Microstructure of Oxides on Zircaloy-4, 1.0Nb Zircaloy-4, and Zircaloy-2 Formed in 10.3-MPa Steam at 673 K

Abstract

The microstructure of ZrO2 formed on sheet materials of Zircaloy-2 (Zr2), Zircaloy-4 (Zr4), and an alloy of 1.0% Nb added to Zircaloy-4 (1Nb-Zr4) was analyzed using HRTEM (high-resolution transmission electron microscopy). The relationship between the corrosion behavior of the alloys and the microstructure is discussed. Stress-relieved sheet specimens of the three alloys were prepared and corrosion tested under static conditions in steam at 673 K and 10.3 MPa for a total of 220 days. The order of corrosion resistance in 673-K steam was Zr2, 1Nb-Zr4, and Zr4. Several transitions were observed in the corrosion kinetic curve of 1Nb-Zr4 and Zr2. However, only the first transition was observed in the curve of Zr4. Oxide structure in the pre-transition region on Zr4 was analyzed to be in the following order from the outside surface: columnar m-ZrO2, t-ZrO2 layer, substoichiometric Zr oxide layer, and α-Zr matrix. The t-ZrO2 layer was approximately 50 to 80 nm thick, and the substoichiometric Zr oxide layer was approximately 100 to 200 nm. These layers were absent in the microstructure of the oxide in the post-transition region. The substoichiometric Zr oxide layer consisted of m-ZrO2 grains that were less than 10 nm in diameter and some as yet unidentified grains that had lattice parameters similar to distorted and significantly oriented α-Zr. However, the t-ZrO2 layered structure and the substoichiometric Zr oxide layer structure were observed in the post-transition oxides on Zr2 and 1Nb-Zr4. It was also observed that transformation of columnar grains to fine equiaxed grains had occurred near the lateral cracks and the incorporated intermetallic precipitates in post-transition oxides. It is implied from these results that the t-ZrO2 layer and the substoichiometric Zr oxide layer structures play an important role as a barrier layer in controlling the occurrence of kinetic transitions.

The influence of alloying elements on the corrosion of Zr alloys

“…this process is thought to be associated with the periodic transitions observed in the oxidation rate during corrosion [12]. Since it is considered to be key to corrosion behaviour of Zr-alloys, the simulation portion of this work will focus primarily on the defect behaviour in t-ZrO 2 .…”

Ab initio study of hydrogen diffusion in zirconium oxide

“…There has been considerable research about the oxide layer including its microstructure [1][2][3], crystallographic structure [4,5], stress distribution [6][7][8], and hydrogen diffusivity [9][10][11][12]. Transmission electron microscopy observations revealed that oxide layers near metal/oxide interfaces include tetragonal zirconium oxide [4,5], which is metastable at the cladding temperature and pressure.…”

“…Transmission electron microscopy observations revealed that oxide layers near metal/oxide interfaces include tetragonal zirconium oxide [4,5], which is metastable at the cladding temperature and pressure. This implies that the oxide layer is subjected to a high compressive stress.…”

Influence of water vapour on the zircaloy-4 oxidation at high temperature - Stress determination by in situ X-Ray diffraction

“…It has been mentioned that the transition from a protective to a more porous oxide could be associated to a phase transitions from tetragonal (t-ZrO 2 ) to monoclinic (m-ZrO 2 ), close to the metal/oxide interface [8]. From the literature, it has been experimentally demonstrated by X-ray diffraction (XRD) [9][10][11][12][13], Raman spectroscopy [14][15][16][17] or transmission electron microscopy [18,19] that t-ZrO 2 can be detected at the metal/oxide interface in pre-transition scales while it is hardly detected in the scale during the post-transition regime. Various factors such as a deviation from stoichiometry, nanometric grain size, chemical doping, and compressive stresses can explain the stabilization of the t-ZrO 2 for temperatures below its thermodynamic stability domain.…”

“…It has been confirmed by thermodynamic calculations that compressive stresses can stabilize the t-ZrO 2 form of zirconia [20], due to the high Pilling-Bedworth ratio of the Zr/ZrO 2 system. It is generally observed that zirconia scales exhibiting the highest compressive stresses and thus a better stability of the t-ZrO 2 compound exhibit a better corrosion resistance [10][11][12]18]. Most of the authors consider that the t-ZrO 2 to m-ZrO 2 transformation have a detrimental effect on the corrosion resistance.…”