The corrosion of Zr(Fe, Cr)2 and Zr2Fe secondary phase particles in Zircaloy-4 under 350 °C pressurised water conditions

Kirsty Annand,Magnus Nord,Ian Maclaren,Mhairi Gass

doi:10.1016/j.corsci.2017.09.014

Abstract

Using Scanning Transmission Electron Microscopy (STEM) coupled with Dual Electron Energy Loss Spectroscopy (DualEELS) and scanned diffraction, the corrosion and incorporation of Secondary Phase Particles (SPPs) in the oxide layer of Zircaloy-4 material has been investigated. This study focuses on mapping the corrosion of Zr2Fe and Zr(Fe, Cr)2 precipitates during the oxidation process and depicting their morphology as the oxidation front advances through the material. It has been found that Zr2Fe SPPs retain the same general shape as in their pre oxidation stage, and transform to a nanocrystalline homogeneous mixed oxide, with a strong crystallographic texture, but hitherto unknown structure. The Zr(Fe, Cr)2 Laves-phase SPPs however, oxidise in a notably more complicated manner. As the α-Zr around an SPP begins to oxidise, the SPP is completely encapsulated by the ZrO2 whilst much of the SPP remains initially unoxidised. But, on oxidation, significant elemental segregation takes place, usually leaving a Cr2O3-rich cap, a nanocrystalline Zr,Cr mixed oxide body and veins of well-crystallised metallic iron. Both forms of SPP have a different expansion on oxidation compared to the Zr, resulting in cracking of the ZrO2.

Highlights

Worldwide, Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr) has been a popular material of choice for the containment of nuclear fuel and other structural components within commercial Pressurised Water Reactors (PWRs), due to their high corrosion resistance [1], low thermal neutron cross section [2], superior mechanical properties, and favourable chemical stability in highly aggressive environments
This study has focussed on the morphology and chemical composition of both Zr2Fe and Zr(Fe, Cr)2 Secondary Phase Particles (SPPs) found within samples of Zircaloy-4, as the oxidation front advances through the metal matrix
Whilst Zr2Fe SPPs have rarely been reported in the literature, in the work of which this publication forms a part, more than 30 different Zircaloy-4 specimens have been prepared and analysed and Zr2Fe-type precipitates have been observed on multiple occasions

Summary

Introduction

Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr) has been a popular material of choice for the containment of nuclear fuel and other structural components within commercial Pressurised Water Reactors (PWRs), due to their high corrosion resistance [1], low thermal neutron cross section [2], superior mechanical properties, and favourable chemical stability in highly aggressive environments. A number of recent studies have elucidated the nanoscale details of this process in the alpha-Zr phase in the Zircaloy [4,5,6]. In this case, with the addition of alloying elements such as Sn, Fe and Cr to the Zr base metal, up to a value of about 2 wt.%, the corrosion behaviour of the alloy becomes more predictable and less likely to suffer catastrophic breakaway corrosion. The addition of Cr to a Zr base alloy improves the alloy’s resistance to oxidation [9], this improvement is offset by the fact that the introduction of ZrCr2 laves phase precipitates to the matrix can result in embrittlement of the material [9]. Precipitates in the Zr1%Fe system have been seen to show an increased oxidation rate when in contact with the gas:oxide surface [11]

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