ° C. At high HFA concentrations of 60-120 g / L, improvement in surface finish most typically occurs at a lower temperature of 21 ° C.

Table 4C

Highest citric acid concentrations

Citric acid (g / l) HFA (g / L) Temperature (° C) Current density (A / m ² ) Deleting material (mm / h) Change in surface finish quality (%) 780 ten 85 1076 0.168 39, 2

- 18 036890

780 ten 71 53, 8 0.100 30, 4 780 ten 71 10, 8 0.108 30.2 780 ten 85 538 0.132 28.8 780 ten 54 538 0.024 21.2 780 ten 85 10, 8 0.136 17.9 780 ten 54 53, 8 0.088 15.3 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 ten ten ten ten 20 20 20 20 20 20 20 20 20 20 20 20 20 20 60 60 60 120 120 120 120 120 120 120 120 120 120 21 71 71 71 71 21 54 71 71 21 54 85 54 85 85 85 71 21 21 54 21 21 85 71 71 54 21 54 85 21 54 53, 8 0 215 538 538 1076 1076 1076 0 538 538 10, 8 215 53, 8 215 0 53, 8 53, 8 538 1076 1076 538 10, 8 10, 8 53, 8 10, 8 1076 0 0 10, 8 53, 8 0.040 0.200 0.048 0.064 0.100 0.044 0.228 0.196 0.176 0.016 0.100 0.244 0.108 0.216 0.260 0.232 0.172 0.032 0.088 0.160 0.204 0.116 0.004 0.000 0.008 0.032 0.196 0.008 0.016 0.004 0.020 14.5 14.3 9, 8 5.4 44.8 30, 9 28, 6 27.1 22.1 20.3 16, 7 15.7 15.6 7,7 7,7 5.7 2.0 1.7 42.0 36, 1 34.6 49.1 30, 0 25.0 23,7 16, 4 16, 3 13.5 11, 1 7.8 5.9 780 120 85 53, 8 0.016 2.2

From a comparison of the table. 4C with tab. 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 citric acid concentrations of 780 g / L. As shown in table. 4C, at high citric acid concentrations of 780 g / L, a significant improvement in surface finish can be obtained at almost all current densities of 10.8-1076 Am ² and from low to high temperatures of 21-85 ° C and as at low concentrations of HFA 10- 20 g / l and high HFA concentrations of 120 g / l.

FIG. 5A and 5B show the rates of material removal and change in surface finish at a characteristic low temperature of 21 ° C and a characteristic high current density of 538 A / m ² . As can be seen in FIG. 5B, the deterioration in surface finish is most moderate at all citric acid concentrations below 600 g / L, for HFA concentrations below 60 g / L, and that surface finish quality actually improves for all HFA concentrations from 10 to 120 g / L at high concentrations citric acid above 600 g / l and especially at 780 g / l. In addition to FIG. 5A shows that

- 19 036890 material removal rate under these operating conditions is relatively low. Therefore, operation in this range of composition, temperature and current density would be desirable to achieve the most moderate controlled material removal with minimal surface degradation, or perhaps the most moderate improvement in surface finish, but would not be particularly effective for large scale material removal.

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

FIG. 7A and 7B show that under certain conditions, controlled material removal and improved surface finish can be achieved simultaneously. In particular, at an HFA concentration of about 10 g / L, FIG. 7A shows the sustained most moderate material removal rates at all citric acid concentrations when the preform 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, FIG. 7B shows the actual improvement in surface finish at all citric acid concentrations greater than or equal to 60 g / L. Even at higher HFA concentrations of 20 to 120 g / L, the material removal obtained can be directly related to the HFA concentration, without actually degrading the surface finish. However, at maximum citric acid concentrations of 600 g / L or more, material removal rates are significantly reduced.

Several operating ranges have been identified where controlled material removal can be achieved with only the most moderate deterioration in surface finish, typically with less than about 50% increase in roughness. FIG. 8A-8B, 9A9B, and 10A-10B illustrate exemplary modes of operation in this category.

FIG. 8A shows that under conditions of high temperature (85 ° C) and low current density (10.8 A / m ² ) a near constant material removal rate can be achieved at all HFA concentrations for citric acid concentrations ranging from about 60 g / L up to about 300 g / l, with high rates of material removal, which are in direct proportion to the concentration of HFA. FIG. 8B shows that for these ranges of citric acid and HFA concentrations, degradation in surface finish is consistently the most moderate, with little or no regard for specific citric acid and HFA concentrations. Citric acid concentrations of 600 g / L and above greatly reduce or even eliminate the material removal capacity of the electrolyte solution, and, except in the case of 60 g / L HFA, moderate deterioration in surface finish and may even tend to slightly improve surface finish. FIG. 9A and 9B show very similar results under the conditions of high temperature (85 ° C) and high current density (538 A / m ^2), and FIG. 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 .

Based on the test results disclosed here, it becomes clear that by adjusting the temperature and current density, the same bath with an aqueous electrolyte solution can be used in a multi-stage process that involves first removing the most moderate and controlled amount of material at a relatively low current density, and then restoration of the damaged surface by increasing the current density to a high level while maintaining or slightly reducing the temperature. For example, using a solution containing 300 g / l of citric acid and 120 g / l HFA can obtain the most moderate material removal rate at a temperature of 85 ° C and a current density of 53.8 A / m ² (see. FIG. 3D), when less than 30% deterioration in surface finish and then an improvement in surface finish can be obtained at the same temperature and current density of 1076 A / m ² (see FIGS. 7A and 7B) with less material removed.

Numerous different combinations of multistage processing conditions can be matched by varying the citric acid concentration in addition to temperature and current density due to the strong material removal softening effect that occurs when the citric acid concentration is increased to 600 g / L or more. For example, from FIG. 8A and 8B, it can be seen that using a 120 g / L HFA electrolyte solution at 85 ° C and a current density of 10.8 A / m ² , aggressive material removal can be achieved with the most moderate surface degradation at a citric acid concentration of 300 g / L at the first stage of processing, and then simply by increasing the concentration

- 20 036890 citric acid up to 780 g / l in the second stage of processing, the removal of material can be practically stopped, while the quality of the surface finish is significantly improved. Similar results can be obtained using the high temperature, higher current density conditions of FIG. 9A and 9B or the moderately high temperature, moderate current density conditions of FIG. 10A and 10B.

Very low concentrations of ammonium bifluoride have been found to be effective in both material removal and micro-polishing. As shown in FIG. 1A, material removal rates are greatest at elevated temperatures, so lower concentrations of ammonium bifluoride is 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 bifluoride concentrations of 2 g / L, material removal and surface finish 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 quality by -156%. At 0 A / m ² , material removal rates of 0.0035 mm / h were recorded, with a corresponding change in surface finish of -187%.

Similarly, when treated in an aqueous solution of 2 g / L HFA and without citric acid with an applied current of 271 A / m ² , material removal rates of 0.004 mm / h were recorded, with a corresponding change (deterioration) in the quality of 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 of -168%.

While it would be preferable to use as little HFA as possible necessary to be effective, concentrations well above 120 g / L can be used, including concentrations of ammonium bifluoride at levels as high as 240 to 360 g / L, and even concentrations exceeding the saturation concentration in water. The effectiveness of electrolyte solutions at high concentrations of HFA was tested by stepwise addition of HFA to a 179.9 g / L citric acid solution with a temperature fixed at 67 ° C and current densities in the range from 10.8 to 255,000 A / m ² . Since this solution has a relatively low electrical resistance, it was expected that higher concentrations of HFA would be able to provide higher solution conductivity, especially at higher current density levels. The temperature was also raised above room temperature to lower the electrolyte resistance. Samples of both PM titanium and nickel-based alloy 718 were exposed to electrolyte and, as HFA was added, continued removal of bulk material and micropolishing. HFA was added up to and beyond its saturation point in the electrolyte. The saturation point of HFA (which varies with temperature and pressure) at these parameters was between about 240 g / L and about 360 g / L. The data given 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.

A test was conducted to determine the effectiveness of electrolyte solutions for micropolishing and bulking metal removal at relatively high current densities, including those approaching 255,000 A / m ² . It is clear from the literature that electrolytes with low resistance values can withstand high current densities. Certain combinations of citric acid concentration and HFA concentration exhibit particularly low resistance. For example, an electrolyte solution containing about 180 g / L citric acid in a temperature range of about 7185 ° C was tested at high current densities. Samples of commercially 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 to 255,000 A / m ² . The data given in table. 5 indicate that bulk material removal and micropolishing were achieved at all investigated current densities in this range, including 255,000 A / m ² . Compared to the machining of titanium and titanium alloys, higher current densities can be used for machining nickel-based alloys, in particular at about 5000 A / m ² .

While PM titanium is efficiently processed using relatively low voltages of less than or equal to about 40 V, higher voltages can also be used. In one exemplary test, PM titanium was treated in a bath of 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 53,160 A / m ² ... Under these conditions, a bulk metal removal rate of 5 mm / h was achieved along with a 37.8% improvement in surface roughness as measured by the profilometer, resulting in a uniform, visually shiny reflective appearance. The electrolyte of the same chemical composition remained effective for PM titanium specimens for removing bulk metal after increasing the voltage to 150 VDC, and lowering the current density up to 5067 A / m ^2, but under these conditions, the metal removal rate slowed to about 0.3 mm / h, and the quality of surface finish has slightly deteriorated to a matte appearance.

For some metals and alloys, higher stresses may be equally or even more effective in achieving improved volumetric material removal, surface finish

- 21 036890 or both. In particular, certain metals, including but not limited to nickel-based alloys (such as Waspaloy and Nickel Alloy 718), 18K gold, pure chromium, and Nitinol alloys, appear to benefit from higher stress machining in terms of faster bulky metal removal 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 150 VDC potential and density current 4934 A / m ² , led to a volumetric metal removal rate of only 0.09 mm / h, but based on surface measurements with a filometer, a uniform improvement in surface finish quality by 33.8%.

Table 5

Material LC (g / l) HFA (g / L) Initial temperature (° C) Final potential (V) Current density (A / m ² ) Removal rate (mm / h) Δ surface finish quality,% (- worse, + better) Comments Ti PM2 179.9 20 89.4 64.7 11,227 1.20 62.9% Uniform, glossy finish Ti PM2 179.9 20 85.0 64.7 8.027 1.15 29.4% Uniform, glossy finish Ti PM2 179.9 20 83.9 64.7 7,901 5.68 21.2% Uniform, glossy finish Ti PM2 179.9 60 82.8 64.7 36,135 4.24 26.6% Uniform, glossy finish Ti PM2 179.9 60 81,7 64.7 34,576 4.34 47.6% Uniform, glossy finish Ti PM2 179.9 60 79.4 24.5 40,219 6, 12 47.2% Uniform, glossy finish Ti PM2 179.9 120 85.0 64.7 15,175 4.16 -169.8% The end most deeply immersed in the solution is shiny, the rest is "frozen" Ti PM2 179.9 120 85.0 64.7 15,379 3.44 -183.9% The end most deeply immersed in the solution is shiny, the rest is "frozen" Ti PM2 179.9 120 85.6 64.7 53,160 5.00 37.8% Uniform, glossy finish Ti PM2 179.9 120 90.0 150 5.067 0.30 -22.6% Matte appearance, slight oxidation Ti PM2 179.9 240 71.1 14.3 160,330 21.42 -33.3% Uniform, glossy finish Ti PM2 179.9 240 70.0 14.4 255,733 22.08 -103.0% Uniform, glossy finish Ti 179.9 360 57.8 11.4 146,728 27.72 -179.5% Uniform, lustrous finish with HFA concentration beyond the point saturation Ti 179.9 360 66.7 9.6 164,876 24.36 -191.2% Uniform, lustrous finish with HFA concentration beyond the point saturation Ti PM2 179.9 360 28.3 0,4 10.8 0, 08 29.6% Uniform, lustrous finish with HFA concentration beyond saturation point Ti 179.9 360 25.0 0.3 53.8 0, 10 7.3% Uniform, lustrous finish with HFA concentration beyond the point saturation Ti PM2 179.9 360 22.2 0.2 215 0, 11 9.3% Uniform, lustrous finish with HFA concentration beyond saturation point Corroded, Ti PM2 179.9 360 20.6 0.1 538 0, 13 -346.9% unsatisfactory finish HFA concentration beyond the point saturation

- 22 036890

Ti PM2 179.9 360 20.6 0.6 1,076 0.16 -988.6% Very corroded, unsatisfactory finish HFA concentration beyond saturation point Nickel 718 179.9 20 81,7 64.7 68,585 4.01 -12.5% Uniform, glossy finish Nickel 718 179.9 20 81.1 39.9 79,301 4.85 55.0% Uniform, glossy finish Nickel 718 179.9 20 80.6 36, 3 39,828 4.75 48.3% Uniform, glossy finish Nickel 718 179.9 60 80.0 64.7 42,274 3.42 11.1% Uniform, glossy finish Nickel 718 179.9 60 80.0 64.7 35,066 3.69 -11.1% Uniform, glossy finish Nickel 718 179.9 60 81,7 14.8 39,484 4.86 -20.0% Uniform, glossy finish Nickel 718 179.9 120 85.0 64 33.945 3.84 8.3% Uniform, glossy finish Nickel 718 179.9 120 83.3 65 34,818 3.96 13.0% Uniform, glossy finish Nickel 718 179.9 120 82.2 9.7 39,984 6, 08 -57.1% Uniform, glossy finish Nickel 718 179.9 120 86.7 150 4.934 0.09 33.8% Uniform matte appearance Nickel 718 179.9 360 67.2 11.5 140,005 12.90 -16.0% Uniform, lustrous finish with HFA concentration beyond saturation point

To assess the effect of the accumulated dissolved metal in the electrolyte solution, a batch of Ti-6Al-4V rectangular rods with dimensions of 6.6 cm by 13.2 cm by approximately 3.3 m in an amount of 21 pieces. were subsequently processed in a bath of approximately 1135 liters. The treatment was intended to demonstrate highly controlled metal removal on typical rolled products. A total of 70.9 kg of material was removed from 21 rectangular bars 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 treated with a dissolved metal content of more than 60 g / L. From start to finish, no deleterious effects on metal surface conditions or metal removal rates were found, and as a result of the increase in dissolved metal 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 / HNO3, in which the solution becomes much less effective even at a titanium concentration in the solution of 12 g / L. Similarly, electrochemical sizing is hindered by high levels of dissolved metal in the electrolyte solution, since metal particles can clog the gap between the cathode and the anode workpiece and, if the solid is electrically conductive, can even cause a short circuit.

Crack modulation and oxide removal.

The aqueous electrolyte solution disclosed herein can be used to modulate or round the bottom of surface cracks, thereby eliminating pointed crack tips that form at varying depths along the surface of metals and are most hazardous 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 arise during cooling from processes including, but not limited to, hot working (eg, forging, rolling, superplastic forming, etc.), welding, and heat treatment. Moreover, an aqueous electrolyte solution can modulate these cracks with relatively low yield losses. The rounded or modulated end of the crack is suitable for subsequent hot working of the metal, since such a crack is able to heal during subsequent hot working, whereas a typical crack formed during metal cooling, if not completely removed, creeps and the metal breaks or cracks during subsequent processing.

FIG. 11 illustrates a conventional crack removal process that involves mechanical removal, typically by grinding or machine grinding, of a completely uniform layer of material to expose the bottom of the deepest surface crack. As shown schematically, this results in significant material loss. In contrast, FIG. 12 illustrates the process of modulating a crack using an electrolyte solution in combination with the application of an electric current to widen the crack and round the end of the crack at its bottom so that the crack does not propagate when the workpiece is hot worked or put into service.

Accordingly, the preferred conditions for the fracture tip modulation or removal process are such that very high removal rates are obtained while minimizing or eliminating hydrogen saturation. As discussed further below, these conditions are mainly obtained when

- 23 036890 the concentration of carboxylic acid (eg citric acid) is low, the concentration of fluoride ions (eg in the form of ammonium bifluoride) is high and the temperature is high. Although efficient large-scale removal can be performed at all power densities, good results have been achieved when the power density is low and the power is cycled to have as much OFF period as the process allows to minimize the amount of hydrogen incorporated into the material.

When processing within these technological parameters, the ability of an aqueous electrolyte solution to modulate the crack 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 is ground away flush with the surrounding metal. In addition, processing and consumable costs are significantly lower when using the electrolyte solution and method disclosed herein compared to current mechanical removal methods.

Another significant advantage of the process, achieved with an aqueous electrolyte solution, is the removal of the reactive metal oxide layer, or in the case of titanium and titanium alloys, the α-shell. Like the oxide layers of other reactive metals, the α shell is an oxygen-rich phase that occurs when titanium and its alloys are exposed to heated air or oxygen. The α-shell is brittle and tends to create a series of surface microcracks that will degrade performance, including the strength, fatigue, and corrosion resistance of the metal part. Titanium and titanium alloys are reactive metals, which means they react with oxygen and form a brittle cohesive oxide layer (TiO2 for Ti, ZrO2 for Zr, etc.) whenever they are heated in air or in an oxidizing atmosphere at a higher or higher temperature at which the natural oxide layer is formed, which depends on the specific alloy and the oxidizing atmosphere. As noted above, an α-shell oxide layer or oxide layer can be created by any heating of the metal to temperatures required for roll forging or rolling in a rolling mill, by welding, or by heating to forge a finished part or thermoform a part. The α-shell is brittle and full of microcracks that penetrate the bulk of the metal, potentially causing premature fracture due to tension or fatigue and making the surface more susceptible to chemical attack.

Therefore, the α-shell layer must be removed prior to any subsequent hot or cold pressure working or operation of the finished product. The aqueous electrolyte solutions and methods described herein using these solutions can remove the α-shell prior to the appearance of an unaffected base metal. Removal of the α-shell is a difficult problem in the processing of titanium and titanium alloys, since the α-shell is extremely resistant to attack, and conventional wisdom dictates that some mechanical intervention is required before electrochemical processing.

This difficulty was evident in an early trial in which the titanium α-shell was first blasted and / or lightly ground as a pretreatment prior to electrochemical treatment. Once abraded in this manner, the remainder of the oxide layer is most easily removed by an ON cycle, followed by a simple hard brushing, followed by a DC OFF cycle, in a cyclic process that is then repeated several times. It should be noted that an alternative to rough brushing would be a high pressure high volume injection system. Once the α-shell layer is removed, the rates of material removal in electrochemical etching are increased. However, the implementation of such a multi-stage technological cycle is very expensive and low income.

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

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 α-shell from the surface of a reactive metal in a controlled, reproducible manner. This is in contrast to the modern acid etching of HF-HNO3, in which a highly exothermic reaction occurs, which in the case of titanium uses the released titanium as a catalyst in the reaction, thereby causing continuous changes in acid concentration and reaction rates, making reproducibility difficult and almost

- 24 036890 impossible to accurately remove surface metal. Moreover, the disclosed aqueous electrolyte solution and method do not introduce harmful hydrogen into the bulk of the metal. Indeed, the disclosed method can be carried out in such a way as to remove hydrogen from the bulk of the metal. In contrast, the modern process by its very nature introduces harmful hydrogen into the bulk of the metal, making additional costly degassing steps necessary to remove the hydrogen.

The aqueous electrolyte solution is environmentally friendly and does not create hazardous waste, while the modern method uses the environmentally threatening and hazardous hydrofluoric acid (HF) and nitric acid (HNO3), which are extremely difficult to handle and which can only be used in strictly regulated programs ... As a result, processes using such an aqueous electrolyte solution can be performed without the involvement of significant air handling equipment and does not require the operator to wear protective clothing to protect against hazardous chemicals as is required with a modern HF-HNO3 based process.

Although the surface finish is not particularly important for modulating cracks and removing the α-shell, and improving the surface finish is not necessary due to subsequent processing steps to be carried out, it is still desirable not to seriously damage the surface of the material or cause pitting or other deep defects in material.

FIG. 3D, 4E-4J, 7A, 8A, 9A and 10A, it can be seen that the fastest material removal rates are obtained by maximizing the HFA concentration and temperature (i.e. at 120 g / L HFA concentration and 85 ° C temperature as shown in the graphs) while maintaining the citric acid concentration at or below 300 g / L. In addition, as emphasized in some of the figures, including FIG. 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 material removal rate tends to increase dramatically as the citric acid concentration approaches 0 g / L. However, under near zero citric acid conditions, FIG. 4H, 7B, 8B, 9B, and 10B indicate that surface pitting and severe deterioration in surface finish can occur.

Therefore, when using an aqueous electrolyte solution for modulating cracks, where it is preferable to avoid serious damage to the surface, at least a small amount of citric acid, for example 1 g / L or 10 g / L, should be used to mitigate the most severe effects of fluoride ions. However, when using an aqueous electrolyte solution to remove the α-shell, where serious surface damage would not be particularly harmful, fluoride ions can be as aggressive as possible without severe pitting, so the citric acid concentration can be reduced to near zero.

In both the crack modulation situation and the α-shell removal situation, it is desirable to avoid introducing hydrogen into the material in order to prevent embrittlement. Since, as seen in particular in FIG. 1A-1C, 2A-2B, and 4E-4G, material removal rates are relatively insensitive to current density, and since higher current densities tend to cause hydrogen to be incorporated into the material, it is preferable to operate at the lowest effective current densities.

While the description has been given in connection with exemplary embodiments, those skilled in the art will appreciate that not specifically described additions, deletions, modifications, and substitutions may 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 specific embodiments disclosed.

Claims (3)

1. A method of removing a metal oxide from a substrate surface of a non-ferrous metal preform, comprising the steps of exposing the surface to a bath with an aqueous electrolyte solution containing citric acid with a concentration of about 1 to about 180 g / l and ammonium bifluoride with a concentration of about 1 up to about 120 g / L and having no more than about 3.35 g / L of strong acid; adjusting the temperature of the bath so that it is greater than the freezing point of the bath and less than or equal to about 85 ° C; connect the workpiece to the anode of the DC power supply and immerse the cathode of the DC power supply in the bath and pass a current with a density less than or equal to about 255,000 A / m ² through the bath. 2. A method according to claim 1, wherein the non-ferrous metal is selected from titanium, nickel and their alloys. 3. A method for removing the α-shell from the surface of a substrate of a titanium or titanium alloy workpiece, comprising the steps of exposing the surface to a bath with an aqueous electrolyte solution containing citric acid with a concentration of about 1 to about 180 g / l and ammonium bifluoride with a concentration from about 1 to about 120 g / L and having no more than about 3.35 g / L strong ki - 25 036890 slots; adjusting the bath temperature to be greater than the freezing point of the bath and less than or equal to about 85 ° C; connect the workpiece to the anode of the DC power supply and immerse the cathode of the DC power supply into the bath and pass a current with a density less than or equal to about 5000 A / m ² through the bath.