3D reconstruction and interconnectivity quantification of the nano-porosity in the oxide layer of corroded Zr alloys

Abstract

Nano-porosity development in thermally grown oxides plays a significant role in oxidation kinetics as nanopores may provide pathways for oxidizing species. As an example, Zr alloys are the most commonly used cladding materials in pressurized and boiling water nuclear reactors and are known to develop significant oxide nano-porosity. Corrosion results in the formation of an oxide layer containing nanopores while still being protective. It is still under debate whether the nanopores can provide fast diffusion paths for the oxidizing and hydriding species, and possibly accelerate corrosion. In the current study, we precisely quantify the nano-porosity in different regions of the oxide layer using a machine-learning-based method reported in a previous publication. In addition, the entire oxide layer depth and each pore within the layer are imaged, and a TEM-based 3D tomographic reconstruction is obtained. We further investigate the interconnectivity of the pores and the shortest path through the pores as functions of oxide depth, exposure time, and corrosion rate. Results demonstrate that interconnectivity is highest in the proximity of the W/O interface and gradually decreases towards the M/O interface. Specifically, higher temperatures, longer exposure times, and higher corrosion rates are correlated to increased interconnectivity among pores. This work provides essential evidence that pores in the near water/oxide interface region can provide paths for oxidizing species.