cleaning, pretreatment & surface preparation

ELECTROPOLISHING

BY KENNETH B. HENSEL

ELECTRO POLISH SYSTEMS INC., MILWAUKEE; WWW.EP-SYSTEMS.COM

The electropolishing system smoothens, polishes, deburrs, and cleans steel, stainlesssteel, copper alloys, and aluminum alloys in an electrolytic bath. The processselectively 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 todirect current. The metal part is made anodic (+) and a metal cathode (-), usually316L stainless steel or copper, is used. The direct current then flows from

the anode, which becomes polarized, allowing metal ions to diffuse throughthe film to the cathode, removing metal at a controlled rate. The amount ofmetal removed depends on the specific bath, temperature, current density, andthe particular alloy being electropolished. Generally, on stainless steel, 0.0005in. is removed in 1,500 amp-minutes per square foot. Current and time are twovariables 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/ft2 for 21/2 minis 500 amp-minutes. Both pieces of metal would have about the same surfaceprofile. Current densities of 90 to 800 A/ft2 are used in this process dependingupon the part to be polished and other parameters. Electropolishing times varyfrom about 1-15 minutes.

ADVANTAGES

Conventional mechanical finishing systems tend to smear, bend, stress, andeven fracture the crystalline metal surface to achieve smoothness or luster.Electropolishing offers the advantages of removing metal from the surfaceproducing a unidirectional pattern that is both stress- and occlusion-free, microscopicallysmooth, and often highly reflective. Additionally, improved corrosion

resistance and passivity are achieved on many ferrous and nonferrous alloys. Theprocess micro- and macro-polishes the metal part. Micro-polishing accountsfor the brightness and macro-polishing accounts for the smoothness of the

metal part.Deburring is accomplished quickly because of the higher current density onthe burr, and because oxygen shields the valleys, enabling the constant exposureof 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 willremove hydrogen from the surface. This is important to parts placed undertorque.Another benefit is that bacteria cannot successfully multiply on a surfacedevoid of hydrogen, therefore, electropolishing is ideal for medical, pharmaceutical,semiconductor, and food-processing equipment and parts. The combinationof no directional lines due to mechanical finishing, plus a surface relativelydevoid of hydrogen, results in a hygienically clean surface where no bacteria ordirt 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 linesLow-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 electropolishmost metal surfaces successfully:

1. Metal preparation and cleaning

2. Electropolish (electropolish drag-out rinse)

3. Posttreatment (rinse, 30% by volume of 42¡ Baumé nitric acid, rinse, deionizedhot water rinse)

EQUIPMENT NEEDED FOR ELECTROPOLISHING

Electropolishing Tank

The electropolishing tank is generally constructed of 316L stainless steel, doublewelded inside and out. Stainless steel can withstand high temperatures, whichare needed if too much water enters the electrolyte.Polypropylene usually 3/4 to 1-in. thick, is another tank choice. This tank canwithstand temperatures of 180-190¡F.

Power Supply

The direct current source is called a rectifier. The rectifier is generally matched tothe size of the electropolish tank. If the tank is to be cooled by tap water througha 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 areneeded, 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 3,500-10,000requires 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 ratherthan amperage.

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, andbronze, 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 +/÷ 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 electropolishingtank to stir up the solution, preventing temperature stratification. Air is notused directly under the parts to be electropolished because “white wash” can

occur.Mechanical agitation is the optimum method for part agitation. This bringsfresh solution to the surface of the part for faster electropolishing. Other methodsof agitation are mixer, filter-pump, or separate pump.Filtration is used on many electropolishing systems. The solution lasts longerand the tank does not have to be cleaned as often. In high-technology operationsthis may be a requirement.

Temperature

Most electropolishing solutions must be heated and cooled during the operatingperiod. Heating is accomplished by using quartz or Teflon-coated stainless steelelectric heaters with controls. If steam is used, Teflon coils are used. Lead is nolonger used because it is toxic.

Cooling is accomplished with 316L stainless steel plate coils. Stainless steelcannot be used for steam heating as most baths contain sulfuric acid, whichattacks stainless steel at the high-temperature surface of the plate coil. Tankconstruction of 316L stainless steel is all right because excessively high temperatures(above 250OF) are not present.Chillers are used when the tank solution will have 10-15 A/gal 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/inorganicbaths. Some typical formulas are shown below.

Aluminum

Because it is amphoteric in nature, aluminum can be electropolished in bothacid and alkaline electrolytes. The brightening process involves low rate ofattack, use of high-purity aluminum, and requires prefinishing.

Alzac Process: First Stage (Brightening)

Fluoboric acid, 2.5%

Temperature, 85¡

FVoltage, 15-30

VCurrent density, 10-20

A/ft2The polarized film is stripped in hot alkaline solution. Anodizing, as usual, inthe sulfuric acid bath follows. Only superpurity alloys (99.95%) should be used.Polishing and brightening are obtained in concentrated acid-type solutionsthat feature greater stock removal and greater smoothing.

Battelle

Sulfuric acid, 4.7%

Phosphoric acid, 75%

Chromic acid, 6.5%

Al3+ and Cr3+, to 6%

Current density, 150 A/ft2

Temperature, 175-180

¡FVoltage, 10-15 V

Chromic acid decreases the etching rate, but changes from the hexavalentto trivalent form in use. Sulfuric acid drops the cell resistance or voltage, butincreases 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/ft2

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/ft2

S.B. Emery

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)

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

88metals, owing to variations. It has good potential in industrial applications,

as well as for brightening and smoothing; however, results are not consistentbecause of great variations in composition and surface conditions from millsand/or heat treatment.

R. Delaplace and C. Bechard

Pyrophosphoric acid, 400 g

Ethyl alcohol to make 1 L

Temperature, 20¡F

Current density, 300 A/ft2

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/ft2

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/ft2

Temperature, 65-90¡F

Polishing rate is quite low; 1-2 hr are required.

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 itsfinish, 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

ASulfuric acid, 15%

Phosphoric acid, 63%

Current density minimum, 50 A/ft2

Temperature, 80-175¡F

I. Clingan

Phosphoric acid, 56%

Sulfuric acid, 27%

Diethyleneglycolmonobutylether, 7%

Temperature, 125-165¡F

Weisberg and Levin

Lactic acid, 33%

Phosphoric acid, 40%

Sulfuric acid, 13.5%

Current density, 75-300 A/ft2

Temperature, 160-200¡F

C. Faust

Phosphoric acid, 56%

Chromic acid, 12%

Current density, 100-1,000 A/ft2

Temperature, 80-175¡F

J. Kreml

Sulfuric acid, 10-60%

Glycolic acid, 20-80%

Current density minimum, 150 A/ft2

Temperature, 175-212¡F