[en] Highlights: • 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. - Abstract: 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 H_2 evolves on transition metal sites.