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  • 1.
    Anghel, Clara
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Modified oxygen and hydrogen transport in Zr-based oxides2006Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Most metals and alloys in the presence of oxygen and moisture will instantaneously react and form a thin (2-5 nm) surface oxide layer. For further reaction to occur, oxygen ions and/or metal cations often diffuse through the already formed oxide layer. The corrosion resistance of a metal in aggressive environments at high temperatures depends on the properties of the surface oxide scale.

    Zirconium-based alloys represent the main structural materials used in water-cooled nuclear reactors. For these materials, the formation of a thin, adherent oxide scale with long-term stability in high temperature water/steam under irradiation conditions, is crucial. In this thesis, the transport of oxygen and hydrogen through Zr-based oxide scales at relevant temperatures for the nuclear industry is investigated using isotopic gas mixtures and isotope-monitoring techniques such as Gas Phase Analysis and Secondary Ion Mass Spectrometry.

    Porosity development in the oxide scales generates easy diffusion pathways for molecules across the oxide layer during oxidation. A considerable contribution of molecular oxygen to total oxygen transport in zirconia has been observed at temperatures up to 800°C. A novel method for evaluation of the gas diffusion, gas concentration and effective pore size of oxide scales is presented in this thesis. 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 and/or 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 particles composition and size distribution can lead to the formation of dense oxides, which are characterized by low oxygen and hydrogen uptake rates during oxidation.

    Hydrogen uptake in Zr-based materials during oxidation in high temperature water/steam can generate degradation due to the formation of brittle hydrides in the metal substrate. A promising method for the suppression of hydrogen uptake has been developed and is presented in this thesis.

  • 2.
    Anghel, Clara
    KTH, Tidigare Institutioner, Materialvetenskap.
    Studies of transport in oxides on Zr-based materials2004Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    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 generated 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.

  • 3.
    Anghel, Clara
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Dong, Qian
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    A gas phase analysis technique applied to in-situ studies of gas-solid interactions2007Ingår i: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 47, nr 10, s. 3440-3453Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An ultrahigh vacuum technique using mass spectrometry for in-situ investigations of gas-solid interactions is described in this paper. Examples of chemical reactions (oxidation, hydration) between solids and gas mixtures, dissociation of gases on solid surfaces, outgassing of solid materials and permeation of gases through membranes are discussed where the experimental arrangement is explained in detail. This Gas Phase Analysis (GPA) technique can be used at temperatures from room temperature to 1200 degrees C and at pressures up to 1 atm. Aspects related to sample preparation, isotopic gas mixture selection, data acquisition, calibration and interpretation of the experimental data are also addressed.

  • 4.
    Anghel, Clara
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Dong, Qian
    Isotopic investigation of the transport of oxygen species in Y-stabilized zirconiaIngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690Artikel i tidskrift (Övrigt vetenskapligt)
  • 5.
    Anghel, Clara
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Dong, Qian
    Rundgren, John
    Hultquist, Gunnar
    Saeki, Isao
    Limbäck, Magnus
    Gas-tight oxides – Reality or just a Hope2006Ingår i: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 522-523, s. 93-101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A better understanding of the transport properties of gases in oxides is certainly very important in many applications. In the case of metals, a general protection measure against corrosion implies formation of a dense metal oxide scale. The scale should act as a barrier against gas transport and consequently it needs to be gas-tight. This is often assumed but rarely, if ever, confirmed. Hence there is a need for characterization of micro- and/or meso- pores formed especially during the early oxidation stage of metallic materials. This paper presents a novel and relatively straightforward method for characterization of gas release from an oxide previously equilibrated in a controlled atmosphere. The geometry of the sample is approximated to be a plate. The plate can be self-supporting or constitute a scale on a substrate. A mathematical model for calculation of diffusivity and gas content is given for this geometry. A desorption experiment, involving a mass spectrometer placed in ultra high vacuum, can be used to determine diffusivity and amount of gas released with aid of the mathematical model. The method is validated in measurements of diffusivity and solubility of He in quartz and applied in characterization of two Zr-oxides and one Fe oxide. From the outgassed amounts of water and nitrogen the H2O/N-2 molar ratio can be used to estimate an effective pore size in oxides.

  • 6.
    Anghel, Clara
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Hultquist, Gunnar
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Limbäck, M.
    Szakalos, P.
    Effects of Pt surface coverage on oxidation of Zr and other materials2008Ingår i: Journal of ASTM International, ISSN 1546-962X, Vol. 5, nr 2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Certain elements, including noble metals, are identified to influence corrosion behavior of many metals in high-temperature water/steam and O 2. We have previously reported effects of porous Pt coatings on the thermal oxidation of Zr, Fe, Ni, Cr and GaAs in O2. Effects of Pt on oxidation of Zircaloy-2 In H2O have also been observed at temperatures near 400°C. An enhanced oxidation rate Is observed in all of the studied systems upon a sufficiently high surface Pt-particle density. Even more interesting, low Pt-particle density in most cases leads to a decreased oxidation rate. In the case of Zr and Zircaloy-2 the beneficial effect of Pt is stronger when hydrogen is present In the metal substrate and in the oxide layer. From 18O/SIMS experiments it is concluded that, for a sufficiently high Pt-particle density on the surface, enhanced transport of dissociated oxygen, On- (n=0; 2), towards the oxide/metal interface occurs in all of the studied systems. An oxygen spillover from Pt particles to the adjacent surface is a known phenomenon in catalysis and partly explains the results observed also in the current study of oxidation of Zr-based materials. Such a spillover involves a high surface diffusivity of dissociated oxygen, O n-, which results In an increased gradient of On- across the oxide scale. In turn, the high oxygen gradient enhances the transport of oxygen towards the substrate/oxide-interface. This shows that not only diffusivlty but also an effective activity of dissociated oxygen at the external oxide surface influences the oxidation rate. Naturally, the effects of Pt are maximized when small (nm-sized) and evenly distributed Pt particles are present on the surface. In our presentation a summary of obtained results of Pt additions on different materials will be given with suggested interpretations. Especially, the observed beneficial combination of Pt and hydrogen in the oxidation of Zr-based materials Is discussed with a possible mechanistic explanation. Parallels are also drawn to the potential effect of Noble Metal Chemical Application (NMCA), used in a large fraction of the U.S. BWRs, on the corrosion resistance of Zr-based claddings.

  • 7.
    Anghel, Clara
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hultquist, Gunnar
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Limbäck, Magnus
    Influence of Pt, Fe/Ni/Cr–containing intermetallics and deuterium on the oxidation of Zr-based materials2005Ingår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 340, nr 2-3, s. 271-283Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An in situ gas phase analysis technique and the 18O-SIMS technique are used to evaluate the transport of oxygen and hydrogen in oxidation of Zr-based materials. At 400 °C, it is found that oxygen dissociation efficiency decreases in the order: Pt > Zr2Fe > Zr2Ni > ZrCr2 Zircaloy-2. Two Zr-plates partly coated with 200 Å porous Pt, with and respectively without D in the substrate, were oxidized in two stages at 400 °C. SIMS depth profiles in the Pt area show that an enhanced oxidation takes place mainly by inward oxygen transport. A minimum in the oxide thickness was found near the Pt area on both Zr plates. Two Ar-filled Zircaloy-2 tubes with ZrSn liner were exposed at 370 °C to 22 mbar water, filled in from one side. Our experimental results suggest that a proper choice of the SPP composition and size distribution can lead to reduced hydrogen uptake during oxidation of Zr-based materials in water.

  • 8.
    Anghel, Clara
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Hultquist, Gunnar
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Limbäck, Magnus
    Szakalos, Peter
    Effects of Pt Surface Coverage on Oxidation of Zr and Other Materials2009Ingår i: ZIRCONIUM IN THE NUCLEAR INDUSTRY: 15TH INTERNATIONAL SYMPOSIUM, 2009, Vol. 1505, s. 285-302Konferensbidrag (Refereegranskat)
    Abstract [en]

    Certain elements, including noble metals, are identified to influence corrosion behavior of many metals in high-temperature water/steam and O-2. We have previously reported effects of porous Pt coatings on the thermal oxidation of Zr, Fe, Ni, Cr, and GaAs in O-2. Effects of Pt on oxidation of Zircaloy-2 in H2O have also been observed at temperatures near 400 degrees C. An enhanced oxidation rate is observed in all of the studied systems upon a sufficiently high surface Pt-particle density. Even more interesting, low Pt-particle density in most cases leads to a decreased oxidation rate. In the case of Zr and Zircaloy-2 the beneficial effect of Pt is stronger when hydrogen is present in the metal substrate and in the oxide layer. From O-18/SIMS experiments it is concluded that, for a sufficiently high Pt-particle density on the surface, enhanced transport of dissociated oxygen, On- (n=0; 2), towards the oxide/metal interface occurs in all of the studied systems. An oxygen spillover from Pt particles to the adjacent surface is a known phenomenon in catalysis and partly explains the results observed also in the current study of oxidation of Zr-based materials. Such a spillover involves a high surface diffusivity of dissociated oxygen, On-, which results in an increased gradient of On- across the oxide scale. In turn, the high oxygen gradient enhances the transport of oxygen towards the substrate/oxide-interface, This shows that not only diffusivity but also an effective activity of dissociated oxygen at the external oxide surface influences the oxidation rate. Naturally, the effects of Pt are maximized when small (nm-sized) and evenly distributed Pt particles are present on the surface. In our presentation a summary of obtained results of Pt additions on different materials will be given with suggested interpretations. Especially, the observed beneficial combination of Pt and hydrogen in the oxidation of Zr-based materials is discussed with a possible mechanistic explanation. Parallels are also drawn to the potential effect of Noble Metal Chemical Application (NMCA), used in a large traction of the U.S. BWRs, on the corrosion resistance of Zr-based claddings.

  • 9.
    Anghel, Clara
    et al.
    KTH, Tidigare Institutioner, Materialvetenskap.
    Hörnlund, Erik
    KTH, Tidigare Institutioner, Materialvetenskap.
    Hultquist, Gunnar
    KTH, Tidigare Institutioner, Materialvetenskap.
    Limbäck, Magnus
    Gas phase analysis of CO interactions with solid surfaces at high temperatures2004Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 233, nr 1-4, s. 392-401Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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 degreesC). Isotopic exchange rate in H-2 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 N-2 on the uptake rate of H-2 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 C-12, C-13, O-16 and O-18, 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, N-2, and H-2 molecules on surfaces at high temperatures.

  • 10.
    Hultquist, Gunnar
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Anghel, Clara
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Szakalos, Peter
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Effects of hydrogen on the corrosion resistance of metallic materials and semiconductors2006Ingår i: High-Temperature Oxidation And Corrosion 2005 / [ed] Taniguchi, S; Maruyama, T; Yoshiba, M; Otsuka, N; Kawahara, Y, 2006, Vol. 522-523, s. 139-146Konferensbidrag (Refereegranskat)
    Abstract [en]

    For long time it is known that protons in aqueous solutions have a detrimental effect on metallic materials. Relatively recently, it has also been observed in aqueous solution that the pitting corrosion resistance of Cr, stainless steel 304 and 310 decreases and the anodic dissolution rate increases due to the presence of hydrogen in the metal. In gas phase a high oxidation rate has been observed for hydrogen containing Cr and Fe. Hydrogen in the substrate can also enhance the oxidation of Fe in SS 316 and As in GaAs. All these results suggest enhanced dissolution in aqueous solution and enhanced oxide growth at the oxide/gas interface in gas phase oxidation due to hydrogen promoted outward-transport of substrate components. A possible mechanism for such out-transport is an increased metal ion diffusivity in the metal-oxide due to a high abundance of metal ion vacancies generated by hydrogen. In contrast to all the above examples, also positive effects of hydrogen have been identified under certain conditions. In an attempt to understand both the negative and the positive effects the concept of a beneficial, balanced oxide growth is used. In this concept a certain amount of hydrogen can be beneficial in the oxidation by improving the balance between oxygen-ion and metalion transport, leading to more dense and protective oxides. Depending on the temperature, H(2) in air is considered as either a sink or a source for hydrogen in materials.

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