Magnesium Alloys for Aerospace Chemical Engineering

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

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.

III

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

Abstract

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

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

VII

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 32 - EDS for ASTM 7475 without O (Figure 53)...... 66 Table 33 - EDS for ASTM 7475 with O (Figure 54)...... 66 Table 34 - EDS for ASTM 7475 without O (Figure 54)...... 66

VIII

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

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

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

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

XII

ω Angular Frequency

XIII

XIV

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.

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.

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

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 5

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 6

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)

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

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

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

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

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 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 2000 series toughness. Used in decade of 60’s for fuselage skins for commercial and military transport aircraft. 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, 6000 series good manufacturing process. Not very used in aircraft industries because don’t had the balance in the properties. 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 7000 series covers. 7475 has a higher strength, superior fracture toughness and resistance to fatigue crack propagation in air and aggressive environment [25].

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

Tensile 0.2% YS Alloy AMS number % EI Strength(MPa) (MPa)

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 Aluminium alloys, such as aluminium – lithium and influence of Wego Wang 1993 aluminium in mechanicals properties, observance on the entire [23] crack face. Comparison of three most used aluminium alloys series: 2000, J-P Immarigeon 1994 6000 and 7000, improvement in thermodynamic treatment with et. al [24] direct impact in mechanical properties like Tensile Strength. B.B. Verma et. Fatigue behaviour for 7475 aluminium alloys in a T7351 temper 2001 al. [25] with yield stress of 495MPa and elongation of 14%. Importance of zirconium usage in refinement of grain combined Tanaka et. al 2004 with temperature roll; increase 10% in SCC and corrosion [26] resistance by standardizing of electrochemical properties. Effects pressing temperature on the grain formation of Goloborodko et. 2004 microstructures and re-definition in space organization for ultra- al. [27] fine-grain formation, increase in high strain rate super plasticity.

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.

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.

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 Alloy Al Mn Zr Zn Other Key features group grade Low cost extrusion alloy, AZ10 1.2 0.2 - 0.4 - moderate strength and high elongation Mg – AZ21 2.0 0.15 - 1.2 - Extrusions Mn – Al AZ31 3.0 0.3 - 1.0 - Moderate strength - Zn General purpose and AZ61 6.5 0.3 - 1.0 - moderate cost Extruded products and press AZ80 8.5 0.12 - 0.5 - forgings, heat treatable Mg – Moderate strength extrusion ZK21 - - 0.45 2.3 - Zn - Zr alloy

Continuation of Table 9 High yield extrusion, lower ZK40 - - 0.45 4.0 - strength than ZK60 High strength and good ZK60 - - 0.45 5.5 - ductility High temperature creep 4.0Y WE43 - - 0.7 - resistance(300ºC), long term Mg – Y 3.4RE exposure (200ºC) - RE 5.2Y High strength, heat treatable WE54 - - 0.7 - 3.0RE 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 Increase of workability at elevated temperatures in Mohd Ruzi H. et. 2009 improvement of properties by re-crystallization in al. [32] magnesium alloys: ZK and WE alloy system. Effects and influence of Zr in mechanical properties for Jing Liu et. al. 2015 Mg alloys, improvement applications on aerospace and [34] automobile sectors. Peng-Wei Chu et Effects WE43 Mg alloy immersed in 3.5wt% NaCl 2015 solution saturated with Mg(OH)2 and microgalvanic al. [35] effect. Mn addition benefits for the tensile properties of casting Wei Rong et. al. Mg-15Gd-1Zn alloy, high ductility and poor harmful for 2016 [33] 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

14 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 Influence of chloride ions in the resistance of the film R.Pinto et. al. 2010 formed in alloys ZK31, EZ33 and WE54. Decrease of [36] the conductive character of the film. Galvanic potential of Mg alloy in 3% NaCl shown that metallic Mg is not in direct contact with divalent ion R.Pinto et. al. (Mg2+), protected by a passive layer. Contain hydrated 2011 [37] oxides and hydroxides. Non-buffered solutions become alkaline solutions, stabilize magnesium hydroxide species and reduce corrosion rate. Polycrystalline magnesium alloys became ductile at H.Mirzadeh et. al. 2015 elevated temperatures, dynamic recrystallization and [38] 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

15 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).

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.

푂 Using EDS analysis, the contact layer with the surface alloy showed Mg and O with a ratio = 푀푔 1, which reveals the presence of MgO, with traces of other elements. Furthermore, the top of the 푂 film has a ratio = 2 that suggests a presence of Mg(OH)2. This layer is formed by corrosion 푀푔 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

17 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.

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. Strengthening of RE effect in Mg alloys, addiction of yttrium and 1995 al. [41] its effects on microstructural. Influence of the microstructure and crystallographic texture on the mechanical properties influenced by different yttrium contents in D.K. Xu et. al. 2007 Mg alloys; the increasing yttrium content promote spacing of the [42] distributed particles on the fracture surface, influenced the transverse mechanical properties.

Refinement of dendritic size, increase of interdendritic phase Qiang Li et. amount and improvement of thermal stability of interdendridritic 2007 al. [43] phase. Dependence of tensile properties on microstructure: deteriorate strength and elongation.

Continuation of Table 14

Ding Formation of Mg – Zn – Nd particles in the grain boundaries. Mg Wenjiang et. 2008 – Zn – Nd – Zr alloys have better tensile strength compared with al. [44] Mg – Zn – Zr alloys; increase of ductility in presence of Nd and Zr. Texture weakening and recrystallization in rolled Mg alloys; S.A.Farzadfar effects of yttrium as inoculant agent in grain refinement. Re- 2012 et. al. [45] tardiness kinetic of recrystallization and grain coarsening. Hindering effect of Y on boundary mobility. Kristina Neh Rare earth elements of WE43 promote higher strength values; 2014 et. al. [46] higher resistance to corrosion and high potential for applications. Passivation behaviour of WE43C Mg–Y–Nd alloy in chloride Jakraphan et. containing alkaline environments; analysis for pitting corrosion by 2017 al 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.

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]

푀 → 푀푛+ + 푛푒− (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

21 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:

퐹푒 → 퐹푒2+ + 2푒− (2.2)

푁푖 → 푁푖2+ + 2푒− (2.3)

퐴푙 → 퐴푙3+ + 3푒− (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:

+ − 2퐻 + 2푒 → 퐻2 (2.5) − − 2퐻2푂 + 2푒 → 퐻2 + 2푂퐻 (2.6)

+ − 푂2 + 4퐻 + 4푒 → 2퐻2푂 (2.7) − − 푂2 + 2퐻2푂 + 4푒 → 4푂퐻 (2.8)

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.

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) 퐴푢3+ + 3푒− → 퐴푢 +1.498 푃푡2+ 3푒− → 푃푡 +1.118 퐴푔+ + 푒− → 퐴푔 +0.799 퐹푒3+ + 푒− → 퐹푒2+ +0.771 퐶푢2+ + 2푒− → 퐶푢 +0.342 푆푛4+ + 2푒− → 푆푛2+ +0.150 + − 2퐻 + 2푒 → 퐻2 0 푆푛2+ + 2푒− → 푆푛 -0.138 푁푖2+ + 2푒− → 푁푖 -0.250 퐶표2+ + 2푒− → 퐶표 -0.277 퐶푑2+ + 2푒− → 퐶푑 -0.403 퐹푒2+ + 2푒− → 퐹푒 -0.447 퐶푟3+ + 3푒− → 퐶푟 -0.744 푍푛2+ + 2푒− → 푍푛 -0.762 퐴푙3+ + 3푒− → 퐴푙 -1.662 푀푔2+ + 2푒− → 푀푔 -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.

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.

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.

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.

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

+ − 2퐻 + 2푒 → 2퐻 → 퐻2 (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].

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

28 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].

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]

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].

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

Reagent Composition (mL/100mL) Brand

CH3CO2H 20 AppliChem: Panreac

HNO3 1 Sigma - Aldrich

C2H6O2 60 AppliChem: Panreac

H2O 19 Milipore

Table 18 - Keller solution.

Reagent Composition (mL/100mL) Brand

H2O 190 Millipore

HNO3 5 Sigma - Aldrich 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.

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.

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.

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

퐸 (3.1) 푅 = 퐼

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].

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.

퐸 = 퐸0 sin(휔푡) (3.2)

퐼 = 퐼0 sin(휔푡 + ∅) (3.3)

Where 휔 the radial frequency, t is time, ∅ is the phase shift, 퐼0 is the current’s amplitude and 퐸0 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

퐸 퐸0 sin(휔푡) sin(휔푡) (3.4) 푍 = = = 푍0 퐼 퐼0 sin(휔푡 + ∅) sin(휔푡 + ∅)

To simplify the equation, it is a necessary to use a complex function by Euler equations (3.5), (3.6) and (3.7)

푒푗∅ = cos(푗∅) + 푗 sin ∅ (3.5)

푗휔푡 퐸 = 퐸0푒 (3.6)

푗(휔푡+ ∅) 퐼 = 퐼0푒 (3.7)

Obtaining

푗휔푡 퐸 퐸0푒 −푗∅ (4.8) 푍(휔) = = 푗(휔푡+ ∅) = 푍0푒 = 푍0 (cos(∅) − 푗 sin(∅)) = 푅푒 + 푗퐼푚 퐼 퐼0푒

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(휔) or log(푓)) 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].

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= 퐸 Z = R 퐼 Inductor E = L 푑퐼 Z = j휔L 푑푡 1 Capacitor I = C 푑퐸 Z = 푑푡 j휔C

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:

푁 (3.9) 푍푒푞 = ∑ 푍푖 푖=0

And for impedance elements in parallel:

푁 1 1 (3.10) = ∑ 푍푒푞 푍푖 푖=0

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] .

퐸 (5.11) 푅 = ⟺ 퐸 푒푗휔푡 = 퐼 푒푗(휔푡+ ∅) 푅 ⟺ 퐸 푒푗휔푡 = 퐼 푒푗휔푡푅 ⟺ 퐸 = 퐼 푅 ⟺ 푍 = 푅 퐼 0 0 0 0 0 0

For capacitance, C, impedance for capacitor is:

푑퐸 푑(퐸 푒푗휔푡) 1 (3.12) 퐼 = 퐶 ⇔ 퐼 푒푗(휔푡+ ∅) = 퐶 0 ⇔ 퐼 푒푗(휔푡+ ∅) = 퐸 푗휔퐶푒푗휔푡 ⇔ = 푍 푒−푗∅ ⇔ 푍 푑푡 0 푑푡 0 0 푗휔퐶 0 1 = 푗휔퐶

Finally, for inductor, impedance is:

푑퐼 푑(퐼 푒푗(휔푡+ ∅)) (3.13) 퐸 = 퐿 ⇔ 퐸 푒푗휔푡 = 퐿 0 ⇔ 퐸 푒푗휔푡 = 퐼 퐿푗휔푒푗(휔푡+ ∅) ⇔ 푍 푒−푗∅ = 푗휔퐿 ⇔ 푍 푑푡 0 푑푡 0 0 0 = 푗휔퐶

EIS applied to corrosion studies

Considering a general reaction of 푀푒 ⟺ 푀푒푛+ + 푛푒−, or more generally 푅푒푑 ⇔ 푂푥 + 푛푒−, 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).

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

푅푇 (3.14) 푅푐푡 = 2 2 훼 1−훼 푛 퐹 푘0 퐶표푥 퐶푅푒푑

Where R is constant of perfect gases, T absolute temperature in kelvin, 푛 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]:

1 푊 = 휎휔−2 (1 − 푗) (3.15)

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

푅푇 1 1 (3.16) 휎 = ( + ) 2 2 푛 퐹 퐴√2 퐶푂푥√퐷푂푥 퐶푅푒푑√퐷푅푒푑

With 퐷푂푥 and 퐷푅푒푑 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:

1 (3.17) 퐶푃퐸 = 푛 푌0(푗휔)

푌0 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

42 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

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]

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]

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].

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.

-1.715 -1.735 -1.755 -1.775 -1.795 -1.815 First Test

E/v vs vs E/v SCE -1.835 Sixth Test -1.855 Seventh Test -1.875 -1.895 -1.915 0 1000 2000 3000 4000 Time / s

Figure 30 - OCP for WE43C in 0.05 M NaCl solution

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.

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.

-1.2 -1.3 -1.4 -1.5 -1.6 First Test

-1.7 Fourth Test E/v vs vs E/v SCE -1.8 Sixth Test -1.9 Seventh Test -2 -2.1 -9 -7 -5 -3 log ( i / A cm-2)

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

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.

-1.2

-1.4

-1.6

Cathodic -1.8 E/V vs E/Vvs ECS Anodic

-2

-2.2 0.E+00 1.E-03 2.E-03 3.E-03 4.E-03 i/A cm-2

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.

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

2 Solution Ecorr(V) icorr(A/cm ) 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.

-0.2

-0.4

-0.6 Cathodic E/V vs E/Vvs ECS Anodic

-0.8

-1 1.E-09 1.E-07 1.E-05 1.E-03 i/A.cm-2

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.

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 4.89 4.75 - Al – 12 curves: - 5.21 5.34 Al – 13 End of OCP| End test| - 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.

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.

105 -75 -8000 EIS-1.DTA EIS-2.DTA EIS-1.DTA EIS-2.DTA EIS-3.DTA EIS-3.DTA EIS-5.DTA EIS-5.DTA EIS-6.DTA 104 EIS-6.DTA EIS-7.DTA EIS-7.DTA EIS-8.DTA 2 EIS-8.DTA -50 EIS-9.DTA EIS-9.DTA Theta / Degree EIS-10.DTA 2 EIS-10.DTA EIS-11.DTA EIS-11.DTA EIS-12.DTA EIS-12.DTA

103 Z'' / Ohm cm Ohm / Z''

-3000 |Z| Ohm cm Ohm |Z| -25

102

0 5000 Z' / Ohm cm2 101 0 -2 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10 10 Frequency (Hz)

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

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.

2 n-1 -2 2 n-1 -2 2 χ2 Rs(Ω cm ) Y 0,f (F s cm ) nf Rpore(Ω cm ) Y 0,dl(F s cm ) ndl Rct(Ω cm ) 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,

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.

5 -75000 10 -75

Al_EIS_1.DTA Al_EIS_1.DTA Al_EIS_2.DTA Al_EIS_2.DTA Al_EIS_3.DTA Al_EIS_3.DTA Al_EIS_4.DTA 104 Al_EIS_4.DTA Al_EIS_5.DTA Al_EIS_5.DTA Al_EIS_11.DTA Al_EIS_11.DTA -50

-50000 Theta / Degree

2 Al_EIS_12.DTA Al_EIS_12.DTA 2

103

|Z| Ohm cm Ohm |Z| Z'' / Ohm cm Ohm / Z'' -25000 -25 102

1 0 10 0 -2 -1 0 1 2 3 4 5 6 0 25000 50000 75000 10 10 10 10 10 10 10 10 10 2 Frequency (Hz) Z' / Ohm cm

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.

Rs(Ω 2 n-1 -2 2 n-1 -2 2 χ2 cm ) Y 0,f (F s cm ) nf Rpore(Ω cm ) Y 0,dl(F s cm ) ndl Rct(Ω cm ) 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

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.

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]

1 (4.1) (1−푛) 푛 C = [Y0 푅 ]

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 휒2 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.

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.

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.

10 µm

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:

10 µm

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:

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 푂 , for yttrium (regular structure) 푀푔 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

10 µm

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.

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:

1 mm

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:

100 µm 10 µm 10 µm

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

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.

100 µm 10 µm 10 µm

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.

1 mm

Figure 48 - Second SEM for ASTM 7475 alloy

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.

100 µm 10 µm 10 µm

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

100 µm 10 µm 10 µm

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.

1 mm

Figure 51 - Third SEM for ASTM 7475 alloy

100 µm 10 µm 10 µm

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

100 µm 10 µm 10 µm

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.

2 mm

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

100 µm

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

100 µm

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

Table 30 - EDS for ASTM 7475 with O (Figure 55)

Element O Mg Al Cu Zn 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).

Element O Mg Al Cu Zn 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.

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.

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

69 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.

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