SYMPOSIA PAPER Published: 01 April 2012
STP152920120026

Studies Regarding Corrosion Mechanisms in Zirconium Alloys

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Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre- and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.

Author Information

Preuss, M.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Frankel, P.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Lozano-Perez, S.
Dept. of Materials, Univ. of Oxford, Oxford, GB
Hudson, D.
Dept. of Materials, Univ. of Oxford, Oxford, GB
Polatidis, E.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Ni, N.
Dept. of Materials, Univ. of Oxford, Oxford, GB
Wei, J.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
English, C.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB Dept. of Materials, Univ. of Oxford, Oxford, GB National Nuclear Laboratory, Sellafield, Seascale, Cumbria, GB
Storer, S.
Materials Engineering, The Open Univ., Milton Keynes, GB
Chong, K., B.
Materials Engineering, The Open Univ., Milton Keynes, GB
Fitzpatrick, M.
Materials Engineering, The Open Univ., Milton Keynes, GB
Wang, P.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Smith, J.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Grovenor, C.
Dept. of Materials, Univ. of Oxford, Oxford, GB
Smith, G.
Dept. of Materials, Univ. of Oxford, Oxford, GB
Sykes, J.
Dept. of Materials, Univ. of Oxford, Oxford, GB
Cottis, B.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Lyon, S.
School of Materials, The Univ. of Manchester, Materials Performance Centre, Manchester, Lancashire, GB
Hallstadius, L.
Westinghouse Electric, Vasteras/Pittsburgh, Sweden, US
Comstock, R., J.
Westinghouse Electric, Vasteras/Pittsburgh, Sweden, US
Ambard, A.
EDF, R&D, Moret sur Loing, FR
Blat-Yrieix, M.
EDF, R&D, Moret sur Loing, FR
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Details
Developed by Committee: B10
Pages: 649–681
DOI: 10.1520/STP152920120026
ISBN-EB: 978-0-8031-8893-8
ISBN-13: 978-0-8031-7515-0