Evidence of hydrogen trapping at second phase particles in zirconium alloys

Katie L Moore,Mhairi Gass,Michael Preuss,Vidur Tuli,Zaheen Shah,Patrick A Burr,Christopher Jones

doi:10.1038/s41598-021-83859-w

Abstract

Zirconium alloys are used in safety–critical roles in the nuclear industry and their degradation due to ingress of hydrogen in service is a concern. In this work experimental evidence, supported by density functional theory modelling, shows that the α-Zr matrix surrounding second phase particles acts as a trapping site for hydrogen, which has not been previously reported in zirconium. This is unaccounted for in current models of hydrogen behaviour in Zr alloys and as such could impact development of these models. Zircaloy-2 and Zircaloy-4 samples were corroded at 350 °C in simulated pressurised water reactor coolant before being isotopically spiked with 2H2O in a second autoclave step. The distribution of 2H, Fe and Cr was characterised using nanoscale secondary ion mass spectrometry (NanoSIMS) and high-resolution energy dispersive X-ray spectroscopy. 2H− was found to be concentrated around second phase particles in the α-Zr lattice with peak hydrogen isotope ratios of 2H/1H = 0.018–0.082. DFT modelling confirms that the hydrogen thermodynamically favours sitting in the surrounding zirconium matrix rather than within the second phase particles. Knowledge of this trapping mechanism will inform the development of current understanding of zirconium alloy degradation through-life.

Highlights

Second phase particles (SPPs) are formed from Fe, Cr and Ni alloying elements present in Zy-2 and from Fe and Cr alloying elements in Zy-4 as these elements have very low solubility in the α-Zr lattice
Taking a ratio of the 2H− and 1H− signals effectively allows for differences in the ion signal due to differences in the grain to grain sputter rate to be cancelled out as the variation in sputter rates will be consistent between hydrogen isotopes
Both the Zy-4 and Zy-2 samples show the same localisation of 2H− to the SPPs, but lower 2H−/1H− ratios were observed in the Zy-2 sample

Summary

Introduction

Second phase particles (SPPs) are formed from Fe, Cr and Ni alloying elements present in Zy-2 and from Fe and Cr alloying elements in Zy-4 as these elements have very low solubility in the α-Zr lattice. Atom probe tomography (APT) has the potential to detect hydrogen atoms in a sample, and several studies[35,36,37,38,39] have investigated SPPs in various zirconium alloys, both in the zirconium metal and the oxide layer. None of these studies have reported hydrogen interacting with SPPs, either in the bulk of the SPP or the precipitate. The approach adopted here, combining high resolution NanoSIMS (able to directly detect and map hydrogen) with targeted computer modelling, presents a potential model for analysis of hydrogen behaviour in other alloys and safety critical components

Methods

Results

Discussion

Conclusion