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Title:Response of chromium and chromium-aluminum coatings on zircaloy-2 to high temperature steam
Author(s):Zhong, Weicheng
Director of Research:Heuser, Brent J.
Doctoral Committee Chair(s):Heuser, Brent J.
Doctoral Committee Member(s):Stubbins, James F.; Zhang, Yang; Krogstad, Jessica A.
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
High Temperature Oxidation
Microstructure characterization
Accident Tolerant Fuel
Nuclear Structural Materials
Abstract:High temperature steam exposure leads to rapid oxidation of Zr based cladding, which will potentially lead to cladding failure. Our approach is to apply CrAl or Cr coatings on Zircaloy by magnetron sputtering to improve the oxidation resistance of cladding materials. The goal of this research is to characterize the oxidation kinetics and microstructure of CrAl and Cr coatings on Zircaloy-2 in high temperature steam (HTS) environment, and to provide information regarding its functionality during off-normal transient in light water reactors. Pure Cr coating and CrAl coatings with various compositions were exposed to 700C steam environment for up to 20 hours. Weight gain of coated Zircaloy was significantly reduced by two to three orders of magnitude by the CrAl and Cr coatings. Oxidation of Zircaloy substrate was inhibited by the 1um coatings for over 20 hour. Composition of coatings has a significant effect on the oxide formation. CrAl coatings with over 43 at % Al concentration developed a continuous layer of γ-Al2O3, and demonstrated lower weight gain. CrAl coatings with below 33 at % Al concentration formed a outer Cr2O3 scale with inner Al2O3 morphology. Pure Cr coating developed a layer of Cr2O3, and had a higher weight gain than CrAl coatings. Oxidation kinetics was quantified on two of the coatings, 42/58 CrAl and 81/19 CrAl, which represented different oxide structure. The Al2O3 growth on 57/43 CrAl was fitted to two different oxidation kinetics with similar confidence. The Al2O3 growth kinetics up to 20 hours can be described by either power-law oxidation kinetics thickness = 25 x time0.27 or direct logarithmic kinetics thickness = 26 x log(time) + 21.4 with the thickness in the unit of nanometer and time in the unit of hour. The Cr2O3 growth on 81/19 CrAl was quantified by the parabolic kinetics as thickness = 77.6 x time0.49. Coating constituent elements diffused to the substrate and formed intermetallic phases with the Zircaloy substrate. The amount of coating constituents for the diffusion to Zircaloy substrate was compared to that for the oxide formation. For Al rich coatings (42/58 CrAl and 57/43 CrAl coatings), greater amount of Al diffused to Zircaloy substrate than to the oxide formation. However, for Cr rich coatings (67/33 CrAl, 81/19 CrAl and Cr coatings), greater amount of Cr diffused to oxide formation than to Zircaloy substrate. The diffusion of coating constituents to Zircaloy substrate formed intermetallic phases, and the composition and the thickness of the intermetallic phase layers depended on the coating composition. Thicker layers of intermetallic phases developed on the coatings with higher Al composition. The intermetallic phases included Fe and Ni, indicating the dissolution of second phase particles (SPPs) during HTS exposure. The stability of intermetallic SPPs in coated Zircaloy-2 was studied in 700C steam environment. Hydrogen generated from the steam oxidation of uncoated Zr were absorbed and formed δ-hydrides in Zircaloy matrix. Synchrotron XRD demonstrated that longer exposure times increased hydride peak intensity and decreased intermetallic SPPs peak intensity. A concentration of 1000 wppm hydrogen in Zircaloy was estimated using synchrotron XRD after 20 hours exposure in 700C steam environment. Cross-section SEM analysis verified the intermetallic SPPs volume fraction reduction. The volume fraction of intermetallic SPPs was 1.7% in as-received Zircaloy, and it dropped to 1.4% after 5 hours exposure, and to 0.4% after 20 hours exposure. The size distribution of intermetallic SPPs was characterized and larger particles appeared to dissolve at longer exposure. A correlation between the hydrogen concentration and the volume fraction of intermetallic SPPs at 700C steam environment was found, with the volume fraction of SPPs decreasing as hydrogen concentration increases, which could be attributed to the strain from the hydrogen uptake into Zircaloy. Oxidation behavior of CrAl or Cr coated Zircaloy was also examined after 1100C steam exposure for two hours. Coatings with higher Cr composition lead to lower weight gain of Zircaloy. We believed that the inward diffusion of Al perturbed the ZrO2 formation, and exacerbate the oxidation behavior. This lead to thicker ZrO2 formation.
Issue Date:2018-11-07
Rights Information:Copyright 2018 Weicheng Zhong
Date Available in IDEALS:2019-02-07
Date Deposited:2018-12

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