Evaluation of the surface reactivity of beta-Titanium

Click here to load reader

  • date post

    02-Jan-2017
  • Category

    Documents

  • view

    220
  • download

    3

Embed Size (px)

Transcript of Evaluation of the surface reactivity of beta-Titanium

  • Evaluation of the surfacereactivity of -Titanium

    Dissertation

    zur

    Erlangung des Grades

    "Doktor der Naturwissenschaften"

    an der Fakultt fr Chemie und Biochemie

    der Ruhr-Universitt Bochum

    vorgelegt von

    Michael Teka Woldemedhin

    geboren in Addis Abeba, thiopien

    BochumDezember 2011

  • This doctoral study was carried out under the supervision of Prof. Dr. AchimWalter Hassel and Prof. Dr. Ing. Dierk Raabe in the group of Electrochemistryand Corrosion, Department of Interface Chemistry and Surface Engineering,Max Planck Institut fr Eisenforschung, Dsseldorf, Germany in the frame of theInternational Max Planck Research School for Surface and Interface Engineer-ing in Advanced Materials (IMPRS-SurMat) and at the Institut fr ChemischeTechnologie Anorganischer Stoffe (ICTAS), Johannes Kepler University, Linz,Austria in the period between April, 2008 and December, 2011.

    1. Gutachter: Prof. Dr. Achim Walter Hassel2. Gutachter: Prof. Dr. Ing. Dierk Raabe2. Gutachter: Prof. Dr. Wolfgang Schuhmann

  • Acknowledgement

    The lion share of my gratitude goes to Prof. Dr. Achim Walter Hassel andProf. Dr. Ing. Dierk Raabe for their supervision of my research in which theirconstant support, encouragement and complete freedom was always by my side.I would also like to thank Prof. Dr. Wolfgang Schuhmann for reviewing mydissertation.The Intenational Max Planck Research School for Surface and Interface Engi-neering in Advanced Materials (IMPRS-SurMat) is duely acknowledged for thedoctoral fellowship.I would take this opportunity to thank the administrative director of theIMPRS-SurMat school Dr. Rebekka Loschen and the SurMat staff Vanja Wsterand Elke Gattermann for their support during my time as a SurMat student.The group members of the Electrochemistry and Corrosion group of the MaxPlanck Institut fr Eisenforschung and the Institut fr Chemische TechnologieAnorganischer Stoffe (ICTAS) at the Johannes Kepler University are also duelyacknowledged for their support and welcoming me to the group: Dr. AndreiIonut Mardare for passing on his knowledge about the fabrication of scanningdroplet cell and the scanning droplet setup, Dr. Daniel Sanders for the scientificdiscussions we had, Andrea Mingers for the technical support during my stayat MPIE, Karl Kellner for the fast provision of all the chemicals and for all thetechnical help I was in need for.Special thanks goes to the head of the mechanical workshop of the MPIE RalfSelbach and Peter Thyssen for the fabrication of the componenets of the scan-ning droplet cell.I thank my office mates Dr. Genesis Ankah, Dr. Daniel Sanders, Dr. YingChen, Stefanie Drensler, Christian Schnepf for all the laughter we had in thethree offices I sat during my PhD studies.I thank my friends Dr. Faycal Riad Hamou, Dr. Juan Zuo, Dr. Edmanuel Tor-res, Mulda Muldarisnur, Mauro Martin and Kemal Davut for the crazy laughsand good times we had.Friends back home and in far away places Samson Gebremedhin, Anteneh (Abu)Gashaw, Tesfa Solomon, Yohannes Tesfaye, Sefonias Mekonnen, Yonas (Pupi)Abera, Wondwossen Shewangizaw (Baby) thank you for the morale boost.

  • I would like to express my deepest gratitude for all the teachers and instructorsfrom my time in Misrak Dil school till the present day who shared their invalu-able knowledge and made me what I am today.My parents, TekaWoldemedhin and Enanu Alemu (Zeduye), and siblings Sirgut,Addis and Netsi (Mitaye) for your love and support which would have been im-possible to acomplish this. Love you all!!

  • Evaluation of the surface reactivityof -Titanium

    Dissertation

    by

    Michael Teka Woldemedhin

  • The secret is comprised in three words-Work, finish, publish.

    Michael Faraday

  • Abstract

    The surface reactivity of three Ti-Nb alloys is presented in this work. The threealloys are a -type titanium alloy with a composition of Ti-30at.% Nb andtwo +-type titanium alloys with compositions Ti-10at.% Nb and Ti-20at.%Nb. The surface reactivity of these alloys is investigated by using two typesof electrochemical cells: double glass-walled electrochemical cell and a scanningdroplet cell.The double glass walled electrochemical cell is used to evaluate the surface re-activity of all the three alloys in an acetate buffer of pH 6.0. The reactivitystudies were carried out by anodizing a 0.3 cm2 sample surface exposed to theelectrolyte solution. The anodization process was carried out by using cyclicvoltammetry electrochemical technique in steps of 1 V so that the mechanismof oxide growth, reactions involved during oxide growth, oxide growth rate andkinetics behind the oxide growth were studied.Electrochemical impedance measuremets right after each anodic oxide growthenabled for the in situ characterization of the oxide/electrolyte interface. Thedielectric number of the respective oxides is thus determined from the capacitivereactance of these oxides obtained from the impedance measurements. More-over, the semiconducting properties of the mixed oxides of titanium and niobiumgrown on these alloys were assessed by using Mott-Schottky analysis where thetype of the semiconductor, charge carrier concentration and flat band potentialof the respective oxides were determined.Surface reactivity of the microstructures such as grain and grain boundarieswas approached by using scanning droplet cell in which the basic idea entailsbringing a small electrolyte droplet as small as 45 m in diameter to the samplesurface so that localized electrochemical measurements can be carried out. Thelocal reactivity of individual grains and grain boundaries is related to the crys-tallographic orientation data obtained from an electropolished sample surfaceusing electron backscatter diffraction (EBSD) technique. From the anodizationand the subsequent electrochemical impedance measuremets, the oxide growthmechanism, formation factor and dielectric number of the oxides grown on dif-ferently oriented grains and grain boundaries were determined. Moreover, thesemiconducting properties of the oxide spots grown on these grains and grainboundaries were also studied using Mott-Schottky analysis to get a comprehen-sive picture of the surface reactivity of -type Ti-30at.% Nb alloy.

  • Contents

    1 Introduction 1

    2 Theory 52.1 Electrochemical oxide layer formation on metals . . . . . . . . . 12

    2.1.1 The electrochemical behaviour of metals . . . . . . . . . 122.1.2 The electrochemical behaviour of Ti and Nb . . . . . . . 16

    3 Experimental 253.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . 25

    3.1.1 Mechanical grinding and polishing . . . . . . . . . . . . . 253.1.2 Electropolishing . . . . . . . . . . . . . . . . . . . . . . . 26

    3.2 Electrochemical characterization . . . . . . . . . . . . . . . . . . 293.2.1 Electrochemical cells . . . . . . . . . . . . . . . . . . . . 293.2.2 Cyclic voltammetry . . . . . . . . . . . . . . . . . . . . . 333.2.3 Electrochemical impedance spectroscopy . . . . . . . . . 363.2.4 Mott-Schottky Analysis . . . . . . . . . . . . . . . . . . 38

    3.3 Electron Backscatter Diffraction (EBSD) . . . . . . . . . . . . . 413.3.1 Qualitative and quantitative representations of crystal ori-

    entation in EBSD . . . . . . . . . . . . . . . . . . . . . . 433.4 Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

    4 Anodic oxides of Ti-30at.%Nb alloy 514.1 Anodic oxides grown in a conventional electrochemical cell . . . 53

    4.1.1 Anodic oxide growth on Ti-30at.%Nb . . . . . . . . . . . 534.2 Microelectrochemistry of single grains of Ti-30at.%Nb . . . . . . 644.3 Microelectrochemistry at grain boundaries . . . . . . . . . . . . 73

    5 Anodic oxides of (+)-type Ti-Nb alloys 835.1 Oxide growth on Ti-10 wt.% Nb and Ti-20 wt.% Nb . . . . . . . 865.2 Oxide characterization by electrochemial impedance spectroscopy 89

    xiii

  • Contents

    5.3 Mott- Schottky analysis of the oxides of (+)-type Ti-Nb alloys 98

    6 Summary 103

    Bibliography 107

    List of Figures 121

    List of Tables 127

    Glossary 129

    Index 133

    Curriculum Vitae 138

    xiv

  • 1 Introduction

    The quest for biomaterials which can be used in different parts of our body assurgical implants involved different kind of materials in the long run. Duringthis time the implantation technology employed three main material systems:316L stainless steel, cobalt-chromium based alloys and titanium and its alloys[13]. The alloys have been used from cranial plates, maxillofacial reconstruc-tion, dental implants, orthopedic fracture plates, joint replacement prosthesesto ablation catheters. The interface between the surgical implants and the sur-rounding biological environment when these implants are placed inside the bodyhave been of interest to the corrosion and material scientists. With this regardelectrochemistry have lend a big hand in studying the performance of such im-plant materials in biological environments. However, the biological environmentin a human body is not a hospitable environment for an implant as it containsa highly oxygenated saline electrolyte of pH of 7.4 at 37 0C. Moreover the pres-ence of aggresive chloride ions, ionic composition and protein concentration inbody fluids complicates the nascent understanding of biomedical corrosion evenfurther.The main prerequisite for a metallic material to be used as surgical implant isexcellent corrosion resistance and biocompatibility so that it exhibits no toxicityto the biological system. Titanium and its alloys fulfill these requirements toa greater extent than stainless steel and cobalt-chromium alloys. Ti-6Al-4V isone of the many titanium alloys used extensively as a biomaterial. However,these alloys have also some drawbacks reported due to the cytotoxicity of theAl and V. Developments in the field thus led to the introduction of alloys con-taining biocompatible elements instead of Al and V [47]. The stabilization ofthe high temperature phase of titanium with elements like niobium would resultin -type titanium alloys which have superior properties over all the predecessorimplant materials for their excellent corrosion resistance, low modulus of elas-ticity and no cytotoxicity against osteoblastic cells. The corrosion resistanceis attributed to the spontaneous formation of a passive oxide film comprised

    1

  • 1 Introduction

    mainly of titanium dioxide and plays an important role in the integration of theimplant with the surrounding tissue as it is the part which comes in contactwith the biological environment instead of the bare metal surface.The titanium dioxide is of main interest in the electronic industry as well forapplications in dynamic random access memory (DRAM) storage capacitors ormetal-oxide-semiconductor field-effect transistor (MOSFET) gate oxides besidesprotecting the substrate metal from corrosion [8].In this work the surface reactivity of -type and (+)-type Ti-Nb alloys havebeen studied using different electrochemical cells. First, the surface reactivityof these alloys were investigated using a double glass-walled electrochemical cellwhich gives information about the overall reactivity of the surface of these alloysduring potential application. The applied electrochemical potential results inoxide growth on the surface of these alloys where the mechanism and kineticsof the growth was studied with the applied potential. Moreover, the electronicproperties of the oxides such as dielectric number, donor concentration and flatband potentials were determined from electrochemical impedance measurementscarried out right after oxide growth with and without application of a bias po-tential.Conventional electrochemical cells such as the double glass walled electrochem-ical cell expose part of the polycrystalline sample to the electrolyte on a cen-timeter scale. The results obtained with such techniques are a net contibutionfrom all the microstructural elements such as grains, grain boundaries etc. incontact with the electrolyte. The individual contributions of the single grainsand grain boundaries thus need microcells with small working electrode areain the micrometer range. For this reason the scanning droplet cell [9] is usedto investigate the local electrochemical response of the single grains and grainboundaries of Ti-Nb -type titanium alloy. The scanning droplet cell is an mi-croelectrochemical cell which entails the idea of bringing a small drop of an elec-trolyte into contact with the sample surface forming a working electrode diam-eter down to 45 m in this work. The wetted area (working electrode) containsreference and counter electrode, enabling to carry out all electrochemical tech-niques such as cyclic voltammetry, transients and electrochemical impedancemeasurements. The local electrochemical reactivity was studied in relation tothe crystallographic orientation of the substrate grains and grain boundaries.The crystallographic orientation of the polycrystalline sample was determinedby using electron backscatter diffraction (EBSD) technique in a scanning elec-

    2

  • tron microscope.Thus this work encompasses an assessment of the macroscopic surface reactivityof -type and (+)-type Ti-Nb alloys together with the electronic propertiesof the oxides on these alloys using a double glass walled electrochemical cell.Moreover, the local electrochemical reactivity of the single grains and grainboundaries of a -type Ti-Nb alloy addresseed using the scanning droplet cellis also presented.

    3

  • 2 Theory

    Named after the Greek mythological dieties Titans, the sons of the earth god-dess, titanium played and still plays an important role in the day to day activitiesof human beings. It is discovered in 1791 by a British geologist William Gregorin Cornwall, England [10]. It is placed in Group IVB of the periodic table ofelements with an electron configuration of [Ar] 3d2 4s2. The unfilled 3d subshellenables titanium to form solid solutions with most elements having a size factorwithin about 20 % and this fact has opened many alloying possiblities for ex-ploitation [11]. The main source for titanium is its oxide ore rutile. In the firststep of the extraction of titanium, the oxide ore is heated with chlorine in thepresence of coke to form gaseous TiCl4:

    TiO2 (s) + (x+ y)C(s) + 2Cl2 (g) TiCl4 (g) + xCO(g) + yCO2 (g)

    The x and y represent variable quantities in which the ratio depends on thereaction temperature which lies in the range 850 to 1000 0C. Liquified TiCl4then undergoes metal displacement reaction when reduced with magnesium inKroll process or sodium in Hunter process in an argon atmosphere:

    TiCl4 (l)+ 2Mg(l) Ti(s)+2MgCl2 (l)TiCl4 (l)+ 4Na(l) Ti(s)+ 4NaCl(l)

    The titanium produced in the two processes is in the form of a highly porousmaterial called titanium sponge where the chloride salts of sodium or magnesiumare entrapped in the pores.Titanium and its alloys exist in different phases depending on temperature,pressure, cooling rate and alloy composition which will result in equilibriumand non-equilibrium phases. Whether equilibrium or non-equilibrium phasesare obtained depends heavily on the the time allowed to reach steady state

    5

  • 2 Theory

    conditions by minimization of the Gibbs free energy for phase stability. Non-equilibrium phases with a high value of Gibbs free energy can be formed whenthe heating or cooling rate is high enough to cause a cooperative movementof atoms by shuffling or shear type displacive transformation which results ina homogeneous transformation from the body centered cubic lattice into thehexagonal crystal lattice over a given volume. The phase with higher Gibbs freeenergy can eventually transform to stable phases under favourable conditionsby lowering their Gibbs free energy. The most common equilibrium phasesof titanium and its alloys are - and -phase. The -phase of titanium ischaracterized by hexagonal close packed hcp crystal structure. The hexagonalunit cell of titanium is shown in Fig. 2.1 where the lattice parameters a and chave values of 0.295 nm and 0.468 nm respectively at room temperature. Thec/a ratio is then 1.587 compared to the ideal value of 1.633 for the hexagonalclose packed unit cell.

    Figure 2.1: The hexagonal close packed and body centered cubic unit cells of the -and -phase of titanium. (Figure taken from [12]).

    The hexgonal close packed unit cell has three most densely packed latticeplanes; the (0002) basal plane, one of the three {1 0 1 0} which are calledprismatic planes and one of the six {1 0 1 1} planes called pyramidal planes.The three a1, a2 and a3 axes with indices are the close packeddirections. Fig. 2.1 shows the body centered cubic bcc unit cell of the -phase oftitanium indicating the {1 1 0} lattice plane together with the lattice parameterfor pure titanium at 900 0C. The four vectors are the most closepacked directions in the bcc unit cell (see section 3.3.1 for vector and planes) [12].

    6

  • Figure 2.2: Schematic illustration of the effect of alloying elements on the phase di-agrams:(a) stabilizing element (b) eutectoid and (c) isomorphous stabi-lizing elements where is an intermetallic compound and the -transusis for pure titanium [13].

    In Ti based alloys a very important effect of an alloying element pertains to themanner in which its addition affects the allotropic - to -phase transformationtemperature. For pure titanium the allotropic transformation between the- and -phase occurs at 882 0C. The effect of the alloying elements lies onraising or lowering the transformation temperature. Elements which raisethe transformation temperature or bring about little changes in it whendissolved in titanium are referred as stabilizers. These elements are generallynon-transition metals or interstitial elements such as Al, B, Sc, Ga, La, Ge, C,O and N. On the other hand, elements like Nb, V, Mo, Hf, Ta, Mn, Cr, Fe,Cu, Au, Ag and W reduce the transformation temperature and stabilize thecubic crystal strucure and thus are referred to as stabilizers. stabilizingelements are usually transition elements and noble metals with unfilled orjust filled d-electron bands. The stabilizing elements are further classifiedinto isomorphous (Nb, V, Mo, Hf, Ta) and eutectoid (Mn, Cr, Fe, Cu,Au, Ag, W) depending on whether or not a solid solution/eutectoid exists atsufficiently elevated temperature. Hydrogen is a stabilizing element amongthe interstitial elements. In titanium alloys the single phase and the singlephase regions are separated by a two phase (+) region in the temperatureversus composition phase diagram. The width of this region increases withincreasing solute content. The single equilibrium - to -phase transformationtemperature associated with the elemental pure titanium is replaced by twoequilibrium temperatures in the case of an alloy: the transus temperature,

    7

  • 2 Theory

    below which the alloy contains only the -phase, and the -transus temper-ature, above which the alloy contains only the -phase. At temperaturesbetween these two temperatures, both the - and -phases are present. Fig. 2.2shows a simple phase diagram where the effect of and stabilizing elementson the relative stabilities of the - and -phases is illustrated [11, 13].The main non-equilibrium phases in titanium alloys are , and . The

    is a metastable hexagonal martensitic phase that is formed as a result of a veryrapid rate of cooling from the high temperature phase. The martensiteis observed only in pure titanium, in alloys with low solute content and inalloys with high transformation temperature. However, the phase whichhas an orthorhombic crystal structure is formed by martensitic transformationof alloys with high solute content. Unlike the martensite, application ofan external force can cause stress or strain induced martensite besides therapid quenching. The martensitic transformation temperature for pure titaniumis around 850 0C and increases with increasing amount of stabilizer anddecreases with increasing content of stabilizing element in the alloy. The lastnon-equilibrium phase to be discussed here is the phase which could be oftwo types: athermal and isothermal phase. The transformation that involvesdecomposition of the phase by suppressing the martensitic reaction whichresults in either or upon quenching athermally results in an athermal phase. Thus during the formation of the phase shuffling reaction takesplace instead of a martensitic shear transformation. The athermal phase hasa trigonal symmetry in heavily stabilized alloys and a hexagonal symmetry(not hexagonal close packed) in leaner alloys [12]. The isothermal phase isformed during aging of alloys with high solute content in the temperature rangeof 100 to 500 0C and depends heavily on the hold time and the cooling ratewhere the volume fraction of the isothermal phase increases with increasinghold time and decreasing cooling rate [11]. The isothermal phase has thesame crystallographic symmetry as the athermal phase.Titanium alloys which contain - stabilizing and/or -stabilizing elements arebroadly classified into -, (+)- and -type alloys. The (+)-type alloysare further classified into near and near type alloys depending on theircomposition which will place them near the /(+) or the (+)/ phaseboundaries, respectively. -type titanium alloys consist of pure titanium andits alloys which consist of one or more -stabilizing elements where the -phaseis the predominant or the only phase present. (+)-type titanium alloys

    8

  • however, consist a mixture of the - and -phases. These alloys thus consist ofone or more of the - as well as -stabilizing elements alloyed with titanium.In -type titanium alloys, the phase is stabilized by the addition of adequateamounts of -stabilizing elements and can be retained at room temperature[14].Owing to the high corrosion resistance coupled with its exceptional highstrength to weight ratio, titanium and its alloys have found wide spreadapplications in different sectors of application materials since their commercicalinception in the early 1950s. From that time onwards titanium and itsalloys were used in aerospace applications such as jet engines and air frameswhere high temperature performance, creep resistance and superior strengthare needed together with a relatively light weight. In jet engines wide chordtitanium fan blades increase efficiency by reducing noise. Moreover, the use oftitanium and its alloys in place of steel and nickel alloys for landing gear andnacelle applications proved to reduce weight and improve aircraft efficiency.Titanium has been proven by the power industry to be the most reliable surfacecondenser tubing material because of the corrosion resistance that titaniumexhibit to all the processes happening during condenser operation which was aproblem when other metals with less corrosion resistance were used resulting ina damage or a threat to the operational efficiency.Besides having excellent corrosion resistance and high strength to weight ratio,titanium and its alloys have to fulfill additional requirements when intendedto be used as a biomaterial for load bearing surgical implants such as hipand knee prostheses. The excellent corrosion resistance of titanium and itsalloys in aqueous environment is due to the spontaneous formation of a passiveoxide film which mainly consists of titanium dioxide. The physilogical andelectrochemical properties of the passive oxide film and its stability in biologicalenvironments for a very long time plays a decisive role for the biocompatibilityof titanium surgical implants. The passive oxide film which is normally a fewnanometer thick is very stable in vitro [1517]. However, the surface reactivityof titanium implants in vivo showed a marked difference because the passivefilm formed on such kind implants can reach a thickness beyond the nanometerrange after some years inside the human body. During this time not only isthe thickness of the oxide changes but also the composition of the oxide will bechanged due to incorporation of mineral ions from the surrounding biologicalenvironment [15, 18, 19].

    9

  • 2 Theory

    The additional requirements involve properties like high fatigue resistance, lowmodulus of elasticity and low toxicity to name a few. In this regard -typetitanium alloys have proven to be superior over the conventional titanium and(+)-type titanium alloys. Alloying of titanium with biocompatible elementssuch as Nb, Ta, Zr and Mo to stabilize the bcc phase will result in -typetitanium alloys which will fulfill the additional requirements mentioned aboveto be used as a biomaterial. Experimental investigation on such kind of alloysrevealed that these alloys do not show any toxicity against osteoblastic cells,excellent corrosion resistance and good mechanical properties compared to thepredecessor titanium biomaterials. Cytotoxicity is the main problem for otheralloys which have been used as a biomaterial. Among these alloys the mostwidely used titanium surgical implant Ti-6Al-4V exhibited high cytotoxicitydue to the realease of Al and V which may cause neurological disorders andalergic reactions [4, 20]. The other alloys which have been used as biomaterialwere Ti-Ni alloys as orthodontic archwire, endodonthic file and cardiovascularstent due to their shape memory effect and superelasticity. However, the use ofsuch kind of alloys is hampered by their corrosion properties in the human bodydue to the release of metal ions and the risk of Ni induced hypersensitivity [21].The mechanical properties advantage is imparted by the low modulus ofelasticity that -type titanium alloys have compared to the -, (+)-typetitanium alloys and other biomaterials. The modulus of elasticity of some of thealloys used as biomaterials such as pure titanium (105 GPa), Ti-6Al-4V (110GPa), 316L stainless steel (200 GPa) and Co-Cr-Mo (210 GPa) is 4 to 7 timeshigher than the human cortical bone which has a modulus of elasticity around30 GPa. By using -type titanium alloys which has a modulus of elasticityaround 60 GPa the disparity observed in the modulus of elasticity between thehuman bone and the surgical implant can be reduced significantly. The modulusof elasticity is of particular relevance because an important requirement ofa bone replacing surgical implant is to have a modulus of elasticity value asclose as possible to that of the surrounding bone tissue. The principle behindthis is the stress shielding effect. Since bone is a living material subjected tomechanical load emanating from the weight it carries and the motion it creates.A mismatch in elasticity observed when an elastically much stiffer surgicalimplant is placed in place of a bone in the human skeleton system createsuneven distribution of the physiological load. Such uneven distribution in themechanical load shields the bone from the mechanical load that surrounds

    10

  • the surgical implant. The shielding of the bone from the physiological loadresults in resorption of the bone which will lead to undesirable effects such asa decrease in bone density, mineralization state and health. The subsequenteffect would be tissue resorption which increases the danger of formation andmigration of wear debris at the bone/implant interface via body fluid transport.Thus, stress shielding and the subsequent tissue resorption may eventuallyresult in contact loosening, premature implant failure and/or infections inducedby debris transport from the bone/implant system to other parts of the bodyvia biological fluids [22].

    Figure 2.3: The Ti-Nb phase diagram [23].

    Among these type titanium alloys the Ti-Nb system have drawn considerableattention as a biomaterial due to the biocompatibility nature, excellent corro-sion resistance, low elastic modulus and better shape memory effect [2429].The shape memory effect in Ti-Nb alloys results from reversible transformationbetween the martensitic phase and the austenitic phase. Quenching ofbinary Ti-Nb alloys results in a metastable martensitic phase which has thesame hcp structure as the pure unalloyed titanium with the lattice parametersa 0.295 nm and c 0.468 nm, for Nb concentration upto 12 wt.%. Beyondthis concentration, the hcp undergoes rhombic distortion which would resultin an orthorhombic crystal structure with lattice parameters of a 0.313 nm,

    11

  • 2 Theory

    b 0.482 nm and c 0.282 nm of the martensite phase. A hexagonalmetastable phase with a 0.465 nm and c 0.282 nm also appears in the matrix where the bcc structure has a 0.331 nm during quenching. Quenchingof Ti-Nb alloys where the Nb content is above 42 wt.% tends to transform intosingle phase [2937]. The phase diagram of the Ti with Nb which is one ofthe isomorphous stabilizing elements is shown in Fig. 2.3.

    2.1 Electrochemical oxide layer formation onmetals

    2.1.1 The electrochemical behaviour of metals

    Exposure of most metals to an aqueous electrolyte makes a corrosion cell con-taining an anodic site where oxidation of the metal will occur and a cathodic sitewhere either reduction of dissolved oxygen or evolution of hydrogen gas takesplace as given in the following reactions:

    M M z+(aq) + ze

    2H+ + 2 e H2O2 + 4H+ + 4 e 2H2O

    The anodic reaction results in a dissolution of the metal as metallic ions inthe electrolyte or the conversion of these ions into insoluble corrosion products.This destroys the metal and is referred to as corrosion. The active dissolutionof the metal can be hampered if the anodic reaction involved a spontaneousformation of a dense oxide film having limited conductivity as given in thefollowing reaction:

    M + z2H2O MO z2 + zH+ + ze

    The formation of such kind of oxide film with low conductivity will passivatethe substrate metal from further electrochemical reactions. The passive oxidefilm can be stabilized or dissolved by various kinds of reactions which involveion transfer reaction (ITR), electron transfer reaction (ETR) or a combination

    12

  • 2.1 Electrochemical oxide layer formation on metals

    Figure 2.4: Schematic diagram of the processes taking place at the oxide film oftitanium [38, 39].

    of the two. The schematic diagram of the reactions taking place at passivatedtitanium is shown in Fig. 2.4.

    The passivation reaction given above involves two half ion transfer reactions:

    M M z+(ox) + ze

    which take place at the metal/oxide interface and:

    H2O OH ox + H+

    at the oxide/electrolyte interface. The subsequent process involves migrationof cations and/or anions within the oxide. The electrochemical reactions might

    13

  • 2 Theory

    be steady state reactions which take place at constant oxide film thickness orinstationary processes involving oxide growth, corrosion or oxide reduction ormodification. For example, if the only reactions taking place at constant oxidefilm thickness involve the above two half reactions and an ion transfer reaction:

    M z+(ox) M z+(aq)

    the net reaction would be the active corrosion of the metal which falls to asteady state reaction since the reaction takes place at constant film thickness.In contrast, reaction which involve redox reaction:

    MO y2

    + ( (zy)2 )H2O MO z2 + (zy)H+ + (zy)e

    or hydration of the oxide:

    MO z2

    + z2H2O M(OH)z

    are examples of instationary processes. Moreover, the oxide might undergoreduction into the metal or other soluble product as shown in the followingreactions:

    MO z2

    + zH+ + ze M + z2H2OMO z

    2+ zH+ + (zy)e M y+ + z2H2O

    The current measured under electrochemical conditions however is the totalcurrent from all the half reactions taking place in the metal/oxide/electrolytesystem shown in Fig. 2.4 and the capacitive charging (iC) and is given by:

    i = iox + iredox + icorr + iC (2.1)

    where iox is the oxide formation current, iredox is the current for redox reactionsinvolving hydrogen or oxygen evolution, icorr is corrosion current and iC is dueto capacitive charging.

    Among the reaction involved in a metal/oxide/electrolyte system, the elec-tron transfer reactions are of particular interest in corrosion and electrochemicaltechnology as the formation, reduction and modification of oxide films involvessuch kind of reactions. For electron transfer reaction to take place the oxide film

    14

  • 2.1 Electrochemical oxide layer formation on metals

    must exhibit electronic conductivity. Electron transfer reactions are consideredas elastic (isoenergetic) exchange of electrons between occupied and empty en-ergy states in the oxide and electrolyte, respectively. Unlike metals where theelectron transfer takes place at the Fermi level, electron transfer reactions inoxide films take place at higher or lower energy states as shown schematicallyfor different systems in Fig. 2.5.

    Figure 2.5: Schematic diagram of the different mechanisms of electron exchange be-tween redox systems:(a) H2 evolution on stable oxide (TiO2, Nb2O5)(b) Reduction before hydrogen evolution (PtO, Bi2O3)(c) Oxidation before oxygen evolution (Cu2O)[38, 39].

    Electron transfer reaction can take place via the conduction band or valenceband or between the metal and the electrolyte. The ETR via the conductionband takes place at the oxide surface via the edge of the conduction band or bydirect or resonance tunneling to the conduction band of the oxide. The ETRvia the valence band at the surface with following tunnel process to the metalor via hole diffusion to the valence band. The ETR between the metal and theelectrolyte takes place by resonance or direct tunneling [38]. The later happensfor suffiently thin oxide films ( 2-3 nm) so that electrons can be exchangedbetween the metal and the electrolyte. When the oxide film is too thick forelectron tunneling to take place, the intrinsic electronic properties of the oxidefilm dictate the kinetics of the electron transfer at the oxide film/electrolyteinterface. The electronic properties of the oxide films can vary greatly fromone oxide to the other. The energetics of the valence and conduction bandsof different oxides relative to the vacuum level is given in Fig. 2.6. Thus the

    15

  • 2 Theory

    mechanism of the ETR depends on the band structure of the oxide, the filmthickness and the electrode potential.

    Figure 2.6: Band energetics of the different oxides. (Figure taken from [38]).

    2.1.2 The electrochemical behaviour of Ti and Nb

    Titanium and niobium, the two metals involved in this work, are not corrosionresistant in the same way as gold and other noble metals are. The corrosionresistance of these two metals relies on the formation of a strongly protectiveoxide film that prevents the bare metal from further reaction. The oxide filmis formed spontaneously as it is evident from the negative value of the Gibbsenergy of formation of the corresponding oxides in water and oxygen as shownin the following reactions. The Gibbs energy values were calculated consideringatmospheric conditions having a relative humidity of 80 %, T = 298 K, pO2 =21.28 kPa, pH2O = 2.57 kPa and pH2 = 6.24 1037 Pa [4042].

    Ti + O2 TiO2 G = -885.6 kJ mol1

    Ti + 4H2O TiO2 + 4H2 G = -884.6 kJ mol1

    2Nb + 52O2 Nb2O5 G = -1768.4 kJ mol1

    2Nb + 5H2O Nb2O5 + 5H2 G = -1766.8 kJ mol1

    An understanding of the corrosion behaviour of titanium and niobium andthe thermodynamic stability of the corresdponding oxides of the two metals

    16

  • 2.1 Electrochemical oxide layer formation on metals

    can be obtained from their Pourbaix diagrams. Fig. 2.7 and 2.8 represent thePourbaix diagram of titanium and niobium respectively at 25 oC. The Pourbaixdiagram of titanium is established by considering the anhydrous oxides Ti2O3and rutile TiO2. The reactions and the corresponding equations for the relativestability of the titanium and niobium and their oxides the following equationswere considered:

    Ti + H2O TiO +2H+ + 2 e E = -1.306 - 0.0591 pH2TiO + H2O Ti2O3 +2H+ + 2 e E = -1.123 - 0.0591 pHTi2O3 + H2O 2TiO2+2H+ + 2 e E = -0.556 - 0.0591 pHNb + H2O NbO +2H+ + 2 e E = -0.733 - 0.0591 pHNbO + H2O NbO2 +2H+ + 2 e E = -0.625 - 0.0591 pH2NbO2+ H2O Nb2O5+2H+ + 2 e E = -0.289 - 0.0591 pH

    The following reactions and the corresponding equations are for the corrosionand solubility of titanium:

    Ti 2+ + H2O TiO + 2H+ log[Ti2+] = 10.91 - 2pHTi Ti 2+ + 2 e E = -1.63 + 0.0295log[Ti2+]

    2Ti 2+ + 3H2O Ti2O3 + 6H+ + 2 e E = -0.478 - 0.1773pH - 0.0591log[Ti2+]Ti 2+ + 2H2O TiO2 + 4H+ + 2 e E= -0.502 - 0.1182pH - 0.0295log[Ti2+]

    Figure 2.7: Pourbaix diagram of titanium in water at 25 oC [43].

    17

  • 2 Theory

    Figure 2.8: Pourbaix diagram of niobium in water at 25 oC [43].

    From Fig. 2.7 and 2.8 it is seen that both titanium and niobium apear to be basemetals as their domain of thermodyamic stability does not have any portion incommon with the domain of the thermodynamic stability of water and lies atpotentials well below the potential which correspond to the stability of water.Titanium monoxide (TiO) is unstable in the presence of water and dissolves re-sulting in the evolution of hydrogen in the presence of acidic solutions free fromoxidizing agent. Likewise, titanous oxide (Ti2O3) dissolves in acidic solutionsforming titanous ion (Ti3+). In some cases the dissolution is accompanied bythe reduction of the acid. Moreover, the Pourbaix diagram of titanium showsthat the domain of stability of rutile TiO2 cover the domain stability of waterwhich shows that TiO2 is thermodynamically stable in water or aqueous solu-tions. Thus the TiO2 is the most stable of the oxides of titanium which is thereason behind the protection of titanium from further electrochemical reactiononce it is formed spontaneously on the surface. Similarly, niobium monoxide(NbO) and niobium dioxide (NbO2) are thermodynamically unstable in aqueoussolutions of any pH as the domain of stability of the two oxides lies below theline which corresponds to the equilibrium of the reaction of the reduction ofwater to hydrogen at atmospheric pressure. These oxides become oxidized tohigher oxides when they react with water resulting in the evolution of hydrogen.Niobium pentoxide (Nb2O5) is the thermodynamically stable oxide of niobiumin water, non-complexing acid, alkaline and neutral solutions as its domain ofstability covers the entire domain of stability of water.

    18

  • 2.1 Electrochemical oxide layer formation on metals

    High field model

    Oxide formation on valve metals like Ti and Nb follows the high field mech-anism where the development and the concept of the model will be discussedthroughly in this subsection. Valve metals bear the name because they do notpass current in both directions (current rectification). The current rectificationcan be explained for the cathodic and anodic current where the cathodic cur-rent is almost zero and anodic current is recorded when the potential appliedexceeds the oxide formation potential of the oxide film present on the surface ofthe metal. The anodic oxide formation on valve metals is governed by the highfield equation [44] :

    i = i0 exp(E) (2.2)

    where i is the oxide formation current, i0 and are material dependent constantsand E is the electric field strength inside the oxide. An ideal valve metal canbe described by the following points:

    The metal surface is covered by a 2-5 nm thick native oxide from air orelectrolyte passivation

    Anodization increases the oxide thickness

    The oxide layer is not reduced by cathodic current

    The oxide has a small ionic conductivity in steady state conditions or atpotential lower than the oxide formation potential of the oxide alreadypresent on the surface of the metal

    Al, Bi, W, Ti, Nb, Ta, Hf and Zr are examples of typical valve metals.Gntherschulze and Betz [45] forwarded the first oxide formation models in1934 from their work on the anodization of Al and Ta where they reported thedependence of the oxide formation current i on the electric field strength E asgiven in eqn. (2.2). Verwey [46] reported the oxide formation involved cationsreleased from the metal electrode which jump from one interstitial position to thenext because of the high electric field strength present within the oxide. the nextcontribution comes from Mott and Cabrera [4749] where their model assumesthe movement of cations by a thermally activated field assisted ion hoppingmechanism like Verwey suggested. The combination of the theories and modelsproposed by Guntherschulze, Betz, Verwey, Mott and Cabrera is referred to asthe high field model. The model is based on the ion hopping mechanism where

    19

  • 2 Theory

    ions positioned at regular sites or interstitial positions hop along a distance a tovacancies and interstitial positions located in their neighbourhood as shown inFig. 2.9. The hopping distance a is typically a lattice parameter. The high fieldmodel neglects the quantum effect associated with the ion hopping mechanismdue to the high mass of the ions. Thus the model will be discussed soley fromthe mechanistic view point. In order for the hopping of ions to undergo anapplication of an activation energy A would be necessary.

    Figure 2.9: Figure taken from [42] showing the ion hopping between two adjacentlattice planes within the oxide film.

    Figure 2.10: A plot showing the effect of the electric field strength on the activationenergy of hopping ions.

    The derivation of the high field equation starts by assuming to lattice planesseparated by a distance a at x and x+a positions as shown in Fig. 2.9. Thenumber of moles of mobile species in 1 cm2 of the lattice planes placed at xand x+a respectively is designated by nx and na+x. Thus the amount of ions

    20

  • 2.1 Electrochemical oxide layer formation on metals

    exchanged in a certain time interval can be written as the difference betweenthe transfer rates of the ions to the left and to the right:

    dndt =

    dndt

    dndt = nxP nx+aP (2.3)

    where P is the transition probability given by the relation:

    P = exp( ART ) (2.4)

    where is the attempt frequency where the ion is treated as a harmonic oscilla-tor, R is the gas constant and T is absolute temperature. The activation energyfor ion hopping in both directions is equal in the abscence of an electric fieldas shown in the symmetrical activation energy curve of Fig. 2.10. However, theactivation energy for the ion hopping in opposite directions become differentwhen an electric field is applied. This results in a change in the shape of theactivation energy curve as shown in Fig. 2.10. Thus in the presence of an elec-tric field the activation energy depends on the ion hopping direction. It needs asmaller activation energy when the electric field supports the ion hopping andhigher in the opposite direction. The activation energy for the forward andreverse directions is thus given by:

    A = A azFE (2.5)

    A = A+ (1 )azFE (2.6)

    where is the transfer coefficient which explains the symmetry of the activationenergy barrier, z is the charge number of the mobile species and F is the Faradayconstant. Introducing the ion concentration c as:

    c = na

    (2.7)

    and the concentration at the position x+a can be calculated from the concen-tration gradient dc/dx as:

    cx+a = cx +dcdxa (2.8)

    Inserting the corresponding terms of eqn. (2.7) and (2.8) in eqn. (2.3):

    dndt = acxP acx+aP = acxP a

    (cx +

    dcdxa

    )P (2.9)

    21

  • 2 Theory

    Inserting the corresponding transition probability terms of eqn. (2.4) ineqn. (2.9) yields:

    dndt = a

    [cxexp

    (ART

    )(cx +

    dcdxa

    )exp

    (ART

    )](2.10)

    Substituting the corresponding relation of the activation energy for the forwardand reverse hopping:

    dndt = a exp

    ( ART

    ) [cx exp

    (azFERT

    )(cx +

    dcdxa

    )exp

    ((1 )azFE

    RT

    )](2.11)

    In the absence of an electric field (E=0) eqn. (2.11) reduces to:

    dndt = a

    2 exp( ART

    ) dcdx (2.12)

    Eqn. (2.12) corresponds to Ficks first law of diffusion where the diffusion fluxis driven by concentration gradient (dc/dx) in the direction from the higherconcentration to the lower direction where the diffusion coefficient D is givenby:

    D = a2 exp( ART

    )(2.13)

    However, when an electric field is applied the ion transport is assisted by the fieldand the effects from diffusion due to concentration gradient will be supressed.Thus the case becomes electrochemical in nature where the dn/dt term willbe replaced by current density i and the charge number z by mobile chargeconcentration given in Coulomb per unit volume as:

    i = zFdndt (2.14)

    = czF (2.15)

    In the absence of diffusion eqn. (2.11) can be rewritten as:

    i = a exp( ART

    )exp

    (azFERT

    )[1 exp

    (azFERT

    )](2.16)

    The current density i depends linearly on the electric field strength and alsothe applied potential like Ohms law as far as the thickness d remains constant

    22

  • 2.1 Electrochemical oxide layer formation on metals

    for low values of the electric field strength and eqn. (2.16) thus reduces to eqn.(2.17) if azFE/RT < 1:

    i = a exp( ART

    )azFERT (2.17)

    In the case where the electric field strength is high enough to make ion transferin the opposite direction to what is assumed to be in eqn. (2.3) where the effectfrom diffusion is supressed then eqn. (2.16) can be rewritten as:

    i = a exp( ART

    )exp

    (azFERT

    )(2.18)

    Eqn. (2.18) is equivalent to the experimentally determined high field equationeqn. (2.2) which relates the current with the electric field during anodic oxidegrowth on valve metals [50] where:

    i0 = P exp( ART

    )(2.19)

    = azFRT (2.20)

    Taking the natural logarithm of eqn. (2.2) will modify it to a Tafel equationwhere the material dependent constant can be determined from the slope ofthe Tafel plot assuming constant layer thickness d:

    ln i = ln i0 + E (2.21)

    The ion hopping mechanism assumes the ion transfer between the lattice planesin Fig. 2.10a to take place regardless of the position where the lattice planesare located within the metal/oxide/electrolyte system. The planes might rep-resent either the metal/oxide interface or the oxide/electrolyte interface orplanes located in random position with the oxide film. Experiments fulfillingeqn. (2.2) cannot determine where the rate determining step is located withinthe metal/oxide/electrolyte system. Previous arguments as to the location ofthe rate determining step is located is proposed by different authors. Gther-schulze and Betz [45] proposed for the rate determining step to be located withinthe oxide, Mott and Cabrera [49] assumed it to be at the metal/oxide inter-face and Fehlner and Mott [51] assumed to be at the oxide/gas interface inmetal/oxide/oxygen gas system. However, transient experiments indicate the

    23

  • 2 Theory

    rate determining movement of ions is located within the oxide and also the valid-ity of eqn. (2.2). This infers that the processes taking place at the metal/oxide,oxide/electrolyte or oxide/gas interface are not rate determining step. Secondly,the oxide is homogeneous and hence the electric field strength within the oxide isconstant. The constant field strength thus can be calculated from the potentialdrop E within the oxide and the layer thickness:

    E = Ed

    (2.22)

    Thus eqn. (2.2) can also be rewritten as [44]:

    i = i0 exp(E) = i0 exp(Ed

    ) (2.23)

    24

  • 3 Experimental

    3.1 Sample preparation

    The first crucial step in studying the surface reactivity of a material involvespreparation of a sample surface following certain procedures that will ultimatelyresult in the true microstructure of the surface. The microstructure of the sur-face of Ti alloys in this work is revealed by Electron Backscattered Diffraction(EBSD) (explained in detail in section 3.3). The EBSD technique relies heavilyon the information obtained from the very top surface of crystalline samples.Thus sample surfaces must be as defect free as possible to obtain good results.The most critical defects for EBSD are roughness, deformation of crystal struc-ture through mechanical damage, surface contamination, surface relief, residualstrains, etc. [52]. The sample preparation techniques and procedures followedin this work are described in the following subsections.

    3.1.1 Mechanical grinding and polishing

    Silicon carbide, aluminum oxide, emery, diamond and boron carbide are someof the many grinding and polishing abrasives that can be used for grinding andpolishing of sample surfaces in metallography. Graded abrasive is bonded toa paper or cloth in a variety of forms, for example, as sheets, belts or discsof varying size. Alternatively, loose abrasive particles can be applied to a lapfor grinding. Each abrasive size and type produces different scratch and de-formation depth on different samples. Silicon carbide is the most widely usedabrasive used in metallographic laboratories due to its hardness (Mohs 9.5),cost and excellent cutting characteristics. The surface of the Ti-Nb alloys weresuccesively ground with silicon carbide papers, starting with a coarse abrasive of220 grit and then progressing through to finer abrasives of 2500 and 4000 grit in

    25

  • 3 Experimental

    the presence of flowing water. The water helps to minimize the heat produceddue to friction between the sample surface and the abrasive which might havean effect in altering the microstructure of the sample surface. Moreover, thewater helps to reduce metal entrapment between abrasive particles (clogging)and avoids smearing and burnishing and thereby increase the cutting efficienyby exposing more of the abrasive to the sample surface[53, 54].The next step after grinding is polishing. Polishing involves sprinkling or spray-ing of polishing abrasives on to a polishing disc which can be made of differentkind of clothing to produce a flat and reasonably scratch free surface with highreflectivity. Polishing can also be classified as coarse or fine. Coarse polishinguses abrasive size in the range of 30-3 m, while fine polishing involves abrasiveswith a size of 1 m and below. Coarse polishing is usually done using diamondpaste or aerosol together with a lubricant and final polishing mostly employsabrasives like aluminum oxide, magnesium oxide and colloidal suspension of sil-icon oxide[54, 55].The Ti-Nb sample surfaces which were ground with 2500 and 4000 grit siliconcarbide papers were then polished with 40 nm silicon oxide abrasive (Struers)on a nylon cloth disc (Struers) to remove the damage from the grinding processand thereby getting a smooth surface with less deformation to the crystal struc-ture of the sample surface. One fraction by volume of 30 % hydrogen peroxideis added to five parts of colloidal silica solution for polishing of the + typeTi-Nb alloys for better results. The last seconds of the polishing procedure weredone in the presence of a running water to clean the sample surface from silicaresiduals.

    3.1.2 Electropolishing

    Electropolishing is an electrochemical sample preparation technique to obtaina sample surface smooth enough and free from lattice deformations and hencereveals the true microstructure of the sample surface. Such kind of surface isattained by removal of the outer most surface of a metal/alloy in an electrolytesolution under controlled current, voltage, electrolyte temperature and stirringrate of the electrolyte. The electrolyte composition is selected based on theelectrochemical activty of the metal/alloy components and on the solubility ofthe salts of the metal or components of the alloy [56]. Fundamental aspectsof electropolishing were reviewed by Landolt [57]. Electropolishing of titanium

    26

  • 3.1 Sample preparation

    and other valve metals can be carried out in an acidic media containing fluorideor perchlorate ions, a typical example being concentrated acetic acid-perchloricacid solution [58, 59]. The use of such kind of electrolytes, however, posed asafety hazard. A common feature of the electrolytes used for electropolishingis the presence of aggressive ions in order to facilitate the attack of the passiveoxide films and a relatively low water content. The presence of a high amountof water increases the chemical stability of the passive film on titanium andvalve metals in general and thus prevents electropolishing.

    Figure 3.1: Schematic diagram of mass transport mechanisms involving:(a) anodically formed metal ions Maq - salt film mechanism(b) acceptor anion A or(c) H2O as transport limiting species. MAy is a complex ion and Csat issaturation concentration [57].

    Electropolishing usually involves anodic dissolution at a limiting current and ismass transport controlled process. The mechanism of electropolishing can beexplained by either of the two theories: salt film and acceptor mechanism. Insalt film mechanism, at the limiting current a thin salt film is present on the an-ode and the rate of the reaction depends on the rate at which the metal ions are

    27

  • 3 Experimental

    transported away from the sample surface into the bulk of the electrolyte. Theacceptor mechanism instead assumes diffusion limited transport of an acceptor(water or complexing agent) necessary for the solvation [60, 61]. Fig. 3.1 showsthe schematic representation of the transport mechanisms involved where in allcases mass transport being the rate determining step.A perchlorate ion free electrolyte such as sulfuric acid-methanol solution havebeen reported for electropolishing of titanium and its alloys [62, 63]. Effectivepolishing of titanium and its alloys was obtained in a 3 M sulfuric acid- methanolsolution using 8 V applied potential. Mass transport controlled limiting currentplateaus were observed for sulfuric acid concentration in the range of 2 - 4 M.In this work as well a 3 M methanolic sulfuric acid solution was used for elec-tropolishing all the Ti-Nb alloys. The electropolishing experiments were donepotentiostatically at 8 V using a DC power supply, a platinum sheet as cathodein an electrolyte stirred with a magnetic stirrer at 100 rpm and temperaturerange of 251 - 263 K. For achieving the desired electrolyte temperature andkeeping it constant through out the experiment two types of cryostats (LAUDARC 20-CP and JULABO FT 901) were used.Ultrasonic baths were used in order to effectively clean the sample surfaces

    from leftovers of the sample preparation procedures. Ultrasonic cleaning resultsfrom cavitation, a phenomenon that occurs in many liquids. When an ultrasonicwave passes through a liquid it produces fluctuations in hydrostatic pressure. Anacoustic wave consists of an alternating pressure front that moves at a particularvelocity in the liquid. In an ultrasonic cleaner, a standing wave is produced as aresult of reflections, leading to a stationary wave front. However, the alternatingpressure phase persists. Most liquids exhibit an amplitude threshold level abovewhich cavitation occurs. The frequency and amplitude of the acoustic wave andcleaning medium all influence cleaning action[53]. All the samples were cleanedin an ultrasonic bath of ethanol and distilled water to remove remainings ofthe sample preparation procedures from the samples surface. Finally, the sam-ple surface was rinsed with ethanol and dried with compressed nitrogen gas tocomplete the sample surface preparation.

    28

  • 3.2 Electrochemical characterization

    3.2 Electrochemical characterization

    3.2.1 Electrochemical cells

    Double glass-walled electrochemical cell

    A double glass walled electrochemical cell with seven outlets as shown in Fig.3.2was used for carrying out the electrochemical measurements. From the sevenoutlets two of them were used to insert the gas inlet used for purging the elec-trolyte with an inert gas like argon or nitrogen to drive the dissolved oxygenout from the electrolyte and a gas outlet. A gold plate of 2 cm2 area counterelectrode and Ag/AgCl/3 M KCl (Metrohm) reference electrode placed insidea luggin capillary were inserted through the other two inlets. The sample forthe experiment is placed in a sample holder made of teflon exposing an areaof 0.3 cm2 to the electrolyte. It is inserted through the centraal inlet into theelectrochemical cell. The electrochemical cell has also two inlets besides theseven outlets which can be connected to cryostat to keep the temperature ofthe electrolyte inside the electrochemical cell constant by circulating a coolingsolution between the glass walls.

    Figure 3.2: Picture of the double glass-walled electrochemical cell.

    29

  • 3 Experimental

    Scanning droplet cell

    Conventional electrochemical cells like the double glass walled cell explainedabove are well suited for a relatively big working electrode area. Many effortswere made in the past in reducing the working electrode area using differenttechniques. Of these attempts the first main effort involves the usage of ultra-microelectrodes with different geometries and critical dimension in the microm-eter range for voltammetric and polarographic measurements [64, 65]. Otherapproaches included covering the sample surface partially with insulating film[6467] or completely with photoresist materials and produce the microelec-trodes by using a radiation source [68, 69]. However the usage of such kind ofmasks might result in undesirable effects like surface modification of the sampleunder consideration. The effort to minimize the working electrode area laterinvolved different probe techniques such as scanning microelectrodes [70, 71]especially the scanning electrochemical microscope [7276], ion sensitive cap-illary electrodes [77] scanning tunnel microscope (STM) [78, 79] and Kelvinprobe [80, 81]. The probe techniques showed impressive resolution to analyzestructured surfaces, but lack the wide spectrum of the conventional macroscopicmethods [82].The limitations mentioned above for the different microelectrodes is overcomedby positioning a tiny electrolyte droplet at the tip of a capillary based microcelland making a contact with the sample surface with it. The wetted area from thecontact of the tiny droplet with the sample surface defines the working electrodearea. The droplet formed between the capillary tip and sample surface is heldby its own surface tension. The droplet can also be moved across the sample sur-face and thus referred Scanning Droplet Cell (SDC) [9, 83]. The SDC containsreference and counter electrodes which completes the three electrode configu-ration. By using SDC a complete range of electrochemical techniques such ascyclic voltammetry, transients and impedance measurements can be performed.Schematic drawing of the SDC used in this work is shown in Fig. 3.3a. The basicidea entails pumping a small amount of electrolyte through the electrolyte inletin the acrylic block and bringing it at the tip of the outer capillary to makeelectrical contact with the sample when the tip of the cell is pressed against thesample surface. The tip of the outer capillary made of borosilicate glass withouter diameter of 2.5 mm and inner diameter of 1.5 mm was first pulled usinga capillary puller (PC-10, Narishige) and ground with a microgrinder (EG-400,Narishige) to achieve the desired diameter for the capillary tip.

    30

  • 3.2 Electrochemical characterization

    Figure 3.3: (a) Schematic diagram of scanning droplet cell(b) Picture of scanning droplet cell setup.

    31

  • 3 Experimental

    The tip of the capillary is then dipped in silicone and blown with compressed ni-trogen gas to form the silicone gasket at the rim of the capillary tip. The siliconegasket helps to seal off the electrolyte from contact with the atmosphere andthereby preventing the evaporation of the electrolyte. In addition the changein the wetted area when a potential is applied resulting from the change inthe contact angle due to electrocapillarity effects can be prevented and therebydefining the working electrode area precisely [82]. The scanning droplet cell isthen fixed to a force sensor ( KD45 2N, ME-Messsysteme) which helps to applya precise amount of force through the capillary tip on the sample surface. Theapplied force should be small enough to guarantee only a small elastic defor-mation to the silicone gasket. A constant force of 3 mN was applied throughout all of the experiments so that the crucial part in microelectrochemistry, theworking electrode area will be highly reproducible. The reproducibilty of theworking electrode area is demonstrated in detail for anodic oxide spots grownon Hafnium thin film by Maradare et al.[84]. The other end of the force sensoris connected to a computer controlled 3D scanner for spatial movement of thecell tip across the sample surface. The position of the cell tip in relation to thesample surface is controlled by two cameras; a top camera to control the posi-tion of the tip in the sample surface and grazing view to determine the heightof the tip in relation to the sample surface.A precise amount of electrolyte is pumped through the electrolyte inlet with

    a resolution of 50 pl/step using a computer controlled microsyringe pump (Mi-cro 4, World precision instruments) actuating a 100 l syringe. A microrefer-ence electrode AuHg/Hg2(CH3COO)2/CH3COONa placed inside the scanningdroplet cell through one of the openings at the top of the cell and brought asclose as possible to the outer capillary tip to decrease potential drop between thereference electrode tip and the working electrode. The microreference electrodeis prepared by two step electrodeposition of mercury followed by acetate salt ona very pure (99.999 %) 100 m thick gold wire. The result is mercury acetatesalt on the surface of an amalgamated gold wire. The wire together with thedeposited salt is placed inside of a flexible microfiller (World precision instru-ments) with an outer diameter of 350 m and inner diameter of 250 m. Thespace between the wire and the capillary is filled by sucking from a hot solu-tion of NaCH3COO and agar with a syringe. Solidification of the agar at roomtemperature helps to preserve the NaCH3COO salt solution inside the capillary[85]. The three electrode configuration is completed by inserting a 100 m in

    32

  • 3.2 Electrochemical characterization

    diameter gold wire wound around the reference electrode and brought out of thecell for electrical contact through the inlet on the left side of the acrylic blockas shown in Fig. 3.3b. A tungsten wire pressed against the sample surface wasused to serve as an electrical contact for the working electrode.The SDC have been used since its introduction for different kinds of micro-electrochemical measurements. It was used in microbiology for determining themembrane potential in living cells [86]. In material science research it has beenused to study the local electrochemical response of single grains of technicallyrelevant samples [8790], for microelectrochemical lithography [91] and com-binatorial electrochemistry of binary and ternary alloys [42, 9296]. The SDCwas used in this work as well to study the local electrochemical response of Ti-Nb alloys in relation to the crystallographic orientation of the microstructuresselected for the measurements.

    3.2.2 Cyclic voltammetry

    Cyclic voltammetry is one of the voltammetric techniques that developed afterthe discovery of polarography by the Czech scientist Jaroslav Heyrovsky in 1922and received 1959 Nobel prize in chemistry for his contribution. The techniqueinvolves varying the potential applied to the working electrode in an electro-chemical cell at a certain scan rate towards a certain potential and reversing thescan of potential towards the starting potential while measuring the current inthe mean time. The task of applying a potential to an electrochemical cell atthe certain scan rate and measuring the current in the mean time is done byusing a potentiostat.

    The basic circuit components of a potentiostat connected to an electrochemi-cal cell is shown in Fig. 3.4a. A potentiostat measures the potential differencebetween the working electrode and reference electrode, applies current throughthe counter electrode and measures the current by measuring the voltage acrossthe serial resistor Rm. The applied current passes through the electrolyte andpolarizes the working electrode exactly so that the voltage between the refernceand working electrode is as close as possible to the voltage of the input sourceEin. This is achieved by the operational amplifier (OPA). The OPA continouslyadjusts its output voltage Eout to control current passsing through the elec-trochemical cell so that the input feedback potential Er is equal to the inputpotential Ein. Fig. 3.4b shows the electrochemical cell and the serial resistor

    33

  • 3 Experimental

    Figure 3.4: (a) Basic components of a three-electrode potentiostat connected to anelectrochemical cell(b) Circuit of the potentiostat after the electrochemical cell and thecurrent measuring circuit were replaced by two impedances Z1 and Z2.

    Rm replaced by two impedances (explained in section 3.2.3) Z1 and Z2. Z1 isthe impedance which includes the serial resistor Rm in series with the interfa-cial impedance of the counter electrode and the electrolyte solution resistancebetween the counter electrode and reference electrode and Z2 includes the inter-facial impedance of the working electrode in series with the electrolyte resistancebetween the reference and working electrode. Based on the function of the OPAwhich includes the amplification of the potential difference between the non-inverting and inverting inputs of the OPA:

    Eout = A (Ein Er) (3.1)

    where A is the amplification factor of the OPA. The current passing throughthe reference electrode is zero or negligible because it is connected to a resistor

    34

  • 3.2 Electrochemical characterization

    Rr. This helps to protect the OPA from being destroyed by static high voltageshocks when the input is open. Thus the current passing through the cell Ic isgiven by:

    Ic =Eout

    Z1 + Z2= Er

    Z2(3.2)

    Solving for Er:Er =

    Z2Z1 + Z2

    Eout = Eout (3.3)

    where is the feedback factor.

    = Z2Z1 + Z2

    (3.4)

    Inserting eqn. (3.1) in eqn. (3.3) and rearranging yields:

    ErEin

    = A1 + A (3.5)

    When A 1, eqn. (3.5) becomes:

    Er = Ein (3.6)

    Eqn. (3.6) shows how the OPA works to keep the voltage between the referenceelectrode and working electrode as close as possible to the voltage of the inputsource Ein.In this work a PAR Potentiostat/ Galvanostat Model 283 (Princeton AppliedResearch) potentiostat was used to grow anodic oxides on Ti-Nb alloys usingcyclic voltammetry as an electrochemical technique. The oxides were grown bysweeping the potential at a rate of 100 mV s1 starting from 0 V to 8 V with 1V step in eight cycles as shown in Fig. 3.5.A potentiostat can also be used to apply a constant potential to the workingelectrode for a certain time. Such kind of experiments are referred to as poten-tiostatic. The above potentiostat was used also to grow anodic oxides on Ti-Nballoys potentiostatically at 3 V for 1000 s for Mott-Schottky analysis (See sec-tion 3.2.4). The oxide growth on Ti-Nb alloys were carried out in an acetatebuffer of pH 6.0 at room temperature.

    35

  • 3 Experimental

    Figure 3.5: Excitation signals for successive cyclic voltammetric sweeps.

    3.2.3 Electrochemical impedance spectroscopy

    The opposition of components of an electrical circuit to the passage of an al-ternating current (AC) at a given frequency is regarded as impedance. Theterm electrochemical impedance spectroscopy (EIS) is used when dealing withthe impedance of an electrochemical system. EIS measurement is carried out byapplying an AC voltage to an electrochemical system and measuring the currentpassing through the system. Mathematically, the AC voltage E and current Ican be written as[97]:

    E(t) = E0 exp (it) (3.7)I(t) = I0 exp (i(t )) (3.8)

    where E0 and I0 are amplitudes of the AC voltage and current respectively, is the phase shift between the AC current and voltage, t is time, is the radialfrequency expressed in radians per second and related to frequency f as:

    = 2f (3.9)

    Analogous to Ohms law the impedance (Z) can be written as:

    Z = E(t)I(t) =

    E0exp(it)I0exp(i(t ))

    = E0I0expi (3.10)

    36

  • 3.2 Electrochemical characterization

    Eulers equation is given by:

    ei = cos+ isin (3.11)

    Thus the impedance relation in eqn. (3.10) can be rewritten as:

    Z = E0I0

    (cos + isin ) = Z + iZ (3.12)

    where Z and Z are the real and imaginary part of the impedance Z. Eqn (3.12)is a complex number which can be represented vectorially as:

    Figure 3.6: Vectorial representation of impedance.

    EIS experiments employ frequency response analyzer (FRA) to impose a smallamplitude AC signal to the working electrode of an electrochemical cell. TheFRA analyzes the current measured at the counter electrode by comparing itwith the applied signal to determine the impedance at a certain frequency. AnImpedance/gain-phase analyzer (Solartron SI 1260) and FRA (NF ElectronicInstruments, S 5720C) were used in this work for electrochemical impedancemeasurements. The EIS measurements were carried out right after each oxidegrowth in a wide frequeny range (100 kHz - 10 mHz) with a perturbation ACvoltage of 10 mV. Besides EIS measurements were also carried out with theapplication of a bias voltage to assess the semiconducting properties of theoxides using Mott-Schottky analysis (See section 3.2.4).

    37

  • 3 Experimental

    3.2.4 Mott-Schottky Analysis

    The qualitative and quantitative information regarding the semiconductingproperties of a material can be revealed by Mott-Schottky analysis [98, 99]. Theconcept could be explained easily by considering an ideal interface between asemiconductor electrode and an electrolyte. In order for the semiconductor andthe electrolyte be in equilibrium, the Fermi level of the semiconductor must beon the same energy level as to the redox potential of the electrolyte. If these twoenergy levels are not equal there will be an exchange of charge between the semi-conductor electrode and the electrolyte solution until equilibrium is attained.Space charge region extending 1-1000 nm into the electrode is formed due to theexcess charge. For an n-type semiconductor, electrons will be transferred fromthe electrode to the electrolyte because the Fermi level is high in energy thanthe redox potential of the electrolyte. This results in a positive charge for thespace charge region leading to the bending of the band edge upwards as shownin Fig. 3.7. From Fig. 3.7 it is obvious that the concentration of the elec-

    Figure 3.7: Semiconductor/electrolyte interface.

    trons in the space charge region varies with the potential difference between thesemiconductor electrode (working electrode) and the reference electrode whichdictates the energy level of the electrolyte. It is worth mentioning at this point

    38

  • 3.2 Electrochemical characterization

    that there are two double layers at the semiconductor/electrolyte interface: thespace charge and the Helmholtz double layer. The later is formed due to accu-mulation of nonadsorbed counter ions at the interface. The capacitance of thespace charge region Csc is much smaller than the Helmholtz capacitance CH (i.eCsc CH) hence the potential drop in the Helmholtz layer EH is constant andany possible change in the applied potential between the semiconductor and thereference electrode is directly observed in the potential drop in the space chargeregion Esc. Thus the potential in the bulk of the semiconductor E can bewritten as:

    E = Esc + Efb (3.13)

    where Efb is the flat band potential, the potential where there is no net transferof charge and hence no band bending as shown in Fig 3.8b. Any potentialpositive or negative of the flat band potential results in band bending as shownin Fig 3.8 a & c for an n-type semiconductor.

    Figure 3.8: Effect of applied potential (E) on the band edges in the interior of ann-type semiconductor: (a) E > Efb, (b) E = Efb and (c) E < Efb [98].

    39

  • 3 Experimental

    With the assumption of constant charge density within the space charge region,Poissons equation is given by:

    d2Edx2 =

    eNDr0

    (3.14)

    where e is the electronic charge, ND is the donor concentration in the spacecharge region, r is the relative permitivity of the semiconductor and 0 is thevacuum permitivity. Integrating eqn. (3.14) twice yields;

    Esc =eND2r0

    (dsc)2 (3.15)

    where dsc is the width of the space charge region. Eq. (3.15) is obtained assum-ing the electric field at x0 and the potential in the bulk of the semiconductor iszero. The width of the space charge region can be written as:

    dsc =r0Csc

    (3.16)

    where Csc is the capacitance of the space charge region. Inserting eqn. (3.13) andeqn. (3.16) in eqn. (3.15) and rearranging yields the Mott-Schottky equation;

    C2sc =2

    eNDr0(E Efb) (3.17)

    A linear region in a Mott-Schottky plot (C2sc vs. E) gives information as to thetype of the semiconductor, charge carrier density and flat band potential. Thesign of the slope of the linear region indicates the type of the semiconductor;positive slope indicates an n-type and negative slope a p-type. The value ofthe slope ( 2eNDr0 ) gives the charge carrier concentration. The intercept of thepotential axis with the extrapolation of the linear region gives the flat bandpotential. In this work Mott-Schottky analysis were carried out on an oxidegrown potentiostatically at 3 V for 1000 s in an acetate buffer of pH = 6.0on Ti-Nb alloys. Electrochemical impedance spectroscopy measurements werecarried out by applying bias potential to follow the variation of the capacitanceof the space charge region with the applied potential.

    40

  • 3.3 Electron Backscatter Diffraction (EBSD)

    3.3 Electron Backscatter Diffraction (EBSD)

    Electron backscattered diffraction is a vital technique in microstructure researchwhich provides information regarding crystallographic orientation, phase identi-fication, grain size measurement, strain measurements etc. In EBSD acceleratedelectrons of the primary beam of a scanning electron microscope (SEM) are di-rected onto the surface of a crystalline sample tilted at 700 from horizontal asshown in Fig. 3.9.

    Figure 3.9: Schematic diagram of a typical EBSD system.

    Upon hitting the sample surface, the primary electrons are diffusely and elasti-cally scattered through large angles within the sample; so that electrons divergefrom a point source just below the sample surface and impinge upon crystalplanes in all directions. Part of the scatterd electrons which are incident on setof lattice planes and satisfying Braggs law (eqn. 3.18 ) undergo elastic scatter-ing to form Kossel cones for every diffraction lattice plane as shown in Fig. 3.11.Two diffraction cones, one from diffraction from the upper side and the otherfrom the lower side of of the crystal planes are formed for each set of crystalplanes[100].

    n = 2dsin (3.18)

    41

  • 3 Experimental

    Figure 3.10: Schematic diagram for Bragg diffraction [101].

    where n is an integer, is the wavelength of the electrons, d is the interplanarspacing of the scattered atoms and is the angle of incidence. When thesecones are intercepted by the phosphor screen the result are Kikuchi bands ofthe electron backscatter pattern (EBSP).

    Figure 3.11: Formation of the Kikuchi lines from a tilted sample [102].

    The collected EBSP is then indexed for determinination of the crystal orien-tation of the part of the sample from where the pattern is obtained. Indexingis a process which involves detecting the bands, determining the angle betweenthe bands and determining the phase. Once the bands are detected, the crystalorientation and phase can be determined from the angles between the bands in

    42

  • 3.3 Electron Backscatter Diffraction (EBSD)

    EBSP as these angles represent the angles between the diffracting lattice planes.Fig. 3.12 a & b shows the EBSP of Ti-30at.% Nb alloy before and after index-ing.EBSD experiments can be carried out either in manual or automatic mode. Inmanual mode, specific points on the sample surface where crystal orientation in-formation is needed are selected by the operator, whereas in an automatic modethe electron beam is scanned over the sample surface to collect and index EBSPat each grid point and obtain crystal orientation of a wider area of the sample.A plot of the orientation at each grid point will give the crystal orientation map.

    Figure 3.12: (a) An example of EBSP and(b) Indexed EBSP of Ti-30at.% Nb -type Ti alloy.

    3.3.1 Qualitative and quantitative representations of crystalorientation in EBSD

    Direction vectors and planes

    Direction vectors in real lattice, where a, b and c are base vectors as shown inFig. 3.13 is given by:

    r = ua+ vb+ wc = [uvw] (3.19)

    Likewise the vector representation of a crystal plane or set of planes can be doneusing Miller indices (h k l). Miller indices were developed by William Miller andfollows the simple rules:

    Determine the intercepts of the plane with the crystallographic axes.

    43

  • 3 Experimental

    Figure 3.13: Base vectors and direction vectors in real lattice.

    Take the reciprocals of the intercepts

    change the fractions to integers by multiplying the fractions with the leastcommmon multiplier of the denominators of the fractions

    reduce to lowest terms

    i.e (hkl) =(m

    um

    vm

    w

    )(3.20)

    where u , v , w are intercepts of the planes with the axes a, b and c respectivelyand m is the number used to change the fractions to integers. If all members ofa symmetry family are meant then the notation and {hkl} are usedfor vectors and crystal planes respectively.

    Figure 3.14: Miller indices for low index planes of a cubic crystal.

    Description of crystallographic orientation

    An example of a relationship between sample coordinate system (ND,TD andRD) and the crystal coordinate system of a cubic unit cell is shown in Fig. 3.15.

    44

  • 3.3 Electron Backscatter Diffraction (EBSD)

    Figure 3.15: Relationship between sample coordinate system and crystal coordinatesystem ([100], [010], [001]) [102].

    The orientation is defined as the rotation that transforms the sample coordinatesystem onto the cystal coordinate system. Mathematically,

    C = S.R (3.21)

    where C and S are the crystal and sample coordinate systems respectively andR is the orientation given by the square matrix:

    R =

    cos1 cos1 cos1

    cos2 cos2 cos2

    cos3 cos3 cos3

    =R11 R12 R13

    R21 R22 R23

    R31 R32 R33

    (3.22)

    where 2, 2 and 2 are the angles between [010] and RD, TD and ND respec-tively and likewise 3, 3 and 3 are the angles between [001] and RD, TD andND respectively [102104]. Crystal orientation is commonly described by theuse of pole figures, inverse pole figures, Euler space and metallurgical description((h k l)).

    Pole figures Pole figure is a plot of crystal orientations of a given plane normal(pole) with respect to the sample axes and constructed by means of stereographicprojection. Stereographic projection involves projection of points on the surfaceof a sphere onto its equatorial plane as shown in Fig. 3.16a. Point P is the

    45

  • 3 Experimental

    intersection of the normal to the crystal plane placed at the center of the spherewith the surface of the sphere. The intersection point P of the line joining thesouth pole S and P with the equatorial plane is the projection point and thusthe pole figure for the crystal plane under consideration. Spatial arrangementof the poles in pole figure are characterized by angles (00 900) and (00 3600) and give the crystallogrpahic orientation of the crystal underconsideration as shown in Fig. 3.16b.

    Figure 3.16: (a) Streographic projection of the normal to a crystal plane {hkl} onto the equatorial plane(b) Pole figure as described by the two angles and .

    46

  • 3.3 Electron Backscatter Diffraction (EBSD)

    Inverse pole figure In inverse pole figure the orientation of the sample co-ordinate system is projected into crystal coordiante system unlike pole figureand thus the name "inverse" is used. Thus the reference system in inverse polefigures is the crystal coordinate system C, and the orientation is defined by theaxes of the sample coordinate system S. Because of crystal symmetry a completeinverse pole figure usually consists of many areas where the same information isrepeated. Fig. 3.17a shows the inverse pole figure for a cubic system where thereare 24 symmetric sections. Thus practically only the well known unit triangleshown in Fig. 3.17b which is 1/24th of the complete inverse pole figure is used.Inverse pole uses a basic Red-Green-Blue (RGB) color mode where full red,green and blue are assigned to grains whose , or axes,respectively are parallel to the projection direction of the inverse pole figure.The other orientations are colored by the RGB mixture of the primary colors[105].

    Figure 3.17: (a) Complete inverse pole figure(b) Unit triangle of the inverse pole figure for a cubic system.

    Euler space Euler space is another method for expressing the crystal orienta-tion using Euler angles. Euler angles represent three angles involved in succesiverotation of a crystal which helps to transform the sample coordinate system ontocrystal coordinate system and thereby defining the crystal orientation. Euler

    47

  • 3 Experimental

    angles as defined by Bunge [106, 107] are shown in Fig. 3.18a and follows thesequence:

    a) 1 about normal direction (ND), transforming the transverse direction(TD) and rolling direction (RD) into TD and RD respectively;

    b) about RD ;

    c) 2 about ND in its new orientation

    where 1, and 2 are the Euler angles that will coincide the sample and crystalcoordinate systems and satisfy the relations:

    00 1 3600, 00 1800, 00 2 3600 (3.23)

    Figure 3.18: (a) Euler angles(b) Representation of orientation in Euler space [108].

    48

  • 3.4 Chemicals

    Using the Euler angles the orientation can be expressed as a point in a three-dimensional coordinate system whose axes are the Euler angles as shown in Fig.3.18b. The resulting three-dimensional coordinate system is called Euler space.

    Goss orientation - (h k l) A more intutive description of orientationis given by Goss orientation. In Goss orientation, the orientation is described bythe crystal plane (h k l) parallel to the ND plane of the sample and the vectordirection parallel to the RD of the sample.

    Figure 3.19: Schematic illustration of Goss orientation.

    3.4 Chemicals

    Reagent grade chemicals purchased from VWR international were used withoutany modifications throughout this work:

    CH3COONa . 3H2O CH3COONa CH3COOH H2SO4 (95-97%) CH3OH C2H5OH Hg2(NO3)2 Agar H2O2 (30%)

    Deionized water produced in the laboratory with a water purification system wasused for preparing of aqueous solutions and for rinsing the sample surfaces.

    49

  • 4 Anodic oxides of Ti-30at.%Nballoy

    Owing to the excellent corrosion resistance combined with their light weightcharacteristics and high strength Ti and its alloys have found wide spread ap-plications in aerospace, chemical industries, power plants, medical prostheses[109, 110], military [111] and sporting goods [112] in a relatively short timesince their inception in the early 1950s. Based on their crystal structures Tialloys are categorized into -, ( + )- and -type alloys.The -type Ti alloys which are characterized by body centered cubic crystalstructure excel the other classes of Ti alloys for applications as biomaterialdue to their nontoxicity against osteoblastic cells, high corrosion resistance andimproved mechanical properties due to solid solution and second phase strength-ening while preserving the light weight characteristics of titanium [22, 113, 114].The main advantage in using -type Ti alloys is seen in the favourable mechan-ical properties. For crystallographic reasons, the body centered cubic crystalstructure shows higher symmetry as compared to the -type Ti alloys whichare characterized by a hexagonal close packed crystal structure resulting in anisotropic mechanical behaviour. Moreover, -type Ti alloys have low modu-lus of elasticity which makes them premium choices for load bearing surgicalimplants where the aim consists of providing surgical implant materials withstiffness similar to that of the human bone. This strategy lowered the loadshielding effect which in the conventional -type Ti alloys results in bone decayfollowed by abrasion, infection and eventual implant failure [22]. The excellentcorrosion resistance is attributed to the spontaneous formation of a dense andpassive thin oxide film on the surface which protects the material underneatheffeciently from deterioration despite the high affinity of Ti for oxygen [115, 116].This dense passive oxide layer which consists of TiO2 has a thickness of 1.3-5.4nm [39] has a low electronic conductivity [117], great thermodynamical stabil-ity [118] and low ion formation tendency in aqueous environments [119, 120]

    51

  • 4 Anodic oxides of Ti-30at.%Nb alloy

    which is responsible for the observed corrosion resistance of Ti alloys. Besides,the passive oxide layer has a high relative permitivity value which makes it aninteresting material for electronic application such as dynamic random accessmemory (DRAM) storage capacitors or metal-oxide-semiconductor field effecttransistor (MOSFET) gate oxides [8].Ti-30at.%Nb alloy has also found applications in hydrogen storage devices wherethe hydrogen forms a solid solution with Ti-30at.%Nb alloy. The hydrogen dif-fusion coeffcient which depends strongly on the hydrogen content where its de-pendence on temperature follows the Arrhenius type behaviour in samples withhydrogen content between 0.03 and 0.37 H/M (Hydrogen/Metal atoms ratio) atdifferent temperatures. The diffusion coefficient at room temperature decreasesbetween 0 and 0.1 H/M, but has a significantly higher value for 0.37 H/M. Thedecrease of the diffusion coefficient upto 0.1 H/M can be related to the mutualblocking of Ti4 sites by hydrogen. These sites are prefentially occupied at lowhydrogen contents. The increase at 0.37 H/M is attributed to the occupationof Ti3Nb sites which have a lower activation energy for hydrogen diffusion incomparison to the Ti4 sites [121].Despite their potential application, only few papers addressed the passivity andelectronic properties of the oxides grown on Ti-Nb alloys. Semoboshi et al. [122]studied the structural and dielectric properties of anodic oxide films on Ti-Nballoys with Ti contents in the alloy ranging from 0 to 15 at.%. They reportedthat the oxide films contained amorphous and partly crystalline niobium and ti-tanium oxides. The increase of titanium content in the alloy lead to an increasein the amount of titanium oxide in the mixed oxide film. The growing content oftitanium oxide in the oxide increases the capacitance of the mixed oxide film dueto the higher dielectric number of the titanium oxide up to a composition of 3at.%Ti in the substrate alloy. Further increase in Ti content in the alloy beyond3 at.% decreases the capacitance due to the formation of a thicker oxide filmwhich suppresses the effect of the higher dielectric number of titanium oxide.Mardare et al. [95] reported the high-throughput growth and characterizationof anodic oxides on the surface of Ti-Nb thin film libraries where issues like thepassivity of the surface and electrical properties of the anodic oxides grown werediscussed for a large spread of alloys.Ti-30at.% Nb alloy is one of the -type Ti alloys which draw considerable atten-tion for potential applications as a biomaterial [29]. Theory-guided bottom-updesign of Ti-30at.%Nb alloy as a biomaterial is reported by Raabe et al. [22]

    52

  • 4.1 Anodic oxides grown in a conventional electrochemical cell

    using parameter free density funtional theory calculations. The study of the pas-sivity of Ti-30at.%Nb -Ti all