- Email: [email protected]
Electropolishing
SURFACE TREATMENTS ELECTROPOLISHING by Kenneth B. Hensel Electro Polish Systems Inc., Milwaukee The electropolishing system smoothens, polishes, deburrs, and cleans steel, stainless steel, copper alloys, and aluminum alloys in an electrolytic bath. The process selectively removes high points on metal surfaces, giving the surface a high luster.
HOW IT WORKS The metal part is immersed in a liquid media (electrolyte) and subjected to direct current. The metal part is made anodic (+) and a metal cathode ( - ) , usually 316L stainless steel or copper, is used. The direct current then flows from the anode, which becomes polarized, allowing metal ions to diffuse through the film to the cathode, removing metal at a controlled rate. The amount of metal removed depends on the specific bath, temperature, current density, and the particular alloy being electropolished. Generally, on stainless steel, 0.0005 in. is removed in 1,500 amp-minutes per square foot. Current and time are two variables that can be controlled to reach the same surface finish. For example, 100 A/ft2 electropolished for 5 min is 500 amp-minutes; 200 A/ft 2 for 21/2 min is 500 amp-minutes. Both pieces of metal would have about the same surface profile. Current densities of 90 to 800 A/ft2 are used in this process depending upon the part to be polished and other parameters. Electropolishing times vary from about 1-15 minutes.
ADVANTAGES Conventional mechanical finishing systems tend to smear, bend, stress, and even fracture the crystalline metal surface to achieve smoothness or luster. Electropolishing offers the advantages of removing metal from the surface producing a unidirectional pattern that is both stress- and occlusion-free, microscopically smooth, and often highly reflective. Additionally, improved corrosion resistance and passivity are achieved on many ferrous and nonferrous alloys. The process micro- and macro-polishes the metal part. Micro-polishing accounts for the brightness and macro-polishing accounts for the smoothness of the metal part. Deburring is accomplished quickly because of the higher current density on the burr, and because oxygen shields the valleys, enabling the constant exposure of the tip of the burr. Because the metal part is bathed in oxygen, there is no hydrogen embrittlement to the part. In fact, electropolishing is like a stress-relieve anneal. It will remove hydrogen from the surface. This is important to parts placed under torque.
440
Another benefit is that bacteria cannot successfully multiply on a surface devoid of hydrogen, therefore, electropolishing is ideal for medical, pharmaceutical, semiconductor, and food-processing equipment and parts. The combination of no directional lines due to mechanical .finishing, plus a surface relatively devoid of hydrogen, results in a hygienically clean surface where no bacteria or dirt can multiply or accumulate.
SUMMARY OF UNIQUE QUALITIES AND BENEFITS • • • • • • • • • • • • •
Stress relief of surface Removes oxide Passivation of stainless steel, brass, and copper Superior corrosion resistance Hygienically clean surfaces Decarbonization of metals No hydrogen embrittlement No direction lines Low-resistance welding surface Reduces friction Both polishes and deburrs odd-shaped parts Radiuses or sharpens edges depending upon rack position Reduces annealing steps
SIMPLICITY OF THE SYSTEM Practically speaking, three major process steps are necessary to electropolish most metal surfaces successfully: 1. Metal preparation and cleaning 2. Electropolish (electropolish drag-out rinse) 3. Posttreatment (rinse, 30% by volume of 42 ° Baum6 nitric acid, rinse, deionized hot water rinse)
EQUIPMENT NEEDED FOR ELECTROPOLISHING
Eleetropolishing Tank The electropolishing tank is generally constructed of 316L stainless steel, double welded inside and out. Stainless steel can withstand high temperatures, which are needed if too much water enters the electrolyte. Polypropylene usually 3/4 to 1-in. thick, is another tank choice. This tank can withstand temperatures of 180-190°F.
Power Supply The direct current source is called a rectifier. The rectifier is generally matched to the size of the electropolish tank. If the tank is to be cooled by tap water through a plate coil, no more than 5.0 A/gal should be used, therefore, in a 500-gal tank, the capacity of the rectifier should not be more than 2,500 A. If 3,500 A are needed, then the tank size must be increased to compensate for the increased wattage going into the tank (amps × volts = watts). Voltage is also determined by the number of amperes needed to electropolish the part. Generally, 600-3,000 A requires an 18-V DC output, and 3,500-10,000 requires a 24-V rectifier. Optimum running voltage is 9-13 V for stainless steel. Aluminum requires a 30-40 V" rectifier. Aluminum is run by voltage rather than amperage.
442
Racks Electropolish racks for most metals are made of copper spines and crosspieces, which have been pressed in a thin skin of titanium. Copper, phosphor-bronze, or titanium clips are used and can be bolted on with titanium nuts and bolts. Some racks are made of copper and copper spines and are coated with PVC. These racks are generally for electropolishing of aluminum, copper, brass, and bronze, although titanium can be used here instead. When building a rack, remember that 1 in. 2 of copper carries 1,000 A; therefore, if you use two spines of 1 × 1/4 in., this rack will carry 500 A. When large volumes of parts are to be processed, a specially constructed barrel may be used, or a tray.
Agitation An air line is usually placed diagonally on the bottom,of the electropolishing tank to stir up the solution, preventing temperature stratification. Air is not used directly under the parts to be electropolished because "white wash" can occur. Mechanical agitation is the optimum method for part agitation. This brings fresh solution to the surface of the part for faster electropolishing. Other methods of agitation are mixer, filter-pump, or separate pump. Filtration is used on many electropolishing systems. The solution lasts longer and the tank does not have to be cleaned as often. In high-technology operations this may be a requirement.
Temperature Most electropolishing solutions must be heated and cooled during the operating period. Heating is accomplished by using quartz or Teflon-coated stainless steel electric heaters with controls. If steam is used, Teflon coils are used. Lead is no longer used because it is toxic. Cooling is accomplished with 316L stainless steel plate coils. Stainless steel cannot be used for steam heating as most baths contain sulfuric acid, which attacks stainless steel at the high-temperature surface of the-plate coil. Tank construction of 316L stainless steel is all right because excessively high temperatures (above 250°F) are not present. Chillers are used when the tank solution will have 10-15 Algal from the rectifier. Heat exchangers are used when input amperage is above 5 A/gal.
TYPICAL SOLUTIONS There are organic electropolishing baths, inorganic baths, and organic/inorganic baths. Some typical formulas are shown below.
Aluminum Because it is amphoteric in nature, aluminum can be electropolished in both acid and alkaline electrolytes. The brightening process involves low rate of attack, use of high-purity aluminum, and requires prefinishing. Alzac Process: First Stage (Brightening) Fluoboric acid, 2.5% Temperature, 85°F Voltage, 15-30 V Current density, 10-20 Alft 2
443
The polarized film is stripped in hot alkaline solution. Anodizing, as usual, in the sulfuric acid bath follows. Only superpurity alloys (99.95%) should be used. Polishing and brightening are obtained in concentrated acid-type solutions that feature greater stock removal and greater smoothing. Battelle Sulfuric acid, 4.7% Phosphoric acid, 75% Chromic acid, 6.5% A13+ and Cr 3+, to 6% Current density, 150 A/ft 2 Temperature, 175-180°F Voltage, 10-15 V Chromic acid decreases the etching rate, but changes from the hexavalent to trivalent form in use. Sulfuric acid drops the cell resistance or voltage, but increases the etching rate.
Copper and Alloys
R.W. Manuel Water, 100 parts by wt Chromic acid, 12.5 parts by wt Sodium dichromate, 37.5 parts by wt Acetic acid, 12.5 parts by wt Sulfuric acid, 10.0 parts by wt Current density, 250-1,000 A/ft 2 Temperature, 86°F H.J. Wiesner • • • • •
Sodium tripolyphosphate, 14-16 oz/gal Boric acid, 4-5 oz/gal pH, 7-7.5 oz/gal Temperature, 125-135°F Current density minimum, 100 A/ft 2
S.B. E m e r y Ammonium phosphate, 100 parts Citric acid, 100 parts Potassium phosphate, 25 parts Water, 1,000 parts Voltage, 6-25 V Current density, 75-575 A]ft2 (AC)
444
Nickel and Alloys Sulfuric acid, 60% minimum Chromic acid, to saturation Water, as required Sulfuric acid, 60% minimum Glycerin, 200 ml/L Water, as required Nickel sulfate, 240 g/L Ammonium sulfate, 45 g/L Potassium chloride, 35 g/L Orthophosphoric acid, 15-70% Sulfuric acid, 15-60% Water, balance
Steel Steel is more difficult to electropolish to the same degree of perfection as other metals, owing to variations. It has good potential in industrial applications, as well as for brightening and smoothing; however, results are not consistent because of great variations in composition and surface conditions from mills and/or heat treatment.
R. Delaplace and C. Beehard Pyrophosphoric acid, 400 g Ethyl alcohol to make 1 L Temperature, 20°F Current density, 300 A/ft 2 Cooling of the electrolyte is required, and water must be absent.
C. Faust Sulfuric acid, 15% Phosphoric acid, 63% Chromic acid, 10% Current density, 50-1,000 A/ft 2 Temperature, 125°F This solution has a finite life.
Weisberg and Levin Lactic acid, 33% Phosphoric acid, 40% Sulfuric acid, 15.5% Current density, 100 A/ft z Temperature, 65-90°F Polishing rate is quite low; 1-2 hr are required.
446
Hammond, Edgeworth, and Bowman Phosphoric acid, 55-85% Trialkali metal phosphate, 1-15% Alkali metal sulfate, 0.5% minimum
Stainless Steel Stainless steel is the most popular electropolished metal today. It retains its finish, and no aftertreatment is required.
H. Uhlig Phosphoric acid and glycerine, 90% Glycerin, >50% Current density, >20 A/ft2 Temperature, >200°F J. Ostrofsky Citric acid, 55% Sulfuric acid, 15% Current density minimum, 100% A/ft2 Temperature, 200°F This solution freezes below 130°F. Alcohol is recommended to reduce the freezing point.
C. Faust Sulfuric acid, 15% Phosphoric acid, 63% Current density minimum, 50 AJft 2 Temperature, 80-175°F
I. Clingan Phosphoric acid, 56% Sulfuric acid, 27% Diethyleneglycolmonobutylether, 7% Temperature, I25-165°F
Weisberg and Levin Lactic acid, 33% Phosphoric acid, 40% Sulfuric acid, 13.5% Current density, 75-300 A/ft2 Temperature, 160-200°F 447
C. F a u s t
Phosphoric acid, 56% Chromic acid, 12% Current density, 100-1,000 A/ft 2 Temperature, 80-175°F J. K r e m l Sulfuric acid, 10-60% Glycolic acid, 20-80% Current density minimum, 150 A/ff 2 Temperature, 175-212°F
Electroplating byELowenheim 594
pages $150.00 An an~horitative, comprehensive introduction to the theory and practice of electroplating, this volume offers a thorough understating of materials and processes. Among the topics: metallurgy and corrosion, preplating procedures, rinsing and water supplies, direct current, and a detailed discussion of plating metals. This book is ideal for those considering a metal finishing position, as well as an opportunity to ill/ gaps in the practicing dectroplaters knowledge of fundamentals. Send Orders to:
METAL FINISHING, 660 White Plains Rd., T a r r y t o w n , N Y 10591-5153 For faster service, call (914) 333-2578 or F A X y o u r order to (914) 333-2570 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.
Pro EP
MEDICAL & SEMI.CONDUCTOR SPECIALISTS
CALL T H E PRO
PRO BRITE BATH Premixed Electropolishing Solution
Electropolishing Console System • Stock Systems • Custom Designs • All System Sizes - Self Contained or Modular & All Related Processes • Most Experienced Design Staff in the Industry
• • • • • • • •
Guaranteed Longest Bath Life Unparalled Finishing Results Low Temperature Operation Most Controlled Metal Removal Lowest Fuming Process NO WASTE TREATMENT Most Economic E.P. Process Shortest Cycle Times Available
Also PASSIVATION- VENTILATION & PARTSDRYINGSYSTEMS
2330 Euclid Hts. Blvd., Unit #408, Cleveland, OH 44106 USA Phone: 216-707-1933 • Fax: 216-707-1934 www.proep.com • E-maih [email protected]
448
HOW IT WORKS The metal part is immersed in a liquid media (electrolyte) and subjected to direct current. The metal part is made anodic (+) and a metal cathode ( - ) , usually 316L stainless steel or copper, is used. The direct current then flows from the anode, which becomes polarized, allowing metal ions to diffuse through the film to the cathode, removing metal at a controlled rate. The amount of metal removed depends on the specific bath, temperature, current density, and the particular alloy being electropolished. Generally, on stainless steel, 0.0005 in. is removed in 1,500 amp-minutes per square foot. Current and time are two variables that can be controlled to reach the same surface finish. For example, 100 A/ft2 electropolished for 5 min is 500 amp-minutes; 200 A/ft 2 for 21/2 min is 500 amp-minutes. Both pieces of metal would have about the same surface profile. Current densities of 90 to 800 A/ft2 are used in this process depending upon the part to be polished and other parameters. Electropolishing times vary from about 1-15 minutes.
ADVANTAGES Conventional mechanical finishing systems tend to smear, bend, stress, and even fracture the crystalline metal surface to achieve smoothness or luster. Electropolishing offers the advantages of removing metal from the surface producing a unidirectional pattern that is both stress- and occlusion-free, microscopically smooth, and often highly reflective. Additionally, improved corrosion resistance and passivity are achieved on many ferrous and nonferrous alloys. The process micro- and macro-polishes the metal part. Micro-polishing accounts for the brightness and macro-polishing accounts for the smoothness of the metal part. Deburring is accomplished quickly because of the higher current density on the burr, and because oxygen shields the valleys, enabling the constant exposure of the tip of the burr. Because the metal part is bathed in oxygen, there is no hydrogen embrittlement to the part. In fact, electropolishing is like a stress-relieve anneal. It will remove hydrogen from the surface. This is important to parts placed under torque.
440
Another benefit is that bacteria cannot successfully multiply on a surface devoid of hydrogen, therefore, electropolishing is ideal for medical, pharmaceutical, semiconductor, and food-processing equipment and parts. The combination of no directional lines due to mechanical .finishing, plus a surface relatively devoid of hydrogen, results in a hygienically clean surface where no bacteria or dirt can multiply or accumulate.
SUMMARY OF UNIQUE QUALITIES AND BENEFITS • • • • • • • • • • • • •
Stress relief of surface Removes oxide Passivation of stainless steel, brass, and copper Superior corrosion resistance Hygienically clean surfaces Decarbonization of metals No hydrogen embrittlement No direction lines Low-resistance welding surface Reduces friction Both polishes and deburrs odd-shaped parts Radiuses or sharpens edges depending upon rack position Reduces annealing steps
SIMPLICITY OF THE SYSTEM Practically speaking, three major process steps are necessary to electropolish most metal surfaces successfully: 1. Metal preparation and cleaning 2. Electropolish (electropolish drag-out rinse) 3. Posttreatment (rinse, 30% by volume of 42 ° Baum6 nitric acid, rinse, deionized hot water rinse)
EQUIPMENT NEEDED FOR ELECTROPOLISHING
Eleetropolishing Tank The electropolishing tank is generally constructed of 316L stainless steel, double welded inside and out. Stainless steel can withstand high temperatures, which are needed if too much water enters the electrolyte. Polypropylene usually 3/4 to 1-in. thick, is another tank choice. This tank can withstand temperatures of 180-190°F.
Power Supply The direct current source is called a rectifier. The rectifier is generally matched to the size of the electropolish tank. If the tank is to be cooled by tap water through a plate coil, no more than 5.0 A/gal should be used, therefore, in a 500-gal tank, the capacity of the rectifier should not be more than 2,500 A. If 3,500 A are needed, then the tank size must be increased to compensate for the increased wattage going into the tank (amps × volts = watts). Voltage is also determined by the number of amperes needed to electropolish the part. Generally, 600-3,000 A requires an 18-V DC output, and 3,500-10,000 requires a 24-V rectifier. Optimum running voltage is 9-13 V for stainless steel. Aluminum requires a 30-40 V" rectifier. Aluminum is run by voltage rather than amperage.
442
Racks Electropolish racks for most metals are made of copper spines and crosspieces, which have been pressed in a thin skin of titanium. Copper, phosphor-bronze, or titanium clips are used and can be bolted on with titanium nuts and bolts. Some racks are made of copper and copper spines and are coated with PVC. These racks are generally for electropolishing of aluminum, copper, brass, and bronze, although titanium can be used here instead. When building a rack, remember that 1 in. 2 of copper carries 1,000 A; therefore, if you use two spines of 1 × 1/4 in., this rack will carry 500 A. When large volumes of parts are to be processed, a specially constructed barrel may be used, or a tray.
Agitation An air line is usually placed diagonally on the bottom,of the electropolishing tank to stir up the solution, preventing temperature stratification. Air is not used directly under the parts to be electropolished because "white wash" can occur. Mechanical agitation is the optimum method for part agitation. This brings fresh solution to the surface of the part for faster electropolishing. Other methods of agitation are mixer, filter-pump, or separate pump. Filtration is used on many electropolishing systems. The solution lasts longer and the tank does not have to be cleaned as often. In high-technology operations this may be a requirement.
Temperature Most electropolishing solutions must be heated and cooled during the operating period. Heating is accomplished by using quartz or Teflon-coated stainless steel electric heaters with controls. If steam is used, Teflon coils are used. Lead is no longer used because it is toxic. Cooling is accomplished with 316L stainless steel plate coils. Stainless steel cannot be used for steam heating as most baths contain sulfuric acid, which attacks stainless steel at the high-temperature surface of the-plate coil. Tank construction of 316L stainless steel is all right because excessively high temperatures (above 250°F) are not present. Chillers are used when the tank solution will have 10-15 Algal from the rectifier. Heat exchangers are used when input amperage is above 5 A/gal.
TYPICAL SOLUTIONS There are organic electropolishing baths, inorganic baths, and organic/inorganic baths. Some typical formulas are shown below.
Aluminum Because it is amphoteric in nature, aluminum can be electropolished in both acid and alkaline electrolytes. The brightening process involves low rate of attack, use of high-purity aluminum, and requires prefinishing. Alzac Process: First Stage (Brightening) Fluoboric acid, 2.5% Temperature, 85°F Voltage, 15-30 V Current density, 10-20 Alft 2
443
The polarized film is stripped in hot alkaline solution. Anodizing, as usual, in the sulfuric acid bath follows. Only superpurity alloys (99.95%) should be used. Polishing and brightening are obtained in concentrated acid-type solutions that feature greater stock removal and greater smoothing. Battelle Sulfuric acid, 4.7% Phosphoric acid, 75% Chromic acid, 6.5% A13+ and Cr 3+, to 6% Current density, 150 A/ft 2 Temperature, 175-180°F Voltage, 10-15 V Chromic acid decreases the etching rate, but changes from the hexavalent to trivalent form in use. Sulfuric acid drops the cell resistance or voltage, but increases the etching rate.
Copper and Alloys
R.W. Manuel Water, 100 parts by wt Chromic acid, 12.5 parts by wt Sodium dichromate, 37.5 parts by wt Acetic acid, 12.5 parts by wt Sulfuric acid, 10.0 parts by wt Current density, 250-1,000 A/ft 2 Temperature, 86°F H.J. Wiesner • • • • •
Sodium tripolyphosphate, 14-16 oz/gal Boric acid, 4-5 oz/gal pH, 7-7.5 oz/gal Temperature, 125-135°F Current density minimum, 100 A/ft 2
S.B. E m e r y Ammonium phosphate, 100 parts Citric acid, 100 parts Potassium phosphate, 25 parts Water, 1,000 parts Voltage, 6-25 V Current density, 75-575 A]ft2 (AC)
444
Nickel and Alloys Sulfuric acid, 60% minimum Chromic acid, to saturation Water, as required Sulfuric acid, 60% minimum Glycerin, 200 ml/L Water, as required Nickel sulfate, 240 g/L Ammonium sulfate, 45 g/L Potassium chloride, 35 g/L Orthophosphoric acid, 15-70% Sulfuric acid, 15-60% Water, balance
Steel Steel is more difficult to electropolish to the same degree of perfection as other metals, owing to variations. It has good potential in industrial applications, as well as for brightening and smoothing; however, results are not consistent because of great variations in composition and surface conditions from mills and/or heat treatment.
R. Delaplace and C. Beehard Pyrophosphoric acid, 400 g Ethyl alcohol to make 1 L Temperature, 20°F Current density, 300 A/ft 2 Cooling of the electrolyte is required, and water must be absent.
C. Faust Sulfuric acid, 15% Phosphoric acid, 63% Chromic acid, 10% Current density, 50-1,000 A/ft 2 Temperature, 125°F This solution has a finite life.
Weisberg and Levin Lactic acid, 33% Phosphoric acid, 40% Sulfuric acid, 15.5% Current density, 100 A/ft z Temperature, 65-90°F Polishing rate is quite low; 1-2 hr are required.
446
Hammond, Edgeworth, and Bowman Phosphoric acid, 55-85% Trialkali metal phosphate, 1-15% Alkali metal sulfate, 0.5% minimum
Stainless Steel Stainless steel is the most popular electropolished metal today. It retains its finish, and no aftertreatment is required.
H. Uhlig Phosphoric acid and glycerine, 90% Glycerin, >50% Current density, >20 A/ft2 Temperature, >200°F J. Ostrofsky Citric acid, 55% Sulfuric acid, 15% Current density minimum, 100% A/ft2 Temperature, 200°F This solution freezes below 130°F. Alcohol is recommended to reduce the freezing point.
C. Faust Sulfuric acid, 15% Phosphoric acid, 63% Current density minimum, 50 AJft 2 Temperature, 80-175°F
I. Clingan Phosphoric acid, 56% Sulfuric acid, 27% Diethyleneglycolmonobutylether, 7% Temperature, I25-165°F
Weisberg and Levin Lactic acid, 33% Phosphoric acid, 40% Sulfuric acid, 13.5% Current density, 75-300 A/ft2 Temperature, 160-200°F 447
C. F a u s t
Phosphoric acid, 56% Chromic acid, 12% Current density, 100-1,000 A/ft 2 Temperature, 80-175°F J. K r e m l Sulfuric acid, 10-60% Glycolic acid, 20-80% Current density minimum, 150 A/ff 2 Temperature, 175-212°F
Electroplating byELowenheim 594
pages $150.00 An an~horitative, comprehensive introduction to the theory and practice of electroplating, this volume offers a thorough understating of materials and processes. Among the topics: metallurgy and corrosion, preplating procedures, rinsing and water supplies, direct current, and a detailed discussion of plating metals. This book is ideal for those considering a metal finishing position, as well as an opportunity to ill/ gaps in the practicing dectroplaters knowledge of fundamentals. Send Orders to:
METAL FINISHING, 660 White Plains Rd., T a r r y t o w n , N Y 10591-5153 For faster service, call (914) 333-2578 or F A X y o u r order to (914) 333-2570 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.
Pro EP
MEDICAL & SEMI.CONDUCTOR SPECIALISTS
CALL T H E PRO
PRO BRITE BATH Premixed Electropolishing Solution
Electropolishing Console System • Stock Systems • Custom Designs • All System Sizes - Self Contained or Modular & All Related Processes • Most Experienced Design Staff in the Industry
• • • • • • • •
Guaranteed Longest Bath Life Unparalled Finishing Results Low Temperature Operation Most Controlled Metal Removal Lowest Fuming Process NO WASTE TREATMENT Most Economic E.P. Process Shortest Cycle Times Available
Also PASSIVATION- VENTILATION & PARTSDRYINGSYSTEMS
2330 Euclid Hts. Blvd., Unit #408, Cleveland, OH 44106 USA Phone: 216-707-1933 • Fax: 216-707-1934 www.proep.com • E-maih [email protected]
448