Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/02—Etching
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/16—Polishing
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
  • C25F1/02—Pickling; Descaling
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
  • C25F1/02—Pickling; Descaling
  • C25F1/04—Pickling; Descaling in solution
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/02—Etching
  • C25F3/08—Etching of refractory metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/16—Polishing
  • C25F3/22—Polishing of heavy metals
  • C25F3/26—Polishing of heavy metals of refractory metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F5/00—Electrolytic stripping of metallic layers or coatings
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F7/00—Constructional parts, or assemblies thereof, of cells for electrolytic removal of material from objects; Servicing or operating
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/02—Etching
  • C25F3/04—Etching of light metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/16—Polishing
  • C25F3/18—Polishing of light metals

Definitions

• the solutions and methods relate to the general field of electropolishing non- ferrous metal parts and surfaces, and more specifically to surface treatment by
• electropolishing including crack modulation and oxide removal, for non-ferrous and reactive metals, particularly titanium and titanium alloys.
• titanium and titanium alloys are among the reactive metals, meaning that they react with oxygen and form a brittle tenacious oxide layer (Ti0 2 for Ti, Zr0 2 for Zr, etc.) whenever heated in air or an oxidizing atmosphere above about 480 °C (900 °F), depending on the specific alloy and oxidizing atmosphere.
• the oxide layer is created by heating of the metal to necessary temperatures for typical mill forging or mill rolling, as a result of welding, or by heating for finished part forging or hot part forming.
• Reactive metal oxides and alpha case are brittle, and upon forming are routinely accompanied by a series of surface microcracks which penetrate into the bulk metal, potentially causing premature tensile or fatigue failures, and making the surface more susceptible to chemical attack. Therefore, the oxide or alpha case layer must be removed before any subsequent hot or cold working, or final component service.
• Reactive metals are typically heated, hot processed (e.g., forged, rolled, drawn, extruded), cooled, and reheated for additional hot processing between 4 and 8 times to turn an ingot into a finished mill product.
• the mill product is often again heated for finished part fabrication using teachniques including, but not limted to, hot spin forming, ring rolling, superplastic forming, and closed die forming. Each time the metal is cooled after hot processing, cracks form at the surface and extend into the workpiece.
• these cracks are removed by grinding, which involves mechanically removing, or chemical milling in a strong acid, typically HF-HN0 3 , a uniform thickness layer or amount of material from the workpiece until the bottom of the deepest crack is exposed and removed. Grinding or chemical milling to this depth ensures that all of the cracks are removed, but takes a significant amount of time and labor and also results in a significant and costly loss of material. This is because the cracks sometimes extend into the workpiece to a depth of 5% or more of the thickness or diameter of the workpiece or finished part. But removal of the cracks is necessary, because if the cracks are not removed prior to a subsequent hot working step, or use of a finished part in service, the cracks can propagate and ruin the workpiece or finished part.
• a strong acid typically HF-HN0 3
• electrolysis is a method of using direct electrical current (DC) to drive an otherwise non-spontaneous chemical reaction.
• Electropolishing is a well known application of electrolysis for deburring metal parts and for producing a bright smooth surface finish.
• the workpiece to be electropolished is immersed in a bath of electrolyte solution and subjected to a direct electrical current.
• the workpiece is maintained anodic, with the cathode connection being made to one or more metal conductors surrounding the workpiece in the bath.
• Electropolishing relies on two opposing reactions which control the process.
• the first of the reactions is a dissolution reaction during which the metal from the surface of the workpiece passes into solution in the form of ions. Metal is thus removed ion by ion from the surface of the workpiece.
• the other reaction is an oxidation reaction during which an oxide layer forms on the surface of the workpiece.
• ECM electrochemical machining processes
• a high current (often greater than 40,000 amperes, and applied at current densities often greater than 1.5 million amperes per square meter) is passed between an electrode and a metal workpiece to cause material removal.
• Electricity is passed through a conductive fluid (electrolyte) from a negatively charged electrode "tool" (cathode) to a conductive workpiece (anode).
• the cathodic tool is shaped to conform with a desired machining operation and is advanced into the anodic workpiece.
• a pressurized electrolyte is injected at a set temperature into the area being machined.
• Material of the workpiece is removed, essentially liquefied, at a rate determined by the tool feed rate into the workpiece.
• the distance of the gap between the tool and the workpiece varies in the range of 80 to 800 microns (0.003 to 0.030 inches).
• the electrolyte fluid carries away metal hydroxide formed in the process from the reaction between the electrolyte and the workpiece. Flushing is necessary because the
• Electrolyte solutions for metal electropolishing are usually mixtures containing concentrated strong acids (completely dissociated in water) such as mineral acids. Strong acids, as described herein, are generally categorized as those that are stronger in aqueous solution than the hydronium ion (H 3 0 + ).
• strong acids commonly used in electropolishing are sulfuric acid, hydrochloric acid, perchloric acid, and nitric acid
• weak acids include those in the carboxylic acid group such as formic acid, acetic acid, butyric acid, and citric acid.
• Organic compounds, such as alcohols, amines, or carboxylic acids are sometimes used in mixtures with strong acids for the purpose of moderating the dissolution etching reaction to avoid excess etching of the workpiece surface. See, for example, U.S. Patent No. 6,610,194 describing the use of acetic acid as a reaction moderator.
• Citric acid has previously become accepted as a passivation agent for stainless steel pieces by both Department of Defense and ASTM standards.
• prior studies have shown and quantified the savings from using a commercial citric acid passivation bath solution for passivating stainless steel, they have been unable to find a suitable electrolyte solution in which a significant concentration of citric acid was able to reduce the concentration of strong acids.
• Alpha case removal and crack modulation have not typically been remedied by electropolishing processes.
• the strong acid components found in typical electrolyte solutions used in the prior art of electropolishing result in hydrogen migration into the metal surfaces, and aggressive uncontrolled etching that may deepen the cracks.
• the inventors have discovered that both alpha case removal and crack modulation can be effectively remedied through
• an aqueous electrolyte solution including about 0.1% by weight to about 59% by weight of a carboxylic acid, and about 0.1% by weight to about 25% by weight of a fluoride salt, and being substantially free of a strong acid.
• an aqueous electrolyte solution including about 1.665 g/L citric acid to about 982 g/L citric acid, and about 2 g/L ammonium bifluoride to about 360 g/L ammonium bifluoride of a fluoride salt, and being substantially free of a strong acid.
• a method of treating the surface of a non-ferrous metal workpiece including exposing the surface to a bath of an aqueous electrolyte solution including a concentration of citric acid less than or equal to about 300 g/L and a concentration of ammonium bifluoride greater than or equal to about 10 g/L, and having no more than about 3.35 g/L of a strong acid, controlling the temperature of the bath to be greater than or equal to about 54 °C, connecting the workpiece to the anode of a DC power supply and immersing a cathode of the DC power supply in the bath, and applying a current across the bath.
• a method of modulating cracks in the surface of a non- ferrous metal workpiece including exposing the surface to a bath of an aqueous electrolyte solution including a concentration of citric acid less than or equal to about 300 g/L and a concentration of ammonium bifluoride greater than or equal to about 60 g/L, and having no more than about 3.35 g/L of a strong acid, controlling the temperature of the bath to be greater than or equal to about 54 °C, connecting the workpiece to the anode of a DC power supply and immersing a cathode of the DC power supply in the bath, and applying a current across the bath of less than about 53.8 amperes per square meter.
• a method of metal oxide removal from the surface of a non-ferrous metal workpiece including exposing the surface to a bath of an aqueous electrolyte solution including a concentration of citric acid less than or equal to about 60 g/L and a concentration of ammonium bifluoride greater than or equal to about 60 g/L, and having no more than about 3.35 g/L of a strong acid, controlling the temperature of the bath to be greater than or equal to about 54 °C, connecting the workpiece to the anode of a DC power supply and immersing a cathode of the DC power supply in the bath, and applying a current across the bath of less than about 53.8 amperes per square meter.
• a method of alpha case removal from the surface of a titanium or titanium alloy workpiece including exposing the surface to a bath of an aqueous electrolyte solution including a concentration of citric acid less than or equal to about 60 g/L and a concentration of ammonium bifluoride greater than or equal to about 60 g/L, and having no more than about 3.35 g/L of a strong acid, controlling the temperature of the bath to be greater than or equal to about 54 °C, connecting the workpiece to the anode of a DC power supply and immersing a cathode of the DC power supply in the bath, and applying a current across the bath of less than about 53.8 amperes per square meter.
• Figs. 1 A-1B are graphs of data showing the rate of material removal and the change in surface finish as a function citric acid concentration in an aqueous electrolyte solution having a moderately low concentration of 20 g/L ammonium bifluoride a high current density of 1076 A/m over a range of temperatures.
• Figs. 2A-2B are graphs of data showing the rate of material removal as a function of ammonium bifluoride concentration in an aqueous electrolyte solution including 120 g/L citric acid at representative low and high temperatures, respectively, over a range of current densities.
• Figs. 2C-2D are graphs of data showing the change in surface finish as a function of ammonium bifluoride under conditions corresponding to Fig. 2A-2B, respectively.
• Figs. 2E-2F are graphs of data showing the rate of material removal and the change in surface finish, respectively, as a function of current density in an aqueous electrolyte solution substantially without citric acid at a temperature of 85 °C.
• Figs. 3 A-3D are graphs of data showing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution for several concentrations of ammonium bifluoride at a current density of 53.8 A/m and temperatures of 21 °C, 54 °C, 71 °C, and 85 °C, respectively.
• Figs. 4A-4D are graphs of data showing the rate of material removal as a function of citric acid concentration in an aqueous electrolyte solution for several concentrations of ammonium bifluoride at a temperature of 54 °C and current densities of 10.8 A/m 2 , 215 A/m 2 , 538 A/m 2 , and 1076 A/m 2 , respectively.
• Figs. 4E-4G are graphs of data showing the rate of material removal as a function of current density at a temperature of 85 °C in an aqueous solution having 120 g/L, 600 g/L, and 780 g/L of citric acid, respectively, for several concentrations of ammonium bifluoride.
• Figs. 4H-4J are graphs of data showing the change in surface finish as a function of current density under conditions corresponding to Figs. 4E-4G, respectively.
• Figs. 5 A-5B are graphs of data showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a low temperature (21 °C) and high current density (538 A/m 2 ).
• Figs. 6A-6B are graphs of data showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a low temperature (21 °C) and high current density (1076 A/m 2 ).
• Figs. 7A-7B are graphs of data showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a high temperature (85 °C) and high current density (1076 A/m 2 ).
• Figs. 8A-8B are graphs of data showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a representative high temperature (85 °C) and low current density (10.8 A/m 2 ).
• Figs. 9A-9B are graphs of data showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a representative high temperature (85 °C) and high current density (538 A/m 2 ).
• Figs. 10A-10B are graphs of data showing the amount of material removed and the change in surface finish, respectively, at various combinations of citric acid and ammonium bifluoride concentrations at a representative moderately high temperature (71 °C) and moderate current density (215 A/m 2 ).
• FIG. 11 is schematic representation of a sequence that occurs in prior art process for removing a crack extending into a workpiece from a surface of the material.
• Fig. 12 is a schematic representation of a sequence that occurs in a process using an electrolyte as disclosed herein for modulating a crack extending into a workpiece form a surface of the material.
• Aqueous electrolyte solutions that are particularly useful for surface treatment of reactive metals including, but not limited to, titanium and titanium alloys are disclosed herein. Relatively small amounts of a fluoride salt and a carboxylic acid are dissolved in water, substantially in the absence of a strong acid such as a mineral acid, such that the solution is substantially free of a strong acid.
• This electrolyte solution is a notable departure from earlier attempts at electrolyte baths for surface treatment of reactive metals, including but not limited to titanium and titanium alloys, which typically use strong acids and require that the amount of water in the electrolyte solution be kept to an absolute minimum.
• the fluoride salt provides a source of fluoride ions to the solution and may be, but is not limited to, ammonium bifluoride, NH 4 HF 2 (sometimes abbreviated herein as "ABF").
• NH 4 HF 2 sometimes abbreviated herein as "ABF”
• carboxylic acid moderates the fluoride ion attack on the reactive metal surface to be treated and may be, but is not limited to, citric acid. No amount of strong acid or mineral acid is deliberately added to the solution, although a trace amount of strong acid may be present.
• the terms “substantially in the absence of and “substantially free of are used to designate concentrations of strong acid less than or equal to about 3.35 g/L, preferably less than or equal to about 1 g/L, and more preferably less than about 0.35 g/L.
• Test coupons of commercially pure (CP) titanium were immersed in a bath of aqueous solution including 60 g/L of citric acid and 10 g/L ABF at 54 °C, and a current was applied at 583 A/m .
• a coupon cut from mill-surface titanium strip (0.52 ⁇ surface roughness) exposed to this solution for 15 minutes was uniformly smooth (0.45 ⁇ surface roughness) and cosmetically reflective.
• small quantities of 42° Be HN0 3 (nitric acid) were incrementally added, and the prepared test coupon was processed repeatedly until surface changes were detected.
• Nitric acid is considered to be a borderline strong acid with a dissociation constant not much greater than that of the hydronium ion. Therefore, it is expected that for other stronger acids having the same or greater dissociation constants than nitric acid, a similar electrolyte solution would be similarly effective at controlled material removal and micropolishing at concentrations of strong acid less than approximately 3.35g/L.
• Extensive electropolishing testing has been conducted on titanium and titanium alloy samples using a range of chemistry concentrations, current densities, and temperatures. In particular, testing has been performed on "clean" mill products
• metals in addition to titanium and titanium alloys including, but not limited to, gold, silver, chromium, zirconium, aluminum, vanadium, niobium, copper, molybdenum, zinc, and nickel.
• alloys such as titanium-molybdenum, titanium-aluminum-vanadium, titanium-aluminum-niobium, titanium-nickel (Nitinol®), titanium-chromium (Ti 17®), Waspaloy, and Inconel® (nickel base alloy) have also been positively processed.
• An electrolyte solution containing citric acid and ammonium bifluoride has proven to be effective at etching non-ferrous metals and metal alloys in surprisingly dilute concentration of both components.
• etching is understood to encompass substantially uniform surface removal.
• improvements in surface finish have been shown over a wide range of both citric acid and ammonium bifluoride
• citric acid concentrations While any concentration of citric acid up to saturation point with water (59% by weight, or about 982 g/L of aqueous solution at standard temperature and pressure) could be used, there appears to be a correlation between citric acid concentration and ammonium bifluoride concentration at which the citric acid sufficiently mitigates the etching effects of the fluoride ion generated by dissociation of the ammonium bifluoride that the rate of material removal is dramatically curtailed while micropolishing of the material surface is enhanced. For both etching and micropolishing, several mixtures having amounts of citric acid concentration as low as 3.6 wt.
• etch rates and surface micropolishing results on titanium comparable to concentrations of citric acid well above that amount, including up to about 36 wt. % or about 600 g/L of solution.
• the etch rate is apparently more directly influenced by the concentration of ABF than by the concentration of citric acid.
• Effective etching and micropolishing has even been shown at extremely low citric acid concentrations of less than about 1 wt. %, or about 15 g/L of solution. The presence of even the smallest amount of fluoride ion, however, appears to be sufficient for some metal removal to occur.
• the etch rate falls substantially at concentrations of citric acid above about 600 g/L.
• the surface finish results improve while the etch rate falls.
• the more dilute mixtures of citric acid enable greater rates of surface material removal, while the more concentrated mixtures of citric acid, up to mixtures as high as about 42% by weight, or about 780 g/L of solution, provide a smoother and more lustrous finish, with uniform fine grain and no corona effect as compared to pieces finished with less concentrated citric acid mixtures.
• Highly controlled metal removal can be achieved using the bath solutions and methods described herein.
• the level of control is so fine that bulk metal can be removed in thicknesses as small as 0.0001 inches and as large and precise as 0.5000 inches.
• Such fine control can be achieved by regulating a combination of citric acid and ABF concentrations, temperature, and current density, as well as by varying the duration and cyclical application of direct current. Removal can be performed generally uniformly on all surfaces of a workpiece, or can be selectively applied only on certain selected surfaces of a mill product or manufactured component. Control of removal is a achieved by fine tuning several parameters, including but not limited to temperature, power density, power cycle, ABF concentration, and citric acid concentration.
• Removal rate depends on the manner in which DC power is applied. Contrary to what might be expected, removal rate appears to be inversely related to continuously applied DC power, and when continuously applied, increasing the DC power density decreases the removal rate. However, by cycling the DC power, removal rates can be hastened. Consequently, when significant material removal rates are desired, DC power is cycled from OFF to ON repeatedly throughout a treatment operation. Conversely, when fine control of removal rates is desired, DC power is continually applied.
• Figs. 8A and 9A demonstrate that at 85 °C, 300 g/L citric acid, 10 g/L ammonium bifluoride, material removal rates increase as current density increases from 10.8 A/m 2 to 538 A/m 2 .
• Figs. 8B and 9B demonstrate that at the same conditions, surface finishes degrade when current density increases from 10.8 A/m to 538 A/m .
• a net result can be achieved that is better than operating solely at either one of the current densities for the entire process.
• the process time to remove a specific amount of material can be reduced as compared to operating solely at 10.8 A/m .
• DC switching rates as fast as 50 kHz to 1 MHz, or as slowly 15 to 90 minutes cycles, may be beneficial depending on the surface area to be processed, the mass of the workpiece, and the particular surface condition of the workpiece. Additionally, the DC switching cycle itself may optimally require its own cycle. For example, a large mass workpiece with a very rough initial surface finish may benefit the greatest from a slow switching cycle initially, followed by a switching cycle of increased frequency as material is removed and the surface finish improves.
• testing electrolytic baths of the type described herein also revealed that electropolishing takes place in certain embodiments without increasing hydrogen concentration in the surface of the metal, and in some instances decreases the hydrogen concentration.
• the oxygen barrier at the material surface may be responsible for the absence of hydrogen migration into the matrix of the metal. Data suggests that this oxygen barrier may also be removing hydrogen from the metal surface.
• Higher fluoride ion concentrations result in faster removal rates, but have an unknown impact on hydrogen adsorption to the metal matrix.
• Higher citric acid concentrations tends to slow removal rates and demand higher power densities during electropolishing, but also act to add 'smoothing' or 'luster' to the surface.
• aqueous electrolyte solution of ABF and citric acid as compared with prior art solutions for finishing and/or pickling metal products.
• the disclosed electrolyte solutions enable a precisely controlled finish gauge to be achieved. Finishing of conventional producer alloy flat products (sheet and plate) involves multi-step grinding to finished gauge using increasingly fine grinding media, typically followed by "rinse pickling" in an acid bath including hydrofluoric acid (FiF) and nitric acid ( ⁇ 0 3 ) to remove residual grinding materials, ground-in smeared metal, and surface anomalies.
• SOF hydrofluoric acid
• ⁇ 0 3 nitric acid
• FiF-FiN0 3 acid pickling is exothermic and is therefore difficult to control, and often results in the metal going under gauge, resulting in a higher scrap rate or lower-value repurposing of the metal.
• the typical secondary and tertiary grinds can be eliminated, as can the need for the rinse pickle.
• a precise predetermined finished gauge can be reached that cannot be achieved with current state of the art grinding and pickling.
• the disclosed electrolyte solutions do not introduce stresses into the part being treated. By comparison, any mechanical grinding process imparts significant surface stresses, which can cause material warping and results in some percentage of material being unable to meet typical or customer stipulated flatness specifications.
• a typical process using FiF-FiN0 3 acid pickling will charge hydrogen into the target material which often must be removed by costly vacuum degassing to prevent embrittlement of the material.
• Testing conducted using an aqueous electrolyte bath containing citric acid and ABF on typical mill production full-size sheets of Ti-6A1-4V and on coupons of CP titanium, 6A1-4V titanium, and nickel base alloy 718 has shown reduced hydrogen impregnation results as compared with samples exposed to conventional strong acid pickling solutions.
• Micropolishing or microsmoothing of components, and in particular micro- smoothing of already relatively smooth surfaces, can be achieved using solutions and methods described herein with a superior precision as compared with manual or machine polishing. Micropolishing occurs without generating detrimental residual stresses in the target workpiece or material, and without smearing of metal in the workpiece, both of which are problems inherent in current mechanical methods. Additionally, by eliminating human variability, the resulting levels of polish are specific and reproducible. Cost savings can also be achieved using the disclosed electrolyte solution versus existing methods.
• alpha stabilizing element which in the case of most alpha-beta alloys (including Ti-6A1-4V) is aluminum anodizing to A1 2 0 3 rather than being polished.
• titanium-molybdenum (all beta phase metallurgy) and commercially pure (CP) titanium (all alpha phase) get brighter with increasing DC power densities without apparently being bound by a similar upper voltage limit.
• higher voltages up to at least 150 volts can be used, for example with the nickel base alloy 718 to produce beneficial results in electropolishing, micropolishing, and surface treatment using electrolyte solutions as disclosed herein.
• the solutions and method disclosed herein can be used to deburr machined parts by preferentially processing the burrs on machined metal components, especially when the parts are made from difficult to machine metals such as titanium and nickel base alloys.
• deburring of machined components is typically performed as a manual operation, and thus suffers from many problems associated with human error and human inconsistency.
• Testing with the disclosed solutions has shown that deburring is most effective when citric acid concentration is low, due to the resistive nature of citric acid in the electrochemical cell, and best when fluoride ion from ABF, is high.
• Similar solutions can also be used to remove surface impurities or to clean a workpiece after machining, such as might otherwise be done using a strong acid pickling with an HF-HN0 3 bath.
• Non-ferrous and especially reactive metals demonstrate an effective rate of chemical etch in a wide range of dilute citric mixtures, as described above. This allows customization of a finishing process for a particular non-ferrous metal workpiece that may include a selected dwell time in the bath before applying electric current to remove and react some of the surface metal before electropolishing begins to selectively reduce peak areas.
• the citric acid based electrolyte has a much lower viscosity than traditional electropolishing mixtures, in part due to the much lower dissociation constant of citric acid as compared with the strong acids normally used in electropolishing electrolytes.
• the lower viscosity aids in material transport and lowers electrical resistance, so that lower voltages can be used than in conventional electropolishing.
• the electropolishing finish ultimately obtained is substantially influenced by the viscosity and resistivity of the electrolyte employed. It has been found that the finest surface finishes (highly
• micropolished can be achieved using a highly resistive electrolyte solution in combination with a high electropolishing voltage (and thus a moderate to high current density).
• a somewhat more conductive (less highly resistive) electrolyte solution is employed, fine micropolishing can still be achieved at high voltages and high current densities.
• electrochemical machining it is expected that electrolyte baths having compositions as described herein can be used effectively in place of conventional electrochemical machining and/or pickling solutions, with substantial environmental and cost benefits. Because the electrolyte solutions disclosed herein are essentially free of strong acid, the problems of hazardous waste disposal and handling are minimized. Moreover, the required current densities are far less than required for conventional electrochemical machining.
• ammonium bifluoride increases the electrical conductivity of the electrolyte solution
• citric acid or increasing the concentration of citric acid relative to the concentration of ammonium bifluoride
• the electrical resistance of the electrolyte solution can be beneficially controlled to achieve desired levels of micropolishing of the surface of a workpiece.
• the proximity of the workpiece (anode) to the cathode need not be precise, in contrast to conventional electropolishing or
• Electropolishing of a metallic workpiece is performed by exposing the workpiece and at least one cathodic electrode to a bath of an electrolyte solution, and connecting the workpiece to an anodic electrode.
• the electrolyte solution includes an amount of carboxylic acid in the range of about 0.1% by weight to about 59% by weight.
• the electrolyte solution may also include about 0.1% by weight to about 25% by weight of a fluoride salt selected from alkali metal fluorides, alkali earth metal fluorides, silicate etching compounds and/or combinations thereof.
• Current is applied from a power source between the at least one anodic electrode connected to the workpiece and the cathodic electrode immersed in the bath to remove metal from the surface of the workpiece.
• Citric acid is a preferred carboxylic acid, although other carboxylic acids may be used, including but not limited to formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, palmitic acid, and stearic acid.
• ABF is a preferred fluoride salt.
• the current is applied at a voltage of about 0.6 VDC to about 150 VDC.
• the current may be applied at a current density of less than or equal to about 255,000 amperes per square meter ((A/m ) (roughly 24,000 amperes per square foot), where the denominator represents the total effective surface area of the work piece.
• the electropolishing processes using the electrolyte solution may be operated between the freezing and boiling points of the solution, for example at a temperature of about 2 °C to about 98 °C, and preferably in the range of about 21 °C to about 85 °C.
• material may removed from the metallic substrate at a rate of about 0.0001 inches (0.00254 mm) to about 0.01 inches (0.254 mm) per minute. The following examples show the effectiveness of the electrolyte at varying concentrations and operating conditions.
• Example 1 Etching Commercially Pure Titanium
• Example 3 Electropolishing 6A1-4V Coupon
• tests were conducted at temperatures of about 21 °C, about 54 °C, about 71 °C, and about 85 °C, and at current densities of about 0 A/m 2 , about 10.8 A/m 2 , about 52.8 A/m 2 , about 215 A/m 2 , about 538 A/m 2 , and about 1076 A/m 2.
• No amount of a strong acid was intentionally added to any of the tested solutions, although trace amounts would likely not impact the results significantly.
• Figs. 1 A- IB show the material removal rate and change in surface finish, respectively, at four different temperatures using an aqueous electrolyte solution including a moderately low concentration of ammonium bifluoride of 20 g/L and concentrations of citric acid from about 0 g/L to about 780 g/L and a current density of 1076 A/m .
• Fig. 1 A shows that material removal rate varies directly with temperature, particularly at lower concentrations of citric acid. As the bath temperature increases, so does the removal rate.
• Fig. IB shows that at lower citric acid concentrations, particularly at or below 120 g/L to 180 g/L, the surface finish is degraded at all but the lowest temperature.
• the fluoride ion that is responsible for significant material removal at lower citric acid concentrations also creates surface damage, but the presence of citric acid in sufficient concentrations appears to act as a beneficial barrier to fluoride ion attack.
• the surface finish actually improves, particularly at citric acid levels of 600 g/L and greater where the rate of material removal is significantly reduced.
• improvements in surface finish can be achieved
• the anodic oxide layer forms, because the applied DC power can no longer attack the material surface, it hydrolyzes the water.
• the resulting nascent oxygen that is formed quickly finds another mono-atomic oxygen and is given off at the anode as 0 2 gas.
• Figs. 2A-2B and 2C-2D show the rate of material removal and the change in surface finish, respectively, using an aqueous electrolyte solution including a
• Figs. 2A and 2C show data at a representative low temperature of 21 °C and Figs. 2B and 2C show data at a representative high temperature of 71 °C.
• Figs. 2A-2B show that material removal is strongly correlated to ammonium bifluoride concentration and temperature, but is minimally impacted by current density. Higher rates of material are generally obtained by increasing one or both of the ammonium bifluoride concentration and the temperature.
• Figs. 2C-2D show that material removal comes along with some surface degradation.
• Figs. 2E-2F show that the rate of material removal and the change in surface finish, respectively, using an aqueous electrolyte solution consisting essentially of ammonium bifluoride in water, with no intentionally added citric acid, as a function of current density when operated at a high temperature of 85 °C. Fiigh rates of material removal can be achieved with an ABF-only electrolyte, but this material removal comes at the expense of surface finish, which is often moderate to significantly degraded by the electrolyte solution. Nevertheless, at certain operating conditions (not shown in the figures), minimal degradation or modest improvement in surface finish was achieved.
• improvements in surface finish from ABF-only electrolyte solutions were achieved with a 10 g/L ABF solution at 21 °C and 215 - 538 A/m 2 and at 54 - 71 °C and 1076 A/m 2 , with a 20 g/L ABF solution at 21 °C and 215 - 1076 A/m 2 , and with a 60 g/L ABF solution at 21 °C and 538 - 1076 A/m 2 .
• Figs. 3A-3D depict, at a representative current density of 53.8 A/m 2 , that the rate of material removal can be varied in direct relationship to temperature, so that for the same mixture of citric acid, ammonium bifluoride, and water, greater material removal occurs at higher temperatures. Similar trends were observed at all current densities from 0 A/m 2 to 1076 A/m 2 .
• Figs. 4A-4D depict, at a representative temperature of 54 °C, that the rate of material removal is relatively constant with current density, so that for the same mixture of citric acid and ammonium bifluoride at any given bath temperature, the rate of material removal is relatively insensitive to changes in current density. Similar trends were observed at all temperatures from 21 °C to 85 °C, and it is believed that those trends hold below 21 °C (but above the freezing point of the solution) and above 81 °C (but below the boiling point of the solution). As occurs at nearly all temperature and current conditions, regardless the ABF concentration, when the citric acid concentration rises above a certain level, typically between 600 g/L and 780 g/L, the rate of material removal is significantly curtailed. Therefore, to maintain the ability to achieve some level of material removal, when shaping a workpiece is desired, the citric acid concentration should generally be maintained at less than 600 g/L.
• Figs. 4E-4G depict, at a representative high temperature of 85 °C and three different concentrations of citric acid, the impact of current density on material removal rates, and Figs. 4H-4J depict the impact of current density on surface finish under the same sets of conditions.
• Fig. 4E shows, as do Figs. 4F and 4G but to a lesser extent, that the material removal capabilities of the electrolyte solution are greatest at the highest concentrations of ammonium bifluoride, and are quite significant at high temperature. It should be noted that although Fig. 4E shows data only at 120 g/L citric acid, essentially the same rates of material removal are seen at citric acid concentrations at 60 g/L, 120 g/L, and 300 g/L.
• Fig. 4H shows that at high temperature and modest citric acid concentration, a moderate amount of surface finish degradation is experienced at nearly all ammonium bifluoride concentrations and current densities. However, when viewing Figs. 4E and 4H together, one process condition stands out. At 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 , material removal is suppressed and a significant improvement in surface finish results.
• the elevated current density may be creating enough excess oxygen at the material surface to fill the "valleys" in the surface morphology such that the "peaks" are preferentially attacked by the fluoride ion generated by dissociation of the ammonium bifluoride.
• This effect combined with the possible micro-barrier effect of citric acid, can be seen even more strongly in Fig. 41 (at 600 g/L citric acid) and Fig. 4 J (at 780 g/L citric acid), which show a reduced degradation in surface finish, and in some cases an improvement in surface finish, at higher citric acid concentrations and higher current densities alone, and even more so at a combination of higher citric acid concentrations and higher current densities.
• Tables 3A-3C and 4A-4C do not include electrolyte consisting essentially of water and ammonium bifluoride, and substantially free of citric acid, because those conditions were discussed separately with reference to Figs. 2A-2D.
• Tables 3 A-3C are separated by levels of surface finish refinement, and are then organized in order of increasing ABF concentration.
• Tables 4A- 4C are separated by levels of citric acid concentration and are then organized in order of increasing ABF concentration.
• aqueous solutions of citric acid and ABF in the substantial absence of a strong acid, can produce fine surface finishes with minimal material loss in concentrations as low as 60 g/L citric acid and 10 g/L ABF, and concentrations as high as 780 g/L citric acid and 120 g/L ABF, and at several combinations in between.
• concentrations of 120 - 780 g/L and generally at lower ABF concentrations of 10 - 20 g/L.
• ABF concentration is lower, in the range of 10 - 20 g/L, higher temperatures of 71 - 85 °C tend to produce better surface finishes at the higher citric acid concentrations of 600 - 780 g/L, while more moderate temperature of 54 °C produced fine surface finishes at moderate citric acid concentrations of 120 - 300 g/L.
• the best surface finish improvements were obtained at low ABF concentrations of 10 - 20 g/L and at moderate to high temperatures of 54 - 85 °C.
• Low and moderate surface finish improvement was achieved at ABF concentrations of 10 - 60 g/L and low temperatures of 21 °C.
• Figs. 5 A and 5B show rates of material removal and changes in surface finish at a representative low temperature of 21 °C and a representative high current density of 538 A/m". It can be seen in Fig. 5B that surface finish degradation is modest at all citric acid concentrations below 600 g/L for ABF concentrations below 60 g/L, and that the surface finish actually improves for all ABF concentrations from 10 -120 g/L at high citric acid concentrations above 600 g/L, and specifically at 780 g/L. In addition, Fig. 5 A shows that the rate of material removal at these process conditions is relatively low. Therefore, operating at this range of composition, temperature, and current density would be desirable to achieve modest controlled material removal with minimal surface degradation or perhaps modest surface finish improvement, but would not be particularly effective for large scale material removal.
• Figs. 6A and 6B show rates of material removal and changes in surface finish at a representative low temperature of 21 °C and a high current density of 1076 A/m". It can be seen in Fig. 6B that the small to modest surface finish improvement is achieved at all citric acid concentrations below 600 g/L for ABF concentrations greater than 10 g/L and less than 120 g/L, and that the surface finish improves most significantly at citric acid concentrations of 600 g/L and above.
• Fig. 6A shows that the rate of material removal at these process conditions is relatively low, except for compositions near 300 g/L citric acid and 120 g/L ABF, where the material removal rate is higher without causing any significant surface degradation. Therefore, operating at these ranges of composition, temperature, and current density would be desirable to achieve modest controlled material removal with minimal surface degradation or perhaps modest surface finish improvement, but would not be particularly effective for large scale material removal.
• Figs. 7A and 7B show that under certain conditions controlled material removal and surface finish improvement can be achieved simultaneously.
• Fig. 7 A shows a consistent modest material removal rates across all citric acid concentrations when a workpiece is exposed to the electrolyte solution at a high temperature of 85 °C and at a high current density of 1076 A/m .
• Fig. 7B shows a substantial improvement in surface finish at all citric acid concentrations equal to or greater than 60 g/L. Even at higher ABF concentrations, from 20 g/L to 120 g/L ABF, material removal can be obtained in direct relation to ABF concentration without a substantial degradation of surface finish.
• FIGs. 8A-8B, 9A-9B, and 10A- 10B illustrate exemplary operating conditions in this category.
• Fig. 8A shows that at a high temperature (85 °C) and low current density (10.8 A/m ) condition, a fairly constant rate of material removal can be achieved at all ABF concentrations for citric acid concentrations in the range of about 60 g/L to about 300 g/L, with greater material removal rates being obtained in direct relation to ABF concentration.
• Fig. 8B shows that for these citric acid and ABF concentration ranges, surface finish degradation is consistently modest almost without regard to the specific citric acid and ABF concentrations. Citric acid concentrations of 600 g/L and higher greatly reduce or even stop the material removal capability of the electrolyte solution and also, except at an ABF concentration of 60 g/L, moderate surface finish degradation and even may tend to slightly improve the surface finish.
• Figs. 9A and 9B show very similar results at a high temperature (85 °C) and high current density (538 A/m 2 ) condition
• Figs. 10A and 10B show that similar results can be approached even at a somewhat lower temperature of 71 °C and at a modest current density of 215 A/m 2 .
• the same aqueous electrolyte solution bath could be used in a multi-step process that includes first removing a modest and controlled amount of material at a relatively low current density and then healing the surface by raising the current density to a high level while maintaining or slightly lowering the temperature.
• a solution having 300 g/L citric acid and 120 g/L ABF modest material removal rates can be obtained at a temperature of 85 °C and a current density of 53.8 A/m (see Fig. 3D) while degrading the surface finish by less than 30%, and then surface improvement can be obtained at the same temperature and a current density of 1076 A/m (see Figs. 7A and 7B) while removing less material.
• concentrations of ammonium bifluoride would be more effective at higher temperatures, such as at 85 °C or greater.
• electrolyte solution having both citric acid and ammonium bifluoride concentrations of 2 g/L, material removal and surface finish changes were observed. At 285 A/m , material removal rates of 0.008 mm/hr were recorded, with a corresponding surface finish change (degradation) of -156%. At 0 A/m , material removal rates of 0.0035 mm/hr were recorded with a corresponding surface finish change of -187%.
• concentrations significantly in excess of 120 g/L including concentrations of ammonium bifluoride at levels as high as 240 g/L to 360 g/L, and even concentrations in excess of saturation in water, can be used.
• concentrations significantly in excess of 120 g/L including concentrations of ammonium bifluoride at levels as high as 240 g/L to 360 g/L, and even concentrations in excess of saturation in water, can be used.
• concentrations significantly in excess of 120 g/L including concentrations of ammonium bifluoride at levels as high as 240 g/L to 360 g/L, and even concentrations in excess of saturation in water, can be used.
• concentrations significantly in excess of 120 g/L including concentrations of ammonium bifluoride at levels as high as 240 g/L to 360 g/L, and even concentrations in excess of saturation in water, can be used.
• concentrations significantly in excess of 120 g/L including concentrations of ammonium bifluoride at levels as high
• micropolishing were achieved at all tested current densities in the range, including at 255,000 A/m . In comparison to processing titanium and titanium alloys, higher current densities, particularly at about 5000 A/m may be useful for processing nickel base alloys.
• CP titanium is effectively processed using relatively low voltages of less than our equal to about 40 volts, higher voltages can also be used.
• CP titanium was processed in a bath of an aqueous electrolyte solution including of about 180 g/L citric acid and about 120 g/L ABF at 85.6 °C applying a potential of 64.7 VDC and a current density of 53,160 A/m . Under these conditions, a 5 mm/hr bulk metal removal rate was achieved along with a 37.8% improvement of surface profilometer roughness, resulting in a surface with a uniform visually bright, reflective appearance.
• certain metals included but not limited to nickel base alloys (such as Waspaloy and nickel alloy 718), 18k gold, pure chrome, and Nitinol alloys, appear to benefit from higher voltage processing, either with more rapid bulk metal removal and/ or better surface finish improvement.
• nickel base alloys such as Waspaloy and nickel alloy 718
• 18k gold, pure chrome, and Nitinol alloys appear to benefit from higher voltage processing, either with more rapid bulk metal removal and/ or better surface finish improvement.
• aqueous electrolyte including about 180 g/L citric acid and about 120 g/L ABF at 86.7 °C using a potential of 150 VDC and 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 improvement of 33.8% based on surface profilometer measurements.
• An aqueous electrolyte solution as disclosed herein can be used to modulate or round out the bottom of surface cracks, thereby eliminating the sharp pointed crack tips that form at varying depths across the surface of metals, and most detrimentally in the family of reactive metals including but not limited to titanium and titanium alloys, nickel base alloys, zirconium and the like, when those metals are cooled from elevated temperatures in an oxygen-containing atmosphere.
• reactive metals including but not limited to titanium and titanium alloys, nickel base alloys, zirconium and the like, when those metals are cooled from elevated temperatures in an oxygen-containing atmosphere.
• such surface cracks can occur upon cooling from processes including but not limited to hot processing (e.g., forging, rolling, superplastic forming, and the like), welding, and heat treating.
• the aqueous electrolyte solution can modulate these cracks with relatively little yield loss.
• a rounded or modulated crack tip is suitable for subsequent hot metal processing because such a crack is able to "heal” on subsequent hot working, whereas a typical metal-cooling crack, if not fully removed, "runs” and the metal breaks apart or fractures on subsequent processing.
• Fig. 11 illustrates the conventional process for removing cracks, which involves mechanically removing, typically by grinding or machining away an entire uniform layer of material, to expose the bottom of the deepest surface crack. As is shown schematically, this results in a substantial loss of material.
• Fig. 12 illustrates a crack modulation process using an electrolyte solution combined with the application of an electric current to widen the crack and round out the tip of the crack at its bottom, so that the crack will not propagate when the workpiece is subjected to further hot processing operations or put into service.
• preferred process conditions for crack tip modulation or removal are those that produce very fast removal rates, while minimizing or eliminating hydrogen pickup.
• these conditions are generally obtained when the concentration of carboxylic acid (e.g., citric acid) is low, the concentration of fluoride ion (e.g., in the form of ammonium bifluoride) is high, and temperature is high.
• carboxylic acid e.g., citric acid
• fluoride ion e.g., in the form of ammonium bifluoride
• temperature is high.
• the crack modulation capabilities of the aqueous electrolyte solution provide significant overall metal yield improvement over current process methods in which the metal surface is uniformly or locally mechanically removed (ground or machined) until the deepest crack bottom is ground flush with the surrounding metal.
• processing and consumables costs are significantly lower using the electrolyte solution and process disclosed herein as compared with the current mechanical removal methods.
• alpha case is an oxygen-enriched phase that occurs when titanium and its alloys are exposed to heated air or oxygen.
• Alpha case is brittle, and tends to create a series of surface microcracks which will reduce the performance, including strength, fatigue properties, and corrosion resistance of a metal part.
• Titanium and titanium alloys are among the reactive metals, meaning that they react with oxygen and form a brittle tenacious oxide layer (Ti0 2 for Ti, Zr0 2 for Zr, etc.) whenever heated in air or an oxidizing atmosphere at or above a temperature at which the natural oxide layer forms, which depend on the specific alloy and oxidizing atmosphere.
• an oxide layer or alpha case oxide layer can be created by any heating of the metal to necessary temperatures for mill forging or mill rolling, as a result of welding, or by heating for finished part forging or hot part forming.
• the alpha case is brittle and full of micro-cracks which penetrate into the bulk metal, potentially causing premature tensile or fatigue failures, and making the surface more susceptible to chemical attack.
• alpha case layer must be removed before any subsequent hot or cold working, or final component service.
• Aqueous electrolyte solutions, and processes using those solutions as described herein can remove alpha case to reveal the non-affected base metal.
• Alpha case removal is a challenging problem in titanium and titanium alloy processing because the alpha case is extremely resistant to attack, and the conventional wisdom is that some mechanical intervention is required prior to electrochemical processing.
• a workpiece with an oxide or alpha case layer is treated in a bath of aqueous electrolyte solution having low carboxylic acid (e.g., citric acid) concentrations, high fluoride ion (e.g., ammonium bifluoride) concentrations, high temperatures, and preferably low power densities.
• carboxylic acid e.g., citric acid
• fluoride ion e.g., ammonium bifluoride
• high temperatures preferably low power densities.
• Low citric acid concentrations, high ammonium bifluoride concentrations, and high temperatures maximize material removal rates, and because removal rates are relatively insensitive to current density when power is applied cyclically, a lower power density is used to cause less hydrogen to impregnate the material surface.
• an aqueous electrolyte solution and process using such a solution enables removal of oxide layers and titanium alpha case from a reactive metal surface in a controlled repeatable fashion.
• This is in contrast to the current FIF- FIN0 3 acid pickle in which a strongly exothermic reaction, which in the case of titanium uses the evolved titanium as a catalyst in the reaction, thereby causing continual changes in acid concentration and reaction rates, making repeatability difficult and accurate surface metal removal nearly impossible.
• the disclosed aqueous electrolyte solution and method does not charge detrimental hydrogen into the bulk metal. Indeed, the disclosed process may be operated in a manor so as to remove hydrogen from the bulk metal. In contrast, the current process, by its very nature, introduces detrimental hydrogen into the bulk metal, necessitating additional costly degassing steps to remove the hydrogen.
• the aqueous electrolyte solution is environmentally friendly and produces no hazardous wastes, whereas the current process employs environmentally challenging and hazardous hydrofluoric acid (HF) and nitric acid (HN0 3 ) acids which are extremely difficult to handle, and which can be used only under stringent permitting programs.
• HF hydrofluoric acid
• HN0 3 nitric acid
• Figs. 4E-4G because citric acid tends to mitigate the attack of fluoride ions on the material surface, it can be seen that the material removal rate tends to spike upward as the citric acid concentration approaches 0 g/L. However, at near zero citric acid conditions, Figs. 4H, 7B, 8B, 9B, and 10B indicate that surface pitting and severe surface finish degradation may result.
• the aqueous electrolyte solution for crack modulation where severe surface degradation is preferably avoided, at least a small amount of citric acid, for example 1 g/L or 10 g/L, should be used to mitigate the most severe effects of the fluoride ion attack.
• the fluoride ion attack can be allowed to be as aggressive as possible without causing serious pitting, and therefore the citric acid concentration can be reduced to near zero.

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