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ELECTROPOLISHING

A User's Guide toApplications, Quality Standards and

Specifications

For more information please contactDELSTAR METAL FINISHING, INC.

11501 Brittmoore Park DriveHouston, TX 77041 US

Toll-free: 1-800-433-5782Telephone: 713-849-2090

Fax: 713-849-2091Email: [email protected]

Website: www.delstar.com

January 2003Ninth Edition

Delstar Metal Finishing, Inc.s User's Guide to Electropolishing summarizes much of what has beenwritten about electropolishing in textbooks, magazines, technical journals and handbooks, and it isbelieved to be accurately represented. However, Delstar makes no guarantees, warranties orrepresentations regarding its accuracy and use.

In the absence of nationally accepted electropolishing finish standards, Delstar's research anddevelopment teams created recommended standards and specifications representing the variousneeds of industry. This Users Guide is provided as a service to engineers, designers and industry inan effort to promote the use of electropolishing in various applications.

Copyright 2003 Delstar


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CONTENTS________________________________________________________________________

WHAT IS ELECTROPOLISHING?HOW IS IT ACCOMPLISHED? 1

HISTORY OF ELECTROPOLISHING 2

BENEFITS OF ELECTROPOLISHING 3

WHAT METALS CAN BE ELECTROPOLISHED? 5

PHYSCIAL CHARACTERISTICS OF THE

ELECTROPOLISHED SURFACE 6Basic Mill Plate Finishes for Stainless Steel

Surface Roughness Comparison Table

APPLICATIONS 12

GOOD AND BAD ELECTROPOLISHING:

How to Recognize Quality Work 16

HOW TO SPECIFIY ELECTROPOLISHING

The Standards of Quality 18

ELECTROPOLISH FINISH STANDARDS (EFS) 19

PHOTOMICROGRAPHS:

The Proof of Quality Assured Electropolishing 21


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WHAT IS ELECTROPOLISHING?HOW IS IT ACCOMPLISHED?________________________________________________________________________

Electropolishing streamlines the microscopic surface of a metal object byremoving metal from the object's surface through an electrochemicalprocess similar to, but the reverse of, electroplating.

In electropolishing, the metal is removed ion by ion from the surface of themetal object in question. Electrochemistry and the fundamental principlesof electrolysis (Faraday's Law) replace traditional mechanical finishingtechniques, including grinding, milling, blasting and buffing as the finalfinish. In very basic terms, the object to be electropolished is immersed inan electrolyte and subjected to a direct electrical current. The object is

maintained anodic, with the cathodic connection being made to anearby metal conductor.

During electropolishing, the polarized surface film is subjected to theeffects of gassing (oxygen), which occurs with electrochemical metalremoval, saturation of the surface with dissolved metal and the agitationand temperature of the electrolyte.

Smoothness of the metal surface is a primary and very advantageouseffect of electropolishing. During the process, a film of varying thickness

covers the surfaces of the metal. This film is thickest overmicrodepressions and thinnest over microprojections. Electricalresistance is at a minimum wherever the film is thinnest, resulting in thegreatest rate of metallic dissolution. Electropolishing selectively removesmicroscopic high points or "peaks" much faster than the correspondingrate of attack on the corresponding micro-depressions or "valleys." Stockis removed as metallic salt. Metal removal under certain circumstances iscontrollable and can be held to 0.0001 to 0.0025 inch.

In summary, electropolishing removes metal. It does not move it or wipe

it. As a result, the surface of the metal is microscopically featureless, withnot even the smallest speck of a torn surface remaining. The basic metalsurface is subsequently revealed -- bright, clean and microscopicallysmooth. By contrast, even very fine mechanically finished surfaces willshow smears and other directionally oriented patterns or effects.


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HISTORY OF ELECTROPOLISHING________________________________________________________________________

History's first reference to electropolishing occurred in 1912 when the

Imperial German government issued a patent for the finishing of silver in acyanide solution.

Further experimentation with the process continued, but the nextsignificant advancement was not made until 1935 when copper wassuccessfully electropolished. This leap forward was followed by other newdevelopments in 1936 and 1937, when Dr. Charles Faust and othersdiscovered solutions for electropolishing stainless steels and other metals.

During World War II, extensive research and process development by both

Allied scientists yielded a substantial number of new formulas and results.Data from these projects was published during the post-war period inhundreds of articles describing electropolishing's applications and itstheoretical basis. Dozens of new patents were registered between 1940and 1955. Important applications were developed for the military duringWorld War II and the Korean conflict.

Today, electropolishing is being rediscovered as a replacement formechanical finishing. In addition to making a surface smoother, it is amore visible means of brightening, deburring, passivating, stress relievingand otherwise improving the physical characteristics of most metals and

alloys.


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BENEFITS OF ELECTROPOLISHING________________________________________________________________________

Better Physical Appearance

No fine directional lines from abrasive polishing

Excellent light reflection and depth of clarity

Bright, smooth polish; uniform luster of shaped parts

Enhanced Mechanical Properties

Less friction and surface drag

Increased production and duty cycles in process equipment.Electropolishing greatly reduces fouling, plugging, scaling andproduct build-up

Surface retains the true grain structure and properties of the bulkmetal

Fatigue strength is not reduced. Electropolishing allows the truefatigue strength of a part to be accurately determined

Higher fatigue strength can be promoted by particle-blasting thesurface to reintroduce compressive stress without losingelectropolishing's advantages

Stress-relieving of the surface

Reduces galling of threads on stainless and carbon steel and otheralloys

Better Corrosion ProtectionElectropolishing yields maximum tarnish and corrosion resistance in many metalsand alloys. Stainless steel contains metallic and non-metallic inclusions, whichare unavoidably included during manufacture. Mechanical polishing not onlyfails to remove inclusions, but also tends to push them further into the surface andeven increase them by further pick-up of abrasive materials. These inclusionseventually can become points of corrosion.


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Ease of Cleaning

Substantially reduces product contamination and adhesion due tothe microscopic smoothness of an electropolished surface (muchlike a glass surface)

Decreases cleaning time. Electropolished surfaces can beeffectively hydroblasted in less time and with less pressure. Somecompanies report that electropolished process equipment surfaceshave reduced cleaning time by more than 50 percent

Improves sterilization and maintenance of hygienically cleansurfaces for food, drug, beverage and chemical processingequipment

Provides best passivation of stainless steel. Unipotentializes stainlesssteel with the oxygen absorbed by the surface, creating a

monomolecular oxide film

Decarburizes metals

Removes cold-worked metal oxides

For Special Effects

Simultaneously deburrs as it polishes

Radiuses or sharpens edges, depending on racking position

Polishes areas inaccessible by other methods

Reveals flaws in metal surfaces undetectable by other means.Electropolishing is one of the most effective inspection tools for

judging metal surfaces

Provides a correct and reproducible microhardness on the metalsurface

Increases magnetism of parts by approximately 20 percent

Allows micromachining of metal and alloy surfaces

Processes large numbers of parts simultaneously

Enables metal to be formed with fewer passes and annealing steps.


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WHAT METALS CAN BE ELECTROPOLISHED?________________________________________________________________________

Most metals can be electropolished successfully, but the best results are

obtained with metals with fine grain boundaries that are free of non-metallic inclusions and seams. Those metals having a high content ofsilicon, lead or sulfur are usually troublesome.

Stainless steels are the most frequently electropolished alloys, and all canbe processed. Castings will polish to a bright finish but not to the samebrightness or smoothness produced by wrought alloys.

Other Commercially Electropolished Metals include:

Low and High Carbon SteelsTool SteelsHigh Temperature Alloys (Molybdenum, Nimonic, Waspaloy,Tungsten)

Aluminum TitaniumCopper KovarCupronickel InconelBrass ColumbiumBronze Leaded Steel (Low-Lead)Nickel Silver BerylliumMonel VanadiumHastelloy TantalumBeryllium Copper Silver and Gold

Many of the above metals can only be electropolished in largeproduction runs and in controlled environments. This is due to thesometimes-costly set-up, tooling and special environmental and safetyequipment requirements associated with many of these processes.


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PHYSICAL CHARACTERISTICS OFTHE ELECTROPOLISHED SURFACE________________________________________________________________________

Important Considerations

A smooth, highly reflective electropolished surface is determined to alarge extent by the surface conditions of the metal and the processcontrols.

Surface Conditions. Base metal conditions that can result in less thanoptimum electropolishing finishes include the presence of non-metallicinclusions, improper annealing, overpickling, heat scale, large grain size,directional roll marks, insufficient cold reduction or excessive cold

working. These conditions may be inherent in the metal as it comes fromthe mill. During electropolishing, metal is removed, revealing these flaws.

Process Controls. The degree to which the electropolishing process iscontrolled determines the final quality and consistency of the finish. Manyelectropolishing shops employ, at best, a minimum number of controls, orno controls at all. A lack of process controls produces inconsistent andunpredictable quality. While some variables are functions of technology,others fall under what many call "the art of electropolishing."

It is imperative that the proper electrolyte be used, that its temperature bemaintained precisely (heated or cooled as necessary) and that itschemistry be monitored constantly. Critical factors in the chemistry arethe specific gravity (an indicator of water content and a majorconsideration in hygroscopic electrolytes), the acid concentration andthe metals content. A supply of clean, ripple-free DC power must beavailable to drive the process, as well as appropriately sized cables andconnectors to the anodes and cathodes. The DC power must be appliedat the correct voltage and current density (amperes/square foot).

The "art" of electropolishing is the ability of a technician with many years

of experience to configure a cathode for optimum polishing ininaccessible areas, corners, and areas of low current density. Equallyimportant is the knowledge of where, when and how to agitate either theelectrolyte or the part in order to prevent gassing streaks, flow marks andsimilar undesirable markings.

For many products, electropolishing's mirror-like luster is the goal. Manyothers, however, require additional functional or technical advantages of


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electropolishing not attainable by mechanical means. Because there is nomechanical disturbance of the surface during the metal removal process, theelectropolished surface possesses the true grain structure and properties ofthe bulk metal.

In contrast, all mechanical polishing procedures leave a layer of disturbed

structure. The mechanically finished surface will not have the propertiesreported for the bulk metal, regardless of the thickness of the disturbedsurface or the mechanical process employed.

The surface of a metal is often described as that place where the metalceases to exist. This certainly can be said of an electropolished surface, butnot for a surface that has been mechanically finished by cutting, smearing,skin rolling, buffing, drilling, boring, reaming, broaching or grinding.

Electropolishing vs. Mechanical Polishing

A mechanically polished metal surface yields an abundance of scratches,strains, metal debris and embedded abrasives. In contrast, anelectropolished surface is completely featureless. It reveals the true crystalstructure of the metal without the distortion produced by the cold-workingprocess that always accompanies mechanical finishing methods.

The difference between electropolishing and mechanical finishing is oftennot readily obvious to the unaided human eye, particularly if both arepolished to the same micro-inch finish. The extraordinary advantages of thesurface finish produced by electropolishing leap forward, however, when themetal is viewed under high magnification. In contrast, finishing processes

using abrasives or other cutting or burnishing action, regardless of how smallthe amount of work, always distorts the metal surface.

The differences are much more than simply topographical. The damageassociated with cold working penetrates deeply into the metal; likewise,abrasives become embedded in the surface. The mechanical strength of thesurface is lowered significantly by the cold working that accompanies simplecutting operations. For example, the application of a mechanical finishingprocess to steel having a tensile strength of 100,000 psi can leave a surfaceskin of worked metal possessing only a tensile strength of only 35,000 psi.

Burnishing metal by lapping, buffing or coloring processes decreasesmicroinch roughness and improves the image-defining quality of a surface,but it never completely removes the debris and damaged metal caused bymechanical polishing. Burnishing cannot be viewed as an adequatesubstitute for electropolishing in most applications.

Surface finish is confirmed by high-magnification scanning electronmicrographs. Several examples of these SEM photomicrographs areprovided in this Guide, using representative stainless steel surfaces in tubingand plate.


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The tubing photomicrographs show 316L tubing before and afterelectropolishing at various magnifications. The photomicrographs of platesurfaces (taken at 2,000X and enlarged) show plate in the as-receivedcondition (No.1 hot rolled, annealed and pickled finish), as well as thecold-rolled (2-B finish), ground (No. 4 finish) and electropolishedconditions. SEM photomicrographs showing inadequate and intermediate

quality electropolishing also are included for reference.

Basic Mill Plate Finishes for Stainless Steel

There are eight basic stainless steel mill plate finishes available. Finishes 3,4, 6, 7 and 8 are produced mechanically by using abrasive compositionsand buffing wheels.

Unpolished No. 1. This is a very dull finish produced by hot-rolling thesteel on hand sheet mills to specified thicknesses, followed by annealingand descaling. The surface is microscopically very porous and uneven,

allowing a level of contamination pick-up that is many times its geometricsurface area. Used in industrial applications where resistance to heat orcorrosion, not a smooth finish, is desired.

Unpolished No. 2D. This is a dull finish produced on hand sheet mills orcontinuous mills by cold rolling the metal to the specified thickness,followed by annealing and descaling. The dull finish may result from thedescaling operations or may be developed by a final light cold roll passon dull rolls. This finish is favorable to the surface retention of lubricants indeep drawing operations, and it is generally used in forming deep-drawn

articles that may be polished after fabrication.Unpolished No. 2B. This is a bright, cold-rolled finish produced similarly tothe No. 2D finish, except the annealed and descaled sheet receives afinal light cold roll pass on polished rolls. The metal grains are flattened,which facilitates removal of smearable contamination. However, theetched boundaries between the grains are only partially sealed, resultingin a network of sub-surface crevices. Contaminants lodging in thesecrevices are protected from contact with cleaning agents, leading topossible subsequent migration of trapped contaminants onto the cleanedsurface (bleeding). No. 2B is a general purpose finish used for all but the

most difficult deep-drawing applications, and it is more readily polishedthan No. 1 or No. 2D.

Polished No. 3. This is an intermediate polished finish for use where asemi-finished polished surface is required for subsequent finishingoperations following fabrication, or as a final finish with a 50 or 80 gritabrasive compound. (In the case of metal sheets or articles that will notbe subject to additional finishing and polishing, a No. 4 finish is ideal.)


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Polished No. 4. This is a general purpose mechanically polished stainlesssteel finish that is widely used for architectural panels and trim as well as forrestaurant, dairy and kitchen equipment. Initial grinding is followed bycoarser abrasives, with sheets eventually finished out with 100-150 meshabrasives. Although microscopically flat, the grain of Polished No. 4 stainlesssteel still contains deep grooves and other microscopic cavities that entrap

and retain contaminants.Buffed No. 6. A dull satin finish possessing low reflectivity, a Buffed No. 6finish is produced with a greaseless compound and 200-grit abrasive, and istop-dressed with chrome rouge.

Buffed No. 7. This very reflective finish is produced by buffing a surface thatfirst was refined with 220-grit abrasives to approximate a No. 6 finish. It is thenbuffed lightly with a white chrome rouge without removing the satin finishlines.

Buffed No. 8. This is the most reflective mechanical finish. It is obtained bypolishing with progressively finer abrasives (320-grit and finer), followed byextensive buffing with very fine white chrome bar buffing compounds. To theunaided eye, the surface appears free of grit lines from previous grindingoperations.

Electropolished Surfaces. Electropolished surfaces are extremely smooth,macro-scopically flat, microscopically featureless, and exhibit high highluster, reflectivity and brightness. Electropolishing substantially reduces thesurface area available for contamination pick-up and eliminates all micro-cracks and internal crevices.________________________________________________________________________

COMPARISON TABLE

Differences Between Polished and Buffed Milled Finishes, Abrasive GritNumbers and Surface Roughness (Before and After Electropolishing)

Before Electropolishing After Electropolishing*Mill Grit Surface Roughness, Ra Surface Roughness, RaNo. No. micrometer microinches micrometer microinches

3 60 3.56 max 140 max 1.78-2.25 70-90

4 120 1.14 max 45 max 0.57-0.75 23-30180 0.64 max 25 max 0.32-0.40 13-16

7 240 0.20-0.51 8-20 0.10-0.26 4-108 320 0.15-0.38 6-15 0.08-0.19 3-88 500 0.10-0.25 4-10 0.05-0.13 2-5

* Values are approximate. Electropolishing generally reduces surfaceroughness readings of a non-electropolished surface by 50 percent.

________________________________________________________________________


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Smoothness is not an independent variable in surface definition. It is onefactor of an important subject that is referred to as "surface metallurgy."Smoothness specifications, based on gages, can be achieved by bothelectropolishing and mechanical abrasive finishing techniques.

Surface Roughness

Surface roughness is commonly measured or classified as Ra (Roughnessaverage) or Rq (the equivalent of RMS -- Root Mean Square). Both aremeasured in microinches and denote the smoothness of ground ormachined surfaces. For comparison, an Ra reading is approximately 87.5percent of an Rq (RMS) reading.

Roughness measurements have no real relationship to how

easily an electropolished surface can be cleaned after use

or to its non- contaminating, non-particulating or non-stick

properties.

Surface roughness is usually measured with a profilometer. This instrumentcannot accurately read the distances between the "peaks" and "valleys."Electropolishing may reduce the peaks from substantial points toinsignificant mounds without changing the peak-to-peak distance at thesame ratio. However, microscopic examination of the surface will showup to a 90 percent reduction in surface area and up to a 50 percentimprovement in profilometer readings.

Surface Chemical Analysis

One of electropolishing's primary benefits is the chromium enrichment ofthe surface resulting from properly controlled processing.

A consistent chromium-rich oxide layer only is attained when the atomicconcentration of chrome exceeds the iron in the surface layer asdemonstrated by the Auger Electron Spectroscopy (AES). AES analysisalso measures depth and extent of surface passivation. Electropolishingmaximizes surface passivation because the surface contains very lowlevels of iron (Fe) in zero oxidation states.

Other surface chemistry analysis can be made for sulfide inclusions,precipitated carbides and other similar impurities, all of which affect thefinal appearance of electropolished surfaces. The end-grain surfaces offree-machining stainless grades such as Types 303 and 416 will appearfrosty after electropolishing due to the removal of the sulfide inclusions.Type 302 stainless steel will show pitting if the annealing process failed toredissolve the precipitated carbides.


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Friction Reduction

Electropolishing reduces the coefficient of friction of metals. The processremoves or rounds off the small surface asperities, yielding a coefficient offriction that measures approximately one-fourth of the coefficientregistered by a mechanically finished surface.

Electropolished Castings

Various alloys are used in most castings, making this particular productless well suited to electropolishing. A major exception to this rule isstainless steel.

Electropolished stainless steel castings will be brightened but will not be assmooth as strip stock nor obtain the same mirror finish. The processdecontaminates and passivates the metal surface. Note: During electro-

polishing, the surface of the casting will be removed and may exposesub-surface porosity.

Investment castings are better candidates for electropolishing than sandcastings, primarily because of the inherently smoother surface ofinvestment grade castings. However, sand castings will electropolish to aclean and bright surface.

Limitations

Electropolishing cannot smear over or otherwise conceal defects such asseams and non-metallic inclusions in metals. In addition, heavy orangepeel, mold-surface texture and rough scratches are not removed by apractical amount of electropolishing and thus require an initial "cutdown"with abrasives. Multiphase alloys in which one phase is relatively resistantto anodic dissolution usually are not well suited to an electropolishingtreatment.


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APPLICATIONS________________________________________________________________________

Today, electropolishing is successfully applied to an expanded range of

new applications:

Pipe/Tubing Valves Fittings Sheet MetalStampings Spinnings Weldments CastingsWire Goods Forgings Fasteners Drawings

Major benefits of electropolishing continue to be demonstrated in reactorvessels, heat exchangers, blenders, storage tanks, piping and tubing,clean rooms, food and beverage processing equipment, medicalequipment, machined parts and nuclear applications. See the sections

below to find the advantages electropolishing offers for your application.

Reactor Vessels, Heat Exchangers, Mixers,Agitators, Blenders & Storage Tanks

Extends operating times and reduces wear and tear on parts

Reduces adhesion and contamination on the surfaces of processequipment (Electropolished surfaces perform as well as glass

surfaces in many applications)

Promotes easy cleaning and reduces associated downtime

Significantly reduces a metal's tendency to corrode

Reduces friction between moving metal parts

Improves heat transfer efficiency in heat exchangers

Enhances flow characteristics of piping and tubing

Eliminates burrs from machined parts

Reduces surface stresses in formed metal parts

Lengthens metal life against sour gas


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Experience tells us that any container used in mixing, blending or storageprocesses for liquids or powders is an excellent candidate forelectropolishing. The non-stick qualities of an electropolished surfacehave obvious advantages in these applications. Companies who haveused electropolished parts successfully in the above applications includeExxon, Shell, Union Carbide, Goodyear, Ethyl, Occidental, BASF, Allied,

Dow Chemical, Mobay and others.

Piping & Tubing

In recent years, electropolishing has been shown to provide the ultimateI.D. and O.D. finish for piping and tubing. Electropolishing is necessarywhenever non-contaminating, non-particulating and anti-fouling surfacesare required. In addition, pipe and tubing also benefit from the minimalfriction and maximum purity aspects of electropolishing. Majorbeneficiaries of electropolished piping and tubing include thepetrochemical, nuclear, pharmaceutical, semi-conductor and food andbeverage industries.

Clean Rooms

Clean rooms demand non-contaminating and non-particulating surfaces.Electropolishing is the ultimate finish for clean room tables, chairs, wastecontainers, light fixtures, exposed electrical conduit and outlet boxes,manufacturing and processing equipment, and other metalliccomponents used in this application.

Food & Beverage ProcessingElectropolishing provides the smooth, easy cleaning and cosmeticallypleasing surfaces demanded by this industry coupled with unmatchednon-contamination and sanitary qualities. The process reduces oxidationand contamination of stainless steel components used in kitchen, dairyand automatic food processing equipment, and descales food andbeverage containers. Overall, one can expect a significantly cleanersurface that resists the contaminating build-up of undesirable bacteriaand product particles.

Medical Applications

For many years, the medical field has been a beneficiary ofelectropolishing. All hospital, medical and surgical equipment (scalpels,clamps, saws, bone and joint implants, prosthetic devices, burn beds andrehabilitation whirlpools) should be electropolished to facilitate cleaningand achieve high levels of non-contamination. All metal articles exposedto radiation and requiring regular decontamination are prime candidatesfor electropolishing.


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Machined Parts

All screws, bolts, washers, valve stems and bodies, and other machinedparts benefit from electropolishing. The process provides a number ofadvantages, including deburring and stress relieving of the surface, easyclean up, a non-stick, non-contaminating, non-particulating finish, and a

pleasing cosmetic appearance. It has been demonstrated that machinedparts of electropolished 304 stainless steel exhibit superior corrosionresistance when compared to the same parts fashioned from non-electropolished 316 stainless steel. Precise machining usingelectropolishing techniques is an advantage in many applications.

Nuclear

Electropolishing is a critical finish in the nuclear industry. Used to polishsurfaces located in radioactive environments, electropolishing reducescontamination pick-up and increases the effectiveness of conventionaldecontamination techniques. The process can be used to decontaminateradioactive metallic surfaces to non-detectable levels. Anycontamination located on or embedded in the surface can be removedby the electropolishing process. In addition, residual contaminatedelectrolyte can be removed in the rinsing operation. A very effectiveapplication for electropolishing is the polishing of nuclear plantrecirculation piping for stress-relief of internal surfaces that have alreadybeen mechanically polished.

More Industrial Applications

Dry Product Delivery Systems

Filters, Screens and Strainers

Product Trays and Dryers

Thermowells

Pumps and Valves

Compressors and CondensersCooling and Plate Coils

Turbine Blades, Wheels and Impellers

Vacuum Chambers and EquipmentSignificantly reduces outgassing on surfaces in a vacuum environment


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Paper Mill EquipmentPaper slurry pipe systems and head boxes are two of many applications

Electronic and Communications Parts

Offshore Oil field Applications

Instrumentation, down-hole safety equipment and other similar systems

Fouling Reduction of Screens and Springs

Reduction of Oxidation and CorrosionCrucial for preserving mechanical seals and parts located in chlorine

environments

ElectromachiningOccasionally, parts are made to improper tolerances or to tolerances

needing a slight change. Electropolishing can be a valuable tool to alter

tolerances by precisely controlling the removal of a small amount ofmetal.

PassivationElectropolishing serves as an excellent passivation process. The process

removes contamination located on or just beneath the surface and

passivates stainless steel to a much greater extent than any other

treatment.

Electropolishing also helps preserve carbon steel parts scheduled forextended storage. Electropolished carbon steel has been stored at 60-70

percent relative humidity for more than six months without visible rust.Electropolished brass tarnishes only slightly, if at all, under similarcircumstances a sharp contrast to buffed brass which turns dark.


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GOOD & BAD ELECTROPOLISHING:How to Recognize Quality Work________________________________________________________________________

What Does High Quality Electropolishing Look Like?

High quality electropolishing should exhibit brilliant luster and reflectivity.The smoother the surface to be electropolished, the higher the brillianceand reflectivity will be after the process is complete. High qualityelectropolishing should be free of the following flaws:

"Frosting" Streaks or Stains Orange PeelShadows Water Spots ErosionIrregular Patterns Pitted Surfaces Pebbly Surfaces

Under high magnification, the electropolished surface should show noevidence of grain boundaries and should be essentially featureless. (SeeSEM Photomicrographs section, p. 21.)

What Contributes to Good (and Bad) Electropolishing?

High quality electropolishing begins with materials possessing superiorelectropolishing properties. Quality materials, coupled with propertechniques and conditions, generally will yield superb results.

However, there are times when even the best efforts of the Electropolisherdo not produce the desired results. A stainless steel part produced from aspecific alloy or possessing a unique history may present certainproblems. Pits, exposed seams, a grainy or dull luster, "patchy luster" anda generally "unfinished" look are examples of situations where theproblem may be rooted in the material.

One or more of these effects can be attributed to non-metallic inclusionsthat trace back to hot-rolling of slabs, under- or over-pickling, excessiveor undue temperature rise during cold rolling (as by too heavy a reductionper pass), under- or over-annealing, surface decarburization duringannealing, excessive grinding prior to cold-rolling, burnishing actionduring the last stage of cold rolling, and contamination from exposure toindustrial process materials, pollutants, lubricants and other materials usedin manufacturing. End-grain surfaces of free-machining stainless steelgrades such as Types 303 and 416 will appear "frosty" afterelectropolishing due to removal of the sulfide inclusions.


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A uniform, fine crystal homogeneous structure produces the bestelectropolishing results. However, certain structural characteristics canvary in stainless steel without affecting the nominally specified properties,yet influence the electropolishing results. Examples include broken down,highly oriented structures, grain boundary precipitation of carbides, andother non-homogeneities all of which cause a lower quality

electropolished finish.

Recognizing Quality

Learning to recognize quality electropolishing is like learning to distinguishbetween a real one-dollar bill and a counterfeit. The best way to tell thedifference is to learn how the real things looks, feels and performs. Inferiorelectropolishing then can be readily identified, either by visualexamination or by photomicroscopy.

Electropolishing is preferred in many industries as a final finish for manymetals because of its surface enhancement, non-contaminating, non-particulating, non-sticking or cosmetic qualities. If the goal is a pleasingcosmetic finish, high quality electropolishing usually can be judged byvisual inspection. A microscopic evaluation of the surface is necessary,however, where a "maximum" finish is required and where it is imperativethat the surface condition be documented for future evaluation.

For example, a buffed surface (such as a No. 8 finish) often appears to beelectropolished to the uneducated eye. Either finish can yield the samesurface finish profilometer readings (Ra or RMS). Only photomicrographs

of the two surfaces would show the substantial difference between the twosurfaces. The electropolished surface would be seen as featureless, whilethe buffed surface would show layers of smeared, disturbed anddamaged metal, as well as embedded abrasives and buffing compound.

Photomicroscopy is highly recommended whenever substantial capitalinvestment is involved and where an electropolished surface plays a keyrole in the results of a research project or manufacturing process. In orderto maintain a rigorous quality control program, the use of comparativesample coupons (previously electropolished and thoroughly proven with

photomicroscopy) is recommended. In addition to being cost-effective,electropolishing establishes an objective evaluation and permanentrecord for future reference.

Photomicrographs offer the only positive proof of the quality of theelectropolishing. Your electropolisher should be willing to certify the finalfinish with a photomicrograph.


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HOW TO SPECIFY ELECTROPOLISHING:The Standards of Quality________________________________________________________________________

For many years, the lack of accepted specifications for electropolishing leftmany uncertain as to how to specify the finish desired on the metal surface.There are four primary and generally accepted finish standards forelectropolishing. (See next page for specifics.)

To achieve quality electropolishing, Step One must be the specification of thecorrect metal alloy. For example, a general materials specification tomanufacture a part from "300 series stainless steel" alloys can createproblems. Some 300 series stainless steels Electropolish better than others.Type 316 generally polishes better than Type 304. Type 303 stainlesspossesses sulfide inclusions, which presents an obstacle to high-grade

electropolishing. Selection of the material should be discussed with theElectropolisher.

Step Two is the selection of the final finish required for the application beingconsidered. An intermediate step of mechanical abrasive polishing may berequired to achieve the final finish desired. Generally, electropolishing canreduce the Ra or RMS reading on a mechanically polished surface byapproximately 50 percent. For example, if the existing mill or mechanicallypolished surface shows a surface reading of 50 Ra, one can expect anelectropolished surface reading of approximately 25 Ra. There are factors,however, that can depress or elevate the final Ra reading. Quality

Electropolishers use sample coupons of the metals to be electropolished inorder to help determine the final finish.

Step Three: Any mechanical polishing required to achieve the specified finalfinish must be considered. Multiple passes of ever-finer abrasive grits arerecommended as mandatory for best results. Very coarse grits (less than 80grit) should be avoided. Surface quality obtained by Electropolishing isdirectly related to the quality of the pre-electropolished surface.Electropolishing cannot remove digs, gouges, scratches or other similarsurface distortions. Discussion of the mechanical polishing requirements withthe Electropolisher will help assure the quality of the desired final finish.

Delstar Corporation recognized some time ago that the creation andimplementation of nationally accepted standards regarding electropolishedfinishes was desperately needed in order to establish and maintain quality.Working closely with an independent research institute, our research anddevelopment teams led the industry's response to this challenge byestablishing the Electropolish Finish Standards (EFS). Each standard containsspecifications in order to enable customers to accurately specify the finalfinish desired. Details on grades EFS-1, EFS-2, EFS-3 and EFS-4 are describedon the following page.


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ELECTROPOLISH FINISHSTANDARDS (EFS)________________________________________________________________________

EFS-1(Critical Applications Requiring Surface Verification)

"Part is to be pre-cleaned if necessary, and electropolished to uniformoverall finish with maximum brightness, luster and reflectivity on requiredsurfaces as shown by symbol on print. If indicated, final finish must be tothe Raor RMSfinish noted.

"Specified electropolished surfaces to be free of frosting, shadows, streaks,erosion, stains, water spots and irregular patterns in the finish. Specifiedsurfaces also to be free of pebbly, orange peel or pitted appearance.

"Part is to be cleaned to acid-free condition throughout, D.I. water rinsed,dried and packaged to protect the electropolished surface. Surface shallbe confirmed by one or more standard surface quality tests specifiedbelow."

EFS-1a (Verification by SEM Photomicroscopy)

"Electropolisher to provide photomicrographs (100x and 1,000X

magnification) of representative surface areas to assure maximumelectropolished surface condition. Surface will show no evidence ofgrain boundaries."

EFS-1b (Verification by Auger Electron Spectroscopy)

"Electropolisher to provide results of Auger Electron Spectroscopy(AES) taken from representative surface areas to assure maximumelectropolished surface condition. Depth of chrome oxide layer shallnot be less than 20 (Angstroms)."

EFS-1c (Verification by ESCA Analysis)

"Electropolisher to provide results of Electron Spectroscopy forChemical Analysis (ESCA) taken from representative surface areas toassure that the maximum electropolished condition has beenachieved. ESCA will be conducted in tandem with AES to confirm thequality of the Cr/Fe oxide profile.


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EFS-2 (Critical Applications Not Requiring Surface Verification)

"Part is to be pre-cleaned if necessary, and 100 percent electropolished touniform overall finish with maximum brightness, luster and reflectivity onrequired surfaces as shown by symbol on print. If indicated, final finish mustbe to the Ra or RMS finish noted. Specified electropolished surfaces to be freeof frosting, shadows, streaks, erosion, stains, water spots and irregular patternsin the finish. Specified surfaces also to be free of pebbly, orange peel orpitted appearance. Part is then to be cleaned to acid-free conditionthroughout, D.I. water rinsed, dried and packaged to protect theelectropolished surface."

EFS-3 (Technical Applications Allowing Variations in Finish)

Where an EFS-2 electropolished surface is not required and where a moreeconomical electropolishing finish is acceptable, the following specificationcan be used. Less stringent than the one above, this specification will allowsome irregularities in the finish.

"Part to be pre-cleaned if necessary, and bright electropolished to a visuallyuniform finish on required surfaces as designated by symbol on print.Specified electropolished surfaces must be free of pebbly, orange peel orpitted appearance. Part is then to be cleaned to an acid-free conditionthroughout."

EFS-4 (Non-Reflective Applications)

An EFS-4 electropolished finish is used where a non-reflective surface isrequired.

"Part to be pre-cleaned if necessary, and electropolished to a uniform satinfinish. Surfaces shall exhibit no bright, reflective areas and shall be free ofpebbly, orange peel or pitted appearance. Part is to cleaned to an acid-free condition throughout."

EFS-X (Specification for Electrical Connection to Part)

Where location of "rack marks" or other evidence of DC power anodecontact is important, the following specification should be included:

"Electrical anode connections to part shall be at those points or within thoseareas indicated on the part drawing, or at alternate locations as approved

by Purchaser."

Special Note

Specifications should be included in any written description of work as wellas on the part drawing. The electropolishing specification should be a partof, and in addition to, any required mechanical polishing specification.Special post-electropolishing cleaning and/or packing instructions shouldalso be included.


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SEM PHOTOMICROGRAPHS:The Proof of Quality Assured Electropolishing________________________________________________________________________

A Scanning Electron Microscope is an invaluable tool when evaluatingsurface finishes. SEM photomicrographs illustrate in graphic detail thedifferences between the surface qualities of mill and mechanicallypolished finishes and an electropolished surface. The followingphotomicrographs represent typical evaluations of various surfaces duringthe polishing process.

SEM photomicroscopy is highly recommended whenever substantialcapital investment is involved and where an electropolished surface playsa key role in the results of a research and development project or

manufacturing process.

(Note: When the actual surface of a large object cannot physically beloaded into a SEM and viewed, a thin film of clear acetate often is used tomake an exact replica of metal surfaces. Film is pressed against themetal surface in order to create a "mirror image." The processed replicacan then be sputtered with gold and subsequently viewed andphotographed using a SEM.)


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Comparative Photomicroscopy Showing Stainless Steel Plate AsReceived and in Successive States of Electropolishing

This photomicrograph represents

the surface of HRAP No. 1

stainless steel plate as received

from the mill. Note the grainboundaries, porosity, crevices,

cavities and breaks in the surface.

If left intact, these surface flaws

will entrap contaminants that may

subsequently migrate onto thecleaned surface.

HRAP #1 S.S. Plate As Received


the same surface after inadequate

Electropolishing. Note that many

of the serious surface flawspersist. Metal that is inadequately

electropolished will not yield theresults expected from high quality

electropolished surfaces.

Inadequate Electropolishing

Comparative photomicroscopy of stainless steel plate in successive statesof electropolishing continues on next page


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Comparative Photomicroscopy Showing Stainless Steel PlateAs Received and in Successive States of Electropolishing (contd.)


intermediate quality

Electropolishing. While most of the

surface flaws have been removed orconsiderably reduced, the surface

can continue to entrap

contaminants. As a result, it will

perform inadequately in manyapplications.

Intermediate Quality Electropolishing

This photomicrograph represents a

premium high-grade electropolished

surface. The surface is completelyfeatureless when viewed under high

magnification. This electropolished

surface is non-contaminating, non-

particulating, non-outgassing and

non-sticking.

Premium-Grade Electropolishing


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Comparative Photomicroscopy of Two StandardMill Finishes Before and After Electropolishing


the surface of 304 stainless steel

with a No. 2B mill finish, before

Electropolishing. Note that theetched boundaries between the

grains are only partially sealed,

resulting in a network of sub-

surface crevices. Contaminants

which lodge in these crevices areprotected from contact with

cleaning agents. As a result,

extensive migration of trapped

contaminants onto the cleaned

surface can easily occur.

Before Electropolishing

This photograph represents the

same No. 2B surface afterElectropolishing. Featureless

under high magnification, the

surface has the desired non-contaminating, non-particulating,

non-outgassing and non-stick

properties.

After Electropolishing

Comparative photomicroscopy of mill finishes continues on next page.


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Comparative Photomicroscopy of Two StandardMill Finishes Before and After Electropolishing (contd)

This photograph represents a 304

stainless steel surface with a

mechanically polished No. 4 mill

finish before Electropolishing.Note the deep grooves, cavities,

torn metal and microscopic

imperfections that entrap

contaminants.



the same No. 4 finish after

Electropolishing. When viewed bythe naked eye, the surface may still

show some of the abrasive

produced topography, but will be

microscopically featureless under

high magnification. This surface isnon-contaminating, non-

particulating, non-outgassing and

non-sticking.



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Comparative Photomicroscopy of Stainless Steel TubingSurfaces

This 1000X photomicrograph

shows the surface of seamless

316L stainless steel tubing as

received from the mill. Note theelongated grain structure,

cavities and surface distortions

resulting from the mandrel used

in the tubings manufacture.


This 1000X photomicrograph

shows the same surface after

electropolishing. Note thedisappearance of grainboundaries and other

imperfections that were in the

pre-electropolished surface.


Comparative photomicroscopy of stainless steel tubing before and after

Electropolishing continues on next page


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Comparative Photomicroscopy of Stainless Steel TubingSurfaces (contd.)

This photograph shows the same

pre-electropolished surface viewed

at 3500X. Note the large fissure

that is typical of the larger surface.Contaminants will inevitably lodgein these crevices making the

process of maintaining purity of

surface virtually impossible.


This 3500X photomicrographrepresents the surface of the same

tubing sample afterelectropolishing. Note the absence

of the fissure of the pre-

electropolished surface shown

above.



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DELSTAR: THE QUALITY LEADER________________________________________________________________________

Delstar Metal Finishing, Inc. is capable of electropolishing all components

mentioned in this User's Guide to Electropolishing, including tanks, vessels,heat exchangers, blenders, pipe, tubing, clean room equipment, foodprocessing equipment, medical items, as well as parts for many otherindustrial applications.

Delstar operates a very modern, state-of-the-art plant facility thatencompasses 30,000 square feet of production space. In addition,Delstars Mobile Electropolishing Teams (METs) can process large vesselson-site and, in some cases, actually perform the process from within thevessel.

Delstar's technicians and executives have worked on nuclear projects(Louisiana Power and Light's Waterford III Nuclear Power Station,Westinghouse Savannah River Company, Peachbottom Nuclear Station)and process reactor vessels and heat exchangers for BASF, Shell, Hymont,Georgia-Pacific, Tenneco, Union Carbide, Shamrock, Dow Chemical,Mobil, B.F. Goodrich and many others.

Delstar is the electropolisher for major industry leaders such as Intel,Motorola, Applied Materials, Lockheed Martin, NASA, Texas Instruments,General Electric and Merck Pharmaceuticals. Other customers include

the U.S. Navy Special Warfare Center, Los Alamos National Laboratories,and the U.S. Army.

Delstar Metal Finishing, Inc. is a diversified Texas-based corporationoperating in markets worldwide. In addition to electropolishing, Delstaroffers a full range of precision mechanical polishing, passivating, pickling,chemical cleaning, corrosion and wear resistant coatings.

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