School of Materials Science & Engineering

Second Year Undergraduate Experiment

MS2081 Laboratory

Formal Report

For

Experiment EA1

Electrochemical Polishing

Name: Fairuz Nabilah Bnte Muzafar

Matric: U1321254C

Lab Group: GA3

Academic Year 2014-2015

Nanyang Technological University

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Table of Contents

1. OBJECTIVES .......................................................... 3

2. INTRODUCTION .................................................... 3

3. THEORY .................................................................. 3

3.1 Parameters Affecting Electropolishing .............. 5

4. EXPERIMENT ......................................................... 6

4.1 Equipment .......................................................... 6

4.2 Procedures .......................................................... 7

5. REPORT ................................................................... 8

5.1 Experimental Results .......................................... 8

5.2 Discussion .......................................................... 9

6. CONCLUSION ...................................................... 14

7. REFERENCES ....................................................... 14

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1. OBJECTIVES

The objectives of this experiment are to study the electrochemical reactions accuring in

an electrolytic call for electropolishing, to understand the principles of electrolytic

cells and to examine some of the factors that affect electropolishing.

2. INTRODUCTION

Electroplating is a process that smoothes metal surfaces anodically through the

chemical reaction resulting from the passage of DC current in the presence of an

electrolytic fluid which is a concentrated acid or alkaline solution. There are numerous

applications of electropolishing, both in the laboratory and in industries such as the

electronics, aerospace, automotive, and medical/surgical industries. Metals are

electrochemically polished for various purposes like to improve appearance and

reflectivity and to prepare metals for plating, anodising or coating. Many commercial

products made from different materials are electrochemically polished. These include

aluminium alloys, brass and stainless steels Generally, pure metals and simple solid

solution alloys polish well. The more heterogenous alloys are much less responsive to

electropolishing.

3. THEORY

Electropolishing is one of the numerous applications of electrochemistry. When DC

electricity is passed through a solution of an electrolyte, chemical reactions occur. This

is also known as electrolysis. A typical electrolysis apparatus, called an electrolytic

cell is shown in figure 1. The DC source acts as an 'electron pump', pulling electrons

away from the anode and pushing them through the external wiring onto the cathode.

When electricity starts to flow, chemical reactions begin to happen. At the anode,

metal is oxidised as electrongs are pulled away and dissolved into the solution. At the

same time, reduction reactions take place at the cathode.

Electropolishing is a procedure in which electrolysis is used to selectively dissolve a

metal surface in order to create a smooth surface and in some instances, a mirror finish.

It is achieved in an electrolytic cell where micro-elevations on a specimen surface are

removed by selective electrochemical dissolution (Figure 2). The specimen to be

polished is connected to the positive DC supply and becomes an anode of the

electrolytic cell (Figure 1). This process is often referred to as reverse plating process.

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Figure 1: Schematic illustration of a typical electrolytic cell

used for electroplating a copper specimen.

Figure 2: Illustrating the dissolution of micro-elevations by electropolishing

The key to achieve electropolishing is the selective dissolution of micro-elevations on

a specimen surface. The mechanism of the process can be briefly described as follows:

With the passage of DC current, two types of layers form over the specimen surface:

(i) a viscous liquid layer that is nearly saturated (or is supersaturated) with the

dissolution products; (ii) an anodically discharged gas, usually oxygen, layer. Neither

type of layer conforms closely to the micro-elevations of the specimen surface. That

is, the layers are effectively thinner over the micro-elevations and thicker at the micro-

depressions. As a result, resistance to the flow of electric current is less at micro-

elevations; hence, more current can flow there than in the micro depressions. This

situation is augmented by the relatively shorter diffusion paths for acceptor anions to

the elevations and by products of dissolution from the elevations. The result is more

rapid dissolution of elevations to cause micro-leveling of the surface. However, this

electropolishing mechanism is true only under certain conditions. If parameters such

as the voltage of the DC source and electrolyte composition are not selected properly,

the anodic dissolution mechanism could be different, resulting in etching or pitting on

the specimen surface. It is thus important to investigate the effects of electrolytic cell

parameters and to establish the optimum polishing conditions.

DC Source

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3.1 Parameters Affecting Electropolishing

Usually several attempts are required before the optimum polishing conditions are

established. Parameters that could affect the polishing are listed below:

1. Voltage / Current

It is important to adjust voltage / current in order to produce the current density

required to obtain polishing. Low voltage/current values would produce etching,

which normally occurs in the 2 - 4 volts range. Too high voltage/current values

produce pitting, gassing and heat. Voltage VS current density curves such as those

shown in Figure 3, can be used to aid the selection of optimum polishing voltage /

current.

2. Specimen pre-preparation

Before electropolishing, a surface should be manually abraded to a 600 grit finer

finish.

3. Polishing time

Use the shortest time needed to produce acceptable results. If the polishing time is

too short, the specimen may not be adequately polished and scratches may remain.

If the polishing time is too long, pitting may appear.

4. Electrolyte flow rate

Must be sufficiently adequate to cover the specimen surface without any

turbulence.

5. Electrolyte composition

The selection of electrolyte depends on specimen composition and thermal history.

A simple acidic solution is used in this experiment.

Figure 3: Typical voltage vs current density curves

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6. Temperature

Keep electrolyte temperature reasonably constant to maintain uniform results and

avoid unsafe conditions.

7. Mask size

Select the smallest aperture to provide the surface area required.

4. EXPERIMENT

In this experiment, we will determine how the voltage/current and polishing time will

affect the polishing results of the copper sample.

4.1 Equipment

A typical electropolishing system which includes the following components:

• Electrolyte tank

• D.C. power source

• Cathodes made from corrosion resistant materials such as stainless steel

• Specimen/Sample (workpiece)

• Electrolyte circulating system - pump stirrer

• Cooling coils and cables for connection to power source

The equipment shown in Figure 4 has the following additional working principles:

• Electronic touch-pad on/off, timing, voltage, polish/etch select and pump

controls

• Digital LED displays of time, voltage, current and electrolyte temperature,

current and pump speed.

• Electrolyte temperature monitoring to prevent operational hazards.

• Parameter memory to automatically reset operating conditions.

• Current overload shut-off and LED fault indicator.

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Using the equipment shown in Figure 4, an electrolytic cell is formed by pumping an

appropriate electrolyte through the stainless steel cathode tube to the copper specimen

surface (as anode).

Note: Always ON the electrolyte pump before start of experiment and always

OFF the pump before removing the Cu Sample from the white mask.

4.2 Procedures

1) Pre-preparation.

• Abrade specimen surfaces to a 600 grit or finer finish using SiC paper of

various grades.

2) Set up the electrolytic cell for electropolishing.

• Fill in the cell with appropriate electrolyte.

• Mount the specimen into position on the polishing cell.

• Connect the electrodes.

3) Determine the current/voltage relationship for the system over a range of

voltages (usually 3 – 12 volts). Plot voltage/current curve on the report sheet. It

can be seen that the curve falls into three natural regimes.

4) Try to establish the optimum polishing conditions. Parameters such as electrolyte

composition, temperature and mask size have already been selected for you.

You should investigate the effects of the following parameters on polishing

results:

a) Voltage: Choose a typical voltage from each of the three natural regimes in

the voltage/current curve and polish the specimens at each of these voltages.

Figure 4: Electropolishing equipment

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b) Polishing time: Usually 1 – 3 minutes.

If time allows, you may also try to investigate the effects of,

Electrolyte flow rate: Adjusting the pump speed can change the rate and

pattern of electrolyte flow.

Specimen pre-preparation: Usually 800 grit is adequate. Finer pre-preparation

is required in some cases.

Note: Only select and try a few conditions that interest your group the most. It is

unrealistic to perform a full investigation on the effects of various parameters in

a short experiment session.

5) Record all experimental phenomena observable during electropolishing.

6) Wash/Rinse each of the samples with water and use paper wiper to gently tap

dry. Examine each specimen under optical microscope. Select the best specimen

and record your observations.

5. REPORT

5.1 Experimental Results

Sample 1:

VOLTAGE/V CURRENT/A Remarks (Observations)

3 0.1 no visible observation

4 0.4 bubbles forming from coil

5 0.5 more bubbles forming & blue colour under copper

forming

6 0.8 more bubbles forming & blue colour under copper

forming

7 1.1 more bubbles forming, blue colour under copper

forming & electrolyte turning pale greenish-blue

8 1.8 more bubbles forming, blue colour under copper

forming & electrolyte turning pale greenish-blue

9 2.0 more bubbles forming, blue colour under copper

forming, electrolyte turning pale greenish-blue &

few bubbles coming out from the sample

10 2.4 more bubbles forming, blue colour under copper

forming, electrolyte turning pale greenish-blue &

few bubbles coming out from the sample

11 2.8 electrolyte turning more blue

12 3.9 electrolyte turning more blue

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All

Samples

Sample preparation before

electro-chemical polish

Manually grind the smooth side with 800 to 1000

grit sand paper

Sample Set Voltage Set Time Description

1 3V to 12V 1 min Record the current for each voltage.

To generate pitting surface.

2 3V 1 min To generate etching surface.

3 4V 1min 30s To generate polishing surface.

4 5V 1min 30s To generate polishing surface.

Rinse the sample with water after polished and use paper wiper to gently tap dry.

Observe the samples surface using the optical micrpscope.

Sketch the microstructure for the Polished, Etched and Pitting specimens.

Select from sample 3 to 4, which is the optimum polishing conditions.

5.2 Discussion

1. Plot the current – voltage curves:

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

3 4 5 6 7 8 9 10 11 12

Cu

rren

t (A

)

Voltage (V)

Current - Voltage Graph

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2. According to the microscopic observation, what is the best polishing

condition for your specimens? Sketch the microstructure of the polished,

etched and pitted specimens.

Hence, from the above pictures, it can be seen that the best polishing conditions

should be at 5V for 2 minutes as this will produce a better finished surface.

Polishing at 4V for 1min 30s

Polishing at 5V for 1min 30s

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Polished specimens:

Etched Specimens:

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Pitted specimens:

3. Draw a schematic diagram of the electrolytic cell you have used for

electropolishing. Label the anode, cathode and other components of the cell.

Indicate the flow directions of the electrons and ions in the cell.

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4. Describe all experimental phenomena you have observed during the

electropolishing process. Explain using chemical and electrochemical

reactions and principles.

The solution gradually turns blue as the process is prolonged. This is due to the

presence of Cu2+ in the electrolyte solution.

In the electrolytic cell used for electropolishing, the sample is a piece of Cu which is

placed as the anode. As the current is passed through the cell, the Cu anode undergo

oxidation reaction, producing Cu2+ ions which in turn dissolved into the electrolyte

solution giving the blue coloured solution.

The reaction is as followed:

Cu (s) Cu2+ (aq) + 2e-

3Cu2+(aq) + 2PO4

3-(aq) Cu3(PO4)2 (s)

As the anode undergoes oxidation producing 2 extra electrons, the system has to

balance out the charges in the solution. The H+ ions present in the electrolyte, the

phosphoric acid, will undergo reduction reaction to produce H2 gas. This reaction can

be observed during the process as presence of effervescence or bubbles formed on the

surface of the stainless steel used as cathode. In addition, slight amount of brown

deposits could be observed on the steel cathode due to reduction of Cu2+.

The reaction is as followed:

2H+ (aq) + 2e- H2(g)

Cu2+(aq) + 2e-

Cu (s)

Due to these reduction – oxidation (Redox) reaction, can we achieve polished surface.

As the rough surface of the copper will gradually be dissolved into the solution,

leaving behind a smooth surface if a correct condition is applied. However, on the case

where the condition applied is below the ideal condition (3V), it will produce an etched

surface. This is due to insufficient current that passed through the surface, resulting in

inability for redox reaction to occur effectively. On the other end of spectrum, when

the current applied is larger than that of the ideal condition (12V), the surface becomes

pitted. Hence, to achieve ideal polished surface, experiments need to be conducted and

from this experiment, we can observe that 5V is the most ideal condition for it.

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6. CONCLUSION

From this experiment, I have understood the method of using the electrochemical to

polish a surface and also the principles behind it, namely using Redox reaction to cause

dissolution of the micro-elevations on the surface in order to polish the surface.

Additionally, the experiment also teaches the consequences of applying non-ideal

condition to polish surface which produces either etched surface or pitted surface. This

experiment has successfully demonstrated the use of electropolishing to produce

polished surfaces.

7. REFERENCES

James E. Brady, Joel W. Russell and John R. Holum, Chemistry: the study of

matter and its changes, 3rd edition, John Woley & Sons, Inc. (2000)

Raymond Chang, Chemistry, 7th edition, McGraw-Hill Company (2002)

C. H. Hamann, A. Hamnett and W. Vielstich, Electrochemistry, Wiley-VCH,

1998

Metals handbook / prepared under the direction of the ASM Handbook

Committee, Taylor Lyman (editor), Vol.2, American Society for Metals, 1976

http://en.wikipedia.org/wiki/Electrochemistry

http://en.wikipedia.org/wiki/Electropolishing