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.

No abstract

Capillary condensation measurements of the pore size distribution, transmission electron microscopy using ion micromilling to thin the oxide, and a–c impedance measurements in aqueous electrolytes have been used to obtain information on the porosity and grain structure of post-transition oxide films grown in oxygen at 600°C on Zircaloy-2. The results indicate that the films contain important amounts of very fine porosity which is interconnected to the exterior surface. The impedance measurements suggest the presence of a nonporous sublayer near the oxide-metal interface. The oxide has a fine columnar grain structure at some distance from the metal and an extremely fine structure or substructure near the oxide-metal interface.

No abstract