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Effect of Bi Addition on the Corrosion Resistance of Zr-1Nb Alloy

Author: ZhuLi
Tutor: ChenWenJue;YaoMeiYi
School: Shanghai University
Course: Materials Science
Keywords: Zr-Nb alloy Bi oxidation of SPPs corrosion resistance microstructure
CLC: TL341
Type: Master's thesis
Year: 2013
Downloads: 1
Quote: 0
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Abstract


Zirconium alloys was used as fuel cladding material in PWRs because of thelow thermal neutron absorption cross-section, good corrosion resistance, andadequate mechanical properties. In order to meet the demand for higher burn-up andlonger refueling period in reactor, great efforts have been made to improve thecorrosion resistance of zirconium alloys during the past several decades. Alloying isone of the main methods to improve effectively the corrosion resistance of zirconiumalloys. Bi can be considered as an alloying element because of the low thermalneutron absorption cross-section(0.034×10-24cm2), high solid solution content inα-Zr and meet the vacancy diffusion theory proposed by Wagner. In this study,Zr-1Nb alloy was adopted to prepare the zirconium alloys containing Bi of0.05%~0.5%in mass fraction. These alloys were denoted as Zr-1Nb-xBi. The effectof Bi addition on the microstructure and corrosion resistance of Zr-1Nb alloys wasinvestigated by TEM and autoclave corrosion tests in superheated steam at400℃/10.3MPa and deionized water at360℃/18.6MPa; the corrosion behavior ofZr-1Nb-xBi alloys in lithiated water at360℃/18.6MPa was also investigated byshortening every corrosion period. At the same time, the microstructure of oxide filmformed on the zirconium alloys with Bi was also investigated by HRSEM andHRTEM. The main experimental results and conclusions are as follows.（1）There are two types of second phase particles (SPPs) in the Zr-1Nb-xBialloys: one is ZrNbFe type with100nm in size and the other is β-Nb type with50nm in size. No Bi-containing SPP was detected. This illustrates that the maximiumsolid solution content of Bi in α-Zr matrix is not less than0.3%.（2）The addition of0.05%~0.3%Bi exhibits a beneficial effect on thecorrosion resistance of Zr-1Nb alloy and the corrosion resistance is markedlyimproved with the increase of Bi content whether in deionized water at360℃/18.6MPa, in superheated steam at400℃/10.3MPa and in lithiated water at360℃/18.6MPa. （3）The oxide layers of Zr-1Nb-xBi alloys, corroded in three different waterchemistries, consist of equiaxed and columnar ZrO2grains. The addition of Bi slowsdown the formation process of pores and micro-cracks. The morphologies of crosssection and inner surface also become smoother with the increase of Bi content.（4）The oxidation rate and oxidation products of-Nb SPPs are different indionized water and lithiated water at360℃/18.6MPa. The oxidation rate is faster inthe former one than the latter one. Only T-NbO2was detected after-Nb wasoxidized in deionized water, while T-NbO2and M-Nb2O5were detected after-Nbwas oxidized in lithiated water. The above results can reasonably explain the reasonthat the corrosion resistance of Zr-Nb alloys in lithiated water with0.01M LiOH at360℃/18.6MPa is obviously inferior to Zr-Sn and Zr-Sn-Nb alloys.（5）By comparing the oxidation behavior of β-Nb in Zr-Nb alloys corroded indionized water at360℃/18.6MPa, which reported by other researchers, it isindicated that the oxidation of β-Nb in Zr-1Nb-0.3Bi alloy is much slower. Thisillustrates that the solid solution of Bi in-Zr matrix can delay the oxidation processof β-Nb and the microstructural evolution process of oxide film to influence thestress in the oxide film, thereby improving the corrosion resistance.（6）The corrosion resistance of Zr-1Nb alloys in lithiated water with0.01MLiOH at360℃/18.6MPa can be improved by shortening every autoclave corrosionperiod, which is maybe related to the stress in the oxide film. This need be furtherstudied.

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