Precipitate Behavior in Zircaloy-2 Oxide Films and Its Relevance to Corrosion Resistance

Abstract

The a-c impedance measurements were made for oxide films formed in high temperature steam on Zry-2 with varying precipitate distributions. TEM and EDX analyses were also carried out on precipitates in oxide films. The electrical resistivity of oxide films tends to increase with an increase in nodular corrosion resistance, with the β-quenched samples having the highest resistivity. The resistivity tended to increase with a decrease in average precipitate size. EDX analyses showed that precipitates in oxide films dissolved into oxide matrix during the corrosion process in high temperature steam. Very fine precipitates in β-quenched Zry-2 would have disappeared early in the corrosion process. The improvement in corrosion resistance by β-quenching was attributed not to short-circuit effects of precipitates but to a change in oxide properties induced by an increase in the amount of alloying elements retained in the matrix.

Investigating the Effect of Zirconium Oxide Microstructure on Corrosion Performance: A Comparison between Neutron, Proton, and Nonirradiated Oxides

“…Nano-scale porosity is commonly observed in non-irradiated zirconium oxide films using Fresnel contrast in the TEM [10], [13], [31]- [33]. These pores are often seen to be aligned along the oxide grain boundaries, 1-5 nm in size and disconnected close to the metal/oxide interface at the pre-transition stage [10], [29], [34]- [38]. In posttransition oxides, the pores tend to become larger and interconnected providing a fast track for diffusing species.…”

The progress of SPP oxidation in zircaloy-4 and its relation to corrosion and hydrogen pickup

“…The migration of Fe ahead of the main MO into metallic Zr has been seen previously using atom probe tomography [39] [40]. The migration of Fe and Cr either ahead of the Zr oxidation front in SPPs or through ZrO2, has been seen on a number of previous occasions, with the metals collecting at SPP boundaries, in fissures in the SPPs, on the surfaces of cracks close to the SPPs or on the OW surface near the SPPs [14] [15] [25] [27] [28]. The Cr and Fe are expected to oxidise after the Zr, since their oxidation requires a higher partial pressure of oxygen, but this work has quantified their behaviour in terms of the oxide thickness.…”

“…For example, the SPPs in Zircaloy-4 are more noble than the matrix so, when SPP and matrix are in electrical contact with each other and the environment, the SPPs provide cathodic sites and remain metallic [11]. While the SPPs remain metallic and the oxide is thinner than the SPP diameter, the SPPs could act as short-circuit metallic conduction paths across the insulating oxide [12] [13] [14]. When the SPPs oxidise, the alloying elements tend to migrate out of the original SPP bounds, partitioning to islands rich in Fe and Cr.…”

Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

“…In the oxide layer of zirconium alloys, some alloying elements (Fe, Cr, Ni and Nb) precipitated in secondphase particles are known to be oxidized slower than the surrounding matrix [8][9][10][11][12]. This phenomenon has been reported as the delayed oxidation and would be a key factor in both the corrosion and hydrogen absorption mechanism as mentioned in the previous section.…”