Microscopic and electrochemical impedance spectroscopy analyses of Zircaloy oxide films formed in highly concentrated L1OH solution

Corrosion layers of Zircaloy-4 were formed in highly concentrated (HC) and lowly concentrated (LC) LiOH solution and, for comparison, in steam and pure water. These oxide layers were characterized by different methods. SEM images display a rather homogeneous oxide for specimens corroded in HC LiOH solution and steam, whereas the other specimens reveal extended lateral pores in the oxide. TEM images of specimens corroded in HC LiOH solution show a porous outer oxide composed of large equiaxed grains and a narrow band of rather dense oxide close to the metal/oxide interface. The specimens corroded in steam and water are composed of dense columnar oxide grains. A SIMS depth profile through the oxide of a specimen corroded in HC LiOH solution displays a rapid decrease of lithium concentration before the metal/oxide interface is reached by the primary ions. EIS and anodization studies in aqueous electrolyte demonstrate that the specimens are penetrated by the electrolyte. For specimens corroded in HC and LC LiOH solutions the rapid penetration is stopped at a very thin compact oxide layer close to the metal/oxide interface revealing an oxide which is composed of two layers. At higher temperatures the electrolyte penetrates the compact layer and this process is reversible. The specimens corroded in steam and water reveal complex dissipative characteristics in their EIS spectra whereas a multiple layer structure of the oxide cannot be deduced. Nevertheless, effective dielectric constants can be evaluated and all elements of the designed equivalent electrical circuits can be related to specific aspects of microstructure. Conclusions are drawn to a better understanding of Zircaloy corrosion.