This study aims to precise and to quantify the link between the development of the metal/oxide roughness, the localization of cracks in the oxide layer, and the transition in Zircaloy-4 and Zr-1Nb-O corrosion kinetics. The morphological characteristics of oxide scales formed in autoclave and thermobalance at 415 degreesC were obtained through quantitative analysis of cross-sectional SEM images. During pre-transition stage, the metal/oxide interface roughness is observed to increase in accordance with the stress configuration iii the oxide layer. Cracks are formed at the metal/oxide interface, preferentially on the delayed parts of the oxidation front, before any kinetic transition. It is proposed that their connection to the outer porous part of the oxide layer is the reason for the kinetic transition. The differences between the morphological characteristics of the Zy4 and Zr-1Nb-O oxide scales are in accordance with their respective corrosion kinetics.