The cyclic nature of corrosion of zircaloy-4 in 633 K water

Optical and electron microscopic examination of cathodic-vacuum-etched cross-sections of corrosion films on zircaloy-4, produced during 108.86-Ms (1260-day) tests in 633 K water, revealed alternate dense columnar-grained layers and narrow porous layers lying parallel to the metal surface, but no through-thickness openings except at corners. Correlation of layer structures with corrosion test data indicates that the corrosion process consists of repeated cycles of cube-root kinetics. During each cycle, a new protective layer grows at the metal-oxide interface. At a critical thickness, the layer suddenly becomes non-protective, causing transition to the initially rapid corrosion rate of a new cycle. Porous layers mark the location of the interface at the time of each transition. Variations in post-transition corrosion rates are related to variations in the cube-root rate constant, average film density, and critical layer thickness. A stress-controlled corrosion mechanism is proposed.