Understanding Corrosion and Hydrogen Pickup of Zirconium Fuel Cladding Alloys: The Role of Oxide Microstructure, Porosity, Suboxides, and Second-Phase Particles

Abstract

We used a range of advanced microscopy techniques to study the microstructure, nanoscale chemistry, and porosity in zirconium alloys at different stages of oxidation. Samples from both autoclave and in-reactor conditions were available, including ZIRLO™, Zr-1.0Nb, and Zr-2.5Nb samples with different heat treatments. Scanning transmission electron microscopy (STEM), transmission Kikuchi diffraction (TKD), and automated crystal orientation mapping with TEM were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidized in the autoclave and in-reactor, with shorter, less well-aligned monoclinic grains and more tetragonal grains in the neutron-irradiated samples. A combination of energy-dispersive X-ray mapping in STEM and atom probe tomography analysis of second-phase particles (SPPs) can reveal the main and minor element distributions respectively. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of β-niobium precipitates but encourages the dissolution of iron from Laves-phase precipitates. An electron energy-loss spectroscopy (EELS) analysis of the oxidation state of niobium in β-niobium SPPs in the oxide revealed the fully oxidized Nb5+ state in SPPs deep into the oxide but Nb2+ in crystalline SPPs near the metal-oxide interface. EELS analysis and automated crystal orientation mapping with TEM revealed Widmanstatten-type suboxide layers in some samples with the hexagonal ZrO structure predicted by ab initio modeling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlated well to the instantaneous oxidation rate, suggesting that this oxygen-rich zone is part of the protective oxide that is rate limiting in the transport processes involved in oxidation. Porosity in the oxide had a major influence on the overall rate of oxidation, and there was more porosity in the rapidly oxidizing annealed Zr-1.0Nb alloy than in either the recrystallized alloy or the similar alloy exposed to neutron irradiation.

Hydrogen pickup during oxidation in aqueous environments: The role of nano-pores and nano-pipes in zirconium oxide films

“…% and a layer thickness of about 200 nm. Similar studies on other commercial alloys corroded in pressurized water showed varying thicknesses of the same metastable ZrO suboxide at different stages of the oxidation process [12][13][14][15]. All these authors also observed an oxygen concentration gradient over a distance of 200 -400 nm into the metal from a level close to the sub-oxide of about 30 at.…”

Effect of neutron and ion irradiation on the metal matrix and oxide corrosion layer on Zr-1.0Nb cladding alloys

“…These SPPs are 20 nm to 60 nm in diameter and roughly spherical in the metallic matrix. They oxidise rapidly when incorporated into the growing oxide layer and become amorphous as previously reported [43,44], and at the same time they expand in the direction of oxidation and become elliptical in shape as oxygen atoms are incorporated ( Figure 7). In bright field TEM images, like those shown in Figure 3, SPPs can be hard to detect, but highangle annular dark field (HAADF) imaging is able to differentiate between metallic and oxidised SPPs by the change in average atomic mass (z-contrast), as seen in Figure 7.…”

“…Moving on to the h-ZrO suboxide phases at the metal-oxide interface, we have previously shown a correlation between instantaneous oxidation rate and the thickness of the combined suboxide/oxygen-saturated area layer [70]. This feature of the metal-oxide interface can be best studied by STEM HAADF and scanning precession electron diffraction mapping in the TEM, as shown in Figure 4.…”

The characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fe alloys: A critical review

“…The corrosion and mechanical performance of Zr alloys is intrinsically related to the nature, morphology and behaviour of second phases and the underlying microstructure [1][2][3][4][5][6][7][8][9]. It is therefore essential to characterise second phases with clarity and consistency in order to accurately assess their evolving behaviour during the lifetime of nuclear components.…”