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Gas-tight oxides – Reality or just a Hope
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
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2006 (English)In: Materials Science Forum, ISSN 0255-5476, Vol. 522-523, 93-101 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2006. Vol. 522-523, 93-101 p.
Keyword [en]
gas diffusion; porosity; gas content; gas phase analysis; zircaloy; iron; oxides
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-6091ISI: 000241416500011ScopusID: 2-s2.0-37848999932OAI: diva2:10702
QC 20100629Available from: 2006-09-08 Created: 2006-09-08 Last updated: 2010-06-29Bibliographically approved
In thesis
1. Modified oxygen and hydrogen transport in Zr-based oxides
Open this publication in new window or tab >>Modified oxygen and hydrogen transport in Zr-based oxides
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
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.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 48 p.
Zirconia, Zirconium, Zircaloy, hydrogen and oxygen diffusion, SPP, oxygen dissociating elements, oxidation, dissociation, hydration, CO adsorption, molecular transport, porosity.
National Category
Materials Engineering
urn:nbn:se:kth:diva-4095 (URN)91-7178-429-2 (ISBN)
Public defence
2006-09-22, F3, Lindstedtsvägen 26, Stockholm, 10:00
QC 20100629Available from: 2006-09-08 Created: 2006-09-08 Last updated: 2011-10-05Bibliographically approved

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