Abstract:
An electrochemical machining system for metals and alloys having a strongly passivating character including an electrolyte solution that is free of hydrofluoric acid, an electrode in contact with the electrolyte solution, a workpiece spaced apart from the electrode and in contact with the electrolyte solution and a power source including a first electrical lead electrically coupled to the electrode and a second electrical lead electrically coupled to the workpiece, the power source being configured to pass an electric current between the electrode and the workpiece, wherein the electric current includes anodic pulses and cathodic pulses, and wherein the cathodic pulses are interposed between at least some of the anodic pulses.

Description:
GOVERNMENT RIGHTS 
       [0001]    This Disclosure was developed under DOE P.O, No. 594128 and SBIR DOE Contract No. DE-SC0004588. 
     
    
     FIELD 
       [0002]    This application relates to electrochemical machining or polishing of metals and metal alloys and, more particularly, to an electrically mediated electropolishing process of metals such as niobium and niobium alloys, titanium and titanium alloys (such as titanium and molybdenum alloys, and titanium and nickel alloys also know as nitinol), zirconium and zirconium alloys, hafnium and hafnium alloys, tantalum and tantalum alloys, molybdenum and molybdenum alloys and tungsten and tungsten alloys, and aluminum and aluminum alloys, and other metals and metal alloys having a strongly passivating character, in a hydrofluoric acid and fluoride salt free aqueous electrolyte. 
       BACKGROUND 
       [0003]    Niobium and certain alloys of niobium become superconductors at relatively high temperatures (e.g., 2 to 9 Kelvin). Therefore, niobium and niobium alloys are attractive materials for use in various superconductor applications, including magnetic resonance imaging (MRI) machines and particle accelerators. Additionally, these metals and similar metals and alloys such as cobalt chromium alloys have promising applications for medical devices such as medical stents and implants. However in all these applications, the metal surface must be polished without detracting from the properties which make the metal desirable for use in these applications. One of the vehicles that is often used in electropolishing these metals is hydrofluoric acid. However, the introduction of fluoride to a metal may make that metal unsuitable for certain medical applications. Accordingly, there is a need for a method for polishing niobium and other strongly passivating metals and cobalt chromium alloys that does not result in the introduction of possibly contaminating substances. 
         [0004]    Electrochemical polishing or electrolytic polishing or electropolishing is a process whereby metal)(M 0 ) is selectivity removed from a surface by an electrochemical reaction, generally of the form 
         [0000]      M 0           M n+   +ne   −   Eq. 1
 
         [0005]    As illustrated in  FIG. 1 , during electropolishing, the current distribution is controlled so that the peaks or asperities of the surface are preferentially removed relative to the recesses or valleys in the subject surface. In the case of primary or geometric current distribution as depicted in  FIG. 2 , the resistive path length from the cathode to the surface asperity (Ωp) is shorter than the distance from the cathode to the recess (Ωr). Consequently, the peaks are preferentially dissolved. The difference in the current distribution between the peak and recess is greater as the electrolyte resistance increases. Highly resistive electrolytes and low electrolyte temperatures are desirous to increase the differential between the current at the peak and the recess. Decreasing temperature increases resistivity. 
         [0006]    In the case of tertiary or mass transport controlled current distribution as depicted in  FIG. 3 , the diffusion distance from the peak to the bulk solution (Dp) is less than the diffusion distance from the recess to the bulk solution (Dr). Since one skilled in the art would understand that the diffusion limited current is based on either dissolved metal ions diffusing away from the peaks or acceptor ions diffusing to the peaks, the diffusion limiting current for metal dissolution at the peaks is higher than the diffusion limiting current at the recesses. Consequently the peaks are preferentially dissolved. The difference in the diffusion limited current distribution between the peaks and recesses is higher for viscous solutions. Viscous solutions have the effect of slowing down the diffusion process. Consequently, highly viscous electrolytes (e.g., about 15 to 30 cP) and low temperatures (e.g., 10° C. to 30° C.) leading to higher viscosity are desirable to increase the differential between the current at the peak and the recess. An example of a highly resistive electrolyte used to electropolish titanium, INCONEL® (Specialty Metals Corp.), and stainless steel based on alkylene glycol is disclosed in U.S. Pat. No. 6,835,300. 
         [0007]    Consequently, electropolishing solutions used in the systems discussed above are generally highly resistive (e.g., 10 mS/cm to 200 mS/cm) and high viscosity (e.g., about 15 cP to 30 cP) solutions, in some cases operating at low temperatures as disclosed by D. Ward “Electropolishing” in  Electroplating Handbook  ed. L. Durney 4 th  edition pg. 108, Van Nostrand Reinhold, New York (1984). One skilled in the art would also understand that additional additives and complexing agents are often added to electropolishing solutions knowin the prior art. However, such electropolishing solutions are difficult to handle, difficult to control, and often present environmental concerns. 
         [0008]    U.S. Pat. No. 6,402,931 to Zhou discloses a pulse/pulse reverse process for electrochemical machining of metals and alloys that form passive surface layers. The disclosed process is useful for electrochemical shaping or electrochemical polishing or electrochemical deburring of metal and alloy surfaces. The disclosed process uses aqueous salt solutions of sodium nitrate and/or sodium chloride and the like. The entire disclosure is incorporated by reference. 
         [0009]    U.S. Pat. No. 6,558,231 to Taylor further discloses a two-step electrochemical process comprising a sequence of pulse/pulse reverse electric waveforms for electromachining and electropolishing. The two step process is tuned to account for the beginning stages of the electromachining process where the surface roughness could be large and consequently the diffusion boundary follows or conforms to the surface profile and the later stages of the electromachining process where the diffusion boundary layer is larger than the surface profile due to removal of surface asperities. The entire disclosure is incorporated herein by reference. 
         [0010]    Niobium and niobium alloys, titanium and titanium alloys (such as titanium and molybdenum alloys, and titanium and nickel alloys also know as nitinol), molybdenum and molybdenum alloys, tungsten and tungsten alloys, and aluminum and aluminum alloys tend to form passivation layers that are strongly bonded to the underlying material. For example, niobium metal forms an oxide passivation layer in the presence of a source of oxygen such as water, wherein the oxide passivation layer is several atoms thick and is strongly bonded to the underlying niobium metal. These strongly-bonded passivation layers are difficult to remove and, therefore, render these metal and alloys significantly resistant to electrochemical treatment processes. 
         [0011]    Despite the obstacles presented by strongly-bonded passivation layers, various techniques have been developed for electrochemically processing such metals as niobium and niobium alloys. In addition to highly resistive and high viscosity electrolytes, these techniques typically require high voltages and/or hydrofluoric acid in the electrolyte solution. For example, U.S. Pat. No. 3,689,388 discloses a method for electrolytically polishing niobium parts in an electrolyte composed of H 2 SO 4  and HF in water. The concentrated sulfuric acid is resistive and viscous and thereby facilitates preferential electrochemical dissolution at the peaks relative to the recesses. The hydrofluoric acid in such electrolyte solutions has long been considered an essential component due to its ability to dissolve the niobium oxide passivation layer. However, as indicated earlier, the presence of fluoride in these metals has its own disadvantages particularly from the standpoint of integrating tissue growth for medical applications as well as difficulty in handling, control, worker exposure and disposal. U.S. patent application Ser. No. 11/379,792 discloses an electrolyte for electropolishing niobium and niobium alloys, titanium and titanium alloys (such as titanium and molybdenum alloys, and titanium and nickel alloys also know as nitinol), zirconium and zirconium alloys, hafnium and hafnium alloys, tantalum and tantalum alloys, molybdenum and molybdenum alloys and tungsten and tungsten alloys based on organic dissolvent and fluoride containing salts. Other approaches to electropolishing titanium include electropolishing solutions based on sulfuric acid and alcohol with low water concentration or no water. (D. Landolt, P. Chauvy, and O. Zinger  Electrochimica Acta  3185-3201 48 (2003)). The electrochemical conditions which drive the reaction shown in Eq. 1 above also drive the following reaction which results in the formation of passivating oxides. 
         [0000]      M 0   +x H 2 O         MO x +2 x H + +2 xe   −   Eq. 2
 
         [0000]    By electropolishing in non-aqueous or minimally aqueous electrolytes, the source of the oxygen that forms these passivating oxides is eliminated. However, maintaining low water content presents a new set of control challenges. Using reverse current pulse conditions in accordance with this disclosure provides the means to manage the formation of this layer of passivating oxides, even in the presence of substantial water, so that the oxides do not interfere with electropolishing. 
         [0012]    The use of highly viscous and high resistance electrolytes as well as nonaqueous or low water content electrolytes introduces considerable handling, control, and disposal issues. The use of hydrofluoric acid presents various health and environmental challenges due to its highly corrosive nature, thereby increasing processing costs accordingly. Furthermore, residual hydrofluoric acid on electrolytically processed workpieces, even in trace amounts, can render such workpieces unsuitable for their intended purpose, particularly when such workpieces are used in particle accelerators and surgical implants (e.g., stents). 
         [0013]    Accordingly, those skilled in the art continue to seek alternatives to using highly resistive and high viscosity electrolytes and/or hydrofluoric acid-containing electrolyte solutions and non-aqueous or minimally aqueous electrolyte solutions in connection with electrochemical processing of metals which tend to form a strongly passive layer, such as niobium and niobium alloys, titanium and titanium alloys (such as titanium and molybdenum alloys, and titanium and nickel alloys also know as nitinol), zirconium and zirconium alloys, hafnium and hafnium alloys, tantalum and tantalum alloys, molybdenum and molybdenum alloys and tungsten and tungsten alloy. 
       SUMMARY 
       [0014]    In one aspect, the disclosed electrochemical system may include an electrolyte solution, an electrode in contact with the electrolyte solution, a workpiece spaced apart from the electrode and in contact with the electrolyte solution, and a power source including a first electrical lead electrically coupled to the electrode and a second electrical lead electrically coupled to the workpiece, the power source being configured to pass an electric current between the electrode and the workpiece, wherein the electric current is comprised of anodic pulses and cathodic pulses, and wherein the cathodic pulses are interposed between at least some of the anodic pulses. The electrolyte is characterized in that it is hydrofluoric acid and/or fluoride salt free. In one embodiment it is an aqueous solution of sulfuric acid at concentrations of 70% by weight and less. In certain embodiments, the amount of water in the electrolyte may be greater than 5% by weight, or greater than 10%, or greater than 20%, or greater than 70%. 
         [0015]    According to this disclosure, an electrically mediated approach is used to eliminate the need for hydrofluoric acid and/or fluoride salts and to reduce the effect of hydrogen in finishing these highly passive metal surfaces. The electrically mediated process is environmentally benign and retains the advantages of electrochemical processes in terms of speed and investment. Depending on the flow rate of the electrolyte across the work piece, a waveform is selected to polish the surface. As the surface roughness is reduced and macroroughness is reduced to microroughness, the waveform may be changed as required. These distinct waveforms can be preprogrammed into the rectifier. 
         [0016]    In another aspect, the disclosed electrochemical machining method may include the steps of positioning an hydrofluoric acid free electrolyte solution that may optionally be an aqueous electrolyte solution between a workpiece and an electrode, and passing an electric current between the workpiece and the electrode, wherein the electric current is comprised of anodic pulses and cathodic pulses, and wherein the cathodic pulses are interposed between at least some of the anodic pulses. 
         [0017]    In another aspect the electrolyte solution contains a surfactant such as Triton-X to facilitate the release of oxygen bubbles generated in the electrolytic process. 
         [0018]    Other aspects of the disclosed electrochemical machining system and method will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic representation of electropolishing. 
           [0020]      FIG. 2  is a schematic representation of electropolishing with resistive electrolytes. 
           [0021]      FIG. 3  is a schematic representation of electropolishing with viscous electrolytes. 
           [0022]      FIG. 4  is a schematic illustration of one particular implementation of the disclosed electrochemical machining system. 
           [0023]      FIGS. 5A and 5B  are graphical illustrations of anodic pulse-cathodic pulse waveforms used in connection with the electrochemical machining system of  FIG. 4 . 
           [0024]      FIG. 6  is an example of polarization curves for metal in different electrolytes. 
           [0025]      FIG. 7  is a two electrode polarization curve for Niobium in different electrolytes. 
           [0026]      FIG. 8  is a graph showing the effect of Vf on the average surface roughness of Nb after electropolishing in 31% by weight H 2 SO 4  electrolytes at ambient temperature (˜20° C.) using (a) Vf=20 V; (b) Vf=30 V; and (c) Vf=40 V. 
           [0027]      FIG. 9  is a graph showing the effect of time on the average surface roughness of Nb after electropolishing in 31% by weight H 2 SO 4  electrolytes at room temperature (˜20° C.) using Vf=30 V, Vr=8 V, frequency=1000 Hz, Df/Dr=10%/90% and different Rao. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    As used herein, “electrochemical machining” broadly refers to any electrochemical process that involves the removal of material from a workpiece, regardless of the extent of removal. For example, electrochemical machining includes electrochemical polishing, electrochemical etching, electrochemical through-mask etching, electrochemical shaping and electrochemical deburring, among other processes. 
         [0029]    As used herein the term “hydrofluoric acid free” includes electrolytes that may be formulated to be substantially free of fluoride acids and salts as well as hydrofluoric acid. 
         [0030]    Disclosed herein is an electrolyte solution that may be used with an anodic pulse-cathodic pulse waveform to electrochemically machine niobium and alloys thereof, as well as other metals and metal alloys, including other metals and metal alloys that tend to have strongly bonded passivation layers. For example, the disclosed electrolyte solution may be used with an anodic pulse-cathodic pulse waveform to electrochemically machine niobium and niobium alloys, titanium and titanium alloys (such as titanium and molybdenum alloys, and titanium and nickel alloys also know as nitinol), zirconium and zirconium alloys, hafnium and hafnium alloys, tantalum and tantalum alloys, molybdenum and molybdenum alloys, and tungsten and tungsten alloy. In another embodiment the process may be used to polish cobalt chrome alloys of a type used in medical applications such as vascular and other stents. 
         [0031]    The disclosed hydrofluoric acid free electrolyte solution in one embodiment may be an electrolyte having a low viscosity such as a viscosity of about 1 cP to 15cP or about 1 cP to 8 cP, or about 1 cP to 4 cP. In a particular embodiment it may be an aqueous solution comprised of low concentrations of sulfuric acid, for example, concentrations of about 1% by weight to 70% by weight, more particularly about 15% by weight to 40% by weight, still more particularly about 20% by weight to 40% by weight. In other embodiments of the invention, other acidic, hydrofluoric acid and fluoride acid and salt free electrolytes may be used such as combinations of sulfuric/chromic/phosphoric acids, phosphoric/chromic acids, phosphoric/sulfuric acids, phosphoric acid, phosphoric/sulfuric/chromic acids, phosphoric/sulfuric/hydrochloric acids, sulfuric/glycolic acids, phosphoric/sulfuric acids, sulfuric/chromic acids, sulfuric/citric, and others. Generally, it will be desirable to select electrolytes having a high conductivity such as greater than 200 mS/cm, or greater than 400 mS/cm, or greater than 600 mS/cm or greater than 800 mS/cm. 
         [0032]    In one implementation of the invention, aqueous electrolytes containing substantial water as disclosed above may be use. However, another implementation may employ non-aqueous or minimally aqueous electrolytes containing less than 15%, less than 10% or less than 5% water. When water-containing electrolytes are used, oxygen is generated according to the equation: 
         [0000]      H 2 O         2H + +O 2 +2 e   −   Eq. 3
 
         [0000]    It has been found that the addition of a surfactant facilitates electropolishing. One possible reason for this that the surfactant promotes the formation of small bubbles that do not interfere with the diffusion process by stirring the electrolyte. Conventional surfactants may be used for this purpose such as Triton X (polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether), a product of Rohm and Haas, in conventional amounts. 
         [0033]    Referring to  FIG. 4 , one particular implementation of the disclosed electrochemical machining system, generally designated  200 , may include a working chamber  202  defined by a tank  204  and a cover  208 , an electrolyte holding tank  222 , a conduit  218 , a pump  220 , an electrode  304 , a workpiece  302 , a power source  228  and the disclosed electrolyte solution. The working chamber  202  may be in fluid communication with the electrolyte holding tank  222  by way of a gravity drain  206 . A filter  224  may be associated with the drain  206  to filter the electrolyte solution flowing from the working chamber  202  to the electrolyte holding tank  222 . The working chamber  202  may also be in fluid communication with the electrolyte holding tank  222  by way of the conduit  218 , wherein the pump  220  may pump the electrolyte solution from the electrolyte holding tank  222  to the working chamber  202 , as shown by arrow  234 . 
         [0034]    Within the working chamber  202 , a workpiece holder  210  may be mounted near the bottom of the tank  204  with adequate spacing from the walls and bottom of the tank  204  to allow for drainage of the electrolyte solution into the electrolyte holding tank  222 . The workpiece  302  may be supported on the workpiece holder  210  and may be connected to a first electrical lead  302  of the power source  228 . 
         [0035]    The workpiece  302  may be any apparatus or thing capable of being electrochemically machined. In one embodiment the work piece can be a medical stent. In another embodiment it may be a dental implant. In another embodiment it may be RF superconducting cavities that are components of linear particle accelerators. In a first expression, the workpiece  26  may be any apparatus or thing that is formed from or includes a metal or a metal alloy. In a second expression, the workpiece may be any apparatus or thing that is formed from or includes a metal or a metal alloy that forms a strongly-bonded passivation layer. Examples of metals that form strongly-bonded passivation layers include niobium, titanium and tantalum. In a third expression, the workpiece may be any device that is formed from or includes niobium or a niobium alloy. For example, the workpiece may be a niobium structure or a portion of a niobium structure that defines RF superconducting cavities in a particle accelerator. In a fourth expression, the workpiece may be any apparatus or thing that is formed from or includes titanium or a titanium alloy. For example, the workpiece may be a nitinol tube coated with a resist material, wherein the resist material is patterned to define a surgical stent after electrochemical machining. In another expression the workpiece may be any device that is formed from or includes cobalt chromium alloy. 
         [0036]    An electrode holder  214  may be positioned in the working chamber  202  above the workpiece holder  210 . The electrode holder  214  may be supplied with the electrolyte solution by way of the conduit  218 . The electrode  304  may be connected to a second electrical lead  232  (opposite polarity than the first electrical lead  302 ) of the power source  228  and may be supported by the electrode holder  214  such that the tool holder  214  may move the electrode  304  in a vertical axis under control of an electrode feed controller  226 . 
         [0037]    In one particular aspect, the electrode  304  may include a central bore and the electrode  304  may be connected to the electrode holder  214  such that the central bore of the electrode  304  is directed at the workpiece  302 . During electrochemical processing, the electrolyte solution may be pumped by pump  220  from the electrolyte holding tank  222  to the electrode holder  214  and, ultimately, to the electrode  304  by way of the conduit  218 . The rate of the electrolyte solution flow is herein referred to as E v . The electrolyte solution may flow through the central bore of the electrode  304  and may exit between the electrode  304  and the workpiece  302  before returning to the electrolyte holding tank  222  by way of the drain  206 . The power source  228  may supply electric current to the workpiece  302  and the electrode  304  by way of the first and second electrical leads  230 ,  232  in accordance with the disclosed anodic pulse-cathodic pulse waveform. 
         [0038]    The spacing between the electrode and workpiece  302  during processing may be considered an optimizeable parameter and may depend on the composition of the electrolyte solution and the type of electrochemical machining process being performed, among other factors. For example, the spacing between the electrode and workpiece may range from about 0.5 to 20 millimeters or more particularly 0.5 to 10 millimeters for an electrochemical shaping process, about 5 to about 12 millimeter for an electrochemical polishing process and about 5 to about 50 millimeters for an electrochemical deburring process. 
         [0039]    As shown in  FIG. 5A , an anodic pulse-cathodic pulse waveform, generally designated  50 , may include a plurality of anodic pulses  52  and a plurality of cathodic pulses  54 . One particular implementation is shown in  FIG. 5B . 
         [0040]    The period T of the waveform is the sum (T=t 1 +t 2 +t o +t i ) of the anodic on-time t 1 , cathodic on-time t 2 , relaxation period t o  and intermediate period t i . The inverse (1/T) of the period T of the waveform is the frequency f of the waveform. The ratio (t 1 /T) of the anodic on-time t 1  to the period T is the anodic duty cycle D 1  and the ratio (t 2 /T) of the cathodic on-time t 2  to the period T is the cathodic duty cycle D 2 . The current density (i.e., current per unit area of the electrode) during the anodic on-time t 1  and cathodic on-time t 2  may be referred to as the anodic peak pulse current density and cathodic peak pulse current density, respectively. The anodic charge transfer density Q 1  is the product (I 1 t 1 ) of the anodic current density I 1  and the anodic on-time t 1 , while the cathodic charge transfer density Q 2  is the product (I 2 t 2 ) of the cathodic current density I 2  and the cathodic on-time t 2 . 
         [0041]    In a first expression of the anodic pulse-cathodic pulse waveform, the anodic peak current I 1  may range from about 2 to 6 A/cm 2 , the cathodic peak current I 2  may range from about 8 to 15 A/cm 2 . 
         [0042]    In one embodiment, the voltage and on-time of the anodic pulse is adjusted to remove metal from the micropeaks in the surface via Eq. 1 above without accumulating a passivating layer thickness via Eq. 2 above that cannot be effectively removed during the cathodic duty cycle. Accordingly, the anodic voltage and on-time are adjusted to oxidize the metal on the micropeaks while generating only that amount of passivating oxides that can be removed by the cathodic pulse. If the passivating oxide layer cannot be removed, it prevents or terminates polishing. The appropriate conditions will vary depending on the nature of the metal. In one embodiment a relatively short anodic pulse t 1 , typically from about 0.01 ms to about 100 ms, preferably from about 0.05 ms to about 10 ms, and an anodic duty cycle from about 1 to 60% or from about 5% to about 60%, preferably from about 1 to 40% or from about 10% to about 40%. The cathodic pulse t 2  may have a pulse width from about 0.01 ms to about 900 ms, preferably from about 0.1 ms or from about 0.5 ms to about 90 ms, and a duty cycle from about 40% to about 99% or about 95%, preferably from about 60% to about 99% or about 90%. The relaxation period t o  may range from about 0 to about 600 s and the intermediate off period t i  may range from about 0 to about 1000 ms. The frequency f of the waveform  50  may range from about 1 Hertz to about 5000 Hertz, preferably from about 10 Hz to about 2000 Hz and more preferably from about 100 Hz to about 2000 Hz or about 100 Hz to 1000 Hz. 
         [0043]    At this point, those skilled in the art will appreciate that the parameters of the pulse waveform  50  can be selected to provide uniform metal removal from the workpiece and thereby provide more accurate conformity of the workpiece. Additionally, the field can be implemented to reduce or anodically consume some of the hydrogen generated at the workpiece surface and reduce or eliminate the effects of a nonuniform oxide film. The anodic peak current I 1 , the anodic on-time t 1 , the cathodic peak current I 2 , the anodic on-time t 2 , the relaxation period t o  and the intermediate off period t i , may be varied depending on the composition of the workpiece  26 , the composition of the electrode, the composition of the electrolyte solution and the type of electrochemical machining process being performed, among other factors to achieve these objectives. Furthermore, those skilled in the art will appreciate that the voltage and current may be proportional under the circumstances of the disclosed system and method and, therefore, the ordinate in  FIG. 2  could represent either current or voltage, although it is generally more convenient in practice to control the voltage. Furthermore, the waveform  50  need not be rectangular as illustrated. The anodic pulses and the cathodic pulses may have any voltage-time (or current-time) profile. Rectangular pulses are assumed merely for simplicity. Again, one skilled in the art will recognize that the point in time chosen as the initial point of the pulse train is entirely arbitrary. Either the anodic pulse or the cathodic pulse (or any point in the pulse train) could be considered as the initial point. The representation with the anodic initial pulse is introduced for simplicity in discussion. In accordance with one embodiment, the cathodic voltage is about 4 to 40 volts or about 4 to 15 volts or about 8 to 35 volts or about 6 to 12 volts, and in one embodiment about 35 volts. This is in contrast to processes in which the workpiece is not made from a strongly passivating metal in which case a cathodic voltage of 4 volts or less may be satisfactory. The cathodic voltage is used to depassivate the surface, and for strongly passivating materials the cathodic voltage needs to be greater than 4 volts, one skilled in the art can determine the anodic voltage required for the desired electrochemical dissolution, i.e. etching and/or polishing. 
         [0044]    The disclosed processes are particularly useful in polishing medical stents. Such stents are often laser cut from a tube of a strongly passivating metal, such as nickel-titanium shape memory alloys. This process imparts thermal damage to the surface often leaving burrs and other defects that must be removed before the stent can be implanted. Prior art electropolishing often uses sulfuric acid-phosphoric acid polishing solutions which, due to process instability results in high reject rates of the electropolished stents. 
         [0045]    Without being limited to any particular theory, it is believed that the introduction of cathodic pulses between the anodic pulses has the effect of cathodically consuming the nascent oxygen or cathodically reducing the oxide film, thereby reducing or eliminating the adverse effects due to the formation of a non-uniform oxide film. Consequently, when the next anodic pulse is applied, any passive layer that may have formed will be more easily broken down, and therefore less capable of forming local islands of passivity that tend to resist erosion of the underlying metal. 
       EXAMPLES 
       [0046]    Niobium foil, 99.9% pure, was purchased from GoodFellow (GoodFellow, Oakdale, Pa.) ( FIG. 131 ) and cut into two different coupon sizes to use for the electropolishing studies. Final coupon sizes had the following dimensions: 1) 25.4 mm×25.4 mm×3 mm, and 2) 30 mm×10 mm×3 mm. 
         [0047]    As a simple, efficient, and cost-effective screening method, the polarization curve can be used to select candidate electrolytes. In  FIG. 6 , curve  1  shows the behavior of an active metal, and curve  2  shows the behavior of a passivated metal. Before the electric field is applied, the metal anode immersed in the electrolyte has a steady-state voltage (E ss ). When the power is applied, the electrode voltage will shift in the positive direction from E ss  to E ab  (the breakdown voltage). Above E ab , the current density rises abruptly due to the dissolution reactions occurring on the anode (region AB). The dissolution rate of the anode metal stops increasing when a limiting current density I lim  is reached (BC region), where the metal atoms form metal ions and compounds with the activating anions and pass into the electrolyte. The limiting current density I lim  and the ratio of ΔI to ΔE (the slope of AB on the polarization curve) can be defined as the metal dissolution rate and current efficiency in the electrolyte, respectively. In region BC, the current density remains constant (curve  1 ) or drops to a lower value (curve  2 ) indicating mass transport phenomena that limit the rate of metal ion removal. The products of metal dissolution reach their solubility limit and form a loose deposit or passive film on the electrode surface. If the metal dissolution is conducted in a passive electrolyte, the passive film can grow faster than metal ions pass into the electrolyte, with the result that the current density falls to lower values (curve  2 ). Generally, the limiting current decreases with increasing electrolyte concentration, due to the decrease in the solubility of the reaction products. Since the limiting current is strongly related to diffusion, it can be increased in the pulse/pulse reverse process by increasing the electrolyte flow rate. When the anode voltage increases to region CD of the polarization curve, the higher voltage can breakdown or remove the passive film and deposits, and increase the ionization rate of the metal to increase the current density. 
         [0048]    The metal brightness and smoothness in different electrolytes can be directly observed from polarization tests, providing information as to the effect of electrolytes on the etching process. In region AB of the polarization curve metal is eroded. The metal surface roughness is high due to the different dissolution rates of the various microscopic areas on the surface. At high anode voltages (region BC), the metal surface becomes smoothed or even polished, as in the case of curve  1 . If the anode voltage reaches the CD region, the metal dissolution at higher voltages will lead to a polished surface with macrodefects (such as fine lines, striations and pits). The optimal polarization curve should (1) indicate a low breakdown voltage (E ab ), (2) have a high ratio of ΔI/ΔE, and (3) provide a smooth and shiny surface. 
         [0049]    DC polarization studies were carried out in order to select an electrolyte that would enable the pulse/pulse reverse process for electropolishing Nb coupons. The 2-electrode DC polarization studies were performed on 25.4 mm×25.4 mm×3 mm Nb coupons to study the electrochemical activity (e.g. total current density) of Nb in different electrolyte type and concentrations. A platinum coated Nb mesh was used as the cathode. All polarization curve experiments were performed at room temperature (−20° C.). A TecNu power supply was used for this study (Model SPR-300/100/48-3). The cell voltage was raised by increments of five volts per minute. Total current densities were read from the oscilloscope trace recorded on a FLUKE 196C Scopemeter color system. 
         [0050]      FIG. 7  summarizes the electrochemical activity of Nb substrates in different electrolytes, 200 and 300 g/L sodium chloride (NaCl), 31% by weight sulfuric acid (H 2 SO 4 ), 200 g/L sodium bromide (NaBr), 50 g/L sodium fluoride (NaF) and 21% by weight phosphoric acid (H 3 PO 4 ). In all cases breakdown of the Nb was not observed; any current measured is assumed to be associated with water oxidation (2H 2 O→O 2 +2H + +2e − ) and Nb anodization. The highest and lowest total current density observed for voltages up to 70 V was in the 31% by weight H 2 SO 4  and 50 g/L NaF electrolytes, respectively. 
         [0051]    This data demonstrated the tenacity of the Nb oxide film; DC polarization studies were unable to shed any light on the conditions that would be required to break down the oxide film without the use of hydrofluoric acid. Electrochemical cells with variable flow as shown in  FIG. 4  were used to test the efficacy of pulse/pulse reverse waveforms in electropolishing Nb. As described in the prior art, variable flow channel cells are used to successfully perform metal removal of passive metal and alloys. 1, 2  An advantage of electrolyte flow is the removal of undesired byproducts from the surface of the substrate being electropolished, such as Nb ions, heat and bubbles (resulting mainly due to oxygen and hydrogen generation from water electrolysis).  1  A. Lozano-Morales, A. Bonifas, M. Inman, P. Miller and E. J. Taylor,  J. Appl. Surf Finish.,  2 (3), 192-197 (2007). 2  J. J. Sun, E. J. Taylor, R. Srinivasan,  J. Materials Processing Technology,  108 356-368 (2001). 
         [0052]    As reported in the prior art, a 300 g/L NaCl electrolyte has been successfully used for pulse/pulse reverse electropolishing different passive materials such as nickel based alloys and stainless steel, and therefore it was used to initially study the electrochemical activity of Nb. An initial design of experiments set was developed using statistical software called MINITAB®. Frequency, duty cycle and reverse (cathodic) voltage were varied with three levels for each variable. Three different frequencies were varied at 10, 100 and 1000 Hz at three different forward (anodic) duty cycles (D f =10, 50 and 90%), and three different reverse voltages (V r =2, 4 and 8 V). A Nb coupon of the same dimension as the anode was used as the cathode. The electrolyte velocity was kept constant at 12 m/s and a forward (anodic) voltage (V f ) of 48 V was used in all the experiments at ambient temperature (˜20° C.). The temperature of the electrolyte was not controlled. The total run time in each case was 10 minutes. 
         [0053]    While uniform etching was not achieved in 300 g/l NaCl, there was evidence of Nb breakdown at the coupon edges, where the electrolyte flow entered and exited the cell. Oxides were also formed on the surface, indicating electrochemical activity. These oxides were not tenacious, able to be removed using a scotch-brite pad, soap and water. However, high electrolyte flows and very close electrode gaps are not likely to be realistically accommodated. 
         [0054]    In initial experiments in 31% by weight H 2 SO 4 , the constant parameters were electrolyte velocity (E v )=0.4 m/s, V f =20 V, V r =8 V, run time=10 minutes, anode to cathode distance=5 mm, and ambient temperature (˜20° C.). Since the low flow channel cell did not have temperature control built in, the electrolyte temperature rose from an initial value of 20° C. to around 27° C. by the end of every experiment. In conventional electropolishing, temperatures above 40° C. must be avoided in order to prevent etching pits on the Nb substrate. 3  A design of experiments set was performed using MINITAB®. Two different frequencies were used (10 and 1000 Hz) at two different duty cycles (D f =10 and 90%), (see Table 1), for a total number of 3 experiments.  3  L. Lilje, E. Kako, D. Kostin, A. Matheisen, W.-D Moller, D. Proch, D. Reschke, K. Saito, P. Schmuser, S. Simrock, T. Suzuki, and K. Twarowski, Nuclear Instruments and Methods in Physics Research A 524 1-12 (2004). 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Design of experiments using reverse pulse waveforms to electropolish 
               
               
                 Nb in a 31% by weight H 2 SO 4  electrolyte for 10 minutes using a 
               
               
                 V f  = 20 V and anode-cathode distance of 5 mm. 
               
             
          
           
               
                   
                   
                 Forward (Anodic) 
                   
               
               
                 Run Order 
                 Frequency (Hz) 
                 Duty Cycle (%) 
                 Reverse Voltage (V r ) 
               
               
                   
               
             
          
           
               
                 1 
                 10 
                 90 
                 8 
               
               
                 2 
                 1000 
                 10 
                 8 
               
               
                 3 
                 1000 
                 90 
                 8 
               
               
                   
               
             
          
         
       
     
         [0055]    Run 1 showed no evidence of etching at all. The different colors observed represent Nb oxide layers formed on the substrate. Run 2 showed some degree of etching, which suggested that Nb substrates could be uniformly electrochemically etched in an electrolyte free of hydrofluoric acid. Run 3 also showed some degree of etching, but much lower compared to Run 2. 
         [0056]    Based on these preliminary results, the pulse/pulse reverse waveform used for Run 2 was further explored. Specifically, the same waveform parameters as Run 2 were used, but the coupon was electropolished for 37 minutes instead of 10 minutes. 100 μm of Nb was successfully removed uniformly from the coupon at an average removal rate of 2.7 μm/min in an area approximately 161 mm 2 . 
         [0057]    The effect of raising V f  from 20 to 30 to 40 V on Nb electropolishing performance was studied. In all cases, the other pulse/pulse reverse process parameters were kept constant: V, =8 V, frequency=1000 Hz, D f /D r =10%/90%.  FIG. 8  summarizes the effect of anodic peak voltage on Nb surface finish. The roughest surface finish was obtained at 20 V. When V f  was increased to 30 V, the surface finish of Nb dropped from 1.38 μm to 0.29 μm. At 40 V the Nb surface finish started getting rougher again and surface discoloration was observed. 
         [0058]    The effect of Nb initial surface roughness, Ra o  on final surface roughness, Ra f  was also studied by performing electropolishing time studies at different Ra o .  FIG. 9  compares the effect of time on the average surface roughness of a Nb coupon after electropolishing in a 31% by weight H 2 SO 4  electrolyte for Ra o =0.56 μm (Run #4) and Ra o =1.53 μm (Run #5). For the higher initial surface roughness, there was a significant decrease in Ra o  after 10 minutes, from 1.53 μm to ˜0.85 μm. Thereafter, Ra decreased further by increasing electropolishing time up to 60 minutes down to an Ra f  of 0.33 μm. Further decreases in surface finish did not occur after 50 minutes. 
         [0059]    Accordingly, electrochemical machining systems and methods employing the disclosed electrolyte solution with the disclosed anodic pulse-cathodic pulse waveform may be used to process niobium and niobium alloys, as well as other metals and metal alloys, without the need for fluoride acids or salts, such hydrofluoric acid. 
         [0060]    Although various aspects of the disclosed electrochemical system and method for machining niobium and other metals have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.