Studies of transport in oxides on Zr-based materials

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(1)Studies of transport in oxides on Zr-based materials. Clara Anghel. Licentiate Thesis. Division of Corrosion Science Department of Material Science and Engineering Royal Institute of Technology, KTH SE-10044 Stockholm, Sweden. Stockholm 2004 ISRN KTH/MSE--04/71--SE+CORR/AVH ISBN 91-7283-921-X.

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(3) Abstract Zr-based materials have found their main application in the nuclear field having high corrosion resistance and low neutron absorption cross-section. The oxide layer that is formed on the surface of these alloys is meant to be the barrier between the metal and the corrosive environment. The deterioration of this protective layer limits the lifetime of these alloys. A better understanding of the transport phenomena, which take place in the oxide layer during oxidation, could be beneficial for the development of more resistant alloys. In the present study, oxygen and hydrogen transport through the zirconia layer during oxidation of Zr-based materials at temperatures around 400°C have been investigated using the isotope-monitoring techniques Gas Phase Analysis and Secondary Ion Mass Spectrometry. The processes, which take place at oxide/gas and oxide/metal interface, in the bulk oxide and metal, have to be considered in the investigation of the mechanism of hydration and oxidation. Inward transport of oxygen and hydrogen species can be influenced by modification of the surface properties. We found that CO molecules adsorbed on Zr surface can block the surface reaction centers for H2 dissociation, and as a result, hydrogen uptake in Zr is reduced. On the other hand, coating the Zr surface with Pt, resulted in increased oxygen dissociation rate at the oxide/gas interface. This resulted in enhanced oxygen transport towards the oxide/metal interface and formation of thicker oxides. Our results show that at temperatures relevant for the nuclear industry, oxygen dissociation efficiency decreases in the order: Pt > Zr2Fe > Zr2Ni > ZrCr2 ≥ Zircaloy-2. Porosity development in the oxide scales generates easy diffusion pathways for molecules across the oxide layer during oxidation. A novel method for evaluation of the gas diffusion, gas concentration and effective pore size of oxide scales is presented in this study. Effective pore sizes in the nanometer range were found for pretransition oxides on Zircaloy-2. A mechanism for densification of oxide scales by obtaining a better balance between inward oxygen and outward metal transport is suggested. Outward Zr transport can be influenced by the presence of hydrogen in the oxide/metal substrate. Inward oxygen transport can be promoted by oxygen dissociating elements such as Fe-containing second phase particles. The results suggest furthermore that a proper choice of the second-phase particle composition and size distribution can lead to the formation of dense oxides, which are characterized by low oxygen and hydrogen uptake rates during oxidation. Keywords: Zirconium, hydrogen, oxygen diffusion, second-phase particle, oxidation, dissociation, hydration, carbon monoxide, adsorption, porosity. i.

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(5) Preface. This thesis includes the following papers: I. Gas phase analysis of CO interactions with solid surfaces at high temperatures. Clara Anghel, Erik Hörnlund, Gunnar Hultquist, Magnus Limbäck Applied Surface Science 233 (2004) p. 392–401 DOI number: doi:10.1016/j.apsusc.2004.04.001. II. Influence of Pt, Fe/Ni/Cr–containing intermetallics and deuterium on the oxidation of Zr-based materials. Clara Anghel, Gunnar Hultquist and Magnus Limbäck Accepted for publication in Journal of Nuclear Materials. III. A method for characterization of gas transport in porous oxides C. Anghel, Q. Dong, J. Rundgren, G. Hultquist, I. Saeki and M. Limbäck Manuscript. iii.

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(7) Table of content 1. Introduction .................................................................................................……...... 1 2. Oxidation of Zr-based materials 2.1 Oxidation in dry environment …………………………………………….. 3. 2.1.1 Thermodynamic considerations …………………….……….... 4. 2.1.2 Zirconium-oxygen phase diagram …………………………….... 6. 2.1.3 Kinetic considerations………..…………………………………. 7. 2.2 Oxidation in wet environment …………………………………………….. 8. 2.3 Influence of second-phase particles (SPP)……………………………….... 10. 2.4 Diffusion ………………………………………………………………….. 11. 3. Experimental 3.1 Materials …………………………………………………………………. 12. 3.2 Gas Phase Analysis Technique (GPA).………………………….……….. 12. 3.2.1 Outgassing ……………………………………………..………. 13. 3.2.2 Hydration and oxidation ………….………..…….…………….. 14. 3.2.3 Dissociation and exchange ………….….……….…………….. 15. 3.2.4 Permeation ………………………..……………..…………….. 18. 3.3 Secondary Ion Mass Spectroscopy (SIMS).….………………..….……... 18. 3.4 X-ray Photoelectron Spectroscopy (XPS)……………………….……….. 19. 3.5 Scanning Electron Microscopy (SEM)…….……………………..…….... 19. 4. Summary of appended papers 4.1 Gas phase analysis of CO interactions with solid surfaces at high temperatures. …………………………………………………………... 20. 4.2 Influence of Pt, Fe/Ni/Cr–containing intermetallics and deuterium on the oxidation of Zr-based materials…………………………………. 22. 4.3 A method for characterization of gas transport in porous oxides …….... 26. 5. Conclusions ......................................................................................................…. 29. 6. Future work ......................................................................................................…. 30. 7. Acknowledgments ............................................................................................…. 31. 8. References .......................................................................................................….. 32. Papers I-III. v.

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(9) 1. Introduction Martin Heinrich Klaproth, a German chemist, discovered zirconium metal in 1789 by analyzing the composition of the mineral jargon (ZrSiO4) [1]. In 1824, Jöns Jacob Berzelius, a Swedish chemist, isolated for the first time zirconium metal in impure form and after 90 years, in 1914, pure Zr was prepared by D. Lely and L. Hamburger [1]. Zirconium is usually associated with hafnium, being difficult to be obtained in pure form [2]. The high resistance to corrosion in aggressive environments at high temperatures makes the Zr metal to be appropriate in many applications, such as: heat exchangers, pumps, reactor vessels, valves, surgical appliances [1,2]. Furthermore, Zr metal has a low neutron absorption crosssection (unlike Hf), therefore the nuclear power industry became the major application of the purified Zr [1]. The necessity for better in-reactor performance generated further materials development, and Zr-based alloys containing Sn, Fe and Cr with or without Ni, called Zircaloy-2 and –4, have been designed for use in water-cooled nuclear reactors [2]. Other alloys like Zr-2.5%Nb have also been developed for use as pressure tube material. A more efficient use of the nuclear fuel is limited by the corrosion of the fuel cladding. Further development of the existing alloys by optimization of the alloying elements chemical composition, size distribution and density for higher burn-up applications is the subject of many recent studies [3,4,5]. Fe, Cr and Ni have a low solubility in the Zr matrix and are therefore mainly incorporated in the form of second-phase particles (SPP) [6]. Nodular corrosion was observed in systems were initially large size SPPs or agglomerated small size SPPs were present [5]. In studies related to the Pt influence on the oxidation of other metals than Zr, the same effect of enhanced oxidation in the Pt area has been observed [7,8]. The presence of Pt is connected to the generation of more oxygen in the form of On- (n = 0, 2) [7,8]. In the case of zirconium alloys, each alloying element has certain effects on the oxidation and it seems plausible that one effect is related to efficiency for oxygen dissociation. In this study, oxygen dissociation efficiency on preoxidized ZrCr2, Zr2Fe and Zr2Ni intermetallics as well as preoxidized Zircaloy-2 and Pt is investigated upon exposure to a mixture of oxygen isotopes at relevant temperatures for the nuclear industry. One aim is to study the possible influence of O2 dissociation rate on the corrosion rate of zirconium alloys. The influence of Pt on the oxidation of Zr in 20 mbar O2 at 400°C is also investigated.. 1.

(10) Another problem in the use of Zr-based materials in wet environments at high temperatures is related to hydrogen uptake [9]. A certain fraction of the hydrogen produced during the reaction of Zr with water, diffuses through the oxide layer and is accumulating in the bulk metal. When the hydrogen content exceeds the solubility limit, hydrides starts to form, and as a result, hydrogen embrittlement may occur [9-11]. Many parameters influencing hydrogen uptake in zirconium alloys have been identified, such as: composition and size distribution of alloying elements, corrosive environment, oxide microstructure, temperature and irradiation [9-12]. T. Laursen et al. [10] suggested that hydrogen ingress in Zr-based materials depends on the surface preparation. By modifying the surface properties, hydrogen uptake rate can be influenced [10]. In this work, the modification of the surface of preoxidized Zr by blocking the active sites for hydrogen dissociation is investigated. The effect of CO and N2 is hereby considered. In recent publications dealing with other metals than Zr [13-15], it has been shown that the presence of hydrogen in the metal substrate or in the gas phase has an effect on oxidation by increasing the metal outward transport, which eventually can generate voids at the metal/oxide interface. One possible mechanism is based on a proton-induced high concentration of metal ion vacancies in the oxide, which is likely to result in an increased metal ion transport [15]. Oxidation of Zr by cation outward diffusion via cation vacancies was already taken into consideration in early 50’s by E. A. Gulbransen and K.F. Andrew [16]. In this study, the influence of hydrogen, present in the substrate, on the oxidation of Zr in O2 at 400°C is investigated. Evaluation of the hydrogen uptake upon exposure of 1m long Ar-filled Zr-based tubes to water from one end of the tubings at 370°C is also presented. G. Hultquist et al. [13] indicated that the oxidation rate decreases if a better balance between Zr ion and oxygen ion transport is obtained, resulting in adherent oxide growth, with low density of pores and other defects. Obviously, healing of pores by oxide formation needs outward Zr transport. The rate of metal consumption in corrosion is dramatically dependent on the transport of species like oxygen and hydrogen through the oxide layer, so defects like pores can have a significant influence on the corrosion rate. Even pores with nm-size and their abundance, distribution and interconnectivity may affect the corrosion rate. Naturally where one is aiming at obtaining a gas-tight barrier, the size of the interconnected pores needs to be smaller than the size of the diffusing molecules. N. Ramasubramanian et al. [11] suggested that the pretransition zirconium oxide layer is porous not only in the outer part, but also within the barrier layer. Therefore, determination of the open porosity of pretransition oxides on Zr-based materials is crucial for a better understanding of the transport processes, which take place during oxidation. 2.

(11) Finally, a novel and relatively straightforward method to quantify the effective pore size, gas diffusion and amount of gas present in oxide scales, with known oxide thickness, on samples equilibrated in controlled atmosphere is presented. The method is validated in measurements of diffusivity and solubility of He in quartz at 80°C and also applied for characterization of an Fe-oxide and two pretransition oxides on Zircaloy-2. The aim of this thesis is to improve the knowledge related to the transport of species like oxygen and hydrogen through the growing Zr-oxide layer using Gas Phase Analysis and Secondary Ion Mass Spectrometry.. 2. Oxidation of Zr-based materials 2.1 Oxidation in dry environment The formation of the oxide layer on Zr-based materials has been extensively studied during the last decade with a wide variety of techniques [16-19]. Generally, starting with a virtually clean metal surface, the formation of the oxide layer can be divided in stages as illustrated in Figure 2.1 [20].. Figure 2.1- General mechanism for metal-oxygen reaction, after Kofstad [20] In the case of Zr, in the first stage, dissociative adsorption of oxygen molecules on the Zr metal surface takes place. The sticking probability of oxygen molecules on the clean Zr 3.

(12) surface is close to 1 for coverages up to 1 monolayer (ML) [21]. Oxidation proceeds with the oxide nucleation and growth followed by a continuous thin film formation [20]. C. -S. Zhang et al. [21] have studied the initial stage of oxidation of Zr by Auger Electron Spectroscopy (AES) and nuclear reaction analysis and at 90, 293 and 473 K. They concluded that Zr oxide is growing layer by layer at 90 K, layer by layer for approximately 2 layers followed by island growth into the metal at 293 K, and by an island growth mechanism at 473 K [21]. Yoshitaka Nishino et al. [22] have studied the formation of suboxides on Zr and Zircaloy-2 in the early stages of oxidation in low pressure O2 and water vapor exposures at room temperature using AES and X-ray Photoelectron Spectroscopy (XPS). They found that three suboxides of Zr2O, ZrO and Zr2O3 as well as ZrO2 are formed on the respective surfaces. The last three stages in Figure 2.1 define the oxide growth, in which oxidation requires that species like oxygen and/or metal are transported through the already existing oxide film. It is well known that Zr oxide is growing mainly by inward oxygen transport [16,17]. The oxide is highly stressed as a result of lattice parameters mismatch and thermal expansion difference between Zr oxide and Zr metal [23]. There is a stress gradient across the oxide thickness: maximum stress at oxide/metal (O/M) interface and minimum at gas/oxide (G/O) interface [23]. As a result of the high compressive stress, the tetragonal ZrO2 phase is stabilized at O/M interface [5,6]. Away from the O/M interface, the tetragonal to monoclinic phase transformation takes place as a result of stress relief with a volume expansion of 7% and generates defects like cracks and pores (easy diffusion pathways) [5]. This transformation can occur inside the barrier layer, possibly inducing porosity within the barrier layer [11]. The porosity development in the zirconium oxide scale is considered to be the main reason that leads to the transition from parabolic to linear oxide growth kinetics [24]. Oxidation of metals and alloys can be considered from thermodynamic and kinetic point of view. Both are important to consider therefore some of the most important aspects for the case of Zr-based materials oxidation are addressed below. 2.1.1 Thermodynamic considerations Thermodynamically, to determine the stability of Zr oxide at different temperatures and O2 pressures, the values of the free energy, G, enthalpy, H, and entropy, S, are required [20]. The chemical reaction for Zr oxidation is presented in equation (1). Zr(s) + O2 (g) = ZrO2 (s). (1) 4.

(13) The standard free energy change of this reaction, ∆G0T(ZrO2), for temperatures T > 298K can be expressed as: ∆G0T = ∆H0298 – T∆S0298. (2). where ∆H0298 and ∆S0298 are the standard enthalpy and entropy change of the reaction at 298K. This is a linear equation, and the slope of this line is constant as long as no phase transformation takes place [2]. A chemical reaction can occur only if ∆G0T is negative. When a metal is exposed to an oxygen-containing atmosphere, the metal oxide will start to form only if the partial pressure of oxygen is higher than the dissociation pressure of the oxide at the temperature of exposure [20]. In reaction (1), Zr can only be oxidized if:  ∆G 0 ( ZrO2 )  PO2 ≥ exp− T  RT  . (3). The diagram of the standard free energy of formation of oxides per mole of oxygen versus temperature with the corresponding dissociation pressures of the oxides, called Ellingham/Richardson diagram, is a useful tool to predict oxide stability for different temperatures and oxygen pressures [20]. In Figure 2.2, the Ellingham/Richardson diagram for some oxides is presented [20].. * The line for ZrO2 was added. to. the. original. diagram using calculated values. for. ∆G0T(ZrO2). using eq. (2) and standard enthalpy and entropy of formation from ref. [25].. *. Figure 2.2 - Ellingham/Richardson diagram for some oxides, after Kofstad [20]. 5.

(14) It can be seen in Figure 2.2 that Zr oxide is stable, even at low pressures obtained in ultra high vacuum, and as a result, a thin oxide layer (2-4 nm) will form on the surface [24]. In gas mixtures of H2/H2O or CO/CO2 the required low partial pressure of oxygen may be achieved and under these conditions the Zr metal became stable. In Figure 2.2, the stability of Fe, Cr and Ni oxides are also presented. In the figure, lines of constant PO2 are shown. For example, at 500°C, ZrO2, Cr2O3, NiO and Fe2O3 respectively are stable if the partial pressure of oxygen (atm O2) is higher than approx. 10-65, 10-42, 10-23 and 10-19 respectively (only for zirconia is presented in Figure 2.2). These results are valuable in the case of oxidation of alloys in low oxygen pressure to predict which alloy component will be oxidized [20]. 2.1.2 Zirconium-oxygen phase diagram Phase diagrams are important in interpretation of the mechanism of oxidation. These diagrams show the phases that can form as a function of temperature and composition of the system. Correlated phase equilibrium and crystallographic information are obtained [26]. In Figure 2.3, the Zr-O phase diagram is presented [24]. It can be seen that oxygen can be in solid solution with αZr up to 28.6 at-% oxygen at temperatures around 500°C.. Figure 2.3 – Zr-O phase diagram, after Oskarsson [24] Kinetic factors and other parameters like, stress, impurities also influence the phases that will appear at the respective temperatures, and therefore have to be taken in consideration. 6.

(15) 2.1.3 Kinetic considerations During the oxidation of Zr-based materials, in different stages of oxidation, different processes can be rate limiting. Raspopov et al. [18] investigated the initial stage of Zr oxidation (formation of Zr-O solid-solutions) in atomic and molecular oxygen at low pressures in the temperature range 873-1123K. They [18] concluded that oxygen chemisorption is likely to be a rate limiting step for the oxidation process at the early stages of oxidation. Linear oxidation rates have been reported [18]. It is suggested that the formation of Zr-oxide takes place after the surface of Zr metal is saturated in dissolved oxygen [18]. Generally, after the formation of a continuous thin oxide layer on the Zr surface, the oxidation rate becomes parabolic, so called pretransition period, which is followed by a pseudo-linear regime (three short parabolic regimes which can be approximated with a linear regime) [27]. The transition between the parabolic and linear oxidation kinetics is called “break away” point being associated with the change from a protective to a non-protective oxide scale [27]. The rate of the oxidation reaction is influenced by many parameters such as: temperature, oxygen pressure, surface preparation and metallurgical characteristics of the metal [20, 27]. Parabolic oxidation rate equation is given by [20]: ' dx k p = dt x. where x is the oxide thickness and. (4). k p' is the parabolic rate constant.. Parabolic oxidation rate can be interpreted as diffusion limited oxidation. Linear oxidation rate means that the time evolution of the oxide thickness, x, is given by the equation (5) [20]:. dx = k1 t dt. (5). where k1 is the linear rate constant. Linear oxidation rate can be interpreted as regular oxide failure in combination with new oxide formation [27]. The rate-determining step could be: a surface or grain boundary process or a chemical reaction. If the rate-limiting step is the adsorption of oxygen molecules on the surface, a great dependence of the reaction with the oxygen pressure is observed [20].. 7.

(16) 2.2 Oxidation in wet environment When oxidation takes place in aqueous environment, hydrogen formed during the reaction of Zr with water plays an important role in the oxidation kinetics [22, 28]. The oxidation proceeds also in stages, the difference between wet and dry oxidation being related to the effect of hydrogen uptake. Hydrogen accumulates in the metal substrate and forms solid solutions with Zr metal until the hydrogen content reaches the solubility limit. Further hydrogen uptake generates formation of hydrides, and as a result, hydrogen embrittlement may occur [9-11]. Hydrogen solubility in α-zirconium at different temperatures has been extensively studied [29]. A summary of the available data is presented in Figure 2.4.. Figure 2.4 – Hydrogen solubility in Zr, after Dupin et al. [29] The presence of alloying elements in the Zr matrix influences the solubility of hydrogen [30]. This might cause problems for example in the case of fuel claddings in which the liner is made by a different Zr alloy. Takagi et al. have studied the redistribution of hydrogen in Zr-lined Zircaloy-2 claddings [31]. They suggested that a significant amount of hydrogen is moving from Zircaloy-2 to Zr during slow cooling, although the difference in their hydrogen solubility limit is quite small. It is reported that among the Zr alloys, Zircaloy-4 has the highest solubility for hydrogen in the α region [30]. Hydrogen solubility also depends on interstitially dissolved oxygen [32]. For evaluation of the hydrogen solubility in zirconium-oxygen solid solutions, the Zr-O-H ternary system, exemplified for 700°C in Figure 2.5, can be used [32]. For small oxygen content, the thermal solubility of hydrogen in α phase increases and then decreases at higher oxygen content [30, 32]. 8.

(17) I.. α+β. II.. α+β+δ. III.. β+δ. IV.. α+δ. V.. α+δ+ε. VI.. α+ε. VII.. δ+ε. VIII.. α + ε + ZrO2. IX.. ε+ α ZrO2. X.. α + α ZrO2. Figure 2.5 - Isothermal Zr-O-H ternary system at 700°C, after Miyake et al. [32] The driving force for hydrogen diffusion through the oxide layer is the concentration gradient. Using a thermal desorption technique, Miyake et al. have found that the solubility of hydrogen in zirconia is in the range of 10-5 – 10-4 mol H/mol oxide and decreases with increasing temperature [32]. The variation of hydrogen solubility in monoclinic zirconia as a function of temperature is shown in Figure 2.6. The diffusion of hydrogen through the oxide scale depends on the fraction of the tetragonal ZrO2 present in the oxide [33].. [32]. Figure 2.6 – Hydrogen solubility in some oxides versus 1/T, after Miyake et al. [32] 9.

(18) 2.3 Influence of second-phase particles (SPP) Fe, Cr and Ni have a low solubility in the Zr matrix and are therefore mainly incorporated in the form of SPP [3,5,6,34] and also partly incorporated in the oxide during oxidation. The size distribution of SPPs in the Zr-based materials has an important effect on the in-reactor performance of these materials [34]. The formation rate of protective oxides and also the thickness of the barrier layer are found to be dependent on the initial SPP size distribution [34]. Nodular corrosion was observed in systems were initially large size SPPs or agglomerated small size SPPs were present [5]. Dissolution of small-size SPPs in Zr-based materials during in-reactor operation under high neutron flux in combination with hydrogen pick-up can enhance the deterioration of the barrier layer [34]. In Zircaloy-2, for example, two types of SPPs are reported: Zr2(Fe,Ni) and Zr(Fe,Cr)2 particles [3,5,34]. The Crcontaining particles were found to be less resistant to dissolution in the matrix during irradiation than the Ni-containing particles [5,34]. Abolhassani et al. [17] performed in situ studies of the oxidation of Zircaloy-4 at 700°C using environmental scanning electron microscopy (ESEM). The samples were also characterized with atomic force (AFM) and transmission electron microscopies (TEM). They found that after the uppermost precipitates start to oxidise, the surface oxide, above the respective precipitates, became depleted in Zr, showing a lens-type feature. This is due to outward diffusion of Cr and Fe probably via grain boundaries, which will then be oxidised at the O/G interface. AFM revealed granular morphology for the pretransition oxide and the presence of randomly distributed pores with non-uniform size and geometry [17]. In the vicinity of SPPs, cracks lying parallel to the interface were identified. The un-oxidized SPPs, which are found deeper in the oxide layer, may influence the electron and hydrogen transport across the oxide layer [5]. An overall view of possible roles of the SPPs on the oxidation of Zr alloys is illustrated in Figure 2.7 [33].. Figure 2.7 – Possible effects of SPPs on the oxidation of Zr alloys, after Lim [33] 10.

(19) 2.4 Diffusion Crank [35] defined diffusion as: “the process by which matter is transported from one part of a system to another as a result of random molecular motion”. Fick expressed the mathematical equation for diffusion in isotropic materials as shown in equation (6), also called Fick’s first law of diffusion:. F =− D. ∂C ∂x. (6). where F is the flux of diffusing species, C their concentration, D diffusivity and x the space coordinate measured normal to the section [35]. The diffusivity, or the diffusion coefficient, is related to the rate of movement of species. The solubility represents the dissolved gas concentration per unit of applied pressure of gas [36]. Experimentally, to determine the diffusion parameters, the differential equation of diffusion is also used. For a plate-like geometry, if the gradient is only along the x-axis, Fick’s second law of diffusion is expressed as:. ∂C ∂ 2C =D 2 ∂t ∂x. (7). This equation can be applied to other geometries using the appropriate coordinates. To obtain a solution of the diffusion equation, the initial and boundary conditions have to be considered. Isotope tracing techniques like SIMS [37], nuclear microprobe technique [38], ion beam induced nuclear reactions [39], are widely used for diffusion studies. Analysing the concentration profiles of the diffusing isotopes the mechanism of diffusion can be identified. The saturation/outgassing method is also a useful technique for determination of gas solubility and diffusivity, using solutions of the diffusion equation corresponding to the shape and properties of the respective samples [36]. The gas phase is continuously analysed by a mass spectrometer and the obtained results give average values for the diffusivity of gases in solids.. 11.

(20) 3. Experimental 3.1 Materials The materials used in this study together with the methods used for the characterization of the samples are summarized in Table 3.1. Table 3.1 – Materials used in this study Dissociation. Oxidation. SIMS. XPS. SEM. Zr plate. x. x. -. x. x. x. 1, 2. Zircaloy-2 plate. x. x. x. -. -. -. 1, 2, 3. 1m long Zr-based tubes. -. x. -. x. -. -. 2. Fe plate. x. x. x. -. -. x. 3. Pt plate. x. -. -. -. -. -. 1, 2. Ni plate. x. x. -. -. -. -. 1. Al plate. x. x. -. -. -. -. 1. Cr plate. x. x. -. x. -. -. 1. Cu plate. x. x. -. -. -. -. 1. Cu-8Al. x. x. -. -. -. -. 1. SS 304. x. x. -. -. -. -. 1. SS 304-0.1Pt. x. x. -. -. -. -. 1. Zr2Fe. x. x. -. -. -. -. 2. Zr2Ni. x. x. -. -. -. -. 2. ZrCr2. x. x. -. -. -. -. 2. Quartz plate. x. -. x. -. -. -. 3. Material. Gas diffusivity, pore size estimation. Paper. 3.2 Gas Phase Analysis Technique (GPA) The schematic diagram of the GPA equipment presented in Figure 3.1 consists of:. •. A 70 cm3 reaction chamber made of a silica tube and a stainless steel cross. The reaction chamber is pumped with an ion pump via a leak valve. In oxidation of 1m long Zircaloy-2 tubes, the silica tube in Figure 3.1 is replaced by the Zircaloy-2 tube (paper 2).. 12.

(21) In the experiments applied for evaluation of effective pore size of oxide scales (paper 3), the silica tube has been removed giving too high background levels. For permeation studies, the silica tube is replaced by a double wall tube.. •. A gas handling system where pressures up to 1 atm can be used.. •. A pressure gauge to measure the total pressure inside the reaction chamber.. •. A mass spectrometer (MS), with a quadruple analyser, placed in an UHV chamber. Hydrogen and different isotopes can be detected.. •. A tube furnace, which can be used up to 1200°C.. The GPA equipment can be used for different types of measurements: outgassing, hydration, oxidation, dissociation, exchange and permeation. These different measurements are shortly described bellow with some examples. Printer. Leak valve. Pressure gauge. Tube furnace. UHV QMS. Quartz tube 400°C. Ion pump Sample Rough pump. P in MS, mbar. Pressure gauge Time, s. Computer. Gas handling system. Figure 3.1 – Gas Phase Analysis setup. 3.2.1 Outgassing Heating of a sample in vacuum generates release of gases that can be quantified. The sample is placed in the reaction chamber and pumped continuously by the ion pump via a fully open leak valve. The MS analyses the composition of the gases released by outgassing and the computer graphically reproduces the time evolution of the partial pressure of the gas constituents. By careful calibration of the mass spectrometer signal and ion pump rate, the amount of gas released from the sample can be quantified. This enables, for example, the hydrogen content in a material to be determined. In Figure 3.2, the outgassing of hydrogen from a zirconium plate at temperatures up to 700°C is shown. 13.

(22) Hydrogen pressure in MS, mbar. 1.E-06. 1.E-07. 1.E-08. 700°C 600°C 200°C 400°C. 1.E-09 0. 100. 200. 300. Time, h Figure 3.2 – Outgassing of hydrogen from a Zr plate The data obtained by outgassing from air-equilibrated oxide samples at low temperatures (20-80°C) can be used for evaluation of gas diffusivity, solubility and effective pore size in respective oxides.. 3.2.2 Hydration and oxidation The most common way to study oxidation of metals is to measure the weight gain of the sample after oxidation. With GPA, the changes in the gas phase are measured. The pressure decrease of the gas in the reaction chamber upon time is continuously monitored and is conveniently recorded on an x-t plotter. A 1.8 cm2 Zr plate covered with a naturally formed oxide was exposed to 7.5 mbar deuterium at temperatures 200-550°C. Calculated from the decrease of D2-pressure in the reaction chamber, the deuterium uptake versus time in the Zr plate is shown in Figure 3.3. After charging, the D content in the Zr plates was 600wt ppm. As seen in the figure, the uptake rate of D in Zr is increasing considerably at temperatures above 400°C. For study of the oxidation mechanism, the oxidation can be performed in two stages, first in “normal” O2 followed by exposure in. 18. O-enriched oxygen gas. In a subsequent analysis. with SIMS the position of the oxide growth can be found if the exchange rate between O in the oxide and O in O2 is slow compared with O-uptake rate in the oxidation. The previously D-charged Zr plate, labelled Zr(D), was partly coated with Pt and exposed at 400°C for 12 14.

(23) hours to 20 mbar O2 in two stages. The oxidation kinetics presented as oxygen uptake from the gas phase is shown in Figure 3.4.. 25. 550°C 15. 400. 10. 500°C. 5. 200. D uptake, wt ppm. D uptake, µmol D cm. -2. 600 20. 400°C. 200°C. 0. 0 0. 20. 40. 60. Time, h. Figure 3.3 – The kinetics of deuterium uptake in a Zr plate. 5 1.5 4 1. 3 2. 0.5. 18. Adition of O. 1. 0 0. 200. 400. 600. -2. 6. Oxygen uptake, µmolO cm. Oxygen uptake, mbar. 2. 0 800. Time, min.. Figure 3.4 - Oxidation of a partly Pt coated Zr sample containing 600 wt ppm D, in 20 mbar O2 at 400°C. 3.2.3 Dissociation and exchange Dissociation measurements The interaction between gas molecules and a surface can lead to dissociation (molecules split into smaller constituents). The surface activity of a material for dissociation can be quantified [40]. To measure the dissociation rate of a molecule, a mixture of its isotopes are 15.

(24) used, for ex.. 1,1. H2+2,2H2 or. 16,16. O2+18,18O2. The dissociation process and a subsequent. association of the dissociated species result in the formation of the mixed molecules 1,2H2 or 16,18. and. O2. The partial pressures of the gas components, 18,18. 1,1. H2,. 1,2. H2 and. 2,2. H2 or. 16,16. O2,. 16,18. O2. O2, are obtained via a negligible inlet to the MS. By considering the measured. formation rate of the mixed molecules and the distance from the statistical equilibrium (no more changes in the gas composition), the dissociation rate can be calculated. The background dissociation rate of the silica tube must of course also be taken into account. The composition at statistical equilibrium depends on the isotopic abundance in the gas phase. In the case of hydrogen isotopes, for an initial 50% 1,1H2 + 50% 2,2H2 gas mixture, the statistical equilibrium composition is: 25% 1,1. H2 + 50%. 2,2. 1,1. H2, 50%. 1,2. H2, 25%. 2,2. H2. Starting with 50%. H2 and assuming a constant dissociation rate, vdiss, the kinetics of. 1,2. H2. formation is shown in Figure 3.5.. Figure 3.5 - Time evolution of 1,2H2 molecules, with an initial composition of the hydrogen gas 50%. 1,1. H2 + 50%. 2,2. H2. fE is the fraction of the mixed molecules at statistical. equilibrium (t→ ∞ ). In this figure the preferable region for measurement with good accuracy is indicated. In the following, dissociation refers to molecular dissociation. vdiss can be calculated from [40]:. v diss = 0.04 ⋅ n ⋅. dP −1 ⋅Vch ⋅ ( f E ⋅ B ⋅ As ) dt. (6). where  f   B = 1  fE . (7) 16.

(25) In equation (6), the factor 0.04 (mol atm-1 dm-3) comes from the molar volume of an ideal gas at standard pressure and temperature, P is the partial pressure of the “mixed” molecules (1,2H2 or. 16,18. O2) (atm), As is the sample area (cm2), Vch is the volume of the reaction. chamber (dm3). The factor n is 1 for molecules of the form XY (one X and one Y atom per molecule) and 2 for molecules of the form X2 (two X atoms per molecule). The dissociation rate, νdiss, given by equation (6) is expressed in mol X cm-2h-1. Equation (7) defines a compensation factor, B, where f is the fraction of the mixed molecules in the gas phase at time t and fE is the fraction of the mixed molecules in the gas phase at statistical equilibrium. As an example, in Figure 3.6, the time evolution for isotopic mixtures of hydrogen in exposure to a preoxidized Zircaloy-2, Zr2ox, (approximately 2µm oxide thickness) at 300°C is presented.. Figure 3.6 - Pressure in reaction chamber during exposure of an 18 cm2 Zircaloy-2 sample at 300°C in a gas mixture with 1H and 2H isotopes. The indicated statistical equilibrium among the hydrogen molecules is based on the abundances of 1H and 2H at 2 hours. From Figure 3.6, hydrogen dissociation rate was calculated using eq. (1). The resulted dissociation rate is in the range 2.7 – 7.7 µmol H cm-2 h-1. The dissociation rate decreased in time, which shows that the activity of the surface can change in time. Exchange measurements Exchange may take place between oxygen in the gas phase and oxygen in the oxide. This exchange can be measured in exposure of a 16O-containing oxide to rate of. 16,18. 18,18. O2. The formation. O2 in the gas phase will then be a result of exchange between 18O from the gas. phase and 16O from the oxide lattice. 17.

(26) 3.2.4 Permeation Permeation of gases from the high-pressure side of a membrane to the low-pressure side, which is evacuated by an ion pump, can be investigated using isotopic gas mixtures. Special setup with double wall tubes is used for this purpose. 3.3 Secondary Ion Mass Spectroscopy (SIMS). SIMS analyses were performed with a Cameca IMS-6f apparatus, 10 keV, 50-200 nA primary beam of Cs+ ions, which was rastered over 200x200 µm2, where ions where detected from a centered area with a diameter of 70 µm. The oxidation mechanism can be investigated by using two stage oxidation, fist in. 16,16. O2. followed by exposure to 18O-enriched oxygen gas, in combination with SIMS depth profiles. OnMen+. O2. a. Mainly inward oxygen transport. Men+ Men+. Men+. 18O. 18O. 18O. Sputter time, s. O2. On-. count rate. count rate. On-. Men+. count rate. On-. metal. O2. On-. oxide. analysis. The oxide growth mode can be retrieved as can be seen in Figure 3.7.. Sputter time, s. b. Balanced transport. Sputter time, s. c. Mainly outward metal transport. Figure 3.7 – Mechanism of oxide growth obtained by investigations of 18O SIMS depth profiles The exchange with the lattice has to be considered when profiles like b. and c. (Figure 3.7) are obtained. When the exchange rate is negligible related to the oxidation rate, it can be concluded that the mechanism of oxide growth for case b. is by balanced transport and for case c. is mainly by outward metal transport.. 18.

(27) The oxide thickness can be obtained based on sputter time when the 90Zr ion count rate has decreased to half of its maximum value. SIMS depth profiles of. 90. Zr18O, and. 90. Zr16O ions were used in addition to. 18. O and. 16. O. profiles to investigate the oxide layer formation on Zr surface. An advantage of SIMS technique is the capability of detecting hydrogen-containing species. Deuterium depth profiles give valuable information about the mechanism of hydrogen uptake.. 18. OD SIMS profiles were considered to elucidate the possible connection between. hydrogen and oxygen diffusion. 3.4 X-ray photoelectron spectroscopy (XPS) XPS analyses have been performed with a Kratos AXIS HS spectrometer using a monochromatic Al Kα X-ray source (1486.6 eV). The area of analysis was approximately 0.4 mm2. The method is based on the photoelectric effect. Upon exposure of a sample to photons with relatively high incident energy, electrons from the internal shell of the atoms are released. The energy of these photoelectrons can be expressed as:. E kin = hν − E B −Φ. (8). where hν is the X-ray incoming energy, EB is the binding energy of the electron and Φ is the work function of the material. The kinetic energy of the electron depends on the incident photon energy, but the binding energy is a physicochemical constant. Therefore, XPS can be used to identify which elements are present at the outermost surface of a sample and also to characterize the nature of the chemical bonds, which connects these atoms. 3.5 Scanning Electron Microscopy (SEM) SEM uses electrons to form the image of the surface of a sample. When the incoming beam of electrons hits the surface, photons, secondary and backscattered electrons are ejected from the sample. The ejected electrons are used in SEM to obtain high-resolution images of the topography or morphology of the surface. FEG-SEM analysis were performed with a Leo 1530 Field Emission Scanning Electron Microscope equipped with a GEMINI field emission column.. 19.

(28) 4. Summary of appended papers 4.1 Gas phase analysis of CO interactions with solid surfaces at high temperatures In Paper 1, the deactivation of a surface related to adsorption and dissociation of molecules has been examined and related to problems, which are common in industrial applications. Using the GPA technique, the dissociation of hydrogen on preoxidized-Zr and of carbon monoxide on oxidized Cr have been investigated. Carbon monoxide is well known for its poisoning effect on different catalysts. The dissociation rates of carbon monoxide on various materials exposed to 20 mbar CO at different temperatures have been studied. The results are presented in Figure 4.1.. Figure 4.1 – CO dissociation rate on different materials exposed to 20 mbar CO at different temperatures High dissociation rates have been measured on materials such as pure Cr, Ni, Fe and stainless steel SS 304. It is interesting to note that these materials often suffer from high carbon uptake in CO containing atmospheres, a negative consequence of the high CO dissociation rates. On the other hand, relatively low dissociation rates were found on Zr, Cu, Al, Cu-8Al alloy, and Pt. CO adsorption on the surface of these materials can have a 20.

(29) positive effect. Carbon monoxide is identified as an effective site blocker for hydrogen adsorption and dissociation on preoxidized Zircaloy-2 (Zr2ox) at temperatures around 400°C. The uptake of hydrogen, measured as a pressure decrease in the reaction chamber upon exposure to Zr2ox, is shown in Figure 4.2.. Figure 4.2 – Influence of carbon monoxide addition to the gas phase on the hydrogen pressure upon exposure to Zr2ox and time evolution of hydrogen pressure in the reaction chamber without sample at 400°C. It can be seen that the uptake rate of hydrogen is substantially lowered (approximately 40 times) when 2 mbar carbon monoxide is added to the gas phase. No influence of nitrogen gas on the hydrogen uptake by Zircaloy-2 at 400°C was seen. When Cr2O3 was exposed to carbon monoxide at 600°C, a decrease in dissociation rate was observed upon addition of water. This can be interpreted as a blocking effect of carbon monoxide by water. Upon a subsequent removal of water from the gas phase, the carbon monoxide dissociation rate increased again. The results suggest that the surface activity for carbon monoxide dissociation should be a key factor for carbon uptake in certain applications and can be reduced by adsorbed water. Based on the results of the present work the following rating for the tendency of adsorption on Zr2ox and Cr2O3 at temperatures in the range 400-600°C has been made: N2 < H2 < CO < H2O.. 21.

(30) 4.2 Influence of Pt, Fe/Ni/Cr–containing intermetallics and deuterium on the oxidation of Zr-based materials In Paper 2, the transport of oxygen and hydrogen through the growing zirconia layer in the oxidation of Zr-based materials in O2 at 400°C and in water vapour at 370°C was investigated using GPA and SIMS. The paper is divided in three sections. I.. Oxygen dissociation on Zr-based materials and on Pt. II. Influence of Pt and deuterium on the oxidation of Zr in O2 at 400°C III. Oxidation of Zr-based tubes in water at 370°C I. Oxygen dissociation on Zr-based materials and Pt Oxygen dissociation rate on Pt as well as on preoxidized Zr2Fe, Zr2Ni, ZrCr2 and Zircaloy-2 in exposure to 20 mbar O2 has been measured and the results are summarized in Figure 4.3.. T,°C 600. 500. 400. 350. Dissociation rate, mol O cm s. -2 -1. 10-6. 700. Pt. 10-8. Zr Fe 2 Zr Ni. 10-10. 2. 10-12. Zry-2 oxide. ZrCr. 2. 10-14 0.0009. 0.00105. 0.0012. 0.00135. 0.0015. 0.00165. 1/T,1/K Figure 4.3 – Oxygen dissociation rate on Pt, preoxidized Zr2Fe, Zr2Ni, ZrCr2 and Zircaloy-2 in 20 mbar O2 At 400°C it can be seen that the oxygen dissociation efficiency decreases in the order: Pt > Zr2Fe > Zr2Ni > ZrCr2 ≥ Zircaloy-2. It is important to note that the dissociation rate of O2 on preoxidized-Zr2Fe is 103 times higher than on preoxidized-Zircaloy-2 at 400°C. 22.

(31) II. Influence of Pt and deuterium on the oxidation of Zr in O2 at 400°C Two 1.8 cm2 Zr plates (99.9% metal basis excl. Hf) were used in this study. One sample was charged with deuterium to 600 wtppm D. Both samples were then partly coated with 200Å porous Pt. The samples were oxidized at 400°C in two stages, first in. 16,16. O2 then in. 18. O-. enriched oxygen gas for totally 12 hours. After oxidation, the samples were characterized with SIMS, XPS, SEM and optical microscopy. A summary of the 18O SIMS results, expressed as integrated oxygen-counts from the second. 1 1012. 8 1011. Integrated O-count rates from 2. nd. stage of oxidation. stage of oxidation, is presented in Figure 4.4.. 6 1011 Effect of Pt on the oxidation of Zr. Zr. 4 1011 Effect of Pt on the oxidation of Zr(D). 2 1011. O-spill over ~ 10 nm. O-spill over ~ mm. 0. Area with Pt particles. Zr(D). 0. 2000. 4000. 6000. 8000. Distance from Pt-area, µm. Figure 4.4 – Effect of Pt and D on the oxide growth on Zr in 20 mbar O2 at 400°C It can be seen that an enhanced oxidation in the area with Pt particles takes place on both samples. In the vicinity of the Pt-enriched surface area a local minimum in oxide thickness is obtained for both samples and the thinnest oxide is obtained on the Zr sample with D in the substrate. At the interface between the Pt-enriched area and the area without Pt-particles, the oxygen dissociation rate decreases about 105 times as can be seen in Figure 4.3. There is also a spillover effect by surface diffusion of the dissociated oxygen from the area with Pt particles, which is identified as having mm-range effect. These observations suggest that in the vicinity of the Pt-enriched area, which has a lower activity of On- compared with the area with Pt particles, an outward Zr diffusion induced by D from the substrate balances the On23.

(32) inward flux and can explain the formation of a more dense oxide. Diffusion profiles for oxygen and hydrogen were investigated and related to the latest findings in the field of Zr oxidation. We suggest that in the oxidation of Zr-based materials in water containing atmospheres, hydrogen accumulation in the oxide near the oxide/metal interface could explain the frequently observed deterioration of the “barrier” layer. This also indicates that Zr could move easier in the deteriorated barrier when hydrogen is present. Our XPS results show that at the oxide/gas interface, the Zr oxidation state varies, being high in the Pt area (probably ZrO2 or ZrO2+x), and some suboxides are probably present away from the area with Pt particles. For the sample with D in the substrate, the presence of D in the oxide at the oxide/gas interface generates higher binding energies for O1s and Zr 3d5/2, especially outside the Pt-enriched area. A general illustration of the influence of Pt and D on the oxidation of zirconium in O2 at 400°C is presented in Figure 4.5. For simplicity, the molecular transport is not shown in the figure.. Zr. Zr(D) Porous Pt coating. O2. Zr oxide Zr metal Zr metal with D in the substrate. Zrn+. Dn+ On-. Figure 4.5 - Illustration of the influence of Pt and D on the oxide growth on Zr. 24.

(33) III. Oxidation of Zr-based tubes in water at 370°C The exposure of two Ar-filled 1 m long Zr-based tubes to 22 mbar water vapour at 370°C for 305 and respectively 630 minutes was investigated. The oxide thickness and the hydrogen content were measured at different positions from the water inlet using SIMS depth profiles. Integrated SIMS count rates of oxygen and deuterium along the length of the tubes are shown in Figure 4.6. Related to the water inlet, the thickest oxide was obtained at 330-400 mm. Local minima in the oxide thickness were found at 430 mm and 478 mm respectively. At these positions there is some hydrogen in the substrate, as a result of the reaction with water. In this case, both oxygen and hydrogen diffuse inward towards the oxide/metal interface. At 996 mm no significant oxidation takes place but the deuterium content is slightly higher than at the water inlet position (20°C). Deuterium, resulting from the reaction of D2O with Zr at 370°C, diffuses faster than D2O through the Ar filled tubes.. D-counts 630 min.. 250. 200 O - counts 630 min. 150. 100. 50. O - counts 305 min.. O - counts 20°C. 100. Oxygen counts, a.u.. Counts in SIMS, a.u.. Oxygen counts, a.u.. 300. 305 min. Ar+H2O. 50. 0 415. 420. 425. d, mm. 150. 430. 435. 630 min. Ar+D2O. 100 50 0 400. 450. 500. d, mm. 550. 600. 0 0 D - counts 20°C. 200. 400. 600. 800. 1000. Distance from water inlet (d), mm. Figure 4.6 – SIMS integrated oxygen and deuterium count rates along the length of the Zrbased tubes after oxidation in 20 mbar water vapour for 305 and 630 minutes respectively The experiment shows that the hydrogen content in the substrate influences the formation of the oxide layer. 25.

(34) A balance between inward oxygen and outward metal diffusion has been previously proposed as an important condition for obtaining oxides, such as zirconium oxide, with high protective ability [13]. The inward oxygen flux can be promoted by depositing Pt or other oxygen dissociating elements on the Zr surface. We propose that Fe-containing secondphase particles act as oxygen dissociating elements in analogue way as Pt. The outward. zirconium flux, on the other hand, can be promoted by introduced hydrogen in the oxide lattice. We suggest that this phenomenon is beneficial for healing defects in the zirconium oxide scale, such as pores and microcracks, which are mainly formed during oxidation caused by the inward oxygen diffusion. Hence, a proper choice of second-phase particles composition and size distribution can be beneficial for reducing the negative effect of hydrogen uptake. 4.3 A method for characterization of gas transport in porous oxides In Paper 3, a theoretical method based on gas release is presented, which allows calculation of gas diffusivity and concentration in samples with specific geometries such as plate, cylinder and sphere. The model is also used to estimate the effective pore size of microporous and/or mesoporous oxides, previously equilibrated in controlled humid air. Low outgassing temperatures are used for this purpose. This method is non-destructive and can also be applied to characterize metal oxide scales, with known oxide thickness. The characterization of the open micro- and/or meso- pores formed in the oxide during early oxidation stages of metals is crucial. The gas release during outgassing is quantitatively evaluated using a mass spectrometer having a quadrupole analyser, placed in ultra high vacuum. The method is validated in measurements of diffusivity and solubility of He in quartz at 80°C. Two pretransition oxides on Zircaloy-2 and one Fe-oxide have been characterized.. Diffusion of nitrogen and water in Zircaloy-2 and Fe oxide scales. Using the solution of the differential equation of diffusion for plate-like geometry and assuming that the initial concentration is uniform and equal with C1, and there is no flux across L=0 (metal substrate), the mathematical equation for flux during outgassing was obtained. 26.

(35) oxide. metal substrate. The outgassing from an oxide scale of thickness L is illustrated in the Figure 4.7:. F=. UHV. 2 DC1 L. ∞. . ∑ exp − (2n − 1) π 2. 2. n =1. Dt  4 L2 . L Figure 4.7 - Outgassing from an oxide scale of thickness L The experimental data for water and nitrogen flux evolution in time and the integrated flux over time are used for determination of the diffusivity (D) and concentration (C1) of these gases in Zircaloy-2 and Fe-oxide scales. The results are presented in Table 4.1 and 4.2 respectively.. Table 4.1 – Transport parameters for H2O and N2 in oxide scales on Zircaloy-2 at 80°C H2 O 1.3 µm. Oxide thickness, L 2. D, cm /s. N2. 3.4 - 5.2 10 3. C1, µmol/cm. 3 µm -14. 8.6 10. 190. -14. - 1.1 10. 119. 1.3 µm -13. 2.4 - 2.8 10. 3 µm -13. 2.1 10. 3.8. -13. 5.0. Table 4.2 - Diffusivity and concentration of H2O and N2 in Fe oxide scale at 20°C. Oxide thickness, L 2. H2 O. N2. 300 µm. 300 µm. 3-6 10. D, cm /s 3. C1, µmol/cm. -9. 90. 5 10-9. 10. Effective pore size estimation We assume that the pores are long cylinders with circular cross section of diameter Φ. The samples used for this study were equilibrated with controlled humid air at room temperature. Under these conditions, the water molecules, which have higher tendency for adsorption than N2 molecules, will stick to all surfaces, including internal pore surfaces. N2 molecules will predominantly be found in the pore’s gas phase. The H2O/N2 molar ratio,. 27.

(36) which can be associated with the ratio between the surface area and the volume of the pores, can be used to calculate the effective pore size. The C1 values for water and nitrogen from Table 4.1 and 4.2 were used to calculate the H2O/N2 molar ratio for the Zr and Fe oxide scales. The estimated effective pore size for the above-mentioned oxide scales is presented in Figure 4.8. 1000000. 1.0 ML. 100000. 0.04 ML. 1000 100 10. 0.1. Fe oxide. 1. Zry-2 3 µ m oxide. Zry-2 1.3 µ m oxide. H2 O/N2 molar ratio. 10000. 0.01 0.1. 1. 10. 100. 1000. Effective pore diameter, Φ, nm. Figure 4.8 - Effective pore size estimation for oxide scales on Zircaloy-2 and Fe Effective pore sizes in the range of 1-10nm and 100-1000nm for the Zircaloy-2 and Fe oxides have been found.. 28.

(37) 5. Conclusions The transport of oxygen and hydrogen in zirconia oxide scales at temperatures around 400°C has been investigated using the isotope monitoring technique Gas Phase Analysis and Secondary Ion Mass Spectroscopy, and the following conclusions have been drawn:. Modification of the oxide surface properties (oxide/gas interface) can influence the. inward transport of species such as hydrogen and oxygen. We can distinguish two main effects: o Inhibitor effect: CO is identified as an effective site blocker for hydrogen adsorption and dissociation on preoxidized Zircaloy-2 at temperatures around 400°C. No influence of N2 on the hydrogen uptake in Zircaloy-2 is found. Based on the results of the present work the following rating for the tendency for adsorption on oxidized Zircaloy-2 and Cr2O3 at temperatures in the range 400-600°C has been made: N2 < H2 < CO < H2O. o Catalytic effect: porous Pt coated on the Zr surface, catalyses the oxygen dissociation at the gas/oxide interface, promoting the inward oxygen transport. At 400°C, oxygen dissociation efficiency decreases in the order: Pt > Zr2Fe > Zr2Ni > ZrCr2 ≥ Zircaloy-2. The dissociation rate of O2 on preoxidized-Zr2Fe is 103 times higher than on preoxidized-Zircaloy-2 at 400°C. Hydrogen present in the oxide generates enhanced outward diffusion of Zr. Combining the effect of Pt (enhanced inward oxygen transport) and D (enhanced outward Zr diffusion), a better balance in the oxide growth was obtained, and as a result, a more protective oxide layer was formed. We propose that Fe-containing second-phase particles act as oxygen dissociating elements in the analogue way as Pt does. Hence, a proper choice of second-phase particles composition and size distribution can be beneficial for reducing the negative effect of hydrogen uptake. Open porosity with effective pore diameter in nanometer range was found in thermally-grown pretransition oxides on Zircaloy-2.. 29.

(38) 6. Future work. The results obtained opened new perspectives for understanding of the phenomena involved in the transport of hydrogen and oxygen species in the Zr-oxide scales. Further investigation of the hydrogen transport in zirconia, including new ways to reduce the hydrogen uptake needs to be considered. Dual transport mode (atomic and molecular) for hydrogen transport in silica was identified in studies performed by E. Hörnlund and G. Hultquist [41]. One question arises: Can atomic and/or molecular hydrogen be transported through the zirconia layer, and under which conditions? We found in pretransition oxides in Zircaloy-2 porosity in the nanometer range, which can easily accommodate molecular species like hydrogen and/or water. Formation of hydrides is also an important issue that needs to be further clarified. To become hydride, hydrogen needs to receive electrons, which is not difficult inside metals, but for oxides, special reducing conditions are necessary. Close to the oxide/metal interface, these reducing conditions are possible to appear, and thus, the oxide becomes thermodynamically instable, oxygen can dissolve in the metal underneath the oxide, and hydrides start to from. The formation of “substitutional hydroxides” could be the prerequisite for hydride formation inside the oxide layer. The correlation between the second-phase particles composition, size distribution and the hydrogen uptake could be interesting to investigate using the GPA technique. Better knowledge of hydrogen uptake rate by Zr intermetallics exposed in hydrogen gas or aqueous environment could also be useful to consider for optimisation of the existing alloys. Oxygen transport in zirconia, investigations of dual transport mode is certainly still another subject of interest not only for nuclear energy production but also for fuel cell technology.. 30.

(39) 7. Acknowledgments I would like to thank: My supervisor, Doc. Gunnar Hultquist for guiding me in using the GPA technique, for sharing with me valuable knowledge and for the long discussions and scientific support. Prof. Christofer Leygraf, as an excellent leader for the “corrosion family”, for the friendly and fruitful working atmosphere, for reviewing my manuscript and for his guiding and moral support in difficult moments. Magnus Limbäck, my supervisor from Westinghouse Electric Sweden AB, for organising this project, for his kindness and friendship and for all the stimulating discussions. Thanks also to Mats Dahlbäck and Per Tägtström for their suggestions and fruitful discussions. Erik Hörnlund for all his help related to computers, animations and experimental questions and for the friendly working atmosphere. Dr. John Rundgren for his suggestions about animations and for sharing his knowledge about diffusion. All Zr Experts Group Friends for interesting discussions and scientific advice. Qian Dong, Wenle He, Andreas Pettersson and Jan Sandberg for being really nice roommates and friends. All my colleagues from the corrosion division for their friendship. It is a real pleasure to work with you! Anna, Johan, Qian and Gunilla thanks for the relaxing dinners after hardworking days. Klara, Inger and Sofia for the moral support and friendship. Babak for help during my party. Jinshan and Bo for suggestions and fruitful discussions. Peter Szakalos for helping me with the SEM and LOM and being a good friend. Johan Angenete for guiding me in Boston and for fruitful discussions. Financial support from Westinghouse Electric Sweden AB is gratefully acknowledged. Finally I would like to thank my family, especially my husband, for making everything easier, even being kilometres away.. 31.

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(47) Applied Surface Science 233 (2004) 392–401. Gas phase analysis of CO interactions with solid surfaces at high temperatures Clara Anghela,*, Erik Ho¨rnlunda, Gunnar Hultquista, Magnus Limba¨ckb a. Division of Corrosion Science, Department of Materials Science and Engineering, Royal Institute of Technology, Drottning Kristinas va¨g 51, S-100 44 Stockholm, Sweden b Westinghouse Electric Sweden AB, S-721 63 Va¨stera˚s, Sweden Received in revised form 5 April 2004; accepted 5 April 2004 Available online 28 May 2004. Abstract An in situ method including mass spectrometry and labeled gases is presented and used to gain information on adsorption of molecules at high temperatures (>300 8C). Isotopic exchange rate in H2 upon exposure to an oxidized zicaloy-2 sample and exchange rate in CO upon exposure to various materials have been measured. From these measurements, molecular dissociation rates in respective system have been calculated. The influence of CO and N2 on the uptake rate of H2 in zirconium and oxidized zicaloy-2 is discussed in terms of tendency for adsorption at high temperatures. In the case of oxidized Cr exposed to CO gas with 12 C, 13 C, 16 O and 18 O, the influence of H2O is investigated with respect to dissociation of CO molecules. The presented data supports a view of different tendencies for molecular adsorption of H2O, CO, N2, and H2 molecules on surfaces at high temperatures. # 2004 Elsevier B.V. All rights reserved. PACS: 82.65.P; 82.30.L Keywords: CO; Chromium; Zirconium; Zircaloy-2; Adsorption; Dissociation; High temperature; Gas phase analysis. 1. Introduction The importance of the fundamental interactions of gas molecules with surfaces is reflected by the vast quantity and diversity of papers published on adsorption [1–8]. Traditional surface science techniques such as AES, XPS, UPS, LEED, HREELS, IRAS and TPD continue to be used to reveal new insights of molecular–solid surface interactions. However, in most of *. Corresponding author. Tel.: þ46-8-7906670; fax: þ46-8-208284. E-mail address: anghel@kth.se (C. Anghel).. the published papers the temperatures considered are scarcely above room temperature. This reflects the experimental difficulties to measure surface–molecular interaction at elevated temperatures. For example, there is a large amount of data on surface coverage and sticking probabilities of molecules on surfaces below room temperature while there is a striking lack of this information for higher temperatures. While waiting for experimental methods to directly measure adsorption at high temperatures, theoretical calculations on the subject have been performed [9–12]. Our experimental approach to the high temperature problem is to focus not directly on the sample itself but instead on. 0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.04.001.

(48) C. Anghel et al. / Applied Surface Science 233 (2004) 392–401. the gas phase surrounding the sample. Isotopic labeled gases are used in the technique called gas phase analysis (GPA), where processes on a sample surface are monitored by analyzing changes in the gas phase with mass spectrometry. By mixing isotopic labeled gases and measure changes in the gas phase over time, information about surface reactions such as dissociation and exchange can be retrieved. Information about these processes is crucial in many scientific fields involving elevated temperatures such as high temperature oxidation [13], hydration, metal dusting (catastrophic carburization) and catalysis [14] to name a few. In this paper, the following two issues are considered: 1. The influence of CO on the hydrogen uptake in Zrbased materials is examined and relates to the important technological problem of hydrogen embitterment of the Zr-based materials used in the nuclear power industry. 2. A summary of dissociation rates of CO on some materials is presented. Specifically the influence of H2O on CO dissociation on oxidized Cr is investigated and relates to applications, where metal dusting is at risk on stainless steel.. 393. 2.2. GPA equipment Fig. 1 shows the experimental apparatus used in the GPA studies in this work. The equipment consists of essentially three parts. A mass spectrometer (MS) placed in ultrahigh vacuum (UHV), an approximately 70 cm3 virtually closed reaction chamber (consisting of a quartz tube and a cross made of stainless steel) and a gas handling system. By means of a pressure gauge the total pressure is measured in the reaction chamber. The equipment allows inlet of gas mixtures with various isotopes from the gas handling system into the reaction chamber. The partial pressures of the gas constituents in the reaction chamber can be analyzed over time with the MS (placed in UHV) via a leak valve. Gas consumption in reaction of a sample with a gas is measured as pressure decrease in the closed reaction chamber. Heating of a sample in the reaction chamber during evacuation with the ionpump via a fully opened leak valve provides a method for out-gassing samples from mostly hydrogen. A calibration of mass spectrometer signal and ionpump rate gives the opportunity also to quantify the amount of hydrogen and other gases released in out-gassing [15]. 2.3. Secondary ion mass spectroscopy analysis SIMS analyses were performed with a 10 keV, 50– 200 nA primary beam of Csþ ions, which was rastered over 200 mm 300 mm, where ions where detected from a centered area with a diameter of 70 mm.. 2. Experimental 2.1. Materials All samples were polished down to 2400 mesh SiC paper and rinsed in ethanol before the experiments. All pure metals investigated had purities higher than 99.9% and the composition of the alloys are presented in Table 1.. Leak valve. Enclosed volume P. Table 1 Composition of alloys. MS. Alloys. Composition (wt.%). Zircaloy-2 Cu–8Al SS 304. 1.5Sn, 0.14Fe, 0.1Cr, 0.06Ni, balance Zr 8Al, balance Cu 18.5Cr, 10.1Ni, 1.2Mn, 0.43Si, 0.41Mo, 0.25Cu, balance Fe 18.5Cr, 10.1Ni, 1.2Mn, 0.43Si, 0.41Mo, 0.25Cu, 0.1Pt, balance Fe. SS 304-Pt. Quartz tube. 10 -12 atm Ion pump. Tube Sample furnace. Rough pump Gas handling system. Fig. 1. Equipment used in gas phase analysis (GPA) experiments..

(49) 394. C. Anghel et al. / Applied Surface Science 233 (2004) 392–401. 2.4. Molecular dissociation 2.4.1. Calculation method The GPA technique described above has been used to quantify molecular dissociation rates in the following way. A mixture of isotopic labeled gases (1;1 H2 þ 2;2 H2 or 12 C16 Oþ13 C18 O or 16;16 O2 þ 18;18 O2 ) is introduced into the reaction chamber. The exchange of isotopes between the molecules is then measured over time. After a sufficiently long exposure time, the composition of the gas phase will reach to statistical equilibrium (fully scrambled system where no further changes in the gas composition takes place). The composition at statistical equilibrium depends on the isotopic abundance in the gas. The composition at statistical equilibrium of CO molecules, 12 16 C O, 13 C16 O, 12 C18 O, 13 C18 O, is calculated by combinatorial analysis using the abundance of 12 C, 13 C, 16 O, and 18 O. For an initial 50% 12 C16 O þ 50% 13 18 C O gas mixture, the statistical equilibrium composition is: 25% 12 C16 O, 25% 13 C16 O, 25% 12 C18 O, 25% 13 C18 O. In the case of hydrogen isotopes, 1 H and 2 H are combined in the form of 1;1 H2 , 1;2 H2 and 2;2 H2 . For an initial 50% 1;1 H2 þ 50% 2;2 H2 gas mixture, the statistical equilibrium composition is: 25% 1;1 H2 , 50% 1;2 H2 , 25% 2;2 H2 . Starting with 50% 1;1 H2 þ 50% 2;2 H2 and assuming a constant dissociation rate, ndiss , the kinetics of 1;2 H2 formation is shown in Fig. 2. In this figure the preferable. Fig. 2. Time evolution of 1;2 H2 molecules, with an initial composition of the hydrogen gas 50% 1;1 H2 þ 50% 2;2 H2 . fE is the fraction of the mixed molecules at statistical equilibrium (t ! 1).. region for measurement with good accuracy is indicated. In the following, dissociation refers to molecular dissociation. ndiss can be calculated from [16]: ndiss ¼ 0:04n. dP Vch ðfE BAs Þ1 dt. (1). where B¼1. f fE. In Eq. (1), the factor 0.04 (mol atm1 dm3) comes from the molar volume of an ideal gas at standard pressure and temperature, P is the partial pressure of the ‘‘mixed’’ molecules (1;2 H2 , 12 C18 O, 13 C16 O and 16;18 O2 respectively) (atm), As the sample area (cm2) and Vch is the volume of the reaction chamber (dm3). The factor n is 1 for molecules of the form XY (one X and one Y atom per molecule) and 2 for molecules of the form X2 (two X atoms per molecule). The dissociation rate, ndiss , given by Eq. (1) is expressed in mol cm2 h1. B defines a compensation factor, where f is the fraction of the mixed molecules in the gas phase at time t and fE is the fraction of the mixed molecules in the gas phase at statistical equilibrium. A thorough description of the method used can be found in [16]. 2.4.2. Experimental examples The time evolution for isotopic mixtures of hydrogen in exposure to a preoxidized zicaloy-2, Zr2ox, (approximately 2 mm oxide thickness) at 300 8C and isotopic mixtures of CO in exposure to Cr with native oxide at 600 8C is shown in Figs. 3 and 4, respectively. After the gas consumption in the MS measurements was taken into account, the uptake rate of carbon was just about to be measurable (approximately 0.1 mmol C cm2 h1) as seen in Fig. 4. Slopes of the ‘‘mixed’’ molecules are measured at different times in respective graph. The compensation factor, B, is calculated and by the use of Eq. (1) the dissociation rates of H2 and CO are evaluated. The values for the dissociation rate of H2 on Zr2ox and the hydrogen uptake rate in Zr2ox at 3008 at different exposure times are presented in Table 2. The data from Table 2 shows that the hydrogen dissociation rate exceeds the hydrogen uptake rate, which is plausible since the hydrogen should be absorbed in atomic form. For CO on Cr it is shown in Fig. 4 that the dissociation rate is about.

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