Pre-transition Corrosion Behavior of SZA-4 and ZIRLO Alloys in Dissolved Oxygen Aqueous Condition at 360℃

doi:10.3901/JME.2019.08.088

Abstract

Abstract: Development of high-performance zirconium alloys with improved corrosion resistant is vital to meet the demands of higher fuel duty, increased cycle length and more aggressive water chemistries, such as potential dissolved oxygen (DO) in some advanced boil water reactor's (ABWR) and small module reactor's (SMR) environment. It is therefore necessary to consider the corrosion behavior of zirconium alloys in DO condition. In the present study, the corrosion behavior of a new-designed SZA-4 alloy (Zr-0.85n-0.25Nb-0.35Fe-0.1Cr-0.05Ge, wt.%) (China) and a reference commercial ZIRLO alloy (Zr-1.0Nb-1.0Sn-0.1Fe) (America) was estimated using an autoclave loop in 360oC/20.0 MPa pure water with approximately 2.0 mg/L DO. TEM was employed to characterize the microstructure of the Zr metal and the oxide film and EDS to give corresponding composition analysis. The results show that, the partial-recrystallized annealing SZA-4 alloy mainly consists of large completely recrystallized grains and small recovered equiaxed grains, with dominant Nb-rich Zr(Fe,Cr)₂ phase and a small amount of Zr₃Fe phase. The stress-relieved annealing ZIRLO alloy is composed of uniformly distributed short-lath grains, with dispersed Nb-containing β-Nb and Zr(Nb,Fe)₂ phase. The weight gain of SZA-4 alloy is obviously lower than that of ZIRLO alloy, and the discrepancy in weight gain increases with prolonged corroded time, which suggests the SZA-4 alloy has better corrosion resistance. After corroded for 30 days, the oxide of both alloys are consists of loose quiaxed grains in outer layer and dense columnar grains in inner layer, which indicates the corrosion is at its pre-transition stage. The oxide of SZA-4 alloy (1.0-1.2μm) was significantly thinner than the ZIRLO alloy (1.3-2.0μm), and contains less transverse cracks. The presence of transverse cracks may be a result of the rapid release of stress in the oxide film during sample preparation. The second phase precipitates (SPPs) in SZA-4 and ZIRLO alloys was not sufficiently oxidized and retained into the oxide film at a certain distance from the oxide/metal boundary. In SZA-4 alloy, Cr tends to "bind" Fe into Zr(Fe,Cr)₂ phase for in-situ oxidation, while in ZIRLO alloy, Fe is readily to diffuse into the surrounding oxide from the Zr(Nb,Fe)₂ phase on its onset of oxidation. The different Fe and Nb content in solid solution is more possibly responsible for the difference in corrosion weight gain for the two Zr alloys at the early stage of corrosion.

Key words: corrosion, dissolved oxygen, microstructure, oxide film, zirconium alloy

CLC Number:

TB37
TL341

LIU Qingdong, ZHANG Hao, ZENG Qifeng, LU Junqiang, LI Cong, ZHANG Lefu. Pre-transition Corrosion Behavior of SZA-4 and ZIRLO Alloys in Dissolved Oxygen Aqueous Condition at 360℃[J]. Journal of Mechanical Engineering, 2019, 55(8): 88-96.

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