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.