Microstructural characterization of ZrO2 layers formed during the transition to breakaway oxidation

Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

“…Oxide morphology appeared to be linked to undulation of the metal-oxide interface as well as oxide damage: oxide crystallites are shorter and wider with a larger density of micro-pores in the spalled sublayer and in delayed oxidation regions of the adherent sublayer of postbreakaway oxides than in pre-breakaway oxide layers or in the advanced oxidation regions of the adherent sublayer of post-breakaway oxides. These observations are consistent with those made in [40] for Zircaloy-4 oxidized at 1000°C, showing, on average, larger grains, better defined and less tortuous grain boundaries and more numerous micro-pores in post-breakaway oxide layers than in pre-breakaway oxide layers. With larger grains and flatter grain boundaries, the grain boundary surface area and therefore the interface energy are reduced and the overall energy is minimized.…”

“…Indeed, in those cases, the oxide is dense, adherent and protective with respect to hydrogen pickup. However, in some conditions, at temperatures between 600 and 1050°C typically, an increase of the oxidation rate and a hydrogen pickup is sometimes observed after an incubation period [3][4] [5] [9][10] [11] [12] [13] [14] [15] [16] [17] [18][19]. This transition is commonly called breakaway oxidation.…”

The use of micro-Raman imaging to measure 18 O tracer distribution in thermally grown zirconia scales

“…The m-ZrO 2 phase has a rather well defined Raman spectrum and is always the main crystallographic phase in zirconia TGOs for temperatures relevant to corrosion in normal conditions [4,6,[9][10][11][12][13][14][31][32][33][34][35] as well as for oxidation at higher temperatures [7,8,36]. As it is the case for tetragonal or even cubic zirconia [29], 16 O substitution by the higher mass isotope 18 O induces strong downshifts of the Raman modes involving O-O or Zr-O vibrations [37].…”

Parallel mechanism of growth of the oxide and α-Zr(O) layers on Zircaloy-4 oxidized in steam at high temperatures

“…A recent study using HT XRD analysis demonstrated that this phase transformation is not the main cause of the breakaway oxidation [18]. Other hypotheses suggest that the stress relaxation in the oxide after a certain incubation time [19,20] and the formation of an interconnected network of micropores can cause a direct access of the oxidizing gas to the metal/oxide interface [21,22].…”