characterization of zircaloy-4 oxide layers by scanning electron microscopy

Open Access
- Author:
- Pantano, Michael Andrew
- Area of Honors:
- Nuclear Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Arthur Thompson Motta, Thesis Supervisor
Seungjin Kim, Thesis Honors Advisor - Keywords:
- zircaloy-4
cladding
nuclear
oxide
zirconium
corrosion - Abstract:
- This study aims to investigate oxide layers formed during corrosion of Zircaloy-4 samples, ranging in thickness from 1 µm to 100 µm. The aim is to establish trends between oxide thickness and the nature of the oxide/metal interface as well as the cracking patterns in the oxide. By taking scanning electron microscopy (SEM) pictures of cross-sectional oxide samples, qualitative comparisons are made between samples. Because the reaction rate is controlled by a processes taking place at the oxide-metal interface, the nature of the interface can play a significant role in corrosion kinetics. The variation of the interface with oxide thickness was examined. Furthermore, in the oxide, horizontal cracks form. The possible periodicity in the occurrence of these cracks was studied, as was the presence of oxide “veins,” which may be connected to oxide growth. Additionally, a sample of an oxide layer formed in crystal bar zirconium (as opposed to Zircaloy-4) was also studied. The oxide layer in this sample has partially flaked away, a characteristic of a non-protective oxide layer. There are still some sections of adherent oxide, the structure of which was examined and compared. In this study, the stability of the oxide formed in Zircaloy-4 was seen, especially in contrast with the unstable pure zirconium oxide. The oxide/metal interface in the Zircaloy-4 has a flat average front, with smooth, low-amplitude oscillations. Due to the volumetric expansion of the oxide, stresses form. At a critical level, the oxide begins to crack to relieve the stress. These cracks are parallel to the oxide/metal interface. After cracking, further oxidation occurs, stress builds, and another layer of cracks forms to relieve it. In Zircaloy-4, these cracks are roughly 2 µm apart. Perpendicular to the interface are oxide “veins”, paths from the interface to the surface free of cracks, that may be used to transport oxygen to the interface to cause further corrosion.