Electropolishing

SURFACE TREATMENTS ELECTROPOLISHING by Kenneth B. Hensel E/e&o

Pobsh

Systems

Inc.,

Milwaukee

The electropolishing system smoothens, polishes. dcburrs, and clean\ steel, stainless steel, copper alloys, and aluminum alloys in an electrolytic bath. The process selectively removes high points on metal surfaces, giving the ~rface 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 3 l6L stainless steel or copper. is used. The direct current then flows from the anode, which hecomeb 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 partudar alloy being electropolished. Generally, on stainless steel, 0.0005 in. is removed in I.500 amp-minute& per square foot. Current and time are two variables that can he controlled to reach the same surface finish. For example, IO0 A/ft’ elcctropolishcd for S 200 A/ft’ for 2% min is 500 amp-minutes. Both piece\ of metal min is 500 amp-minutes; would have about the \ame surfxe profile. Current densities of 90 to 800 A/ft’ are used in this proceu depending upon the part to be polished and other parameters. Elcctropoli\hing times vary from about l-15 minute\.

ADVANTAGES Conventional mechanical finishing systems tend to smear. bend, stress, and ecen fracture the crystalline metal surface to achieve smoothness or luster. Electropolishing offers the advantages of removing metal from the surfxe producing a unidirectional pattern that is both stress- and occlusion-free, microscopically smooth, and often highly reflective. Additionally, improved corroGon 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. Dcburring 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

Approach Makes Electropolishing Even Brighter

Rotative Carrousel Machine. 4owI amps

Rotative Ann Machine. MN amps per station.

ElEC7ROPUUSHSYS7EMS’Nc I 800 9594868 North 81 West 12920 Leon Road, Suite 221 Menomonee Falls, WI 530.51 0ffice(414)253~9780/FAx’(414)253-1773 www ep-systems cum E-mai/: khensel@aol cam

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 line\ 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 l l

. l l l l l

. . . . .

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

Practically speaking, surfaces successfully:

OF THE

three ma.jor process

SYSTEM

steps are necessary

1. Metal preparation and cleaning 2. Elrctropolish (electropolish drag-out rinse) 3. Posttreatment (rinse, 30% by volume of 42” BaumC water rinse) EQUIPMENT

NEEDED

FOR

to electropolish

nitric

acid, rinse,

most metal

deionized

hot

ELECTROPOLISHING

Electropolishing Tank The clectropolishing tank is generally constructed of.31 6L 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 % to I-in. thick, is another tank choice. This tank can withstand temperatures of I 80-l 90°F.

Power

Supply The dir-ect current source is called a rectifier. The rectifier is generally matched to the sire of the electropolish tzank. If the tank is to be cooled by tap water through a plate coil, no Inore than 5.0 A/gal should be used, therefore, in a 500.gal tank, the capacity of the rectifier should not bc 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 X 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.’ of copper carries 1,000 A; therefore, if you use two spines of I X ‘A in., this rack will carry 500 A. When large volumes of part\ 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 electropoliahing. 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 3 l6L 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 3 l6L stainless steel is all right because excessively high temperatures (above 250°F) are not present. Chillers are used when the tank solution will have IO-15 A/gal from the rectifier. Heat exchangers are used when input amperage is above S A/gal.

TYPICAL There are organic Some typical formulas

SOLUTIONS

electropolishing baths, are shown below.

inorganic

baths,

and organic/inorganic

Aluminum Because it is amphoteric in nature, aluminum can be electropolished alkaline electrolytes. The brightening process involves low rate of attack, aluminum, and requires prefinishing. Alzac

Process:

First

Stage

Fluoboric acid, 2.5% Temperature, 85°F Voltage, 15-30 V Current density, 1620

baths.

in both acid and use of high-purity

(Brightening)

A/f? 443

The polarioxi film is htrippcd in hot alkaline solution. AnodiLin,, 0 as usual. in the sulfuric acid bath t’ollow~. Only wperpurity alloy!, (9c).c)S%‘/r) should be uxd. Polishing und hriehtening are obtained in conccntratcd acid-type solution\ that l’eaturc grtxtcr stock rrmo\ al and geater smoothing.

Battelle Sulfuric acid. 4.7% Pho@orlc acid. 75% Chromic acid. 6.5% Al” rind cr*+. to 6% Current density, I SO A/Ii’ Temperature. 17.5 -I 80°F Vr,ltage. IO-15 v Chromic in LM.

form

Copper

R.W.

and

acid dccreascs the etching rate. hut changes from the hexavalcnt to trivalalt Sulfuric acid drops the cell I-csistance or voltage, but increaw the etching rate.

Alloys

Manuel Water, 100 parts hy wt Chromic acid, 12.5 parts by wt Sodium dichromate, 373 pal-ts by wt Acetic acid. 13.5 parts by wt Sulfuric acid. 10.0 parts by wt Current density. 2X-1,000 A/Ii’ Temperature. 86°F

H.J. Wiesner l l l l l

Sodium tr.ipolypho\phate, Boric acid, 4-S w/gal pH, 7-7.5 od$ Temperature. 125-l 35°F Current density mimmunl.

14-10

100 A/t’?

S.B. Emery Ammonium phosphate, IO0 parts Citric acid, I00 parts Potnsslum phosphate. 25 parts Water. 1,000 parts Voltage. 6-25 v Current density, 75-575 A/ft’ (AC)

444

odgnl

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, IS-70%’ Sulfuric acid, 15~60% water, balance

Steel Steel is more difficult to electropoli\h to the same dcgrcc of perfection as other mctnls. owing to variations. It has good potenlial in industrial applications, us 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. Bechard

Pyrophosphoric acid, 400 g Ethyl alcohol to make I L Temperature, 20°F Current density. 300 A/f? Cooling

of the electrolyte

is required,

and water

c. Faust Sulfuric acid, 15%~ Phosphoric acid. 63% Chromic acid. IO5+ Current density. SO&l ,000 A/ft’ Temperature. I2S’F This solution ha\ a finite life.

Weisherg

and Ixvin

Lactic acid. 33% Phwphoric acid, 10% Sulfuric acid, 15.5% Current density. 100 A/f? Tcmpcrature, 6S-C)O”F Polishing rate 1s quite low;

446

l-2

hr are ~rcquit-cd

must

be absent

Hammond,

Edgeworth,

and Bowman

Phosphoric acid, 55-85% Trialkali metal phosphate, Alkali metal sulfate. 0.5%

l-15% minimum

Stainless Steel Stainless steel is the most popular no aftertreatment is required.

electropolished

metal today.

It retains

its finish,

and

H. Uhlig Phosphoric acid and glycerine, Glycerin, >50% Current density, >20 A/f? Temperature, >200”F

90%

J. Ostrofsky Citric acid, 55% Sulfuric acid, 15% Current density minimum, Temperature, 200°F This solution

freezes

100% A/f?

below

130°F. Alcohol

is recommended

to reduce

the freezing

point.

C. Faust Sulfuric acid, 15% Phosphoric acid, 63% Current density minimum, Temperature, 8Gl75”F

50 A/ft’

I. Clingan Phosphoric acid, 56% Sulfuric acid, 27% Diethyleneglycolmonobutylether, Temperature, 125-l 65°F Weisherg

and

7%

Levin

Lactic acid, 33% Phosphoric acid, 40% Sulfuric acid, 13.5% Current density, 75-300 Temperature, 166200°F

A/f?

447

C. Faust Phosphoric acid, 56%~ Chromic acid, 12% Current density, I O&I ,000 A/t’? Temperature, X&l 75°F J. Kreml Sulfuric acid, I O-(10%’ Glycolic acid, 20-X0% Current density minimum, Temperature, 175-2 12°F

I50 A/R’

PRO BRITE Premixed Electropolishing Solution

l

Guaranteed Longest Bath Life Unparalled Finishing Results Low Temperature Operation Most Controlled Metal Removal Lowest Fuming Process NO WASTE TREATMENT

l

Shortest Cycle limes Available

l l l l l

Electropolishing l l

l

COIlSOk

BySbXll

Stock Systems . Custom Designs All System Sizes - Self Contained or Modular & All Related Processes Most Experienced Design Staff in the industry 2330 Euclid

. Most EconomicE.p. process

Also PASS/VAT/ON - VENTILATION & PARTS DRYING SYSTEMS

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USA