Meiyi Yao

Effect of Water Chemistry and Composition on Microstructural Evolution of Oxide on Zr Alloys

The microstructure of oxide films formed on Zircaloy-4 and Alloy No. 3, which has a composition similar to ZIRLO™, was investigated by high resolution transmission and scanning electron microscopy, and by scanning probe microscopy after corrosion tests performed at 360°C/18.6 MPa in deionized water or lithiated water with 0.01 M LiOH. The microstructural evolution of the oxide films was analyzed by comparing the microstructure at different depths in the oxide layer. The defects, consisting of vacancies and interstitials, such as points, lines, planes, and volumes, were produced during the oxide growth. Monoclinic, tetragonal, cubic, and amorphous phases were detected and their coherent relationships were identified. The characteristic of oxide with such microstructure had an internal cause, and the temperature and time were the external causes that induced the microstructural evolution during the corrosion process. The diffusion, annihilation, and condensation of vacancies and interstitials under the action of stress, temperature, and time caused stress relaxation and phase transformation. It was observed, in the middle of the oxide layer, that the vacancies absorbed by grain boundaries formed pores to weaken the bonding strength between grains. Pores formed under compressive stress lined up along the direction parallel to the compressive stress. Thus, cracks developed from the pores were parallel to the oxide/metal interface. Li+ and OH− incorporated in oxide films were adsorbed on the wall of pores or entered into vacancies to reduce the surface free energy of the zirconium oxide during exposure in lithiated water. As a result, the diffusion of vacancies and the formation of pores were enhanced, inducing the degradation of the corrosion resistance. The relationship between the corrosion resistance of zirconium alloys and the microstructural evolution of oxide films affected by water chemistry and composition is also discussed.

Study of the Initial Stage and Anisotropic Growth of Oxide Layers Formed on Zircaloy-4

An in situ investigation of the epitaxial oxide layer formed on a thin specimen heated in a transmission electron microscope was carried out. Some dot-like grains about 10 nm in size were formed on the surface of a relatively thin area. The dot-like grains are monoclinic zirconium oxide and have an orientation relationship of ( 001 ) m / / ( 0 1 ¯ 11 ) α -Zr , ( 111 ) m / / ( 1 1 ¯ 01 ) α -Zr with the α-Zr matrix. Some long strip-like grains, probably a new kind of zirconium suboxide, were formed on the surface of a relatively thick area. The strip-like grains have a bcc structure with a lattice parameter a=0.66 nm and have an orientation relationship of ( 110 ) bcc / / ( 10 1 ¯ 0 ) α -Zr , [ 1 1 ¯ 0 ] bcc / / [ 0001 ] α -Zr with the α-Zr matrix. The relationship between the thickness of oxide layers and the grain orientations of the α-Zr matrix was studied with coarse-grained Zircaloy-4 specimens through autoclave corrosion tests at 500 and 400°C in superheated steam, and at 360°C in both lithiated and deionized water for long time exposure. The results show that the anisotropic growth of oxide layers on the grain surface with different orientations is considerable. However, the relationship between the thickness of oxide layers and the grain orientations of the α-Zr matrix varies with corrosion temperature and water chemistry. The largest variation of oxide thickness developed during corrosion tests at 500°C. The thickest oxide layers were formed on those grains whose surface orientations were distributed around the planes from ( 01 1 ¯ 0 ) to ( 1 ¯ 2 1 ¯ 0 ) . The thicker oxide layers on these grains were further developed into nodular corrosion. When the specimens were corroded at 360°C in lithiated water, the thickest oxide layers formed on those grains, whose surface orientations tilted from 15 to 30° away from the (0001) plane. When the specimens were corroded at 400°C in superheated steam and at 360°C in deionized water, the difference between the thickness of oxide layers on different grain surfaces was less prominent.

Corrosion-related defects in Zircaloys: a preliminary study with slow positron beam

Corrosion-related microstructure and defects in Zircaloy-4 and N18 alloys were investigated by variable energy positron annihilation spectroscopy. The specimens were corroded in 0.01mol/L LiOH aqueous solution at 360 °C/18.6 MPa and in super heated steam at 400 °C/10.3 MPa, respectively. Defect profiles were analyzed by measuring the S parameter as a function of incident positron energy from 0.25 to 27 keV. Results indicated that Zircaloys corroded in LiOH aqueous solution contained more defects in the oxide layer than that in superheated steam, which implies that formation of defects in oxide layer may relate to the effects of Li+ ions in corrosion solution.

Effect of heat treatment on the Nb distribution and corrosion resistance of Zr-Sn-Nb-Fe zirconium alloy

Abstract After being treated in different ways, Zr-Sn-Nb-Fe alloy specimens are exposed in 0.01mol/L LiOH aqueous solution at 350°C under 16.8 MPa. The examination of microstructures and second phase particles (SPPs) of these specimens was carried out by high-resolution transmission electron microscopy (HR-TEM). The specimens treated at 800°C before the final cold rolling have a better corrosion resistance than those treated at 680°C, and the specimens treated at 500°C, after the final cold rolling, have a better corrosion resistance than those treated at 560°C. TEM examination shows that the SPPs existing in the 800°C/500°C specimen, which has the best corrosion resistance, contains a lot of Nb element, which results in the reduction of the niobium content in the α-Zr solid solution.

Effects of Prior-Deformation and Water Chemistry on Stress Corrosion Cracking of Austenitic Alloys in High Temperature Water

The interactive effects of prior-deformation and water chemistry on stress corrosion cracking of austenitic alloys in simulated nuclear power plant coolants were quantitatively investigated. Experimental results showed that increasing material yield strength tends to increase stress corrosion cracking growth rates. Increasing electrode potential tends to increase stress corrosion cracking growth rates of austenitic stainless steels. There is a maximum stress corrosion cracking growth rate for Nickel-base alloys and weld metals at electrode potentials near the Ni-NiO equilibrium line. Crack growth rate of prior-deformed austenitic alloys become less dependent on electrode potential than that of their non-deformed counterparts. The modes of prior-deformation and electrode potential affect the stress corrosion cracking path and growth kinetics. The interactive effects between prior deformation and water chemistry on stress corrosion cracking are analyzed.Copyright