Electrochemical Behaviour of Magnesium Alloys Study on the influence of Rare Earths as alloying elements

Ricardo Augusto de Almeida Pinto

Dissertação para obtenção do Grau de Mestre em

Engenharia de Materiais

Júri Presidente: Professor Luís Guerra Rosa

Orientador: Doutora Maria de Fátima Grilo da Costa Montemor

Vogal: Professora Maria João Pedroso Carmezim

Setembro de 2008

Acknowledgements

The author acknowledges all the colleagues and Professors at the Grupo de Estudos de Corrosão

e Efeitos Ambientais of Instituto Superior Técnico - Technical University of Lisbon. Their suggestions

and technical discussions and, more important their support, constituted an essential contribution to

this work. They have all been my teachers during this past year and I am very thankful for their

patience.

A special thank you is due to Professor Rogério Colaço who was always available to clarify any

question that I might have and especially for all the time he spent training me in the use of the Atomic

Force Microscope. His contribution greatly enriched this work.

Finally, I would like to extend my greatest appreciation and esteem to Fátima Montemor, PhD. for

all her orientation and support during this period. I would also like to thank her for the confidence she

has deposited in my work and for giving me the possibility to continue my academic studies.

Agradecimentos

O autor gostaria de estender um sincero agradecimento a todos os colegas e Professores do

Grupo de Estudos de Corrosão e Efeitos Ambientais do Instituto Superior Técnico – Universidade

Técnica de Lisboa. As discussões técnicas que tivemos, as suas sugestões e fundamentalmente o

apoio que demonstraram constituiriam uma contribuição essencial para este trabalho. Todos eles

foram meus professores este ano que passou e eu estou muito agradecido por toda a paciência que

demonstraram para comigo.

Um agradecimento especial é devido ao Professor Rogério Colaço pela disponibilidade que

demonstrou na clarificação de todas as minhas dúvidas e em especial por todo o tempo que

dispensou para me treinar no uso do Microscópio de Força Atómica.

Finalmente, gostaria de declarar a minha maior estima e apreço para com a Dr.ª Fátima Montemor

por toda a sua orientação e apoio durante este período. Gostaria de agradecer em particular toda a

confiança que ela depositou no meu trabalho e por me ter proporcionado a possibilidade de

prosseguir com os meus estudos académicos.

i

Abstract Magnesium alloys present a very high specific strength and good processing capabilities making

them interesting alternative materials for the aerospace and automotive industries. However, the low

corrosion resistance, caused by a high surface reactivity, has impaired the widespread application of

these alloys. The corrosion resistance of magnesium alloys is limited by a passive oxide/hydroxide layer formed

spontaneously in air that is poorly protective and very unstable under neutral or acidic media. To

increase the corrosion resistance of magnesium alloys it is usual to apply a coating to or modify the

chemical and physical properties of the surface. However, it has been reported in literature that the

addition of rare earths as alloying elements improves the corrosion behaviour of magnesium alloys.

The objective of this work was to investigate the influence of the rare earths as alloying elements

on the corrosion behaviour of magnesium alloys. Another objective was to determine the influence of

the presence of chlorides in the formation of the passive film.

This was achieved by dividing the study into two parts: i) evaluation of the passive film behaviour in

strongly alkaline passivating media; ii) the study of the corrosion resistance of the magnesium alloys in

corrosive solutions with different pH levels.

The alloys used in this study were the ZK31, EZ33 and WE54; where the ZK31 alloy was chosen

as control sample since it does not contain rare earths.

In the passivation study the electrochemical techniques revealed that the presence of rare earths

affected the properties of the passive films. The results from the Electrochemical Impedance

Spectroscopy indicated a bi-layer MgO/Mg(OH)2 structure for the passive film formed at alkaline pH.

The influence of the rare earths in the passive films was established; however, their concentrations

were always inferior to the detection level of the X-Ray Photoelectron Spectroscopy. The presence of

chlorides affected the morphology of the passive film, as revealed by Atomic Force Microscopy, but

not the electrochemical properties of the passive films.

The corrosion study confirmed that the presence of rare earths increases the corrosion resistance

of magnesium alloys. The WE54 alloy that has the highest amount of rare earth elements presented a

corrosion resistance one order of magnitude higher. The corrosion resistance of this alloy is the result

of two different processes: the stabilization of the surface layer provided by the rare earths and the

formation of a thick layer of corrosion products rich in Yttrium.

The importance of alloy microstructure was also revealed by Scanning Electron Microscopy and by

chemical analysis provided by the Energy Dispersive X-Ray Spectroscopy. The EZ33 alloy, which

contains rare earths showed a poorer behaviour than the ZK31 alloy. This is the result of the thermal

treatment that provoked the migration of the rare earths to the grain boundaries and the onset of a

strong microgalvanic effect, increasing the corrosion rate.

Keywords Magnesium, Rare Earths, Yttrium, Electrochemical Impedance Spectroscopy, X-Ray Photoelectron

Spectroscopy, Atomic Force Microscopy.

ii

Resumo

As ligas de magnésio constituem materiais interessantes e alternativos para a indústria automóvel

e aeroespacial graças a uma combinação de propriedades com uma elevada resistência específica e

boa aptidão para o processamento. No entanto, a fraca resistência à corrosão destes materiais

resultante de uma elevada reactividade tem limitado fortemente a sua aplicação.

A resistência à corrosão das ligas de magnésio é condicionada pela camada passiva de

óxidos/hidróxidos que se forma espontaneamente na superfície e que é pouco protectora e muito

instável em meios neutros ou ácidos. Para proteger o magnésio da corrosão é habitual aplicar

revestimentos ou modificar as propriedades químicas ou físicas da sua superfície. Contudo, a

literatura refere que as ligas que contêm terras raras como elementos de liga apresentam, em geral,

melhor resistência à corrosão.

O presente trabalho visou estudar a influência das terras raras enquanto elementos de liga, quer

no comportamento face à corrosão quer no comportamento passivo. Simultaneamente pretendeu-se

também estudar a influencia da presença de cloretos na formação do filme passivo. Para concretizar

estes objectivos o trabalho foi dividido em duas partes: i) estudo das propriedades dos filmes

passivos formados em meio alcalinos (pH13), ii) avaliação da resistência à corrosão das ligas de

magnésio em meios agressivos a diferentes valores de pH. As ligas usadas no estudo foram as ligas:

ZK31, EZ33 e WE54. A liga ZK31 foi a amostra de referencia dado que não contém terras raras.

No estudo de passivação as técnicas electroquímicas revelaram que a presença da terras raras

melhora as propriedades dos filmes passivos. Os resultados da Espectroscopia de Impedância

Electroquímica indicaram que os filmes passivos formados em meio alcalino são constituídos por uma

bi-camada de MgO/Mg(OH)2. As terras raras têm influência no comportamento do filme passivo,

sendo que a sua concentração se situa abaixo do nível de detecção da Espectroscopia de

Fotoelectrões de Raios-X. A Microscopia de Força Atómica revelou que a presença de cloretos afecta

a morfologia do filme passivo, apesar de não influenciar o comportamento do filme passivo.

O estudo de corrosão confirmou que as terras raras aumentam a resistência à corrosão das ligas

de magnésio. A liga WE54, que contém a maior quantidade de terras raras, apresentou uma

resistência à corrosão uma ordem de grandeza superior às duas outras ligas. A resistência desta liga

aparenta ser o resultado de dois factores: a estabilização do filme superficial resultante da presença

de terras raras e a formação de uma camada de produtos de corrosão espessa e rica em Ítrio. A

análise por Microscopia Electrónica de Varrimento demonstrou a importância da microestrutura da

liga no comportamento face à corrosão. A liga EZ33 que contém terras raras revelou um

comportamento inferior à liga ZK31 como resultado do tratamento térmico que provocou a migração

das terras raras para os limites de grão, onde provocaram um efeito microgalvânico e o consequente

aumento da velocidade de corrosão.

Palavras-Chave Magnésio, Terras Raras, Ítrio, Espectroscopia de Impedância Electroquímica, Espectroscopia de

Fotoelectrões de Raios-X, Microscopia de Força Atómica.

iii

Index

Pag. Chapter 1 Introduction to Magnesium Science and Technology 1.1 History of Magnesium 1 1.2 General Properties of Magnesium 2 1.3 Magnesium Alloys and Alloying Behaviour 1.3.1 Nomenclature and Identification of Magnesium alloys 5 1.3.2 Alloy Development 6 1.4 Summary 10 Chapter 2 Basic Corrosion Science 2.1 Definition of Corrosion 11 2.2 Principles of Metallic Corrosion 11 2.2.1 Electrochemical Nature of Corrosion 11 2.2.2 Thermodynamics of Corrosion 13 2.2.3 Kinetics of Corrosion 17 2.2.4 Passivation Behaviour of Metals 21 2.2.5 Forms of Corrosion 23 2.3 Summary 26 Chapter 3 The Electrochemistry of Magnesium and Magnesium Alloys 3.1 Introduction 27 3.2 Corrosion Mechanisms 3.2.1 Pure Magnesium 27 3.2.2 The Negative Difference Effect 29 3.2.3 Impurity Elements 31 3.2.4 Magnesium Alloys 31 3.3 Aspects in the Corrosion of Magnesium Alloys 3.3.1 Environmental Corrosion 35 3.3.2 Galvanic Corrosion 36 3.3.3 Environmental Induced Cracking of Magnesium 36

3.4 Corrosion Protection of Magnesium Alloys 37

3.5 Summary 37

iv

Pag. Chapter 4 Experimental Techniques 4.1 Electrochemical Techniques 4.1.1 Potentiodynamic Polarization 38 4.1.2 Electrochemical Impedance Spectroscopy 38

4.2 Microscopic and Surface Characterization Techniques 4.2.1 Optical Microscopy 45 4.2.2 Scanning Electron Spectroscopy 45 4.2.3 X-Ray Photoelectron Spectroscopy 46 4.2.4 Atomic Force Microscopy 47 Chapter 5 Experimental Procedure 5.1 Samples 50 5.2 Test Solutions 50 5.3 Electrochemical Tests 51 5.4 Analytical and Surface Characterization 52 5.5 Metallographic Characterization 53 Chapter 6 Results and Discussion 6.1 Metallographic Characterization 6.1.1 ZK31 Alloy 54 6.1.2 EZ33 Alloy 55 6.1.3 WE54 Alloy 56

6.2 Passivation Study 57 6.2.1 Potentiodynamic Polarization Results 57 6.2.2 Passive Film Analysis by Electrochemical Impedance Spectroscopy 58 6.2.3 Microscopic and Quantitative Analysis 65 6.2.4 Compositional Analysis by X-Ray Photoelectron Spectroscopy 67 6.2.5 Topographical Characterization 68

6.3 Corrosion Study 70 6.3.1 Potentiodynamic Polarization Results 70 6.3.2 Electrochemical Impedance Spectroscopy Results 73 6.3.3 Microscopic and Quantitative Analysis 77 6.3.4 Compositional Analysis by X-Ray Photoelectron Spectroscopy 80

v

Pag. Chapter 7 Conclusions 83 Chapter 8 Future Work 85 References 86

Appendix 1 – EIS Fitting Results A1-1

Appendix 2 – List of Figures and Tables A2-1

vi

Chapter 1

Introduction to Magnesium Science and Technology

1.1 History of Magnesium

Throughout history mankind has been able to identify and beneficiate form the use of several

magnesium compounds such as hydroxides, sulphates and carbonates. However, the first person to

put forward the notion of magnesium as a separate chemical element was J. Black in 1755 [1]. The

credit for first having identified this element is normally attributed to Sir Humphrey Davy, who was able

to isolate magnesium from a mixture of magnesia (MgO) and mercuric oxide (HgO) in 1808. The first

magnesium metal was produced in 1833 by Michael Faraday by the electrolysis of fused anhydrous

magnesium chloride. Nevertheless, the method for producing magnesium in commercial quantities is

attributed to Robert Bunsen who designed a cell for the electrolysis process. The production of

commercial magnesium began in 1886 in Germany and the production process was further improved

in 1896 by Chemische Fabrik Griesheim-Elektron, which was the sole supplier of magnesium until

1916 [2]. The Magnesium Elektron Company is still the world’s largest magnesium supplier.

The name of magnesium derives from the ancient Greek city of Magnesia. Magnesium constitutes

the eighth most common element (about 2.5% of the Earth’s crust) and the sixth most abundant metal.

In addition seawater contains 0.14% (approximately 1.1 kg.m-3) of magnesium, thus creating an

almost endless reserve [3,4].

During the first half of the 20th century, the magnesium alloys experienced an initial development

which was parallel to other light alloys. However, due to early technological difficulties related with the

production of magnesium, that did not allow producers to maintain competitive prices, and the difficulty

in improving mechanical and corrosion properties through alloy development, the magnesium alloys

were replaced, in all but a few applications, by aluminium alloys and from the 1960s on by plastics [4].

The initial driving force for the development and application of magnesium alloys was the weight

reduction of components. This reduction was especially attractive to the military industry and the early

development of airplanes, and it is without surprise that we find that the greatest peaks in magnesium

production coincide with the two World Wars (WW), and particularly with WW II. At this point in time

the consumption of magnesium achieved the maximum of 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 [4,5].

From this point on, magnesium remained in use in niche applications in the military, aerospace and

nuclear industries, but the greatest single application was in the Volkswagen Beetle that between

1939, when it started production, and 1972 used magnesium in several different parts that included

the crank case, camshaft sprockets and gearbox housing, among others. The application of

magnesium in this case generated a 50 kg reduction in mass, as compared with steel [4].

Currently, with the development of high purity alloys in the 1980s that present superior corrosion

resistance, magnesium production has reached and surpassed WW II levels (360 000 t in 1998 at a

price of US$3.6 per kg) [5]. The principal consumer of magnesium today is the automotive industry,

with the main goal of achieving significant reductions in fuel consumption and gas emissions, so as to

conform to the growing demands for a more sustainable use of resources and the reduction of

environmental impact from human activities.

1

1.2 General Properties of Magnesium

Besides seawater, magnesium occurs in nature in three main mineral forms: magnesite MgCO3,

dolomite MgCO3.CaCO3 and carnallite KCl.MgCl2.6H2O. The most used technologies for production of

metallic magnesium are the electrolysis of molten anhydrous MgCl2, the thermal reduction of the

dolomite mineral or the extraction of magnesium oxide from seawater [3]. In addition to these traditional

methods, magnesium is recyclable, characteristic that opens the possibility of great cost reductions in

extraction and production.

The main physical properties of pure magnesium are depicted in Table 1.

Table 1 – Physical properties of pure magnesium [4,6].

Crystalline Structure Hexagonal dense packed

Density (ρ) 1.74 kgm-3 (a)

Young Modulus (E) 45 GPa

Yield Tensile Strength (σY) 21 MPa

Ultimate Tensile Strength (σU) 80 – 180 MPa

Fracture Elongation (εf) 1 – 12 %

Melting Point (Tm) 650 ºC

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

Fusion Heat 195 kJkg-1

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

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

Solidification Shrinkage 4.2 %

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

(a) at room temperature

The most important characteristic of magnesium in terms of physical metallurgy is its hexagonal

dense packed (hdp) crystalline structure. As in other hdp metals, this feature makes magnesium

unsuitable for cold forming. 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) [7], combined with the hdp structure, accounts for an

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

The majority of those elements generate eutectic systems and only cadmium presents complete

solubility as revealed in Table 2. The most common elements used in commercial magnesium alloys

are: Al, Ag, Be, Ca, lanthanides (Ce and Nd), Li, Mn, Si, Th, Y, Zn and Zr [4].

2

The result of the alloying of magnesium with different solutes is the occurrence of a wide range of

intermetallic compounds. The most frequent types of intermetallics in magnesium present the following

structures:

• AB – CsCl simple cubic structure; ex.: MgTi, CeMg, SnMg.

• AB2 – RA/RB ratio of 1.23 preferred for Laves phases; ex.: MgCu2 (f.c.c., abcabc stacking

sequence), MgZn2 (hexagonal, ababab stacking sequence), MgNi2 (hexagonal, abacaba

stacking sequence).

• CaF2 – f.c.c. structure, ex.: Mg2Si, Mg2Sn.

Table 2 – Solubility of solute elements in binary magnesium alloys [7].

Element At. % wt. % System Element At. % wt. % System Titanium 0.1 0.2 Peritectic Tin 3.4 14.5 Eutectic Cerium 0.1 0.5 Eutectic Yttrium 3.8 12.5 Eutectic Gold 0.1 0.8 Eutectic Silver 3.8 15.0 Eutectic Thorium 0.5 4.8 Eutectic Terbium 4.6 24.0 Eutectic Calcium 0.8 1.4 Eutectic Thulium 6.3 31.8 Eutectic Manganese 1.0 2.2 Peritectic Lead 7.8 41.9 Eutectic Neodymium ~1 ~3 Eutectic Aluminium 11.8 12.7 Eutectic Zirconium 1.0 3.8 Peritectic Scandium ~15 ~24.5 Peritectic Samarium ~1.0 ~6.4 Eutectic Thallium 15.4 60.5 Eutectic Bismuth 1.1 8.9 Eutectic Lithium 17.0 5.5 Eutectic Ytterbium 1.2 8.0 Eutectic Indium 19.4 53.2 Peritectic Zinc 2.4 6.2 Eutectic Cadmium 100 100 S. S. Gallium 3.1 8.4 Eutectic

Magnesium alloys possess a series of characteristics that make them very exciting and promising

materials for structural applications in the 21st Century.

The central point of the renewed and growing interest in these alloys is the low density that they

present. Magnesium alloys are lighter than all the construction metals including the most popular like

steel, aluminium alloys and titanium alloys. Low density and high mechanical resistance are an

important association that produces a high specific strength, which opens the possibility of important

weight reductions in structural components.

Concerning production technologies, magnesium alloys show a great aptitude for a large number of

casting processes, including high pressure die casting. In addition, magnesium is readily recyclable

through the traditional methods of direct scrap reinsertion into the melt, with an added bonus arising

from the recovery of certain valuable alloying elements such as rare earths.

Components built with magnesium alloys do not have to be produced in final form by casting

processes. These alloys comply very well with the properties required for machining, including high

cutting speeds, and are also weldable under a protective inert gas atmosphere.

The alloys are generally more resistant to corrosion than the pure metal, and this characteristic can

be enhanced through alloy design, subject that will be addressed in Chapter 3.

3

In section 1.1, the great abundance and availability of magnesium was reported. This is an

important advantage for the use of magnesium, but the energy necessary for its extraction from the

raw minerals is higher than that required for the extraction of other comparable metals, when

calculated as a function of the mass obtained. However, taking into account the low density of

magnesium, the same calculation made on the basis of volume of material becomes favourable over

other structural metals like Al and Zn.

There is another advantage which favours magnesium comparatively to other metallic alloys:

magnesium alloys are casted at relatively low temperatures, between 650 ºC and 680 ºC. This implies

that the energy necessary to melt a magnesium alloy can be up to 70 % less then the necessary for

an Al alloy [4].

Frequently, magnesium alloys have to compete with plastics and polymers in several types of

applications. But, compared with plastics, magnesium generally presents enhanced mechanical

properties, improved aging resistance, better electrical and thermal conductivities and simpler

recycling technology.

Despite the very interesting possibilities that support magnesium alloys as substitutes, in some

applications, of conventional structural metals and plastics, there are serious drawbacks limiting a

more widespread use of this material.

In terms of mechanical behaviour, a great disadvantage is the limited number of wrought alloys

available. This fact, combined with a generally low deformability and toughness at room temperature

and a low creep resistance at high temperatures, effectively hinders their use in a very large number

of applications with specific requirements.

Casting technology is the most common for the production of magnesium. Therefore, the

solidification shrinkage and the relatively high linear expansion coefficient (αL) are a major issue. To

the 4 % solid-liquid shrinkage one must ad up to 5 % shrinkage in cooling [4].

The high chemical reactivity of magnesium is most important factor limiting its extensive use.

Although magnesium alloys show comparatively better resistance to corrosion then the pure element,

in some environments the level of deterioration is still unacceptable for some applications. A deeper

and more detailed analysis of the corrosion behaviour of magnesium and its alloys will be given in

Chapter 3.

The high reactivity also creates some security hazards related to handling and processing of

magnesium.

There are also some concerns related with recycling of magnesium alloys. If the technological part

of the equation is simple, the lack of a circuit for retrieval of magnesium limits its recycling to an

internal process in production plants.

Finally, there are some economic issues leading to a limited use of magnesium, since the

producers are in small number and a low and stable price for the metal has not been reached.

4

1.3 Magnesium Alloys and Alloying Behaviour

1.3.1 Nomenclature and Identification of Magnesium Alloys

Several systems exist for the codification and identification of magnesium alloys, none of them

accepted universally. In this work, the preferred nomenclature will be the one defined by ASTM for the

non-ferrous alloys, either for the identification of the alloys or for the heat treatments they might

endure. However, since other important systems exist, like the DIN or the British specifications, that

commonly identify some magnesium alloys and because some of the alloys mentioned in this work are

experimental and need to be formally classified, a case by case strategy will applied. When the need

arises to refer a non ASTM nomenclature, it will be accompanied by the necessary explanation.

In the ASTM standard, magnesium alloys are identified by two letters that correspond to the two

main alloying elements. Those letters are followed by numbers that correspond to the nominal

composition in weight percentage of that particular alloy element, rounded to the nearest unit. After

this sequence a final letter might be present. This final letter is attributed sequentially (A, B, C…) and

represents the stage of development of a particular alloy. To the first alloy registered for a particular

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, including the ones considered as impurities that might have a harmful effect on

alloy performance. The information about alloying elements and the corresponding abbreviation letters

is summarized in Table 3.

Table 3 – ASTM codes for magnesium alloys [4,7,8].

Code letter Chemical Element Code letter Chemical Element A Aluminium N Nickel B Bismuth P Lead C Copper Q Sliver D Cadmium R Chromium E Rare Earths S Silicon F Iron T Tin H Thorium V Gadolinium J Strontium W Yttrium K Zirconium X Calcium L Lithium Y Antimony M Manganese Z Zinc

The alloy AZ91D that is one of the most employed and studied among magnesium alloys.

According to the ASTM standard, this is an alloy that contains Al in the quantity of 9 % (wt) and a

maximum global composition of 1 % (wt) of Zn. The letter D indicates that it is the fourth alloy

registered with this global composition. The DIN equivalent is the MgAl9Zn1 alloy.

5

1.3.2 Alloy Development

In the early stages of magnesium alloy development, the main focus of researchers was placed on

the enhancement of mechanical properties. For this purpose a great effort was made on the

description and understanding of metallurgical and alloying behaviour.

In the case of magnesium, because of the hdp structure, there are only two mechanisms possible

for this improvement: solid-solution hardening and precipitation hardening. While the effectiveness of

solid solution hardening is dependent on the differences in atomic radii between magnesium and

solute elements, precipitation hardening is only possible in alloying systems where the solubility of the

solute atoms in the magnesium matrix decreases greatly with temperature, and consequently, triggers

the formation of intermetallic phases [4,7].

As reported on section 1.2, the formation of intermetallics is a common event when dealing with

magnesium. The efficiency of the hardening effect is related with the reduced solubility at low

temperatures, the magnesium content of the intermetallic compound and the stability of said

intermetallics at operational temperatures. The precipitation mechanisms in magnesium alloys can be

very complex; however, the importance of the physical metallurgy justifies the abundance of literature

related to these mechanisms and their influence in microstructure and properties [7, 9-39].

The first elements used commercially in magnesium alloys were Al, Zn and Mn; with Al constituting

the principal alloying element. Thus the AZ and AM alloy series were created. The improved physical

properties resulted from the Mg-Al eutectic system and from the formation of the dendritic intermetallic

phase - Mg17Al12. The main process used for production of these alloys was casting. These alloys

were important not only because of the improvement in their mechanical properties, but also for the

progress in corrosion resistance. As a result, the AZ91 is still the most important and employed alloy

for die-casting products, mainly because of its splendid castability for complex shapes and thin walled

components.

In 1937 the positive effect of the addition of Ce was discovered giving birth to the AE alloy series.

A major landmark in the development of magnesium alloys was the discovery of the influence of Zr.

This innovation was so important that currently magnesium alloys can be classified as either

zirconium-free or zirconium-containing alloys.

Even though the solubility of Zr in magnesium is very low (less then 1 %), this element has a very

positive impact on its properties. The key factor determining this positive impact is the capability for

grain refinement of the alloy structure by the introduction of Zr in the alloy composition. These alloys

also respond better to heat treatment and ageing than the Al based alloys.

Of the several types of alloys created after this finding it is useful to mention the EK, EZ, ZE and ZK

series.

6

A further advance occurred after 1940 with the recognition of the role of the lanthanides in the

improvement of the mechanical properties of magnesium alloys. The Mg-RE (rare earth) alloys display

better castabillity due to the formation of eutectic intermetallics with low melting point. The eutectics

are also responsible for an added grain refinement effect that enhances creep resistance and tensile

properties. Rare earths are somewhat expensive, and for that reason they are usually added to

magnesium in form of cheaper Ce based mishmetal.

In 1959 [4], Payne and Bailey [7] discovered that by adding Ag to the Mg-RE-Zr alloys they could

increase significantly the tensile properties of these alloys. When heat treated with the right procedure

(typically solution treatment followed by cold water quenching), the precipitation mechanism in the

alloys form the QE and EQ series are affected, leading to an increase in the fraction of Mg-Nd

precipitates.

The superior properties of these alloys allowed for their use very critical applications in the

aerospace industry such as: aircraft landing wheels, gearbox housings and helicopter rotors, among

others.

In view of the alloying behaviour of magnesium, the continuous development of new systems is a

rather predictable fact. However, the alloy group currently regarded has having the best and the most

balanced behaviour regarding mechanical properties and corrosion resistance started its development

in the early 1980’s. The addition of Y to Mg-RE alloys, taking advantage of its relatively high solubility

(Table 2), created an excellent combination of high strength at room temperature, good creep

resistance (50 to 100 ºC superior to the QE and EQ alloys) and corrosion resistant capabilities in the

form of the WE magnesium alloy series.

The RE based systems are well suited for both solid solution hardening and precipitation

hardening. Nonetheless, Y also enhances the aging capabilities due to the formation of more stable

and coherent precipitates then Ce based alloys [9]. Table 4 summarizes the effect of the alloying

elements in the behaviour of the alloys and its production technologies.

Nobler elements such as Cu, Fe and Ni are not usual alloying elements in magnesium alloys. On

solidification, they usually precipitate into cathodic compounds that are extremely detrimental for

corrosion resistance. For the same reason, the levels of these elements have to be reduced to very

low values to avoid increased corrosion rates and alloy deterioration.

There is a noticeable exception to this behaviour in the ZC alloy series. These alloys show tensile

properties similar to the AZ alloys, presenting at the same time improved ductility as well as a good

response to age hardening. The unexpected stability in the corrosion resistance is attributed to the

incorporation of all the Cu into a eutectic phase Mg(Cu,Zn)2 [7].

7

Table 4 – Influence on the behaviour of Mg alloys of the most important alloying elements [4,7].

Element Benefits Disadvantages

Al General improvement of the mechanical properties. Excellent castabillity.

Low temperature and creep resistance. Tendency for microporosity. Weak response to aging.

Ag Combined with RE increases high temperature and creep resistance. Low corrosion resistance.

Be It is only used in low quantities (<30 ppm) to reduce melt oxidation. Not reported.

Ca Improves grain refinement and creep resistance.

Can lead to sticking to the tool during casting and hot cracking.

Li Solid solution hardening at room temperatures. Reduced density and increased ductility.

Burning and vapour behaviour of molten metal. Bad corrosion behaviour.

Mn

Increase in tensile strength (>1.5 % wt). Corrosion resistance (removes Fe atoms into harmless intermetallics). Improves grain refinement and weldability.

Not reported.

RE and Y

Precipitation and age hardening (for Y). Creep resistance and high temperature strength. Corrosion resistance.

Low castabillity. High cost.

Si Creep resistance. Lowers the castabillity.

Th High temperature strength and creep resistance.

Radioactivity (is currently being substituted for other elements).

Zn

Same behaviour as Al in terms of castabillity and strengthening. Increase in tensile strength. Reduction in shrinkage.

Tendency for microporosity. Hot cracking (above 2 % wt).

Zr

Increase in tensile strength without compromising ductility. Grain refinement through the formation of stable oxides that act as nuclei.

Cannot be added to melts containing Al or Si. Reduction in welding capability (ZK alloys).

Today, the largest consumers of casting alloys are the members of the automotive industry. The

most employed casting alloys are the ones in which Al is the principal alloying element, namely: AZ,

AM, AS and AE series. For the aerospace industry, the requirements for high temperature strength

and creep resistance force the substitution of these alloys by the alloys from the QE and WE series,

even though their limited castabillity only allows for sand casting technology to be employed.

Despite the hdp lattice and the formation of twins during cold work, the market for wrought

magnesium alloys has been growing and their use is today comparable to casting alloys.

8

As it happens for many other metals and alloys, there are still very few truly optimized systems;

however, some magnesium alloys show good potentialities that allow for their widespread application.

Among these alloys it is possible to name examples from the AZ series and derivatives, and also from

the ZK, ZM, HM, HK and the WE series.

As the interest in magnesium alloys grows, so does the effort of the scientific community and the

resources applied to the improvement of their properties. In the area of research and development,

there is a continuous demand, from several technological applications, in order to improve the

performance of magnesium alloys.

The most common approach is the development of new alloys through standard metallurgy.

Resorting to compositional changes, alloying with new chemical elements, altering production

sequences and thermal treatments, it is possible to achieve enhanced properties from new and old

alloy systems. A good example of the effectiveness of this practice is the recent development of

ternary Mg-Sc and Mg-Gd that show heat resistance and creep characteristics superior to the WE

series, which represents the current state of the art in creep resistant magnesium alloys [9].

Great advances can also be obtained though the development and application of new processing

technologies. Alloys obtained by rapid solidification [4,40,41] are a clear example of this link between

alloy development and production technology. The process of spray-forming, in which the molten

metal is cooled in non thermodynamic equilibrium conditions, creates a refined structure with

significant impact in the mechanical and corrosion properties, since the element distribution is much

more homogeneous. This technology also encompasses the possibility of creating new alloy

compositions, which cannot be obtained by traditional metallurgy, like the bulk amorphous alloys [42,43].

In relation with production technologies, several methods developed for other metals have been

successfully adapted to magnesium that allowed for a greater flexibility and increased market

coverage. Some of these processes are: squeeze-casting, rheo-casting, thixo-casting and thixo-

forming [4].

Finally, these new production methods also permitted the design of magnesium based composite

materials with superior mechanical properties such as hardness, tensile strength, hot working abilities

and creep resistance. The control of the characteristics of these alloys allows engineers to design

them to match specific requirements. By spray-forming it is possible to produce composite magnesium

alloys with a homogeneous distribution of SiC or Al2O3 particles. On the other hand, the squeeze-

casting makes it possible to impregnate a magnesium alloy with ceramic fibres, of different lengths or

materials (SiC, Al oxide or carbon) [4,44,45].

9

1.4 Summary

This first chapter that now ends is intended only as an introduction to the general properties of

magnesium and its alloys. It aims at familiarizing the reader with the basic principles and terms related

with the physical metallurgy of magnesium, in order to facilitate the comprehension of their influence

on the corrosion behaviour, as those questions are addressed in the following chapters.

The cited literature contains a much more profound and detailed analysis of all of these concepts,

however, since the object of this work is orientated towards corrosion behaviour, a deeper discussion

would fall out of the scope of this dissertation.

The need for a more intelligent and sustainable management of the natural resources is the major

driving force of our times. The environmental awareness by the public and the economic

repercussions of past and present misuse of raw materials and energy is forcing the transportation

industry to seek alternative technologies. With the price of the oil barrel showing a tendency to

increase and with the new European Directive that will limit CO2 emissions on newly designed cars to

110 g per 100 km, there is a true urgency in the automotive and aerospace industries to find

technologies allow important weight reductions in structural components. Magnesium and magnesium

alloys are expected, and have the potential, to fill those requirements.

Despite the good perspectives enclosed by magnesium alloys, their corrosion behaviour is a major

impediment to their widespread application. Although the electrochemistry of magnesium and

especially magnesium alloys is quite complex, it can be said, with all fairness, that its corrosion

behaviour is generally inferior to its competitors, with few exceptions.

This aspect supports and motivates the research presented in this work. A deeper understanding of

the corrosion behaviour of magnesium and magnesium alloys is necessary. This work intends to

contribute to achieve these objectives by investigating the behaviour of passive films formed on the

surface of different magnesium alloys and its corrosion resistance.

10

Chapter 2

Basic Corrosion Science

2.1 Definition of Corrosion

The definition of “corrosion” is a relatively complicated issue since the very scope of the term is

constantly being extended. One of the most successful definitions found in literature is: “Corrosion is

the deterioration of a substance (usually a metal) or its properties because of a reaction with its

environment” [46]. This is a broad definition, not only because it encompasses all types of materials like

ceramics, polymers and natural materials, but because it also relates corrosion with the alteration of

the properties beyond the deterioration of the materials themselves.

However, as Shreir et al. [47] have pointed out, the definitions supported on the concept of

deterioration do not describe all phenomena pertaining corrosion, especially regarding metallic

corrosion. A clear example of this is the formation of a surface oxide layer on metals, in the presence

of specific environments, that in some cases protects the material from deterioration. In this sense, a

definition based on the concepts of interaction between material and media and transformation of

properties could prove more adequate.

The subject of metallic corrosion is, nevertheless, the major issue in corrosion science. This

concern is even more critical when dealing with metal surfaces exposed or in contact with moisture,

aqueous environments or even more aggressive solutions like sea water. For this reason, this chapter

aims at giving an overview of the basic electrochemical concepts that govern metallic corrosion, as

discussed in numerous text books [46-52]. It aims at preparing for a more detailed analysis on the

corrosion behaviour of magnesium and its alloys, forthcoming in Chapter 3.

2.2 Principles of Metallic Corrosion

The reason why metals corrode in contact with an aqueous solution is very easy to understand and

pertains to 2nd Principle of Thermodynamics. The ordered state of the atoms that constitute a metal is

not the most stable form, as all matter tends to maximum disorder. In order to reduce their chemical

potential, the surface atoms leave the metal crystalline structure and dissolve into the solution as ions,

until the chemical potential (or free energy) of both states becomes equal [48].

2.2.1 Electrochemical Nature of Corrosion

The corrosion of metals is almost always the result of two or more electrochemical reactions,

occurring at the metal surface. For the electrochemical reaction to take place, four main factors are

required: an anode (or anodic process in which an atom is oxidized and passes into the solution as a

positive ion leaving the negative charge of its electrons at the metallic surface), a cathode (or cathodic

reaction that consumes the electrons released in the anodic reaction by reducing a species present in

the solution), an electrolyte (the solution itself that is responsible for the transport of ionic species to

and from the metallic surface) and an electronic circuit (to conduct the electrons produced in the

anodic reaction).

11

For the corrosion process to occur spontaneously, the two electrochemical reactions (reactions in

which the valence or oxidation state of a substance changes) must take place simultaneously, and the

number of electrons produced by the anodic reaction (oxidation) has to be equal to the number of

electrons consumed in the cathodic reaction (reduction). This represents one of the most important

principles in corrosion: the oxidation rate and the reduction rate must be equal, which defines the

coupling of both events as one cannot occur without the other.

As metallic ions enter into the solution, the metallic surface is left with an excess of negative

charges that creates an electric field affecting the solution in its proximity. To the surface formed

between the metal and the ion containing solution is attributed the name of electrode. The electrode

reactions can be divided in two groups: oxidation and reduction reactions, and can be described by

their partial equation reactions:

(1) −+ +→ neMM n

(2) 222 HeH →+ −+

(3) OHeHO 22 244 →++ −+

(4) −− →++ OHeOHO 442 22

(5) 1−+−+ →+ nn MeM

(6) MeM →+ −+

The anodic reaction in metallic corrosion is always the oxidation of a metal into its ion form

represented by Eq. 1; however, there are several different cathodic reactions possible depending on

the particular conditions.

The reactions depicted in Equations 2, 3 and 4 are the normal cathodic reaction occurring in

aqueous environments. Because one of the components of the reaction is in the gas form, they are

called gas electrodes and are very important in corrosion. The hydrogen reduction (Eq. 2) is a

common cathodic reaction in acidic and neutral media because of the availability of hydrogen ions.

The reduction of oxygen is also possible since most aqueous media contains some percentage of

dissolved O2. In acidic solutions oxygen is reduced according to Eq. 3, while in neutral or basic media

follows Eq. 4. The metallic ion reduction (Eq. 5) and metal deposition (Eq. 6) are not common

reactions and normally are not associated with spontaneous events, but rather with chemical and

industrial processes.

As the metal ions pass into the electrolyte, the negative charge of the surface attracts them and

keeps them close to the surface, establishing what is known as a double layer. The potential of this

double layer is measurable, and being specific to an electrode reaction can be used to classify the

different metals in a scale known as the Electromotive Series. An example of this series containing

some of the most important metals is presented in Table 5.

12

Table 1 – The Electromotive Force Series (adapted) [49].

Electrode reaction Standard Potential (V) 1

AueAu →+ −+ 33 1.50

AgeAg →+ −+ 0.80 Noble or cathodic

CueCu →+ −+ 22 0.34

Reference 222 HeH →+ − 0.00

NieNi →+ −+ 22 -0.25

FeeFe →+ −+ 22 -0.44

CreCr →+ −+ 33 -0.74

ZneZn →+ −+ 22 -0.76

TieTi →+ −+ 22 -1.63

AleAl →+ −+ 33 -1.66

MgeMg →+ −+ 22 -2.37

Active or Anodic

LieLi →+ −+ -3.05 1potentials are relative to the Hydrogen Saturated Electrode (HSE) as an arbitrary reference; at 25 ºC, 1 atm

of pressure for gas phases, the ion concentrations are 1 molal and the solid phases are pure.

The very restrictive conditions in which the Electromotive Series was achieved are generally

unpractical. In addition, the alteration of the test solution may also change the relative potentials of

different materials. For this reason, more practical series were created based on real solutions: the

Galvanic Series. In general the Electromotive and Galvanic series are in good agreement, for metals

as well as alloys, so the Electromotive Series is a good guideline for corrosion engineers. However,

there are some relevant exceptions: in a 3 % NaCl water solution for example, titanium develops a

cohesive and adherent oxide layer that effectively shields it from corrosion and transforms the

potential from negative (anodic) to positive (cathodic), making it the most corrosion resistant structural

metal known, having a potential more noble then silver.

Concerning to the position in the Electromotive or Galvanic series, the lower (more negative) the

potential the higher the tendency of that metal to corrode. However, since cathodic and anodic

activities are coupled, a cathodic reaction with a nobler (more positive) potential has to be possible in

that system for corrosion to progress spontaneously.

2.2.2 Thermodynamics of Corrosion

The potential of a specific electrode reaction is called the Standard Electrode Potential (Eº). There

are several variables that affect the potential of an electrode such as: concentration of chemical

species, temperature and partial pressure. The variation of potential as a function of concentration in

particular, can be calculated using the Nernst equation:

(7) [ ][ ]redox

nFRTEE ln0 −=

13

where E is the electrode potential, Eº is the standard electrode potential, R is the gas constant, T is

the absolute temperature, n is the number of moles of electrons exchanged in the half-cell reaction, F

is the Faraday constant, and [ox] and [red] are the activities (or concentrations when the activities are

not known) of the oxidized and reduced species, respectively.

Because gas electrodes are very difficult to construct and handle, the Saturated Hydrogen

Electrode (SHE) based on the hydrogen reduction potential is not normally used in potential

measurements. Instead, liquid electrodes with known and constant potentials in relation with the SHE

are used. The two most employed are the Saturated Calomel Electrode (SCE) and the Copper-Copper

Sulphate Electrode, with a relative potential to the SHE of +0.241 V and +0.318 V [50], respectively.

When performing electrochemical experiments it is normal to use a Three-Electrode

Electrochemical Cell. For measuring the potential of a system at rest (also called the open circuit

potential - OCP), since the currents are typically null, a reference electrode and test electrode with an

electrical contact are the only two components necessary.

On the other hand, if current flows freely because of a galvanic cell (cell formed by two metals in

contact with different potentials) or an externally imposed current or potential, the reactions at the

electrodes will be out of equilibrium. The measured potential will include two overpotentials (of the two

electrodes) that cannot be separated and a measurement of the potential of the test electrode

becomes impossible.

The potential value can be obtained by adding a third electrode, the auxiliary electrode, and

connecting the reference electrode to a high resistance voltmeter. In this way, current only flows

between the test and the auxiliary electrodes and not in the reference electrode, which remains stable

and unpolarized, being able to perform the potential measurement.

One of the most important tools in corrosion engineering is the Potential-pH diagram (E-pH), also

known as Pourbaix diagram. These diagrams are found profusely in the cited literature, however, the

reference for authors is still the original work by Pourbaix and his co-workers [53].

The basis of the Pourbaix diagram is the Nernst equation (Eq. 7) and it is computed form

thermodynamic data such as the standard chemical potentials. It is actually a graphical representation

of the Nernst equation and of the domains of stability of metal ions, oxides and other species in

solution; following their variation as a function of potential (represent in the y axis), pH (represented in

the x axis) or both.

14

There are three reactions to consider in a E-pH diagram:

• Electrochemical reactions of pure charge transfer; which involve only electrons and

reduced and oxidized species (Eq. 1), are independent of pH and represented by

horizontal lines in the diagram.

• Electrochemical reactions involving electrons and protons (H+ ions); they represent

reactions with the general equation depicted by Eq. 8 and depend on the value of potential

as well as pH. They are represented by sloped lines in the diagram.

(8) −+ ++→+ eHMOOHM 222

• Pure acid-base (redox) reactions; they are independent of potential and represented by

vertical lines in the diagram.

In metals, these lines delimit the domains in which the metals are immune (do not corrode),

passive (due to the formation of an oxide and/or hydroxide layer at the surface that prevents

corrosion) or actively corroding through anodic dissolution.

Because Pourbaix diagrams represent the equilibrium reactions that take place in water, the water

decomposition reactions (into hydrogen and oxygen) are also represented. They are very important as

the common cathodic reactions.

At negative potentials water decomposes with the evolution of hydrogen gas according to Eq. 1, in

acidic media, and Eq. 9 in alkaline solutions.

(9) −− +→+ OHHeOH 222 22

When the potential rises to values sufficiently positive, other reactions are thermodynamically

possible, namely, the reactions represented by Eq. 3 (acidic solutions) or Eq. 4 (alkaline solutions).

It can be deduced from what was explained so far, that the reactions of hydrogen and oxygen

evolution have different potentials that depend upon the pH values. However, it is easily demonstrated

that the pH dependency, given by the slope of a straight line, is the same for both and has the value of

-0.059 ( ). This happens because the activity of a reduced species (Nernst equation) has a

unitarian value by convention.

nFRT /

In Figure 1 depicts the Pourbaix diagrams of four important structural materials with very different

behaviours. Note that all the diagrams have the water reactions depicted (line a represents hydrogen

reduction and line b corresponds to water oxidation).

The copper diagram can be used to demonstrate the importance of these lines. Although Cu is a

very noble metal, it can corrode in highly acidic or highly basic solutions. However, because of its

nobility, the corrosion potential (the potential for anodic dissolution) is always higher than the potential

for cathodic reduction of hydrogen, as corrosion can only proceed if the cathodic reaction is the

reduction of the O2 dissolved in solution. If the solution is deaerated, Cu will not corrode

spontaneously.

15

The domains of instability are the shadowed areas in Figure 1 while the blank areas represent

either passivity or immunity to corrosion. The four materials have somewhat dissimilar behaviours: Al

will corrode in acidic and basic media remaining passive in neutral solutions; Fe is nobler than Al and

the passivity domain extends from neutral to alkaline solutions; Cu has, as mentioned earlier, a

diagram that is representative of a noble metal; finally, Ti shows why it is a material with excellent

corrosion resistance revealing an extended passivation domain, only corroding in very acidic solutions.

a) b)

c) d)

Figure 1 – Pourbaix diagrams of pure metals with representative corrosion behaviour: a) aluminium, b) iron, c) copper and d) titanium (adapted) [53].

Although the Pourbaix diagrams are an extremely important tool for corrosion engineers and supply

important information, they have several limitations.

The reactions depicted are the ones obtained by conditions of thermodynamical equilibrium,

ignoring unstable and metastable compounds that might form. Also, the information obtained is purely

thermodynamical and no information about the kinetics of the different reactions is available, and more

specifically, about the corrosion rates.

Finally, the term passivation is used broadly and sometimes abusively, when referring to the zones

where a layer of oxide and/or hydroxide forms. This layer only has passivation capabilities, and is

16

therefore capable of protecting a metal from corrosion, if it covers the surface completely forming a

coherent and adherent barrier, which is not always the case.

2.2.3 Kinetics of Corrosion

An electrochemical reaction will either produce or consume electrons. The total current (I), called

exchange current, generated by the electron flow between the anodic and cathodic half reactions is an

effective measurement of the corrosion rate, since it is directly proportional to mass loss during anodic

dissolution.

The relation between the measured current and the quantity of corroded metal is given by

Faraday’s law:

(10) nFtIMm =

where m (g) is the mass that reacted, t (s) is the time elapsed, I (A) is the current generated, M (g/mol)

is the atomic weight, n is the number of electrons exchanged in a reaction and F is the Faraday

constant.

It should be noticed that both cathodic and anodic reactions can evolve as the result of more than

one electrode process. In this case, the sum of the rates of all the partial anodic reactions has to be

equal to the sum of the rates of all the partial cathodic reactions.

Another important factor in corrosion kinetics is the area of the electrodes. A current density (i),

with the usual units of A/cm2, can be defined for electrode processes. In uniform corrosion, anodic

and cathodic areas, and consequentially the respective current densities, are said to be equal, while in

localized attack or pitting, the cathodic areas are bigger than the anodic areas, leading to a higher

anodic current density (ia) and a lower cathodic current density (ic).

The direct calculation of current densities isn’t, however, as straightforward as is it may appear at a

first glance. On one hand, in uniform corrosion the cathodic and anodic sites may be small enough to

become indistinguishable; and on the other hand, just because a surface is available for an electrode

process that doesn’t mean it participates actively in an electrochemical reaction.

The potential of a half-cell electrode can change due to changes in the net surface reaction. The

changes to electrode potential will cause a polarization (η) or overpotential at the surface. When

electrons are supplied in excess to the surface, constituting a cathodic polarization (ηc), the potential

becomes more negative making ηc negative by definition. In the opposite manner, the removal of

electrons from the surface provokes a positive overpotential or anodic polarization (ηa). In both cases

the polarizations are brought on by slow reaction rates.

There are two types of mechanisms that govern polarization phenomena. According to which one

is the limiting factor, polarization is said to have either activation control or mass transport control.

17

There are several intermediate steps underlining every electrochemical reaction. Even though they

are not always experimentally separable, these reactions are essential for both the anodic and

cathodic processes. An example of these reactions is given in Eq. 11 [49] and Eq. 12 [50].

(11) +++ →→→ nsolution

nsurfacesurfacelattice MMMM

(12) 2HHHHeH adsorbedadsorbedadsorbedsolution →+→→+ −+

These overall reactions represent the charge transfer processes that are the stepping stone of

activation control. The net corrosion rate can be controlled either by the anodic or the cathodic

process and both can be studied individually by the construction of experimental current-potential

curve or polarization curve. The polarization curve represented in Figure 2 can be obtained by

imposing a potential and measuring the flow of current or vice-versa.

Figure 2 – Theoretical current-potential diagram for a metallic electrode [49].

The curve follows the Butler-Volmer equation:

(13) ( )⎭⎬⎫

⎩⎨⎧

⎟⎠⎞

⎜⎝⎛ −−−⎟

⎠⎞

⎜⎝⎛= ηβηβ

RTnF

RTnFii 1expexp0

where R is the gas constant, T is the absolute temperature, β is a symmetry coefficient (normally

close to 0.5) and η is the overpotential relative to the equilibrium potential (Ee).

At equilibrium there is no overpotential, but since equilibrium is dynamic there are anodic and

cathodic current densities that are equal of opposite signs ( ia = -ic = i0 ). At small overpotentials the

two reactions also oppose each other.

When the potential departs sufficiently from equilibrium, one reaction is favoured over the other

whose rate becomes negligible, the Tafel region. Giving as example an anodic overpotential of

metallic dissolution (point 1 in Figure 2), the second term can be removed from Eq. 13 and the anodic

current density becomes:

(14) ⎟⎠⎞

⎜⎝⎛= ηβ

RTnFiia exp0

18

Applying the logarithmic function and rearranging the terms, one obtains:

(15) ⎟⎟⎠

⎞⎜⎜⎝

⎛=

0

logiib a

aaη

where ba is the Tafel coefficient given by:

(16) nF

RTba β303.2

=

The Tafel coefficient can be obtained by plotting η versus log i, creating what is known as an Evans

diagram. In the Tafel region the variation will be given by a straight line in which the slope is precisely

ba. The intercept also point yields a value for i0, which means that these values can be obtained

experimentally.

The same calculation can be made for cathodic overpotentials, the metal deposition reaction,

yielding the following value for the Taffel coefficient:

(17) ( )nFRTbc β−

−=

1303.2

In a natural spontaneous corrosion process, the anodic reaction is always coupled with a cathodic

reaction with a more positive (nobler) potential, as discussed earlier. Figure 3 represents a current-

potential curve and the corresponding Evans diagram of the two electrode processes.

Figure 3 – Theoretical current-potential diagram (left) and Evans diagram (right) for a corrosion process with two coupled electrode processes [49].

Again, the anodic current generated must be counter-balanced by a cathodic current of equal

dimension ( ia = -ic = icorr ), the corrosion current. The only way this can be achieved is if the corrosion

potential (Ecorr), a mixed potential of the two half-cell reactions, lies somewhere between the

equilibrium potential for the anodic (Ee)a and the cathodic (Ee)c electrode processes. This is the basic

principle of the mixed potential theory.

19

The metal dissolution reaction is driven by a positive anodic overpotential (ηa) and the reduction

reaction is driven by a negative cathodic overpotential (ηc). In this way, the activation overpotential can

be considered a measure of how far from equilibrium the reactions must be driven to produce the

corrosion current.

It should be noted that the two polarization curves are not necessarily symmetrical and are

seldomly identical in shape. The shape of the curves will be determined by the Taffel coefficient and

the exchange current.

Figure 4 depicts a practical example, the polarization of zinc in acidic media, which contains all of

the concepts discussed in the last paragraphs.

Figure 4 – Polarization diagram for the corrosion of zinc in an acidic solution [50] (adapted).

Since the two electrode processes in corrosion are coupled, the overall corrosion rate will be

controlled by the reaction which is kinetically slower. This means that the limiting reaction will be the

one with the smallest exchange current and/or the largest Taffel coefficient.

The Evans diagrams for three different types of control are depicted in Figure 5.

Figure 5 – Schematic Evans diagram of three electrode reactions with the same icorr but different types of

control: anodic (left), mixed (centre) and cathodic (right) [48].

When reaction rates are very high, the cathodic reaction can deplete the solution adjacent to the

surface of species to be reduced, limiting the evolution of the corrosion process. This can also happen

in solutions where the cathodic reagent is in short supply, for example deaerated solutions.

20

In this case we are in the presence of a mass transport control. There will be a diffusion layer, with

a profile similar to the one presented in Figure 6, in which the diffusion of species in steady-state

complies with Fick’s first law.

(18) ⎟⎠⎞

⎜⎝⎛∂∂

−=x

CDJ 0

where D is the diffusion coefficient and C0 is the concentration of reagent at a point x.

Combining Eq. 18 with the Nernst equation and applying the appropriate boundary conditions, the

limiting current density (iL) can be calculated:

(19) δ

BZL

nFCDi =

where DZ is the diffusivity of the reactant species, n is the number of electrons exchanged, F is the

Faraday constant, CB is the solution concentration and δ is thickness of the diffusion layer.

Mass transport control is usually only considered for the cathodic reactions and ignored for the

anodic process due to the limitless supply of metallic atoms emanating from the surface. However,

some polarization of the oxidation reaction can occur at very high corrosion rates.

In sum, anodic dissolution can be said to occur normally under pure activation control, disregarding

concentration effects; however, in the case of cathodic reduction the concentration of reactant species

must be taken into account. In this case, it is more correct to refer to a combined or total polarization

control (ηT,c) that is the direct algebraic sum of activation polarization (ηact,c) and mass transport

polarization (ηmass).

2.2.4 Passivation of Behaviour of Metals

The concept of passivation has been mentioned earlier in this discussion without a more profound

discussion of the characteristics of the phenomena.

Passivity is a condition found in some metals and alloys that are capable of resisting corrosion due

to the formation of a surface film. These films form under strongly oxidizing conditions or high anodic

polarizations. This definition excludes metals possessing a simple barrier film with reduced corrosion

at active potential and small anodic polarization. Also, it should be pointed out that insoluble

compounds formed by dissolution and reprecipitation are less tenacious and less protective than oxide

films formed in situ at the metal surface.

This is an exceedingly important characteristic found in several structural metals such as iron,

chromium, nickel titanium and aluminium, and its respective alloys being the most noticeable example

that of stainless steels.

21

Metals and alloys that exhibit passive behaviour display a very distinct evolution on the Evans

diagram, as exemplified in Figure 6.

As the potential is increased from the corrosion potential, so the current increases according to

normal dissolution behaviour until a critical value (icrit). This point also defines the beginning of stability

for passive films, which occurs at potentials higher than Epp (primary passive potential). Beyond this

point, the current measured can fall several orders of magnitude to a residual current (ipass). At higher

potentials (Et) breakdown of the passive film might occur with an increase in anodic activity, as the

metal enters the transpassive state. As always, for self-passivation to occur there must be a cathodic

reaction with a nobler potential relative to the anodic reaction and, in this case, superior also to Epp.

Figure 6 – Schematic polarization diagram displaying transitions from active corrosion to passive behaviour and to the transpassive state [50].

One of the strategies employed in the protection of metals is the anodization process. By applying

a potential in the passive state (between Epp and transpassivation), and choosing the right media and

applied current, it is possible to grow very thick oxide layers to protect the metal surface from

oxidation. This constitutes one of the most used techniques in the protection of aluminium alloys.

There are several models that try to explain the formation and structure these oxide films; however,

much is still uncertain. There are two basic theories: the crystalline oxide model and hydrated

polymeric oxide model.

In the crystalline oxide model, as the name implies, passivation depends on the formation of an

oxide/hydroxide layer. The exact structure of the oxides is very uncertain and seems to vary from

crystalline all the way to completely amorphous. The oxides may contain oxygen and/or hydrogen

under several different forms (H+, OH- or H2O), and the number of layers may change according to

specific systems as well as the stoichiometry of the oxide.

In polymeric model, on the other hand, water molecules have an important role in passivation as

they connect chains of polymeric oxide, whose structure varies from partially crystalline to amorphous.

22

2.2.5 Forms of Corrosion

The classification of the different types of corrosion presented in this section follows the usual

arrangement found in literature [47,48,50]. But, if the authors agree generally in the organization of the

most common processes, they usually disagree in the classification and nomenclature of some

specific types. For this reason, the listing presented here, that is mostly arbitrary in its order of

presentation, may not agree with some classifications.

The main forms of corrosion are the following:

• Uniform Corrosion

• Galvanic Corrosion

• Crevice Corrosion

• Pitting Corrosion

• Intergranular Corrosion

• Dealloying or Selective Leaching

• Erosion Corrosion

• Environmentally Induced Cracking

• Biological Corrosion

In the above listing of corrosion forms the topic of corrosion of polymers and natural materials is

absent. Because this discussion is centred on the subject of metallic corrosion, those aspects will not

be addressed.

In Uniform Corrosion, the dissolution provokes the loss of a regular layer of atoms throughout the

surface due to dissolution. It’s the form of corrosion that accounts for the greatest loss of mass but,

due to the homogeneous nature of the attack, it is possible to predict accurately the life time of the

components or parts exposed.

For this process, anodes and cathodes are considered to be randomly distributed along the surface

and continuously changing places in a stochastic manner. Because it requires a great metallurgical

and compositional uniformity, it is not the most usual form of attack; however, some heterogeneity is

allowed within the definition of uniform corrosion.

The most mentioned example of uniform attack is the corrosion of carbon steel in acid media.

Galvanic Corrosion may occur when two different metals or alloys are coupled together (in direct

contact or with electrical contact established between them) in the presence of a corrosive solution. In

result of different corrosion potentials, one of the metals (the most active) will corrode while the other

(with a nobler potential) will serve as a site for cathodic reactions (gas reductions processes or

metallic reduction and deposition), being effectively protected from corrosion.

To achieve a more efficient design of parts made from dissimilar metals or alloys, it is very useful to

consult a Galvanic Series (section 2.2.1). The metals chosen to work in direct contact must be as

23

close to each other as possible in the series, keeping in mind that each series is only valid for a

specific solution, as different solutions will alter the relative positions of metals and alloys.

Beside this macroscopic effect, the galvanic process can also occur at a microscopic level.

Heterogeneities, defects and phases with different potentials can force corrosion to occur at localized

and well defined sites.

This is a frequent form of corrosion between carbon steel and stainless steel in welded or riveted

joints.

Another form of localized corrosion is Crevice Corrosion. This type of attack is brought on by the

creation of a crevice or recess where a volume of solution remains in contact with the metal surface

but relatively shielded from contact with the bulk solution. A crevice may be created by the

accumulation of an inert material like mud, sand or an insoluble material, or may be part of a fastener

component (like the space between a screw and a bolt, rivet or washer).

When corrosion starts within a crevice, anodic and cathodic reactions occur normally. Since the

solution is stagnant, it is a matter of time until all the oxygen is consumed and the cathodic reaction

stops inside the crevice (differential aeration). Meanwhile, the anodic dissolution of metal continues

inside the crevice because outside the reduction reactions are still possible and capable of consuming

the electrons generated by the oxidation reaction. The accumulation of metallic ions causes an

electrical unbalance that forces the migration of aggressive ions, such as chlorides, into the crevice

forming metal chlorides. The metal chlorides hydrolyze to produce metal hydroxide and free acid,

decreasing the pH of the solution inside the crevice. The acidified solution causes the oxidation of

more metallic ions that causes the migration of more chlorides accelerating the rate of reaction, which

becomes autocatalytic.

There is a period of incubation before the beginning of a crevice, but once the process starts it

proceeds with an ever increasing pace.

There are two elements necessary to the occurrence of Pitting Corrosion: a passive state provided

by a surface oxide layer and the presence in the electrolyte of aggressive ions, namely, halides. Pitting

cannot take place bellow a critical potential (Epit), specific of the system material-solution. At potentials

above Epit, current density increases dramatically as the protective oxide breaks down at very localized

sites, the pits, that concentrate all the anodic activity while the rest of the surface remains cathodic.

The polarization curves for Pitting present a very distinctive profile as seen in Figure 7.

Chlorides increase the level of anodic current at all potentials, but their influence on the pitting

potential is clearly visible.

The mechanism by which a pit grows after it is initiated is generally accepted as being the same as

for Crevice Corrosion. It involves differential aeration, hydrolysis of metallic ions and localized

acidification.

The characteristics of the initiation step, however, are much less known and give birth to several

conflicting theories [48], that attribute the formation of pits to such different effects as: migration of

24

aggressive ions through the oxide film, chemical-mechanical rupture of the oxide film, formation of a

salt film or localized acidification.

As in the case of Crevice Corrosion, once a pit is initiated it becomes autocatalytic due to the

localized acidification.

Figure 7 – Theoretical polarization curve displaying pitting behaviour [50] (adapted).

The phenomenon of Intergranular Corrosion is associated with the segregation of impurities or

passivating alloying elements at the vicinity of grain boundaries. The depletion in those elements of

the grain boundaries or the adjacent regions makes them more susceptible to corrosion.

One of the most relevant examples of Intergranular Corrosion is the attack suffered by heat treated

stainless steels, where chromium has been removed from the grain boundaries and formed chromium

carbide.

These forms of localized corrosion (Crevice, Pitting and Intergranullar Corrosion) are more

dangerous than generalized corrosion, in the sense that they are more difficult to predict and to detect.

Even tough the loss of mass can be very small, pits and crevices can grow in depth and cause the

failure of a component much sooner than predicted.

Dealloying or Selective Leaching can occur in alloys when a more active alloying element dissolves

preferably in relation to a nobler element. It’s a common mechanism in the corrosion of brass where

the less noble elements like zinc, nickel, aluminium and silicon leach out leaving behind a sponge like

structure of pure copper with very poor mechanical properties.

In this discussion Erosion Corrosion includes three different phenomena that have in common the

existence of motion: Erosion Corrosion, Cavitation and Fretting.

In Erosion Corrosion, a high velocity fluid physically erodes and removes protective films and

corrosion products accelerating the corrosion process.

When high velocity fluids are combined with pressure reductions, the nucleation of water vapour

bubbles can be nucleated. These bubbles will implode at free surfaces with enormous pressure,

removing material and resulting in the process of Cavitation.

25

Fretting will occur in cases were to metallic surfaces are in contact, sustaining a load and moving

relative to each other in very small cyclic movements. The continuous destruction of surface oxides by

the attrition of the surfaces and the particle debris generated provokes a great erosion of the surfaces.

Environmental Induced Cracking (EIC) groups three different processes where brittle fracture may

occur instead of the normal process of ductile fracture. These processes are: Stress Corrosion

Cracking (SCC), Corrosion Fatigue Cracking (CFC) and Hydrogen Induced Cracking (HIC).

Both SCC and CFC are related with the existence of a passive film and an aggressive solution.

While SCC happens during tensile load, CFC happens during cyclic loading but in both cases the

corrosion currents are very small. The process of SCC is solution specific; this means that alloys that

suffer one of the processes when subjected to the presence of a determined solution will be immune

to that process when exposed to another aggressive electrolyte. CFC however, does not show the

same specificity and any solution capable of attack to an alloy will make it susceptible to CFC.

This is a very important type of corrosion, especially in the aeronautical industry, for several

reasons: the combined effect of environment and applied load will be greater than the sum of both

effects individually, leading to failure of components much sooner than predicted; the failure of the

components is catastrophic; and it is very hard to detect and predict.

HIC occurs as the result of infiltration of atomic hydrogen into the metallic matrix due to processing

(melting, welding) or electrochemical processes (corrosion, electroplating, cathodic protection). The

low solubility of hydrogen in the solid phase promotes the grouping of the atoms in the form of gas

molecules. The voids thus created inside the metal can serve as stress concentrators or they can

provoke themselves the localized rupture of the component due to high gas pressures inside the

voids.

The processes classified as Biological Corrosion are the result of the action of living beings: micro

organisms such as bacteria or macro organisms like algae or fungi. Even though the range of ambient

conditions (pH, temperature and pressure), type of organisms, metabolic reactions (aerobic or

anaerobic) and corrosive substances produced can vary, the mechanism is almost always the same

and involves the creation of very acidic conditions at the metallic interface.

2.3 Summary

The Chapter that now ends intends to provide a background in Corrosion Science and to introduce

the basic terminology to be used in the discussion about the corrosion behaviour of magnesium and

its alloys. This subject, which is the central core of this work, will be addressed in the next Chapter.

26

Chapter 3

The Electrochemistry of Magnesium and Magnesium Alloys

3.1 Introduction

In light of the great interest in the improvement of the properties of magnesium alloys, and

considering that corrosion resistance is their major weakness, it is without surprise that it is possible to

find very active research in this field. A search in the main scientific databases reveals an increasing

number of publications on the electrochemistry and corrosion behaviour of magnesium alloys.

Despite the abundance of publications, the corrosion mechanisms of magnesium and its alloys are

very complicated and much is still unknown, which justifies the ongoing effort of the scientific

community to increase the understanding of the processes involved.

Two of the most active researchers in this field are G. Song and A. Atrens, with a significant

number of publications that spans for more than a decade [54-60]. Among their publications, there is an

article that revises the principal concepts in this field [58], and that will serve as a guideline for this

discussion.

It should be noticed that the articles on the behaviour of magnesium and magnesium alloys

immersed in corrosive solutions are quite abundant in the published literature [54-81], but articles related

with the electrochemical behaviour of magnesium in passivating media [82,83] or the properties of the

passive film formed at the surface [84] (spontaneously or in alkaline solutions) are rarer.

3.2 Corrosion Mechanisms 3.2.1 Pure Magnesium

As for other metals that have negative corrosion potentials, the stability of magnesium depends on

the formation of a surface film able to inhibit the attack of the surface when it is exposed to a corrosive

environment. In the case of magnesium, the film formed shows poor protective performance and it is

very susceptible to breakdown.

The magnesium corrosion reaction follows the overall equation:

(20) 222 )(2 HOHMgOHMg +→+

This simplified equation can be broken down into the cathodic (Eq. 21) and anodic (Eq.22) partial

reactions:

(21) 222 HeH →+ −+

(22) −+ +→ eMgMg 222

The evolution of hydrogen is very important in the corrosion mechanism of magnesium. H2

molecules can be generated in the reduction reaction (Eq. 21) and by a chemical reaction between

magnesium and water:

(23) 22

2 22222 HOHMgOHMg ++→+ −++

27

The main corrosion product formed is magnesium hydroxide (Eq. 24). However, different

atmospheres and solutions will produce other types of corrosion products such as carbonates and

hydrated carbonates (in the presence of carbonic acid or CO2 dissolved in water) or sulphites and

sulphates (in the presence of diluted sulphuric acidic or sulphur containing contaminants) [49].

(24) 22 )(2 OHMgOHMg →+ −+

There are two interesting conclusions that can be drawn from the analysis of the above equations

and the hydrogen evolution: a) the corrosion of magnesium will not show a significant dependency to

oxygen concentration in the media [49,58,62]; b) in the presence of a small electrolyte volume there is a

significant alkalinization of the solution.

Magnesium has a very active standard potential of -2.37 V (SHE) [47,49,58], assuming direct contact

and equilibrium between the bare metallic surface and the divalent ion. However, the standard

potential is much nobler in 3% NaCl solutions -1.63 V (SCE), or -1.38 V (SHE) [47], which indicates that

the metallic surface is not in direct contact with the solution and is covered by an hydroxide layer. This

layer covers the surface conferring some corrosion protection in alkaline solutions.

The thermodynamics of aqueous solution are described by the Pourbaix diagram (Figure 8). The

diagram shows that the divalent ion is stable in solutions until a pH value of about 11. After this point

the hydroxide Mg(OH)2 becomes the stable species.

Figure 1 – Pourbaix Diagram of pure magnesium [53].

The corrosion processes in pure magnesium are normally associated with Localized Corrosion [58].

This special denomination intendeds to differentiate this process from other forms of localized

corrosion like pitting, because it evolves very differently.

The surface film that covers magnesium is metastable and only slightly protective. For this reason,

magnesium seems to have a pitting potential (Epit) more negative than the free corrosion potential

(Ecorr).

28

When the corrosion process begins, irregular and shallow pits are formed. These pits behave very

differently comparatively to those formed in other systems like stainless steels or aluminium and

instead of growing in depth, they spread laterally covering the entire surface, therefore not forming

deep pits. This behaviour is consequence of the alkalinization of the solution caused by the cathodic

reaction. The increase in pH stabilizes the magnesium hydroxide Mg(OH)2 and slows down the

dissolution process inside the pit. The attack then proceeds at the edges of the pits where the pH is

low enough for anodic dissolution to occur. This constitutes a self-limiting characteristic of the

corrosion process; this mechanism is very different form other pitting mechanisms that are self-

catalytic and generally accelerate the corrosion activity.

As the corrosion process spreads laterally, it provokes the undermining and formation of particles

that fall away from the surface. The process of second phase undermining is very common in

magnesium alloys.

3.2.2 The Negative Difference Effect

In the corrosion of magnesium there is a very controversial phenomenon that even defies basic

electrochemical theory. This phenomenon has been named the Negative Difference Effect (NDE).

For most metals when the potential of the surface increases (to more positive values) the rate of

anodic dissolution in acidic media increases. This is accompanied by the predictable reduction of the

hydrogen evolution rate (HER). In the case of magnesium, however, the HER increases as the

potential becomes more noble in a clear contradiction with the basic electrochemistry. This process

has been verified experimentally in anodic polarization experiments where the gas evolved is

recovered to estimate corrosion rates, according to the principle (Eq. 20) that the dissolution of one

atom of magnesium will provoke the release of one mol of hydrogen gas [54-59,68].

The NDE can be explained more clearly by the analysis of Figure 9.

Figure 2 – Schematic representation of the NDE in magnesium [58].

29

When a noble potential (Eappl) is applied, above Ecorr, the anodic partial reaction should follow the

line Ia and increase until the point IMg,e. Consequentially, the cathodic partial reaction should decrease

along the line Ic, which represents the normal electrochemical behaviour, until the point IH,e. The

experimental verification of the increase in HER implies that the cathodic partial reaction must follow a

line along IH until the point IH,m, corresponding to a cathodic current much higher than the predicted by

normal polarization behaviour.

A second experimental evidence of NDE is manifested by another aberrant behaviour of

magnesium corrosion: the anodic dissolution of magnesium, verified in experiments on lost mass

during dissolution [54-59,70,74,75,77,80], increases faster than expect according to the polarization curve Ia.

The weight loss has been found to exceed the predicted by Faraday’s Law (Eq. 10), and should follow

the curve IMg until the point IMg,m that corresponds to a higher anodic current than expected.

If the experimental verification of NDE is fairly simple, the explanation of the mechanisms by which

it occurs is much more difficult. Investigators have been trying to explain this phenomenon for

decades, and so far, four possible mechanisms have been proposed [54], but there is no agreement on

which is the correct one because there are controversial experimental evidences [54-56,71,83].

Mechanism I describes NDE in terms of the breakdown of the partially protective oxide/hydroxide

film that covers the magnesium surface. The higher the potential, the more it disrupts the film. This

mechanism is not satisfactory in explaining the corrosion of magnesium in acidic and neutral media,

where the film is not very stable.

Mechanism II attributes NDE to the undermining and falling away of second phase particles during

corrosion. If these phases are cathodic relative to the magnesium matrix, they will accelerate anodic

dissolution around them. When enough mass has been lost around them, the particles will fall away,

justifying the added weight loss.

Mechanism III explains the abnormal HER by the formation of the unstable monovalent

magnesium ion (Eq. 22) in an intermediate step. The monovalent ion reacts with hydrogen ions to

from H2 according to the equation:

(25) 22222 HMgHMg +→+ +++

Equation 25 provides a chemical reaction to be added to electrochemical hydrogen evolution.

Mechanism IV justifies NDE through the formation of magnesium hydride by electrochemical

reduction (Eq. 26), predicted by thermodynamical data. The hydride, which is not stable, reacts with

water to form hydrogen.

(26) 222 MgHeHMg →++ +

These four mechanisms, as all the above discussion about the NDE, also apply to magnesium

alloys, which show identical behaviour. The NDE is closely related with corrosion resistance: a strong

NDE is usually observed in alloys with low corrosion resistance [58].

It is also believed that if the surface films formed on magnesium could be improved the NDE would

be prevented, reducing the corrosion rate of magnesium.

30

3.2.3 Impurity Elements

The final issue that needs to be approached in the corrosion of pure magnesium is the influence of

contaminants. In terms of their influence on the corrosion behaviour of magnesium, different elements

have different effects: some improve the resistance while others are extremely pernicious. There are

also elements whose influence is insignificant or uncertain.

In general, there are four elements that must be controlled: Fe, Ni, Cu and Co. They have been

found to posses a very deleterious effect on the corrosion of magnesium when present at contents

superior to 0.2% wt [58]. This is known as the Tolerance Limit. The mechanism has been associated

with the segregation of these elements that due to their low solubility in the magnesium matrix, that

serve as active sites for corrosion.

None of the models put forward to describe this mechanism has been largely accepted, nor has a

relation been found between the values for the tolerance limits of the elements and their respective

solubility in liquid or solid magnesium. However, there are two main explanations for the accelerated

corrosion of magnesium in the presence of the elements, both related with microgalvanic effects:

- As the magnesium matrix corrodes, the impurity elements also dissolve into the solution. The

elements will later reprecipitate as metallic Fe, Ni, Cu and Co, at the surface, accelerating corrosion.

- The impurities at concentrations beyond their solubility limits precipitate as second phases that

serve as cathodic sites.

Regardless of the mechanism, it is certain that the NDE and HER will play significant roles in the

corrosion process.

3.2.4 Magnesium Alloys

Two factors are crucial for the understanding of the corrosion behaviour of magnesium alloys: film

resistance and hydrogen evolution.

The increase in corrosion resistance due to alloying elements has been associated with a greater

stability of the natural protective film formed on the surface. Elements usually associated with this

effect are: aluminium, zirconium, rare earth elements and yttrium. However the mechanisms that might

lead to this improvement are largely unknown and unreported in literature.

In section 3.2.2 it was mentioned that magnesium has a Pitting potential (Epit) more negative than

its free corrosion potential (Ecorr). This behaviour is also verified for many magnesium alloys, but not all

as there is a great dispersion in pitting behaviour reported in literature [54-58,62,63,65,66,69-71,73-75,77,78,80-83]

that seems to vary not only with the alloy tested, but also with test solution and testing parameters.

The alloys from the AZ series follow the behaviour of pure magnesium presenting a Epit lower than

Ecorr. However, the addition of 1 % wt of Nd stabilizes the alloy, moving the Epit to a value higher than

Ecorr [74] as depicted on Figure 10.

31

Figure 3 – Polarization curve of AZ91 magnesium alloy with increasing amounts of Nd [74].

In alloys the discussion becomes even more difficult because the different phases will present

dissimilar pitting behaviour, as reported by G. Song and A. Atrens [58] and depicted in Figure 11.

Figure 4 – Independent polarization curves of α-phase and β-phase in AZ91 magnesium alloy [58].

Despite the lack of a consistent model to explain the different Epit in magnesium alloys and the

possible relation with a more stable surface layer due to alloying elements one trend seems to emerge

from the referred literature: alloys containing Zr always present a Epit above Ecorr.

The HER is closely related with galvanic effects in the surface, and more specifically, with the

electrochemical behaviour of the α matrix in relation with second phase particles.

In the alloys based in the Mg-Al system (AM, AS and AE alloy series) it can be hard to distinguish

from the effect of the Al in solid solution (α phase) and the influence of the β phase (intermetallic

phase Mg17Al12). The increase in Al content leads to a significant decrease in corrosion rate, as

depicted in Figure 12.

However, for aluminium contents above 2 % wt an increasing amount of β phase is formed at the

grain boundaries. The β phase is more stable than the α matrix because it is nobler (has a higher

corrosion potential), passive over a wider pH range and, contrary to magnesium, shows a pitting

potential above the free corrosion potential. Table 6 presents the corrosion potentials of several

phases formed in magnesium alloys.

32

Figure 5 – Influence of Al content on the corrosion rate of magnesium alloys immersed in 5 % NaCl [58].

Although the β phase itself is very stable, it is very detrimental for the resistance of the magnesium

matrix since it creates a strong microgalvanic effect. Being a nobler phase, it becomes an active

cathode accelerating HER and the corrosion of the α phase.

Table 1 – Corrosion potentials for common magnesium second phases in 5% NaCl (pH 10.5) [58].

Metallic Phase Ecorr (VSCE)

Mg -1.65

Mg2Si -1.65

Al6Mn -1.52

Al4Mn -1.45

Al8Mn5 -1.25

Mg17Al12 (β) -1.20

Al8Mn5(Fe) -1.20

Beta-(Mn) -1.17

Al6Mn(Fe) -1.10

Al6(MnFe) -1.00

Al3Fe(Mn) -0.95

Al3Fe -0.74

As it can be concluded from the values depicted in Table 6, because second phases in magnesium

are normally nobler than the matrix, the microgalvanic effect is always a possibility. For this reason, no

matter how complicated the corrosion process of a specific alloy might be, the α matrix will always

corrode preferably over other constituents in a multiphase alloy. The noble second phases will be

undermined by the corrosion of the matrix and eventually be released from the surface.

Another relevant information obtained in Table 6 is that iron based phases are the most powerful

cathodes, which supports the discussion presented in section 3.2.3.

33

There are also metallurgical effects that affect the microgalvanic corrosion of magnesium alloys.

For example, it has been reported [58] that in the alloy AZ91D a high volume fraction of continuous β

phase surrounding fine grains of α phase retards corrosion. This happens because as the α phase

corrodes, the continuous β phase isn’t undermined and does not fall away from the surface. Once the

matrix is completely corroded at the surface, the corrosion of the β phase can be stopped by the

accumulation of corrosion products. This process is called β phase protection and it is represented

schematically in Figure 13. The opposite effect will occur when the morphology of the phases is one of

large α grains and the β phase is agglomerated in particles separated from each other. In this case,

the second phase will be undermined and the corrosion rate will not be affected by accumulation of

corrosion products. The second phase will actually accelerate the corrosion of the matrix.

Figure 6 – Schematic representation of β phase protection: (A) initial surface, (B) obstruction of corrosion by accumulation of corrosion products [58].

In conclusion, the metallurgical and thermal processing is fundamental to explain the microgalvanic

corrosion of magnesium alloys. This knowledge is critical when analyzing a specific system due to the

duality of second phases, which can either protect or enhance the corrosion of the surface, depending

on the phase morphology.

34

3.3 Aspects in the Corrosion of Magnesium Alloys 3.3.1 Environmental Corrosion

Exposing magnesium to an aqueous solution will provoke the immediate corrosion attack of the

surface; however, the corrosion rate will depend on the type, volume, flux and temperature of the

solution.

The corrosion rate in cold pure water is typically low due to the natural hydroxide layer, yet the

introduction of aggressive ions such as chlorides will cause an increase in the attack proportional to

the quantity added. In chloride containing media, such as sea water, the attack proceeds as pitting

corrosion. As mentioned in section 3.2.1 the corrosion mechanism of magnesium alters the pH of the

solution making it more alkaline. If the pH value reaches values above 10.5 [58] the Mg(OH)2 becomes

the stable surface film and the corrosion rate is negligible. This occurs, for example, in samples

suffering atmospheric corrosion where the amount of solution is typically small, or in the presence of

stagnant solutions.

In the case of flowing water, the corrosion rate of magnesium and its alloys is much higher, with

some dependence on velocity and direction of flow. This has been attributed to the mechanical

removal of the hydroxide layer and the impediment of the increase in local pH.

Neutral or alkaline solutions containing fluorides do not provoke a significant attack on magnesium.

In this media a layer of insoluble MgF2 is formed that protects the surface from corrosion. This film is

so stable that protects the metal in the presence of concentrated hydrofluoric acid. Fluorides are

known as good corrosion inhibitors for magnesium [58].

Dilute sulphuric acid provokes a strong dilution of magnesium; however, in concentrated solutions

a layer of magnesium sulphate is formed that protects the surface from corrosion.

In general all inorganic acids attack magnesium except the above mentioned hydrochloric acid and

also the chromic acid that induces a low corrosion rate.

Solutions containing sulphates, phosphates and nitrates are able to attack the surface of

magnesium, but not as much as chlorides. Magnesium also corrodes in water saturated with CO2.

Humidity has an important role in the atmospheric corrosion of magnesium. In damp conditions the

attack is mainly superficial but the corrosion rate increases with relative humidity. In the absence of

humidity “dry” gases present at the metal surface such as chlorine, iodine and bromine do not cause

significant attack; on the other hand, in the presence of water bellow the dew point they can cause

severe corrosion.

Magnesium is normally very resistant to corrosion in soils due to the presence of calcium that

inhibits the corrosion process. However, the corrosion rate will vary upon the type and environment of

the soil.

35

3.3.2 Galvanic Corrosion

Magnesium is a very active metal and its alloys, in particular, have the most negative Ecorr of all

structural materials. For this reason, when in contact with another metal or alloy, magnesium will

corrode preferentially. This process is also called Macrogalvanic Corrosion, in opposition to the

microgalvanic process.

Due to this effect, which has been used for more then a century in cathodic protection systems

where magnesium sacrificial anodes protect other alloys like steel from corrosion, any parts build from

a magnesium alloy cannot be in direct contact with another alloy in the presence of a corrosive

environment. A great care and consideration has to be put in the design of these types of elements to

prevent catastrophic element failures.

3.3.3 Environmental Induced Cracking of Magnesium

Because of its corrosion mechanism magnesium is susceptible to EIC, and especially to SCC and

HIC. Despite this susceptibility and the importance of EIC in the aerospace industry, one of the major

fields of application for magnesium alloys, very little work has been published on the EIC of

magnesium and its alloys, as a result of a low incidence of EIC in service parts. This tendency will

change as magnesium alloys are developed with capabilities to withstand greater mechanical loads.

The EIC process in magnesium is transgranular and magnesium alloys containing Al in a

concentration superior to 1.5 % wt are susceptible to SCC. This means that the alloys from the series

AZ, AM and AS will show this behaviour. Very little work has been published on the Zr containing

alloys and their vulnerability to this process is mostly unknown. Most of the published work has been

developed on wrought or rapid solidification alloys, with casting alloys being largely ignored in the

research.

Several mechanisms based on conflicting experimental results have been proposed to explain the

EIC process in magnesium alloys. They have been summarized [58] as follows:

• Crystallographic adsorption-decohesion at the crack tip.

• Strain-induced film rupture with hydrogen production; crack advance due to hydrogen

decohesion ahead of the crack tip.

• Brittle hydride forming ahead of the crack tip.

• Corrosion tunnels.

• Rupture of a brittle surface film.

Unfortunately, these mechanisms are somewhat out of date because of new advances made on

the understanding of EIC phenomena based on research on other metals and alloys.

A greater investigation is needed on the EIC process of magnesium alloys, not only to clarify the

mechanism that might cause it, but also to obtain a more complete scenario on the influences of

alloying elements, metallurgical processing and environmental aspects.

36

3.4 Corrosion Protection of Magnesium Alloys The previous discussion shows that the corrosion performance of magnesium alloys, unprotected

and exposed to corrosive environmental conditions, is not compliant with the requirements of most

applications. For this reason a great effort is placed in the research and development of protective

systems [85-108] to reduce the corrosion rate of magnesium alloy to acceptable levels. The simplest way

of protecting magnesium from corrosion is to physically separate the metal surface from the

environment by applying a coating or modifying the surface though physical or chemical processes.

Basically, all the protective systems developed for other structural metals and alloys have been

applied to magnesium, with more or less success. An extensive review article has been published by

J.E. Gray and B. Luan [109] that encompasses the fundamental aspects of these technologies and their

applications. The following list is a summary of the most important systems mentioned:

• Electrochemical Plating (electroplating [85] and electroless plating [86,87]).

• Conversion Coatings (chromate, phosphate-permanganate [88-90], fluorozirconate, stannate [91,92], rare earths [93-97]).

• Hydride Coatings [98,99].

• Anodizing (modified acid fluoride anodizing, Dow 17 process, Anomag process, Magoxid-

coat process or Plasma Electrolytic Anodization [100], Tagnite surface treatment, other

miscellaneous processes [89,101-103]).

• Gas Phase Deposition Coatings (thermal spray, chemical vapour deposition, physical

vapour deposition).

• Ion Implantation [98,104].

• Laser Cladding.

• Organic/Polymer Coatings (painting systems, powder coatings such as the E-coat system,

sol-gel process [105,106], silanes [107,108]).

Regardless of the specific system chosen to protect a magnesium alloy, the main objective is

always to obtain the highest performance to enhance the lifetime under service.

3.5 Summary Arriving at the end of this chapter, it is now possible to understand why magnesium alloys had, so

far, a limited application: not only is their corrosion behaviour generally inferior to other structural

materials, but also their electrochemical properties are so complex and mostly unknown that true and

significant improvements of its corrosion resistance are scarce and predominantly random.

This study aims at contributing to understand how alloying elements influence the properties of

surface films, and if their influence is related or not with the experimentally verified improvements of

corrosion resistance.

37

Chapter 4

Experimental Techniques

4.1 Electrochemical Techniques

4.1.1 Potentiodynamic Polarization

This technique offers a graphical representation, the Evans plot, of the changes in current density

as function of an applied potential. The electrochemical theory necessary for the comprehension of

this technique has been explained in section 2.2.3. Through it, is possible to study the behaviour of

electrode processes at anodic (more positive than Ecorr) and cathodic (more negative than Ecorr)

potentials.

There are two types of Potentiodynaminc Polarization (PP) tests that can be performed: a fast

polarization scan, where the polarization rates are about 60 V/h and a slow polarization scan, where

the polarization rates are around 1 V/h [110]. The high polarization rates are used to study situations

where surface films are thin or non-existing and high anodic activity is expected. The slow scan rate is

desirable to study processes with film formation, because it allows for film stability. The later method

was the one chosen method for this study.

Several parameters are obtained through the analysis of a polarization graph: Ecorr and icorr by

extrapolating straight lines in the Tafel region (the Tafel slope extrapolation method) as showed on

Figure 3; or the behaviour of the system in terms of kinetics of the anodic and cathodic processes as

well as passivation and passivation breakdown phenomena.

4.1.2 Electrochemical Impedance Spectroscopy

The Electrochemical Impedance Spectroscopy (EIS) is one of the most powerful techniques

available for the characterization of electrochemical systems as it obtains all the information possible

to obtain by electrical means. The spectroscopic nature of the technique allows for the study of

surface processes that occur in a system at the same time, but at different velocities. This is especially

suited for electrochemical systems that often depend on two or more electrode processes developing

simultaneously, offering kinetic and mechanistic information on those processes.

The concept of Impedance is based on Ohm’s law (Eq. 27) that correlates the resistance to the

passage of an electrical current in a circuit element:

(27) IVR =

where R is the electrical resistance, V the is the potential and I the electrical current in the circuit.

Since Ohm’s law is only applicable to ideal resistances, where the resistance obeys Eq. 27 for any

voltage value, the resistance is always independent of the frequency and remains in phase with an

applied AC voltage signal; the more generalized concept of Impedance (Z) was created:

(28) ( )( )tItVZ =

which is not affected by the same limitations as the electrical resistance.

38

In the EIS technique a small perturbation is applied to the system under the form of a potential

sinus wave signal:

(29) ( ) tsenVtV ω0=

where V0 is the signal amplitude and ω is angular frequency. The current response of the system is

then registered:

(30) ( ) ( )δω += tsenItI 0

where I0 is the amplitude of the current signal and δ is the difference in phase between the two

signals.

The total Impedance of the system will be given by the equation:

(31) ( )δωω+

==tsentsen

IV

IVZ

0

0

The two signals can be represented as sinus waves with the respective amplitudes V0 and I0, the

same frequency, but out of phase by an angle of δ; or they can be represented by two vectors rotating

at the same frequency with the phase shift δ.

This implies that the impedance of a system can be represented by a vector that corresponds to

the division of the two other vectors:

(32) IV

Z =

and a phase angle:

(33) ( ) ( ) δδωωθ −=+−= tt

The impedance vector can also be represented as a complex number with real and imaginary

components:

(34) imaginaryreal jZZZ +=

with the usual treatment for vector module and angle:

(35) 22imagreal ZZZ += ; (36)

real

imag

ZZ

=θtan

There are two usual ways of representing an impedance spectrum: the Nyquist plots and the Bode

plots.

A Nyquist plot is a Cartesian representation of the coordinates defined by Zreal and Zimag. These

diagrams contain information that can be assessed directly such as polarization resistance and some

kinetic data. In these plots it’s usual to invert the YY axis to facilitate the interpretation; since most

electrochemical system posses some type of capacitive behaviour the points will appear in the 4th

quadrant. The inversion of the axis transports these points to the 1st quadrant.

39

In complex electrochemical systems, where the impedance values and frequencies of the electrode

processes span several orders of magnitude, the correct interpretation of a Nyquist plot can become

difficult. For these cases the representation of the results in polar coordinates, the Bode plots, is much

more useful. The Bode plots represent log|Z| or phase angle (θ) as a function of logω, where the

angular frequency (ω) is the independent variable. In this representation, every electrode process will

appear as a maximum in the phase angle plot. The maximum in the phase angle is referred to the

absolute value, since phase angle values are normally negative and for that reason the YY axis is

represented inverted.

Other types of representations are used in more limited application such as: Zreal vs logω, Zimag vs

logω and Zreal vs ωZimag; or resorting to the property of Admittance (Y), the inverse of impedance

(Y=1/Z).

The response of an electrochemical system to the potential perturbation applied during an EIS

experiment is similar to the response of the passive components of an electric circuit: resistors,

capacitors and inductors.

For a pure resistance the Ohm’s law is directly applicable. The conditions stated at the beginning of

this section are verified and so the impedance of a resistor is directly proportional to the resistance:

(36) RZR =

In Figure 14 it is possible to see that a resistance is a single point in the Nyquist and two straight

lines, with constant values, in the |Z| and phase angle Bode plots, respectively.

Figure 1 – Schematic representation of the Nyquist and Bode plots for a resistor [111].

In the case of a capacitor the analysis must start with the equation for the parallel plate capacitor:

(37) CVq =

After some mathematical manipulation it is possible to obtain the expression for the impedance of a

capacitor:

(38) CCj

ZC ωω11

−==

40

The capacitor has only the imaginary component and the impedance is inversely proportional to the

frequency and the capacitance (C). In the Nyquist diagram a capacitor is a straight line converging to

the origin and in the Bode plot of log|Z| a line with a slope of -1. It is also evident from Figure 15 that a

capacitor reacts to the perturbation with a phase delay of -90º.

Figure 2 – Schematic representation of the Nyquist and Bode plots for a capacitor [111].

In the case of an inductor, the impedance is directly proportional to the frequency of the

perturbation and the inductance (L), having only an imaginary component:

(39) LjZL ω=

The Nyquist and Bode plots are the inverse of the capacitor plots, as the impedance increases with

frequency, the slope of the log|Z| plot is 1 and the phase angle is +90º.

These might be simple examples, but the same concepts can be extended to the analysis of

complex electrochemical systems simply by associating several resistors and capacitors in series.

Figure 16 gives an example of a system commonly found in corrosion science. The plots represent the

impedance response of an R(RC), a resistor linked in series with another resistor and a capacitor

connected in parallel.

Figure 3 – Nyquist and Bode plots for a R(RC) circuit [111].

41

The impedance of the system is given by Equation (40):

(40) CRj

RRZω+

+=10

The data collected in EIS measurements may be difficult to analyse, and in particular, experimental

errors can cause the appearance of “artefacts” in the plots not representative of the electrochemical

behaviour of the system. The validity of EIS experiments is subject to four [111] conditions:

• Causality – the response of the system must be only the result of the imposed perturbation.

• Linearity – the relation between perturbation and system response must be able to be

described by linear differential equations, in practice the impedance must be independent

from the perturbation amplitude. This means that EIS experiments must be conducted at

potentials close to OCP where the response is linear, typically the perturbation shall be

around ±10 to ±20 mV.

• Stability – the system must be stable, returning to its original state upon ceasing of the

imposed perturbation.

• Impedance must have finite values when ω→0 and ω→∞, it must be a continuous function

and have finite values for all intermediate frequencies.

A failure in the verification of any one of these conditions may invalidate the results leading to an

erroneous interpretation of the processes and mechanisms at the interface.

In terms of interpretation of the results there are two approaches: the creation of an “equivalent

circuit” of passive components to modulate the electrochemical behaviour of the system or the

development of mathematical models based on the kinetics of the involved reactions. In this work, the

approach chosen for the data analysis will be the based on equivalent circuits.

An example of this procedure can be illustrated for the impedance plots depicted in Figure 16. The

shape of the different curves is, as stated earlier, very common in corrosion. The R(RC) circuit

depicted in Figure 17 can model, for instance, a corrosion process controlled by charge transfer on a

metallic surface.

Figure 4 – Equivalent circuit for the interpretation of the impedance plots of Figure 16.

In this case RΩ will represent the Ohmic resistance of the system (also represented by R0 or Rs)

comprised of three components: the electrolyte resistance, the resistance of the electrical wires and

the intrinsic resistance of the electrodes.

42

The two last components are normally ignored because the wires and the electrodes, usually

metallic, have resistances several orders of magnitude inferior to an electrolyte.

The diameter of the semi circle R will be the charge transfer resistance (Rtc), the Faradic

component of the current, which corresponds to the resistance associated with the exchange of

electrons (see Chapter 2).

There is another parameter that can be calculated directly from the plots: the capacitance created

by the charge separation of the electrochemical double layer (also explained in Chapter 2). The

double layer capacitance (Cdl) is the non-Faradic or discontinuous current in the system and can be

calculated by the expression:

(41) tc

dl RC *

1ω

=

where ω* is the angular frequency at the point of maximum YY coordinate, in absolute value, in the

Nyquist diagram.

This analysis procedure for the impedance data is very simple, but it is obviously only possible for

very simple systems. In more complicated systems it is common to have several processes spanning

and overlapping in the frequency range of study. For these cases, several softwares for fitting

procedures exist, using different algorithms, which can deconvolute the different overlapping curves

and calculate the parameters automatically.

The major challenge in the fitting of an electrochemical process with an electrical equivalent circuit

is to choose a circuit that accurately represents the system. This is a process that requires experience

and some knowledge on the electrochemical reactions that should be occurring at the electrode

surface.

One of the characteristics of an impedance spectrum that makes the interpretation of the data even

more difficult is the deviation from ideal behaviour that is always present. In fact, in an impedance

experiment the semi circle obtained in the Nyquist is never perfect, like in Figure 16, and generally

appears depressed, as depicted in Figure 18. The level of depression varies from system to system.

Figure 5 – Nyquist plot with and without depression [111].

43

This depression is called the Cole-Cole depression, after the authors that first addressed this issue

in their study of dielectric constants. The concept was later on adapted to electrochemical systems by

several different authors [111].

If we go back to the impedance plots in Figure 16, there is only one electrochemical process

modelled by the equivalent circuit depicted on Figure 17. For this process it is possible to define a time

constant (τ):

(42) dltcCR=τ

The circle depression is explained by considering that the time constant of the process is not in fact

a constant, but rather a distribution of time constants centred on an average value (τ0). This dispersion

of time constants has been related with the heterogeneity of the surface by considering it as a

collection of very small adjacent electrodes, each with its own Rtc and Cdl and consequently its own

time constant [111].

An alternative approach to the problem of the depression in the EIS results is the Constant Phase

Element (CPE). A CPE is an element very similar to a capacitor, in the sense that it as a constant

phase angle, but whose phase can be different from -90º.

The two approaches give similar mathematical answers to the problem of depression, as it can be

seen in the resolution for the impedance of an R(RC) circuit presented in Eq. 43 (considering the Cole-

Cole depression) and Eq. 44 (substituting the capacitor by a CPE).

(43) ( ) αωτ −Ω +

+= 101 j

RRZ ; (44) ( )njRYRRZ

ω01++= Ω

where α varies between 0 and 1 and n can assume any value in the interval of -1 to 1.

In this work the CPE will be used during the fitting of the experimental results; however, the CPE

must be used with some care and attention to the results.

Even though a CPE can be used to substitute a capacitor, it does not represent true capacitance;

instead, the property represented is admittance.

More importantly, because the phase of a CPE can be changed, the element is so versatile that it

can simulate the behaviour of any other passive component in an electric circuit. If the power n is

equal to 1, then the CPE will give the same response as a capacitor; for n=0 the CPE will simulate the

behaviour of a resistor and for and inductor n as the value of -1. A CPE can also simulate diffusion

controlled processes normally fitted with elements called Warburg diffusion elements, when n=0.5. In

fact, it is possible to build an equivalent circuit comprised only of CPEs that will return a perfect

mathematical result for any electrochemical process, even if lacking in any physical meaning.

Finally, it should be noted that even though the CPE seems to lack the physical meaning of the

Cole-Cole depression, the fact that both have the same mathematical result, the versatility of the CPE

and the inclusion of the CPE in most fitting algorithms, makes the CPE an useful tool for impedance

analysis, if used with caution.

44

4.2 Microscopic and Surface Characterization Techniques

4.2.1 Optical Microscopy

Optical Microscopy (OM) is often used in materials characterization for metallurgical analysis as a

complement to more advanced techniques. OM is a very limited technique: it cannot distinguish

characteristics with sub micrometric size and the accurate chemical and structural identification of the

phases observed is also not possible.

Despite these limitations, OM is a useful technique because it offers a general overview of the

metallurgical characteristics of a metal or alloy, from which it is possible to get some information, for

example, about the thermal and mechanical processing of the sample.

To make an observation it is first necessary to polish the sample until it becomes totally reflective,

as a mirror. The smooth surface is then chemically attacked by an etching reagent. The choice of the

right etching agent will permit the visualization of the phases with the highest chemical reactivity to

that specific reagent, which will grow darker, in contrast to the more immune that will remain brighter.

Consulting the specialized literature [112] it is possible to find etching reagents capable of revealing

phases in basically every metallic system.

4.2.2 Scanning Electron Microscopy

The Scanning Electron Microscopy (SEM) is probably the most used technique by engineers and

researchers today. The need to observe and to analyse materials at the micrometer and sub

micrometer scale has been met by the evolution of this technique, which is capable of revealing local

characteristics such as surface topography, chemical composition and crystallography in

heterogeneous organic and inorganic samples.

SEM images are created by focusing an electron beam of primary electrons (excitation beam) on

the surface and collecting the emitted electrons with appropriate detectors, in high vacuum or ultra

high vacuum environment. The beam provokes the emission of secondary electrons, backscattered

electrons, Auger electrons, characteristic X-rays and phonons of various energies.

The primary electrons are generated in a cathode, normally tungsten filament, by thermionic

emission at high temperatures; the high temperature of the cathode as several disadvantages

including low brightness, evaporation of the filament material and thermal drift during operation. A

great improvement in cathode design was the field emission electron gun (FEG); comprised of a

single-crystal of tungsten with a very sharp edge (100 nm or less), the potential barrier to the passage

of electrons becomes so much smaller that a FEG can produce a current density five orders of

magnitude higher than a conventional tungsten hairpin filament [113].

The electron beam generated needs to be accelerated and focused on the surface; these tasks are

performed by electromagnetic lens and an anode. The final spot size of the electron beam can vary

from 1 µm down to 1 nm [113].

The SEM images are created using the secondary electrons or the backscattered electrons. The

secondary electrons are released from the surface as a result of inelastic interaction between the

45

primary electrons and the atoms of the material while the backscattered electrons result from elastic

interaction in which there is practically no loss in energy. Secondary electrons create higher resolution

images; this occurs because they have lower energy and a lower escape probability being generated

primarily around the electron beam, although secondary electrons are also generated by the

interaction between backscattered electrons and the sample. Secondary electrons have a maximum

escape path of about 10 nm while the backscattered electrons can escape the surface form a depth

up to 450 nm [113].

The maximum spatial resolution of images in SEM commercial devices is around 2 to 5 nm, but

resolutions higher than 1 nm have been achieved in advanced research instruments [113].

The ionization of the electronic inner-shell in an atom, in inelastic scattering, can lead to the

production of characteristic X-rays, specific of the element, which can be used for compositional

analysis in a process called Energy Dispersive X-Ray Spectroscopy (EDS). The X-ray radiation is

released by the relaxation of an electron to a lower energy state, left vacant by the ejection of another

electron caused by the electron beam. The nature of the effect means that only elements with an

atomic number of four (beryllium) or greater can be detected.

In terms of quantitative analysis, the detection limit of the EDS technique is around 0.01 %wt or

100 ppm [113]. The depth of the EDS analysis is harder to define since it is closely related with the

escape distance of the radiation. The escape distance is severely on the density of the sample that

increases the probability of readorpstion. Although the term mass depth (the product of density and

depth) is more broadly used, it is safe to say that the X-rays detected in EDS can be produced at

distances of some microns inside the sample [113].

4.2.3 X-Ray Photoelectron Spectroscopy

The X-ray Photoelectron Spectroscopy (XPS) is a well known and documented technique with a

widespread application in the surface analysis of organic (including biological) and inorganic materials.

It is a technique well suited for the study of a great variety of samples such as: fibres, films, powders

and particles, as well as bulk materials.

The basic principle of the XPS technique is the photoelectric effect: when a sample is irradiated

with X-ray radiation, the atoms at the surface will release electrons, known as the photoelectrons, in a

photoionization process. The energy of the electrons emitted is characteristic of the element and the

electronic level from which it was released, allowing for a qualitative elemental identification simply by

the measurement of the kinetic energy. The relation between the kinetic energy of the emitted

electrons (Ek), the energy of the incident radiation (hν) and the bonding energy of the electron (Eb) was

derived by Einstein:

(44) bk EhE −= ν

Only electrons with enough energy to escape the surface will contribute to the photoelectric effect,

the others will be reabsorbed by inelastic scattering. Normally, the escape depth for the

photoelectrons is between 2 and 10 atomic [114] layers (below 5 nm) making this a true surface

analysis technique.

46

As for other surface analysis techniques, an ultra high vacuum environment is mandatory.

The X-ray photons are emitted by anodes that are usually made of magnesium or aluminium.

These materials are among the few that comply with the necessary requirements: radiation with

enough energy to excite core electrons for every element, quasi-monochromatic radiation with a

narrow peak and without satellite peaks and good material suitability in terms of conductivity and

melting point. The use of a monochromated source may improve the quality of the spectra by

removing possible satellite and ghost peaks.

The emitted photoelectrons are analysed in the electron energy analyser that is the heart of an

XPS system. The identification of the elements present is done by peak assignment, and it is normally

a simple procedure in which the peaks in the experimental spectra are compared with a peak library.

Quantitative analysis in XPS is also possible, but the procedure may be somewhat complex due to

the need of high sensitivity factors and fitting procedures.

The poor spatial or lateral resolution of several millimetres was, until recently, one of the major

shortcomings of XPS. However, recent advances in instrumentation have allowed for some

constructors to offer values of spatial resolution as low as 100 µm or even 20 µm [114].

4.2.4 Atomic Force Microscopy

Of all the techniques employed in this study the Atomic Force Microscopy (AFM) is, undoubtedly,

the most recent. The AFM is one of several techniques that belong to the group of methods known as

Scanning Probe Microscopes (SPM), in which information is drawn from the interaction between a

very small moving tip or electrode and the sample surface. Being an SPM technique, its origins can be

traced back to the birth of the Scanning Tunnelling Microscope (STM); a technique that can form

images of a surface with atomic resolution by measuring the current exchanged between the sample

and a very small conductive tip, scanning the surface at the distance of a few atomic diameters.

The STM, with all its different operating modes [115], is an incredibly powerful and successful

technique, but is has an important limitation: it can only be used to investigate conductive materials or

samples coated with conductive layers. This shortcoming was surpassed when the idea of using

atomic forces, instead of electrical current, to characterize the surface was put forward by Binnig and

co-workers [115]. They presented the first working AFM in 1986 [115,116]. The AFM uses several different

types of forces to characterize the surface: Van der Waals, electrical, magnetic, short range, capillary

forces and other type of electromagnetic interactions. This means that AFM is very versatile being

capable of much more than just topographical characterization. It can also characterize mechanical,

electrical and electromechanical, magnetic and even chemical properties by employing a diverse

range of tips with different electrical and magnetic properties or even chemically functionalized tips.

For this reason the technique has established itself as an important characterization tool, idea

reinforced by the fact that publications referring AFM related techniques have achieved approximately

half the number of publications referring to SEM [116].

47

The AFM has a basic setup which is common to all the different operating modes, it comprises:

• A tip with a very small radius that interacts with the surface.

• A cantilever with a very low spring constant to which the tip is attached. The forces

produced between the tip and the surface are measured by measuring the deflection of the

cantilever.

• A three dimensional scanning capability. There are two types of scanners: either the

scanner is mounted on the cantilever, which moves in relation with an immobile sample, or

the scanner is attached to the sample that is mobile while the cantilever is fixed.

• A feedback system that interprets cantilever deflection to obtain the topographical data and

operate the system.

• Software that can interpret the data and reconstruct the images.

Figure 19 presents a schematic representation of a current AFM device.

Figure 6 – Schematic representation of an AFM apparatus [116].

The tips used in AFM are normally made of silicon or silicon nitride. The silicon tips have a radius

of about 10 nm while the radius of silicon nitride varies between 10 and 20 nm [116].

The cantilevers are also made of silicon (spring constants between 0.1 N/m to 40 N/m) and silicon

nitride (spring constants from 0.01 N/m to 0.58 N/m more suitable for contact mode) [116].

The cantilever deflection is measured by one of three techniques: the laser reflection method, the

interferometric method and piezoresistive method. In the laser reflection method, normally used in

most commercial devices, a laser is reflected from the back of the cantilever, coated with a reflective

metal, towards a photodetector. The detector is dived into quadrants (see Figure 19) and the current

measured from the top (1+2) minus the current from the bottom half (3+4) will be proportional to the

vertical movement of the tip. The interferometric method uses light interference between a beam

reflected on a fiber air interface and the beam reflected of the cantilever. The minute difference in

signal is still sufficient to trace cantilever defection. The piezoresistive method uses cantilevers coated

with piezoresistive materials. The deflection is proportional to the change in electric resistivity

produced by the deformation of the material.

48

To obtain a topographical image the tip must follow a fixed sample or vice-versa. The three

dimensional scanning capability is achieved but employing piezolelectric materials. The piezoscan is

controlled by the application of voltage that provokes the extension or contraction of a piezoelectric

material in the three space dimensions. It is a system that allows for an incredibly precise positioning

of the tip and/or sample.

The feedback system regulates tip-sample distance. The system interprets signal alterations

generated as the tip interacts with the sample. The perturbation of cantilever creates a signal that is

subtracted from the original setpoint. This creates an error signal, which is the input for the feedback

calculation. The task of the feedback system is to maintain the error signal equal to zero; to do that it

commands the piezoscanner to change its position until the original setpoint is achieved. The

feedback calculation, or in practical terms, the alterations in height provoked by the extension or

contraction of the piezoelectric material are the signals interpreted as topography or height. In contact

mode the signal that serves for comparison is the cantilever deflection, while in tapping the error signal

is generated by the alterations in the vibration amplitude of the tip.

In the previous paragraph were mentioned two of the three operation modes of AFM: contact

mode, non-contact mode and tapping. All these operation modes have several different techniques

with specific research applications, but all are suitable for topographical analysis.

Contact mode measurements are performed in the region of strong atomic repulsion forces, with

permanent contact between the tip and the sample. There are several different contact mode

techniques [115] with advantages and disadvantages for specific measurements: constant height mode,

constant force mode and lateral force imaging among others. With diamond tips contact mode is a

very useful tool for tribological, wear and abrasion experiments.

In the non-contact mode the scans are conducted in the region of weak attraction forces, which in

the absence of magnetic and other type of electrostatic interaction will be the sole result of Van der

Waals forces. The tip is excited by a piezoelectric at a frequency close to its resonant frequency; the

alterations to vibration amplitude due to the interaction with the weak forces, at a constant driving

frequency, are registered as topographical aspects. Due to the lower forces involved, it is the ideal

process for the analysis of soft samples like polymer surfaces.

In tapping mode the tip is made to vibrate in the same manner that in non-contact mode but the

distance between the tip and the substrate is lowered. In this case the tip makes intermittent contact

with the sample and the forces that it suffers vary from the low attractive Van der Waals interactions to

the high repulsive forces. Like in the non-contact mode, as the vibration amplitude is altered due to the

sample-tip interaction, the feedback system changes the distance until the amplitude is reset,

recording the alterations as topography. The tapping mode offers additional information by recording

the deviations in the vibration frequency of the tip. Those deviations are directly related with the elastic

properties of the material and the phase contrast images derived can map heterogeneities in the

surface.

49

Chapter 5

Experimental Procedure

5.1 Samples

The samples used in this work were obtained from the Magnesium Elektron Company. The three

alloys selected are wrought alloys belonging to the zirconium containing group. Wrought alloys were

chosen over casting alloys to avoid the heterogeneities in chemical composition usually associated

with casting alloys.

Table 8 summarizes the chemical composition of the magnesium alloys used.

Table 1 – Chemical composition of the tested magnesium alloys [3,7].

Nominal Composition (wt %) ASTM designation

British designation Zn Zr RE (a) Y RE (b) Nd

ZK31 ZW3 3.0 0.6 - - - -

EZ33 ZRE1 2.0 – 3.0 0.4 – 1.0 2.5 – 4.0 - - -

WE54 - - 0.4 (c) - 4.8 – 5.5 1.0 – 2.0 1.5 – 2.0 (a) Rare earth mishmetal containing La, Ce and Nd. (b) Heavy rare earths containing fractions of Yb, Er, Dy and Gd. (c) Minimum content.

The test coupons were obtained from extruded cylinders. The ZK31 alloy did not suffer any heat

treatment upon extrusion while the EZ33 and WE54 were aged by the T5 (10 to 16 hours at 170 – 200

ºC and air cooled) and T6 (8 hours at 525 ºC, hot water or polymer quench or air cooled, aging for 16

hours at 250 ºC and air cooled) heat treatments [3], respectively.

5.2 Test Solutions

The passivation studies and the properties of the passive films formed at the surface of the

samples were performed by immersing the alloys in NaOH solutions with a pH of 13. Two different

solutions were prepared by adding NaOH to water until the desired pH was achieved: one without

chlorides and a second with 0.05 M of NaCl.

The corrosion studies on these alloys were carried out in two different solutions: a sodium borate

buffer solution with a pH of 9 and a 0.5 M NaCl solution.

All solutions were prepared with Millipore® water and p.a. reagents.

50

5.3 Electrochemical Tests

For the electrochemical tests, coupons were cut from the extruded cylinders and electrical contact

was established by gluing a copper wire to them with silver based glue. The samples were them

mounted on cold curing resin. This sequence is depicted in Figure 20.

Figure 1 – Sample preparation sequence.

The tests were carried out in a three electrode electrochemical cell, in which the sample constitutes

the work electrode (WE). The reference electrode (RE) was the Saturated Calomel Electrode (SCE)

and the counter or auxiliary electrode (CE) was a platinum coil. In the NaCl solutions all the electrodes

were immersed in the test cell, but in the NaOH and borate solutions a sodium nitrate saline bridge

was used to prevent the contamination of the test solutions with chlorides. Figure 21 depicts the

testing apparatus.

Figure 2 – Three electrode experimental setup.

In order to obtain comparable surfaces and decrease surface area effects, the samples were

polished with SiC paper up to 4000 grit. After the polishment the samples were cleaned in acetone for

5 minutes with ultrasonic agitation and dried with a stream of air.

To prevent the onset o crevice corrosion and the formation of partial aeration cells the test surface

was delimited with epoxy glue (in the case of hydroxide solutions) or bee wax.

51

The Potentiodynamic Polarization tests were performed in a VoltaLab PGZ 100 potentiostat.

Anodic and cathodic polarization curves were ran separately after 10 minutes of immersion starting at

-20 mV (for anodic polarization) or +20 mV (in the case of cathodic polarization) in relation to the Open

Circuit Potential (OCP). Due to the 10 minute stabilization period, a stable OCP was achieved with a

variation that was always inferior to 5 mV/s. The scanning rate was 1 mV/s.

The Electrochemical Impedance Spectroscopy measurements were carried out in a Solartron SI

1286 Electrochemical Interface with a 1250 Frequency Response Analyser. The scans were

performed at different immersion times. The scanning frequency ranged from 60 kHz to 5 mHz with

7.13 points per decade. The amplitude of the applied signal was 10 mV RMS.

The results were analysed with the ZView2 Software in which the fitting parameters were obtained.

All the electrochemical tests were repeated until three concurring replicates were obtained.

5.4 Analytical and Surface Characterization

For these tests the surface of the samples was prepared according to the same basic polishing

sequence as the sample for the electrochemical tests. However, because the AFM requires a very

smooth surface, after the SiC polishing the samples were also polished in 5000 alumina paste.

The XPS, SEM and AFM analysis were performed after a period of immersion in the test solution of

24 hours.

The XPS analysis was performed in a Microlab 310F (Thermo Electron former VG Scientific)

apparatus. The spectra were obtained in Constant Analyzer Energy or CAE mode (20 eV) with a non

monochromated aluminium anode and an acceleration voltage of 15 kV.

The device used for the acquisition of SEM-FEG images was a JEOL JSM-7001F with an Oxford

INCA EDS unit for chemical composition analysis. Images and chemical composition spectra were

acquired at different magnifications and points on the samples.

The AFM analysis was carried out in a Veeco-di CPII equipment. Unfiltered images of topography,

vibration amplitude, phase contrast and error signal were obtained in tapping mode, with silicon

probes at room temperature. The analysis comprised scans at different magnifications and positions

on the sample surface.

52

5.5 Metallographic Characterization

After the polishing procedure (the same as in section 5.4), the samples were etched and observed

under an optical microscope. The contrasting agents and procedure were selected for each case,

upon consulting the specialized literature [112], to reveal specific alloy characteristics. Table 7

summarizes the relevant information.

Table 2 – Summary of etching reagents used, their characteristics and applications.

Etching reagent Characteristics and use Alloys

Nital: 5 ml HNO3 (conc), 100 ml ethanol (95%).

Shows general structure. ZK31, EZ33, WE54

Acetic glycol: 20 ml acetic, 1 ml HNO3 (conc.), 60 ml ethylene glycol, 20 ml water.

Shows general structure and grain boundaries in heat treated castings. Reveals grain boundaries in Mg-RE and Mg-Th alloys.

EZ33, WE54

2 ml HF (48%), 2 ml HNO3 (conc.), 96 ml water.

Reveals grain structure and coring in Mg-Zn-Zr alloys.

ZK31

53

Chapter 6

Results and Discussion

6.1 Metallographic Characterization

6.1.1 ZK31 Alloy

Figure 22 depicts the optical micrography images obtained for the ZK31 magnesium alloy after

etching in Nital and HF solutions.

Figure 1 – Micrographs of ZK31 alloy after 20 s etching in the Nital (left) and HF (right) solutions.

The images show severe directional deformation, confirming that the alloy did not suffer any kind of

thermal treatment upon extrusion, as indicated in the supplier information.

The Nital solution, as indicated in Table 7, is useful to reveal the general structure in magnesium

alloy such as grain boundaries and second phases, which are clearly visible but hard to delimitate

because of the high amount of deformation. The HF solution is capable of revealing coring of Zn and

Zr elements in magnesium alloys; consequentially, the darker areas should correspond to Mg-Zn and

Mg-Zn-Zr intermetallic compounds.

In this case, and due to the absence of a thermal treatment the main intermetallic formed should be

the eutectic MgZn2 commonly reported in literature [7,23,26,32,33,35,38]. Nevertheless, the precipitation

sequences in magnesium alloys are very complex and not fully characterized. Since a detailed

structural analysis of this precise alloy and processing condition was not found in the literature, the

presence of variable amounts of other type of intermetallics such as: MgZn [26,32,33,35,38], Mg2Zn3 [7,26,32,33,35,38], Mg4Zn7 [32,33,35,38], and Zn-Zr intermetallics [23] normally associated with ageing treatments,

cannot be discarded without a complete crystallographic characterization.

The heterogeneity presented by the alloy may increase the dispersion of experimental results. To

minimize the possible impact of this metallurgical factor, the area exposed to the tests was always in

the direction illustrated in Figure 22, parallel to the direction of extrusion. As result of this procedure,

the consistency and reproducibility of the results obtained for ZK31 was comparable to the other

alloys.

54

6.1.2 EZ33 Alloy

The most representative results of alloy EZ33 are presented in Figure 23.

Figure 2 - Micrographs of EZ33 alloy after 20 s etching in the Nital solution (left) and 10 s etching in the

Acetic Glycol solution (right).

The first noticeable feature of this alloy is the large dimensions of the grain boundaries. Their

dimensions that seem to be in the order of a few microns are visible even prior to etching. The grain

size was calculated by measuring their dimensions against the intersections with a grid composed of

30 lines (15 horizontal and 15 vertical). The value obtained for the average grain size was 20.3 µm

with a standard deviation of 8.7 µm.

The structure presented by the alloy is consistent with the thermal treatment (T5 ageing) reported.

The abnormal size of the grain boundaries occurs due to the precipitation of the rare earth elements.

These grain boundaries are responsible for the increased tensile strength and creep resistance of

these alloys [7]. The intermetallic phase forming at the grain boundary is believed to be the Mg3RE

phase [7,11,36,40]. Other precipitates have also been suggested as possible intermediate metastable or

stable phases in these systems: Mg27RE2 [7], Mg12RE [19,31].

Despite not being reported in literature [112], the etching reagents were also capable of revealing a

coring effect in the EZ33 alloy. It is possible to assume that if the RE elements diffused into the grain

boundaries during ageing, decreasing the amount present in solid solution in the magnesium matrix,

the remaining alloying elements (Zn and Zr) should be found in higher concentrations at the inner part

of the grain, possibly causing the coring effect.

55

6.1.3 WE54 Alloy

The WE54 alloy revealed itself to be much less reactive to the etching reagents (Figure 24) than

the other alloys, including when the recommended immersion times [112] were exceeded.

Figure 3 - Micrographs of WE54 alloy after 50 s etching in the Nital solution (left) and 20 s etching in the

Acetic Glycol solution (right).

The grain size was evaluated by the same procedure applied to the EZ33 alloy: the average grain

size measured was 10.8 µm with a standard deviation of 4.9 µm.

As reported earlier, the WE series represents the current pinnacle in alloy evolution in several

characteristics, including mechanical properties. For this reason, the crystallographic structure of

these alloys, especially the WE43 and WE54, has been studied profusely, with a great focus on the

precipitation mechanisms responsible for the mechanical resistance they present. Despite this

interest, the complex precipitation mechanisms that contain several intermediate steps result in a

certain degree of uncertainty in the crystallographic description of these alloys.

In general the superior tensile strength and creep resistance of these alloys is attributed to the

formation of intermetallic compounds. Several different types of intermetallics and metastable phases

have been identified in literature: the Mg3RE [11,12], Mg12NdY [12,21], Mg14Nd2Y isomorphous to Mg5Gd [12,14,21,28], Mg24Y5 [14,15,20,21,25,28] and Mg12RE [28].

Several controversial conclusions reported in literature are supported on experimental evidences.

The sequence of precipitation during thermal treatment and in particular ageing, the exact structure of

the precipitates formed and even if the principal factor in the improvement of the mechanical

properties is precipitation hardening or solid solution hardening, all constitute characteristics still under

debate.

It cannot be concluded from this analysis if the black phases visible on Figure 24 are compatible

with one or more of the phases described in the literature.

56

6.2 Passivation Study

In order to investigate the properties of the passive films formed on the surface, the samples were

exposed to alkaline solutions with pH 13. This high pH value was chosen upon consulting the literature [53] to create conditions in which the magnesium matrix would be in a passive state due to the

formation of an oxide/hydroxide surface layer. At this pH all the alloying elements of the three alloys

are also in the passive state, except Zn and Zr.

The stabilized passive layer obtained at this pH level permits a better characterization of the

involved processes.

6.2.1 Potentiodynamic Polarization Results

From the polarization curves presented in Figure 25, it can be concluded that the RE elements and

Y have some influence in the electrochemical response of the film formed at the surface. Although all

the samples presented a well defined passive behaviour extending for more than 2 V, the ZK31 alloy,

that does not contain RE, showed a higher ipass.

The WE54 sample presented the lowest ipass but not the lowest icorr; however, the differences in icorr

between the alloys were very small.

All the alloys showed a transpassivation region at approximately the same potential, but WE54

revealed a different behaviour by showing a two step transpassivation rather than a direct process.

The cathodic branches of the polarization curves did not reveal any relevant features. The main

data extracted from the polarization curves is summarized in Table 9.

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03i (Acm-2)

E (V

SCE)

ZK31EZ33WE54

Figure 4 – Polarization curves obtained for magnesium alloys immersed for 10 minutes in NaOH – pH 13.

One of the most relevant characteristics of the polarization curves is the fact that the WE54 alloy

presented an Ecorr more negative than the ZK31 alloy. This fact seems to challenge not only basic

electrochemical theory, but also experimental findings [59,74,75] where the addition of RE to magnesium

alloys provoked an increase in the OCP values to nobler (more positive) values.

57

This trend may be explained by the principles exposed in previous chapters. In Chapter 1 it was

reported that the alloying elements in magnesium lead invariably to the formation of intermetallic

compounds; in Chapter 3 it was shown that the intermetallics are always nobler than the magnesium

matrix and that those intermetallics create a microgalvanic effect by promoting the cathodic reaction; if

the cathodic reaction occurs more rapidly than the anodic, the process is under anodic control and the

OCP should become more positive, as depicted in Figure 5 (Chapter 2).

The reason for the more negative OCP presented by WE54 could be related with the distribution of

Zr in the magnesium matrix. Several authors [57,70] propose that Zr can act as cathodic inhibitor; in this

case, a more homogeneous distribution of this element would be responsible for the OCP decrease.

The other two alloys also have Zr in their composition however, the extrusion process (ZK31) and the

thermal treatment (EZ33) lead to an uneven distribution of the Zr content, revealed by the

metallographic analysis. This behaviour can also be found in literature [71,77] where some alloys

containing Zr and RE are usually reported as having better corrosion resistance in the as-cast

condition than in the aged condition.

Table 9 summarizes the main data extracted form the polarization curves.

Table 1 – Polarization parameters for the alloys immersed for 10 minutes in NaOH – pH13; current density units are µAcm-2 and potential units are VSCE.

Alloy Ecorr icorr Epp icrit ipass Et

ZK31 -1.49 0.2 - - 16.1 1.13

EZ33 -1.48 0.1 -0.78 11.9 4.6 1.47

WE54 -1.68 0.5 -1.43 5.3 2.9 1.38

All the values presented were obtained by direct graphical analysis.

58

6.2.2 Passive Film Analysis by Electrochemical Impedance Spectroscopy

In the EIS tests, all the alloys showed an identical trend when immersed in the NaOH – pH13

solution, as depicted in Figures 26, 27 and 28.

The impedance spectra presented a capacitive response with a phase angle close to -90º, which is

consistent with the presence of a passive film. This capacitive response was present in a wide

frequency range.

The middle frequency capacitive response was followed by a low frequency resistive behaviour.

The impedance at low frequencies increased by one order of magnitude in the 24 hours test period,

denoting an increase in the corrosion resistance provided by the barrier film.

0 250000 500000 750000 1000000

-1000000

-750000

-500000

-250000

0

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105101

102

103

104

105

106

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 5 – Impedance plots of ZK31 alloy immersed in NaOH – pH 13.

0 100000 200000 300000 400000 500000

-500000

-400000

-300000

-200000

-100000

0

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105101

102

103

104

105

106

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 6 – Impedance plots of EZ33 alloy immersed in NaOH – pH 13.

59

0 250000 500000 750000 1000000

-1000000

-750000

-500000

-250000

0

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105100

101

102

103

104

105

106

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 7 – Impedance plots of WE54 alloy immersed in NaOH – pH 13.

One of the objectives of this work was to investigate the role of ions in the formation of the passive

film. Therefore, EIS measurements were also performed in 0.05 M NaCl at pH 13.

The shape of the impedance plots was identical for all samples. The introduction of chlorides into

the test solution, at this concentration, did not provoke changes on the behaviour of the passive films

and on the electrochemical processes, as shown by the EIS spectra presented in Figures 29, 30 and

31, which are similar to the ones obtained in the absence of chlorides.

0 250000 500000 750000 1000000

-1000000

-750000

-500000

-250000

0

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105101

102

103

104

105

106

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 8 – Impedance plots of ZK31 alloy immersed in NaOH – pH 13 with 0.05 M NaCl.

60

0 250000 500000 750000

-750000

-500000

-250000

0

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105100

101

102

103

104

105

106

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 9 – Impedance plots of EZ33 alloy immersed in NaOH – pH 13 with 0.05 M NaCl.

0 250000 500000 750000 1000000

-1000000

-750000

-500000

-250000

0

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105101

102

103

104

105

106

107

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 10 – Impedance plots of WE54 alloy immersed in NaOH – pH 13 with 0.05 M NaCl.

The variation of the OCP during the EIS measurements is depicted in Figure 32. The general trend

is that OCP becomes nobler as the immersion time increases, for solutions with and without chlorides,

as expected due to the growth of the passive film.

The WE54 alloy starts with more negative OCP and reaches, after 24 hours, the noblest potential,

which could indicate a more protective passive film.

The sample that seems to be more affected by the presence of chlorides is ZK31 that presents the

widest potential variation.

61

-1.6

-1.5

-1.4

-1.3

-1.2

0 3 6 9 12 15 18 21 24time (hours)

OC

P (V

SCE

)

ZK31EZ33WE54ZK31 w/ ClEZ33 w/ ClWE54 w/ Cl

Figure 11 – OCP evolution during EIS measurements.

The experimental results of the EIS experiments were modelled with an equivalent circuit in order

to obtain a clearer understanding of the mechanisms involved. The fitting routine also permits the

acquisition of numerical data for sample comparison.

The first fitting attempt was made with a simple R(RC) circuit, like the one described in section

4.1.2. This circuit fits a system where a single electrochemical process with a faradic (Rtc) and a non-

faradic (Cdl) response. The numerical adjustment was very unsatisfactory with values of χ2 above 10-3.

This was a clear indication that the system is composed of two time constants in the middle frequency

region.

The next step was the fitting of the experimental data with equivalent circuits composed of two time

constants. The circuits tested are depicted in Figure 33.

Figure 12 – Equivalents circuits used for fitting of experimental results: “ladder” circuit (left) and Voight circuit (right).

Considering the electrochemical system being studied, the circuits represent two different

responses with different mathematical solutions.

62

The “ladder” circuit can model a system where an imperfect or porous film exists. In this case

CPE1 will represent the capacitance of the film and R1 the electrolyte resistance inside the pore, while

CPE2 can be either a double layer capacitance of a corrosion process occurring inside the pore

(consequentially R2 will represent charge transfer resistance) or the capacitance of a film formed

inside the pore (where R2 will be the film resistance).

The application of the Voight circuit to this system implies that a complete and non-porous barrier

film must cover the entire metallic surface. More importantly, it considers that the passive film presents

two distinct layers.

The results presented in Figures 34 and 35 were obtained with the Voight equivalent circuit. This

circuit was the best suited for the interpretation of the experimental data as the numerical adjustment

was generally better.

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0 3 6 9 12 15 18 21 24time (hours)

Y0 o

f CPE

1 (O

hm-1

cm-2

s-n)

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06R

1 (O

hm c

m2 )

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0 3 6 9 12 15 18 21 24time (hours)

Y0 o

f CPE

2 (O

hm-1

cm-2

s-n)

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

R2

(Ohm

cm

2 )

Figure 13 – Fitting results for alloys immersed in NaOH –pH 13: ZK31 – red, EZ33 – green, WE54 – black; admittance – full lines, resistance – dashed lines.

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0 3 6 9 12 15 18 21 24time (hours)

Y0 o

f CPE

1 (O

hm-1

cm-2

s-n)

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

R1

(Ohm

cm2 )

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0 3 6 9 12 15 18 21 24time (hours)

Y0 o

f CPE

2 (O

hm-1

cm-2

s-n)

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

R2

(Ohm

cm2 )

Figure 14 – Fitting results for alloys immersed in NaOH – pH 13 with the addition of 0.05 M NaCl: ZK31 – red, EZ33 – green, WE54 – black; admittance – full lines, resistance – dashed lines.

The results showed only slight differences between the samples. The introduction of chlorides also

seemed to have very little effect on the impedance of the system. There is some variation in the fitting

results at low immersion time which are the result of a not fully stabilized OCP, as depicted in Figure

32.

63

The analysis of the fitting parameters revealed that the first time constant (CPE1 and R1) doesn’t

show a marked trend in admittance or resistance values. The variation stays within one order of

magnitude and the values are consistent with a passive barrier film.

The values obtained for the second time constant (CPE2 and R2) are also consistent with a

passive film. This indicates that the two layers of the film are very similar in nature. However, for the

second time constant there is a visible trend as the admittance decreases from values above 100

Ohm-1cm-2s-n to values around 10 Ohm-1cm-2s-n. The resistance values also increased with immersion

time until they stabilized just below 1 MΩcm2.

The Cole depression for all the samples and solutions remained very close or above 0.9, for both

time constants, reinforcing the idea that the film presents a structure with two layers with very similar

properties. The complete fitting parameters for all EIS experiments are presented in Appendix 1.

A global analysis of the impedance results suggests that the passive film formed on the surface for

all the magnesium alloys has the same structure: an outer layer of constant thickness represented by

the higher frequency time constant and an inner layer whose thickness increases with immersion time.

This has been concluded form the decrease in the admittance values for the lower frequency time

constant. The analogy with a parallel plate capacitor tells us that, if the capacitance (or in this case the

admittance) decreases, the separation between the electrodes (or in this case the thickness of the

layer) increases.

This bi-layer structure for magnesium passive films has been proposed independently in literature [84] by analysing initial film formation on pure magnesium using two different spectroscopic techniques:

XPS and photocurrent spectroscopy (PCS). In their model, they propose that an inner MgO film is

formed spontaneously at the surface. This layer is ultra-thin with a thickness of approximately 2.5 nm,

which remains constant. The MgO layer is completely covered by an external Mg(OH)2 layer whose

thickness is a function of immersion time. The structure is depicted in Figure 36.

Figure 15 – Schematic representation of the Mg/passive film/electrolyte interface [84].

The verification, through the EIS technique, of the existence of the MgO/Mg(OH)2 bi-layer in

magnesium passive films is, to the authors best knowledge, the first independent confirmation of this

model. However, the EIS results suggest a different structure for the film: a thin external Mg(OH)2

layer with constant thickness covering a MgO layer with a thickness that increases during immersion.

64

6.2.3 Microscopic and Quantitative Analysis

The magnesium alloys presented a non-porous film after an immersion period of 24 hours in

NaOH-pH 13. This feature was verified in both solutions, with and without chlorides. The absence of

porosity the passive film is an argument in favour of the model used for the fitting of the EIS results.

The ZK31 and the EZ33 alloys revealed the presence of some microcracks in the film. The

microckracing seems to be more severe in the presence of chlorides.

Although it is difficult to analyse the texture of the different surfaces by SEM, the ZK31 and EZ33

seem to have similar surfaces while WE54 appeared to be the most dissimilar of the three alloys.

Figure 16 – SEM micrographs of ZK31 alloy after 24 hours of immersion in NaOH pH13: without NaCl (left) and with 0.05 M NaCl

1

2

Figure 17 – SEM micrographs of EZ33 alloy after 24 hours of immersion in NaOH pH13: without NaCl (left) and with 0.05 M NaCl

65

Figure 18– SEM micrographs of WE54 alloy after 24 hours of immersion in NaOH pH13: without NaCl (left) and with 0.05 M NaCl

The micrograph of Figure 38 showed again the very large grain boundaries that characterize the

EZ33 alloy. Those grain boundaries are the result of the thermal processing and it was speculated in

section 6.1.2 that there is a coring effect caused by the diffusion of the different alloying elements. To

verify this effect an EDS analysis was preformed at two sites: in the inner part of the grain (point 1 in

Figure 38) and in the grain boundary (point 2 in Figure 38), as depicted on Figure 40.

Figure 19 – EDS analysis of EZ33 alloy after 24 hours of immersion in NaOH-pH13.

The EDS analysis confirmed that all the main alloying elements (RE and Zn) diffused into the grain

boundary. No Zr was detected in any of the two sites. Both the magnesium and oxygen contents are

higher in the inner part of the grain, revealing that the magnesium matrix is more oxidized, probably

due to a greater amount of Mg(OH)2. This is an expectable characteristic, considering that the alloying

elements present at the grain boundary have probably formed a nobler intermetallic compound.

66

6.2.4 Compositional Analysis by X-Ray Photoelectron Spectroscopy

All the samples presented similar XPS spectra after an immersion period of 24 hours in NaOH-

pH13, both without and with chlorides. The spectra also showed a predominance of magnesium oxide

over the magnesium hydroxide species, which was verified not only in the magnesium ionization

spectra but also in the oxygen ionization spectra. Figure 41 depicts the magnesium ionization spectra

for the samples immersed in NaOH without the addition of chlorides.

This analysis also revealed that the quantity of alloying elements present in the surface layer,

whose presence influenced the electrochemical response, is very small, staying bellow the detection

limit of the technique that is in the order of the hundreds of ppm.

Figure 20 – Magnesium ionization spectra for the ZK31 (left), EZ33 (middle) and WE54 (right) alloys after

24 hours of immersion in NaOH-pH13.

As expected, the presence of chlorides was detected in the samples immersed in NaOH-pH13 with

0.05 M NaCl, but the composition of the film remained identical.

The XPS method revealed the presence of both MgO and Mg(OH)2 in the surface film. This leads

to the conclusion that the MgO/Mg(OH)2 bi-layer film structure described in the previous sections is

possible. Due to its own nature, this technique can only verify the presence of the chemical species,

not its structural arrangement, and therefore it cannot give definitive evidence on this matter.

The XPS results also confirm that there is more oxide than hydroxide in the passive film. If the MgO

layer is thinner than the Mg(OH)2 layer, the intensity of the XPS peaks should be inversed, with the

hydroxide showing a stronger signal then the oxide as demonstrated by M. Santamaria et all. [84]. This

difference might be related with the high pH of the test solutions. It is possible, if not yet experimentally

verified, that the strongly alkaline solution has a stabilizing effect over the magnesium oxide. In this

way it is the MgO rather than the Mg(OH)2 that grows in thickness.

The film should grow by diffusion of magnesium from the base metal to the film/electrolyte interface

and by the diffusion of oxygen in the inverse direction.

67

6.2.5 Topographical Characterization

The AFM was used in order to assess the morphology of the surface films, and more specifically to

verify if there were differences between the alloys and the influence of chlorides in film formation.

Figures 42, 43 and 44 depict the most representative topographical images obtained for the three

alloys.

The structures of the surface passive film formed in NaOH without chlorides are characterized, in

the case of the ZK31 and EZ33 alloys by the presence of two different structures.

In the ZK31 alloy the largest “grains” present approximate widths between 300 and 600 nm and

heights between 40 and 70 nm. The smaller structures have widths between 20 and 70 nm and

heights that vary between 2 and 8 nm.

The structures visible in the EZ33 alloy have widths between 450 and 550 nm and heights between

50 and 100 nm. The finer structures shows have a wideness interval of 50 to 70 nm with a height

interval of 5 to 10 nm.

The WE54 alloy, on the other hand, seems to have only one type of structure, somewhat smaller

than the other alloys, with widths between 280 and 300 nm and heights in the interval of 20 to 40 nm.

The basic effect of adding NaCl to the hydroxide solution seemed to be the same for all the alloys:

the structures grow larger in dimension and more disordered.

The most affected alloy appeared to be the ZK31, where the structures grew into “scales”

approximately 100 nm wide and 5 to 10 nm tall.

In the EZ33 alloy the largest structures showed less definition and the smaller structures grew to

widths of 100 nm and heights of 20 nm, approximately.

In the presence of chlorides the “grains” of the surface film of the WE54 alloy grew to widths

between 400 and 650 nm and heights of 20 to 50 nm.

No decoupling was found between the topography images and the phase contrast images. This

means that the films are completely homogeneous in nature.

Several differences were registered between the surface films formed at this pH on the different

alloys. It was also apparent that the presence of chlorides in the solution increases the growth rate of

film. Nevertheless the differences are very difficult to relate with the electrochemical behaviour of the

alloys, which was, at this high pH, very similar.

68

Figure 21 – Topographical images of ZK31 alloy after 24 hours of immersion in NaOH-pH13: without

chlorides (left), with 0.05 M NaCl (right).

Figure 22 – Topographical images of EZ33 alloy after 24 hours of immersion in NaOH-pH13: without

chlorides (left), with 0.05 M NaCl (right).

Figure 23 – Topographical images of WE54 alloy after 24 hours of immersion in NaOH-pH13: without

chlorides (left), with 0.05 M NaCl (right).

69

6.3 Corrosion Study

The corrosion study was performed in two different solutions: NaCl 0.5 M and a sodium borate

(Na2B4O7) buffer solution with a constant pH value (pH 9).

The NaCl solution was chosen because it is the test solution commonly used in corrosion studies,

since it has approximately the same amount of NaCl as sea water. The borate solution, on the other

hand, was chosen specifically with the intention of magnifying any effects resulting from the presence

of the RE elements. At a pH of 9, magnesium is well into the active dissolution region as depicted in

Figure 8; however the Pourbaix diagrams [53] for the alloying elements indicate that Y, Zn and Zr are

completely passive and that RE are either passive or in the transition region between passive and

active behaviour.

6.3.1 Potentiodynamic Polarization Results

Figure 45 depicts the polarization curves obtained for the magnesium samples immersed in the

sodium borate solution.

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02i (Acm-2)

E (V

SCE)

ZK31EZ33WE54

Figure 24 – Polarization behaviour of the magnesium alloys immersed for 10 minutes in Na2B4O7 – pH 9.

The first noticeable feature is that the relations between the OCP values for the three alloys are

maintained, which further reinforces the hypotheses advanced in section 6.2.1 about the role and

distribution of the RE elements and Zr.

As the potential departs from OCP to more positive values, all the alloys evolve under an activation

controlled process. As the potential becomes nobler the current density increases to levels

corresponding to corrosion (10-3) and enters in a range of diffusion controlled kinetics.

As the corrosion products accumulate at the surface, the area available for the anodic reactions

decreases thus limiting the current generated in the process. The depletion at the surface of species

necessary for the cathodic reaction means that they can only proceed at a rate controlled by the

diffusion coefficients, also limiting the corrosion process.

70

No pitting potential was detected in this electrolyte either in the anodic or the cathodic branches of

the polarization curve. No relevant information was revealed in the cathodic polarization curves of the

three alloys, and for that reason they are not depicted in the figure.

The WE54 presented the lowest icorr and limiting current; however, the values are very similar for

the three alloys.

The polarization curves obtained in NaCl 0.5 M are in Figure 46.

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.E-07 1.E-05 1.E-03 1.E-01 1.E+01i (Acm-2)

E (V

SCE)

ZK31EZ33WE54

Figure 25 – Polarization behaviour of the magnesium alloys immersed for 10 minutes in NaCl 0.5 M.

The same premises applied for the explanation of the borate results are valid for NaCl test with

some noticeable exceptions.

A pitting potential (Epit) was clearly visible at potential values nobler than the OCP. The WE54

presented the largest difference between Ecorr and Epit (around 300 mV for WE54 against the 100 mV

for the ZK31 and EZ33 alloys), probably resulting from a more stable surface film.

The anodic currents are higher than in the borate solution, as expected, but are limited not only by

the same diffusion control mentioned earlier but also by a steep rise in the solution pH that increases

from neutral values to values above 10, which cannot occur in the borate buffered solution. This

severe alkalinization was experimentally verified.

The summary of the relevant data for the polarization curves in the sodium borate and sodium

chloride solutions is presented in Table 10.

Table 2 – Polarization parameters for the alloys immersed for 10 minutes in Na2B4O7 solution (pH9) and NaCl solution (0.5 M); current density units are µAcm-2 and potential units are VSCE.

Na2B4O7 solution (pH9) NaCl solution Alloy

Ecorr icorr Ecorr icorr Epit

ZK31 -1.55 10 -1.55 4 -1.43

EZ33 -1.64 30 -1.58 10 -1.49

WE54 -1.92 9 -1.85 7 -1.54

71

The results presented in Table 10 were again obtained by direct graphical analysis and Tafel

analysis was not applied in this study because the samples do not seem to show a Tafel region.

Although it is possible to find in literature Tafel analysis of [55,104], it is debatable if magnesium and

magnesium alloys possess a Tafel region. Furthermore, it has been verified experimentally [57] that in

the corrosion of magnesium the cathodic reaction occurs with two types of hydrogen evolution, which

contradicts one of the basic conditions for the Tafel analysis that requires one single cathodic and

anodic reaction. This was also verified in our experiments by the release of hydrogen gas produced by

two separate cathodic reactions: at the counter electrode, as expected, and in the sample surface.

The values obtained for icorr of the samples immersed in the NaCl solution were very similar. They

were also very similar, but consistently smaller, than the values obtained in the borate solution. This

trend is unexpected but also explainable by the alkalinisation of the solution.

72

6.3.2 Electrochemical Impedance Spectroscopy Results

Figures 47, 48 and 49 depict the impedance spectra obtained for the alloys immersed in the

sodium borate buffer solution with a pH of 9.

The shape of these plots is consistent with the results found in literature [55,61-63,65,66,69-72].

Magnesium usually presents capacitive loops at high and/or middle frequencies followed by an

inductive loop at low frequencies. The high and middle frequency capacitive loops are normally

associated with either the corrosion process (possessing a double layer capacitance and a charge

transfer resistance) or with mass transport through a corrosion product layer, according to the

capacitance and resistance values obtained. The low frequency inductive loop as been linked with the

relaxation of species absorbed at the interface, namely MgOH+ or Mg(OH)2 [62].

0 50 100 150 200

-150

-100

-50

0

50

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104101

102

103

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 26 – Impedance plots of ZK31 alloy immersed in Na2B4O7 – pH 9.

25 50 75 100 125 150

-100

-75

-50

-25

0

25

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104101

102

103

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 27 – Impedance plots of EZ33 alloy immersed in Na2B4O7 – pH 9.

73

0 100 200 300 400 500

-400

-300

-200

-100

0

100

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104101

102

103

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 28 – Impedance plots of WE54 alloy immersed in Na2B4O7 – pH 9.

For the fitting of the experimental results, several equivalent circuits were experimented, but with

very little success. For the ZK31 and EZ33 alloys the numerical adjustment was not satisfactory and

for the WE54 alloy no circuit was found capable of modelling the experimental data.

Considering that more work is needed to obtain a better understanding of the corrosion

mechanisms of these alloys, a decision was made to fit only the middle frequency time constant that

should be more related with the corrosion resistance of the alloys than the rest of the spectra.

As a general analysis of the impedance plots, it is visible that impedance values, and

consequentially corrosion resistance, decreases with immersion and stabilizes at 24 hours of

immersion. The WE54 alloy revealed the highest impedance and therefore the highest corrosion

resistance.

The EIS results for the immersion tests in NaCl 0.5 M are presented in Figures 50, 51 and 52.

The first noticeable characteristic is that the impedance of the three alloys is higher when

immersed in NaCl than in the borate solution. This was unexpected, but might be easily explained

considering pH alteration during testing.

The borate solution of pH 9 was chosen with the intention of determining if the RE elements, in

their passive state, could provide sufficient corrosion protection to an active magnesium surface. This

was not the case.

Because the borate solution is buffered, its pH cannot change like the pH in the NaCl solution. After

the EIS tests in NaCl, the pH was measured and was found to be above 9.5, which is higher than the

buffer solution. Also, if the bulk pH was around 9.5 it reasonable to assume that at 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 form protective oxide/hydroxide layers of

these elements at pH values where MgO/Mg(OH)2 is still not stable.

74

0 250 500 750 1000

-750

-500

-250

0

250

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-2 10-1 100 101 102 103 104 105101

102

103

104

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

30

phase angle (degrees)

Figure 29 – Impedance plots of ZK31 alloy immersed in NaCl 0.5 M.

0 50 100 150 200

-150

-100

-50

0

50

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105100

101

102

103

Frequency (Hz)

|Z| (

Ohm

cm

2 )1h3h6h12h24h

-90

-60

-30

0

30

phase angle (degrees)

Figure 30 – Impedance plots of EZ33 alloy immersed in NaCl 0.5 M.

0 2500 5000 7500

-5000

-2500

0

2500

Z' (Ohm cm2)

Z'' (

Ohm

cm

2 )

1h3h6h12h24h

10-3 10-2 10-1 100 101 102 103 104 105100

101

102

103

104

Frequency (Hz)

|Z| (

Ohm

cm

2 )

1h3h6h12h24h

-90

-60

-30

0

phase angle (degrees)

Figure 31 – Impedance plots of WE54 alloy immersed in NaCl 0.5 M.

75

The shape of the impedance spectra obtained in NaCl was also similar with the results of the

borate solution, having less definition of the low frequency inductive loop. The impedance evolution,

however, was very different for the ZK31 and WE54 alloys as the impedance levels increased until

they stabilized at 24 hours of immersion.

Figure 53 presents the fitting values for the middle frequency time constant in borate and NaCl

solutions.

1.E-05

1.E-04

1.E-03

0 3 6 9 12 15 18 21 24time (hours)

Y0 o

f CPE

(Ohm

-1cm

-2s-n

)

0

50

100

150

200

250

300

R (O

hmcm

2 )

1.E-05

1.E-04

1.E-03

0 3 6 9 12 15 18 21 24

time (hours)

Y0 o

f CPE

(Ohm

-1cm

-2s-n

)1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

R (O

hmcm

2 )

Figure 32 – Fitting results for alloys immersed in Na2B4O7 – pH 9 (left) and NaCl 0.5 M (right); ZK31 – red, EZ33 – green, WE54 – black; admittance – full lines, resistance – dashed lines.

These fitting results were obtained with the circuit presented in Figure 17 for a process under

charge transfer control. Relating admittance with double layer capacitance (Cdl) and resistance with

charge transfer resistance (Rtc), WE54 presented the best results in both features. The EZ33 alloy,

despite the RE elements, presented the worst results. In Figure 53 it also becomes more visible that

the Rtc, for all samples, is one order of magnitude higher in the NaCl than in borate solution.

Table 11 presents the charge transfer resistance values obtained for the three alloys in the borate

and NaCl media at an immersion time of 24 hours.

Table 3 – Rtc values (Ωcm2) obtained by the fitting of the EIS results for the alloys immersed for 24 hours in Na2B4O7 (pH9) and NaCl (0.5 M).

Alloy Na2B4O7 solution (pH9) NaCl 0.5 M

ZK31 41.0 879.9

EZ33 27.8 74.9

WE54 200.1 3881.0

The WE54 alloy showed the highest corrosion resistance in both media by presenting the highest

values of charge transfer resistance. By comparison the EZ33 alloy, despite of the RE elements, is the

alloy that revealed the worst behaviour due to the microgalvanic effect. When comparing the EIS plots

obtained for the two corrosive media, the corrosion resistance of the WE54 alloy seems to be related

not only with a more stable film but also with the stabilization of the low frequency process.

76

6.3.3 Microscopic and Quantitative Analysis

The SEM micrographs of the alloys immersed in the borate buffer solution (pH9) are depicted in

Figures 54, 55 and 56.

The ZK31 alloy revealed a generalized corrosion aligned in the direction of extrusion. No other

relevant structures were observed.

Figure 33 – SEM micrographs of ZK31 alloy after 24 hours of immersion in Na2B4O7 at different magnifications: x300 (left) and x2000 (right).

The corrosion process of the EZ33 alloy occurred with the anodic dissolution of the grains while the

grain boundaries remained somewhat intact, creating a cellular structure.

An EDS analysis was preformed on the sample at two separate points: the grain boundary (point 1

on Figure 55) and the interior of what appeared to be a lees corroded grain (point 2 on Figure 55). The

results of this analysis are presented in Table 12.

2

1

Figure 34 – SEM micrographs of EZ33 alloy after 24 hours of immersion in Na2B4O7 at different magnifications: x300 (left) and x950 (right).

77

Table 4 – EDS analysis of EZ33 alloy after 24 hours of immersion in Na2B4O7 (pH9).

Weight % Chemical Element Grain Boundary

(Point 1) Grain Centre

(Point 2)

O 20.8 52.9

Mg 28.8 19.3

Zn 32.5 10.6

La 6.2 0.9

Ce 9.2 2.7

Nd 2.5 –

Zr – 13.6

From the analysis of the micrograph and EDS it is clear that the EZ33 alloy corroded by a

microgalvanic process. The EDS showed that the RE diffused into the grain boundaries in large

quantities, which in the case of Ce exceed the solid solubility of the element (see Table 2), with the

probable formation of intermetallic compounds. Those compounds are nobler and act as cathodic

sites, catalysing the anodic dissolution of the magnesium matrix. This is visible in the amount of

oxygen present at the two sites that showed a higher degree of oxidation at the centre of the grain.

Another noticeable characteristic is that due to the coring of Zn and Zr, the grains that have larger

amounts of these elements seemed more stabilized and resistant to corrosion.

The microgalvanic effect should be responsible for the fact that, despite having RE in its chemical

composition, the EZ33 alloy showed the worst results in the electrochemical tests.

The WE54 alloy revealed a different corrosion mechanism from the other two alloys. Very large

features with a “plateau” like appearance were found randomly distributed throughout the corroded

surface.

2

1

Figure 35 – SEM micrographs of WE54 alloy after 24 hours of immersion in Na2B4O7 at different magnifications: x100 (left) and x600 (right).

78

Two EDS spectra were obtained on the sample surface: one in the “plateau” area (signalled as

Point 1 in Figure 56) and on the corroded surface (Point 2 in Figure 56). The results are presented in

Table 13.

Table 5 – EDS analysis of WE54 alloy after 24 hours of immersion in Na2B4O7 (pH9).

Weight % Chemical Element

Point 1 Point 2

O 63.4 15.4

Na 1.8 0.3

Mg 7.11 76.1

Ca 0.4 –

Y 20.4 6.4

Nd 6.8 2.2

The “plateau” region showed to have a large amount of Y and a very small quantity of magnesium,

which could be consistent with one of the intermetallic compounds that might occur in alloys of this

series. However the large number and size of these structures and the fact that they are severely

oxidized (the oxygen level is 36.4 % wt) suggests that they are in fact the remainder of a layer of

corrosion products.

This layer was probably formed by anodic dissolution of the magnesium matrix. Because the Y

solubility limit is relatively high there is a great amount of this element in solid solution that, together

with the probable existence of intermetallic compounds, is capable of forming a metastable layer by

precipitation of corrosion products under the form of oxides and hydroxides..

As this layer is attacked and the corrosion proceeds, it will fragment and break-up. The process of

second phase undermining will cause the falling away of most of this layer.

At this point in time, SEM analysis has not yet been preformed on the corrosion products formed

upon immersion in the NaCl 0.5 M media.

79

6.3.4 Compositional Analysis by X-Ray Photoelectron Spectroscopy

The comparative analysis of the magnesium ionization spectra after immersion in the NaOH-pH13

and borate solution is presented in Figures 57, 58 and 59.

Figure 36 – Magnesium ionization spectra for the ZK31 alloy after 24 hours of immersion: NaOH-pH13

(left) and Na2B4O7-pH9 (right).

Figure 37 – Magnesium ionization spectra for the EZ33 alloy after 24 hours of immersion: NaOH-pH13

(left) and Na2B4O7-pH9 (right).

Figure 38 – Magnesium ionization spectra for the WE54 alloy after 24 hours of immersion: NaOH-pH13 (left) and Na2B4O7-pH9 (right).

80

In the ZK31 alloys there was a predominance of magnesium oxide over the magnesium hydroxide

after the immersion in the borate solution. The signal originated by the magnesium metal was also

present indicating that some areas of the surface are covered by a thinner layer. In the EZ33 and

WE54 alloys the strongest signal was the main corrosion product of magnesium the Mg(OH)2, which

was found in greater quantities than the oxide.

The level of the signal was much smaller after the immersion in borates as compared with the

hydroxide (the left and right spectres have different scales).

The oxidized form of Zr, (Zr(OH)2), was found in the ZK31 and EZ33 alloys, characterized by the

appearance of a duplet that is typical for this element, as depicted in Figure 60. The presence of Zr in

the EZ33 alloy was only vestigial.

Figure 39 – Zirconium ionization spectra for the ZK31 alloy (left) and EZ33 alloy (right) after 24 hours of

immersion in Na2B4O7-pH9.

The fact that no Zr corrosion products were found in the WE54 alloy may be simply related with a

more homogeneous distribution along the surface that does not allow for the accumulation of the

element in quantities that can be measured by the XPS technique.

81

The analysis also revealed the corrosion products of the rare earth elements (Figure 61). In the

case of alloy EZ33 there was a vestigial amount of Ce, represented by its two main peaks and

corresponding satellites; in the WE54 the peaks corresponding with the oxidized forms of Y were also

present.

Figure 40 – Ce ionization spectra for the EZ33 alloy (left) and Y ionization spectra for the WE54 alloy (right) after 24 hours of immersion in Na2B4O7-pH9.

The ZK31 and EZ33 also revealed the presence of Zn corrosion products, as expected since both

have approximately 3 % wt of this element in their chemical composition. In the ZK31 alloy two peaks

appeared at 1022 and 1023 eV with a total atomic percentage of 4.9 %. For the EZ33 alloy only one

peak was visible at 1025 eV with corresponding to an atomic percentage of 0.8 %. There seems to be

less corrosion products of Zn in the EZ33 alloy than in the ZK31 alloy. This fact is probably related

with a the distribution of the element; in the EZ33 alloy there is a great amount of Zn present at less

corroded areas including the grain boundaries.

The most significant corrosion product, responsible for the strongest signal, should be, in both

cases, the Zn(OH)2. The second peak in the ZK31 alloy is due to another corrosion product present in

very small quantities. The identification of this second peak is usually complicated since zinc

hydroxides, oxides and chlorides (that in this case can only be the result of solution contamination)

have very similar binding energies.

82

Chapter 7

Conclusions

The main objective of this work was to analyse the influence of the RE as alloying elements in

magnesium alloys either in the passive or in the corrosion behaviour. Thus, the study was divided into

two parts: a study on the properties of surface films formed in strongly alkaline passivating media and

a study on the corrosion of magnesium alloys in aggressive solutions with different pH levels. Another

objective pursued in the passivation study was to try to evaluate the role of chlorides in passive film

formation.

The electrochemical tests in passive media demonstrated that the WE54 alloy presents the more

marked passive behaviour due to its high RE content (about 9 % wt in total). In the passivating media,

WE54 showed the lowest ipass followed by the EZ33 alloy. The ZK31 alloy that does not contain RE

was used as control sample and showed a lower transpassivation potential than the other alloys.

Despite the differences found in the polarization behaviour, the EIS results were very similar for all

the samples. The WE54 showed the highest impedance levels but the EZ33, despite the RE, revealed

the lowest impedance values. The EIS experiments didn’t reveal significant differences in the alloy

behaviour in the presence of chlorides, at the concentration and pH tested.

An important conclusion of the EIS analysis is that the passive surface film on the magnesium

alloys has a bi-layer structure, partially confirming a previous model proposed in literature. However,

contrary to that model, the XPS technique revealed a predominance of MgO over Mg(OH)2 in the

surface layers. The XPS did not detect the presence of any alloying elements in the MgO/Mg(OH)2

film; this means that the elements can influence the electrochemical properties of the alloys even at

very small concentration (bellow the XPS detection limits).

The AFM images revealed that the morphology of the surface film is different for each alloy.

Although no major differences were registered in the electrochemical behaviour of the alloys with the

addition of NaCl, the AFM clearly showed an increase in grain size and disorder in the presence of

chlorides.

The electrochemical tests performed in corrosive media confirmed that the WE54 alloy has a

superior corrosion resistance, which was clearly verified in the EIS experiments by higher impedance

values. The EZ33 alloy, however, didn’t beneficiate from the presence of RE in its chemical

composition and has shown consistently weaker corrosion resistance compared to the ZK31 alloy,

resulting in higher icorr and lower impedance, in both sodium borate and sodium chloride solutions.

All the samples showed higher corrosion rates in the borate solution than in the NaCl media. This

was the result of solution alkalinization, associated with the normal corrosion process of magnesium

alloys, which increased the pH of the NaCl solution to a level higher than the borate media.

The SEM and in particular the EDS analysis clarified the reason why the EZ33 alloy behaves worst

than the ZK31 alloy. The EZ33 alloy suffers from a strong microgalvanic effect, suggested by the

metallographic analysis and confirmed by EDS, which provokes the preferential dissolution of the

magnesium matrix catalysed by the presence of large grain boundaries rich in the RE elements.

83

The metallurgical structure has a great influence on the corrosion behaviour of these alloys: in

order 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 WE54 the SEM images suggested that upon the dissolution of the magnesium in the

matrix, the Y and RE formed a layer of corrosion products, either directly or by secondary

precipitation, in the exposed surface. This layer seems to be poorly attached to the surface and

eventually falls away by undermining.

If this layer could be more stabilized, it is possible that the corrosion resistance of this alloy could

improve. This idea derives from the comparative analysis of the EIS spectra obtained for the borate

and the NaCl solutions. Not only did the Rtc increase by one order of magnitude, but the impedance

level also increased with time. The low frequency part of the Nyquist diagram also showed a

stabilization of the corrosion products caused by the increase in pH.

84

Chapter 8

Future Work

The study demonstrated the importance of the structure of the alloys on the corrosion behaviour.

For this reason an X-Ray Diffraction (XRD) analysis will be conducted in order to fully characterize

these alloys, and in particular to verify the presence intermetallic compounds.

It was shown that the RE elements have an important influence in the behaviour of the passive film.

To characterize this influence more effectively, capacitance measurements will be preformed using the

Mott-Shottky technique complemented with Photocurrent Spectroscopy (PCS) measurements.

The EIS results pointed towards a bi-layer structure of the passive films. Nevertheless, the actual

structure of these films is still uncertain and a direct observation for a complete characterization is

fundamental. This should be accomplished by Transmission Electron Microscopy (TEM) that is

capable of achieving the very large magnifications required.

The Tafel extrapolation doesn’t seems to be a reliable method to determine the corrosion rate in

magnesium alloys. To assess the corrosion rate more exactly, experiments will conducted with

analytical techniques. In this case, the technique employed will be Inductively Coupled Plasma Atomic

Emission Spectroelectrochemistry (ICP AES), capable of determining in real time the corrosion rate of

the alloys.

The precipitation of the alloying elements and the possible creation of surface layers composed of

corrosion products needs to be more explained, as well as their influence on the corrosion behaviour.

This task will be performed by the conclusion of the SEM-EDS and XPS studies of the samples

immersed in NaCl. The corrosion products formed in the borate and sodium chloride solutions will be

analysed though XRD.

Due to its particular grain morphology, the EZ33 alloy is a prime candidate to corrosion testing

using localized techniques. Two main techniques will be applied: Scanning Vibrating Electrode

Technique (SVET) and Scanning Ion Electrode Technique (SIET). SVET can map the electric fields

created by the ions as they dissolve into solution and SIET can make a bidimensional map of the

variation in pH at the local level. With the appropriate electrodes SIET can also map the distribution of

ions in the solution; in this case the relevant ions would be Mg and the main alloying elements.

Finally, since the WE54 alloy has proved to be the alloy with better corrosion resistance, it will

serve as a substrate for anodization experiments. The anodization will be conducted in different

conditions of media and current and subjected to corrosion testing.

The objective of this task is to verify if it is possible to create a barrier film rich on Y but compact

and adherent to the alloy, capable of superior corrosion resistance.

85

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89

Appendix 1 – EIS Fitting Results

Table 1 – EIS results for immersion in NaOH – pH13.

CPE1 CPE2 Alloy Time

(hours) RΩ

(Ohm) Y0 (a) n

R1 (b)

Y0 n R2 (b) χ2

1 20.2 7.99 0.94 45.5 238.8 0.92 10.5 5.0x10-4

3 20.7 7.49 0.92 91.7 333.4 0.88 25.4 7.5x10-4

6 22.0 7.16 0.90 187.7 190.9 0.89 50.1 5.9x10-4

12 24.6 6.96 0.89 350.0 85.1 0.90 127.5 7.8x10-4

ZK31

24 28.7 7.59 0.88 530.7 27.7 0.89 542.9 1.1x10-3

1 16.9 11.9 0.93 57.7 284.6 0.87 25.2 8.8x10-4

3 16.5 12.1 0.91 109.8 227.8 0.90 54.0 4.0x10-4

6 17.3 12.2 0.90 153.6 120.1 0.90 86.8 5.7x10-4

12 19.2 12.0 0.89 243.9 84.1 0.90 170.6 1.1x10-3

EZ33

24 21.5 49.6 0.97 45.9 11.6 0.88 493.7 1.2x10-3

1 3.7 9.6 0.94 60.7 89.6 0.95 27.3 5.8x10-4

3 6.8 8.9 0.93 171.3 68.5 0.94 103.1 1.0x10-3

6 6.9 9.5 0.93 237.2 33.9 0.93 238.7 1.4x10-3

12 7.2 28.3 0.93 31.7 87.8 0.93 648.6 1.1x10-3

WE54

24 7.6 35.9 0.911 27.8 77.2 0.91 849.0 4.4x10-4

(a) admittance (Y0) units are µΩ-1cm-2s-n.

(b) resistance units (R1 and R2) are kΩcm2.

A1-1

Table 2 – EIS results for immersion in NaOH – pH13 with the addition of 0.05 M NaCl.

CPE1 CPE2 Alloy Time

(hours) RΩ

(Ohm) Y0 (a) n

R1 (b)

Y0 n R2 (b) χ2

1 27.5 8.0 0.95 79.8 211.0 0.90 37.4 1.7x10-3

3 28.3 7.6 0.94 164.1 140.1 0.92 81.1 1.2x10-3

6 29.2 7.2 0.93 381.6 73.7 0.93 210.9 9.2x10-4

12 32.5 8.3 0.89 531.7 40.6 0.91 444.9 8.3x10-4

ZK31

24 30.1 8.3 0.92 474.3 24.4 0.92 554.4 7.6x10-4

1 6.7 19.0 0.93 38.3 254.9 0.78 19.8 1.2x10-3

3 7.0 12.0 0.92 66.5 142.9 0.75 45.8 9.2x10-4

6 7.1 14.7 0.93 101.3 49.5 0.77 139.7 1.4x10-3

12 7.5 16.7 0.93 135.1 47.0 0.75 216.5 1.3x10-3

EZ33

24 4.9 36.8 0.94 11.7 13.0 0.90 611.6 1.6x10-3

1 16.2 9.8 0.94 76.7 116.2 0.95 30.7 7.2x10-4

3 16.1 76.6 0.94 111.0 9.1 0.94 207.8 9.2x10-4

6 16.4 36.5 0.94 215.3 9.7 0.93 261.6 9.4x10-4

12 15.57 18.9 0.86 498.0 11.7 0.97 328.2 1.4x10-3

WE54

24 21.57 52.5 0.75 694.5 8.9 0.95 643.2 1.9x10-3

(a) admittance (Y0) units are µΩ-1cm-2s-n.

(b) resistance units (R1 and R2) are kΩcm2.

A1-2

Table 3 – EIS results for immersion in Na2B4O7 – pH9.

Alloy Time (hours)

RΩ (Ohm) Y0

(a) n R (b) χ2

1 43.5 40.8 0.94 153.7 3.6x10-4

3 43.3 61.2 0.93 99.6 3.1x10-4

6 42.1 113.9 0.92 74.7 3.4x10-4

12 41.3 230.8 0.92 50.8 2.4x10-4

ZK31

24 39.4 290.4 0.91 41.0 2.6x10-4

1 37.6 53.5 0.91 79.6 2.3x10-4

3 36.9 118.8 0.90 45.0 2.8x10-4

6 36.3 311.9 0.93 29.4 3.9x10-4

12 35.6 669.9 0.92 25.2 2.7x10-4

EZ33

24 34.3 768.2 0.92 27.8 3.3x10-4

1 39.6 25.0 0.87 270.2 4.6x10-4

3 39.6 25.1 0.88 264.0 3.8x10-4

6 39.7 26.5 0.87 259.9 3.6x10-4

12 39.8 28.1 0.87 239.4 3.6x10-4

WE54

24 37.2 31.4 0.86 200.1 3.7x10-4

(a) admittance (Y0) units are µΩ-1cm-2s-n.

(b) resistance units (R) are Ωcm2.

A1-3

Table 4 – EIS results for immersion in NaCl.

Alloy Time

(hours) RΩ

(Ohm) Y0 (a) n R (b) χ2

1 16.8 44.5 0.92 311.1 9.5x10-4

3 16.7 50.8 0.89 562.4 1.9x10-3

6 14.6 91.5 0.92 325.5 5.0x10-4

12 15.3 57.1 0.90 964.9 1.8x10-3

ZK31

24 14.8 56.3 0.88 879.9 6.4x10-4

1 7.6 28.6 0.95 147.3 7.2x10-4

3 7.3 53.9 0.95 132.5 5.3x10-4

6 6.0 104.3 0.96 99.6 2.7x10-4

12 6.5 112.3 0.95 67.82 6.3x10-4

EZ33

24 6.8 117.1 0.94 74.89 5.7x10-4

1 6.5 19.5 0.93 1262 9.6x10-4

3 6.8 17.9 0.93 2054 9.2x10-4

6 6.7 17.4 0.93 2465 8.7x10-4

12 6.6 17.0 0.93 2935 7.6x10-4

WE54

24 6.8 16.4 0.92 3881 6.9x10-4

(a) admittance (Y0) units are µΩ-1cm-2s-n.

(b) resistance units (R) are Ωcm2.

A1-4

A2-1

Appendix 2 – List of Figures and Tables

pag. Figure 1 – Pourbaix diagrams of pure metals with representative corrosion behaviour: a) aluminium, b) iron, c)

copper and d) titanium (adapted) [53] ........................................................................................................ -16-

Figure 2 – Theoretical current-potential diagram for a metallic electrode [49]. .................................................... -17-

Figure 3 – Theoretical current-potential diagram (left) and Evans diagram (right) for a corrosion process with two coupled electrode processes [49]. ............................................................................................................. -19-

Figure 4 – Polarization diagram for the corrosion of zinc in an acidic solution [50] (adapted). ............................ -20-

Figure 5 – Schematic Evans diagram of three electrode reactions with the same icorr but different types of control: anodic (left), mixed (centre) and cathodic (right) [48]. ................................................................................ -20-

Figure 6 – Schematic polarization diagram displaying transitions from active corrosion to passive behaviour and to the transpassive state [50]. ........................................................................................................................ -22-

Figure 7 – Theoretical polarization curve displaying pitting behaviour [50] (adapted). ........................................ -25-

Figure 8 – Pourbaix Diagram of pure magnesium [53]. ....................................................................................... -28-

Figure 9 – Schematic representation of the NDE in magnesium [58]. ................................................................. -29-

Figure 10 – Polarization curve of AZ91 magnesium alloy with increasing amounts of Nd [74]. ........................... -32-

Figure 11 – Independent polarization curves of α-phase and β-phase in AZ91 magnesium alloy [58]................ -32-

Figure 12 – Influence of Al content on the corrosion rate of magnesium alloys immersed in 5 % NaCl [58]. ...... -33-

Figure 13 – Schematic representation of β phase protection: (A) initial surface, (B) obstruction of corrosion by accumulation of corrosion products [58]..................................................................................................... -34-

Figure 14 – Schematic representation of the Nyquist and Bode plots for a resistor [111]. .................................. -40-

Figure 15 – Schematic representation of the Nyquist and Bode plots for a capacitor [111]................................. -41-

Figure 16 – Nyquist and Bode plots for a R(RC) circuit [111]. ............................................................................. -41-

Figure 17 – Equivalent circuit for the interpretation of the impedance plots of Figure 16. ................................... -42-

Figure 18 – Nyquist plot with and without depression [111]................................................................................. -43-

Figure 19 – Schematic representation of an AFM apparatus [116]...................................................................... -48-

Figure 20 – Sample preparation sequence.......................................................................................................... -51-

Figure 21 – Three electrode experimental setup. ................................................................................................ -51-

Figure 22 – Micrographs of ZK31 alloy after 20 s etching in the Nital (left) and HF (right) solutions. .................. -54-

Figure 23 - Micrographs of EZ33 alloy after 20 s etching in the Nital solution (left) and 10 s etching in the Acetic Glycol solution (right). ................................................................................................................................ -55-

Figure 24 - Micrographs of WE54 alloy after 50 s etching in the Nital solution (left) and 20 s etching in the Acetic Glycol solution (right). ................................................................................................................................ -56-

Figure 25 – Polarization curves obtained for magnesium alloys immersed for 10 minutes in NaOH – pH 13. .... -57-

Figure 26 – Impedance plots of ZK31 alloy immersed in NaOH – pH 13. ........................................................... -59-

Figure 27 – Impedance plots of EZ33 alloy immersed in NaOH – pH 13. ........................................................... -59-

Figure 28 – Impedance plots of WE54 alloy immersed in NaOH – pH 13. .......................................................... -60-

Figure 29 – Impedance plots of ZK31 alloy immersed in NaOH – pH 13 with 0.05 M NaCl. ............................... -60-

Figure 30 – Impedance plots of EZ33 alloy immersed in NaOH – pH 13 with 0.05 M NaCl. ............................... -61-

Figure 31 – Impedance plots of WE54 alloy immersed in NaOH – pH 13 with 0.05 M NaCl. .............................. -61-

Figure 32 – OCP evolution during EIS measurements. ....................................................................................... -62-

Figure 33 – Equivalents circuits used for fitting of experimental results: “ladder” circuit (left) and Voight circuit (right).......................................................................................................................................................... -62-

A2-2

Figure 34 – Fitting results for alloys immersed in NaOH –pH 13: ZK31 – red, EZ33 – green, WE54 – black; admittance – full lines, resistance – dashed lines. ..................................................................................... -63-

Figure 35 – Fitting results for alloys immersed in NaOH – pH 13 with the addition of 0.05 M NaCl: ................... -63-

Figure 36 – Schematic representation of the Mg/passive film/electrolyte interface [84]. ..................................... -64-

Figure 37 – SEM micrographs of ZK31 alloy after 24 hours of immersion in NaOH pH13: without NaCl (left) and with 0.05 M NaCl........................................................................................................................................ -65-

Figure 38 – SEM micrographs of EZ33 alloy after 24 hours of immersion in NaOH pH13: without NaCl (left) and with 0.05 M NaCl........................................................................................................................................ -65-

Figure 39– SEM micrographs of WE54 alloy after 24 hours of immersion in NaOH pH13: without NaCl (left) and with 0.05 M NaCl........................................................................................................................................ -66-

Figure 40 – EDS analysis of EZ33 alloy after 24 hours of immersion in NaOH-pH13. ........................................ -66-

Figure 41 – Magnesium ionization spectra for the ZK31 (left), EZ33 (middle) and WE54 (right) alloys after 24 hours of immersion in NaOH-pH13. ........................................................................................................... -67-

Figure 42 – Topographical images of ZK31 alloy after 24 hours of immersion in NaOH-pH13: without chlorides (left), with 0.05 M NaCl (right). ................................................................................................................... -69-

Figure 43 – Topographical images of EZ33 alloy after 24 hours of immersion in NaOH-pH13: without chlorides (left), with 0.05 M NaCl (right). ................................................................................................................... -69-

Figure 44 – Topographical images of WE54 alloy after 24 hours of immersion in NaOH-pH13: without chlorides (left), with 0.05 M NaCl (right). ................................................................................................................... -69-

Figure 45 – Polarization behaviour of the magnesium alloys immersed for 10 minutes in Na2B4O7 – pH 9. ....... -70-

Figure 46 – Polarization behaviour of the magnesium alloys immersed for 10 minutes in NaCl 0.5 M................ -71-

Figure 47 – Impedance plots of ZK31 alloy immersed in Na2B4O7 – pH 9........................................................... -73-

Figure 48 – Impedance plots of EZ33 alloy immersed in Na2B4O7 – pH 9........................................................... -73-

Figure 49 – Impedance plots of WE54 alloy immersed in Na2B4O7 – pH 9. ........................................................ -74-

Figure 50 – Impedance plots of ZK31 alloy immersed in NaCl 0.5 M. ................................................................. -75-

Figure 51 – Impedance plots of EZ33 alloy immersed in NaCl 0.5 M. ................................................................. -75-

Figure 52 – Impedance plots of WE54 alloy immersed in NaCl 0.5 M................................................................. -75-

Figure 53 – Fitting results for alloys immersed in Na2B4O7 – pH 9 (left) and NaCl 0.5 M (right); ......................... -76-

Figure 54 – SEM micrographs of ZK31 alloy after 24 hours of immersion in Na2B4O7 at different magnifications: x300 (left) and x2000 (right). ...................................................................................................................... -77-

Figure 55 – SEM micrographs of EZ33 alloy after 24 hours of immersion in Na2B4O7 at different magnifications: x300 (left) and x950 (right). ........................................................................................................................ -77-

Figure 56 – SEM micrographs of WE54 alloy after 24 hours of immersion in Na2B4O7 at different magnifications: x100 (left) and x600 (right). ........................................................................................................................ -78-

Figure 57 – Magnesium ionization spectra for the ZK31 alloy after 24 hours of immersion: NaOH-pH13 (left) and Na2B4O7-pH9 (right). .................................................................................................................................. -80-

Figure 58 – Magnesium ionization spectra for the EZ33 alloy after 24 hours of immersion: NaOH-pH13 (left) and Na2B4O7-pH9 (right). .................................................................................................................................. -80-

Figure 59 – Magnesium ionization spectra for the WE54 alloy after 24 hours of immersion: NaOH-pH13 (left) and Na2B4O7-pH9 (right). .................................................................................................................................. -80-

Figure 60 – Zirconium ionization spectra for the ZK31 alloy (left) and EZ33 alloy (right) after 24 hours of immersion in Na2B4O7-pH9. ....................................................................................................................... -81-

Figure 61 – Ce ionization spectra for the EZ33 alloy (left) and Y ionization spectra for the WE54 alloy (right) after 24 hours of immersion in Na2B4O7-pH9. .................................................................................................... -82-

A2-3

Table 1 – Physical properties of pure magnesium [4,6]......................................................................................... -2-

Table 2 – Solubility of solute elements in binary magnesium alloys [7]. ................................................................ -3-

Table 3 – ASTM codes for magnesium alloys [4,7,8]. ........................................................................................... -5-

Table 4 – Influence on the behaviour of Mg alloys of the most important alloying elements [4,7]. ........................ -8-

Table 5 – The Electromotive Force Series (adapted) [49]. .................................................................................. -13-

Table 6 – Corrosion potentials for common magnesium second phases in 5% NaCl (pH 10.5) [58]................... -33-

Table 7 – Chemical composition of the tested magnesium alloys [3,7]. .............................................................. -50-

Table 8 – Summary of etching reagents used, their characteristics and applications.......................................... -53-

Table 9 – Polarization parameters for the alloys immersed for 10 minutes in NaOH – pH13; current density units are µAcm-2 and potential units are VSCE. .................................................................................................... -58-

Table 10 – Polarization parameters for the alloys immersed for 10 minutes in Na2B4O7 solution (pH9) and NaCl solution (0.5 M); current density units are µAcm-2 and potential units are VSCE. ........................................ -71-

Table 11 – Rtc values (Ωcm2) obtained by the fitting of the EIS results for the alloys immersed for 24 hours in Na2B4O7 (pH9) and NaCl (0.5 M). .............................................................................................................. -76-

Table 12 – EDS analysis of EZ33 alloy after 24 hours of immersion in Na2B4O7 (pH9)....................................... -78-

Table 13 – EDS analysis of WE54 alloy after 24 hours of immersion in Na2B4O7 (pH9). .................................... -79-

Table 14 – EIS results for immersion in NaOH – pH13. .....................................................................................A1-1

Table 15 – EIS results for immersion in NaOH – pH13 with the addition of 0.05 M NaCl. ..................................A1-2

Table 16 – EIS results for immersion in Na2B4O7 – pH9.....................................................................................A1-3

Table 17 – EIS results for immersion in NaCl. ....................................................................................................A1-4