Electropolishing solution and methods for its use and recovery

Abstract

An electrolyte solution containing an alkylene glycol and a chloride salt is useful for electropolishing metal substrates. An electropolishing method using the electrolyte solution is effective for polishing a varied of metals, including titanium and noble metals. The method avoids the handling and disposal challenges associated with previous methods using etching solutions containing hydrofluoric acid. A method of recycling the spent electrolyte solution is also described.

1 . An electropolishing solution, comprising: 
 about 75 to about 99 weight percent of an alkylene glycol; and    about 1 to about 25 weight percent of a chloride salt selected from the group consisting of alkali metal chlorides, alkaline earth metal chlorides, and combinations thereof.    
     
     
         2 . The electropolishing solution of  claim 1 , wherein the alkylene glycol is selected from the group consisting of ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, glycerol, 1,3-butylene glycol, 1,4-butylene glycol, 2,3-butylene glycol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, 2-methyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, dibutylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and combinations thereof.  
     
     
         3 . The electropolishing solution of  claim 1 , wherein the alkylene glycol comprises ethylene glycol or propylene glycol.  
     
     
         4 . The electropolishing solution of  claim 1 , wherein the chloride salt is selected from the group consisting of lithium chloride, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, and combinations thereof.  
     
     
         5 . The electropolishing solution of  claim 1 , wherein the chloride salt comprises potassium chloride.  
     
     
         6 . The electropolishing solution of  claim 1 , comprising less than or equal to 5 weight percent water.  
     
     
         7 . The electropolishing solution of  claim 1 , comprising less than 0.5 weight percent fluoride ion.  
     
     
         8 . An electropolishing solution, comprising: 
 about 75 to about 99 weight percent of ethylene glycol; and    about 1 to about 25 weight percent of potassium chloride.    
     
     
         9 . An electropolishing solution, consisting essentially of: 
 about 75 to about 99 weight percent of ethylene glycol; and    about 1 to about 25 weight percent of potassium chloride.    
     
     
         10 . An electropolishing method, comprising: 
 disposing a metallic substrate and at least one electrode in an electrolyte solution; wherein the electrolyte solution comprises    about 75 to about 99 weight percent of an alkylene glycol; and    about 1 to about 25 weight percent of a chloride salt selected from the group consisting of alkali metal chlorides, alkaline earth metal chlorides, and combinations thereof; and    applying a current from a power source between the at least one electrode and the metallic substrate to remove metal from the metallic substrate.    
     
     
         11 . The electropolishing method of  claim 10 , wherein the current is applied at a voltage of about 3 to about 100 volts.  
     
     
         12 . The electropolishing method of  claim 10 , wherein the current is applied at a current density of about 0.1 to about 20 amperes per square-centimeter.  
     
     
         13 . The electropolishing method of  claim 10 , wherein the electrolyte solution has a temperature of about 50 to about 200° C.  
     
     
         14 . The electropolishing method of  claim 10 , wherein metal is removed from the metallic substrate at a rate of about 1 to 200 micrometers per minute.  
     
     
         15 . A method of recovering an electropolishing solution, comprising: 
 agitating a spent electropolishing solution comprising    an alkylene glycol,    a chloride salt, and    a metal-containing solid; and    separating the agitated, spent electropolishing solution to yield the metal-containing solid and a solid-depleted solution.    
     
     
         16 . The method of  claim 15 , wherein the spent electropolishing solution comprises about 75 to about 98 weight percent of the alkylene glycol.  
     
     
         17 . The method of  claim 15 , wherein the spent electropolishing solution comprises about 2 to about 25 weight percent of the chloride salt.  
     
     
         18 . The method of  claim 15 , wherein the metal-containing solid comprises a metal selected from the group consisting of aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, niobium, molybdenum, silver, halfnium, tungsten, platinum, gold, and combinations thereof.  
     
     
         19 . The method of  claim 15 , wherein the metal-containing solid comprises titanium.  
     
     
         20 . The method of  claim 15 , wherein the metal-containing solid comprises about 10 to about 25 weight percent titanium, about 5 to about 20 weight percent carbon, and about 1 to about 5 weight percent hydrogen, on a dry weight basis.  
     
     
         21 . The method of  claim 15 , wherein the metal-containing solid has an average particle size of less than 20 micrometers prior to agitating.  
     
     
         22 . The method of  claim 15 , wherein agitating the spent electropolishing solution is conducted at a temperature greater than 100° C.  
     
     
         23 . The method of  claim 15 , wherein agitating comprises an input of about 10,000 to about 1,000,000 joules per kilogram of spent electropolishing solution.  
     
     
         24 . The method of  claim 15 , wherein agitating comprises sparging with a pressurized gas having a pressure of about 0.01 kg/cm 2  to about 1000 kg/cm 2 .  
     
     
         25 . The method of  claim 24 , wherein the pressurized gas comprises air.  
     
     
         26 . The method of  claim 24 , further comprising adding water to the spent electropolishing solution in an amount of about 0.001 to about 5 weight percent based upon the total weight of the spent electropolishing solution.  
     
     
         27 . The method of  claim 24 , wherein the pressurized gas comprises a water concentration effective to create a water concentration of about 0.001 to about 5 weight percent water in the spent electropolishing solution during agitation.  
     
     
         28 . The method of  claim 15 , wherein the metal-containing solid has an average particle size greater than 20 microns after agitating.  
     
     
         29 . The method of  claim 15 , further comprising distilling the solid-depleted solution.  
     
     
         30 . A method of recovering an electropolishing solution, comprising: 
 sparging a spent electropolishing solution with pressurized air, wherein the spent electropolishing solution comprises    about 75 to about 98 weight percent of ethylene glycol,    about 2 to about 25 weight percent of potassium chloride, and    at least 1 weight percent of a titanium-containing solid comprising about 10 to about 25 weight percent titanium, about 5 to about 20 weight percent carbon, and about 1 to about 5 weight percent hydrogen, on a dry weight basis; and    separating the sparged, spent electropolishing solution to yield the titanium-containing solid and a solid-depleted solution.