Electro-Polishing

Presented By : Naman Soni (12BME051) Nimesh Suthar (12BME052) Naishal Thakkar

(12BME054) Aditya Vaghasia

(12BME055)

Contents

Introduction

Principle

Processes

Equipments

Process parameters

Advantages

Limitations

Commercially Electropolished metals

Properties of Electrolytes

Examples of Electrolytes

Applications

Introduction

• Electropolishing is a chemical surface finishing technique by which metal is electrolytically removed ion by ion from the surface of the metal object.

PrincipleElectropolishing removes metal from a workpiece by

passing an electric current while the workpiece is submerged in an electrolyte of a specific composition.

In an electropolishing system, the workpiece itself is eroded, adding metal ions to the solution.

As shown in figure, the metal part is charged positive (anodic) and immersed in the chemical bath. A series of lead, copper or stainless steel cathode plates are lowered into the bath to the –ve side of the power source. When current is applied, the electrolyte acts as a conductor (tool) to allow metal ions to be removed.

PrincipleWhile the ions are drawn towards the cathode, the

majority of the dissolved metals remain in solution. Some ions are deposited in sludge form on the cathodes which require regular cleaning for efficiency.

In the course of electropolishing, burrs and other projections become areas of very high current density and are preferentially eroded.

The process is essentially the reverse of electroplating.

Principle

Processes

Alkaline Soak CleanThe purpose of the alkaline soak cleaner is to remove

any oil, grease, shop dirt, fingerprints, or similar films left on the parts after manufacturing.

Surface contaminants present on the parts during electropolishing can lower the quality of the resulting finish, particularly for critical applications such as medical, pharmaceutical, and semi-conductor products.

Cold Water RinseRemove the chemical residue from the previous

operation by dilution.

As the alkaline cleaner solution is removed from the parts, the concentration of alkali in Station 2 gradually increases.

In order to minimize the effect of drag-out, fresh make-up water must be fed to Station 2 on a continuous or regular intermittent basis.

Acid DescaleThe purpose of the Acid Descale is to remove light

oxidation from the work and to neutralize the alkaline film left by the cleaner tank.

Surface oxidation present on the parts during electropolishing can affect the quality of the resulting finish, particularly for critical applications such as semiconductor, medical, and pharmaceutical products.

Oxide scales, such as welding scale, may also be removed by mechanical means. Sanding, grinding, and bead blasting of scaled or discolored areas may eliminate the need for acid descaling.

Cold Water RinseThe principles of operation for the acid descale rinse

are essentially the same as those for the alkaline cleaner rinse.

The main difference is that acidic solutions are generally much easier to remove by rinsing than alkaline residues, and lower flow rates and/or shorter rinsing times may often be used.

Electropolish

Applications 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 process

Significantly reduces a metal's tendency to corrode

Reduces friction between moving metal parts

Eliminates burrs from machined parts

Reduces surface stresses in formed metal parts

Lengthens metal life against sour gas

Companies who have used electropolished parts successfully in the above applications include Exxon, Shell, Union Carbide, Goodyear, Ethyl, Occidental, BASF, Allied, Dow Chemical, Mobay and others.

Piping & Tubing

Major beneficiaries of electropolished piping and tubing include the petrochemical, nuclear, pharmaceutical, semi-conductor and food and beverage industries.

Improves heat transfer efficiency in heat exchangers

Enhances flow characteristics of piping and tubing

Medical Applications

All hospital, medical and surgical equipment (scalpels, clamps, saws, bone and joint implants, prosthetic devices, etc) should be electropolished to facilitate cleaning and achieve high levels of non-contamination. All metal articles exposed to radiation and requiring regular decontamination are prime candidates for electropolishing.

Machined Parts

All screws, bolts, washers, valve stems and bodies, and other machined parts benefit from electropolishing. The process provides a number of advantages, including de-burring and stress relieving of the surface, easy clean up, a non-stick, non-contaminating, non-particulating finish, and a pleasing cosmetic appearance.

High purity chemical tanks

Nuclear

Electropolishing is used to polish surfaces located in radioactive environments, electropolishing reduces contamination pick-up. Any contamination located on or embedded in the surface can be removed by the electropolishing process.

A very effective application for electropolishing is the polishing of nuclear plant recirculation piping for stress-relief of internal surfaces that have already been mechanically polished.

Food & Beverage Processing

Electropolishing provides the smooth, easy cleaning and cosmetically pleasing surfaces demanded by this industry coupled with unmatched non-contamination and sanitary qualities. The process reduces oxidation and contamination of stainless steel components used in kitchen, dairy and automatic food processing equipment, and descales food and beverage containers.

Pharmaceutical processing

Inside surfaces of pharmaceutical mixers, dry product delivery systems, filters, strainers, vessels, dryers, choppers, cooling coils, platecoils, heat exchangers etc. are electropolished. For microbiology or other contamination problems, electropolishing offers ultimate solution for stainless steel pharmaceutical components.

Aerospace

Aircraft parts which are commonly electropolished include turbine blades, struts and other landing gear parts, skin panels and components, vacuum chambers, and various piping and tubing systems. In addition, a wide range of space vehicle parts is routinely electropolished, including entire satellite exterior surfaces.

EquipmentsMCP “Console” units usually consist of the Electropolishing Tank,

Rinse Tank etc. Console systems are the smallest, least expensive systems available, and are intended for small-scale production, process development,and laboratory applications.Console units are available in sizes ranging from 5-gallons to 45-gallons tank capacity.

MCP Standard Electropolishing System consisting of twelve processing stations,60-gallons nominal capacity, with power supply equipped for precision polishing,ventilating system, atmospheric evaporator, mist eliminator, and remote operator control panel.The line produces extremely high quality technical finishes for clean gas distribution systems.

Tank Sizes Available in “Standard” Series EquipmentMCP: Metal Coating Process Corporation

MODEL No. Of tanks

Tank Volume

Power Supply

(ampere)

Floor space PRICE

1060 10 60 gallons 300 100 $41760

10100 10 100 gallons 500 150 $53850

10150 10 150 gallons 1000 175 $64790

Photo shown at left is the air blower, located on a concrete pad outside the building ,part of ventilation system to control air quality.

This twelve-station line features a 1500-gallon electropolishing tank with a 6000-ampere power supply. The system is designed for handling large volumes of flat sheets for food manufacturing equipment.

ELECTROPOLISHING OF STAINLESS STEEL

Process parameters:

1)Effect of Varying Current Density and Temperature

2)Effect of Varying Stirring Speed

3)Effect of Varying Electrolyte Concentration

4)Effect of Varying Anode-Cathode Separation

With sufficient agitation, placing the electrodes closer together has been shown to decrease surface roughness, as the current becomes more spatially directed. At large separations current density is more uniform over the piece and therefore a smoother surface is achieved.

Relatively high surface roughness resulted by diluting the electrolytic solution with deionized water.

The mechanical stirring of the electrolyte by use of a hot plate with stirring capability, can reduce the incidence of pitting and decrease bubble tracks on the surface of the anode . By stirring the electrolyte, the evolved gas on the anode in the form of bubbles is flushed away by agitation before it can leave tracks on the surface. Which results into better finish.

Increasing the current density results in a smoother and more polished surface while sacrificing more material loss, due to an increase in the mass transport rate.

The best polishing and the most thickness reduction takes place at higher temperatures and higher current densities.

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 brilliance and reflectivity will be after the process is complete. High quality electropolishing should be free of the following flaws:

♦ Frosting♦ Streaks or Stains ♦ Erosion♦ Irregular Patterns ♦ Pitted Surfaces

SEM PHOTOMICROGRAPHS:The Proof of Quality Assured

Electropolishing

A Scanning Electron Microscope is an invaluable tool when evaluating surface finishes. SEM photomicrographs illustrate in graphic detail the differences between the surface qualities of mill and mechanically polished finishes and an electropolished surface.

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

S.S. Plate As Received Inadequate Electropolishing

Intermediate Quality Electropolishing Premium-Grade Electropolishing

Electropolishing vs Mechanical Polishing

Mechanical polishing

Mechanical grinding , polishing and buffing of stainless steel are typically used to produce attractive mirror-like results on consumer goods or decorative architectural.

However, process can induce stress in surface layer, which may cause their metallurgical properties to deteriorate and make them less suitable for particular demand.

Mechanical polished metal surface involves microscopic scratched, strains, metal debris and embedded abrasives. In contrast, an electropolished surface is completely featureless. It reveals original crystal structure of metal without the distortion produced by the cold working process that always accompanies mechanical finishing.

Limitations of ElectropolishingRough surface defects can not be removed;

Electropolishing multiphase alloys may cause roughening due to selective dissolution of different phases.

In addition, Heavy orange peel, mold-surface texture and rough scratches are not removed by a practical amount of electopolishing and thus requre an initial cutdown with abrasives.

Multiphase alloys in which one phase is relatively resistant to anodic dissolution usually are not well suited to an electropolishing treatment.

Because the chemicals and combinations of chemicals used in electropolishing are poisonous and many are highly flammable or potentially explosive ,only well trained personnel who are thoroughly familiar with chemical laboratory procedures should be permitted to handle or mix the chemicals ,or to operate the polishing baths.

A large number of electrolytes may be needed to polish the variety of metals encountered by a given laboratory.

Plastic or metal mounting materials may react with the electrolyte.

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 PropertiesLess friction and surface drag.

Electropolishing greatly reduces fouling, plugging,and scaling.

Surface retains the true grain structure and properties of the bulk metal.

Fatigue strength is not reduced.

Stress-relieving of the surface.

Better Corrosion Protection

Electropolishing yields maximum tarnish and corrosion resistance in many metals and alloys. Stainless steel contains metallic and non-metallic inclusions, which are unavoidable during manuafacture.

Mechanical polishing not only fails to remove inclusions but also tends to push them further into the surface and even increase them by further pick-up of abrasive materials. These inclusions eventually can become points of corrosion.

Ease Of Cleaning

Substantially reduces product contamination and adhesion due to the microscopic smoothness of an electropolished surface (much like a glass surface)

Decreases cleaning time. Electropolished surfaces can be effectively hydroblasted in less time and with less pressure.

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 nonmetallic inclusions.

Those metals having a high content of silicon, lead or sulfur are usually troublesome.

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

Commercially Electropolished Metals

Low and High Carbon Steels

Tool Steels

High Temperature Alloys (Molybdenum, Tungsten)

Aluminum

Titanium

Copper

Cupronickel

Inconel

Brass

Bronze

Leaded Steel (Low-Lead)

Nickel Silver

Beryllium

Vanadium

Tantalum

Beryllium Copper

Silver and Gold

Many of the above metals can only be electropolished in large production runs and in controlled environments.

This is due to the sometimes-costly set-up, tooling and special environmental and safety equipment requirements associated with many of these processes.

Characteristics of Electrolytes:

Act as a good solvent for the anode metal during electrolysis conditions.

Should not attack the anode metal when no current is flowing.

Simple to mix,stable and safe to handle .

Effectively functions at room temperature and not sensitive to room temperature.

Examples of Electrolytes

Electrolyte USE

800ml ethanol + 200ml perchloric acid Stainless steel , Aluminium

940ml ehanol + 6ml distilled water + 54ml perchloric acid Stainless steel , Thorium

760ml ethanol + 30ml distilled water + 190ml ether + 20ml perchloric acid

Aluminium , Aluminium – silicon alloys , Iron – silicon alloys

900ml acetic acid + 100 ml perchloric acid Zr , Ti , U , Superalloys

175ml distilled water + 825ml phosphoric acid Pure copper

Electropolishing various metals

Electrolyte composition

Metal Voltage (V)

9ml perchloric acid + 91ml butyl cellosolve

Steel , Iron & Alloys Aluminium & Alloys Beryllium & Alloys

35 – 4030 – 3543 – 46

54ml Phosphoric acid + 22ml Ethanol + 3ml Distilled water + 21ml Butyl cellosolve

Copper & it’s Alloys 4 – 6

11ml Perchloric acid + 65ml Methanol + 24ml Butyl cellosolve

Titanium Alloys 26 – 28

Referenceshttp://www.delstar.com/assets/pdf/epusersguide.pdf

http://www.ableelectropolishing.com

ASM metal reference book

WWW.Balco.co.uk

WWW.euro-inox.org

http://www.electropolish.com/

WWW.youtube.com

Research Paper :PULSE AND DC ELECTROPOLISHING OF STAINLESS STEEL FOR STENTS AND OTHER DEVICES by Anshuman Bhuyan, Brandon Gregory, Howard Lei, Seow Yuen Yee, Yogesh B. Gianchandani , Department of Electrical Engineering and Computer Science ,University of Michigan, Ann Arbor, MI 48109-2122, USA,year:

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