JP5913349B2 - Electrolyte solution and electrochemical surface modification method - Google Patents

Description

Cross-reference to related application This application is related to an application by the same applicant and entitled “Electrolyte Solution and Electropolishing Method” filed with the present application.

  The solutions and methods of the present invention relate to the general field of electropolishing non-ferrous metal parts and surfaces. More specifically, with respect to surface treatment by electropolishing for non-ferrous and reactive metals, particularly titanium and titanium alloys, the treatment includes crack conditioning and oxide removal.

After processing a reactive metal from a metal ingot to a finished drawn material or after hot working the finished part, certain surface layer materials of metal oxides (in the case of titanium and titanium alloys) It is necessary to remove (usually called α case). These oxygen-rich phases occur when the reactive metal is heated in air or in an atmosphere containing oxygen. The oxide layer can affect the material strength, fatigue strength, and corrosion resistance of the metal. Titanium and titanium alloys belong to reactive metals. Reactive metals depend on the particular alloy and oxidizing atmosphere, but when heated above about 480 ° C. (900 ° F.) in air or oxidizing atmosphere, it always reacts with oxygen and becomes hard. A brittle oxide layer (TiO _{2 for} Ti, ZrO _{2 for} Zr, etc.). The oxide layer heats the metal to the temperature required as a result of forge welding, or forging of the finished part or heating for forming the hot part, in a typical mill forging or mill rolling. It is caused by doing. Reactive metal oxides and α cases are brittle and usually have a series of surface microcracks during molding. And microcracks penetrate into the bulk metal, causing potentially immature tension and fatigue failure loss, making the surface more susceptible to chemical attack. Therefore, the oxide layer or α-case layer must be removed before any subsequent hot or cold processing or final part use.

In addition, when forming an ingot into a finished part by processing reactive metals such as titanium and titanium alloys, it is also important to remove cracks formed by heat treatment or mechanical treatment. As described above, these cracks may penetrate deeper than the α case and may penetrate into the bulk metal. Typically, the reactive metal is heated, hot worked (eg, forging, rolling, drawing, extruding, etc.), cooled, reheated for further hot working, etc. 4-8 times, and ingot Into a finished drawn material. The stretched material is often reheated for finished part processing, and techniques such as, but not limited to, hot spinning, ring rolling, superplastic forming, and closed die forging are used. Each time the metal is cooled after hot working, cracks are formed on the surface and expand into the work piece. In conventional processing, these cracks have been removed by grinding. This involves mechanical removal and chemical milling in a strong acid (typically HF-HNO ₃ ). Then, a layer of uniform thickness, or an amount of material until the bottom of the deepest crack is exposed and removed is removed or milled from the workpiece. Grinding or chemical milling to such depths ensures that all cracks have been removed. However, a considerable amount of time and labor is consumed, and significant material loss occurs in terms of cost. This is because cracks sometimes extend into the workpiece at a depth of 5% or more relative to the thickness or diameter of the workpiece or finished part. However, the removal of cracks is essential because if cracks are not removed before the subsequent hot working process or before the finished part is used in service, the crack will expand and the workpiece or finished part Because it destroys.

  In chemistry and manufacturing, electrolysis is a method that uses direct current (DC), and without it, it induces a chemical reaction that occurs spontaneously. Electropolishing is a well-known application of electrolysis for deburring metal parts and producing glossy smooth surface finishes. The workpiece to be electropolished is immersed in an electrolyte solution bath and receives a direct current. While the cathode connection is made with one or more metal conductors surrounding the workpiece in the bath, the workpiece maintains its state as an anode. Electropolishing controls the process and relies on two opposing reactions. The first reaction is a dissolution reaction, during which the metal moves from the surface of the workpiece into the solution in the form of ions. Thus, the metal is removed from the workpiece surface for each ion. Another reaction is an oxidation reaction, during which an oxide layer is formed on the surface of the workpiece. When the oxide thin film is formed, the progress of the ion removal reaction is limited. The thin film is thickest with respect to micro depressions and thinnest with respect to micro projections. Then, because the electric resistance is proportional to the thickness of the oxide thin film, the metal dissolution rate is the fastest at the minute protrusions and the slowest at the minute depressions. Thus, electropolishing selectively removes microscopic high points or “tops” at a faster rate than attacking the corresponding micro-dents or “valleys”.

There is another application of electrolysis in the electrochemical machining (ECM) process. In ECM, a high current (often over 40,000 A, often giving current at a current density of over 1.5 million A / m ² ) is passed between the electrode and the metal workpiece to cause material removal. Electricity flows from the negatively charged electrode “tool” (cathode) to the conductor workpiece (anode) via a conductive fluid (electrolyte). The cathode tool is configured according to the desired machine operation and leads to the anode workpiece. The pressurized electrolyte is injected into the machine site at a set temperature. The workpiece material is removed and essentially liquefied at a rate determined by the feed rate of the tool to the workpiece. The gap distance between the tool and the workpiece varies from 80 to 800 microns (0.003 to 0.030 inches). As the electrons pass through the gap, the material on the workpiece melts and the tool forms the desired morphology for the workpiece. The electrolyte fluid removes the metal hydroxide formed in the process from the reaction between the electrolyte and the workpiece. Flushing is essential because the electrochemical machining process is less tolerant to metal complexes that accumulate in the electrolyte solution. On the other hand, in the method using the electrolyte solution disclosed in the present specification, even if a high concentration of titanium is present in the electrolyte solution, the stability and the effect are maintained.

The electrolyte solution for metal electropolishing is usually a mixture and contains concentrated strong acid such as mineral acid (completely dissolved in water). The strong acids described herein are generally categorized as being stronger acids than oxonium ions (H ₃ O ⁺ ) in aqueous solution. Examples of strong acids often used in electropolishing include sulfuric acid, hydrochloric acid, perchloric acid, and nitric acid, while examples of weak acids include those of the carboxylic acid group (eg, formic acid, acetic acid, Butyric acid and citric acid). Also, organic compounds (eg, alcohols, amines, or carboxylic acids) are sometimes used with strong acids in the mixture for the purpose of adjusting the solution etching reaction to avoid excessive etching of the workpiece surface. See, for example, US Pat. No. 6,610,194, which describes the use of acetic acid as a reaction modifier.

US Pat. No. 6,610,194

  There are incentives to reduce the use of these strong acids in metal finishing tanks, mainly due to health hazards and disposal costs of used solutions. Citric acid has been accepted by both the Department of Defense and ASTM standards as a passivating agent for stainless steel pieces. However, previous studies have shown and quantified the use of commercially available citric acid passivator solutions to passivate stainless steel, but reducing the strong acid concentration with an effective concentration of citric acid. An appropriate electrolyte solution could not be found. For example, in the publication “Citric Acid & Pollution Prevention in Passion & Electropolishing” (2002), some benefits of reducing the amount of strong mineral acid by replacing it with a certain amount of weaker organic acid (especially citric acid). This is due to the low cost, availability and relatively low risk of waste. However, in the end, other electrolytes are being evaluated that contain mainly a mixture of phosphoric acid and sulfuric acid with a small amount of organic acid (not citric acid).

Overview Normally, alpha case removal and crack control were not improved by the electropolishing method. Strong acid components found in typical electrolyte solutions used in prior art electropolishing result in aggressive and uncontrolled etching that can move hydrogen into the metal surface and deepen cracks. . In the development of a new electropolishing bath chemical that uses a weak acid and ABF solution without the presence of a strong acid component, the present inventor can effectively improve both α case removal and crack control by electropolishing. I found out. Therefore, an oxide removal and crack adjustment method by an electropolishing method is disclosed in this specification. These methods use new tank chemicals suitable for these methods.

In one embodiment, the disclosed aqueous electrolyte solution is as follows:
An aqueous electrolyte solution comprising a carboxylic acid in a concentration range of about 0.1 wt% to about 59 wt%; and a fluoride salt in a concentration range of about 0.1 wt% to about 25 wt% and substantially free of strong acid .

In one embodiment, the disclosed aqueous electrolyte solution is as follows:
An aqueous electrolyte solution comprising:
About 1.665 g / L or more and about 982 g / L or less of citric acid; and about 2 g / L or more and about 360 g / L or less of ammonium hydrogen fluoride as a fluoride salt substantially free of strong acid.

In one embodiment, the disclosed method is as follows:
A method for treating a surface of a non-ferrous metal workpiece, comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid in a concentration range of about 300 g / L or less; and ammonium difluoride in a concentration range of about 10 g / L or more; and a strong acid is about 3.35 g / L or less);
Controlling the temperature of the bath to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current to the vessel.

In one embodiment, the disclosed method is as follows:
A method of adjusting cracks in the surface of a non-ferrous metal workpiece, the method comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid at a concentration of about 300 g / L or less; and ammonium difluoride at a concentration of about 60 g / L or more; and a strong acid is about 3.35 g / L or less);
Controlling the bath temperature to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of less than about 53.8 A / m ^{2 to the} vessel.

In one embodiment, the disclosed method is as follows:
A method of removing metal oxide from the surface of a non-ferrous metal workpiece, the method comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid at a concentration of about 60 g / L or less; and ammonium difluoride at a concentration of about 60 g / L or more; and a strong acid is about 3.35 g / L or less);
Controlling the bath temperature to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of less than about 53.8 A / m ^{2 to the} vessel.

In one embodiment, the disclosed method is as follows:
A method of removing alpha case from the surface of a titanium or titanium alloy workpiece, the method comprising the following steps:
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid at a concentration of about 60 g / L or less; and ammonium difluoride at a concentration of about 60 g / L or more; and a strong acid is about 3.35 g / L or less);
Controlling the temperature of the vessel to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of less than about 53.8 A / m ^{2 to the} vessel.

FIG. 4 is a graph of data representing the rate of material removal at surface finish as a function of citric acid concentration in an aqueous electrolyte solution, the solution having a moderately low concentration of 20 g / L ammonium bifluoride; It has a high current density (1076 A / m ² ) in a certain temperature range. FIG. 5 is a graph of data representing changes in surface finish as a function of citric acid concentration in an aqueous electrolyte solution, the solution having a moderately low concentration of 20 g / L ammonium bifluoride and in a temperature range. And a high current density (1076 A / m ² ). FIG. 4 is a graph of data representing the rate of material removal as a function of ammonium bifluoride concentration in an aqueous electrolyte solution, the solution containing 120 g / L citric acid, which is a specific current at a typical low temperature. It is the speed in the density range. FIG. 3 is a graph of data representing the rate of material removal as a function of ammonium bifluoride concentration in an aqueous electrolyte solution, the solution containing 120 g / L citric acid, which is a specific current at a typical elevated temperature. It is the speed in the density range. 2B is a graph of data representing the change in surface finish as a function of ammonium hydrogen difluoride under conditions corresponding to FIG. 2A. 3 is a graph of data representing the change in surface finish as a function of ammonium hydrogen difluoride under conditions corresponding to FIG. 2B. 6 is a graph of data representing the material removal rate at surface finish as a function of current density density in an aqueous electrolyte solution with substantially no citric acid and a temperature of 85 ° C. 6 is a graph of data representing changes in surface finish as a function of current density density in an aqueous electrolyte solution with substantially no citric acid and a temperature of 85 ° C. FIG. 7 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 53.8 A / m ² . Represents the speed when the temperature is 21 ° C. FIG. 7 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 53.8 A / m ² . Represents the speed when the temperature is 54 ° C. FIG. 7 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 53.8 A / m ² . Represents the speed when the temperature is 71 ° C. FIG. 7 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 53.8 A / m ² . Represents the speed when the temperature is 85 ° C. FIG. 4 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 10.8 A / m ² . Represents the speed when the temperature is 54 ° C. FIG. 5 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 215 A / m ² , temperature Represents the rate when is 54 ° C. FIG. 5 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 538 A / m ² , temperature Represents the rate when is 54 ° C. FIG. 5 is a graph of data representing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution, for several concentrations of ammonium hydrogen difluoride, with a current density of 1076 A / m ² , temperature Represents the rate when is 54 ° C. FIG. 4 is a graph of data representing the rate of material removal as a function of current density, the temperature in aqueous solution is 85 ° C., citric acid is 120 g / L, and the rate for ammonium bifluoride at several concentrations. Represents. FIG. 5 is a graph of data representing the rate of material removal as a function of current density, with an aqueous solution temperature of 85 ° C., citric acid at 600 g / L, and rates for ammonium bifluoride at several concentrations. Represents. FIG. 4 is a graph of data representing the rate of material removal as a function of current density, with an aqueous solution temperature of 85 ° C., citric acid at 780 g / L, and rates for ammonium bifluoride at several concentrations. Represents. FIG. 4 is a graph of data representing changes in surface finish as a function of current density, representing changes under conditions corresponding to FIG. 4E. FIG. 5 is a graph of data representing changes in surface finish as a function of current density, representing changes under conditions corresponding to FIG. 4F. 4 is a graph of data representing changes in surface finish as a function of current density, representing changes under conditions corresponding to FIG. 4G. FIG. 7 is a graph of data representing the amount of material removed in a surface finish, with various concentrations for citric acid and ammonium bifluoride, at low temperature (21 ° C.), and high current density (538 A / m ² ). A graph of data representing changes in surface finish, at various concentrations for citric acid and ammonium bifluoride, at low temperature (21 ° C.), and at a high current density (538 A / m ² ). is there. FIG. 6 is a graph of data representing the amount of material removed in a surface finish, with various concentrations for citric acid and ammonium bifluoride, at low temperature (21 ° C.), and high current density (1076 A / m ^2). ). A graph of data representing changes in surface finish, at various concentrations for citric acid and ammonium difluoride, at low temperature (21 ° C.), and at a high current density (1076 A / m ² ). is there. FIG. 5 is a graph of data representing the amount of material removed in a surface finish, with various concentrations for citric acid and ammonium bifluoride at high temperature (85 ° C.) and high current density (1076 A / m ^2). ). Graph of data representing changes in surface finish, at various concentrations for citric acid and ammonium bifluoride, at high temperature (85 ° C.), and at high current density (1076 A / m ² ). is there. FIG. 6 is a graph of data representing the amount of material removed in a surface finish, with a representative high temperature (85 ° C.), low current density (10 ° C., with various concentration combinations for citric acid and ammonium bifluoride. .8 A / m ² ). FIG. 5 is a graph of data representing changes in surface finish, with representative high temperatures (85 ° C.) and low current densities (10.8 A / m ² ) for various concentration combinations for citric acid and ammonium bifluoride. ). FIG. 5 is a graph of data representing the amount of material removed in a surface finish, with a combination of various concentrations for citric acid and ammonium bifluoride, typical high temperature (85 ° C.), and high current density (538 A / M ² ). FIG. 5 is a graph of data representing changes in surface finish, with various concentrations combinations for citric acid and ammonium bifluoride at a typical high temperature (85 ° C.) and at a high current density (538 A / m ² ). It is a graph of. FIG. 6 is a graph of data representing the amount of material removed in a surface finish, with various sub-concentration combinations for citric acid and ammonium bifluoride, typical sub-high temperatures (71 ° C.), medium current density ( 215 A / m ² ). Graph of data representing changes in surface finish, representative sub-high temperature (71 ° C.), medium current density (215 A / m ² ) at various concentration combinations for citric acid and ammonium bifluoride. It is a graph in. 1 conceptually represents the order that occurs in a prior art method for removing cracks extending from the surface of a material into a workpiece. FIG. 2 conceptually represents the order that occurs in the method using the electrolyte described herein for adjusting cracks that extend into the interior of the workpiece at the surface of the material.

Detailed Description Disclosed herein is an aqueous electrolyte solution that is particularly useful for surface treatment of reactive metals, including but not limited to titanium and titanium alloys. A relatively small amount of fluoride salt and carboxylic acid is dissolved in water and is substantially devoid of strong acid (eg, mineral acid) so that the solution is substantially free of strong acid. The electrolyte solution of the present invention has been used in an electrolyte bath for surface treatment of reactive metals (including but not limited to titanium and titanium alloys) (typically using a strong acid, and the amount of water in the electrolyte solution is If it was necessary to maintain the absolute minimum amount), it is a remarkable development.

The fluoride salt provides a fluoride ion source for the solution. The fluoride salt may be, but is not limited to, ammonium hydrogen difluoride, NH ₄ HF ₂ (sometimes abbreviated as “ABF” herein). Without being bound by theory, it is believed that the carboxylic acid mitigates the attack of fluoride ions on the reactive metal surface to be treated. The carboxylic acid may be citric acid, but is not limited thereto. Although a trace amount of strong acid may be present, no strong acid or mineral acid is intentionally added to the solution. As used herein, the terms “substantially deficient” and “substantially absent” mean that the concentration of strong acid is about 3.35 g / L or less, preferably about 1 g / L or less, and further Preferably used to mean less than about 0.35 g / L.

An industrial pure (CP) titanium test material was immersed in a 54 ° C. aqueous solution bath containing 60 g / L citric acid and 10 g / L ABF, and a current was applied at 583 A / m ² . The coupon cut (0.52 μm surface roughness) from the mill surface titanium strip exposed to the solution for 15 minutes was uniformly smooth (0.45 μm surface roughness) and visually reflective. Then, a small amount of 42 ° C. BeHNO ₃ (nitric acid) was added incrementally, and the prepared test material was repeatedly processed unless a change in the surface was detected. Until the nitric acid concentration reached 3.35 g / L, the test material was not affected by the processing after each nitric acid addition. At this concentration point, the test panel shows a visually non-uniform appearance including pitting and spalling, and is not around the test material having a surface roughness in the range of 0.65 to 2.9 μm or more. Had a regular attack. Nitric acid is believed to be a borderline strong acid with a dissociation constant that does not significantly exceed the dissociation constant of hydronium ions. Therefore, for other strong acids with the same or higher dissociation constant than nitric acid, a similar electrolyte solution can be used for controlled material removal and micropolishing at strong acid concentrations below about 3.35 g / L. It is expected to have a similar effect. However, other electrolyte solutions disclosed herein have different concentrations of citric acid and ABF and different ratios of citric acid and ABF concentrations, but may be less tolerant with respect to the presence of strong acids. Yes, it is expected to depend not only on operating parameters such as temperature and current density but also on specific strong acids. Therefore, in order to effectively use aqueous electrolyte solutions for material removal and surface refinement over a wide range of citric acid and ABF concentrations, and within a wide range of temperatures and current densities, The abundance should be about 1 g / L or less, preferably about 0.35 g / L or less.

  Extensive electropolishing tests were performed on titanium and titanium alloy samples using a range of chemical concentrations, current densities, and temperatures. Specifically, the test is representative of a typical mill product as a “washed” mill product (American Society for Testing and Materials (ASTM), or Aerospace Materials Standard (AMS) and “shipped” condition metal). For the following purposes: measuring the ability of various solutions and methods to remove bulk metal; improving surface finish on sheet metal products at low material removal rates Or refine; and / or microclean the metal surface to make the surface finish very fine with a very low material removal rate. In addition, although most tests focus on titanium and titanium alloys, the tests also show that the same solution and method are more universally applicable to the processing of many non-ferrous metals. In addition to titanium and titanium alloys, metals that provide good results include, but are not limited to, gold, silver, chromium, zirconium, aluminum, vanadium, niobium, copper, molybdenum, zinc, and nickel. Moreover, the following alloys are also reliably processed: titanium-molybdenum, titanium-aluminum-vanadium, titanium-aluminum-niobium, titanium-nickel (Nitinol®), titanium-chromium (Ti 17 (Registered trademark), Waspaloy, and Inconel® (nickel-based alloy).

  Electrolyte solutions containing citric acid and ammonium bifluoride have been shown to be effective in etching non-ferrous metals and metal alloys even at surprisingly low concentrations of both components. In that context, etching is understood as including substantially uniform surface removal. Furthermore, improvements in surface finish are shown over a wide range of both citric acid and ammonium bifluoride concentrations. While it can be used from any concentration of citric acid to the saturation point with water (59 wt%, or about 982 g / L aqueous solution, standard temperature and pressure), citric acid concentration and ammonium bifluoride concentration There is a correlation between. At this concentration, citric acid sufficiently reduces the etching effect of fluoride ions generated by the dissociation of ammonium hydrogen fluoride, and the micropolishing of the material surface is enhanced, while the material removal rate is significantly reduced. For both etching and micropolishing, some mixtures are solutions with a citric acid concentration of 3.6% by weight or an amount of about 60 g / L, but with a concentration of citric acid that is significantly above that amount (maximum Etch rate and surface micropolishing results comparable to about 36% by weight or solutions up to about 600 g / L) on titanium. Therefore, in these solutions, it is clear that the ABF concentration more directly affects the etching rate than the citric acid concentration. Effective etching and micropolishing have been demonstrated with solutions with extremely low citric acid concentrations of less than about 1 wt% or about 15 g / L. However, it has been found that even when a minimal amount of fluoride ions is present, it is sufficient to cause some metal removal.

  At concentrations of citric acid above about 600 g / L, the etch rate is substantially reduced. However, although the etching rate is reduced, the surface finish results are improved at least at moderate to high current densities with the high concentration of citric acid. Thus, when a direct current is applied, the surface material removal rate can be increased with a more diluted citric acid mixture. On the other hand, a more concentrated mixture of citric acid (up to about 42 wt% or about 780 g / L of solution) results in a smoother and more glossy finish. And compared with the piece finished using the citric acid mixture with low concentration, it has a uniform and fine grain and no corrosive effect.

  Using the bath solutions and methods described herein, it is possible to achieve highly controlled metal removal. Specifically, because of the fine control level, bulk metal can be removed in thickness as small as 0.0001 inches and as large as 0.5000 inches and accuracy. Such precise control can be achieved by controlling the combination of citrate and ABF concentrations, temperature, and current density, as well as changing the duration and periodic dosing of the direct current. In general, removal can be performed uniformly over the entire surface of the workpiece. Alternatively, it can be applied selectively only to certain selected surfaces on the mill product or manufactured parts. Removal control is achieved by precisely tuning several parameters, including but not limited to temperature, power density, power cycle, ABF concentration, and citric acid concentration.

  The removal rate varies directly with temperature. Thus, if all other parameters are constant, removal will be slower if the temperature is lowered and faster if the temperature is higher. Nevertheless, although contrary to expectations, by maintaining the citric acid concentration and ABF concentration within certain preferred ranges, a high degree of micropolishing can also be achieved at high temperatures.

  The removal rate depends on the administration of DC power. Contrary to expectation, the removal rate is inversely related to continuously administered DC power. When administered continuously, the removal rate decreases as the DC power density increases. However, the removal speed can be increased by making the DC power periodic. Thus, if an effective material removal rate is desired, the DC power is cycled from OFF to ON during the processing operation. Conversely, when it is desired to control the removal rate with high accuracy, the DC power is continuously applied.

  Without being bound by theory, removal is slowed in proportion to the thickness of the oxide layer formed on the metal surface, and the metal surface becomes more oxidized when higher DC power is applied. And it is thought that this oxidation may act as a barrier against attack of fluoride ions on the metal. Therefore, cycling the DC power on and off at the stated rate can overcome the oxygen barrier, creating a mechanism that promotes the periodic stripping of thick oxide from the surface. As described herein, changing the bath temperature, dosing voltage, and operating parameters of citric acid concentration and ammonium bifluoride concentration allow the electrolyte to produce a useful result, i.e. advanced Controlled bulk metal removal and micropolishing are provided for specific applications. In addition to changing operating conditions within a given process, the set of operating parameters can change or enhance the ability to tune and control accurately for metal removal and surface finishing.

For example, as FIGS. 8A and 9A are, 85 ° C., 300 g / L citric acid, and under the condition of ammonium hydrogen difluoride of 10 g / L, the current density is increased from 10.8A / m ² to 538A / m ² It shows that the material removal rate is increasing. At the same time, FIG. 8B and 9B, the same conditions, when the current density is increased from 10.8A / m ² to 538A / m ^2, shows that the surface finish is degraded. By cycling the DC power source between these two current densities, better results (net result) can be achieved than when operating at only one current density during the entire process. Specifically, the process time for removing a specific amount of material can be reduced compared to simply operating at 10.8 A / m ² . Furthermore, due to the smoothing effect at lower current densities, the overall surface finish of the final product is superior to that obtained by simply treating at 538 A / m ² . Thus, cycling between two or more power settings (indicated by current density) allows for excellent results in both surface and precise bulk metal removal, which can be either surface enhancement or bulk metal removal. Less total time is required than individual processes.

  In addition to changing the duty cycle, electricity can be delivered through the electrolyte solution and through the workpiece, taking various waveforms obtained from a DC power source, including the following: Including but not limited to: half-wave rectification; full-wave rectification; square wave; and others that produce additional useful results and / or process speed enhancements without sacrificing the final surface finish Intermediate rectification. DC switching speeds (50 kHz to 1 MHz speed, or 15 to 90 minute period slowness) may be useful depending on the surface area being processed, the mass of the workpiece, and the specific surface conditions of the workpiece. Furthermore, the DC switching period itself may require its own period as appropriate. For example, a massive workpiece with a very rough initial surface finish can get the most benefit by first increasing the period with a slow switching cycle and then increasing the material as the surface is removed. improves.

  Also, in certain embodiments, when an electrolytic cell of the type described herein is tested, it is apparent that electropolishing is performed without increasing the hydrogen concentration on the metal surface, and in some instances, reducing the hydrogen concentration. Became. The oxygen barrier at the material surface may be responsible for the lack of hydrogen migration into the metal matrix. The data suggests that the oxygen barrier may also remove hydrogen from the metal surface. A higher fluoride ion concentration results in a faster removal rate, but has the unknown effect of adsorbing hydrogen on the metal matrix. When the citric acid concentration is high, the removal rate is low, and a high power density tends to be required during electropolishing. However, it also acts to add “smoothness” or “gloss” to the surface.

Compared to prior art solutions, the use of an aqueous electrolyte solution of ABF and citric acid provides several advantages with respect to metal product finishing and / or pickling. The electrolyte solution of the present disclosure makes it possible to achieve a strictly controlled finish specification. The finishing of conventional producer alloy flat products (sheets and plates) involves a number of steps and is ground to the finished standard using fine grinding media. Typically followed by “rinse pickling” in an acid bath containing hydrofluoric acid (HF) and nitric acid (HNO ₃ ) to form residual cutting material, ground-in deformed metal (ground) -Remove in-smeared metal) and surface abnormalities. HF-HNO ₃ pickling is exothermic and is therefore difficult to control and often results in metals below specification. As a result, the scrap rate of the metal is further increased, or the numerical value at which the metal serves again is further decreased. Although the rinse pickling may be required, typical secondary grinding and tertiary grinding can be omitted by using the electrolyte solution of the present disclosure. Predetermined finishing specifications can be reached that could not be achieved with the current state of cutting and pickling techniques. Furthermore, the electrolyte solution of the present disclosure does not introduce stress into the treated portion. By comparison, any mechanical grinding process can cause significant surface stress and cause warping of the material, resulting in a certain degree of probability for a typical or customer specified flatness specification. Will not be able to meet.

In a typical process using HF-HNO ₃ pickling, the target material may be hydrogenated and often must be removed by costly vacuum degassing to prevent embrittlement of the material. Using a water-based electrolyte bath containing citric acid and ABF for Ti-6A1-4V full-size sheet typical mill products and test materials of CP titanium, 6A1-4V titanium, and nickel base alloy 718 Tests that have been performed have shown that the hydrogen permeation results are reduced compared to the case where the sample is exposed to a conventional pickling solution, which is a strong acid. In particular, when treating Ti-6A1-4V and CP titanium to achieve the same alpha case-free, clean surface end results normally achieved by strong pickling include ammonium bifluoride and citric acid. Using an aqueous electrolyte solution composition, a range of temperatures, and current density conditions, the workpiece material is not loaded with hydrogen, and in many of these operating conditions, hydrogen is actually out of the material. Turned out to be out. While continuing testing and refining the preferred operating range for all metals and alloys, the same operating conditions using a strong pickling bath will be burdened even under conditions that may not be optimal. The results consistently showed that less hydrogen was loaded on the material compared to In general, as the concentration of ammonium hydrogen fluoride decreases, hydrogen removal from the material exposed to the electrolyte solution increases and hydrogen permeation into the material decreases.

  Highly controlled metal removal, surface finish, and micropolishing

  Micro-polishing or micro-smoothing of components, and especially micro-smoothing of already relatively smooth surfaces, is superior to manual or mechanical polishing using the solutions and methods described herein. Can be achieved. Both are problems in current mechanical methods, but micropolishing is performed without creating detrimental residual stress in the target workpiece or target material and without deforming the metal in the workpiece. Furthermore, by excluding human variation, the resulting polishing level is specific and reproducible. Moreover, cost reduction can also be achieved when the electrolyte solution of the present disclosure is used as compared with existing methods.

The test gave good results for micropolishing at high concentrations of citric acid, low to medium concentrations of ABF, high temperature, and high DC current density (which can be given continuously or periodically). However, the DC power density should be adjusted based on the alloy being processed. Titanium alloys containing aluminum (typically alpha-beta metallurgical alloys including the common Ti-6A1-4V alloy) tend to lose luster at dose DC voltages in excess of 40 volts. However, for these metals, it is possible to re-achieve the material gloss even when applying a voltage of about 40 volts and applying a higher current (ie to achieve higher power density). Without being bound by theory, this may be the result of an alpha stabilizing element, which in the case of many alpha beta alloys (including Ti-6Al-4V) is polished. Rather, it is aluminum that is anodized to Al ₂ O ₃ . But more specifically, titanium-molybdenum (all beta phase metallurgy) and industrial pure (CP) titanium (all alpha phase) are brighter with increasing DC power density without being clearly bound by similar upper voltage limits. Increase. In particular, higher voltages up to at least 150 volts can be used for other metals, such as electropolishing, micropolishing, and surface with the electrolyte solution disclosed herein for nickel-based alloy 718, for example. In processing, it has been found to produce useful results.

The solutions and methods described herein can be used to perform deburring of machined parts, preferably by processing burrs on metallized parts that are machined, especially machined. This is done when the parts are made from difficult metals (eg titanium and nickel based alloys). In the current state of the art, deburring of components on a machine is usually performed as a manual operation. Therefore, it suffers from many problems associated with human error and lack of human consistency. Testing with the solution of the present disclosure shows that deburring is most effective when the citric acid concentration is low (due to the resistance properties of citric acid in an electrochemical cell), and Best when fluoride ions from ABF were high. A similar solution can also be used to remove surface impurities after application to the machine or to clean the workpiece, for example using a HF-HNO ₃ bath if it is not. Would have been done with a strong pickling.

  As noted above, non-ferrous and especially reactive metals exhibit effective rates of chemical etching in a wide range of dilute citric acid mixtures. This makes it possible to customize the finishing process for a particular non-ferrous metal work piece, including the application of additional current before starting electropolishing and selectively reducing the tip area. Selected dwell time in the bath may be included before removing and reacting some of the surface metal.

  Citric acid-based electrolytes have a much lower viscosity than conventional electropolishing mixtures, partly due to the considerably lower dissociation constant of citric acid compared to the strong acids normally used in electropolishing electrolytes. Is given. The lower viscosity assists in moving the material and lowering the electrical resistance, so that a lower voltage can be used than in conventional electropolishing. The final electropolished finish is substantially affected by the viscosity and resistance of the electrolyte used. It has been found that the finest surface finish (high micropolishing) can be achieved using a highly resistive electrolyte solution combined with a high electropolishing voltage (and therefore medium to high current density). Furthermore, if an electrolyte solution having a somewhat higher conductivity (low resistance) is used, fine micropolishing can still be achieved even at high voltages and high current densities.

  Accordingly, the corresponding advantages may apply to electrolytic machining. It is particularly anticipated that an electrolyte bath having the composition described herein is an effective alternative to conventional electrolytic processing and / or pickling solutions, with substantial environmental and cost advantages. Can be used. The electrolyte solution disclosed herein is essentially free of strong acids, minimizing the risk of hazardous waste disposal and handling. Furthermore, the required current density is much lower than that required in conventional electrochemical machining.

  In general, as the concentration of ammonium difluoride increases, the electrical resistance of the electrolyte solution tends to decrease (ie, ammonium difluoride increases the electrical conductivity of the electrolyte solution). On the other hand, the presence of citric acid or increasing the concentration of citric acid relative to the concentration of ammonium bifluoride tends to moderate 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 and promote micropolishing, the ammonium hydrogen difluoride concentration should be kept low or even higher than ammonium difluoride concentration. Further, it is desirable to use it in combination with a higher concentration of citric acid. Therefore, by varying the concentration of ammonium bifluoride and the relative concentration of ammonium bifluoride and citric acid, the electrical resistance of the electrolyte solution is usefully controlled to achieve the desired level of micropolishing of the workpiece surface. Can be achieved.

  In contrast to conventional electropolishing or electromachining, the process disclosed herein does not require accuracy for the workpiece (anode) to be close to the cathode. The process is successful when the cathode is processed at a distance of about 0.1 cm to about 15 cm from the workpiece. Practical limits on the maximum distance between the cathode and the anode workpiece have arisen mainly from the industrial ones, including cell size, workpiece size, and electrical resistance of the electrolyte solution. Since the overall current density is often lower and much lower than that required for electrochemical machining, use a larger distance from the workpiece to the cathode, and simply increase the capacity of the power supply accordingly It is possible to make it. In addition, the lower viscosity electrolyte solution disclosed herein enables highly controlled bulk metal removal, surface finish, and micropolishing, so the same solution is also effective in electrolytic processing. It is expected that.

  Electropolishing of the metal workpiece is performed by exposing the workpiece and at least one cathode to an electrolyte solution bath and connecting the workpiece to the anode. The electrolyte solution includes carboxylic acid in an amount ranging from about 0.1% to about 59% by weight. The electrolyte solution may be a fluoride salt selected from an alkali metal fluoride, an alkaline earth metal fluoride, a silicate etching compound, and / or a combination thereof in a range of about 0.1 wt% to about 25 wt%. It can also be included. A current is applied from a power source between at least one anode connected to the workpiece and a cathode immersed in the bath to remove metal from the surface of the workpiece. The current is applied at a voltage in the range of about 0.6 millivolt direct current (mVDC) to about 100 volts direct current (VDC). Other carboxylic acids 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) can also be used, but citric acid is the preferred carboxylic acid. ABF is a preferred fluoride salt.

In another aspect of the electropolishing method, the current is applied at a voltage of about 0.6 VDC to about 150 VDC. The current can be applied at a current density of about 255,000 A / m ² or less (approximately 24,000 amps / square foot). Here, the denominator represents the entire effective surface area of the workpiece. For some non-ferrous metals such as nickel-based alloys, current densities up to and including about 5,000 A / m ² (about 450 A / ft ² ) can be used. For titanium and titanium alloys, a current density of about 1 to about 1100 A / m ² (about 0.1 to 100 A / ft ² ) is preferred. The electropolishing process using the electrolyte solution can be performed between the freezing point and the boiling point of the solution. For example, the temperature ranges from about 2 ° C to about 98 ° C, preferably from about 21 ° C to about 85 ° C. Also good.

  Indeed, material can be removed from the metal substrate at a rate of about 0.0001 inch (0.00254 mm) to about 0.01 inch (0.254 mm) / min. The following examples show the effect of the electrolyte when the concentration and operating conditions are changed.

Example 1: Etching of industrial pure titanium

  An electrolyte (consisting essentially of, by weight, about 56% water, about 43% citric acid (716 g / L), and about 1% ammonium bifluoride (15.1 g / L) at 185 ° F. In order to improve the surface finish of the material (ie to make the mill-standard finish more smooth), an industrial pure titanium plate sample was processed. The material starts with a surface finish of about 160 microinches, and after processing, the surface finish is reduced by 90 microinches, reduced to a final reading of 50 microinches, or by about 69%. Improved. The process was run for only 30 minutes, resulting in a reduction in material thickness to 0.0178 inches.

  Although a key feature of titanium plate products for many end-use applications, cold formability is highly dependent on the surface finish of the product. Using the electrochemical process embodiments disclosed herein, the material surface finish can be improved at a lower cost compared to conventional grinding and pickling methods. The finishes obtained using the solution and method embodiments of the present disclosure showed a much higher improvement in the cold formability of the plate product than conventional methods.

Example 2: Etching of 6A1-4V test material

The following example was performed on a 6A1-4V titanium alloy sheet stock test material having a measured value of 52 mm × 76 mm. The electrolyte was composed of water (H ₂ O), citric acid (CA), and ammonium hydrogen difluoride (ABF) with varying concentrations and temperatures. The results and measured values obtained as a result are shown in Table 1 below.

Example 3: Electropolishing of 6A1-4V test material

The following example was performed on a 6A1-4V titanium alloy sheet stock test material having a measured value of 52 mm × 76 mm. The electrolyte was composed of water (H ₂ O), citric acid (CA), and ammonium hydrogen difluoride (ABF) with varying concentrations and temperatures. The resulting findings and measurements were recorded in Table 2 below.

Further extensive testing was conducted under the following conditions: aqueous electrolyte solution (about 0 g / L to about 780 g / L (about 0% to about 47% by weight) of citric acid, and about 0 g / L to about 120 g / L. L (ammonia difluoride in the range of about 0% to about 8% by weight) and substantially free of strong acid (ie, less than about 1 g / L or less than 0.1% by weight)); Temperature range from about 21 ° C. to about 85 ° C .; current density applied, range from about 0 A / m ² to about 1076 A / m ² with respect to the surface area of the workpiece (citric acid has a saturation concentration of 780 g / L at 21 ° C. in water) Note that this is the case). A current density as high as at least 225,000 A / m ² can be used with a dosing voltage of 150 volts or higher. Test metals include industrial pure titanium and several spot tests on 6A1-4V titanium and nickel base alloy 718. Based on these results, similar electropolishing, micropolishing, and surface treatment results are expected to be obtained with the above classes of non-ferrous metals and alloys. The results are summarized in the following table and description, with reference to the drawings. Unless otherwise noted, temperatures are about 21 ° C., about 54 ° C., about 71 ° C., and about 85 ° C., and current densities are about 0 A / m ² , about 10.8 A / m ² , about 52.8 A / Tests were conducted at m ² , about 215 A / m ² , about 538 A / m ² , and about 1076 A / m ² . Even if it was a trace amount, it would not have a significant effect on the results, but no strong acid was intentionally added to any of the test solutions.

1A-1B show the material removal rate and variation at the surface finish, performed at four different temperatures, with quasi-low concentrations of 20 g / L ammonium bifluoride, and from about 0 g / L to about This was carried out using an aqueous electrolyte solution containing citric acid at a concentration of 780 g / L, and at a current density of 1076 A / m ² . FIG. 1A shows that the material removal rate varies directly with temperature, especially at lower concentrations of citric acid. As the bath temperature increases, the removal rate also increases. At lower temperatures of 21 ° C., 54 ° C., and 71 ° C., 180 g / L of citric acid is sufficient to begin to mitigate the material removal effect of ammonium hydrogen fluoride. On the other hand, at the higher temperature of 85 ° C., relatively rapid material removal continues up to a range of about 300 g / L of citric acid. At higher concentrations of citric acid above 300 g / L, the removal rate decreases at all temperatures. Conversely, FIG. 1B shows that the surface finish at all temperatures except the lowest temperature is degraded at lower citric acid concentrations, especially 120 g / L to 180 g / L or less. In other words, fluoride ions that contribute to significant material removal at lower citric acid concentrations also produce surface damage. However, the presence of citric acid at a sufficient concentration clearly acts as a useful barrier to fluoride ion attack. However, when the citric acid concentration is increased to 180 g / L or more, the surface finish is actually improved, and particularly the citric acid level is improved at 600 g / L. Above that, the material removal rate is significantly reduced. Furthermore, even with citric acid levels of about 120 g / L to 600 g / L where material removal still occurs, surface finish enhancement can be achieved simultaneously.

Tests have shown that a fluoride ion source, such as ammonium dihydrogen fluoride, is required to achieve the desired material removal and surface finish improvement. Electrolyte solutions that consist essentially of citric acid in water and are essentially free of ammonium hydrogen difluoride do not actually provide material removal and minimal changes in surface finish regardless of bath temperature or current density It is the limit. When the titanium or another reactive metal is treated in an aqueous electrolyte containing only citric acid, the surface of the material is essentially an oxide layer that is very thin (ie, about 200 nm to about 600 nm thick) and forms quickly. It is considered to be anodized. Once the anodized layer is formed, the administered DC power cannot attack the material surface any more and therefore hydrolyzes the water. The initial oxygen that forms quickly as a result finds another single atom of oxygen and is released as O ₂ gas at the anode.

  FIGS. 2A-2B and 2C-2D show materials in surface finish when using an aqueous electrolyte solution containing ammonium difluoride at a concentration of about 0 g / L to about 120 g / L of citric acid at a concentration of 120 g / L. The removal rate and the change are shown respectively. 2A and 2C show data at 21 ° C., a typical low temperature. 2B and 2C show data at 71 ° C., which is a typical high temperature. 2A-2B show that material removal is strongly correlated with ammonium bifluoride concentration and temperature, but the effect of current density is minimal. In general, higher material rates are obtained by increasing one or both of ammonium bifluoride concentration and temperature. 2C-2D show that material removal is accompanied by some degree of surface degradation. Surprisingly, however, the degree of surface finish degradation decreases as the temperature increases and the material removal rate increases. As shown in FIG. 2C, at 21 ° C., which is a low temperature, the surface deterioration effect is reduced if the current density is given, and it was proved that the surface finish is improved to some extent at the highest current density. As shown in FIG. 2D, at a higher temperature of 71 ° C., there was no significant change in the surface finish with the change in current density.

FIGS. 2E-2F show the surface as a function of current density when operated at a high temperature of 85 ° C. with an aqueous electrolyte solution consisting essentially of ammonium difluoride in water and without the intentional addition of citric acid. The material removal rate and the change in finishing are shown respectively. High material removal rates can be achieved using an ABF-only electrolyte, but such material removal is at the expense of surface finish and is often moderately degraded by the electrolyte solution. Nevertheless, under certain operating conditions (not shown), minimal degradation or the most gradual change in surface finish was achieved. For example, surface finish enhancement from an ABF-only electrolyte solution is 20 g / L at 10 g / L ABF solution, 21 ° C., and 215-538 A / m ² , and 54-71 ° C., and 1076 A / m ² . ABF solution, 21 ° C., and 215-1076 A / m ² and 60 g / L ABF solution, 21 ° C., and 538-1076 A / m ² were achieved.

  Without being bound by theory, a possible explanation for the ability to improve the surface finish by increasing the current density with minimal impact on the material removal rate is that the function of the current as a function of the current The purpose is to naturally generate an oxide layer. This excess oxygen, in combination with citric acid, is thought to act as a useful barrier against attack on the material surface. Therefore, it can be considered that as the current density increases, the concentration of oxygen at the anode increases and gradually acts as a large transport barrier. Or, from the simplest point of view of the surface morphology of the material as a series of “tops” and “valleys”, citric acid and oxygen are expected to stay in the valleys and expose only the tops of the surface morphology to fluoride ions. . As the strength of the citric acid and oxygen barrier increases (ie, higher concentrations of citric acid and higher current density), only the highest apex of the surface can be chemically attacked. Under these theories, low current density and low citric acid concentration are expected to provide the least possible process for surface smoothing, while high current density and high citric acid concentration provide surface smoothing. It is expected to provide the most capable process in terms of conversion. Regardless of whether these theories are correct, the data clearly produces results consistent with the above analysis.

  With the understanding that oxygen (caused by current) and citric acid clearly act as a micro-barrier to the removal process, the ABF concentration and temperature can be used for the purpose of controlling material removal and micro-polishing results. It becomes clear that it is the most likely variable to follow. Thus, in the process described herein, current density clearly acts primarily as producing oxygen and is rarely an important factor in increasing overall material removal. Rather, material removal is apparently almost exclusively induced by fluoride ions, and the material removal activity is governed to some extent by the thermodynamic effects of temperature. Overall, the current density as a control variable is surprising, but obviously its importance is relatively minor. The presence of fluoride ions exceeds the effect of current density.

3A-3D, at a typical current density of 53.8 A / m ² , the material removal rate can vary directly with temperature, resulting in citric acid, ammonium hydrogen difluoride, and water. For the same mixture, the material removal is further increased at higher temperatures. Similar trends were observed at all current densities (0 A / m ^{2 to} 1076 A / m ² ).

  4A-4D, at a typical temperature of 54 ° C., the rate of material removal is relatively constant with respect to current density, so that citric acid and ammonium bifluoride at any given bath temperature. For the same mixture of materials, the material removal rate is relatively insensitive to changes in current density. Similar trends were observed at all temperatures (21 ° C. to 85 ° C.). These tendencies are considered to be maintained even at temperatures below 21 ° C. (however, above the freezing point of the solution) and above 81 ° C. (however, below the boiling point of the solution). Although occurring at almost all temperature and current conditions, when the citric acid concentration exceeds a certain level (typically 600 g / L to 780 g / L), the rate of material removal is significant regardless of the ABF concentration. To decrease. Thus, in general, the citric acid concentration should be maintained below 600 g / L when it is desired to shape the workpiece to maintain the ability to achieve a certain level of material removal. .

  4E-4G show the effect of current density on material removal rate at a typical high temperature of 85 ° C. and three different concentrations of citric acid. 4H to 4J show the influence of the current density on the surface finish under the same condition group. 4E and 4G, although to a lesser extent, FIG. 4E shows that the material removal capability of the electrolyte solution is greatest when ammonium bifluoride is at the highest concentration, and is significantly more noticeable at higher temperatures. Is shown. FIG. 4E only shows data with 120 g / L of citric acid, but essentially the same material removal rate is seen at citric acid concentrations of 60 g / L, 120 g / L, and 300 g / L. Please note that. However, as shown in FIG. 4F, 600 g / L of citric acid clearly protects the surface to some extent from large-scale attacks at that concentration, and the material removal rate is lower than that of lower concentrations of citric acid. falling. As shown in FIG. 4G, the removal rate further decreases at 780 g / L. Regardless of the concentration of ammonium hydrogen fluoride and citric acid, material removal is clearly almost unaffected by current density.

FIG. 4H shows that at high temperatures and moderate citric acid concentrations, moderate surface finish degradation occurs at almost all ammonium bifluoride concentrations and current densities. However, referring to FIGS. 4E and 4H together, one process condition is outstanding. With a citric acid concentration of 120 g / L, a low level of 10 g / L ammonium bifluoride, and a high current density of 1076 A / m ² , material removal is suppressed and the surface finish results are significantly improved. Yes. This may be further evidence of the theory described above in the following respects. The increased current density produces a sufficient excess of oxygen at the material surface, filling the “valley” in the surface morphology, and as a result, the “top” is preferably attacked by fluoride ions resulting from the dissociation of ammonium bifluoride. The These effects are further emphasized in FIG. 4I (600 g / L citric acid) and FIG. 4J (780 g / L citric acid) in combination with the possible microbarrier effect of citric acid. In the figure, reduced degradation in surface finish, and in some cases improved surface finish, are shown with higher concentrations of citric acid and higher current density alone, higher concentrations of citric acid, and higher The combination of current densities is even more prominent. For example, in the range of 600 g / L to 780 g / L of citric acid, the surface finish is significantly improved when ammonium difluoride is 10 g / L and 20 g / L.

  However, in the citric acid range of 120 g / L to 600 g / L, and even to 780 g / L, at the highest concentration of 120 g / L ammonium bifluoride and higher current density, the surface finish is significantly worse. These effects are clearly limited. Similar results were obtained with 60 g / L ammonium bifluoride at least in the citric acid concentration range of 600 g / L to 780 g / L.

  As shown in Tables 3A-3C and 4A-4C below, process conditions for finishing sheet products (requires minimal material removal and conditions where medium to high surface finish enhancement is desired, and With regard to micropolishing, virtually no material removal is required, and very good surface finish enhancement is desired) over a wide range of electrolyte mixtures, temperatures and current densities. Tables 3A-3C and 4A-4C show that from water and citric acid, even though essentially no material removal and a moderate to high surface improvement could be achieved over a wide range of temperatures and current densities. It is essentially free of electrolytes that are substantially free of ammonium hydrogen fluoride. The reason is that these conditions were discussed separately in connection with FIGS. 1A-1C. Similarly, Tables 3A-3C and 4A-4C consist essentially of water and ammonium hydrogen difluoride and do not contain electrolytes that are substantially free of citric acid. The reason is that these conditions are discussed separately in connection with FIGS. 2A-2D. Tables 3A-3C are separated at the surface finish accuracy level and are summarized in ascending order of ABF concentration. Tables 4A-4C are separated by citric acid concentration level and summarized in ascending order of ABF concentration.

In Tables 3A-3C, several trends appear from the data. First, low or near zero material removal and improved surface finish have been obtained over the following ranges: citric acid concentration (60 g / L to 780 g / L), ammonium hydrogen fluoride concentration ( 10 g / L to 120 g / L), temperature (21 ° C. to 85 ° C.), current density (10.8 A / m ^{2 to} 1076 A / m ² ). Thus, an aqueous solution of citric acid and ABF that is substantially deficient in strong acid can produce a fine surface finish with minimal material loss at the following concentrations: 60 g / L of citric acid Acids and low concentrations of 10 g / L ABF; and 780 g / L citric acid and high concentrations of 120 g / L ABF; and some combinations between these.

In general, as shown in Table 3A, the most advanced surface finish enhancement (ie,> 30% reduction in surface roughness) is at a higher current density of 538-1076 A / m ² and medium Higher concentrations of citric acid at 120-780 g / L and generally lower ABF concentrations of 10-20 g / L. When the ABF concentration is lower, that is, in the range of 10 to 20 g / L, the citric acid concentration of 600 to 780 g / L, which is a higher concentration, is even better at a higher temperature of 71 to 85 ° C. Tend to produce a smooth surface finish. On the other hand, at a medium citric acid concentration of 120 to 300 g / L, a fine surface finish was produced at 54 ° C., which is a more moderate temperature. Nevertheless, significant surface finish improvement was also obtained under the following conditions: low concentration of ABF, medium concentration of citric acid, and high temperature conditions (10 g / L ABF, 120 g / L citric acid, 85 ° C. ); And low concentrations of ABF, medium concentrations of citric acid, and low temperature conditions (20 g / L ABF, 180 g / L citric acid, 54 ° C.). When the ABF concentration is higher, that is, in the range of 60 to 120 g / L, at a higher concentration of citric acid of 600 to 780 g / L, and at a higher current density, a lower temperature of 21 to 54 ° C., Furthermore, it tends to produce a good surface finish. Further, as shown in FIG. 4H, for both the low ABF concentration of 10 g / L and the high ABF concentration of 120 g / L, an even lower current density of 10.8 to 53.8 A / m ² , A significant surface finish refinement (improvement) could be achieved at an acid concentration of 780 g / L and high temperatures of 71-85 ° C.

As shown in Table 3B, generally, but not the highest, a high degree of surface finish improvement (ie, a reduction of about 15% to about 30% in surface roughness) was obtained at the following conditions: even lower ABF concentration of 10-20 g / L; and medium-higher temperatures of 54-85 ° C .; and many but not limited to the following current densities, many of which are even higher current densities: 538-1076 A / m ² . Typically, these results were achieved with high citric acid concentrations of 600-780 g / L. For example, ABF concentrations of 10-20 g / L typically produced fine results at higher current densities and higher citric acid concentrations. On the other hand, fine results could also be obtained using the following conditions: 60-300 g / L for lower citric acid concentration: 10.8 A / m ² for lower current density, and A high temperature of 85 ° C .; and a low current density of 53.8 A / m ² and a medium temperature of 54 ° C. High improvements in surface finish are also achieved with high levels of 120 g / L ABF, high temperature and low current density (71-85 ° C. and 10.8-53.8 A / m ² ), and low temperature and high current density. It was achieved at both (21 ° C. and 1076 A / m ² ) and was obtained in all cases with a high citric acid concentration of 780 g / L. In this respect, similar surface finish results can be achieved for solutions with high concentrations of citric acid using higher current densities and lower temperatures or using lower current densities and higher temperatures. There is clearly some complementary activity between temperature and current density. See also FIGS. 4H-4J. These figures show that high temperature conditions combined with high current density tend to produce the best surface finish improvement.

As shown in Table 3C, in general, the most moderate level of surface finish improvement (ie, less than about 15% reduction in surface roughness) was obtained under the following conditions: lower ABF concentration 10-20 g / L; and even higher temperatures of 71-85 ° C .; and many of them were obtained over a current density of 10.8-1076 A / m ² overall. Typically, these results were achieved with high citric acid concentrations of 600-780 g / L. One notable exception to this trend is that medium to high surface finish improvements were also obtained under the following conditions: total ABF concentrations in the range of 10 to 120 g / L; low to moderate citric acid Concentration 60-300 g / L; low temperature 21 ° C .; and high current density 1076 A / m ² .

  As shown in Table 4A, at low citric acid concentrations of 60-180 g / L, the improvement in surface finish clearly requires a high current density. Typically, the best surface finish improvement was obtained at a low ABF concentration of 10-20 g / L and a medium to high temperature of 54-85 ° C. Low and moderate surface finish improvements were achieved at ABF concentrations of 10-60 g / L and a low temperature of 21 ° C.

As shown in Table 4B, medium citric acid concentrations of 300-600 g / L generally require a higher current density of 538-1076 A / m ² for significant improvement in surface finish, It occurs at 10-20 g / L ABF, which is a very low ABF concentration. At the lowest ABF concentration of 10 g / L, the best results are achieved at 54 to 85 ° C., which is a higher temperature, while at the ABF concentration of 20 g / L, it is in the range of 21 to 85 ° C. Good results are achieved. At higher ABF concentrations of 60-120 g / L, improved surface finish occurs more typically at a lower temperature of 21 ° C.

Comparing Tables 4A and 4B with Table 4C, most process conditions to obtain virtually no material loss or minimal surface improvement occur at a high citric acid concentration of 780 g / L. I understand that. As shown in FIG. 4C, at a high citric acid concentration of 780 g / L, a significant surface finish improvement was obtained under the following conditions: almost all current densities in the range of 10.8 to 1076 A / m ² ; Low to high temperature 21-85 ° C; and both low ABF concentration 10-20 g / L ABF and high ABF concentration 120 g / L ABF.

FIGS. 5A and 5B show the material removal rate and change in surface finish at a typical low temperature of 21 ° C. and a typical high current density of 538 A / m ² . As can be seen from FIG. 5B, the surface finish degradation was moderate for all citric acid concentrations below 600 g / L for ABF concentrations below 60 g / L. And in fact, the surface finish was improved above 600 g / L, which is a high citric acid concentration, and specifically at 780 g / L for all ranges where the ABF concentration was 10-120 g / L. In addition, FIG. 5A shows that the rate of material removal at these process conditions is relatively low. Thus, operating in these ranges of compositions, temperatures, and current densities would be desirable to achieve the most reasonably controlled material removal with perhaps minimal surface degradation, or perhaps the most appropriate surface finish improvement. However, large-scale material removal will not be particularly effective.

Similarly, FIGS. 6A and 6B show material removal rates and changes in surface finish at a typical low temperature of 21 ° C. and a typical high current density of 1076 A / m ² . As can be seen from FIG. 6B, for the case where the ABF concentration is greater than 10 g / L and less than 120 g / L, a small to moderate surface finish improvement is achieved in all cases where the citric acid concentration is less than 600 g / L. The surface finish is most significantly improved when the citric acid concentration is 600 g / L or more. Furthermore, FIG. 6A shows that the rate of material removal under these process conditions is relatively low, except for compositions where citric acid is near 300 g / L and ABF is near 120 g / L. Here, the material removal rate is even higher without causing significant surface degradation. Thus, operating in these ranges of compositions, temperatures, and current densities would be desirable to achieve the most reasonably controlled material removal with perhaps minimal surface degradation, or perhaps the most appropriate surface finish improvement. However, large-scale material removal will not be particularly effective.

7A and 7B show that controlled material removal and surface finish enhancement can be achieved simultaneously under certain conditions. In particular, FIG. 7A shows a consistent ABF concentration of about 10 g / L over all citric acid concentrations when the workpiece is exposed to an electrolyte solution at a high temperature of 85 ° C. and a high current density of 1076 A / m ^2. Moderate material removal rate. Under the same conditions, FIG. 7B shows a substantial improvement in surface finish in all cases of citric acid concentrations above 60 g / L. Even with an ABF having a higher ABF concentration of 20 g / L to 120 g / L, material removal can be obtained directly in relation to the ABF concentration without substantially deteriorating the surface finish. However, at the highest citric acid concentration of 600 g / L or higher, the material removal rate was significantly reduced.

  Several ranges of operating conditions have been identified, in which controlled material removal can be achieved while moderately degrading the surface finish and usually increasing the roughness by less than about 50%. . 8A-8B, 9A-9B, and 10A-10B show exemplary operating conditions in these categories.

FIG. 8A shows a fairly constant at all ABF concentrations for citric acid concentrations ranging from about 60 g / L to about 300 g / L at high temperature (85 ° C.) and low current density (10.8 A / m ² ) conditions. The material removal rate can be achieved, indicating that a higher material removal rate is obtained in direct relation to the ABF concentration. FIG. 8B shows that the surface finish degradation is consistently moderate for these citric acid and ABF concentration ranges with little regard to specific citric acid and ABF concentrations. If the citric acid concentration is 600 g / L or more, the material removal ability of the electrolyte solution is greatly reduced or even stopped. Moreover, except for the case where the ABF concentration is 60 g / L, there is a tendency to alleviate the surface finish deterioration and slightly improve the surface finish. 9A and 9B show very similar results at high temperature (85 ° C.) and high current density (538 A / m ² ) conditions. 10A and 10B show that similar results can be approached even at a somewhat lower temperature of 71 ° C. and a moderate current density of 215 A / m ² .

Clearly based on the test data disclosed herein, by controlling temperature and current density, the same aqueous electrolyte solution bath can be used in a method that includes multiple steps. Yes, in this method, a moderate controlled amount of material is first removed at a relatively low current density, and the current density is then raised to a high level while maintaining or slightly lowering the temperature. Repairing the surface. For example, using a solution with 300 g / L citric acid and 120 g / L ABF, a moderate material removal rate, a temperature finish of less than 30%, a temperature of 85 ° C., and a current density of 53. 8 A / m ² (see FIG. 3D). Thereafter, surface improvement can be obtained at the same temperature and a current density of 1076 A / m ² (see FIGS. 7A and 7B) with less material removal.

By changing the citric acid concentration in addition to temperature and current density, more combinations of conditions for the multi-step process are found. This is due to the strong material removal mitigation effect that occurs when the citric acid concentration increases by 600 g / L or more. For example, referring to FIGS. 8A and 8B, when an electrolyte solution containing 120 g / L of ABF is used at a temperature of 85 ° C. and a current density of 10.8 A / m ² , the citric acid concentration is 300 g in the first processing step. At a concentration of / L, aggressive material removal with moderate surface degradation can be achieved. Then, in the second processing step, the material removal can be virtually stopped while the surface finish is significantly improved by simply increasing the citric acid concentration to 780 g / L. Similar results can be obtained using the high temperature, higher current density conditions of FIGS. 9A and 9B, or the moderately high temperature, medium current density conditions of FIGS. 10A and 10B.

Very low concentrations of ammonium hydrogen difluoride have been found to be effective in both material removal and micropolishing. As shown in FIG. 1A, the material removal rate is greatest at the elevated temperature. Therefore, it is expected that even lower concentrations of ammonium hydrogen difluoride will be more effective at higher temperatures (eg, 85 ° C. or higher). In one exemplary electrolyte solution having both a citric acid concentration and an ammonium hydrogen difluoride concentration of 2 g / L, material removal and surface finish changes were observed. At 285 A / m ² , a material removal rate of 0.008 mm / hr was recorded, which was −156% of the corresponding surface finish change (deterioration). At 0 A / m ² , a material removal rate of 0.0035 mm / hr was recorded, which was -187% of the corresponding surface finish conversion.

Similarly, a material removal rate of 0.004 mm / hr was recorded when processed in an aqueous solution of 2 g / L ABF without citric acid at a current of 271 A / m ² , correspondingly. It was -162% of the surface finish change (deterioration). At 0 A / m ² , a material removal rate of 0.0028 mm / hr was recorded, which was −168% of the corresponding surface finish change.

It is preferred to use the minimum amount of ABF required to have an effect, but concentrations significantly exceeding 120 g / L (ammonium difluoride concentrations as high as 240 g / L to 360 g / L) In addition, including concentrations exceeding saturation in water). By fixing the temperature at 67 ° C. and adding to the 179.9 g / L citric acid solution with increasing ABF at a current density in the range of 10.8 A / m ^{2 to} 255,000 A / m ² , The effect of the electrolyte solution at a concentration of ABF was tested. Since the solution has a relatively low electrical resistance, it was expected that higher ABF concentrations would result in higher conductivity in the solution (especially at higher levels of current density). Also, the temperature rises above room temperature, reducing the resistance of the electrolyte. Both samples of CP titanium and nickel base alloy 718 were exposed to the electrolyte, and bulk material removal and micropolishing continued while ABF was added. ABF was added above the saturation point in the electrolyte. The saturation point of ABF (which varies with temperature and pressure) under these parameters was between about 240 g / L and about 360 g / L. The data in Table 5 shows that the ABF concentration was above the saturation concentration in water and the electrolyte solution was effective for both bulk metal removal and micropolishing.

Tests were performed to determine the effect of the electrolyte solution on micropolishing at relatively high current densities (including those close to 255,000 A / m ² ) and bulk metal removal. In text, it should be understood that electrolytes with low resistance values can tolerate high current densities. Certain combinations of citrate and ABF concentrations exhibit particularly low resistance values. For example, an electrolyte solution containing about 180 g / L citric acid was tested at a high current density at temperatures ranging from about 71 ° C. to 85 ° C. Commercially Pure (CP) Titanium, and a sample of nickel-based alloy 718, while gradually increasing the current density in the range up to ^{10.8A / m 2 ~255,000A / m 2} , were exposed to electrolyte solution. The data in Table 5 shows that bulk material removal and micropolishing were achieved in all current density ranges tested (including 255,000 A / m ² ). Compared to processing titanium and titanium alloys, even higher current densities, particularly about 5000 A / m ² , may be useful in processing nickel-based alloys.

While CP titanium is effectively processed using relatively low voltages of about 40 volts or less, higher voltages can also be used. In one exemplary test, at 85.6 ° C., a voltage of 64.7 VDC and a current density of 53,160 A / m ² were applied to give about 180 g / L citric acid and about 120 g / L ABF. CP titanium was processed in the aqueous electrolyte solution bath containing it. Under these conditions, a bulk metal removal rate of 5 mm / hr was achieved with a 37.8% improvement in surface profilometer roughness. The result was a uniform, visually glossy surface with a reflective appearance. The same chemical electrolyte maintained the effect on the CP titanium sample for bulk metal removal even after raising the voltage to 150 VDC and reducing the current density to 5,067 A / m ² . However, under these conditions, the metal removal rate dropped to 0.3 mm / hr and the finish was slightly degraded to a satin appearance.

For some metals and alloys, higher voltages can provide an equivalent or further effect in achieving one or both of bulk material removal and surface finish enhancement. In particular, certain metals (including but not limited to nickel-based alloys (eg, Waspaloy and Nickel Alloy 718), 18k gold, pure chromium, Nitinol alloys) can be used for faster bulk metal removal, and / or With respect to good surface finish enhancement, the benefits of processing at higher voltages are clearly obtained. In one exemplary experiment at a relatively high voltage for nickel alloy 718, a voltage of 150 VDC at 86.7 ° C. in an aqueous electrolyte containing about 180 g / L citric acid and about 120 g / L ABF, And specimens processed with a current density of 4,934 A / m ² resulted in a bulk metal removal rate of only 0.09 mm / hr, but a uniform surface finish of 33.8% (surface (Based on profilometer measurements) improved.

To evaluate the effect of dissolved metal accumulated in the electrolyte solution, a batch of 21Ti-6A1-4V rectangular bars measuring 6.6 cm × 13.2 cm × about 3.3 meters in a tank of about 1135 liters. Processed continuously. The processing showed highly controlled metal removal in a typical mill product form. More than 21 rectangular rod pieces (70.9 kg material in total) have been removed from the rod and suspended in the electrolyte solution. Initially, the bar started processing with 0 g / L of dissolved metal in the solution, and finally the bar was processed with a dissolved metal amount greater than 60 g / L. From the start of processing to the end of processing, no detrimental effects on the metal surface condition or metal removal rate were detected. And, as a result of the increased amount of dissolved metal in the electrolyte solution, no significant changes were required in any operating parameters. This is in contrast to the result of pickling titanium with HF / HNO ₃ , where the effect of the solution is substantially reduced even when the titanium concentration in the solution is 12 g / L. Similarly, electrolytic processing is hindered by metals dissolved at high levels in the electrolyte solution. The reason is that the metal particles can interfere with the gap between the cathode and the anode workpiece. If the solid is conductive, it can even cause a short circuit.

Crack Control and Removal of Oxide Layer The aqueous electrolyte solution disclosed herein can be used to adjust or round the bottom of a crack on the surface, thereby eliminating the sharp tip of the crack. Can do. The tip is formed at various depths on the surface of the metal. And, the most harmful substances in the family of reactive metals include, but are not limited to: titanium and titanium alloys, nickel-base alloys, zirconium and the like. These metals form cracks at the time when cooling is performed from an elevated temperature in an oxygen-containing atmosphere. For example, such surface cracks may occur during cooling from the following processes, but are not limited to: processes involving heat (eg, forging, rolling, superplastic forming, etc.), welding, and heat treatment. Furthermore, the aqueous electrolyte solution hardly causes a loss, and can control these cracks. The tip of the rounded or conditioned crack is suitable for subsequent metal heat treatment. This is because such cracks can be healed in subsequent hot working. On the other hand, typical metal cooling cracks, if not completely removed, run in subsequent processing, causing the metal to split or crack.

  FIG. 11 illustrates a conventional processing method for removing cracks. Here, the processing method involves performing mechanical removal (typically grinding or mechanically removing the entire uniform layer of material) to expose the bottom of the deepest surface of the crack. As conceptually shown, such processing results in substantial loss of material. On the other hand, in the crack adjustment processing shown in FIG. 12, an electrolyte solution is used, and a current is applied to expand the crack at the bottom and round the tip of the crack in a form combined with the electrolyte solution. As a result, cracks will not propagate when the workpiece is further heat treated or serviced.

  Therefore, preferable processing conditions for adjusting and removing the crack tip are conditions that produce a rapid removal rate while minimizing or eliminating hydrogen pickup. As will be described later, these conditions are usually obtained when the carboxylic acid (eg, citric acid) concentration is low, the fluoride ion concentration is high (eg, in the form of ammonium hydrogen difluoride), and the temperature is high. Effective and extensive removal can be done at all power densities. However, good results can be achieved when the power density is low. Then, to minimize the amount of hydrogen added to the material, the power cycle is adjusted to have as much OFF operation as the process can tolerate.

  By processing within the range of these processing parameters, the ability of the aqueous electrolyte solution to control cracks is the same as the current processing method (the deepest crack bottom is ground with the surrounding metal (ground flush), the metal surface is uniform or Compared to local and mechanical removal (grinding or mechanical polishing)), it leads to a significant overall metal yield improvement. Also, when compared to current mechanical removal, the costs associated with processing and consumables are significantly lower using the electrolyte solutions and methods described herein.

Another important treatment advantage obtained with aqueous electrolyte solutions is that the reactive metal oxide layer (or alpha case in the case of titanium and titanium alloys) can be removed. Like other reactive metal oxide layers, the alpha case is an oxygen rich phase and occurs when titanium and titanium alloys are exposed to heated air or heated oxygen. The α case is brittle and tends to cause a series of microcracks on the surface and degrade performance including: strength, fatigue properties, and corrosion resistance of metal parts. Titanium and titanium alloys belong to reactive metals. This means that titanium and titanium alloys react with oxygen to form brittle and hard oxide layers (TiO _{2 for} Ti, ZrO _{2 for} Zr, etc.). This occurs whenever heated in air or in an oxidizing atmosphere above the temperature at which the oxide layer naturally forms and depends on the particular alloy or oxidizing atmosphere. As described above, the oxide layer or α-case oxide layer may be formed at a temperature required for mill forging or mill rolling, a temperature required as a result of welding, or forging of a finished part or forming a hot part. Can be caused by heating the metal to the temperature of the heating. The α-case is brittle and filled with microcracks that penetrate into the bulk metal, potentially causing immature tension or fatigue failure and making it more susceptible to chemical attack on the surface.

  Accordingly, the α case layer must be removed before subsequent hot or cold processing or final part service. The aqueous electrolyte solutions described herein and the treatment with these solutions remove the alpha case and expose a base metal that is not affected at all. Alpha case removal is a very difficult problem in the treatment of titanium and titanium alloys. The reason is that the α-case is extremely resistant to attack, and according to conventional knowledge, it has been necessary to perform several mechanical treatments before the electrochemical treatment.

  This difficulty became apparent in early trials. In the test, the titanium α case is initially grid-blasted and / or lightly ground as a pre-treatment for the electrochemical treatment. Once worn, the remaining oxide layer is most easily removed with a DC power cycle ON, followed by a simple scrubbing wear, and the DC power cycle is turned off. Thereafter, the cycle process is repeated several times. Note that high pressure, high volume pump systems are an alternative to scrub brush wear. Once the α-case layer is removed, the material removal rate from electrochemical etching increases. However, these multi-stage processing cycles are very expensive to operate and generate low revenue.

  In order to overcome these difficulties, the workpiece having the oxide layer or α-case layer is treated in a bath of aqueous electrolyte solution. Here, the solution contains a low concentration of carboxylic acid (eg, citric acid), a high concentration of fluoride ions (eg, ammonium hydrogen difluoride), is at a high temperature, and preferably has a low power density. The low citric acid concentration, the high ammonium difluoride concentration, and the high temperature maximize the material removal rate. And when power is applied periodically, the removal rate is relatively insensitive to current density, so a lower power density is used to reduce hydrogen penetration into the material surface.

In short, as disclosed herein, the aqueous electrolyte solution and method using the solution remove the oxide layer and remove the titanium alpha case from the reactive metal surface in a controlled and repeatable manner. Enable. This is in contrast to current HF-HNO ₃ pickling. The pickling is a powerful exothermic reaction. In the case of titanium, evolved titanium is used as a catalyst during the reaction, and thus the acid concentration and reaction rate are continuously changed and repeated. Therefore, accurate surface metal removal is almost impossible. Furthermore, the disclosed aqueous electrolyte solutions and methods do not penetrate harmful hydrogen into the bulk metal. Indeed, the disclosed method can be carried out in such a manner as to remove hydrogen from the bulk metal. In contrast, current methods, by their very nature, introduce harmful hydrogen into the bulk metal and require an additional costly step of degassing to remove the hydrogen.

The aqueous electrolyte solution is environmentally friendly and does not produce harmful waste. On the other hand, the current treatment method places a burden on the environment, produces harmful hydrofluoric acid (HF) and nitric acid (HNO ₃ ), and is extremely difficult to handle. It can only be used under very strictly authorized programs. As a result, the treatment with the aqueous electrolyte solution can be performed without significant air conditioning equipment. There is also no need for operators of protective gear for hazardous chemicals required for use in current HF-HNO ₃ treatment.

  For crack conditioning and alpha case removal, the surface finish is not particularly important and may not need to be improved as further processing steps follow. However, it is still desirable that the surface of the material is not severely degraded and does not cause pits or other deep defects in the material.

  As can be seen from FIGS. 3D, 4E-4J, 7A, 8A, 9A and 10A, when the ABF concentration and temperature are maximized (ie, the ABF concentration is 120 g / L and the temperature is 85 ° C. as shown in the graph), The highest material removal rate was obtained. On the other hand, the citric acid concentration was maintained at 300 g / L or less. Also highlighted in several figures, including FIGS. 4E-4G, citric acid tends to mitigate fluoride ion attack on the material surface, so as the citric acid concentration approaches 0 g / L. The material removal rate tended to increase sharply upward. However, near citric acid conditions can result in surface pits and severe surface finish degradation, as shown in FIGS. 4H, 7B, 8B, 9B, and 10B.

  Therefore, when using an aqueous electrolyte solution for crack control, it is preferable to avoid serious surface degradation, but at least a small amount of citric acid (e.g., to reduce the effectiveness of the most severe fluoride ion attack) 1 g / L to 10 g / L) should be used. However, when the aqueous electrolyte solution is used for the purpose of removing the α case, since serious surface deterioration is not particularly harmful, attack of fluoride ions can be allowed to the extent that no serious pits are generated. Thus, the concentration of citric acid can be reduced to near zero.

  In both crack control and alpha case removal environments, it is desirable to avoid introducing hydrogen into the material to prevent embrittlement. As seen particularly in FIGS. 1A-1C, 2A-2B, and 4E-4G, the removal rate of the material is relatively insensitive to current density, and as the current density increases, hydrogen is introduced into the material. Therefore, it is preferable to operate at the lowest density among effective current densities.

Although described in connection with exemplary embodiments of the invention, additions, deletions, and modifications not specifically described herein do not depart from the spirit and scope of the invention as defined in the appended claims. And those skilled in the art will appreciate that substitutions can be made and that the invention is not limited to the specific disclosed embodiments.
In one aspect, the present invention includes the following inventions.
(Invention 1)
A carboxylic acid in a concentration range of about 0.1% to about 59% by weight; and
Fluoride salt in a concentration range of about 0.1% to about 25% by weight
An aqueous electrolyte solution containing substantially no strong acid.
(Invention 2)
The electrolyte solution according to invention 1, wherein the fluoride salt is selected from the group consisting of alkali metal fluorides, alkaline earth metal fluorides, silicate etching compounds, and combinations thereof.
(Invention 3)
The electrolyte solution according to invention 2, wherein the fluoride salt is ammonium difluoride.
(Invention 4)
The electrolyte solution according to invention 1, wherein the carboxylic acid is citric acid.
(Invention 5)
An aqueous electrolyte solution comprising:
From about 1.665 g / L to about 982 g / L of citric acid; and
Ammonium difluoride as a fluoride salt of about 2 g / L or more and about 360 g / L or less
The electrolyte solution substantially free of strong acid.
(Invention 6)
A method for treating a surface of a non-ferrous metal workpiece, comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath
(Here, the aqueous electrolyte solution is:
Citric acid in a concentration range of about 300 g / L or less; and
Including ammonium difluoride in a concentration range of about 10 g / L or more; and
Strong acid is about 3.35 g / L or less);
Controlling the temperature of the bath to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and
Applying a current to the bath;
(Invention 7)
The method according to claim 6, wherein the temperature of the tank is controlled to about 71 ° C or higher.
(Invention 8)
The method according to invention 6, wherein the current applied in the vessel is about 538 A / m ² or less.
(Invention 9)
The method according to invention 8, wherein the current applied in the vessel is about 53.8 A / m ² or less.
(Invention 10)
The method of claim 6, wherein the citric acid concentration is less than about 60 g / L.
(Invention 11)
The method according to invention 6, wherein the ammonium hydrogen difluoride concentration is about 60 g / L or more.
(Invention 12)
A method of adjusting cracks in the surface of a non-ferrous metal workpiece, the method comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath
(Here, the aqueous electrolyte solution is:
Citric acid at a concentration of about 600 g / L or less; and
Containing ammonium difluoride at a concentration of about 60 g / L or more; and
Strong acid is about 3.35 g / L or less);
Controlling the bath temperature to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and
Applying a current of less than about 53.8 A / m ² to the vessel .
(Invention 13)
A method of removing metal oxide from the surface of a non-ferrous metal workpiece, the method comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath
(Here, the aqueous electrolyte solution is:
Citric acid at a concentration of about 60 g / L or less; and
Containing ammonium difluoride at a concentration of about 60 g / L or more; and
Strong acid is about 3.35 g / L or less);
Controlling the bath temperature to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and
Applying a current of less than about 53.8 A / m ² to the vessel .
(Invention 14)
A method of removing alpha case from the surface of a titanium or titanium alloy workpiece, the method comprising the following steps:
Exposing the surface to an aqueous electrolyte solution bath
(Here, the aqueous electrolyte solution is:
Citric acid at a concentration of about 60 g / L or less; and
Containing ammonium difluoride at a concentration of about 60 g / L or more; and
Strong acid is about 3.35 g / L or less);
Controlling the temperature of the vessel to be about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and
Applying a current of less than about 53.8 A / m ² to the vessel .

Claims (11)

An aqueous electrolyte solution comprising:
1.65 g / L or more and 982 g / L or less of citric acid; and 2 g / L or more and 360 g / L or less of ammonium hydrogen difluoride as a fluoride salt.
The electrolyte solution for performing electrochemical surface treatment on a workpiece as an anode. A method for treating a surface of a non-ferrous metal workpiece, comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid in a concentration range of 300 g / L or less; and containing ammonium difluoride in a concentration range of 10 g / L or more; and a strong acid of 3.35 g / L or less);
Controlling the temperature of the bath to be 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current to the vessel. It is a method of Claim 2 , Comprising: This method of controlling the temperature of the said tank to 71 degreeC or more. 3. The method according to claim 2 , wherein an electric current applied to the tank is 538 A / m < ² > or less. 5. The method according to claim 4 , wherein an electric current applied to the tank is 53.8 A / m < ² > or less. 3. The method according to claim 2 , wherein the citric acid concentration is less than 60 g / L. The method according to claim 2 , wherein the ammonium hydrogen fluoride concentration is 60 g / L or more. A method of adjusting cracks in the surface of a non-ferrous metal workpiece, the method comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid at a concentration of 600 g / L or less; and ammonium difluoride at a concentration of 60 g / L or more; and a strong acid of 3.35 g / L or less);
Controlling the bath temperature to be 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of less than 53.8 A / m ^{2 to the} vessel. A method of removing metal oxide from the surface of a non-ferrous metal workpiece, the method comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid at a concentration of 60 g / L or less; and ammonium difluoride at a concentration of 60 g / L or more; and a strong acid of 3.35 g / L or less);
Controlling the bath temperature to be 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of less than 53.8 A / m ^{2 to the} vessel. A method of removing alpha case from the surface of a titanium or titanium alloy workpiece, the method comprising the following steps:
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid at a concentration of 60 g / L or less; and ammonium difluoride at a concentration of 60 g / L or more; and a strong acid of 3.35 g / L or less);
Controlling the temperature of the tank to be 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of less than 53.8 A / m ^{2 to the} vessel. A method for treating a surface of a non-ferrous metal workpiece, comprising the following steps;
Exposing the surface to an aqueous electrolyte solution bath (wherein the aqueous electrolyte solution is:
Citric acid in a concentration range of 0.1% to 59% by weight; and ammonium difluoride in a concentration range of 10 g / L or more; and a strong acid of 0.335% by weight or less);
Controlling the temperature of the vessel in the range of freezing point and boiling point;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the vessel; and applying a current of 1 A / m ^{2 to} 255,000 A / m ^{2 to} the vessel.

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