i CORROSION INHIBITION OF 6061 ALUMINIUM ALLOY IN ...umpir.ump.edu.my/id/eprint/1877/1/Mohd_Zulkarnain_B...klorida. Kaedah kehilangan berat dan pengukuran polarisasi potensiodinamik

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CORROSION INHIBITION OF 6061 ALUMINIUM ALLOY IN MARINE

ENVIRONMENTS BY MILK

MOHD ZULKARNAIN BIN ZULKIFLI

Thesis submitted in fulfilment of the requirement

for the award of the degree of

Bachelor of Mechanical Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

DECEMBER 2010

ii

UNIVERSITI MALAYSIA PAHANG

FACULTY OF MECHANICAL ENGINEERING

We certify that the project entitled “Corrosion Inhibition of 6061 Aluminium Alloy in

Marine Environments by Milk” is written by Mohd Zulkarnain Bin Zulkifli. We have

examined the final copy of this project and in our opinion; it is fully adequate in terms

of scope and quality for the award of the degree of Bachelor of Engineering. We

herewith recommend that it be accepted in partial fulfilment of the requirements for the

degree of Bachelor of Mechanical Engineering.

Luqman Hakim Bin Ahmad Shah

Examiner Signature

iii

SUPERVISOR’S DECLARATION

I hereby declare that I have checked this project and in my opinion this project is

adequate in terms of scope and quality for the award of the degree of Bachelor of

Mechanical Engineering.

Signature

Name of Supervisor : JULIAWATI BINTI ALIAS

Position : LECTURER

Date :

iv

STUDENT’S DECLARATION

I hereby declare that the work in this thesis is my own except for quotations and

summaries which have been duly acknowledged. The project has not been accepted for

any degree and is not concurrently submitted for award of other degree.

Signature

Name : MOHD ZULKARNAIN BIN ZULKIFLI

ID Number : MA07067

Date :

vi

ACKNOWLEDGEMENTS

Praise be to Allah S.W.T, the Most Gracious, the Most Merciful for all the

blessings and guidance upon me through my study. Thank you so much dear Allah for

giving me strength and answering my prayers.

This thesis would not have been possible without the guidance and the help of

several individuals who in one way or another contributed and extended their valuable

assistance in the preparation and completion of this research.

First and foremost, I would like to record my sincere gratitude to my supervisor,

Madam Juliawati Binti Alias for her supervision, advice and guidance from the very

early stage of this research as well as giving me extraordinary experiences throughout

the work. Above all and the most needed, she provides me unflinching encouragement

and support in various ways. I am indebted to her more than she knows. One simply

could not wish for a better or friendlier supervisor.

Many thanks go to all FKM laboratory instructor and technicians who are

willingly landing their hands in laboratory and experimental work and all these staff of

the Mechanical Engineering Department, UMP, who helped me in many ways and

made my stay in UMP pleasant and unforgettable. Special thanks from me also go to

my friends who are under same supervisor. I would like to acknowledge their comments

and suggestions which were crucial for the successful completion of this study.

Where would I be without my family? My parents deserve special mention for

their inseparable support and prayers. My father, Zulkifli Bin Adlan, in the first place is

the person who put the fundament by learning character, showing me the joy of

intellectual pursuit ever since I was a child. My mother, Jusnani Binti Hj Abdul Ghani,

is my special one who sincerely raised me with her caring and gentle love. Angah and

adik, thanks for being supportive and caring siblings.

Finally, I would like to thank everybody who was important to the successful

realization of this thesis, as well as expressing my apology that I couldn’t mention

personally one by one.

vii

ABSTRACT

6061 aluminium alloy is an important material with wide ranges of industrial

applications and marine technology. This study examines the use of milk for

improvement of corrosion resistance of 6061 aluminium alloy in sodium chloride.

Weight loss method and potentiodynamic polarization measurement were employed to

study the corrosion behavior of 6061 aluminium alloy in sodium chloride. The weight

loss method showed that the presence of milk significantly decreases the corrosion rates

of 6061 aluminium alloy 6061 in the test solutions. Pitting corrosion take places in all

the specimens during immersion test and the pitting can be seen through surface

analysis by using optical microscope 200x magnifications. With the presence of milk in

the sodium chloride, the pitting growth is smaller and lesser compare to no milk added.

The electrochemical measurements also showed the similar finding that the presence of

milk reduces the corrosion rates, and that corrosion current densities (icorr)

simultaneously increases the values of polarization resistance Rp. The inhibition

efficiencies increase with increasing of milk concentration. The nature of adsorption of

milk on the metal surface has also been examined. Scanning electron microscope (SEM)

and energy dispersive x-ray spectroscopy (EDS) confirmed the formation of thin film

on the metal surface, which reduces the overall corrosion reaction.

viii

ABSTRAK

Aloi aluminium 6061 merupakan bahan penting yang diaplikasikan secara meluas

dalam industri dan teknologi marin. Kajian ini meneliti penggunaan susu untuk

peningkatan ketahanan pengaratan aloi aluminium 6061 di dalam larutan natrium

klorida. Kaedah kehilangan berat dan pengukuran polarisasi potensiodinamik digunakan

untuk mempelajari perilaku pengaratan aloi aluminium 6061 di dalam natrium klorida.

Kaedah kehilangan berat menunjukkan bahawa kehadiran susu secara signifikan

mengurangkan kadar pengaratan aloi aluminium 6061 di dalam larutan uji. Pengaratan

jenis lubang mengambil tempat di semua spesimen selama ujian perendaman dan

lubang dapat dilihat melalui analisis permukaan dengan menggunakan mikroskop 200x

pembesaran. Dengan kehadiran susu dalam larutan natrium klorida, pertumbuhan

lubang adalah kurang dan saiz lubang lebih kecil berbanding dengan tanpa susu

ditambah ke dalam larutan uji. Pengukuran elektrokimia juga menunjukkan penemuan

yang sama bahawa kehadiran susu mengurangkan kadar pengaratan, arus kakisan (icorr),

secara bersamaan meningkatkan nilai rintangan polarisasi Rp. Kecekapan penghalangan

meningkat seiring dengan peningkatan kepekatan susu. Sifat serapan susu pada

permukaan logam juga telah diperiksa dengan menggunakan mikroskop pengimbas

electron (SEM) dan spektroskopi x-ray pemancar tenaga (EDS) mengesahkan

pembentukan lapisan tipis di permukaan logam, yang mengurangkan proses pengaratan

secara keseluruhan.

ix

TABLE OF CONTENTS

Page

EXAMINER’S DECLARATION ii

SUPERVISOR’S DECLARATION iii

STUDENT’S DECLARATION iv

DEDICATION v

ACKNOWLEDGEMENTS vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF TABLES xii

LIST OF FIGURES xiii

LIST OF SYMBOLS xvi

LIST OF ABBREVIATIONS xvii

CHAPTER 1 INTRODUCTION

1.1 Background of Study 1

1.2 Problem Statement 2

1.3 Objectives of Study 3

1.4 Scopes of Project 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.2 Forms of Corrosion 5

2.3 Marine Corrosion 8

2.4 Corrosion Control 9

2.5 Corrosion Inhibitor

2.5.1 Inhibitor Classification

9

11

2.6 Passivity 12

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CHAPTER 3 METHODOLOGY

3.1 Introduction

3.1.1 General Procedure for Experiment

21

22

3.2 Material Preparation

3.2.1 Specimen Preparation

3.2.2 Solution Preparation

3.2.2.1 Procedure for NaCl Solution Preparation

3.2.2.2 Procedure for Inhibitor Preparation

23

23

27

27

27

3.3 Weighing Before and After Testing

3.3.1 Before Immersion

3.3.2 After Immersion

28

28

28

3.4

3.5

3.6

3.7

3.8

3.9

Immersion Test in Sodium Chloride Solutions

Optical Microscope Analysis

SEM-EDS Analysis

Cleaning Specimen After Immersion

Corrosion Rate Analysis

Electrochemical Test

3.9.1 Electrochemical Cell Set-up

3.9.2 General Parameters

3.9.3 Inhibition Efficiency

29

30

31

32

33

34

34

36

37

2.7 Corrosion Rate Measurement 14

2.7.1 Tafel extrapolation 14

2.8

Aluminium and Its Alloy

2.8.1 Properties of Aluminium

2.8.2 Classification of Aluminium Alloy

2.8.3 6061 Aluminium Alloy

15

15

16

16

2.9 Potential of Milk as Inhibitor

2.9.1 Physical and Chemical Status of Milk

2.9.2 Milk Constituents

2.9.3 Milk Fat

2.9.4 Milk Carbohydrates

17

17

18

19

20

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CHAPTER 4 RESULT AND DISCUSSIONS

4.1

4.2

4.3

4.4

4.5

4.6

4.7

Introduction

Visual Inspection After Exposure of Specimens

Surface Analysis

Corrosion Rate Determination

Energy Dispersive X-Ray Spectroscopy (EDS)

Inhibition Mechanism

Potentiodynamic Test

38

38

40

44

46

48

50

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.0 Introduction 57

5.1 Conclusion 57

5.2 Recommendations 58

REFERENCES

59

APPENDICES 61

A Figure of Specimens for Immersion Test

Figure of Specimens for Electrochemical Test

61

61

Figure of Sodium Chloride as Test Solution

Figure of Sodium Milk used as Inhibitor

62

62

B Example Calculation for Corrosion Rate

Example Calculation for Inhibition Efficiency

63

69

C Gantt Chart /Project Schedule FYP 1

Gantt Chart /Project Schedule FYP 1

70

71

xii

LIST OF TABLES

Table No. Title Page

2.1 Typical composition of 6061 aluminium alloy 17

2.2 Form presence in milk 18

2.3 Composition of cow’s milk 18

3.1 Procedure for prepare inhibitor 28

3.2 Procedure for corrosion product removal 32

4.1 Corrosion rate result 44

4.2 Electrochemical parameters of 6061 aluminium alloy in 3.5%

NaCl solution at various concentration of milk

53

4.3 Value of inhibition efficiency at various concentration of milk 55

xiii

LIST OF FIGURES

Figure No. Title Page

2.1 Forms of corrosion 7

2.2 Pourbaix diagram for aluminium in aqueous environment 13

2.3 Tafel slope calculations 15

2.4 Molecular structure of β Glycol bond 17

2.5 Fat globules in milk 19

2.6 Structure of a lactose molecule 20

3.1 Experimental procedures 22

3.2 Dimension of specimen for immersion test 23

3.3 Dimension of specimen for electrochemical test 24

3.4 Specimen preparation process 25-26

3.5 Four decimal micro weighing scales 29

3.6 Immersion test of specimen 30

3.7 Optical microscope 30

3.8 SEM-EDS device 31

3.9 Corrosion products cleaning 33

3.10 Electrochemical cell interface with WPG 100 potentiostat

and computer

34

3.11

Electrochemical cell 36

4.1 Specimen after being immersed 39

4.2 Surface morphology of aluminium immersed in sodium

chloride without inhibitor

40


chloride with 20% concentration inhibitor

41

xiv



41



42



42



43

4.8 Relation between corrosion rate and inhibitor

concentration for 6061 aluminium alloy in NaCl for 28

days immersion period

45

4.9(a) SEM image obtained from 6061 aluminium alloy sample

after being immersed 28 days in 3.5% NaCl with the

presence of 20% concentration milk

46

4.9(b) EDS spectrum 2 in well away from pitting 46

4.9(c) EDS spectrum 5 acquired on the pitting 46

4.10(a)

Compositional features on the pitting 47

4.10(b) Compositional features away from pitting 47

4.11 Surface morphology of 6061 aluminium alloy after 28

days immersion in sodium chloride with the presence of

milk

48

4.12 Schematic diagrams of surface film 48

4.13 Tafel extrapolation plot obtained in 3.5% sodium chloride

for 6061 aluminium alloy

50


mixed with 20% inhibitor for 6061 aluminium alloy

50



51



51

4.17

Tafel extrapolation plot obtained in 3.5% sodium chloride


52

xv

4.18

Tafel extrapolation plot obtained in 3.5% sodium chloride


52

4.19 Polarization curves of 6061 aluminium alloy in 3.5%

NaCl solution at various concentrations of milk

53

4.20 Effect of milk concentration on corrosion rate 54

4.21 Effect of inhibitor concentration on the inhibition

efficiency

55

6.1 Figure of specimens for immersion test 61

6.2 Figure of specimens for electrochemical cell 61

6.3 Figure of sodium chloride as test solution 62

6.4 Figure of milk used as inhibitor 62

xvi

LIST OF SYMBOLS

A Area

Al Aluminium

Al3+

Aluminium dissolves to 3 electron

Al2O3 Aluminium oxide

Al(OH)3 Hydroxide

AlOOH Oxyhydroxide

ba Anodic Tafel slopes

bc Cathodic Tafel slopes

C Carbon

-COOH Carboxyl

d Diameter

e electron

Ecorr Corrosion potential

H2 Hydrogen gas

H+ Hydrogen ion

H2O Water

icorr Corrosion current density

K Corrosion constant

Si silicon

W Weight loss

& And

°C Celcius

xvii

LIST OF ABBREVIATIONS

ASTM American Standard Testing Method

EDS Energy Dispersive X-Ray Spectroscopy

FYP

NaCl

Final year project

Sodium Chloride

SEM Scanning Electron Microscope

UMP University Malaysia Pahang

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CHAPTER 1

INTRODUCTION

1.1 BACKGROUND OF STUDY

The corrosion of aluminium and its alloy is the subject of critical technological

importance due to the increasing industrial application of these materials. Aluminium

and its alloy represent an important category of materials due to their high technological

value and wide range of industrial applications, especially in aerospace, household

industries, automotive, transportation and marine technology. Mainly is because of their

good specific strength, excellent formability and corrosion resistance. Therefore the

understanding of the corrosion resistance and electrochemical behavior of aluminium

for the future industrial applications and development is vital.

Corrosion is defined as destruction or deterioration of a material because of its

chemical reaction with its environment. One example of aggressive environments is

seawater. Seawater systems are used by many industries such as shipping, offshore oil

and gas production, power plants and coastal industrial plants. Exposure of these

structures in marine environments will cause corrosion that finally leads to total

damage. Therefore, it is very important to study on corrosion prevention in this

environment.

With the fact of corrosion represent a tremendous economic loss and much can be

done to reduce it. There are many ways to reduce corrosion rate and one of the most

popular and acceptable practice is the use of inhibitors. Large numbers of organic

compound was studied and are being studied to investigate their corrosion inhibition

2

potential. All these studies reveal that organic compounds especially those with N, S,

and O showed significant inhibition efficiency.

An inhibitors is a substance that, when added in small concentrations to an

environment, decreases the corrosion rate. In a sense, an inhibitor can be considered as a

retarding catalyst. There are numerous inhibitor types and compositions. Most inhibitors

have been developed by empirical experimentation, and many inhibitors are proprietary

in nature and thus their composition is not disclosed. Inhibition is not completely

understood because of these reasons, but it is possible to classify inhibitors according to

their mechanism and composition.

The safety and environmental issues of corrosion inhibitors arisen in industries

has always been global concern. In recent days, many alternative eco-friendly corrosion

inhibitors have been developed. The growing needs for the corrosion inhibition

becomes increasingly necessary to delay or stop the attack of metal in aggressive

solution. Many efforts made to find suitable natural source to be used as corrosion

inhibitor in various corrosion media. This study considered this particular issue when

applying selected 6061 aluminium alloy to its application which would be suitable with

our natural environment for instance tropical seawater. Milk offers interesting

possibilities for corrosion inhibitor due to its safe use, low cost, availability and the

most important is the potential usages of milk discussed in this research are in line with

the recent trend of environment-friendly concept (Rosliza et. al., 2009).

1.2 PROBLEM STATEMENT

This organic inhibitor can be applied in some practical areas such as:

(i) The main uses of seawater are for cooling purpose, fire fighting, oil field

water injection and desalination plants. 6061 aluminium alloy can be used as

container for these applications and the organic inhibitor may add to the

container to retard the corrosion cause by the seawater.

(ii) Marine corrosion includes the immersion of components in seawater,

equipment and piping that use seawater or brackish water, and corrosion in

3

marine atmospheres. Exposure of components can be continuous or

intermittent. Ships, marinas, pipelines, offshore structures, desalination

plants, and heat exchangers are some examples of system that experience

marine corrosion.

The corrosion problems in these systems have been well studied over many years

despite several published information on materials behavior in seawater, failures still

occur. Therefore, more investigations need to carry out to obtain better understanding

on material corrosion behavior.

1.3 OBJECTIVES OF STUDY

The objectives of this study are:

(i) To study the corrosion behavior of 6061 aluminium alloy in NaCl.

(ii) To investigate the effect of variation concentration of milk as inhibitor, on

the corrosion rate of 6061 aluminium alloy in NaCl.

1.4 SCOPES OF PROJECT

The scope of this study includes:

(i) Preparation for specimen and inhibitor.

(ii) Exposure of specimen in sodium chloride (NaCl).

(iii) Cleaning process of corrosion product.

(iv) Weighing sample by digital weighing scale.

(v) Analysis corrosion rate by using weight loss method and electrochemical

technique.

(vi) Surface morphology examination using scanning electron microscope.

(vii) Compositional features characterization using energy dispersive x-ray

spectroscopic

4

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Corrosion is a very serious problem. There are several examples that show how

corrosion cost is very high when doing the maintenance. Corrosion of bridge is a major

problem as they age and require replacement, which costs billions. One large chemical

company have to spent a lot of budget for corrosion maintenance in its sulfuric acid

plants, the petroleum industry spends a million dollar per day to protect underground

pipelines and another spends on painting steel to prevent rusting by a marine

atmosphere. Corrosion engineering is the application of science and art to prevent or

control corrosion damage economically and safely. In solving corrosion problems, the

corrosion engineer must select the method that will maximize profit.

Corrosion is the chemical transformation of metal due to chemical reactions. The

most common form of corrosion is oxidation, where metal atoms combine with oxygen

atoms to form oxides. Iron rust is the most recognizable form of corrosion, and appears

when iron oxide forms on iron or steel components that are exposed to air or water,

however, virtually all metals and alloys are susceptible to corrosion. Technically,

corrosion can occur in other types of materials, such as ceramics or polymers, but the

process is either rare or different enough that the term ―corrosion‖ is generally not used.

5

2.2 FORMS OF CORROSION

Engineers often take their specific environment into effect and try to understand

the types of possible corrosion when designing metal components and structures. There

are several types of corrosion, depending on the metal, corrosive agent, geometry, and

environment: (Jones et. al., 1982)

(i) General Corrosion: Uniform corrosion is characterized by corrosive attack

proceeding evenly over the entire surface area, or a large fraction of the total

area. General thinning takes place until failure. On the basis of tonnage

wasted, this is the most important form of corrosion. However, uniform

corrosion is relatively easily measured and predicted, making disastrous

failures relatively rare. In many cases, it is objectionable only from an

appearance standpoint. As corrosion occurs uniformly over the entire surface

of the metal component, it can be practically controlled by cathodic

protection, use of coatings or paints, or simply by specifying a corrosion

allowance.

(ii) Pitting Corrosion: Pitting corrosion is a localized form of corrosion by which

cavities or holes are produced in the material. Pitting is considered to be

more dangerous than uniform corrosion damage because it is more difficult

to detect, predict and design against. Corrosion products often cover the pits.

A small, narrow pit with minimal overall metal loss can lead to the failure of

an entire engineering system. Pitting corrosion, which, for example, is

almost a common denominator of all types of localized corrosion attack.

Pitting is initiated by:

a. Localized chemical or mechanical damage to the protective oxide film;

water chemistry factors which can cause breakdown of a passive film are

acidity, low dissolved oxygen concentrations which tend to render a

protective oxide film less stable and high concentrations of chloride such

as in seawater.

b. Localized damage to, or poor application of, a protective coating.

http://corrosion-doctors.org/Forms-pitting/pitted-pot.htm

6

c. The presence of non-uniformities in the metal structure of the

component, for instance nonmetallic inclusions.

(iii) Galvanic Corrosion: Galvanic corrosion is refers to corrosion damage

induced when two dissimilar materials are coupled in a corrosive electrolyte.

When a galvanic couple forms, one of the metals in the couple becomes the

anode and corrodes faster than it would all by itself, while the other becomes

the cathode and corrodes slower than it would alone. For galvanic corrosion

to occur, three conditions must be present:

a. Electrochemically dissimilar metals must be present.

b. These metals must be in electrical contact.

c. The metals must be exposed to an electrolyte.

The relative nobility of a material can be predicted by measuring its

corrosion potential. The well known galvanic series lists the relative

nobility of certain materials in sea water. A small anode or cathode area

ratio is highly undesirable. In this case, the galvanic current is concentrated

onto a small anodic area. Rapid thickness loss of the dissolving anode tends

to occur under these conditions. Galvanic corrosion problems should be

solved by designing to avoid these problems in the first place.

(iv) Stress Corrosion: Stress corrosion cracking is the cracking induced from the

combined influence of tensile stress and a corrosive environment. The

impact of this type of corrosion on a material usually falls between dry

cracking and the fatigue threshold of that material. The required tensile

stresses may be in the form of directly applied stresses or in the form of

residual stresses.

(v) Crevice Corrosion: Crevice corrosion is a localized form of corrosion

usually associated with a stagnant solution on the micro-environmental level.

Such stagnant microenvironments tend to occur in crevices shielded areas

such as those formed under gaskets, washers, insulation material, and

clamps.

http://corrosion-doctors.org/Definitions/galvanic-corrosion.htm

http://corrosion-doctors.org/Definitions/galvanic-series.htm

http://corrosion-doctors.org/Forms-SCC/scc.htm#See

http://corrosion-doctors.org/Forms-crevice/CUI.htm

7

(vi) Intergranular Corrosion: The microstructure of metals and alloys is made up

of grains, separated by grain boundaries. Intergranular corrosion is localized

attack along the grain boundaries, or immediately adjacent to grain

boundaries, while the bulk of the grains remain largely unaffected. This form

of corrosion is usually associated with chemical segregation effects which

means impurities have a tendency to be enriched at grain boundaries or

specific phases precipitated on the grain boundaries. Corrosion then occurs

by preferential attack on the grain-boundary phase, or in a zone adjacent to it

that has lost an element necessary for adequate corrosion resistance. In any

case the mechanical properties of the structure will be seriously affected.

Figure 2.1 Forms of corrosion

Source: Roberge (1999)

http://corrosion-doctors.org/MatSelect/corrmetals.htm

http://corrosion-doctors.org/MatSelect/corralloys.htm

8

2.3 MARINE CORROSION

The corrosion of metals and alloys in chlorinated seawater has long presented

challenges for those responsible for materials selection and has been much studied.

Marine corrosion is of particular interest to designers of ships and shoreline facilities

because most metals used in these structures are vulnerable to damage from seawater.

Maintenance cost for ships, offshore structures and other related equipment are

dependent on how marine corrosion issues and failures are managed.

In addition to the salt (NaCl) in seawater, there are other commonly occurring

constituents, dissolved gases, living organism, and various other materials found in

seawater. Rives, temperature, dissolved oxygen and pollutants are some examples of

issues that may affect the corrosion of a given component in seawater. Marine

atmospheric corrosion is generally considered to be one of the more aggressive

atmospheric corrosion environments. Some factors that affect corrosion rates in marine

atmosphere are humidity, wind, temperature, location, airborne contaminants and

biological organism. Alloy selection, metallic coatings, organic coatings (inhibitor) and

cathodic protection are commonly used methods for providing proper corrosion

protection to various components.

The choice of an appropriate material for seawater service is a difficult decision

that has to be mad by a designer prior to specification of the system. A number of alloys

have been successfully used in seawater services. Marine grade aluminium can form an

oxide on the surface that excludes contaminants and prevents corrosion. Marine grade

here mean 5000 or 6000 series aluminium alloys, such as 5058 and 6061. Aluminium

forms an oxide on the surface thus it will not corrode unless the oxide is damage or

washed away. In marine environments, the presence of aggressive anions which is

chloride leads to pit formations and film breakdown. Pitting is one of extreme localized

attack that results in holes in the metal. These holes may be small or large in diameter,

but most cases they are relatively small. Pitting is particularly vicious because it is a

localized and intense form of corrosion, and failures often occur with extreme

suddenness (Fontana, 1986).

i CORROSION INHIBITION OF 6061 ALUMINIUM ALLOY IN ...umpir.ump.edu.my/id/eprint/1877/1/Mohd_Zulkarnain_B...klorida. Kaedah kehilangan berat dan pengukuran polarisasi potensiodinamik

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