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Study of Anisotropic Oxidation for Zirconium Alloys

Author: SunGuoCheng
Tutor: ZhouBangXin
School: Shanghai University
Course: Materials Science
Keywords: zirconium alloys texture anisotropic oxidation corrosion resistance alloy element niobium
CLC: TL341
Type: Master's thesis
Year: 2012
Downloads: 1
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Abstract


Zirconium alloys being important structural materials in water-cooled nuclearpower reactors have been used for fuel cladding, due to their combination of lowthermal neutron capture cross section, adequate mechanical properties and goodcorrosion resistance at high temperature. In order to increasing the economy ofnuclear power plants, high fuel duties with high burn-up and extended fuel cycleoperation are required, meanwhile, zirconium alloys with superior properties havebeen developed. Zirconium alloys of a hexagonal close-packed crystal structure haveprominent anisotropic characteristic and a strong texture is produced in sheet ortubular materials during the fabrication process. The anisotropic characteristic isbound to be reflected on the corrosion behavior and the texture will influence thecorrosion resistance of zirconium alloys. Zr-4, N18and ZIRLO zirconium alloys withdifferent contents of Nb were adopted for the experimental materials in present work.All the plate specimens of zirconium alloys with2mm in thickness have a similartexture. Corrosion tests were carried out in static autoclaves in lithiated water with0.01mol/L LiOH at360℃/18.6MPa, in super heated steam at400℃/10.3MPa and500℃/10.3MPa. Based on the oxides thickness on the rolled surface (SN), thesurface (SR) perpendicular to rolling direction and the surface (ST) perpendicular totransversal direction as a function of exposure time for specimens and the differentmicrostructure of oxides formed on different surfaces, the anisotropic oxidation ofzirconium alloys with different contents of Nb were investigated and the effect oftexture and chemical composition on corrosion resistance of zirconium alloys werealso discussed. The main experimental results and conclusions are as follows.1) The Zr-4specimens showed evident anisotropic oxidation characteristiccorroded in lithiated water with0.01mol/L LiOH at360℃/18.6MPa. The growthrate of oxide formed on SNsurface with maxium (0001) texture factors markedlyincreased after90~100days exposure showing a worse corrosion resistance, but thegrowth rate of oxides formed on SRand STsurfaces with minimum (0001) texturefactors showed no variation up to280days exposure. The corrosion resistance of SR and STsurfaces is much better than that of SNsurface.2) For N18and ZIRLO specimens corroded in lithiated water with0.01mol/LLiOH at360℃/18.6MPa, the anisotropic growth of oxide layers is restrained by theaddition of Nb. The growth rate of oxides formed on SN, SRand STsurfaces withdifferent (0001) texture factors is the same, therefore the corrosion resistance of SNsurface was improved. The corrosion resistance of tubular or sheet specimens for N18and ZIRLO alloys is superior to that of Zr-4corroded in lithiated water at360℃/18.6MPa, because the oxide layers grow mainly on SNof the specimens. If making acomparison among Zr-4, N18and ZIRLO specimens about the growth rate of oxidelayers only on SRand STsurfaces, it is shown that the growth rate of oxide layersincreases with the increase of Nb content in these alloys. However, the growth rate ofoxide layers for N18specimen is close to that of Zr-4specimen.3) None of the Zr-4, N18and ZIRLO specimens showed evident anisotropicoxidation characteristics corroded in super heated steam at400℃/10.3MPa. Thegrowth rate of oxides formed on SN, SRand STsurfaces with different (0001) texturefactors are the same. The growth rate of oxide layers formed on these three differentsurfaces increases with the increase of Nb content in these alloys. However, thegrowth rate of oxide layers for N18specimen is close to that of Zr-4specimen.4) The Zr-4specimens showed evident anisotropic oxidation characteristicscorroded in super heated steam at500℃/10.3MPa. As to Zr-4specimens heattreated in present work, there is no appearance of nodular corrosion on SNsurfacewith maxium (0001) texture factors, exihibiting superior nodular corrosion resistance,while nodular corrosion easily occurred on SRand STsurfaces, showing poor nodularcorrosion resistance.5) For N18and ZIRLO specimens corroded in super heated steam at500℃/10.3MPa, the anisotropic growth of oxide layer is restrained by the addition of Nb.There is no occurrence of nodular corrosion on SN, SRand STsurfaces with different(0001) texture factors. The oxide layers formed on these three surfaces showedsimilar growth behavior, while the growth rate of oxide layers increases with theincrease of Nb content in these alloys. 6) From a point of view for improving corrosion resistance, it is not very suitablefor adding Nb of1%(in mass fraction) in Zr-Sn-Nb alloys for commercial application.The addition of Nb should not be too high.

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CLC: > Industrial Technology > Nuclear technology > Engineering of Nuclear Reactors > Reactor materials and their properties > Structural materials
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