A Mechanism for the Effect of Heat‐Treatment on the Accelerated Corrosion of Zircaloy‐4 in High Temperature, High Pressure Steam

Abstract

Potential differences which develop across growing oxide films and effects of applied electric fields on oxide growth have been measured on specimens of Zircaloy-4 corroding in high temperature, high pressure steam. From these results it is concluded that the accelerated corrosion process observed in this environment is associated with the development of metal-negative potential differences across the oxide (analogous to potential differences associated with cathodic polarization in aqueous corrosion). Initiation of this form of attack is inhibited on suitably heat-treated material because the relatively high effective electronic conductivity of the oxide film growing on this material shifts these potential differences toward the metal-positive direction. The effect of alloy heat-treatment on the oxide conduction characteristics is explained in terms of the microstructure of the metal, and it is suggested that hydrogen absorption into the oxide under metal-negative potential differences leads to initiation of the accelerated attack.

Localized oxidation of zirconium alloys in high temperature and pressure oxidizing environments of nuclear reactors

“…This may also be one of the main reasons for the non‐existence of a standard laboratory test for assessing nodular corrosion susceptibility of Zr alloys. This paper discusses these complex phenomena of Zr oxidation and the role of oxidants in the light of our results on nodular corrosion (parametric studies on the currently used laboratory tests and morphological investigation), long term oxidation kinetics of Zircaloy‐2 and Zircaloy‐4 and our earlier results on local enhanced oxidation of Zr–Nb alloys and similar observations in the literature .…”

“…In addition, different alloys respond differently to a given reactor environment . Localized enhanced oxidation has been observed in different Zr alloys under different water chemistry conditions and after varied exposure periods . These include nodular corrosion and shadow corrosion phenomena in Zircaloys (Zr–Sn alloys with Fe, Cr, and Ni) prevalent in boiling water reactors (BWRs) and local enhanced oxidation in two‐phase Zr–Nb alloys .…”

“…These phenomena have been observed both in‐pile and out of pile . However, the mechanism of nodular and shadow corrosion of Zircaloys in reactors in a multi oxidant scenario has been hypothesized to be different than that occurring out of reactor. An obvious demonstration of the above statement may lie in the context of accelerated laboratory tests available for assessing nodular corrosion susceptibility of Zircaloys.…”

Effect of Alloying Elements on Hydrogen Pickup in Zirconium Alloys

“…Clearly, if electron transport controls the oxidation kinetics, the precipitates embedded into the growing oxide and the extrinsic compensating defects due to aliovalent cations in solid solution could have a significant effect on the corrosion kinetics by affecting the oxide electronic conductivity [35][36][37].…”

“…Metallic precipitates would likely enhance the electronic conductivity of the oxide layer, which would in turn reduce the hydrogen pickup relative to pure Zr. It is believed that a material with a homogeneous distribution of fine precipitates has a higher oxide electronic conductivity than pure Zr [34,37].…”