The effect of Sn–VO defect clustering on Zr alloy corrosion

Abstract

Density functional theory simulations were used to study Sn defect clusters in the oxide layer of Zr-alloys. Clustering was shown to play a key role in the accommodation of Sn in ZrO 2 , with the {Sn Zr :V O } × bound defect cluster dominant at all oxygen partial pressures below 10-20 atm, above which Sn Zr × is preferred. {Sn Zr :V O } × is predicted to increase the tetragonal phase fraction in the oxide layer, due to the elevated oxygen vacancy concentration. As corrosion progresses, the transition to Sn Zr × , and resultant destabilisation of the tetragonal phase, is proposed as a possible explanation for the early first transition observed in Sn-containing Zr-Nb alloys.

Modelling and experimental analysis of the effect of solute iron in thermally grown Zircaloy-4 oxides

“…The effects of iron additions on the types of defects and their concentrations in ZrO 2 are investigated by carrying out density functional theory (DFT) simulations of iron defects in isolation and in clusters with other iron and intrinsic defects. The need to consider the effect of bound defects on oxidation state, defect populations and therefore overall oxide behaviour was shown by Bell et al for both tin and niobium [38,39]. It was shown for tin that including paired defects significantly increased the oxygen partial pressure at which the 2+ state was stable [39].…”

“…The need to consider the effect of bound defects on oxidation state, defect populations and therefore overall oxide behaviour was shown by Bell et al for both tin and niobium [38,39]. It was shown for tin that including paired defects significantly increased the oxygen partial pressure at which the 2+ state was stable [39]. Paired defects in Zr-Nb alloys showed why niobium can exist in tetragonal ZrO 2 below the 5+ oxidation state and explained why X-ray absorption near-edge structure (XANES) measurements consistently showed 2+, 3+ and 4+ oxidation states [38].…”

Unraveling different influences of the fraction of the tetragonal phase in oxide films on the corrosion resistance of Zr alloys from the phase transition mechanism

“…Using the results of Brouwer diagrams, the changes of the corrosion resistance of Zr alloys were extensively discussed. 34–38…”

“…For example, the binding of solid solution Nb or Sn with atomic vacancies will change the valence state of solid solution Nb or Sn, respectively. 34…”

“…For example, the binding of solid solution Nb or Sn with atomic vacancies will change the valence state of solid solution Nb or Sn, respectively. 34 The phase stability of ZrO 2 should be related to the solid solution state of the alloying element that is embodied in the ZrO 2 instead of precipitates. The valence state of Nb and Sn in different chemical environments in oxide films is complicated, and the experimental results of the valence state include many forms of alloying elements, such as the solid solution state and different precipitates.…”