The breakaway oxidation of zirconium and its alloys a review

Microstructural analysis and XPS investigation of nodular oxides formed on Zircaloy-4

“…These different stoichiometric structures of black and whitish nodular oxides are consistent with earlier results reported by Cheng and Adamson [2]. In previous studies on the mechanical properties of oxide formed on Zr and alloys, it has been also reported that there is a decrease in the plasticity when it changes from an anion-deficient and non-stoichiometric composition to a stoichiometric composition [15][16][17]. Furthermore, near stoichiometric ZrO 2 showed more cracks radiating outward than substoichiometric oxide.…”

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

“…1-4), we conclude in partial consistence with Keys et al [14] that the breakaway oxidation phenomena is associated with the change of non-stoichiometric (anion-deficient) and protective black oxide into stoichiometric and non-protective white (or grey) oxide. This change accompanies a decrease in the plasticity of the oxide layer and leads to cracks in the outer stoichiometric region and a mechanical breakdown under the growth stress originating from a high value (1.56) for the Pilling-Bedworth ratio [4]. Consequently, this resultant stoichiometric layer of oxide is separated from the underlying non-stoichiometric layer.…”

“…1d) was formed during the transition to breakaway oxidation. Based on results of the previous report [4], it seems likely that the oxide layer below the arrow is the black hypostoichiometric oxide (anion-deficient) and the upper one is the white (or grey) stoichiometric oxide. Oxygen-stabilized alpha phase was formed beneath the oxide layer in all samples because of the oxygen diffusion along the cladding thickness and excess of oxygen solubility limit in prior beta region.…”

“…This phenomenon could severely promote embrittlement of the cladding, therefore it is very important to explain precisely why this breakaway oxidation takes place during the LOCA for safety analysis. However, no mechanism has been generally accepted for the explanation of pre-to post-breakaway kinetics and its actual process is still a matter of controversy, although extensive investigations have been carried out [3][4][5][6][7][8]. Furthermore, most investigations of them were performed at a lower temperature than a real LOCA condition.…”

Corrosion characteristics of zirconium alloy with a high temperature pre-formed oxide film

“…However, the oxide layer that forms on the surface of Zry used in nuclear reactors exhibits the oxidation rate transition (from cubic to linear). Due to this rate transition, the oxide layer thickness increases linearly with time and eventually peels off [1,2]. In addition, in a failed fuel element, oxidation of the cladding inner surface by steam produces hydrogen in the fuel-cladding gap.…”