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STUDIES OF ZINC CORROSION INHIBITION USING Zr- MCM-41 INORGANIC MEMBRANE

BY

NORAINI BT MOHAMED NOOR

A dissertation submitted in fulfilment of the requirement for the degree of Master of Science (Materials Engineering)

Kuliyyah of Engineering

International Islamic University Malaysia

JUNE 2014

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ii

ABSTRACT

Zinc corrosion in alkaline potassium hydroxide electrolyte (KOH) (1 – 8 M) has been studied using three different methods, namely, weight loss method, Tafel extrapolation and open circuit potential (OCP)-time transient profile. In weight loss method, zinc corrosion rates are measured from various immersion times i.e. one until seven days. It is observed that zinc corrosion rates data obtained after an exposure time of 120 hours is the most optimum. The accuracy of Tafel extrapolation method relies on the accurate determination of the linear regions from the polarization branches. In order to eliminate the ambiguity in the linear extrapolation, this work develops a quantitative method based on piecewise linear regression analysis. In this method, the entire polarization data is divided into small sub-intervals and variation in the regression coefficient and linearity coefficient values are observed. Consistency in these values serves as a quantitative indicator of the linear Tafel regions. Using the Tafel extrapolation method, the trend of zinc corrosion in KOH electrolyte is found to be comparable to that obtained from the weight loss method, except in 8-M KOH. For KOH molarity of 1-5 M, the zinc corrosion rates are measured 1.6 to 1.8 times higher than the values obtained from the weight loss measurement. Comparing with the OCP-time transient profiles, a slightly different trend in the tendency of zinc to corrode is observed. The efficacy of zirconia as zinc corrosion inhibitor in KOH electrolyte is studied using the OCP relaxation profiles. Rather than employing the corrosion inhibitor either into the electrolyte or as one of the components in the electrode formulation, zirconia is incorporated into the structure of MCM-41 membrane separator. MCM-41 material consists of arrays of silica-based, hexagonal nano-channels. The corrosion inhibition activity of Zr-MCM-41 membrane is studied with varying zirconia content i.e. 3 wt.%, 7 wt.% and 11 wt. %. The efficiency of the zinc corrosion inhibition increases with higher zirconia wt. %, however, in KOH of 1 M and 7 M, higher amount of zirconia is required to supress the zinc corrosion.

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iii

ﺚﺤﺒﻟا ﺔﻣﺪﻘﻣ

رﺪﻴﻫ لﻮﻠﳏ ﰲ ﻚﻧﺰﻟا ﻞﻛﺂﺗ ) ﻦﻣ يﻮﻠﻘﻟا مﻮﻴﺳﺎﺗﻮﺒﻟا ﺪﻴﺴﻛو

- 1

ثﻼﺛ ﺔﻄﺳاﻮﺑ سرد ﺪﻗ (م 8

, نزﻮﻟا ناﺪﻘﻓ ﺔﻘﻳﺮﻃ ﻲﻫو قﺮﻃ .ﺔﺣﻮﺘﻔﳌا ﺔﻴﺋﺎﺑﺮﻬﻜﻟا ةﺮﺋاﺪﻟا ﺔﻘﻳﺮﻃ و ﻢﺘﻳ ،ةرﺎﺴﳋا ﺔﻘﻳﺮﻃ ﺪﺣاو يأ ﺔﻔﻠﺘﺨﳌا ﺮﻤﻐﻟا تاﺮﻣ ﻚﻧﺰﻟا ﻦﻣ ﻞﻛﺂﺘﻟا تﻻﺪﻌﻣ سﺎﻴﻗ ﱴﺣ Tafel ﺔﻘﻳﺮﻃ ﺔﻴﻄﺧ ﻖﻃﺎﻨﻤﻠﻟ ﻖﻴﻗد ﺪﻳﺪﲢ ﻰﻠﻋ ﺪﻤﺘﻌﻳ ءاﺮﻘﺘﺳﻻا ءﺎﻀﻘﻟا ﻞﺟأ ﻦﻣ .بﺎﻄﻘﺘﺳﻻا عوﺮﻓ ﻦﻣ

ﻲﻤﻜﻟا ﻞﻴﻠﺤﺘﻟا ﻰﻠﻋ ﺪﻤﺘﻌﺗ ﺔﻘﻳﺮﻃ رﻮﻄﻳ ﻞﻤﻌﻟا اﺬﻫ و ،ﺔﻴﻄﳋا ءاﺮﻘﺘﺳا ﰲ ضﻮﻤﻐﻟا ﻰﻠﻋ

piecewise ﰲ ﻞﻣﺎﻛ بﺎﻄﻘﺘﺳﻻا تﺎﻧﺎﻴﺒﻟا ﻢﻴﺴﻘﺗ ﻢﺘﻳ ، ﺔﻘﻳﺮﻄﻟا ﻩﺬﻫ ﰲ .راﺪﳓﻻا ﺔﻴﻄﺧ ﳋا ﻞﻣﺎﻌﻣ ﻢﻴﻗ ﻆﺣﻼﻳو راﺪﳓﻻا ﻞﻣﺎﻌﻣ ﰲ ﻦﻳﺎﺒﺘﻟا و ةﲑﻐﺻ ﺔﻴﻋﺮﻓ تاﱰﻓ ﰲ قﺎﺴﺗﻻا . ﻲﻄ

ﻖﻃﺎﻨﻤﻠﻟ ﻲﻤﻛ ﺮﺷﺆﻣ ﺔﺑﺎﺜﲟ ﻢﻴﻘﻟا ﻩﺬﻫ Tafel ءاﺮﻘﺘﺳﻻا ﺔﻘﻳﺮﻃ ماﺪﺨﺘﺳﺎﺑ .ﺔﻴﻄﳋا Tafel ، ﺎﻬﻴﻠﻋ لﻮﺼﳊا ﰎ ﱵﻟا ﻚﻠﺘﻟ ﺔﻠﺛﺎﳑ نﻮﻜﺘﻟ ءﺎﺑﺮﻬﻜﻟﺎﺑ ﻩﻮﻛ ﰲ ﻚﻧﺰﻟا ﻞﻛﺂﺗ ﻩﺎﲡا ﻰﻠﻋ رﻮﺜﻌﻟا ﰎ

ﰲ ﻻإ ،نزﻮﻟا ناﺪﻘﻓ ﺔﻘﻳﺮﻃ ﻦﻣ

8 -M KOH . 5 - 1 ﻦﻣ ﺔﻴﻟﻮﳌا ﻩﻮﻛ ل M سﺎﻴﻗ ﻢﺘﻳ ، ﻚﻧﺰﻟا ﻞﻛﺂﺘﻟا تﻻﺪﻌﻣ

1 . 6

ﱃإ سﺎﻴﻗ ﻦﻣ ﺎﻬﻴﻠﻋ لﻮﺼﳊا ﰎ ﱵﻟا ﻢﻴﻘﻟا ﻦﻣ ﻰﻠﻋأ ةﺮﻣ 1.8

ﻊﻣ ﺔﻧرﺎﻘﻣ .نزﻮﻟا ناﺪﻘﻓ OCP ﻩﺎﲡا ﰲ ﻼﻴﻠﻗ ﻒﻠﺘﳐ ﻩﺎﲡا دﻮﺟو ﻆﺣﻮﻟ ،ةﺮﺑﺎﻋ تﺎﶈ ﺖﻗو ءﺎﺑﺮﻬﻜﻟﺎﺑ ﻩﻮﻛ ﰲ ﻚﻧﺰﻟا ﻞﻛﺂﺘﻠﻟ ﻊﻧﺎﳌا ﺎﻤﻛ ﺎﻴﻧﻮﻛرز ﺔﻴﻟﺎﻌﻓ ﺔﺳارد ﻢﺘﻳ . ﻞﻛﺂﺗ ﱃإ ﻚﻧﺰﻟا

ﻼﻣ ماﺪﺨﺘﺳﺎﺑ

ءﺎﺧﱰﺳﻻا ﺢﻣ OCP . ﰲ وأ ءﺎﺑﺮﻬﻜﻟﺎﺑ ﺎﻣإ ﻞﻛﺂﺘﻠﻟ ﻊﻧﺎﳌا ماﺪﺨﺘﺳا ﻦﻣ ﻻﺪﺑ ﻞﻜﻴﻫ ﰲ ﺎﻴﻧﻮﻛرز جرﺪﻣ ﻮﻫو ، ﺐﻄﻘﻟا ﺔﻏﺎﻴﺻ ﰲ تﺎﻧﻮﻜﳌا ﻦﻣ ةﺪﺣاو MCM -41 ﻞﺻﺎﻓ نﻮﻜﺘﻳ .ءﺎﺸﻐﻟا MCM -41 ﻮﻧﺎﻧ ﺔﻴﺳاﺪﺳ ، ﺎﻜﻴﻠﻴﺴﻟا ﻰﻠﻋ ﻢﺋﺎﻘﻟا ﻒﺋﺎﻔﺻ ﻦﻣ ةدﺎﻣ ﻞﻛﺂﺘﻟا ﻂﻴﺒﺜﺗ طﺎﺸﻨﻟا ﺔﺳارد ﻢﺘﻳ .تاﻮﻨﻘﻟا Zr- MCM -41 ىﻮﺘﶈا فﻼﺘﺧا ﻊﻣ ءﺎﺸﻐﻟا يأ ﺎﻴﻧﻮﻛرز 3

٪ ، 7

٪ و 11 .

٪ ﺎﻴﻧﻮﻛرز عﺎﻔﺗرا ﻊﻣ ﺪﻳﺰﻳ ﻚﻧﺰﻟا ﻞﻛﺂﺗ ﻂﻴﺒﺜﺗ ةءﺎﻔﻛ .

.نزﻮﻟﺎﺑ

٪ ﻦﻣ ﻩﻮﻛ ﰲ ﻦﻜﻟو ،

1 M و 7 M ﻊﻤﻘﺗ ل ﺎﻴﻧﻮﻛرز ﻦﻣ ﻰﻠﻋأ ﻎﻠﺒﻣ بﻮﻠﻄﻣ ،

ﻚﻧﺰﻟا ﻞﻛﺂﺗ.

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APPROVAL PAGE

I certify that I have supervised and read this study and that in my opinion; it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a thesis for the degree of Master of Science (Materials Engineering).

………

Raihan Othman

Supervisor

………

Mohd Hanafi Ani Co-Supervisor

I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a thesis for the degree of Master of Science (Materials Engineering).

………

Suryanto

Internal Examiner

………

Ahmad Azmin Mohamad External Examiner

This thesis was submitted to the Department of Materials and Manufacturing Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science (Materials Engineering).

………

Mohammad Yeakub Ali

Head, Department of Materials and Manufacturing Engineering This thesis was submitted to the Kulliyyah of Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science (Materials Engineering).

……….

Md. Noor Salleh

Dean, Kulliyyah of Engineering

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v

DECLARATION

I hereby declare that this thesis is the result of my own investigations, except where otherwise stated. I also declare that it has not been previously or concurrently submitted as a whole for any other degrees at IIUM or other institutions.

Noraini binti Mohamed Noor

Signature……….. Date………...

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vi

INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA

DECLARATION OF COPYRIGHT AND AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH

Copyright © 2014 by International Islamic University Malaysia.

All rights reserved.

STUDIES ON ZINC CORROSION INHIBITION USING Zr-MCM-41 INORGANIC MEMBRANE

I hereby affirm that The International Islamic University Malaysia (IIUM) holds all rights in the copyright of this Work and henceforth any reproduction or use in any forms or by means whatsoever is prohibited without the written consent of IIUM. No part of this unpublished research may be produced, stored in a retrieval system, or transmitted, in any form or by means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder.

Affirmed by Noraini binti Mohamed Noor

……… ………

Signature Date

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vii

ACKNOWLEDGEMENTS

In the name of Allah, the Most Gracious and the Most Merciful. First and foremost, all praises to Allah for the strengths and His blessing in completing this thesis.

First of all, I would like to express my gratitude and deepest thanks to my supervisor, Dr. Raihan Othman, who guided me through my master. Dr Raihan prompted me into developing bold arguments and helped me improve my stream of thought. His talent to teach, patience, experience and scientific excellence make him an irreplaceable person to work with.

I would also like to thank my Co supervisor Dr Mohd. Hanafi Ani for the valuable advice and succinct comments that he provided me when needed, for which I am indebted.

One of the most important factors influencing your mood during the masters work is probably the people that you work with in the lab. They can “make your day”

just by smiling to you and being kind. Therefore, I would also like to express my thanks to the PhD and master students in the Energy Laboratory; Bro Shahrul, Sis Akmal, Sis Norliza, Sis Sharifah, Sis Aimi, and Sis Hanisah. My research would not have been possible without their helps. Not to forget, my acknowledgement also goes to all lab technicians who have guided me to do the experiments and allowed me to use the equipments.

Special thanks to the Department of Materials Engineering which has provided the support and equipment needed to complete my thesis. I would also like to thank the Ministry of Higher Education (MOHE) and International Islamic University Malaysia (IIUM) for the financial support throughout my studies.

Finally, I would like to thank my husband and my family. They were always there supporting and encouraging me with their best wishes. They were cheering me up and stood by me through the good times and bad. Thanks to them for being supportive and listening to me all these years.

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TABLE OF CONTENTS

Abstract……… ii

Abstract in Arabic……… iii

Approval Page………. iv

Declaration Page……….. vi

Copyright Page……… vii

Acknowledgements ……… viii

List of Tables ...……….. xi

List of Figures...……….. xii

List of Abbreviations ……….. xiv

CHAPTER 1: INTRODUCTION... 1

1.1 Overview... 1

1.2 Problem Statement and Its Significance... 2

1.3 Research Objectives... 3

1.4 Research Methodology... 3

1.5 Scope of Research... 4

1.6 Thesis Organization... 5

CHAPTER 2: LITERATURE REVIEW... 6

2.1 Introduction... 6

2.2 Basic concept of corrosion... 6

2.3 Corrosion measurements... 8

2.3.1 Polarization curves... 8

2.3.1.1 Tafel extrapolation method... 9

2.3.1.2 Linear polarization method ... 13

2.3.2 Open circuit potential decay... 14

2.3.3 AC impedance measurement... 16

2.3.4 Weight loss measurement... 19

2.4 Zinc as anode in batteries...………... 20

2.5 Properties of zinc in alkaline electrolyte………... 22

2.6 Zinc corrosion inhibition... 28

2.7 MCM-41 membrane synthesis and its potential to host corrosion inhibitor additives... 31

2.8 Summary………...……... 34

CHAPTER 3: MATERIALS AND EXPERIMENTAL METHODS... 35

3.1 Introduction... 35

3.2 Zinc corrosion in KOH elctrolyte... 35

3.2.1 Weight loss method... 37

3.2.2 Tafel extrapolation metohd... 39

3.2.3 Open circuit potential method... 40

3.3 Zinc corrosion inhibition using Zr-MCM-41 membrane... 41

3.3.1 Zr-MCM-41 membrane preparation ... 42

3.3.2 Physical characterization... 42

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3.4 Flowchart ………..….. 43

3.5 Summary...44

CHAPTER 4: RESULTS AND DISCUSSION... 45

4.1 Introduction... 45

4.2 Improvement of Tafel analysis... 45

4.3 Zinc corrosion behaviour in KOH electrolyte... 51

4.3.1 Weight loss method... 51

4.3.2 Tafel polarization method... 56

4.3.3 Open circuit potential method... 61

4.4 Zinc corrosion inhibition using Zr-MCM-41 inorganic membrane...… 66

4.4.1 Zr-MCM-41 characterization ... 68

4.4.2 OCP variation of Zr-MCM-41coated zinc... 70

4.5 Summary... 75

CHAPTER 5: CONCLUSION AND RECOMMENDATIONS FOR FUTURE WORK... 76

5.1 Conclusion... 76

5.2 Recommendation... 77

REFERENCES…... 78

PUBLICATION... 83

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x

LIST OF TABLES

Table No. Page No.

2.1 The criteria for selection of the materials for construction in 20 industries are based on corrosion rates

2.2 Rate of dissolution of zinc in g/m3.hour in alkalis solution at 200C 23 2.3 Parameters derived from potentiodynamic polarization curves 28

(10mV/s) for the corrosion of zinc in pure KOH

2.4 Summary of various inhibitors used 29

3.1 Conversion between Current, Mass Loss, and Penetration 38 Rates for all Metals

4.1 Weight loss data of zinc specimen under exposure to KOH 52 electrolyte under varying duration and KOH concentration.

4.2 Zinc corrosion data obtained from the Tafel linear polarization 59

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xi

LIST OF FIGURES

Figure No. Page No.

2.1 Schematic experimental polarization curves (solid curves) 10 assuming Tafel behavior for the individual oxidation and

cathodic-reactant reduction polarization curves (dashed curves) 2.2 Schematic representation of a linear polarisation diagram 13 2.3 Open circuit potential variation with time for (a) Sn/Cu 15

(b) Sn-9Zn/Cu and (c) Zn/Cu

2.4 Equivalent electronic circuit for a simple electrochemical cell 17

2.5 Nyquist plot 19

2.6 Rate of dissolution of zinc as a function of the concentration 23 of solutions of NaOH, KOH, and LiOH at 200C

2.7 Corrosion rate of zinc in solutions of potassium hydroxide 25 at various rates of mixing

2.8 Rate of hydrogen evolution from nonamalgamated zinc at 25°C 26 2.9 Rate of hydrogen evolution from nonamalgamoted zinc us 26

a function of KOH concentration and temperature

2.10 Variations of corrosion potential of zinc in pure KOH electrolyte 27

2.11 Formation mechanisms of MCM-41 material 33

3.1 Schematic diagram of zinc mounted on acrylic resin 36 3.2 Schematic diagram of weight loss method experiment 37 3.3 Schematic diagram of the arrangement for potentiodynamic 39

Polarization/Tafel Polarization

3.4 Schematic diagram of open circuit potential measurement 40

3.5 Schematic diagram of dip coating process 42

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xii

4.1 Bar chart distribution for m and R2 values across the entire 49 polarization curve of zinc in 1 M KOH.

4.2 Anodic curve of zinc in 1M KOH 50

4.3 Cathodic curve of zinc in 1M KOH 50

4.4 Variation of zinc weight loss with time in various 52 concentrations of KOH electrolyte

4.5 Bar chart of corrosion rate of zinc in various molarities 54 4.6 Bar chart of corrosion rate of zinc in various molarities 55

plotted in different day

4.7 Piecewise linear region analysis of zinc corrosion polarization 57 curves

4.8 Extended piecewise linear region analysis of zinc corrosion 58 Polarization curve in 3M KOH

4.9 Variation of zinc corrosion rates against the KOH molarity 60 determined using the Tafel method

4.10 Variation in zinc corrosion rates against KOH molarity until 7 M 60 4.11 Open-circuit potential-time transient of zinc electrode in 61

KOH electrolyte

4.12 The similarity between the OCP relaxation profiles 64 4.13 A typical cyclic voltammogram of zinc in alkaline electrolyte 65 4.14 Passivation potential of zinc against KOH molarity 65 4.15 Corrosion inhibition activity due to zirconia in compressed 67

zinc electrode in 6-M KOH

4.16 XRD pattern of Zr-MCM-41 powder 68

4.17 EDX elemental mapping of 3% Zr-MCM-41 69

4.18 EDX elemental mapping of 7% Zr-MCM-41 69

4.19 EDX elemental mapping of 11% Zr-MCM-41 70

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4.20 OCP relaxation profiles of zinc electrode coated with 72 Zr-MCM-41 membrane

4.21 Bar chart of Zr-MCM-41 membrane on the passivation potential 73 with 3 wt. %

4.22 Bar chart of Zr-MCM-41 membrane on the passivation potential 73 with 7 wt. %

4.23 Bar chart of Zr-MCM-41 membrane on the passivation potential 74 with 11 wt. %

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xiv

LIST OF ABBREVIATIONS

ASTM American Society for Testing and Materials

cm centimetre

cm² centimetre squared

°C Degree Celsius

EDX Energy Dispersive X-Ray

Eeq Equilibrium potential

Ecorr Corrosion Potential

icorr Corrosion Current

Mw Molecular Weight of Water Vapour

mA miliampere

OCP Open Circuit Potential

SEM Scanning Electron Microscope

V Voltage

XRD X-Ray Diffraction

Zn Zinc

θ theta

η Overpotential

KOH Potassium Hydroxide

MCM Mobil Composition of Matter

TEOS Tetraethylorthosilicate

Zr Zirconium

ZrPr Zirconium Propoxide

CTAB Cethyltrimethylammonium Bromide

RM Corrosion Rate

SCE Standard Calomel Electrode

SHE Standard Hydrogen Electrode

AC Alternating Current

DC Direct Current

EIS Electrochemical Impedance Spectroscopy

HCl Hydrochloric Acid

H2O Water

EtOH Ethanol

WE Working Electrode

RE Reference Electrode

CE Counter Electrode

mmy-1 Milimetre peryear

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1

CHAPTER ONE INTRODUCTION

1.1 OVERVIEW

Corrosion refers to the breaking down of essential properties in a material due to chemical reactions with its surroundings. In simple words, this means a loss of electrons of metal reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion.

This is commonly known as rusting. This type of damage typically produces oxides and/or salts of the original metal. Corrosion can also refer to the degradation of ceramic materials as well as the discoloration and weakening of polymers by the sun's ultraviolet light.

In battery research, corrosion studies are one of the core areas. Battery components (anodic and cathodic active materials, separator, current collector and sealant) are subjected to prolong exposure to acidic or alkaline corrosive medium. The ability of battery components, in particular the metallic active materials, to resist corrosion would determine the capacity retention of the battery upon extended shelf life. Zinc the most widely used active material in primary battery industry (Ein-Eli, et.al, 2003). Its presence in the commercial market has been over a century. The main commercial zinc battery is alkaline zinc-manganese oxide cell. This is due the increase in high powered portable electronic gadgets in the consumer market. Mercury has traditionally been an effective zinc corrosion inhibitor in alkaline electrolyte (Snyder and Lander, 1965). However, its use has been phased out due to green technology initiatives.

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2

From the electrochemical aspect, corrosion of metallic substances can be mitigated to an acceptable level by incorporating corrosion inhibiting agents. These additives reduce the corrosion rates either by increasing the hydrogen over voltage or by forming a protective film on the metallic surface. In battery application, the corrosion inhibiting agent in typically added into the electrolyte or formed on the components of the electrode formulation. The present work investigates the zinc corrosion in alkaline KOH electrolyte and the corrosion inhibition using zirconia additive. Unlike the customary approaches described above, the additive is incorporated into structure of the membrane separator i.e. MCM-41.

MCM-41 belongs to a class of mesoporous materials which having hexagonally ordered, uniform nano-sized pore channels. It possesses excellent thermal and hydrothermal stability. The structure is basically composed of silica framework which is almost catalytically inactive (Kresge et.al, 2004)

1.2 PROBLEM STATEMENT AND ITS SIGNIFICANCE

Zinc is the most widely used anode or negative material in primary battery industries.

Besides its low cost and abundance, zinc is the most electropositive metal that is relatively stable in alkaline electrolyte. Nonetheless, corrosion of zinc is still one of the leading issues in battery research especially when highly concentrated potassium hydroxide (KOH) aqueous electrolyte (6 – 7 M) is normally used in an alkaline cell (Bagotsky, 2006). Corrosion of zinc predominantly reduces the shelf life of the batteries. Among the approach taken by battery technologist to mitigate the zinc corrosion is to incorporate corrosion inhibiting additives either in the electrolyte or in the zinc powder paste formulation. The approach proposed in the present work is different. Corrosion inhibiting additive shall be incorporated into the novel MCM-41

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inorganic membrane separator. MCM-41 is reported to be one of the best barriers against oxygen diffusion since it contains TEOS (tetraethoxysilane) (Zakaria, et.al., 2011). It is also relatively stable in the alkaline solutions at pH 9–12 when a small amount of Zirconia (Zr) is added (Nishiyama et.al, 2003)

1.3 RESEARCH OBJECTIVES

The main objectives of this work can be stated as follow:

i. To utilize various methods, namely, the weight loss method, Open Circuit Potential (OCP) profile and Tafel polarization method in quantifying the zinc corrosion rate in potassium hydroxide (KOH) aqueous electrolyte with concentrations from 1 M to 8 M.

ii. To incorporate zirconia as zinc corrosion inhibitor into the MCM-41 membrane structure

iii. To investigate the efficacy of Zr-MCM-41 membrane as zinc corrosion inhibitor in potassium hydroxide aqueous electrolyte.

1.4 RESEARCH METHODOLOGY

Zinc corrosion was investigated in KOH molarity of 1 M to 8 M. The amount of KOH electrolyte was fixed at 30 ml for all measurements. A thermostatic controlled water bath was used to maintain the temperature of the corrosion cell at 30°C. Zinc specimen was prepared from a 99.9% purity zinc foil, 250 µm thick.

The corrosion measurement was performed using three different methods i.e.

weight loss method, Tafel extrapolation and OCP profile. In weight loss method, the immersion time was varied from 24 hours to 168 hours (7 days). For Tafel polarization, measurement was conducted was performed in the scan range of 2 V to -

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2 V at a rate of 0.1 V/s using an AUTOLAB PGSTAT302N potentiostat/galvanostat (Eco Chemie B.V.). The open circuit potential (OCP) profiles of the zinc electrode were monitored for 400 minutes against a Hg/HgO reference electrode.

MCM-41 membrane was synthesized from the sol gel parent solution comprising of tetraethiylortosilicate (TEOS) as the silica construction material and cethyltrimethylammonium bromide (CTAB) cationic surfactant as the organic template for mesoporous structure. A controlled amount of zirconia dopant, 3 – 11 wt.

%, was incorporated into the parent solution using zirconium (IV) propoxide (ZrPr) suspension. The Zr-MCM-41 membrane was finally applied onto the zinc electrode using din coating technique.

The zinc corrosion inhibiting activity of Zr-MCM-41 membrane was measured from the Open Circuit Potential (OCP) profile.

1.5 SCOPE OF RESEARCH

This study mainly investigates the corrosion of zinc electrode in alkaline KOH medium or molarity 1 M to 8 M. Three most adopted methods are used i.e. weight loss, Tafel polarization and OCP profile, and the zinc corrosion data obtained are then compared and discussed. Furthermore, the role of zirconia in inhibiting zinc corrosion in KOH electrolyte is studied using the OCP profile. Zirconia as the corrosion inhibiting agent is incorporated into the silica ordered network of MCM-41 membrane. Zirconia dopant content is varied from 3 wt. %, 7 wt. % and 11 wt. %.

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5 1.6 RESEARCH ORGANIZATION

The present thesis is organized into five chapters. Chapter 1 gives an overview of the thesis which includes the problem statement and its significance, research objectives, Research methodology and scope of research.

Chapter 2 presents the literature review, providing details on the properties of zinc in potassium hydroxide (KOH), the usage of zinc as anode in batteries, the problems occurred in zinc anode batteries, as well as experimental approaches for measuring corrosion rate reported by various researchers.

Chapters 3 highlights the details on the sample preparation for the experimental work as well as characterization techniques which include the MCM-41 material preparation and characterizations, corrosion measurement using various techniques and its physical characterizations and finally the corrosion inhibition characterizations. In addition, details of the specimens’ preparation, the electrochemical measurement equipment used, and the electrochemical measurement techniques chosen for the experiments are documented in this section.

Chapter 4 covers the results and discussion of the experimental findings. The zinc corrosion rates obtained from the three methods are compared. The effectiveness of Zr-MCM-41 in inhibiting the zinc corrosion in KOH electrolyte is discussed.

Finally, chapter 5 concludes this work and presents some recommendations for further studies to improve this research.

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CHAPTER TWO LITERATURE REVIEW

2.1 INTRODUCTION

Zinc is a metallic chemical element with atomic symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 of the periodic table. The crystal structure of zinc is hexagonal as shown in Figure 2.1. The physical appearance of zinc is bluish pale gray in color. Its oxidation states are 1+ (rare) and 2+. Zinc is one of the most abundance elements in the earth’s crust but it is the fourth worldwide metal production and consumption after iron, aluminium and copper. Sphalerite, a zinc sulfide, is the most important zinc ore. Corrosion-resistant zinc plating of steel is the major application of zinc due to its position in electromotive series of metals which provides not only a protective layer to prevent direct contact between the coated steel and the environment but also a sacrificial protection if discontinuities in the coating occur (Zhang, 1996).

2. 2 BASIC CONCEPT OF CORROSION

Corrosion is commonly known as rust, an undesirable phenomena which destroys the lustre and beauty of objects and shortens their life. Corrosion is either chemical or electrochemical in nature. The distinction between chemical and electrochemical corrosion is based on the corrosion causing mechanism. Chemical and electrochemical corrosion are not mutually exclusive and can occur simultaneously. Basically, the main difference between chemical and electrochemical corrosion is the involvement

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of electrical current in electrochemical current which applied in electrochemical corrosion.

From the electrochemical aspect, corrosion can be defined as the dissolution of metal through oxidation process. Both oxidation and reduction process must occur simultaneously in electrochemical corrosion. However, corrosion will only occur in oxidation process where the metals will loss their electrons. The area where oxidation process takes place is known as the anode. At the anode, positively charged ions leave the solid surface and enter into the electrolyte. The ions then travel towards the location of the cathode through the conducting electrolyte. At the cathode, the opposite reaction takes place. The positive ions such as hydrogen ions in electrolyte consume the free electrons and reduced as hydrogen. These redox couple reactions can be summarized as follow:

MMn+ +ne (Eq.2.1)

2 2

2nH+ + nenH (Eq.2.2)

On the other hand, chemical corrosion is defined as the deterioration of materials by chemical interaction with their environment (Fontanna, 1986; Jones, 1996; McCafferty, 2010). The term chemical corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.

Chemically, corrosion can be fast or slow depending on the environment itself. For example, stainless steel pipe line under the sea may take few months to corrode compared to stainless steel railroad track exposed in oxygen may take longer time to corrode.

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8 2. 3 CORROSION MEASUREMENTS

Monitoring corrosion characteristics of a proposed or existing structure can lead to a proper selection of extended life materials, durable and protective coatings and corrosion control measures. Due to the nature of highly time-dependent of corrosion damage, modern corrosion monitoring technologies provide early warning of costly corrosion damage and also indicate where the damage is taking place.

Several chemical and electrochemical experimental techniques are used to characterize the corrosion behaviour of metals and their alloys. Some corrosion measurement techniques can be used on-line, constantly exposed to the process stream, while others provide off-line measurement, such as that determined in a laboratory analysis. Some techniques give a direct measure of metal loss or corrosion rate, while others are used to infer that a corrosive environment may exist. Corrosion of metal occurs through electrochemical reactions at the metal-solution interface involving charge (electrons) transfer through an electrolyte (concrete pore solution).

Therefore different electrochemical techniques can be employed to evaluate the state of a specimen from a corrosion point of view. Currently there are four established techniques to measure the corrosion of metal-based specimens i.e.

• Polarization curves

• Open Circuit Potential Decay

• AC Impedance Measurement

• Weight Loss Measurement

2.3.1 Polarization Curves

Due to the electrochemical nature of most corrosion processes, electrochemical methods are useful tools for studying corrosion (Badea et al., 2010; El-Sayed, et.al.,

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2011; Hegazy & Zaky, 2010; Shi, et.al., 2010; Walsh et.al., 2008). Besides, electrochemical techniques can also determine the kinetics of electrochemical processes as well as measure and control the oxidizing power of a specific environment.

In this method, external potential or current is applied to the three-electrode configuration cell which are; working electrode, counter/auxiliary electrode, and reference electrode. The metal sample which is the working electrode and cathodic current is supplied to it through an auxiliary electrode comprised of an inert material such as platinum. Reference electrode provides a fixed potential which does not change during the experiment. Reference electrode used is depending on the type of electrolyte.

Polarization method is a non-destructive test and offers quick results and high sensitivity. The main objective is to determine the corrosion current density under steady-state conditions from the polarization curve. There are two approaches in this technique i.e. the Tafel extrapolation which is mainly for laboratory measurements and the electrochemical linear polarization which is used for laboratory and in-service equipment.

2.3.1.1 Tafel Polarization Method

The Tafel polarization method for determining corrosion rate was first used by Friedmann (1938/2006) to verify the mixed-potential theory. This technique uses data obtained from cathodic or anodic polarization measurements, however, cathodic data are preferred, since these are easier to measure experimentally. In Figure 2.1 the total anodic and cathodic polarization curves corresponding to hydrogen evolution and metal dissolution are superimposed as dotted lines. It can be seen that at relatively

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high-applied current densities the applied current density and that corresponding to hydrogen evolution have become virtually identical. To determine the corrosion rate from such polarization measurements, the Tafel region is extrapolated to the corrosion potential, as shown.

Figure 2.1 Schematic experimental polarization curves (solid curves) assuming Tafel behavior for the individual oxidation and cathodic-reactant reduction polarization

curves (dashed curves) (Stansbury & Buchanan, 2000).

Tafel polarization method is closely related to The Butler-Volmer equation.

The Butler-Volmer equation is one of the most fundamental relationships in electrochemistry. It describes how the electrical current on an electrode depends on the electrode potential, considering that both a cathodic and an anodic reaction occur on the same electrode:

Rujukan

DOKUMEN BERKAITAN

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