The early stage of aqueous corrosion of Zr-2.5 wt% Nb and Zircaloy-2 pressure tube materials

The early stage of aqueous corrosion has been studied in Zr-2.5 wt% Nb and Zircaloy-2 pressure tube materials by immersing specimens of these alloys in heavy water for 1 day at temperatures ranging from 293 to 573 K. The deuterium and oxygen distributions were measured by nuclear reaction analysis. The oxide layer appeared to have a non-uniform thickness. It was measured to be 20 to 40 nm over the temperature range from 293 to 473 K. Above 523 K the oxide thickness was greater than 100 nm. Deuterium was present in the oxide layer at 373 K and above, whereas deuterium levels of ~ 5 mg/kg are measured in the metallic phase above 373 or 423 K depending on whether the specimen was Zircaloy-2 or Zr-2.5 wt% Nb. It is suggested that the deuterium ingress in this early stage is controlled mainly by deuterium diffusion in the oxide.