Magnesium Alloys for Aerospace - ULisboa · Magnesium Alloys for Aerospace Miguel Alexandre Cardoso...

Magnesium Alloys for Aerospace

Miguel Alexandre Cardoso Dias

Dissertation developed for the award of Master of Science Degree in

Chemical Engineering

Supervisor(s): João Carlos Salvador Santos Fernandes

Maria de Fátima Grilo da Costa Montemor

Maria Luís Vieira Rodrigues

Examination Committee

Chairperson): Francisco Manuel da Silva Lemos

João Carlos Salvador Santos Fernandes

Maria Teresa Oliveira de Moura e Silva

June 2017

III

Acknowledgments Firstly, I would like to thank to my mentors: Prof. João Salvador and Prof. Maria de

Fátima Montemor, for every time they advised me during the development of this work. Secondly,

but not less important, a special thanks to Miguel Ferreira for every time he spent explaining me

procedures and guiding me through what I was supposed to use in the laboratory and for some

jokes that were very important to relax my mind. A special thanks to all my colleagues - Lenia,

Kayxa and Yegor.

After that I could not forget the support, the patience, the belief and above all the love

that my parents David and Maria Rosario had for me until now so desired that no matter how I

write to them, I will never come to thank them. To my closest friends João Artur and Jorge Araujo

who put up with me and supported me during these years, not only for the words but also for the

conversations that illuminated through the path I travelled until here, thank you very much. I

cannot forget also my friends who have accompanied me always and who gave me strength and

scolded when necessary: João Marzia and Ana Milheiro.

Lastly but not the least, I want to thank the woman who listened to me, who encouraged

me, who shared with me my sorrows, victories and joys over these past months, the best girlfriend

I could have: my better half Angela Santos.

V

Resumo

The WE43C alloy is a magnesium alloy, Mg is the lightest and most abundant metal present on

the earth's surface is an alternative to commercial aluminum alloys. The WE43C league has in its

constitution earth rare. Some published studies increase the corrosion resistance in magnesium

alloys.

The comparison of WE43C with ASTM 7475 was performed. This comparison was based on

electrochemical tests in 0.05M NaCl solution, namely a first step with the determination of

polarization curves and impedance spectroscopy. Both were preceded by an open circuit potential

determination that allowed the determination of the corrosion potential for both alloys. For the

surface characterization, both samples were polished in SiC sandpaper and buffed diamond

paste polishing (macro characterization) and were taken to the electron microscope coupled with

EDS, which allowed a quantitative percentage evaluation of the elements present in the alloys.

We used RAMAN spectroscopy to understand what happened to the anodic curve in the WE43C

alloy.

It has been found by electrochemical testing that WE43C has a corrosion potential lower than that

of ASTM 7475; By performing EIS, and fitting to an equivalent circuit, a model based on a porous

film (WE43C) or with bites (7475) was considered. The surface analysis techniques allow us to

observe that in the case of the WE43C rare earths besides being segregated along the grain

boundaries, where they can promote matrix corrosion due to their cathodic character.

Palavras-chave: WE43C, ASTM 7475, corrosão, EIS, RAMAN, filme de óxido

VI

Abstract

The WE43C alloy is a magnesium alloy, Mg is the lightest and most abundant metal present on

the earth's surface is an alternative to commercial aluminum alloys. The WE43C league has in its

constitution earth rare. Some published studies increase the corrosion resistance in magnesium

alloys.

The comparison of WE43C with ASTM 7475 was performed. This comparison was based on

electrochemical tests in 0.05M NaCl solution, namely a first step with the determination of

polarization curves and impedance spectroscopy. Both were preceded by an open circuit potential

determination that allowed the determination of the corrosion potential for both alloys. For the

surface characterization, both samples were polished in SiC sandpaper and buffed diamond

paste polishing (macro characterization) and were taken to the electron microscope coupled with

EDS, which allowed a quantitative percentage evaluation of the elements present in the alloys.

We used RAMAN spectroscopy to understand what happened to the anodic curve in the WE43C

alloy.

It has been found by electrochemical testing that WE43C has a corrosion potential lower than that

of ASTM 7475; By performing EIS, and fitting to an equivalent circuit, a model based on a porous

film (WE43C) or with bites (7475) was considered. The surface analysis techniques allow us to

observe that in the case of the WE43C rare earths besides being segregated along the grain

boundaries, where they can promote matrix corrosion due to their cathodic character..

Keywords: WE43C, ASTM 7475, corrosion, EIS, RAMAN, oxide film

VII

Contents

Acknowledgments ..................................................................................................................... III

Resumo ........................................................................................................................................ V

Abstract ....................................................................................................................................... VI

List of Tables ............................................................................................................................ VIII

List of Figures ............................................................................................................................. IX

Acronyms .................................................................................................................................... XI

Chapter 1 ..................................................................................................................................... 1

Introduction .............................................................................................................................. 1

1.1 Magnesium in History .................................................................................................. 1

1.2 Objectives ..................................................................................................................... 2

1.3 Thesis outline ............................................................................................................... 2

Chapter 2 ..................................................................................................................................... 5

Background ............................................................................................................................... 5

2.1 Theoretical Overview ................................................................................................... 5

2.2 Development of Magnesium Alloys ........................................................................... 5

2.3 Magnesium General Properties and Nomenclature ............................................... 6

2.4 Aluminium and Aluminium alloys ............................................................................... 9

2.5 Magnesium Alloys Alloying Elements ..................................................................... 13

2.6 Corrosion ..................................................................................................................... 20

2.7 Applications of Mg alloys .......................................................................................... 29

Chapter 3 ................................................................................................................................... 33

3.1 Overview ..................................................................................................................... 33

3.2 Materials and Solutions ............................................................................................. 33

3.3 Sample Preparation ................................................................................................... 34

3.4 Electrochemical Measurements .............................................................................. 35

3.5 Microscopy and Surface Analysis Techniques ...................................................... 43

Chapter 4 ................................................................................................................................... 47

4.1 Electrochemical measurements results .................................................................. 47

4.2 Surface Analysis Results .......................................................................................... 58

Chapter 5 ................................................................................................................................... 69

Conclusions and Future Work ........................................................................................ 69

Bibliography ............................................................................................................................... 71

VIII

List of Tables

Table 1- Physical properties of pure magnesium [7] ..................................................................... 7

Table 2 - Chemical Properties of Magnesium [20] ........................................................................ 7

Table 3 - ASTM codes for magnesium alloys [7, 21, 22] ............................................................... 8

Table 4 - Three most used families of aluminium alloys [24] ........................................................ 9

Table 5 - Minimum Properties for 2024 and 7475 alloys compared with conventional Al alloys [24]

..................................................................................................................................................... 10

Table 6 –Case studies on Aluminium alloys and improvement of their properties. .................... 10

Table 7- ASTM 7475 element alloy composition [28]................................................................. 11

Table 8- Mechanical properties influenced by alloy elements [30] ............................................. 12

Table 9 – Most used families of Mg alloys [32]. .......................................................................... 13

Table 10 - Addiction of zirconium and manganese to magnesium alloys ................................... 14

Table 11 - Experiments in rare earth alloys. ............................................................................... 15

Table 12 - General Physical Properties of WE43C [39] .............................................................. 16

Table 13 - General Mechanical Properties of WE43C [39] ......................................................... 16

Table 14 - Influence of recrystallization on corrosion behaviour of Mg alloys with RE ............... 19

Table 15 - Standard Electrochemical Series [56]. ....................................................................... 23

Table 16 - Nital solution. .............................................................................................................. 33

Table 17 - Acetic Glicol ............................................................................................................... 33

Table 18 - Keller solution. ............................................................................................................ 34

Table 19 - Common electrical elements for impedance [91] ....................................................... 40

Table 20 - Anodic and Cathodic curves parameters for WE43C alloy. ....................................... 51

Table 22 - pH values for Polarization curves .............................................................................. 52

Table 23- Fitting the EIS to the equivalent circuit for WE43C alloy. ........................................... 54

Table 24 - Fitting the EIS to the equivalent circuit for ASTM 7475. ............................................ 55

Table 25 - EDS without O for regular structure ........................................................................... 59

Table 26 - EDS without O for regular structure ........................................................................... 60

Table 27 - EDS with O for irregular structure .............................................................................. 60

Table 28 - EDS without O for irregular structure ......................................................................... 60

Table 29 - EDS for ASTM 7475 with O (Figure 52) ..................................................................... 65

Table 30 - EDS for ASTM 7475 without O (Figure 52)................................................................ 65

Table 31 - EDS for ASTM 7475 with O (Figure 53) ..................................................................... 66


Table 33 - EDS for ASTM 7475 with O (Figure 54). .................................................................... 66


IX

List of Figures

Figure 1 - Potentiodynamic curves: Al 7475 [31] ........................................................................ 12

Figure 2- Images of 7475 surface alloys by scanning electron microscopy, (a) and (b) Al7Cu2Fe

precipitated adjacent to pits formed [31]. ................................................................ 13

Figure 3 - SEM images of as-polished surfaces from the (a)solution-treated and (b) peak-aged

WE43 showing Y-rich and Zr-rich particles. (c)Back-scattered electron (BSE) SEM

image of peak-aged WE43 showing the fine scale precipitates throughout the grains

and along grain boundaries. [35] ............................................................................ 17

Figure 4 - Corrosion Example [51]. ............................................................................................. 21

Figure 5 -Schematic diagram of the dissolution of a metal in acidic medium. [48] ..................... 21

Figure 6 - Schematic of a Standard Electrode Potential [55]. ..................................................... 23

Figure 7 - Uniform Corrosion [59, 60] .......................................................................................... 24

Figure 8 - Galvanic Corrosion [4] ................................................................................................ 25

Figure 9 - Crevice Corrosion [64, 65] .......................................................................................... 26

Figure 10 - Pitting Corrosion [66, 67] .......................................................................................... 26

Figure 11 - Environmentally Induced Cracking [69] .................................................................... 27

Figure 12 - Intergranular Corrosion [72, 73, 74] .......................................................................... 28

Figure 13 - Erosion Corrosion [75] .............................................................................................. 28

Figure 14 - Examples of magnesium alloy applications on automobiles [78]. ............................ 29

Figure 15 - Aerospace examples for military and civil applications [79]. .................................... 30

Figure 16 - Magnesium implants in the arm (left) and femur (right) [80] ..................................... 30

Figure 17 - Examples for electronics applications [82, 83] ......................................................... 31

Figure 18 - Sports and disabled utilities examples [84]. ............................................................. 31

Figure 19 – Struers Cut-off machine. .......................................................................................... 34

Figure 20 - Preparation sample sequence. ................................................................................. 35

Figure 21 – Sample polishing sequence. .................................................................................... 35

Figure 22 - Work Cell in a Faraday cell. ...................................................................................... 36

Figure 23 - Sinusoidal current response to sinusoidal potential. ................................................ 38

Figure 24 - Nyquist impedance plot. ........................................................................................... 39

Figure 25 - Magnifying glass (left) and optical microscope (right). ............................................. 43

Figure 26 - Analytical JEOL 7001F FEG-SEM ............................................................................ 44

Figure 27 - Simple Block Diagram of SEM [94] ........................................................................... 44

Figure 28 - Spectrum for eds with peaks for maximum element adsorption [98] ........................ 45

Figure 29 - Raman Equipment and schematics [100]. ................................................................ 46

Figure 30 - OCP for WE43C in 0.05 M NaCl solution ................................................................ 47

Figure 31 - OCP for ASTM 7475 in 0.05 M NaCl solution .......................................................... 48

Figure 32 – Illustration of the potential transients’ due to the breakdown and repassivation of pits

[102] ........................................................................................................................ 49

Figure 33 - Polarization curves for WE43C in 0.05 M NaCl (log scale) ...................................... 49

X

Figure 34 - Linear scale plot for WE43C alloy in 0.05 M NaCl. ................................................... 50

Figure 35 - Polarization curves for ASTM 7475 alloy in 0.05 M NaCl. ........................................ 51

Figure 36 - Theoretical explanation of Potentiodynamic data for Al. .......................................... 51

Figure 37 - (a) Nyquist diagram, (b) Bode diagram for WE43C in 0.05M NaCl. ......................... 53

Figure 38- Proposal of Equivalent Circuit .................................................................................... 54

Figure 39 - (a) Nyquist diagram, (b) Bode diagram for ASTM 7475 in 0.05M NaCl. .................. 55

Figure 40 - Left: WE43C alloy with acetic glycol etching; right: WE43C alloy with nital etching. 58

Figure 41 - ASTM 7475 alloy with Keller's reagent. .................................................................... 58

Figure 42 – (Left)SEM-(right) EDS analysis for WE43C alloy. .................................................... 59

Figure 43 - (Left)SEM-(right) EDS analysis for regular structure of WE43C alloy. ..................... 59

Figure 44 - (Left)SEM-(right) EDS analysis for irregular structure of WE43C alloy. ................... 60

Figure 45 –SEM for ASTM 7475 alloy ......................................................................................... 61

Figure 46 – First SEM image for top mark on ASTM 7475 alloy ................................................. 61

Figure 47 - First SEM image for left mark on ASTM 7475 alloy. ................................................. 62

Figure 48 - Second SEM for ASTM 7475 alloy ........................................................................... 62

Figure 49 - Second SEM image for top mark on ASTM 7475 alloy ............................................ 63

Figure 50 - Second SEM image for left mark on ASTM 7475 alloy ............................................ 63

Figure 51 - Third SEM for ASTM 7475 alloy ............................................................................... 63

Figure 52 - Third SEM image for top mark on ASTM 7475 alloy ................................................ 64

Figure 53 - Third SEM image for left mark on ASTM 7475 alloy ................................................ 64

Figure 54 - (Left)SEM-(right) EDS analysis for ASTM 7475 alloy in matrix ................................ 65

Figure 55 - (Left)SEM-(right) EDS analysis for ASTM 7475 alloy in dark zone. ......................... 65

Figure 56 - (Left)SEM-(right) EDS analysis for ASTM 7475 alloy in clear zone. ........................ 65

Figure 57 - Raman spectra of the passivated WE43C samples in 0.1 M NaCl [47]. .................. 67

Figure 58 - XPS high resolution Y 3d spectra of the WE43C specimens potentiostatically

passivated (Solution treated specimen prior to Ar-ion sputtering (as passivated

surface) [47] ............................................................................................................ 67

XI

Acronyms

A Electrode Surface Area

AA Aluminium alloy

Al Aluminium

ASTM American Society for Testing Materials

BSE Back scattered electron

C Capacitance

CAA Civil Aviation Authority

CdL Double-layer Capacitance

Cox Concentration of the oxidized species

Cr Chromium

Cred Concentration of the reduced species

CPE Constant Phase Element

Cu Copper

E Voltage

EDS Energy-Dispersive X-ray

EIS Electrochemical Impedance Spectroscopy

Ep Critical Potential

ESA European Space Agency

eV Electron Volt

Ea Anodic Potential

Ec Cathodic Potential

Ecorr Corrosion Potential

Enp Pit nucleation Potential

E0 Voltage Amplitude

FEA Flight Experiment Apparatus

GBP Ground Boundary Precipitates

HCl Hydrochloric acid

hcp Hexagonal Close Packed

Hz Hertz

Gd Gadolinium

I Current

Icritc Critical Intensity

I0 Current Amplitude

J Joule

K Kelvin

Kg Kilogram

K0 Kinetic Constant

Mg Magnesium

XII

m meter

MgO Magnesium oxide

Mg(OH)2 Magnesium hydroxide

mol Mole

Mn Manganese

NACE National Association of Corrosion Engineers

NaCl Sodium chloride

Ni Nickel

Nd Neodymium

OCP Open Circuit Potential

Pa Pascal

RE Rare Earth

SCC Stress Corrosion Cracking

SCE Standard Calomel electrode

SE Secondary electron

SEM Scanning Electron Microscopy

SHE Standard Hydrogen Electrode

Si Silicon

R Resistance

Rs Solution Resistance

Rct Charge-transfer resistance

R’’ Gas constant

t Time

Ti Titanium

Ti Thermal Treatment i; i=1,…,9

UTS Ultimate Tensile Strength

V Volt

W Watt

W Warburg Impedance

XRD X-ray Diffraction

Y Yttrium

Ys Yield Strength

Y0 Admittance

Zn Zinc

Zr Zirconium

ºC Degree Celsius

%EI Efficiency index

Ϭ Warburg Coefficient

Φ Phase shift

Ω Ohm

XIII

ω Angular Frequency

1

Chapter 1

Introduction

1.1 Magnesium in History

Magnesium (Mg) is the lightest of all metals having a density of 1.74 g.cm-3, when compared with

two of the most used metals, Aluminium (Al) with 2.7 g.cm-3 and steel (Fe) with 7.8 g.cm-3,

however apart from some exotic types of construction it is not used frequently [1].

Mg was clearly identified as a chemical element separated by J.Black, when he, in 1755,

distinguished between magnesia and lime, by showing that from the former, it formed a soluble

sulphate while from the lather it formed a slightly soluble sulphate. However, it was Sir Humphrey

Davy, in 1808, who was able to isolate magnesium from a mixture of magnesia (MgO) and

mercuric oxide (HgO) [2] . In 1833, Michael Faraday produced the first magnesium metal, by

using electrolysis on fused anhydrous magnesium chloride (MgCl2). In the following decades

Germany started the production of commercial Mg (1886), being perfected in 1896 by Chemische

Fabrik Greisheim-Elektron, who until nowadays, is still the world’s largest magnesium supplier [3,

4].

The “Magnesia” was the Greek word for the district of Thessaly [2] . It’s the eight most abundant

elements in Earth’s crust and the sixth most dominant metal [5] .

At the beginning of the 20th century, Mg alloys had an experimental development in simultaneous

with odder alloys but difficulties related with the production of magnesium did not allow producers

to maintain competitive prices. Along with the difficulty in improving mechanical and corrosion

properties through alloy development, the magnesium alloys were replaced. Mg alloys only have

been used in certain industries and there is no tendency to use them outside those. However,

during the World Wars, mostly during the World War II, Mg alloys had an increased demand in

the military industry, as they were looking to produce lighter airplanes what could only be achieved

by weight reduction of the components. At this period, the consumption of Mg increased 228 000

t/year in 1944, coinciding with the greatest period in alloy development (1930-1950) but, after the

war, the production values were reduced to 10 000 t per year [1, 6, 7] .

Until the second half of the 20th century, Mg applications remained in possession of the military,

aerospace and nuclear industries [7] but, with the need and with the constant research for

improvement technologies with superior corrosion resistance, magnesium production has

reached and surpassed World War II levels (360 000 t in 1998 at a price of US$3.6 per kg) [6].

Nowadays, the principal consumer of magnesium is the automotive industry, with the main goal

of achieving significant reductions in fuel consumption and gas emissions and achieving the goal

of reducing environmental impact derived from human activities.

http://nautilus.fis.uc.pt/st2.5/scenes-p/biog/b0022.html

http://nautilus.fis.uc.pt/st2.5/scenes-p/biog/b0022.html

2

1.2 Objectives

The main objective of this work is to understand the effects of the alloying elements on the

corrosion of WE43C in saline medium and compare it with one of the most used in commercial

airplane productions.

The focus of this dissertation was to compare the current aluminium alloy (ASTM 7475) used for

aerospace industry and a new, more recent magnesium alloy (ASTM WE43C). With the purpose

of testing the replacement of the first alloy for the second, by comparing the corrosion behaviour

of untreated material in a specific medium at room temperature and normal pressure. To better

understanding the corrosion resistance of the two specimens, electrochemical measurements

were performed on both alloy samples with the same electrochemical treatments. The

electrochemical characterization consisted in the determination of polarization curves, open

circuit potentials and electrochemical impedance spectroscopy, carried out in a NaCl solution at

room temperature.

For completion, surface analysis was performed to examine the surface layers’ composition using

Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectroscopy (EDS).

1.3 Thesis outline

The present work is divided in 5 chapters.

In the first chapter, there is a brief overview of magnesium in history, with a summary of its

physical and chemical properties. There is also a brief mention to what drives to development of

magnesium alloys, and the objectives of this master thesis.

In chapter 2 there is a small description of magnesium and aluminium alloys and a comparison

between them, a brief review of some articles, some ideas about corrosion and applications for

magnesium alloys.

The experimental methods are described in chapter 3, including the comprising of the samples,

preparation of solutions and a brief description of electrochemical measurements and surface

analysis techniques.

In the chapter 4 are described the results of open- circuit potential, polarization curves and

electrochemical impedance spectra of treated Mg and Al alloys and they are compared with SEM

results for the respective alloys to perform a surface characterization. Also, RAMAN were

performed to determinate oxide formation on surface area for WE43C alloy.

3

The concluding chapter presents the general conclusions for this work and some ideas for future

work.

5

Chapter 2

Background

2.1 Theoretical Overview

For a production of a certain application or product the material choice must be framed according

to the required properties. In the aerospace sector, it’s important that materials selection and

application are adequate. It’s very important to secure the high corrosion resistance during a cycle

life time (25-30 years) and to maintain the resistance flammability and stress as a goal permissive

to achieve new goal and new applications. WE43C it’s a combined alloy with a matrix of

magnesium with extrusions of yttrium, neodymium and zirconium.

This alloy has been known since the beginning of 2000’s but more recently has had an increased

research due to his properties, like it’s density that is less than the one of aluminium and or iron.

Magnesium was used for the first time in large scale during the WWII in car engines due to its low

density, that reduces the consumption of oil [1] . Since then, magnesium had a restricted use,

until recently when, the oil crisis and environmental issues brought it back to the spotlight of

technologies. [8, 3, 9, 10] .

2.2 Development of Magnesium Alloys

Over the decades and with the aim of being at the best of the state of the art technology, the

concerns about improvement and legalization for use, the Mg alloys must be approved by entities

like ANAC (National civil aviation authority – in Portugal), FAA (Federal Aviation Administration in

USA), EASA – (European Advertising Standards Alliance) and CAA (Civil aviation authority - in

the United Kingdom) [11] .

With increases in fuel prices, the objective of getting greater efficiency and reductions in

CO2 emissions for environmental reasons became the reason for developing lighter alloys. As it

was said before, this type of alloys had mostly been used by military aerospace industries but

nowadays the development extended to commercial aerospace industry mostly in aircraft interiors

[3, 10] . For that reason and due to the very important chemical properties mentioned on Table

1Table 2, some of the most import tests are performed on those properties. According to

magnesium-Elektron [9] and FAA in a published report [12] 1 , Mg alloys are high-performance

materials that are designed to withstand elevated temperatures and be resistant to corrosion and

proved long-term performance records, including critical applications in jet engines and military

aircrafts.

1 https://www.fire.tc.faa.gov/pdf/AR11-13.pdf

6

The use of Mg alloys in military aircrafts requires different performance tests because the military

planes and helicopters operate in more demanding environments than most commercial aircrafts.

Weight control is important, but not necessarily for fuel efficiency. Military aircrafts are required to

perform at the very edge of technical limitations and must therefore reduce mass wherever

possible [10] .

Some of the main properties that were tested by entities previously mentioned:

• Stress Analysis – viability of the structure: analyses surface forces acting on the

structure's surface and forces acting on the structure's volume (gravitational and inertial

effects) [11] ;

• Primary structures – critical load bearing structure of an aircraft that in case of severe

damage will fail the entire aircraft, Secondary structures – structural elements of an

aircraft that carry only air and inertial loads generated on or in the secondary structure

[13] ;

• Lifetime (between 25-30 years in service) and corrosion: resistance to corrosion and

metal choice according the type of corrosion (galvanic or contact), influence in

maintenance costs [14, 15] ;

• Identify promising materials technologies, design issues and performance parameters,

achieve fatally fire-resistance and fire-safety interior in future aircrafts [16] ;

• Evolutionary response of materials in response to fires, mass loss rate, oxygen

Temperature Index, toxicity [17, 18],;

• The influence of extra weight and fluctuation in fuel prices [15] .

Those properties are essential for safety and security and must be fulfilled before utilization.

2.3 Magnesium General Properties and Nomenclature

Magnesium occurs in nature at three known forms: dolomite (MgCO3.CaCO3), MgCl2, derived

from brine (solution salt) and from carnallite (KCl.MgCl2.6H2O). The technologies most used to

obtain metallic Mg are electrolysis of molten magnesium chloride, such as the one that is used to

separate aluminium from alumina or thermal reduction of magnesium oxide2 [1, 8] . Although the

first one is more environmentally friendly, it’s very expensive and when compared with the

aluminium extraction and it has the disadvantage that magnesium is less dense than the

electrolyte from which it has been separated and so it floats on the surface of the cell and due to

that it must be protected from the atmosphere [1].

The most important properties are listed in Table 1:

2 Thermal reduction of magnesium oxide by Pidgeon [1] : 2(CaO.MgO) + FeSi → Mg(g) + 2CaO.SiO2 + Fe

7

Table 1- Physical properties of pure magnesium [7]

Crystalline Structure Hexagonal dense packed

Density (ρ) 1.74 kgm-3 (a)

Young Modulus (E) 45 GPa

Yield Tensile Strength (Ys) 21 MPa

Ultimate Tensile Strength (UTS) 80 – 180 MPa

Fracture Elongation (εf) 1 – 12 %

Melting Point (Tm) 650 ºC

Specific Heat Capacity (c) 1.05 kJkg-1K-1

Fusion Heat 195 kJkg-1

Heat Conductivity (K) 156 Wm-1K-1 (a)

Coefficient of Linear Expansion (αL) 2.6x10-7 K-1 (a)

Solidification Shrinkage 4.2 %

Electrical Conductivity (σ) 217 kΩ-1cm-1 (a)

Vanderwaals radius 0.16 nm (b)

Ionic radius 0.065 nm (b)

Isotopes 5 (b)

Electronic shell [Ne] 3s2 (b)

Energy of first ionisation 737.5 kJ.mol -1 (b)

Energy of second ionisation 1450 kJ.mol -1 (b)

Standard potential - 2.34 V (b)

(a) at room temperature

(b) according [19]

The most important chemical properties are listed in Table 2 :

Table 2 - Chemical Properties of Magnesium [20]

Chemical Formula Mg

Compounds

Oxide, hydroxide, chloride, carbonate and sulphate. Also, Epsom salts

(magnesium sulphate heptahydrate) and Milk of Magnesia (magnesium

hydroxide).

Flammability Burns in air with a bright white light

Reactivity Upon heating, magnesium reacts with halogens to yield halides.

Alloys Magnesium alloys are light, but very strong

Oxides It combines with oxygen at room temperature to form a thin skin of

magnesium oxide.

8

For physical metallurgy, the most important characteristic of magnesium is its hexagonal dense

packed (hcp) crystalline structure. The pure metal, obtained by casting, is generally brittle

presenting both transcrystalline and intercrystalline failure, but at higher temperatures (above 225

ºC) it shows good deformation behaviour.

The atomic diameter of magnesium (0.320 nm) [21], combined with the hcp structure, accounts

for an excellent alloying behaviour, as the size factors are favourable with a very large number of

elements.

The alloys have a specified nomenclature and in this work, the ASTM (American Society for

Testing and Materials) nomenclature was adopted. When the need arises to refer a non-ASTM

nomenclature, it will be accompanied by the necessary explanation.

Magnesium alloys are identified by two letters that correspond to the two main alloying elements.

Those letters are followed by numbers corresponding to the nominal composition in weight

percentage of that alloy element, rounded to the nearest unit. After this sequence, a final letter

might be present. To the first alloy registered for a composition will be attributed the letter A, to

the second the letter B and so on.

The ASTM regulations also define the composition intervals admissible for the other elements not

present in the name and the codes are listed in Table 3:

Table 3 - ASTM codes for magnesium alloys [7, 21, 22]

Code Chemical Element Code Chemical Element

A Aluminium M Manganese

B Bismuth N Nickel

C Copper P Lead

D Cadmium Q Silver

E Rare Earths R Chromium

F Iron S Silica

G Magnesium T Tin

H Thorium W Yttrium

K Zirconium Y Antimony

L Lithium Z Zinc

9

2.4 Aluminium and Aluminium alloys

Since the first time it was used in aircrafts, the relevance of aluminium has increased. Because

of that, the United States was extruding water heater anodes and fabricating aluminium truck

bodies with the supply of electron Ltd. It was the earlier development in the extrusions process

that achieved the future in use of materials.

Aluminium is one the most used elements in aerospace and automotive sectors due to its

properties such as Lightweight, electrical conductivity, corrosion resistance, low melting point and

malleability and has cfc structure, so for a long time, aluminium was the most used metal in the

transport industry, as most of the constructions were made in aluminium. Apart from this use, the

scientific community started to look through new alternatives, new alloys that could be used to

improve mechanical properties without forgetting properties like corrosion behaviour [23]. To

improve the modern aircraft, three types of alloys were considered: the 2000 series (Al – Cu –

Mg), the 6000 series (Al – Si – Mg) and the 7000 series (Al – Zn – Mg – Cu). All of them are

precipitation – hardnable alloys, leading to precipitation of the precipitated and dispersoids fines

for reinforcement, morphological characterization, mechanical properties and environmental

response of the materials.

Table 4 - Three most used families of aluminium alloys [24]

Alloy Brief description

2000 series

The most used alloy is 2024 – T3: takes the advantage of cold

working and natural aging, moderate yield strength, good

resistance to fatigue crack growth and good fracture

toughness.

Used in decade of 60’s for fuselage skins for commercial and

military transport aircraft.

6000 series

Better corrosion resistance than the 2000 series, most used

6013-T6 alloy: increase 12% strength over 2024 – T3 with

comparable toughness and resistance to fatigue crack growth,

good manufacturing process.

Not very used in aircraft industries because don’t had the

balance in the properties.

7000 series

The most used are 7075 and 7475 alloys.

The 7075 have the highest strengths by far, could be used for

fuselage skins, stringers bulkheads, wing skins, panels and

covers.

7475 has a higher strength, superior fracture toughness and

resistance to fatigue crack propagation in air and aggressive

environment [25].

10

Table 5 - Minimum Properties for 2024 and 7475 alloys compared with conventional Al alloys [24]

Alloy AMS number Tensile

Strength(MPa)

0.2% YS

(MPa) % EI

2024 4037 441 290 15

7475 4084 517 455 9

Tanaka et. al. [26] affirm that to use aluminium alloys for structural components it is very important

to improve their mechanical properties on resistance to corrosion, as well as strength for high

reliability. The suggestion consists in the use of grain refinement. Different than grain refinement

of Aluminium Alloy (AA) 7075, where the grain refinement has a disadvantage to the resistance

to stress corrosion cracking (SCC), the AA 7475 has a different grain refinement by using

zirconium and roll temperature. The results show an increase of 10% for fatigue strength that is

proportional to tensile strength and an improvement of resistance to SCC correlated to uniformity

of electrochemical between grain interior and his boundary area. On a parallel case study

Goloborodko et. al. [27] concluded that by increasing pressing temperature, it slowed down the

transformation rate from low angle boundaries that resulted in a rapid development of new grains.

Combining these results in presence of RE (zirconium) lead to grain refinement.

Table 6 –Case studies on Aluminium alloys and improvement of their properties.

Author Year Study

Wego Wang

[23] 1993

Aluminium alloys, such as aluminium – lithium and influence of

aluminium in mechanicals properties, observance on the entire

crack face.

J-P Immarigeon

et. al [24] 1994

Comparison of three most used aluminium alloys series: 2000,

6000 and 7000, improvement in thermodynamic treatment with

direct impact in mechanical properties like Tensile Strength.

B.B. Verma et.

al. [25] 2001

Fatigue behaviour for 7475 aluminium alloys in a T7351 temper

with yield stress of 495MPa and elongation of 14%.

Tanaka et. al

[26] 2004

Importance of zirconium usage in refinement of grain combined

with temperature roll; increase 10% in SCC and corrosion

resistance by standardizing of electrochemical properties.

Goloborodko et.

al. [27] 2004

Effects pressing temperature on the grain formation of

microstructures and re-definition in space organization for ultra-

fine-grain formation, increase in high strain rate super plasticity.

11

Aluminium alloy ASTM 7475 has a following composition (%wt):

Table 7- ASTM 7475 element alloy composition [28]

Si Fe Cu Mn Mg Cr Ni Zn Ti Zr Al

0.1 0.12 1.2-1.9 0.06 1.9-2.6 0.2 - 0.25 - 5.2-6.2 0.06 - Base

This alloy has a large modulus of elasticity and great fracture toughness. Considering the

elements alloy, those with most influence are Cu, Mg, Mn and Zn which are responsible for

increasing mechanical and specific resistance. In the other hand, all of them produce harmful

intermetallic for corrosion: they form a thin oxide conductive layer, responsible for cathodic

reactions.

This layer provides inert protection and is resistant. Although resistant, if put in acidic medium

(pH < 4) or basic medium (pH > 9), destabilizes the oxide leading to its rupture. When this

happens, greasy ions, such as chlorides and sulphides promote pit corrosion and intergranular

corrosion.

Looking to alloy elements ASTM 7475, such as Fe and Si, it is observed that they precipitate in

cell boundaries or in dendrite form. During the corrosion process, they form intermetallic

structures (1-20µm) like Al7Cu2Fe or Al23CuFe4. While this is observed, Andretta et al. [28] explain

the function of Cr, Zr and Mn. On this alloy, they act like discontinuities by controlling grain size.

As they are not uniformly distributed through the matrix. Furthermore, Francesco [28] reported

that the potential difference between intermetallic and matrix is the driving force and due to this,

intermetallics are the initiators for pit corrosion. On the other side, there are MgZn2 particles that

promote localized attacks in the grain boundary because of their anodic behaviour when

compared to the matrix, driving to the anodic intergranular attack. This shows that microstructure

is directly related with localized corrosion.

When potentiodynamic analysis was performed, breakdowns that were related to pitting corrosion

have been observed in the cathodic curve. Andretta [28] suggested that intermetallic are the

starters for localized corrosion, with more impact on surface while intergranular corrosion is driven

by a penetrating attack in intermetallic location.

In another study, Tsai and Chuang [29] mentioned that stress corrosion cracking (SCC, described

below in section “Environmentally Induced Cracking”) could be minimized by elements that induce

grain refining. This decreasing of SCC is explained by the plane spacing reduction. Another

important observation was that the decreasing of planar glide expresses a reduction of H2

transported through the grain boundaries, which captures H2 in bubbles, inhibiting embrittlement

weakening the alloy. The study shows that SCC decreases when homogeneous planar glide and

minor Ground Boundaries Precipitates treatments are applied.

12

According Payandeh et al. [30] the influence of alloy elements was different, depending on their

use and they were also responsible for decreasing electrical conductivity by promoting impurities

in the alloys.

Table 8- Mechanical properties influenced by alloy elements [30]

Si increasing Increasing UTS and decreasing elongation

Mg increasing Increasing yield stress

Mn increasing Increasing ductility

Fe decreasing Decreasing ductility

In his study Chemin et.al. [31] said that alloying elements such as Cu, Fe and Si are the promoters

of pitting corrosion due to anodic behaviour matrix. In their work, they observed that potential

corrosion and pitting potentials are relatively close and that the dissolution of the matrix occurs

around the precipitates, showing that the matrix presents an anodic behaviour.

While performing tests, Chemin et.al. [31] prepared samples were placed in a NaCl solution. Once

pitting corrosion started to occur due to the breakdown of the passivation layer, chlorites

penetrated the layer and once inside they occupied the empty space and promoted further

destruction of the layer. By analysis of the polarization curves, it was observed that pitting starts

when the potential exceeds a critical value.

Figure 1 - Potentiodynamic curves: Al 7475 [31]

Using SEM examination, Chemin et al. [31] observed precipitates rich in Fe.

13

Figure 2- Images of 7475 surface alloys by scanning electron microscopy, (a) and (b) Al7Cu2Fe

precipitated adjacent to pits formed [31].

Chemin et al. [31] concluded that Al3Fe intermetallics show cathodic behaviour relative to the

matrix, promoting matrix dissolution and pitting corrosion. The cathodic activity was supported by

pH increase due to the formation of hydroxyl ions.

2.5 Magnesium Alloys Alloying Elements

Magnesium alloys have a higher affinity to oxygen, which lead them to an easy oxidation [32].

Mohd [32] in his study, classified magnesium alloys in three families: The Mg – Mn – Al – Zn

alloys, the Mg – Zn – Zr alloys and Mg - Y – RE alloys and describes the influences of each

alloying according to the table below.

Table 9 – Most used families of Mg alloys [32].

Alloy

group

Alloy

grade Al Mn Zr Zn Other Key features

Mg –

Mn – Al

- Zn

AZ10 1.2 0.2 - 0.4 -

Low cost extrusion alloy,

moderate strength and high

elongation

AZ21 2.0 0.15 - 1.2 - Extrusions

AZ31 3.0 0.3 - 1.0 - Moderate strength

AZ61 6.5 0.3 - 1.0 - General purpose and

moderate cost

AZ80 8.5 0.12 - 0.5 - Extruded products and press

forgings, heat treatable

Mg –

Zn - Zr ZK21 - - 0.45 2.3 -

Moderate strength extrusion

alloy

14

Continuation of Table 9

ZK40 - - 0.45 4.0 - High yield extrusion, lower

strength than ZK60

ZK60 - - 0.45 5.5 - High strength and good

ductility

Mg – Y

- RE

WE43 - - 0.7 - 4.0Y

3.4RE

High temperature creep

resistance(300ºC), long term

exposure (200ºC)

WE54 - - 0.7 - 5.2Y

3.0RE

High strength, heat treatable

applied to 300ºC

According Mohd et al. [32] the first alloy group was Mg – Mn – Al – Zn. Magnesium turned in to

the main element in the matrix of the alloy. For this group, Wei Rong [33] proposed to use the

addiction of manganese as it’s beneficial tensile properties in alloys with high ductility without

harmful properties in the extruded alloys due un-recrystallized grains. In the case of magnesium

alloys, it is very important in improving ductility and strength. That is possible thanks to the effect

of microalloying. It has been shown that the influence in microalloying with the use of manganese

results in a delay of recrystallization.

The second group is related to the introduction of zirconium in magnesium alloys containing zinc.

Combining zirconium is important as it supresses the grain growth of zinc in magnesium alloys

[33, 34].

Table 10 - Addiction of zirconium and manganese to magnesium alloys

Author Year Study

Mohd Ruzi H. et.

al. [32] 2009

Increase of workability at elevated temperatures in

improvement of properties by re-crystallization in

magnesium alloys: ZK and WE alloy system.

Jing Liu et. al.

[34] 2015

Effects and influence of Zr in mechanical properties for

Mg alloys, improvement applications on aerospace and

automobile sectors.

Peng-Wei Chu et

al. [35] 2015

Effects WE43 Mg alloy immersed in 3.5wt% NaCl solution saturated with Mg(OH)2 and microgalvanic effect.

Wei Rong et. al.

[33] 2016

Mn addition benefits for the tensile properties of casting

Mg-15Gd-1Zn alloy, high ductility and poor harmful for

the properties of the extruded alloys due to the coarse

un-recrystallized grains.

In recent studies, the role of rare earths (RE) in the alloy has been discussed. For example, R.

Pinto [36] found that Mg alloys can have serious improvements in corrosion resistance by adding

15

aluminium, zirconium, rare earths (RE) or lanthanides and yttrium. Upon analysis of results, it is

observed MgO formation in the surface film covered by Mg(OH)2 layer. In this study, it is

concluded that the highest charge transfer resistance values and lowest anodic current densities

were observed in absence of chloride. That revealed a more protective passive film.

In another study using the same alloys, R. Pinto [37] observed the corrosion behaviour of rare-

earth containing magnesium alloys in borate buffer solution. Using an alkaline solution (pH 13),

Mg with RE shown evidences, with and without chlorides, of an inner MgO layer and outer

Mg(OH)2. Mass spectrometry shown the amount of secondary ion was too small when compared

with the amount of MgO and Mg(OH)2. Other important observation was, when added, the alloy

elements revealed positive influence in the corrosion resistance such as Zr and RE. To support

this, X-ray diffraction (XRD) results were analysed and revealed diffraction peaks on alloys of

crystalline phases where could be seen corrosion products: MgO and Mg(OH)2 for ZK31 and

EZ33. No diffraction peaks are observed in WE54, which is constituted by RE elements in the

alloy. The conclusion of the experiment was that the presence of RE elements improves the

corrosion resistance in Mg-RE alloys.

Mirzadeh et al. [38] applied gadolinium (Gd) to explain the dislocation glide and climb, as well as

for recrystallization and mechanical properties improvement. The use of RE Gd allowed to

conclude that dislocation glide occurs in form of a viscous drag that interacted with Gd atoms and

increased resistance; the mechanical twinning in low temperatures and high strain rates were a

bigger determination parameter for stress level and some deviation from theoretical values were

not significant for strains closer to the peak point.

Table 11 - Rare earth alloying influence in Mg alloys experiments. Author Year Study

R.Pinto et. al.

[36] 2010

Influence of chloride ions in the resistance of the film

formed in alloys ZK31, EZ33 and WE54. Decrease of

the conductive character of the film.

R.Pinto et. al.

[37] 2011

Galvanic potential of Mg alloy in 3% NaCl shown that

metallic Mg is not in direct contact with divalent ion

(Mg2+), protected by a passive layer. Contain hydrated

oxides and hydroxides. Non-buffered solutions become

alkaline solutions, stabilize magnesium hydroxide

species and reduce corrosion rate.

H.Mirzadeh et. al.

[38] 2015

Polycrystalline magnesium alloys became ductile at

elevated temperatures, dynamic recrystallization and

mechanical twinning for Mg – 3Gd - 1Zn.

Magnesium alloys have been produced to provide cost reduction but there were some concerns,

one of them was related with its corrosion behaviour. Recently, the groups with more interest in

Magnesium alloys application were brought always with a very big concern: high corrosion

16

behaviour tendency. More recently, alloys such as WE43 and, particularly, WE43C have gained

more interest. This alloy has in its composition rare earth elements such as yttrium, neodymium

and zirconium. According to Elektron-Magnesium®, the available alloy has, in terms of weight, the

most relevant influence with 4% of yttrium, followed by neodymium with 2.25% and zirconium with

0.5% (the molecular weight is 88.906, 144.240 and 91.224 g/mol respectively). Yttrium and

neodymium are Rare Earths (RE). The WE designation group suggests that yttrium is one of the

most essential elements in the Mg alloy, since it took the higher percentage in the elements that

were contained in the alloy, followed by the remaining rare earth elements.

Table 12 - General Physical Properties of WE43C [39]

Density 1.8 g/cm3

Melting point 540-640°C

Table 13 - General Mechanical Properties of WE43C [39]

Tensile strength 250 MPa

Poisson’s ratio 0.27

Elongation 2%

Hardness, Vickers 85-105

Thermal conductivity 51.3 W/mK

Thermal expansion co-efficient 26.7 µm/m°C

In a study of WE43 alloy, Peng-Wai Chu [35], observed the evolution of hydrogen bubbles and

Mg(OH)2 that protrude from the surface with a hemispherical shape (“domes”) formed on Zr-rich

impurity particles by a micro galvanic effect.

WE43 is of interest because if in combination with high specific strength, a good creep resistance

and good castability. Achieving high strengthening is usually accomplished by adding alloy

elements. This addition will change the chemical and electrochemical properties of the alloy. For

WE43 the main alloy elements added are Y and Zr – grain refiner that is responsible for localized

corrosion. Y has an uniform distribution, which increases WE43´s corrosion resistance. Zr, a grain

refiner, is responsible for its localized corrosion.

According to Peng-Wai Chu et al. [35], Mg and Mg alloys have a by-layer Mg(OH)2/ MgO on the

top of an inner MgO layer. In microstructure characterization, it is observed a well-defined

rectangular Y structure and a less defined Zr element (Figure 3).

17

Figure 3 - SEM images of as-polished surfaces from the (a) solution-treated and (b) peak-aged WE43 showing Y-rich

and Zr-rich particles. (c) Back-scattered electron (BSE) SEM image of peak-aged WE43 showing the fine scale

precipitates throughout the grains and along grain boundaries. [35]

Peng-Wai Chu et al. [35] observed an OCP increase that suggests passivation and layer

formation on the surface. After reaching a maximum, the OCP curve shows a decrease that

suggests a breakdown of the surface layer. Peng-Wai Chu et al. [35] also suggested that there is

no significant crevice corrosion around the edge samples, which could contribute for

overestimation of the corrosion rate for Tafel method.

Using EDS analysis, the contact layer with the surface alloy showed Mg and O with a ratio 𝑂

𝑀𝑔 =

1, which reveals the presence of MgO, with traces of other elements. Furthermore, the top of the

film has a ratio 𝑂

𝑀𝑔 = 2 that suggests a presence of Mg(OH)2. This layer is formed by corrosion

reactions on the alloy. Meanwhile, it is known there is a H2O formation and formation of H2 that

leads to a pH increase.

Looking at Zr rich particles, their cathodic effect leads to a protection zone in neighbourhood of

Mg(OH)2 formation. Those effects have a dominant galvanic connection with Zr particles, turning

it into cathode zones.

The corrosion layer shows two types of reactions: oxidation front and hydration front. This last

one propagates while corrosion occurs. Microstructure analysis also shown a porous layer, where

H2O and solvated ions can penetrate it. This layer increasing mechanism is dominated by internal

transport of H2 diffusion. Peng-Wai Chu et al. [35] also concludes that combining Y and Nd RE

elements could improve corrosion behaviour, because those RE elements are able to form a more

efficient and protective layer that decreases the influence localized corrosion promoted by Zr.

To obtain better corrosion behaviour, Zr and Mn were added and undesirable elements were

removed [35].

WE43 is a high strength Magnesium Alloy which offers good mechanical properties both at

ambient and elevated temperatures. The alloy mainly contains yttrium and neodymium. WE43

can be used successfully in temperatures up to 300°C and benefits from good corrosion

resistance [40]. I.J Polmear [21] gives a description about the main rare earth elements in Mg

alloy: yttrium, neodymium and zirconium. Yttrium has a maximum solid solubility in Mg of 12.5%

which, is greater than another RE. That shown superior creep resistance and an acceptable

ductility of 6%. On the other hand, neodymium promotes an inoculant behaviour on the grain

18

refinement that is responsible for good casting properties. Zirconium was added to the alloy

because of his high resistance to corrosion.

Lou [41] in his work, has shown the influence of adding rare earths to improve the strength in Mg

alloys. The alloy investigated was Mg – Zn – Zr – Y. According to the optical micrograph results,

some fine grains that showed RE elements have suppressed effect in the dynamic

recrystallization. This recrystallization promotes the increase of strength on Mg alloys. In what

concerns the analysis of the structural matrix, it was observed that Mg matrix was surrounded the

Zr particles and the RE rich phase was formed in the grain boundaries.

Few years later, it has reported that Mg alloys including Y and Zr, had higher tensile properties

without homogenization. In his study, Xu [42] concludes that an increasing of Y content promotes

a decreasing in the grain size on the alloy. Moreover, the increase content in Y let to a change in

failure nodes of the tensile samples from ductile – fragile failure to ductile failure – the “woody

fracture” with large number of precipitates

.

According Qiang Li [43] when combined Nd and Y in Mg alloy, there were observed changes

affecting the microstructure and mechanical properties in the alloy. The addition of the rare-earth

metals also results in a significant improvement castability and elevated temperature strength. In

terms of mechanical properties, it is observed that the increase of Y could recover the tensile

strength lost by adding Zr.

This brief analysis led him to conclude that when small values of Nd (1wt%) were added in to the

alloy, the interdendritic phase crystalized into a continuous network in two morphologies: ribbon

– shaped precipitates and lamellar eutectics with α-Mg, but when increased more than 2 wt%, the

continuous lamellar eutectics became predominant; when Y were added in a range of 0.5-1 wt.%,

led to a lamellar eutectic with α-Mg. He also concluded that by adding Nd and Y to Mg alloys

dendritic size was refined, increased interdendritic phase amount and improved the thermal

stability between interdendritic phases. Moreover, he also concludes there was a dependence on

tensile properties on microstructure: less continuity of intergranular phases would favour the

strength and elongation.

In another investigation, Ding [44] studied microstructure and mechanical properties of hot-rolled

Mg alloy with Zn, Nd and Zr elements. On microstructure evolution, some of the grains shown a

different distribution and others an abnormal growth in the alloy. There were also observed

intermetallic particles distributed uniformly along the rolling direction. The addition of Nd results

in Mg – Zn – Nd intermetallics. This causes a reduction of solubility of Zn in Mg – matrix. Other

influence of the Nd addition was observed in tensile properties by increasing the ultimate tensile

strength (UTS), even so as more influence in the yield strength. In a range of increasing

temperatures, UTS would decrease while ductility increased and promoted better tensile strength.

Although a grain refinement is observed in presence of Nd, there are still large particles in the

boundaries that behave as crack sources reducing tensile strength.

19

RE elements are largely soluble in Mg, Farzadfar et al. [45] considered Y an excellent element to

explore the weakening effect. The presence of Y in Mg solid solution inhibits recrystallization. By

taking results on XRD, it is observable a suppression of Y resulting in absence of necklacing and

soft regions on XRD. Moreover, yttrium suggests hindering effect on boundary mobility; Y also

retards kinetics in grain recrystallization and coarsening in the Mg.

On a more recent study, Kristina [46] studied for the first-time magnesium alloy WE43 as twin-

roll-cast. Twin-roll-casting enables magnesium production strip in an economic way due to the

greatest properties of magnesium. In the author’s experiment, the commercial WE43 alloy used

contained RE (Y, Nd and Zr). After the process was applied, typical dendritic microstructures were

not observed; small grains appeared near the surface and in the mid-thickness. In the

interdendritic areas eutectic and intermetallic compounds were observed but grains shown

irregular forms with serrated grain boundaries. Due to solidification conditions, the eutectic β-

phase was located along the grain boundaries and segregation was observed in the mid-

thickness. Since the eutectic phase was based in Nd/Y ratio and was consisted in Mg, Y and Nd,

the eutectic β-phase conforms to ternary Mg14Nd2Y phase. It was achieved 410MPa for UTS and

376 MPa for YS and an increase of elongation of 2.8% when compared with other commercial

alloys. It was also concluded that hot rolling allowed a better intermediate heat treatment that

leads to improvement of grain refinement and directly affects the recrystallization of WE43. The

results of hot rolling shown an increase in corrosion resistance that was been proven with surface

analysis.

Table 14 - Influence of recrystallization on corrosion behaviour of Mg alloys with RE

Author Year Study

Z.P.Lou et.

al. [41] 1995

Strengthening of RE effect in Mg alloys, addiction of yttrium and

its effects on microstructural.

D.K. Xu et. al.

[42] 2007

Influence of the microstructure and crystallographic texture on the

mechanical properties influenced by different yttrium contents in

Mg alloys; the increasing yttrium content promote spacing of the

distributed particles on the fracture surface, influenced the

transverse mechanical properties.

Qiang Li et.

al. [43] 2007

Refinement of dendritic size, increase of interdendritic phase

amount and improvement of thermal stability of interdendridritic

phase. Dependence of tensile properties on microstructure:

deteriorate strength and elongation.

Continuation of Table 14

20

Ding

Wenjiang et.

al. [44]

2008

Formation of Mg – Zn – Nd particles in the grain boundaries. Mg

– Zn – Nd – Zr alloys have better tensile strength compared with

Mg – Zn – Zr alloys; increase of ductility in presence of Nd and Zr.

S.A.Farzadfar

et. al. [45] 2012

Texture weakening and recrystallization in rolled Mg alloys;

effects of yttrium as inoculant agent in grain refinement. Re-

tardiness kinetic of recrystallization and grain coarsening.

Hindering effect of Y on boundary mobility.

Kristina Neh

et. al. [46] 2014

Rare earth elements of WE43 promote higher strength values;

higher resistance to corrosion and high potential for applications.

Jakraphan et.

al 2017

Passivation behaviour of WE43C Mg–Y–Nd alloy in chloride

containing alkaline environments; analysis for pitting corrosion by

heat treatment condition with NaOH maximum concentration up

to 0.1M.

In a recent study Jakraphan et. al [47] analyse passivation behaviour of WE43C Mg–Y–Nd alloy

in chloride containing alkaline environments and conclude localized corrosion was observed to

be initiated on the secondary phases. He also concludes the Zr-rich intermetallic particle having

spherical morphology in treated conditions and the irregular shaped were susceptible to localized

corrosion.

The passive layer of the WE43C formed under potentiostatic condition contained MgO, Mg(OH)2,

and RE2O3 phases.

2.6 Corrosion

Significance of corrosion

One of the definitions of corrosion is the destructive result of chemical reactions between the

materials and the environment. In general, corrosion occurs in metals and normally by oxidation

due the electrochemical process but there are some other types of materials that can suffer

corrosion such as wood, ceramics or concretes [48] . In United States of America, in 1976, the

costs associated with corrosion are estimated between $8 billion and $126 billion and in 1982 it

rises to approximately $126 billion what becomes too expensive [49]. In 2016, the NACE

international (National Association Corrosion engineers) estimated global corrosion in $2.5 trillion

[50]. Figure 4 shows some corrosion examples.

21

Figure 4 - Corrosion Example [51].

Mechanisms of Corrosion

In metallic materials, the corrosion could be explained by an electrochemical process where a

transfer of electric charges occurs over an aqueous environment, self-denominated electrolyte.

The standard reaction for oxidation is showed in equation (2.1) [52]

𝑀 → 𝑀𝑛+ + 𝑛𝑒− (2.1)

According to D.A. Jones [48] the oxidation reaction, also known as anodic reaction, involves an

increase of oxidation state in the metal (from 0 to +n) in the electrode that is called anode. The

loss of electrons in the anodic reaction is earned by another specimen, decreasing the oxidation

number. This reaction is called reduction or cathodic reaction. The electrode where is observed

a decrease of oxidation number is called cathode.

Figure 5 -Schematic diagram of the dissolution of a metal in acidic medium. [48]

The example on Figure 5 shows the dissolution of metal M according oxidation reaction (2.1)

where the electrons being released went to the bulk of the metal and M2+ ions into the surrounding

22

solution of HCl. The electrons move through the metal, working as cathode, where they will reduce

the H+ acidic ions and form molecular hydrogen. The following equations show some examples

of ion oxidation:

𝐹𝑒 → 𝐹𝑒2+ + 2𝑒− (2.2)

𝑁𝑖 → 𝑁𝑖2+ + 2𝑒− (2.3)

𝐴𝑙 → 𝐴𝑙3+ + 3𝑒− (2.4)

There are reactions with high importance for corrosion, such as the reduction of water into

hydrogen gas [53] or the reduction of dissolved oxygen. In both cases these reactions may be

written in different forms for acidic and neutral/alkaline media:

Electrode Potentials

When there is more than one metallic specimen in contact with an electrolyte, it’s necessary to

know the tendency of the metals to suffer corrosion in a determinate environment. The zero-

reference point of the Electrochemical Series has been chosen by selecting a hydrogen cell at

standard state – 25oC and atmospheric pressure [54], known as standard hydrogen electrode

(SHE). SHE consists of a platinum specimen immersed in unity activity acid solution where H2

gas is bubbled at standard state.

2𝐻+ + 2𝑒− → 𝐻2 (2.5)

2𝐻2𝑂 + 2𝑒− → 𝐻2 + 2𝑂𝐻− (2.6)

𝑂2 + 4𝐻+ + 4𝑒− → 2𝐻2𝑂 (2.7)

𝑂2 + 2𝐻2𝑂 + 4𝑒− → 4𝑂𝐻− (2.8)

23

Figure 6 - Schematic of a Standard Electrode Potential [55].

As shown on the schematic of a Standard Electrode Potential, in Figure 6, it is possible to know

the tendency of a metal to oxidize.

Table 15 - Standard Electrochemical Series [56].

Reaction Standard Reduction Potential (V)

𝐴𝑢3+ + 3𝑒− → 𝐴𝑢 +1.498

𝑃𝑡2+ 3𝑒− → 𝑃𝑡 +1.118

𝐴𝑔+ + 𝑒− → 𝐴𝑔 +0.799

𝐹𝑒3+ + 𝑒− → 𝐹𝑒2+ +0.771

𝐶𝑢2+ + 2𝑒− → 𝐶𝑢 +0.342

𝑆𝑛4+ + 2𝑒− → 𝑆𝑛2+ +0.150

2𝐻+ + 2𝑒− → 𝐻2 0

𝑆𝑛2+ + 2𝑒− → 𝑆𝑛 -0.138

𝑁𝑖2+ + 2𝑒− → 𝑁𝑖 -0.250

𝐶𝑜2+ + 2𝑒− → 𝐶𝑜 -0.277

𝐶𝑑2+ + 2𝑒− → 𝐶𝑑 -0.403

𝐹𝑒2+ + 2𝑒− → 𝐹𝑒 -0.447

𝐶𝑟3+ + 3𝑒− → 𝐶𝑟 -0.744

𝑍𝑛2+ + 2𝑒− → 𝑍𝑛 -0.762

𝐴𝑙3+ + 3𝑒− → 𝐴𝑙 -1.662

𝑀𝑔2+ + 2𝑒− → 𝑀𝑔 -2.372

Table 15 shows the tendency of oxidation elements increase as they descend in table. Elements

that are in the top of the table are considered noble as gold or platinum.

Standard conditions are not usually seen due to several reasons: temperature different from 25°C,

ion concentration different from unity, purity of metals, possible gaseous electrodes forming on

the surface. Thus, the reduction potentials are conditioned by environment, and relative positions

of each reaction on the series may vary. As an example and according to [53], titanium, despite

being practically at the bottom of the table in the electrochemical series, will shift to a much higher

position in a galvanic series obtained for seawater.

24

Passivity

Metals such as iron, nickel, chromium, cobalt and titanium suffer a decrease in corrosion rate

above a critical potential, Ep, in situations where is expected the corrosion dissolves the material,

which is defined as passivity. This phenomenon occurs under certain environmental conditions

for these metals. This passive film as about a nm of thickness of hydrated oxide, that is not enough

for suppression of the corrosion but it helps to reduce it considerably. Passive corrosion rates are

low, estimated in 103 to106 times below the corrosion rate, in the active state is not unusual [57,

58].

Passivity is, in some constructions, a factor to consider because without this thin layer that isolates

surface from the environment, many of the structures wouldn’t resist to violent environmental

conditions [53].

Although an important characteristic, passivity brings some problems. A so thin film could

breakdown and result in an unpredictable localized form of corrosion.

2.6.1 Types of corrosion

There are many types of corrosion which will be succinctly described next.

2.6.1.1 Uniform corrosion

Figure 7 - Uniform Corrosion [59, 60]

Uniform corrosion refers to the relatively uniform reduction of thickness over the surface of a

corroding material. The corrosive environment should be able to access the same form to the

entire metal surface [60]. Some typical examples are rusting steel or iron. This type of corrosion

is the principal responsible for the greatest metal decay and the one who as much more

predictability, therefore should be avoided whenever it is possible. However, it is not considered

the most problematic type of corrosion. Localized corrosion is more difficult to predict and avoid.

25

2.6.1.2 Galvanic Corrosion

Figure 8 - Galvanic Corrosion [4]

Galvanic Corrosion, also known as bimetallic, happen when two dissimilar alloys are coupled in

the presence of a corrosive electrolyte. While one is protected (being positive or noble) from the

corrosion, the other one is preferentially corroded (active in the Galvanic series) [61, 53].

One of the greatest examples in the world is the Statue of Liberty in New York. The skin of the

statue was made from copper (cathodic) and the structure from cast iron (anodic). Extensive

galvanic corrosion occurred, leading to a major repair in 1984. The entire cast iron interior was

removed and replaced with a low-carbon, corrosion resistant stainless steel [62].

This corrosion type also depends on the relative anode-to-cathode surface areas exposed to the

electrolyte. In a small anodic area, when compared with the cathode, the last one will continue to

receive electrons, which may lead to complete consumption, so is advisable to use larger anodic

areas than cathodic ones.

Another way to protect metals is cathodic protection where there need to use two different

materials close together in to the galvanic series and a use of a third metal, less noble, that as

the purpose to be sacrificed.

2.6.1.3 Crevice Corrosion

According to Recognition, Mechanisms & Prevention [63], crevice corrosion is referred to the

localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two

joining surfaces. Between the crevices and the gaps of two metallic surfaces there are stagnated

volumes of the solution that don’t allow oxygen renovation, while outside the metal gap both

metals could be resistant to corrosion.

26

Figure 9 - Crevice Corrosion [64, 65]

The most efficient environmental attack (in presence of sulphates, nitrates and chloride) occurs

in the presence of H+ and Cl- that destroys the passive protective film. The use of welded butt

joints instead of riveted or bolted joints in new equipment or to eliminate crevices in existing lap

joints by continuous welding or soldering are two forms to preventing crevice corrosion.

2.6.1.4 Pitting Corrosion

Figure 10 - Pitting Corrosion [66, 67]

Pitting corrosion it is a localized form of attack in passive surface. It occurs by forming a pit that

could be deep, shallow or undercut and results in one of the most severe forms of corrosion that

occurs in the presence of aggressive ions like chlorides [67].

The pit is intrinsically related with crevice corrosion, they share the same mechanism. The pit is

serving crevice to restrict transport between the bulk solution and the acid chloride pit anode.

27

As galvanic corrosion, pitting corrosion also depends on the relation between anodic/cathodic

areas. Other important aspect, according NACE [68], is the density of the pit that shows that a

single pit can grow up and deep very quickly rather than the small ones.

2.6.1.5 Environmentally Induced Cracking

Figure 11 - Environmentally Induced Cracking [69]

As a form of corrosion environmentally induced cracking, takes in account the effect of corrosion

and the applied tensile strength on the surface of the material in three distinct types of failure: (1)

stress corrosion cracking (SSC), (2) corrosion fatigue cracking and (3) hydrogen-induced cracking

(HIC) [70].

In SSC, prevention can be achieved by decreasing stress with the elimination the main corrosive

species by using processes like degasification, demineralization, and distillation or by using

coatings. To achieve corrosion fatigue cracking, the material must be subjected to a stress-cycle

frequency where the lower frequencies result in a more severe corrosion, explained by the great

contact time between material and environment with aggressive conditions like pH, temperature

and oxygen content. Prevention, is achieved by reducing cyclic forces and using redesigned parts

of the material structure or even replacing the alloy for other one less susceptible to corrosion

and/ or fatigue. HIC is about atomic presence of hydrogen. In normal conditions hydrogen

combines forming molecular hydrogen gas

2𝐻+ + 2𝑒− → 2𝐻 → 𝐻2 (2.8)

However, hydrogen itself tends to combine with sulphide, cyanide or even antimony ions that

inhibit the molecular hydrogen formation and increases the hydrogen concentration on the surface

of the material, facilitating the penetration into material, leaving it more susceptible to acidic attack

for example by hydrogen sulphide. It is advised to protect the material by using cathodic protection

[71].

28

2.6.1.6 Intergranular Corrosion

Figure 12 - Intergranular Corrosion [72, 73, 74]

Intergranular corrosion is one of the localized forms of corrosion seen in adjacent zones of grain

boundaries of a metal alloy. Its formation occurs due to corrosion microcells at the grain

boundaries and propagates all over the material’s mechanical properties and promotes fracture

under mechanical load.

Intergranular corrosion is most common observed in the austenitic stainless steels. It could occur

in areas where the temperature is between 450-800oC, where chromium is close to the grain

boundaries and combined with carbon, forming Cr23C6 what creates depletion of chromium (below

10% (w/w)), shows the quantity needed to turn into stainless [73].

2.6.1.7 Erosion Corrosion

Figure 13 - Erosion Corrosion [75]

Corrosion by erosion is a phenomenon caused by the relative movement between a corrosive

fluid and a metal surface. The mechanical aspect of the movement is important and friction and

wear phenomena can be involved. Turbulence phenomena can destroy protective films and

cause very high corrosion rates in materials otherwise highly resistant under static conditions. In

the laminar flow regime, the fluid flowrate has a variable effect depending on the material

29

concerned. To protect the metal, it is necessary to choose a material more resistant and regulate

the process conditions (temperature, pH, flow-rate, etc.) [70]

Cavitation-corrosion is a form of erosion caused by the "implosion" of gas bubbles on a metal

surface. It is often associated with sudden variations in pressure related to the hydrodynamic

parameters of the fluid. One of the main forms to prevent cavitation-erosion is the use of rubber

or plastic coatings [76].

2.7 Applications of Mg alloys

2.7.1 Automotive sector

Nowadays, there are five key areas of application for Mg alloys. In the automotive sector, Mg

alloys combine the high strength properties and low density for innovative applications, weight

reduction will improve the performance of a vehicle by reducing the rolling resistance and energy

is used in acceleration, thus reducing fuel consumption and, moreover, a reduction in the

greenhouse gas CO2 can be achieved [77].

Figure 14 - Examples of magnesium alloy applications on automobiles [78].

30

2.7.2 Aerospace industry

In the aerospace industry, it’s essential to reduce weight of air and space craft, as well as

projectiles, to achieve decreases in emissions and greater fuel efficiency. Spacecraft and missiles

also contain magnesium in its alloys. Magnesium can withstand the extreme elevated

temperatures, exposure to ozone and the impact of high energy particles and matter [77].

Figure 15 - Aerospace examples for military and civil applications [79].

2.7.3 Medicine

In the medical sector, it is also of great importance too, because since the beginning of the XX

century Mg alloys have been used as an orthopaedic biomaterial. Its properties made Mg more

attractive for use in implants and other applications where common implant materials have

densities in range from 3.1-9.2g/cm3, but the density of natural bone is 1.8-2.1g/cm3. Magnesium

alloys are much more comparable, at a density of 1.74-2.0g/cm3. Magnesium mechanical

properties match well to those of the natural bone compared to other materials regarding fracture

toughness, elastic modulus and compressive yield strength [77].

Figure 16 - Magnesium implants in the arm (left) and femur (right) [80]

31

2.7.4 Electronics

Nowadays on the electronics market, Mg alloys are used to replace plastics. This is very useful

in increase the durability and robustness of the small and portable devices and itis present in

most multinationals in the world such as Sony® and IBM® [77] and some emergent multinationals

like Xiaomi in Asia [77] [81] .

Figure 17 - Examples for electronics applications [82, 83]

2.7.5 Sports

Magnesium is valued for its use in sports equipment due to its lightweight and impact resistance.

It can be shaped into intricate shapes, which is ideal for use in field sports like tennis. The damping

effects of the alloys also make them a good candidate for bicycle frames and the chassis of

skates, where the magnesium can absorb shock and vibration [77]. It also could be an advantage

in the day to day of people with motor disabilities due to the lightness of the Mg alloys.

Figure 18 - Sports and disabled utilities examples [84].

2.7.6 Other Applications

Apart from previous mentioned areas of application, Mg alloys could be shaped in many small

things like optical and hand-held design tools, hand-held work tools, and small household

appliances such as vacuums. All of this because there are huge benefits for its use, also the

demand for sustainable, lightweight and recyclable materials continues to increase [1] [77].

33

Chapter 3

Experimental Method

3.1 Overview

The materials and techniques used during this investigation are briefly described throughout this

chapter.

3.2 Materials and Solutions

The material used was block-shape for both alloys, ASTM 7475 and ASTM WE43C, which was

cut in samples of 14 mm x 10 mm x 10 mm. The first sample has 88.5 - 91.5 % of aluminium and

the second sample has 4% of yttrium and 2.25% of neodymium. Both materials were supplied by

Material Property Data – MatWeb. All percentages are weight percentages.

Acetic glycol, Nital and Keller etching solutions were prepared in the laboratory to be used in

metallographic contrasting. Acetic glycol and Nital were both used in ASTM WE43C and Keller

solution was used in ASTM 7475. The reagents used are listed in Table 16, Table 17 and Table

18 .

All electrochemical experiments were performed in 0.05 M NaCl solutions.

Table 16 - Nital solution.

Reagent Composition (mL/100mL) Brand

HNO3 5 Sigma - Aldrich

C2H6O 95 AppliChem: Panreac

Table 17 - Acetic Glicol


CH3CO2H 20 AppliChem: Panreac


C2H6O2 60 AppliChem: Panreac

H2O 19 Milipore

34

Table 18 - Keller solution.


H2O 190 Millipore


HF 2 Sigma - Aldrich

HCl 3 AppliChem: Panreac

3.3 Sample Preparation

Both alloys were previously cut from the block-shaped material, resulting in 14 mm x 10 mm x 10

mm samples; the cut was performed using a Minitom cut-of machine from Struers.

Figure 19 – Struers Cut-off machine.

3.3.1 Untreated Samples

After the cutting treatment, samples were connected to a wire copper tip, with help of a little drop

of superglue, as in excess it could interfere with the wire-metal connection, and then they were

left to dry. A small coverage of silver paint was applied and once again allowed to dry, this time

for 24 hours. After that, a minimum ohmic resistance was checked (less the one ohm) with a

multimeter. When the samples were completely dry, the next step was to attach the samples to a

plastic mold and let them soak in with a mix of epoxide resin and hardener (EpoxiCureTM 2 epoxi

resin and EpoThin TM 2 Epoxide Hardener, by Buehler) and they were left to dry for at least 12

hours. Once the samples were dry, the mould was removed resulting on a free-surface

surrounding by the insulating resin Figure 20.

35

Figure 20 - Preparation sample sequence.

Both alloy surfaces were mechanically polished using a sequence of grind paper of SiC with

different grains sizes, starting with P360 to P4000 in the Struers LaboPool-25 machine. After this

procedure, the samples were polished by cloth in a 3 and 1µm diamond paste (METADI® II from

Buehler and Diamond Compound Sparkling from Microdiamant, respectively). Then the main

impurities on the surface were removed by a cleaning ultrasonic bath in 2-propanol (in a Branson

1200 Ultrasonic Cleaner) for 10 minutes. Finally, beeswax was used for isolation of the edges

and for confining the active area of the specimen.

Figure 21 – Sample polishing sequence.

3.4 Electrochemical Measurements

During this investigation, the different electrochemical preparations were performed in a 3-

electrode cell containing a working electrode (sample of threated alloy), a reference electrode

(Saturated Calomel electrode) and a Pt coil acting for counter-electrode. In the working cell, it

was used a NaCl solution with 0.05 M. The working cell was then put in a Faraday cell to provide

protection against external interferences and the data was recovered by Gamry Instruments and

registered in Gamry Framework.

36

Figure 22 - Work Cell in a Faraday Cage.

Open Circuit Potential

Open Circuit Potential (OCP) is related to a difference that exists in the electrical potential and it

normally occurs between two device terminals when detached from a circuit involving no external

load. It is the initial step of electrochemical tests of this investigation and it is very relevant to

determine the corrosion potential, Ecorr [85] , evolution through time and measurements using the

potential difference between immersed metal and the Saturate Calomel Electrode.

When the system achieves the “steady state”, it is assumed that corrosion reactions (anodic and

cathodic reactions) have the same rate and the Ecorr corresponds to the potential value on the

surface of metal due to the interaction and reactivity of the metal and the solution. The use of the

Open Circuit Potential is of great significance because it precedes any electrochemical

measurement and provides qualitative information about the tendency for metal corrosion in

contact with the environment.

Polarization curves

Polarization measurements are destructive and couldn’t be repeated before previous mechanical

treatment [86].

The corresponding rates of corrosion reactions establish a relationship between the metal

potential and current density allowing to understand the corrosion behaviour of a specific

electrode- electrolyte combination by changing the potential of the metal with a stable value Ecorr,

through the flow of current.

37

In potentiodynamic polarization, it is possible obtain information such as the possibility of

passivate the metal in the environment where it works. Using curves allows, in a limited region,

to obtain parameters as Tafel coefficients or critical parameters (Enp or icrit) for phenomena such

as passivation or pitting corrosion. The current density in passive region, ipass, stays almost

immutable with the increasing potential until is possible to break down the layer and let the current

pass again – transpassive region [87, 88].

To obtain the polarization curves, after previously stabilization of the Open Circuit Potential

(OCP), a potential scan was performed, which started 10 mV below to Ecorr and upward to -0.25

V/ECS, with a scan rate to 0.167 mV/s (cathodic curve) and -10 mV above to Ecorr and down to

0.5 V/ECS, with a scan rate to 0.167 mV/s anodic curve). To collect data, it was used a

potentiostat by Gamry Instruments, INTERFACE1000/ ZRA 07087 connected to a computer.

Electrochemical Impedance Spectroscopy (EIS)

Introduction

The circuit ability to resist to a flow of an electric circuit is known as resistance. Resistance could

be defined by the ratio of voltage, E and current, I, also known as Ohm’s law

𝑅 =

𝐸

𝐼

(3.1)

Equation (3.1) it’s a simple relationship limited to ideal resistor, composed only by one circuit

element and has properties such as obeying Ohm’s Law at all current and voltage levels,

independence of frequency from resistance value or even with AC current and voltage signals

though a resistor are in phase simultaneous [89].

38

Theory behind EIS

To understand EIS, it is necessary to measure using a small excitation signal. The response of

the system to a sinusoidal potential will be a sinusoid at the same frequency but shifted in phase

observed in Figure 23 and measured by following equations:

Figure 23 - Sinusoidal current response to sinusoidal potential.

𝐸 = 𝐸0 sin(𝜔𝑡) (3.2)

𝐼 = 𝐼0 sin(𝜔𝑡 + ∅) (3.3)

Where 𝜔 the radial frequency, t is time, ∅ is the phase shift, 𝐼0 is the current’s amplitude and 𝐸0

is the amplitude of the signal. With both equations (3.2) and (3.3), it is possible, similarly deduce

Ohm’s law in a new equation

𝑍 =

𝐸

𝐼=

𝐸0 sin(𝜔𝑡)

𝐼0 sin(𝜔𝑡 + ∅)= 𝑍0

sin(𝜔𝑡)

sin(𝜔𝑡 + ∅)

(3.4)

To simplify the equation, it is a necessary to use a complex function by Euler equations (3.5),

(3.6) and (3.7)

𝑒𝑗∅ = cos(𝑗∅) + 𝑗 sin ∅ (3.5)

𝐸 = 𝐸0𝑒𝑗𝜔𝑡 (3.6)

39

𝐼 = 𝐼0𝑒𝑗(𝜔𝑡+ ∅) (3.7)

Obtaining

𝑍(𝜔) = 𝐸

𝐼=

𝐸0𝑒𝑗𝜔𝑡

𝐼0𝑒𝑗(𝜔𝑡+ ∅) = 𝑍0𝑒−𝑗∅ = 𝑍0 (cos(∅) − 𝑗 sin(∅)) = 𝑅𝑒 + 𝑗𝐼𝑚 (4.8)

Looking close into the impedance equation, there is a real and an imaginary part. Those parts are

essential to represent Nyquist plot, where real part is plotted in the X-axis and the imaginary part

is plotted in the Y-axis; each point that has Co-ordinarily defined represents one point of

impedance at different frequency.

Figure 24 - Nyquist impedance plot.

Another representation on EIS measurements is the Bode diagram where the impedance is

plotted the X-axis (as log(𝜔) or log(𝑓)) along with values of the impedance (|Z|=Z0) and the phase-

shift on the Y-axis.

Electrical Circuit Elements and Equivalents Circuit

The data of EIS is analysed by fitting it to an equivalent electrical circuit model with three electrical

elements: resistors, capacitors and inductors. The main purpose is to collect data after introducing

different frequencies to build a model with same response based in physical electrochemical of

the system, for example: models that contain a resistor which models the cell’s solution resistance

[90, 91].

40

Formulas that were used and obtained for the three electrical elements are listed in Table 19 and

present 3 important factors: the impedance of a resistor is independent of the frequency, has no

imaginary part and the current stays in phase; the impedance of an inductor increases with the

frequency, has only imaginary part and the current is phase shifted in phase with +/- 90o ; and

the impedance of a capacitor (inductor) decreases (increases) with the raise of frequency, with

only imaginary part [91].

Table 19 - Common electrical elements for impedance [91]

Component Current (vs) Voltage Impedance

Resistor R= 𝐸

𝐼 Z = R

Inductor E = L 𝑑𝐼

𝑑𝑡 Z = j𝜔L

Capacitor I = C 𝑑𝐸

𝑑𝑡 Z =

1

j𝜔C

The EIS model has several circuit elements, connected between them, in a network: in parallel or

series. In order to determine the impedance of a circuit, it is necessary to know how the circuit is

organized and the impedance of each element value [90, 91]. To calculate the impedance

elements in series:

𝑍𝑒𝑞 = ∑ 𝑍𝑖

𝑁

𝑖=0

(3.9)

And for impedance elements in parallel:

1

𝑍𝑒𝑞

= ∑1

𝑍𝑖

𝑁

𝑖=0

(3.10)

Using Ohm’s law, the resistor would be in direct proportion to the applied potential difference and

current; the potential sinusoidal wave response is a sinusoidal current with the same phase and

frequency (∅ = 0) [90] .

41

𝑅 =

𝐸

𝐼 ⟺ 𝐸0𝑒𝑗𝜔𝑡 = 𝐼0𝑒𝑗(𝜔𝑡+ ∅) 𝑅 ⟺ 𝐸0𝑒𝑗𝜔𝑡 = 𝐼0𝑒𝑗𝜔𝑡𝑅 ⟺ 𝐸0 = 𝐼0𝑅 ⟺ 𝑍 = 𝑅

(5.11)

For capacitance, C, impedance for capacitor is:

𝐼 = 𝐶

𝑑𝐸

𝑑𝑡⇔ 𝐼0𝑒𝑗(𝜔𝑡+ ∅) = 𝐶

𝑑(𝐸0𝑒𝑗𝜔𝑡)

𝑑𝑡⇔ 𝐼0𝑒𝑗(𝜔𝑡+ ∅) = 𝐸0𝑗𝜔𝐶𝑒𝑗𝜔𝑡 ⇔

1

𝑗𝜔𝐶= 𝑍0𝑒−𝑗∅ ⇔ 𝑍

= 1

𝑗𝜔𝐶

(3.12)

Finally, for inductor, impedance is:

𝐸 = 𝐿

𝑑𝐼

𝑑𝑡⇔ 𝐸0𝑒𝑗𝜔𝑡 = 𝐿

𝑑(𝐼0𝑒𝑗(𝜔𝑡+ ∅))

𝑑𝑡⇔ 𝐸0𝑒𝑗𝜔𝑡 = 𝐼0𝐿𝑗𝜔𝑒𝑗(𝜔𝑡+ ∅) ⇔ 𝑍0𝑒−𝑗∅ = 𝑗𝜔𝐿 ⇔ 𝑍

= 𝑗𝜔𝐶

(3.13)

EIS applied to corrosion studies

Considering a general reaction of 𝑀𝑒 ⟺ 𝑀𝑒𝑛+ + 𝑛𝑒−, or more generally 𝑅𝑒𝑑 ⇔ 𝑂𝑥 + 𝑛𝑒−, only

involving a charge transfer process, makes the explanation of the total impedance analysed in

three different elements: the ohmic resistance of the solution, the double layer capacitance and

the charge transfer resistance.

Electrolyte resistance can define ohmic resistance. The solution resistance (Rs) is one significant

element in an electrochemical cell’s impedance since the electrical wire resistance and the

internal resistance of the electrodes are negligible (the actual potentiostats can compensate the

solution resistance between reference and counter electrodes). Although they are negligible, the

solution resistance between the reference and the working electrode must be considered in the

model’s formulation [90].

In the double layer capacitance, separation of charges in both sides in the metal-electrolyte

interface, acts like a parallel plate capacitor by assuming a non-faradaic current (assuming

successive charges and discharges that allow the current to pass through in a discontinuous form.

Double layer capacitance, Cdl , depends on: thickness, ionic concentration and dielectric constant

of the electrolyte. In the present case, linearity could only be found at the double layer for small

perturbation amplitudes (normally with ∆V < 20 mV).

42

The charge- transfer resistance (Rct) defined as the resistance to electron addition or removal, on

the reactional or faradaic component of the system. It is related with the kinetic constant, k0,

symmetry coefficient, α and concentrations for oxidized and reduced species, Cox and Cred ,

respectively and it is related using

𝑅𝑐𝑡 =

𝑅𝑇

𝑛2 𝐹2 𝑘0 𝐶𝑜𝑥𝛼 𝐶𝑅𝑒𝑑

1−𝛼 (3.14)

Where R is constant of perfect gases, T absolute temperature in kelvin, 𝑛 the number of electrons

for the reaction and F the Faraday’s constant. In this approach, it is normal to consider the use of

two other elements, which are the Warburg Impedance, W, and Constant Phase Element (CPE).

The Warburg Impedance is related with the mass transfer (diffusion) of electro-active species and

W has a small value in high frequencies. Diffusing species don’t need to move far way; in low

frequencies, the reactants should diffuse further which implies the increase of the Warburg

Impedance [90]:

𝑊 = 𝜎𝜔−21 (1 − 𝑗) (3.15)

Where 𝜎 is the Warburg coefficient and is defined by equation (3.16):

𝜎 =

𝑅𝑇

𝑛2 𝐹2 𝐴√2 (

1

𝐶𝑂𝑥√𝐷𝑂𝑥

+ 1

𝐶𝑅𝑒𝑑√𝐷𝑅𝑒𝑑

) (3.16)

With 𝐷𝑂𝑥 and 𝐷𝑅𝑒𝑑 being the diffusion coefficients for oxidation and reduction, respectively, A is

the electrode surface area. In this case, Warburg shows up on Nyquist diagram as a 45º sloop

diagonal line for low frequencies.

CPE, is considered to be similar to a capacitor but, although constant, its phase angle differs from

90º on the ideal conditions; equation (3.17) describing a non-ideal capacitor:

𝐶𝑃𝐸 =

1

𝑌0(𝑗𝜔)𝑛

(3.17)

𝑌0 is the admittance and is not equivalent to a capacitance and the n exponential value, that is

limited between 0 to 1 (n=0 represent the resistor response, n=1 represent a capacitor); for non-

ideal capacitor n= 0.9 – 1. In the Warburg element (n=0.5), or an inductor (n= -1). The exponential

43

value of n is an important feature for the CPE and depends on the surface roughness and the

integrity of the oxide film for the corrosion system.

With all this supporting information, it is possible to formulate an adequate equivalent circuit model

to answer for the system under study. To analyse the data, ZView® from Scriber Associates was

used.

Measurement

To perform Electrochemical Impedance Spectroscopy, EIS software from Gamry Instruments was

used, connected and controlled by a computer. The setup was performed with a stabilization of

the open-circuit potential and by imposing a sinusoidal perturbation of 10 mV (rms), maintaining

DC potential of -20mV versus OCP, from 100kHz to 10 mHz.

3.5 Microscopy and Surface Analysis Techniques

To ensure the alloy was according the specifications intended for use, these must be confirmed

by knowing the both the substantial presence of rare earths in alloy. For this purpose and

clarification, surface analysis was used, this is a very important tool to provide microstructural

characterization of the materials. In a first approach pre-treated samples, after etching, were

submitted to a magnifying glass and posteriorly to an optical microscope, both Leica® [92].

Figure 25 - Magnifying glass (left) and optical microscope (right).

The technique used to evaluate surface of Mg alloy: Field- emission Gun - Scanning Electron

Microscope (FEG-SEM) with Energy Dispersive X-ray Spectroscopy (EDS) in Figure 26

44

Figure 26 - Analytical JEOL 7001F FEG-SEM

Scanning Electron Microscopy

The scanning electron microscope is one of the first most important analytical instrument that

quickly gives a view over the surface material with in much depth resolution (1-50 nm) and shows

a high magnification image and a composition map (elementary) and a is also a non-destructive

technique.

SEM is an electron microscope that produces images by using a focused beam of electrons. As

the electrons penetrate the surface, electrons or photons are emitted from the material surface

and a fraction is collected and used to modulate a cathode ray tube (CRT). The SEM images can

be of three types: secondary electron (SE) images, backscattered electron (BSE) images and

elemental-X ray maps. The SE and BSE are separated according energies, since it is easier to

rip out the first electron than the second one [93].

Figure 27 - Simple Block Diagram of SEM [94]

45

Energy - Dispersive X-Ray Spectroscopy

EDS makes use of the X -ray spectrum emitted by a solid sample bombarded with a focused

beam of electrons to obtain a localized chemical analysis. X-ray intensities are measured by

counting photons. The incident beam may excite an electron in an inner shell, ejecting it from the

shell while creating an electron hole where the electron was. An electron from an outer, higher-

energy shell then fills the hole, and the difference in energy between the higher-energy shell and

the lower energy shell may be released in the form of an X-ray. As the energies of the X-rays are

characteristic of the difference in energy between the two shells and of the atomic structure of the

emitting element, EDS allows the elemental composition of the specimen to be measured [93,

95].

Energy – Dispersive X-Ray Spectroscopy (EDS) is a very practical technique that can detect X

rays from all elements in periodic table above beryllium, Z=4. Detection and measurement of the

energy permits elemental analysis EDS and can provide rapid qualitative, or with adequate

standards, quantitative analysis of elemental composition with a sampling depth of 1-2 microns.

X-rays may also be used to form maps or line profiles, showing the elemental distribution in a

sample surface [93] [96].

The surface must be highly polished to perform a quantitative analysis, since surface roughness

could cause undue absorption for the x-ray signal. X-ray range covers low, medium and high-

density materials and as a limit detection range 100-200ppm and depth sample from 0.02 to µm,

depending on Z and keV applied. The data is then represented in a plot: energy (eV – x-axis) vs

intensity (cps – y- axis) where peaks represent the quantitative portion of an element [96, 97].

Figure 28 - Spectrum for eds with peaks for maximum element adsorption [98]

.

46

Raman Spectra

Raman spectrum occurs when a sample is irradiated with monochromatic light and the incident

radiation could be: absorbed, simulate emission or even scattered [99]. This last physical process

is the one involved in Raman spectroscopy, allowing, based on conservation energy and

differences at vibrational or rotational energy, to know the vibrational and rotational states of the

molecules. This results from the induced polarization of scattering molecules [100].

Figure 29 - Raman Equipment and schematics [100].

Raman Spectra is used, for mineral identification and structural characterization, analyses of

gemstones and archaeometric objects, mineral inclusion, speciation and concentration,

characterization of thermal maturity, OH content and impurities. This method is non-destructive

and uses small samples without previous preparation [101].

47

Chapter 4

Results and Discussion

The Open Potential Circuit tests precede both, polarization curves (anodic and cathodic curves)

and electrochemical impedance spectroscopy. The surface analysis techniques (SEM and EDS)

allowed surface characterization. The results are compared with the analysed papers in chapter

2.

4.1 Electrochemical measurements results

4.1.1 OCP and Polarization curves

To evaluate the corrosion behaviour of Mg and Al alloys, open circuit potential and polarization

curves measurements were performed. These procedures were performed in 0.05 M NaCl

solution and the results are depicted in Figure 30 and Figure 31, respectively for WE43C and

ASTM 7475.

Figure 30 - OCP for WE43C in 0.05 M NaCl solution

-1.915

-1.895

-1.875

-1.855

-1.835

-1.815

-1.795

-1.775

-1.755

-1.735

-1.715

0 1000 2000 3000 4000

E/v

vs S

CE

Time / s

First Test

Sixth Test

Seventh Test

48

Figure 31 - OCP for ASTM 7475 in 0.05 M NaCl solution

The analysis of the OCP variation with time reveals some differences between the two alloys,

which may be related with their different type of activity. This is consistent with Andreatta [28]

observations about localized pitting corrosion. Due to the low solubility of most of the alloying

elements in aluminium, during solidification of the alloys they tend to be segregated, forming

precipitates. Most of these precipitates (especially those containing Cr and Fe) are nobler than

the Al matrix and will behave as cathodic relatively to the matrix. Cu, one of the most important

alloying element of the so called duraluminium, also tends to form precipitates more noble than

Al, introducing a similar behaviour as Cr and Fe. The presence of these local cathodes will

promote anodic activity in the adjacent Al matrix, leading to trenching around the particles or being

the “driving force” for pitting corrosion. Thus, the OCP evolution of ASTM 7475 is typical of a

passive system suffering pitting corrosion, showing potential transients that may be related with

the onset of pitting and subsequent repassivation. In fact, the cathodic activity of a passive

material is spread by a large area (all the passive surface) and is not significantly affected by the

nucleation of repassivation of pits. On the contrary, the nucleation of a peak leads to a significant

increase in the anodic area, thus also increasing the corresponding anodic current. As a result,

and as depicted in, the onset of a pit leads to a sudden drop in the corrosion potential (or OCP),

followed by a recovery to the initial values, due to repassivation.

This behaviour of ASTM 7475 has also been observed by Andretta et al. [28], who concluded that

this alloy was suffering corrosion while the OCP was performed.

49

Figure 32 – Illustration of the potential transients’ due to the breakdown and repassivation of pits [102]

Contrarily to ASTM 7475, WE43C is not supposed to form a highly protective passive film, as the

oxides resulting from its corrosion are not expected to confer a strong protection. Thus, the

respective OCP values are much steadier (not affected by the typical transients’ due to pitting),

but much more negative, indicating a higher activity of the material.

As already mentioned, the OCP was measured during the first hour of immersion and prior to

polarization curves. The values recorded after 1 hour of OCP (-1.745 V for WE43C and -0.663 V

for ASTM 7475), were used as reference for the calculation of the starting point for the polarization

curves. For both alloys, anodic curves were performed from -0.01 V vs the OCP up to +0.5 V vs

the OCP, whereas and cathodic polarization curves started at +0.01 vs OCP down to -0.5 V vs

OCP, being presented in Figure 33 and Figure 35.

Figure 33 - Polarization curves for WE43C in 0.05 M NaCl (log scale)

-2.1

-2

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-9 -7 -5 -3

E/v

vs S

CE

log ( i / A cm-2)

First Test

Fourth Test

Sixth Test

Seventh Test

50

Starting with WE43C, Figure 33 shows, for low anodic polarization values, a Tafel-like behaviour,

probably associated to corrosion under a poorly protective oxide layer, followed by an irregular

slope change above -1.5 V. From the logarithmic polarization curve, the behaviour above this

potential could be ascribed to diffusion control or even to the formation of a new film, but the very

high current densities are not consistent with these phenomena.

To better understand the meaning of this observation, the same data was analyzed in a linear

scale as shown in Figure 34. For low polarization values, the low current densities (but higher

than those normally observed in passive materials) confirm the presence of a poorly protective

oxide layer, as there is a slight increase of current density with increasing potentials. However,

when the potential is close to -1.5, a sudden increase of the current intensity is observed. This

may be due to the rupture of the above-mentioned oxide layer, leading to a catastrophic

dissolution of the specimen. In fact, the current would be expected to increase sharply, resulting

in a horizontal E vs i line, but at these current density values an ohmic drop is normally present,

leading to a higher slope of the polarization curve.

Figure 34 - Linear scale plot for WE43C alloy in 0.05 M NaCl.

According to the Tafel extrapolation method, straight lines were drawn in the anodic and cathodic

Tafel zones and their interception corresponds to Ecorr and icorr.

-2.2

-2

-1.8

-1.6

-1.4

-1.2

0.E+00 1.E-03 2.E-03 3.E-03 4.E-03

E/V

vs

ECS

i/A cm-2

Cathodic

Anodic

51

Table 20 - Anodic and Cathodic curves parameters for WE43C alloy.

Solution Ecorr(V) icorr(A/cm2) bc(V) ba(V)

0.05 M NaCl -1.74 1.2E-5 0.224 0.190

The anodic and cathodic polarization curves obtained for the ASTM 7475 alloy are shown in

Figure 35.

Figure 35 - Polarization curves for ASTM 7475 alloy in 0.05 M NaCl.

A sudden increase of current is observed at the corrosion potential, indicating breakdown of the

passive film due to pitting, typical for ASTM 7475 alloy, since the matrix is made in aluminium.

This behaviour confirms F. Andreatta et al. [28] analysis, which shown that aluminium alloys are

already corroded when corrosion potential is achieved. For that, the polarization parameters are

obtained by crossing anodic and cathodic lines, but anodic line was drawn starting in corrosion

potential value.

Figure 36 - Theoretical explanation of Potentiodynamic data for Al.

-1

-0.8

-0.6

-0.4

-0.2

1.E-09 1.E-07 1.E-05 1.E-03

E/V

vs

ECS

i/A.cm-2

Cathodic

Anodic

52

Figure 36 explains why ASTM 7475 alloy shows this behaviour.

When compared, both curves show significant differences as expected. Looking at the current

densities, ASTM 7475 alloy show a lower value (one order in magnitude, taken as the passive

current from the cathodic curve) when compared with the WE43C alloy, which is an indication of

the enhanced protection conferred by the passive film in the ASTM 7475 alloy. The more negative

value for WE43C could be explained due to introduction of RE elements – Y and Nd. However,

pitting of 7475 occurs at the corrosion potential, with a drastic increase in the current.

While polarization curves were performed, the pH was measured three times: in the beginning of

the OCP, in the end of OCP and in the end of the test, whether it is anodic or cathodic.

Table 21 - pH values for Polarization curves

WE43C alloy ASTM 7475 alloy

Sample Anodic Cathodic Anodic Cathodic Sample

Mg - 1 - 4.97 5.28 - 5.19 5.37 Al – 1

Mg - 2 4.73 4.54 - 4.76 4.72 - Al – 2

Mg - 3 - 5.06 5.30 - 5.21 5.35 Al – 3

Mg - 4 4.81 4.62 - 4.89 4.74 - Al – 4

Mg - 5 - 5.17 5.37 - 5.17 5.31 Al – 5

Mg - 6 4.87 4.64 - 4.86 4.69 - Al – 6

Mg - 7 - 5.13 5.29 - 5.19 5.35 Al – 7

Mg - 8 4.84 4.67 - 4.84 4.70 - Al – 8

Mg - 9 - 5.11 5.27 - 5.12 5.34 Al – 9

Mg - 10 4.71 4.64 - 4.89 4.76 - Al – 10

Mg - 11 - 5.16 5.31 - 5.25 5.37 Al – 11

pH values measured for Cathodic and Anodic

curves:

End of OCP| End test|

4.89 4.75 - Al – 12

- 5.21 5.34 Al – 13

- 5.21 5.37 Al – 14

- 5.19 5.38 Al – 15

As expected, in cathodic measurements pH increases, due to O2 reduction and production of

hydroxyl ions, while in anodic measurements pH values decreases due to hydrolysis of Mg

cations. The bulk pH before OCP was around 4.97. It is reasonable to assume that the local pH

at the surface was even higher. This has important implications to the corrosion mechanisms of

the magnesium alloys containing RE: the local alkalinization may lead to the formation of

protective oxide/hydroxide layers of these elements at bulk pH values where MgO/Mg(OH)2 is still

not stable.

53

Looking for WE43C alloy, both reactions (anodic and cathodic) occur on the surface of the

material by solution contact, which results in its cancellation in the OCP. On the other side, for

ASTM 7475 alloy, the external surface works as cathode while the anodic process occurs in

confined environment, inside the pits. Thus, pH is mainly expected to decrease inside the pits,

little affecting bulk pH values, which explains why ASTM 7475 is more affected by the cathodic

reaction - Table 21.

On the other side, WE43C showed distinct zones, where the current increases and after that

around -1.55 V (Figure 33) a zone that in a first analysis seems to be related to the presence of

a possible protective layer. To better understand this, a linear scale plot was made and what

should be a straight horizontal shows a slight slope (Figure 34). This observation, combined with

pH values indicates that both semi-equations (reduction and oxidation) have the same speed,

leads to a continuous production/destruction of the thin oxide layer – autocatalytic reaction – as

mentioned previously.

4.1.2 Electrochemical Impedance Spectroscopy

To have a better understanding of the behaviour of the alloys, EIS measurements were performed

in 0.05 M NaCl solution, at room temperature, after a brief stabilization of the open circuit potential

for one hour. In this procedure, all samples were treated in same conditions. Surface areas were

the changing characteristic for both alloys: for WE43C they vary in a range from 0.49 to 0.64 cm2,

for ASTM 7475 from 0.56 to 0.80 cm2. The Nyquist and Bode plots for WE43C and ASTM 7474

alloys, represent the collected data.

Figure 37 - (a) Nyquist diagram, (b) Bode diagram for WE43C in 0.05M NaCl.

0 5000

-8000

-3000

Z' / Ohm cm2

Z''

/ O

hm

cm

2

EIS-1.DTAEIS-2.DTAEIS-3.DTAEIS-5.DTAEIS-6.DTAEIS-7.DTAEIS-8.DTAEIS-9.DTAEIS-10.DTAEIS-11.DTAEIS-12.DTA

10-2 10-1 100 101 102 103 104 105 106101

102

103

104

105

Frequency (Hz)

|Z| O

hm

cm

2

EIS-1.DTAEIS-2.DTAEIS-3.DTAEIS-5.DTAEIS-6.DTAEIS-7.DTAEIS-8.DTAEIS-9.DTAEIS-10.DTAEIS-11.DTAEIS-12.DTA

-75

-50

-25

0

Theta

/ Degre

e

54

The impedance spectra of Figure 37 show the presence of two-time constants, one at high

frequencies, normally attributed to an oxide layer, and the second at low frequencies, normally

assigned to the corrosion reaction. It is therefore assumed that a porous and poorly protective

oxide is formed on the WE43C. The behaviour of this systems may be modelled through the

electrical equivalent circuit proposed in Figure 38. Here, a porous film with capacitance Cf is

considered, with pores that contain electrolyte (with an additional solution resistance Rpore) and

expose the metal at their base, thus allowing for the formation of a double-layer with capacitance

Cdl and for the dissolution of the alloy, represented by the charge-transfer resistance Rct. The

corresponding fitted values are presented in Table 22.

Figure 38- Proposal of Equivalent Circuit

Table 22- Fitting the EIS to the equivalent circuit for WE43C alloy.

χ2 Rs(Ω cm2) Y 0,f (F sn-1 cm-2) nf Rpore(Ω cm2) Y 0,dl(F sn-1 cm-2) ndl Rct(Ω cm2)

Mg 1 3.27E-04 79.6 1.65E-05 0.92 2877 9.76E-04 0.70 2223

Mg 2 3.60E-04 94.1 1.22E-05 0.92 3891 7.69E-04 0.63 3844

Mg 3 6.84E-04 57.2 1.32E-05 0.92 3390 6.46E-04 0.75 2529

Mg 5 4.59E-04 100.0 1.32E-05 0.92 3945 7.42E-04 0.67 3561

Mg 6 8.36E-04 119.2 1.38E-05 0.92 3492 7.13E-04 0.75 2759

Mg 7 6.09E-04 77.3 1.23E-05 0.92 3672 7.62E-04 0.75 2969

Mg 8 5.31E-04 105.3 1.26E-05 0.92 3393 7.78E-04 0.65 3203

Mg 9 4.83E-04 95.5 1.51E-05 0.92 2925 7.27E-04 0.80 2335

Mg 10 7.99E-04 90.6 1.61E-05 0.92 3053 8.42E-04 0.73 2313

Mg 11 4.45E-04 107.4 1.47E-05 0.92 3207 7.71E-04 0.72 2722

Mg 12 8.91E-04 99.9 1.66E-05 0.91 2967 8.19E-04 0.78 2298

The impedance spectra of Figure 39, relative to the ASTM 7475 allow, also show the presence

of two time constants, one at high frequencies, normally attributed to an oxide layer, and the

second at low frequencies, normally assigned to the corrosion reaction. However, in this case,

55

based on the large literature results for impedance of aluminium alloys and also based on the

polarization results resented above, this behaviour is mainly attributed to the spontaneous

formation of pits, even at the corrosion potential. Thus, the same electrical equivalent circuit

(Figure 38) may be used for the fitting of the spectra, leading to the fitted parameters depicted in

Table 23.

Figure 39 - (a) Nyquist diagram, (b) Bode diagram for ASTM 7475 in 0.05M NaCl.

Table 23 - Fitting the EIS to the equivalent circuit for ASTM 7475.

χ2 Rs(Ω cm2) Y 0,f (F sn-1 cm-2) nf Rpore(Ω cm2) Y 0,dl(F sn-1 cm-2) ndl Rct(Ω cm2)

Al 1 2.94E-04 128.6 9.77E-06 0.88 13336 6.38E-05 0.86 76756

Al 2 2.45E-04 116.7 9.99E-06 0.89 13195 6.32E-05 0.95 46982

Al 3 8.23E-04 111.8 8.81E-06 0.89 14239 6.59E-05 0.92 59364

Al 4 6.28E-04 54.71 7.84E-06 0.91 9481 1.13E-04 0.92 37311

Al 5 6.18E-04 60.01 1.01E-05 0.92 6920 2.34E-04 0.97 9937

Al 6 6.59E-04 48.43 1.10E-05 0.91 7645 1.61E-04 0.93 19147

Al 7 6.04E-04 77.27 8.92E-06 0.90 9735 9.32E-05 0.97 38192

Al 8 7.87E-04 53.01 9.56E-06 0.92 8250 1.25E-04 0.93 29006

Al 9 2.34E-04 49.77 1.15E-05 0.89 9262 1.02E-04 0.90 41384

Al 10 3.90E-04 74.03 8.95E-06 0.92 8301 1.42E-04 0.90 31104

Al 11 2.71E-04 51.10 1.22E-05 0.91 7690 1.48E-04 0.93 30093

Al 12 5.44E-04 56.75 9.92E-06 0.91 8310 1.04E-04 0.91 48629

Al 13 5.79E-04 55.14 1.21E-05 0.88 9159 9.99E-05 0.92 35125

Al 14 3.81E-04 60.31 1.15E-05 0.91 6108 1.61E-04 0.85 47146

Al 15 5.01E-04 58.72 1.05E-05 0.90 9034 1.02E-04 0.94 35540

0 25000 50000 75000

-75000

-50000

-25000

0

Z' / Ohm cm2

Z''

/ O

hm

cm

2

Al_EIS_1.DTAAl_EIS_2.DTAAl_EIS_3.DTAAl_EIS_4.DTAAl_EIS_5.DTAAl_EIS_11.DTAAl_EIS_12.DTA

10-2 10-1 100 101 102 103 104 105 106101

102

103

104

105

Frequency (Hz)

|Z| O

hm

cm

2

Al_EIS_1.DTAAl_EIS_2.DTAAl_EIS_3.DTAAl_EIS_4.DTAAl_EIS_5.DTAAl_EIS_11.DTAAl_EIS_12.DTA

-75

-50

-25

0

Theta

/ Degre

e

56

The pits also behave as pores, although in this case they are not so open to the outer

environment, allowing for the establishment of much more aggressive conditions inward.

However, the surface area occupied by pits on the ASTM 7475 is surely much lower than those

occupied by the open pores in the WE43C, thus both the Rpore and the Rct values are higher for

the aluminium alloy.

In a first approach, the comparative analysis of the spectra obtained both alloys shown that they

presented different resistance values in low frequencies: WE43C had values closer to 104 Ω.cm2

while ASTM 7475 had values of resistance closer to 105 Ω.cm2; hence the value is higher for

ASTM 7475 alloys, it corrodes slower than WE43C. However, it is important to stress that the

type of attack is different for these alloys, with corrosion through a poorly protective oxide layer in

the case of Mg and pitting corrosion on a passive material in the case of ASTM 7475.

As mentioned in subchapter 3.4, an electrochemical system can be represented by applying the

concept of electrical equivalent circuit. In the present case, the model that fits better the EIS

spectra of both materials is that represented in Figure 38.

The proposed model is constituted by 3 resistances and 2 constant phase elements: Rs , the

solution resistance measured between the working and the reference electrodes; Rpore , the

additional resistance inside the pores and Rct, the charge-transfer resistance corresponding to the

corrosion of the active metal (at the bottom of pores or in the pits, respectively for WE43C and

ASTM 7475); Cf, related to the capacitance of the oxide films and Cdl, related to the capacitance

of the double layer formed in the active material, so again at the bottom of pores or inside pits.

The constant phase elements (CPE) are used instead of pure capacitances in the fitting, to show

the deviation from the ideal behaviour.

According to this proposed equivalent circuit, the flow of current through both systems may occur

by two different ways:

• involving faradaic processes, related with oxidation or reduction of species, such as the

oxidation of the material. Current flows through the overall solution (Rs), and then through

the inner pore or pit solution (Rpore) due to ionic motion, and then by charge transfer at

the interface (Rct).

• involving the non-faradaic processes, as the charge and discharge of a capacitor: this is

the current flowing through the oxide film (where in principle the electrical resistance of

the film is too high to allow for electron transfer by conduction, so the corresponding Rf

resistor is normally discarded) and represented by the constant phase element Cf, or the

current due to charge and discharge at the double layer, represented by the constant

phase element Cdl.

57

Analysing the fitting results for both materials (Table 22 and Table 23), first of all it is clear that a

good reproducibility exists in each one of them, with each parameter showing similar values for

all the samples. Comparing the two alloys, some differences may be found:

• On which concerns Rs, all the values lie in the same magnitude, as expected, as the

testing electrolyte was the same. The individual differences found may be mainly related

with the geometry of the cell and with the distance between electrodes, which was not

controlled in the experiments;

• Rpore values are higher for ASTM 7475 than for WE43C, as the surface area of the pits

in the aluminium alloy;

• The above-mentioned area effect is also important for the difference between the Rct

values of both alloys, but in this case the intrinsic charge transfer resistance, so the

activity of the bare alloy surface is also very important. Thus, WE43C shows values of

Rct which are one order of magnitude lower than those for ASTM 7475.

• The values of Cf are quite similar for both materials. Assuming that the oxide layer

capacitance is expressed by C=ԑԑ0 A/d, where d is the oxide thickness, A the surface

area, ԑ0 the absolute permittivity in vacuum and ԑ oxide’s dielectric constant, and taking

into account that both oxides show similar dielectric constants, at least in the non-

hydrated form, (9-10 for Al2O3 [103], and 9.0-10.1 for MgO, [104], the only important

difference between them could only be the surface area. However, the effect of porosity

or pitting in the oxide coverage is normally reduced, as the oxide surface area will be

given as A=Ageom (1-), where is the area of pores or pits. Thus, for lower than 10%,

its effect on A will be not significant.

• Finally, Cdl depend on the values of and on the characteristics of the electrolyte. In this

case, will strongly affect the value of Cdl. As discussed above, it is expected that the

area of exposed WE43C at the bottom of the pores will be higher than the pitted area,

so the corresponding Cdl values are also expected to be higher, as confirmed.

This discussion was performed based on capacitances, although they were based on the Y0

values and corresponding CPE’s. According to the Brug’s equation [105]

C = [Y0 𝑅(1−𝑛)]

1

𝑛 (4.1)

and taking into account that the n values are relatively close to 1 and that the values of R are

quite low, the Y0 value will not be too different from C.

In what concerns to 𝜒2 shown for both alloys (Table 22 and Table 23) gave us the idea of the

good fitness of the model chosen, and once again for both alloys, they have similar 10-4 range,

defining a good proposition model.

58

4.2 Surface Analysis Results

Superior tensile strength and creep resistance of these alloys is attributed to the formation of

intermetallic compounds. For surface analysis, the previous essays were performed for

macroanalysis, using the etching solutions for both alloys. Starting with WE43C alloy, both acetic

glycol and nital solutions were used for etching (Figure 40).

Figure 40 - Left: WE43C alloy with acetic glycol etching; right: WE43C alloy with nital etching.

As ASM [106] mentioned, acetic glycol etching gave a view of grain boundary and relevant

precipitates. In other hand, nital etching only shown the general structure of the alloy. But none

of then gave any other information.

Using the same method for ASTM 7475 alloy, the results with Keller’s etching solution are shown

in Figure 41.

Figure 41 - ASTM 7475 alloy with Keller's reagent.

Even with suggestion of ASM [106], for ASTM 7475 alloy, there is no great visible significance.

To evaluate and support future conclusions, the next step was related with SEM analysis.

59

By taking samples of both alloys to SEM (Scanning Electron Microscope), it was possible to

perform microanalysis on surface, showing microstructures. The SEM was combined with EDS

(Energy Dispersive x-ray Spectroscopy) which give quantitative information about specific point

in surface alloy. With no great surprises, the first general observation was that the matrix is mainly

composed by Mg and RE are minority Figure 42.

Figure 42 – (Left)SEM-(right) EDS analysis for WE43C alloy.

In this general observation was possible observe some light-grey structures along grain

boundaries, and as Peng-Wai -Chu et al. [35] observed, the most regular structures were yttrium

and the other ones were neodymium. By zooming a specific spot from the initial area, the analysis

was performed over this type of structure:

Figure 43 - (Left)SEM-(right) EDS analysis for regular structure of WE43C alloy.

Table 24 - EDS without O for regular structure

Element Mg Y Nd O

Atomic % 22.82 50.09 2.49 24.60

The EDS data was analysed and recalculated without the influence of oxygen:

10 µm

10 µm

60

Table 25 - EDS without O for regular structure

Element Mg Y Nd

Atomic % 30.27 66.71 3.30

For the analysis of the composition values it is important to take into account that the thickness

of the Y particles may be lower than the sampling thickness of EDS and that the EDS sampling

area may be larger than the particle’s area, so the spectrum may be affected by the composition

of the matrix under or around the particle. In fact, it is our opinion that the Y particle may be

constituted by pure Y, without any Mg or Nd. Also the ratio of 𝑂

𝑀𝑔 , for yttrium (regular structure)

is equal to 1, providing relevant information. Following Peng-Wai Chun [35], he suggests when

the ratio is equal to 1, the sample consists MgO layer, so both the oxygen and Mg content may

be due to MgO not included in the Y particle.

Moving to the irregular structure, the SEM-EDS analysis is represented in Figure 44

Figure 44 - (Left)SEM-(right) EDS analysis for irregular structure of WE43C alloy.

The same procedure was taken to irregular structure

Table 26 - EDS with O for irregular structure

Element Mg Y Nd O

Atomic % 94.51 1.60 1.76 2.13

Table 27 - EDS without O for irregular structure

Element Mg Y Nd

Atomic % 96.57 1.63 1.80

SEM analysis was also applied to the ASTM 7475 alloy.

10 µm

61

For ASTM 7475, the procedure was the same, however, this time the Keller’s solution was used

for etching on this alloy. This etching solution [106], was applied for three times before the sample

was taken in SEM-FEG. For the first two times, only SEM was performed with purpose to observe

the influence of etching solution on surface area of the alloy and the observation method was the

same: starting with a SEM image between four marks made with a diamond knife - Figure 45:

Figure 45 –SEM for ASTM 7475 alloy

Looking to this image, it seems there are two similar zones: the first one is related with bottom

and top mark’s, where the matrix around clearest then in the second zone. The first zone is related

with marks that are on the left and right: they present a larger darker area. In the consequence of

this observation, it was chosen two areas for better resolution observation: the top and the left

marks:

Figure 46 – First SEM image for top mark on ASTM 7475 alloy

1 mm

10 µm

10 µm

100 µm

62

In the first image from Figure 46 is observed some black points and white zones. As the resolution

is increased, it seems clear that black point could be precipitates, since they are very well

observed in the matrix; in the right image, it seems to be observed some grain boundaries and

once again, the black points.

Figure 47 - First SEM image for left mark on ASTM 7475 alloy.

On Figure 47, can still be observed clear and dark points but is more obvious the ground

boundaries in centre and right images. In this last two images, the points are mainly over the grain

boundaries, what is mentioned by Chemin et al. [31]. In the image on the right, the dark points

are surrounded by a brighter zone. . Secondary electron images in scanning electron microscopy

are used in order to reveal the topography of the samples, as steep surfaces and edges tend to

be brighter than flat surfaces. Thus, the brighter zones should correspond to zones where

dissolution (trenching) of the matrix has occurred, due the galvanic effect of the more noble

precipitates – Chemin et al. [31].

This procedure was repeated in the same way (with the same etching solution), and the

observation has proceeded as describe before.

Figure 48 - Second SEM for ASTM 7475 alloy

100 µm 10 µm 10 µm

1 mm

63

In this second SEM image, the matrix seems to become more homogenous in clear zones; the

darker zones are now less visible than in the first SEM image, this could be a result of the

successive use of etching solution. However, the chosen points for better resolution were the

same as used before – to maintain, as much as possible, space reference in this test.

Figure 49 - Second SEM image for top mark on ASTM 7475 alloy

Figure 50 - Second SEM image for left mark on ASTM 7475 alloy

The procedure and the space dislocation was the same as the first two previous observations.

Figure 51 - Third SEM for ASTM 7475 alloy

100 µm 10 µm 10 µm

100 µm 10 µm 10 µm

1 mm

64

Figure 52 - Third SEM image for top mark on ASTM 7475 alloy

Figure 53 - Third SEM image for left mark on ASTM 7475 alloy

For this last test, SEM was coupled with EDS technique what allowed to know and quantify the

atomic percentage in alloy. For that, EDS was performed in three different points in the surface

alloy: in the matrix Figure 54, a dark point near the left mark Figure 55 and a clear point near the

left mark on the surface sample Figure 56. The corresponding composition of the identified points

is shown in Table 28 - Table 33, and they are shown with and without the oxygen percentage.

In these images is possible to observe that, as Payandeh and Chemin [30, 31] describes, ASTM

7475 alloy could present Al3Fe intermetallic in the matrix with shapes like the ones in Figure 51 -

Figure 53 since they might introduce cathodic behaviour, the matrix starting to dissolve and

promoting pits, shown by Figure 50 and Figure 53. In the EDS analysis, Fe percentage is not

observed since the atomic percentage is low the detection limit of the SEM-FEG and is supported

by data on Table 28 - Table 33.

100 µm 10 µm 10 µm

10 µm 10 µm 100 µm

65

Figure 54 - (Left)SEM-(right) EDS analysis for ASTM 7475 alloy in matrix

Figure 55 - (Left)SEM-(right) EDS analysis for ASTM 7475 alloy in dark zone.

Figure 56 - (Left)SEM-(right) EDS analysis for ASTM 7475 alloy in clear zone.

Table 28 - EDS for ASTM 7475 with O (Figure 54)

Element O Mg Al Cu Zn

Atomic % 3.76 2.59 89.37 1.04 3.24

Table 29 - EDS for ASTM 7475 without O (Figure 54)

Element Mg Al Cu Zn

Atomic % 2.69 92.86 1.08 3.37

2 mm

100 µm

100 µm

66

Table 30 - EDS for ASTM 7475 with O (Figure 55)


Atomic % 3.86 1.77 58.11 0.86 2.63

Table 31 - EDS for ASTM 7475 without O (Figure 55).

Element Mg Al Cu Zn

Atomic % 1.84 60.43 0.89 2.74

Table 32 - EDS for ASTM 7475 with O (Figure 56).


Atomic % 2.55 2.69 90.36 1.05 3.35

Table 33 - EDS for ASTM 7475 without O (Figure 56)

Element Mg Al Cu Zn

Atomic % 2.76 92.72 1.08 3.44

Once the ASTM 7475 has Al as main element and shown pits on ground boundary, it is supported

by observation of Figure 31, where the alloy already suffers pit corrosion during the OCP.

4.2.1 RAMAN ANALYSIS

Raman spectroscopy was performed with the purpose to analyse anodic curve consistence in

WE43C alloy - Figure 34. For starting, the first assay was based in a different alloy not used in

any part of the present work, but has essential for sow specific and well-known peaks. This pre-

work was not possible, and it would have referred in Conclusions and Future Work in final chapter.

Once that was not possible perform RAMAN analysis with the wanted purpose due to technical

problem with the equipment, this description will be based in Jakraphan Ninlachart et al. [47]

where passivation behaviour of WE43C Mg–Y–Nd alloy in chloride containing alkaline

environments.

This work took more extreme environment because it was performed in 0.1M of NaCl instead of

what was used in this work. Jakraphan Ninlachart et al. [47] present this RAMAN spectroscopy

Figure 57. This observation allowed to saw corresponding peaks of Mg(OH)2 which are observed

along with the Mg solid solution peaks. The XRD pattern did not reveal presence of either oxides

or hydroxides of rare earth elements.

67

Figure 57 - Raman spectra of the passivated WE43C samples in 0.1 M NaCl [47].

Figure 58 - XPS high resolution Y 3d spectra of the WE43C specimens potentiostatically passivated

(Solution treated specimen prior to Ar-ion sputtering (as passivated surface) [47]

This could be one answer in anodic behaviour of WE43C alloy that explain what was observed in

Figure 34. The Raman spectra suggested that the passive layer consisted in MgO and RE2O3,

where RE represents Y, Nd, and Gd. Raman peaks corresponding to Mg(OH)2 were not observed.

Among the RE2O3 possible, in this surface layer, Nd2O3 has a lower force constant than that of

Gd2O3 and Y2O3 has a higher force constant than that of Gd2O3, and therefore the Raman shift of

the given vibration mode will be lower for the Nd2O3 than the other two oxides.

The resolved Y 3d doublet was not observed, but the broad peak observed at 158.0 eV and a

shoulder at 159.75 eV with the as-received specimen after sputtering could be assigned to the

presence of Y3+ species in the surface layer. Jamesh et al. [107] saw a single peak of Y 3d signal

at 158.7 eV on the WE43C sample exposed to Ringer’s solution, and assigned the peak to the

presence of Y2O3 [107] . The RE elements were also present in the matrix of the WE43C alloy as

solid solution and therefore the surface layer consisted of MgO, Mg(OH)2, and RE2O3 phases.

69

Chapter 5

Conclusions and Future Work

The corrosion behaviour of WE43C and ASTM 7475 alloys were analysed in an aggressive 0.05M

NaCl solution, where electrochemical measurements were taken and compared. This comparison

was based on previous case studies in the literature. The samples were also characterized for

surface analysis with SEM-EDS, to understand the positioning of RE (for WE43C) as complement

of electrochemical data, and to explain the corrosion type of ASTM 7475 and its consequences.

Magnesium alloys, in special WE43C, were recently introduced in the markets as a viable

alternative to aluminium alloys since magnesium is lighter than aluminium and their use brings

economic benefits.

The open circuit potential evolution showed that ASTM 7475 alloy suffered pitting corrosion as

expected and that its corrosion potential is -0.663V, while WE43C presents a corrosion potential

of -1.74V. This confirms that WE43C alloys have more tendency to oxidize compared to ASTM

7475 - Table 15 comparison between pure Mg and Al.

Potentiodynamic data for polarization curves showed differences in anodic curves for both alloys,

the WE43C alloy exhibits a different slope when compared to the ASTM 7475 alloy. While the

WE43C alloy shows a positive slope that might indicate corrosion under a poorly protective film

of corrosion products, the ASTM 7475 alloy exhibits an almost horizontal line due to pitting that

start to occur at the open circuit potential.

EIS results show that even with RE elements, the WE43C alloy has lower impedance values

when compared to the ASTM 7475 alloy. The results show differences in pH values, increasing

on the cathodic curve due to hydrogen consumption and decreasing on the anodic curves as

expected. EIS analysis reveals that the passive surface film on the magnesium alloys has an

oxide layer structure.

The surface analysis showed that the metallurgical structure has a great influence on the

corrosion behaviour of these alloys: for the RE elements to have a more protective effect they

must be homogeneously distributed in the alloy, preferably in solid solution in the magnesium

matrix. For the alloy WE43C the SEM images suggested that upon the dissolution of the

magnesium in the matrix, the Y and the RE formed a layer of corrosion products. This layer seems

to be poorly attached to the surface and eventually falls away by undermining.

The intermetallic leads to micro-galvanic corrosion and can lead to an increased corrosion rate.

Although the redox potentials of both elements are similar, Y is thermodynamically stable in

slightly alkaline solutions at lower pHs and lower potentials than Nd. If Y-containing intermetallics

are nobler than the matrix, they will generate the observed micro-galvanic effect. Thus, the effect

70

of Yttrium depends on whether it is incorporated in the Mg matrix or segregated as precipitates:

in the first case, increasing Y content may decrease the corrosion rate due to the formation of an

increasingly protective surface film by the incorporation of more Y in that film; on the contrary, if

segregated, the presence of Y will increase the corrosion rate by a micro-galvanic effect.

Finally, as a conclusion, both alloys shown limitations. While WE43C exhibits a weak oxide layer,

ASTM 7475 suffers pitting corrosion.

As a suggestion for future work, RAMAN spectroscopy should by performed and compared with

results of Jakraphan et al. [47]; another important assay should be XRD technique. Finally, a

coating protection must be considered to prevent pitting corrosion and its propagation, in principle

involving the joint use of anodization and organic coatings.

71

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