Corrosion Behavior Of TiAIBN, TiN And AIN Coating Produced By Magnetron Sputtering On Magnesium Substrate (Using Hank's Solution).

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

CORROSION BEHAVIOR OF TiAlBN, TiN AND AlN COATING

PRODUCED BY MAGNETRON SPUTTERING ON

MAGNESIUM SUBSTRATE (USING HANK’S SOLUTION)

This report submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials) (Hons.)

by

KHAIRUN NAZIHAH BINTI KHALID B050810081

890503105312

FACULTY OF MANUFACTURING ENGINEERING 2012

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: Corrosion Behavior of TiAlBN, TiN and AlN Coating Produced by Magnetron Sputtering on Magnesium Substrate (Using Hank’ s Solution)

SESI PENGAJIAN: 2011/ 12 Semest er 2

Saya KHAIRUN NAZIHAH BINTI KHALID

mengaku membenarkan Laporan PSM ini disimpan di Perpust akaan Universit i Teknikal Mal aysia Mel aka (UTeM) dengan syarat -syarat kegunaan sepert i berikut : 1. Laporan PSM adal ah hak mil ik Universit i Teknikal Mal aysia Mel aka dan penul is. 2. Perpust akaan Universit i Teknikal Mal aysia Mel aka dibenarkan membuat sal inan

unt uk t uj uan pengaj ian sahaj a dengan izin penul is.

3. Perpust akaan dibenarkan membuat sal inan l aporan PSM ini sebagai bahan pert ukaran ant ara inst it usi pengaj ian t inggi.

4. **Sil a t andakan (√)

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi makl umat yang berdarj ah kesel amat an at au kepent ingan Mal aysiasebagaimana yang t ermakt ub dal am AKTA RAHSIA RASMI 1972)

(Mengandungi makl umat TERHAD yang t el ah dit ent ukan ol eh organisasi/ badan di mana penyel idikan dij al ankan)

Al amat Tet ap:

233 A, Tasek Chempedak 14120, Simpang Ampat

Seberang Prai Sel at an, P. Pinang

Tarikh: _________________________

Disahkan ol eh:

Cop Rasmi:

Tarikh: _______________________

** Jika Laporan PSM ini SULIT at au TERHAD, sil a l ampirkan surat daripada pihak berkuasa/ organisasi berkenaan dengan menyat akan sekal i sebab dan t empoh l aporan PSM ini perl u dikel askan sebagai SULIT at au TERHAD.

DECLARATION

I hereby, declared this report entitled “Corrosion Behavior of TiAlBN, TiN and AlN Coating Produced by Magnetron Sputtering on Magnesium Substrate (Using

Hank’s Solution)” is the results of my own research except as cited in references.

Signature :

Author’s Name : KHAIRUN NAZIHAH BINTI KHALID

APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the degree of Bachelor of Manufacturing Engineering (Engineering Materials) (Hons.). The member of the supervisory is as follow:

……… (Project Supervisor)

i

ABSTRAK

Matlamat projek ini adalah untuk membandingkan sifat kakisan/karat salutan (coating) TiN, AlN dan TiAlBN ke atas aloi magnesium (Mg). Selain itu, sifat-sifat umum salutan yang mempengaruhi kakisan di dalam Mg akan ditentukan. Dalam projek ini, ujian elektrokimia akan dilakukan untuk mendapatkan kesan kakisan oleh salutan ke atas Mg. Tambahan pula, ujian ini memberi persekitaran yang sama seperti persekitaran kakisan/karat. Cecair Hank (Hank’s solution) akan digunakan sebagai elektrolit dan kadar kakisan/karat akan dikira melalui ujian ini. Peralatan pencirian bahan seperti ‘Scanning Electron Microscopy’ (SEM) dan ‘Energy Dispersive X-ray Spectroscopy’ (EDX) diperlukan untuk melihat permukaan Mg yang terkakis/karat.

ii

ABSTRACT

The aim of this project is to compare the corrosion behavior of TiN, AlN and TiAlBN coating on magnesium (Mg) alloy. Besides, the general properties of coating which influnced the corrosion of Mg are going to be determined. Electrochemical test will be utilized in order to get the effect of corrosion as this test is giving a similar environment as a corrosion environment. Furthermore, Hank’s solution will be used as an electrolyte and the corrosion rate will be calculated through the test. Material characterization equipment such as Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) will be used in order to observe the morphology of corroded surface of Mg sample.

iii

DEDICATION

To my beloved parents and friends for all the time I spent away from them while preparing this final year project’s report.

iv

ACKNOWLEDGEMENT

Alhamdulillah, I have successfully completed my final year project within the time given. In that period of time, I have gained so much experiences, memories and knowledge through my project. A lot of experiences and unforgettable memories are captured along the way in finishing my project. May those experiences help me in future either in my working life or in my further studies.

I would like to give the greatest gratitude to my supervisor, Dr Zulkifli Bin Mohd Rosli who was always helping and guiding me in order to complete this project. Although he always busy as he is the Head Department of Engineering Materials Department, he tried to spend his time for me and my friends regarding to this final year project.

In addition, I would like to tribute to Zainab Mahamud and Kwan Wai Loon, master students under Dr Zulkifli who were always giving me some additional information about my project. Besides, they make my understanding into detail on my project as their project is on the same field as mine.

Lastly, million thanks to friends and others who were directly or indirectly in helping and guiding me during finishing this project. Thank you so much for your endless support.

v

TABLE OF CONTENT

Abstrak i

Abstract ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Tables viii

List of Figures ix

List Abbreviations and Symbols xii

CHAPTER 1: INTRODUCTION 1

1.1 Background 1

1.2 Problem Statement 2

1.3 Objectives 3

1.4 Scope 3

1.5 Project Organization 4

CHAPTER 2: LITERATURE REVIEW 5

2.1 Magnesium and Its Alloy 5

2.2 Corrosion 7

2.2.1 Corrosion of Magnesium Alloy 8

2.2.2 Corrosion of AZ91 Magnesium Alloy in Hank’s Solution 11

2.3 Coating 12

2.3.1 TiN Coating 13

2.3.2 AlN Coating 15

2.3.2 TiAlBN Coating 18

2.4 Physical Vapour Deposition (PVD) 19

2.4.1 Sputtering 21

2.4.1.1 Magnetron Sputtering 22

vi 2.4.1.3 Direct Current (DC) Sputtering 23

2.5 Hank’s Solution 24

2.5.1 Biomedical Applications 24

2.5.2 pH of Human Blood 25

2.6 Electrochemical Test Technique 26

2.6.1 Potentiodynamic Polarization Method 28 2.7 Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray 30

Spectroscopy (EDX

CHAPTER 3: METHODOLOGY 33

3.1 Introduction 33

3.2 Electrochemical Cell 34

3.3 Experimental 35

3.3.1 Coating Sample Preparation 35

3.3.1 Electrochemical Testing Preparation 37 3.3.3 Experimental Setup for Electrochemical Testing 39

3.3.4 Surface Characterization 40

CHAPTER 4: RESULT AND DISCUSSION 41

4.1 Morphology and Chemical Composition 41

4.1.1 Uncoated AZ91 Magnesium Alloy 41

4.1.2 TiAlBN Coated AZ91 Magnesium Alloy 44

4.1.3 AlN Coated AZ91 Magnesium Alloy 47

4.1.4 TiN Coated AZ91 Magnesium Alloy 50

4.2 Electrochemical Behaviour 54

4.2.1 Corrosion Behavior of Coated and Uncoated AZ91 Magnesium 55 Alloy

4.2.2 Penetration Rate, CR and Mass Loss Rate, MR 59

vii

CHAPTER 5: COCLUSION AND FUTURE WORK 62

5.1 Coating Corrosion Rate 62

5.2 Biomedical Applications 63

5.3 Duplex Coating in Future 63

viii

LIST OF TABLES

2.1 2.2 2.3

Classification of magnesium alloys Anodic index (v) of metal

Summary for the deposition and diffusion techniques 6

8 20 3.1 Coating Deposition (Reactive Sputtering) 36 4.1a

4.1b

Elemental Analysis of crack of uncoated AZ91 magnesium alloy by using EDX

Elemental analysis of corrosion products of uncoated

AZ91 magnesium alloy by using EDX 43 43 4.2a 4.2b 4.3a 4.3b 4.4a 4.4b 4.5 4.6 4.7

Elemental analysis of crack of TiAlBN coated AZ91 magnesium alloy by using EDX

Elemental analysis of corrosion products of TiAlBN coated AZ91 magnesium alloy by using EDX

Elemental analysis of crack of AlN coated AZ91 magnesium alloy by using EDX

Elemental analysis of corrosion products of AlN coated AZ91 magnesium alloy by using EDX

Elemental analysis of crack of TiN coated AZ91 magnesium alloy by using EDX

Elemental analysis on surface of TiN coated AZ91 magnesium alloy by using EDX

Total anodic current, corrosion potential and corrosion current of AZ91

Penetration rate, CR and mass loss rate, MR of coated and

uncoated AZ91 magnesium alloy pH change of AZ91 magnesium alloy in Hank’s Solution

46 46 49 49 52 52 58 60 61

ix

LIST OF FIGURES

2.1 Microstructure of AZ91 10

2.2 Evolution of corrosion potential (Ecorr) in electrochemical test

using saturated calomel electrode (SCE) with time for the TiNx

films

14

2.3 Surface morphologies and aspect of cross-section of TiNx films 14

2.4 The XRD patterns of AlN-coated magnesium alloys 16

2.5a 2.5b 2.6 2.7 2.9 2.10 2.11 2.12 2.13a 2.13b The SEM micrograph of fracture section of AlN coating deposited on a glass; magnification x 2000 The SEM micrograph of fracture section of AlN coating deposited on a glass; magnification x 5000 The SEM micrograph of coating defects Classification of surface treatment Chemical composition of Hank’s solution compared with the simulated body fluid SBF5 and SBF27, inorganic part of human blood plasma and NaCl solution Corrosion process showing the anodic and cathodic current components Classic Tafel analysis Scanning Electron Microscopy (SEM) Surface appearance of samples after corrosion in Hank’s solution before and product removal using SEM Surface appearance of samples after corrosion in Hank’s solution after product removal using SEM 16 16 17 19 24 28 29 30 31 31

3.1 Summary of experimental procedures 33

3.2 Experimental setup for electrochemical techniques in laboratory 34

3.3 Flow chart of coating sample preparation 36

x

3.5 AZ91 magnesium alloy mounted sample 38

3.6 Flow chart of experimental setup for electrochemical testing 39

3.7 Electrochemical testing in progress 40

4.1 SEM micrograph of uncoated AZ91 magnesium alloy in Hank’s 41

Solution (magnification x 3000) 4.2a EDX spectra measured the element composition for crack 42

of uncoated AZ91 magnesium alloy 4.2b EDX spectra measured the element composition for corrosion 42

products of uncoated AZ91 magnesium alloy 4.3 SEM micrograph of TiAlBN coated AZ91 magnesium alloy in 44

Hank’s Solution (magnification x 3000) 4.4a EDX spectra measured the element composition for crack 45

of TiAlBN coated AZ91 magnesium alloy 4.4b EDX spectra measured the element composition for corrosion 45

products of TiAlBN coated AZ91 magnesium alloy 4.5 SEM micrograph of AlN coated AZ91 magnesium alloy in 47

Hank’s Solution (magnification x 3000) 4.6a EDX spectra measured the element composition for crack 48

of AlN coated AZ91 magnesium alloy 4.6b EDX spectra measured the element composition for corrosion 48

products of AlN coated AZ91 magnesium alloy 4.7 SEM micrograph of TiN coated AZ91 magnesium alloy in 50

Hank’s Solution (magnification x 3000) 4.8a EDX spectra measured the element composition for crack 51

of TiN coated AZ91 magnesium alloy 4.8b EDX spectra measured the element composition for corrosion 51

xi

4.9a SEM micrograph at magnification x 500: Uncoated AZ91 53 Magnesium Alloy

4.9b SEM micrograph at magnification x 500: Coated AZ91 53 Magnesium Alloy

4.9c SEM micrograph at magnification x 500: AlN Coated AZ91 53 Magnesium Alloy

4.9d SEM micrograph at magnification x 500: TiN Coated AZ91 53 Magnesium Alloy

4.10 Potentiodynamic polarization of AZ91 magnesium alloy in Hank’s 55 Solution

4.11a Potentiodynamic polarization of TiAlBN coated and uncoated 57 AZ91 magnesium alloy in Hank’s Solution

4.11b Potentiodynamic polarization of TiN coated and uncoated 57 AZ91 magnesium alloy in Hank’s Solution

4.11c Potentiodynamic polarization of AlN coated and uncoated 57 AZ91 magnesium alloy in Hank’s Solution

xii

LIST OF ABBREVIATIONS, SYMBOLS AND

NOMENCLATURE

Ag-AgCl - Argentum-ArgentumChloride

Al - Aluminium

AlN - Aluminium nitride

ASTM - American Society for Testing Material BSS - Balanced Salt Solution

CVD - Chemical Vapor Deposition (CVD)

DC - Direct current

EDX - Energy Dispersive X-ray Spectroscopy HBSS - Hank’s Balanced Salt Solution

Mg - Magnesium

N - Nitrogen

PVD - Physical Vapor Deposition (PVD)

RF - Radio frequency

SBF - Simulated body fluid SCE - Saturated calomel electrode SEM - Scanning Electron Microscopy TEM - Transmission Electron Microscopy TiAlBN - Titanium aluminium boron nitride TiN - Titanium nitride

UTeM - Universiti Teknikal Malaysia Melaka XRD - X-ray Diffraction

Zn - Zinc

Ainitial - Exposed sample surface area, cm2 CR - Penetration Rate, mm/y

EW - Equivalent Weight, g

Ecorr - Corrosion Potential, V

xiii K1 - 3.27 x 10 -3_{, mm g/µA cm yr}

K2 - 8.954 x 10-3_{, g cm}2_{/µA m}2 _d

MR - Mass Loss Rate, g/m2_d

M - Mass, g

v - Volume, cm3

icorr - Corrosion current density, µA/cm2

- Density, g/cm3

α-phase - Primary α-phase in magnesium alloy β-phase -eutectic β-phase in magnesium alloy

1

1.1 Background

In the early of 1950s, magnesium alloys has been widely used in aerospace and automotive industries due to its low density and high strength-to-weight ratio. As the lightest structural metallic materials, their application has been extensively increase especially in vehicle construction and aeronautics field.

It is well known that magnesium and its alloys having the hexagonal crystal structure. Minchen and Weiheim (2000) as cited in Wei Guo K., (2010) claims that their formability at ambient temperature has been affected by the multi-slip planes causing the deforming of the grain boundaries due to stresses from localized slip. In addition, the formation of a thin surface film which is not dense indicates that the underlying metal cannot be completely covered. Thus, they are highly influenced to corrosion (Wei Guo, 2010).

Since the application of magnesium in industries is limited by the corrosion, it is important to improve this weakness by surface modification. In this paper, various coating has been done on magnesium AZ91 by using physical vapor deposition (PVD) methods. PVD is a process where the material is vaporized from solid or liquid form into atom or molecules form. The main categories of PVD methods are vacuum deposition, sputter deposition arc deposition and ion platting. In this project,

INTRODUCTION

2 the coatings will be deposited by using sputter deposition. There are three types of coating which will be focused in this work, TiN, AlN and TiAlBNcoating.

In order to determine the corrosion behavior for each coating, an electrochemical test will be done. The sample will be immersed in simulated body fluid; Hank’s solution and the corrosion rate and corrosion potential will be determined. Higher the corrosion potential, better the corrosion resistance for magnesium AZ91. In order to observe the surface morphology of the corroded sample, analytical techniques will be used. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) is used in order to see the surface morphology of corroded sample.

1.2 Problem Statement

Over the years, magnesium (Mg) and its alloy has been developed as widely used structural materials in many fields due to their mechanical properties such as low density, high strength-to-weight ratio (Wang et al., 2010). Besides, increased in ductility and castability also make them being an industrially accepted for the numerous application.

However, one major problem with the application of magnesium and its alloy is poor corrosion resistance even in moderately aggressive environments (Sudholz et al., 2009). Therefore, to increase the resistance of corrosion on magnesium, surface treatment is needed. Coating is one of the surface treatments which can be done on magnesium. Coating tends to improve surface properties of the substrate such as corrosion resistance, wear resistance, appearance and wettability.

In this works, three types of coating would be deposited on magnesium AZ91 which are TiN, AlN and TiAlBN coating. The most preferable coating due to several properties which increases the corrosion resistance on magnesium will be determined.

3

1.3 Objectives

The objectives of this project are:

• To compare the corrosion behavior of TiAlBN, TiN and AlN coating produced by magnetron sputtering on magnesium substrate.

• To understand the general properties of coating including surface morphology and chemical composition.

• To evaluate surface morphology of general corrosion on different coatings.

1.4 Scope

This project is to compare the corrosion behavior between TiAlBN, TiN and AlN coating. Only the general properties of corrosion in coating will be discussed. Test method which relates with this project paper, electrochemical testing will be used. The solution which will be used as an electrolyte in the electrochemical testing is a simulated body solution, Hank’s Solution. Besides, an observation will be done on surface topography of the sample in order to prove the form of corrosion.

4

1.5 Project Organization

This project was organized into five chapters:

Chapter 1 is the introduction to this report. This includes background, problem statement, objectives and scope of the project.

Chapter 2 is on literature review which describing the findings of the project, theoretical and methodology before the experimental works start.

Chapter 3 is on methodology which includes experimental procedures and apparatus setup required for the corrosion test. The micrographic observation procedure also will be included.

Chapter 4 includes the results and discussion of the experiment and the observation on the morphology.

Chapter 5 is would like to conclude on overall project. Recommendation for further study also will be discussed in this chapter.

5

2.1 Magnesium and its Alloy

Having a quarter weights of the steel and a third lighter than aluminium makes magnesium becomes the lightest structural of metal in the world. Magnesium was found 2.8% in sea water and other forms, including Dolomite (CaMg(Co3)2), Magnesite (MgCo3) and Carnallite (KMgCl3.6H2O). These forms have been found in USA, England, Australia, Germany, Russia and Italy (Tapany, 2007). Magnesium with 99.8% purity is readily available but it was rarely used in engineering applications.

According to the ASTM (American Society for Testing Material) system, magnesium and its alloy has been designated by two capital letters followed by two or three numbers.

LITERATURE REVIEW

6

Table 2.1: Classification of magnesium alloys (Tapan, 2007).

Letters: Stands for the two major alloying elements.

*First letter: The highest amount

*Second letter: The second highest amount

Example : AZ91

A-Aluminium is the highest amount in AZ91.

Z-Zinc is the second highest. Numbers: Stands for the amount of the two

major alloying elements.

*First number: The wt% of the first letter element.

*Second number: The wt% of the second letter element.

Example: AZ91

9-9 wt% of Aluminium. 1-1 wt% of Zinc.

Some of the commercial magnesium alloys: 1. Mg-Al casting alloys

2. Mg-Al-Zn casting alloys

3. Mg-Zn and Mg-Zn-Cu casting alloys 4. Mg-Zn-Zr and Mg-RE-Zn-Zr casting alloy 5. High temperature Mg casting alloys 6. Wrought Mg alloys

In this project, magnesium with AZ91 will be focused as Mg-Al-Zn system being the most widely used. This is because cast magnesium alloys dominate 85-90% of all magnesium products (Tapany, 2007).

Alloying elements in magnesium intends to produce alloys with high-strength-to-weight ratios. This may suitable in many fields of application such as aerospace, electronics and automobile field. Besides, they have high specific strength modulus and excellent anti shock resistance as they are able to form compounds with negative valence ion rather than solid solution due to strong electropositive in the elements. However, low strength and toughness, low corrosion resistance and easily flammable with oxygen has limits the application of magnesium alloy (Tapany, 2007). Poor atmospheric corrosion resistance and very reactive in the air makes magnesium will

7 be corroded easily. Thus, to improve the adhesion, wear and corrosion resistance of magnesium alloy, a proper surface treatment is necessarily done (Sudagar et al., 2011).

2.2 Corrosion

Low corrosion resistance is one of the limitations in the application of magnesium alloy. This purpose of this project is to determine which of the coating produced by magnetron sputtering is giving better surface treatment for magnesium alloy. Corrosion can be defined as the degradation of a material due to a reaction with its environment. Corrosion is a destructive attack of a metal by chemical or

electrochemical reaction with its environment (Revie, 2008). Up to 1960s, the term corrosion is restricted only to metals and its alloy without incorporating ceramics, polymers, composites and semiconductors. However, in recent years all type of natural and man-made materials is having connection with corrosion, not metal and its alloy alone (Ahmad, 2006).

2 the coatings will be deposited by using sputter deposition. There are three types of coating which will be focused in this work, TiN, AlN and TiAlBNcoating.

In order to determine the corrosion behavior for each coating, an electrochemical test will be done. The sample will be immersed in simulated body fluid; Hank’s solution and the corrosion rate and corrosion potential will be determined. Higher the corrosion potential, better the corrosion resistance for magnesium AZ91. In order to observe the surface morphology of the corroded sample, analytical techniques will be used. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) is used in order to see the surface morphology of corroded sample.

1.2 Problem Statement

Over the years, magnesium (Mg) and its alloy has been developed as widely used structural materials in many fields due to their mechanical properties such as low density, high strength-to-weight ratio (Wang et al., 2010). Besides, increased in ductility and castability also make them being an industrially accepted for the numerous application.

However, one major problem with the application of magnesium and its alloy is poor corrosion resistance even in moderately aggressive environments (Sudholz et al., 2009). Therefore, to increase the resistance of corrosion on magnesium, surface treatment is needed. Coating is one of the surface treatments which can be done on magnesium. Coating tends to improve surface properties of the substrate such as corrosion resistance, wear resistance, appearance and wettability.

In this works, three types of coating would be deposited on magnesium AZ91 which are TiN, AlN and TiAlBN coating. The most preferable coating due to several properties which increases the corrosion resistance on magnesium will be determined.

3

1.3 Objectives

The objectives of this project are:

• To compare the corrosion behavior of TiAlBN, TiN and AlN coating produced by magnetron sputtering on magnesium substrate.

• To understand the general properties of coating including surface morphology and chemical composition.

• To evaluate surface morphology of general corrosion on different coatings.

1.4 Scope

This project is to compare the corrosion behavior between TiAlBN, TiN and AlN coating. Only the general properties of corrosion in coating will be discussed. Test method which relates with this project paper, electrochemical testing will be used. The solution which will be used as an electrolyte in the electrochemical testing is a simulated body solution, Hank’s Solution. Besides, an observation will be done on surface topography of the sample in order to prove the form of corrosion.

4

1.5 Project Organization

This project was organized into five chapters:

Chapter 1 is the introduction to this report. This includes background, problem statement, objectives and scope of the project.

Chapter 2 is on literature review which describing the findings of the project, theoretical and methodology before the experimental works start.

Chapter 3 is on methodology which includes experimental procedures and apparatus setup required for the corrosion test. The micrographic observation procedure also will be included.

Chapter 4 includes the results and discussion of the experiment and the observation on the morphology.

Chapter 5 is would like to conclude on overall project. Recommendation for further study also will be discussed in this chapter.

5

2.1 Magnesium and its Alloy

Having a quarter weights of the steel and a third lighter than aluminium makes magnesium becomes the lightest structural of metal in the world. Magnesium was found 2.8% in sea water and other forms, including Dolomite (CaMg(Co3)2), Magnesite (MgCo3) and Carnallite (KMgCl3.6H2O). These forms have been found in USA, England, Australia, Germany, Russia and Italy (Tapany, 2007). Magnesium with 99.8% purity is readily available but it was rarely used in engineering applications.

According to the ASTM (American Society for Testing Material) system, magnesium and its alloy has been designated by two capital letters followed by two or three numbers.

LITERATURE REVIEW

6 Table 2.1: Classification of magnesium alloys (Tapan, 2007).

Letters: Stands for the two major alloying elements.

*First letter: The highest amount

*Second letter: The second highest amount

Example : AZ91

A-Aluminium is the highest amount in AZ91.

Z-Zinc is the second highest. Numbers: Stands for the amount of the two

major alloying elements.

*First number: The wt% of the first letter element.

*Second number: The wt% of the second letter element.

Example: AZ91

9-9 wt% of Aluminium. 1-1 wt% of Zinc.

Some of the commercial magnesium alloys: 1. Mg-Al casting alloys

2. Mg-Al-Zn casting alloys

3. Mg-Zn and Mg-Zn-Cu casting alloys 4. Mg-Zn-Zr and Mg-RE-Zn-Zr casting alloy 5. High temperature Mg casting alloys 6. Wrought Mg alloys

In this project, magnesium with AZ91 will be focused as Mg-Al-Zn system being the most widely used. This is because cast magnesium alloys dominate 85-90% of all magnesium products (Tapany, 2007).

Alloying elements in magnesium intends to produce alloys with high-strength-to-weight ratios. This may suitable in many fields of application such as aerospace, electronics and automobile field. Besides, they have high specific strength modulus and excellent anti shock resistance as they are able to form compounds with negative valence ion rather than solid solution due to strong electropositive in the elements. However, low strength and toughness, low corrosion resistance and easily flammable with oxygen has limits the application of magnesium alloy (Tapany, 2007). Poor atmospheric corrosion resistance and very reactive in the air makes magnesium will

7 be corroded easily. Thus, to improve the adhesion, wear and corrosion resistance of magnesium alloy, a proper surface treatment is necessarily done (Sudagar et al., 2011).

2.2 Corrosion

Low corrosion resistance is one of the limitations in the application of magnesium alloy. This purpose of this project is to determine which of the coating produced by magnetron sputtering is giving better surface treatment for magnesium alloy. Corrosion can be defined as the degradation of a material due to a reaction with its environment. Corrosion is a destructive attack of a metal by chemical or

electrochemical reaction with its environment (Revie, 2008). Up to 1960s, the term corrosion is restricted only to metals and its alloy without incorporating ceramics, polymers, composites and semiconductors. However, in recent years all type of natural and man-made materials is having connection with corrosion, not metal and its alloy alone (Ahmad, 2006).