Barrier oxide chemistry and hydrogen pick-up mechanisms in zirconium alloys

• The evolution of secondary phase chemistry in the oxide scale during corrosion is investigated for Zircaloy-2 and Zr-2.5Nb.
• Oxide grain boundaries are shown to be transport paths for hydrogen pick-up through hydroxylation.
• Fe and Ni are shown to decorate oxide grain boundaries.
• Porosity is shown to be present along oxide grain boundaries.
• Hydrogen pick-up is proposed to be mitigated by cathodic reactions at transition metal sites in oxide grain boundaries.

The chemistry of barrier oxide layers formed on zirconium alloys was investigated using atom probe tomography (APT). Grain boundary segregation of Fe and Ni was observed. The Zr(Fe, Cr)2 particles maintain a constant Zr:Fe:Cr ratio through the oxide until they reach an oxygen content of approximately 50 at%, when Fe depletion occurs. Enrichment of hydrogen along oxide crystallite boundaries was observed, which is interpreted as a sign of ingress being localized to grain boundaries. TEM revealed porosity networks around particles and oxide grain boundaries. It is proposed that pores are local reduction sites where H2 evolves on transition metal sites.