Zirconium alloy corrosion proceeds in a series of cyclical steps in which passivating oxide grows into the metal at a decreasing rate until critical thickness is reached. At the point where critical thickness is reached, the protective property in the oxide is suddenly lost and a new cycle of oxide growth begins. The loss of passivation at the end of each cycle is commonly thought to result from the formation of cracks which appear as each new layer of oxide reaches a thickness of approximately 2 microns, allowing ingress of the environment to the metal/oxide interface. Microstructural characterization shows parallel cracks in the oxide that do appear in a cyclic pattern, but concentrated effort has failed to identify any connected pathway through the thickness of the film. This lack of connectivity has motivated a new hypothesis that ingress of the environment may actually result from the development of connected porosity in the film, most likely associated with columnar grain boundaries.
The objective of this study is to lay the groundwork toward a better understanding of porosity development in zirconium oxides as a function of corrosion time and temperature, and how this porosity development may affect corrosion and hydrogen pickup kinetics. Proper identification and understanding of the microstructural mechanism of film breakdown is central to the development of a fundamentally based corrosion model.
Motta, A., A. Couet, and R.J. Comstock, Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding. Annual Review of Materials Research, 2015. 45(1): p. 311-343.
Hu, J., et al. Understanding corrosion and hydrogen pickup of zirconium fuel cladding alloys: The role of oxide microstructure, porosity, suboxides, and second-phase particles. in 18th International Symposium on Zirconium in the Nuclear Industry, May 15, 2016 – May 19, 2016. 2018. Hilton Head, SC, United states: ASTM International.